Blending Carbon Intensity for Ethanol in Gasoline

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.