Pretreatment of Lignocellulosic Biomass Using Supercritical Carbon Dioxide as a Green Solvent

Abstract

Only concerted efforts utilizing various forms of energy can relieve today’s energy crunch that threatens world economy and stability. Renewable bioenergy is an integral part of the solution. Lignocellulosic biomass is attractive for bioenergy production because it is cheap and ubiquitous. Unlike corn, its use does not interfere with the human and farm animal food supply chain. Unfortunately, by nature’s design, lignocellulosic biomass is recalcitrant. It is difficult and costly to release the fermentable sugars from lignocellulosic biomass for ethanol fermentation. Thus, pretreatment is necessary. In the production of lignocellulosic ethanol, the biomass pretreatment step is often the most difficult and expensive part of the entire process. Many pretreatment methods have been proposed in the literature. Some of them require harsh chemicals that are not suited for a mobile or on-farm biomass processing unit. Supercritical CO2 (SC-CO2) explosion pretreatment uses CO2, which is a green solvent, to treat biomass prior to enzyme hydrolysis. In glucose fermentation for bioethanol production, each mole of ethanol is accompanied by one mole of CO2 by-product. Some of the CO2 can be used for biomass processing without a net increase in CO2 emission into the atmosphere. SC-CO2 can diffuse into the crystalline structure of cellulose. The subsequent explosion action weakens the biomass cell wall structure and increases accessible surface areas for cellulase enzymes. SC-CO2 also introduces acidity in moist biomass that helps pretreatment. This chapter discussed various aspects of the SC-CO2 explosion pretreatment of lignocellulosic biomass including corn stover, wood, and switchgrass. Operating conditions, glucose yields for different types of lignocellulosic biomass, and pretreatment mechanisms were investigated.