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 | Â | |
 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 | Â | |
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 | Â |