Table 2 Advantages and disadvantages of lignocellulose biomass pretreatment methods. (
Category | Pretreatment | Advantages | Disadvantages |
|---|---|---|---|
Physical | Milling | Control of final particle size Reduces cellulose crystallinity Cost-effective especially for agricultural residues | High consumption of power and energy High energy required for hardwood biomass |
| Â | Steam explosion | Cost-effective for hardwood High concentrated sugars Lignin transformation and hemicellulose solubilization Low capital investment, moderate energy requirements and low environmental impacts | Hemicellulose is partly degraded Sugar degradation might happen Less effective for softwood Efficiency is affected by particle size |
| Â | Liquid hot water | Enhance cellulose digestibility, sugar extraction, and pentose recovery, No need for additional acid and size reduction low-cost reactors low or no inhibitor production | Water and energy demanding are higher |
| Â | Microwave | Less reaction time, Selectively heats for polar part Low inhibitor production | High cost Low effective for materials with low dielectric loss factor |
| Â | Ultrasonication | No external reagents are needed | Increase of cost for larger scales |
Chemical | Acid hydrolysis | Hemicellulose and partly lignin are removed High reaction rate | Corrosion problem of reactor. High inhibitory formation from sugars degradation Requirement of neutralization |
| Â | Alkaline hydrolysis | Decrease in the polymerization of carbohydrates Efficient removal of lignin Low inhibitor formation Low temperature and pressure | Relatively long reaction time Low digestibility enhancement in softwood Requires alkali removal High cost of alkaline catalyst |
| Â | Ozonolysis | Reduces lignin content Low inhibitor formation Room temperature and atmospheric pressure | High cost of large amount of ozone needed Flammability and toxicity |
| Â | Organic solvents | Solubilization of lignin and hemicellulose Pure cellulose yield High glucose yield Lignin recovery | High cost of energy and catalysts Inhibitor generation Fire and explosion hazard Recycling of solvent and/or catalysts. |
| Â | Ionic liquids | Mild reaction conditions Requires no catalyst and low-cost reactor Ionic liquids are recyclable and reusable Lignin extraction can be achieved | Toxicity and inhibitory effects on enzyme activity High ionic liquids costs Requirement of ionic liquids recovery. |
| Â | Deep eutectic solvent (DES) | Green solvent, biodegradable and biocompatible High-purity lignin | Poor stability under higher pretreatment temperatures |
Physicochemical | Wet oxidation | Efficient removal of lignin Low formation of inhibitors Reduced crystallinity of cellulose | High cost of oxygen Cellulose degradation High cost of corrosive resistant reactor Low hemicellulose recovery |
| Â | Ammonia fiber expansion (AFEX) | Cellulose crystallinity can be reduced Short reaction time High efficiency and selectivity for lignin Lower inhibition | Requires ammonia recycling system Less effective for softwood High cost of large amount of ammonia Environmental concerns |
| Â | Supercritical fluid | High solid load Low sugar degradation Output controllable by some factors Increases accessible surface area | High costs of energy consumption and reactor High pressure requirement |
| Â | Sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) | Effective for hardwood and softwood Cost efficient Low inhibitor | Pretreatment is preceded by biomass size reduction |
| Â | Co-solvent enhanced lignocellulosic fractionation (CELF) | Highly efficient for lignin extraction Nearly pure lignin production | High cost of solvents High temperature requirement |
Biological | Enzymes | Mild reaction conditions Environment friendly Selective degradation of lignin | Very long reaction time Low hydrolysis rate High environmental requirements Inactivate easily High cost of enzymes |
| Â | Microbes | Have better tolerance for the environment than enzyme | Long pretreatment time Requires careful control of growth conditions |