Studies on slag deposit formation in pulverized-coal combustors: 6. Sticking behaviour of slag drops from three Pennsylvania steam coals

Abstract

The Moza-Austin sticking test has been used to investigate the sticking behaviour of slag drops formed from low-temperature ash from three eastern US coals of similar total ash analysis. While the total ash was relatively infusible, separation of the pulverized coal into specific gravity fractions showed that the 2.8 sink specific gravity fraction gave ash which formed fusible and sticky drops. The 2.8 fraction ash with the worst tendency for slag deposit formation had the necessary combination of pyrite, quartz and alkalis to form a low melting glass which did not crystallize on contact with the substrate, whereas the others showed crystallization in the drops. Synthetic mixtures of pyrite/quartz and pyrite/clays showed similar behaviour with 25 wt% addition of quartz or clay to pyrite. Adhesion strength was ranked in the order py rite/quartz pyrite/kaolinite pyrite/illite. Pyrite in mixtures was more effective in forming sticky drops than the equivalent amount of iron oxides. Fly ashes from the three coals burned in a laboratory pulverized coal test combustor did not form sticky drops, but the bottom ash gave iron-rich sticky drops. Deposition initiating particles on a controlled-temperature probe in the same pulverized coal furnace were also composed of iron rich ash spheres. Microscopic analysis showed that the coal with the worst tendency for slag deposit formation contained pyrite and quartz or clays in initimate mixture, so that the burning pulverized coal would contain a greater number of particles with this poor performance composition.