For release: July 16, 2002
Contacts:
Iver Anderson, Metallurgy and Ceramics, (515) 294-9791
Kerry Gibson, Public Affairs, (515) 294-1405
CLEAN ELECTRIC POWER FROM
DIRTY COAL
Metallic filter holds key to clean-burning, coal-fired power
generation
AMES, Iowa - As Iowa and other parts of the nation scramble to boost electric power generation capacity, researchers at the U.S. Department of Energy's Ames Laboratory may hold the key that would allow some of those power plants to cleanly burn high-sulfur, dirty coal. A thin metal filter material developed at Ames Laboratory would overcome the final barrier to commercial application of new clean-burning, coal-fired electric generation technology, resulting in lower generating costs, cleaner air and a potential economic boom for Iowa.
"The technology to burn dirty coal cleanly has existed for some time," said 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 high temperature volatilize or burn off most of the pollutants, even those in the exhaust gases," drastically reducing the potential for acid rain and other pollution related problems.
But there's 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 these particles from reaching the turbine blades (and the atmosphere), the hot gas is passed through clusters, or banks, of cylindrical "candle" filters. These 3-inch-diameter filter tubes are about 4 feet long and currently made from a ceramic material that can trap fly ash particles as small as one micron.
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 about 850° Celsius, even the abrupt 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 said, "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 filters.
"Power companies are in the business of generating power and making money, not constantly changing filters," Anderson explained. "You want a filter assembly that is rugged enough and has a long enough life that you can essentially forget about it. It's the last big hurdle to seeing this technology take off."
To find those properties, Anderson's research team looked at developing rugged metal filters from nickel-, cobalt-, and iron-based "superalloys" developed for the aerospace industry. The researchers selected a nickel-based alloy that maintains its strength at high temperatures and is unaffected by thermal shock, but more importantly, develops a protective scale when it oxidizes.
"The nickel-chromium-aluminum-iron alloy we chose contains a sufficient amount of aluminum to form a tough, protective film of aluminum oxide," Anderson said. "Once an aluminum-oxide layer forms, it also prevents further oxidation. It's why structural aluminum can be left unpainted without rusting away."
While ceramic filters need to be thick for strength, a superalloy metal filter may be quite thin, giving it an airflow efficiency advantage. To create these thin, permeable sheets of metal, Anderson uses a process called tap-densified loose powder sintering.
He starts by converting high-purity molten superalloy into a fine powder using a high-pressure gas atomization system. As the hot metal passes through a nozzle, a high-pressure jet of nitrogen gas breaks up the liquid superalloy into millions of tiny metal spheres. The resulting powder is sorted, by screening, and spread out as a thin layer (.5 millimeters) in a shallow "cookie sheet," then heated in a vacuum furnace. This "sintering" bonds the particles together, forming strong, smooth joints between the spheres, but leaving air gaps as well.
Tests show the material experiences only a moderate drop in yield strength going from room temperature to operating temperature (850° C). It's also about six times stronger at operating temperature than an iron-aluminide material being developed by another research laboratory.
Given those encouraging results, the researchers tried a series of bend radius tests to see how well the metal could be formed. According to Anderson, the material was ductile enough to enable it to be formed into corrugated tubes, an important feature not only for strength, but for dramatically increasing the amount of filter surface area. The next big step will be to perfect a technique for welding or crimping the longitudinal seam to close off the tube and for adding a mounting flange and cap on the open ends.
As that work progresses, Anderson hopes to try out the sintering process on high-capacity commercial equipment with the help of Mott Metallurgical, a Connecticut-based metal filter manufacturer. He also hopes to test the filters at a DOE demonstration power plant run by the University of North Dakota. The prospects of what could happen if that testing is successful bring an excited grin to Anderson's face.
"I think the filter could have a great impact on the electric power industry worldwide," he said. "I'm a conservationist at heart, but of the resources available, we have a much greater reserve of coal than anything else. Iowa, for example, is sitting on top of huge coal deposits, but it's high-sulfur coal. Making it possible to burn dirty coal cleanly would provide us the stop-gap measure we need until we can develop the ultra-clean hydrogen conversion (fuel-cell based) power plants or use completely renewable resources, such as wind or solar."
The research is funded by the DOE's Fossil Energy Advanced Research and Technology Development program. Ames Laboratory is operated for the DOE by Iowa State University. The Lab conducts research into various areas of national concern, including energy resources, high-speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
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