Corrosion mechanisms in fluidized bed combustion environments.

摘要

Fluidized bed combustion (FBC) is an effective method for using coal as a fuel in an environmental acceptable manner. Although in-bed corrosion for FBC was not expected because of the low combustion temperatures, it has been shown that alloys may undergo a serious sulfidation/oxidation type of corrosion in FBC when calcium oxide is used as sulfur getter. Oxygen probes indicate that there is a low oxygen partial pressure in the bed. Relevant thermodynamic stability diagrams show that these low oxygen activities can generate significantly high sulfur activities by calcium sulfate decomposition and, as a consequence, may cause corrosion. Calcium oxide may also react with the protective oxides to make them less protective.; A laboratory apparatus was constructed to test the corrosion mechanism in such environments. Carbon monoxide/carbon dioxide gases of known oxygen partial pressures are passed over calcium sulfate/calcium oxide or calcium sulfide/calcium oxide to generate a gas with oxygen and sulfur activities similar to those in FBC. Two groups of specimens are exposed: in-bed samples are placed in contact with the calcium-containing mixture; gas-exposed samples are downstream in a gas ambient.; The corrosion morphologies of the test specimens are similar to those developed in FBC pilot plants. As predicted from thermodynamic stability diagrams, there is an oxygen activity window in which sulfidation occurs. We observe an increase in sulfur attack for IN800 as the temperature is lowered.; Other gas mixtures were used with or without calcium salt to establish whether equilibrium is attained between calcium salts and the gas atmosphere. These gas atmospheres also allowed a study of the interaction between oxidation, sulfidation and carburization. Similar corrosion morphologies for those in coal-burning FBC systems were also found using these gas mixtures. It was observed that, carbon-containing atmospheres promote grain-boundary penetration of sulfides for IN800, and it can also accelerate the carburization of 304SS. Severe carburization occurs for 304SS in CO{dollar}sb2{dollar}/CO gas mixtures, especially at high oxygen partial pressures, due to the local high carbon activity established upon rapid oxidation, but there is no significant influence of carburization on either oxide thickness of sulfur penetration for 304SS. (Abstract shortened with permission of author.)