A sensitivity analysis of the lifecycle and global land use change greenhouse gas emissions of U.S. corn ethanol fuel.

摘要

Recent studies argue that corn ethanol fuel is becoming more sustainable and has less direct lifecycle greenhouse gas (GHG) emissions than previously estimated (Wang, Wu and Huo, 2007; Kim and Dale, 2008; Liska et al., 2008). In the U.S. Energy Independence and Security Act of 2007 (U.S. EISA) corn ethanol fuel produced from new facilities are required to have at least 20% less GHG emissions than conventional gasoline. However, it also specifies that the GHG emissions from global land use changes should be considered when assessing corn ethanol fuel emissions. Global land use change emissions from increased U.S. corn ethanol production therefore play a crucial role in determining whether this renewable fuel can meet the U.S. EISA requirement.;This study examines the range of overall GHG emissions of U.S. corn ethanol fuel when the direct lifecycle and global land use change emissions are estimated using various data and assumptions. The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation (GREET) model by Wang (1996) is used to determine the direct lifecycle emissions of corn ethanol. These emissions are calculated using recent data on energy and chemical input usage during corn farming from Liska et al. (2008) and using new energy use and yield assumptions during ethanol production from selected studies (Wu, 2008; Liska et al., 2008; Christianson, 2008; Perrin, Fretes and Sesmero, 2009).;The global land use change emissions are generated using the Land Use Change Emissions (LUCE) module. These emissions are calculated using the land use carbon emission factors and the global land use change data. The global land use change data is based on the simulation results of a special version of the Global Trade Analysis Project (GTAP) model (Hertel, 1997) by Taheripour, Hertel, and Tyner (2008) for several assumed increases in U.S. ethanol production and trade restrictions scenarios. The land use carbon emissions factors are the estimated GHG emissions incurred for each year of ethanol consumption when a hectare of forest or grassland undergo cropland conversion. These factors are calculated using the Woods Hole and the Intergovernmental Panel on Climate Change (IPCC) land carbon data and using different assumptions on soil and vegetation carbon losses during cropland conversion, wood product decay and duration of ethanol consumption.;The results show that the direct lifecycle GHG emissions of corn ethanol fuel can exceed the 20% GHG reduction requirement in the U.S. EISA given the new data and assumptions during corn farming and ethanol production. However, the overall GHG emissions of corn ethanol are considerably higher due to global land use change emissions. Global land use change emissions rise with greater soil and vegetation carbon loss assumptions and when wood product decay is considered. A longer duration of ethanol consumption reduces the global land use change emissions since these emissions are distributed over a longer time period. The findings of this study indicate that it is highly uncertain if corn ethanol fuel has less GHG emissions than conventional gasoline when global land use change emissions are considered. It is possible to argue that corn ethanol fuel has more GHG emissions than conventional gasoline by increasing the soil and vegetation carbon loss assumptions, shortening the duration of ethanol consumption and using the estimates of corn ethanol direct lifecycle emissions based on previous corn farming and ethanol production data. Likewise, it is possible to conclude that corn ethanol can satisfy the 20% GHG reduction requirement in the U.S. EISA by adopting conservative assumptions on soil and vegetation carbon losses, lengthening the duration of ethanol consumption and adopting corn ethanol direct lifecycle emissions estimated using recent corn farming and ethanol production data.