Consortium Tests Alloys for Advanced Ultrasupercritical Boilers

A two-year project begun this April by Southern Co. and a consortium of partners including boiler vendors Alstom, Babcock & Wilcox, Foster Wheeler, and Riley Power; the Electric Power Research Institute (EPRI); and the Oak Ridge National Laboratory will gather data on the performance of alloy materials under advanced ultrasupercritical (USC) temperatures of about 1,400F.

The project is critical to the advancement of USC technologies, because the materials under evaluation could prove essential for construction of coal-fired boilers with advanced steam cycles involving much higher temperatures and pressures than those currently used in conventional pulverized coal power plants, the developers say.

As they point out, USC steam plants have been operated in the U.S. since the late 1950s, but materials performance problems forced the reduction of steam temperatures in these plants and discouraged further development efforts on low–heat rate units. Advanced USC coal-fired power plants promise to achieve much higher efficiencies, lower emissions of carbon dioxide and other effluents, and less solid waste products. However, their successful operation depends on a scientific understanding of the effects of high temperatures and pressures on advanced materials.

The project funded by the Department of Energy, along with the Ohio Coal Development Office and the consortium partners, primarily seeks to identify improved alloys, fabrication processes, and coating methods that would allow coal boilers to operate at 1,400F, but it also seeks to explore and define issues affecting the design and operation of USC plants operating at even higher temperatures—up to 1,600F. A key goal is to develop cost targets and promote commercialization of alloys and processes expected to emerge from the effort.

The phase begun in April at Southern Co.’s Plant Barry in Alabama follows nearly a decade of research sponsored by the DOE’s National Energy Technology Laboratory (NETL), which has already tested several alloys for creep strength (the ability to resist strain under constant load over time). These included austenitic steels specifically for superheater/reheater tubes, ferritic alloys for waterwall tubing, and nickel-based alloys for heavy wall piping (Figure 4).

4. An alloy alliance. Boiler vendors Babcock & Wilcox, Alstom, Foster Wheeler, and research groups the Electric Power Research Institute and Oak Ridge National Laboratory began a two-year project to test the performance of alloy materials under ultrasupercritical temperatures of about 1,400F. This image shows the steam-cooled test loop installed in Alabama Power Co.’s Plant Barry Unit 4 furnace prior to initial operation. Courtesy: Southern Co.

According to NETL, the limiting factor for the ferritic steels in use “is their high-temperature corrosion behavior (fireside and steam side),” while the austenitic steels in use are “approaching their mechanical property limits at the higher-temperature range of U.S. steam generating practice.” The federal laboratory concludes that “A move to steam temperatures and pressures significantly higher than those employed in current U.S. power plants will necessitate use of the newer ferritic and austenitic steels developed (and in service) in Europe and Japan, as well as the introduction of nickel (Ni)-based alloys for the highest-temperature components.”

Testing at Plant Barry, which currently focuses on fireside corrosion, will now enable selection of appropriate alloys and coatings required for economical construction and operation at coal-fired power plants capable of operating under varying steam conditions and using coals containing different levels of sulfur.

According to Southern Co., the materials will be assembled into a continuous steam tube inside the boiler that will be the focus of the demonstration. There, they will be exposed to flue gas and steam, allowing researchers monitoring the materials to learn more about oxidation and corrosion under actual operating conditions.

For an in-depth look at advanced USC technology development, see COAL POWER’ s three-part series “Design Features of Advanced Ultrasupercritical Plants” at

—Sonal Patel is POWER’s senior writer.