Life Cycle Assessment of Biofuels Produced from Short Rotation Woody Crops

Abstract

In an effort to find a substitute for petroleum based liquid transportation fuels many countries are turning to biofuels. The newest form of these biofuels, also known as the second generation or cellulosic biofuels, have received considerable attention. Short rotation woody crops (SRWC) such as willow and poplar have been proposed as possible sources of biomass to produce these biofuels via biochemical conversion. Before moving to commercial scale, the impacts these biofuels could place on environment must be investigated. In this thesis two research projects to assess environmental performance of SRWC biofuels using Life Cycle Assessment (LCA) are presented. In the first project an LCA of ethanol production via bioconversion of willow biomass crop feedstock is investigated. Willow crop data are used to assess feedstock production impacts. The bioconversion process is modeled with an Aspen simulation that predicts an overall conversion yield of 310 liters of ethanol per tonne of feedstock (74 gal per US short ton). Vehicle combustion impacts are assessed using greenhouse gases, regulated emissions, and energy use in transportation (GREET) model. The impacts of bioconversion produced ethanol are compared with those of gasoline on an equivalent energy basis. Results of the LCA show that the life-cycle global warming potential of ethanol is slightly negative. Carbon emissions from ethanol production and use are balanced by carbon absorption in the growing willow feedstock and the displacement of fossil fuel produced electricity with renewable electricity produced in the bioconversion process. The fossil fuel input required for producing 1 MJ of energy from ethanol is 141 percent less than that from gasoline. More water is needed, however, to produce 1 MJ of ethanol fuel than 1 MJ of gasoline. The life-cycle water use for ethanol is 169 percent greater than that for gasoline. The largest contributors to water use are the conversion process itself and the production of chemicals and materials used in the process, such as enzymes and sulfuric acid. In the second project, LCA for two lignocellulosic bioethanol production pathways are simulated using hybrid poplar as a feedstock are developed and compared. Both processes use a dilute acid pretreatment followed by enzymatic hydrolysis. The processes differ in the fermentation process. In the first pathway an ethanologen is used to produce ethanol. The second pathway is fermented with an acetogen to produce acetic acid. Acetic acid undergoes hydrogenation to produce ethanol. Both bioconversion pathways are modeled in ASPEN-plus simulations. In both processes lignin is recovered and burned onsite to produce electricity. The critical difference between the two processes is that the acetic acid pathway has a higher product yield but requires hydrogen for the process. Steam methane reforming is assumed to be the source of hydrogen. Greenhouse gases, regulated emissions, and energy use in transportation (GREET) is used to model combustion of ethanol from each scenario in a flex fuel vehicle. All necessary chemicals, transportation, and processes required by each production pathway are included within the LCAs. Each pathway is assessed to determine the global warming potential (GWP), fossil fuel use, and freshwater use. Compared to gasoline the ethanologen pathway has a GWP that is 97 percent lower, uses 97 percent less fossil fuels, and 180 percent more water. The acetogen pathway has a GWP 53 percent lower than gasoline, reduces fossil fuel use by 55 percent, and increases water use by 81 percent. In regards to the GWP and fossil fuel use the ethanologen pathway achieves larger reductions compared to gasoline. However, the acetogen pathway will produce more ethanol per unit of land and this may play a crucial role in choosing a lignocellulosic ethanol production method if land is a limited resource.