Coal-Derived Fuel Burns Cleaner

DME, dimethyl ether, is normally produced by dehydration of methanol, but DME production from natural gas and from coal derived syngas may open up this clean fuel for broader use, the researchers suggest.

In a study of the emissions produced when burning DME as a substitute for n-butane or propane published recently in Energy & Fuels, researchers found that DME had lower carbon monoxide emissions and the same or lower nitric oxide emissions than either of these commercially available fuels.

"In China and India, propane and butane are used in great quantities for utility purposes such as cooking," says Andre L. Boehman, director of the Penn State Combustion Laboratory and assistant professor of fuel science. "Switching to a fuel that generally has lower emissions and is coal derived could make cleaner fuel available from local resources."

The researchers, who also included Christopher Frye, Penn State graduate student in fuel science and Peter J. A. Tijm of Air Products and Chemicals, Allentown, Pa., found that in a series of tests, carbon monoxide emissions were lower when burning DME than for either butane or propane. They also found that in most cases nitric oxide emissions were lower, but that even in the worst case, nitric oxide emissions were no higher than when burning butane and propane.

"We concluded that in terms of its comparative carbon monoxide and nitric oxide emissions, DME is a viable alternative utility fuel," says Boehman.

With a favorable emission profile, Boehman is now investigating DME as a replacement fuel in diesel engines. While burning DME produces fewer emissions than burning diesel fuel, DME has very poor lubricating properties.

"Diesel engines rely on the lubricating properties of diesel fuel to lubricate the fuel injection system and other components," says Boehman. "DME has no natural lubricating properties and we will need to add a lubricating additive for diesel engine use."

Boehman and his team are planning to test DME-blend fuel in a Defender model, Champion motor coach with an International Corp. chassis and a Navistar engine. The shuttle bus, which seats 22 passengers and is handicapped accessible, was purchased jointly by Penn State's fleet services and Boehman's grants. The motor coach is in daily use as a faculty/staff shuttle bus.

The researchers have begun to determine the baseline characteristics of the motor coach when run on standard diesel fuels. At the same time they are testing a variety of DME fuel blends on a test-bed engine.

According to Boehman, DME would make a poor replacement for gasoline because its octane rating is very low. Octane rating measures a fuel's resistance to spontaneous sparking. However, diesel fuels are cataloged by cetane rating, a measure of a fuel's ease of spontaneous combustion once the proper temperature and pressure are reached. Methanol, an alternative fuel being investigated for automotive spark-ignited internal combustion engines, would make a poor diesel fuel as it has a high octane rating, but only a 5-cetane rating. Diesel fuel has a cetane of 45 and dimethyl ether has a cetane of 55.

Once the baseline and laboratory fuel tests are completed sometime next year, the researchers will adapt the motor coach for use with the DME fuel and will evaluate engine response, emissions and wear under normal usage.

Boehman's research is being funded by the U.S. Department of Energy, the Pennsylvania Department of Environmental Protection and Air Products and Chemicals, Inc.