A number of feedstocks can be used to produce biofuels, all of which are derived from plants. The most common biofuel in recent years has been ethanol. In the United States, approximately 1.8 billion gallons (6.8 billion liters) of liquid ethanol were manufactured in 1996-1997 from the starch in corn kernels (RFA, 1999); in Brazil, approximately 14 billion liters of ethanol were produced from sugarcane in 1996-1997 (PCAST, 1999). Trees, as well as switchgrass, are being developed for cellulose-to-ethanol manufacture and as a fuel source for electric power generation. Eventually, crops might be grown for the sole purpose of producing fuels. For example, poplar or willow trees might be grown on energy plantations. In the near term, OFD appears to consider poplars as coproduct systems, in which the fiber can be used for material production and the residuals for bioenergy production.
Rather than growing a dedicated energy crop for fuels manufacture, residues from various production processes could be used as biomass feedstocks. Large amounts of biomass are left in the field after conventional food crops have been harvested or after trees have been cut by the forest products industry. These residues range from sugarcane bagasse, rice straw, wood mill residues, and corn stover to forest residues from logging and other activities. Although estimates vary with region and local soil conditions, approximately 100 million metric tons of corn residue in the United States are potentially available as biomass feedstock for ethanol manufacture. This estimate is based on the assumption that 30 percent of the corn residues are left in the field to conserve soil and water (NRC, 1999c). Residues that have already been collected have the advantage of low cost. Municipal waste, which contains organic matter, such as paper, paper products, wood, and other organic materials, is another potential source of feedstock. Because some wastes come from many sources, the composition of municipal waste can be heterogeneous (NRC, 1999c). Therefore, the economic viability of using municipal waste is limited because solid wastes often contain materials that could be hazardous or that could increase processing costs.
Producing a liquid fuel from biomass entails several processing steps. If a dedicated crop is used, it must be planted, fertilized, possibly irrigated, and harvested, much like a conventional food crop. Feedstock collection costs can increase exponentially with distance, sharply constraining the optimal size of a plant (Sperling, 1988). Therefore, the costs of collection will limit the distance and area over which a crop might be harvested and collected. The collected biomass constitutes a cellulosic biomass feedstock that must then be pretreated.
Many pretreatments, including biological, chemical, physical, and thermal processes, have been investigated, but none has been demonstrated at a commercial scale. OFD's current pretreatment breakdown involves milling and exposure to acids and heat to reduce the size of the plant fibers, break down sugars from a portion of the material to yield fermentable sugars, and make their component parts more accessible to conversion processes. During hydrolysis, feedstock components, primarily polymers of glucose and pentoses, are hydrolyzed by acids and/or enzymes to fermentable sugar monomers to produce sugars that can be fermented into ethanol. Because cellulose polymers are more difficult to hydrolyze than pentosan polymers, in current practice cellulose is hydrolyzed after pentose. The NREL model under development includes a simultaneous saccharification and fermentation (SSF) process, in which hydrolysis and fermentation take place in the same reactor. The process of fermentation involves using yeast or other microorganisms to convert sugar into ethanol, carbon dioxide, and other minor components. The fermented mixture is then distilled to remove the ethanol from the water and then dewatered via azeotropic distillation or an adsorption process.
The ethanol must then be transported to service stations for distribution by pipeline, truck, barge, or railroad. Obviously, each step, from the planting to final distribution, will entail some cost, and much of OFD's R&D is intended to reduce the costs of the steps that contribute most to the cost of the overall process (see Figure 1-1).
Another approach to producing ethanol from cellulosic biomass is gasification of the biomass to synthesis gas followed by microbial fermentation to form ethanol. Methanol can also be produced from the gasification of biomass using inorganic catalysts. The dominant cost factors in the production of methanol are associated with the production of synthesis gas. The OFD program is not currently developing gasification technologies for cellulosics-to-methanol conversion (Lynd, 1996; Wyman et al., 1992). The projected costs of producing ethanol from biological processes and methanol from gasification using current technologies are comparable. No significant cost reductions are projected for producing methanol by mature gasification technologies. After more than two decades of R&D on gasification, DOE concluded in the mid-1990s that biomass-based methanol would not be competitive with methanol manufactured from natural gas.
The motivation for developing bioethanol as a transportation fuel is based on concerns about energy security, environmental quality, economic competitiveness, and stabilization of the agricultural sector. Congress has addressed environmental and energy security concerns through several mandates, including the Alternative Motor Fuels Act of 1988, the Clean Air Act Amendments of 1990, and the Energy Policy Act of 1992.
The Alternative Motor Fuels Act of 1988 encourages the development and widespread use of alternate fuels, including methanol, ethanol, and natural gas, as transportation fuels. It directs DOE to work with federal agencies to administer programs to encourage the development of alternative fuels and the production of alternative-fueled vehicles.