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All these factors argue against the promise of corn ethanol as a solution to the energy problem. “My take,” says Polasky, “is that [ethanol] is only going to be a bit player in terms of energy supplies.” He calculates that even if all the corn planted in the United States were used for ethanol, the biofuel would still displace only 12 percent of gasoline consumption. “If I’m doing this for energy policy, I don’t see the payback,” he says. “If we’re doing this as farm support policy, there may be more merit there. But we’re going to have to go to the next generation of technology to have a significant impact on the energy markets.”

Since the oil crisis of the 1970s, when the price of a barrel of petroleum peaked, chemical and biological engineers have chased after ways to turn the nation’s vast reserves of “cellulosic” material such as wood, agricultural residues, and perennial grasses into ethanol and other biofuels. Last year, citing another of President Bush’s goals–reducing U.S. gasoline consumption by 20 percent in 10 years–the U.S. Department of Energy (DOE) announced up to $385 million in funding for six “biorefinery” projects that will use various technologies to produce ethanol from biomass ranging from wood chips to switchgrass.

According to a 2005 report by the DOE and the U.S. Department of Agriculture, the country has enough available forest and agricultural land to produce 1.3 billion tons of biomass that could go toward biofuels. Beyond providing a vast supply of cheap feedstock, cellulosic biomass could greatly increase the energy and environmental benefits of biofuels. It takes far less energy to grow cellulosic materials than to grow corn, and portions of the biomass can be used to help power the production process. (The sugarcane-based ethanol produced in Brazil also offers improvements over corn-based ethanol, thanks to the crop’s large yields and high sugar content.)

But despite years of research and recent investment in scaling up production processes, no commercial facility yet makes cellulosic ethanol. The economic explanation is simple: it costs far too much to build such a facility. Cellulose, a long-chain polysaccharide that makes up much of the mass of woody plants and crop residues such as cornstalks, is difficult–and thus expensive–to break down.

Several technologies for producing cellulosic ethanol do exist. The cellulose can be heated at high pressure in the presence of oxygen to form synthesis gas, a mixture of carbon monoxide and hydrogen that is readily turned into ethanol and other fuels. Alternatively, industrial enzymes can break the cellulose down into sugars. The sugars then feed fermentation reactors in which microörganisms produce ethanol. But all these processes are still far too expensive to use commercially.

Even advocates of cellulosic ethanol put the capital costs of constructing a manufacturing plant at more than twice those for a corn-based facility, and other estimates range from three times the cost to five. “You can make cellulosic ethanol today, but at a price that is far from perfect,” says Christopher Somerville, a plant biologist at the University of California, Berkeley, who studies how cellulose is formed and used in the cell walls of plants.

“Cellulose has physical and chemical properties that make it difficult to access and difficult to break down,” explains Caltech’s Arnold, who has worked on and off on the biological approach to producing cellulosic ethanol since the 1970s. For one thing, cellulose fibers are held together by a substance called lignin, which is “a bit like asphalt,” Arnold says. Once the lignin is removed, the cellulose can be broken down by enzymes, but they are expensive, and existing enzymes are not ideal for the task.

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Credit: Steve Hebert/Polaris

Tagged: Energy

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