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Keywords:

  • bioenergy;
  • biomass;
  • energy balance;
  • fossil fuels;
  • life-cycle assessment (LCA);
  • renewable energy

Summary

The use of biomass as a raw material for production of fuels and commodity chemicals is attracting increasing attention motivated by the possibility of positive contributions to a sustainable resource supply, enhanced national security, and macroeconomic benefits for rural communities and society at large. Fossil fuel displacement exclusive of product recovery can be estimated for biological processing of biomass in the absence of product-specific information other than the product yield and whether fermentation is aerobic or anaerobic. Based on this observation, a framework is proposed for estimating fossil fuel displacement on a per-unit-product or per-unit-biomass basis. Use of a per-unit-biomass basis offers somewhat different insights as compared to a per-unit-product basis and appears particularly appropriate for consideration of the efficacy of resource or land use. Using the proposed framework, the following feedstock and process factors are shown to be particularly important in determining the extent of fossil fuel displacement via biological processes: feedstock (corn or cellulosic) and, for corn, harvest mode (e.g., with or without stover recovery); biological conversion (aerobic or anaerobic); product yield; and the energy requirements for product recovery. When all of these factors are favorable, as in the case of the cellulosic ethanol scenario examined, significant fossil fuel displacement can be achieved. When all of these factors are unfavorable, as in the case of a scenario involving polyhydroxyalkanoate (PHA) production from corn without stover recovery, no net displacement is achieved. The proposed framework provides a means to screen processes with respect to potential for fossil fuel displacement in the absence of product-specific information, to gain general insights into feedstock and process features important in determining the extent to which fossil displacement is realized, and to rapidly incorporate product-specific information into a preexisting evaluative rubric.