Coal: The cool fuel for future jets

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.

Tina Hesman Saey is the senior staff writer and reports on molecular biology. She has a Ph.D. in molecular genetics from Washington University in St. Louis and a master’s degree in science journalism from Boston University.

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