78
3 Advanced Technologies (Biological and Thermochemical) …
Methanogenesis : CO 2 + 4H 2 → CH 4 + H 2 O
( 3 . 4 )
The AD reactions can take place in either a single- or a two-stage digester. In
a single-stage digester, all four steps of hydrolysis, acidogenesis, acetogenesis, and
methanogenesis occur in the same reactor. However, in a two-stage digester, the first
two steps of hydrolysis and acidogenesis take place in the first reactor and the rest
two steps occur in the second reactor. It was reported that two-stage digesters have
better performance compared to the single-stage ones [70]. There are also standard
rate (cold) and high rate digesters. The standard rate digesters are usually used for
small systems and have a long Solid Retention Time (SRT) of 30–60 days and the
contents are usually unheated and unmixed, while the high rate digesters have an
SRT of 10–20 days and are completely mixed and heated [69].
Application of AD on lignocellulosic biomass is usually limited by a long retention
time (20–30 days) and low degradation efficiency of the organic dry solids (30–
50%), due to the inefficient hydrolysis stage [69]. During hydrolysis, plant cell walls
need to be ruptured to release organic materials readily available for the acidogenic
microorganisms [69]. Applying pre-treatment to disintegrate plant cell walls could
effectively enhance the anaerobic biodegradability and make the feedstock more
accessible to the anaerobic microorganisms [71]. Different pre-treatment methods
such as thermal, mechanical, chemical have been developed and applied in the AD
of biomass, which are briefly described below.
3.2.1.1 Thermal Pre-treatments
Thermal pre-treatments destroy the cell walls using heat to make the organic
compounds accessible for biological degradation [72–74]. Thermal pre-treatments at
a wide range of temperatures have been studied, ranging from 60 to 270 °C, which can
be divided into low-temperature (<100 °C) and high-temperature treatments (>100
°C). The most common pre-treatment temperatures were between 60 and 180 °C
[70, 75, 76]. Temperatures above 200 °C were found responsible for refractory and
toxic compound formation and thus reduction in the biodegradability [72, 74, 77,
78]. Lower pre-treatment temperatures are more advantageous due to lower energy
requirements and applicability, where a longer treatment time is required than that at
a higher pre-treatment temperature [79]. Although high-temperature pre-treatment
requires higher energy, it can be offset by the increased biogas production, increased
biodegradability that reduces the digester heating requirements, which increase the
overall energy production in the entire AD process.
3.2.1.2 Chemical Pre-treatments
Chemical pre-treatments are used to hydrolyze the cell wall and membranes by
means of strong acids, alkalis or oxidants to increase the solubility of the organic
3 Advanced Technologies (Biological and Thermochemical) …
Methanogenesis : CO 2 + 4H 2 → CH 4 + H 2 O
( 3 . 4 )
The AD reactions can take place in either a single- or a two-stage digester. In
a single-stage digester, all four steps of hydrolysis, acidogenesis, acetogenesis, and
methanogenesis occur in the same reactor. However, in a two-stage digester, the first
two steps of hydrolysis and acidogenesis take place in the first reactor and the rest
two steps occur in the second reactor. It was reported that two-stage digesters have
better performance compared to the single-stage ones [70]. There are also standard
rate (cold) and high rate digesters. The standard rate digesters are usually used for
small systems and have a long Solid Retention Time (SRT) of 30–60 days and the
contents are usually unheated and unmixed, while the high rate digesters have an
SRT of 10–20 days and are completely mixed and heated [69].
Application of AD on lignocellulosic biomass is usually limited by a long retention
time (20–30 days) and low degradation efficiency of the organic dry solids (30–
50%), due to the inefficient hydrolysis stage [69]. During hydrolysis, plant cell walls
need to be ruptured to release organic materials readily available for the acidogenic
microorganisms [69]. Applying pre-treatment to disintegrate plant cell walls could
effectively enhance the anaerobic biodegradability and make the feedstock more
accessible to the anaerobic microorganisms [71]. Different pre-treatment methods
such as thermal, mechanical, chemical have been developed and applied in the AD
of biomass, which are briefly described below.
3.2.1.1 Thermal Pre-treatments
Thermal pre-treatments destroy the cell walls using heat to make the organic
compounds accessible for biological degradation [72–74]. Thermal pre-treatments at
a wide range of temperatures have been studied, ranging from 60 to 270 °C, which can
be divided into low-temperature (<100 °C) and high-temperature treatments (>100
°C). The most common pre-treatment temperatures were between 60 and 180 °C
[70, 75, 76]. Temperatures above 200 °C were found responsible for refractory and
toxic compound formation and thus reduction in the biodegradability [72, 74, 77,
78]. Lower pre-treatment temperatures are more advantageous due to lower energy
requirements and applicability, where a longer treatment time is required than that at
a higher pre-treatment temperature [79]. Although high-temperature pre-treatment
requires higher energy, it can be offset by the increased biogas production, increased
biodegradability that reduces the digester heating requirements, which increase the
overall energy production in the entire AD process.
3.2.1.2 Chemical Pre-treatments
Chemical pre-treatments are used to hydrolyze the cell wall and membranes by
means of strong acids, alkalis or oxidants to increase the solubility of the organic