Energy and Environmental Contributions of Corn-Ethanol

Previous analyses of the net energy yield of corn-ethanol systems have estimated a relatively low efficiency of 1.2:1 (energy output:input) and a 13% reduction in GHG emissions (Farrell 2006). These studies were based on aggregate average crop production data and average biorefinery efficiencies to estimate the performance of the standard USA corn grain ethanol production system. In contrast, we analyzed the potential energy efficiency of the most advanced corn-ethanol systems that now represent the majority of the USA ethanol production capacity, which comes from plants built within the past 2-3 years. Net energy yield of corn-ethanol systems were estimated based on the methods of Farrell et al. (2006) substituting data from long-term field experiments (rainfed and irrigated) with progressive crop and soil management practices, in conjunction with updated biorefinery energy efficiencies, and theoretical technological improvements in crop nitrogen use-efficiency (+10% grain yield per unit N fertilizer), maize genetics for high-starch (7275%), biorefinery engineering (closed-loop systems reduce life-cycle energy needs by ~55%), and fermentation efficiency (91-97%). In addition, we evaluated a closed-loop facility in which coproduct distiller’s grains are not dried (wet distiller’s grains) and fed to cattle in a feedlot adjacent to the ethanol plant, where the associated livestock manure and urine is used for anaerobic digestion to produce methane and replace natural gas in the ethanol plant. All of the above improvements