Cellulose dreams: the search for new means and materials for making ethanol

Science News,  August 25, 2007  by Corinna Wu

Tags: Department of Energy, gasoline, NETWORKING, researcher

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At the third center, the Joint Bioenergy Institute in Berkeley, Calif., chief executive officer Jay Keasling has a different point of view on alternatives to gasoline and diesel fuel. He says he's not fond of ethanol because of its low energy density--the modest amount of energy that 1 kilogram of the fuel yields.

He says that his focus is on generating "biogasoline" from plant matter. His group is attempting to engineer microbes to turn cellulose into hydrocarbons that can go directly into a gas tank without the need for mixing in petroleum.

ETHANOL, ENVIRONMENT The success of cellulosic ethanol as a fuel depends on whether experimental processing methods can be scaled up in an economically sound way. Because it's risky to try these technologies on a large scale, DOE is sharing the cost with industry, according to Davison. The six biorefineries funded in February represent a "great survey of the technology available now," he says, adding that he's "confident that more than half of these biorefineries will work." The pilot plants are being built by six companies across the United States. Some will focus on feedstocks grown specifically for energy, such as switchgrass, while others will use agricultural waste.

Still, many questions remain about the long-term practicality and environmental value of large-scale ethanol production. "Can it be produced efficiently and economically?" Davison wonders. "Can we produce enough plant matter to make a substantial impact? Then, can we do it sustainably?" He's confident about answers to the first and second questions, he says, but not so confident about the third.

A sustainable crop, he says, can ideally be grown on the same land for 100 years with no inputs--that is, no fertilizer or irrigation. By contrast, even collecting existing waste biomass for ethanol may not be as environmentally benign as it seems. Farmers often plow cornstalks back into their fields to keep the soil fertile, so "if we immediately start taking cornstalks off the field, we need to worry about how the soil is going to be affected by the loss of carbon or other nutrients," says Donohue.

Then there's a much bigger question: Is ethanol worth pursuing at all? The answer depends on how it's produced, says Daniel Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley. Large-scale farming of feedstocks would involve heavy machinery that burns energy and produces greenhouse gases. Transporting those feedstocks to a refinery and converting them into ethanol would also require energy.

Kammen and his colleagues have developed a model--published in the Jan. 27, 2006 Science and modified since--that takes these factors into account. They find that ethanol from corn requires 95 percent less petroleum to produce than gasoline does but cuts greenhouse-gas emissions by only about 18 percent.

Cellulosic ethanol, by contrast, cuts greenhouse-gas emissions by more than 90 percent. That's mainly because producers of corn ethanol burn fossil fuels to heat fermentation tanks, while producers of cellulosic ethanol burn the lignin from their feedstocks.