7.1 Processes for fermentative hydrogen production

The methods that are investigated widely for fermentative hydrogen production are dark fermentation, photo-fermentation, and a coupling system comprising dark fermentation and photo-fermentation [204]. Dark fermentation is an acidogenic fermentation process conducted under anaerobic conditions in the absence of light. Dark fermentation, as compared to photo-fermentation, is regarded as a more promising method [42], owing to its ability to utilize a wide range of biomass, its high hydrogen production rate, and its independence of lighting conditions [109]. Microorganisms used in dark fermentation are strictly anaerobic bacteria, particularly those in the genus Clostridium, and facultative anaerobic bacteria, e.g., Enterobacter spp. [205]. Mixed cultures, for example, sludge compost and sewage sludge, are also used [204]. In theory, the maximum HY obtained under dark fermentation is 4 mol-H2/mol-glucose when acetic acid is produced as a co-product (Eq. (9)). This is roughly equivalent to one third of energy recovery from the biomass [204]. The HY of 2 mol-H2/mol-glucose can also be obtained when butyric acid is produced as the co-product (Eq. (10)). However, when mixed culture is used, mixed acids are often produced, leading to a lower HY of 2.5 mol-H2/molglucose (Eq. (11)).

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 + 6\text{H}\_2\text{O} \rightarrow 2\text{CO}\_2 + 2\text{CH}\_3\text{COOH} + 4\text{H}\_2\tag{9}$$

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 + 6\text{H}\_2\text{O} \rightarrow 2\text{CO}\_2 + \text{CH}\_3\text{CH}\_2\text{CH}\_2\text{COOH} + 2\text{H}\_2\tag{10}$$

$$\text{\textbullet \text{C}\_6\text{H}\_{12}\text{O}\_6 + \text{\textbullet H}\_2\text{O} \rightarrow \text{\textbullet C}\_3\text{CH}\_2\text{CH}\_2\text{COOH} + 2\text{CH}\_3\text{COOH} + \text{\textbullet CO}\_2 + 10\text{ H}\_2\text{ (11)}$$

Photo-fermentation is another process being investigated widely for hydrogen production from biomass. Unlike dark fermentation, photo-fermentation is a process that requires light to drive the conversion of organic substrates into hydrogen. Purple non-sulfur bacteria are a group of microorganisms responsible for hydrogen production under photo-fermentation. Examples of PNSB include Rhodobacter spp., Rhodopseudomonas spp., and Rhodospirillum sp. Photo-fermentation is a process known for its high substrate conversion efficiencies [206]. In theory, photofermentation can completely convert organic compound into hydrogen, i.e., 12 moles of hydrogen can be obtained from a mole of glucose (Eq. (12)), which is much higher than that obtained through dark fermentation (4 mol-H2/molglucose). However, when VFAs are used as the substrate, lower HYs in a range 1–10 mol-H2/mol-VFA are obtained (Eqs. (13)–(17)). In photo-fermentation, it was reported that PNSB showed an affinity toward VFAs, with malate and lactate being the most preferable substrate. Nevertheless, a good yield is also reported using acetate as the substrate [206].

$$\text{C}\_6\text{H}\_{12}\text{O}\_6 + \text{6H}\_2\text{O} + \text{Light} \rightarrow \text{6CO}\_2 + \text{12H}\_2\tag{12}$$

$$\text{HCOOH} + \text{Light} \rightarrow \text{CO}\_2 + \text{H}\_2\tag{13}$$

before acetic acid and hydrogen are consumed to produce methane [213]. AD process have been used to produce methane from a wide variety of lignocellulosic biomass, e.g., corn stover, barley straw, rice straw, wheat straw, sugarcane bagasse, and yard waste [200, 214]. Biochemical methane potential (BMP) of a selected biomass with a formula CaHbOc can be estimated using Buswell's equation (Eq. (18)), while Boyle's equation (Eq. (19)) is used to estimate BMP of biomass with a formula CaHbOcNdSe, where a, b, c, d, e is the molar fraction of C, H, O, N, S, respectively. It should be noted that Eqs. (18) and (19) are used assuming the total stoichiometric conversion of organic matter into methane and carbon dioxide [215].

Using cellulose (C6H10O5) as an example, BMP estimated using Eq. (18) is

a <sup>2</sup> <sup>þ</sup> <sup>b</sup> <sup>8</sup> � <sup>c</sup> 4 ð Þ 12a þ b þ 16c

 ð Þ 12a þ b þ 16c þ 14d þ 32e

Alternatively, organic fraction composition of biomass can be used to estimate

BMP <sup>¼</sup> <sup>415</sup> � %carbohydrate <sup>þ</sup> <sup>ð</sup><sup>496</sup> � %proteinÞ þ ð Þ <sup>1014</sup> � %lipid (20)

AD process can be divided, based on the percentage of total solids (TS) in the system, into liquid-AD (L-AD) and solid-state AD (SS-AD). Although the criteria for this classification is not clear, it is generally accepted that systems containing less than 15% TS are called L-SD and those containing 15% TS or higher are called SS-AD. While L-AD is a traditional process being used extensively for waste treatment, SS-AD is relatively new, being developed in the past decades for municipal solid waste treatment [217]. Comparing between the two, SS-AD has many advantages over L-AD, including a smaller reactor volume, thus higher volumetric productivity of methane, higher organic loading rate, lower water consumption, lower energy input for operation (heating and mixing), and no problems of floating and stratification of fats [218]. However, due to a relatively high TS content of the system, limitation of mass and heat transfers can occur during the process, leading to a low fermentation yield. The use of SS-AD on wheat straw, corn stover, switch grass, and grass silage was reported to produce 55–197 L-CH4/kg-volatile solids [219], while methane production of 45–290 L/kg-volatile solids were obtained from

8. Bioconversion process for lignocellulosic materials to bio-hydrogen

Based on average composition of lignocellulose, 35–50% cellulose, 20–35% hemicellulose, and 10–25% lignin [220], bioconversion processes for cellulose into hydrogen and methane through dark fermentation, photo-fermentation, sequential dark fermentation-photo-fermentation, and AD are presented (Figure 4). Starting

193.4–276.3 m3 of hydrogen is obtained by dark fermentation, 580–828.8 m<sup>3</sup> of hydrogen is obtained by photo-fermentation and a sequential dark fermentation-

with 1000 kg of lignocellulosic biomass containing 35–50% cellulose,

photo-fermentation, and 145.0–207.2 m<sup>3</sup> of methane is obtained by AD.

� 22, 400 (18)

� 22, 400 (19)

BMP ¼

Bio-hydrogen and Methane Production from Lignocellulosic Materials

DOI: http://dx.doi.org/10.5772/intechopen.85138

the theoretical methane production using Eq. (20) [216]:

rice straw, corn straw, wheat straw, and yard waste [200].

a <sup>2</sup> <sup>þ</sup> <sup>b</sup> <sup>8</sup> � <sup>c</sup> <sup>4</sup> � <sup>3</sup><sup>d</sup> <sup>8</sup> � <sup>e</sup> 4

BMP ¼

415 mL/g-VS:

and methane

123

$$\text{CH}\_3\text{COOH} + 2\text{H}\_2\text{O} + \text{Light} \rightarrow 2\text{CO}\_2 + 4\text{H}\_2\tag{14}$$

$$\text{CH}\_3\text{CH}(\text{OH})\text{COOH} + \text{3H}\_2\text{O} + \text{Light} \rightarrow \text{3CO}\_2 + \text{6H}\_2\tag{15}$$

$$\text{HO}\_2\text{CCH}(\text{OH})\text{CH}\_2\text{COOH} + \text{3H}\_2\text{O} + \text{Light} \rightarrow \text{4CO}\_2 + \text{6H}\_2\tag{16}$$

$$\text{CH}\_3\text{CH}\_2\text{CH}\_2\text{COOH} + \text{6H}\_2\text{O} + \text{Light} \rightarrow \text{4CO}\_2 + \text{10H}\_2\tag{17}$$

Due to the ability of photo-fermentation to utilize VFAs as the substrate for hydrogen production, in recent years, much attention has been paid on improvement of hydrogen production from biomass using coupling systems comprising dark fermentation and photo-fermentation. Anaerobic bacteria and PNSB can be co-cultivated in a single bioreactor, so that VFAs produced as the co-products during dark fermentation are instantly converted into hydrogen by photofermentation. Several co-cultivation of anaerobic bacteria, either pure or mixed culture, and PNSB have been reported in literatures with better HYs compared with the use of single-strain cultivation, for example, C. butyricum and Rhodobacter sp. M-19 [207], C. butyricum and R. sphaeroides [208], and Lactobacillus delbrueckii and R. sphaeroides RV [209], and heterotrophic consortium and R. sphaeroides N7 [210]. However, the implementation of this integrated dark fermentationphoto-fermentation system is still hindered by the great differences in growth rate and acid tolerance between anaerobic bacteria and PNSB [211]. Alternatively, dark fermentation and photo-fermentation can be performed sequentially in separated reactors. In this process configuration, dark fermentation effluent containing VFAs is fed, after some adjustments such as dilution and neutralization [204], into photo-fermentation reactor to allow the conversion of VFAs to hydrogen by PNSB. This sequential process is generally easier to operate and control compared with the co-cultivation system as dark fermentation and photo-fermentation are operated separately. Recently, the sequential dark fermentation-photo-fermentation process was tested at a pilot scale using corn stover hydrolysate as a substrate in 11 m<sup>3</sup> reactor (3 m<sup>3</sup> for dark fermentation and 8 m<sup>3</sup> for photo-fermentation). Results showed that 59.7 m<sup>3</sup> /d of hydrogen was produced, of which 22.4 m<sup>3</sup> /d was from dark fermentation and 37.3 m<sup>3</sup> /d was from photo-fermentation [47]. This demonstrates clearly that the sequential dark fermentation-photo-fermentation process is more efficient in conversion of biomass into hydrogen, compared with a single-stage dark fermentation or photofermentation process.

#### 7.2 Process for methane production

A process for fermentative production of methane is generally called AD. AD is a microbiologically mediated process, in which organic compounds are converted into methane and carbon dioxide in the absence of oxygen [212]. AD process consists of four sequential stages, hydrolysis, acidogenesis, acetogenesis, and methanogenesis, and involves several groups of microorganisms. The hydrolysis is a stage that macromolecules (protein, fat, carbohydrate) are degraded to water soluble monomers (amino acids, fatty acids, and sugars). These monomers are then fermented to VFAs (acetic, propionic, lactic, butyric, and valeric acids) during the acidogenesis stage. The fermentation products after acidogenesis are subsequently converted into acetic acid, carbon dioxide, and hydrogen in the acetogenesis stage

Bio-hydrogen and Methane Production from Lignocellulosic Materials DOI: http://dx.doi.org/10.5772/intechopen.85138

C6H12O6 þ 6H2O þ Light ! 6CO2 þ 12H2 (12)

CH3COOH þ 2H2O þ Light ! 2CO2 þ 4H2 (14)

CH3CH OH ð ÞCOOH þ 3H2O þ Light ! 3CO2 þ 6H2 (15)

CH3CH2CH2COOH þ 6H2O þ Light ! 4CO2 þ 10H2 (17)

HO2CCH OH ð ÞCH2COOH þ 3H2O þ Light ! 4CO2 þ 6H2 (16)

Due to the ability of photo-fermentation to utilize VFAs as the substrate for hydrogen production, in recent years, much attention has been paid on improvement of hydrogen production from biomass using coupling systems comprising dark fermentation and photo-fermentation. Anaerobic bacteria and PNSB can be co-cultivated in a single bioreactor, so that VFAs produced as the co-products during dark fermentation are instantly converted into hydrogen by photofermentation. Several co-cultivation of anaerobic bacteria, either pure or mixed culture, and PNSB have been reported in literatures with better HYs compared with the use of single-strain cultivation, for example, C. butyricum and Rhodobacter sp. M-19 [207], C. butyricum and R. sphaeroides [208], and Lactobacillus delbrueckii and R. sphaeroides RV [209], and heterotrophic consortium and R. sphaeroides N7 [210]. However, the implementation of this integrated dark fermentationphoto-fermentation system is still hindered by the great differences in growth

rate and acid tolerance between anaerobic bacteria and PNSB [211].

Biomass for Bioenergy - Recent Trends and Future Challenges

m<sup>3</sup> for photo-fermentation). Results showed that 59.7 m<sup>3</sup>

duced, of which 22.4 m<sup>3</sup>

fermentation process.

122

7.2 Process for methane production

Alternatively, dark fermentation and photo-fermentation can be performed sequentially in separated reactors. In this process configuration, dark fermentation effluent containing VFAs is fed, after some adjustments such as dilution and neutralization [204], into photo-fermentation reactor to allow the conversion of VFAs to hydrogen by PNSB. This sequential process is generally easier to operate and control compared with the co-cultivation system as dark fermentation and photo-fermentation are operated separately. Recently, the sequential dark fermentation-photo-fermentation process was tested at a pilot scale using corn stover hydrolysate as a substrate in 11 m<sup>3</sup> reactor (3 m<sup>3</sup> for dark fermentation and 8

photo-fermentation [47]. This demonstrates clearly that the sequential dark fermentation-photo-fermentation process is more efficient in conversion of biomass into hydrogen, compared with a single-stage dark fermentation or photo-

/d was from dark fermentation and 37.3 m<sup>3</sup>

A process for fermentative production of methane is generally called AD. AD is a microbiologically mediated process, in which organic compounds are converted into methane and carbon dioxide in the absence of oxygen [212]. AD process consists of four sequential stages, hydrolysis, acidogenesis, acetogenesis, and methanogenesis, and involves several groups of microorganisms. The hydrolysis is a stage that macromolecules (protein, fat, carbohydrate) are degraded to water soluble monomers (amino acids, fatty acids, and sugars). These monomers are then fermented to VFAs (acetic, propionic, lactic, butyric, and valeric acids) during the acidogenesis stage. The fermentation products after acidogenesis are subsequently converted into acetic acid, carbon dioxide, and hydrogen in the acetogenesis stage

HCOOH þ Light ! CO2 þ H2 (13)

/d of hydrogen was pro-

/d was from

before acetic acid and hydrogen are consumed to produce methane [213]. AD process have been used to produce methane from a wide variety of lignocellulosic biomass, e.g., corn stover, barley straw, rice straw, wheat straw, sugarcane bagasse, and yard waste [200, 214]. Biochemical methane potential (BMP) of a selected biomass with a formula CaHbOc can be estimated using Buswell's equation (Eq. (18)), while Boyle's equation (Eq. (19)) is used to estimate BMP of biomass with a formula CaHbOcNdSe, where a, b, c, d, e is the molar fraction of C, H, O, N, S, respectively. It should be noted that Eqs. (18) and (19) are used assuming the total stoichiometric conversion of organic matter into methane and carbon dioxide [215]. Using cellulose (C6H10O5) as an example, BMP estimated using Eq. (18) is 415 mL/g-VS:

$$\text{BMP} = \frac{\left(\frac{a}{2} + \frac{b}{8} - \frac{c}{4}\right)}{\left(12a + b + 16c\right)} \times 22,400 \tag{18}$$

$$\text{BMP} = \frac{\left(\frac{a}{2} + \frac{b}{8} - \frac{c}{4} - \frac{3d}{8} - \frac{e}{4}\right)}{\left(12a + b + 16c + 14d + 32e\right)} \times 22,400 \tag{19}$$

Alternatively, organic fraction composition of biomass can be used to estimate the theoretical methane production using Eq. (20) [216]:

$$\text{BMP} = \left( 415 \times \text{@carbohydrate} \right) + \left( 496 \times \text{@protein} \right) + \left( 1014 \times \text{@iplid} \right) \tag{20}$$

AD process can be divided, based on the percentage of total solids (TS) in the system, into liquid-AD (L-AD) and solid-state AD (SS-AD). Although the criteria for this classification is not clear, it is generally accepted that systems containing less than 15% TS are called L-SD and those containing 15% TS or higher are called SS-AD. While L-AD is a traditional process being used extensively for waste treatment, SS-AD is relatively new, being developed in the past decades for municipal solid waste treatment [217]. Comparing between the two, SS-AD has many advantages over L-AD, including a smaller reactor volume, thus higher volumetric productivity of methane, higher organic loading rate, lower water consumption, lower energy input for operation (heating and mixing), and no problems of floating and stratification of fats [218]. However, due to a relatively high TS content of the system, limitation of mass and heat transfers can occur during the process, leading to a low fermentation yield. The use of SS-AD on wheat straw, corn stover, switch grass, and grass silage was reported to produce 55–197 L-CH4/kg-volatile solids [219], while methane production of 45–290 L/kg-volatile solids were obtained from rice straw, corn straw, wheat straw, and yard waste [200].
