Submitted to: American Society of Agricultural and Biological Engineers
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/16/2011
Publication Date: 6/10/2011
Citation: Pradhan, A., Shrestha, D.S., Mcaloon, A.J., Yee, W.C., Haas, M.J., Duffield, J.A. 2011. Energy life-cycle assessment of soybean biodiesel revisited. American Society of Agricultural and Biological Engineers. 54:1031-1039. Interpretive Summary: As the nation assesses various routes for the production of renewable energy to replace fossil fuels, an important consideration is the amount of energy a fuel releases relative to the amount of fossil fuel energy required for its production. This ratio of output to input is termed the Fossil Energy Ratio, or FER. The FER for petroleum diesel fuel itself has been previously calculated to be 0.83, meaning that it yields less energy than is required in its production. A 1998 calculation of the FER for biodiesel produced from soybean oil yielded a value of 3.2. The purpose of the present study was to assess the impact on this value of recent changes in agriculture and biodiesel production technologies. By applying contemporary process modeling approaches we estimated fossil energy input requirements for the production of soybeans, their transport to a processing facility, extraction of their oil, its conversion to biodiesel, and transport of the biodiesel to sites of use. Especially due to recent advances in agricultural and chemical process efficiencies it was calculated that the FER for soybean-based biodiesel has risen to 4.5, a substantial increase. An accounting for the energy used in construction of agricultural production machinery and biodiesel production facilities, terms not factored into the original analysis, reduced the FER only slightly to 4.4. As agricultural and chemical-industry technology become increasingly efficient in the future further increases in the FER for soy-based biodiesel production are likely.
Technical Abstract: A life-cycle assessment (LCA) was conducted to quantify the energy flows associated with biodiesel production. A similar study conducted previously (Sheehan et al., Life Cycle Inventory of Biodiesel and Petroleum Diesel for Use in an Urban Bus, Publication NREL/SR-580-24089, National Renewable Energy Laboratory, Golden CO, 1998, www.nrel.gov/docs/legosti/fy98/24089.pdf) calculated that biodiesel produced from soybean oil had a fossil energy ratio (FER, the ratio of the energy output of the final biofuel product to the fossil energy required for its production) of 3.2. The purpose of the present analysis was to update those calculations in light of intervening advances in agricultural and biodiesel production technologies. The assessment modeled an operation with an annual production of 9.8 million gallons of biodiesel, 151,515 tons of soybean meal, 9,000 tons of soybean hulls, and 4,380 tons of crude glycerin. The analysis was comprised of four subsystem energy calculations: feedstock production, feedstock transportation, soybean processing with biodiesel production, and product distribution. For feedstock production, the analysis used updated USDA agricultural inputs and energy use figures. It expanded on the previous report by including the energies required for the production of agricultural machinery and building materials. Recent changes in agricultural practices, such as the widespread adoption of no-till agriculture, new seed varieties, improved fertilizer and pesticide application routines, and of genetically engineered soybeans, were also taken into account. All significant energy inputs were accounted for, including not only fuels for agriculture and transportation but also energy use for the production of fertilizers, pesticides, and other required petrochemical-based inputs. Transportation-related energy requirements were estimated using the previously published Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model. Oil recovery from soybeans was modeled to occur via hexane extraction, the predominant technology for vegetable oil recovery in the United States. Energy savings that have occurred in the soybean crushing/extraction industry as new, more efficient plants replace older ones were accounted for. The model for biodiesel production employed conventional alkaline transesterification technology representative of that used by the contemporary biodiesel industry. A mass-based allocation method was used to account for the energy associated with the meal and glycerin coproducts of biodiesel production. The analysis indicated fossil energy inputs of approximately 27,000, 4,000, 42,000, and 1,000 Btu/gal of biodiesel product for soybean agriculture, soybean transport, crushing and conversion, and biodiesel distribution, respectively. Upon mass-based allocation of energy use to coproducts the respective portions allotted to biodiesel were approximately 4,500, 1000, 19,500 and 1,000, respectively, for a total of 26,000 Btu of input energy per gallon biodiesel. The total energy output of a gallon of biodiesel is approximately 117,000 Btu/gallon, resulting in a calculated FER of 4.5 for biodiesel under this new analysis. This is an increase of about 41% compared to the 1998 analysis. In a further step, secondary energy inputs such as those for the production of farm machinery and production facilities, omitted from the earlier analysis, were taken into consideration. The FER declined, but only slightly: to 4.40. In addition, the 1998 analysis did not consider the use of lime, a prevalent component of soybean agriculture. Here lime use was accounted for, but its impact was slight, lowering the FER by only 0.22 percent. In considering expected future increases in soybean yields it was calculated that the FER will reach 4.69 in 2015, an increase of 4.2%. Further increases can