Studies on slag deposit formation in pulverized-coal combustors. 4. Comparison of sticking behaviour of minerals and low-temperature and ASTM high-temperature coal ash on medium carbon steel substrates

Abstract

Sticking test results indicate that high-temperature melts containing iron sulphide (pyrrhotite) spread well over an oxidized mild steel surface and the adhesion forces are comparatively high even at low metal temperatures. This sulphide phase originates in the coal as pyrite. Alkalis substituted in the clay structure of both illite and montmorillonite led to the formation of lower-melting slag drops compared to kaolinite. Potassium and chlorine present in the slag drops, formed from the low-temperature ash (LTA) of a Wilcox belt Texas lignite, also led to enhanced sticking properties. The sticking behaviour of slag drops formed by the rapid melting of coal minerals, either LTA residue or synthetic mineral combinations, differed from that of the ASTM ash or synthetic metal oxide mixtures. Any tests, including the sticking test using the bulk minerals or ash composition of a specific coal, may not correlate with slag deposit formation in a utility boiler, as indicated by non-agreement between the test results with the Upper Freeport ash residues and operating experience at Keystone Generating Station.