Materials Performance of Structural Alloys in Simulated Oxy-fuel 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 including alkalis 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, last year we tested coupon specimens of several candidate alloys in pure CO2 and in CO2 plus steam environments at temperatures between 650 and 950°C for times up to 10,000 h. Materials selected for the study include intermediate-chromium ferritic steels, Fe-Cr-Ni heatresistant alloys, and nickel-based superalloys. We presented detailed results on the corrosion performance of various alloys after exposure at 750°C. We also addressed the mechanism for the oxidation of various alloys in these environments and also evaluated the long-term performance of the alloys from the standpoint of scaling and internal penetration. The oxidation test results showed that the total corrosion (scaling plus internal penetration) rates are <0.05 mm/y at 750°C, in the absence of ash or sulfur. During the current year, we conducted tests to evaluate the corrosion performance of the structural alloys in the presence of simulated coal ash consisting of alumina, silica, and iron oxide along with sodium and potassium sulfates. Detailed results are presented on weight change, scale thickness, internal penetration, microstructural characteristics of corrosion products, and cracking of scales for the alloys after exposure at 750°C. To establish the role of steam in the exposure environment, tests were also conducted in environments with and without steam in the oxy-fuel gas atmospheres. Results from these tests are used to address the role of steam in the long-term corrosion performance of alloys.