Photograph courtesy USDA-ARS via PNAS
Published January 8, 2008
Ethanol made from a prairie grass shows promise as a viable fuel that could be much more environmentally friendly and energy-efficient than corn ethanol, a new study says.
Ethanol is often touted as a cleaner-burning gasoline alternative that lessens dependence on oil. (Get the basics on greenhouse gases and global warming.)
But a key criticism of the biofuel is that large amounts of fossil fuels are required to farm and refine it.
Switchgrass ethanol, though, can yield 540 percent more energy than is required to produce it, the new study says.
Part of the reason switchgrass ethanol is more energy efficient is that the whole plant is used. Corn ethanol, by contrast, is made only from kernels.
In addition, producing and burning switchgrass ethanol releases 94 percent less greenhouse gas than burning gasoline does, the researchers found.
Corn ethanol production and use emits 22 percent less greenhouse gas than gasoline, according to the October 2007 issue of National Geographic magazine.
In any form, ethanol is far from being a silver bullet for environmental and energy woes, however.
A UN study published last year, for example, found that the booming liquid-biofuel industry could spawn deforestation, deplete soil nutrients, and undermine food security.
The new study, published today in the Proceedings of the National Academy of Sciences, highlights some of the ways that switchgrass ethanol may help defuse these concerns.
For example, unlike corn—the most common ethanol source in the U.S.—switchgrass shouldn't threaten food availability.
The grass wouldn't likely compete for the same farmland as food crops and therefore may not be likely to drive up food prices. Switchgrass can flourish on marginal lands unsuited for food crops yet yield as much energy on the same amount of acreage and for much less fossil fuel input, said Kenneth Vogel, a geneticist with the U.S. Department of Agriculture in Lincoln, Nebraska, and a co-author of the study.
"In other words, it takes less energy to produce ethanol via switchgrass than corn," he said.
The so-called net energy calculation, he noted, includes factors such as the diesel burned to harvest the crop and the gas burned to transport materials.
"We even took into account the energy required to produce the seed that was used to plant the crop," he said. "That was something that had not been done before."
Vogel and colleagues based their work on a previous study led by Daniel Kammen, who directs the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley.
Kammen's results have been cited as evidence in favor of ethanol from switchgrass and other cellulose-rich plants.
He praised the new finding—based on a five-year study of switchgrass grown on ten farm sites in the U.S. Midwest—as a key step in bioenergy research.
"Their result is very important—namely that yields can be higher and inputs lower than had previously been expected," he commented via email.
"This translates to a much greener biofuel in the end."
But David Pimentel, a professor of ecology and agriculture at Cornell University, is skeptical.
He questioned the energy inputs for refining the ethanol.
The process requires the use of acids or enzymes to separate the starches and sugars in the plant from lignin, a substance that gives support to cellulosic plants, he noted. The lignin is typically discarded as waste.
The new study, though, assumes the use of experimental refining technologies that reportedly separate lignin easily and then use the by-product to power the refineries.
The process would therefore require no fossil fuels, which are used to power many of the refineries that convert corn grain to ethanol.
But such biorefinery technologies are expensive and not as energy efficient as touted, according to Pimentel.
"It's the reason why there's not a commercial plant in the world making ethanol from cellulosic material," he said.
UC Berkeley's Kammen, however, believes that one of the experimental technologies currently under development is "very, very likely to work."
(Related: "Ethanol Plant 'Brews' Grass Into Gas" [May 16, 2006].)
Even if one of the refining technologies is proven commercially viable, Cornell's Pimentel doubts the biofuels industry will ever dent U.S. dependence on oil.
For example, in 2007 the U.S. produced 3 billion gallons (23 billion liters) of ethanol from corn, which amounted to one percent of U.S. oil consumption, Pimentel said.
"That's obviously not making us oil independent," he said.
Nor, he said, is a biofuel industry boosted by cellulosic plants likely to improve the situation, as the new study suggests.
The study cites earlier research that found cellulosic ethanol could potentially displace 30 percent of current U.S. petroleum consumption.
Doing so, Pimentel said, would require using 65 percent of all the crops, grasses, and trees harvested each year in the U.S. just for ethanol.
According to study co-author Vogel, it is too early to know what percent of the U.S. energy mix could come from switchgrass, but the potential is significant and would benefit the environment and economy.
The study also says that switchgrass breeds will continue to improve—as will techniques for converting the grass into fuel.
Vogel also touts advantages that go beyond energy savings.
"You can get the conservation benefits [from growing switchgrass to prevent soil erosion], you can also get an energy crop, and you can also get something that can be used to bring income to a rural area," Vogel said.
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