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.
Schobert and his colleagues make the fuel using refined coal oil, which is a byproduct of coke manufacture; the byproduct is mixed at an oil refinery with a product of crude oil called light cycle oil. This mix is then hydrogenated using equipment that already exists at refineries, and then it’s distilled into various products – mostly diesel fuel and jet fuel (about 40 percent of each), as well as some gasoline and heating oil.
Other potential benefits of the coal-based fuel: it can replace the three or four different jet fuels used by the military for aircraft and missiles, and the same fuel can be used in diesel engines if those engines are modified slightly. The fuel could also be used without modification in high-temperature stationary fuel cells for generating electricity, Schobert says.
But significant hurdles remain before the fuel can see widespread use. So far, only 500 gallons of it have been produced, far too little to assess production costs, Schobert says. Nevertheless, he suspects that the coal-based fuel could compete with other fuels.
One cost-related problem, however, is that supply of refined coal oil used in the current process is limited, and prices of it would likely go up sharply with increased demand. “Frankly, we’d probably soak up the entire byproduct market, and the folks that sell those byproduct chemicals are not dopes,” says Schobert; “they know what they could do to the price under those circumstances.” Schobert is now working on other methods of producing the fuel using oil refinery products.
Before the economics of the process can be evaluated, the fuel will need a significant production run – probably around 50,000 barrels, Schobert estimates, which could cost tens of millions of dollars. He hopes to raise money for the trial run from the private sector. To this end he’s organizing a “summit” this spring to bring together parties such as engine makers and oil companies. Schobert also hopes that airlines will be interested: “They don’t need the superior thermal stability that this fuel has, but what they do need is a reliable source of fuel that’s at a pretty steady price level.”
If the money for such a run does come together, one important step still remains. While they’ve tested the fuel in a stationary jet engine, eventually, “Somebody’s got to put this in an airplane and fly it,” Schobert says.
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