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Amreica’s Enery Future: Technology and Transformation
cess is important for stimulating public awareness and enhancing the industrial infrastructure for fuel ethanol, the committee considers grain-based ethanol to be a transition to cellulosic ethanol and other so-called advanced biofuels, because grain-based ethanol does not meet the sustainability criteria discussed above. The biomass supplies likely to be available by 2020 could technically be converted into ethanol by biochemical conversion, thereby displacing a significant proportion of petroleum-based gasoline and reducing greenhouse gas emissions, but the conversion technology has to be demonstrated first and developed into a commercially deployable state.
Over the next decade or two, cellulosic ethanol could be the main product of the biochemical conversion of biomass into fuels. Further research and development could also lead to commercial technologies that convert sugars into other biofuels such as butanol and alkanes, which have higher energy densities and could be distributed by means of the existing infrastructure. Although the committee focused on cellulosic ethanol as the most deployable technology over the next 10 years, it sees a long-term transition to conversion of cellulosic biomass to higher-energy alcohols or hydrocarbons—so-called advanced biofuels—as having significant long-term potential.
The challenge in biochemical conversion of biomass into fuels is to first break down the resistant structure of a plant’s cell wall and then to break down the cellulose into five-carbon and six-carbon sugars fermentable by microorganisms; the effectiveness with which this sugar is generated is critical to economic biofuel production. The process for producing cellulosic ethanol, as shown in Figure 5.2, includes (1) preparation of the feedstock to achieve size reduction by grinding or other means; (2) pretreatment of the feedstock with steam, liquid hot water, or an acid or base to release cellulose from the lignin shield; (3) saccharification, by which cellulase hydrolyzes cellulose polymers into cellobiose (a disaccharide) and glucose (a monosaccharide), and hemicellulase breaks down hemicellulose into monosaccharides; (4) fermentation of the sugars into ethanol; and (5) distillation to separate the ethanol. The CO2 generated by the conversion process and the combustion of the fuel is mostly offset by the CO2 uptake during the growth of the biomass. The unconverted materials are burned in a boiler to generate steam for the distillation; some surplus electricity can thus be generated.
As of the end of 2008, no commercial-scale cellulosic ethanol plants were in operation. However, the U.S. Department of Energy (DOE) announced in February 2007 that it would invest up to $385 million for six biorefinery projects (two of them based on gasification) over 4 years to help bring cellulosic ethanol to