*2.1.1.1 Hydrothermal gasification*

Hydrothermal gasification is a biomass treatment that involves the use of water at high temperatures and pressures. Products formed during this process is as a result of different reactions that takes place in the biomass which mainly depends on factors like temperature, pressure, and time of treatment. To understand the process, behavior of water at high temperature and pressure must be known. **Figure 3** indicated the phase diagram of water, where at 273.15 K and atmospheric pressure (0.101325 MPa), ice melts to liquid water, while at 373.15 K liquid water boils and vapourized to steam. However, boiling point of water is affected by pressure and this means at high pressure the boiling point decreases, while at low pressure it increases. Likewise, pressure has

**7**

**Figure 4.**

*Biomass Conversion Technologies for Bioenergy Generation: An Introduction*

are below the critical values, the water is called subcritical [9].

carbon residues is left as bio-char (**Figure 6**) [20].

*Pressure effect on volume change when water changes into steam [9].*

effect on volume of water when it changes to steam. The volume of water increases greatly when it changes to steam. This change in volume is as high as 1600 times under

to steam volume. Therefore, under increased pressure, the increase in volume associated with the phase change becomes smaller (**Figure 4**). The volumes for both water and steam were found to be equal at 22.1 MPa at the phase change. Also, when the pressure is higher than this value, no noticeable phase change is seen. At this point, the pressure is called the critical pressure of water, while the temperature is called critical temperature of water which corresponds to 647 K. This point on the phase diagram, is called the critical point. If the temperature and pressure are above these critical values, the water is called supercritical water, while when the values

At increased pressure, the volume of liquid water is not affected when compared

Hydrothermal treatment of biomass can be carried out in either supercritical or subcritical water. That is when the temperature and pressure of the water is high. The process employs low temperatures ranging between 150 and 250°C. Under these conditions, the polymeric components of the biomass such as hemicellulose and lignin are dissolved together with small fraction of cellulose [15]. This process is mainly physical and requires harsh reaction conditions since the decomposition of the polymeric substances is limited. The process is often employed for saccharification of cellulose (**Figure 5**) or for an increased biomethanation of lignocellulosic

The term pyrolysis is defined as the thermal depolymerization of organic matter in the presence of nitrogen or absence of oxygen. Pyrolysis is an exothermic reaction with heat requirements that ranges between 207 and 434 kJ/kg of which many wood based and agricultural biomass were heated in an inert atmosphere to produce vapours and a carbon rich residue. The vapours composed of fragments from cellulose, hemicellulose and lignin polymers. These vapours can be condensed into free flowing organic liquid known as the bio-oil. On the other hand, the remaining

The polymeric substances distribution in bio-oil largely depends on the lignocellulosic contents of the biomass feed [21]. Many researchers investigated the individual pyrolysis characteristics of cellulose, hemicellulose and as well lignin. Hemicellulose was observed to decomposes at 220-315°C, cellulose decomposes between the range of 314-400°C, while lignin decomposition takes place from 160 to 900°C and it

*DOI: http://dx.doi.org/10.5772/intechopen.93669*

atmospheric pressure.

biomass [16–18].

*2.1.2 Pyrolysis*

**Figure 3.** *Phase diagram of water [9].*

#### *Biomass Conversion Technologies for Bioenergy Generation: An Introduction DOI: http://dx.doi.org/10.5772/intechopen.93669*

effect on volume of water when it changes to steam. The volume of water increases greatly when it changes to steam. This change in volume is as high as 1600 times under atmospheric pressure.

At increased pressure, the volume of liquid water is not affected when compared to steam volume. Therefore, under increased pressure, the increase in volume associated with the phase change becomes smaller (**Figure 4**). The volumes for both water and steam were found to be equal at 22.1 MPa at the phase change. Also, when the pressure is higher than this value, no noticeable phase change is seen. At this point, the pressure is called the critical pressure of water, while the temperature is called critical temperature of water which corresponds to 647 K. This point on the phase diagram, is called the critical point. If the temperature and pressure are above these critical values, the water is called supercritical water, while when the values are below the critical values, the water is called subcritical [9].

Hydrothermal treatment of biomass can be carried out in either supercritical or subcritical water. That is when the temperature and pressure of the water is high. The process employs low temperatures ranging between 150 and 250°C. Under these conditions, the polymeric components of the biomass such as hemicellulose and lignin are dissolved together with small fraction of cellulose [15]. This process is mainly physical and requires harsh reaction conditions since the decomposition of the polymeric substances is limited. The process is often employed for saccharification of cellulose (**Figure 5**) or for an increased biomethanation of lignocellulosic biomass [16–18].

#### *2.1.2 Pyrolysis*

*Biotechnological Applications of Biomass*

feedstock of uniform size were utilized [7].

and filtering equipment [14].

*2.1.1.1 Hydrothermal gasification*

Feedstocks for biomass gasification exists in different forms with each type having peculiar issues. Therefore, it is vital to predict suitable type of biomass for a specific gasifier type under defined conditions. Although, characteristics within specific biomass feedstock species is identical, the shape and size of the feedstock particles are useful in determining the difficulties that may arise during movement, delivery and as well as the feedstock behavior in the gasifier. The size and size distribution of the feedstock affect the gasification zone thickness, pressure drop in the bed and the maximum hearth load. To overcome some of this problems, biomass

Gasifier operation depends on moisture content of the biomass feed used. The use of feedstock with high moisture content reduces biomass conversion efficiency and as well the production rate. This is because the process discharges more fuel or heat in order to vapourize the excess moisture to the temperature of the syngas [13]. During the pyrolysis/gasification process, water need about 2.3 MJ/kg to vapourize and as well 1.5 MJ/kg to raise it to 700°C. Also, high moisture content in a biomass reduces the achieved temperature in the oxidation zone which results in incomplete cracking of the products released in the pyrolysis zone. Consequently, high moisture content in the biomass feedstock affect the syngas composition or quality due to production of CO2 from reaction between the moisture. Furthermore, using feedstock that has high moisture content results in the production of syngas with high moisture, which subsequently course additional stress on downstream cooling

Hydrothermal gasification is a biomass treatment that involves the use of water at high temperatures and pressures. Products formed during this process is as a result of different reactions that takes place in the biomass which mainly depends on factors like temperature, pressure, and time of treatment. To understand the process, behavior of water at high temperature and pressure must be known. **Figure 3** indicated the phase diagram of water, where at 273.15 K and atmospheric pressure (0.101325 MPa), ice melts to liquid water, while at 373.15 K liquid water boils and vapourized to steam. However, boiling point of water is affected by pressure and this means at high pressure the boiling point decreases, while at low pressure it increases. Likewise, pressure has

**6**

**Figure 3.**

*Phase diagram of water [9].*

The term pyrolysis is defined as the thermal depolymerization of organic matter in the presence of nitrogen or absence of oxygen. Pyrolysis is an exothermic reaction with heat requirements that ranges between 207 and 434 kJ/kg of which many wood based and agricultural biomass were heated in an inert atmosphere to produce vapours and a carbon rich residue. The vapours composed of fragments from cellulose, hemicellulose and lignin polymers. These vapours can be condensed into free flowing organic liquid known as the bio-oil. On the other hand, the remaining carbon residues is left as bio-char (**Figure 6**) [20].

The polymeric substances distribution in bio-oil largely depends on the lignocellulosic contents of the biomass feed [21]. Many researchers investigated the individual pyrolysis characteristics of cellulose, hemicellulose and as well lignin. Hemicellulose was observed to decomposes at 220-315°C, cellulose decomposes between the range of 314-400°C, while lignin decomposition takes place from 160 to 900°C and it

**Figure 4.** *Pressure effect on volume change when water changes into steam [9].*

**Figure 5.** *Reaction network for hydrothermal gasification of cellulose [9].*

**Figure 6.** *Carbonization reaction scheme of a carbonaceous material [19].*

generates a solid residue with highest percentage of about 40% [22]. From energy view point, cellulose pyrolysis was observed to be an endothermic reaction, while the reactions of hemicellulose and lignin is an exothermic. The gaseous products obtained from pyrolysis of these three components were similar and mainly comprises of CO2, CO, CH4 and other organic gases. Micro-GC was employed to analyzed the releasing behaviour of the H2 and total gases released when the three gases were pyrolyzed in a packed bed. Hemicellulose was observed to have higher yield for CO2, cellulose gives higher yield for CO with high presence of aromatic ring and methoxyl, while the lignin cracking and deformation yields higher H2 and CH4. Cellulose pyrolysis involves the cleavage of glycosidic groups via dehydration which is followed by the breakdown of anhydroglucose units. The dehydration and breakdown of sugar molecules at lower temperatures, results in the formation of char. Shafizadeh and Fu [23] reported char yield of 34.2% for the pyrolysis of pure cellulose in the absence of air and at 300°C. At high temperatures, there is enough energy to initiate the rapid cleavage of glycosidic bonds and evaporation of gaseous products was favoured. However, the distribution of cellulose, hemicellulose and lignin in a bio-oil is predominantly determined by the interactions between these components rather than just their quantities. Rowell [24] suggested that hemicellulose and cellulose were bonded through hydrogen bond, while hemicellulose and lignin were covalently bonded via ester bonds. The bonds that exist between these polymeric substances influence the pyrolytic behaviour of the biomass which may bring about a difference in products distribution when

**9**

*Biomass Conversion Technologies for Bioenergy Generation: An Introduction*

compared to a sample prepared synthetically by physical mixing. Couhert *et al.* [25] pyrolyzed two mixtures at 950°C containing the three components. One of the mixtures was prepared by simple mixing, while the other was prepared by intimate mixing. He discovered that, the yield for CO2 increases with an increase in intimacy of the mixture. Hence, the effect of components interaction may differ in a physical mixture in comparison with the actual biomass sample, because the structure of the biomass can affect pyrolysis outcome which alter selectivity for certain products [26]. The necessary conditions for pyrolysis are temperature, pressure, heating rate, residence time, environment, catalyst, etc. This conditions greatly determines the nature of the products formed after pyrolysis [27]. Therefore, the pyrolysis conditions can be adjusted to obtain a desired product. It is well known from literatures that high temperature and short residence time favours formation of condensable fractions, high temperatures and longer residence time favours non-condensable gaseous products, and as well solids fractions are only favoured at low temperatures [28]. Depending on the pyrolysis conditions, the process can be classified as follows;

Recently, fast pyrolysis which is an advanced technology is gaining attention because of an increasing need for the production of fuel oil from biomass. As a continuous process, fast pyrolysis is aimed to prevent further cracking of the pyrolytic fractions to non-condensable compounds. During the process, the parameters that give high oil yield were carefully controlled in which the primary parameter is high rates of heat transfer. This parameter could be achieved by grinding the biomass feed finely. The finely ground biomass feed is heated rapidly at high temperatures between 450–600°C for a very short residence time of typically less than 2 seconds. The liquid yield for wood fast pyrolysis was reported to be as high as 75% [29, 30]. Since the process takes place in a very short period, not only chemical kinetics, but rate of heat and mass transfer, and as well transition phenomena plays an important role in determining the chemistry of the end products. Tailored products could be

In comparison with fast pyrolysis, intermediate pyrolysis is operated at optimum temperature range of 300–500°C. The liquid products obtained during the process is less viscous and contains low tar. However, the chemical reactions taking place during intermediate pyrolysis are more controlled and thus the process offers a wide range of parameter variations for process optimization. Although low yield for liquids of up to 55% were obtained during this operation, large sizes for biomass

feed are acceptable that may be coarse, chopped, shredded or ground [31].

Slow pyrolysis is the carbonization of a biomass feed without condensing the pyrolysis products. The process is carried out in batches at low temperatures, slow heating rate and for a long residence time. Though, most of the literatures present about the process were based on its use to produce solid fuels such as charcoal and bio-char, but it can also be used to produce liquid fuels and bio-gas [32]. Temperatures as low as 0.1–2°C were reported by literatures. Slow pyrolysis is the oldest technique used for biomass conversion when the desired end product is charcoal or biochar. The vapours produced during the process were not condensed usually, but they could be used in the process to directly or indirectly provide heating. Moisture of about

obtained by setting the necessary parameters at optimum [29].

*DOI: http://dx.doi.org/10.5772/intechopen.93669*

*2.1.2.1 Fast pyrolysis*

*2.1.2.2 Intermediate pyrolysis*

*2.1.2.3 Slow pyrolysis*

#### *Biomass Conversion Technologies for Bioenergy Generation: An Introduction DOI: http://dx.doi.org/10.5772/intechopen.93669*

compared to a sample prepared synthetically by physical mixing. Couhert *et al.* [25] pyrolyzed two mixtures at 950°C containing the three components. One of the mixtures was prepared by simple mixing, while the other was prepared by intimate mixing. He discovered that, the yield for CO2 increases with an increase in intimacy of the mixture. Hence, the effect of components interaction may differ in a physical mixture in comparison with the actual biomass sample, because the structure of the biomass can affect pyrolysis outcome which alter selectivity for certain products [26].

The necessary conditions for pyrolysis are temperature, pressure, heating rate, residence time, environment, catalyst, etc. This conditions greatly determines the nature of the products formed after pyrolysis [27]. Therefore, the pyrolysis conditions can be adjusted to obtain a desired product. It is well known from literatures that high temperature and short residence time favours formation of condensable fractions, high temperatures and longer residence time favours non-condensable gaseous products, and as well solids fractions are only favoured at low temperatures [28]. Depending on the pyrolysis conditions, the process can be classified as follows;

#### *2.1.2.1 Fast pyrolysis*

*Biotechnological Applications of Biomass*

*Reaction network for hydrothermal gasification of cellulose [9].*

*Carbonization reaction scheme of a carbonaceous material [19].*

**Figure 5.**

**Figure 6.**

generates a solid residue with highest percentage of about 40% [22]. From energy view point, cellulose pyrolysis was observed to be an endothermic reaction, while the reactions of hemicellulose and lignin is an exothermic. The gaseous products obtained from pyrolysis of these three components were similar and mainly comprises of CO2, CO, CH4 and other organic gases. Micro-GC was employed to analyzed the releasing behaviour of the H2 and total gases released when the three gases were pyrolyzed in a packed bed. Hemicellulose was observed to have higher yield for CO2, cellulose gives higher yield for CO with high presence of aromatic ring and methoxyl, while the lignin cracking and deformation yields higher H2 and CH4. Cellulose pyrolysis involves the cleavage of glycosidic groups via dehydration which is followed by the breakdown of anhydroglucose units. The dehydration and breakdown of sugar molecules at lower temperatures, results in the formation of char. Shafizadeh and Fu [23] reported char yield of 34.2% for the pyrolysis of pure cellulose in the absence of air and at 300°C. At high temperatures, there is enough energy to initiate the rapid cleavage of glycosidic bonds and evaporation of gaseous products was favoured. However, the distribution of cellulose, hemicellulose and lignin in a bio-oil is predominantly determined by the interactions between these components rather than just their quantities. Rowell [24] suggested that hemicellulose and cellulose were bonded through hydrogen bond, while hemicellulose and lignin were covalently bonded via ester bonds. The bonds that exist between these polymeric substances influence the pyrolytic behaviour of the biomass which may bring about a difference in products distribution when

**8**

Recently, fast pyrolysis which is an advanced technology is gaining attention because of an increasing need for the production of fuel oil from biomass. As a continuous process, fast pyrolysis is aimed to prevent further cracking of the pyrolytic fractions to non-condensable compounds. During the process, the parameters that give high oil yield were carefully controlled in which the primary parameter is high rates of heat transfer. This parameter could be achieved by grinding the biomass feed finely. The finely ground biomass feed is heated rapidly at high temperatures between 450–600°C for a very short residence time of typically less than 2 seconds. The liquid yield for wood fast pyrolysis was reported to be as high as 75% [29, 30]. Since the process takes place in a very short period, not only chemical kinetics, but rate of heat and mass transfer, and as well transition phenomena plays an important role in determining the chemistry of the end products. Tailored products could be obtained by setting the necessary parameters at optimum [29].

#### *2.1.2.2 Intermediate pyrolysis*

In comparison with fast pyrolysis, intermediate pyrolysis is operated at optimum temperature range of 300–500°C. The liquid products obtained during the process is less viscous and contains low tar. However, the chemical reactions taking place during intermediate pyrolysis are more controlled and thus the process offers a wide range of parameter variations for process optimization. Although low yield for liquids of up to 55% were obtained during this operation, large sizes for biomass feed are acceptable that may be coarse, chopped, shredded or ground [31].

### *2.1.2.3 Slow pyrolysis*

Slow pyrolysis is the carbonization of a biomass feed without condensing the pyrolysis products. The process is carried out in batches at low temperatures, slow heating rate and for a long residence time. Though, most of the literatures present about the process were based on its use to produce solid fuels such as charcoal and bio-char, but it can also be used to produce liquid fuels and bio-gas [32]. Temperatures as low as 0.1–2°C were reported by literatures. Slow pyrolysis is the oldest technique used for biomass conversion when the desired end product is charcoal or biochar. The vapours produced during the process were not condensed usually, but they could be used in the process to directly or indirectly provide heating. Moisture of about

15–20% were reported and it affects the properties of the solid fuels produced during the process [20]. The biomass feed sizes can vary from ground to a whole log.

#### *2.1.3 Torrefaction*

Torrefaction is a slow and mild pyrolysis process that is usually carried out at low temperatures between 225°C-300°C. The process is aimed at increasing the biomass energy density and as well its fuel properties [33]. This is achieved by removal of biomass moisture content and other superfluous volatiles. During the process, the biopolymeric substances such as cellulose, hemicellulose and lignin were partly decomposed to release organic volatiles. The product obtained at the end of the process is a dry and black residual solid regarded as torrified biomass. The torrified biomass is hydrophobic and soft which can easily be crush, grind or pulverized [20, 33].

#### *2.1.4 Combustion*

The process of combustion is a widely applied biomass conversion technology that was functional to a sizeable portion of human race since the advent of human civilization. It is widely applied even today for burning of wood and agricultural residues to make pot fires and stoves in order to provide heat and light energy for cooking and heating. Combustion process is frequently used for the conversion of lignin-rich biomass. The process could be applied in two broad ways, that is either by direct conversion of the whole biomass feedstock or by biochemical conversion in which some portions of the biomass remained. Compared with the other biomass conversion technologies, the process is largely non-selective in terms of the biomass feedstock. During the process, biomass feedstock is converted to CO2 and water including smaller amount of other species which depends on the composition of the biomass and the process parameters. However, combustion of biomass largely depends on energy content of the feedstock. The amount of heat energy released during the process depends on feedstock energy content and as well as the conversion efficiency of the reaction. The fact that biomass feedstock composition plays a vital role in the combustion process was well established by many researchers worldwide in various reports [34–36]. The major share of energy in the biomass is formed by the assembly of organic matter during photosynthesis and respiration in plants. However, the inorganic fractions in the biomass are important in design and operation of the combustion system, especially when using the fluidized bed reactor. The amount of volatile matter in biomass feedstock is higher when compared with its fossil counterpart in which it is around 70–80%. The presence of this high volatile matter, greatly influence the thermal decomposition of the biomass feedstock and as well as the combustion performance of the solid fuels. This is because, large portion of the biomass feedstock has to be vapourized before the homogeneous combustion reaction takes place and the remaining char will then undergo heterogeneous combustion reaction.

The main elements that constitutes the biomass feedstock are C, H, and O, while herbaceous feedstock such as agricultural waste and grasses contain higher amounts of ash forming minerals [37, 38]. Biomass is more oxygenated compared to the conventional fossil fuel. This is due to the biomass carbohydrate structure and its dry mass usually contains about 30–40% oxygen [37]. During the combustion process, part of the oxygen required is supplied by the organically bonded oxygen from the biomass, while the rest is supplied through air injection into the system. The primary constituent of a biomass is carbon which made up about 30–60% by weight of dry matter depending on its ash content. The carbon present in biomass

**11**

**Figure 7.**

*Various reactors for combustion process [41].*

*Biomass Conversion Technologies for Bioenergy Generation: An Introduction*

feedstock is in partly oxidized form and this justifies the low gross calorific value of biomass feedstock when compared to coal. Of the biomass organic components, hydrogen is the third most important constituent that made up of about 5–6% of the dry matter. Other elements that are found in smaller quantities in the biomass (less than 1%) are Nitrogen, Sulfur and Chlorine, with the exception of agricultural residues where their figures are sometimes above 1% [39, 40]. The presence of high amount of such inorganic elements in a biomass feedstock leads to serious operational problems such as agglomeration, deposition, fouling, sintering and corrosion or erosion. Combustion process, unlike biochemical and other thermochemical conversion technologies, is largely nonselective in terms of biomass feedstock selection and the process aims to reduce the entire fuel to simple products. However, this shows that the complex nature of the biomass has substantial influence on its combustion performance. Inorganic elements such as Si, K, S, Cl, P, Ca, Mg and Fe are associated with reactions that leads to ash fouling and slagging (**Figure 7**) [36].

Biochemical biomass conversion technologies refer to conversion of biomass through biological pre-treatments. These pre-treatments were aimed to turn the biomass into a number of products and intermediates through selection of different microorganisms or enzymes. The process provides a platform to obtain fuels and chemicals such as biogas, hydrogen, ethanol, butanol, acetone and a wide range of organic acids [42]. However, this process was aimed at producing products that could replace petroleum-based products and as well as those obtained from the grains. Biomass biochemical conversion technologies are clean, pure, and efficient

Anaerobic digestion (AD) is one of the most sustainable and cost-effective technology for lignocellulosic and other form of waste treatment for energy recovery in form of biofuels. This process does not only minimize the amount of waste, but also transforms such waste into bioenergy. Also, the digestates produced during the process are rich in nutrients, which can serve as fertilizer for agricul-

when compared with the other conversion technologies [43].

*DOI: http://dx.doi.org/10.5772/intechopen.93669*

**2.2 Biochemical methods**

*2.2.1 Digestion*

tural purposes [44].

*Biomass Conversion Technologies for Bioenergy Generation: An Introduction DOI: http://dx.doi.org/10.5772/intechopen.93669*

feedstock is in partly oxidized form and this justifies the low gross calorific value of biomass feedstock when compared to coal. Of the biomass organic components, hydrogen is the third most important constituent that made up of about 5–6% of the dry matter. Other elements that are found in smaller quantities in the biomass (less than 1%) are Nitrogen, Sulfur and Chlorine, with the exception of agricultural residues where their figures are sometimes above 1% [39, 40]. The presence of high amount of such inorganic elements in a biomass feedstock leads to serious operational problems such as agglomeration, deposition, fouling, sintering and corrosion or erosion. Combustion process, unlike biochemical and other thermochemical conversion technologies, is largely nonselective in terms of biomass feedstock selection and the process aims to reduce the entire fuel to simple products. However, this shows that the complex nature of the biomass has substantial influence on its combustion performance. Inorganic elements such as Si, K, S, Cl, P, Ca, Mg and Fe are associated with reactions that leads to ash fouling and slagging (**Figure 7**) [36].
