Life Cycle Assessment (LCA) of ethanol from corn grain is a cradle to grave evaluation of energy and environmental issues associated with the production and use of ethanol made from corn grain. A number of studies have examined the net energy use of producing corn and converting it to ethanol (Farrell 2006; Shapouri 1995, 2000; Pimentel 1991, 2002, 2005). A LCA goes beyond these analyses in that it examines both energy and environmental impacts of producing and transporting corn, converting the corn into ethanol, and distributing and using the ethanol in cars and trucks. Corn grain ethanol LCAs frequently include an assessment of gasoline, the petroleum derived product that ethanol will displace, as a means to compare the two products. Energy and environmental issues examined include crude oil used, nonrenewable energy consumption, greenhouse gas emissions, photochemical smog formation, acidification, and eutrophication.
LCA methodologies have been standardized by the International Organization for Standardization (ISO 1997, 1998, 2000a, 2000b) and include guidelines to establish the goal and define the scope of the analysis (i.e., define methodologies, reference condition, system boundary, etc.), to conduct the inventory analysis (i.e., collect inputs/outputs and environmental burdens associated with the processes and normalize the environmental impacts to the reference conditions), to conduct the impact assessment, and to interpret the results.
In 2006, nearly 4 billion gallons of ethanol were used as a transportation fuel in the U.S. Most ethanol used today is added to gasoline to reduce smog and enhance octane in a mixture of 10% ethanol and 90% gasoline by volume (E10 or gasohol), but flex-fueled automobiles are able to use gasoline or ethanol, and the ethanol used in these vehicles is a mixture of 85% ethanol and 15% gasoline by volume (E85 ethanol). Most ethanol produced in the U.S. is made from corn grain using either a dry grind or a wet milling process. Early in the development of the ethanol industry, wet milling predominated, but now most of the ethanol is produced using dry grind technology.
In addition to ethanol, the corn dry grind process produces distillers’ dried grains and solubles (DDGS). To estimate the energy and environmental performance of the ethanol only, the energy and environmental impacts must be allocated to each product based on the equivalent product it is replacing. Thus the energy and environmental impacts of DDGS are estimated based on their displacement of corn grain and soybean meal in livestock feed rations (Wang, 1999).
The corn production practices and location of production are important considerations in the LCA. Environmental impacts associated with changes in soil characteristics (i.e., soil organic carbon; N2O, NOx, NO3- emissions from the soil) vary with soil type and physical characteristics (e.g., slope), climate, and tillage and other management practices. Corn grain yields and fertilizer and chemical input levels are also important considerations.
Kim and Dale (2005) evaluated the production of ethanol from corn grain for Hardin County, IA and its adjacent counties. Corn production assumed no-till planting and average yields, fertilizer and chemical inputs, and fuel use for the years 2001 to 2003 (USDA NASS; USDA ERS). The analysis included the transportation of the corn on site and to the conversion facility. Impacts on soil attributes (i.e., soil organic carbon dynamics, inorganic nitrate losses due to leaching, and nitrous oxide and nitrogen oxide emissions from soil) in each county were estimated using the DAYCENT model, the daily time step version of the CENTURY model which simulates long-term (100 to 1,000 year) soil carbon and nitrogen impacts for different ecosystems (e.g. agricultural crop production, prairie grass systems, etc.) resulting from changes in climate, land use, and management (Del Grosso 2000, 2001; Natural Resource Ecology Laboratory, 2005). The DAYCENT model simulates N2O, NOx and N2 emissions from soil resulting from nitrification and denitrification. DAYCENT requires information regarding temperature and precipitation, site-specific soil properties (i.e., soil texture, soil organic content, soil moisture content, and soil mineral content), and the current and historical cropping system. The characterization factors for acidification, eutrophication and photochemical smog formation are adapted from the TRACI model (Tools for the Reduction and Assessment of Chemical and Other Environmental Impacts) (Bare, 2003). The dry grind technology was used with an assumed conversion rate of 0.32 kg ethanol/kg of dry corn grain (McAloon, 2000). DDGS were assumed to displace corn grain and soybean meal in livestock feed rations. Transportation of the ethanol to retailers and use of the ethanol assume that it is used as E85 in a compact passenger car (Lyons, 2002). Results are estimated based on changes per kilometer traveled and are summarized in table 1.
The analysis estimated that driving one km on E85 consumed 86.4 g of ethanol and 14.2 g of gasoline compared to 73.3 g of gasoline when using pure gasoline. For each km driven, E85 reduced the amount of petroleum used (60.9 g), other fossil fuels used (1.5 MJ), and the amount of greenhouse gas emissions (122 g CO2 equivalent) compared to using gasoline. The greenhouse gas credits come from the displacement of gasoline. Corn production and conversion to ethanol result in greenhouse gas production (N2O from soil in corn production and CO2 from energy use in converting corn to ethanol). Photochemcial smog forming compounds are also reduced. Acidification (mainly from NOx emissions from soil during corn production) and eutrophication (from nitrogen and phosphorus fertilizer losses) increase relative to gasoline use.