THEORY AND EXPERIENCE ON CORROSION OF WATERWALL AND SUPERHEATER TUBES OF WASTE-TO-ENERGY FACILITIES

Corrosion of boiler tubes remains an operational and economic constraint in waste-to-energy (WTE) facilities. Despite the long experience of the WTE industry in reducing corrosion, current methods of protection against corrosion remain costly. Understanding the theoretical mechanism and analyzing empirical data and related factors may help reduce corrosion. The research objective of this study was to analyze the theoretical data and empirical experience of the US WTE industry with corrosion, in the hope that this information may assist operators in reducing corrosion in waste-to-energy facilities. The mechanism involving high temperature corrosion is discussed in detail in Chapter 2. The effects of municipal solid waste fuel (MSW) to corrosion and the corrosion sensitive areas in WTE facilities are presented. These are the waterwall and the superheater tubes. Corrosion in superheater tubes is due to either gaseous chloride or sulfate attack or to chloride or sulfate salt deposits. The mechanism of gaseous chloride/sulfate attack, also known as active oxidation, is discussed in detail. Each step in the mechanism of active oxidation of boiler tubes is described, with accompanying thermodynamic calculations. In general, the mechanism of active oxidation involves the formation and transport of chlorine in the gas into the metal/scale interface, chemical reaction with the metal surface and outward transport of gaseous metal chlorides. The mechanism of corrosion of waterwall and superheater tubes by chloride or sulfate deposits is also discussed in detail. In general, corrosion by chloride or sulfates salts occurs at much lower temperatures and is mostly experienced at the waterwall tubes. Corrosion is due to the formation of low melting temperature salts of chloride-chloride, chloride-sulfate or sulfate-sulfate mixtures. The salt melts react with and dissolve the protective oxide films on the metal surface (fluxing). After fluxing, the oxide scale is porous and barely protective. Current methods of protection are: refractory lining or coating of waterwall tubes with highly alloyed, corrosion-resistant materials; and use of surface coatings, such as weld overlay of nickel-chromium based alloys of superheater tubes. The methods of protection against corrosion in the lower and upper furnace, i.e. the waterwall tubes, and the heat exchanger sections, where the primary, secondary or final superheater tubes are located, are discussed in detail. The most popular method of protection is the use of nickel-chromium alloys applied as weld overlay on alloyed steel or as solid tubes, in particular the use of Alloy 625 (Inconel 625). A list of corrosion resistant …

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