Conversion of Lignocellulosic Biomass to Ethanol: Recent Advances

Cellulosic ethanol produced from lignocellulosic biomass (LCB) is a promising biofuel to reduce the use of petrol. The production of cellulosic ethanol through biochemical route using biocatalysts has been investigated widely. The lignin-carbohydrate complex (LCC) in LCB makes it highly recalcitrant to degradation by biocatalysts. Hence, a thermochemical pretreatment of LCB is employed to disintegrate the LCC structure and facilitate cellulase's biocatalytic hydrolysis of cellulose fibers. There are several pretreatment methods based on mechanical, hydrothermal, and thermochemical treatments. The pretreated LCB is subsequently enzymatically hydrolyzed to simple sugars and fermented to ethanol. The non-hydrolytic lytic polysaccharide monooxygenases (LPMO) and expansins like proteins enhance the efficiency of the enzymatic hydrolysis of cellulose. While LPMO oxidatively cleaves the glucosidic bonds, the expansin-like proteins break the hydrogen bonds in the crystalline cellulose to enable rapid action of cellulases. The heterogeneous xylan polysaccharide is the major hemicellulose in conventional feedstocks like crop residues. A mixture of enzymes hydrolyze xylan to xylose for subsequent fermentation. Genetically engineered microbes are used to efficiently co-ferment glucose and xylose to ethanol. The enzymatic hydrolysis of LCB at high solids loading (HSL) and co-fermentation of glucose and xylose are important to achieve high ethanol concentration. This chapter describes various aspects of biochemical processing of LCB to produce ethanol.