Say the words “coal-powered engine,” and images of men shoveling black rock into the fiery
belly of a steam engine come to mind. However, liquid fuel made from coal instead of oil may
shoot the next generation of supersonic jets across the sky.
If flight speeds are to increase, jets will require new fuels that don’t fall apart chemically in
engines that become extremely hot, says fuel chemist John M. Andrésen of the Energy Institute
at Pennsylvania State University in State College.
Currently, the F-15 Eagle jet fighter can reach speeds two and a half times the speed of
sound. The U.S. Air Force is working to build a jet that will fly at eight to nine times the speed
of sound, Andrésen said last week at the American Chemical Society’s spring national meeting
in San Francisco. The engines in such future supersonic jets could get hotter than 450C.
Today’s petroleum-based jet fuels can handle the 300C temperatures of a normal jet
engine, but they tend to “crack” when the going gets really hot, Andrésen says. At high temperatures,
these fuels break down into solid carbon waste called coke that can plug up fuel systems
and cause catastrophic engine failure.
“Before we can put Luke Skywalker in the cockpit, we have to make sure the Force is with
him,” Andrésen says. The Penn State scientists are creating coal-derived fuels to deliver the
punch needed to get new jets safely up to speed.
Using reactors that simulate conditions inside a jet engine, the researchers heated chemicals
modeling petroleum-derived or coal-derived jet fuels. The coal fuel, decahydronaphthalene,
stood up to temperatures as high as 500C. That’s about 50C higher than
the temperature that reduced the petroleum fuel, n-tetradecane, to coke, says Andrésen.
Coal’s secret to keeping its cool is its structure, Andrésen says. The carbon molecules in
coal-derived fuels form rings that join into chainlike molecules called cycloalkanes, while
petroleum fuels have mainly linear molecules. Cycloalkane chains are much harder to break
apart than petroleum’s shoestringlike molecules, Andrésen says. Some coal-derived jet fuels
may be able to withstand temperatures up to 800C, he speculates.
The next step is to test the model fuels in actual engines, the researchers say.
The new jet fuels will not only have to withstand engine heat but also serve as a coolant for
the jet’s mechanical and electrical systems, says Harold H. Schobert, the director of the coaljet-fuel project at Penn State. Because fast planes must be light, engineers strip them down to
bare essentials. “You need something that can soak the heat up, and about the only thing
you’ve got on the aircraft is the fuel,” says Schobert.
So far, methods aren’t available for making large amounts of coal-derived fuels cheaply. “It’s
all a question of economics,” says fuel scientist Gerald P. Huffman of the University of
Kentucky in Lexington.
To combat the cost, Anne E. Fickinger of Penn State and her team have converted a mixture
of petroleum and coal into a fuel that contains heat-resistant cycloalkanes.
