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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

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