Materials Considerations for Supercritical CO2 Turbine Cycles

Abstract

Proposed open and closed Brayton-type cycle systems employing supercritical CO2 (sCO2) as the working fluid develop hot gas path environments that present challenging requirements of strength and environmental resistance for the materials of construction. The likely materials properties required by different cycle configurations are examined and compared with the capabilities of available materials. In some instances where peak operating conditions in the turbines in closed-cycle systems approach 500°–700°C at 200 bar, or 1150°C at 300 bar for open cycles, the range of available alloys with the requisite temperature-strength and capabilities is limited. The cycles are highly recuperated: hot, lower-pressure turbine exhaust is used to pre-heat high-pressure CO2 entering the external heater or combustor just upstream of the turbine. Obviously, there is a need to match alloy capabilities with the performance requirements of individual components. Where alloys are employed under conditions beyond current experience, or in unusual configurations (the unique properties of sCO2 may impact the design of some components), practical acceptance will depend on successful qualification testing involving close collaboration among equipment manufacturers, materials suppliers, and materials research and development groups. Examination of the range of scenarios published for sCO2 turbine systems suggests that approaches used in the U.S. Advanced, Ultra-Supercritical (A-USC) Steam Boiler program, and some of the data generated for fabrication, joining, mechanical properties, and performance in simulated service environments in that program, will be applicable to some of the components in these systems. An attempt to elaborate and prioritize the materials property and qualification activities required for the successful realization of these sCO2 cycles is presented as a guide to activities needed to facilitate materials selection.