As the US moves towards lowering greenhouse gas (GHG) emissions from its transportation fleet, states may evaluate policies, such as low carbon fuel standards (LCFSs), to lower GHG emissions from transportation fuels. Policies designed to reduce the average fuel carbon intensity (AFCI) can achieve significant GHG emissions reductions in the transportation sector when combined with strategies to increase vehicle efficiency and lower the rate of growth of vehicle activity. Previous analyses have indicated that without policies such as LCFSs, GHG emissions from transportation would not be significantly reduced even if economy-wide GHG cap and trade policies were enacted.

...Fuels are often evaluated by lifecycle analyses (LCAs) using average national data, but more accurate and specific methods and data are available to perform LCAs for fuels on a state and individual refinery basis. Localized differences in electrical generation, crude oil sources, biomass production practices, refinery efficiencies, and other parameters may cause the fuel carbon intensity (FCI) of specific fuel pathways to differ significantly from the national average. Development of FCIs using state- or refinery-specific data could aid in the development of policies that can lower transportation emissions of states in accordance with their unique profiles.

—Boies et al.

The study examined the FCI of transportation fuels on a lifecycle basis within a specific state, Minnesota, and compared the results to FCIs using national averages. Using data compiled from 18 refineries over an 11-year period, the team found found find that ethanol production is highly variable, resulting in a 42% difference between carbon intensities.

Historical data suggested that lower biofuel FCIs are possible through incremental improvements in refining efficiency and the use of biomass for processing heat. Stochastic modeling of the corn ethanol FCI showed that gains in certainty due to knowledge of specific refinery inputs are overwhelmed by uncertainty in parameters external to the refiner, including impacts of fertilization and land use change.

The authors then incorporated the LCA results into multiple policy scenarios to demonstrate the effect of policy configurations on the use of alternative fuels. These results provided a contrast between volumetric mandates—i.e., renewable fuel standards—and LCFSs.

Primary conclusions of the study with respect to LCAs are:

1. LCA should be done at as local a level as possible.

2. Uncertainty analysis is important to determine the risk of fuels’ environmental impacts.

3. Refiners are improving efficiency.

4. The efficiency improvements of refiners are being dwarfed by uncertainties such as land-use change and NO₂ emissions from fertilizers.

Policies such as low carbon fuel standards that seek to reduce fuel GHG emissions on a state or regional scale require that fuels be evaluated on an equivalent basis. The work presented here demonstrates that fuels within a state can have different FCI even when produced using similar pathways. The variation in corn ethanol FCIs was found to be largely a result in differences in the source of process heat, energy efficiency of production and ethanol yield. The historic trends in fuel production show that it is likely that reductions in corn ethanol FCI can be made through incremental improvements if given the proper incentives. These improvements, however, are insignificant when compared to the current range of uncertainties beyond the refiners’ control (e.g. emissions associated with LUC [land use change] and N₂O from fertilizers) and thus highlight the need for further study of critical para- meters. While it is unlikely that policies will be crafted to include uncertainty analysis, these results highlight that FCI point estimates alone do not guarantee accurate accounting of possible GHG reductions from different fuels and will thus hamper effective choices of fuel alternatives.

—Boies et al.

With respect to policy modeling, the authors concluded that:

• Carbon-intensity based policies (such as LCFS) outperform existing volume mandates (such as RFS) in carbon reductions.

• Specific policy configurations matter—i.e., not every LCFS is the same. Some might be optimized for conventional oil, some for cellulosic ethanol.

Ultimately, the rate at which alternative fuels are produced and decarbonized will largely be driven by policies such as an LCFS. This work demonstrates how different structures of an LCFS can affect the levels of biofuel production and their resulting emissions.

—Boies et al.

Resources

• Adam M. Boies, Dane McFarlane, Steven Taff, Winthrop F. Watts, David B. Kittelson (2011) Implications of local lifecycle analyses and low carbon fuel standard design on gasohol transportation fuels, Energy Policy, Volume 39, Issue 11, Pages 7191-7201, doi: 10.1016/j.enpol.2011.08.040