Advanced Process Options for Bioethanol Production, Chapter 14

Biomass promises to be an efficient and environmentally sound raw material for producing fuels and commodity chemicals in the future. Lignocellulosic biomass, an abundant resource, provides a potentially cheap feedstock to convert to fuel ethanol and other chemicals. Notwithstanding the advances in the lignocellulose-based process for ethanol production, considerable improvements are needed inbasic research and engineering to make ethanol a viable competitor to gasoline and to produce quantities that are sufficient to meet the country's current energy demands. Based on the extensive research that has spanned more than 3 decades, a base-case process consisting of the following steps may be constructed for ethanol production from biomass: 1) Biomass pretreatment to achieve partial orcomplete hydrolysis; 2) Conversion of the liquid fraction containing pentose sugars; 3) Conversion of the hexose fraction as either glucose or cellulose; and 4) Product purification. The important process factors in cost reduction are seed requirement, feed solids concentration, ethanol yield and volumetric productivity in cellulosic conversion, and ethanol tolerance of microorganisms. Newprocess schemes for improving the base-case technology should be investigated in parallel with ongoing base-case process improvement. These schemes include cofermentation of xylose and cellulose, immobilized systems for enhanced productivity and cell retention, simultaneous fermentation and product recovery to prevent ethanol from inhibiting the biocatalyst, and novel reactor systems sych asfluidized beds for biomass conversion. An increased emphasis on innovative approaches for ethanol production would help explore avenues for increasing the competitiveness of ethanol as a transportation fuel.