As every pilot and mechanic knows, high temperatures are the bane of any engine. But as engineers strive to design smaller, more powerful turbine engines by adding compressor stages, the compression ratio goes up and so does the temperature of the ram air. At a compression ratio of about 20 to 25, ram-air cooling becomes inefficient. The solution, researchers believe, lies with cooling the air before it goes into the combustor. One way to do this involves adding a heat-exchanger using a heat-sink fluid.
That fluid could be jet fuel, if it remains stable at high enough temperatures. But jet-A and JP8 (its military equivalent), become unstable at about 300 degrees F, meaning that the molecular structure of the fuel begins to decompose. This creates carbon deposits that foul valves, nozzles and other parts inside turbine engines.
In the search for new fuels for engines to power aircraft such as the F-35 joint strike fighter, the Air Force is funding research under its VAATE (versatile, affordable, advanced turbine engines) program. One such project at Penn State University in State College, Pa., involves converting bituminous coal to jet fuel. According to Dr. Harold Schobert, head of the project at Penn State’s Energy Institute, coal-based fuel can absorb significant amounts of heat. “Coal has a molecular structure that has properties necessary for making high-temperature-stable fuel,” he said. “We have produced coal-based jet fuel that has remained stable to 900 degrees Fahrenheit.”
Civilian Use Possible
Although civil use of the fuel is not an aim of the project, Schobert, an organic chemist and professor of fuel science, said the prototype fuel is expected to meet or exceed the military specifications for JP8. “There’s good reason to anticipate that coal-based fuel could be used to replace jet-A,” he said, but cautioned that the fuel is still in development.
A side benefit of coal-based fuel is strategic, Schobert told AIN. “Our country’s dependence on imported oil is at an all-time high,” he said. “Yet there are vast quantities of coal in the U.S.; in fact, we have the third-largest coal reserves in the world. An efficient way to convert coal to jet fuel would provide us with a source of jet fuel for centuries.”
Penn State’s Energy Institute is working on two processes to create “JP900,” so named because of its stability to 900 degrees F. The first is a variation of a standard process used in petroleum refining, but involves bituminous coal. When mixed with decant oil (a petroleum byproduct) and heated, the coal becomes fluid and the liquid portion that distills off is JP900.
The second process combines light-cycle oil, another petroleum byproduct, and coal-derived chemical oil, a byproduct of coke production. After chemists add hydrogen to change the molecular structure and remove sulfur, the mixture becomes distilled. The distillate is again JP900. Penn State researchers believe existing oil refineries could accomplish both processes.
Later this year, Schobert said his team will run JP900 in an Allison T63 turboshaft engine at Wright Patterson AFB in Ohio. Following this, it will perform further testing in other engines designated by the Air Force.
The Air Force Office of Scientific Research and the Department of Energy have provided funding for the project, which began in 1989.