Life Cycle Greenhouse Gas Emissions and Water Footprint of Next Generation Cellulosic Ethanol.

Federal legislation has set into motion a trajectory of increased domestic renewable fuel production, with an increasingly larger share coming from cellulosic sources each year through the year 2022. Displacing a significant share of petroleum-based transportation fuels with renewable biomass-based fuels is not without risk. Questions have been raised regarding the marginal environmental costs and potential benefits of planting and harvesting enough biomass to produce billions of gallons of ethanol annually. This study used life cycle assessment to quantify the cradle-to-grave greenhouse gas emissions and water footprint of eight renewable feedstock-to-ethanol pathways. Feedstocks and conversion pathways evaluated in this study were: corn grain via dry mill conversion, corn stover via simultaneous saccarification and fermentation (SSF), wheat straw via SSF, switchgrass via SSF, forage sorghum via SSF, miscanthus via SSF, forest residues via indirect gasification and mixed alcohol synthesis (IGMAS), and short-rotation willow via IGMAS. Feedstock supply chain and end-use modules were developed in SimaPro v.7.1 using the EcoInvent v2.0 database; all processes from field preparation to vehicle endues were evaluated. Additionally, all embodied energy and material flows were included. Green and blue water footprint was evaluated for each feedstock using Stella v 9.1.3 dynamic modeling software. The green water footprint was calculated using the modified Penman-Monteith equation. Blue water foot print was calculated as the difference between the green water footprint and the mean regional precipitation. Results from this study including a rigorous sensitivity and uncertainty analysis will be presented.