Sustainability metrics and life cycle assessment for thermochemical conversion of woody biomass to mixed alcohols.

This study quantifies selected sustainability metrics including the life cycle greenhouse gas (GHG) emissions for ethanol production via thermochemical conversion of biomass. Results are based on the process detailed in the National Renewable Energy Laboratory's 2011 conceptual design for mixed alcohols via indirect gasification of woody biomass. The impacts of biomass feedstock moisture and ash contents on the biofuel production efficiency and the life cycle GHG emissions are investigated. Field drying the feedstock from 50 wt% to 30 wt% moisture content lowers the life cycle GHG emissions by more than 13%. Reducing ash content in the feedstock preprocessing step from 7 wt% to 1 wt% decreases the overall GHG emissions by about 7% due to improved biofuel yield. The life cycle GHG emissions for the cases evaluated range from 39.1 to 48.7 g CO2-equivalent (i.e., CO2e) per kilometer driven or 12.4 to 15.5 g CO2e per MJ. For all cases evaluated here, the GHG emissions are reduced by more than 83% compared to the baseline gasoline life cycle GHG emissions. This reduction exceeds the 60% threshold necessary to qualify as a cellulosic biofuel as specified in the U.S. Renewable Fuel Standard under the Energy Independence and Security Act of 2007.