Thin metal filter material solves green power problem
18 Jul 2001
Researchers at Ames Laboratory have developed a thin metal filter material that may overcome the final barrier to the commercial application of new clean-burning, coal-fired electric generation technology.
'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 pressurised-fluidised bed combustion and integrated gasification combined cycles are highly efficient, low-emission power plant concepts.
'The high pressure and high temperature volatilise 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, the performance of these systems is said to suffer from the amount of fine particle fly ash in flue gases. High in sulphides, 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 the hot gas is passed through clusters 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 the 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.
Since the filters' operating temperature is about 850 degrees Celsius, even the abrupt change in temperature caused by the compressed air can crack the fragile ceramic material.
'You want a filter assembly that is rugged enough and has a long enough life that you can essentially forget about it,' said Anderson. 'It's the last big hurdle to seeing this technology take off.' 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 oxidises.
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 used a process called tap-densified loose powder sintering.
He started by converting high-purity molten superalloy into a fine powder using a high-pressure gas atomisation 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 millimetres) 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.
The researchers then 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 also for increasing the amount of filter surface area.
