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 (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 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.
In addition to ethanol, the production of ethanol from switchgrass produces electricity and steam (from the lignin) which can be used by the conversion facility or sold to the electrical grid. To estimate the energy and environmental performance of the ethanol only, the total energy and environmental impacts are allocated to each product based on the equivalent product it is replacing. The energy and environmental impacts of steam and electricity are estimated based on their displacement of electricity and steam generated from fossil fuels.
The switchgrass 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.
Kim and Dale (2004, 2005) 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. 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. Conversion of switchgrass to ethanol assumes pretreatment by the ammonia fiber expansion (AFEX) process and the impacts are estimated using the ASPEN PLUS model (Laser, 2005). 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).
A conversion rate of 0.32 kg ethanol/kg switchgrass was assumed and reflects future rather than current yields (Wu, 2006). Electricity generated from converting switchgrass to ethanol displaces electricity generated in a coal-fired power plant and the generated steam displaces steam generated by petroleum and natural gas (Aden, 2002). Excess electricity and steam are sold to the electrical grid and to a district heating system. The ethanol was assumed to be used as E85 fuel in a compact passenger car. Results are estimated based on changes per kilometer traveled and are summarized in table 1.
The analysis estimated that ethanol from switchgrass could decrease nonrenewable energy consumption by 4.3 MJ km-1 driven, crude oil consumption by 68.5 g km-1 driven, and reduce greenhouse gas by 336 g CO2 equivalent km-1 driven compared to gasoline use. The greenhouse gas reduction is due to the sequestration of carbon in the soil by switchgrass, and the export of greater quantities of surplus electricity and steam (0.33MJ km-1 of electricity and 1.15 MJ km-1 steam generated and 0.16 MJ km-1 of electricity and 0.59 MJ km-1 of steam exported) than for ethanol produced from corn stover due to a higher assumed lignin content of switchgrass relative to corn stover (Thammasouk, 1997). Eutrophication increases relative to gasoline and is higher than for ethanol from corn stover due to high nitrogen fertilizer applications assumed by the authors. If current ethanol conversion yields (0.22 kg ethanol/kg of dry switchgrass) are used instead of future yields (figure 1), surplus electricity and steam quantities increase providing greater reductions in crude oil and energy use, and greenhouse gas emissions. Acidification, eutrophication, and smog forming chemicals increase.
Spatari, 2005 evaluated the production of ethanol from switchgrass in Canada and concluded that it can reduce greenhouse gas emissions when used as liquid fuel. The study assumed a dilute sulfuric acid pretreatment followed by fermentation and use of the lignin to produce electricity and steam.
