Greenhouse gas emissions reduction from the light-duty transportation fleet is
urgent and should address both electric and conventional powertrain
technologies. Internal combustion engines will continue to be employed for
vehicle propulsion and fleet turnover is slow, encouraging reduction of carbon
content in gasoline. Currently ethanol, a renewable fuel, is blended at the 10%
level into petroleum to produce finished market gasoline. Ethanol enables a less
carbon-intensive petroleum blendstock composition, providing for additional
reduction, but this is often overlooked in studies. Carbon intensity, as a ratio
of CO2 mass to heat released upon combustion, is a measure of
well-to-wheels greenhouse gas production. The well-to-wheels carbon intensity of
ethanol does not include its chemical carbon content because it arises from a
renewable source, but does consider all upstream farming, production, and
transportation carbon impacts. The well-to-wheels carbon intensity of the
petroleum fraction includes the chemically bound carbon, as well as production
and transportation impact. Carbon intensity modeling results for ethanol vary
widely, primarily due to differences in land-use change assessment. The GREET
model has gained wide acceptance and provides a present-day carbon intensity for
pure ethanol that is 43% lower than for petroleum gasoline. Ethanol exhibits a
high blending octane number so that the petroleum component has a lower octane
rating than required for purely petroleum gasoline. Fuel trends and modeling
suggest that a 10% (by volume) ethanol addition enables a 9% reduction of
aromatics, which have a high carbon intensity. If the carbon reduction benefits
of the aromatic reduction are assigned to the agency of the ethanol, the
blending carbon intensity of ethanol is 56% lower than for petroleum gasoline.
Increase in ethanol blending therefore offers substantial immediate climate
change reduction.
