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Table 2 Advantages and disadvantages of lignocellulose biomass pretreatment methods. (

From: Recent advances in the valorization of plant biomass

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
  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
  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
  Microwave Less reaction time,
Selectively heats for polar part
Low inhibitor production
High cost
Low effective for materials with low dielectric loss factor
  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
  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
  Ozonolysis Reduces lignin content
Low inhibitor formation
Room temperature and atmospheric pressure
High cost of large amount of ozone needed
Flammability and toxicity
  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.
  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.
  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
  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
  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
  Sulfite pretreatment to overcome recalcitrance of lignocellulose (SPORL) Effective for hardwood and softwood
Cost efficient
Low inhibitor
Pretreatment is preceded by biomass size reduction
  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
  Microbes Have better tolerance for the environment than enzyme Long pretreatment time Requires careful control of growth conditions