Just because the prefix “bio-” is tacked on the word “fuel” doesn't necessarily mean it creates less pollution.

Conventional fossil fuels sometimes result in less overall carbon dioxide emissions than biofuels, points out a Massachusetts Institute of Technology study recently published online in the journal Environmental Science and Technology.

Many airlines, like Virgin Atlantic, Continental and Lufthansa, have started using blends of conventional jet fuel with fuel produced from plants. The biofuels can help the companies cut costs, but are they really better for the environment?

Only if the plants they come from are grown in ecologically sensitive ways, says James Hileman, principal research engineer in the department of aeronautics and astronautics at MIT.

"What we found was that technologies that look very promising could also result in high emissions, if done improperly," reports Hileman in an MIT press release.

"You can't simply say a biofuel is good or bad — it depends on how it's produced and processed, and that's part of the debate that hasn't been brought forward,” says Hileman.

Hileman and MIT graduate students Russell Stratton and Hsin Min Wong examined the carbon dioxide produced during the life cycle of 14 fuel sources, including conventional petroleum-based jet fuel and "drop-in" biofuels: alternatives that can directly replace conventional fuels with little or no change to existing infrastructure or vehicles.

Growing crops for jet fuel production often entails clearing forests or using machines that also burn fuel. Drilling for oil takes energy, too. Then the raw materials from both sources have to be transported and processed.

"All those processes require energy," Hileman says, "and that ends up in the release of carbon dioxide.

The biggest problem with biofuels is where the crops are grown. Oil palm plantations in particular are devastating if rain forests are cleared to make way for palms.

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The MIT study found that fuel derived from palm plantations on recently deforested lands result in 55 times more carbon dioxide emissions than fuel from plantations grown on previously cleared land.

Biofuel crops grown without an eye to their overall impact can produce 10 times more carbon dioxide than conventional fuels.

People don't often think of coal-to-liquid fuel production as a green option, remarks Hileman. But "severe cases of land-use change could make coal-to-liquid fuels look green," he says.

On the other hand, the study points out many forms of biofuel that are more environmentally sensitive and produce less pollution.

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Many of these truly “green” biofuels have common characteristics. They can grow on marginal lands and don't compete with food for prime fertile land, and they also create useful by-products.

Hileman notes that many of these by-products can further reduce the overall carbon dioxide release from the biofuels.

For example, converting jatropha, a shrub that can grow in poor soils and dry areas, to biofuel also yields solid biomass: For every kilogram (2.2 pounds) of jatropha oil produced, 0.8 kilograms (1.8 pounds) of meal, 1.1 kilograms (2.4 pounds) of shells and 1.7 kilograms (3.7 pounds) of husks are created. These by-products can be used to improve soil, prevent erosion or feed animals, or be burned for heat or electricity production.

All in all, the transition to biofuels is a complicated subject. Hileman notes that this research is only one lens through which biofuels can be viewed. The costs involved and crop yields are important considerations as well.

IMAGE 1: Aviation Boatswain's Mate 3rd Class Michael Mott, from Minot, N.D., checks a JP-5 jet fuel sample (Wikimedia Commons).

IMAGE 2: Palm oil plantation in Cigudeg, Indonesia (Wikimedia Commons).

IMAGE 3: A former gas station selling firewood. (Wikimedia Commons).