Soil Greenhouse Gas Emissions in the Life Cycle Assessment of Cellulosic Ethanol From Crop Residue

Concerns about climate change and the increasing cost of fossil fuels have led to interest in the development of renewable biofuel pathways for reducing greenhouse gas (GHG) emissions. The use of corn residue as a potential source of biomass feedstock for cellulosic ethanol production has been favored as cost effective. Previous research has shown that crop residue removal can cause a loss of soil organic carbon (SOC), which potentially is a source of GHG emissions in the form of CO2. Using a life cycle assessment (LCA) approach, this study investigated the impact of corn residue removal on total GHG emissions for several biochemical conversion technologies at two different removal rates (50% & 90%). An inventory of total emissions from energy use in crop production and residue harvest was used with micrometeorological measurements and biomass processing data to determine the total life cycle GHG emissions for cellulosic ethanol. Due to hail damage of the crop in 2010, a modeling approach was used to estimate CO2 fluxes for both removal levels based on 9 years of previous eddy covariance flux data for continuous corn. Model prediction on average was iii within 7% of both the measured change in SOC and the measured CO2 flux from soil using the eddy covariance tower. The average GHG intensity based on five years of change in SOC with the combination of field measurements and the different conversion technologies was 53 g CO2 MJ-1 (45% reduction compared to gasoline) for 50% removal, and 57 g CO2 MJ-1 (40% reduction compared to gasoline) for 90% removal. Emissions of CO2 associated with change in SOC as a result of residue removal were demonstrated to be a large and important contribution to the overall GHG emission intensity of cellulosic ethanol. Compared to gasoline, the assessment results indicate that none of the conversion technologies would meet the 60% GHG emission reduction required for cellulosic ethanol from corn residue, unless soil carbon is better managed.