Environmental life cycle impacts of cellulosic ethanol in the Southern U.S. produced from loblolly pine, eucalyptus, unmanaged hardwoods, forest residues, and switchgrass using a thermochemical conversion pathway

Abstract

The cradle-to-grave environmental impacts of thermochemical ethanol from loblolly pine, eucalyptus, unmanaged hardwoods, forest residues, and switchgrass biomass feedstocks were determined and compared to gasoline. The Tool for the Reduction and Assessment of Chemical and Other Impacts (TRACI) method was implemented in SimaPro 7.3 to calculate midpoint environmental impacts. Two normalization value sets were used and weighting was performed to produce a single environmental score.Greenhouse gas (GHG) emission reductions of cellulosic ethanol as compared to gasoline were 65%–77%, depending on the biomass feedstock, qualifying these biofuels as cellulosic ethanol under the Renewable Fuel Standards (RFS2). Effects of direct land-use change were significant (~18%) and could increase the GHG emissions for switchgrass derived ethanol above the federal GHG reduction thresholds for cellulosic ethanol. The production and use of cellulosic ethanol reduced fossil fuel consumption by between 95% and 97% and by 81% for forest and switchgrass derived ethanol, respectively. Cellulosic ethanol, however, did not reduce all environmental impact categories (e.g., eutrophication, ozone depletion respiratory effects, acidification, and smog) compared to gasoline. The fuel scenario ranking from lowest impact to highest impact consistently remained the same for GHG emissions, fossil fuel use, and the two single weight score analysis methods and was, in ascending order, forest residues with no forest establishment burdens, forest residues with forest establishment burdens, natural hardwood, pine, switchgrass, and finally gasoline. The GHG emission reductions from the use of cellulosic ethanol at the renewable fuel standards mandated production volume of 16 billion gallons of cellulosic ethanol per year by 2020 would result in 9–10 billion metric tonnes of GHG emissions avoided.