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Liquid Transportation Fuels from Coal and Biomass: Technological Status, Costs, and Environmental Impacts
produced when wild-type yeast ferments six-carbon sugars. Sugar can be obtained directly from sugarcane (Brazil) and sugar beets (Europe) or indirectly from the hydrolysis of starch-based grains, such as corn (United States) and wheat (Canada and Europe). In the latter case, the starch feedstock needs to be ground to a meal that is hydrolyzed to glucose by enzymes. The resulting mash is fermented by natural yeast and bacteria. Finally, the fermented mash is separated into ethanol and residues (for feed production) via distillation and dehydration (Figure 3.2).
Corn grain is the major source of ethanol in the United States, and its potential for growth is defined by production efficiencies, food-versus-fuel debates, and the question of sustainability and carbon footprint. Developments aimed at future processes are targeting cellulose conversions that could address those issues by providing a growth potential, a low carbon footprint, and sustainability. The infrastructure that was established by the corn grain ethanol industry will benefit the future cellulosic-ethanol industry because the use of ethanol as a transportation fuel has been proved to be feasible, a distribution system exists, and automobiles with internal-combustion engines that use ethanol efficiently are on the road.
Recent analyses of the full life cycle of corn grain ethanol have indicated
FIGURE 3.2Schematic representation of bioprocessing elements.