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MATERIALS PERFORMANCE OF STRUCTURAL ALLOYS IN CO 2 AND IN CO 2 STEAM ENVIRONMENTS

MATERIALS PERFORMANCE OF STRUCTURAL ALLOYS IN CO 2 AND IN CO 2 STEAM ENVIRONMENTS,K. Natesan,Z. Zeng,D. L. Rink

MATERIALS PERFORMANCE OF STRUCTURAL ALLOYS IN CO 2 AND IN CO 2 STEAM ENVIRONMENTS   (Citations: 1)
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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 H 2 O and CO 2 that facilitates sequestration of CO 2 or use it in a turbine to generate electricity, thereby leading to reduction in CO 2 emissions. Also, as the oxidant is bereft of N 2 , NO x 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 H 2 O/CO 2 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 H 2 O/CO 2 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 CO 2 , steam, and in steam-CO 2 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 CO 2 and to CO 2 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 CO 2 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. Background An increase in carbon dioxide gas in the atmosphere is identified as one of the major causes for the global climate change and one of the sources of carbon dioxide is the exhaust from fossil fuel fired combustion power plants. The energy production, in particular electricity generation, is expected to increase globally due to population increase and per capita increase in energy consumption. To meet the energy needs, fossil fuels (coal, oil, and gas) will play a major part in production even with a projected increase in alternate renewable sources. However, to minimize the carbon dioxide emission, the current systems emphasize its capture from power plants and subsequent sequestration. The oxy-fuel combustion systems (without the diluent nitrogen gas) can enable recycling of the carbon dioxide to the compressor, use of novel gas turbines, and advance reuse.
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