Abstract
In Japan, pulverized-coal-fired thermal power plant boilers are operated with a high two-stage combustion ratio to reduce NOx emissions. This combustion ratio forms a strongly reducing atmosphere containing H2S at the burner zone. Water-wall tube materials, such as low-alloy steel, sulfidize severely in such an atmosphere. In the past few years, wastage of the tubes observed in limited areas. However, the sulfidation area tends to extend and/or to shift along with changes in combustion conditions and coal properties. As a result, large-area inspections are necessary to identify the sulfidation areas. Several methods for identifying sulfidation areas have been developed, but these methods have various drawbacks, including (a) long inspection times to adequately assess large areas and (b) a need for descaling, which can act to accelerate tube wastage. Accordingly, a new approach is required for quick identification of sulfidation areas in large commercial boilers without any need for wall descaling. Toward this end, we investigated coal ash chemistry, scale structure/composition, and combustion conditions at several pulverized-coal-fired boilers in Japan. The results show that severe sulfidation occurs in areas where coal-ash deposits contain a comparatively high Zn content. In a reducing atmosphere with sufficient sulfur potential, only a limited amount of Zn contained in coal is sulfidized to form ZnS. At the combustion temperature (about 1300 °C), ZnS is present as a gaseous phase. It presumably solidifies on the tube surface, the temperature of which is about 500 °C. Continuous deposition of ZnS acts to increase the Zn content in the coal ash on tube surfaces. A relatively high level of Zn on the tube surface can be an “indicator” for a sulfidation area. With this as an indicator, the entire region of actual boilers can be easily evaluated without descaling and within several hours by the use of a handheld X-ray fluorescence analyzer.
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