Gasoline or Ethanol: Which Is More Polluting?

by Marlo Lewis on August 7, 2015

in Blog

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Adding ethanol to gasoline makes it cleaner and reduces air pollution, right? So biofuel lobbyists would have us believe. However, a study published in Proceedings of the National Academy of Sciences (PNAS) comes to the opposite conclusion.

Lead author Jason Hill of the University of Minnesota Department of Bioproducts and Biosystem Engineering summarized key findings of the study at a recent House Science Committee hearing. To determine which is cleaner, gasoline or ethanol, researchers must “compare these fuels over their full life cycle.” Hill explains:

That is, we need to consider the damage caused by producing them in addition to using them. For gasoline, the life cycle includes extracting and refining crude oil, and distributing and combusting the gasoline itself. The life cycle of corn ethanol includes growing and fermenting grain, and distilling, distributing, and combusting the ethanol itself.

Although combustion emissions from ethanol are lower than those of gasoline, production emissions from ethanol are higher. So much so that on a life-cycle basis, ethanol is the larger source of five different air pollutants: primary fine particulate matter (PM2.5), nitrogen oxides (NOx), amonia (NH3), volatile organic compounds (VOCs), and sulfur oxides (SOx).

Ethanol vs Gasoline, Jason Hill, House Science, July 2015









Those results are an additional reason EPA should resist pressure from biofuel lobbyists to increase Renewable Fuel Standard (RFS2) blending targets for 2014-2016. As Hill cautions:

RFS2 will continue to damage air quality as long as it supports corn grain ethanol regardless of how the cellulosic biofuel industry develops. Increasing the efficiency of corn grain ethanol production may lessen its negative health effects, but even dramatic improvements would be unlikely to make it a less damaging alternative to gasoline. Likewise, even ideal development of the cellulosic biofuel industry would likely result in only marginal improvements in the health impacts of transportation.

Hill and colleagues used a “spatially and temporally explicit life cycle model” to “estimate total fuel supply chain air pollutant emissions for scenarios where 10% of US projected vehicle miles traveled in year 2020 are driven in 1 of 11 types of passenger cars,” including automobiles powered by conventional gasoline, corn ethanol, and cellulosic ethanol made from corn stover. The higher-than-gasoline production emissions associated with ethanol are “caused by fertilizer application and nitrification emissions during the spring planting season.”

The figure below shows projected PM2.5 impacts for each of the 11 vehicle/fuel options in 2020.

Ethanol PM2.5 impacts of 11 vehicle & fuel options, Hill et al 2015






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