CAMBRIDGE, MA — Airline companies like Virgin and Continental have been testing biofuel alternatives to jet fuel to reduce their dependence on oil and limit greenhouse gas emissions. In some cases, though, it's possible for biofuels to have a larger carbon footprint if the land they came from had been drastically converted.

Researchers at the Massachusetts Institute of Technology looked at the life cycle GHG emissions related to 14 conventional fuels and biofuels, taking into account different land use change scenarios, and recently published their findings on Environmental Science and Technology's website. 

For the most part, biofuels come out on top. But a few bring with them bigger footprints. Palm oil that comes from forests or rainforests that were clear-cut, and soy oil from former rainforests can cause up to 10 times more carbon emissions than jet fuels.

The researchers note that all fuels have a range of possible life cycle emissions, but on the low end, all other biofuels are lower than conventional fuels. Some, like fuels from switchgrass and salicornia (as well as palm oil that came from land that had not been converted), have the lowest carbon footprints of all, coming nowhere near the emissions of normal fuels.

Others lower their footprints by creating byproducts that can be used elsewhere. When jatropha oil, a popular choice so far by airlines, is produced, it leaves behind shells, husks and meal, which can be used as animal feed and fertilizer, or burned to produce energy.

Air New Zealand has tried out a 50-50 mix of jatropha fuel and conventional oil, while Continental tested a combination of jatropha, algae and regular jet fuel. Japan Airlines has also trialed jatropha in a mix with camelina and algae. Algae has potential to put out fairly low GHG emissions as well, and has tested by Virgin Airlines. 

"The most significant challenge is not in developing viable alternative fuels that could reduce aviation's GHG emissions — the  technology exists," says the report [PDF], published by MIT Department of Aeronautics and Astronautics principal research engineer James Hileman, "Rather, the challenge lies in developing and commercializing the large scale production of next generation of biomass feedstocks that could be grown in a sustainable manner."

What that means for companies is that they can't just make blanket statements that their use of biofuels is inherently a greener choice, but they need to look closely at their supply chain, and find out where the feedstock for the biofuels comes from, and what had been done to the land to grow it. 

_{Airplane - CC license by Joshua Davis}