Materials for Ultrasupercritical Coal Power Plants - Boiler Materials: Part 1

The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). The results of the review show that with respect to boilers, high-strength ferritic 9–12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni-based alloys are needed. Advanced austenitic stainless steels for use as super and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni-based superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher-strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high-strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper will present the results of the EPRI review in detail, relating to boiler material. Results relating to turbine materials are presented in a companion paper as Part 2.