Effect of key minerals on the ash melting behavior in a reducing atmosphere

A coal sample was ashed at 600 °C in air for 12 h, then pressed into pellets, and further treated in a reducing mixture of 60% CO and 40% CO ₂ for 1 h at different temperatures ranging from 700 to 1200 °C in 100 °C intervals. Scanning electron microscopy linked with energy-dispersive X-ray analysis and X-ray diffraction were used to investigate the effects of key minerals on the ash melting behavior. No significant change was found in color and size of ash samples treated at 700-900 °C. However, from 1000 to 1100 °C, substantial liquid-phase formation resulted in a greater shrinkage level and closed spherical pore formation in the pellet. A Na illite most likely existed in the parent coal. Most of Na and K coalesced with Si and Al during heating. A small number of Na-K-enriched aluminosilicate particles were responsible for the formation of a liquid phase at 900 °C. The Fe-O phase formed aluminosilicates less readily than lime. The temperature significantly influenced the fragmentation extent of calcite. The formation of substantial Fe minerals and Fe-Ca minerals required a particular temperature condition. In this study, this temperature condition was about 1000 °C. Fe minerals and Fe-Ca minerals played an important role in determining the molten percentage of coal ash at relatively low temperatures. Evidence for this was that some molten Fe-Ca aluminosilicate particles were identified at 1000 and 1100 °C and most Fe minerals and Fe-Ca minerals formed liquid phases.