Life Cycle Environmental Impacts of Selected U.S. Ethanol Production and Use Pathways in 2022

Abstract

Projected life cycle greenhouse gas (GHG) emissions and net energy value (NEV) of high-ethanol blend fuel (E85) used to propel a passenger car in the United States are evaluated using attributional life cycle assessment. Input data represent national-average conditions projected to 2022 for ethanol produced from corn grain, corn stover, wheat straw, switchgrass, and forest residues. Three conversion technologies are assessed: advanced dry mill (corn grain), biochemical (switchgrass, corn stover, wheat straw), and thermochemical (forest residues). A reference case is compared against results from Monte Carlo uncertainty analysis. For this case, one kilometer traveled on E85 from the feedstock-to-ethanol pathways evaluated has 43%-57% lower GHG emissions than a car operated on conventional U.S. gasoline (base year 2005). Differences in NEV cluster by conversion technology rather than by feedstock. The reference case estimates of GHG and NEV skew to the tails of the estimated frequency distributions. Though not as optimistic as the reference case, the projected median GHG and NEV for all feedstock-to-E85 pathways evaluated offer significant improvement over conventional U.S. gasoline. Sensitivity analysis suggests that inputs to the feedstock production phase are the most influential parameters for GHG and NEV. Results from this study can be used to help focus research and development efforts.

Exploring impacts of process technology development and regional factors on life cycle greenhouse gas emissions of corn stover ethanol

“…The life cycle GHG emissions in the current study, based on the 2011 NREL process design, are lower than those reported by Hsu et al (2010) for the 2002 NREL process design (Table 6). One key process design difference arises with stover pre-processing: the current study does not include drying or densification, resulting in overall pre-processing electricity consumption of 42 kWh/tonne of feedstock.…”

“…System expansion is the most common approach used in previous studies (e.g., Sheehan et al 2004;Kim et al 2009;Spatari and MacLean 2010;Hsu et al 2010). In the system expansion approach, only emissions related to the additional activities required to harvest stover are assigned to the stover.…”

“…There are a number of LCAs of the corn stover to ethanol process (e.g., Sheehan et al, 2004, Luo et al, 2009, Hsu et al, 2010, Spatari and MacLean, 2010, Kaufman et al, 2010. These studies all reported that replacing gasoline with ethanol produced from corn stover would reduce GHG emissions; however, the percentage reductions (compared to gasoline) reported in the studies vary between 58% and 106%.…”

“…Spatari and MacLean (2010) studied both National Renewable Energy Laboratory (NREL) (Aden et al, 2002) and Michigan State University conversion process design models, but most other studies relied only on technical process information for conversion of stover to ethanol reported by NREL (Aden et al, 2002). Hsu et al(2010), Spatari and MacLean (2010) and Kaufman et al (2010) used US-average data for corn yield, fertilizer application rate and electricity grid intensity. Sheehan et al (2004) determined the aforelisted data based on a specific location in Iowa.…”

“…We additionally provide a comparison of NREL 's 2011 and 2002 process designs, and compare our life cycle results with those of Hsu et al (2010), a study based on NREL's 2002 design. To our knowledge this is the first life cycle study comparing the 2002 and 2011 NREL designs.…”