81
5.3.1.2.1 Acid Pretreatment
Acid pretreatment is desirable for anaerobic digestion of rice straw because it breaks
down the lignin structure and helps methanogens acclimatize to low pH conditions.
• Hydrolysis by weak acid: acid pretreatment at low concentration is one of the
most effective methods for treating lignocellulosic biomass. This method
includes two types of hydrolysis: (1) continuous hydrolysis at high temperature
(above 160 °C) and a low loading rate (about 5–10% TS), and (2) batch hydrolysis at low temperatures (below 160 °C) with a high loading rate (about 10–40%
TS). Acid sulfuric—sprayed into the lignocellulose, mixed, and then kept at from
160 to 200 °C for a few minutes—will remove hemicellulose. As a result, the
efficiency of the hydrolysis process will be improved. Acid pretreatment showed
the significant improvement of enzyme activity and removed hemicellulose
effectively (Chen et al. 2007).
• Strong acid pretreatment: High concentrations of H 2 SO 4 and HCl are widely
used for pretreatment of lignocellulose because these acids are powerful and they
help hydrolyze cellulose (Sun and Cheng 2002) without the involvement of
enzymes in the hydrolysis process. The disadvantage of this method is the corrosive properties of these acids and they should be recycled to reduce pretreatment costs.
5.3.1.2.2 Alkaline Pretreatment
The main effect of alkaline pretreatment is lignin removal from rice straw, therefore
improving the degradability of polysaccharides. In addition, alkaline pretreatment
also eliminates acetyl and replaces uronic acid on hemicellulose, thereby increasing
the enzyme activities on hemicellulose and cellulose surfaces.
• Pretreatment with ammonia solution: This is an effective treatment for lignocellulose. An ammonia solution is highly selective in reactions with lignin in comparison to carbohydrates. One of the reactions in an ammonia solution with
lignin is the breakdown of the C-O-lignin as well as the ether and ester bonds in
the lignin complex (Binod et al. 2010). However, this solution is an
environmentally- polluting compound and a corrosive chemical.
• Pretreatment with calcium and sodium hydroxide: Lime (Ca(OH) 2 ) and sodium
hydroxide (NaOH) are commonly used for the pretreatment process. During the
process. Salts can be formed and incorporated into materials (González et al.
1986). The condition of this process is quite simple but the reaction time is relatively long. This pretreatment results in high solubility of lignin, especially for
materials with low lignin content such as softwoods and weeds. The addition of
air or oxygen during the pretreatment process can improve the efficiency of the
lignin decomposition (Chang and Holtzapple 2000).
5 Anaerobic Digestion of Rice Straw for Biogas Production
5.3.1.2.1 Acid Pretreatment
Acid pretreatment is desirable for anaerobic digestion of rice straw because it breaks
down the lignin structure and helps methanogens acclimatize to low pH conditions.
• Hydrolysis by weak acid: acid pretreatment at low concentration is one of the
most effective methods for treating lignocellulosic biomass. This method
includes two types of hydrolysis: (1) continuous hydrolysis at high temperature
(above 160 °C) and a low loading rate (about 5–10% TS), and (2) batch hydrolysis at low temperatures (below 160 °C) with a high loading rate (about 10–40%
TS). Acid sulfuric—sprayed into the lignocellulose, mixed, and then kept at from
160 to 200 °C for a few minutes—will remove hemicellulose. As a result, the
efficiency of the hydrolysis process will be improved. Acid pretreatment showed
the significant improvement of enzyme activity and removed hemicellulose
effectively (Chen et al. 2007).
• Strong acid pretreatment: High concentrations of H 2 SO 4 and HCl are widely
used for pretreatment of lignocellulose because these acids are powerful and they
help hydrolyze cellulose (Sun and Cheng 2002) without the involvement of
enzymes in the hydrolysis process. The disadvantage of this method is the corrosive properties of these acids and they should be recycled to reduce pretreatment costs.
5.3.1.2.2 Alkaline Pretreatment
The main effect of alkaline pretreatment is lignin removal from rice straw, therefore
improving the degradability of polysaccharides. In addition, alkaline pretreatment
also eliminates acetyl and replaces uronic acid on hemicellulose, thereby increasing
the enzyme activities on hemicellulose and cellulose surfaces.
• Pretreatment with ammonia solution: This is an effective treatment for lignocellulose. An ammonia solution is highly selective in reactions with lignin in comparison to carbohydrates. One of the reactions in an ammonia solution with
lignin is the breakdown of the C-O-lignin as well as the ether and ester bonds in
the lignin complex (Binod et al. 2010). However, this solution is an
environmentally- polluting compound and a corrosive chemical.
• Pretreatment with calcium and sodium hydroxide: Lime (Ca(OH) 2 ) and sodium
hydroxide (NaOH) are commonly used for the pretreatment process. During the
process. Salts can be formed and incorporated into materials (González et al.
1986). The condition of this process is quite simple but the reaction time is relatively long. This pretreatment results in high solubility of lignin, especially for
materials with low lignin content such as softwoods and weeds. The addition of
air or oxygen during the pretreatment process can improve the efficiency of the
lignin decomposition (Chang and Holtzapple 2000).
5 Anaerobic Digestion of Rice Straw for Biogas Production