Coal Source Of Jet Fuel For Next Generation Aircraft

"On a pilot scale, we have produced thermally stable coal-based jet fuel," says Dr. Harold H. Schobert, professor of fuel science and director of Penn State's Energy Institute. "This coal-based fuel can absorb significant amounts of heat and remain stable to 900 degrees Fahrenheit."

The new fuel will not decompose at high temperatures to create the deposits of carbon, which foul valves, nozzles and other engine parts. The fuel will be provisionally designated jet propulsion 900 or JP900 because of this high temperature stability. The researchers are designing the fuel for the new generation of high performance engines in aircraft such as the F35 joint strike fighter and the U.S. Air Forces' VAATE program – versatile, affordable, advanced turbine engines. However, according to the researchers, it may be possible to use this fuel in conventional jet engines in current aircraft.

The front portion of a jet engine is an air compressor and the new engines compress air at higher and higher pressures generating larger amounts of heat. The outside air is not sufficient as a cooling medium, so the designers use the fuel itself as a heat sink, so high temperature stability is necessary.

"While power generation will remain the mainstay of coal use for many decades, coal does supply a molecular structure that has properties necessary for making high-temperature stable fuel," says Schobert.

Schobert; Suchada Butnark, former graduate student in fuel science; and Leslie R. Rudnick, senior scientist at the Energy Institute, worked on two processes to create JP900 from coal-based materials. One method relies on bituminous coal becoming fluid when heated. The researchers mixed bituminous coal with decant oil, a byproduct of petroleum refining, at normal pressures. When heated, the mixture becomes fluid and the liquid portion distills off and is collected as JP900. The remaining solid is coke, a valuable byproduct for making anodes for aluminum smelting or in making graphite.

"This process is a variant of a standard process used in petroleum refining," says Schobert. "We would really just need a mixer for the two components and then the process could be done in normal refinery operations."

The second process uses light cycle oil, another petroleum byproduct, and coal-derived refined chemical oil, a byproduct of the coke industry. The researchers mix the two components and add hydrogen. When distilled, jet fuel comes off as a distillate.

The Penn State researchers believe that they can carry out both processes in existing refineries. They plan in the next year to test the fuel in a jet engine at Wright Patterson Air Force base. Currently, the researchers are producing JP900 in 55-gallon barrel lots, but they hope in the future to test manufacturing with a run at United Refining in Warren, Pa.

The researchers are also working with the Air Force to develop an official specification for JP900. "Without a specification, no one will put this fuel in an engine," says Schobert.

One potential benefit with manufacturing these fuels in existing refineries is that small amounts of the leftover components will feed into various portions of the petroleum stream. The lighter portions will go to the pool of chemicals that make gasoline and the heavier ones go to the diesel or fuel oil streams.

"The inclusion of coal-based compound in the petroleum steam will probably be beneficial in making gasoline and probably will not make any difference at all in the fuel oil stream," says Schobert. "What we do not know is how it will affect the diesel stream."

In addition to its high temperature properties, JP900 has a 10-degree Fahrenheit lower cloud point – the temperature at which a cloud forms over a liquid. This is a better cold weather fuel than either the Jet A or JP8 currently in use.

These coal-derived fuels also have no ash and very low sulfur. Refined chemical oil, derived from coal, has already had the ash removed. In the decant oil process, the coal would need to be pre-cleaned but would also produce a low-ash coke byproduct.

When it comes to coal, sulfur is often the most troublesome pollutant, but these processes can be as low sulfur as three parts per million, depending on the original sulfur content of the coal and the amount of hydrogen used. For higher sulfur coal, more hydrogen will allow fuels that are still low sulfur.

"We do not have much doubt now that we can do this," says Schobert. "We have a lot more to do and it will be expensive, but there is not much doubt that it will work."

The U.S. Air Force Office of Scientific Research funded this work. The U.S. Department of Energy is also funding some of the ongoing research.