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 bio-energy 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 bio-mass 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 CO 2 (SC-CO 2) explosion pretreatment uses CO 2, 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 CO 2 by-product. Some of the CO 2 can be used for biomass processing without a net increase in CO 2 emission into the atmosphere. SC-CO 2 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-CO 2 also introduces acidity in moist biomass that helps pretreatment. This chapter discussed various aspects of the SC-CO 2 explosion pretreatment of lignocellulosic bio-mass including corn stover, wood, and switchgrass. Operating conditions, glucose yields for different types of lignocellulosic biomass, and pretreatment mechanisms were investigated.