Metallic filter holds key to clean-burning, coal-fired power generation
Iver Anderson thinks the solution to the rolling power blackouts in California and parts of the East Coast may lie under the rolling black soil of Iowa's farm country. "Iowa is sitting on top of huge deposits of coal," says Anderson, an Ames Laboratory senior metallurgist. "The problem is that it's high-sulfur, dirty coal."
That's why trainloads of cleaner-burning coal from western states pass by every day on the Union Pacific's east-west line, just a couple blocks away from Anderson's lab in the Metals Development building. As those trains rumble past, Anderson and colleagues Bob Terspstra and Brian Gleeson are closing in on a new material to filter the nasty ashes and dust that result from burning "dirty" coal.
"The technology to burn dirty coal cleanly has existed for some time," Anderson says. "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."
But there's a catch, quite literally, with these systems. 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. Open on the bottom end, these three-inch-diameter filter tubes are about four feet long and currently made out of a ceramic material that can trap particles as small as one micron.
"The 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 can 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. Switching to a fresh filter bank is much better than shutting down the whole power plant to change one filter tube."
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 fly ash is periodically knocked off by an internal blast of compressed air, a process called backflushing. Since the flue gases are about 850 degrees Celsius (1,562 degrees Fahrenheit), the abrupt change in temperature caused by the compressed air can also crack the ceramic material. For new combustion technologies to move from demonstration plants to widespread use in the power industry, a new, tougher filter is needed.
"It's the last big hurdle to seeing this technology take off," Anderson explains. "Power companies are in the business of generating power and making money, not constantly changing filters. You want a filter assembly that is rugged enough and has a long enough life that you can essentially forget about it."
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. These high-strength metals can withstand high temperatures and aren't affected by thermal shock during backflushing. The researchers selected a nickel-based alloy that maintains its strength at high temperatures, 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 protective film of aluminum oxide," Anderson says. "Once an aluminum oxide layer forms, it 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 stream of liquid superalloy into millions of tiny metal spheres. The resulting powder is then sorted, by screening, into spheres only 25-45 microns in diameter.
After spreading the sorted metal powder out as a thin layer (0.5 millimeters) in a shallow "cookie sheet," Anderson heats the metal in a vacuum furnace. This sintering process bonds the powder particles together, forming strong, smooth joints between the spheres, but leaving air gaps as well.
"In tests we conducted, our alloy experienced only a moderate drop in yield strength going from room temperature to operating temperature (850 C)," Anderson says. "And our porous sample turned out to be about six times stronger at operating temperature than a porous sample of an iron-aluminide material being developed by another research laboratory. That strength allows us to go with a very thin filter body."
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.
While the group will tinker with the alloy mixture and conduct additional corrosion testing, 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.
"The key to getting the best bonded neck structures — the fused joints between the powder particles that give the material its ductile strength — is keeping the powder material clean," Anderson says. "You have to use pure material and keep it that way during atomizing and sintering to permit proper bonding of the spheres."
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 brings an excited grin to Anderson's face.
"I think the filter could have a great impact on the electric power industry worldwide," he says. "I'm a conservationist at heart, but of the resources available, we have a much greater reserve of coal than anything else. It's even right here in Iowa.
"Making it possible to burn dirty coal cleanly would provide us the stopgap that we need until we can develop the ultraclean hydrogen conversion (fuel-cell based) power plants or use completely renewable resources, such as wind or solar." Such filters could have an impact on diesel combustion engines as well.
"The technology exists to produce highly efficient diesel engines — two to three times more efficient than gasoline engines," Anderson says, "and you'll probably see diesel hybrid cars very soon. The problem is that the emissions are either high in particulates and low in nitrous oxides or low in particulates and high in nitrous oxides — but not low in both.
"Ideally, you'd use a particulate filter in combination with the low- nitrous-oxide-producing engine," he adds. "Exhaust emissions from burning common diesel fuels are high in sulfur. We've already addressed that problem, so our filter could possibly work in this application as well. And because diesel fuels can be formulated in so many blends, including soy oil, we'd have all sorts of options."
The Department of Energy's Office of Science is the single largest supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time.