Materials Performance of Structural Alloys in Co2 and in Co2-steam Environments

The U.S. Department of Energy (DOE) Office of Fossil Energy is intensely promoting research and development of oxy-fuel combustion systems that employ oxygen, instead of air, for burning the fuel. The resulting flue gas primarily consists of H2O and CO2 that facilitates sequestration of CO2 or use it in a turbine to generate electricity, thereby leading to reduction in CO2 emissions. Also, as the oxidant is bereft of N2, NOx emissions are minimized to a great extent from the exhaust gas. Studies at NETL have indicated that oxy-fuel combustion can increase efficiency in the power plants from the current 30-35% to 50-60%. However, the presence of H2O/CO2 and trace constituents like sulfur and chlorine in the gas environment and coal ash deposits at the operating temperatures and pressures can have adverse effects on the corrosion and mechanical properties of structural alloys. Thus, there is a critical need to evaluate the response of structural and turbine materials in simulated H2O/CO2 environments in an effort to select materials that have adequate high temperature mechanical properties and long-term environmental performance. As a first step, we have evaluated the corrosion performance of the materials in CO2, steam, and in steam-CO2 mixtures. Materials selected for the study include intermediate-chromium ferritic steels, Fe-Cr-Ni heat-resistant alloys, and nickel-based superalloys. Coupon specimens of several of the alloys were exposed to pure CO2 and to CO2 plus steam environments at temperatures between 650 and 950°C for times up to 10,000 h. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, mechanical integrity and cracking of scales for the several structural alloys after long term exposure at 750°C. The information is used to address the long-term performance of the alloys for use in oxy-fuel combustion environments. In the future experiments, it is planned to incorporate low levels of sulfur and chlorine compounds (in addition to CO2 and steam) in the exposure environment to establish the role of second/third reactant on the scaling, internal penetration, and long term performance of the structural alloys.