9-09: Ethanologens vs acetogens: Comparing environmental impacts of two fermentation pathways to produce bioethanol

Monday, April 29, 2013
Exhibit Hall
Erik L. Budsberg, Rick Gustafson, Renata Bura and Jordan Crawford, School of Environmental and Forest Sciences, University of Washington, Seattle, WA
Life Cycle Assessments (LCA) for two lignocellulosic bioethanol production pathways using hybrid poplar as a feedstock are developed and compared.  Both processes use a SO2 catalyzed steam pretreatment followed by enzymatic hydrolysis.  The processes differ in the fermentation methods.  In the first pathway, an ethanologen is used to produce ethanol.  In the second pathway, sugars are fermented with an acetogen to produce acetic acid.  Acetic acid is then reduced using hydrogen to produce ethanol.  The critical difference between the two processes is that the acetic acid pathway has a much higher product yield but requires hydrogen to reduce the acid to an alcohol. Natural gas reforming is assumed to be the source of hydrogen. In both processes lignin is assumed to be recovered and burned onsite to produce process steam and electricity. 

In the LCA, the bioconversion processes are modeled with rigorous Aspen simulations.  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 using TRACI impact indicators for global warming potential, acidification, and eutrophication to determine environmental performance.  The amount of water used for each process is also assessed.  LCA results for the two pathways are compared. Comparisons are also made with fuel produced using conventional petroleum feedstock and conversion technologies.