The Role of Coal Ash in the Corrosion 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. During the past few years, we have conducted corrosion tests on a variety of candidate alloys in CO2 and steam containing environments in the presence of SO2, typical of oxy-fuel combustion systems. Materials selected for the study include intermediate-chromium ferritic steels, Fe-Cr-Ni heat-resistant alloys, and nickel-based superalloys. We presented detailed results on the corrosion performance of various alloys after exposure at 750°C. During the current year, we continued testing of alloys to evaluate their performance in a simulated oxy-fuel environment with low pO2 and the presence of simulated coal ash consisting of alumina, silica, and iron oxide along with sodium and potassium sulfates. In addition, we have examined the role of CaO in the ash (typical of US Western coal ash) in laboratory exposure environments. 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.