Microbial Roles in Second Generation Bioethanol

Abstract

Fossil fuel reserve is declining gradually which results in serious scarcity of fuels and chemicals across the globe. In addition, fossil fuels’ versatile usage is also responsible for the ever-increasing global warming. All together a potential solution to those two problems could be the use of lignocellulosic biomass for fuel production because this alternative energy source is renewable, inexpensive, and readily available. Moreover, this biomass has neutral emission during the total biomass conversion process for fuels and chemicals coproduction. This chapter discusses bioethanol production from lignocellulosic biomass in two conversion routes - biochemical and hybrid conversion (combination of thermochemical and biochemical conversion) route. In the first biorefinery route, non-woody lignocellulosic biomass is converted into bioethanol through a biochemical conversion process that involves the conversion of biomass carbohydrates to sugars and subsequent fermentation by genetically modified Saccharomyces cerevisiae to bioethanol. The second biorefinery route uses gasification and consecutive synthesis gas fermentation in presence of Clostridium species for bioethanol production from the woody lignocellulosic biomass. In each route, the biomass pretreatment process, biomass sugar conversion process, and down-stream separation process are discussed in detail. Pretreatment processes by microbial consortia are discussed to show their impact on biomass crystallinity reduction and surface area increase in first route. Then separate hydrolysis and fermentation (SHF) process is utilized to hydrolyze lignocellulosic sugar and finally fermentation of all soluble sugars to bioethanol is discussed in the biochemical conversion process. On the contrary, in hybrid conversion process, gasification of woody biomass is carried out at first and later synthesis gas fermentation is discussed for bioethanol production. As a result, this chapter enables one to understand the details of process design steps required for second generation bioethanol production based on two large microbial consortia, genetically modified S. cerevisiae yeast and Clostridium bacteria species.