A Life Cycle Assessment of Biodiesel Produced from Waste Vegetable Oil

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2019
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Swarthmore College. Dept. of Engineering
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en
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Biofuels have received considerable attention as a more sustainable solution for heating applications. Used vegetable oil, normally considered a waste product, has been suggested as a possible candidate. Herein we perform a life cycle assessment to determine the environmental impact of using waste vegetable oil to produce biodiesel. We present two cradle to fuel models. One includes the following unit processes: farming, soybean transport. soy oil processing, transport to distributor, transport to restaurant, cooking oil usage, WVO transport, WVO refining, and transesterification. The other process considers the “cradle” as starting after cooking oil usage, so only considers the last 3 steps of the process. For soybean farming, national historical data for yields, energy required for machinery, fertilizers (nitrogen, phosphorous and potassium), herbicides, pesticides and nitrous oxide production are considered. In soy oil refining, steam production using natural gas and electricity for machinery are considered inputs. Preprocessing, extraction using hexane and post processing are considered. In order to determine a mass balance for the cooking operation, oil carryout and waste oil removal are estimated. During waste oil processing, oil is filtered and water removed. Biodiesel is produced from the refined waste oil through transesterification, which considered inputs of methanol and sodium methoxide as well as natural gas and electricity as energy inputs. Data from GREET is used to compute global warming potential (GWP) and energy consumption in terms of cumulative energy demand (CED). The two methods of mass-allocation and displacement-allocation are applied to the soy meal produced in refining, oil utilized for cooking, and glycerol produced in transesterification. Results are discussed with emphasis on improving sustainability. A comparison is made to traditional fuels, e.g., petroleum diesel and gasoline, as well as biodiesel made from fresh oil. The results of this paper show that the production of biodiesel from WVO, when its life cycle starts at the farming process, has a higher CED than other similar fuels, while its and GWP is higher than other biofuels but lower than petroleum fuels. When the life cycle started after the cooking process, the CED and GWP of biodiesel from WVO was among the lowest of any of the fuels compared, including biofuels. The study should provide useful information on the sustainability of using waste cooking oil as a a source oil for biodiesel production.
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