Transportation's Role in Reducing U.S. Greenhouse Gas Emissions ...

Transportation’s Role in Reducing U.S. Greenhouse Gas Emissions: Volume 2
The proportion of GHG use associated with vehicle operation versus fuels production and
transport can vary significantly by the type of fuel. For example, hydrogen fuel cells and
electric vehicles have no operating emissions since there is no fuel combustion in the
vehicle.
Summary of Impacts
Ethanol benefits vary substantially depending upon the feedstock used to produce the fuel
and the production pathway. For instance, using low carbon energy sources to process
the feedstock into fuel produces lower greenhouse gas emissions than high carbon energy
sources. Cellulosic ethanol, such as ethanol from switchgrass, shows much greater GHG
benefits than first generation biofuels. Cellulosic ethanol is produced from the structural
material that comprises much of the mass of plants. As EPA’s renewable fuel standard
rulemaking was ongoing at the time of writing, this report to Congress does not include
analysis of biofuels, at EPA’s request. Readers are instead referred to EPA’s renewable
fuels website, which includes draft life cycle analysis of the greenhouse gas impacts of
renewable fuels: http://www.epa.gov/OMS/renewablefuels/.
Biodiesel provides the potential for GHG reductions in heavy-duty vehicles, and a blend
with conventional diesel of up to 20 percent biodiesel (B20) is compatible with existing
engines and infrastructure. Biodiesel can be used in light and heavy duty road vehicles
and marine vessels, although further research is required on material compatibility issues
for rail applications. For more information, readers are referred to EPA’s renewable fuels
website, which includes draft life cycle analysis of the greenhouse gas impacts of
renewable fuels: http://www.epa.gov/OMS/renewablefuels/.
Natural gas is a clean-burning fossil fuel that has a well-established pipeline system for
distribution and provides moderate life-cycle GHG reductions relative to light-duty
gasoline vehicles (about 15 percent compared to gasoline vehicles, roughly equivalent to
diesel vehicle benefits). A GHG reduction of 7 to 13 mmt per year (0.3 to 0.6 percent of
transportation emissions in 2030) is estimated assuming a 3 to 6 percent LDV market share
for compressed natural gas (CNG) in 2030. The relatively low potential benefit is not
constrained so much by natural gas supplies as by the ability to move natural gas away
from other end uses such as electricity production and residential heating. Natural gas
would not provide significant GHG benefits in heavy-duty vehicles (HDV) or the off-road
sector, since it would primarily displace diesel fuel rather than gasoline.
CNG vehicles have a proven performance and safety record, although range penalties can
be significant. CNG also is one of the few alternative fuels available that offers potentially
lower fuel costs relative to conventional fuels, with the break-even point estimated to be
about $2.50 per gallon of gasoline (pretax). Preferred applications for CNG vehicles are
fleets with high mileage and centralized refueling. Primary barriers to implementation
include the limited number of CNG vehicle offerings, limited refueling infrastructure, and
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