Biodiesel made from vegetable oil seems to have good green credentials.
Several musicians fill their tour buses with it, and environmental entrepreneurs brew it themselves from recycled kitchen grease.
However, expectations that this renewable fuel will deliver significant reductions in greenhouse gases may be too high.
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"The general belief is that biodiesel offers a huge benefit from a global warming standpoint," said Russell Heinen, vice president of SRI Consulting, which last month released a biofuel report for the chemical industry. "We found that it's not necessarily that great."
Biodiesel and other biofuels such as ethanol release carbon dioxide when burned, but part of this is offset by the carbon dioxide absorbed by the biofuel crop.
However, Eric Johnson, the report's author and editor of the journal Environmental Impact Assessment Review, found the environmental benefits of biofuels depend on what crops are planted and how the land might be used otherwise.
Surprisingly, European farmers currently growing rapeseed for biodiesel could reduce their carbon footprint by more than half if they planted trees and let regular diesel be burned instead.
This is partly because commonly used fertilizers emit nitrous oxide, a greenhouse gas that has nearly 300 times more warming effect than carbon dioxide, which tends to get all the attention.
Using vegetable oil or animal fats for fuel is hardly a new idea. The first diesel car more than a century ago ran on peanut oil.
Current diesel engines can be modified slightly to run on straight vegetable oil, but it's more common to chemically modify the oil to make biodiesel.
Pure biodiesel can gel up in cold temperatures, so it is often blended with ordinary petroleum-based diesel.
Biodiesel is politically attractive as a way to lessen dependence on oil, while also supporting local farmers.
Government incentives and rising petroleum prices have helped the biodiesel industry grow by more than a factor of 20 in the past decade.
The European Union has the largest biodiesel production worldwide with nearly 1.5 billion gallons last year, mostly from rapeseed oil.
The United States comes in second in the groupings, with about 250 million gallons last year, mostly from soybean oil.
Plant forests instead
The United States, the European Union and other countries have mandated that biofuels make up certain fractions of their fuel consumption in the coming years, as part of programs to combat global warming and increase energy independence.
To determine the environmental impact of these policies, Johnson analyzed various biofuel scenarios.
Generally, he found that emissions of nitrous oxide and CO2 from farming practices negated much of the carbon dioxide soaked up by the plants.
This was especially true for rapeseed, but environmental benefits could be reaped from other crops.
"Soy biodiesel and palm-oil biodiesel are generally better than petrodiesel on greenhouse-gas emissions," Johnson told LiveScience. "It is mainly a function of crop yield and fertilizer amount."
Carbon-footprint reductions of as much as 40 percent were possible from soy-derived biodiesel.
However, in most cases that Johnson looked at, planting trees on the farmland and using regular diesel made a larger dent in carbon-dioxide levels than producing and using biodiesel.
The results do not agree with other studies.
"The overwhelming evidence is that most biofuels offer a greenhouse-gas savings," said Greg Archer, director of the Low Carbon Vehicle Partnership, a U.K. group that promotes eco-friendly cars and fuels.
The amount of savings depends on how the biofuel is produced.
Smart practices — such as limiting fertilizer amount, heating with biomass instead of coal and generating electricity from waste heat — can make practically any biofuel less polluting than petroleum, Archer said.
One way to be sure to reduce the carbon footprint is to reuse vegetable oil or animal fat to make biodiesel.
Johnson estimated that waste-derived biodiesel releases 60 percent less greenhouse gas in its lifetime than normal diesel.
Recycling kitchen grease into biodiesel has become popular in small pockets around the country, where "the cars can smell like French fries when they drive down the street," Heinen said.
The U.S. Environmental Protection Agency estimates that restaurants generate some 3 billion gallons of waste cooking oil each year.
"The biggest problem is how dispersed it is at all the McDonald's and the like," Heinen said. "You need a concentrated supply to have an economy of scale."
One solution might be to locate a biodiesel factory next to a meat-packing plant or a potato chip manufacturer.
"Then at least you would have all the fat you need right next door," Heinen said.
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