(384b) Environmental Life Cycle Evaluation of Biofuels Produced Via Biomass Fast Pyrolysis

The Energy Independence and Security Act (EISA) of 2007 in the United States mandates the production and use of 36 billion gallons of biofuels by 2022 comprised of 16 billion gallons of cellulosic biofuels along with 15 billion gallons of corn ethanol and 5 billion gallons of other advanced biofuels. Cellulosic and advanced biofuels requirement can be met with biomass derived hydrocarbon (HC) biofuels. HC biofuels are biomass derived fuels that can act as drop-in replacement for petroleum based gasoline, diesel, and jet fuels. Biomass derived HC biofuels are attractive because of their higher energy density and their compatibility with existing fuel infrastructure. Holistic evaluation of emerging HC biofuel pathways via systems analysis that considers the resource consumption, emissions, and their impact across the entire life cycle is critical to ensure transition to a sustainable bioeconomy. Several biofuel pathways for converting biomass feedstocks into HC biofuels are currently under development. There are primarily three major pathways for converting biomass into transportation fuels: chemical, biochemical, and thermochemical. This work focuses on the life cycle environmental evaluation of HC biofuels produced via biomass fast pyrolysis. Pyrolysis converts biomass into an intermediate product called pyrolysis oil which can be catalytically converted to yield high octane hydrocarbon fuels in the gasoline range.

We have developed original life cycle inventory (LCI) modules for the production of liquid transportation biofuels via fast pyrolysis of corn stover. The process is modeled based on available literature, best available engineering information, and complemented with material and energy balance calculations. Two process scenarios are analyzed; the first scenario considers natural gas derived hydrogen for bio-oil upgrading and hydrotreating, while the second scenario considers separating a fraction of the bio-oil to generate hydrogen onsite for bio-oil upgrading. LCI for the biofuel production pathway is developed by combining the process specific data for biomass fast pyrolysis with life cycle data for individual raw materials and energy inputs. The LCI is utilized to perform life cycle energy and greenhouse gas (GHG) emissions analysis. Preliminary results of the traditional and multiscale hybrid life cycle assessment (LCA) indicate that hydrogen required for hydrotreating and upgrading bio-oil into finished fuel constitutes roughly 60% of the net GHG emissions. It is further observed that hydrogen production via a portion of the produced bio-oil reduces the net life cycle GHG emissions by almost 50% compared to the natural gas derived hydrogen scenario. It is observed that the choice of co-product allocation method significantly influences the LCA results. Traditional LCA is enhanced by utilizing the thermodynamic methods at multiple scales. Full fuel cycle energy analysis is performed for the emerging HC biofuel pathways along with an estimation of a variety of metrics such as energy return on investment (EROI) and thermodynamic efficiency. This is then supplemented by exergy analysis of the pyrolysis pathway at multiple scales. While the LCA results help identify the life cycle phases with the highest environmental impact, exergy analysis helps in identifying sources of thermodynamic inefficiencies for these emerging HC biofuel processes. A variety of hierarchical sustainability metrics spanning the detailed level data to aggregate metrics based on thermodynamics will be presented. This will include metrics such as EROI based on traditional energy analysis. Unlike energy analysis, exergy analysis permitted the calculation of unique metrics such as exergetic return on investment (ExROI) and renewability index for the selected fuels. The implications of the results in formulating effective renewable energy policies will also be described.