Effect of Quenching Temperatures on Melting Characteristics of Coal Ash in a Reducing Atmosphere

Two representative Chinese bituminous coals (one with rich Ca content, having a low melting temperature, and the other with rich Fe content, having a high melting temperature) were selected for this study to investigate the effect of quenching temperatures on melting characteristics of coal ash in a reducing atmosphere (6:4 CO/CO2). The final structures were characterized by X-ray diffraction (XRD), scanning electron microscopy coupled with energy-dispersive X-ray (SEM–EDX), and differential thermal analysis (DTA). The results show that, with the quenching temperature increasing, the crystalline volume percentage experiences a maximum value. However, the thermal properties (the glass transition temperature and the melting temperature) determine the maximum crystalline volume percentage temperature. There is a large variation in the phase compositions with an increase of the quenching temperatures, but the maximum diffraction peak intensities of most minerals simultaneously occur. A comparison of the coal ash samples after quench and direct heat treatment shows that there is a low crystalline volume percentage and simple phase compositions for the quenching process at corresponding temperatures, which are due to a preheat treatment at high temperatures prior to quench. Anorthite can form a better crystal shape in the coal ash sample with a low melting temperature and high Ca content for the quenching process. The diffusion controls the dissolution of the Fe–O particle, especially for the high melting temperature coal ash with a high Fe content.