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Ceramic filter technology breathes life into turbines

Just as people need air to breathe, high-temperature gas turbines need clean fuel to burn to prevent damage to sensitive internal parts.

The Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) and 3M Company have developed an award-winning ceramic technology used in the new 3M Ceramic Composite Filter, an important breakthrough in coal-fired power generation.

Because natural gas is currently plentiful and relatively inexpensive, it is being used extensively for new gas turbine power generation. However, the supply of natural gas is limited, and its use will likely decline in the longer term. For these long-term fuel supply considerations and economic reasons, systems are being developed that use coal to power high-temperature gas turbines. However, coal does not burn cleanly, and when it is converted into a gas, particles of char and ash are formed. If these impurities reach sensitive parts inside the turbine, they could cause severe erosion and corrosion.

To address the problem, several designs for ceramic filters have been developed. However, these filters tend to break, allowing ash and char to stream into the turbine.

Advanced technologies for using coal to generate electricity take advantage of the vast coal resources available in the United States.

"Coal-fired power plants will supply a significant portion of world demand for electricity in the coming decades," said Ed Fischer, leader of the 3M Stationary Hot Gas Filtration team. "Products such as the 3M Ceramic Composite Filter are critical to the success of advanced coal-fired power generation technologies."

The most effective style of filter for this purpose, called a candle filter, resembles a 5-foot-high, 2-1/2-inch diameter laboratory test tube. The gas is forced through the porous material, leaving impurities clustered as a filter "cake" on the outside of the filter.

Conventional candle filters consist of silicon carbide grains held together by a clay or glassy binder, which is comparatively heavy and brittle. Gas being filtered is at a very high temperature, and the blasts of air used to blow the filter clean are usually at room temperature. Because of the extreme temperature and pressure changes, conventional filters tend to weaken and break, sending a stream of impurities into the turbine.

"The demanding conditions in these advanced power systems have extended conventional materials beyond their limits of durability," said Rod Judkins, manager of ORNL's Fossil Energy Program. "We simply had to create a material that could better handle those demands."

The 3M Ceramic Composite Filter is made from a combination of continuous and chopped ceramic fibers reinforced with a silicon carbide matrix.

Because of the design, the 3M Composite Ceramic Filter is stronger, lighter and less brittle than its predecessors. The continuous fibers not only strengthen the filter, but remain intact if the filter breaks, keeping impurities from damaging the turbine. The thin wall of the filter makes it less susceptible to breakage from extreme changes in temperature and pressure.

Because the filter is much lighter than previous designs, the structures built to support hundreds of them, called tubesheets, cost less and are not as complex.

These improved characteristics create many other uses for the technology, such as filters for industrial waste incinerators, metal smelters and advanced diesel engines.

ORNL Metals and Ceramics Division and 3M developers won a 1995 R&D 100 Award for developing the technology. The awards are presented annually by R&D Magazine in recognition of the year's most significant technological innovations.

Funding for this research was provided by DOE's Office of Fossil Energy: Advanced Research and Technology Development Materials Program, Morgantown Energy Technology Center.

You can learn more about this research and many other exciting projects by visiting ORNL on Oct. 21, 1995, during its Community Day event. Many of our facilities will be open to the public that day. For additional information, call ORNL Public Affairs, 865-574-4160.

ORNL, one of DOE's multiprogram national research and development facilities, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.