derivatives (furfural and 5-hydroxy methyl furfural (5-HMF)) [47, 48]. The relative content of 5-HMF in the hydrolysates of some lignocellulosic substrates
was corn cob > corn stalk > wheat and barley while the concentration of furfural
in media was corn cob > wheat straw > barley straw > corn stalk [48].
1.2.2 Fermentation/Acidogenesis
This stage is carried out by a group of microorganisms called acidogens. They
convert the simple monomers produced in hydrolysis into volatile fatty acids
(VFAs) through fermentative reactions (Table 8.2: Eqs. 1 to 9). Acidogenesis is a
complex process that produces VFAs (acetate, propionate, butyrate, lactate,
caproate, valerate, heptoate, etc.), H 2 , alcohols, and CO 2 [49]. Acidogens have a
higher growth rate than those of acetogens and methanogens [50, 51]. Acidogens
and acetogens produce H 2 , while hydrogenotrophic and acetotrophic methanogens
consume it to produce CH 4 in anaerobic digestion. Therefore, acidogens are very
important to H 2 fermentation.
Table 8.2 Some hydrogen producing and consuming anaerobic bioreactions during dark
fermentation
Acidogenic reactions
ΔG
o’
(kJ)
Eq. No.
C 6 H 12 O 6 + 2H 2 O ! 2CH 3 COOH + 4H 2 + 2CO 2
À206.0 1
C 6 H 12 O 6 ! CH 3 CH 2 CH 2 COOH +2CO 2 + 2H 2
À254.0 2
3C 6 H 12 O 6 ! 4CH 3 CH 2 COOH + 2CH 3 COOH + 2CO 2 + 2H 2 O
À694.7 3
C 6 H 12 O 6 + 2H 2 ! 2CH 3 CH 2 COOH + 2H 2 O
À279.4 4
C 6 H 12 O 6 + 2H 2 ! 2CH 3 COOH + 2H 2 O
À198.1 5
C 6 H 12 O 6 ! 2CH 3 CHOHCOOH + H
+
À225.4 6
C 6 H 12 O 6 ! 2CH 3 CH 2 OH + 2CO 2
À164.8 7
3CH 3 CHOHCOOH! 2CH 3 CH 2 COOH+ CH 3 COOH + HCO 3
À + H
+
À165.0 8
2CH 3 CHOHCOOH + 2H 2 O! CH 3 CH 2 CH 2 COOH+ 2HCO 3
À + H
+ +2H 2
À56.3 9
Acetogenic reactions
Propionate: CH 3 CH 2 COOH + 2H 2 O ! CH 3 COOH + 3H 2 + CO 2
+76.2 10
Butyrate: CH 3 CH 2 CH 2 COOH + 2H 2 O ! 2CH 3 COOH + 2H 2
+48.4 11
Lactate: CH 3 CHOHCOOH + 2H 2 O ! CH 3 COOH + HCO 3
À + 2H 2
À4.2 12
Ethanol : CH 3 CH 2 OH + H 2 O ! CH 3 COOH + 2H 2
+9.6 13
Homoacetogenesis: 4H 2 + 2CO 2 ! CH 3 COOH + 2H 2 O
À104.0 14
Methanogenic reactions
Hydrogen: 4H 2 + CO 2 ! CH 4 + 2H 2 O
À135.0 15
Acetate: CH 3 COOH ! CH 4 + CO 2
À31.0 16
Sulfidogenic reactions
SO 4
2À + 4H 2 + H
+ ! HS
À + 4H 2 O
À152.2 17
330
A. Hajizadeh et al.
was corn cob > corn stalk > wheat and barley while the concentration of furfural
in media was corn cob > wheat straw > barley straw > corn stalk [48].
1.2.2 Fermentation/Acidogenesis
This stage is carried out by a group of microorganisms called acidogens. They
convert the simple monomers produced in hydrolysis into volatile fatty acids
(VFAs) through fermentative reactions (Table 8.2: Eqs. 1 to 9). Acidogenesis is a
complex process that produces VFAs (acetate, propionate, butyrate, lactate,
caproate, valerate, heptoate, etc.), H 2 , alcohols, and CO 2 [49]. Acidogens have a
higher growth rate than those of acetogens and methanogens [50, 51]. Acidogens
and acetogens produce H 2 , while hydrogenotrophic and acetotrophic methanogens
consume it to produce CH 4 in anaerobic digestion. Therefore, acidogens are very
important to H 2 fermentation.
Table 8.2 Some hydrogen producing and consuming anaerobic bioreactions during dark
fermentation
Acidogenic reactions
ΔG
o’
(kJ)
Eq. No.
C 6 H 12 O 6 + 2H 2 O ! 2CH 3 COOH + 4H 2 + 2CO 2
À206.0 1
C 6 H 12 O 6 ! CH 3 CH 2 CH 2 COOH +2CO 2 + 2H 2
À254.0 2
3C 6 H 12 O 6 ! 4CH 3 CH 2 COOH + 2CH 3 COOH + 2CO 2 + 2H 2 O
À694.7 3
C 6 H 12 O 6 + 2H 2 ! 2CH 3 CH 2 COOH + 2H 2 O
À279.4 4
C 6 H 12 O 6 + 2H 2 ! 2CH 3 COOH + 2H 2 O
À198.1 5
C 6 H 12 O 6 ! 2CH 3 CHOHCOOH + H
+
À225.4 6
C 6 H 12 O 6 ! 2CH 3 CH 2 OH + 2CO 2
À164.8 7
3CH 3 CHOHCOOH! 2CH 3 CH 2 COOH+ CH 3 COOH + HCO 3
À + H
+
À165.0 8
2CH 3 CHOHCOOH + 2H 2 O! CH 3 CH 2 CH 2 COOH+ 2HCO 3
À + H
+ +2H 2
À56.3 9
Acetogenic reactions
Propionate: CH 3 CH 2 COOH + 2H 2 O ! CH 3 COOH + 3H 2 + CO 2
+76.2 10
Butyrate: CH 3 CH 2 CH 2 COOH + 2H 2 O ! 2CH 3 COOH + 2H 2
+48.4 11
Lactate: CH 3 CHOHCOOH + 2H 2 O ! CH 3 COOH + HCO 3
À + 2H 2
À4.2 12
Ethanol : CH 3 CH 2 OH + H 2 O ! CH 3 COOH + 2H 2
+9.6 13
Homoacetogenesis: 4H 2 + 2CO 2 ! CH 3 COOH + 2H 2 O
À104.0 14
Methanogenic reactions
Hydrogen: 4H 2 + CO 2 ! CH 4 + 2H 2 O
À135.0 15
Acetate: CH 3 COOH ! CH 4 + CO 2
À31.0 16
Sulfidogenic reactions
SO 4
2À + 4H 2 + H
+ ! HS
À + 4H 2 O
À152.2 17
330
A. Hajizadeh et al.