How green is ethanol as a renewable fuel?
In the U.S. quest for greater energy security, ethanol stands poised to begin displacing gasoline. A new ES&T analysis shows that ethanol’s energy output is greater than the inputs required to make it.
Even as ethanol production is ramping up in the U.S., energy policy analysts are at odds over whether fueling cars with the biologically derived fuel is better for the environment than gasoline. Now, research published today on ES&T’s Research ASAP website (doi 10.1021/es052024h) attempts to put this debate to rest.
Assessing the benefits of ethanol over gasoline is complicated by the fact that this biofuel is produced in a series of steps that themselves require energy, typically in the form of fossil fuels. A wide range of studies have come to different conclusions about whether more energy is saved or wasted in the complete process, which starts with the cultivation of the various crops that can be used to produce the fuel.
By first normalizing and then comparing the data used in 10 of the most prominent studies, Roel Hammerschlag, president of the Institute for Lifecycle Environmental Assessment and author of the ES&T paper, draws a much clearer picture: Producing and burning ethanol are better for the environment than producing and burning gasoline. However, not all ethanol is created equal.
Fuel produced from corn via the more traditional approach may yield only marginal renewable energy returns. But the ethanol obtained from cellulose with a developing technology that uses fibrous materials—such as wood chips, switchgrass, or farm residues—as opposed to corn kernels has a clear advantage over gasoline.
“Unfortunately, there’s no single item that you can pin the differences on, which is one of the reasons why the debate has been so murky,” Hammerschlag says. Widely varying assumptions were made about farming practices. He found a particularly glaring difference in one of the cellulosic studies: The researchers obtained a higher fossil-fuel investment than anyone else because they assumed that coal or natural gas would be used to power the facility converting the cellulose material to a liquid fuel. In practice, though, one of the biggest reasons cellulosic ethanol has a much better energy balance than corn ethanol is that the whole plant can be used, Hammerschlag notes. The cellulose or fermentable component is separated from the lignin or nonfermentable component, which itself has a high energy value. Consequently, the lignin can be burned to power the facility, so no external power source is needed.
Hammerschlag’s results are similar to those of a recent independent publication in Science by a research team led by Alexander Farrell with the University of California, Berkeley. However, the ES&T paper goes further in showing the advantages of cellulosic ethanol over corn ethanol, says Lee Lynd, a biochemical engineer at Dartmouth College. One of Lynd’s own ethanol studies is analyzed in the ES&T paper.
“What you see very clearly from both [the ES&T and Science] studies is that those [ethanol studies] that have found negative returns are decided outliers from a very large and solid set of alternative studies,” says Nathaniel Greene, a senior policy analyst for the environmental group Natural Resources Defense Council (NRDC), which provided funding for Hammerschlag’s study. “We need to move on now and start figuring out how we [can] use this technology and advance it to get as much out of it as possible.”
NRDC and the Union of Concerned Scientists, an independent alliance of scientists and citizens working on environmental solutions, are on record as saying that cellulosic ethanol is at least as attractive as hydrogen as a sustainable transportation fuel in the long term.
Joseph Romm, who is executive director of the Center for Energy and Climate Solutions and also served as the acting assistant secretary of the U.S. Department of Energy under President Clinton, agrees. “The most important thing for people to realize is the environmental impact of not doing anything, which is global warming,” he says. “Corn ethanol isn’t the endgame, [but] we need a fuel to replace gasoline that doesn’t use so much carbon, and improving the process of conversion on the cellulosic side is a critical task at this point.”
Large-scale ethanol production raises other environmental concerns, including water-pollution and soil-quality problems associated with growing energy crops, but by far the biggest are the air-quality issues associated with using ethanol, Greene says. The “low blends” predominantly used today, which contain only 5–10% ethanol and are mixed with regular unleaded gas, have higher evaporative emissions than regular gasoline in warm climates, and this contributes to ozone problems.
“The best way to mitigate that is to use ethanol in high blends,” such as E85 (which contains 85% ethanol), Greene says. “So, from our perspective, the question is how quickly can we make the transition to really using ethanol as an alternative fuel as opposed to just a fuel additive?”
The U.S. EPA’s newly established Renewable Fuel Standard final rule will help. It requires nearly 3% of gasoline sold this year to be produced from renewable fuels, with a doubling by 2012. And in his recent State of the Union address, President Bush promised additional research funding into cutting-edge methods of producing ethanol, especially from cellulosic materials.
