A fully coupled electromagnetic, heat transfer and multiphase porous media model for microwave heating of coal

Microwave heating has gained widespread popularity in coal processing. Numerical simulation is a promising tool to visualize and quantize microwave-coal interactions. However, coal is always assumed as a solid continuum in the current models, which cannot accurately predict the thermodynamic behavior. In this study, a fully coupled electromagnetic, heat transfer and multiphase porous media model was developed to investigate microwave heating of coal. Results show that microwave absorption by coal induces significant redistribution of the electromagnetic field in the cavity, forming high- and low-energy regions. The nonuniform electromagnetic distribution and microwave selective heating generate hot and cold spots, whereas heat convection tends to homogenize the thermal field. The temperature rise during microwave heating is characterized by fast-slow-fast. In addition, water evaporation and surface heat convection can exert profound impacts on the thermal evolution of coal. When water evaporation is included in the model, the temperature increases nonlinearly because the dielectric property of coal is constantly changing during microwave heating. Outcomes of this study can be used to identify the heating behaviors of coal during microwave processing and thereby help to optimize the microwave applicators.