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Pluses: Made from vegetable oils like soy and canola and animal fat, biodiesel provides 90% more energy than is required to produce it. Compared with petroleum-based diesel fuel, biodiesel is estimated to cut greenhouse-gas emissions 40% to 80%.

Minuses: Like corn ethanol, biodiesel's production from food crops boosts "agflation." European demand has been blamed for inducing farmers in Southeast Asia to burn and replant the rain forest with palm plantations, which has released large amounts of greenhouse gases. Production is limited at the moment_ just 250 million gallons in 2006.

NEXT: Sugar-cane ethanol
Last updated April 22 2008: 7:09 AM ET