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bioweb.sungrant.org » Technical » Environmental » Life Cycle Analysis » Integrated Biorefinery
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Life Cycle Assessment (LCA) of transportation fuels from biomass resources is a cradle to grave evaluation of energy and environmental issues associated with producing, collecting, and transporting the biomass, converting the biomass into transportation fuels, and distributing and using the transportation fuel in motor vehicles. Biomass transportation fuel LCAs frequently include an assessment of the petroleum derived product that they will displace (e.g., gasoline, diesel), 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.
An integrated biorefinery is a facility in which bioenergy and bioproducts are produced from both lignocellulose (e.g., corn stover, switchgrass) and starch (corn grain) biomass resources. Operations within a biorefinery are synergistic—the starch processing component can utilize the surplus energy generated (through the combustion of the lignin in the cellulose resources) in processing the lignocellulose feedstock thus reducing the overall energy needs of the facility. No integrated biorefineries exist at this time, but a program to fund six pilot facilities to demonstrate the concept is underway.
The production of ethanol from corn grain, corn stover, and switchgrass produces byproducts such as distillers’ dried grains and solubles (DDGS) from the conversion of corn grain to ethanol and electricity and steam from the conversion of corn stover and switchgrass to ethanol. To estimate the energy and environmental performance of the ethanol only, the energy and environmental impacts are allocated to each product based on the equivalent product it is replacing. 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) and the energy and environmental impacts of steam and electricity are estimated based on their displacement of electricity and steam generated from fossil fuels. Additionally, corn stover is produced jointly with corn grain and the impacts associated with the production of the corn grain are subtracted from the combined impacts of corn stover and grain production and collection.
The corn and switchgrass production practices and location, and the quantities of corn stover that can be removed for bioenergy use are important considerations. 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. Crop yields and fertilizer and chemical input levels are also important. The removal of corn stover may increase soil erosion and decrease soil organic matter levels (Mann, 2002) and sufficient quantities must be left in the field to maintain soil quality.
Kim and Dale (various publications) evaluated two integrated biorefinery scenarios—one based on corn grain and corn stover to produce ethanol (biorefinery A) and one based on switchgrass and corn grain to produce ethanol (biorefinery B). The analysis assumed that the biorefinery was located in 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) and assumed that only 50 percent of the corn stover produced could be collected for conversion to ethanol (Nelson, 2002). The decay of corn stover releases nutrients to the soil, and the analysis included nutrients loss by adjusting the fertilizer application rates in the following growing season.
The analysis included the transportation of the biomass resource 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 assumed annual ethanol production rates are 90,000 m3 yr-1 from corn grain and 640,000 m3 yr-1 from corn stover and from switchgrass (McAloon, 2000; Laser, 2005) and reflect future rather than current yields for stover and switchgrass. Electricity generated from converting corn stover or 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.
Table 1 presents the results of the analysis for the two integrated biorefinery scenarios and for a stand-alone corn grain to ethanol facility for comparison. Both biorefinery scenarios provided increased benefits relative to gasoline and the stand-alone corn grain facility with respect to crude oil use, non-renewable energy use, and greenhouse gas emissions as a result of the use of the surplus energy produced in converting the lignocellulose material. Relative to gasoline and the stand-alone corn grain facility, ethanol production in both integrated biorefinery scenarios resulted in increased production of compounds involved in acidification and photochemical smog formation. This result is due to the assumption that the nitrogen contained in the corn stover or switchgrass is converted to NOx in the combustion process. Ethanol production from the combined corn grain and switchgrass biorefinery (biorefinery B) displaces greater quantities of crude oil, nonrenewable energy, and greenhouse gases relative to the other ethanol production options due to the higher lignin content of switchgrass relative to corn stover which is used to produce electricity, heat, and steam. The lowest rates of eutrophication occurred in the corn grain-corn stover biorefinery (biorefinery A) due to lower nitrogen related emissions from the soil resulting from corn stover removal and the assumption that phosphorus is the only additional nutrient that must be added in the following growing season and assumed high rates of fertilizer applications for switchgrass.
Results of changing the production capacity of the integrated biorefinery from 85 to 120 percent of the cellulose feedstock base capacity (640,000 m3 yr-1), while keeping the corn grain production capacity unchanged are presented in figure 1. Increasing the lignocellulose capacity reduces crude oil and nonrenewable energy use, and greenhouse gas emissions. Acidification and smog formation increase with both integrated biorefineries relative to the stand-alone corn grain facility. Eutrophication is higher for ethanol from all facilities relative to gasoline, but when compared to the stand-alone corn grain facility, eutrophication is lower for the corn grain-corn stover biorefinery and higher for the corn grain-switchgrass biorefinery resulting from the assumption that the nitrogen in the switchgrass is converted to NOx in the combustion process.
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Aden, M.; Ruth, K.; & 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. (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. (EPA/600/R-02/052). United States Environmental Protection Agency.
Del Grosso, S.J.; Parton, W.J.; & 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.; & 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.
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Kim, S.; & Overcash, M. (2000). Allocation procedure in multi–output process – an illustration of ISO 14041. International Journal of Life Cycle Assessment, 5, 221 – 228.
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Laser, M.; & Lynd, R.L., personal communications. May, 2005.
Mann, L.; Tolbert, V.; & 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.
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Wang, M.; Saricks, C.; & Santini, D. (1999). Effects of fuel ethanol use on fuel-cycle energy and greenhouse gas emissions. *ANL/ESD-38). Argonne National Laboratory.
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.
U.S. Department of Agriculture, Economic Research Service (http://www.ers.usda.gov/Data/CostsAndReturns/testpick.htm).
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Wang, M.; Saricks, C.; & Santini, D. (1999). Effects of fuel ethanol use on fuel-cycle energy and greenhouse gas emissions. *ANL/ESD-38). Argonne National Laboratory. |
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