A thin metal filter material developed at Ames Metallurgy and Ceramics Laboratory 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. Pressurized-fluidized bed combustion and integrated gasification combined cycles are highly efficient, low-emission power plant concepts.
However, even though combustion in these plants is more complete, the flue gases contain fine particles of flyash. 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 flyash particles as small as one micron.
Ceramic filters stand up well to the heat and oxidizing-sulfidizing environment created by the gases, but ceramics crack easily.
Ames Lab 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.
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, a process called tap-densified loose powder sintering is used.
High-purity molten superalloy is converted 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 (0.5 millimeters) in a shallow "cookie sheet," then heated but not melted 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. 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 steps 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, researchers hope to try out the sintering process on high-capacity commercial equipment with the help of Mott Metallurgical, a Connecticut-based metal filter manufacturer and to test the filters at a DOE demonstration power plant run by the University of North Dakota.