Table 3.

Pre-treatment methods for lignocellulosic biomass conversion to bio-based products advantage and disadvantages as adapted from Cheah et al. [66].

Treatment method	Cost	Toxic byproducts	Range of biomass applicable	Advantage/disadvantages
Acid pre-treatment	√	x	√	Inhibitors limited by dilute acid use
Freezing	√	√	x	Freezing/thawing cycles
Milling	√	√	√	Used for bioethanol and biogas production
Liquid hot water	√	√	x	High water and energy inputs
Organic solvent (Organosolv)	x	x	√	Low boiling point of the solvent. Solvent recycling is required
Oxidation	x	√	√	High cost of ozone generation. Ozone handling is required
Steam explosion	x	x	√	High cost of steam generation
Extruction	√	√	√	Hydrolysis efficiency is improved
Wet oxidation	x	√	x	Less water use as no washing is required
CO2 explosion	x	√	√	High cost for pressure maintenance
Microwave irradiation	x	√	√	More effective than conventional heating
Ultrasound	x	√	√	Low temperature and time required
Ammonium fiber expansion	x	√	√	Less effective for biomass with high lignin contents
Ionic liquid	x	√	√	Stability and reuse. Instability may cause contamination
Biological pre-treatment	√	√	√	Increases delignification. Able to reduce polymerisation
Hydrothermal liquefaction	x	√	√	Lignocellulosic materials are depolymerised into bio-oil, biogas, biochar and water-soluble compounds