Melting-dissolving kinetics and mechanism of high silica-alumina coal ash particles deposited to liquid slag wall in entrained flow gasification

The depositing and subsequent physicochemical behaviors of coal ash particles to the liquid slag wall in an entrained flow gasifier are essential to the wall reaction, slag flow and discharge. In this study, the meltingdissolving behavior and kinetics of high silica-alumina ash on the typical molten slag surface were investigated at a particle scale through in-situ melting experiments. The multifactorial effect on the ash fusibility was further evaluated. Experimental results revealed that the presence of molten slag significantly reduced the melting temperatures of high silica-alumina ash particles, with an initial melting temperature of around 1270 degrees C. Dissolvability was linearly correlated with temperature, and the melting rate increased with increasing temperature or decreasing particle size. However, complete melting was difficult to achieve at 1400 degrees C. Consequently, the effect of adding flux (CaO) in the molten slag on the ash fusibility was further evaluated. The ash particles achieved complete melting at a 4 wt% CaO addition, and the flux addition increased the melting rate and solubility. Besides, the mechanism-level explanation of the melting-dissolving process showed that Ca ion migration led to the transformation of aluminum to higher coordination sites, which destabilized the silicaaluminium tetrahedra structure and manifested in the generation of Bytownite macroscopically. Comparative analysis showed the melting rate depended on the diffusion process, and the melting-dissolution kinetics was established to predict the dissolution rate, which could provide a reference for the slag discharge of coals with high ash fusion temperatures in the industry.