For large-scale, economically viable use of lignocellulose there will be two input streams of sugars, one from hydrolysis of pretreated cellulose (C6 sugars such as glucose) and one from the hydrolysis of pretreated hemicellulose (C5 sugars such as xylose) since the common fermentation yeast (Saccharomyces cerevisiae) can only utilize C6 sugars, an additional technology is required for lignocellulose compared to starch or sucrose based ethanol production. The fermenting process for lignocellulosic ethanol production will include either two fermentation pro­cesses (S. cerevisiae for glucose and bacteria or other yeast for pentoses) or one C5 and C6 co-fermentation process (e. g. genetically-engineered microorganisms with specifically-designed metabolic pathways). To-date, several microbial species have been engineered to ferment both glucose and pentoses, including E. coli, Zymomonas mobilis, Pichia stipitis, Thermoanaerobacterium saccharolyticum and S. cerevisiae [56-58]. While these metabolically-engineered microbes show C6 and C5 fermentation, the ethanol yields have been too low for commercial applica­tions [57]. In addition, many engineered organisms are susceptible to inhibitory compounds generated during pretreatment, and are not as tolerant to high ethanol concentration as the typical S. cerevisiae strains. Research continues to explore the possibilities for economic fermentation of both C6 and C5 sugars.

1.4 Butanol and Other Chemicals

Once hemicelluloses and celluloses in biomass feedstock have been hydrolyzed, the sugar “platform” can be utilized to generate a range of chemicals, including other fuels such as butanol [59]. Butanol has several advantages over ethanol as an alternative fuel (but not as an oxygenate) and may be a better choice for the large volume liquid transport fuel market. However, if other chemicals are produced in an ethanol plant, the final product separation process (distillation and dehydration) would be problematical. Separate down-stream production paths will be required in future biorefineries to accommodate the potential product flows, which may result in different designs and configurations [15].