Two coals (one coal with high-melting temperature ash and one with low-melting temperature ash) were chosen to investigate the effect of residence time at high temperatures in a reducing atmosphere on coal ash behavior. Scanning electron microscopy linked with energy dispersive X-ray analysis (SEM-EDX) and X-ray diffraction (XRD) were used to characterize the ash behavior. The residence time has a great influence on the amount and composition of minerals, especially for the high-melting temperature ash. The diffraction peak intensities for most minerals formed in the high-melting temperature coal ash after heat treatment reach their maximum value after 2 min, and then, they decrease. This trend is not apparent for the low-melting temperature ash. The high-melting temperature ash also has a higher mineral content than the low-melting temperature ash for the same residence time. Formation of a large amount of mullite in the high-melting temperature ash after heat treatment indicates that mullite formation is rapid, but a high (Na2O + K2O + CaO) content can inhibit the formation of mullite. Different mineral compositions have been found in the ash samples under nonthermal equilibrium and thermal equilibrium conditions. Most minerals formed under thermodynamic equilibrium conditions are not observed under the nonequilibrium conditions. This is attributed to the low stability of minerals at high temperatures. The kinetic limitations of the dissolution of FeO particles must also be considered, especially for larger FeO particles.