Friday, March 31, 2006

Coal-Powered Jets

A new process using jet fuel made from coal could reduce oil dependence, and improve fuel performance in advanced aircraft.

By Kevin Bullis

Researchers have powered a turboshaft jet engine, the type used to drive helicopter rotors, with a coal-based fuel that could eventually replace military and commercial jet fuels, says Harold Schobert, director of the Energy Institute at Pennsylvania State University. The successful development of the coal-based fuel, which was described this week at the American Chemical Society meeting in Atlanta, could also have uses in diesel engines and fuel cells, Schobert says.

Coal-powered aircraft are not new -- Germany used fuels derived from coal to power planes in World War II. But the high cost of building production plants to turn coal into liquid fuel has prevented the technology's widespread use. Now Schobert and colleagues have developed a way to make jet fuel containing as much as 75 percent coal products using existing oil refineries, eliminating the need to build costly new plants -- and potentially making coal-derived fuel an economically viable alternative to oil.

"In the current formulation this would displace half the petroleum, which is very close to the fraction of petroleum that we import. We've actually tested, at a smaller scale, 75 percent replacement," with success, says Schobert.

Coal, the cheapest of fossil fuels, which also has the steadiest prices, is abundant in the United States. John Grasser, a U.S. Department of Energy spokesperson, cites estimates that the amount of recoverable coal in the country is enough for 250-300 years. "You hear a lot about renewables, and certainly renewables have a part to play in making us self sufficient," says Grasser. "But they're not going to have an impact on petroleum coming in. You're going to have to take something like coal, which we have in huge quantities here, and turn it into a petroleum component."

In addition to reducing dependence on oil, the new fuel might, in fact, also have benefits for advanced aircraft. Today's high-performance military aircraft generate a lot of heat, which can damage hydraulics and electronics, Schobert says. As a result, engineers design these planes to use the onboard fuel as a heat sink. As fuels absorb heat, however, they can begin to break down, which can lead to carbon deposits that clog fuel lines and nozzles. Future advanced aircraft could generate even more heat -- too much for today's fuels to handle. Schobert and colleagues methodically tested about 50 compounds to discover thermally stable ones -- and the best, they found, could readily be made from coal. Their fuel can handle temperatures around 600 degrees Fahrenheit (315 degrees Celsius), higher that today's fuels.