Coal fired power generation accounts for more than 50% of the world's electricity output. However, it has long been viewed as the bane of the power industry because of its effect on the environment. As a consequence of its high level of emissions, new technologies and methods of combustion are being researched and developed to make coal a more efficient, cleaner fuel for power generation.
Clean Coal Burning
According to the International Energy Agency, abundant and clean energy supplies will come from new technologies such as clean coal burning from 2020 (IEA, World Energy Outlook: 2001 Insights, October 2001). Research by the US Department of Energy's Ames Laboratory into clean coal technology has led to the development of a new thin metal filter material, which could hold the key to the commercial application of new clean burning, coal fired electricity generation technology.
Clean coal technology has existed for some time in the form of pressurised-fluidised bed combustion and integrated gasification combined cycles incorporated into existing power plants. The high pressure and high temperature burns off most of the pollutants reducing the amount of greenhouse emissions produced. However, these systems use fragile ceramic filters, which prevent fine particles of fly ash from the flue gases reaching the turbines that drive the power plant's generators, but are susceptible to cracking as a result of periodic back flushing - an internal blast of compressed air that clears accumulated fly ash.
“Ceramic filters do a good job of standing up to the heat and the nasty oxidising-sulfidising environment created by the gases,” says Iver Anderson, a senior metallurgist with Ames Laboratory’s Metallurgy and Ceramics programme, “but they're very delicate. You want a filter 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 these properties, Anderson and his research team looked at developing rugged 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, it develops a protective scale when it oxidises.
“The nickel chromium aluminium iron alloy we chose contains a sufficient amount of aluminium to form a tough, protective film of aluminium oxide,” explains Anderson. “Once the aluminium oxide layer forms it also prevents further oxidation.”
Manufacturing Superalloy Filters
While ceramic filters need to be thick for strength, a superalloy metal filter can be quite thin. Anderson uses a process called tap-densified loose powder sintering to create the thin, permeable sheets of metal.
The high purity molten superalloy is converted 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 then sorted and spread out as a thin 0.5mm layer in a shallow ‘cookie sheet’, then heated in a vacuum furnace. This bonds the particles together, forming strong, smooth joints between the spheres, but leaving air gaps as well.
Properties of Superalloy Filters
The researchers have used the process to carry out a series of bend radius tests to see how well the metal can be formed. According to Anderson, the material was ductile enough to enable it to be formed into corrugated tubes, which is particularly significant not only for strength, but for dramatically increasing the amount of filter surface area.
The Future for Superalloy Filters
Anderson hopes to test the process on high capacity commercial equipment and test the filters at the Department of Energy’s demonstration power plant run by the University of North Dakota, USA. “I think the filter could have a great impact on the electric power industry worldwide,” he says. “Making it possible to burn dirty coal cleanly would provide a stop gap measure until we can develop the ultra clean hydrogen conversion (fuel cell based) power plants or use completely renewable resources such as wind or solar.”