Ceramic Filter Is Key to
Advanced Coal Technology

The road to technology transfer has its twists and turns. The design of the commercially available 3M ceramic composite filter, which earned 3M and ORNL an R&D 100 Award in 1995, is a case in point. The story behind this development underscores the significance of networking to perceive market needs and of "people skills" to convince companies that needs for specific products exist. Also important is teamwork in the use of technical skills for developing products. Putting money in the right places also helps.

Developers at ORNL of the 3M ceramic composite filter for combined-cycle fossil fuel plants include, from left, Jerry McLaughlin, Rod Judkins, and David Stinton. Not pictured is Rick Lowden, another ORNL developer of the filter, which received an R&D 100 award for 3M and ORNL in 1995.

In 1984, David Stinton of ORNL's Metals and Ceramics Division was asked to develop a process to fabricate a special type of ceramic composite for DOE's Fossil Energy Materials Program. The project had a long-term goal: develop highly dense silicon carbide composite tubes that have high thermal conductivity so they can be used as heat exchangers. It was thought that such tubes would be needed in advanced combined-cycle fossil plants in 10 years or so.

So Stinton worked on developing a dense fiber-reinforced silicon carbide composite tube that would meet the long-term need. As he conducted his research, ORNL's fossil technology program managers, Ron Bradley and Rod Judkins, visited a number of experimental combined-cycle power plants--integrated coal gasification and pressurized-bed coal combustion combined cycles. These are facilities in which coal is burned to produce hot gases that drive a gas turbine as well as heat that makes steam to spin a steam turbine. This so-called combined cycle is expected to be 35 to 50% more efficient than today's coal-fired steam power plants. It is believed that, as natural gas becomes less abundant and more expensive, many combined-cycle fossil plants will be built to generate electricity.

During these visits in 1985 and 1986, Bradley and Judkins kept hearing about an immediate problem facing operators of combined-cycle fossil plants. The operators rarely mentioned heat exchangers. But they talked a great deal about the breakage of candle filters that remove coal ash from the hot gases produced in coal combustion or gasification. The ORNL managers also heard about the filter problem through their interactions with DOE's Morgantown Energy Technology Center (METC). METC was sponsoring a project at Westinghouse's Waltz Mill site, which had been experiencing filter problems at its coal gasification pilot plant.

These filters are important because they prevent the ash from striking the blades of the gas turbine. "When the filters break," Stinton says, "ash particles get through and erode the metallic blades. Also, the blades can be corroded by chemicals in the ash."

The filters now in use are made of silicon carbide particulates bonded together by a glassy matrix consisting of aluminosilicate clay. These filters can break because of thermal shock--large and rapid changes in temperature--which occurs when the filters are cleaned during plant operation.

"The gas flows from the outside of the filter tube to the inside," Stinton says. "The ash particles in the gas cannot get through the filter material, so they are trapped as a cake on the outside of the tube. To clean the outside of the tube, air or gas inside the filter is blown repeatedly to the outside. Because the cleaning gas is colder than the gas being filtered, the filters undergo thermal shock."

I came up with the concept of changing
the process to make tubes that are
60% dense--porous enough
to serve as filters.

In 1986, Judkins informed Stinton that the advanced combined-cycle plants have an immediate need for ceramic filters that don't break at temperatures as high as 850°C. Says Stinton: "I told him that the process for producing fiber-reinforced silicon carbide tubes that are 90% dense for heat exchangers could be modified to make tubes that are 60% dense--porous enough to serve as filters."

This collection of ceramic filters in a combined-cycle power plant removes ash particles from hot gases generated by burning coal, protecting the plant's gas turbine from erosion and corrosion.

The filter tubes were made using chemical vapor infiltration and deposition of a ceramic matrix into a structural form (fibrous preform). Special gases are introduced to the preform, which is heated to cause silicon carbide to form and deposit uniformly throughout the preform.

Using a small amount of funding from DOE's Office of Fossil Energy Advanced Research and Technology Development Materials Program, Stinton explored the concept and found it promising. Subsequently, with funding obtained through METC, Stinton worked on the filter development with Judkins, Rick Lowden, Laura Riester, and Jerry McLaughlin, all of the Metals and Ceramics Division. He later talked to filter manufacturers to find out ways to improve the filter. One suggestion was to make its wall thinner so the filter is lighter and creates less pressure drop. The ORNL scientists found a way to do this.

"If lighter filters are used," Stinton says, "then the structures built to support hundreds of them, called tubesheets, would be simpler and cost less. Also, less cooling water would be needed for the tubesheets, which would save energy."

ORNL lacks the facilities to fabricate and test candle filters, which are each 1.5 meters (5 feet) long and 6.4 centimeters (2.5 inches) in diameter. So in 1989 the Laboratory issued a request for proposals to filter companies to work with ORNL to design and manufacture filters for testing. Of the eight responses by the competitive bidders, the ORNL scientists were favorably impressed with three. They chose the 3M Company of St. Paul, Minnesota, because of the strength of its proposal and because of its experience in manufacturing and marketing bag filters. Says Stinton: "We thought they were most likely to improve and commercialize our filter concept."

Stinton and his ORNL colleagues began work initially with 3M Company scientist Lloyd White and subsequently worked on improving the filter design with 3M Company scientists Bob Smith, Ed Fischer, Joe Eaton, Bill Weaver, Larry Kahnke, and Doug Pysher. "I initially wanted to coat felt with silicon carbide," Stinton says, "but because of 3M's experience we settled on using a combination of chopped fibers to control porosity and continuous fibers to provide strength and prevent breakage."

The resulting filter is light, strong,
tough, resistant to thermal shock,
and relatively inexpensive.

The oxide fiber used to fabricate each layer of the filter is a 3M product called NextelTM 312 that consists of alumina, boria, and silica. The relatively fragile chopped fiber filter surface is covered by an open braid of NextelTM 312 to protect it during installation and handling. The resulting filter is light, strong, tough, resistant to thermal shock, relatively inexpensive, and easily retrofitted in filter holders.

"In 1990 and 1991, our project with 3M stagnated," Stinton adds. "It was not an initial success. But Rod convinced them that there was this need out there and that our ORNL-3M team was close to having a good filter worth testing. So the work picked up on the project and we produced a filter to test."

In 1992, Stinton and his colleagues received a patent for their filter invention. In September 1994, 3M obtained rights to the ORNL technology through a licensing agreement with Lockheed Martin Energy Systems.

The main U.S. supplier of filter systems for combined-cycle pilot plants is Westinghouse Electric Corporation. But how do you motivate this giant firm to consider testing new filters in its combined-cycle pilot plants? Says Stinton, "With the encouragement and aid of METC's Richard Dennis and the DOE program manager, Jim Carr, ORNL provided money through 3M Company for Westinghouse to test filters on a bench scale at the Westinghouse Science and Technology Center. In March 1993, Westinghouse researchers conducted tests and found that the 3M filters performed as well as other filters. However, the test showed a need for improved filter construction. All parties saw the value of these tests; Westinghouse, 3M, American Electric Power, METC, DOE, and ORNL collaborated to test the filters in a Clean Coal Technology project at the Tidd pressurized fluidized-bed combustor demonstration plant in Brilliant, Ohio. The filters will also be tested in a pilot plant in Alabama, and 60 of the 3M filters were provided for a demonstration plant in Karhula, Finland."

Westinghouse agreed to test three filters in Tidd for 2000 hours. Although two of the three filters failed, it was determined that the failures were not caused by any flaws in the filters. So 3M modified the attachments. Ten newly installed filters were then tested for over 1000 hours ending in March 1995. They performed well with no significant problems.

Improved high-temperature candle filters to remove particulates from hot gases produced from burning coal are needed to increase efficiency in the operation of coal-fired combined-cycle systems.

3M has now made sales in the United States, Europe, and Asia. Improvements in the filters continue to be made, and production capacity is being increased. The filter business is expected to earn $200 million per year by 1998 with a total market of about $7 billion from now to 2003, according to independent estimates.

"Many companies don't like to take risks," Stinton observes. "They have to be helped along. Rod helped 3M see the market, which he learned about from his visits to the combined-cycle pilot plants. He played an important role because he kept encouraging 3M to finish developing a product for which there was a need.

"Technology transfer doesn't mean simply transferring an idea from a national laboratory to a private company. It means having teams from both work together and encourage each other to improve the final product."

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

"Coal-fired power plants will supply a significant portion of world demand for electricity in the coming decades," says Ed Fischer, leader of the 3M hot gas filtration team. "Products such as the 3M ceramic composite filter are critical to the success of advanced coal-fired power generation technologies."

Superconducting Cable Goal of Southwire-ORNL CRADA

ORNL has teamed with the largest U.S. manufacturer of electrical transmission cable to develop a "superconducting cable" that will deliver electricity efficiently. ORNL and Southwire Company of Carrollton, Georgia, are working together to provide this "third-millenium technology."

Superconductivity is the phenomenon in which substances cooled to very low temperatures lose all resistance to electrical current, virtually eliminating energy losses during transmission.

Underground cables are being installed increasingly in dense urban areas and places where right-of-way space is at a premium. Superconducting cables will deliver at least twice as much current as conventional cables, providing energy more efficiently.

This is one of the most exciting and challenging
projects ever undertaken by Southwire.

The first Southwire Company superconducting cable for an underground transmission line is prepared to go into a dewar for critical current and ac-loss measurements at liquid nitrogen temperature (77 K). The measurements are performed by the Magnetics and Superconductivity Group of ORNL's Fusion Energy Division. The cable is would with four layers of thin high-temperature superconducting tape manufactured by Intermagnetics General Company.

"This is one of the most exciting and challenging projects ever undertaken by Southwire," says Roy Richards, Jr., chairman and chief executive officer of company. "The efficient use of energy is critical to our country's future competitiveness, and we are confident we can make a major contribution toward that goal--confident enough, in fact, to invest our own money in the program."

The two organizations seek to demonstrate the commercial viability of the technology. Under a cooperative research and development agreement (CRADA), DOE will provide $95,000 and Southwire will contribute at least $775,000 in the first phase of what could be a multiphase project.

ORNL and Southwire are both bringing
unique facilities to the project.

"This agreement demonstrates DOE's role as broker of one of tomorrow's exciting energy technologies," says Christine A. Ervin, DOE's Assistant Secretary for Energy Efficiency and Renewable Energy. "Through the marriage of two unique American strengths--U.S. private sector entrepreneurial spirit and the unique technological resources of DOE's Oak Ridge National Laborator--your private-sector partners can become world leaders in high-temperature superconductivity wire and coil manufacture."

Southwire's assignment is to design and fabricate a one-meter (1-m) length of superconducting cable for testing and verification of the project's concept. ORNL will use its unique testing facilities to measure the performance of the cable and will provide cryogenic, or low-temperature, systems support to Southwire.

"If the test cable works, then it should be possible to fabricate much longer lengths and thus commercialize what is now an experimental technology," says R. L. Hughey, project manager for Southwire.

Electrical current will be carried by silver-clad tapes containing dozens of filaments of "high-temperature" superconductor made of a copper oxide ceramic. When chilled by liquid nitrogen to approximately 77 K (-320° F), the tapes lose their resistance to electricity and conduct direct current (dc) with virtually no energy loss. (The technology is considered "high temperature" when compared to the temperature of the much more expensive coolant, liquid helium, which is 4.2 K.)

"Superconductors are loss-free only when carrying dc current," says Martin S. Lubell, an ORNL expert on superconducting magnets. "The transmission cable we are developing with Southwire will be carrying alternating current. Because there will be ac losses, these must be measured and kept low for the cable to be ultimately viable."

Southwire is the nation's largest manufacturer of copper and aluminum rod, wire, and cable for the transmission and distribution of electricity. The company will handle the experimental fabrication of the cable and work with ORNL to design and build test equipment and perform scanning electron microscopy to study the characteristics of materials used in the project.

Bob Hawsey, director of ORNL's Superconductivity Technology Center, is project manager for the ORNL effort. The researchers working on the project are Don Kroeger and Patrick Martin, both of ORNL's Metals and Ceramics Division, and Lubell, Winston Lue, and Ed Jones, a postdoctoral research fellow, all with ORNL's Fusion Energy Division. The DOE funding source is Energy Efficiency and Renewable Energy, Office of Utility Technologies.

The superconducting cable will be fashioned from dozens of individual silver-clad ceramic tapes, purchased by Southwire from Intermagnetics General Corporation. These tapes will be wound around a pipe that carries liquid nitrogen, and the insulating material will be wound around the superconducting tapes.

The silver-clad tapes are made by first drawing silver tubes packed with powders of a ceramic compound containing bismuth, strontium, calcium, copper, and oxygen (BSCCO, pronounced "bisco"). The thin silver-clad BSCCO tubes that result are chopped into filaments and stacked into a hollow silver tube that is drawn, rolled, and heated to align the superconducting grains and produce a thin, flat tape.

"The silver-clad BSCCO tapes conduct current well at liquid nitrogen temperatures if they are not exposed to large magnetic fields, such as those required to operate electric motors and generators," Hawsey says. "BSCCO is well suited for the transmission cable application because its current-carrying ability is not greatly affected by the very low fields generated by neighboring wires in the cable."

ORNL's unique testing facilities that will be used for this project include (1) a facility in the Metals and Ceramics Division for measuring the electrical performance of the individual purchased tapes, and (2) a cable alternating-current (AC) loss test facility in the Fusion Energy Division that is capable of measuring the ac losses in 1.25-m-long cables. Says Hawsey: "The cable test facility that has been recently developed and built in the Fusion Energy Division is unique among the national labs."

ORNL's Fusion Energy Division is designing the cryogenic system for the liquid nitrogen-based transmission cables. This system includes the cable cryostat, or insulated jacket, as well as electrical leads and terminations, insulation, and structural designs for operation at 77 K.

"Southwire is also bringing unique facilities to the project," Hawsey says. "These include a high-voltage test lab and a polymer lab for dielectrics-insulation research. Southwire is even lending ORNL a 2000-ampere ac power supply for the cable testing to be done in Oak Ridge."

If the test cable works, will there be any problems with manufacturing longer lengths for commercial purposes?

"Several issues must be resolved to scale-up the superconducting cable to longer lengths,"Hawsey says. "Challenges in the second phase of the project include cable manufacturing machinery design (because the superconducting tapes are fragile), cryogenic dielectric and insulation design. In addition, the cost of today's research-grade tapes must decrease 10 to 100 times, and tensile strength must increase 3 to 5 times, for the cable to be commercially viable.However, there are no known technical show-stoppers, at this time. We hope to have a Southwire utility customer on the team for the second phase to help us design and test the long-length cables."

Teamwork will remain a key to developing new technologies for the third millenium.

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