Chapter 8 - Pretreatment process and its effect on enzymatic hydrolysis of biomass

Abstract

Bioethanol from lignocellulosic biomass (LCB) is considered as an alternative solution to replace the fossil-based liquid fuels and thus causes the reduction in the environmental pollution in a sustainable manner. LCB is predominantly composed of cellulose, hemicellulose, lignin, ash, and extractives, which are interconnected with each other intricately leading to a rigid and recalcitrant structure. In its native form, enzymes could not access the carbohydrates and pretreatment is the key to disintegrate the highly ordered LCB structure and expose the cellulose and hemicellulose for enzymatic hydrolysis and release fermentable sugars. However, pretreatment is the high CAPEX (capital expenditure) and energy intensive step and it may represent up to 20%–30% of the ethanol costs. Thus, cost-effective and energy efficient pretreatment is the key to harness the potential of this largely untapped natural bioresource. Among various pretreatment strategies only a few have been validated at commercial scale (such as dilute acid, steam explosion, and hydrothermal) while most of them have only been evaluated at lab, pilot, or demonstration scale only (e.g., ammonia, alkali, biological, ionic liquids) majorly due to the poor economics and process complexity challenges. This study provides a detailed insight of the mechanism of action of different pretreatments, its impact on structural and functional properties of LCB with a focus on enzymatic hydrolysis yields. The analytical tools and techniques used to evaluate the substrate properties affected by the pretreatment and its correlation with enzymatic hydrolysis is discussed. Finally, the current status of pretreatment and enzymatic hydrolysis of different biomass at varied temperatures and solid loadings is illustrated to understand their impacts and implications.