Ethanol from cellulosic biomass

Abstract

The major objective of this project is to achieve the direct microbiological conversion of cellulosic biomass to liquid fuel, ethanol. Within the scope of this objective, it is also the intent to maximize the conversion efficiency of ethanol production from biomass. This can be achieved through the effective utilization of both the cellulosic (six-carbon sugar) and hemicellulosic (five-carbon sugar) fractions in biomass. The degradation of cellulosic biomass is achieved through the use of a thermophilic and anaerobic bacterium, Clostridium thermocellum . This microorganism is unique in that it is able to hydrolyse both the cellulosic and hemicellulosic fractions of biomass but, unfortunately, it is not able to metabolize the pentoses. Therefore, to achieve total utilization of biomass, a second thermophilic and anaerobic microorganism, Clostridium thermosaccharolyticum , has been under study owing to its ability to convert pentoses to ethanol. Mutation, selection and adaption programmes have yielded ethanol tolerant strains of both organisms. A fermentation process using mutant strains of the anaerobic, thermophilic bacteria Clostridium thermocellum and Clostridium thermosaccharolyticum has been investigated for the direct production of ethanol from agricultural cellulosics. Through strain improvements for increased ethanol tolerance and catabolite selectivity, alcohol yields of 85% of the theoretical maximum have been obtained from solka floc with mixed culture. The method of isolation and the performance of these improved strains on both refined cellulosics and a realistic biomass, corn [maize] stover, is presented in detail.