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Table 4 Some common pre-treatment methods for lignocellulosic biomass

From: Biomass waste-to-energy valorisation technologies: a review case for banana processing in Uganda

Pre-treatment method Advantages Disadvantages
Physical
 Mechanical: Physical reduction in substrate particle size by grinding, milling, etc. Reduced cellulose crystallinity and degree of polymerization Usually negative energy balance
Increased surface area
 Irradiation: Biomass undergoes high-energy radiation (i.e. γ-ray, ultrasound, electron beam, pulsed electrical field, UV, microwave heating) Results in one or more changes to biomass Slow
Increased surface area Energy intensive
Reduced cellulose crystallinity and polymerization Prohibitively expensive
Partial depolymerization of lignin  
 Steam explosion: Substrate particles rapidly heated by high-pressure saturated stream. Explosive decompression caused by quick release of pressure acids released aid in hemicellulose hydrolysis Causes hemicellulose solubilization and lignin transformation Destruction of a portion of the xylan fraction
Cost-effective Generation of toxin compounds
 Hydrothermal: Substrate is subject to high-temperature/high-pressure water Hemicellulose solubilization High water and energy demand
Partial delignification
Chemical
 Alkaline: Addition of base causes swelling, increasing internal surface of cellulose which provokes lignin structure disruption (NaOH, KOH, Lime, Mg(OH)2, NH4OH) Lignin solubilization Relatively long residence times required
Reduced cellulose crystallinity and degree of polymerization Irrecoverable salts formed and incorporated into biomass
Increased surface area  
Can be done at ambient temperature
Relatively inexpensive
 Acid: Addition of dilute or concentrated acid solutions result in hemicellulose hydrolysis (H2SO4, HCl, HNO3, H3PO4) Hemicellulose hydrolysis and converted to fermentable sugars Relatively expensive
Alters lignin structure Corrosive
With high acid concentration can be done at room temp. High operational and maintenance costs
Some inhibitory compounds formed
 Catalysed stream explosion: Similar to steam explosion with addition of acid catalyst (SO2, H2SO4, CO2, oxalic acid) Hemicellulose solubilization Some inhibitory compounds formed
Portion of xylan fraction lost
Incomplete disruption of lignin-carbohydrate matrix
 Ammonia fibre explosion (AFEX): Substrate is exposed to hot liquid ammonia under high pressure. Pressure is released suddenly breaking open biomass structure Delignification Hemicellulose not significantly removed
Increases surface area Very high-pressure requirements
Reduced cellulose crystallinity Expensive
Low formation of inhibitors
 Wet oxidation: Dissolved oxygen oxidises substrate Efficient removal of lignin High cost of oxygen and alkaline catalyst
Low formation of inhibitors High temps and pressures
Exothermic
 Organo-solvent extraction: Organic solvents are applied, with or without addition of an acid or alkali catalyst to degrade internal lignin and hemicelluloses bonds Delignification Solvent removal is necessary
Some hemicellulose solubilization Relatively expensive
Recovery of relatively pure lignin as by-product
Biological
 Fungi and actinomycetes: Microorganisms degrade/alter biomass structure (white-, brown-, soft-rot fungi ) Degrades lignin and hemicellulose Low rate of hydrolysis
Low energy consumption