Anaerobic Digestion Improvement and Evaluation

1999;159(2):145-163. DOI: 10.1016/

[41] El-Mashad HM. Kinetics of methane production from the codigestion of switchgrass and Spirulina platensis algae. Bioresource Technology. 2013;

S0025-5564(99)00020-6

Anaerobic Digestion

132:305-312. DOI: 10.1016/j.

biortech.2012.12.183

88

Chapter 5

Abstract

technologies

91

1. Introduction

Production

Biomass Pretreatment for

Tamilarasan Karuppiah and Vimala Ebenezer Azariah

Biomass is a renewable energy source developed from living or recently living

plant and animal materials, which can be used as fuel. The main components present in biomass are polymers such as carbohydrate, protein, cellulose, lignin and fat. Biogas is produced when the biomass is anaerobically degraded by microorganisms. The process of anaerobic digestion (AD) takes place in four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The hydrolysis step is rate limiting due to the presence of complex polymers in biomass. Pretreatment is a process in which the biomass is made ready for microbial attack. This pretreatment can be physical operations such as communition, irradiation etc.; chemical treatment with alkali, acids, wet oxidation etc.; biological pretreatment, by fungi or enzymes; or a combination of these processes. During the pretreatment process, the

Keywords: biomass, biogas production, anaerobic digestion, pretreatment,

Rapidly increasing energy demands worldwide has resulted in tremendous depletion of fossil fuel resources. This makes it necessary to find alternative energy sources which have a minimum impact on the environment. In this context, biogas is one of the sustainable energy sources that can be produced from many types of biomass including waste. AD technology is one of the most promising technologies, having the potential to convert various biomass into methane-rich biogas, a carbonneutral alternative to fossil fuels. In addition, AD technology has a number of benefits including solids reduction, decreased odor, reduced greenhouse gas emissions, and increased income from non-market benefits compared to conventional waste treatment systems [1, 2]. In Germany, which is the leading country in this field, greater than 50% of the biogas potential results from energy crops treated in over 7000 biogas plants [3]. AD has wide application in sludge stabilization due to

AD system utilizes anaerobic microorganisms to convert the organic matter in

the biomass, into biogas in an oxygen free environment. Biogas is the main byproduct of AD and contains about 60% methane by volume. Digestate is produced as a byproduct, which after an appropriate treatment can have agricultural

compact structure of biomass is disrupted and exposed which.

its low cost, energy recovery and minimized biosolids production.

Enhancement of Biogas

### Chapter 5

## Biomass Pretreatment for Enhancement of Biogas Production

Tamilarasan Karuppiah and Vimala Ebenezer Azariah

### Abstract

Biomass is a renewable energy source developed from living or recently living plant and animal materials, which can be used as fuel. The main components present in biomass are polymers such as carbohydrate, protein, cellulose, lignin and fat. Biogas is produced when the biomass is anaerobically degraded by microorganisms. The process of anaerobic digestion (AD) takes place in four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The hydrolysis step is rate limiting due to the presence of complex polymers in biomass. Pretreatment is a process in which the biomass is made ready for microbial attack. This pretreatment can be physical operations such as communition, irradiation etc.; chemical treatment with alkali, acids, wet oxidation etc.; biological pretreatment, by fungi or enzymes; or a combination of these processes. During the pretreatment process, the compact structure of biomass is disrupted and exposed which.

Keywords: biomass, biogas production, anaerobic digestion, pretreatment, technologies

### 1. Introduction

Rapidly increasing energy demands worldwide has resulted in tremendous depletion of fossil fuel resources. This makes it necessary to find alternative energy sources which have a minimum impact on the environment. In this context, biogas is one of the sustainable energy sources that can be produced from many types of biomass including waste. AD technology is one of the most promising technologies, having the potential to convert various biomass into methane-rich biogas, a carbonneutral alternative to fossil fuels. In addition, AD technology has a number of benefits including solids reduction, decreased odor, reduced greenhouse gas emissions, and increased income from non-market benefits compared to conventional waste treatment systems [1, 2]. In Germany, which is the leading country in this field, greater than 50% of the biogas potential results from energy crops treated in over 7000 biogas plants [3]. AD has wide application in sludge stabilization due to its low cost, energy recovery and minimized biosolids production.

AD system utilizes anaerobic microorganisms to convert the organic matter in the biomass, into biogas in an oxygen free environment. Biogas is the main byproduct of AD and contains about 60% methane by volume. Digestate is produced as a byproduct, which after an appropriate treatment can have agricultural applications as fertilizer [4]. It reduces organic matter to more stable solids by complex biochemical reactions. There are three consecutive steps of biological process in AD. The first step involves hydrolysis of complex organic matter into simpler compounds. The second step is the acidogenesis, which involves conversion of these organics to form organic acids and hydrogen. The final step is methane and carbon dioxide production from organic acids and hydrogen, by methanogens. The high methane content makes biogas a useful fuel that can displace natural gas in pipelines or be converted to electricity and heat. AD typically require long residence times, as certain anaerobic microorganisms have slow rate of growth. Long residence times lead to large volumes of tanks. Therefore, to improve digestion efficiency, the most efficient approach is to disrupt the chemical bonds in the material prone to hydrolysis [5]. Other factors limiting its performance are slow hydrolysis, low biodegradability, inhibition due to toxic compounds and toxic intermediates formed and poor methanogenesis. To overcome this recalcitrant property and to improve the degradation rate, a pretreatment prior to the AD process is introduced. Thus the goal of a pretreatment is to open up the structure of the substrate, making it more accessible for enzymatic attack [6] which aids in increasing biogas yield. The effects of various pretreatment methods highly differ, depending on the characteristics of the substrates and the pretreatment type. Recently, a lot of interest has been devoted to biomass disintegration and solubilization techniques in order to overcome the biological limitations of anaerobic digestion. The pretreatment techniques include mechanical treatment [7], ultrasonic treatment [8, 9] and biological hydrolysis with enzymes [10–12], alkaline treatment [13], oxidative treatments using ozone [14, 15], microwave irradiation [5, 16, 17], thermal treatment [18] thermochemical [19], sono-thermal [20–22] etc.

C6H12O6 ! 2C2H5OH þ 2CO2 (1)

CH3COOH ! CH4 þ CO2 (2)

CO2 þ 4H2 ! CH4 þ 2H2O (4)

2C2H5OH þ CO2 ! CH4 þ 2CH3COOH (3)

The digestion efficiency and its stability can vary significantly depending upon the wastewater characteristics and type and design of the treatment system. The longer a substrate is kept under proper reaction conditions, the more complete its degradation will become. Longer retention time demands the provision of reactor with large volume for a given amount of substrate to be treated. On the other hand,

Schematic representation of anaerobic digestion (source: https://www.e-education.psu.edu/egee439).

In the last step of the process, methanogens use acetic acid or carbon dioxide and hydrogen, to produce methane and carbon dioxide. For mesophilic bacteria, the optimal methane production rate is mostly reached at 35–37°C. The thermophilic methanogens differ from the mesophilic one and their maximum methanogenic activity is reached at about 55°C. A thermophilic digestion process can sustain a higher organic loading compared to a mesophilic one. But the thermophilic process produces gas with a lower methane concentration [25] and is more sensitive to toxicants [26]. Methanogens are also sensitive towards changes in temperature than the other species, because of their slower growth rate in the reactor environment. Methanogenesis occurs at neutral pH- in the range of 6.5–7.5, although optimum lies at pH 7.0–7.2 [26]. If, for example, a temperature shift affects the methanogens negatively, there can be a build-up of volatile fatty acids (VFAs). This lowers the pH which further affects the methanogens in a negative way which leads to a vicious circle of negative feedback. The methanogenesis reactions can be expressed

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

as follows [27] in Eqs. (2)–(4):

Figure 1.

93

### 2. Microbiology of anaerobic digestion

AD process is mediated through four main steps—hydrolysis, acidogenesis, acetogenesis and methanogenesis. These are carried out by a consortium of microorganisms: acidogenic bacteria, acetogenic bacteria and methanogenic bacteria [23]. The microbial community of the anaerobic process is very complex. There are two prokaryotic kingdoms that closely interact with each other: Bacteria and Archaea. The first step involves hydrolysis of complex organic matter into simpler compounds. In the second step, the acidogenesis of these organics take place to form organic acids and hydrogen. In the final step, methane and carbon dioxide are produced from organic acids and hydrogen by archael methanogens.

Figure 1 summarizes the overall process of AD. Organic matter consists of particulate, water-insoluble polymers such as carbohydrates, lipids and proteins. Insoluble polymers cannot penetrate cellular membranes and are therefore not directly available to the microorganisms. During hydrolysis, appropriate strains of hydrolytic bacteria excrete hydrolytic enzymes [23] which break up the insoluble polymers to soluble mono and oligomers. Carbohydrates are converted to sugars, lipids are broken down to long-chain fatty acids and proteins are split into amino acids [24]. These soluble molecules are converted by acidogens to acetic acid and other longer volatile fatty acids, alcohols, carbon dioxide and hydrogen on acidogenesis. The foremost acids produced are acetic acid (CH3COOH), propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), and ethanol (C2H5OH). Other acid formers are Clostridium, Peptococcusanerobus, Lactobacillus, and Actinomyces. The next process is acetogenesis during which, the longer volatile fatty acids and alcohols are oxidized by proton-reducing acetogens to acetic acid and hydrogen. An acetogenesis reaction is shown below:

Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

applications as fertilizer [4]. It reduces organic matter to more stable solids by complex biochemical reactions. There are three consecutive steps of biological process in AD. The first step involves hydrolysis of complex organic matter into simpler compounds. The second step is the acidogenesis, which involves conversion of these organics to form organic acids and hydrogen. The final step is methane and carbon dioxide production from organic acids and hydrogen, by methanogens. The high methane content makes biogas a useful fuel that can displace natural gas in pipelines or be converted to electricity and heat. AD typically require long residence times, as certain anaerobic microorganisms have slow rate of growth. Long residence times lead to large volumes of tanks. Therefore, to improve digestion efficiency, the most efficient approach is to disrupt the chemical bonds in the material prone to hydrolysis [5]. Other factors limiting its performance are slow hydrolysis, low biodegradability, inhibition due to toxic compounds and toxic intermediates formed and poor methanogenesis. To overcome this recalcitrant property and to improve the degradation rate, a pretreatment prior to the AD process is introduced. Thus the goal of a pretreatment is to open up the structure of the substrate, making it more accessible for enzymatic attack [6] which aids in increasing biogas yield. The effects of various pretreatment methods highly differ, depending on the characteristics of the substrates and the pretreatment type. Recently, a lot of interest has been devoted to biomass disintegration and solubilization techniques in order to overcome the biological limitations of anaerobic digestion. The pretreatment techniques include mechanical treatment [7], ultrasonic treatment [8, 9] and biological hydrolysis with enzymes [10–12], alkaline treatment [13], oxidative treatments using ozone [14, 15], microwave irradiation [5, 16, 17], thermal treatment [18]

AD process is mediated through four main steps—hydrolysis, acidogenesis, acetogenesis and methanogenesis. These are carried out by a consortium of microorganisms: acidogenic bacteria, acetogenic bacteria and methanogenic bacteria [23]. The microbial community of the anaerobic process is very complex. There are two prokaryotic kingdoms that closely interact with each other: Bacteria and Archaea. The first step involves hydrolysis of complex organic matter into simpler compounds. In the second step, the acidogenesis of these organics take place to form organic acids and hydrogen. In the final step, methane and carbon dioxide are

Figure 1 summarizes the overall process of AD. Organic matter consists of particulate, water-insoluble polymers such as carbohydrates, lipids and proteins. Insoluble polymers cannot penetrate cellular membranes and are therefore not directly available to the microorganisms. During hydrolysis, appropriate strains of hydrolytic bacteria excrete hydrolytic enzymes [23] which break up the insoluble polymers to soluble mono and oligomers. Carbohydrates are converted to sugars, lipids are broken down to long-chain fatty acids and proteins are split into amino acids [24]. These soluble molecules are converted by acidogens to acetic acid and other longer volatile fatty acids, alcohols, carbon dioxide and hydrogen on

acidogenesis. The foremost acids produced are acetic acid (CH3COOH), propionic acid (CH3CH2COOH), butyric acid (CH3CH2CH2COOH), and ethanol (C2H5OH). Other acid formers are Clostridium, Peptococcusanerobus, Lactobacillus, and Actinomyces. The next process is acetogenesis during which, the longer volatile fatty acids and alcohols are oxidized by proton-reducing acetogens to acetic acid and

produced from organic acids and hydrogen by archael methanogens.

hydrogen. An acetogenesis reaction is shown below:

92

thermochemical [19], sono-thermal [20–22] etc.

Anaerobic Digestion

2. Microbiology of anaerobic digestion

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 \rightarrow 2\text{C}\_2\text{H}\_5\text{OH} + 2\text{CO}\_2 \tag{1}$$

In the last step of the process, methanogens use acetic acid or carbon dioxide and hydrogen, to produce methane and carbon dioxide. For mesophilic bacteria, the optimal methane production rate is mostly reached at 35–37°C. The thermophilic methanogens differ from the mesophilic one and their maximum methanogenic activity is reached at about 55°C. A thermophilic digestion process can sustain a higher organic loading compared to a mesophilic one. But the thermophilic process produces gas with a lower methane concentration [25] and is more sensitive to toxicants [26]. Methanogens are also sensitive towards changes in temperature than the other species, because of their slower growth rate in the reactor environment. Methanogenesis occurs at neutral pH- in the range of 6.5–7.5, although optimum lies at pH 7.0–7.2 [26]. If, for example, a temperature shift affects the methanogens negatively, there can be a build-up of volatile fatty acids (VFAs). This lowers the pH which further affects the methanogens in a negative way which leads to a vicious circle of negative feedback. The methanogenesis reactions can be expressed as follows [27] in Eqs. (2)–(4):

$$\text{CH}\_3\text{COOH} \rightarrow \text{CH}\_4 + \text{CO}\_2 \tag{2}$$

$$\text{2C}\_2\text{H}\_5\text{OH} + \text{CO}\_2 \rightarrow \text{CH}\_4 + \text{2CH}\_3\text{COOH} \tag{3}$$

$$\text{CO}\_2 + 4\text{H}\_2 \to \text{CH}\_4 + 2\text{H}\_2\text{O} \tag{4}$$

The digestion efficiency and its stability can vary significantly depending upon the wastewater characteristics and type and design of the treatment system. The longer a substrate is kept under proper reaction conditions, the more complete its degradation will become. Longer retention time demands the provision of reactor with large volume for a given amount of substrate to be treated. On the other hand,

with shorter retention time washout of microorganism takes place with a lower overall degradation [25]. Therefore, these two effects have to be balanced in the design of AD for the efficient and proper operation of the full scale reactor. This needs operation of AD through skilled supervision for optimal performance.

### 3. Need for pretreatment

Several renewable matters have been tried for biogas production which are classified into crop biomass such as maize, wheat, barley, sweet sorghum, etc.; organic wastes such as municipal solid waste, municipal and industrial wastewater sludge, animal manure, and residues from various processing; energy crops like sunflower, rape, jatropha, etc.; crop residues which include banana stem, barley straw, rice straw, softwood spruce, etc.; and non-conventional biomass like glycerol, microalgae, etc. [28–34]. Figures 2–4 show the effect of pretreatment of lignocellulosic, sludge and macroalgal biomass respectively.

The diverse composition of lignocellulose biomass and the interactions between fractions make its structure very complex and resistant to deconstruction. Cellulose and hemicellulose are polysaccharides that can be hydrolyzed to simple sugars. Lignin which acts as a support to the cell structure, embedding cellulose and hemicellulose, hinders the susceptibility to microbial attack during hydrolysis process [35]. The aim of pretreatment is to break the lignin layer that protects the cellulose and hemicellulose, in order to make the biomass more accessible for digestion [6]. Pretreatment also helps to decrease the crystallinity of cellulose and to increase the porosity. Furthermore, biomass such as fruit wastes is easily degraded but result in low yield due to the presence of inhibitors.

Keratin, which is present in horns and feathers, is an insoluble protein in which the polypeptides chain is tightly packed and highly cross-linked with disulfide bonds, hydrogen bonds, and hydrophobic interactions [36]. This insoluble protein is extremely resistive to the proteolytic enzyme action, which is a major hindrance in the biological processing of these wastes. For such biomass, if the crosslinking between the polypeptides chain breaks, the keratin becomes more accessible and easier to digest. Contrarily, while keratin-rich waste is pretreated using a strong

acid, alkali, or other harsh physicochemical methods, severe degradation and

Activated sludge, a bio product of aerobic wastewater treatment, can be a better raw material for generating energy because of its high organic content [38]. Secondary wastewater sludge consists of numerous microbial cells, the cell walls of which act as barriers against exo-enzyme degradation. Besides microbial cells, exocellular polymeric substances (EPS) comprise a major organic fraction in activated sludge floc structure and binding mechanisms of EPS to cations appear to be a significant factor determining the digestibility of activated sludge. Hence hydrolysis becomes the rate-limiting step and degree of degradation achieved is limited to 30– 35% chemical oxygen demand (COD) reduction in conventional anaerobic sludge treatment [23]. Pretreatment of sludge is required to rupture the cell wall and to facilitate the release of intracellular matter into the aqueous phase, which improves the biodegradability thereby enhancing the AD with lower retention time and with

The macroalgal cell envelope made of thick and hard layer composed of complex proteins and carbohydrates with more mechanical power and high chemical resistance, restricts the attack of the biopolymers by methanogenic bacteria during

destruction of the keratin occurs [37].

Effect of pretreatment of macroalgal biomass.

Figure 4.

95

Figure 3.

Effect of pretreatment on sludge biomass.

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

higher biogas production [20].

Figure 2. Effect of pretreatment on lignocellulosic biomass (source: https://www.e-education.psu.edu/egee439/node/653).

Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

Figure 3. Effect of pretreatment on sludge biomass.

with shorter retention time washout of microorganism takes place with a lower overall degradation [25]. Therefore, these two effects have to be balanced in the design of AD for the efficient and proper operation of the full scale reactor. This needs operation of AD through skilled supervision for optimal performance.

Several renewable matters have been tried for biogas production which are classified into crop biomass such as maize, wheat, barley, sweet sorghum, etc.; organic wastes such as municipal solid waste, municipal and industrial wastewater sludge, animal manure, and residues from various processing; energy crops like sunflower, rape, jatropha, etc.; crop residues which include banana stem, barley straw, rice straw, softwood spruce, etc.; and non-conventional biomass like glycerol, microalgae, etc. [28–34]. Figures 2–4 show the effect of pretreatment of

The diverse composition of lignocellulose biomass and the interactions between fractions make its structure very complex and resistant to deconstruction. Cellulose and hemicellulose are polysaccharides that can be hydrolyzed to simple sugars. Lignin which acts as a support to the cell structure, embedding cellulose and hemicellulose, hinders the susceptibility to microbial attack during hydrolysis process [35]. The aim of pretreatment is to break the lignin layer that protects the cellulose and hemicellulose, in order to make the biomass more accessible for digestion [6]. Pretreatment also helps to decrease the crystallinity of cellulose and to increase the porosity. Furthermore, biomass such as fruit wastes is easily degraded but result in

Keratin, which is present in horns and feathers, is an insoluble protein in which

Effect of pretreatment on lignocellulosic biomass (source: https://www.e-education.psu.edu/egee439/node/653).

the polypeptides chain is tightly packed and highly cross-linked with disulfide bonds, hydrogen bonds, and hydrophobic interactions [36]. This insoluble protein is extremely resistive to the proteolytic enzyme action, which is a major hindrance in the biological processing of these wastes. For such biomass, if the crosslinking between the polypeptides chain breaks, the keratin becomes more accessible and easier to digest. Contrarily, while keratin-rich waste is pretreated using a strong

lignocellulosic, sludge and macroalgal biomass respectively.

low yield due to the presence of inhibitors.

Figure 2.

94

3. Need for pretreatment

Anaerobic Digestion

Figure 4. Effect of pretreatment of macroalgal biomass.

acid, alkali, or other harsh physicochemical methods, severe degradation and destruction of the keratin occurs [37].

Activated sludge, a bio product of aerobic wastewater treatment, can be a better raw material for generating energy because of its high organic content [38]. Secondary wastewater sludge consists of numerous microbial cells, the cell walls of which act as barriers against exo-enzyme degradation. Besides microbial cells, exocellular polymeric substances (EPS) comprise a major organic fraction in activated sludge floc structure and binding mechanisms of EPS to cations appear to be a significant factor determining the digestibility of activated sludge. Hence hydrolysis becomes the rate-limiting step and degree of degradation achieved is limited to 30– 35% chemical oxygen demand (COD) reduction in conventional anaerobic sludge treatment [23]. Pretreatment of sludge is required to rupture the cell wall and to facilitate the release of intracellular matter into the aqueous phase, which improves the biodegradability thereby enhancing the AD with lower retention time and with higher biogas production [20].

The macroalgal cell envelope made of thick and hard layer composed of complex proteins and carbohydrates with more mechanical power and high chemical resistance, restricts the attack of the biopolymers by methanogenic bacteria during

AD [39]. Pretreatment leads to improvement in the liquefaction process, enhancing the biopolymer release [28]. Several pretreatment methods have been reported in detail, aiming to make these biomass viable to digestion by microorganisms, and increase the biogas yield. It is necessary to carry out the pretreatment at mild conditions to prevent excessive sugar degradation.

Several pretreatment processes such as ball mill [40], microwave irradiation [2], sodium hydroxide [13], steam explosion [41], ultrasonic [42], biological [43], ozonation [14] have been shown to enhance biodegradability of biomass by promoting the hydrolysis process. Since most available articles are addressed based on pretreatment of lignocellulosic biomass, this chapter is mainly focused towards sludge pretreatment.

### 4. Pretreatment technologies

The lower hydrolysis rates during conventional AD process, results in higher hydraulic retention time (HRT) in the digester and larger digester volume, constitutes the prime drawbacks of the conventional AD [6]. The non-availability of the readily biodegradable, soluble organic matters and lower digestion rate constantly necessitates the pretreatment of sludge. Pretreatment of biomass enhances the AD, with lower retention time and with higher biogas production [17]. With the advancements in various pretreatment techniques like thermal, chemical, mechanical, biological and physical and several combinations such as physicochemical, biological–physicochemical, mechanical–chemical and thermal–chemical, biodegradability of sludge can be enhanced by several orders. Extensive research has been carried throughout the world to establish the best economically feasible pretreatment technology to enhance the digestibility of biomass [12]. Tables 1 and 2 show the specific energy consumed and methane yield with various chemo-mechanical and physico-chemical pretreatment.

### 4.1 Physical

In physical pretreatment, the structure of the biomass gets altered and the size of the particles reduced, by the application of physical force. This leads to an increase in the surface area of the particles thereby making it susceptible to microbial and enzymatic attacks, which enhances the AD process for methane production [61]. Physical pretreatment may be done by employing microwave irradiation, sonication, mechanical beating, deflaking, dispersing, extruding, refining, milling, and cavitation etc. [62].

### 4.1.1 Milling

Milling pretreatment is carried out, especially for lignocellulose and algal biomass to reduce the size of the substrate to break open the cellular structure, and improve their bio accessibility to the cell tissues, by increasing the specific surface area of the biomass [40]. Particle size reduction not only increases the rate of enzymatic degradation, but also reduces viscosity in digesters thus making mixing easier and can reduce the problems of floating layers. For effective hydrolysis of lignocellulose, beta particle size of 1–2 mm has been recommended [63]. Using three batch reactors, Motte et al. [40] demonstrated, treating straw particle milled to different sizes 0.25 mm, 1 mm and 10 mm followed during 62 days. They achieved the highest methane production for straw with 10 mm particle size (192 25 Nm L/g VS) which was associated with a straw biodegradability of 43%.

4.1.2 Cavitation

Table 1.

97

S. No. Name of the pretreatment

1 Disperser + alkali

2 Thermo chemo disperser

5 Thermo chemo sonic

6 Citric acid + ultrasonic

7 Fenton +

8 Thermo chemo sonic

9 Disperser + microwave

10 Chemo

11 Sonic mediated biological

12 Chemo thermo disperser

14 Chemo

15 Surfactant + sonic

16 Disperser + bacterial

17 Ultrasound + microwave

18 Surfactant + sonic

disperser

ultrasonic

mechanical

disperser

3 Chemo

Specific energy consumed (KJ/kg TS)

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

4 Sono alkaline 4172 59 0.108 ml/g VS

Solubilization achieved (%)

5290.5 27 0.413 g COD/

641 34.4 0.3 g COD/g

5500 35 0.60 g COD/g

18,000 22 0.28 g COD/g

7377 38 50 ml/g VS

174 60 0.84 g COD/g

3312.6 15 0.14 g COD/g

9.5 22.4 0. 279 g COD/

9600 23.9 0.239 g/g

13 Surfactant sonic 5120 24.7 0.24 g/g COD Ushani et al.

4544 24 1391 ml Rani et al. [21]

3360.94 18.6 0.455 L/g VS Kavitha et al.

5013 20 0.522 L/g VS Poornima Devi

171.9 22.7 0.435 L/g VS Gayathri et al.

2.45 23 0.19 d1 Kavitha et al.

5400 26 0.6 g/g COD Santhi et al.

16,700 33.2 0.3 L/g COD Kavitha et al.

Biomethane yield

removed

g COD

COD

COD

COD

removed

COD

COD

g COD

COD

References

[44]

et al. [45]

Rani et al. [46]

Kavitha et al. [47]

[29]

Kavitha et al. [48]

Kavitha et al. [49]

Kavitha et al. [50]

Kavitha et al. [51]

[52]

Kavitha et al. [43]

[53]

Tamilarasan et al. [28]

[54]

Banu et al. [55]

[56]

Tamilarasan et al. [57]

The most frequently applied cavitation techniques include acoustic cavitation, which is produced by passing ultrasonic waves through the liquid medium and hydrodynamic cavitation produced using hydraulic systems. In acoustic cavitation, microbubbles called cavitation were developed when the ultrasound waves propa-

gate in a liquid medium, due to a repeating pattern of compressions and

Specific energy consumed and methane yield with various chemo-mechanical pretreatment.


### Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

### Table 1.

AD [39]. Pretreatment leads to improvement in the liquefaction process, enhancing the biopolymer release [28]. Several pretreatment methods have been reported in detail, aiming to make these biomass viable to digestion by microorganisms, and increase the biogas yield. It is necessary to carry out the pretreatment at mild

Several pretreatment processes such as ball mill [40], microwave irradiation [2],

The lower hydrolysis rates during conventional AD process, results in higher hydraulic retention time (HRT) in the digester and larger digester volume, constitutes the prime drawbacks of the conventional AD [6]. The non-availability of the readily biodegradable, soluble organic matters and lower digestion rate constantly necessitates the pretreatment of sludge. Pretreatment of biomass enhances the AD, with lower retention time and with higher biogas production [17]. With the advancements in various pretreatment techniques like thermal, chemical, mechanical, biological and physical and several combinations such as physicochemical, biological–physicochemical, mechanical–chemical and thermal–chemical, biodegradability of sludge can be enhanced by several orders. Extensive research has been carried throughout the world to establish the best economically feasible

In physical pretreatment, the structure of the biomass gets altered and the size of the particles reduced, by the application of physical force. This leads to an increase in the surface area of the particles thereby making it susceptible to microbial and enzymatic attacks, which enhances the AD process for methane production [61]. Physical pretreatment may be done by employing microwave irradiation, sonication, mechanical beating, deflaking, dispersing, extruding, refining, milling, and

Milling pretreatment is carried out, especially for lignocellulose and algal biomass to reduce the size of the substrate to break open the cellular structure, and improve their bio accessibility to the cell tissues, by increasing the specific surface area of the biomass [40]. Particle size reduction not only increases the rate of enzymatic degradation, but also reduces viscosity in digesters thus making mixing easier and can reduce the problems of floating layers. For effective hydrolysis of lignocellulose, beta particle size of 1–2 mm has been recommended [63]. Using three batch reactors, Motte et al. [40] demonstrated, treating straw particle milled to different sizes 0.25 mm, 1 mm and 10 mm followed during 62 days. They achieved the highest methane production for straw with 10 mm particle size (192 25 Nm L/g VS) which was associated with a straw biodegradability of 43%.

pretreatment technology to enhance the digestibility of biomass [12]. Tables 1 and 2 show the specific energy consumed and methane yield with

various chemo-mechanical and physico-chemical pretreatment.

sodium hydroxide [13], steam explosion [41], ultrasonic [42], biological [43], ozonation [14] have been shown to enhance biodegradability of biomass by promoting the hydrolysis process. Since most available articles are addressed based on pretreatment of lignocellulosic biomass, this chapter is mainly focused towards

conditions to prevent excessive sugar degradation.

sludge pretreatment.

Anaerobic Digestion

4.1 Physical

cavitation etc. [62].

4.1.1 Milling

96

4. Pretreatment technologies

Specific energy consumed and methane yield with various chemo-mechanical pretreatment.

### 4.1.2 Cavitation

The most frequently applied cavitation techniques include acoustic cavitation, which is produced by passing ultrasonic waves through the liquid medium and hydrodynamic cavitation produced using hydraulic systems. In acoustic cavitation, microbubbles called cavitation were developed when the ultrasound waves propagate in a liquid medium, due to a repeating pattern of compressions and


4.1.3 Microwave irradiation

4.1.4 Extrusion

the subsequent step [61].

was 405 m3

4.2 Thermal

99

During microwave irradiation the destruction of the microbial cells is caused by the disruption of the chemical (hydrogen) bonds in the cell walls and membranes, by polarized parts of macromolecules aligning with the poles of the electromagnetic field, which results in denaturation. Microwaves can induce an athermal effect in addition to their thermal effect due to dipole orientation, which results in possible breakage of hydrogen bonds and subsequently leads to the disintegration of the floc matrix [17]. They observed that, microbial cells exposed to MW showed greater damage at similar applied temperatures compared to conventional heating. Rincón et al. [67] studied the effect of a MW pre-treatment on olive mill solid residue to enhance its anaerobic digestibility. They carried out the experiment at a power of

800 W and temperature 50°C and observed a maximum methane yield of

MW treatment proved to increase the methane yield.

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

395 1 ml CH4/g VS for an applied specific energy 7660 kJ/kg TS. Beszédes et al. [16] focused on the effects of MW irradiation at different power levels on biodegradation and subsequent AD of sludge from the dairy and meat industry. Compared to their results obtained from conventional heat treatment of the same sludge, the

In extrusion pretreatment, the biomass is allowed to experience heat, compression and shear force, which creates physical damage and chemical alterations of biomass cells while passing through the extruder. The extruder arrangement consists of single or twin screws that spin into a tight barrel, which is equipped with temperature control. When a biomass material passes through the barrel, it is exposed to friction and vigorous shearing causing an increase in temperature and pressure. When it exits the finishing end, the biomass material experiences a pressure release, which causes structural changes in the processed biomass enabling easy digestion in

Maroušek [68] evaluated extrusion parameters of pelleted hay for maximal cumulative biogas production, and reported that, at optimal conditions of pressure 1.3 MPa, reaction time 7 min, and 8% dry matter, the maximal biogas production

biogas yield of control. Novarino and Zanetti [69] employed extrusion pretreatment to improve biogas production from the organic fraction of municipal solid waste, resulting in a biogas yield of 800 L/kg VS containing about 60% methane content.

Thermal pretreatment improves hydrolysis, with increased methane yield during subsequent anaerobic digestion. A wide range of temperatures has been studied, ranging from 60 to 270°C, but temperatures above 200°C have been found responsible for the production of recalcitrant soluble organics or toxic/inhibitory intermediates during the pretreatment process [70]. Many studies employed at an optimum thermal range of 160–180°C for hydrolysis of wastewater sludge have proved an increase in methane yield during AD. Higher temperatures lead to a sharp reduction in biodegradability of sludge hydrolysate, due to production of recalcitrant soluble organics or toxic/inhibitory intermediates during the process [71]. The effect of thermal treatment of anaerobic sludge on the disintegration of the remaining organic fraction was evaluated by Borges and Chernicharo [18]. At 75°C, they observed an increase of 30–35 times increase in the concentrations of protein,

/ton TS (with 52.3% methane), which was about a 33% increase over the

### Table 2.

Specific energy consumed with various physico-chemical pretreatment.

rarefactions. These cavitation expand to unstable size, and then rapidly collapse resulting in temperatures up to 5000 K and pressures up to 180 MPa. The rapid collapse of a numerous microbubbles generates powerful shear forces in the surrounding liquid, which damages the cell walls of microorganisms [21, 53]. However, higher sonication power level is reported to adversely affect the pretreatment process. At higher power level, bubbles are formed near the tip of the ultrasound transducer, which hinders the transfer of energy to the liquid medium [64].

In the ultrasonic pretreatment study on waste activated sludge (WAS), Apul & Sanin [7] investigated an improvement in anaerobic biodegradability at 15 min of sonication. They achieved an increase in daily biogas production and methane production by 49 and 74% respectively compared to control in semi continuous reactors at a solid retention time (SRT) of 15 days and organic loading rate of 0.5 kg/m<sup>3</sup> d. Zeynali et al. [42] studied the efficiency of ultrasonic pretreatment in improving biogas production from fruits and vegetable waste. They adopted three sonication times of 9, 18, 27 min operating at 20 kHz and amplitude of 80 μm on the substrate. The highest methane yield they obtained was at 18 min sonication with specific energy 2380 kJ/kg TS (Total solids) for a 12 d batch period, while longer exposure to sonication led to lower methane yield. The energy content of the biogas obtained by them was twice that of input energy for sonication. Alzate et al. [65] reported that, the sonication applied to macro algae at a specific energy input of 75 MJ/kg TS produced just 20% of the methane production. Upon increasing the specific energy to about 100–200 MJ/kg TS, they reported an increase in the methane production rate between 80 and 90%.

In hydrodynamic systems, cavitation is generated by forcing fluid flow through cavitating devices, where pressure substantially drops. Many microbubbles formed as a consequence of this pressure drop subsequently collapse. The collapse of the cavitation, results in release of large magnitudes of energy which helps in dissolution of biomass and makes it more suitable for subsequent bacterial decomposition, improving biogas yield during the AD process [66]. They investigated the application of hydrodynamic cavitation (HC) for the pretreatment of wheat straw with an objective of enhancing the biogas production. They observed the methane yields of 31.8 ml with untreated wheat straw, 77.9 ml with HC pre-treated wheat straw and a maximum yield of 172.3 ml with the combined pre-treatment using KOH and HC.

### 4.1.3 Microwave irradiation

During microwave irradiation the destruction of the microbial cells is caused by the disruption of the chemical (hydrogen) bonds in the cell walls and membranes, by polarized parts of macromolecules aligning with the poles of the electromagnetic field, which results in denaturation. Microwaves can induce an athermal effect in addition to their thermal effect due to dipole orientation, which results in possible breakage of hydrogen bonds and subsequently leads to the disintegration of the floc matrix [17]. They observed that, microbial cells exposed to MW showed greater damage at similar applied temperatures compared to conventional heating. Rincón et al. [67] studied the effect of a MW pre-treatment on olive mill solid residue to enhance its anaerobic digestibility. They carried out the experiment at a power of 800 W and temperature 50°C and observed a maximum methane yield of 395 1 ml CH4/g VS for an applied specific energy 7660 kJ/kg TS. Beszédes et al. [16] focused on the effects of MW irradiation at different power levels on biodegradation and subsequent AD of sludge from the dairy and meat industry. Compared to their results obtained from conventional heat treatment of the same sludge, the MW treatment proved to increase the methane yield.

### 4.1.4 Extrusion

rarefactions. These cavitation expand to unstable size, and then rapidly collapse resulting in temperatures up to 5000 K and pressures up to 180 MPa. The rapid collapse of a numerous microbubbles generates powerful shear forces in the surrounding liquid, which damages the cell walls of microorganisms [21, 53]. However, higher sonication power level is reported to adversely affect the pretreatment process. At higher power level, bubbles are formed near the tip of the ultrasound transducer, which hinders the transfer of energy to the liquid medium [64].

4. Microwave + H2O2 18,600 56 0.323 L/g VS Eswari

5. H2O2 + microwave 18,910 46.6 250 ml/g VS Eswari

6. Thermo ozone 141.02 30.4 0.32 g COD/g

Specific energy consumed with various physico-chemical pretreatment.

Specific energy consumed (KJ/kg TS)

1. Microwave 1844 18.6 0.162 ml/g VS

Solubilization achieved (%)

14,000 31 0.615 L/g VS Ebenezer

14,000 28 0.47 L/g VS Ebenezer

Biomethane yield

removed

COD

References

Rani et al. [33]

et al. [38]

et al. [58]

et al. [59]

et al. [60]

Kannah et al. [1]

In the ultrasonic pretreatment study on waste activated sludge (WAS), Apul & Sanin [7] investigated an improvement in anaerobic biodegradability at 15 min of sonication. They achieved an increase in daily biogas production and methane production by 49 and 74% respectively compared to control in semi continuous reactors at a solid retention time (SRT) of 15 days and organic loading rate of 0.5 kg/m<sup>3</sup> d. Zeynali et al. [42] studied the efficiency of ultrasonic pretreatment in improving biogas production from fruits and vegetable waste. They adopted three sonication times of 9, 18, 27 min operating at 20 kHz and amplitude of 80 μm on the substrate. The highest methane yield they obtained was at 18 min sonication with specific energy 2380 kJ/kg TS (Total solids) for a 12 d batch period, while longer exposure to sonication led to lower methane yield. The energy content of the biogas obtained by them was twice that of input energy for sonication. Alzate et al. [65] reported that, the sonication applied to macro algae at a specific energy input of 75 MJ/kg TS produced just 20% of the methane production. Upon increasing the specific energy to about 100–200 MJ/kg TS, they reported an increase in the meth-

In hydrodynamic systems, cavitation is generated by forcing fluid flow through cavitating devices, where pressure substantially drops. Many microbubbles formed as a consequence of this pressure drop subsequently collapse. The collapse of the cavitation, results in release of large magnitudes of energy which helps in dissolution of biomass and makes it more suitable for subsequent bacterial decomposition, improving biogas yield during the AD process [66]. They investigated the application of hydrodynamic cavitation (HC) for the pretreatment of wheat straw with an objective of enhancing the biogas production. They observed the methane yields of 31.8 ml with untreated wheat straw, 77.9 ml with HC pre-treated wheat straw and a maximum yield of 172.3 ml with the combined pre-treatment using

ane production rate between 80 and 90%.

S. No. Name of the

Anaerobic Digestion

2. Microwave + citric acid

3. Microwave +

Table 2.

pretreatment

surfactant

KOH and HC.

98

In extrusion pretreatment, the biomass is allowed to experience heat, compression and shear force, which creates physical damage and chemical alterations of biomass cells while passing through the extruder. The extruder arrangement consists of single or twin screws that spin into a tight barrel, which is equipped with temperature control. When a biomass material passes through the barrel, it is exposed to friction and vigorous shearing causing an increase in temperature and pressure. When it exits the finishing end, the biomass material experiences a pressure release, which causes structural changes in the processed biomass enabling easy digestion in the subsequent step [61].

Maroušek [68] evaluated extrusion parameters of pelleted hay for maximal cumulative biogas production, and reported that, at optimal conditions of pressure 1.3 MPa, reaction time 7 min, and 8% dry matter, the maximal biogas production was 405 m3 /ton TS (with 52.3% methane), which was about a 33% increase over the biogas yield of control. Novarino and Zanetti [69] employed extrusion pretreatment to improve biogas production from the organic fraction of municipal solid waste, resulting in a biogas yield of 800 L/kg VS containing about 60% methane content.

### 4.2 Thermal

Thermal pretreatment improves hydrolysis, with increased methane yield during subsequent anaerobic digestion. A wide range of temperatures has been studied, ranging from 60 to 270°C, but temperatures above 200°C have been found responsible for the production of recalcitrant soluble organics or toxic/inhibitory intermediates during the pretreatment process [70]. Many studies employed at an optimum thermal range of 160–180°C for hydrolysis of wastewater sludge have proved an increase in methane yield during AD. Higher temperatures lead to a sharp reduction in biodegradability of sludge hydrolysate, due to production of recalcitrant soluble organics or toxic/inhibitory intermediates during the process [71]. The effect of thermal treatment of anaerobic sludge on the disintegration of the remaining organic fraction was evaluated by Borges and Chernicharo [18]. At 75°C, they observed an increase of 30–35 times increase in the concentrations of protein,

carbohydrate, lipid and COD and an increase of 50% in the biogas production, thus characterizing a higher biodegradability of the remaining organic fraction.

as well as to help increase the concentration of dissolved oxygen, and thus oxidation rate. Chandra et al. [76] employed wet air oxidation to enhance the biodegradability of the complex biomethanated distillery effluent. They reported an enhanced biogas yield of pretreated effluent up to 2.8 times higher than the untreated effluent with

Ozone is a strong oxidant and hence powerful in oxidizing substrates. It has potential to degrade lignin in diverse feedstocks. It reacts with the polysaccharides, proteins, lipids and other recalcitrant compounds and transform them into biodegradable molecules. The ozonation process can result in efficient cell wall rupture and release of more soluble and easily biodegradable organics, which can be easily accessed and assimilated by anaerobic microorganisms. Thus it leads to improve-

AD of ozone pretreated excess sludge was studied by Goel et al. [14] through

pretreatment was effective in partially solubilizing the sludge solids and leading to subsequent improvement in anaerobic degradability. The extent of solubilization and digestion efficiency depended on the applied ozone doses. At 0.05 g O3/g TS, the AD efficiencies improved to about 59% as compared to 31% for the control run. Different process indicators like specific methane production and ammonia concentration in the reactor, also specify the higher observed solid degradation rates for

The biological mediated pretreatment process is based on the function of multi-

pretreatment lies in the fact that is solubilizes the organic compounds present in the biomass with minimum energy, with no severe changes in substrate environment. Biological pretreatment is done with or without enzyme addition some of which can be produced endogenously by microorganisms present in the sludge. Some of the enzymes like protease, lipase, cellulase, alpha-amylase and dextranase [11] can effectively improve the hydrolysis rate and release of biopolymers to a large extent. Contrarily, these enzymes are more costly and difficult to preserve. Bonilla et al. [77] evaluated the potential for enzymatic pretreatment of pulp mill biosludge with protease from B. licheniformis for biodegradability. Carrying out BMP test, they

ple form of heterotrophic microbes. Complex biopolymers such as protein and carbohydrate can be transformed into simpler end products due to the action of various enzymes produced by the bacteria. The significance of biological

Saranya et al. [10] studied the impacts of phase separated disintegration pretreatment using calcium chloride (CaCl2) and bacteria. For their study a pH of 6.5, temperature of 40°C and treatment period of 42 h were the optimum conditions for pretreatment. In the initial phase, they achieved the floc disruption (deflocculation) with 0.06 g/g SS of CaCl2 and in the latter phase, cell disintegration through potent biosurfactant producing bacteria, Planococcus jake 01. They were

deflocculated and bacterially pretreated sludge, which were comparatively higher than for sludge treated with bacteria alone. They observed a biogas yield potential for pretreated sludge of 0.322 L/g VS as against 0.145 L/g VS for control. Kavitha et al. [43] investigated the bacterial-based biological pretreatment on liquefaction of microalga Chlorella vulgaris with cellulase-secreting bacteria prior to anaerobic

able to achieve 17.14% SS reduction and 14.14% COD solubilization for

arrived at a maximum improvement of 26% in biogas yield.

long-term operation of laboratory-scale reactors. They found that ozone

methane content up to 64.14%.

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

ment in the AD process [15].

4.3.4 Ozonation

ozonated sludge.

4.4 Biological

101

### 4.3 Chemical

### 4.3.1 Acid

Acid pretreatment causes sludge disintegration and cell lysis which releases the intracellular organics, which become more bioavailable and thus increases the rate and efficiency of the digestion process [17]. In lignocellulosic biomass, the pretreatment results in the disruption of the Van der Waals forces, hydrogen bonds and covalent bonds that hold together the biomass components, which consequently causes the breaking of hemicellulose and the reduction of cellulose [72]. Devlin et al. [73] showed the improved effects of HCl pretreatment at pH 2 on subsequent digestion of WAS. In semi-continuous digestion experiments conducted for 12 day hydraulic retention time at 35°C, they found a 14.3% increase in methane yield compared to untreated WAS. Taherdanak et al. [74] used dilute sulfuric acid pretreatment, to improve the biomethane production from wheat plant under mesophilic anaerobic digestion. At 121°C, they obtained a maximum methane yield of 15.5% higher than that of the untreated wheat plant after pretreatment for 120 min.

### 4.3.2 Alkali

The mechanism of alkaline pretreatment mainly induces swelling of particulate organics at elevated pH, enabling the biomass cellular substances more susceptible to enzymatic action [24]. The complex cell gets damaged by the hydroxyl anions available in the alkali. In macroalgae, it enhances hydrolysis of RNA, organic liquefaction of proteins and saponification [28]. In lignocellulosic biomass, it causes swelling, delignification and de-esterification of intermolecular ester bonds. With the disintegration of the bonds the porosity and internal surface area of the biomass increases, the degree of polymerization and crystallinity decreases. This makes it more accessible for enzymes and bacteria [6]. Regarding WAS, at higher pH, the microbial cell walls are broken and intracellular material is released into the liquid phase.

Studies were explored by Banu et al. [13] to evaluate the advantage of sodium hydroxide (NaOH) for its higher sludge solubilization potential and lime. They conducted experiments at a fixed alkali strength (35 meq/l) and varying concentration of NaOH and lime to demonstrate the role of alkalis in solubilizing sludge. The highest solubilization they achieved, was at an optimum dosage of NaOH and lime 1.6 and 0.7 g/l respectively at time 3 h. Sambusiti et al. [75] investigated the effect of alkaline (NaOH) pretreatment on ensiled sorghum forage in semi continuous digesters. They observed that pretreatment with 10 g NaOH/100 g TS increased the methane yield by 25% compared to untreated sorghum without experiencing any inhibition of the process.

### 4.3.3 Oxidative

Wet air oxidation is a pretreatment option that enhances contact between molecular oxygen and organic matter for the complete degradation of organic compounds into carbon dioxide and water. In order to achieve this, high temperature (and subsequently high pressure) conditions are required [22]. The correspondingly high pressure required is to maintain the high temperature conditions, as well as to help increase the concentration of dissolved oxygen, and thus oxidation rate. Chandra et al. [76] employed wet air oxidation to enhance the biodegradability of the complex biomethanated distillery effluent. They reported an enhanced biogas yield of pretreated effluent up to 2.8 times higher than the untreated effluent with methane content up to 64.14%.

### 4.3.4 Ozonation

carbohydrate, lipid and COD and an increase of 50% in the biogas production, thus

Acid pretreatment causes sludge disintegration and cell lysis which releases the intracellular organics, which become more bioavailable and thus increases the rate

pretreatment results in the disruption of the Van der Waals forces, hydrogen bonds and covalent bonds that hold together the biomass components, which consequently causes the breaking of hemicellulose and the reduction of cellulose [72]. Devlin et al. [73] showed the improved effects of HCl pretreatment at pH 2 on subsequent digestion of WAS. In semi-continuous digestion experiments conducted for 12 day hydraulic retention time at 35°C, they found a 14.3% increase in methane yield compared to untreated WAS. Taherdanak et al. [74] used dilute sulfuric acid pretreatment, to improve the biomethane production from wheat plant under mesophilic anaerobic digestion. At 121°C, they obtained a maximum methane yield of 15.5% higher than that of the untreated wheat plant after pretreatment for

The mechanism of alkaline pretreatment mainly induces swelling of particulate organics at elevated pH, enabling the biomass cellular substances more susceptible to enzymatic action [24]. The complex cell gets damaged by the hydroxyl anions available in the alkali. In macroalgae, it enhances hydrolysis of RNA, organic liquefaction of proteins and saponification [28]. In lignocellulosic biomass, it causes swelling, delignification and de-esterification of intermolecular ester bonds. With the disintegration of the bonds the porosity and internal surface area of the biomass increases, the degree of polymerization and crystallinity decreases. This makes it more accessible for enzymes and bacteria [6]. Regarding WAS, at higher pH, the microbial cell walls are broken and intracellular material is released into the liquid

Studies were explored by Banu et al. [13] to evaluate the advantage of sodium hydroxide (NaOH) for its higher sludge solubilization potential and lime. They conducted experiments at a fixed alkali strength (35 meq/l) and varying concentration of NaOH and lime to demonstrate the role of alkalis in solubilizing sludge. The highest solubilization they achieved, was at an optimum dosage of NaOH and lime 1.6 and 0.7 g/l respectively at time 3 h. Sambusiti et al. [75] investigated the effect of alkaline (NaOH) pretreatment on ensiled sorghum forage in semi continuous digesters. They observed that pretreatment with 10 g NaOH/100 g TS increased the methane yield by 25% compared to untreated sorghum without experiencing any

Wet air oxidation is a pretreatment option that enhances contact between molecular oxygen and organic matter for the complete degradation of organic compounds into carbon dioxide and water. In order to achieve this, high temperature (and subsequently high pressure) conditions are required [22]. The correspondingly high pressure required is to maintain the high temperature conditions,

characterizing a higher biodegradability of the remaining organic fraction.

and efficiency of the digestion process [17]. In lignocellulosic biomass, the

4.3 Chemical

Anaerobic Digestion

4.3.1 Acid

120 min.

phase.

inhibition of the process.

4.3.3 Oxidative

100

4.3.2 Alkali

Ozone is a strong oxidant and hence powerful in oxidizing substrates. It has potential to degrade lignin in diverse feedstocks. It reacts with the polysaccharides, proteins, lipids and other recalcitrant compounds and transform them into biodegradable molecules. The ozonation process can result in efficient cell wall rupture and release of more soluble and easily biodegradable organics, which can be easily accessed and assimilated by anaerobic microorganisms. Thus it leads to improvement in the AD process [15].

AD of ozone pretreated excess sludge was studied by Goel et al. [14] through long-term operation of laboratory-scale reactors. They found that ozone pretreatment was effective in partially solubilizing the sludge solids and leading to subsequent improvement in anaerobic degradability. The extent of solubilization and digestion efficiency depended on the applied ozone doses. At 0.05 g O3/g TS, the AD efficiencies improved to about 59% as compared to 31% for the control run. Different process indicators like specific methane production and ammonia concentration in the reactor, also specify the higher observed solid degradation rates for ozonated sludge.

### 4.4 Biological

The biological mediated pretreatment process is based on the function of multiple form of heterotrophic microbes. Complex biopolymers such as protein and carbohydrate can be transformed into simpler end products due to the action of various enzymes produced by the bacteria. The significance of biological pretreatment lies in the fact that is solubilizes the organic compounds present in the biomass with minimum energy, with no severe changes in substrate environment. Biological pretreatment is done with or without enzyme addition some of which can be produced endogenously by microorganisms present in the sludge. Some of the enzymes like protease, lipase, cellulase, alpha-amylase and dextranase [11] can effectively improve the hydrolysis rate and release of biopolymers to a large extent. Contrarily, these enzymes are more costly and difficult to preserve. Bonilla et al. [77] evaluated the potential for enzymatic pretreatment of pulp mill biosludge with protease from B. licheniformis for biodegradability. Carrying out BMP test, they arrived at a maximum improvement of 26% in biogas yield.

Saranya et al. [10] studied the impacts of phase separated disintegration pretreatment using calcium chloride (CaCl2) and bacteria. For their study a pH of 6.5, temperature of 40°C and treatment period of 42 h were the optimum conditions for pretreatment. In the initial phase, they achieved the floc disruption (deflocculation) with 0.06 g/g SS of CaCl2 and in the latter phase, cell disintegration through potent biosurfactant producing bacteria, Planococcus jake 01. They were able to achieve 17.14% SS reduction and 14.14% COD solubilization for deflocculated and bacterially pretreated sludge, which were comparatively higher than for sludge treated with bacteria alone. They observed a biogas yield potential for pretreated sludge of 0.322 L/g VS as against 0.145 L/g VS for control. Kavitha et al. [43] investigated the bacterial-based biological pretreatment on liquefaction of microalga Chlorella vulgaris with cellulase-secreting bacteria prior to anaerobic

biodegradation. The biomethanation studies implied that bacterial pretreatment increased the bioavailability of biomass and hence methane generation. They arrived at a methane yield of nearly twice that of control.

ultrasonic assisted microwave disintegration (UMWD) when compared to microwave disintegration MWD (20.9%). Their results of BMP test showed that UMWD

Ammonia fiber expansion is a promising method especially to pretreat agricultural materials for bioenergy production. Ammonia can be easily recovered and presents a high selectivity towards the lignin reactions, while preserving the carbohydrates. Ammonia can also penetrate the crystalline structure of cellulose and causes swelling [30]. The method involves treating the lignocellulosic biomass with liquid ammonia under mild temperature (70–200°C) and pressure (100–400 psi) for a specific time. This explosion results in several physical and chemical alterations in the structure of biomass. Jurado et al. [32] studied the effect of aqueous ammonia soaking (AAS) as a method to disrupt the lignocellulosic structure and increase the methane yield of wheat straw, miscanthus and willow. In all three cases, with AAS they observed an increase in methane yield from 37 to 41%, 25 to 27% and 94 to 162% for wheat straw, miscanthus and willow, respectively. Antonopoulou et al. [30] employed AAS as a pretreatment method, for the AD of three lignocellulosic biomass—poplar sawdust, sunflower straw and grass. In their study, they arrived at an increase in the ultimate methane yield being 148.7, 37.7 and 26.2% of poplar, sunflower straw and grass, respectively. They did not observe

any toxic compounds such as furaldehydes, during AAS pretreatment.

In this pretreatment, a very short burst (100 μs) of rapidly pulsed (several kHz), high voltage (about 20 kV) electric field is utilized to disrupt and break up the cell membrane of microorganism. This focused pulse (FP) induces a critical electrical potential across the cell membrane, causing cell lysis by direct attack on phospholipids and the peptidoglycan, respectively. Once the cell membranes get

has better amenability towards AD with 50% higher methane production representing enhanced liquefaction potential of disaggregated sludge biomass. Jang and Ahn [5] determined the effect of MW irradiation with NaOH pretreatment on AD of thickened municipal WAS in semi-continuous mesophilic digesters at HRT of 15, 10, 7, and 5 days. They combined MW pretreatment at temperature of 135°C with the input power of 1000 W with 60 ml of alkaline (20 meq NaOH/l) pretreated sludge. The degree of substrate solubilization arrived was 18 times higher in pretreated sludge (53.2%) than in raw sludge (3.0%). With HRT reduced to 5 days, they observed an improvement in biogas production (205% higher) for pretreated sludge compared with the control. The results show that MW irradiation combined with alkali pretreatment is effective in increasing mesophilic anaerobic biodegradability of sewage sludge. Ebenezer et al. [58] reported an increased COD and biopolymers release of WAS treated with Sodium citrate, a cationic binding agent, followed by microwaves pretreatment. They also concluded that the above pretreatment made the biomass more amenable for batch AD and hence higher biogas production with a methane content of 60–70% of biogas volume. Tamilarasan et al. [28] has made an attempt, by coupling a mechanical disperser with a chemical Sodium tripolyphosphate (STPP) for pretreatment of macroalgal biomass. They arrived at a 15% liquefaction and more than 5 times higher methane production compared to control at an optimal disperser-specific energy input of about 3312.67 kJ/kg TCOD (total COD) and an STPP dosage of about 0.04 g/g COD. Thus the combined pretreatment showed a greater biodegradability

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

and biomethanation properties.

4.5.3 Ammonia fiber expansion

4.6 Electrical

103

Fungal pretreatment improves degradation of lignin and hemicellulose and hence result in increased digestibility of cellulose, which is preferably essential for AD process. Several fungal classes, including brown-, white- and soft-rot fungi, have been used for pretreatment of lignocellulosic biomass for biogas production, with white-rot fungi being the most effective. Amirta et al. [78] employed four fungal species to pretreat Japanese cedar wood chips in the presence of wheat bran which supplements nutrition for fungal growth. They revealed that wood chips pretreated by Ceriporiopsis subvermispora ATCC 90467 produced the highest methane yield, which was 4 times higher than that of the control biomass at the end of 8 weeks.

### 4.5 Combined treatment

### 4.5.1 Steam explosion

Steam explosion pretreatment is an effort to expose the biomass to high temperature and pressure for short period of time and then reducing the pressure rapidly. This stops the reactions, causing the biomass to decompose explosively. This pretreatment condition may involve temperatures as high as 260°C and pressure up to 4.5 MPa. A study was investigated by Nges et al. [79] to improve the anaerobic biodegradability of Miscanthus lutarioriparius for biogas production. Employing steam explosion pretreatment with 0.3 M NaOH with particle size reduced to 0.5, they achieved a methane yield of 57% higher than that for the untreated samples. Their result was estimated to be 71% of theoretical methane yield of the biomass. Wang et al. [80] achieved a 24% higher methane yield than untreated bulrush at 1.72 MPa steam pressure, 8.14 min residence time, and 11% moisture content employing steam-explosion treatment of bulrush. During the pretreatment they observed the breakage, disruption, and redistribution of the rigid lignin structure which was proved by thermos gravimetric analysis. Srisang and Chavalparit [81] optimized a pre-treatment condition of 1.0% acetic acid, 17.45 min reaction time of sugarcane bagasse using steam explosion at 180° C. They achieved a maximum biogas production (434.47 L/kg VS) which was 91.88% higher than that of control (226.42 L/kg VS).

### 4.5.2 Physico chemical

The combination of thermal and chemical pre-treatments have been investigated in a number of studies in which the enhancement of the anaerobic digestibility of sludge was reported. Yi et al. [19] has used combined alkaline and lowtemperature thermal pretreatment to enhance the subsequent AD of WAS. Different combinations of these two methods were investigated and biochemical methane potential (BMP) test was used to assess the anaerobic digestibility of pretreated WAS. With the combined treatment of adding 0.05 g NaOH/g TS and temperature maintained at 70°C for 9 h, they achieved a ratio of 72.8% soluble carbohydrate/ total carbohydrate. Biogas production achieved through their BMP experiment was six times higher than the control and the average value of methane content of the produced biogas was 64%. In another study, Kavitha et al. [56], employed microwave irradiation to disintegrate the dairy WAS biomass after deagglomerating the sludge using a mechanical device, ultrasonicator. The outcomes of their study revealed that a higher biomass lysis efficiency of about 33.2% was possible through

### Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

biodegradation. The biomethanation studies implied that bacterial pretreatment increased the bioavailability of biomass and hence methane generation. They

Fungal pretreatment improves degradation of lignin and hemicellulose and hence result in increased digestibility of cellulose, which is preferably essential for AD process. Several fungal classes, including brown-, white- and soft-rot fungi, have been used for pretreatment of lignocellulosic biomass for biogas production, with white-rot fungi being the most effective. Amirta et al. [78] employed four fungal species to pretreat Japanese cedar wood chips in the presence of wheat bran which supplements nutrition for fungal growth. They revealed that wood chips pretreated by Ceriporiopsis subvermispora ATCC 90467 produced the highest methane yield, which was 4 times higher than that of the control biomass at the end of

Steam explosion pretreatment is an effort to expose the biomass to high temperature and pressure for short period of time and then reducing the pressure rapidly. This stops the reactions, causing the biomass to decompose explosively. This pretreatment condition may involve temperatures as high as 260°C and pressure up to 4.5 MPa. A study was investigated by Nges et al. [79] to improve the anaerobic biodegradability of Miscanthus lutarioriparius for biogas production. Employing steam explosion pretreatment with 0.3 M NaOH with particle size reduced to 0.5, they achieved a methane yield of 57% higher than that for the untreated samples. Their result was estimated to be 71% of theoretical methane yield of the biomass. Wang et al. [80] achieved a 24% higher methane yield than untreated bulrush at 1.72 MPa steam pressure, 8.14 min residence time, and 11% moisture content employing steam-explosion treatment of bulrush. During the pretreatment they observed the breakage, disruption, and redistribution of the rigid lignin structure which was proved by thermos gravimetric analysis. Srisang and Chavalparit [81] optimized a pre-treatment condition of 1.0% acetic acid, 17.45 min reaction time of sugarcane bagasse using steam explosion at 180° C. They achieved a maximum biogas production (434.47 L/kg VS) which was 91.88% higher than that

The combination of thermal and chemical pre-treatments have been investigated in a number of studies in which the enhancement of the anaerobic digestibility of sludge was reported. Yi et al. [19] has used combined alkaline and lowtemperature thermal pretreatment to enhance the subsequent AD of WAS. Different combinations of these two methods were investigated and biochemical methane potential (BMP) test was used to assess the anaerobic digestibility of pretreated WAS. With the combined treatment of adding 0.05 g NaOH/g TS and temperature maintained at 70°C for 9 h, they achieved a ratio of 72.8% soluble carbohydrate/ total carbohydrate. Biogas production achieved through their BMP experiment was six times higher than the control and the average value of methane content of the produced biogas was 64%. In another study, Kavitha et al. [56], employed microwave irradiation to disintegrate the dairy WAS biomass after deagglomerating the sludge using a mechanical device, ultrasonicator. The outcomes of their study revealed that a higher biomass lysis efficiency of about 33.2% was possible through

arrived at a methane yield of nearly twice that of control.

8 weeks.

4.5 Combined treatment

of control (226.42 L/kg VS).

4.5.2 Physico chemical

102

4.5.1 Steam explosion

Anaerobic Digestion

ultrasonic assisted microwave disintegration (UMWD) when compared to microwave disintegration MWD (20.9%). Their results of BMP test showed that UMWD has better amenability towards AD with 50% higher methane production representing enhanced liquefaction potential of disaggregated sludge biomass.

Jang and Ahn [5] determined the effect of MW irradiation with NaOH pretreatment on AD of thickened municipal WAS in semi-continuous mesophilic digesters at HRT of 15, 10, 7, and 5 days. They combined MW pretreatment at temperature of 135°C with the input power of 1000 W with 60 ml of alkaline (20 meq NaOH/l) pretreated sludge. The degree of substrate solubilization arrived was 18 times higher in pretreated sludge (53.2%) than in raw sludge (3.0%). With HRT reduced to 5 days, they observed an improvement in biogas production (205% higher) for pretreated sludge compared with the control. The results show that MW irradiation combined with alkali pretreatment is effective in increasing mesophilic anaerobic biodegradability of sewage sludge. Ebenezer et al. [58] reported an increased COD and biopolymers release of WAS treated with Sodium citrate, a cationic binding agent, followed by microwaves pretreatment. They also concluded that the above pretreatment made the biomass more amenable for batch AD and hence higher biogas production with a methane content of 60–70% of biogas volume. Tamilarasan et al. [28] has made an attempt, by coupling a mechanical disperser with a chemical Sodium tripolyphosphate (STPP) for pretreatment of macroalgal biomass. They arrived at a 15% liquefaction and more than 5 times higher methane production compared to control at an optimal disperser-specific energy input of about 3312.67 kJ/kg TCOD (total COD) and an STPP dosage of about 0.04 g/g COD. Thus the combined pretreatment showed a greater biodegradability and biomethanation properties.

### 4.5.3 Ammonia fiber expansion

Ammonia fiber expansion is a promising method especially to pretreat agricultural materials for bioenergy production. Ammonia can be easily recovered and presents a high selectivity towards the lignin reactions, while preserving the carbohydrates. Ammonia can also penetrate the crystalline structure of cellulose and causes swelling [30]. The method involves treating the lignocellulosic biomass with liquid ammonia under mild temperature (70–200°C) and pressure (100–400 psi) for a specific time. This explosion results in several physical and chemical alterations in the structure of biomass. Jurado et al. [32] studied the effect of aqueous ammonia soaking (AAS) as a method to disrupt the lignocellulosic structure and increase the methane yield of wheat straw, miscanthus and willow. In all three cases, with AAS they observed an increase in methane yield from 37 to 41%, 25 to 27% and 94 to 162% for wheat straw, miscanthus and willow, respectively. Antonopoulou et al. [30] employed AAS as a pretreatment method, for the AD of three lignocellulosic biomass—poplar sawdust, sunflower straw and grass. In their study, they arrived at an increase in the ultimate methane yield being 148.7, 37.7 and 26.2% of poplar, sunflower straw and grass, respectively. They did not observe any toxic compounds such as furaldehydes, during AAS pretreatment.

### 4.6 Electrical

In this pretreatment, a very short burst (100 μs) of rapidly pulsed (several kHz), high voltage (about 20 kV) electric field is utilized to disrupt and break up the cell membrane of microorganism. This focused pulse (FP) induces a critical electrical potential across the cell membrane, causing cell lysis by direct attack on phospholipids and the peptidoglycan, respectively. Once the cell membranes get

damaged, the intracellular organic material are released, making complex organic macromolecules more biodegradable [82]. They evaluated the effects of FP treatment and SRT on WAS in laboratory-scale digesters operated at SRTs of 2–20 days. They achieved an increased methane production rate and TCOD removal efficiency of about 33% and 18%, respectively, at a SRT of 20 days. They also concluded that, an increase in the hydrolysis rate was caused by FP-treatment of WAS, particularly at lower SRTs. Salerno et al. [83] applied FP to WAS and pig manure for increasing the production of methane during AD. In their work, methane production increased 200% for sludge and 80% for pig manure as compared to untreated sludge and manure. Thus PEF technology is advantageous due to low energy requirement for very short pulse time.

methods are expensive or have a high energy demand. The performance of any pretreatment method is quantified based on the economic feasibility of the method in terms of the cost of pretreatment versus the value of added methane yield. The effect of the pretreatment is however mostly dependent on the biomass composition and operating conditions. The investment costs for pretreatment of recalcitrant substrates are high at the moment due to high expenditure in process engineering. Biological disintegration is devoid of chemical contamination and energy inputs and the use of an enzyme secreting bacterial consortium for biomass is beneficial, as commercial enzymes are expensive [55]. But the need for long reaction times renders biological pretreatment unsuitable for large scale plants where land space is

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

Most studies reviewed assessed the impact of pretreatment processes on the biogas yield on a laboratory scale with a few determining the net energy gain/loss obtained after pretreatment [11, 28, 58]. Most studies in the literature are conducted

This chapter concludes the effect of various biomass pretreatment for enhancement of biogas production and the future challenges for an energy efficient and ecofriendly manner. Therefore, optimizing the pretreatment conditions in order to lower production costs, improving the process performance and production of fewer residues is needed. A pretreatment method optimized based on the above situations may enhance the performance of individual pretreatments and achieve technical, environmental and financial feasibility. However, a further research on combined pretreatments is necessary in the future to get useful information that

\*

as lab scale experiments and do not represent the same output that could be achieved through large scale biogas production facilities. Hence, there is a continuous need for newer and cleaner methods of biomass processing with less energy

may lead to the necessary improvements in the AD industry.

Tamilarasan Karuppiah<sup>1</sup> and Vimala Ebenezer Azariah<sup>2</sup>

\*Address all correspondence to: vimalajulius1@gmail.com

2 V V College of Engineering, Tirunelveli, India

provided the original work is properly cited.

1 Department of Civil Engineering, Regional Centre of Anna University,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

expensive or restricted.

Author details

Tirunelveli, India

105

demand and lower waste generation.

### 5. Future challenges and conclusion

The global energy supply is highly relying on fossil sources (crude oil, coal, natural gas) till now. According to the current energy policies and management, world market energy consumption is forecast to increase by 44% from 2006 to 2030 [84]. At the same time, concentrations of greenhouse gases in the atmosphere are rising rapidly, with fossil fuel-derived CO2 emissions being the most important contributor. Nowadays, increasing attention has been gained on various strategies for the bioconversion of biomass into methane-rich biogas, due to increased global warming, the need for sustainable waste management and high energy costs [41]. The production of biogas through AD offers significant advantages over other forms of bioenergy production. Unlike fossil fuels, biogas from AD is permanently renewable, as it is produced from biomass, which is a living form of storage of solar energy through photosynthesis [85]. It has been evaluated as one of the most energy-efficient and environmentally beneficial technology for bioenergy production [86]. It can drastically reduce GHG emissions compared to fossil fuels by utilization of locally available resources.

Many sources, such as crops, grasses, leaves, manure, fruit, and vegetable wastes or algae can be used, and the process can be applied in small and large scales in many parts of the world. Energy crops digestion requires prolonged HRT of several weeks to month to achieve complete fermentation with high gas yields and minimized residual gas potential of the digestate [4]. For an increased dissemination of biogas plants, further improvements of the process efficiency, and the development of new technologies for mixing, process monitoring, and process control are necessary. Pretreatment of substrates and the addition of micronutrients offers a major potential for increasing the biogas yield. With the increasing number of biogas plants, also an improvement of the effluent quality is necessary, in order to avoid a contamination of ground water with pathogens and nutrients [3]. The choice of a pretreatment should be made not only based on energy balance and economy, but also various environmental factors such as pathogen removal, use of chemicals, and the possibility for a sustainable use of the residues, impacts on human health and the environment [8]. Carballa et al. [87] evaluated the environmental aspects of different pretreatment methods in terms of abiotic resources depletion potential, eutrophication potential, global warming potential, human and terrestrial toxicity potential through a life cycle assessment.

The profitable operation of a biogas plant relies on low capital and operational expenditures [28]. The frequent approaches including physical, thermal and chemical processes have been commercially implemented nowadays with a number of patented technologies. But research on biological techniques is still undergoing investigations from bench scale to full scale applications. Many pretreatment

### Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

damaged, the intracellular organic material are released, making complex organic macromolecules more biodegradable [82]. They evaluated the effects of FP treatment and SRT on WAS in laboratory-scale digesters operated at SRTs of 2–20 days. They achieved an increased methane production rate and TCOD removal efficiency of about 33% and 18%, respectively, at a SRT of 20 days. They also concluded that, an increase in the hydrolysis rate was caused by FP-treatment of WAS, particularly at lower SRTs. Salerno et al. [83] applied FP to WAS and pig manure for increasing the production of methane during AD. In their work, methane production increased 200% for sludge and 80% for pig manure as compared to untreated sludge and manure. Thus PEF technology is advantageous due to low energy requirement for

The global energy supply is highly relying on fossil sources (crude oil, coal, natural gas) till now. According to the current energy policies and management, world market energy consumption is forecast to increase by 44% from 2006 to 2030 [84]. At the same time, concentrations of greenhouse gases in the atmosphere are rising rapidly, with fossil fuel-derived CO2 emissions being the most important contributor. Nowadays, increasing attention has been gained on various strategies for the bioconversion of biomass into methane-rich biogas, due to increased global warming, the need for sustainable waste management and high energy costs [41]. The production of biogas through AD offers significant advantages over other forms of bioenergy production. Unlike fossil fuels, biogas from AD is permanently renewable, as it is produced from biomass, which is a living form of storage of solar energy through photosynthesis [85]. It has been evaluated as one of the most energy-efficient and environmentally beneficial technology for bioenergy production [86]. It can drastically reduce GHG emissions compared to fossil fuels by

Many sources, such as crops, grasses, leaves, manure, fruit, and vegetable wastes

The profitable operation of a biogas plant relies on low capital and operational expenditures [28]. The frequent approaches including physical, thermal and chemical processes have been commercially implemented nowadays with a number of patented technologies. But research on biological techniques is still undergoing investigations from bench scale to full scale applications. Many pretreatment

or algae can be used, and the process can be applied in small and large scales in many parts of the world. Energy crops digestion requires prolonged HRT of several weeks to month to achieve complete fermentation with high gas yields and minimized residual gas potential of the digestate [4]. For an increased dissemination of biogas plants, further improvements of the process efficiency, and the development of new technologies for mixing, process monitoring, and process control are necessary. Pretreatment of substrates and the addition of micronutrients offers a major potential for increasing the biogas yield. With the increasing number of biogas plants, also an improvement of the effluent quality is necessary, in order to avoid a contamination of ground water with pathogens and nutrients [3]. The choice of a pretreatment should be made not only based on energy balance and economy, but also various environmental factors such as pathogen removal, use of chemicals, and the possibility for a sustainable use of the residues, impacts on human health and the environment [8]. Carballa et al. [87] evaluated the environmental aspects of different pretreatment methods in terms of abiotic resources depletion potential, eutrophication potential, global warming potential, human and terrestrial toxicity

very short pulse time.

Anaerobic Digestion

5. Future challenges and conclusion

utilization of locally available resources.

potential through a life cycle assessment.

104

methods are expensive or have a high energy demand. The performance of any pretreatment method is quantified based on the economic feasibility of the method in terms of the cost of pretreatment versus the value of added methane yield. The effect of the pretreatment is however mostly dependent on the biomass composition and operating conditions. The investment costs for pretreatment of recalcitrant substrates are high at the moment due to high expenditure in process engineering. Biological disintegration is devoid of chemical contamination and energy inputs and the use of an enzyme secreting bacterial consortium for biomass is beneficial, as commercial enzymes are expensive [55]. But the need for long reaction times renders biological pretreatment unsuitable for large scale plants where land space is expensive or restricted.

Most studies reviewed assessed the impact of pretreatment processes on the biogas yield on a laboratory scale with a few determining the net energy gain/loss obtained after pretreatment [11, 28, 58]. Most studies in the literature are conducted as lab scale experiments and do not represent the same output that could be achieved through large scale biogas production facilities. Hence, there is a continuous need for newer and cleaner methods of biomass processing with less energy demand and lower waste generation.

This chapter concludes the effect of various biomass pretreatment for enhancement of biogas production and the future challenges for an energy efficient and ecofriendly manner. Therefore, optimizing the pretreatment conditions in order to lower production costs, improving the process performance and production of fewer residues is needed. A pretreatment method optimized based on the above situations may enhance the performance of individual pretreatments and achieve technical, environmental and financial feasibility. However, a further research on combined pretreatments is necessary in the future to get useful information that may lead to the necessary improvements in the AD industry.

### Author details

Tamilarasan Karuppiah<sup>1</sup> and Vimala Ebenezer Azariah<sup>2</sup> \*

1 Department of Civil Engineering, Regional Centre of Anna University, Tirunelveli, India

2 V V College of Engineering, Tirunelveli, India

\*Address all correspondence to: vimalajulius1@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### References

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[2] Kuglarz M, Karakashev D, Angelidaki I. Microwave and thermal pretreatment as methods for increasing the biogas potential of secondary sludge from municipal wastewater treatment plants. Bioresource Technology. 2013; 134:290-297. DOI: 10.1016/j.biortech. 2013.02.001

[3] Weiland P. Biogas production: Current state and perspectives. Applied Microbiology and Biotechnology. 2010; 85:849-860. DOI: 10.1007/s00253-009- 2246-7

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[5] Jang J, Ahn J. Effect of microwave pretreatment in presence of NaOH on mesophilic anaerobic digestion of thickened waste activated sludge. Bioresource Technology. 2013;131: 437-442. DOI: 10.1016/j.biortech. 2012.09.057

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DOI: http://dx.doi.org/10.5772/intechopen.82088

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sludge on the bioavailability and biodegradability characteristics of the organic fraction. Brazilian Journal of Chemical Engineering. 2009;26: 469-480. DOI: 10.1590/S0104-

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Procedia Environmental Sciences. 2013;

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digestion of pulp and paper

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[9] Show KY, Mao T, Lee DJ. Optimization of sludge disruption by sonication. Water Research. 2007;41: 4741-4747. DOI: 10.1016/j.watres. 2007.07.017

[10] Saranya T, Kavitha S, Kaliappan S, Adish Kumar S, Yeom IT, Banu JR. Accelerating the sludge disintegration potential of a novel bacterial strain Planococcus jake 01 by CaCl2 induced deflocculation. Bioresource Technology. 2015;175:396-405. DOI: 10.1016/j. biortech.2014.10.122

[11] Kavitha S, Adish Kumar S, Yogalakshmi KN, Kaliappan S, Banu JR. Effect of enzyme secreting bacterial pretreatment on enhancement of aerobic digestion potential of waste activated sludge interceded through EDTA. Bioresource Technology. 2013; 150:210-219. DOI: 10.1016/j.biortech. 2013.10.021

[12] Merrylin J, Kaliappan S, Adish Kumar S, Yeom IT, Banu JR. Enhancing aerobic digestion potential of municipal waste-activated sludge through removal of extracellular polymeric substance. Environmental Science and Pollution Research International. 2013;21(2): 1112-1123. DOI: 10.1007/s11356-013- 1976-3

[13] Banu JR, Khac UD, Kumar SA, Yeom IT, Kaliappan S. A novel method of sludge pretreatment using the combination of alkalis. Journal of Environmental Biology. 2012;33:249-253 Biomass Pretreatment for Enhancement of Biogas Production DOI: http://dx.doi.org/10.5772/intechopen.82088

[14] Goel R, Tokutomi T, Yasui H. Anaerobic digestion of excess activated sludge with ozone pretreatment. Water Science and Technology. 2013; 47:207-214. DOI: 10.2166/wst.2003. 0648

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2017.08.001

2013.02.001

2246-7

[1] Kannah RY, Kavitha S, Banu JR, Karthikeyan OP, Sivashanmugham OP. Dispersion induced ozone pretreatment of waste activated biosolids: Arriving biomethanation modelling parameters,

2010;101:8984-8992. DOI: 10.1016/j.

[8] Salsabil MR, Prorot A, Casellasa M, Dagot C. Pretreatment of activated sludge: Effect of sonication on aerobic and anaerobic digestibility. Chemical Engineering Journal. 2009;148:327-335.

DOI: 10.1016/j.cej.2008.09.003

Optimization of sludge disruption by sonication. Water Research. 2007;41: 4741-4747. DOI: 10.1016/j.watres.

[10] Saranya T, Kavitha S, Kaliappan S, Adish Kumar S, Yeom IT, Banu JR. Accelerating the sludge disintegration potential of a novel bacterial strain Planococcus jake 01 by CaCl2 induced deflocculation. Bioresource Technology. 2015;175:396-405. DOI: 10.1016/j.

[9] Show KY, Mao T, Lee DJ.

2007.07.017

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2013.10.021

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[13] Banu JR, Khac UD, Kumar SA, Yeom IT, Kaliappan S. A novel method of sludge pretreatment using the combination of alkalis. Journal of Environmental Biology. 2012;33:249-253

Yogalakshmi KN, Kaliappan S, Banu JR. Effect of enzyme secreting bacterial pretreatment on enhancement of aerobic digestion potential of waste activated sludge interceded through EDTA. Bioresource Technology. 2013; 150:210-219. DOI: 10.1016/j.biortech.

biortech.2010.06.128

energetic and cost assessment. Bioresource Technology. 2017;244: 679-687. DOI: 10.1016/j.biortech.

[2] Kuglarz M, Karakashev D,

Angelidaki I. Microwave and thermal pretreatment as methods for increasing the biogas potential of secondary sludge from municipal wastewater treatment plants. Bioresource Technology. 2013; 134:290-297. DOI: 10.1016/j.biortech.

[3] Weiland P. Biogas production: Current state and perspectives. Applied Microbiology and Biotechnology. 2010; 85:849-860. DOI: 10.1007/s00253-009-

agricultural use of digestate.

10.3176/proc.2017.1.10

2012.09.057

106

[4] Kuusik A, Pachel K, Loigu E. Possible

Proceedings of the Estonian Academy of Sciences. 2017;66(1):64-74. DOI:

[5] Jang J, Ahn J. Effect of microwave pretreatment in presence of NaOH on mesophilic anaerobic digestion of thickened waste activated sludge. Bioresource Technology. 2013;131: 437-442. DOI: 10.1016/j.biortech.

[6] Patinvoh RJ, Osadolor OA,

Chandolias K, Horváth IS, Taherzadeh MJ. Innovative pretreatment strategies for biogas production. Bioresource Technology. 2016;224:13-24. DOI: 10.1016/j.biortech.2016.11.083

[7] Apul OG, Sanin FD. Ultrasonic pretreatment and subsequent anaerobic digestion under different operational conditions. Bioresource Technology.

[15] Elliott A, Mahmood T. Pretreatment technologies for advancing anaerobic digestion of pulp and paper biotreatment residues. Water Research. 2007;41:4273-4286. DOI: 10.1016/j. watres.2007.06.017

[16] Beszédes S, László Z, Horváth ZH, Szabó G, Hodúr C. Comparison of the effects of microwave irradiation with different intensities on the biodegradability of sludge from the dairy and meat industry. Bioresource Technology. 2011;102:814-821. DOI: 10.1016/j.biortech.2010.08.121

[17] Eskicioglu C, Kennedy KJ, Droste RL. Enhancement of batch waste activated sludge digestion by microwave pretreatment. Water Environment Research. 2007;79:2304-2317. DOI: 10.2175/106143007X184069

[18] Borges ESM, Chernicharo CAL. Effect of thermal treatment of anaerobic sludge on the bioavailability and biodegradability characteristics of the organic fraction. Brazilian Journal of Chemical Engineering. 2009;26: 469-480. DOI: 10.1590/S0104- 66322009000300003

[19] Yi H, Han Y, Zhuo Y. Effect of combined pretreatment of waste activated sludge for AD process. Procedia Environmental Sciences. 2013; 18:716-721. DOI: 10.1016/j. proenv.2013.04.097

[20] Sahinkaya S, Sevimli MF. Sonothermal pre-treatment of waste activated sludge before anaerobic digestion. Ultrasonics Sonochemistry. 2013;20:587-594. DOI: 10.1016/j. ultsonch.2012.07.006

[21] Rani UR, Kaliappan S, Adish Kumar AS, Banu JR. Combined treatment of alkaline and disperser for improving solubilisation and anaerobic biodegradability of dairy waste activated sludge. Bioresource Technology. 2012;126:107-116. DOI: 10.1016/j.biortech.2012.09.027

[22] Strong PJ, McDonald B, Gapes DJ. Combined thermo alkaline and fermentative destruction of municipal biosolids: A comparison between thermal hydrolysis and wet oxidative pre-treatment. Bioresource Technology. 2011;102:5520-5527. DOI: 10.1016/j. biortech.2010.12.027

[23] Shah FA, Mahmoo Q, Shah MM, Pervez A, Asad SA. Microbial ecology of anaerobic digesters: The key players of anaerobiosis. Scientific World Journal. Article ID 183752. 2014. Open Access. 21 p. DOI: 10.1155/2014/183752

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Do KU, Banu JR. Combined

109

Effect of deflocculation on the efficiency of disperser induced dairy waste activated sludge disintegration and treatment cost. Bioresource Technology. 2014;167:151-158. DOI: 10.1016/j.biortech.2014.06.004

biortech.2017.02.081

03.087

Enhanced biomethane production from Miscanthus lutarioriparius using steam explosion pretreatment. Fuel. 2016;179: 267-273. DOI: 10.1016/j.fuel.2016.

DOI: http://dx.doi.org/10.5772/intechopen.82088

Biomass Pretreatment for Enhancement of Biogas Production

thermo-chemo-sonic disintegration of waste activated sludge for biogas production. Bioresource Technology. 2015;197:383-392. DOI: 10.1016/j.

[48] Kavitha S, Banu JR, IvinShaju CD, Kaliappan S, Yeom IT. Fenton mediated ultrasonic disintegration of sludge biomass: Biodegradability studies, energetic assessment, and its economic viability. Bioresource Technology. 2016; 221:1-8. DOI: 10.1016/j.biortech.2016.

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solubilizing waste activated sludge for biogas production: Energetic analysis and economic assessment. Bioresource Technology. 2016;219:479-486. DOI: 10.1016/j.biortech.2016.07.115

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production. Environmental Science and Pollution Research. 2016;23:2402-2414. DOI: 10.1007/s11356-015-5461-z

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[53] Ushani U, Banu JR, Kavitha S, Kaliappan S, Yeom IT. Immobilized and MgSO4 induced cost effective bacterial

Effect of chemo-mechanical disintegration on sludge anaerobic digestion for enhanced biogas

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chemo-sonic pretreatment in

biortech.2015.08.131

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[37] Barone JR, Schmidt WF, Gregoire NT. Extrusion of feather keratin. Journal of Applied Polymer Science. 2006; 100(2):1432-1442. DOI: 10.1002/

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[39] Kim HT, Yun EJ, Wang D, Chung

temperature and low acid pretreatment and agarose treatment of agarose for the production of sugar and ethanol from red seaweed biomass. Bioresource Technology. 2013;136: 582-587. DOI: 10.1016/j.biortech.

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185:194-201. DOI: 10.1016/j.

JH, Choi IG, Kim KH. A high

biortech.2015.02.102

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[46] Rani RU, Adish Kumar S, Kaliappan S, Banu JR. Enhancing the anaerobic digestion potential of dairy waste activated sludge by two step sonoalkalization pretreatment. Ultrasonics Sonochemistry. 2014;21:1065-1074. DOI: 10.1016/j.ultsonch.2013.11.007

[47] Kavitha S, Kannah RY, Yeom IT, Do KU, Banu JR. Combined

thermo-chemo-sonic disintegration of waste activated sludge for biogas production. Bioresource Technology. 2015;197:383-392. DOI: 10.1016/j. biortech.2015.08.131

[48] Kavitha S, Banu JR, IvinShaju CD, Kaliappan S, Yeom IT. Fenton mediated ultrasonic disintegration of sludge biomass: Biodegradability studies, energetic assessment, and its economic viability. Bioresource Technology. 2016; 221:1-8. DOI: 10.1016/j.biortech.2016. 09.012

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[52] Kavitha S, Brindha GMJ, Gloriana AS, Rajashankar K, Yeom IT, Banu JR. Enhancement of aerobic biodegrdability potential of municipal waste activated sludge by ultrasonic aided bacterial disintegration. Bioresource Technology. 2016;200:161-169. DOI: 10.1016/j. biortech.2015.10.026

[53] Ushani U, Banu JR, Kavitha S, Kaliappan S, Yeom IT. Immobilized and MgSO4 induced cost effective bacterial

disintegration of waste activated sludge for effective anaerobic digestion. Chemosphere. 2017;175:66-75. DOI: 10.1016/j.chemosphere. 2017.02.046

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and pretreatment. Bioresource Technology. 2012;123:488-494. DOI: 10.1016/j.biortech.2012.06.113

Biomass Pretreatment for Enhancement of Biogas Production

DOI: http://dx.doi.org/10.5772/intechopen.82088

Comparison of dilute acid and ionic liquid pretreatment of switchgrass: Biomass recalcitrance, delignification and enzymatic saccharification. Bioresource Technology. 2010;101: 4900-4906. DOI: 10.1016/j.biortech.

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btre.2017.12.009

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Green Energy. 2016;13(11):

1175356

2013.06.095

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[68] Maroušek J. Finding the optimal parameters for the steam explosion process of hay. Revista Técnica de la Facultad de Ingeniería Universidad del Zulia. 2012;35(2):170-178. DOI: 10.1186/

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pretreatment of dairy waste activated sludge on the performance of microbial fuel cell. International Journal of Electrochemical Science. 2014;9:

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R, Scheller HV, Auer M, et al.

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and pretreatment. Bioresource Technology. 2012;123:488-494. DOI: 10.1016/j.biortech.2012.06.113

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[59] Eswari P, Kavitha S, Kaliappan S, Yeom IT, Banu JR. Enhancement of sludge anaerobic biodegradability by

combined microwave-H2O2 pretreatment in acidic conditions. Environmental Science and Pollution Research. 2016;23:13467-13479. DOI:

10.1007/s11356-016-6543-2

2017.07.078

pecs.2014.01.001

cej.2010.11.069

pretreatment processes of

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[65] Alzate ME, Munoz R, Rogalla F, Fdz-Polanco F, Perez-Elvira SI. Biochemical methane potential of microalgae: Influence of substrate to inoculum ratio, biomass concentration

Sivashanmugham P. Effect of surfactant

[55] Banu JR, Kannah RK, Kavitha S, Gunasekaran M, Yeom IT, Kumar G.

Disperser-induced bacterial disintegration of partially digested anaerobic sludge for efficient biomethane recovery. Chemical Engineering Journal. 2018;347:165-172.

DOI: 10.1016/j.cej.2018.04.096

[56] Kavitha S, Banu JR, Kumar G, Kaliappan S, Yeom IT. Profitable ultrasonic assisted microwave disintegration of sludge biomass: Modelling of biomethanation and energy parameter analysis. Bioresource Technology. 2018;254:203-213. DOI: 10.1016/j.biortech.2018.01.072

[57] Tamilarasan K, Arulazhagan P, Uma Rani R, Kaliappan S, Banu JR. Synergistic impact of sonic—Tenside on

biomass disintegration potential: Acidogenic and methane potential studies, kinetics and cost analytics. Bioresource Technology. 2018;253: 256-261. DOI: 10.1016/j.biortech.

[58] Ebenezer AV, Arulazhagan P, Adish Kumar S, Yeom IT, Banu JR. Effect of deflocculation on the efficiency of low energy microwave pretreatment and anaerobic biodegradability of waste activated sludge. Applied Energy. 2015;

145:104-110. DOI: 10.1016/j. apenergy.2015.01.133

2018.01.028

110

2017.02.046

Anaerobic Digestion

2018.05.054

[54] Shanthi M, Banu JR,

assisted sonic pretreatment on liquefaction of fruits and vegetables residue: Characterization, acidogenesis, biomethane yield and energy ratio. Bioresource Technology. 2018;264: 35-41. DOI: 10.1016/j.biortech.

[66] Patil PN, Gogate PR, Csoka L, Dregelyi-Kiss A, Horvath M. Intensification of biogas production using pretreatment based on hydrodynamic cavitation. Ultrasonics Sonochemistry. 2016;30:79-86. DOI: 10.1016/j.ultsonch.2015.11.009

[67] Rincón B, Bujalance L, Fermoso FG, Martín A, Borja R. Biochemical methane potential of two-phase olive mill solid waste: Influence of thermal pretreatment on the process kinetics. Bioresource Technology. 2013;140: 249-255. DOI: 10.1016/j.biortech. 2013.04.090

[68] Maroušek J. Finding the optimal parameters for the steam explosion process of hay. Revista Técnica de la Facultad de Ingeniería Universidad del Zulia. 2012;35(2):170-178. DOI: 10.1186/ s40643-017-0137-9

[69] Novarino D, Zanetti MC. Anaerobic digestion of extruded OFMSW. Bioresource Technology. 2011;104: 44-50. DOI: 10.1016/j.biortech. 2011.10.001

[70] Wilson C, Novak JT. Hydrolysis of macromolecular components of primary and secondary wastewater sludge by thermal hydrolytic pretreatment. Water Research. 2009;43:4489-4498. DOI: 10.1016/j.watres.2009.07.022

[71] Jayashree C, Janshi G, Yeom IT, Kumar SA, Banu JR. Effect of low temperature thermo-chemical pretreatment of dairy waste activated sludge on the performance of microbial fuel cell. International Journal of Electrochemical Science. 2014;9: 5732-5742

[72] Li C, Knierim B, Manisseri C, Arora R, Scheller HV, Auer M, et al.

Comparison of dilute acid and ionic liquid pretreatment of switchgrass: Biomass recalcitrance, delignification and enzymatic saccharification. Bioresource Technology. 2010;101: 4900-4906. DOI: 10.1016/j.biortech. 2009.10.066

[73] Devlin DC, Esteves SRR, Dinsdale RM, Guwy AJ. The effect of acid pretreatment on the anaerobic digestion and dewatering of waste activated sludge. Bioresource Technology. 2011; 102:4076-4082. DOI: 10.1016/j. biortech.2010.12.043

[74] Taherdanak M, Zilouei H, Karimi K. The influence of dilute sulfuric acid pretreatment on biogas production form wheat plant. International Journal of Green Energy. 2016;13(11): 1129-1134. DOI: 10.1080/15435075.2016. 1175356

[75] Sambusiti C, Ficara E, Malpei F, Steyer JP, Carrère H. Benefit of sodium hydroxide pretreatment of ensiled sorghum forage on the anaerobic reactor stability and methane production. Bioresource Technology. 2013;144: 149-155. DOI: 10.1016/j.biortech. 2013.06.095

[76] Chandra TS, Malik SN, Suvidha G, Padmere ML, Shanmugam P, Mudliar SN. Wet air oxidation pretreatment of biomethanated distillery effluent: Mapping pretreatment efficiency in terms color, toxicity reduction and biogas generation. Bioresource Technology. 2014;158:135-140. DOI: 10.1016/j.biortech.2014.01.106

[77] Bonilla S, Choolaei Z, Meyer T, Edwards EA, Yakunin AF, Allen DG. Evaluating the effect of enzymatic pretreatment on the anaerobic digestibility of pulp and paper biosludge. Biotechnology Reports. 2018; 17:77-85. DOI: doi.org/10.1016/j. btre.2017.12.009

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**113**

**Chapter 6**

**Abstract**

Digestion

Techno-Economic Analysis

of Biogas Production from

Microalgae through Anaerobic

*Na Wu, Cesar M. Moreira, Yingxiu Zhang, Nguyet Doan,* 

Microalgae are a promising feedstock for bioenergy due to higher productivity, flexible growing conditions, and high lipid/polysaccharide content compared to terrestrial biomass. Microalgae can be converted to biogas through anaerobic digestion (AD). AD is a mature technology with a high energy return on energy invested. Microalgae AD can bypass energy intensive dewatering operations that are associated with liquid fuel production from algae. A techno-economic assessment of the commercial feasibility of algae-based biogas production was conducted using Cyanothece BG0011 biomass as an example. BG0011 is a naturally occurring, saline cyanobacterium isolated from Florida Keys. It fixes atmospheric nitrogen and produces exopolysaccharide (EPS). Maximum cell density and EPS

g afdw/L) were obtained by air sparging. For an areal cell and EPS productivity of 12.4 and 9.6 g afdw/m2/day, respectively, the biomethane production cost was 14.8 \$/MMBtu using covered anaerobic lagoon and high-pressure water scrubbing for biogas purification. Electricity production from biogas costs 13 cents/kwh. If areal productivity was increased by 33% from the same system, by sparging air enriched with 1% CO2, then biomethane cost was reduced to 12.16 \$/MMBtu and electricity

**Keywords:** microalgae, anaerobic digestion, biogas, techno-economic analysis,

Resource depletion and carbon emissions caused by using fossil fuels have increased interest in alternative fuel sources. Utilization of biomass resources is one option to meet the energy requirements for rapid industrialization and

/L (for total algae biomass concentration of 4.8

*Shunchang Yang, Edward J. Phlips, Spyros A. Svoronos* 

*and Pratap C. Pullammanappallil*

concentration of 2.7 and 2.1 g afdw1

cost to 11 cents/kwh.

*Cyanothece* BG0011

**1. Introduction**

<sup>1</sup> Ash free dry weight.

### **Chapter 6**

[78] Amirta R, Tanabe T, Hondaa Y, Kuwahara M, Watanabe T. Methane fermentation of Japanese cedar wood pretreated with a white rot fungus, Ceriporiopsis subvermispora. Journal of Biotechnology. 2006;123:71-77. DOI: 10.1016/j.jbiotec.2005. 10.004

Anaerobic Digestion

[85] Al Seadi T, Rutz D, Prassl H, Köttner M, Finsterwalder T, Volk S, et al. Biogas Handbook. Esbjerg, Denmark: University of Southern Denmark Esbjerg, Niels Bohrs Vej;

[86] Panwar NL, Kaushik SC, Kothari S. Role of renewable energy sources in environmental protection: A review. Renewable and Sustainable Energy Reviews. 2011;15:1513-1524. DOI: 10.1016/j.rser.2010.11.037

[87] Carballa M, Duran C, Hospido A. Should we pretreat solid waste prior to anaerobic digestion? An assessment of its environmental cost. Environmental Science & Technology. 2011;45: 10306-10314. DOI: 10.1021/es201866u

2008. pp. 9-10

[79] Nges IA, Li C, Wang B, Xiao L, Yi Z, Liu J. Physio-chemical pretreatments for

[80] Wang J, Yue ZB, Chen TH, Peng SC, Yu HQ, Chen HZ. Anaerobic digestibility and fiber composition of bulrush in response to steam explosion. Bioresource Technology. 2010;101:

improved methane potential of Miscanthus lutarioriparius. Fuel. 2016; 166:29-35. DOI: 10.1016/j.fuel.2015.

6610-6614. DOI: 10.1016/j. biortech.2010.03.086

[81] Srisang N, Chavalparit O. Enhancing biogas production from sugarcane bagasse using steam

AMR.856.321

explosion in according with acetic acid pretreatment. Advanced Materials Research. 2013;856:321-326. DOI: 10.4028/www.scientific.net/

[82] Lee I, Rittmann BR. Effect of low solids retention time and focused pulsed pre-treatment on AD of waste activated sludge. Bioresource Technology. 2011;

[83] Salerno MB, Lee HS, Parameswaran P, Rittmann B. Using a pulsed electric field as a pretreatment for improved biosolids digestion and methanogenesis. Water Environment Research. 2009;

[84] IEO, International Energy Outlook. Energy Information Administration. Office of Integrated Analysis and Forecasting. Washington, DC: US Department of Energy; 2009. p. 284

102:2542-2548. DOI: 10.1016/j.

81(8):831-839. DOI: 10.2175/

106143009X407366

112

biortech.2010.11.082

10.108

## Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion

*Na Wu, Cesar M. Moreira, Yingxiu Zhang, Nguyet Doan, Shunchang Yang, Edward J. Phlips, Spyros A. Svoronos and Pratap C. Pullammanappallil*

### **Abstract**

Microalgae are a promising feedstock for bioenergy due to higher productivity, flexible growing conditions, and high lipid/polysaccharide content compared to terrestrial biomass. Microalgae can be converted to biogas through anaerobic digestion (AD). AD is a mature technology with a high energy return on energy invested. Microalgae AD can bypass energy intensive dewatering operations that are associated with liquid fuel production from algae. A techno-economic assessment of the commercial feasibility of algae-based biogas production was conducted using Cyanothece BG0011 biomass as an example. BG0011 is a naturally occurring, saline cyanobacterium isolated from Florida Keys. It fixes atmospheric nitrogen and produces exopolysaccharide (EPS). Maximum cell density and EPS concentration of 2.7 and 2.1 g afdw1 /L (for total algae biomass concentration of 4.8 g afdw/L) were obtained by air sparging. For an areal cell and EPS productivity of 12.4 and 9.6 g afdw/m2/day, respectively, the biomethane production cost was 14.8 \$/MMBtu using covered anaerobic lagoon and high-pressure water scrubbing for biogas purification. Electricity production from biogas costs 13 cents/kwh. If areal productivity was increased by 33% from the same system, by sparging air enriched with 1% CO2, then biomethane cost was reduced to 12.16 \$/MMBtu and electricity cost to 11 cents/kwh.

**Keywords:** microalgae, anaerobic digestion, biogas, techno-economic analysis, *Cyanothece* BG0011

### **1. Introduction**

Resource depletion and carbon emissions caused by using fossil fuels have increased interest in alternative fuel sources. Utilization of biomass resources is one option to meet the energy requirements for rapid industrialization and

<sup>1</sup> Ash free dry weight.

population growth with potential environmental and economic benefits. Energy could be derived from a variety of terrestrial, renewable, bio-based feedstocks like sugar-based biomass (e.g. corn, sugarcane, sugarbeet) and lignocellulosic biomass (e.g. wheat straw, corn stover, sugarcane bagasse, forestry residues, switchgrass, energy cane, sorghum, short rotation woody crops). However, production and conversion of these feedstocks could entail risks associated with disruption of the food chain and biodiversity, depletion of freshwater resources and eutrophication.

Aquatic biomass like microalgae is a promising feedstock with many advantages over terrestrial plants. Its use dates to 1940s [1, 2]. To meet an energy shortage during this period, microalgal biomass was proposed to be used as a source for lipids. Microalgae have higher yield from incident solar energy and higher areal productivity. The photosynthetic efficiency of microalgae (around 3–8%) is substantially higher than that of terrestrial plants (typically 0.5%) due to their simple structure and convenient access to nutrients [3–5, 108]. Therefore, less land area is required and non-arable, non-productive land could be used for their cultivation. Some species could be cultivated using low quality water such as seawater, brackish water, desalination reject water and wastewater. A microalgae production facility could be operated as a closed loop system by allowing for recycling of water, nutrients and energy from downstream production processes [6, 7, 144]. Microalgae are characterized by high lipid/starch/protein content with a lack of lignin, which makes them well-suited for different conversion technologies [8–10]. Besides, microalgae cultivation has less potential to interfere with food and feed production. With such versatility, microalgae appear to be a promising biorenewable resource that has the potential to completely replace fossil resources [11]. Research in microalgae biotechnology has increased dramatically since 2005 and has been a very active field in recent years, especially to produce biomass and biofuels [12, 110, 111, 117, 118, 136, 143].

Though microalgae may demonstrate benefits over terrestrial feedstocks, the major challenges for their production include significant utilization of nutrients, high energy input for harvesting and dewatering, and complex downstream conversion processes for usable fuels like ethanol and biodiesel [6, 8, 100, 109, 131]. An alternative which can potentially decrease the energy footprint could be biogas production through anaerobic digestion [122, 125, 127, 137]. Anaerobic digestion (AD) is a biochemical process that mineralizes organic compounds to biogas through the synergistic and concerted action of microorganisms under anaerobic (O2 free) conditions. Dry biogas is primarily a mixture of methane and carbon dioxide with traces of ammonia, volatile organic compounds and hydrogen sulfide. Methane content of dry biogas usually ranges between 50 and 70% (by volume). Methane has a higher heating value on a mass basis when compared to liquid fuels, such as biodiesel and bioethanol [13, 145]. AD has been recognized as a mature technology to treat organic waste streams and is widely practiced due to its high energy output to input ratio, environmental benefits, as well as for its process simplicity—compared to bioethanol/biodiesel processes [13, 14]. It is suitable for organic feedstock with high moisture content [15] and so can directly be applied to wet algae biomass feedstock with perhaps little dewatering. Besides, no harsh pretreatment is necessary for algal biomass due to the negligible lignin content [14]. The algal biorefinery could be engineered to be resource efficient by recycling phosphorus and nitrogen nutrients in the digestate effluent and carbon dioxide from biogas upgrading processes for microalgae cultivation [13, 14, 16, 17].

In addition to the physical and chemical properties of the fuel as specified by technical standards, the characteristics desired by the stakeholders, distributors

**115**

information.

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

and, consumers could also include sustainability indices related to environmental, social and economic performance. Techno-economic analysis (TEA) establishes a capital and operating cost profile to determine the potential economic viability of the production process for realizing its commercial feasibility. It can be an integral tool to direct research during development of specific technology and assist with investment by averting unnecessary expenditures. A number of techno-economic assessments have been completed to evaluate the economic feasibility of biodiesel derived from microalgae [9, 22, 69, 140, 141]. However, there is a lack of technoeconomic analysis on anaerobic digestion of microalgae for biogas production, especially full-scale production taking the characteristics of algae species into consideration. In this chapter, the entire production process from algae cultivation to biogas upgrading will be discussed emphasizing the key cost drivers. TEA literature is reviewed for methodology and state of art technologies. An example of TEA was conducted based on the biogas production process from a microalgae/cyanobacteria

An anaerobic digestion (AD) process can biochemically convert the whole, wet biomass rather than specific components. The emissions and effluents from the process can be captured for reuse of components like carbon dioxide, ammonia, and phosphorus, and therefore has the potential for economic and environmental benefits. The general biochemical steps in the AD process include: (1) hydrolysis: the breakdown of macromolecules like proteins, lipids, polysaccharides into simpler compounds such as amino acids, sugars, fatty acids and glycerol; (2) acidogenesis and acetogenesis: the hydrolyzed molecules are converted to volatile fatty acids, primarily acetate, hydrogen, and carbon dioxide; (3) methanogenesis: methane production from acetate, hydrogen and carbon dioxide. The hydrolysis step plays a crucial role in determining the successful production of methane [37, 145]. The biochemical processes in AD also occur in nature. AD technology is well established and recognized as a robust technology

Despite the potential, questions related to the economic feasibility and the net energy output are the main hurdles hampering the development of biogas production from microalgae [14, 18–20]. For example, due to the specific structure and composition of the microalgae cell wall, the yield of biogas could be low. Pretreatment to disrupt the cell walls could require high energy inputs. The algae productivity could be low and cultivation cost could be high. Thus, the viability of microalgal biogas production may depend on improvements of efficiency and economic performance. Ongoing efforts include developing inexpensive biomass feedstock, maximizing energy return on investment, and minimizing environmental risks. As only a few studies are available in the literature on the economic feasibility of microalgal biogas exploitation [14], the evaluation and analysis of microalgal biogas production cost will be based on conversion efficiency, technological design aspects as well as available cost

The production of biogas from microalgae feedstock entails a series of steps starting with algae cultivation. Implementation of each step involves capital and

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

species *Cyanothece* BG0011 [82].

to convert biomass to bioenergy [146].

**3. Key drivers of microalgal biogas production cost**

**2. Anaerobic digestion**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

and, consumers could also include sustainability indices related to environmental, social and economic performance. Techno-economic analysis (TEA) establishes a capital and operating cost profile to determine the potential economic viability of the production process for realizing its commercial feasibility. It can be an integral tool to direct research during development of specific technology and assist with investment by averting unnecessary expenditures. A number of techno-economic assessments have been completed to evaluate the economic feasibility of biodiesel derived from microalgae [9, 22, 69, 140, 141]. However, there is a lack of technoeconomic analysis on anaerobic digestion of microalgae for biogas production, especially full-scale production taking the characteristics of algae species into consideration. In this chapter, the entire production process from algae cultivation to biogas upgrading will be discussed emphasizing the key cost drivers. TEA literature is reviewed for methodology and state of art technologies. An example of TEA was conducted based on the biogas production process from a microalgae/cyanobacteria species *Cyanothece* BG0011 [82].

### **2. Anaerobic digestion**

*Anaerobic Digestion*

and eutrophication.

biofuels [12, 110, 111, 117, 118, 136, 143].

population growth with potential environmental and economic benefits. Energy could be derived from a variety of terrestrial, renewable, bio-based feedstocks like sugar-based biomass (e.g. corn, sugarcane, sugarbeet) and lignocellulosic biomass (e.g. wheat straw, corn stover, sugarcane bagasse, forestry residues, switchgrass, energy cane, sorghum, short rotation woody crops). However, production and conversion of these feedstocks could entail risks associated with disruption of the food chain and biodiversity, depletion of freshwater resources

Aquatic biomass like microalgae is a promising feedstock with many advantages

Though microalgae may demonstrate benefits over terrestrial feedstocks, the major challenges for their production include significant utilization of nutrients, high energy input for harvesting and dewatering, and complex downstream conversion processes for usable fuels like ethanol and biodiesel [6, 8, 100, 109, 131]. An alternative which can potentially decrease the energy footprint could be biogas production through anaerobic digestion [122, 125, 127, 137]. Anaerobic digestion (AD) is a biochemical process that mineralizes organic compounds to biogas through the synergistic and concerted action of microorganisms under anaerobic (O2 free) conditions. Dry biogas is primarily a mixture of methane and carbon dioxide with traces of ammonia, volatile organic compounds and hydrogen sulfide. Methane content of dry biogas usually ranges between 50 and 70% (by volume). Methane has a higher heating value on a mass basis when compared to liquid fuels, such as biodiesel and bioethanol [13, 145]. AD has been recognized as a mature technology to treat organic waste streams and is widely practiced due to its high energy output to input ratio, environmental benefits, as well as for its process simplicity—compared to bioethanol/biodiesel processes [13, 14]. It is suitable for organic feedstock with high moisture content [15] and so can directly be applied to wet algae biomass feedstock with perhaps little dewatering. Besides, no harsh pretreatment is necessary for algal biomass due to the negligible lignin content [14]. The algal biorefinery could be engineered to be resource efficient by recycling phosphorus and nitrogen nutrients in the digestate effluent and carbon dioxide from biogas upgrading processes for microalgae

In addition to the physical and chemical properties of the fuel as specified by technical standards, the characteristics desired by the stakeholders, distributors

over terrestrial plants. Its use dates to 1940s [1, 2]. To meet an energy shortage during this period, microalgal biomass was proposed to be used as a source for lipids. Microalgae have higher yield from incident solar energy and higher areal productivity. The photosynthetic efficiency of microalgae (around 3–8%) is substantially higher than that of terrestrial plants (typically 0.5%) due to their simple structure and convenient access to nutrients [3–5, 108]. Therefore, less land area is required and non-arable, non-productive land could be used for their cultivation. Some species could be cultivated using low quality water such as seawater, brackish water, desalination reject water and wastewater. A microalgae production facility could be operated as a closed loop system by allowing for recycling of water, nutrients and energy from downstream production processes [6, 7, 144]. Microalgae are characterized by high lipid/starch/protein content with a lack of lignin, which makes them well-suited for different conversion technologies [8–10]. Besides, microalgae cultivation has less potential to interfere with food and feed production. With such versatility, microalgae appear to be a promising biorenewable resource that has the potential to completely replace fossil resources [11]. Research in microalgae biotechnology has increased dramatically since 2005 and has been a very active field in recent years, especially to produce biomass and

**114**

cultivation [13, 14, 16, 17].

An anaerobic digestion (AD) process can biochemically convert the whole, wet biomass rather than specific components. The emissions and effluents from the process can be captured for reuse of components like carbon dioxide, ammonia, and phosphorus, and therefore has the potential for economic and environmental benefits. The general biochemical steps in the AD process include: (1) hydrolysis: the breakdown of macromolecules like proteins, lipids, polysaccharides into simpler compounds such as amino acids, sugars, fatty acids and glycerol; (2) acidogenesis and acetogenesis: the hydrolyzed molecules are converted to volatile fatty acids, primarily acetate, hydrogen, and carbon dioxide; (3) methanogenesis: methane production from acetate, hydrogen and carbon dioxide. The hydrolysis step plays a crucial role in determining the successful production of methane [37, 145]. The biochemical processes in AD also occur in nature. AD technology is well established and recognized as a robust technology to convert biomass to bioenergy [146].

Despite the potential, questions related to the economic feasibility and the net energy output are the main hurdles hampering the development of biogas production from microalgae [14, 18–20]. For example, due to the specific structure and composition of the microalgae cell wall, the yield of biogas could be low. Pretreatment to disrupt the cell walls could require high energy inputs. The algae productivity could be low and cultivation cost could be high. Thus, the viability of microalgal biogas production may depend on improvements of efficiency and economic performance. Ongoing efforts include developing inexpensive biomass feedstock, maximizing energy return on investment, and minimizing environmental risks. As only a few studies are available in the literature on the economic feasibility of microalgal biogas exploitation [14], the evaluation and analysis of microalgal biogas production cost will be based on conversion efficiency, technological design aspects as well as available cost information.

### **3. Key drivers of microalgal biogas production cost**

The production of biogas from microalgae feedstock entails a series of steps starting with algae cultivation. Implementation of each step involves capital and operational expenditures. The key drivers such as algal biomass productivities, harvesting and dewatering techniques, AD designs, biogas utilization options, integration of algal production, and AD with other bioprocesses were addressed. The production cost breakdown was illustrated in a harmonized framework and a dynamic connection between the technological and economic/environmental assessments was established.

### **3.1 Microalgae cultivation, harvesting and dewatering**

A photobioreactor is the essential component of an algae cultivation facility. An open raceway pond (ORP) and a closed photobioreactor (PBR) are two major cultivation platforms. These two platforms for algae biomass production have been extensively studied [22–27, 83–85, 101]. The main differences are highlighted in **Table 1**. In addition, the steps from inoculum preparation to obtaining the wet algal paste typically include systems for culture circulation, growth medium supply, air/flue gas supply, culture cooling, culture harvesting, and process monitoring. Heat exchangers, pumps, and a piping network are also required. The location and climate are important factors for algae cultivation.

Due to the high methodological variation of TEA in literature, drawing a generic conclusion over the economic feasibility of microalgal cultivation could be impossible. From the technological and economic perspective, the factors presented below are the ones most prominent in the existing literature and identified as important topics in the development of algae fuels.


**117**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

significant capital cost contributors besides the photobioreactors.

4.*Nutrients and carbon dioxide supply*. Higher productivity usually involves higher consumption of nutrients. Thus, nutrient input needs to be adjusted to balance the tradeoff against productivity [23]. Carbon dioxide was found to be the most expensive consumable among the raw materials [26]. Siting algae cultivation facilities on land adjacent to industrial CO2 sources like flue gases may be effective in reducing cost. However, the substantial logistical and practical constraints of using flue gases in facilities of varying sizes are still a

5.*Land and water*. Even though, microalgae can be cultivated on nonarable land, the soil composition, climate, solar radiation have a substantial influence on their growth. The most suitable location should be warm places or close to the equator where insolation is not less than 3000 h/year [24]. Water is required during algae cultivation to compensate for evaporation or for cooling purposes. Availability of water at low cost is critical for process success. Water reuse, wastewater, seawater, brackish water and reasonable distance to the water

6.*Scaling*. It is critical to quantify the economy of scale for algae production to achieve economic viability [23]. However, large uncertainties and unrealistic assumptions will exist in the research where the productivity potential for microalgae at large-scale is being estimated through linear extrapolation for laboratory-based growth data [30]. Data variability and growth modeling considering geographical information should be considered in large-scale

7.*Labor and depreciation*. Tredici et al. [21] performed a TEA of the microalga *Tetraselmis suecica* production based on a 1-ha plant in Tuscany, Italy. Cost data were collected from manufacturers and suppliers as well as operating data from pilot and commercial facilities. This study found that the major fraction of cost was labor at small scales (1 ha) and when the pilot plant is scaled to 100 ha, capital expenses contribute the most to the production cost. This assessment is site and strain-specific, but still provides valuable insights for the

Algae harvesting and dewatering methods include gravity settling, chemical coagulation, flocculation, filtration, centrifuge, and drying. The economic feasibility and energy consumption are two criteria for assessing the performance of unit operations for harvesting and dewatering methods. It was found that the cost of separation takes 20–30% of the biomass production costs [32, 33]. Gravity settling, chemical coagulation, and flocculation usually concentrate the microalgal slurries to 2–7% while filtration and centrifugation concentrate microalgal slurry to 15–25% of total suspended solids [32]. The suitability of microalgal dewatering methods has been investigated for scalability, species flexibility, and downstream processing efficacy [33–36]. Dewatering methods reaching high biomass concentrations are usually associated with high energy input and cost. Thus, a combination of dewatering methods such as flocculation followed by filtration is generally considered to be economical due to the increased harvest efficiency. For downstream processing, methods such as flocculation using flocculants comprised of cationic and anionic

operations requirements and needs to be optimized to determine the minimum energy requirement [27, 28]. Mixing devices such as the paddle wheels are

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

source has the potential to reduce costs.

challenge [23].

assessments.

economic evaluation.

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

operations requirements and needs to be optimized to determine the minimum energy requirement [27, 28]. Mixing devices such as the paddle wheels are significant capital cost contributors besides the photobioreactors.


Algae harvesting and dewatering methods include gravity settling, chemical coagulation, flocculation, filtration, centrifuge, and drying. The economic feasibility and energy consumption are two criteria for assessing the performance of unit operations for harvesting and dewatering methods. It was found that the cost of separation takes 20–30% of the biomass production costs [32, 33]. Gravity settling, chemical coagulation, and flocculation usually concentrate the microalgal slurries to 2–7% while filtration and centrifugation concentrate microalgal slurry to 15–25% of total suspended solids [32]. The suitability of microalgal dewatering methods has been investigated for scalability, species flexibility, and downstream processing efficacy [33–36]. Dewatering methods reaching high biomass concentrations are usually associated with high energy input and cost. Thus, a combination of dewatering methods such as flocculation followed by filtration is generally considered to be economical due to the increased harvest efficiency. For downstream processing, methods such as flocculation using flocculants comprised of cationic and anionic

*Anaerobic Digestion*

assessments was established.

operational expenditures. The key drivers such as algal biomass productivities, harvesting and dewatering techniques, AD designs, biogas utilization options, integration of algal production, and AD with other bioprocesses were addressed. The production cost breakdown was illustrated in a harmonized framework and a dynamic connection between the technological and economic/environmental

A photobioreactor is the essential component of an algae cultivation facility. An open raceway pond (ORP) and a closed photobioreactor (PBR) are two major cultivation platforms. These two platforms for algae biomass production have been extensively studied [22–27, 83–85, 101]. The main differences are highlighted in **Table 1**. In addition, the steps from inoculum preparation to obtaining the wet algal paste typically include systems for culture circulation, growth medium supply, air/flue gas supply, culture cooling, culture harvesting, and process monitoring. Heat exchangers, pumps, and a piping network are also required. The location and

Due to the high methodological variation of TEA in literature, drawing a generic conclusion over the economic feasibility of microalgal cultivation could be impossible. From the technological and economic perspective, the factors presented below are the ones most prominent in the existing literature and identified as important

1.*Microalgae productivity and culture stability*. According to Davis et al. [23], achievable productivity has a strong influence on the economics. Productivity

low minimum biomass selling price. A significant increase in productivity has to be achieved to reduce cost substantially [25]. The cultured strain should have high growth rate and a steady biomass composition. GMOs or extremophiles could provide culture robustness [22]. However, due to lack of regulations for managing GMOs, it is unlikely permits could be obtained for commercial cultivation of GMO algae strains. For commercial outdoor systems, uncertainties could be associated with seasons of the year and across multiple locations. Thus, the productivity data should be integrated with meteorological data for geographically and seasonally resolved assessments

2.*Photobioreactor design, construction, and operating conditions*. For the open pond system, pond liners were found to be one of the primary cost contributors [22, 23, 28]. The location of the pond facilities could be selected according to the nature of the soil. For example, ponds built on soil with high native clay content could avoid full liners to reduce the cost. Acién et al. [26] presented a cost analysis of microalgae production using tubular photobioreactors. In these systems, photobioreactors were found to be one of the significant cost contributors. Generally, open raceway ponds are economically advantageous by more than a factor of 2, compared to closed photobioreactors [29]. However, due to increased productivity and culture stability, closed photobioreactors still have

3.*Energy consumption*. Primary energy consumption is due to the energy required for mixing, circulation, aeration and CO2 sparging. The energy consumption for mixing at experimental scales usually exceeded commercial-scale

/day annual average is critical for maintaining a relatively

**3.1 Microalgae cultivation, harvesting and dewatering**

climate are important factors for algae cultivation.

topics in the development of algae fuels.

of more than 25 g/m<sup>2</sup>

using a robust strain.

the potential for commercial applications.

**116**


### **Table 1.**

*A comparison of the open raceway and closed bioreactor systems for algae cultivation.*

poly-electrolytes, synthetic polyacrylamide polymers and starch-based polymers can be employed. However, the detrimental effect of these flocculants on the subsequent microbial processes need to be considered. For example, anaerobic digester stability and gas production could be affected by metal contamination. Future work should include replacing chemical coagulants with natural and low-cost organic ones for harvesting algal biomass.

### **3.2 Anaerobic digestion systems**

### *3.2.1 Pretreatment*

The efficiency of biogas production has been shown to be species-dependent [39]. One crucial factor is the differences in structure of microalgae cell walls. The role of the cell wall in the microbial degradability of algae biomass is highlighted in many investigations [6, 13, 37, 38, 40–43]. Many microalgae species (e.g. *Chlorella kessleri*, *Scenedesmus*) have recalcitrant cell walls, which make it difficult for anaerobic cultures to hydrolyze microalgal intracellular organic matter. Thus, to improve the biodegradability of microalgal biomass, pretreatments methods have been developed to disrupt or solubilize cell walls [112–116]. General insights from these studies are: (1) pretreatment methods are species-specific and their success depends on the nature of the cell wall; (2) mechanical pretreatments which consume electricity are more energy intensive than thermal, chemical and enzyme pretreatments; (3) chemical pretreatments usually have a low cost but produce inhibitory substances which could hamper the AD process; (4) for pretreatment mechanisms such as disruption of microalgal cell walls, the synergistic effects of the enzymes need further investigation; (5) combined pretreatments may provide energy and cost-effective options; (6) multi-objective optimization techniques could be used to obtain a high biogas yield with a positive energy balance; (7) enzyme/biological pretreatments have high selectivity, low inhibitory effects and higher probability of positive energy return [147]; (8) research on pilot/demonstration scale pretreatments is rare; (9) thermal pretreatments have been employed widely and proven to be the most efficient in microalgae pretreatment for AD; and (10) a detailed economic/energy analysis of microalgal AD for biogas production with pretreatment is still missing.

**119**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

The capital cost of the anaerobic digester could be reduced by using reactors designed for high OLR and low HRT [37]. The OLRs are typically between 1 and 6 g VS/L/d while the HRT varies between 10 and 30 days [37, 38]. Although high OLR will increase the methane productivity, overloading will decrease the biogas production efficiency due to the accumulation of inhibitors such as ammonia and acids [6, 37, 38]. Also, prolonged HRT could lead to ammonia inhibition due to slow liquid removal rate [41], while a low HRT could cause the washout of the anaerobic bacteria community [6]. Thus, an optimized OLR and HRT should be applied to achieve the expected specific methane yield. Possible solutions could be improving anaerobic digester configurations such as using membrane reactors or upflow anaerobic sludge blanket reactors to decouple the OLR and HRT [37, 119] and on-line control of anaerobic digester operation [124]. These have not been applied for digesting algal biomass. Additional costs for land and infrastructure and energy expenditures for

AD microorganisms can grow in three temperature regimes: (1) psychrophilic (5–20°C); (2) mesophilic (25–45°C); and (3) thermophilic (45–65°C). The temperature effect on AD has been discussed [13, 37, 41]. The beneficial temperature regime for AD operation is anaerobic digester is species-specific [44, 45]. The rate of methane generation can be enhanced under mesophilic and thermophilic conditions. The increased temperature could improve enzymatic activity for degrading microorganisms, and at the same time, the photosynthesis activity of viable microalgae within the digester could be reduced [13, 37]. However, an increase in temperature beyond the tolerable range of each temperature regime could cause inactivation of the microbes. Thermophilic temperature may cause increased hydrolysis of nitrogenous compounds which may increase ammonia levels and in turn can cause inhibition [6]. For large-scale biogas productions, the energy required for heating may be more than 1/3 of the total energy output in the form of biogas [46]. Thus, the net energy production from algae biogas may still be limited due to the high heat input

The pH needs to be maintained at an appropriate level for efficient conversion of biomass to biogas. The growth of microbes, enzyme activity, and the biogas compositions are influenced by the pH [47]. The optimum pH level depends on each step of AD [41]. Generally, the pH values are maintained between 7 and 8 for single

Microalgae grown in a saline environment offer a sustainable alternative to other biomass by utilizing non-arable land and seawater. Marine microalgae can usually grow in a salinity range of 35–125 ppt [48]. However, when a highly saline culture is processed in an anaerobic digester, the high salinity could be inhibitory to the AD process. The effects of salinity and concentration of sodium are discussed in previous studies [6, 38]. Adaptation of anaerobic digester microbial consortiums under different saline conditions was investigated by Mottet et al. [121]. In a promising study, methane production was observed from anaerobically digesting *Dunaliella salina* biomass at 35 g/L of salinity. Sulfide is a required micronutrient for anaerobic microorganisms, but high concentrations of sulfide (200 mg/L) could be toxic [6]. For saline microalgal species, the sulfur inhibition may occur due to the presence of oxidized sulfur compounds in saline algae growth medium. Proper inoculum selection for anaerobic digesters could favor the growth of methanogenic bacteria and limit the growth sulfate-reducing bacteria [49].

*3.2.2 Hydraulic retention time (HRT), organic loading rate (OLR), and reactor* 

heating the digesters should be included in the economic analysis.

*3.2.3 Temperature, pH, salinity, sulfur, and lipids content*

associated with a low concentration of algae substrates.

stage anaerobic digesters [13, 41].

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

*configurations*

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

### *3.2.2 Hydraulic retention time (HRT), organic loading rate (OLR), and reactor configurations*

The capital cost of the anaerobic digester could be reduced by using reactors designed for high OLR and low HRT [37]. The OLRs are typically between 1 and 6 g VS/L/d while the HRT varies between 10 and 30 days [37, 38]. Although high OLR will increase the methane productivity, overloading will decrease the biogas production efficiency due to the accumulation of inhibitors such as ammonia and acids [6, 37, 38]. Also, prolonged HRT could lead to ammonia inhibition due to slow liquid removal rate [41], while a low HRT could cause the washout of the anaerobic bacteria community [6]. Thus, an optimized OLR and HRT should be applied to achieve the expected specific methane yield. Possible solutions could be improving anaerobic digester configurations such as using membrane reactors or upflow anaerobic sludge blanket reactors to decouple the OLR and HRT [37, 119] and on-line control of anaerobic digester operation [124]. These have not been applied for digesting algal biomass. Additional costs for land and infrastructure and energy expenditures for heating the digesters should be included in the economic analysis.

### *3.2.3 Temperature, pH, salinity, sulfur, and lipids content*

AD microorganisms can grow in three temperature regimes: (1) psychrophilic (5–20°C); (2) mesophilic (25–45°C); and (3) thermophilic (45–65°C). The temperature effect on AD has been discussed [13, 37, 41]. The beneficial temperature regime for AD operation is anaerobic digester is species-specific [44, 45]. The rate of methane generation can be enhanced under mesophilic and thermophilic conditions. The increased temperature could improve enzymatic activity for degrading microorganisms, and at the same time, the photosynthesis activity of viable microalgae within the digester could be reduced [13, 37]. However, an increase in temperature beyond the tolerable range of each temperature regime could cause inactivation of the microbes. Thermophilic temperature may cause increased hydrolysis of nitrogenous compounds which may increase ammonia levels and in turn can cause inhibition [6]. For large-scale biogas productions, the energy required for heating may be more than 1/3 of the total energy output in the form of biogas [46]. Thus, the net energy production from algae biogas may still be limited due to the high heat input associated with a low concentration of algae substrates.

The pH needs to be maintained at an appropriate level for efficient conversion of biomass to biogas. The growth of microbes, enzyme activity, and the biogas compositions are influenced by the pH [47]. The optimum pH level depends on each step of AD [41]. Generally, the pH values are maintained between 7 and 8 for single stage anaerobic digesters [13, 41].

Microalgae grown in a saline environment offer a sustainable alternative to other biomass by utilizing non-arable land and seawater. Marine microalgae can usually grow in a salinity range of 35–125 ppt [48]. However, when a highly saline culture is processed in an anaerobic digester, the high salinity could be inhibitory to the AD process. The effects of salinity and concentration of sodium are discussed in previous studies [6, 38]. Adaptation of anaerobic digester microbial consortiums under different saline conditions was investigated by Mottet et al. [121]. In a promising study, methane production was observed from anaerobically digesting *Dunaliella salina* biomass at 35 g/L of salinity.

Sulfide is a required micronutrient for anaerobic microorganisms, but high concentrations of sulfide (200 mg/L) could be toxic [6]. For saline microalgal species, the sulfur inhibition may occur due to the presence of oxidized sulfur compounds in saline algae growth medium. Proper inoculum selection for anaerobic digesters could favor the growth of methanogenic bacteria and limit the growth sulfate-reducing bacteria [49].

*Anaerobic Digestion*

ones for harvesting algal biomass.

**3.2 Anaerobic digestion systems**

*3.2.1 Pretreatment*

**Table 1.**

poly-electrolytes, synthetic polyacrylamide polymers and starch-based polymers can be employed. However, the detrimental effect of these flocculants on the subsequent microbial processes need to be considered. For example, anaerobic digester stability and gas production could be affected by metal contamination. Future work should include replacing chemical coagulants with natural and low-cost organic

Net energy ratio (energy output/input) >1 >1 in some cases

Biomass productivities Low High Harvesting biomass concentration Low High Total capital cost (CAPEX) Relatively low High Total operational cost (OPEX) Relatively low High Reliability (low contamination risk, stable yield) Low High

Area required High Low Process control Low High CO2 loss High Low Water evaporation High Low Photosynthesis efficiency Low High Scale-up Easy Difficult

*A comparison of the open raceway and closed bioreactor systems for algae cultivation.*

**Open raceway Closed bioreactor**

The efficiency of biogas production has been shown to be species-dependent [39]. One crucial factor is the differences in structure of microalgae cell walls. The role of the cell wall in the microbial degradability of algae biomass is highlighted in many investigations [6, 13, 37, 38, 40–43]. Many microalgae species (e.g. *Chlorella kessleri*, *Scenedesmus*) have recalcitrant cell walls, which make it difficult for anaerobic cultures to hydrolyze microalgal intracellular organic matter. Thus, to improve the biodegradability of microalgal biomass, pretreatments methods have been developed to disrupt or solubilize cell walls [112–116]. General insights from these studies are: (1) pretreatment methods are species-specific and their success depends on the nature of the cell wall; (2) mechanical pretreatments which consume electricity are more energy intensive than thermal, chemical and enzyme pretreatments; (3) chemical pretreatments usually have a low cost but produce inhibitory substances which could hamper the AD process; (4) for pretreatment mechanisms such as disruption of microalgal cell walls, the synergistic effects of the enzymes need further investigation; (5) combined pretreatments may provide energy and cost-effective options; (6) multi-objective optimization techniques could be used to obtain a high biogas yield with a positive energy balance; (7) enzyme/biological pretreatments have high selectivity, low inhibitory effects and higher probability of positive energy return [147]; (8) research on pilot/demonstration scale pretreatments is rare; (9) thermal pretreatments have been employed widely and proven to be the most efficient in microalgae pretreatment for AD; and (10) a detailed economic/energy analysis of microalgal AD for biogas production with pretreatment is still missing.

**118**

### *Anaerobic Digestion*

Lipids can also be inhibitory to the AD process [6, 18, 50] although lipids have a high theoretical methane potential. Generally, inhibition would occur when lipids concentrations are higher than 30%. In this case, the high-lipid microalgae are suitable for lipid extraction for production of liquid fuels.

### *3.2.4 C/N ratio*

Microalgae biomass generally has a higher composition of protein than terrestrial plants [6, 37]. The degradation of protein will cause ammonia accumulation and inhibit the methanogenesis process. The optimum C/N ratio for AD is between 15 and 30 while this C/N ratio for microalgal AD is generally below 10 [13, 38, 41]. Thus, increasing C/N ratio and reducing the ammonia toxicity are important to enhance the biogas yield and productivity from microalgae. Possible solutions to this issue could be; (1) using ammonia-tolerant inoculum generated either by bioaugmentation or by acclimation [37, 38]; (2) using microalgae biomass that was cultivated under nitrogen-limitation [41, 99, 102, 130]; (3) co-digestion with sludge, oil-greases, waste paper and food wastes [13, 41, 54]; and (4) using a twostage AD for better control of the anaerobic microbial communities [6]. However, these solutions may add more complexity to the system, in which the economic and energetic performance is still clear. For example, the co-substrate needs to be secured for co-digestion; the digester volume and cost may increase due to the loading of the co-substrate; more environmental burdens may be associated with the shipping of biomass, and nitrogen-limitation cultivation may affect microalgae productivity.

### *3.2.5 Other factors*

Many other factors could affect the biogas yield and production of microalgal biomass. For example, the harvesting time influences the composition and biodegradability of algal biomass. Thus, it is essential to harvest algae in the appropriate stage of growth [13]. Storage conditions such as temperature also have an impact on biomass quality like macromolecular distribution and the content of organic compounds. Besides, inoculum to substrate ratio control is instrumental in avoiding inhibition problems such as drop in pH [51].

### *3.2.6 Biochemical methane potential (BMP) of microalgae biomass*

The overall biogas yields depend on the chemical composition of the algae strains. The target strain should be highly digestible. The volatile solids/ash-free dry weight of microalgae plays a significant role in predicting theoretical biogas production potential, which is a critical factor in determining biogas productivities. Theoretically, the methane yield from different components of microalgae is as follows: lipids—1 L CH4/g VS, proteins—0.85 L CH4/g VS, carbohydrates—0.42 L CH4/g VS at standard conditions. Although the lipids have a high theoretical methane yield in AD, a high lipid content (more than 40%) will produce inhibitory substances such as long chain fatty acids [6]. Thus, for high-lipid content microalgae, lipid removal for biofuels production may be a better solution than biomass sent directly to AD.

The impact of the algae cell wall is another critical factor affecting methane yield. Some species either lack cell wall or have cell walls rich in easily-biodegradable proteins as in *Dunaliella salina*, a halophilic microalgae and *Chlamydomonas reinhardtii*, a fresh water green microalgae [38]. Even easily degradable cell wall alone does not ensure efficient methanization. Factors such as the presence of methanogenesis inhibitors, anaerobic microbial community, hydraulic retention

**121**

**4.1 TEA framework**

0.57 m3

**Table 2.**

kg VS<sup>−</sup><sup>1</sup>

Carbohydrates % of DW

*vulgaris, and Nannochloropsis salina [41].*

**4. Techno-economic analysis**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

Protein % of DW 33 65 64 19

Lipids % of DW 22 13 10 36

*Chlamydomonas reinhardtii*

35 23 18 45

*Chlorella vulgaris*

*Nannochloropsis salina*

time, organic loading rates, salinity, carbon to nitrogen ratio, and the concentration

*Approximate compositions of four microalgal species: Scenedesmus sp., Chlamydomonas reinhardtii, Chlorella* 

to 89%. Similarly, *Chlorella vulgaris* shows a significant increase in BMP after pretreatments: from 54 to 85% BMP/TMP ratio [41, 52] under an enzyme pretreatment; and from 62 to 78% BMP/TMP ratio under a biological pretreatment [55, 123]. *Scenedesmus* sp. did not show a BMP/TMP ratio higher than 60%, even after enzyme or thermal pretreatments [56, 57]. The BMP varies from species to species, but no significant difference was found between fresh water microalgae and saline microalgae [58].

In published TEA works, the process complexity was often simplified in terms of limited pathways, few choices of economic drivers and implicit assumptions regarding the growth conditions, process modeling factors and financing of the production facility. Existing reviews in anaerobic digestion of microalgae biomass such as Ward et al. [6] focus on the integration of anaerobic digestion into biodiesel refineries. Considering that diesel or ethanol are more valuable products, anaerobic digestion was suggested to treat the residual biomass to improve the economic viability and sustainability of overall microalgae biodiesel/ethanol stages. Global research in various pathways is going on towards the sustainable development of algae biofuels. The following sections will review these works, highlight the variability of methods of estimating microalgal biogas production cost, find the key drivers of cost contributors, pointing out the convergence and difference in published results, and give a view of the whole value chain towards scaling-up and

commercialization when performing a techno-economic analysis (TEA).

resulting from the interactions between technical advances and financing

To achieve an optimal facility design, it is necessary to evaluate the tradeoff

under batch conditions with a BMP to TMP ratio increasing from 31

of digestible substrate will also affect the final methane yield of microalgae. The microalgal strains which have been investigated extensively include *Scenedesmus*, *Chlamydomonas*, *Chlorella*, and *Nannochloropsis* [12]. The compositions of these four species are shown in **Table 2**. AD conversion process with biochemical methane potential (BMP) to theoretical methane potential (TMP) ratio ≥ 70% are considered highly efficient. *Chlamydomonas reinhardtii* could achieve a 74% BMP (405 ml methane/g VS) to TMP (549 ml methane/g VS) ratio without any pretreatment [52]. Schwede et al. [53] achieved high digestibility of *Nannochloropsis salina* with thermal pretreatment. The methane yield significantly increased from 0.2 to

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

**Components Species**

*Scenedesmus* **sp.**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*


**Table 2.**

*Anaerobic Digestion*

*3.2.4 C/N ratio*

*3.2.5 Other factors*

inhibition problems such as drop in pH [51].

*3.2.6 Biochemical methane potential (BMP) of microalgae biomass*

Lipids can also be inhibitory to the AD process [6, 18, 50] although lipids have a high theoretical methane potential. Generally, inhibition would occur when lipids concentrations are higher than 30%. In this case, the high-lipid microalgae are suit-

Microalgae biomass generally has a higher composition of protein than terrestrial

Many other factors could affect the biogas yield and production of microalgal biomass. For example, the harvesting time influences the composition and biodegradability of algal biomass. Thus, it is essential to harvest algae in the appropriate stage of growth [13]. Storage conditions such as temperature also have an impact on biomass quality like macromolecular distribution and the content of organic compounds. Besides, inoculum to substrate ratio control is instrumental in avoiding

The overall biogas yields depend on the chemical composition of the algae strains. The target strain should be highly digestible. The volatile solids/ash-free dry weight of microalgae plays a significant role in predicting theoretical biogas production potential, which is a critical factor in determining biogas productivities. Theoretically, the methane yield from different components of microalgae is as follows: lipids—1 L CH4/g VS, proteins—0.85 L CH4/g VS, carbohydrates—0.42 L CH4/g VS at standard conditions. Although the lipids have a high theoretical methane yield in AD, a high lipid content (more than 40%) will produce inhibitory substances such as long chain fatty acids [6]. Thus, for high-lipid content microalgae, lipid removal for biofuels production may be a better solution than biomass

The impact of the algae cell wall is another critical factor affecting methane yield. Some species either lack cell wall or have cell walls rich in easily-biodegradable proteins as in *Dunaliella salina*, a halophilic microalgae and *Chlamydomonas reinhardtii*, a fresh water green microalgae [38]. Even easily degradable cell wall alone does not ensure efficient methanization. Factors such as the presence of methanogenesis inhibitors, anaerobic microbial community, hydraulic retention

plants [6, 37]. The degradation of protein will cause ammonia accumulation and inhibit the methanogenesis process. The optimum C/N ratio for AD is between 15 and 30 while this C/N ratio for microalgal AD is generally below 10 [13, 38, 41]. Thus, increasing C/N ratio and reducing the ammonia toxicity are important to enhance the biogas yield and productivity from microalgae. Possible solutions to this issue could be; (1) using ammonia-tolerant inoculum generated either by bioaugmentation or by acclimation [37, 38]; (2) using microalgae biomass that was cultivated under nitrogen-limitation [41, 99, 102, 130]; (3) co-digestion with sludge, oil-greases, waste paper and food wastes [13, 41, 54]; and (4) using a twostage AD for better control of the anaerobic microbial communities [6]. However, these solutions may add more complexity to the system, in which the economic and energetic performance is still clear. For example, the co-substrate needs to be secured for co-digestion; the digester volume and cost may increase due to the loading of the co-substrate; more environmental burdens may be associated with the shipping of biomass, and nitrogen-limitation cultivation may affect microalgae productivity.

able for lipid extraction for production of liquid fuels.

**120**

sent directly to AD.

*Approximate compositions of four microalgal species: Scenedesmus sp., Chlamydomonas reinhardtii, Chlorella vulgaris, and Nannochloropsis salina [41].*

time, organic loading rates, salinity, carbon to nitrogen ratio, and the concentration of digestible substrate will also affect the final methane yield of microalgae.

The microalgal strains which have been investigated extensively include *Scenedesmus*, *Chlamydomonas*, *Chlorella*, and *Nannochloropsis* [12]. The compositions of these four species are shown in **Table 2**. AD conversion process with biochemical methane potential (BMP) to theoretical methane potential (TMP) ratio ≥ 70% are considered highly efficient. *Chlamydomonas reinhardtii* could achieve a 74% BMP (405 ml methane/g VS) to TMP (549 ml methane/g VS) ratio without any pretreatment [52]. Schwede et al. [53] achieved high digestibility of *Nannochloropsis salina* with thermal pretreatment. The methane yield significantly increased from 0.2 to 0.57 m3 kg VS<sup>−</sup><sup>1</sup> under batch conditions with a BMP to TMP ratio increasing from 31 to 89%. Similarly, *Chlorella vulgaris* shows a significant increase in BMP after pretreatments: from 54 to 85% BMP/TMP ratio [41, 52] under an enzyme pretreatment; and from 62 to 78% BMP/TMP ratio under a biological pretreatment [55, 123]. *Scenedesmus* sp. did not show a BMP/TMP ratio higher than 60%, even after enzyme or thermal pretreatments [56, 57]. The BMP varies from species to species, but no significant difference was found between fresh water microalgae and saline microalgae [58].

### **4. Techno-economic analysis**

In published TEA works, the process complexity was often simplified in terms of limited pathways, few choices of economic drivers and implicit assumptions regarding the growth conditions, process modeling factors and financing of the production facility. Existing reviews in anaerobic digestion of microalgae biomass such as Ward et al. [6] focus on the integration of anaerobic digestion into biodiesel refineries. Considering that diesel or ethanol are more valuable products, anaerobic digestion was suggested to treat the residual biomass to improve the economic viability and sustainability of overall microalgae biodiesel/ethanol stages. Global research in various pathways is going on towards the sustainable development of algae biofuels. The following sections will review these works, highlight the variability of methods of estimating microalgal biogas production cost, find the key drivers of cost contributors, pointing out the convergence and difference in published results, and give a view of the whole value chain towards scaling-up and commercialization when performing a techno-economic analysis (TEA).

### **4.1 TEA framework**

To achieve an optimal facility design, it is necessary to evaluate the tradeoff resulting from the interactions between technical advances and financing

parameters. The technical objectives include maximizing microalgal biomass productivity, maximizing biogas yield via AD of biomass, and process stabilization. The economic objectives are to minimize the production cost and maximize the economic benefits. **Figure 1** shows the TEA framework for the sustainability analysis of biogas production from microalgal biomass through anaerobic digestion. The whole biomass processing value chain is determined by the technology framework and progress through experimentally validated process specifics. Economic analysis is based on the process design, which includes the cost assessments and investment analysis. A decision-making platform is built for raw material suppliers, producers and stake holders in an economic perspective. Correspondingly, the economic consequences will direct the research & development of new technologies, which could form a dynamic connection and optimization framework.

Environmental TEA (ETEA) extended the TEA framework with an environmental assessment based on a life cycle analysis [70]. The ETEA is based on the technology readiness level, which means the assessments are performed using the available data based on technology maturity. This would avoid a mismatch between the assessment methodology and the technology readiness level. For example, the whole biogas life cycle includes phases from the biomass cultivation to the final usage and end of life. Under current technology maturity, the whole data set is unavailable, which limits the assessments to certain life cycle phases.

### **4.2 State of the art: TEA of microalgal biogas**

Biorefinery optimization and full utilization of biomass addressing in the economic viability and environmental sustainability of the production of algae biofuels can be found in [39, 71, 72]. Dutta et al. [72] analyzed the sustainability of microalgae-derived biofuel production by performing a TEA and life-cycle assessment and found that coproducts valorization is more energy efficient than the processes focusing on specific components such as lipids. Biorefineries with coproducts and byproducts could have better utilization of the algal biomass and can increase the revenue, thus show greater possibility of achieving economic feasibility. In microalgae biodiesel and bioethanol productions, anaerobic digestion is usually integrated into the biorefinery to treat the residues for energy and nutrient recovery. Sialve et al. [18] compared the energy recovery ratio for two scenarios: direct AD of the whole algae biomass and AD of residue biomass after lipid extraction. Direct AD of

**123**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

the whole biomass was considered to have a higher energetic recovery when the cell lipid content does not exceed 40%. Also, increased lipids content in microalgae is not generally compensated with increased productivity due to nitrogen limitation. The potential of direct AD of microalgae biomass was addressed in their research, taking into account the energetic recovery and necessary nutrient recycle for largescale productions. Chia et al. [73] discussed the economic potential of biohydrogen and biogas production in Germany and Spain. Two processes were compared: direct AD of microalgae biomass (DAD) and coupled hydrogen and biogas production (CHB). In the CHB process, hydrogen was first produced by dark fermentation then effluent from hydrogen fermentation was used for biogas production. The CHB was found to have a lower operating cost due to no additional water and nutrients requirements for the bioreactor feed while the DAD process requires algal biomass in combination with other feedstocks. Both cases have production costs 13–16 times higher than the market price for natural gas. A 1/3 higher biogas yield and a 1/2 lower labor cost did not change the economic status of both processes, due to the high cost of fertilizer and building photobioreactors for microalgae cultivation. Milledge and Heaven [74, 129] performed an energy balance of biogas production from microalgae. Their research emphasized a combination of dewatering methods, as well as the efficient exploitation of the heat generated by the combustion of biogas in combined heat and power (CHP) units to show the energetic viability of

Chew et al. [68] assessed the potential of microalgae biorefineries for producing high-value products such as pigments, proteins, lipids, carbohydrates, and vitamins. The high-value products were added to improve the biorefinery economics. Open pond cultivation and medium recycling were mentioned to have better economic performance than other biorefinery structures. Water, land usage and capital cost were challenging for the economic viability of algal biofuels. The high-value products also need to improve aspects such as separations method, energy consumption, and control of product loss. AD was emphasized to recycle a considerable amount of nutrient usage to make microalgal fuels head towards its large-scale production. Several authors [13, 17, 37, 38, 75, 133, 134] synthesized scientific literature on biogas production from algae and suggested integration of the technology with other technologies as well as co-digestion with other substrates for an optimized biorefinery that sustainably produces biogas. Singh and Gu [76] recommended integrated processes that combine algae cultivation and wastewater treatment for methane production, which could offset the higher cost in compari-

Zamalloa et al. [8] evaluated the techno-economic potential of methane production from microalgae. The assessment was carried out using high rate anaerobic

scale open pond. The energy production cost from microalgal biogas was estimated to be 0.087–0.17 euro/kWh with an algae biomass cost of 86–124 euro/tonne. The result was based on a feed-in tariff of 0.133 euro/kWh and a carbon credit of 30 euro/ton of carbon dioxide. This study is one of the limited works that has been done on a comprehensive technological and economic assessment of electrical and

Collet et al. [77] performed a life-cycle assessment (LCA) of biogas production from the microalgae *Chlorella vulgaris* and found that electricity consumption and the impacts generated by the production of methane from microalgae are strongly correlated. Decreasing mixing and heating cost in different production steps or increasing the efficiency of AD were important to reduce the overall cost.

The studies surveyed show considerable variability in the calculated fuel cost and identifying the significant cost contributors. The varied results come from

/d) and preconcentrated algae biomass from a full-

son to methane production from corn and woody biomass.

thermal energy produced by biogas through AD of microalgae.

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

the whole process.

digesters (10–20 kg COD/m3

**Figure 1.** *TEA framework for biogas production from algae biomass.*

### *Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

the whole biomass was considered to have a higher energetic recovery when the cell lipid content does not exceed 40%. Also, increased lipids content in microalgae is not generally compensated with increased productivity due to nitrogen limitation. The potential of direct AD of microalgae biomass was addressed in their research, taking into account the energetic recovery and necessary nutrient recycle for largescale productions. Chia et al. [73] discussed the economic potential of biohydrogen and biogas production in Germany and Spain. Two processes were compared: direct AD of microalgae biomass (DAD) and coupled hydrogen and biogas production (CHB). In the CHB process, hydrogen was first produced by dark fermentation then effluent from hydrogen fermentation was used for biogas production. The CHB was found to have a lower operating cost due to no additional water and nutrients requirements for the bioreactor feed while the DAD process requires algal biomass in combination with other feedstocks. Both cases have production costs 13–16 times higher than the market price for natural gas. A 1/3 higher biogas yield and a 1/2 lower labor cost did not change the economic status of both processes, due to the high cost of fertilizer and building photobioreactors for microalgae cultivation. Milledge and Heaven [74, 129] performed an energy balance of biogas production from microalgae. Their research emphasized a combination of dewatering methods, as well as the efficient exploitation of the heat generated by the combustion of biogas in combined heat and power (CHP) units to show the energetic viability of the whole process.

Chew et al. [68] assessed the potential of microalgae biorefineries for producing high-value products such as pigments, proteins, lipids, carbohydrates, and vitamins. The high-value products were added to improve the biorefinery economics. Open pond cultivation and medium recycling were mentioned to have better economic performance than other biorefinery structures. Water, land usage and capital cost were challenging for the economic viability of algal biofuels. The high-value products also need to improve aspects such as separations method, energy consumption, and control of product loss. AD was emphasized to recycle a considerable amount of nutrient usage to make microalgal fuels head towards its large-scale production. Several authors [13, 17, 37, 38, 75, 133, 134] synthesized scientific literature on biogas production from algae and suggested integration of the technology with other technologies as well as co-digestion with other substrates for an optimized biorefinery that sustainably produces biogas. Singh and Gu [76] recommended integrated processes that combine algae cultivation and wastewater treatment for methane production, which could offset the higher cost in comparison to methane production from corn and woody biomass.

Zamalloa et al. [8] evaluated the techno-economic potential of methane production from microalgae. The assessment was carried out using high rate anaerobic digesters (10–20 kg COD/m3 /d) and preconcentrated algae biomass from a fullscale open pond. The energy production cost from microalgal biogas was estimated to be 0.087–0.17 euro/kWh with an algae biomass cost of 86–124 euro/tonne. The result was based on a feed-in tariff of 0.133 euro/kWh and a carbon credit of 30 euro/ton of carbon dioxide. This study is one of the limited works that has been done on a comprehensive technological and economic assessment of electrical and thermal energy produced by biogas through AD of microalgae.

Collet et al. [77] performed a life-cycle assessment (LCA) of biogas production from the microalgae *Chlorella vulgaris* and found that electricity consumption and the impacts generated by the production of methane from microalgae are strongly correlated. Decreasing mixing and heating cost in different production steps or increasing the efficiency of AD were important to reduce the overall cost.

The studies surveyed show considerable variability in the calculated fuel cost and identifying the significant cost contributors. The varied results come from

*Anaerobic Digestion*

parameters. The technical objectives include maximizing microalgal biomass productivity, maximizing biogas yield via AD of biomass, and process stabilization. The economic objectives are to minimize the production cost and maximize the economic benefits. **Figure 1** shows the TEA framework for the sustainability analysis of biogas production from microalgal biomass through anaerobic digestion. The whole biomass processing value chain is determined by the technology framework and progress through experimentally validated process specifics. Economic analysis is based on the process design, which includes the cost assessments and investment analysis. A decision-making platform is built for raw material suppliers, producers and stake holders in an economic perspective. Correspondingly, the economic consequences will direct the research & development of new technologies, which

Environmental TEA (ETEA) extended the TEA framework with an environmental assessment based on a life cycle analysis [70]. The ETEA is based on the technology readiness level, which means the assessments are performed using the available data based on technology maturity. This would avoid a mismatch between the assessment methodology and the technology readiness level. For example, the whole biogas life cycle includes phases from the biomass cultivation to the final usage and end of life. Under current technology maturity, the whole data set is

Biorefinery optimization and full utilization of biomass addressing in the economic viability and environmental sustainability of the production of algae biofuels can be found in [39, 71, 72]. Dutta et al. [72] analyzed the sustainability of microalgae-derived biofuel production by performing a TEA and life-cycle assessment and found that coproducts valorization is more energy efficient than the processes focusing on specific components such as lipids. Biorefineries with coproducts and byproducts could have better utilization of the algal biomass and can increase the revenue, thus show greater possibility of achieving economic feasibility. In microalgae biodiesel and bioethanol productions, anaerobic digestion is usually integrated into the biorefinery to treat the residues for energy and nutrient recovery. Sialve et al. [18] compared the energy recovery ratio for two scenarios: direct AD of the whole algae biomass and AD of residue biomass after lipid extraction. Direct AD of

could form a dynamic connection and optimization framework.

unavailable, which limits the assessments to certain life cycle phases.

**4.2 State of the art: TEA of microalgal biogas**

*TEA framework for biogas production from algae biomass.*

**122**

**Figure 1.**

different conversion pathways, technical assumptions (productivity, reactor design, process parameters, etc.) and economic factors (interest rate, raw material cost, etc.), diverse environmental and social conditions (consideration of season and location), and validation of sub-process models (lab/pilot plant/commercial scales). Nevertheless, the contributors to the production cost are mainly identified as microalgal strain selection, biomass cultivation and harvesting, AD operating conditions, biogas upgrading methods, waste management, and type of biorefinery. Thomassen et al. [78] evaluated the methodological reason for the wide variation in the results of multiple environmental and economic assessments. They proposed an environmental techno-economic assessment which can help to solve the challenges for a sustainability assessment: framework for methodology, harmonized assumptions, and integration of different dimensions (stages of technological maturity, technological process). This method is based on the dynamic technological process parameters and the same system boundaries for an integrated TEA and LCA.

### **4.3 Cost management**

Gnansounou and Dauriat [79] investigated TEAs following different types of cost management systems in value engineering, target costing and a combination of value engineering and target costing. Value engineering includes process design via data collection and process flowsheeting. Process simulators such as Aspen Plus enables the evaluation of the whole process chain based on scale up of the pilot plant, state of art technologies and price quotes. For microalgae to biogas technologies, key issues along the process chain include the suitable choice and operation options of the microalgae species, harvesting/dewatering strategies, pretreatment methods, AD configurations, recycling the digestate, and energy integration. Not all the steps are necessary for technologies with simplified processes and high economic potential. Target costing is a market-oriented method, which means a target selling price was set for the cost evaluation based on market and societal values. Following the target price, the target cost of the final product and each step of the supply process will be estimated, which means the cost allowance will play a key role in the process design. Target costing could integrate with value engineering in the cost management activities, so the cost allowance and cost target could be reconciliated. In the case of biogas production, the target costing evaluation seems unfeasible for the whole process due to the weak financial position of the natural gas market [80].

Real options analysis framework was employed by Kern et al. [81] for TEA. The model was adapted to accept stochastic price data for energy and agricultural commodities as well as static operating parameters assumptions for the algal biofuel plants. The TEA work was combined with life cycle analysis in a dynamic system—the fluctuations in market prices for energy and agricultural commodities will influence the operation decisions of the biofuels plants and its associated environmental impacts. Areas such as carbon tax, resource shortage and market forces could be investigated for their impact on biofuel plant design and operations in a dynamic system in the future. This gives the stake holders and suppliers more flexibility in making decisions.

### **4.4 TEA limitations**

The limitations of TEA include the potential competition for resources. For example, the microalgae biomass could have non-energy applications and has the potential for producing high value products besides biofuels. Then the biomass cost for the process will be influenced not only by the biomass production

**125**

m2

cost reductions.

(15.75 g afdw/m2

**5.1** *Cyanothece* **BG0011 cultivation**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

activities but also the market price which is determined by both the suppliers and

**5. Case study: TEA of anaerobic digestion of** *Cyanothece* **BG0011**

The microalgae used for this case study is a cyanobacterium, *Cyanothece* sp. BG0011 isolated from a shallow lake in Florida Keys [82]. Compared to other algal species, this species is endowed with unique features. First, cyanobacterium *Cyanothece* sp. BG0011 is a saline species and can be adapted to a wide range of salinities (10–70 psu). Second, it fixes dinitrogen in the air, which means it does not require nitrogenous nutrients in the culture water. Third, it produces exopolysaccharide (EPS) which can be converted to a variety of bioproducts. The aim of this case study is to assess the economic feasibility of biogas production using *Cyanothece* sp. BG0011 as feedstock by conducting a techno-economic assessment. The analysis investigated alternatives to decrease the cost and energy requirement of the cultivation and anaerobic digestion of algae. Utilization of biogas to produce electrical and thermal energy or upgrading to produce pure methane (renewable natural gas) was also considered. A comprehensive TEA was carried out based on experimental data and a set of operational assumptions which could be conceivably achieved in near term. The process flowsheet for biomass to biogas conversion through anaerobic digestion and biogas purification processes was implemented in Aspen Plus V8.8 to obtain mass balance and energy requirement results. The discussion focused on the preliminary exploration of the conceptual design of a microalgae cultivation and bioconversion system as well as an investigation on improvements that could result in the greatest system flexibility, energy yield and

Results from many experiments [149] conducted in the Bioprocess Engineering Laboratory, Department of Agricultural and Biological Engineering, University of Florida gave an average growth rate of 67.5 mg afdw/L/day (20.25 g afdw/m2

/day), resulting in cell density of 2.7 g/L and EPS concentration of

day) for BG0011 cell biomass and an EPS production rate of 52.5 mg afdw/L/day

2.1 g/L. The areal rates were calculated by assuming that the depth of culture was 30 cm, which is typically the case for open ponds. In the laboratory, the cultures were cultivated under air sparging, a constant illumination of 1200 μmol photons m<sup>−</sup><sup>2</sup>

light and 13 h to 11 h light-dark cycle. Open raceway ponds are generally used for large-scale commercial production of algal biomass [86]. Productivity in industrialscale raceway ponds is generally lower than in small experimental reactors. In literature, algae biomass productivity performance claims range from 7 to 35 g afdw/

/day [23, 87–89] with corresponding net photosynthetic efficiencies from under 1–4%. Among these, for studies involving techno-economic analyses, the baseline

/

s<sup>−</sup><sup>1</sup>

The sustainability of biogas production from microalgae will depend on not only the commercial viability but also environmental improvements such as greenhouse gas emission reduction, lack of direct and indirect impacts on land-use as well as biodiversity and eutrophication. The scope of TEA is limited for the environmental impact assessment, while these impact categories are appropriate for the goals of the overall sustainability analysis. Thus, an ETEA would allow assessing the sustainability of the entire value chain. Besides, TEA is not reflecting social impacts such as social awareness of algal biofuels' non-food competitive characteristics,

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

rural development, and public recognition.

purchase competitors.

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

activities but also the market price which is determined by both the suppliers and purchase competitors.

The sustainability of biogas production from microalgae will depend on not only the commercial viability but also environmental improvements such as greenhouse gas emission reduction, lack of direct and indirect impacts on land-use as well as biodiversity and eutrophication. The scope of TEA is limited for the environmental impact assessment, while these impact categories are appropriate for the goals of the overall sustainability analysis. Thus, an ETEA would allow assessing the sustainability of the entire value chain. Besides, TEA is not reflecting social impacts such as social awareness of algal biofuels' non-food competitive characteristics, rural development, and public recognition.

### **5. Case study: TEA of anaerobic digestion of** *Cyanothece* **BG0011**

The microalgae used for this case study is a cyanobacterium, *Cyanothece* sp. BG0011 isolated from a shallow lake in Florida Keys [82]. Compared to other algal species, this species is endowed with unique features. First, cyanobacterium *Cyanothece* sp. BG0011 is a saline species and can be adapted to a wide range of salinities (10–70 psu). Second, it fixes dinitrogen in the air, which means it does not require nitrogenous nutrients in the culture water. Third, it produces exopolysaccharide (EPS) which can be converted to a variety of bioproducts. The aim of this case study is to assess the economic feasibility of biogas production using *Cyanothece* sp. BG0011 as feedstock by conducting a techno-economic assessment. The analysis investigated alternatives to decrease the cost and energy requirement of the cultivation and anaerobic digestion of algae. Utilization of biogas to produce electrical and thermal energy or upgrading to produce pure methane (renewable natural gas) was also considered. A comprehensive TEA was carried out based on experimental data and a set of operational assumptions which could be conceivably achieved in near term. The process flowsheet for biomass to biogas conversion through anaerobic digestion and biogas purification processes was implemented in Aspen Plus V8.8 to obtain mass balance and energy requirement results. The discussion focused on the preliminary exploration of the conceptual design of a microalgae cultivation and bioconversion system as well as an investigation on improvements that could result in the greatest system flexibility, energy yield and cost reductions.

### **5.1** *Cyanothece* **BG0011 cultivation**

Results from many experiments [149] conducted in the Bioprocess Engineering Laboratory, Department of Agricultural and Biological Engineering, University of Florida gave an average growth rate of 67.5 mg afdw/L/day (20.25 g afdw/m2 / day) for BG0011 cell biomass and an EPS production rate of 52.5 mg afdw/L/day (15.75 g afdw/m2 /day), resulting in cell density of 2.7 g/L and EPS concentration of 2.1 g/L. The areal rates were calculated by assuming that the depth of culture was 30 cm, which is typically the case for open ponds. In the laboratory, the cultures were cultivated under air sparging, a constant illumination of 1200 μmol photons m<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> light and 13 h to 11 h light-dark cycle. Open raceway ponds are generally used for large-scale commercial production of algal biomass [86]. Productivity in industrialscale raceway ponds is generally lower than in small experimental reactors. In literature, algae biomass productivity performance claims range from 7 to 35 g afdw/ m2 /day [23, 87–89] with corresponding net photosynthetic efficiencies from under 1–4%. Among these, for studies involving techno-economic analyses, the baseline

*Anaerobic Digestion*

**4.3 Cost management**

gas market [80].

flexibility in making decisions.

**4.4 TEA limitations**

different conversion pathways, technical assumptions (productivity, reactor design, process parameters, etc.) and economic factors (interest rate, raw material cost, etc.), diverse environmental and social conditions (consideration of season and location), and validation of sub-process models (lab/pilot plant/commercial scales). Nevertheless, the contributors to the production cost are mainly identified as microalgal strain selection, biomass cultivation and harvesting, AD operating conditions, biogas upgrading methods, waste management, and type of biorefinery. Thomassen et al. [78] evaluated the methodological reason for the wide variation in the results of multiple environmental and economic assessments. They proposed an environmental techno-economic assessment which can help to solve the challenges for a sustainability assessment: framework for methodology, harmonized assumptions, and integration of different dimensions (stages of technological maturity, technological process). This method is based on the dynamic technological process parameters and the same system boundaries for an integrated TEA and LCA.

Gnansounou and Dauriat [79] investigated TEAs following different types of cost management systems in value engineering, target costing and a combination of value engineering and target costing. Value engineering includes process design via data collection and process flowsheeting. Process simulators such as Aspen Plus enables the evaluation of the whole process chain based on scale up of the pilot plant, state of art technologies and price quotes. For microalgae to biogas technologies, key issues along the process chain include the suitable choice and operation options of the microalgae species, harvesting/dewatering strategies, pretreatment methods, AD configurations, recycling the digestate, and energy integration. Not all the steps are necessary for technologies with simplified processes and high economic potential. Target costing is a market-oriented method, which means a target selling price was set for the cost evaluation based on market and societal values. Following the target price, the target cost of the final product and each step of the supply process will be estimated, which means the cost allowance will play a key role in the process design. Target costing could integrate with value engineering in the cost management activities, so the cost allowance and cost target could be reconciliated. In the case of biogas production, the target costing evaluation seems unfeasible for the whole process due to the weak financial position of the natural

Real options analysis framework was employed by Kern et al. [81] for TEA. The

The limitations of TEA include the potential competition for resources. For example, the microalgae biomass could have non-energy applications and has the potential for producing high value products besides biofuels. Then the biomass cost for the process will be influenced not only by the biomass production

model was adapted to accept stochastic price data for energy and agricultural commodities as well as static operating parameters assumptions for the algal biofuel plants. The TEA work was combined with life cycle analysis in a dynamic system—the fluctuations in market prices for energy and agricultural commodities will influence the operation decisions of the biofuels plants and its associated environmental impacts. Areas such as carbon tax, resource shortage and market forces could be investigated for their impact on biofuel plant design and operations in a dynamic system in the future. This gives the stake holders and suppliers more

**124**

productivity assumed was 20 g/m2 /day, with an optimistic value of 25–30 g afdw/m2 / day, and a conservative value of 15 g afdw/m2 /day. In this study, which assumed that BG0011 is cultivated in current large commercial open ponds, an average productivity of 12.4 g afdw/m2 /day (corresponding to a net photosynthetic efficiency of under 1%) was used. Similar growth rates were obtained by [148] when the algae was cultivated by air sparging and exposed to a lower light intensity of 122 μmol photons m<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> light and 13 h to 11 h light-dark cycle. Here, laboratory-scale BG0011 cell biomass growth rate is comparable to algae cell growth rates reported from other studies, however, in the case of BG0011, it also produces EPS. The average mass ratio between EPS and cell biomass is 0.778: 1 and also EPS production is cell-growth associated, so for this study it is assumed that in the commercial system, in addition to BG0011 cells, EPS would be concomitantly produced at 0.778 × 12.4 g afdw/ m2 /d = 9.6 g afdw/m2 /day. The total algae biomass productivity used was 22 g/m2 /d. Henceforth, the term "algae biomass" will include both BG0011 cells and EPS.

The scale of algae cultivation in literature for techno-economic analysis ranges from 200 to 700 ktonne afdw/year [22, 27, 72, 89]. In the present study, the scale of algae cultivation was determined based on a hypothetical 20 million gallons per year ethanol plant. The sugar required for such a plant would be 128 ktonnes afdw/ year (assuming yield of around 0.42 g ethanol/g sugar, and 1.1 g sugar/g polysaccharide). Assuming this amount of sugar will be supplied in the form of EPS, the scale of the algae cultivation pond would be 293 ktonnes of algal biomass/year, which also includes BG0011 cell biomass. This scale falls into the range of values found in literature for TEA. To meet a production capacity of 293 ktonnes/year at algal biomass productivity of 22 g afdw/m<sup>2</sup> /day, land area required would be 3660 hectares (approximately 4 × 4 miles). For a sanity check, this cultivation area was compared to land area required to supplying corn grain for a 20 million gallon per year corn-ethanol plant. Based on annual corn grain yield of 7000 kg/ha with starch content of 72% [150], and assuming a conversion of 0.5 kg ethanol/kg of starch, land required would be 23,700 ha. In this case the total above ground biomass productivity of corn, including corn grain, stover and cobs, is 16,700 kg/ha/year [150] whereas for BG0011 it is anticipated to be 80,300 kg/ha/year.

The BG0011 cultivation cost was estimated based on vendor quotes, literature, or engineering estimates. The installed pond capital cost includes civil work, liner, piping, electrical, other pond costs (such as paddlewheels). In addition, pumps for pumping water from ponds to refinery and for refilling the pond and required land also incur significant capital costs. Plastic lined earthen ponds were chosen for its lower cost compared to concrete ponds. Larger pond sizes would enable economically viable algal biomass production [23]. Here, the installed capital cost was estimated based on "dollars/hectare" of growth ponds for simplicity. The installed pond cost was set to be 80,000 \$/ha. Literature value ranges from 46,000 \$/ha to more than 150,000 \$/ha (value adjusted for inflation) due to different liner scenarios (partial or full) and specific design (e.g. with or without equipment to minimize dead zones) [23, 86] which was not included here. A land cost of 3080 \$/acre [90] was used for low-value land. The operation cost for algae cultivation such as utilities, chemicals, labor, overheads, maintenance, insurance tax, etc. were estimated using engineering estimates [91]. BG0011 was assumed to be cultivated in seawater or brackish water. The only fertilizer used for BG 0011 cultivation is phosphorus since it uses dinitrogen in air as a nitrogen source, and seawater would supply rest of micronutrients. From laboratory experiments it was determined that the phosphorous requirement of BG001 is 8.9 mg/L [149], so the annual requirement of phosphorous will be 1186.7 tonnes. Here, triple superphosphate (Ca(H2PO4)2 H2O) which contains 24.6% P is used as phosphorous source with a price of 270 \$/tonne (Source: World Bank, 2017). The requirement of triple superphosphate is 4945 tonne/year.

**127**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

−Coproduct credits)/Annual production

Here, the annual capital charges are calculated as follows:

\* Total capital cost = Total fixed cost + Working capital.

The fixed capital investment was assumed to be borrowed at an interest rate of 10% for 20 years. The plant operates 24 h a day and 360 days annually. The prices were adjusted for Year 2017 using Chemical Engineering Plant Cost Index (CEPCI). These assumptions were also used for the analysis of subsequent biogas production, conversion and upgrading processes. The production cost was calculated as follows:

 =Total capital cost∗Interest rate∗(1 +Interest rate)^Loan period/Interest rate^Loan period−1

The anaerobic digester was designed to treat the un-dewatered whole algae culture from the pond. The energy-intensive steps like algae harvesting and dewatering are avoided in this process which is different from most research [8, 22, 23]. The product biogas was analyzed for economic performance in two different applications: biogas purification or electricity production through combined heat and power. The first step in modeling mass flow rate of reactor outputs and determining energy requirements is to establish the stoichiometry of reactions. The stoichiometry of methane fermentation of algae biomass was developed based on the following assumptions: (1) microbial cells (cyanobacteria and bacteria) can be represented by the empirical formula CH1.8O0.5N0.2 [151]; (2) EPS is pure polysaccharide represented by the empirical formula C6H10O5; (3) algae biomass can be represented by an empirical formula containing the elements C, H, O and N in the mass ratios in which cells and EPS are produced that is 1:1.2; and (4) methane yield from laboratory assays corresponds to complete decomposition of substrate. The empirical formula for algae biomass was CH1.73O0.67N0.1. The stoichiometry for

CH1.73O0.67N0.1 + aNH3 → bCH1.8O0.5N0.2 + cCH4 + dCO2 + eH2O (3)

. This corresponds to 0.35 moles of methane (mole algae bio-

+0.35CH4+0.34CO2+0.04NH3 (4)

Methane yield from algae biomass was measured in the laboratory to be 300 ml

, which is equal to value of 'c' in the above stoichiometry. The other stoichiometric coefficients can now be solved from elemental balances for C, H, O and

CH1.73O0.67N0.1+0.17H2O→0.31CH1.8O0.5N0.2

In the anaerobic digester it was assumed that 98% of the algae biomass is converted. Different scenarios (three anaerobic digester types) were investigated to evaluate the economic and energetic performance. A schematic of biorefinery

=(Annual capital charges+Total operating cost)

(1)

(2)

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

Unit production cost

Annual capital charges

\* Working capital is 10% of fixed capital.

methane formation is written as follows:

at STP (g afdw)<sup>−</sup><sup>1</sup>

N. The stoichiometry is

scenarios are shown in **Figure 2**.

mass)<sup>−</sup><sup>1</sup>

**5.2 Anaerobic digestion**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

The fixed capital investment was assumed to be borrowed at an interest rate of 10% for 20 years. The plant operates 24 h a day and 360 days annually. The prices were adjusted for Year 2017 using Chemical Engineering Plant Cost Index (CEPCI). These assumptions were also used for the analysis of subsequent biogas production, conversion and upgrading processes. The production cost was calculated as follows:


Here, the annual capital charges are calculated as follows:


\* Total capital cost = Total fixed cost + Working capital.

\* Working capital is 10% of fixed capital.

### **5.2 Anaerobic digestion**

*Anaerobic Digestion*

tivity of 12.4 g afdw/m2

/d = 9.6 g afdw/m2

m<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup>

m2

productivity assumed was 20 g/m2

day, and a conservative value of 15 g afdw/m2

algal biomass productivity of 22 g afdw/m<sup>2</sup>

BG0011 is cultivated in current large commercial open ponds, an average produc-

Henceforth, the term "algae biomass" will include both BG0011 cells and EPS.

The scale of algae cultivation in literature for techno-economic analysis ranges from 200 to 700 ktonne afdw/year [22, 27, 72, 89]. In the present study, the scale of algae cultivation was determined based on a hypothetical 20 million gallons per year ethanol plant. The sugar required for such a plant would be 128 ktonnes afdw/ year (assuming yield of around 0.42 g ethanol/g sugar, and 1.1 g sugar/g polysaccharide). Assuming this amount of sugar will be supplied in the form of EPS, the scale of the algae cultivation pond would be 293 ktonnes of algal biomass/year, which also includes BG0011 cell biomass. This scale falls into the range of values found in literature for TEA. To meet a production capacity of 293 ktonnes/year at

hectares (approximately 4 × 4 miles). For a sanity check, this cultivation area was compared to land area required to supplying corn grain for a 20 million gallon per year corn-ethanol plant. Based on annual corn grain yield of 7000 kg/ha with starch content of 72% [150], and assuming a conversion of 0.5 kg ethanol/kg of starch, land required would be 23,700 ha. In this case the total above ground biomass productivity of corn, including corn grain, stover and cobs, is 16,700 kg/ha/year

The BG0011 cultivation cost was estimated based on vendor quotes, literature, or engineering estimates. The installed pond capital cost includes civil work, liner, piping, electrical, other pond costs (such as paddlewheels). In addition, pumps for pumping water from ponds to refinery and for refilling the pond and required land also incur significant capital costs. Plastic lined earthen ponds were chosen for its lower cost compared to concrete ponds. Larger pond sizes would enable economically viable algal biomass production [23]. Here, the installed capital cost was estimated based on "dollars/hectare" of growth ponds for simplicity. The installed pond cost was set to be 80,000 \$/ha. Literature value ranges from 46,000 \$/ha to more than 150,000 \$/ha (value adjusted for inflation) due to different liner scenarios (partial or full) and specific design (e.g. with or without equipment to minimize dead zones) [23, 86] which was not included here. A land cost of 3080 \$/acre [90] was used for low-value land. The operation cost for algae cultivation such as utilities, chemicals, labor, overheads, maintenance, insurance tax, etc. were estimated using engineering estimates [91]. BG0011 was assumed to be cultivated in seawater or brackish water. The only fertilizer used for BG 0011 cultivation is phosphorus since it uses dinitrogen in air as a nitrogen source, and seawater would supply rest of micronutrients. From laboratory experiments it was determined that the phosphorous requirement of BG001 is 8.9 mg/L [149], so the annual requirement of phosphorous will be 1186.7 tonnes. Here, triple superphosphate (Ca(H2PO4)2 H2O) which contains 24.6% P is used as phosphorous source with a price of 270 \$/tonne (Source: World Bank, 2017). The requirement of triple superphosphate is 4945 tonne/year.

[150] whereas for BG0011 it is anticipated to be 80,300 kg/ha/year.

under 1%) was used. Similar growth rates were obtained by [148] when the algae was cultivated by air sparging and exposed to a lower light intensity of 122 μmol photons

 light and 13 h to 11 h light-dark cycle. Here, laboratory-scale BG0011 cell biomass growth rate is comparable to algae cell growth rates reported from other studies, however, in the case of BG0011, it also produces EPS. The average mass ratio between EPS and cell biomass is 0.778: 1 and also EPS production is cell-growth associated, so for this study it is assumed that in the commercial system, in addition to BG0011 cells, EPS would be concomitantly produced at 0.778 × 12.4 g afdw/

/day, with an optimistic value of 25–30 g afdw/m2

/day (corresponding to a net photosynthetic efficiency of

/day. The total algae biomass productivity used was 22 g/m2

/day. In this study, which assumed that

/day, land area required would be 3660

/

/d.

**126**

The anaerobic digester was designed to treat the un-dewatered whole algae culture from the pond. The energy-intensive steps like algae harvesting and dewatering are avoided in this process which is different from most research [8, 22, 23]. The product biogas was analyzed for economic performance in two different applications: biogas purification or electricity production through combined heat and power.

The first step in modeling mass flow rate of reactor outputs and determining energy requirements is to establish the stoichiometry of reactions. The stoichiometry of methane fermentation of algae biomass was developed based on the following assumptions: (1) microbial cells (cyanobacteria and bacteria) can be represented by the empirical formula CH1.8O0.5N0.2 [151]; (2) EPS is pure polysaccharide represented by the empirical formula C6H10O5; (3) algae biomass can be represented by an empirical formula containing the elements C, H, O and N in the mass ratios in which cells and EPS are produced that is 1:1.2; and (4) methane yield from laboratory assays corresponds to complete decomposition of substrate. The empirical formula for algae biomass was CH1.73O0.67N0.1. The stoichiometry for methane formation is written as follows:

$$\text{CH}\_{1.73}\text{O}\_{0.67}\text{N}\_{0.1} + \text{aNH}\_3 \rightarrow \text{bCH}\_{1.8}\text{O}\_{0.5}\text{N}\_{0.2} + \text{cCH}\_4 + \text{dCO}\_2 + \text{eH}\_2\text{O} \tag{3}$$

Methane yield from algae biomass was measured in the laboratory to be 300 ml at STP (g afdw)<sup>−</sup><sup>1</sup> . This corresponds to 0.35 moles of methane (mole algae biomass)<sup>−</sup><sup>1</sup> , which is equal to value of 'c' in the above stoichiometry. The other stoichiometric coefficients can now be solved from elemental balances for C, H, O and N. The stoichiometry is

$$\begin{array}{l}\text{CH}\_{1.73}\text{O}\_{0.67}\text{N}\_{0.1}\text{+0.17}\text{H}\_{2}\text{O} \rightarrow \text{0.31}\text{CH}\_{1.8}\text{O}\_{0.5}\text{N}\_{0.2}\\\text{+0.35}\text{CH}\_{4}\text{+0.34}\text{CO}\_{2}\text{+0.04}\text{NH}\_{3}\end{array} \tag{4}$$

In the anaerobic digester it was assumed that 98% of the algae biomass is converted. Different scenarios (three anaerobic digester types) were investigated to evaluate the economic and energetic performance. A schematic of biorefinery scenarios are shown in **Figure 2**.

Case 1. Above ground mesophilic anaerobic digester. In Aspen, the influent to the reactor was 15 ktonne/h. The temperature was maintained at 37°C. It was operated at an HRT of 25 days.

Case 2. Above ground low-temperature anaerobic digester. Anaerobic digestion at low temperatures (LTAD) was applied to improve the energy balance. In this scenario the digester is operated in the psychrophilic range (12–20°C) [92–94]. However, with the same flow rate, the digester volume is larger to achieve a higher HRT for LTAD than mesophilic and thermophilic anaerobic digestion. Here, the temperature of LTAD is set to 20°C with an HRT of 50 days.

Case 3. Covered anaerobic lagoon. Covered anaerobic lagoon (CAL) does not require additional energy for the biogas production because no heating or mixing processes are involved. Besides, it is economical to construct and operate. The CAL in this research was 6 meters deep and covers an area of 1.5 hectares based on literature data [95]. The HRT was set to 50 days. The cost includes anaerobic lagoon excavation, cut and fill, lagoon liner, inlet and out structures, lagoon cover, ancillaries, pipework & installation, contingencies, design, engineering, etc. Operating costs including utility usage are minimal.

In all three cases above, the capital cost of anaerobic digester was estimated using vendor quotation or literature values. The operating cost was estimated by Aspen Process Economic Analyzer.

### **5.3 Biogas purification**

Several biogas upgrading or purification methods are available such as highpressure water scrubbing, membrane, pressure swing, gas permeation and chemical scrubbing. High pressure water scrubbing and chemical scrubbing (using amine solutions—MEA) are two of the most commonly used processes.

The MEA scrubbing method uses aqueous monoethanolamine (MEA) for acidic gas removal. The concentration of amine for acidic gas absorption is usually below 30% (by weight). The amine process has two main steps, absorption and stripping [96]. The detailed MEA scrubbing process is shown in **Figure 3**. Raw biogas goes

**129**

**Table 3.**

**Figure 3.**

*MEA scrubbing for biogas upgrading.*

Purification cost (\$/kg of

methane)

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

through a scrubbing column in which MEA is flowing counter-current to biogas. The CO2-rich MEA is collected at the bottom of the scrubbing column and pumped into a stripping column to remove CO2 and regenerate MEA by heating. Similar to MEA scrubbing, high pressure water scrubbing was also employed for biogas upgrading: biogas is fed to the bottom of scrubber after compressing it to 10 bar. At the top of scrubber, pressurized water is fed. CO2-rich water is then transferred to a flash column with a lower pressure of 3 bar to release gases for feed recirculation and minimizing methane loss. Then the CO2-rich water goes through a CO2 desorption process from the water stream by air [97]. Both biogas purifying approaches

**Specification MEA High pressure water** 

1.2 bar

8 bar

MAE: 750 kmol/h

Thermodynamic method ELECNRTL PSRK

Solvent recycle rate MEA: 0.99 Water: 0.95 Methane loss 1% 0.3% Product methane purity 95 wt% 99.2 wt% Capacity (raw biogas flow rate) 948.5 kmol/h 948.5 kmol/h Capital cost (million \$) 8.2 12 Operating cost (million \$/year) 20 4.6 Utility cost (million \$/year) 17 2

Scrubbing column RadFrac, 15 stages, pressure:

Stripping column RadFrac, 15 stages, pressure:

*Technical and economic aspects of the biogas purifying systems in ASPEN V 8.8.*

Make up chemicals Water: 150 kmol/h

**scrubbing**

RadFrac, 10 stages, pressure: 10 bar

RadFrac, 10 stages, pressure: 1 bar

Water: 11500 kmol/h

0.3 0.09

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

**Figure 2.** *Schematic diagram showing biorefinery scenarios.*

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

through a scrubbing column in which MEA is flowing counter-current to biogas. The CO2-rich MEA is collected at the bottom of the scrubbing column and pumped into a stripping column to remove CO2 and regenerate MEA by heating. Similar to MEA scrubbing, high pressure water scrubbing was also employed for biogas upgrading: biogas is fed to the bottom of scrubber after compressing it to 10 bar. At the top of scrubber, pressurized water is fed. CO2-rich water is then transferred to a flash column with a lower pressure of 3 bar to release gases for feed recirculation and minimizing methane loss. Then the CO2-rich water goes through a CO2 desorption process from the water stream by air [97]. Both biogas purifying approaches

### **Figure 3.**

*Anaerobic Digestion*

ated at an HRT of 25 days.

Case 1. Above ground mesophilic anaerobic digester. In Aspen, the influent to the reactor was 15 ktonne/h. The temperature was maintained at 37°C. It was oper-

Case 2. Above ground low-temperature anaerobic digester. Anaerobic digestion at low temperatures (LTAD) was applied to improve the energy balance. In this scenario the digester is operated in the psychrophilic range (12–20°C) [92–94]. However, with the same flow rate, the digester volume is larger to achieve a higher HRT for LTAD than mesophilic and thermophilic anaerobic digestion. Here, the

Case 3. Covered anaerobic lagoon. Covered anaerobic lagoon (CAL) does not require additional energy for the biogas production because no heating or mixing processes are involved. Besides, it is economical to construct and operate. The CAL in this research was 6 meters deep and covers an area of 1.5 hectares based on literature data [95]. The HRT was set to 50 days. The cost includes anaerobic lagoon excavation, cut and fill, lagoon liner, inlet and out structures, lagoon cover, ancillaries, pipework & installation, contingencies, design, engineering, etc. Operating

In all three cases above, the capital cost of anaerobic digester was estimated using vendor quotation or literature values. The operating cost was estimated by

Several biogas upgrading or purification methods are available such as highpressure water scrubbing, membrane, pressure swing, gas permeation and chemical scrubbing. High pressure water scrubbing and chemical scrubbing (using amine

The MEA scrubbing method uses aqueous monoethanolamine (MEA) for acidic gas removal. The concentration of amine for acidic gas absorption is usually below 30% (by weight). The amine process has two main steps, absorption and stripping [96]. The detailed MEA scrubbing process is shown in **Figure 3**. Raw biogas goes

solutions—MEA) are two of the most commonly used processes.

temperature of LTAD is set to 20°C with an HRT of 50 days.

costs including utility usage are minimal.

Aspen Process Economic Analyzer.

**5.3 Biogas purification**

**128**

**Figure 2.**

*Schematic diagram showing biorefinery scenarios.*

*MEA scrubbing for biogas upgrading.*


### **Table 3.**

*Technical and economic aspects of the biogas purifying systems in ASPEN V 8.8.*

were simulated in ASPEN Plus to determine the economics of each approach. The technical specification details are shown in **Table 3**. The table shows high pressure water scrubbing to be a more economical alternative and was chosen for the integrated process.

### **5.4 Power generation from biogas**

While the raw biogas can be purified to obtain biomethane, another option is to use the raw biogas to produce heat and power. Steam and electricity can be generated by burning the raw biogas through a combined heat and power (CHP) system. For reference, the CHP system uses General Electric Jenbacher JGS 420 system which is a 1425 kw generator. The total capital cost is \$ 1,150,000 (including installation, tax, etc. 2007), which is 807 \$/kw. The working capital is 10% of the total capital. The operating cost includes direct operating cost such as operating labor, supervised labor, maintenance and repairs, as well as indirect operating cost such as overhead, taxed, insurances. It is assumed that 40% biogas energy is for electricity, 50% for steam, 10% loss.

### **5.5 Techno-economic analysis of integrated system**

### *5.5.1 Biomass cultivation economics*

The BG0011 cultivation economics analysis details are shown in **Table 4**. In the literature algae production costs range from 150 to 6000 \$/tonne [19, 22, 27, 72, 89, 142], however, the studies vary from assumptions (production scale, chemical prices, plant life, etc.) to differences in technical specification (photobioreactor design, algal species, etc.). Some of the estimates also account costs for dewatering of algae [22, 27]. Thus, it is difficult to make a direct comparison between different studies. Besides, specific assumptions in each study could be based on different social-economic conditions, which makes comparisons more complicated [98].


**131**

**Table 5.**

*BG0011 biomass.*

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

Details of the production cost of renewable natural gas for the three anaerobic digestion scenarios are shown in **Table 5**. Case 2 contains two scenarios: The size of anaerobic digester in Case 2(a) is two times of that in Case 1. This is because the hydraulic retention time is longer under lower temperature, the volume of digester needs to be larger to keep the same production scale (the inflow rate). The size of anaerobic digester in Case 2(b) is the same as Case 1. Keeping the digester volume same as Case 1, because the temperature is lower, the productivity will be lower as well. Thus Case 2(b) has a lower production scale compared to other cases. The effect of temperature was incorporated by using the empirical relationship that for every 10°C rise in temperature the degradation rate is doubled. As the difference between the temperature for Case 1 and Case 2 is 17°C, it is expected that in Case 1, the digester has a processing capacity twice as much as that of the digester in Case 2b. The main contributor to the production cost of biogas is the biomass cost. Considering a carbon credit of 10 \$/tonne of CO2, the production cost of biogas only drops 0.5 \$/MMBtu. The results are comparable to Zamalloa et al.'s [8] research (the only paper focusing on the economics of renewable energy through AD, to our best knowledge): 32.2–61.5 \$/MMBtu with the algae biomass cost of 115.4–166.4 \$/tonne (0.087–0.17 euro/kwh with an algae biomass cost of 86–124 euro/tonne, 2011). The methane yield is 0.012 MMBtu/ kg of VS biomass, which is in close agreement to our experimental result 0.0124

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

*5.5.2 Economics of anaerobic digestion*

MMBtu/kg of VS biomass.

Biogas production scale

MMBtu/year)

Fixed capital cost of anaerobic digester (million \$)

Capital cost except anaerobic digester (million \$)

Annual capital charges (million \$/year)

Total raw materials (algae biomass) cost (million

Other operating (labor, utility, indirect, etc.) cost (million \$/year)

Utility cost (million \$/

Renewable natural gas production cost (\$/ MMBtu)

(106

\$/year)

year)

**Item Case 1** 

**(mesophilic anaerobic digester)**

**Case 2(a) (lowtemperature anaerobic digester)**

3.7 3.7 1.85 3.7

67.12 102 67.12 7.5

16.3 16.3 12.3 16.4

9.8 13.9 9.3 2.8

44.8 44.8 44.8 44.8

25.8 7.1 4.4 7.1

21 2.3 1.4 2.3

21.7 17.8 31.6 14.8

*Process and economic assessment for purified biogas production through anaerobic digestion of Cyanothece* 

**Case 2(b) (lowtemperature anaerobic digester**

**Case 3 (covered anaerobic lagoon)**

(including land: \$11400)

### **Table 4.**

*Algae cultivation economics.*

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

### *5.5.2 Economics of anaerobic digestion*

*Anaerobic Digestion*

grated process.

**5.4 Power generation from biogas**

*5.5.1 Biomass cultivation economics*

Production scale

Operating cost

*Algae cultivation economics.*

**5.5 Techno-economic analysis of integrated system**

tions, which makes comparisons more complicated [98].

Capital cost (including fixed, installed and working capital)

were simulated in ASPEN Plus to determine the economics of each approach. The technical specification details are shown in **Table 3**. The table shows high pressure water scrubbing to be a more economical alternative and was chosen for the inte-

While the raw biogas can be purified to obtain biomethane, another option is to use the raw biogas to produce heat and power. Steam and electricity can be generated by burning the raw biogas through a combined heat and power (CHP) system. For reference, the CHP system uses General Electric Jenbacher JGS 420 system which is a 1425 kw generator. The total capital cost is \$ 1,150,000 (including installation, tax, etc. 2007), which is 807 \$/kw. The working capital is 10% of the total capital. The operating cost includes direct operating cost such as operating labor, supervised labor, maintenance and repairs, as well as indirect operating cost such as overhead, taxed, insurances. It is assumed that 40% biogas energy is for electricity, 50% for steam, 10% loss.

The BG0011 cultivation economics analysis details are shown in **Table 4**. In the literature algae production costs range from 150 to 6000 \$/tonne [19, 22, 27, 72, 89, 142], however, the studies vary from assumptions (production scale, chemical prices, plant life, etc.) to differences in technical specification (photobioreactor design, algal species, etc.). Some of the estimates also account costs for dewatering of algae [22, 27]. Thus, it is difficult to make a direct comparison between different studies. Besides, specific assumptions in each study could be based on different social-economic condi-

**Parameters Values**

BG0011 cells production (ktonne/year) 165 BG0011 EPS production (ktonne/year) 128 **Total algae biomass production (ktonne/year) 293**

Pond (million \$) 308 Land (million \$) 26.6 Pump (million \$) 7.85 Total capital cost (million \$) 342.45 **Annual capital charges (million \$/year) 40.22**

Chemicals (P fertilizer: Ca (H2PO4)2 H2O) (million \$/year) 1.3 Other operating cost (including utilities, maintenance and repairs, labor etc.) (million \$/year) 3.26 **Total operating cost (million \$/year) 4.56** BG0011 algae biomass production cost (\$/tonne) 153

**130**

**Table 4.**

Details of the production cost of renewable natural gas for the three anaerobic digestion scenarios are shown in **Table 5**. Case 2 contains two scenarios: The size of anaerobic digester in Case 2(a) is two times of that in Case 1. This is because the hydraulic retention time is longer under lower temperature, the volume of digester needs to be larger to keep the same production scale (the inflow rate). The size of anaerobic digester in Case 2(b) is the same as Case 1. Keeping the digester volume same as Case 1, because the temperature is lower, the productivity will be lower as well. Thus Case 2(b) has a lower production scale compared to other cases. The effect of temperature was incorporated by using the empirical relationship that for every 10°C rise in temperature the degradation rate is doubled. As the difference between the temperature for Case 1 and Case 2 is 17°C, it is expected that in Case 1, the digester has a processing capacity twice as much as that of the digester in Case 2b. The main contributor to the production cost of biogas is the biomass cost. Considering a carbon credit of 10 \$/tonne of CO2, the production cost of biogas only drops 0.5 \$/MMBtu. The results are comparable to Zamalloa et al.'s [8] research (the only paper focusing on the economics of renewable energy through AD, to our best knowledge): 32.2–61.5 \$/MMBtu with the algae biomass cost of 115.4–166.4 \$/tonne (0.087–0.17 euro/kwh with an algae biomass cost of 86–124 euro/tonne, 2011). The methane yield is 0.012 MMBtu/ kg of VS biomass, which is in close agreement to our experimental result 0.0124 MMBtu/kg of VS biomass.


### **Table 5.**

*Process and economic assessment for purified biogas production through anaerobic digestion of Cyanothece BG0011 biomass.*


### **Table 6.**

*The economics of biogas—electricity and steam system.*

### *5.5.3 Electricity production cost*

On an energy potential basis, 40% of total methane produced per year could support a 50 MW power plant. Current residential electricity price is around 12 cents/kwh, while industrial price is around 7 cents/kwh. As shown in **Table 6**, the electricity production cost from biogas is 13 cents/kwh. Renewable energy technologies are usually more expensive than fossil fuel technologies. The reasons could be environmental costs associated with fossil fuels that are not paid by the rate payers, mechanical difficulty in bioenergy production, start-up issues and so on. European countries such as Germany and UK governments subsidize the production of renewable energy by introducing feed-in tariffs. These tariffs may be important to make bioenergy industry profitable.

### **6. Cost minimization approaches**

### **6.1 Nutrient recycling and biogas upgrading**

Nutrient (mostly nitrogen and phosphorous) recycling such as utilizing the digestate or wastewater for microalgae cultivation was highlighted in various studies [59–63, 104–107, 126, 128, 138, 139]. Recycling the effluent from the anaerobic digester for algae cultivation could mitigate the costs associated with supplying nutrient for algal biomass growth and effluent treatment. Erkelens et al. [59] validated that microalgae *Tetraselmis* sp. could utilize its digested effluent as a growth medium and thus form a closed loop system. Also, Prajapati et al. [60] showed that algal liquid digestate have good potential to be utilized as nutrient supplement (30% concentration) in rural sector wastewater for biomass cultivation. The biomass production level is closer to the case in which conventional medium is used. Although there are still technological obstacles when growing microalgae on digestate such as low growth rate due to poor nutrient ratios, shading, ammonia inhibition and bacteria growth, the performance of the nutrient recycling process could be further developed by scale up/optimizing strategies such as controlling inoculum and substrate concentrations, bacteria growth as well as harvesting strategies [59, 61, 64, 132].

One option to increase algae biomass productivity and its concentration in the culture is to enrich the air with CO2. It has been shown that enriching the air with 1% CO2 increases cell concentration to 3.46 g afdw/L and EPS concentration to 2.91 g afdw/L, giving an algae biomass concentration of at least 6.37 g afdw/L

**133**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

Upgrading biogas by fixation of the CO2 in biogas via photosynthesis by microalgae has been investigated with respect to CO2 removal capability, biomass productivity and O2 desorption minimization [16, 63–67]. Toledo-Cervantes et al. [16] optimized the biogas upgrading process by studying the influence of the recycling liquid to biogas ratio. The biomethane produced met specification for injection into natural gas grids. However, this technique requires closed photobioreactors. Hydrogen sulfide (H2S) is another contaminant to be removed from the biogas. Hydrogen sulfide removal was realized by the oxidation of H2S to sulfate by sulfur oxidizing bacteria that used the oxygen produced photosynthetically in situ. In this case, the algae-bacteria symbiosis was employed in the photobioreactors [67]. Nutrient recycling and biogas upgrading provides not only the opportunity for AD of microalgal biomass to be cost-effective, but also the potential to reduce the

To move industrial application of biogas production from microalgal biomass towards commercialization, additional assessment is required regarding large scale operations. These include (1) strain robustness, outdoor productivity, location and seasonal effects, yield from real production systems, and harvesting strategy for algae cultivation (2) for biomass to biogas conversion processes, the conceptual process design needs to take the following factors into consideration: costs associated with digester heating, land, and infrastructure as well as operational parameters such as maintaining pH, temperature, mixing, power consumption, and production

The uncertainty of large-scale algae cultivation is still a challenge which prevents commercialization; process modeling could provide useful information about the performance of microalgae cultivation systems by estimation and optimization of microalgae productivity under different conditions [103]. A growth kinetic model is critical in a process model simulating microalgae cultivation which has a direct impact on downstream conversion processing systems [135] Lee et al. [31] classified the existing kinetic models into three groups: a single limiting substrate (phosphorus, or dissolved CO2 concentration), a physical limiting factor (light intensity or temperature), and multiple factors (e.g. both substrate and light). Based on their study, there was a tradeoff between the accuracy of the model representation and real-world usability. A future modeling framework should consider along with limiting nutrients, integration of light and temperature, and incorporation of species diversity.

[149], which is 1.33 times more than that used in the case study above. The increased productivity of algae biomass will reduce further the cost for biomass production. The CO2 released from the biogas upgrading process or waste gases from biogas combustion containing CO2 could be recycled to the algae growth ponds for enriching the air. The economic analysis for this scenario was also performed assuming algae biomass concentration is 1.33 times the previous value of 293 ktonne/year. The estimated production cost for *Cyanothece BG 0011* algae biomass is 121.6 \$/tonne. This was calculated by accounting for the following additional costs: (1) capital cost associated with pipes and pumps to take CO2 from biogas purification system or biogas combustion output to the pond and (2) operating costs resulting from more nutrient addition to maintain higher cell density and power consumption of compressing CO2 for sparging [152]. Only biogas production from covered anaerobic lagoon as in Case 3 was considered here. Algae production cost was lowered by 20%. The estimated cost of renewable natural gas is now reduced to 12.16 \$/MMBtu and the electricity production

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

cost from biogas is only 10.98 cents/kwh.

environmental impacts.

of coproducts like fertilizer.

**6.2 Dynamic growth models**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

[149], which is 1.33 times more than that used in the case study above. The increased productivity of algae biomass will reduce further the cost for biomass production. The CO2 released from the biogas upgrading process or waste gases from biogas combustion containing CO2 could be recycled to the algae growth ponds for enriching the air. The economic analysis for this scenario was also performed assuming algae biomass concentration is 1.33 times the previous value of 293 ktonne/year. The estimated production cost for *Cyanothece BG 0011* algae biomass is 121.6 \$/tonne. This was calculated by accounting for the following additional costs: (1) capital cost associated with pipes and pumps to take CO2 from biogas purification system or biogas combustion output to the pond and (2) operating costs resulting from more nutrient addition to maintain higher cell density and power consumption of compressing CO2 for sparging [152]. Only biogas production from covered anaerobic lagoon as in Case 3 was considered here. Algae production cost was lowered by 20%. The estimated cost of renewable natural gas is now reduced to 12.16 \$/MMBtu and the electricity production cost from biogas is only 10.98 cents/kwh.

Upgrading biogas by fixation of the CO2 in biogas via photosynthesis by microalgae has been investigated with respect to CO2 removal capability, biomass productivity and O2 desorption minimization [16, 63–67]. Toledo-Cervantes et al. [16] optimized the biogas upgrading process by studying the influence of the recycling liquid to biogas ratio. The biomethane produced met specification for injection into natural gas grids. However, this technique requires closed photobioreactors. Hydrogen sulfide (H2S) is another contaminant to be removed from the biogas. Hydrogen sulfide removal was realized by the oxidation of H2S to sulfate by sulfur oxidizing bacteria that used the oxygen produced photosynthetically in situ. In this case, the algae-bacteria symbiosis was employed in the photobioreactors [67]. Nutrient recycling and biogas upgrading provides not only the opportunity for AD of microalgal biomass to be cost-effective, but also the potential to reduce the environmental impacts.

To move industrial application of biogas production from microalgal biomass towards commercialization, additional assessment is required regarding large scale operations. These include (1) strain robustness, outdoor productivity, location and seasonal effects, yield from real production systems, and harvesting strategy for algae cultivation (2) for biomass to biogas conversion processes, the conceptual process design needs to take the following factors into consideration: costs associated with digester heating, land, and infrastructure as well as operational parameters such as maintaining pH, temperature, mixing, power consumption, and production of coproducts like fertilizer.

### **6.2 Dynamic growth models**

The uncertainty of large-scale algae cultivation is still a challenge which prevents commercialization; process modeling could provide useful information about the performance of microalgae cultivation systems by estimation and optimization of microalgae productivity under different conditions [103]. A growth kinetic model is critical in a process model simulating microalgae cultivation which has a direct impact on downstream conversion processing systems [135] Lee et al. [31] classified the existing kinetic models into three groups: a single limiting substrate (phosphorus, or dissolved CO2 concentration), a physical limiting factor (light intensity or temperature), and multiple factors (e.g. both substrate and light). Based on their study, there was a tradeoff between the accuracy of the model representation and real-world usability. A future modeling framework should consider along with limiting nutrients, integration of light and temperature, and incorporation of species diversity.

*Anaerobic Digestion*

capital)

**Table 6.**

*5.5.3 Electricity production cost*

*The economics of biogas—electricity and steam system.*

make bioenergy industry profitable.

**6. Cost minimization approaches**

**6.1 Nutrient recycling and biogas upgrading**

On an energy potential basis, 40% of total methane produced per year could support a 50 MW power plant. Current residential electricity price is around 12 cents/kwh, while industrial price is around 7 cents/kwh. As shown in **Table 6**, the electricity production cost from biogas is 13 cents/kwh. Renewable energy technologies are usually more expensive than fossil fuel technologies. The reasons could be environmental costs associated with fossil fuels that are not paid by the rate payers, mechanical difficulty in bioenergy production, start-up issues and so on. European countries such as Germany and UK governments subsidize the production of renewable energy by introducing feed-in tariffs. These tariffs may be important to

**Item Value** Electricity capacity (million kwh/year) 435

Capital charges (million \$/year) 6.2 Steam credits (million \$/year) 3.7 Raw biogas cost (million \$/year) 47.7 Other operating cost (million \$/year) 9.5 Electricity production cost (\$/kwh) 0.13

52.4

Total capital cost of the CHP system (million \$) (including fix capital cost and 10% working

Nutrient (mostly nitrogen and phosphorous) recycling such as utilizing the digestate or wastewater for microalgae cultivation was highlighted in various studies [59–63, 104–107, 126, 128, 138, 139]. Recycling the effluent from the anaerobic digester for algae cultivation could mitigate the costs associated with supplying nutrient for algal biomass growth and effluent treatment. Erkelens et al. [59] validated that microalgae *Tetraselmis* sp. could utilize its digested effluent as a growth medium and thus form a closed loop system. Also, Prajapati et al. [60] showed that algal liquid digestate have good potential to be utilized as nutrient supplement (30% concentration) in rural sector wastewater for biomass cultivation. The biomass production level is closer to the case in which conventional medium is used. Although there are still technological obstacles when growing microalgae on digestate such as low growth rate due to poor nutrient ratios, shading, ammonia inhibition and bacteria growth, the performance of the nutrient recycling process could be further developed by scale up/optimizing strategies such as controlling inoculum and substrate concentrations, bacteria growth as well as harvesting strategies [59,

One option to increase algae biomass productivity and its concentration in the culture is to enrich the air with CO2. It has been shown that enriching the air with 1% CO2 increases cell concentration to 3.46 g afdw/L and EPS concentration to 2.91 g afdw/L, giving an algae biomass concentration of at least 6.37 g afdw/L

**132**

61, 64, 132].

### **6.3 Biorefinery concepts**

AD can be integrated to biorefineries which produce high value products from algae such as chemicals for cosmetics, nutraceuticals and pharmaceuticals. This requires diversified business strategies which benchmark the market potential for the total raw materials and alternative products. In the economic perspective, three approaches could be possible for the development of microalgae AD: (1) implementing AD for biogasification of cell debris or waste streams in microalgal based processes such as biodiesel/bioethanol/high-value bioproducts (e.g. PHA)/fuel cell/ hydrothermal liquefaction/hydrogen production [68, 120]; (2) investigation of high-value products from intermediate metabolites produced during AD such as carboxylic acids [37]; (3) electricity production from microalgae derived biogas. In previous sections, the cost of electricity from microalgae derived biogas is comparable with market value while cost of the renewable natural gas from microalgae is much higher than the current market value of natural gas.

### **7. Conclusion and future work**

This chapter reviewed the literature on TEA of biogas production from algae. The key drivers to the overall production cost were identified and possible process improvements to reduce cost were discussed. The need for harmonization of resource, life cycle and techno-economic assessments in the methodology of TEA was highlighted. Modeling efforts, based on well-informed, rigorous engineeringbased process models, should be integrated on a baseline framework such that different process technologies, subprocesses and alternative pathways can be directly compared at a system level. TEA model improvements include strategic planning and using reliable input data from simple mass balance calculations to geographically and seasonally specific assessments, as well as risk analysis for large-scale productivity. Nutrient recycling process has the potential to reduce both cost and environmental burdens.

The cultivation of microalgae BG0011 and its economic feasibility as an energy source through anaerobic digestion was evaluated through a techno-economic analysis. The main contribution to the biogas cost is the biomass production cost. The best-case estimate was a biomethane production cost of 14.8 \$/MMBtu using covered anaerobic lagoon and high-pressure water scrubbing purification. The cost of electricity production from biogas was estimated to be 13 cents/kwh. Even though these costs are higher than commercial prices in the United States, these are much lower than those costs with production of liquid fuels like ethanol or biodiesel from algae.

Improved algal biomass productivities could be essential for lowering the cost of algae-derived biogas. This could be achieved by recycling the CO2 released during biogas upgrading or combustion for algae cultivation. Algal biogas economics could be further improved by marketing the digester sludge as a soil-amendment product, considering that nitrogen in the sludge was fixed from atmospheric dinitrogen.

### **Acknowledgements**

The authors gratefully acknowledge funding provided by Office of Energy, Florida Department of Agricultural and Consumer Services under contract number 92420 for this project.

**135**

**Author details**

Edward J. Phlips<sup>5</sup>

Gainesville, FL, USA

Gainesville, FL, USA

University, Guayaquil, Ecuador

Na Wu1

provided the original work is properly cited.

4 An Giang University, An Giang, Vietnam

\*Address all correspondence to: wuna8703@ufl.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

6 Department of Chemical Engineering, University of Florida, Gainesville, FL, USA

\*, Cesar M. Moreira1,2, Yingxiu Zhang1,3, Nguyet Doan1,4, Shunchang Yang1

1 Department of Agricultural and Biological Engineering, University of Florida,

2 Faculty of Mechanical Engineering and Production Sciences, ESPOL Polytechnic

3 School of Environmental Science and Engineering, Tianjin University/ China-Australia Centre for Sustainable Urban Development, Tianjin, China

5 Department of Fisheries and Aquatic Science, University of Florida,

and Pratap C. Pullammanappallil1

, Spyros A. Svoronos6

,

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

The authors declare that there are no potential financial or other interests that

could be perceived to influence the outcomes of the research.

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

**Conflict of interest**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

### **Conflict of interest**

*Anaerobic Digestion*

**6.3 Biorefinery concepts**

AD can be integrated to biorefineries which produce high value products from algae such as chemicals for cosmetics, nutraceuticals and pharmaceuticals. This requires diversified business strategies which benchmark the market potential for the total raw materials and alternative products. In the economic perspective, three approaches could be possible for the development of microalgae AD: (1) implementing AD for biogasification of cell debris or waste streams in microalgal based processes such as biodiesel/bioethanol/high-value bioproducts (e.g. PHA)/fuel cell/ hydrothermal liquefaction/hydrogen production [68, 120]; (2) investigation of high-value products from intermediate metabolites produced during AD such as carboxylic acids [37]; (3) electricity production from microalgae derived biogas. In previous sections, the cost of electricity from microalgae derived biogas is comparable with market value while cost of the renewable natural gas from microalgae is

This chapter reviewed the literature on TEA of biogas production from algae. The key drivers to the overall production cost were identified and possible process improvements to reduce cost were discussed. The need for harmonization of resource, life cycle and techno-economic assessments in the methodology of TEA was highlighted. Modeling efforts, based on well-informed, rigorous engineeringbased process models, should be integrated on a baseline framework such that different process technologies, subprocesses and alternative pathways can be directly compared at a system level. TEA model improvements include strategic planning and using reliable input data from simple mass balance calculations to geographically and seasonally specific assessments, as well as risk analysis for large-scale productivity. Nutrient recycling process has the potential to reduce both cost and

The cultivation of microalgae BG0011 and its economic feasibility as an energy

Improved algal biomass productivities could be essential for lowering the cost of algae-derived biogas. This could be achieved by recycling the CO2 released during biogas upgrading or combustion for algae cultivation. Algal biogas economics could be further improved by marketing the digester sludge as a soil-amendment product, considering that nitrogen in the sludge was fixed from atmospheric

The authors gratefully acknowledge funding provided by Office of Energy, Florida Department of Agricultural and Consumer Services under contract number

source through anaerobic digestion was evaluated through a techno-economic analysis. The main contribution to the biogas cost is the biomass production cost. The best-case estimate was a biomethane production cost of 14.8 \$/MMBtu using covered anaerobic lagoon and high-pressure water scrubbing purification. The cost of electricity production from biogas was estimated to be 13 cents/kwh. Even though these costs are higher than commercial prices in the United States, these are much lower than those costs with production of liquid fuels like ethanol or biodiesel

much higher than the current market value of natural gas.

**7. Conclusion and future work**

environmental burdens.

from algae.

dinitrogen.

**Acknowledgements**

92420 for this project.

**134**

The authors declare that there are no potential financial or other interests that could be perceived to influence the outcomes of the research.

### **Author details**

Na Wu1 \*, Cesar M. Moreira1,2, Yingxiu Zhang1,3, Nguyet Doan1,4, Shunchang Yang1 , Edward J. Phlips<sup>5</sup> , Spyros A. Svoronos<sup>6</sup> and Pratap C. Pullammanappallil1

1 Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, USA

2 Faculty of Mechanical Engineering and Production Sciences, ESPOL Polytechnic University, Guayaquil, Ecuador

3 School of Environmental Science and Engineering, Tianjin University/ China-Australia Centre for Sustainable Urban Development, Tianjin, China

4 An Giang University, An Giang, Vietnam

5 Department of Fisheries and Aquatic Science, University of Florida, Gainesville, FL, USA

6 Department of Chemical Engineering, University of Florida, Gainesville, FL, USA

\*Address all correspondence to: wuna8703@ufl.edu

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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[19] Ribeiro LA, da Silva PP, Mata TM, Martins AA. Prospects of using microalgae for biofuels production: Results of a Delphi study. Renewable Energy. 2015;**75**:799-804

[20] Suganya T, Varman M, Masjuki HH, Renganathan S. Macroalgae and microalgae as a potential source for commercial applications along with biofuels production: A biorefinery approach. Renewable and Sustainable Energy Reviews. 2016;**55**:909-941

[21] Tredici MR, Rodolfi L, Biondi N, Bassi N, Sampietro G. Techno-economic analysis of microalgal biomass production in a 1-ha Green Wall Panel (GWP®) plant. Algal Research. 2016;**19**:253-263

[22] Hoffman J, Pate RC, Drennen T, Quinn JC. Techno-economic assessment of open microalgae production systems. Algal Research. 2017;**23**:51-57

[23] Davis R, Markham J, Kinchin C, Grundl N, Tan EC, Humbird D. Process Design and Economics for the Production of Algal Biomass: Algal Biomass Production in Open Pond Systems and Processing Through Dewatering for Downstream Conversion. Golden, CO (United States): National Renewable Energy Lab (NREL); 2016

[24] Slade R, Bauen A. Micro-algae cultivation for biofuels: Cost, energy balance, environmental impacts

and future prospects. Biomass and Bioenergy. 2013;**53**:29-38

[25] Richardson JW, Johnson MD, Zhang X, Zemke P, Chen W, Hu Q. A financial assessment of two alternative cultivation systems and their contributions to algae biofuel economic viability. Algal Research. 2014;**4**:96-104

[26] Acién FG, Fernández JM, Magán JJ, Molina E. Production cost of a real microalgae production plant and strategies to reduce it. Biotechnology Advances. 2012;**30**(6):1344-1353

[27] Norsker NH, Barbosa MJ, Vermuë MH, Wijffels RH. Microalgal production—A close look at the economics. Biotechnology Advances. 2011;**29**(1):24-27

[28] Rogers JN, Rosenberg JN, Guzman BJ, Oh VH, Mimbela LE, Ghassemi A, et al. A critical analysis of paddlewheeldriven raceway ponds for algal biofuel production at commercial scales. Algal Research. 2014;**4**:76-88

[29] Quinn JC, Davis R. The potentials and challenges of algae based biofuels: A review of the techno-economic, life cycle, and resource assessment modeling. Bioresource Technology. 2015;**184**:444-452

[30] Moody JW, McGinty CM, Quinn JC. Global evaluation of biofuel potential from microalgae. Proceedings of the National Academy of Sciences. 2014;**21**:201321652

[31] Lee E, Jalalizadeh M, Zhang Q. Growth kinetic models for microalgae cultivation: A review. Algal Research. 2015;**12**:497-512

[32] Barros AI, Gonçalves AL, Simões M, Pires JC. Harvesting techniques applied to microalgae: A review. Renewable and Sustainable Energy Reviews. 2015;**41**:1489-1500

**136**

*Anaerobic Digestion*

[1] Wijffels RH, Barbosa MJ. An outlook

et al. Commercial-scale biodiesel production from algae. Industrial & Engineering Chemistry Research.

Energy Reviews. 2017;**76**:493-506

Reviews. 2016;**58**:180-197

2018;**35**:50-60

2017;**75**:692-709

2015;**43**:961-972

[15] Raheem A, Prinsen P,

Vuppaladadiyam AK, Zhao M, Luque R. A review on sustainable microalgae based biofuel and bioenergy production:

Recent developments. Journal of Cleaner Production. 2018;**181**:42-59

[16] Toledo-Cervantes A, Serejo ML, Blanco S, Pérez R, Lebrero R, Muñoz R. Photosynthetic biogas upgrading to bio-methane: Boosting nutrient recovery via biomass productivity control. Algal Research. 2016;**17**:46-52

[11] Milano J, Ong HC, Masjuki HH, Chong WT, Lam MK, Loh PK, et al. Microalgae biofuels as an alternative to fossil fuel for power generation. Renewable and Sustainable Energy

[12] Garrido-Cardenas JA, Manzano-Agugliaro F, Acien-Fernandez FG, Molina-Grima E. Microalgae

research worldwide. Algal Research.

[14] Montingelli ME, Tedesco S, Olabi AG. Biogas production from algal biomass: A review. Renewable and Sustainable Energy Reviews.

[13] Jankowska E, Sahu AK, Oleskowicz-Popiel P. Biogas from microalgae: Review on microalgae's cultivation, harvesting and pretreatment for anaerobic digestion. Renewable and Sustainable Energy Reviews.

[10] Moreno-Garcia L, Adjallé K, Barnabé S, Raghavan GS. Microalgae biomass production for a biorefinery system: Recent advances and the way towards sustainability. Renewable and Sustainable

2013;**53**(13):5311-5324

[2] Borowitzka MA. High-value products from microalgae—Their development and commercialisation. Journal of Applied Phycology. 2013;**25**(3):743-756

[3] Brennan L, Owende P. Biofuels from microalgae—A review of technologies for production, processing, and

extractions of biofuels and co-products. Renewable and Sustainable Energy Reviews. 2010;**14**(2):557-577

[4] Su Y, Song K, Zhang P, Su Y, Cheng J, Chen X. Progress of microalgae biofuel's commercialization. Renewable and Sustainable Energy Reviews.

[5] Rahimpour MR, Biniaz P, Makarem MA. Integration of microalgae into an existing biofuel industry. In: Bioenergy Systems for the Future. Sawston,

Cambridge: Woodhead Publishing. 2017.

[6] Ward AJ, Lewis DM, Green FB. Anaerobic digestion of algae biomass:

[7] Hise AM, Characklis GW, Kern J, Gerlach R, Viamajala S, Gardner RD, et al. Evaluating the relative impacts of operational and financial factors on the competitiveness of an algal biofuel production facility. Bioresource

Technology. 2016;**220**:271-281

potential of renewable energy through the anaerobic digestion of microalgae. Bioresource Technology.

2011;**102**(2):1149-1158

[8] Zamalloa C, Vulsteke E, Albrecht J, Verstraete W. The techno-economic

[9] Silva C, Soliman E, Cameron G, Fabiano LA, Seider WD, Dunlop EH,

A review. Algal Research.

2017;**74**:402-411

pp. 481-519

2014;**5**:204-214

on microalgal biofuels. Science.

2010;**329**(5993):796-799

**References**

[33] Laamanen CA, Ross GM, Scott JA. Flotation harvesting of microalgae. Renewable and Sustainable Energy Reviews. 2016;**58**:75-86

[34] Fasaei F, Bitter JH, Slegers PM, van Boxtel AJ. Techno-economic evaluation of microalgae harvesting and dewatering systems. Algal Research. 2018;**31**:347-362

[35] Singh G, Patidar SK. Microalgae harvesting techniques: A review. Journal of Environmental Management. 2018;**217**:499-508

[36] Gerardo ML, Van Den Hende S, Vervaeren H, Coward T, Skill SC. Harvesting of microalgae within a biorefinery approach: A review of the developments and case studies from pilot-plants. Algal Research. 2015;**1**(11):248-262

[37] Gonzalez-Fernandez C, Sialve B, Molinuevo-Salces B. Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs. Bioresource Technology. 2015;**198**:896-906

[38] Wirth R, Lakatos G, Böjti T, Maróti G, Bagi Z, Rákhely G, et al. Anaerobic gaseous biofuel production using microalgal biomass—A review. Anaerobe. 2018;**52**:1-8

[39] Jones CS, Mayfield SP. Algae biofuels: Versatility for the future of bioenergy. Current Opinion in Biotechnology. 2012;**23**(3):346-351

[40] Passos F, Carretero J, Ferrer I. Comparing pretreatment methods for improving microalgae anaerobic digestion: Thermal, hydrothermal, microwave and ultrasound. Chemical Engineering Journal. 2015;**279**:667-672

[41] Klassen V, Blifernez-Klassen O, Wobbe L, Schlueter A, Kruse O, Mussgnug JH. Efficiency and biotechnological aspects of biogas

production from microalgal substrates. Journal of Biotechnology. 2016;**234**:7-26

[42] Kendir E, Ugurlu A. A comprehensive review on pretreatment of microalgae for biogas production. International Journal of Energy Research. 2018;**42**:3711-3731

[43] Córdova O, Santis J, Ruiz-Fillipi G, Zuñiga ME, Chamy R, Fermoso FG. Microalgae digestive pretreatment for increasing biogas production. Renewable and Sustainable Energy Reviews. 2018;**82**:2806-2813

[44] Kinnunen HV, Koskinen PE, Rintala J. Mesophilic and thermophilic anaerobic laboratory-scale digestion of Nannochloropsis microalga residues. Bioresource Technology. 2014;**155**:314-322

[45] Zamalloa C, Boon N, Verstraete W. Anaerobic digestibility of *Scenedesmus obliquus* and *Phaeodactylum tricornutum* under mesophilic and thermophilic conditions. Applied Energy. 2012;**92**:733-738

[46] Carlini M, Mosconi EM, Castellucci S, Villarini M, Colantoni A. An economical evaluation of anaerobic digestion plants fed with organic agro-industrial waste. Energies. 2017;**10**(8):1165

[47] Saratale RG, Kumar G, Banu R, Xia A, Periyasamy S, Saratale GD. A critical review on anaerobic digestion of microalgae and macroalgae and co-digestion of biomass for enhanced methane generation. Bioresource Technology. 2018;**9**:319-332

[48] Ishika T, Moheimani NR, Bahri PA. Sustainable saline microalgae co-cultivation for biofuel production: A critical review. Renewable and Sustainable Energy Reviews. 2017;**78**:356-368

[49] Aspe E, Marti MC, Roeckel M. Anaerobic treatment of fishery

**139**

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion*

[57] González-Fernández C, Sialve B, Bernet N, Steyer JP. Thermal pretreatment to improve methane production of Scenedesmus biomass. Biomass and Bioenergy. 2012;**40**:105-111

[58] Frigon JC, Matteau-Lebrun F, Abdou RH, McGinn PJ, O'Leary SJ, Guiot SR. Screening microalgae strains for their productivity in methane following anaerobic digestion. Applied

[59] Erkelens M, Ward AJ, Ball AS, Lewis DM. Microalgae digestate effluent as a growth medium for Tetraselmis sp. in the production of biofuels. Bioresource

[60] Prajapati SK, Kumar P, Malik A, Vijay VK. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: A closed loop bioenergy generation process. Bioresource Technology.

[61] Cai T, Park SY, Racharaks R, Li Y. Cultivation of *Nannochloropsis salina* using anaerobic digestion effluent as a nutrient source for biofuel production. Applied Energy. 2013;**108**:486-492

[62] García D, Posadas E, Grajeda C, Blanco S, Martínez-Páramo S, Acién G, et al. Comparative evaluation of piggery wastewater treatment in algalbacterial photobioreactors under indoor and outdoor conditions. Bioresource Technology. 2017;**245**:483-490

[63] Prandini JM, da Silva ML, Mezzari MP, Pirolli M, Michelon W, Soares HM. Enhancement of nutrient removal from swine wastewater digestate coupled to biogas purification by microalgae Scenedesmus spp. Bioresource Technology. 2016;**202**:67-75

[64] Uggetti E, Sialve B, Latrille E, Steyer JP. Anaerobic digestate as substrate for microalgae culture: The role of ammonium concentration on the

Energy. 2013;**108**:100-107

Technology. 2014;**167**:81-86

2014;**158**:174-180

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wastewater using a marine sediment

[50] Cirne DG, Paloumet X, Björnsson L, Alves MM, Mattiasson B. Anaerobic digestion of lipid-rich waste—Effects of lipid concentration. Renewable Energy.

[51] Zhao B, Ma J, Zhao Q, Laurens L, Jarvis E, Chen S, et al. Efficient anaerobic digestion of whole microalgae and lipid-extracted

microalgae residues for methane energy production. Bioresource Technology.

[52] Mahdy A, Mendez L, Ballesteros M, González-Fernández C. Enhanced methane production of *Chlorella vulgaris* and *Chlamydomonas reinhardtii* by hydrolytic enzymes addition. Energy Conversion and Management.

[53] Schwede S, Rehman ZU, Gerber M, Theiss C, Span R. Effects of thermal pretreatment on anaerobic digestion of *Nannochloropsis salina* biomass. Bioresource Technology.

[54] Lu D, Zhang XJ. Biogas production

[55] He S, Fan X, Katukuri NR, Yuan X, Wang F, Guo RB. Enhanced methane production from microalgal biomass by anaerobic bio-pretreatment. Bioresource

[56] Mahdy A, Mendez L, Tomás-Pejó E, del Mar Morales M, Ballesteros M, González-Fernández C. Influence of enzymatic hydrolysis on the biochemical methane potential of *Chlorella vulgaris* and Scenedesmus sp. Journal of

Chemical Technology & Biotechnology.

from anaerobic codigestion of microalgae and septic sludge. Journal of Environmental Engineering.

Technology. 2016;**204**:145-151

2016;**91**(5):1299-1305

2016;**142**(10):04016049

inoculum. Water Research. 1997;**31**(9):2147-2160

2007;**32**(6):965-975

2014;**161**:423-430

2014;**85**:551-557

2013;**143**:505-511

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

wastewater using a marine sediment inoculum. Water Research. 1997;**31**(9):2147-2160

*Anaerobic Digestion*

Reviews. 2016;**58**:75-86

2018;**31**:347-362

2018;**217**:499-508

2015;**1**(11):248-262

2015;**198**:896-906

Anaerobe. 2018;**52**:1-8

[33] Laamanen CA, Ross GM, Scott JA. Flotation harvesting of microalgae. Renewable and Sustainable Energy

production from microalgal substrates. Journal of Biotechnology. 2016;**234**:7-26

comprehensive review on pretreatment of microalgae for biogas production. International Journal of Energy Research. 2018;**42**:3711-3731

[43] Córdova O, Santis J, Ruiz-Fillipi G, Zuñiga ME, Chamy R, Fermoso FG. Microalgae digestive pretreatment for increasing biogas production. Renewable and Sustainable Energy Reviews. 2018;**82**:2806-2813

[44] Kinnunen HV, Koskinen PE, Rintala J. Mesophilic and thermophilic anaerobic laboratory-scale digestion of Nannochloropsis microalga residues. Bioresource Technology.

[45] Zamalloa C, Boon N, Verstraete W. Anaerobic digestibility of

*Scenedesmus obliquus* and *Phaeodactylum tricornutum* under mesophilic and thermophilic conditions. Applied

[46] Carlini M, Mosconi EM, Castellucci

S, Villarini M, Colantoni A. An economical evaluation of anaerobic digestion plants fed with organic agro-industrial waste. Energies.

[47] Saratale RG, Kumar G, Banu R, Xia A, Periyasamy S, Saratale GD. A critical review on anaerobic digestion of microalgae and macroalgae and co-digestion of biomass for enhanced methane generation. Bioresource Technology. 2018;**9**:319-332

[48] Ishika T, Moheimani NR, Bahri PA. Sustainable saline microalgae co-cultivation for biofuel production: A critical review. Renewable and Sustainable Energy Reviews.

[49] Aspe E, Marti MC, Roeckel M. Anaerobic treatment of fishery

2014;**155**:314-322

Energy. 2012;**92**:733-738

2017;**10**(8):1165

2017;**78**:356-368

[42] Kendir E, Ugurlu A. A

[34] Fasaei F, Bitter JH, Slegers PM, van Boxtel AJ. Techno-economic

[35] Singh G, Patidar SK. Microalgae harvesting techniques: A review.

[36] Gerardo ML, Van Den Hende S, Vervaeren H, Coward T, Skill SC. Harvesting of microalgae within a biorefinery approach: A review of the developments and case studies from pilot-plants. Algal Research.

[37] Gonzalez-Fernandez C, Sialve B, Molinuevo-Salces B. Anaerobic digestion of microalgal biomass: Challenges, opportunities and research

needs. Bioresource Technology.

[38] Wirth R, Lakatos G, Böjti T, Maróti G, Bagi Z, Rákhely G, et al. Anaerobic gaseous biofuel production using microalgal biomass—A review.

[39] Jones CS, Mayfield SP. Algae biofuels: Versatility for the future of bioenergy. Current Opinion in Biotechnology. 2012;**23**(3):346-351

[40] Passos F, Carretero J, Ferrer I. Comparing pretreatment methods for improving microalgae anaerobic digestion: Thermal, hydrothermal, microwave and ultrasound. Chemical Engineering Journal. 2015;**279**:667-672

[41] Klassen V, Blifernez-Klassen O, Wobbe L, Schlueter A, Kruse O, Mussgnug JH. Efficiency and biotechnological aspects of biogas

Journal of Environmental Management.

evaluation of microalgae harvesting and dewatering systems. Algal Research.

**138**

[50] Cirne DG, Paloumet X, Björnsson L, Alves MM, Mattiasson B. Anaerobic digestion of lipid-rich waste—Effects of lipid concentration. Renewable Energy. 2007;**32**(6):965-975

[51] Zhao B, Ma J, Zhao Q, Laurens L, Jarvis E, Chen S, et al. Efficient anaerobic digestion of whole microalgae and lipid-extracted microalgae residues for methane energy production. Bioresource Technology. 2014;**161**:423-430

[52] Mahdy A, Mendez L, Ballesteros M, González-Fernández C. Enhanced methane production of *Chlorella vulgaris* and *Chlamydomonas reinhardtii* by hydrolytic enzymes addition. Energy Conversion and Management. 2014;**85**:551-557

[53] Schwede S, Rehman ZU, Gerber M, Theiss C, Span R. Effects of thermal pretreatment on anaerobic digestion of *Nannochloropsis salina* biomass. Bioresource Technology. 2013;**143**:505-511

[54] Lu D, Zhang XJ. Biogas production from anaerobic codigestion of microalgae and septic sludge. Journal of Environmental Engineering. 2016;**142**(10):04016049

[55] He S, Fan X, Katukuri NR, Yuan X, Wang F, Guo RB. Enhanced methane production from microalgal biomass by anaerobic bio-pretreatment. Bioresource Technology. 2016;**204**:145-151

[56] Mahdy A, Mendez L, Tomás-Pejó E, del Mar Morales M, Ballesteros M, González-Fernández C. Influence of enzymatic hydrolysis on the biochemical methane potential of *Chlorella vulgaris* and Scenedesmus sp. Journal of Chemical Technology & Biotechnology. 2016;**91**(5):1299-1305

[57] González-Fernández C, Sialve B, Bernet N, Steyer JP. Thermal pretreatment to improve methane production of Scenedesmus biomass. Biomass and Bioenergy. 2012;**40**:105-111

[58] Frigon JC, Matteau-Lebrun F, Abdou RH, McGinn PJ, O'Leary SJ, Guiot SR. Screening microalgae strains for their productivity in methane following anaerobic digestion. Applied Energy. 2013;**108**:100-107

[59] Erkelens M, Ward AJ, Ball AS, Lewis DM. Microalgae digestate effluent as a growth medium for Tetraselmis sp. in the production of biofuels. Bioresource Technology. 2014;**167**:81-86

[60] Prajapati SK, Kumar P, Malik A, Vijay VK. Bioconversion of algae to methane and subsequent utilization of digestate for algae cultivation: A closed loop bioenergy generation process. Bioresource Technology. 2014;**158**:174-180

[61] Cai T, Park SY, Racharaks R, Li Y. Cultivation of *Nannochloropsis salina* using anaerobic digestion effluent as a nutrient source for biofuel production. Applied Energy. 2013;**108**:486-492

[62] García D, Posadas E, Grajeda C, Blanco S, Martínez-Páramo S, Acién G, et al. Comparative evaluation of piggery wastewater treatment in algalbacterial photobioreactors under indoor and outdoor conditions. Bioresource Technology. 2017;**245**:483-490

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wastewater. Bioresource Technology.

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2009;**177**(4):272-280

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Technology. 2016;**208**:42-48

2015;**215**:44-51

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2016;**13**:195-206

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[130] Klassen V, Blifernez-Klassen O, Wibberg D, Winkler A, Kalinowski J, Posten C, et al. Highly efficient methane generation from untreated microalgae biomass. Biotechnology for Biofuels. 2017;**10**(1):186

[131] Juneja A, Murthy GS. Evaluating the potential of renewable diesel production from algae cultured on wastewater: Techno-economic analysis and life cycle assessment. AIMS Energy. 2017;**5**(2):239-257

[132] Xiang X, Ozkan A, Kelly C, Radniecki T. Importance of microalgae speciation on biogas production and

nutrient recovery from anaerobic digestion of lipid-extracted microalgae biomass. Environmental Engineering Science. 2018;**35**(4):382-389

[133] Mussgnug JH, Klassen V, Schlüter A, Kruse O. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. Journal of Biotechnology. 2010;**150**(1):51-56

[134] Wang X, Nordlander E, Thorin E, Yan J. Microalgal biomethane production integrated with an existing biogas plant: A case study in Sweden. Applied energy. 2013;**112**:478-484

[135] Darvehei P, Bahri PA, Moheimani NR. Model development for the growth of microalgae: A review. Renewable and Sustainable Energy Reviews. 2018;**97**:233-258

[136] Kumar K, Ghosh S, Angelidaki I, Holdt SL, Karakashev DB, Morales MA, et al. Recent developments on biofuels production from microalgae and macroalgae. Renewable and Sustainable Energy Reviews. 2016;**65**:235-249

[137] Santos-Ballardo DU, Rossi S, Reyes-Moreno C, Valdez-Ortiz A. Microalgae potential as a biogas source: Current status, restraints and future trends. Reviews in Environmental Science and Bio/ Technology. 2016;**15**(2):243-264

[138] Wang X, Bao K, Cao W, Zhao Y, Hu CW. Screening of microalgae for integral biogas slurry nutrient removal and biogas upgrading by different microalgae cultivation technology. Scientific Reports. 2017;**7**(1):5426

[139] Lu D, Zhang XJ, Liu X, Zhang L, Hines M. Sustainable microalgae cultivation by using anaerobic centrate and biogas from anaerobic digestion. Algal Research. 2018;**35**:115-124

[140] Harun R, Davidson M, Doyle M, Gopiraj R, Danquah M, Forde G.

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[148] Doan NT. Assessing and Enhancing Methane Productivity from Anaerobic Digestion Using Cyanothece BG0011 as Feedstock [Doctoral Dissertation]:

University of Florida; 2017

[150] Brown RC, Brown TR.

Academic Press; 1995

2006. Available from: https:// escholarship.org/uc/item/1zg00532

[Accessed: 2018-12-05]

Biorenewable Resources: Engineering New Products from Agriculture. Hoboken, New Jersey: John Wiley &

[151] Doran PM. Bioprocess Engineering Principles. Cambridge, Massachusetts:

[152] McCollum DL, Ogden JM. Technoeconomic models for carbon dioxide compression, transport, and storage & correlations for estimating carbon dioxide density and viscosity. [Internet].

Florida; 2018

Sons; 2013

[149] Zhang Y. Cultivation, growth optimization and modeling of a saline Cyanothece species BG0011 for production of biofuels and bioproducts [doctoral dissertation]. University of

*DOI: http://dx.doi.org/10.5772/intechopen.86090*

integrated microalgae photobioreactor, biodiesel and biogas production facility. Biomass and Bioenergy.

Technoeconomic analysis of an

[141] Barlow J, Sims RC, Quinn JC. Techno-economic and life-cycle assessment of an attached growth algal biorefinery. Bioresource Technology.

[142] Batan LY, Graff GD, Bradley TH. Techno-economic and Monte Carlo probabilistic analysis of microalgae biofuel production system. Bioresource

[143] Saharan BS, Sharma D, Sahu R, Sahin O, Warren A. Towards algal biofuel production: A concept of green bio energy development. Innovative Romanian Food Biotechnology. 2013;**12**:1

[144] Stiles WA, Styles D, Chapman SP, Esteves S, Bywater A, Melville L, et al. Using microalgae in the circular economy to valorise anaerobic digestate: Challenges and opportunities. Bioresource Technology.

[145] Kavitha S, Subbulakshmi P, Banu JR, Gobi M, Yeom IT. Enhancement of biogas production from microalgal biomass through cellulolytic bacterial pretreatment. Bioresource Technology.

[146] Kavitha S, Banu JR, Priya AA, Uan DK, Yeom IT. Liquefaction of food waste and its impacts on anaerobic biodegradability, energy ratio and economic feasibility. Applied Energy.

[147] Kavitha S, Kannah RY, Banu JR, Kaliappan S, Johnson M. Biological disintegration of microalgae for biomethane recovery-prediction of biodegradability and computation of energy balance. Bioresource Technology.

Technology. 2016;**219**:45-52

2011;**35**(1):741-747

2016;**220**:360-368

2018;**267**:732-742

2017;**233**:34-43

2017;**208**:228-238

2017;**244**:1367-1375

*Techno-Economic Analysis of Biogas Production from Microalgae through Anaerobic Digestion DOI: http://dx.doi.org/10.5772/intechopen.86090*

Technoeconomic analysis of an integrated microalgae photobioreactor, biodiesel and biogas production facility. Biomass and Bioenergy. 2011;**35**(1):741-747

*Anaerobic Digestion*

SBERA; 2013

Biogas production from microalga biomass. In: Embrapa Suínos e Aves-Artigo em anais de congresso (ALICE). Simpósio Internacional Sobre Gerenciamento de Resíduos Agropecuários e Agroindustriais. Vol. 3. São Pedro, SP. Anais... São Pedro, SP: nutrient recovery from anaerobic digestion of lipid-extracted microalgae biomass. Environmental Engineering

[133] Mussgnug JH, Klassen V, Schlüter A, Kruse O. Microalgae as substrates for fermentative biogas production in a combined biorefinery concept. Journal of Biotechnology. 2010;**150**(1):51-56

[134] Wang X, Nordlander E, Thorin E, Yan J. Microalgal biomethane production integrated with an existing biogas plant: A case study in Sweden. Applied energy. 2013;**112**:478-484

[135] Darvehei P, Bahri PA, Moheimani NR. Model development for the growth of microalgae: A review. Renewable and Sustainable Energy Reviews.

[136] Kumar K, Ghosh S, Angelidaki I, Holdt SL, Karakashev DB, Morales MA, et al. Recent developments on biofuels production from microalgae and macroalgae. Renewable and Sustainable Energy Reviews. 2016;**65**:235-249

[137] Santos-Ballardo DU, Rossi S, Reyes-Moreno C, Valdez-Ortiz A. Microalgae potential as a biogas source: Current status, restraints and future trends. Reviews in Environmental Science and Bio/ Technology. 2016;**15**(2):243-264

[138] Wang X, Bao K, Cao W, Zhao Y, Hu CW. Screening of microalgae for integral biogas slurry nutrient removal and biogas upgrading by different microalgae cultivation technology. Scientific Reports. 2017;**7**(1):5426

[139] Lu D, Zhang XJ, Liu X, Zhang L, Hines M. Sustainable microalgae cultivation by using anaerobic centrate and biogas from anaerobic digestion. Algal Research. 2018;**35**:115-124

[140] Harun R, Davidson M, Doyle M, Gopiraj R, Danquah M, Forde G.

2018;**97**:233-258

Science. 2018;**35**(4):382-389

[126] Passos F, Solé M, García J, Ferrer I. Biogas production from microalgae grown in wastewater: Effect of microwave pretreatment. Applied

[127] Bohutskyi P, Bouwer E. Biogas

[128] Xin C, Addy MM, Zhao J, Cheng Y, Cheng S, Mu D, et al. Comprehensive

techno-economic analysis of wastewater-based algal biofuel production: A case study. Bioresource

Technology. 2016;**211**:584-593

[129] Milledge JJ, Heaven S. Energy balance of biogas production from microalgae: Development of an energy and mass balance model. Current Biotechnology. 2015;**4**(4):554-567

[130] Klassen V, Blifernez-Klassen O, Wibberg D, Winkler A, Kalinowski J, Posten C, et al. Highly efficient methane generation from untreated microalgae biomass. Biotechnology for Biofuels.

[131] Juneja A, Murthy GS. Evaluating the potential of renewable diesel production from algae cultured on wastewater: Techno-economic analysis and life cycle assessment. AIMS Energy.

[132] Xiang X, Ozkan A, Kelly C, Radniecki T. Importance of microalgae speciation on biogas production and

Energy. 2013;**108**:168-175

production from algae and cyanobacteria through anaerobic digestion: A review, analysis, and research needs. In: Advanced Biofuels and Bioproducts. New York, NY: Springer; 2013. pp. 873-975

**144**

2017;**10**(1):186

2017;**5**(2):239-257

[141] Barlow J, Sims RC, Quinn JC. Techno-economic and life-cycle assessment of an attached growth algal biorefinery. Bioresource Technology. 2016;**220**:360-368

[142] Batan LY, Graff GD, Bradley TH. Techno-economic and Monte Carlo probabilistic analysis of microalgae biofuel production system. Bioresource Technology. 2016;**219**:45-52

[143] Saharan BS, Sharma D, Sahu R, Sahin O, Warren A. Towards algal biofuel production: A concept of green bio energy development. Innovative Romanian Food Biotechnology. 2013;**12**:1

[144] Stiles WA, Styles D, Chapman SP, Esteves S, Bywater A, Melville L, et al. Using microalgae in the circular economy to valorise anaerobic digestate: Challenges and opportunities. Bioresource Technology. 2018;**267**:732-742

[145] Kavitha S, Subbulakshmi P, Banu JR, Gobi M, Yeom IT. Enhancement of biogas production from microalgal biomass through cellulolytic bacterial pretreatment. Bioresource Technology. 2017;**233**:34-43

[146] Kavitha S, Banu JR, Priya AA, Uan DK, Yeom IT. Liquefaction of food waste and its impacts on anaerobic biodegradability, energy ratio and economic feasibility. Applied Energy. 2017;**208**:228-238

[147] Kavitha S, Kannah RY, Banu JR, Kaliappan S, Johnson M. Biological disintegration of microalgae for biomethane recovery-prediction of biodegradability and computation of energy balance. Bioresource Technology. 2017;**244**:1367-1375

[148] Doan NT. Assessing and Enhancing Methane Productivity from Anaerobic Digestion Using Cyanothece BG0011 as Feedstock [Doctoral Dissertation]: University of Florida; 2017

[149] Zhang Y. Cultivation, growth optimization and modeling of a saline Cyanothece species BG0011 for production of biofuels and bioproducts [doctoral dissertation]. University of Florida; 2018

[150] Brown RC, Brown TR. Biorenewable Resources: Engineering New Products from Agriculture. Hoboken, New Jersey: John Wiley & Sons; 2013

[151] Doran PM. Bioprocess Engineering Principles. Cambridge, Massachusetts: Academic Press; 1995

[152] McCollum DL, Ogden JM. Technoeconomic models for carbon dioxide compression, transport, and storage & correlations for estimating carbon dioxide density and viscosity. [Internet]. 2006. Available from: https:// escholarship.org/uc/item/1zg00532 [Accessed: 2018-12-05]

Section 4

Biogas

147

Section 4 Biogas

Chapter 7

Abstract

1. Introduction

Table 1. Biogas composition.

149

production is likewise high (Table 1) [2].

a density of approximately 0.75 kg/m<sup>3</sup>

heavier, biogas has a slightly higher density of 1.15–1.25 kg/m<sup>3</sup>

an upper calorific value of 39.8 MJ/m<sup>3</sup> (11.06 kWh/m<sup>3</sup>

Biogas for Clean Energy

Demsew Mitiku Teferra and Wondwosen Wubu

This chapter demonstrates a biogas renewable energy resource potential study for electric power generation from easily available biogas feedstock materials in four selected case study sites. Under this study, the site used in the model is a rural Kebele in Jama Woreda at 10.548° N, 39.33° E. The common biogas feedstocks considered under this study are animal slurry, human feces and jatropha byproducts whereas the biodiesel is considered from jatropha seed.

Keywords: anaerobic digestion, bioenergy, biogas digester, feedstock, Jatropha

Biogas is a byproduct of biomass which contains methane (CH4) and carbon dioxide (CO2) as a main gas component in a 3:2 ratio and it is produced through micro bacterial digestion processes under anaerobic conditions from a variety of organic material from animal, agricultural, industrial and domestic wastes [1]. The biogas production level is depending on the ingredient level in the feedstock. For example; if the material consists of mainly carbohydrates, like glucose and other simple sugars and high-molecular polymers such as cellulose and hemicelluloses, the methane production is low. However, if the fat content is high, the methane

Methane and other additional hydrogen compounds make up the combustible part of biogas. Methane is a colorless and odorless gas with a boiling point of 162°C and it burns with a blue flame. At normal temperature and pressure, methane has

. Due to carbon dioxide being somewhat

) (Table 2) [2].

. Pure methane has

## Chapter 7 Biogas for Clean Energy

Demsew Mitiku Teferra and Wondwosen Wubu

### Abstract

This chapter demonstrates a biogas renewable energy resource potential study for electric power generation from easily available biogas feedstock materials in four selected case study sites. Under this study, the site used in the model is a rural Kebele in Jama Woreda at 10.548° N, 39.33° E. The common biogas feedstocks considered under this study are animal slurry, human feces and jatropha byproducts whereas the biodiesel is considered from jatropha seed.

Keywords: anaerobic digestion, bioenergy, biogas digester, feedstock, Jatropha

### 1. Introduction

Biogas is a byproduct of biomass which contains methane (CH4) and carbon dioxide (CO2) as a main gas component in a 3:2 ratio and it is produced through micro bacterial digestion processes under anaerobic conditions from a variety of organic material from animal, agricultural, industrial and domestic wastes [1]. The biogas production level is depending on the ingredient level in the feedstock. For example; if the material consists of mainly carbohydrates, like glucose and other simple sugars and high-molecular polymers such as cellulose and hemicelluloses, the methane production is low. However, if the fat content is high, the methane production is likewise high (Table 1) [2].

Methane and other additional hydrogen compounds make up the combustible part of biogas. Methane is a colorless and odorless gas with a boiling point of 162°C and it burns with a blue flame. At normal temperature and pressure, methane has a density of approximately 0.75 kg/m<sup>3</sup> . Due to carbon dioxide being somewhat heavier, biogas has a slightly higher density of 1.15–1.25 kg/m<sup>3</sup> . Pure methane has an upper calorific value of 39.8 MJ/m<sup>3</sup> (11.06 kWh/m<sup>3</sup> ) (Table 2) [2].


Table 1. Biogas composition.


### Table 2.

Potential biogas production from various biomass feedstocks on VS based.

### 2. The biogas production process

Anaerobic digestion (AD) is a biochemical process during which complex organic matter is decomposed in absence of oxygen, by various types of anaerobic microorganisms. The result of the AD process is the biogas and the digestate. Biogas is a combustible gas, consisting primarily of methane and carbon dioxide. Digestate is the decomposed substrate, resulted from the production of biogas. If the substrate for AD is a homogenous mixture of two or more feedstock types (e.g., animal slurries and organic wastes from food industries), the process is called "co-digestion" and is common to most biogas applications today.

The process of biogas formation is a result of linked process steps, in which the initial material is continuously broken down into smaller units. Specific groups of micro-organisms are involved in each individual step. The simplified diagram of the AD process, shown in Figure 1, highlights the four main process steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The process steps quoted in Figure 1 run parallel in time and space, in the digester tank. During hydrolysis, relatively small amounts of biogas are produced. Biogas production reaches its peak during methanogenesis [3].

temperature, water content, NH3 concentration and pH are examples of factors

production rate increases with increase the process temperature (Table 3).

Temperature for fermentation will greatly affect biogas production. The AD process can take place at different temperatures, divided into three temperature ranges: psychrophilic (below 20°C), mesophilic (30–42°C), and thermophilic (43–55°C). There is a direct relation between the process temperature and the HRT. The biogas

influencing the methanogenesis process.

Biogas production process by anaerobic digestion.

Figure 1.

Biogas for Clean Energy

DOI: http://dx.doi.org/10.5772/intechopen.79534

151

Methanogenesis is a critical step in the entire anaerobic digestion process, as it is the slowest biochemical reaction of the process. Methanogenesis is severely influenced by operation conditions. Composition of feedstock, feeding rate,

### Figure 1.

2. The biogas production process

Potential biogas production from various biomass feedstocks on VS based.

Substrate HRT

Domestic garbage

Anaerobic Digestion

Food-market waste

Mango processing waste

Tomatoprocessing waste

Mixed feed of fruit waste

Table 2.

150

(days)

Solid concentration (%)

Sewage sludge 25 6 35 0.52 68

Piggery waste 20 6.5 35 0.43 69 Poultry waste 15 6 35 0.5 69 Cattle waste 30 10 35 0.3 58 Canteen waste 20 10 30 0.6 50

Lemon waste 30 4 37 0.72 53 Citrus waste 32 4 37 0.63 62 Banana peel 25 10 37 0.60 55 Pineapple waste 30 4 37 0.37 60

Temperature (°C) Biogas yield

30 5 35 0.47 —

20 4 35 0.75 62

20 10 35 0.45 52

24 4.5 35 0.63 65

20 4 37 0.62 50

(m3 /kg VS) Methane (%)

peak during methanogenesis [3].

Anaerobic digestion (AD) is a biochemical process during which complex organic matter is decomposed in absence of oxygen, by various types of anaerobic microorganisms. The result of the AD process is the biogas and the digestate. Biogas is a combustible gas, consisting primarily of methane and carbon dioxide. Digestate is the decomposed substrate, resulted from the production of biogas. If the substrate for AD is a homogenous mixture of two or more feedstock types (e.g., animal slurries and organic wastes from food industries), the process is called "co-digestion" and is common to most biogas applications today.

The process of biogas formation is a result of linked process steps, in which the initial material is continuously broken down into smaller units. Specific groups of micro-organisms are involved in each individual step. The simplified diagram of the AD process, shown in Figure 1, highlights the four main process steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The process steps quoted in Figure 1 run parallel in time and space, in the digester tank. During hydrolysis, relatively small amounts of biogas are produced. Biogas production reaches its

Methanogenesis is a critical step in the entire anaerobic digestion process, as it is the slowest biochemical reaction of the process. Methanogenesis is severely influenced by operation conditions. Composition of feedstock, feeding rate,

Biogas production process by anaerobic digestion.

temperature, water content, NH3 concentration and pH are examples of factors influencing the methanogenesis process.

Temperature for fermentation will greatly affect biogas production. The AD process can take place at different temperatures, divided into three temperature ranges: psychrophilic (below 20°C), mesophilic (30–42°C), and thermophilic (43–55°C). There is a direct relation between the process temperature and the HRT. The biogas production rate increases with increase the process temperature (Table 3).


Process stages of biogas production:

DOI: http://dx.doi.org/10.5772/intechopen.79534

Main components of biogas plant:

• Feedstock pre-storage tank

• Substrate mixing Tank

• Biogas digester

• Post storage tank

• CHP system

Figure 2.

153

Main components and general process flow of biogas production.

• Gas holder tank and

• Biogas production

Biogas for Clean Energy

• Transport, delivery, storage and pre-treatment of feedstocks

The amount and type of available feedstock can determine the size, type and design structure of the biogas plant. The amount of biogas feedstock could determine the dimensioning of the digester size, storage capacities and CHP unit (Figure 2). The CHP system utilizes the biogas either in heat or electrical energy. The properties of the combustible methane gas (like as shown in Table 4) will affect the operation of the CHP equipment. The combustion nature of the gas must be

• Storage of digestate, conditioning and utilization

• Storage of biogas, conditioning and utilization.

Table 3.

Biogas production thermal stage and their corresponding retention time [4].

In practice most modern biogas plants operate at thermophilic process temperatures because this process provides many advantages, compared to mesophilic and psychrophilic processes:


The metabolic processes in the production of biogas from different biomass feedstocks are hydrolysis, acidogenesis, acetogenesis and methanogenesis and their byproducts in the process is represented in the figure below.

In this study thermophilic biogas temperature process is chosen in order to get higher biogas output and to achieve this target flat plate collector can be used to maintain digester process temperature at 55<sup>o</sup> c.

### 3. Biogas plant

A biogas plant is a complex installation, consisting of a variety of elements. The layout of such a plant depends to a large extent on the types and amounts of feedstock supplied. Now there are several main types of biogas plants all over the world. Each time it is necessary to find the most suitable type in different case. Public acceptance, cost and energy efficiency are the main criteria to install biogas plant and efficiently utilize the biogas production. In smaller areas with scarcity of biogas feedstock or slurry to use low cost clay, concert or stone masonry made biogas digester.

Installation and operation of a biogas plant is a combination of environmental, safety, economic and technical considerations. Acquiring maximum methane output, by complete digestion of feedstock substrate, would require a long fermentation or digestion time of the material inside the biogas digester and a correspondingly large digester size. The ultimate goal of biogas production is getting the highest possible methane output and having justifiable plant economy. Biogas plants have the following main components and operate with four different process stages [3].

### Process stages of biogas production:


### Main components of biogas plant:


In practice most modern biogas plants operate at thermophilic process temperatures because this process provides many advantages, compared to mesophilic and

Thermal stage Process Temperature Minimum HRT Psychrophilic < 20° C 70–80 days Mesophilic 30–42° C 30–40 days Thermophilic 43–55° C 15–20 days

• Minimization of biogas production period, making the process faster and more

The metabolic processes in the production of biogas from different biomass feedstocks are hydrolysis, acidogenesis, acetogenesis and methanogenesis and their

In this study thermophilic biogas temperature process is chosen in order to get higher biogas output and to achieve this target flat plate collector can be used to

A biogas plant is a complex installation, consisting of a variety of elements. The

Installation and operation of a biogas plant is a combination of environmental, safety, economic and technical considerations. Acquiring maximum methane output, by complete digestion of feedstock substrate, would require a long fermenta-

correspondingly large digester size. The ultimate goal of biogas production is getting the highest possible methane output and having justifiable plant economy. Biogas plants have the following main components and operate with four different

layout of such a plant depends to a large extent on the types and amounts of feedstock supplied. Now there are several main types of biogas plants all over the world. Each time it is necessary to find the most suitable type in different case. Public acceptance, cost and energy efficiency are the main criteria to install biogas plant and efficiently utilize the biogas production. In smaller areas with scarcity of biogas feedstock or slurry to use low cost clay, concert or stone masonry made

tion or digestion time of the material inside the biogas digester and a

c.

• Fast grow rate of methanogenic bacteria at higher temperature

• Improve digestibility and availability of substrates

Biogas production thermal stage and their corresponding retention time [4].

• better decomposition and utilization of solid substrates

• Increase the chance to separate liquid and solid fractions

byproducts in the process is represented in the figure below.

maintain digester process temperature at 55<sup>o</sup>

psychrophilic processes:

Table 3.

Anaerobic Digestion

efficient

3. Biogas plant

biogas digester.

process stages [3].

152

• Effective destruction of pathogens


The amount and type of available feedstock can determine the size, type and design structure of the biogas plant. The amount of biogas feedstock could determine the dimensioning of the digester size, storage capacities and CHP unit (Figure 2).

The CHP system utilizes the biogas either in heat or electrical energy. The properties of the combustible methane gas (like as shown in Table 4) will affect the operation of the CHP equipment. The combustion nature of the gas must be

Figure 2. Main components and general process flow of biogas production.


• Specific gas production per day (Gd), which depends on the retention time,

The size of the digester—the digester volume (VD)—is determined by the length of the retention time (RT) and by the amount of fermentation slurry supplied daily (SD). The amount of fermentation slurry consists of the feed material considered in

Daily average collectable biogas feedstock potential from cow dung, oxen dung,

HRT = 20 day, under thermophilic digestion temperature (55°C) the hydraulic

.

Daily average collectable biogas feedstock potential from cow dung, oxen dung,

HRT = 20 day, under thermophilic digestion temperature the hydraulic reten-

. The biogas gas production rate is 9253 kg/day 0.054

Daily average collectable biogas feedstock potential from cattle dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct of site-

/day. Therefore the size of gasholder should account this daily

donkey, mule, and horse waste, chicken waste, human feces and jatropha

/day) = 529 m<sup>3</sup>

byproduct of Site-B in tons/day is 9.253 = 9253 kg/day = 13.22 m<sup>3</sup>

.

/kg of fresh biogas feedstock mix is 1736.4

/kg; the biogas production rate is 10,867 kg/day 0.054

/day. Therefore the size of gasholder should account this daily biogas

.

/day water is required for proper digestion process of biogas

.

. Therefore the size of the digester for

/day, Since the average density of

/day. Since the

/day water is required for proper digestion of biogas feed-

/day. Since

donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct in this study in tons/day is 10.867 = 10,867 kg/day = 15.53 m<sup>3</sup>

/day) = 621 m3

Where, VD = the size of the digester, HRT = hydraulic retention time, and SD is the amount of fermentation slurry (water + feedstock) feed in to the

Therefore the size of the digester for site A could be 621 m3

the digestion temperature and the feed material.

4.1 Sizing of biogas digester and gasholder

DOI: http://dx.doi.org/10.5772/intechopen.79534

this study (e.g., cattle dung) and the mixing water.

the average density of animal slurry mix is 700 kg/m<sup>3</sup>

retention time of the digestion process becomes short. The volume of digester should be, VD = HRT SD.

4.1.1 Sizing of site-A biogas digester and gasholder

stock material to enhance biogas production.

4.1.2 Sizing of site-B biogas digester and gasholder

average density of animal slurry mix is 700 kg/m<sup>3</sup>

feedstock material to enhance biogas production.

tion time of the digestion process becomes short.

4.1.3 Sizing of site-C biogas digester and gasholder

C in tons/day is 8.82 = 8820 kg/day = 12.6 m<sup>3</sup>

The volume of digester should be, VD = HRT SD.

.

Additional 15.53 m<sup>3</sup>

Biogas for Clean Energy

= 20 day (15.53 2 m<sup>3</sup>

digester per day. Biogas yield in m<sup>3</sup>

/31850 kg = 0.054 m<sup>3</sup>

Additional 13.22 m<sup>3</sup>

= 20 day (13.22 2 m<sup>3</sup>

animal slurry mix is 700 kg/m<sup>3</sup>

site-B is 529 m<sup>3</sup>

/kg = 501 m<sup>3</sup>

biogas production.

m3

155

/kg = 588 m<sup>3</sup>

production.

m3

m3

### Table 4.

Biogas minimum requirement used in an electric engine [3].

guaranteed, to prevent damage to the engines. Further treatment and enhancing chemical and physical properties of biogas even possible to use it for other utilizations like as vehicle fuel or in fuel cells application.

### 4. Design of the biogas plant

The design of the biogas plant includes the design of:


To calculate the scale of a biogas plant, certain characteristic parameters are used. These are:


• Specific gas production per day (Gd), which depends on the retention time, the digestion temperature and the feed material.

### 4.1 Sizing of biogas digester and gasholder

The size of the digester—the digester volume (VD)—is determined by the length of the retention time (RT) and by the amount of fermentation slurry supplied daily (SD). The amount of fermentation slurry consists of the feed material considered in this study (e.g., cattle dung) and the mixing water.

### 4.1.1 Sizing of site-A biogas digester and gasholder

Daily average collectable biogas feedstock potential from cow dung, oxen dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct in this study in tons/day is 10.867 = 10,867 kg/day = 15.53 m<sup>3</sup> /day. Since the average density of animal slurry mix is 700 kg/m<sup>3</sup> .

Additional 15.53 m<sup>3</sup> /day water is required for proper digestion of biogas feedstock material to enhance biogas production.

HRT = 20 day, under thermophilic digestion temperature (55°C) the hydraulic retention time of the digestion process becomes short.

The volume of digester should be, VD = HRT SD.

= 20 day (15.53 2 m<sup>3</sup> /day) = 621 m3 .

Therefore the size of the digester for site A could be 621 m3 .

Where, VD = the size of the digester, HRT = hydraulic retention time, and SD is the amount of fermentation slurry (water + feedstock) feed in to the digester per day. Biogas yield in m<sup>3</sup> /kg of fresh biogas feedstock mix is 1736.4 m3 /31850 kg = 0.054 m<sup>3</sup> /kg; the biogas production rate is 10,867 kg/day 0.054 m3 /kg = 588 m<sup>3</sup> /day. Therefore the size of gasholder should account this daily biogas production.

### 4.1.2 Sizing of site-B biogas digester and gasholder

Daily average collectable biogas feedstock potential from cow dung, oxen dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct of Site-B in tons/day is 9.253 = 9253 kg/day = 13.22 m<sup>3</sup> /day. Since the average density of animal slurry mix is 700 kg/m<sup>3</sup> .

Additional 13.22 m<sup>3</sup> /day water is required for proper digestion process of biogas feedstock material to enhance biogas production.

HRT = 20 day, under thermophilic digestion temperature the hydraulic retention time of the digestion process becomes short.

The volume of digester should be, VD = HRT SD.

= 20 day (13.22 2 m<sup>3</sup> /day) = 529 m<sup>3</sup> . Therefore the size of the digester for site-B is 529 m<sup>3</sup> . The biogas gas production rate is 9253 kg/day 0.054 m3 /kg = 501 m<sup>3</sup> /day. Therefore the size of gasholder should account this daily biogas production.

### 4.1.3 Sizing of site-C biogas digester and gasholder

Daily average collectable biogas feedstock potential from cattle dung, donkey, mule, and horse waste, chicken waste, human feces and jatropha byproduct of site-C in tons/day is 8.82 = 8820 kg/day = 12.6 m<sup>3</sup> /day, Since the average density of animal slurry mix is 700 kg/m<sup>3</sup> .

guaranteed, to prevent damage to the engines. Further treatment and enhancing chemical and physical properties of biogas even possible to use it for other utiliza-

11. Hydro carbon HC <0.4 mg/m3 CH4 12. Silicon Si <10 mg/CH4

c

No. Parameter Symbol Value 1. Lower heat value LHV ≥4 kWh/m<sup>3</sup> 2. Sulfur content S ≤2.2 g/m<sup>3</sup> CH4 3. Hydrogen sulfide H2S ≤0.15 Vol. % 4. Chlorine content Cl ≤100 mg/m<sup>3</sup> CH4 5. Fluoride content F ≤50 mg/m<sup>3</sup> CH4 6. Dust (3–10 μm) — ≤10 mg/m<sup>3</sup> CH4 7. Relative humidity ϕ <90% 8. Flow pressure Pgas 20–100 mbar 9. Gas pressure fluctuation — <10% of set value 10. Gas temperature T 10–50<sup>o</sup>

To calculate the scale of a biogas plant, certain characteristic parameters are

• Daily fermentation slurry feeding (Sd), which is an equal mixture of biogas feedstock (animal dung, human feces, poultry waste and jatropha byproduct)

• Retention time (RT), the time by which the fermentation slurry stays in the

• Digester loading (R). This parameter indicates the amount of biogas feedstock material per day is fed to the digester or to be digested. It can be measured in

tions like as vehicle fuel or in fuel cells application.

Biogas minimum requirement used in an electric engine [3].

The design of the biogas plant includes the design of:

4. Design of the biogas plant

• Digester heat maintaining system

with water feed in to the biogas digester.

digester. It is about 2–5 weeks.

• The digester

Table 4.

Anaerobic Digestion

used. These are:

kg/m<sup>3</sup>

154

/day.

• The gas Holder

• Siting of biogas plant

Additional 12.6 m<sup>3</sup> /day water is required for proper digestion of biogas feedstock material to enhance biogas production.

area also depends on the chosen technology [3]. Based on the above criteria of site selection of biogas plant, the location of the biogas plant for each site of the study area is chosen and the detail of it is found in the economic analysis section of the

Various literatures show that methane yield of jatropha fruit hull is 0.438 m<sup>3</sup>

/kg). The biogas yield of Jatropha seed presscake is

0.3 3.15 6 Position Paper on Jatropha

Large Scale Project

Large Scale Project Development, FACT 2007

> Average biogas yield, m<sup>3</sup> /year

Average methane yield, m<sup>3</sup> /year

50,100 25,050-30,060

48,426 27,894–33,414

98,526 52,944–63,474

and use, W. Achten et al., 2008

/kg of presscake. The biogas yield of jatropha fruit hull is better

VS, and the VS is 76% of the TS of the jatropha fruit hull. Methane is 50% of the

than the seedcake [5]. Based on the jatropha fact sheet given in Table 5, the

Parameter Unit Minimum Average Maximum Source

Development, FACT 2007 Fruit hull yield dry ton/

0.2 2.1 4

70 75 80

Methane yield, m<sup>3</sup> /kg

0.69

0.689

Jatropha biomass (from presscake) = seed yield (ton/hectare) % of presscake yield during oil production \* total land

Jatropha biomass (from fruit hull) = hull yield (ton/hectare) total land for Jatropha farming (hectare).

MJ/kg — 37 —

Biogas yield, m<sup>3</sup> /kg

mm/year 600 1000 1500 Position Paper on Jatropha

% of mass \_ 34% 40% Jatropha bio-diesel production

20% 25% 30% Jatropha handbook, 2010

Total biogas yield, m<sup>3</sup>

96,000

4612– 92,240

8812– 188,240 /kg

biogas plant in this paper.

total biogas yield (1.153 m<sup>3</sup>

Seed yield dry ton/

hectare/year

hectare/year

% of mass of seed input

% of mass of seed input

> Average jatropha biomass, tons/year

Presscake 4.2–96 50.1 1 0.5–0.6 4200–

Fruit hull 4–80 42 1.153 0.576–

Total 8.2–176 92.1 1.07 0.575–

Jatropha byproduct biomass potential in the study area.

approximately 1 m<sup>3</sup>

Rainfall requirements for seed production

Oil content of seeds

Oil yield after pressing

Presscake yield after pressing

Energy content of Seed

Table 5. Jatropha fact sheet.

Table 6.

157

Biogas feedstock

Jatropha biomass, tons/year

for Jatropha farming (hectare)

5.1 Biogas potential from jatropha

DOI: http://dx.doi.org/10.5772/intechopen.79534

5. Biogas potential

Biogas for Clean Energy

The volume of digester should be, VD = HRT SD, HRT = 20 day.

= 20 day (12.6 2 m<sup>3</sup> /day) = 504 m<sup>3</sup> .

Therefore the size of the digester for site-C is 504 m<sup>3</sup> .

The gas production rate is 8820 kg/day 0.054 m<sup>3</sup> /kg = 477 m<sup>3</sup> /day. Therefore the size of gasholder should account this daily biogas production also.

4.1.4 Sizing of site-D biogas digester and gasholder

Daily average collectable biogas feedstock potential of Site-D in tons/day is 3.091 = 3091 kg/day = 4.42 m<sup>3</sup> /day, since the average density of animal slurry mix is taken as 700 kg/m<sup>3</sup> . Additional 4.42 m<sup>3</sup> /day water is required.

The volume of digester should be, VD = HRT SD, HRT = 20 day.

= 20 day (4.42 2 m<sup>3</sup> /day) = 179 m<sup>3</sup> .

Therefore the size of the digester for site-D is 179 m<sup>3</sup> .

The gas production rate is 3091 kg/day 0.054 m<sup>3</sup> /kg = 168 m<sup>3</sup> /day. Therefore the size of gasholder should account this daily biogas production.

### 4.2 Location of biogas plant

The next planning step in a biogas plant project idea is to find a suitable site for the establishment of the plant. The list below shows some important considerations to be made, before choosing the location of the plant: [3].


The required site space for a biogas plant cannot be estimated in a simple way. Experience shows that for example a biogas plant of 500 kWel needs an area of approximate 8000 m<sup>2</sup> . This figure can be used as a guiding value only, as the actual Biogas for Clean Energy DOI: http://dx.doi.org/10.5772/intechopen.79534

area also depends on the chosen technology [3]. Based on the above criteria of site selection of biogas plant, the location of the biogas plant for each site of the study area is chosen and the detail of it is found in the economic analysis section of the biogas plant in this paper.

### 5. Biogas potential

Additional 12.6 m<sup>3</sup>

Anaerobic Digestion

= 20 day (12.6 2 m<sup>3</sup>

3.091 = 3091 kg/day = 4.42 m<sup>3</sup>

= 20 day (4.42 2 m<sup>3</sup>

4.2 Location of biogas plant

taken as 700 kg/m<sup>3</sup>

material to enhance biogas production.

/day water is required for proper digestion of biogas feedstock

.

/day, since the average density of animal slurry mix is

.

/kg = 168 m<sup>3</sup>

/day water is required.

/kg = 477 m<sup>3</sup>

/day. Therefore

/day. Therefore

The volume of digester should be, VD = HRT SD, HRT = 20 day.

.

Daily average collectable biogas feedstock potential of Site-D in tons/day is

.

The next planning step in a biogas plant project idea is to find a suitable site for the establishment of the plant. The list below shows some important considerations

• The site should be located at suitable distance from residential areas in order to avoid inconveniences, nuisance and thereby conflicts related to odors and

• The direction of the dominating winds must be considered in order to avoid

• The site should have easy access to infrastructure such as to the electricity grid, in order to facilitate the sale of electricity and to the transport roads in order to

• The soil of the site should be investigated before starting the construction.

• The size of the site must be suitable for the activities performed and for the

• The site should be located relatively close (central) to the agricultural feedstock production (manure, slurry, energy crops) aiming to minimize distances, time

• For cost efficiency reasons, the biogas plant should be located as close as

The required site space for a biogas plant cannot be estimated in a simple way. Experience shows that for example a biogas plant of 500 kWel needs an area of

. This figure can be used as a guiding value only, as the actual

possible to potential users of the produced heat and electricity.

• The chosen site should not be located in a potential flood affected area.

/day) = 504 m<sup>3</sup>

the size of gasholder should account this daily biogas production also.

The volume of digester should be, VD = HRT SD, HRT = 20 day.

/day) = 179 m<sup>3</sup>

Therefore the size of the digester for site-C is 504 m<sup>3</sup>

The gas production rate is 8820 kg/day 0.054 m<sup>3</sup>

. Additional 4.42 m<sup>3</sup>

Therefore the size of the digester for site-D is 179 m<sup>3</sup>

the size of gasholder should account this daily biogas production.

The gas production rate is 3091 kg/day 0.054 m<sup>3</sup>

to be made, before choosing the location of the plant: [3].

increased traffic to and from the biogas plant.

wind born odors reaching residential areas.

facilitate transport of feedstock and digestate.

amount of biomass supplied.

approximate 8000 m<sup>2</sup>

156

and costs of feedstock transportation.

4.1.4 Sizing of site-D biogas digester and gasholder

### 5.1 Biogas potential from jatropha

Various literatures show that methane yield of jatropha fruit hull is 0.438 m<sup>3</sup> /kg VS, and the VS is 76% of the TS of the jatropha fruit hull. Methane is 50% of the total biogas yield (1.153 m<sup>3</sup> /kg). The biogas yield of Jatropha seed presscake is approximately 1 m<sup>3</sup> /kg of presscake. The biogas yield of jatropha fruit hull is better than the seedcake [5]. Based on the jatropha fact sheet given in Table 5, the


### Table 5.

Jatropha fact sheet.


Jatropha biomass (from presscake) = seed yield (ton/hectare) % of presscake yield during oil production \* total land for Jatropha farming (hectare)

Jatropha biomass (from fruit hull) = hull yield (ton/hectare) total land for Jatropha farming (hectare).

### Table 6.

Jatropha byproduct biomass potential in the study area.


Table 7.

Jatropha biogas potential of the study area.


### Table 8.

Summary of Jatropha potential of the study area.

biomass, biogas and methane yield potential of the jatropha byproduct is estimated in Tables 6, 7 and 8.

### 5.2 Biogas energy potential of the study area from animal dung

A wide range of biomass types can be used as substrates (feedstock) for the production of biogas from AD. The most common biomass categories used in biogas production are listed in Table 9 for this thesis work. To produce biogas from animal manure first we have to check whether we have animal livestock potential sufficient for biogas feedstock production or not. The following Table demonstrates the animal livestock potential for each sites of the study area.

The average fresh manure obtained from, cattle is 4.5 kg/day/head [1, 6, 7], donkey, horse and mule is 10 kg/day/head [6, 7], sheep and goat 1 kg/day/head [6, 7], and chicken is 0.08 kg/day/head [6, 7]. The average biogas yield of cattle, horse, mule, and donkey manure is 0.24 m<sup>3</sup> /kg DM [2, 3, 8] and pigs, sheep and goat is 0.37 m<sup>3</sup> /kg DM whereas chicken is 0.4 m<sup>3</sup> /kg of DM [2, 3, 8]. The dry matter


content from the total mass of fresh animal manure and the proportion of methane from the total biogas production is summarized in Table 10 [2, 3, 9] (Table 11). For a given size of plant (rated gas production capacity per day) the amount of

/kWh [1]. This specific

feedstock required can be estimated using the biogas yield data provided. The

fuel consumption value can be used to calculate the requirement for biogas for power generation purposes. The expected biomass potential from animal manure of the case study area is 36.2 tons/day and its biogas production capacity is 1850

consider collection efficiency of 90% for cattle, donkey, mule, horse, pig and chicken manure, 50% for goat and sheep manure and 100% for human feces based

/day. Various literatures show that the collection efficiency of animal manure

Most significantly the collection efficiency varies from 50 to 100% [10]. Let as

specific biogas consumption in biogas engines is 0.6–0.8 m<sup>3</sup>

varies from country to country and region to region.

Summary of expected animal manure potential of the study area.

m3

159

Table 11.

Biomass source

Biogas for Clean Energy

livestock in study area.

Total biogas production, m3

biogas production in m3

(tons/day).

Table 10.

Animal livestock Average fresh manure, kg/day/head

DOI: http://dx.doi.org/10.5772/intechopen.79534

/day = Biogas m3

Summary of fresh manure, biogas and methane yield of animal livestock.

Total no. of livestock in study area

By using biogas generator it is possible to generate 1kWh electricity from 0.7 m<sup>3</sup> biogas [42].

/day.

Ave. fresh manure, kg/day/head m<sup>3</sup> biogas/kg DM

Cattle 4.5 0.24 16.7 65 Pigs 2 0.37 4.4 65 Sheep, goats 1 0.37 30.7 65 Chickens 0.08 0.40 30.7 65 Horse, mule 10 0.24 7 65 Donkey 10 0.24 15 65 Total fresh manure potential of the study area (tons/day) = Average fresh manure (kg/day/head) Total no. of

Total dry mater (DM) from fresh manure = DM % of fresh manure Total fresh manure potential of the study area

Total electricity production in kWh/day = electricity production by biogas generator from 1 m3 biogas in kWh total

Total fresh manure (ton/day)

Cows 4.5 1935 8.708 1455 0.24 350 500 Oxen 4.5 2092 9.414 1573 0.24 378 540 Goats 1 476 0.476 147 0.37 55 79 Sheep 1 5350 5.350 1643 0.37 608 869 Mule 10 29 0.290 24 0.24 6 9 Chicken 0.08 6810 0.545 168 0.40 68 98 Pigs 2 0 0.000 0.00 0.37 0.0 0.0 Horse 10 133 1.330 92 0.24 22 32 Donkey 10 1007 10.070 1511 0.24 363 519 Total animal manure biomass 36.183 6613 0.28 1850 2646

Total DM (kg/ day)

DM % fresh manure

/kg of DM Total dry mater (DM) from fresh manure in kg/day.

Biogas, m<sup>3</sup> /kg of DM

Total biogas, m<sup>3</sup> /day

Electricity production, kWh/day

Methane % biogas

### Table 9.

Jama Woreda, Kebele-8 districts animal livestock potential.


Total fresh manure potential of the study area (tons/day) = Average fresh manure (kg/day/head) Total no. of livestock in study area.

Total dry mater (DM) from fresh manure = DM % of fresh manure Total fresh manure potential of the study area (tons/day).

Total biogas production, m3 /day = Biogas m3 /kg of DM Total dry mater (DM) from fresh manure in kg/day. Total electricity production in kWh/day = electricity production by biogas generator from 1 m3 biogas in kWh total biogas production in m3 /day.

By using biogas generator it is possible to generate 1kWh electricity from 0.7 m<sup>3</sup> biogas [42].

Table 10.

biomass, biogas and methane yield potential of the jatropha byproduct is estimated

16,090–18,774 98,526 18,420 92.1

A wide range of biomass types can be used as substrates (feedstock) for the production of biogas from AD. The most common biomass categories used in biogas production are listed in Table 9 for this thesis work. To produce biogas from animal manure first we have to check whether we have animal livestock potential sufficient for biogas feedstock production or not. The following Table demonstrates the ani-

The average fresh manure obtained from, cattle is 4.5 kg/day/head [1, 6, 7], donkey, horse and mule is 10 kg/day/head [6, 7], sheep and goat 1 kg/day/head [6, 7], and chicken is 0.08 kg/day/head [6, 7]. The average biogas yield of cattle,

HH

/kg DM [2, 3, 8] and pigs, sheep and

/kg of DM [2, 3, 8]. The dry matter

Total livestock in the study area

5.2 Biogas energy potential of the study area from animal dung

Biogas yield, m<sup>3</sup> /kg

Jatropha biogas (m<sup>3</sup> /year)

Biogas yield, m<sup>3</sup>

92.1 1.07 98,526 0.575–0.689 52,944–63,474

Jatropha fertilizer (kg/year)

0.253 1.07 270 0.575–0.689 145–174

Methane yield, m<sup>3</sup> /kg

Methane yield, m3

Jatropha biomass (ton/year)

mal livestock potential for each sites of the study area.

/kg DM whereas chicken is 0.4 m<sup>3</sup>

Source: Jama Woreda rural development and Kebele-8 administration office, Nov 2012.

Jama Woreda, Kebele-8 districts animal livestock potential.

Site-A Site-B Site-C Site-D Ave. no. of animal/

Cows 666 566 535 172 1.7 1935 Oxen 719 612 577 184 1.85 2092 Goats 163 139 131 43 0.42 476 Sheep 1841 1567 1477 472 4.72 5350 Mule 12 10 9 3 0.03 29 Chickens 2340 1992 1878 600 6 6810 Pigs 0 0 0 0 0 0 Horse 48 40 37 12 0.12 133 Donkey 345 295 278 89 0.89 1007

horse, mule, and donkey manure is 0.24 m<sup>3</sup>

in Tables 6, 7 and 8.

Profile Jatropha biomass,

Jatropha biogas potential of the study area.

Yearly average

Anaerobic Digestion

Daily average

Jatropha product

Product yield

Table 8.

Table 7.

tons

Jatropha oil (liter/year)

Summary of Jatropha potential of the study area.

goat is 0.37 m<sup>3</sup>

Animal livestock

Table 9.

158

Summary of fresh manure, biogas and methane yield of animal livestock.


### Table 11.

Summary of expected animal manure potential of the study area.

content from the total mass of fresh animal manure and the proportion of methane from the total biogas production is summarized in Table 10 [2, 3, 9] (Table 11).

For a given size of plant (rated gas production capacity per day) the amount of feedstock required can be estimated using the biogas yield data provided. The specific biogas consumption in biogas engines is 0.6–0.8 m<sup>3</sup> /kWh [1]. This specific fuel consumption value can be used to calculate the requirement for biogas for power generation purposes. The expected biomass potential from animal manure of the case study area is 36.2 tons/day and its biogas production capacity is 1850 m3 /day. Various literatures show that the collection efficiency of animal manure varies from country to country and region to region.

Most significantly the collection efficiency varies from 50 to 100% [10]. Let as consider collection efficiency of 90% for cattle, donkey, mule, horse, pig and chicken manure, 50% for goat and sheep manure and 100% for human feces based


5.4 Total biogas potential of the study area

Biogas potential of study area from human feces.

feces discussed above can be summarized in this section.

Total no. of population

31,850 kg/day and the gas produced is 1736.4 m<sup>3</sup>

tion ratio of biogas feedstock mix is 1736.4 m<sup>3</sup>

Total no. of live stock

The total biogas and collectable feedstock potential of the study area.

Total collectable fresh manure (ton/day)

Cows 4.5 1935 7.837 1309.5 0.24 315 450 Oxen 4.5 2092 8.473 1415.7 0.24 340 486 Goats 1 476 0.238 73.5 0.37 27.3 39 Sheep 1 5350 2.675 821.5 0.37 304 434.3 Mule 10 29 0.261 21.6 0.24 5.2 7.43 Chicken 0.08 6810 0.491 151.2 0.40 60.5 86.43 Pigs 2 0 0.000 0.00 0.37 0.0 0.0 Horse 10 133 1.197 82.8 0.24 19.9 28.43 Donkey 10 1007 9.063 1360 0.24 326.4 466.3 Human 0.12 5675 0.681 170.25 0.2 34.05 48.7 Jatropha byproduct biomass 0.253 253 1.07 270 386 Total 31.85 5829.3 0.3 1736.4 2481.4

kg = 0.0626 kg/kg.

Live stock

Biogas for Clean Energy

Table 14.

Ave. fresh manure, kg/ day/head

DOI: http://dx.doi.org/10.5772/intechopen.79534

Figure 3 given below.

Ave. fresh manure, kg/day/ head

Animal Livestock

Table 15.

161

The total biogas potential from Jatropha byproduct, Animal waste and human

As we have seen from Table 15, animal manure is the major biogas feedstock constitutes which accounts 97% from the total biogas feedstock potential whereas jatropha byproducts and human excreta constitute 1 and 2% of the total biogas feedstock potential of the study area respectively. However, the share of biogas production from, animal manure is 82%, and human excreta is 2% but biogas production from jatropha byproduct is increase to 16% regardless of its low contribution to the biomass potential since the biogas yield of jatropha byproduct is high as compared to both animal and human manure and this can be summarized in

> Total collectable DM (kg/day)

/day. Therefore the gasifica-

/

Total biogas production, m<sup>3</sup> /day

Electricity yield, kWh/day

/31850 kg = 0.0545 m<sup>3</sup>

Biogas, m<sup>3</sup> /kg DM

Taking the density of biogas 1.15 kg/m<sup>3</sup> and calculating the gasification ratio (the mass of biogas produced per unit mass of feed stock consumed) of the biogas system. From Table 15 the mass of biogas feedstock consumed is

Total fresh manure potential (ton/ day)

Human 0.12 5675 0.681 170.25 0.2 34.05 48.7

Total DM (kg/ day)

Biogas, m<sup>3</sup> /kg DM

Total biogas, m<sup>3</sup> /day

Electricity production, kWh/day

### Table 12.

Summary of collectable animal manure potential of the study area.

on their difficulty of collecting it. Therefore the biomass potential available for biogas generation is estimated as follows.

The total collectable fresh animal manure biomass potential of the study area is estimated to be 30.235 tons/day and its biogas production capacity is 1398.3 m<sup>3</sup> /day (Table 12).

### 5.3 Biogas potential of the study area from human feces

Human feces are another feedstock for biogas production in the study area and the potential biogas production from human feces is discussed in this section. Feces are mostly made of water (about 75%). The rest is made of dead bacteria that helped us digest our food, living bacteria, protein, undigested food residue (known as fiber), waste material from food, cellular linings, fats, salts, and substances released from the intestines (such as mucus) and the liver (Table 13).

One person produces on average 100–140 g of feces per day, the dry matter content of which is about 25% and its biogas yield of about 0.2 m<sup>3</sup> /kg DM [11]. The total collectable fresh manure biomass potential of the case study area from humans is estimated to be 0.681 tons/day and its biogas production capacity is 34.05 m<sup>3</sup> /day. This figure accounts the collection efficiency of human excreta. Table 14 demonstrates the biogas potential of the study area from human feces.


Table 13.

Jama Woreda, Kebele-8 districts population data.


Table 14.

on their difficulty of collecting it. Therefore the biomass potential available for

The total collectable fresh animal manure biomass potential of the study area is estimated to be 30.235 tons/day and its biogas production capacity is 1398.3 m<sup>3</sup>

Human feces are another feedstock for biogas production in the study area and the potential biogas production from human feces is discussed in this section. Feces are mostly made of water (about 75%). The rest is made of dead bacteria that helped us digest our food, living bacteria, protein, undigested food residue (known as fiber), waste material from food, cellular linings, fats, salts, and substances released

One person produces on average 100–140 g of feces per day, the dry matter

total collectable fresh manure biomass potential of the case study area from humans is estimated to be 0.681 tons/day and its biogas production capacity is 34.05 m<sup>3</sup>

This figure accounts the collection efficiency of human excreta. Table 14 demon-

Population Site-A Site-B Site-C Site-D Total Number of household 390 332 313 100 1135 Average Family per household 4.39 (5) 4.39 (5) 4.39 (5) 4.39 (5) 4.39 (5) Total population 1950 1660 1565 500 5675

/day

/kg DM [11]. The

/day.

biogas generation is estimated as follows.

Summary of collectable animal manure potential of the study area.

5.3 Biogas potential of the study area from human feces

from the intestines (such as mucus) and the liver (Table 13).

content of which is about 25% and its biogas yield of about 0.2 m<sup>3</sup>

strates the biogas potential of the study area from human feces.

(Table 12).

Table 13.

160

Jama Woreda, Kebele-8 districts population data.

Table 12.

Animal livestock

Anaerobic Digestion

Ave. fresh manure, kg/day/ head

Total no. of livestock in study area

Total collectable fresh manure, tons/day

Cows 4.5 1935 7.837 1309.5 0.24 315 450 Oxen 4.5 2092 8.473 1415.7 0.24 340 486 Goats 1 476 0.238 73.5 0.37 27.3 39 Sheep 1 5350 2.675 821.5 0.37 304 434.3 Mule 10 29 0.261 21.6 0.24 5.2 7.43 Chicken 0.08 6810 0.491 151.2 0.40 60.5 86.43 Pigs 2 0 0.000 0.00 0.37 0.0 0.0 Horse 10 133 1.197 82.8 0.24 19.9 28.43 Donkey 10 1007 9.063 1360 0.24 326.4 466.3 Total animal manure Biomass 30.235 5235.8 0.27 1398.3 1998

Total collectable DM, kg/day

Biogas, m<sup>3</sup> /kg of DM

Total biogas, m<sup>3</sup> /day

Electricity production, kWh/day

Biogas potential of study area from human feces.

### 5.4 Total biogas potential of the study area

The total biogas potential from Jatropha byproduct, Animal waste and human feces discussed above can be summarized in this section.

Taking the density of biogas 1.15 kg/m<sup>3</sup> and calculating the gasification ratio (the mass of biogas produced per unit mass of feed stock consumed) of the biogas system. From Table 15 the mass of biogas feedstock consumed is 31,850 kg/day and the gas produced is 1736.4 m3 /day. Therefore the gasification ratio of biogas feedstock mix is 1736.4 m<sup>3</sup> /31850 kg = 0.0545 m<sup>3</sup> / kg = 0.0626 kg/kg.

As we have seen from Table 15, animal manure is the major biogas feedstock constitutes which accounts 97% from the total biogas feedstock potential whereas jatropha byproducts and human excreta constitute 1 and 2% of the total biogas feedstock potential of the study area respectively. However, the share of biogas production from, animal manure is 82%, and human excreta is 2% but biogas production from jatropha byproduct is increase to 16% regardless of its low contribution to the biomass potential since the biogas yield of jatropha byproduct is high as compared to both animal and human manure and this can be summarized in Figure 3 given below.


### Table 15.

The total biogas and collectable feedstock potential of the study area.

Figure 3. Biogas feedstock contributions for biogas production in the study area.

### 5.5 Monthly variation of the biogas feed stock potential

The variation of jatropha byproduct feedstocks is assumed to be constant throughout the year and the potential biomass obtained from it was divided to each site regardless of the total house hold in each of the study area.

However, the biomass obtained from animal is highly depending on the availability and type of the animal feeding material. The animal feeding materials are varying in type and amount from month to month in the study area. In June and July there is enough root grass in addition to the usual animal food, let as consider this value as the annual average in ton/day (the data obtained by multiplying the biomass obtained per animal live stock in ton/day with the total number of animal live stock for each animal group in the district), as a reference frame. In January, February, and December there is excess dry agricultural farm grass for the animal food in the study area and assuming a 5% biomass resource increment is expected from the reference. March and April is a dry season and there is no enough food for the animal so considering a 5% biomass resource decrement from the reference. May, extremely drought month and August, animal grazing area are not permitted for animal food assuming a 10% animal based biomass resource drop is expected. From September to November there is excess animal food and a 10% biomass growth is assumed. Also assuming chicken manure and human feces are constant throughout the year. Taking in to account the assumption listed above the biogas feedstock potential month to month variation is presented in Tables 16–19.

### 6. Conclusion

The renewable energy potential of the site is estimated based on the primary data collected directly from the study area and secondary data obtained from various sources. The biogas feedstock mix potential of the study area is found to be 10.9 tons/day, 9.25 tons/day, 8.81 tons/day and 3.09 tons/day for Site-A, Site-B, Site-C and Site-D respectively with a gasification ratio of 0.0626 kg/kg. The study result shows that there is a sufficient biogas feedstock potential for all districts of the study area and the feasibility simulation result demonstrates there is an excess biogas after running a biogas generator in a hybrid system. The excess biogas left unused from a hybrid electric generating unit would go to biogas cooking application for the community cooking loads. Also, the biodiesel potential of the study area from Jatropha is estimated to be 18.5 m<sup>3</sup> /year.

Month

163

Cow

Jan Feb Mar Apr May

Jun

Jul Aug

Sep Oct Nov Dec Average

Table 16.

Biomass resource of site-A

—390 families.

 2.693

 2.958

 0.1096

 0.4335

 3.1628

 0.9327

 0.083

2.82

 3.069

 0.114

 0.4536

 3.28

2.954

 3.213

 0.119

 0.475

 3.4353

 1.013 0.967

 0.086

 0.09

0.17 0.17 0.17

0.0633

 0.234

 10.867

0.0633

 0.234

 11.257

0.0633

 0.234

 11.767

2.954

 3.213

 0.119

 0.475

 3.4353

 1.013

 0.09

0.17

0.0633

 0.234

 11.767

2.954

 3.213

 0.119

 0.475

 3.4353

 1.013

 0.09

0.17

0.0633

 0.234

 11.767

2.417

 2.63

 0.0972

 0.3654

 2.811

2.686

 2.921

 0.108

 0.432

 3.123

 0.921

0.83

 0.074

 0.082

2.686

 2.921

 0.108

 0.432

 3.123

 0.921

 0.082

2.417

 2.63

 0.0972

 0.3654

 2.811

0.83

 0.074

2.552

 2.775

 0.1031

 0.4104

 2.97

0.875

 0.08

0.17 0.17 0.17 0.17 0.17

0.0633

 0.234

 9.6912

0.0633

 0.234

 10.740

0.0633

 0.234

 10.740

0.0633

 0.234

 9.693

0.0633

 0.234

 10.233

DOI: http://dx.doi.org/10.5772/intechopen.79534

2.552

 2.775

 0.1031

 0.4104

 2.97

0.875

 0.08

0.17

0.0633

 0.234

 10.233

2.82

 3.069

 0.114

 0.4536

 3.28

0.967

 0.086

2.82

 3.069

 0.114

 0.4536

 3.28

 Oxen

 Mule

 Horse

 Donkey

 Sheep 0.967

 0.086

 Goats

 Chicken

0.17 0.17

0.0633

 0.234

 11.257

0.0633

 0.234

 11.257

Biogas for Clean Energy

 Jatroph

 Human

 Total

Biomass, tons/day



Biomass resource of site-A —390 families.

### Biogas for Clean Energy DOI: http://dx.doi.org/10.5772/intechopen.79534

5.5 Monthly variation of the biogas feed stock potential

Biogas feedstock contributions for biogas production in the study area.

site regardless of the total house hold in each of the study area.

6. Conclusion

162

Figure 3.

Anaerobic Digestion

from Jatropha is estimated to be 18.5 m<sup>3</sup>

The variation of jatropha byproduct feedstocks is assumed to be constant throughout the year and the potential biomass obtained from it was divided to each

However, the biomass obtained from animal is highly depending on the availability and type of the animal feeding material. The animal feeding materials are varying in type and amount from month to month in the study area. In June and July there is enough root grass in addition to the usual animal food, let as consider this value as the annual average in ton/day (the data obtained by multiplying the biomass obtained per animal live stock in ton/day with the total number of animal live stock for each animal group in the district), as a reference frame. In January, February, and December there is excess dry agricultural farm grass for the animal food in the study area and assuming a 5% biomass resource increment is expected from the reference. March and April is a dry season and there is no enough food for the animal so considering a 5% biomass resource decrement from the reference. May, extremely drought month and August, animal grazing area are not permitted for animal food assuming a 10% animal based biomass resource drop is expected. From September to November there is excess animal food and a 10% biomass growth is assumed. Also assuming chicken manure and human feces are constant throughout the year. Taking in to account the assumption listed above the biogas feedstock potential month to month variation is presented in Tables 16–19.

The renewable energy potential of the site is estimated based on the primary data collected directly from the study area and secondary data obtained from various sources. The biogas feedstock mix potential of the study area is found to be 10.9 tons/day, 9.25 tons/day, 8.81 tons/day and 3.09 tons/day for Site-A, Site-B, Site-C and Site-D respectively with a gasification ratio of 0.0626 kg/kg. The study result shows that there is a sufficient biogas feedstock potential for all districts of the study area and the feasibility simulation result demonstrates there is an excess biogas after running a biogas generator in a hybrid system. The excess biogas left unused from a hybrid electric generating unit would go to biogas cooking application for the community cooking loads. Also, the biodiesel potential of the study area

/year.



Biomass resource of site-B—332 families.

Month

165

Cow

Jan Feb Mar Apr May

Jun

Jul Aug

Sep Oct Nov Dec Average

Table 18.

Biomass resource of site-C—313

 families.

 2.183

 2.372

 0.082

 0.449

 2.542

 0.739

 0.067

 0.136

2.263

 2.463

 0.085

 0.755

 2.637

 0.669

 0.069

 0.136

2.37

 2.556

 0.089

 0.418

2.37

 2.556

 0.089

 0.41

2.76 2.76

0.813

 0.072

 0.136

0.813

 0.072

 0.136

2.37

 2.556

 0.089

 0.41

2.76

0.813

 0.072

 0.136

1.94

 2.123

 0.073

 0.30

2.156

 2.35

 0.081

 0.333

 2.511 2.26

0.665

 0.059

 0.136

 0.739

 0.066

 0.136

2.156

 2.35

 0.081

 0.333

 2.511

 0.739

 0.066

 0.136

1.94

 2.13

 0.073

 0.30

2.048

 2.23

 0.077

 0.316

 2.385 2.26

0.665

 0.059

 0.136

 0.702

 0.062

 0.136

2.048

 2.23

 0.077

 0.316

 2.385

 0.702

 0.062

 0.136

2.263

 2.46

 0.085

 0.755

 2.637

 0.776

 0.069

 0.136

2.263

 2.46

 0.085

 0.755

 2.637

 0.776

 0.069

 0.136

 Oxen

 Mule

 Horse

 Donkey

 Sheep

 Goats

 Chicken

 Jatropha

0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633

0.188

 8.820

0.188

 9.328

0.188

 9.457

0.188

 9.457

0.188

 9.457

0.188

 7.812

0.188

 8.618

0.188

 8.618

0.188

 7.812

0.188

 8.206

DOI: http://dx.doi.org/10.5772/intechopen.79534

0.188

 8.206

0.188

 9.431

0.188

 9.434

Biogas for Clean Energy

 Human

 Total

Biomass, tons/day


Biogas for Clean Energy DOI: http://dx.doi.org/10.5772/intechopen.79534

> Table 18.

Biomass resource of site-C—313 families.

Month

164

Cow

Jan Feb Mar Apr May

Jun

Jul Aug

Sep Oct Nov Dec Average

Table 17. Biomass resource of site-B

—332 families.

 2.313

 2.52

 0.09

 0.383

2.40

 2.614

 0.095

 0.378

 2.788

2.69

0.794

 0.071

 0.144

 0.823

 0.073

 0.144

2.513

 2.737

 0.099

 0.469

 2.921

 0.862

 0.077

 0.144

2.513

 2.737

 0.099

 0.469

 2.921

 0.862

 0.077

 0.144

2.513

 2.737

 0.099

 0.469

 2.921

 0.862

 0.077

 0.144

2.056

 2.240

 0.081

 0.324

2.38

0.706

 0.062

 0.144

2.284

 2.489

 0.09

 0.36

2.655

 0.784

 0.070

 0.144

2.284

 2.489

 0.09

 0.36

2.056

 2.240

 0.081

 0.324

 2.390 2.655

 0.784

 0.070

 0.144

 0.706

 0.062

 0.144

2.17

 2.364

 0.086

 0.342

 2.523

 0.744

 0.066

 0.144

2.17

 2.364

 0.086

 0.342

 2.523

 0.744

 0.066

 0.144

2.40

 2.614

 0.095

 0.378

 2.788

 0.823

 0.073

 0.144

2.40

 2.614

 0.095

 0.378

 2.788

 0.823

 0.073

 0.144

 Oxen

 Mule

 Horse

 Donkey

 Sheep

 Goats

 Chicken

 Jatropha

0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633 0.0633

0.183

 9.25

0.183

 9.56

0.183

 10.07

0.183

 10.07

0.183

 10.07

0.183

 8.24

0.183

 9.12

0.183

 9.12

0.183

 8.25

0.183

 8.69

0.183

 8.69

0.183

 9.56

0.183

 9.56

Anaerobic Digestion

 Human

 Total

Biomass, tons/day



Biomass resource of site-D—100 families. Author details

Biogas for Clean Energy

DOI: http://dx.doi.org/10.5772/intechopen.79534

167

Demsew Mitiku Teferra\* and Wondwosen Wubu

provided the original work is properly cited.

\*Address all correspondence to: demsewmitku@gmail.com

Addis Ababa Science and Technology University, Addis Ababa, Ethiopia

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

Biogas for Clean Energy DOI: http://dx.doi.org/10.5772/intechopen.79534

### Author details

Demsew Mitiku Teferra\* and Wondwosen Wubu Addis Ababa Science and Technology University, Addis Ababa, Ethiopia

\*Address all correspondence to: demsewmitku@gmail.com

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Month

166

Cow

Jan Feb Mar Apr May

Jun

Jul Aug

Sep Oct Nov Dec Average

Table 19. Biomass resource of site-D

—100 families.

 0.697

 0.759

 0.027

 0.1094

 0.811

 0.478

 0.043

 0.0432

 0.0633

0.723

 0.787

 0.028

 0.1134

 0.841

 0.496

 0.044

 0.0432

 0.0633

0.757

 0.825

 0.03

 0.1188

 0.881

 0.519

 0.046

 0.0432

 0.0633

0.757

 0.825

 0.03

 0.1188

 0.881

 0.519

 0.046

 0.0432

 0.0633

0.757

 0.825

 0.03

 0.1188

 0.881

 0.519

 0.046

 0.0432

 0.0633

0.620

 0.675

 0.024

 0.0972

 0.721

 0.425

 0.038

 0.0432

 0.0633

0.689

 0.750

 0.027

 0.108

 0.801

 0.472

 0.043

 0.0432

 0.0633

0.689

 0.750

 0.027

 0.108

 0.801

 0.472

 0.043

 0.0432

 0.0633

0.620

 0.674

 0.024

 0.0972

 0.721

 0.425

 0.038

 0.0432

 0.0633

0.654

 0.712

 0.026

 0.1026

 0.761

 0.448

 0.04

 0.0432

 0.0633

0.654

 0.712

 0.026

 0.1026

 0.761

 0.448

 0.04

 0.0432

 0.0633

0.723

 0.787

 0.029

 0.1134

 0.841

 0.496

 0.044

 0.0432

 0.0633

0.723

 0.787

 0.029

 0.1134

 0.841

 0.496

 0.044

 0.0432

 0.0633

 Oxen

 Mule

 Horse

 Donkey

 Sheep

 Goats

 Chicken

 Jatropha

 Human

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

0.06

 3.091

 3.199

 3.344

 3.344

 3.344

 2.766

 3.056

 3.056

 2.766

 2.910

 2.910

 3.199

 3.199

Anaerobic Digestion

 Total

Biomass, tons/day

### References

[1] Hadagu A. Status and Trends of Ethiopian Rural Electrification Fund; 2006. http://www.bgr.de/geotherm/ ArGeoC1/pdf/07%20A.%20Hadgu% 20status%20of%20Ethiopian% 20Electrification%20fund.pdf

[2] Maithel S. Biomass Energy Resource Assessment Handbook—prepared for Asian and Pacific Centre for Transfer of Technology of the United Nations – Economic and Social Commission for Asia and the Pacific (ESCAP); September 2009. https://en.calameo. com/read/001424067e8c637721c50

[3] Jørgensen PJ. Plan Energi and researcher for a day. In: Biogas – green energy. 2nd ed. Digisource Denmark: Faculty of Agricultural Sciences, Aarhus University; 2009. http:// lemvigbiogas.com

[4] Al Seadi T, Rutz D, Prassl H, Köttner M, Finsterwalder T, Volk S, Janssen R. Biogas Handbook. Niels Bohrs Vej 9-10, DK-6700 Esbjerg, Denmark: University of Southern Denmark Esbjerg; 2008

[5] www.fact-foundation.com

[6] Janske van E, André F. Global experience with jatropha cultivation for bioenergy: An assessment of socioeconomic and environmental aspects. Heidelberglaan, The Netherlands: Copernicus Institute, Utrecht University, The Netherlands Eindhoven University of Technology

[7] http://home.t-online.de/home/ 320033440512-0002/downloads/jclmanual.pdf

[8] Jongschaap REE et al. Claims and Facts on Jatropha curcas L., Global Jatropha curcas Evaluation, Breeding and Propagation Programme. Wageningen UR: Plant Research International; 2007

[9] Demirbas A. Conversion of biomass using glycerine to liquid fuel for blending gasoline as alternative engine fuel. Energy Conversion and Management. 2000;41:1741-1748

Chapter 8

Abstract

Combustion

Mario Toledo Torres

filtration combustion.

non-catalytic

169

1. Introduction

reducing GHG emissions to the atmosphere.

cost-effective conversion of biogas into syngas [1–4].

Non-Catalytic Reforming of

Rich combustion of biogas inside an inert porous media reactor was investigated

product gas composition of the combustion waves were analysed, while varying its filtration velocity, for a range of equivalence ratios (φ) from φ = 1.0 to φ = 3.5. A numerical model based on comprehensive heat transfer and chemical mechanisms was found to be in a good qualitative agreement with experimental data. Partial oxidation products of biogas (H2 and CO) were dominant on rich combustion. Different gas mixtures of methane and carbon dioxide, which simulated synthetic biogas, and the addition of a varying fraction of water steam were experimentally analysed. It was observed that an increasing steam to carbon ratio (S/C) improved hydrogen and syngas production. The non-catalytic process investigated results in an effective biogas upgrading, and to be essentially higher than under natural gas

to evaluate hydrogen and syngas production. Temperature, velocities, and

Keywords: biogas reforming, porous media burner, hydrogen, syngas,

Large-scale production of hydrogen (H2) is mainly obtained through the thermochemical conversion of methane (CH4) into H2 and carbon monoxide (CO), a mixture also known as syngas. The main conversion processes are dry reforming, partial oxidation, steam reforming, and autothermal reforming, all of which typically use fossil fuels as main carbonaceous feedstock, the natural gas being the most widely used. However, current efforts have been focused on the development of sustainable, carbon-neutral alternatives for H2 production. Hence, biogas upgrading by partial oxidation of CH4 in the presence of oxygen (O2), steam (H2O), or carbon dioxide (CO2) is considered an interesting alternative to produce syngas while

Accordingly, research has been focused on improving the syngas production from biogas process efficiency, being the use of fluidized bed reactors a promising real alternative to effectively increase efficiency. However, they have presented a major drawback related to catalytic wearing mainly associated to the elevated temperatures (over 1000 K and up to 1300 K) required for the efficient and

Biogas in Porous Media

[10] Nicholson FA, Chambers BJ, Williamsb JR, Unwin RJ. Heavy Metal Contents of Livestock Feeds and Animal Manures in England and Wales. Meden Vale, Mans®eld, Nottinghamshire, NG20 9PF, UK: ADAS Gleadthorpe Research Centre; 1999

[11] Milbrandt A. Assessment of Biomass Resources in Liberia Prepared for the U.S. Agency for International Development (USAID) under the Liberia Energy Assistance Program (LEAP). Technical Report, NREL/TP-6A2-44808; April 2009. http://www.osti.gov/bridge

### Chapter 8

References

Anaerobic Digestion

[1] Hadagu A. Status and Trends of Ethiopian Rural Electrification Fund; 2006. http://www.bgr.de/geotherm/ ArGeoC1/pdf/07%20A.%20Hadgu% 20status%20of%20Ethiopian% 20Electrification%20fund.pdf

[9] Demirbas A. Conversion of biomass using glycerine to liquid fuel for blending gasoline as alternative engine

fuel. Energy Conversion and Management. 2000;41:1741-1748

Research Centre; 1999

U.S. Agency for International Development (USAID) under the Liberia Energy Assistance Program

(LEAP). Technical Report,

NREL/TP-6A2-44808; April 2009. http://www.osti.gov/bridge

[10] Nicholson FA, Chambers BJ, Williamsb JR, Unwin RJ. Heavy Metal Contents of Livestock Feeds and Animal Manures in England and Wales. Meden Vale, Mans®eld, Nottinghamshire, NG20 9PF, UK: ADAS Gleadthorpe

[11] Milbrandt A. Assessment of Biomass Resources in Liberia Prepared for the

[2] Maithel S. Biomass Energy Resource Assessment Handbook—prepared for Asian and Pacific Centre for Transfer of Technology of the United Nations – Economic and Social Commission for Asia and the Pacific (ESCAP); September 2009. https://en.calameo. com/read/001424067e8c637721c50

[3] Jørgensen PJ. Plan Energi and researcher for a day. In: Biogas – green energy. 2nd ed. Digisource Denmark: Faculty of Agricultural Sciences, Aarhus University; 2009. http://

[4] Al Seadi T, Rutz D, Prassl H, Köttner M, Finsterwalder T, Volk S, Janssen R. Biogas Handbook. Niels Bohrs Vej 9-10, DK-6700 Esbjerg, Denmark: University of Southern Denmark

lemvigbiogas.com

Esbjerg; 2008

[5] www.fact-foundation.com

University of Technology

manual.pdf

International; 2007

168

[6] Janske van E, André F. Global experience with jatropha cultivation for bioenergy: An assessment of socioeconomic and environmental aspects. Heidelberglaan, The Netherlands: Copernicus Institute, Utrecht

University, The Netherlands Eindhoven

[7] http://home.t-online.de/home/ 320033440512-0002/downloads/jcl-

[8] Jongschaap REE et al. Claims and Facts on Jatropha curcas L., Global Jatropha curcas Evaluation, Breeding and Propagation Programme. Wageningen UR: Plant Research

## Non-Catalytic Reforming of Biogas in Porous Media Combustion

Mario Toledo Torres

### Abstract

Rich combustion of biogas inside an inert porous media reactor was investigated to evaluate hydrogen and syngas production. Temperature, velocities, and product gas composition of the combustion waves were analysed, while varying its filtration velocity, for a range of equivalence ratios (φ) from φ = 1.0 to φ = 3.5. A numerical model based on comprehensive heat transfer and chemical mechanisms was found to be in a good qualitative agreement with experimental data. Partial oxidation products of biogas (H2 and CO) were dominant on rich combustion. Different gas mixtures of methane and carbon dioxide, which simulated synthetic biogas, and the addition of a varying fraction of water steam were experimentally analysed. It was observed that an increasing steam to carbon ratio (S/C) improved hydrogen and syngas production. The non-catalytic process investigated results in an effective biogas upgrading, and to be essentially higher than under natural gas filtration combustion.

Keywords: biogas reforming, porous media burner, hydrogen, syngas, non-catalytic

### 1. Introduction

Large-scale production of hydrogen (H2) is mainly obtained through the thermochemical conversion of methane (CH4) into H2 and carbon monoxide (CO), a mixture also known as syngas. The main conversion processes are dry reforming, partial oxidation, steam reforming, and autothermal reforming, all of which typically use fossil fuels as main carbonaceous feedstock, the natural gas being the most widely used. However, current efforts have been focused on the development of sustainable, carbon-neutral alternatives for H2 production. Hence, biogas upgrading by partial oxidation of CH4 in the presence of oxygen (O2), steam (H2O), or carbon dioxide (CO2) is considered an interesting alternative to produce syngas while reducing GHG emissions to the atmosphere.

Accordingly, research has been focused on improving the syngas production from biogas process efficiency, being the use of fluidized bed reactors a promising real alternative to effectively increase efficiency. However, they have presented a major drawback related to catalytic wearing mainly associated to the elevated temperatures (over 1000 K and up to 1300 K) required for the efficient and cost-effective conversion of biogas into syngas [1–4].

Moreover, the widely reported effects of sintering and coking on the catalyst both responsible for the generation of depositions that cause catalytic deactivation [5–6], observed to occur on high-temperature regimes (over 1100 K), have prevented the commercial development of the technology. Nevertheless, further studies related to upgrading the thermal properties of catalysts used in biogas reforming, and their resistance to the aforementioned phenomena, have been performed, particularly as a countermeasure to the effects of GHG [7–9].

2. Numerical model

Continuity equation.

Gas phase energy equation.

Solid phase energy equation.

Species conservation equation.

Inlet: Outlet:

171

thermal equilibrium between gas and solid phases.

Experimental temperature measurements showed a minimal radial gradient;

representing the reaction wave inside the porous media. A volume-averaged model [29] was used to solve the two-temperature mathematical model proposed to describe the filtration combustion under isobaric, stationary, and one-dimensional conditions. The combustion wave propagation rate was considered to be at least three orders of magnitude smaller than the filtration velocity of the gaseous mixture. The two-temperature approximation was formulated in order to describe a fully developed stationary reaction wave in a coordinate system moving with the reaction zone [19, 25, 30]. Therefore, this model describes both solid and gaseous

Boundary conditions chosen for the inlet and outlet of the reactor consider

An analytical solution of Eqs. (1)–(4) was used to impose the boundary conditions of the reaction wave temperature at the outlet. This is achieved by assuming

hv <sup>¼</sup> <sup>6</sup>ε=d<sup>2</sup> Nukg and that radiation can be neglected at the outlet since it is far from the reaction zone. Thus, calculation for temperature and species in the reaction wave can be computed for a finite and a well-defined spatial domain. The

the aforementioned conditions for the outlet and adding that ω\_ <sup>k</sup> ¼ 0

convective heat transfer coefficient (hv) was taken from [19] as hv <sup>¼</sup> <sup>6</sup>ε=d<sup>2</sup> Nu kg, whereas the correlation for Nu was considered as ð1Þ

ð2Þ

ð3Þ

ð4Þ

thus a one-dimensional numerical simulation was considered adequate for

Non-Catalytic Reforming of Biogas in Porous Media Combustion

DOI: http://dx.doi.org/10.5772/intechopen.86620

phases through their fluid dynamics and heat transfer interactions.

However, since the catalytic approach has proven to be thermally challenging, an alternative non-catalytic method, such as partial oxidation (POX) in inert porous media (IPM), has proven to be an interesting option for high-temperature biogas conversion. The advantage of using a porous matrix to enhance several reaction processes, such as combustion, partial oxidation, steam reforming, and dry reforming, among others, has been extensively studied by numerous researchers, where the use of a chemically inert porous media enables the propagation of an autothermal reacting wave which benefits from the increased heat transfer on the reaction zone due to the solid matrix. Specifically, this enhanced thermal mechanism, mainly attributed to the heat conduction and radiation, and the highly developed inner surface of the porous media being heated by the reaction wave, acts as a heat recirculation mechanism which distributes the thermal energy up- and downstream of the reacting zone, thus preheating the fresh mixture and homogenising the reaction temperature across the reacting wave. Furthermore, the existence of multiple flow paths for the filtered gas increases its diffusion and heat transfer with the solid phase. Finally, filtration combustion has shown to increase the operational ranges of free-flame combustion on a wide range of filtration velocities, equivalence ratios, and power loads.

When combusting gaseous fuels in porous media, steady and transient systems are the two approaches commonly employed [10–16]. The first approach is widely used in radiant burners and surface combustor-heaters, where the combustion wave maintains its position due to an equilibrium of the heat transfer mechanisms. While the transient operation considers a reaction wave travelling on an upstream or downstream fashion through the porous media, the direction of wave propagation depends mostly on the physical properties of solid and gas, filtration velocity, temperature, and air excess of the mixture. Combining these parameters, it can be noted that the movement velocities of these waves are much lower than free-flame combustion velocities [13].

A transient operation mode is characterised by concentrating or diluting the heat released from the chemical reaction while travelling in a downstream or upstream direction, respectively; thus the reaction front can reach a temperature considerably different from the free-flame adiabatic flame temperature. This phenomenon is mainly attributed to the reaction chemistry and the heat transfer mechanisms. Under a counterflow configuration, the downstream propagation results in superadiabatic reaction waves which effectively increase the conventional free-flame flammability limits for both ultra-lean and ultrarich operation modes.

Superadiabatic filtration combustion of rich and ultrarich mixtures allows the stable operation of both partial oxidation and thermal cracking of hydrocarbons. This technology for hydrogen or syngas production uses an IPM [17–20]. The fuels used in porous combustion systems are basically of gaseous form due to fluidity, volumetric capacity, and shorter mixing length scale [21–28]. Accordingly, this chapter presents the numerical and experimental results obtained for different biogas compositions (CH4 and CO2) on syngas production by filtration combustion in an IPM reactor.

### 2. Numerical model

Moreover, the widely reported effects of sintering and coking on the catalyst both responsible for the generation of depositions that cause catalytic deactivation [5–6], observed to occur on high-temperature regimes (over 1100 K), have prevented the commercial development of the technology. Nevertheless, further studies related to upgrading the thermal properties of catalysts used in biogas reforming, and their resistance to the aforementioned phenomena, have been performed, particularly as a countermeasure to the effects of GHG [7–9].

However, since the catalytic approach has proven to be thermally challenging, an alternative non-catalytic method, such as partial oxidation (POX) in inert porous media (IPM), has proven to be an interesting option for high-temperature biogas conversion. The advantage of using a porous matrix to enhance several reaction processes, such as combustion, partial oxidation, steam reforming, and dry reforming, among others, has been extensively studied by numerous researchers, where the use of a chemically inert porous media enables the propagation of an autothermal reacting wave which benefits from the increased heat transfer on the reaction zone due to the solid matrix. Specifically, this enhanced thermal mechanism, mainly attributed to the heat conduction and radiation, and the highly developed inner surface of the porous media being heated by the reaction wave, acts as a heat recirculation mechanism which distributes the thermal energy up- and downstream of the reacting zone, thus preheating the fresh mixture and homogenising the reaction temperature across the reacting wave. Furthermore, the existence of multiple flow paths for the filtered gas increases its diffusion and heat transfer with the solid phase. Finally, filtration combustion has shown to increase the operational ranges of free-flame combustion on a wide range of filtration velocities, equivalence

When combusting gaseous fuels in porous media, steady and transient systems are the two approaches commonly employed [10–16]. The first approach is widely used in radiant burners and surface combustor-heaters, where the combustion wave maintains its position due to an equilibrium of the heat transfer mechanisms. While the transient operation considers a reaction wave travelling on an upstream or downstream fashion through the porous media, the direction of wave propagation depends mostly on the physical properties of solid and gas, filtration velocity, temperature, and air excess of the mixture. Combining these parameters, it can be noted that the movement velocities of these waves are much lower than free-flame

A transient operation mode is characterised by concentrating or diluting the heat released from the chemical reaction while travelling in a downstream or upstream direction, respectively; thus the reaction front can reach a temperature considerably different from the free-flame adiabatic flame temperature. This phenomenon is mainly attributed to the reaction chemistry and the heat transfer mechanisms. Under a counterflow configuration, the downstream propagation

Superadiabatic filtration combustion of rich and ultrarich mixtures allows the stable operation of both partial oxidation and thermal cracking of hydrocarbons. This technology for hydrogen or syngas production uses an IPM [17–20]. The fuels used in porous combustion systems are basically of gaseous form due to fluidity, volumetric capacity, and shorter mixing length scale [21–28]. Accordingly, this chapter presents the numerical and experimental results obtained for different biogas compositions (CH4 and CO2) on syngas production by filtration combustion

results in superadiabatic reaction waves which effectively increase the conventional free-flame flammability limits for both ultra-lean and ultrarich

ratios, and power loads.

Anaerobic Digestion

combustion velocities [13].

operation modes.

in an IPM reactor.

170

Experimental temperature measurements showed a minimal radial gradient; thus a one-dimensional numerical simulation was considered adequate for representing the reaction wave inside the porous media. A volume-averaged model [29] was used to solve the two-temperature mathematical model proposed to describe the filtration combustion under isobaric, stationary, and one-dimensional conditions. The combustion wave propagation rate was considered to be at least three orders of magnitude smaller than the filtration velocity of the gaseous mixture. The two-temperature approximation was formulated in order to describe a fully developed stationary reaction wave in a coordinate system moving with the reaction zone [19, 25, 30]. Therefore, this model describes both solid and gaseous phases through their fluid dynamics and heat transfer interactions.

Continuity equation.

$$\frac{\partial \langle \dot{\mathsf{M}} \rangle}{\partial \mathbf{x}} = \frac{\partial \langle \mathsf{e} \boldsymbol{\rho} \mathbf{v} \rangle}{\partial \mathbf{x}} = \mathbf{0} \tag{1}$$

Gas phase energy equation.

$$\mathbf{u}\_{\star},\boldsymbol{\uprho}\_{\mathbf{f}},\nu\frac{\partial\mathbf{T}\_{\mathbf{f}}}{\partial\mathbf{x}}=\frac{\partial}{\partial\mathbf{x}}\left(\left[\mathbf{k}\_{\text{f}}+\left\{\mathbf{c},\boldsymbol{\uprho}\_{\text{f}}\right\}\mathbf{D}\_{\text{pr}}\right]\frac{\partial\mathbf{T}\_{\mathbf{f}}}{\partial\mathbf{x}}\right)-\sum\_{\mathbf{k}}\left(\left.\dot{\boldsymbol{\uprho}}\_{\text{c}}\mathbf{h}\_{\text{l}}\mathbf{W}\_{\text{k}}-\boldsymbol{\uprho}\_{\text{c}}\mathbf{Y}\_{\text{k}}\mathbf{V}\_{\text{k}}\mathbf{v}\_{\text{pr}}\right.\left.\frac{\partial\mathbf{T}\_{\mathbf{f}}}{\partial\mathbf{x}}\right)-\frac{\mathbf{h}\_{\text{v}}}{\mathbf{c}}\left(\mathbf{T}\_{\mathbf{f}}-\mathbf{T}\_{\mathbf{c}}\right)\tag{2}$$

Solid phase energy equation.

$$-(\mathbf{1} - \boldsymbol{\varepsilon})\mathbf{c}\_{\mathbf{s}}\rho\_{\mathbf{s}}\mathbf{u}\frac{\partial\mathbf{T}\_{\mathbf{s}}}{\partial\mathbf{x}} = \frac{\partial}{\partial\mathbf{x}}\Big( [\mathbf{k}\_{\mathbf{e}} + \mathbf{k}\_{\mathbf{R}}] \frac{\partial\mathbf{T}\_{\mathbf{g}}}{\partial\mathbf{x}} \Big) - \beta(\mathbf{T}\_{\mathbf{s}} - \mathbf{T}\_{\mathbf{0}}) - \mathbf{h}\_{\mathbf{v}}(\mathbf{T}\_{\mathbf{g}} - \mathbf{T}\_{\mathbf{s}}) \tag{3}$$

Species conservation equation.

$$
\rho\_\text{g} \mathbf{v} \frac{\text{d} \mathbf{Y}\_\text{k}}{\text{dx}} + \frac{\text{d}}{\text{dx}} \{\rho\_\text{g} \mathbf{y}\_\text{k} \mathbf{V}\_\text{k}\} = \dot{\alpha}\_\text{k} \mathbf{W}\_\text{k} \tag{4}
$$

Boundary conditions chosen for the inlet and outlet of the reactor consider thermal equilibrium between gas and solid phases.


An analytical solution of Eqs. (1)–(4) was used to impose the boundary conditions of the reaction wave temperature at the outlet. This is achieved by assuming the aforementioned conditions for the outlet and adding that ω\_ <sup>k</sup> ¼ 0 hv <sup>¼</sup> <sup>6</sup>ε=d<sup>2</sup> Nukg and that radiation can be neglected at the outlet since it is far from the reaction zone. Thus, calculation for temperature and species in the reaction wave can be computed for a finite and a well-defined spatial domain. The convective heat transfer coefficient (hv) was taken from [19] as hv <sup>¼</sup> <sup>6</sup>ε=d<sup>2</sup> Nu kg, whereas the correlation for Nu was considered as

Nu <sup>¼</sup> <sup>2</sup> <sup>þ</sup> <sup>1</sup>:1Re0:6Pr1<sup>=</sup><sup>3</sup> <sup>g</sup> as presented by Wakao and Kaguei [31]. A radiant conductivity model taken from [29] was used to include the effect of radiation, where kR <sup>¼</sup> <sup>4</sup>FσT<sup>3</sup> <sup>s</sup> with F being the radiation exchange factor. This has to be modelled for each material since it is dependent upon its thermal conductivity and emissivity. For the solid phase, composed of 5.6 mm in diameter alumina spheres, values from 0.3 to 0.6 are used. Effective thermal conductivity of the packed bed and its porosity were estimated as ke ¼ 0:005 ks and ε ¼ 0:4. The tortuosity of the porous media and its porosity contributes to flow irregularities which affect the effective mass diffusion of species in the gas phase; this phenomenon is described by an axial gas dispersion coefficient, Dax ¼ 0:5dv [31]. Dispersion coefficients for both thermal and mass diffusivities are considered to be equal according to a heat/mass transfer analogy. As presented by Henneke and Ellzey [30], a sum of molecular diffusion and dispersion is used to represent the effective diffusion.

Finally, all thermophysical properties from the solid phases, such as thermal conductivity, heat capacity, and radiative properties, were obtained from openly available technical reports [32] and verified against the technical specifications from the ceramic manufacturer (Coors, Inc.).

Chemical kinetics of the process was modelled through the implementation of the GRI 3.0 [33] chemical kinetics mechanism, which includes NOX chemistry, alongside the CHEMKIN [34] package. Even though this mechanism is not designed specifically to simulate reactions of ultra-lean nor ultrarich mixtures, it is considered as an acceptable first approach to understand the possible reaction mechanisms occurring under these conditions.

The calculations were performed for a given value of the filtration (interstitial) velocity, and the implemented numerical algorithm in the modified PREMIX [35] code was used to find the wave propagation velocity.

The methane, carbon dioxide, air and steam mixture flowed continuously through the quartz tube, and the reactant concentrations were controlled using Aalborg mass flow controllers. A temperature experimental measurement error was estimated at 50 K, which is considered mainly as a radial error. The reaction wave velocity measurement error was estimated at 10%, and the chemical sampling accuracy was considered close to 10%. The experimental uncertainties were defined based on the accuracy of the laboratory equipment and the repeatability of the

One objective of the numerical simulation was to clarify the effect of interfacial

For natural air mixtures (Figure 2A), upstream wave propagation was observed for the range of equivalence from stoichiometric to 1.7. The velocity of the wave decreases with an increase of the natural gas concentration, approaching zero at 1.7. A stationary combustion wave is formed under these conditions. With further increase of the natural gas content, the regime of propagation changes towards a downstream direction. This regime is observed for the range of equivalence ratios from 1.7 to 3.5, where the speed increases with the natural gas content. It was found experimentally that the maximum combustion temperature remains almost constant (Ts = 1529 K) throughout the rich region and is practically independent of the natural gas content. These combustion temperatures are attributed to the changes of

heat transfer. Calculations performed with the same initial conditions in twotemperature approximations are presented in Figure 2 for natural gas-air and biogas-air mixtures. Thermal non-equilibrium between gas and solid phases needs to be applied for consideration of propagation waves. Downstream, upstream, and standing waves were observed for natural gas-air and biogas-air mixtures, mainly depending on the equivalence ratio. Stable combustion of rich and ultrarich mixtures was observed experimentally for the region of equivalence ratios studied.

3.1 Combustion wave temperature and propagation rate

Non-Catalytic Reforming of Biogas in Porous Media Combustion

DOI: http://dx.doi.org/10.5772/intechopen.86620

experimental data.

Schematic of the experimental setup.

Figure 1.

173

### 3. Syngas production by filtration combustion

In this section, numerical and experimental results are showed for wave velocities, combustion temperature, and H2 and CO concentrations for rich combustion of biogas in porous media. Additionally, results on rich combustion of natural gas in porous media are presented.

The combustion system is shown in Figure 1 and consists on a cylindrical quartz tube. This tube is filled with a bed of alumina spheres (Al2O3) with an average diameter of 5.6 mm, yielding a porosity of �40%. The inner and outer surfaces of the tube were covered with a 2 and 25 mm thick Kaowool insulation material, respectively. System diagnostics were required to assess the temperature profile in the reactor along with emission concentrations in the product gases. The axial temperature distribution of the reactor was acquired using S-type thermocouples. These thermocouples were housed in a long multibore ceramic shell; therefore measured temperatures were considered to be very close to the temperatures of the solid phase. Temperature measurements were digitised using a data acquisition module and transferred to a PC. The reaction wave propagation rates were obtained from thermocouple measurements over time and the known distance of the thermocouples. The concentrations of H2, CO, CH4, and CO2 in the product gases were measured using a gas chromatograph fitted with a thermal conductivity detector (TCD), while gas samples were acquired through an alumina tube immersed at the end of the reactor. To avoid the effect of external air vortices on the composition, the probe was inserted 20 mm into the packed bed.

Non-Catalytic Reforming of Biogas in Porous Media Combustion DOI: http://dx.doi.org/10.5772/intechopen.86620

### Figure 1.

Nu <sup>¼</sup> <sup>2</sup> <sup>þ</sup> <sup>1</sup>:1Re0:6Pr1<sup>=</sup><sup>3</sup>

the effective diffusion.

from the ceramic manufacturer (Coors, Inc.).

code was used to find the wave propagation velocity.

3. Syngas production by filtration combustion

the probe was inserted 20 mm into the packed bed.

occurring under these conditions.

porous media are presented.

172

where kR <sup>¼</sup> <sup>4</sup>FσT<sup>3</sup>

Anaerobic Digestion

<sup>g</sup> as presented by Wakao and Kaguei [31]. A radiant

<sup>s</sup> with F being the radiation exchange factor. This has to be

conductivity model taken from [29] was used to include the effect of radiation,

modelled for each material since it is dependent upon its thermal conductivity and emissivity. For the solid phase, composed of 5.6 mm in diameter alumina spheres, values from 0.3 to 0.6 are used. Effective thermal conductivity of the packed bed and its porosity were estimated as ke ¼ 0:005 ks and ε ¼ 0:4. The tortuosity of the porous media and its porosity contributes to flow irregularities which affect the effective mass diffusion of species in the gas phase; this phenomenon is described by an axial gas dispersion coefficient, Dax ¼ 0:5dv [31]. Dispersion coefficients for both thermal and mass diffusivities are considered to be equal according to a heat/mass transfer analogy. As presented by Henneke and Ellzey [30], a sum of molecular diffusion and dispersion is used to represent

Finally, all thermophysical properties from the solid phases, such as thermal conductivity, heat capacity, and radiative properties, were obtained from openly available technical reports [32] and verified against the technical specifications

Chemical kinetics of the process was modelled through the implementation of the GRI 3.0 [33] chemical kinetics mechanism, which includes NOX chemistry, alongside the CHEMKIN [34] package. Even though this mechanism is not designed specifically to simulate reactions of ultra-lean nor ultrarich mixtures, it is considered as an acceptable first approach to understand the possible reaction mechanisms

The calculations were performed for a given value of the filtration (interstitial) velocity, and the implemented numerical algorithm in the modified PREMIX [35]

In this section, numerical and experimental results are showed for wave velocities, combustion temperature, and H2 and CO concentrations for rich combustion of biogas in porous media. Additionally, results on rich combustion of natural gas in

The combustion system is shown in Figure 1 and consists on a cylindrical quartz

tube. This tube is filled with a bed of alumina spheres (Al2O3) with an average diameter of 5.6 mm, yielding a porosity of �40%. The inner and outer surfaces of the tube were covered with a 2 and 25 mm thick Kaowool insulation material, respectively. System diagnostics were required to assess the temperature profile in the reactor along with emission concentrations in the product gases. The axial temperature distribution of the reactor was acquired using S-type thermocouples. These thermocouples were housed in a long multibore ceramic shell; therefore measured temperatures were considered to be very close to the temperatures of the solid phase. Temperature measurements were digitised using a data acquisition module and transferred to a PC. The reaction wave propagation rates were obtained from thermocouple measurements over time and the known distance of the thermocouples. The concentrations of H2, CO, CH4, and CO2 in the product gases were measured using a gas chromatograph fitted with a thermal conductivity detector (TCD), while gas samples were acquired through an alumina tube immersed at the end of the reactor. To avoid the effect of external air vortices on the composition,

Schematic of the experimental setup.

The methane, carbon dioxide, air and steam mixture flowed continuously through the quartz tube, and the reactant concentrations were controlled using Aalborg mass flow controllers. A temperature experimental measurement error was estimated at 50 K, which is considered mainly as a radial error. The reaction wave velocity measurement error was estimated at 10%, and the chemical sampling accuracy was considered close to 10%. The experimental uncertainties were defined based on the accuracy of the laboratory equipment and the repeatability of the experimental data.

### 3.1 Combustion wave temperature and propagation rate

One objective of the numerical simulation was to clarify the effect of interfacial heat transfer. Calculations performed with the same initial conditions in twotemperature approximations are presented in Figure 2 for natural gas-air and biogas-air mixtures. Thermal non-equilibrium between gas and solid phases needs to be applied for consideration of propagation waves. Downstream, upstream, and standing waves were observed for natural gas-air and biogas-air mixtures, mainly depending on the equivalence ratio. Stable combustion of rich and ultrarich mixtures was observed experimentally for the region of equivalence ratios studied.

For natural air mixtures (Figure 2A), upstream wave propagation was observed for the range of equivalence from stoichiometric to 1.7. The velocity of the wave decreases with an increase of the natural gas concentration, approaching zero at 1.7. A stationary combustion wave is formed under these conditions. With further increase of the natural gas content, the regime of propagation changes towards a downstream direction. This regime is observed for the range of equivalence ratios from 1.7 to 3.5, where the speed increases with the natural gas content. It was found experimentally that the maximum combustion temperature remains almost constant (Ts = 1529 K) throughout the rich region and is practically independent of the natural gas content. These combustion temperatures are attributed to the changes of

combustion chemistry, because all other governing parameters such as flow rates,

For biogas-air mixtures, the combustion in porous media shows similar behaviour (Figure 2B). Upstream wave propagation is observed for the range of equivalence from stoichiometric to 1.5. A standing combustion wave was formed at 1.5. The regime of downstream propagation was observed for the range of equivalence ratios from 1.5 to 3.5. It was found experimentally that the maximum combustion temperature remains almost constant (Ts = 1491 K) throughout the rich region and

Starting from equivalence ratios higher than 1.0, complete combustion of natural gas and biogas will not be achieved because of the low oxygen content in the mixture. Consequently, concentrations of CO2 and H2O decrease, while partial oxidation products such as H2 and CO increase their presence in the product gases (Figure 3). Hydrogen and carbon monoxide concentration on the reaction waves of natural

gas and biogas showed a direct relation to an increase of equivalence ratio.

previous results, under similar conditions, as reported by [10, 16, 20].

Unburned methane was detected in the gaseous products starting from φ ≈ 1.3. Its concentration grew with an increasing equivalence ratio. Natural gas and biogas thermochemical processing could be characterised as fuel reformation or cracking

In comparison, product concentration had a similar behaviour for natural gas and biogas. H2 and CH4 concentrations are highest for natural gas and CO2 and CO for biogas, in the range of equivalence ratios studied, which is consistent with the

Biogas is obtained from the anaerobic digestion of wet biomass which is a relevant component of most urban residual wastes, as well as industrial food and

Composition of chemical products as a function of equivalence ratio for rich and ultrarich mixtures.

porous body properties, and heat content are similar.

Non-Catalytic Reforming of Biogas in Porous Media Combustion

DOI: http://dx.doi.org/10.5772/intechopen.86620

was practically independent of the biogas content.

3.2 Combustion products

rather than combustion.

3.3 Biogas composition

Figure 3.

175

### Figure 2.

Combustion temperatures and wave velocities for natural gas-air mixtures (A) and biogas (60% CH4/40% CO2)-air mixtures (B) varying the equivalence ratio from stoichiometry (φ = 1.0) to φ = 3.5.

### Non-Catalytic Reforming of Biogas in Porous Media Combustion DOI: http://dx.doi.org/10.5772/intechopen.86620

combustion chemistry, because all other governing parameters such as flow rates, porous body properties, and heat content are similar.

For biogas-air mixtures, the combustion in porous media shows similar behaviour (Figure 2B). Upstream wave propagation is observed for the range of equivalence from stoichiometric to 1.5. A standing combustion wave was formed at 1.5. The regime of downstream propagation was observed for the range of equivalence ratios from 1.5 to 3.5. It was found experimentally that the maximum combustion temperature remains almost constant (Ts = 1491 K) throughout the rich region and was practically independent of the biogas content.

### 3.2 Combustion products

Starting from equivalence ratios higher than 1.0, complete combustion of natural gas and biogas will not be achieved because of the low oxygen content in the mixture. Consequently, concentrations of CO2 and H2O decrease, while partial oxidation products such as H2 and CO increase their presence in the product gases (Figure 3).

Hydrogen and carbon monoxide concentration on the reaction waves of natural gas and biogas showed a direct relation to an increase of equivalence ratio. Unburned methane was detected in the gaseous products starting from φ ≈ 1.3. Its concentration grew with an increasing equivalence ratio. Natural gas and biogas thermochemical processing could be characterised as fuel reformation or cracking rather than combustion.

In comparison, product concentration had a similar behaviour for natural gas and biogas. H2 and CH4 concentrations are highest for natural gas and CO2 and CO for biogas, in the range of equivalence ratios studied, which is consistent with the previous results, under similar conditions, as reported by [10, 16, 20].

### 3.3 Biogas composition

Biogas is obtained from the anaerobic digestion of wet biomass which is a relevant component of most urban residual wastes, as well as industrial food and

Figure 3. Composition of chemical products as a function of equivalence ratio for rich and ultrarich mixtures.

Figure 2.

Anaerobic Digestion

174

Combustion temperatures and wave velocities for natural gas-air mixtures (A) and biogas (60% CH4/40%

CO2)-air mixtures (B) varying the equivalence ratio from stoichiometry (φ = 1.0) to φ = 3.5.

agricultural waste. It is known as a gaseous admixture mainly composed of methane (40–65% v/v) and carbon dioxide (35–55% v/v) with traces of hydrogen sulphide (0.1–3.0% v/v), moisture, and other trace contaminants.

This section shows the experimental results of combustion temperatures and syngas production from filtration combustion of synthetic biogas-air mixtures, using different compositions of CH4 and CO2 for equivalence ratio of φ = 1.5 and φ = 2.0. The experimental temperatures reached presented slight differences (<53 K) between φ = 1.5 and φ = 2.0, for the tested biogas-air mixtures (Figure 4A). The maximum combustion temperatures experimentally found were 1564 and 1563 K for 55:45 using φ = 1.5 and φ = 2.0, respectively. A temperature decrease of 31 K (φ = 1.5) and 8 K (φ = 2.0) was observed with an increment of the CO2 content in the biogas mixtures from 100:0 to 40:60. These peaks in the temperature profile could be associated with the partial oxidation of the CH4 component of the biogas (exothermic reaction), while the decline in the temperature profile corresponds to the dry reforming of biogas (endothermic reaction). Also, the different behaviours of temperature profile are attributed to the changes of combustion chemistry, CO2 presence, and filtration velocity, since all other parameters such as heat content (CH4 flow rates), porosity of inert media, geometry, and dimensions of the reactor were kept constant. The effect of decreasing the filtration velocity while reducing the CO2 content in the synthetic biogas could be responsible of the similarity between the peak temperatures recorded for all tested conditions, since it is well known that an increasing filtration velocity is responsible of enhancing the diffusion inside the reactor due to larger turbulence inside the pores of the solid matrix, whereas the peak temperature recorded while operating with a biogas composition of 55:45 using φ = 1.5 and φ = 2.0 could be attributed to a more intense role of the exothermic reactions in comparison to the decreasing filtration velocity.

Figure 4B illustrates the hydrogen and carbon monoxide yields for the biogasair mixtures in the inert bed. The maximum hydrogen yields recorded were 17.68 and 15.30% for φ = 1.5 and φ = 2.0, respectively, using 100:0 (natural gas-air mixtures). On the other hand, the maximum peak of hydrogen yields using biogasair mixtures (at 55:45) were 23.34 and 20.40% to φ = 1.5 and φ = 2.0, respectively, before gradually declining with the biogas mixture of 40:60.

Similar results have been previously reported by Zeng et al. [36] while operating an inert porous media reactor in a stationary regime with a 50:50 CH4/CO2 ratio and a filtration velocity of 25.6 cm/s.

### 3.4 Steam addition

Variations on the steam to carbon (S/C) ratio from 0.0 to 2.0 were performed under constant values of filtration velocity (34.4 cm/s), an equivalence ratio of φ = 2.0, and biogas composition (60:40 CH4/CO2).

Figure 5A depicts peak operational temperatures, reported as the mean value of maximum temperatures recorded inside the reactor, for several S/C ratios. A decreasing temperature in the reaction zone due to an increasing S/C fraction, which went from 1543 K at baseline conditions to 1501 K at an S/C ratio of 2.0, could be associated to an increased contribution of endothermic reactions in the thermochemical conversion of the mixture. However, since biogas is mainly composed by CH4 and CO2, the filtration combustion mode studied could be considered as a trireforming process where thermal partial oxidation (TPOX), dry reforming (DRR), and steam reforming (STR) simultaneously interact with CH4. Thus, this process can be considered as a non-catalytic alternative for biogas valorisation. Regarding the reaction wave propagation rates, Figure 5B shows the values computed for each experimental run, which for all experiments propagated in a downstream direction with no relevant variations.

Figure 4.

177

Combustion temperatures (A) and hydrogen and carbon monoxide yield (B) for equivalence ratio of φ = 1.5

and φ = 2.0, varying biogas composition and filtration velocities.

Non-Catalytic Reforming of Biogas in Porous Media Combustion

DOI: http://dx.doi.org/10.5772/intechopen.86620

Non-Catalytic Reforming of Biogas in Porous Media Combustion DOI: http://dx.doi.org/10.5772/intechopen.86620

agricultural waste. It is known as a gaseous admixture mainly composed of methane (40–65% v/v) and carbon dioxide (35–55% v/v) with traces of hydrogen sulphide

This section shows the experimental results of combustion temperatures and syngas production from filtration combustion of synthetic biogas-air mixtures,

φ = 1.5 and φ = 2.0. The experimental temperatures reached presented slight differences (<53 K) between φ = 1.5 and φ = 2.0, for the tested biogas-air mixtures (Figure 4A). The maximum combustion temperatures experimentally found were 1564 and 1563 K for 55:45 using φ = 1.5 and φ = 2.0, respectively. A temperature decrease of 31 K (φ = 1.5) and 8 K (φ = 2.0) was observed with an increment of the CO2 content in the biogas mixtures from 100:0 to 40:60. These peaks in the temperature profile could be associated with the partial oxidation of the CH4 component of the biogas (exothermic reaction), while the decline in the temperature profile corresponds to the dry reforming of biogas (endothermic reaction). Also, the different behaviours of temperature profile are attributed to the changes of combustion chemistry, CO2 presence, and filtration velocity, since all other parameters such as heat content (CH4 flow rates), porosity of inert media, geometry, and dimensions of the reactor were kept constant. The effect of decreasing the filtration velocity while reducing the CO2 content in the synthetic biogas could be responsible of the similarity between the peak temperatures recorded for all tested conditions, since it is well known that an increasing filtration velocity is responsible of enhancing the diffusion inside the reactor due to larger turbulence inside the pores of the solid matrix, whereas the peak temperature recorded while operating with a biogas composition of 55:45 using φ = 1.5 and φ = 2.0 could be attributed to a more intense role of the exothermic reactions in comparison to the decreasing filtration velocity. Figure 4B illustrates the hydrogen and carbon monoxide yields for the biogasair mixtures in the inert bed. The maximum hydrogen yields recorded were 17.68 and 15.30% for φ = 1.5 and φ = 2.0, respectively, using 100:0 (natural gas-air mixtures). On the other hand, the maximum peak of hydrogen yields using biogasair mixtures (at 55:45) were 23.34 and 20.40% to φ = 1.5 and φ = 2.0, respectively,

(0.1–3.0% v/v), moisture, and other trace contaminants.

Anaerobic Digestion

before gradually declining with the biogas mixture of 40:60.

φ = 2.0, and biogas composition (60:40 CH4/CO2).

a filtration velocity of 25.6 cm/s.

with no relevant variations.

176

3.4 Steam addition

Similar results have been previously reported by Zeng et al. [36] while operating an inert porous media reactor in a stationary regime with a 50:50 CH4/CO2 ratio and

Variations on the steam to carbon (S/C) ratio from 0.0 to 2.0 were performed under constant values of filtration velocity (34.4 cm/s), an equivalence ratio of

Figure 5A depicts peak operational temperatures, reported as the mean value of

maximum temperatures recorded inside the reactor, for several S/C ratios. A decreasing temperature in the reaction zone due to an increasing S/C fraction, which went from 1543 K at baseline conditions to 1501 K at an S/C ratio of 2.0, could be associated to an increased contribution of endothermic reactions in the thermochemical conversion of the mixture. However, since biogas is mainly composed by CH4 and CO2, the filtration combustion mode studied could be considered as a trireforming process where thermal partial oxidation (TPOX), dry reforming (DRR), and steam reforming (STR) simultaneously interact with CH4. Thus, this process can be considered as a non-catalytic alternative for biogas valorisation. Regarding the reaction wave propagation rates, Figure 5B shows the values computed for each experimental run, which for all experiments propagated in a downstream direction

using different compositions of CH4 and CO2 for equivalence ratio of

Combustion temperatures (A) and hydrogen and carbon monoxide yield (B) for equivalence ratio of φ = 1.5 and φ = 2.0, varying biogas composition and filtration velocities.

presence in the mixture is considered to favour the effective thermochemical conversion of biogas by means of a non-catalytic filtration combustion reactor.

Non-Catalytic Reforming of Biogas in Porous Media Combustion

DOI: http://dx.doi.org/10.5772/intechopen.86620

In this chapter numerical and experimental results were presented for filtration combustion of rich biogas-air mixtures, in comparison with methane-air mixtures. Predictions of a numerical model, based on the two-temperature approximation and multistep gas phase combustion mechanism (GRI 3.0), are in good qualitative agreement with experimental data, including combustion temperatures and wave

Applications for the reforming of biogas fuel with different compositions of methane and carbon dioxide into hydrogen and syngas were presented. Also, some improvement as steam addition to biogas-air mixtures allows higher efficiency for

The author wishes to acknowledge the financial support by the DGIIP-USM and

Department of Mechanical Engineering, Universidad Técnica Federico Santa María,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

\*Address all correspondence to: mario.toledo@usm.cl

provided the original work is properly cited.

the technical support of Lorena Espinoza and Nicolás Ripoll.

4. Conclusion

velocities.

hydrogen and syngas production.

Acknowledgements

Author details

Valparaíso, Chile

179

Mario Toledo Torres

Figure 5.

Combustion temperatures (A), wave propagation rate (B), thermal efficiency, and EROI (C) varying steam to carbon (S/C) ratio. Energy evaluation parameters.

Figure 5C presents the computed values for thermal efficiencies and a simplified energy return on investment (EROI) of the process as a function of the S/C ratio. Overall, both values were positively affected by an increasing presence of steam in the reactants, reaching their maximum values at an S/C fraction of 2.0. In particular, the peak thermal efficiency was accounted as 64.2% which represents an upgrade of 69% when compared to baseline conditions, while the EROI, where both the initial heating value of the biogas and the energy required to supply the steam were considered, reached a maximum of 46.3%, which corresponded to an effective increase of 22% relative to baseline conditions. Therefore, an increasing steam

presence in the mixture is considered to favour the effective thermochemical conversion of biogas by means of a non-catalytic filtration combustion reactor.

### 4. Conclusion

In this chapter numerical and experimental results were presented for filtration combustion of rich biogas-air mixtures, in comparison with methane-air mixtures. Predictions of a numerical model, based on the two-temperature approximation and multistep gas phase combustion mechanism (GRI 3.0), are in good qualitative agreement with experimental data, including combustion temperatures and wave velocities.

Applications for the reforming of biogas fuel with different compositions of methane and carbon dioxide into hydrogen and syngas were presented. Also, some improvement as steam addition to biogas-air mixtures allows higher efficiency for hydrogen and syngas production.

### Acknowledgements

The author wishes to acknowledge the financial support by the DGIIP-USM and the technical support of Lorena Espinoza and Nicolás Ripoll.

### Author details

Mario Toledo Torres Department of Mechanical Engineering, Universidad Técnica Federico Santa María, Valparaíso, Chile

\*Address all correspondence to: mario.toledo@usm.cl

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Figure 5C presents the computed values for thermal efficiencies and a simplified energy return on investment (EROI) of the process as a function of the S/C ratio. Overall, both values were positively affected by an increasing presence of steam in the reactants, reaching their maximum values at an S/C fraction of 2.0. In particular, the peak thermal efficiency was accounted as 64.2% which represents an upgrade of 69% when compared to baseline conditions, while the EROI, where both the initial heating value of the biogas and the energy required to supply the steam were considered, reached a maximum of 46.3%, which corresponded to an effective increase of 22% relative to baseline conditions. Therefore, an increasing steam

Combustion temperatures (A), wave propagation rate (B), thermal efficiency, and EROI (C) varying steam to

Figure 5.

Anaerobic Digestion

178

carbon (S/C) ratio. Energy evaluation parameters.

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[9] Artz J, Müller TE, Thenert K, Kleinekorte J, Meys R, Sternberg A, et al. Sustainable conversion of carbon dioxide: An integrated review of catalysis and life cycle assessment. Chemical Reviews. 2018;118:434-504. DOI: 10.1021/acs.chemrev.7b00435

[10] Toledo M, Bubnovich V, Saveliev A, Kennedy L. Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media. International Journal of Hydrogen Energy. 2009;34: 1818-1827

[11] Dhamrat RS, Ellzey JL. Numerical and experimental study of the conversion of methane to hydrogen in a porous media reactor. Combustion and Flame. 2006;144:698-709

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[15] Abdul M, Abdullaha M, Abu Bakarb M. Combustion in porous media and its applications—A comprehensive survey. Journal of Environmental Management. 2009;90:2287-2312

References

Anaerobic Digestion

[1] Chen X, Honda K, Zhang ZG. A comprehensive comparison of CH4-CO2 reforming activities of NiO/Al2O3 catalysts under fixed- and fluidized-bed

biogas-fueled solid-oxide fuel cells for a sewage sludge and food waste treatment facility. Energies. 2018;11:600. DOI:

[8] Zhao X, Li H, Zhang J, Shi L, Zhang D. Design and synthesis of NiCe@m-SiO2 yolk-shell framework catalysts with improved coke- and sinteringresistance in dry reforming of methane. International Journal of Hydrogen Energy. 2016;41:2447-2456. DOI: 10.1016/j.ijhydene.2015.10.111

[9] Artz J, Müller TE, Thenert K, Kleinekorte J, Meys R, Sternberg A, et al. Sustainable conversion of

carbon dioxide: An integrated review of catalysis and life cycle assessment. Chemical Reviews. 2018;118:434-504. DOI: 10.1021/acs.chemrev.7b00435

[10] Toledo M, Bubnovich V, Saveliev A, Kennedy L. Hydrogen production in ultrarich combustion of hydrocarbon fuels in porous media. International Journal of Hydrogen Energy. 2009;34:

[11] Dhamrat RS, Ellzey JL. Numerical

conversion of methane to hydrogen in a porous media reactor. Combustion and

[12] Babkin V, Korzhavin A, Bunev V. Propagation of premixed gaseous explosion flames in porous media. Combustion and Flame. 1991;87:182-190

[13] Babkin V. Filtrational combustion of gases. Present state of affairs and prospects. Pure and Applied Chemistry.

[14] Vogel BJ, Ellzey JL. Subadiabatic and superadiabatic performance of a two-section porous burner. Combustion Science and Technology. 2005;177:

and experimental study of the

Flame. 2006;144:698-709

1993;65:335-344

1323-1338

1818-1827

10.3390/en11030600

operations. Applied Catalysis A:

j.apcata.2005.04.037

06.023

General. 2005;288:86-97. DOI: 10.1016/

[2] Hou Z, Gao J, Guo J, Liang D, Lou H, Zheng X. Deactivation of Ni catalysts during methane autothermal reforming with CO2 and O2 in a fluidized-bed reactor. Journal of Catalysis. 2007;250: 331-341. DOI: 10.1016/j.jcat.2007.

[3] Jing Q, Lou H, Fei J, Hou Z, Zheng X. Syngas production from reforming of methane with CO2 and O2 over Ni/SrO-SiO2 catalysts in a fluidized bed reactor. International Journal of Hydrogen Energy. 2004;29:1245-1251. DOI: 10.1016/j.ijhydene.2004.01.012

[4] Gao J, Guo J, Liang D, Hou Z, Fei J, Zheng X. Production of syngas via autothermal reforming of methane in a fluidized-bed reactor over the combined CeO2-ZrO2/SiO2 supported Ni catalysts. International Journal of Hydrogen Energy. 2008;33:5493-5500. DOI: 10.1016/j.ijhydene.2008.07.040

[5] Moral A, Reyero I, Alfaro C, Bimbela F, Gandía LM. Syngas production by means of biogas catalytic partial oxidation and dry reforming using Rhbased catalysts. Catalysis Today. 2018;

299:280-288. DOI: 10.1016/j.

10.1016/j.cattod.2011.04.021

[7] Kim S, Sung T. Performance and greenhouse gas reduction analysis of

[6] Boullosa-Eiras S, Zhao T, Chen D, Holmen A. Effect of the preparation methods and alumina nanoparticles on the catalytic performance of Rh/ZrxCe1 xO2-Al2O3 in methane partial oxidation. Catalysis Today. 2011;171:104-115. DOI:

cattod.2017.03.049

180

[16] Bingue JP, Saveliev AV, Fridman AA, Kennedy LA. Hydrogen production in ultra-rich filtration combustion of methane and hydrogen sulfide. International Journal of Hydrogen Energy. 2002;27:643-649

[17] Dobrego KV, Zhdanok SA, Khanevich EI. Analytical and experimental investigation of the transition from low velocity to highvelocity regime of filtration combustion. Experimental Thermal and Fluid Science. 2000;21:9-16

[18] Foutko SI, Shabunya SI, Zhdanok SA, Kennedy LA. Superadiabatic combustion wave in a diluted methaneair mixture under filtration in a packed bed. Proceedings of the Combustion Institute. 1996;25:1556-1565

[19] Kennedy LA, Bingue JP, Saveliev AV, Fridman AA, Foutko SI. Chemical structures of methane-air filtration combustion waves for fuel-lean and fuel-rich conditions. Proceedings of the Combustion Institute. 2000;28: 1431-1438

[20] Drayton MK, Saveliev AV, Kennedy LA, Fridman AA, Li Y-E. Syngas production using superadiabatic combustion of ultra-rich methane-air mixtures. Proceedings of the Combustion Institute. 1998;27: 1361-1367

[21] Weinberg FJ, Bartleet TG, Carleton FB, Rimbotti P, Brophy JH, Manning RP. Partial oxidation of fuel rich mixtures in a spouted bed combustor. Combustion and Flame. 1988;72:235-239

[22] Ytaya Y, Oyashiki T, Hasatani M. Hydrogen production by methane-rich combustion in a ceramic burner. Journal of Chemical Engineering of Japan. 2002; 35:46-56

[23] Kennedy LA, Saveliev AV, Fridman AA. Transient filtration combustion. In: Mediterr. Combust. Symp. Vol. 1. 1999. pp. 105-139

[24] Kennedy LA, Saveliev AA, Bingue JP, Fridman AA. Filtration combustion of a methane wave in air for oxygen enriched and oxygen-depleted environments. Proceedings of the Combustion Institute. 2002;29:835-841

[25] Howell JR, Hall MJ, Ellzey JL. Combustion of hydrocarbon fuels within porous medium. Progress in Energy and Combustion Science. 1996; 22:121-145

[26] Gavrilyuk VV, Dmitrienko YM, Zhdanok SA, Minkina VG, Shabunya SI, Yadrevskaya NL, et al. Conversion of methane to hydrogen under superadiabatic filtration combustion. Theoretical Foundations of Chemical Engineering. 2001;35:589-596

[27] Gerasev AP. Hybrid autowaves in filtration combustion of gases in a catalytic fixed bed. Combustion, Explosion, and Shock Waves. 2008;44: 123-132

[28] Schoegl I, Newcomb SR, Ellzey JL. Ultra-rich combustion in parallel channels to produce hydrogen-rich syngas from propane. International Journal of Hydrogen Energy. 2009;34: 5152-5163

[29] Kaviany M. Principles of Heat Transfer in Porous Media. New York: Springer-Verlag; 1991

[30] Henneke MR, Ellzey JL. Modeling of filtration combustion in a packed bed. Combustion and Flame. 1999;117: 832-840

[31] Wakao N, Kaguei S. Heat and Mass Transfer in Packed Beds. New York:

Gordon and Breach Science Publications; 1982

[32] Touloukian YS, Powell RW, Ho CY, Nioolsou MC. Thermophysical Properties of Matter. Vol. 10. New York-Washington: IFI/PLENUM; 1973

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Section 5

Application of Anaerobic

Digestion

183

Section 5
