Regional variations in greenhouse gas emissions of biobased products in the United States—corn-based ethanol and soybean oil

Abstract

Regional variations in the environmental impacts of plant biomass production are significant, and the environmental impacts associated with feedstock supply also contribute substantially to the environmental performance of biobased products. Thus, the regional variations in the environmental performance of biobased products are also significant. This study scrutinizes greenhouse gas (GHG) emissions associated with two biobased products (i.e., ethanol and soybean oil) whose feedstocks (i.e., corn and soybean) are produced in different farming locations. We chose 40 counties in Corn Belt States in the United States as biorefinery locations (i.e., corn dry milling, soybean crushing) and farming sites, and estimated cradle-to-gate GHG emissions of ethanol and of soybean oil, respectively. The estimates are based on 1 kg of each biobased product (i.e., ethanol or soybean oil). The system boundary includes biomass production, the biorefinery, and upstream processes. Effects of direct land use change are included in the greenhouse gas analysis and measured as changes in soil organic carbon level, while the effects of indirect land use change are not considered in the baseline calculations. Those indirect effects however are scrutinized in a sensitivity analysis. GHG emissions of corn-based ethanol range from 1.1 to 2.0 kg of CO2 equivalent per kilogram of ethanol, while GHG emissions of soybean oil are 0.4–2.5 kg of CO2 equivalent per kilogram of soybean oil. Thus, the regional variations due to farming locations are significant (by factors of 2–7). The largest GHG emission sources in ethanol production are N2O emissions from soil during corn cultivation and carbon dioxide from burning the natural gas used in corn dry milling. The second largest GHG emission source groups in the ethanol production system are nitrogen fertilizer (8–12%), carbon sequestration by soil (−15–2%), and electricity used in corn dry milling (7–16%). The largest GHG emission sources in soybean oil production are N2O emissions from soil during soybean cultivation (13–57%) and carbon dioxide from burning the natural gas used in soybean crushing (21–47%). The second largest GHG emission source groups in soybean oil production are carbon sequestration by soil (−29–24%), diesel used in soybean cultivation (4–24%), and electricity used in the soybean crushing process (10–21%). The indirect land use changes increase GHG emissions of ethanol by 7–38%, depending on the fraction of forest converted when newly converted croplands maintain crop cultivation for 100 years. Farming sites with higher biomass yields, lower nitrogen fertilizer application rates, and less tillage are favorable to future biorefinery locations in terms of global warming. For existing biorefineries, farmers are encouraged to apply a site-specific optimal nitrogen fertilizer application rate, to convert to no-tillage practices and also to adopt winter cover practices whenever possible to reduce the GHG emissions of their biobased products. Current practices for estimating the effects of indirect land use changes suffer from large uncertainties. More research and consensus about system boundaries and allocation issues are needed to reduce uncertainties related to the effects of indirect land use changes.