Alternative fuels for cars and trucks are becoming increasingly viable, but there's another area of the transportation sector where they haven't quite taken off: aviation.

Convincing the aircraft industry to start full-scale use of petroleum alternatives won't be easy because of the risks involved with testing new fuels in airplanes. If the switch can be made, though, there are several advantages to using biodiesel over traditional jet fuel, said Robert Dunn, a food and oil researcher at the U.S. Department of Agriculture.

"It's renewable, it can be domestically derived, it's readily biodegradable, it's relatively safe to handle and store �� and it reduces most emissions," Dunn said at the World Congress on Industrial Biotechnology and Bioprocessing held last week in Chicago.

Any concerted effort to decrease global CO2 emissions will have to include the aviation industry, considering its large carbon footprint. The industry consumes 13 percent of the fossil fuels used by the transportation sector worldwide, according to the U.N. Intergovernmental Panel on Climate Change.

And the effects of airplane emissions are much harsher on the atmosphere because of the high altitudes at which they operate, said Max Schauck, chair of the Institute of Air Science at Baylor University in Waco, Texas.

"There's very little exchange with (air near) the ground, so the residence time of the pollutants is much longer and the air is less dense so it's more fragile," Schauck told United Press International. "This means that a substance does much more damage than the same quantity at a lower altitude."

But many factors have made it difficult to develop a workable jet fuel for the industry, including the strict performance standards for jet fuel and the extreme temperatures at which aircraft operate, said Wayne Seames, a chemical engineering professor at the University of North Dakota.

"The annoying thing is that if you're 33,000 feet up, your fuel is going to be solid unless you heat it," said Seames, who currently leads a team of researchers developing biofuels for aviation.

Other factors also create difficulties, such as the low density of many biofuels compared with their petroleum-based counterparts.

"Biodiesel has a lower density than traditional fuels, and in aircraft, especially military aircraft, this is a problem because it reduces" the distance a plane can fly without refueling, Seames said.

By using the newest technologies available, though, a number of these barriers can be overcome, said John Plaza, president of Imperium Renewables, a biodiesel technology company.

"The opportunity exists to make a jet fuel from any feedstock and have it be identical to petroleum," Plaza told UPI.

In February a biofuel created by Imperium powered the first commercial test flight run on bio jet fuel. The biodiesel product created by the company for the flight is molecularly identical to petroleum, eliminating freezing point or density concerns, said Plaza, a former Northwest Airlines pilot.

Oils from coconuts and babassu plants, native to South America, provided the feedstock for the fuel.

While the test flight proved the fuel's ability to work in airplanes, it won't be going commercial any time soon. That's because coconut oil doesn't represent a sustainable resource, although babassu might. But company executives aren't worried about that.

"What we wanted to do (with the test flight) was demonstrate aviation is a huge market for biofuels, and you can make it from existing feedstocks with existing technology," Plaza told UPI. "We're looking at commercialization on a worldwide scale."

Problems with the sustainability of biofuels, evidenced lately by claims that ethanol has contributed to soaring food prices, represent one of the main challenges in trying to use biofuels in the large aviation industry, said Grazia Zanin, research director for Baylor's Institute of Air Science.

"That is the biggest problem," Zanin told UPI. "As it is right now, with what we have available with the biomass base, it would be very hard to �� produce all the jet fuel needed to run all the airplanes in the world."

However, new, non-food feedstock potentials could solve that problem as well, said Terrance Scott, environmental spokesperson for Boeing, the largest manufacturer of commercial jetliners and military aircraft combined.

"We're really focused on next-generation biofuels, so ones that don't compete with food and that don't take a lot of land or water (to grow)," Scott said.

The company is looking at biofuels made from a variety of non-food sources, including halophytes, a saltwater grass; algae; jatropha, a plant that can grow in wastelands; and switchgrass, a North American grass.

The outlook is promising, Scott told UPI.

"You could potentially see (these fuels being used) on a regional basis in probably five to seven years," he said.

Another feedstock option may be leftover animal fats and vegetable oil, according to researchers at North Carolina State University and Diversified Energy Corp. who have developed a technology using fats such as chicken grease, beef tallow and hog lard to produce a biofuel that could be used to power airplanes and other vehicles.

"There's almost 1.3 billion gallons of animal fats produced from rendering animals (in the United States) ever year," said William Roberts, an NCSU professor who helped develop the technology, called Centia fuel.

While this could be converted to more than a billion gallons of fuel, the aviation industry uses 70 billion gallons annually. So although technologies like Centia can help chip away at aviation emissions, in the end, Roberts said, the real answer lies in conservation.

"It's a lot easier to save 20 percent (of the fuel being used) than to make 20 percent more," he told UPI. "But that's not the answer anyone wants to hear."