Clean electric power from dirty coal.

• Position: Energy - Brief Article

As the nation scrambles to boost electric power generation capacity, researchers at the U.S. Department of Energy's Ames (Iowa) Laboratory may hold the key that would allow some of those power plants to burn high-sulfur, dirty coal cleanly. A thin metal filter material developed at the lab would overcome the final barrier to commercial application of new clean-burning, coal-fired electric generation technology, resulting in lower generating costs and cleaner air.

"The technology to burn dirty coal cleanly has existed for some time," notes Iver Anderson, a senior metallurgist with Ames Laboratory's Metallurgy and Ceramics Program. "Demonstration plants have proven that pressurized-fluidized bed combustion and integrated gasification combined cycles are highly efficient, low-emission power plant concepts. The high pressure and temperature volatilize or burn off most of the pollutants, even those in the exhaust gases," reducing the potential for acid rain and other pollution related problems.

However, there is a catch, quite literally, with these systems. Even though the combustion is more complete, the flue gases contain fine particles of fly ash. High in sulfides, chlorides, and sodium compounds, these particles pose an abrasive and corrosive threat to the turbines that drive a power plant's generators, as well as to air quality. To prevent the particles from reaching the turbine blades (and the atmosphere), the hot gas is passed through clusters, or banks, of cylindrical "candle" filters. These three-inch-diameter filter tubes are about four feet long and made from a ceramic material that can trap fly ash particles as small as one micron (0.000039 of an inch).

As more and more particles collect inside the tube-shaped filters, the amount of air passing through decreases. To keep each tube operating efficiently, the accumulated fly ash is periodically knocked off by an internal blast of compressed air, a process called back flushing. Since the filters' operating temperature is approximately 850[degrees]C, even the sudden change in temperature caused by the compressed air can crack the fragile ceramic material.

"Ceramic filters do a good job of standing up to the heat and the nasty oxidizing-sulfidizing environment created by the gases," Anderson says, "but they're very delicate. Ceramics crack easily, and if even a single candle filter breaks, the filtration ability of the whole bank is lost. So, these plants must have several banks of...