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bioweb.sungrant.org » Technical » Environmental » Life Cycle Analysis » Corn Stover to Ethanol

Corn Stover to Ethanol	Printer Friendly

> This section is currently under review.   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 fuel 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 (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). E10 may be substituted for gasoline in most gasoline-operated vehicles, with no system modifications required. Flex-fueled automobiles are able to use gasoline or E85 ethanol, which is a mixture of 85% ethanol and 15% gasoline by volume. 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. Future supplies of ethanol are expected to include production from lignocellulose.   Corn stover is produced jointly with corn grain. To evaluate the energy and environmental implications of ethanol from corn stover only, the impacts associated with the production of the corn grain must be subtracted from the combined impacts of corn stover and grain production/ collection. The impacts allocated to corn grain (e.g., tillage, chemical and nutrient application, grain harvesting) are based on those obtained in corn grain based ethanol systems, in which only corn grain is harvested. In this way, the impacts associated with changes in soil organic carbon levels, soil nitrogen emissions, changes in nutrient requirements, and fuel use associated only with the collection of corn stover are allocated to corn stover ethanol systems.   In addition to ethanol, the production of ethanol from corn stover 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 must be 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.   Corn production practices, location of production, and removal of the corn stover 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, tillage, and other management practices. The removal of corn stover may increase soil erosion and decrease soil organic matter levels (Mann et al., 2002).   Kim and Dale (2005a, 2005b) evaluated the production of ethanol from corn stover for Hardin County, IA and its adjacent counties. The analysis assumed that corn was produced using no-till practices and average yields, fertilizer/  chemical inputs, and fuel use for the years 2001-2003 (USDA-NASS; USDA-ERS). To control for erosion, only 50% of the corn stover was collected for conversion to ethanol (Nelson, 2002). The decay of corn stover releases nutrients to the soil, and these nutrients must be replaced when corn stover is collected. Nutrients 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 (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. This is the daily time step version of the CENTURY model, which simulates long-term (100-1,000 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 corn stover to ethanol assumes pretreatment by the ammonia fiber expansion (AFEX) process and the impacts are estimated using the ASPEN PLUS model (Laser and Lynd, 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 0.34 kg ethanol/kg of dry corn stover (Wu et al., 2006) was assumed, and reflects future rather than current yields.  Electricity generated from converting corn stover to ethanol displaces electricity generated in a coal-fired power plant,  and the generated steam displaces steam generated by petroleum and natural gas (Aden et al., 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 corn stover could decrease crude oil consumption by 65 g km-1 and reduce greenhouse gas emissions by 245.5 g CO2 equivalent km-1 compared to gasoline use. Greenhouse gas reduction is almost twice as large as ethanol from corn grain, due to the reduction in N2O emissions from soil resulting from corn stover removal and the export of surplus electricity and steam (0.25 MJ of electricity km-1 and 0.86 MJ of steam km-1 generated, and 0.09 MJ of surplus electricity km-1 and 0.34 MJ of surplus steam km-1 exported). Photochemcial smog forming compounds are increased relative to gasoline. Acidification from ethanol derived from corn stover increases relative to gasoline (primarily due to the emissions of NOx during the generation of electricity and steam) and are greater than for ethanol derived from corn grain. Eutrophication (from nitrogen and phophorus fertilizer losses) increases relative to gasoline use, but is less than occurs with ethanol production from corn grain, due to lower NO3- and phosphorus leaching with corn stover removal. If current ethanol conversion yields (0.23 kg ethanol/kg of dry corn stover) are used instead of future yields (figure 1), surplus electricity and steam quantities increase providing greater reductions in crude oil, energy use, and greenhouse gas emissions. Acidification, eutrophication, and smog forming chemicals increase.                                                Figure 1—Effect of ethanol conversion rate                            on the environmental impacts of ethanol produced from corn stover                                   Current ethanol yield is 0.23 kg ethanol/kg of dry corn stover and               future ethanol yield is 0.34 kg ethanol/kg of dry corn stover.     Sheehan et al. (2003) evaluated the production of ethanol from corn stover in Iowa and concluded that it could reduce nonrenewable energy consumption and greenhouse gas emissions. The study assumed a dilute sulfuric acid pretreatment, followed by fermentation and use of the lignin to produce electricity and steam.    Spatari, 2005 evaluated the production of ethanol from corn stover in Canada and concluded that it can reduce greenhouse gas emissions when used as used as liquid transportation fuel. The study assumed a dilute sulfuric acid pretreatment, followed by fermentation and use of the lignin to produce electricity and steam.

References
Aden, M.; Ruth, K.; and Ibsen, J. [et al.] (2002). Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute Acid Prehydrolysis and Enzymatic Hydrolysis For Corn Stover. (Report No. NREL/TP-510-32438). National Renewable Energy Laboratory. Bare, J. (2003). Tools for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI): User's Guide and System Documentation. (Report No. EPA/600/R-02/052). United States Environmental Protection Agency. Del Grosso, S.J.; Parton, W.J.; and Mosier, A.R. [et al.] (2001). Simulated interaction of carbon dynamics and nitrogen trace gas fluxes using the DAYCENT model. In Modeling carbon and nitrogen dynamics for soil management (pp. 303–332). Boca Raton, FL: Lewis Publishers. Del Grosso, S.J.; Parton, W.J.; and Mosier, A.R. [et al.] (2000). General model for N2O and N2 gas emissions from soils due to denitrification. Global biogeochemical cycles, 14, 1045–1060. International Organization for Standardization (ISO). (1997). International organization for standardization 14040: Environmental management – Life cycle assessment – principles and framework. International Organization for Standardization. International Organization for Standardization (ISO). (1998). International organization for standardization 14041: Environmental management – Life cycle assessment – Goal and scope definition and inventory analysis. International Organization for Standardization. International Organization for Standardization (ISO). (2000a). International organization for standardization 14042: Environmental management – Life cycle assessment – Life cycle impact assessment. International Organization for Standardization. International Organization for Standardization (ISO). (2000b). International organization for Standardization 14043: Environmental management – Life cycle assessment – Life cycle interpretation. International Organization for Standardization. Kim, S.; and Dale, B.E. (2004). Cumulative energy and global warming impact associated with producing biomass for biobased industrial products. Journal of Industrial Ecology, 7, 147–162. Kim, S.; and Dale, B.E. (2005a). Environmental aspects of ethanol derived from no-tilled corn grain: nonrenewable energy consumption and greenhouse gas emission. Biomass & Bioenergy, 28, 475–489. Kim, S.; and Dale, B.E. (2005b). Life cycle assessment of various cropping systems utilized for producing biofuels: bioethanol and biodiesel. Biomass & Bioenergy, 29, 426 – 439. Kim, S.; and Dale, B.E. (2005c). Life cycle inventory information of the United States electricity system. International Journal of Life Cycle Assessment, 10, 294 – 304. Kim, S.; and Dale, B.E. (2006). Ethanol Fuels: E10 or E85 – Life Cycle Perspectives. International Journal of Life Cycle Assessment, 11, 117 – 121. Laser, M.; and Lynd, R.L.. (2005, May). Personal communications. Mann, L.; Tolbert, V.; and Cushman, J. (2002). Potential environmental effects of corn (Zea mays L.) stover removal with emphasis on soil organic matter and erosion. Agriculture, Ecosystems and Environment, 89, 149-166. Natural Resource Ecology Laboratory. (2005). Century soil organic matter model: user's guide and reference. Colorado State University. Retrieved July 9, 2006, from     http://www.nrel.colostate.edu/projects/century5/reference/index.htm Nelson, R.G. (2002). Resource assessment and removal analysis for corn stover and wheat straw in the Eastern and Midwestern United States - rainfall and wind erosion methodology. Biomass and Bioenergy, 22, 349-363. Sheehan, J.; Aden, A.; and Paustian, K. [et al.] (2003). Energy and Environmental Aspects of Using Corn Stover for Fuel Ethanol. Journal of Industrial Ecology, 7, 117-146. Spatari, S; Zhang, Y.M.; and MacLean, H.L. (2005). Life cycle assessment of switchgrass- and corn stover-derived ethanol-fueled automobiles. Environmental Science & Technology, 39, 9750-9758. Wu, M.; Wu, Y.; and Wang, M. (2006). Energy and Emission Benefits of Alternative Transportation Liquid Fuels Derived from Switchgrass: A Fuel Life Cycle Assessment. Biotechnology Progress, 22, 1012-1024.