A review on key pretreatment approaches for lignocellulosic biomass to produce biofuel and value-added products

Lignocellulosic biomass (LCB) is plentifully and naturally available carbon source produced mostly from agro, food and food processing industries with a global estimate of 1.3 billion tonnes per year. Since LCB is inexpensive and considered as waste, it opens an avenue for cost-effective alternate source of energy. Lignocellulosic biomass can be effectively and efficiently converted to biofuels (e.g. bioethanol, biodiesel and biogas) and value-added products like organic acids, enzymes, biopolymers, biochar etc. However, the bottleneck in using lignocellulosic biomass on industrial scale is its structural complexity and recalcitrance nature. Thus, pretreatment of biomass is an essential step for efficient delignification of biomass. This process separates cellulose and hemicellulose from lignin of the complex polymer matrix. Thereby, reduces the size of the matrix and increases the surface area of cellulose and hemicelluloses to be accessible for enzymes and microbes for hydrolysis and fermentation respectively. The pretreatment process includes physical, chemical, physicochemical or biological. Mechanical milling, ultrasound and microwave radiation as physical; Acid/alkaline hydrolysis, organosolv, ionic liquids and ozonolysis as chemical; ammonia fiber explosion, CO2 explosion steam explosion, liquid hot water treatment as physicochemical methods are established. The use of certain species of bacteria, fungus and yeast in biological methods of pretreatment is yet to establish on large scale.