The following is an excerpt from Sustainable Ethanol: Biofuels, Biorefineries, Cellulosic Biomass, Flex-Fuel Vehicles, and Sustainable Farming for Energy Independence
We should evaluate the ethanol industry not by yesterday’s technology, but by the high-efficiency methods rapidly being implemented. Due to rising costs, producers are realizing the need to lessen dependence on corn kernels as a feedstock and natural gas as a process fuel. This chapter is about the production advances making ethanol make sense—making it less expensive and more environmentally friendly. Advances might even involve producing other biofuels such as butanol. These technologies will steadily improve the sustainability of biofuels.
Large Scale Ethanol Production
Ethanol is generally made like beverage alcohol—sugars are fermented and the resulting “beer” is purified by distillation. Another approach is the syngas (or thermochemical) platform. Cellulosic biomass is gasified, producing a synthesis gas which can be converted into ethanol or other useful substances through chemical catalysis.[i] Pyrolysis of cellulosic biomass also has potential. Pyrolysis oil can be upgraded to ethanol or other biofuels.
Ethanol can be made from just about any organic material. Simple sugars allow the easiest path to ethanol production. Feedstocks such as sugar cane, sweet sorghum, Jerusalem artichoke tops, and sugar beets contain readily fermentable sugars.
North America’s ethanol industry is currently based on starch feedstocks. Starch must be hydrolyzed, releasing sugars prior to fermentation. It can be stored for long periods, facilitating year-round biorefining. Field corn (not sweet corn) is the main starch crop for making ethanol in North America.
Cellulosic feedstocks are on the verge of commercialization. Cellulose and hemicellulose are more abundant than starch or free sugars. The use of cellulosic feedstocks could allow a huge expansion in biofuel production and divert waste streams from landfills. However, it is more difficult to liberate sugars from cellulosic materials than from starch. Cellulosic ethanol is being made on a pilot and demonstration scale. Until production costs come down, however, most North American ethanol will be made from the starch found in corn kernels.
See chapter 8 of Sustainable Ethanol for the following additional topics:
Ethanol from Sugar Feedstocks
Ethanol from Sweet Sorghum
Ethanol from Jerusalem Artichokes
Ethanol from Food Waste
On-Farm Ethanol from Waste Fruit
Ethanol from Beets
Ethanol from Corn Kernels
Adding Value to Coproducts
Ethanol from Field Peas
Ethanol from Grain Sorghum, Wheat, and Barley
Reducing Process Fuel Use
Reducing Water Use
Alternative Process Fuels
Combined Heat and Power
Small-Scale Ethanol Production
Ethanol Transportation & Pipeline Issues
Butanol: The Other Alcohol
Figure 8-1. Ethanol Production Paths
Figure 8-2. Ethanol Production by Feedstock, 2006
Figure 8-3. Estimated Ethanol Yield by Feedstock
Figure 8-4. Estimated Production Cost by Feedstock
Figure 8-5. Ethanol Production from Corn Kernels
Figure 8-6. Conventional vs. Combined Heat & Power
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[i]. USDA Energy Efficiency and Renewable Energy Biomass Program, Integrated Biorefineries, 2005, http://www1.eere.energy.gov/biomass/integrated_biorefineries.html.