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Cellulosic ethanol
Cellulosic ethanol

Pluses: Made by breaking down wood chips, farm waste, and nonfood crops like grasses, cellulosic ethanol wouldn't require diverting the use of cropland. Scientists are making progress at breaking down plants' tough cellulose and lignin molecules, the key to turning nonfood biomass into fuel.

Minuses: Still costly and difficult to make, ethanol produced from nonfood plants is more energy intensive than that made from corn and sugar cane. By one estimate, putting all the grassland in the U.S. into fuel production could replace only about 10% of petroleum.

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Last updated April 22 2008: 7:09 AM ET