Clean Electric Power

Description And Advantages

Researchers at the US Department of Energy's Ames laboratory have developed a method of burning high sulphur, dirty coal. A thin metal filter material may 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 in burning dirty coal cleanly, in the 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 is a disadvantage with these systems. Even though, the combustion is more complete, the flue gases contain fine particles of fly ash. High in sulphides, chlorides and sodium compounds, these particles pose an abrasive and corrosive threat to the turbines that drive power plant's generators as well as to the air quality. To prevent these, hot gases are passed through clusters or banks of cylindrical 'candle' filters. These 3-inch diameter filter tubes are about 1.2m long and currently made from a ceramic material that can trap fly ash particles as small as one micron. 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°C, even the abrupt change in temperature caused by the compressed air can crack the fragile ceramic material.

To avoid this problem, the researchers selected a nickel-chromium-aluminium-iron alloy contains sufficient amount of aluminium to form a protective film of aluminium oxide, which prevents further oxidation. While ceramic filters need to be thick for strength, a super alloy metal filter may be quite thin, giving it an airflow efficiency advantage.

Source: The Chemical Engineer, August 2001