Ethanol and Greenhouse Gas Emissions
Ethanol and Greenhouse Gas Emissions [U.S. Energy Information Administration] Because the carbon in biogenic material is part of the natural carbon cycle, using ethanol in place of gasoline has the potential to reduce greenhouse gas (GHG) emissions. The nature of the impacts could vary greatly, however, depending on the fuels, feedstocks, and processes used to produce the ethanol.
For this report, Argonne National Laboratory produced a life-cycle (“well to wheels”) comparison of GHG emissions for conventional motor gasoline and ethanol per gallon of fuel consumed, on a Btu equivalent basis.
As shown in the figure left, there is substantial variation in the potential GHG savings for ethanol as compared with motor gasoline. The analysis, based on the near future (2010), compared an outcome based on the current industry average with what could be technically feasible for 2010.
Key inputs for the analysis included: corn yield (bushels per acre); nitrogen fertilizer application rate (pounds per acre); nitrogen fertilizer production (Btu per pound); corn ethanol conversion rate (gallons per bushel); ethanol conversion process (Btu per gallon); total energy use (Btu per gallon); and coproduct energy credits (Btu per gallon).
Among the simulations performed, the smallest savings in GHG emissions when ethanol is used are 7 percent (for an ethanol plant using coal as the input fuel, corn as the energy crop feedstock, and a dry mill production process). The comparison based on the projected industry average for ethanol production in 2010b shows savings in GHG emissions of about 18 percent. When a dry mill process is assumed with 100 percent natural gas as the input fuel and corn as the energy crop, the potential savings are about 38 percent.
The higher GHG emissions savings are estimated to occur when the input fuel is renewable and the energy crop is cellulosic rather than corn. With a biogas fuel input and switchgrass as the energy crop, the potential savings are estimated at about 87 percent; with corn stover as the energy crop, the savings are estimated to be more than 90 percent.
The intent of this analysis was not to weigh in on a particular position with regard to the feasibility of the scenarios examined. It is clear, however, that input assumptions are significant in any examination of the potential for GHG emissions savings from the use of ethanol as a transportation fuel. The analysis examined neither economic feasibility nor issues of scale-up to meet a targeted market share, and the future technologies and crop inputs assumed in the analysis remain untested on a national scale.
aThe industry average in 2010 is projected to be 30 percent wet and 70 percent dry process, with an input fuel mix of 72 percent natural gas, 18 percent coal, and 10 percent electricity for a dry mill plant and a fuel mix of 60 percent natural gas and 40 percent coal for a wet mill plant.