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Wet flue gas desulfurization (WFGD) systems have been operating in electric utility coal-fired boiler plants since sulfur dioxide emissions became regulated with a series of amendments to the Clean Air Act from 1971–1977. Since that time, significant improvements have been made in these system designs in terms of scrubbing performance; but scaling and the associated corrosion in limestone forced oxidation (LSFO) based WFGD towers continues to be a concern with scrubber operations. Scaling in a WFGD system is most often caused by operational factors; however, scaling can also form through undesirable side reactions. The formation of MnOx has been observed in the cooling water systems of power plants for a number of decades. MnOx scale deposition is understood through two oxidation mechanisms—one chemical and one biological. MnOx scale can be formed through chemical oxidation with halogens or biocides. Biological oxidation of manganese (Mn) is the result of naturally occurring bacteria from fresh water sources. Understanding how MnOx scale is formed is important; however, the associated corrosion is the deeper issue. Most research on industrial Mn scaling and corrosion has focused on the water source as the leading contributor; however, for WFGD systems, reagent limestone has been found to be a notable contributor of soluble Mn. The proper selection of limestone is important to reducing levels of soluble Mn in the system. This presentation will review several examples of the scaling and corrosion problems and what can be done to mitigate these problems in terms of limestone quality and operational conditions.
flue gas desulfurization, limestone, manganese, electric utility, corrosion, scaling, stainless steel
Carmeuse Lime and Stone, Pittsburgh, PA