Corn stover is the above ground, non-grain portion of the corn plant. It is composed of lignocellulose material which can be converted to ethanol for use as a transportation fuel. A Life Cycle Assessment (LCA) of ethanol from corn stover is a cradle to grave evaluation of energy and environmental issues associated with the production, collection, and transport of corn stover, converting the corn stover into ethanol, and distributing and using the ethanol in cars and trucks. Corn stover 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.
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 currently produced in the U.S. is made from corn grain but ethanol can be produced from lignocellulosic feedstocks such as corn stover, and future supplies of ethanol are expected to include production from lignocellulose.
Corn stover is produced jointly with corn grain. Additionally, the production of ethanol from corn stover also produces electricity and steam which can be used by the ethanol facility or sold to the electrical grid. To evaluate the energy and environmental implications of the ethanol only, the impacts associated with the production of the corn grain and the impacts of displacing electricity and steam generated from fossil fuels are accounted for. Environmental impacts (e.g., soil erosion, soil organic matter, soil nitrogen cycling) associated with changes in soil characteristics vary with soil type and physical characteristics (e.g., slope), climate, and tillage and other management practices. The corn production practices, location of production, and quantities of corn stover removed are also important considerations.
Kim and Dale evaluated the production of ethanol from corn stover for Hardin County, IA and its adjacent counties. The analysis assumed corn was produced using no-till practices and average yields, fertilizer and chemical inputs, and fuel use for the years 2001 to 2003. To control for erosion, only 50 percent of the corn stover was collected for conversion to ethanol. The decay of corn stover releases nutrients to the soil, and these nutrients must be replaced when corn stover is collected and were accounted for by adjusting the fertilizer application rates in the following growing season. The analysis included the transportation of the corn stover on site and to the conversion facility.
Impacts on soil attributes were estimated using the DAYCENT model which simulates changes in soil organic matter (carbon) and nitrogen for different production systems in response to changes in climate, land use, and management practices. The ammonia fiber expansion (AFEX) process is used to break down the cellulose to its constituent sugars. The analysis assumed that the ethanol is used as E85 fuel in a compact passenger car. The results are estimated as changes in miles traveled.
The analysis estimated that the use of ethanol produced from corn stover could decrease crude oil consumption and reduce greenhouse gas emissions for every mile driven compared to the use of gasoline. Greenhouse gas emissions for ethanol produced from corn stover are also less than for ethanol produced from corn grain (due to lower emissions of nitrogen compounds from the soil and the production of electricity and steam). Compounds that cause smog, are a potential source of acid rain (i.e., nitrogen compounds), or cause dead zones in aquatic systems (i.e., nitrogen and phosphorus fertilizers) increase somewhat with the use of ethanol from corn stover relative to gasoline.
Sheehan and Spatari evaluated the production of ethanol from corn stover in Iowa and Canada respectively. The two studies assumed different ethanol conversion processes than Kim and Dale, but also concluded that the production of ethanol from corn stover reduced petroleum and other nonrenewable energy use, and decreased greenhouse gas emissions.