Some airlines have been effective in reducing greenhouse gas emissions. "At Northwest, our greenhouse gas emissions have gone down 25 percent since 2000 and about 5 percent less than 1990," says Ken Hylander, Northwest's senior vice president of safety and engineering. "If Northwest was a country, we would be Kyoto [Protocol on reducing greenhouse gas emissions] compliant."
But emissions from the aviation industry as a whole continue to climb. According to the EPA, from 1990 to 2005 greenhouse emissions from military aircraft slid by 50 percent but those from commercial carriers rose by 16 percent, largely due to growth in the number of carriers.
Efficiency alone—even in the form of aircraft with improved engines and designs such as the Boeing 787, expected to deliver a 20 percent improvement in fuel efficiency over existing big airplanes—is not the answer. "A low-CO2 fuel will help us to address that remaining portion of the pie," says David Daggett, technology leader for energy and emissions at Boeing. "That's why we're interested in biofuels specifically."
One such biofuel—ethanol—is already being used to power a heavily employed commercial fleet: piston-engine propeller crop dusters. Max Shauck, chair of the Baylor Institute for Air Science (who flew an ethanol-powered prop plane at air shows in the 1980s), has converted at least 1,000 such aircraft in Brazil, a country that has weaned itself from foreign oil by embracing ethanol domestically produced from sugarcane.
In addition to being easier on the engine, ethanol costs one quarter to one half as much as the aviation gas typically used in such propeller planes. Ethanol decreases the number of hours or distance such an aircraft can fly, however, due to its lower energy density, but "it develops more power and it's a greenhouse gas–neutral fuel," Shauck says. "There's plenty of ethanol produced in the world to power all the piston-engine aircraft."
The Federal Aviation Administration (FAA) is conducting tests but has yet to certify ethanol as a fuel for piston-engine planes in the U.S., says Lourdes Maurice, chief scientist and technical advisor to the FAA's Office of Environment and Energy. Regardless, ethanol's low energy density makes it unsuitable for jet-turbine engines. "Clearly we can't use ethanol," CAAFI's Altman says. "That's a blessing. We don't want to compete with food crops."
Diesellike fuel derived from plant oils might avoid that problem as well as supply similar greenhouse gas reduction (depending on how the plants are cultivated). Already, a Czechoslovakian L-29 jet—specially built in the 1960s by the Czech military to run on alternative fuels—flew for 37 minutes and reached an altitude of 17,000 feet (5,180 meters) powered entirely by reformulated canola oil. "Would you rather buy your oil from the Middle East," asks BioJet 1 copilot Doug Rodante, president of Green Fuels International (a company that promotes alternative fuels), "or the Midwest?"
But biodiesel solidifies into a gel at the cold temperatures found at high altitude, a fatal flaw for any aircraft fuel. The Czech jet has fuel heaters to get around this problem, and similar solutions could be engineered into other jet engines, argues physicist Rudi Wiedemann, president and CEO of Biodiesel Solutions, Inc., in Sparks, Nev., the flight's fuel provider.
Or the biodiesel itself can be further refined to ensure that it doesn't solidify until at least –40 degrees Celsius (–40 degrees Fahrenheit), the current standard for petroleum-derived jet fuel. UOP, a Honeywell Company, has developed such a "green diesel" by heating vegetable and animal oils to add hydrogen atoms to the long hydrocarbon chains, under the aegis of DARPA. In addition, its "ecofining" process adds kinks in the chains to prevent them from easily stacking—or gelling—at cold temperatures, producing a diesellike fuel with as much as twice the combustion quality of the petroleum-derived variety.