Materials degradation mechanisms in coal-fired boilers

Abstract

INTRODUCTIONThe technical viability of advanced power plants will be critically dependent on the availability of suitable materials. Higher efficiencies can be achieved only by operating at temperatures that will exceed the capabilities of many of the conventional alloys. It is expected that maximum waterwall temperatures in high efficiency units will approach 600 to 625°C, with superheater and reheater outlet temperatures expected to go as high as 750°C. Similar or higher temperatures are expected for piping and headers. These higher operating temperatures may severely limit the serviceable lives of critical operating components due to an increase in the kinetics of oxidation/ corrosion mechanisms. Controls on emissions imposed by government regulatory agencies in the USA promoted the development of various strategies for lowering these emissions through changes in the way the fossil fuel was burned in a boiler. Staged coal firing provided a viable technology to meet stringent requirements on NO x emissions. However, it was found that during staged firing the substoichiometric firing conditions generate relatively high concentrations of total reduced sulfur species (TRS), unburned carbon, and pyrite particulate at the boiler waterwalls. Reduced sulfur species and reducing microclimates then are conducive to accelerated waterwall wastage by various sulfidation mechanisms.Higher operating waterwall temperatures will lead to accelerated scaling on the unprotected ID surface of waterwall tubing. This build-up of oxide will restrict heat transfer, and may, if metal temperatures are high enough, cause local overheating of the tube wall and premature failure. The consequent reduction in boiler availability will jeopardize the economic benefits of the higher efficiency plants. In addition, when the oxide reaches a critical thickness it will exfoliate from the surface of the tube, become entrained in the steam, and enter the turbine, where the high velocity impact of the oxide particles on critical turbine components can lead to significant losses in turbine efficiency. Although alternative materials with higher creep strength are being developed for these applications, the available data suggests that in many cases their oxidation behavior in steam is not satisfactory.Radical changes in the way the fuel is burned in the furnace of present-day steam generator have led to variations in the traditional wastage mechanisms, such as coal-ash corrosion. Standard high-temperature alloys, such as grades 304H or 310H, have been subjected to the effects of high-temperature carburization. Although the number of reported failures related to carburization to date has been small, the frequency of occurrence clearly is increasing, and the phenomenon has been acknowledged officially in a recent study of coal-ash corrosion [1].The purpose of the present report is to review recent test data that has served as the basis for the understanding of the major modes of degradation that affect current and future coal-fired power plants. Recommend...

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