Switchgrass (Panicum virgatum) is a native, perennial prairie grass that is being developed as a bioenergy feedstock. 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 switchgrass is a cradle to grave evaluation of energy and environmental issues associated with producing, harvesting, and transporting switchgrass, converting switchgrass into ethanol, and distributing and using the ethanol in cars and trucks. Switchgrass 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 switchgrass, and future supplies of ethanol are expected to include production from lignocellulose.
The production of ethanol from switchgrass also produces electricity and steam (from the lignin) which can be used by the conversion facility or sold to the electrical grid, and displaces electricity and steam generated from fossil fuels. Switchgrass production practices and production location are important considerations as environmental impacts associated with changes in soil characteristics (i.e., carbon, erosion, and nitrogen cycling ) vary with soil type and physical characteristics (e.g., slope), climate, and tillage and other management practices.
Kim and Dale evaluated the production of ethanol from switchgrass for Hardin County, IA and its adjacent counties. The analysis included the transportation of the switchgrass on site and to the conversion facility. The DAYCENT model was used to estimate impacts on soil characteristics resulting from the production of switchgrass. The breakdown of cellulose into its constituent sugars assumes the use of the ammonia fiber expansion (AFEX) process. Excess electricity generated from the conversion of switchgrass into ethanol and not used by the ethanol facility is sold to the grid. The ethanol was assumed to be used as E85 fuel in a compact passenger car. Results are estimated based on changes per mile driven.
The analysis estimated that use of ethanol produced from switchgrass could decrease nonrenewable energy consumption and crude oil consumption compared with using gasoline. Greenhouse gas emissions were reduced per mile driven relative to using gasoline due to the sequestration of carbon in the soil by switchgrass, and the export of surplus electricity and steam. Compounds that can potentially increase acid rain (i.e., nitrogen compounds), potentially cause dead zones in aquatic systems (i.e., eutrophication by nitrogen and phosphorus fertilizers), and form smog are increased slightly with the use of ethanol produced from switchgrass relative to gasoline.
Spatari evaluated the production of ethanol from switchgrass in Canada. Although the study assumed a different conversion technology than the Kim and Dale study, it also concluded that the use of ethanol produced from switchgrass can reduce greenhouse gas emissions.