Table 1 Typical pretreatment process conditions of pretreatment*
| Â | Dilute Acid | Dilute Alkali | Hot Water | Steam Explosion |
|---|---|---|---|---|
Temperature (°C) | 160-220 | > 100 | 160-230 | 160-290 |
Pressure (MPa) | Saturated vapor pressure | Saturated vapor pressure | Up to 5 | 0.69-4.83 |
Solution (acid/alkali) concentration (%) | 0.05-5 | 0.5-3 | - | - |
Residence Time | 1-60 min | -Few minutes to several hours | 12-60 min | Few seconds-several minutes |
Mode of Action | -Hydrolysis of hemicellulose to soluble sugars -Alteration the lignin structure | -Solublization and extraction of lignin -Swelling of cellulose which increases internal surface area and separation of linkages between lignin and carbohydrates | -Water acts as dilute acid at high temperatures -Release of acetic acid and other acids from hemicellulose hydrolysis helps in further hydrolysis -Hydrolysis of hemicellulose to oligomers | -Water acts as dilute acid at high temperatures -Rapid release of pressure opens up the structure of biomass -Removal of hemicellulose |
Challenges | -Process require Detoxification before fermentation -Acid is corrosive and hazardous so reactor material should be corrosion resistant | -Cost of alkali is very high as compared to other reagents -Longer residence times are required at low temperatures | -Process require high pressure to keep water in liquid state | -Formation of inhibitory compounds due to hemicellulose degradation |