(54b) Optimizing the Sustainability of Ethanol Conversion at the Biorefinery

In response to recent government mandates, the US ethanol industry is expected to increase production by over 24 billion gallons per year (bgy) by 2022. To meet this volume will require an unprecedented expansion in ethanol production capacity, necessitating significant investment in new infrastructure. Despite the government mandates for increased biofuel production, many questions remain with regard to the overall sustainability of such a large increase in the production and use of biofuels. Much of the debate on sustainability has centered on the resource consumption and environmental emissions that occur at the biorefinery. Across the ethanol supply chain, conversion has been shown to be one of the most energy intensive steps; therefore, improvements at the biorefinery could have significant implications to the overall sustainability of the biofuel supply chain. Salient metrics of sustainability are needed to guide the ethanol industry towards sustainable expansion. In response to this need, NREL is currently using process systems engineering and life cycle assessment to investigate novel improvements to reduce resource consumption, wastes and environmental emissions at the biorefinery and to identify specific sustainability drivers, opportunities, and metrics. Key areas under investigation include greenhouse gas (GHG) emissions, water consumption, and refinery waste in both the biochemical conversion of corn stover to ethanol and the thermochemical gasification of mixed woods to ethanol and mixed alcohols . Specifically, improvements in minimizing GHG emissions come from increasing yields in both the biochemical and gasification platforms. The thermochemical conversion process currently as designed uses less water than that of a corn dry mill. The biochemical conversion process as documented in NREL's design report uses more water than a corn dry mill, but DOE and NREL are currently working on reducing water use through energy integration , novel separation schemes, and closed loop processing. Solid waste in the biochemical process model has been reduced by moving from lime to ammonia conditioning. In the thermochemical process model, NREL continues to work on reforming tars to reduce waste as well as improvements in fuel catalysts and elimination of water quench steps for tar removal. Additionally, resource consumption and environmental emissions at the conversion facility are being tracked and quantified using a life cycle assessment approach. A summary of NREL's ongoing research aimed at improving the sustainability of the biorefining process as well as potential sustainability metrics will be presented.