Quantitative investigation of moisture migration during microwave drying of coal slime dough through simulation and tracer analysis

Moisture migration during the microwave drying process of coal slime is critical for improving drying efficiency and reducing energy consumption. Nevertheless, current methods for quantifying these migration behaviors remain insufficient. In this research, we employed a comprehensive approach that combines experimental investigations with multi-physical field simulations to quantitatively characterize the distribution and state of moisture within coal slime dough at different locations during microwave drying. The entire drying process was divided into three distinct stages based on temperature distribution: preheating, constant temperature, and reheating. During the preheating stage, as temperature rose, more than 74.1% of the initial water content within the central region of the slime dough underwent vaporization. creating a pressure gradient between the interior and exterior of the coal slime dough. In the subsequent constant temperature stage, over 45.5% of the remaining water content within the slime dough was driven to the surface by the pressure gradient, where it vaporized and diffused into the surrounding atmosphere. During the reheating stage, moisture was initially vaporized as steam and subsequently diffused into the atmosphere through inter-particle voids, due to the inherent difficulty in forming a continuous liquid bridge between particles at this stage. Furthermore, we examined the influence of particle size and dough diameter of coal slime on moisture migration and diffusion during microwave drying, thereby substantiating the diffusion mechanism. By integrating experimental and simulation data, this study provides a detailed understanding of the moisture migration mechanisms within coal slime dough during microwave drying. These findings are valuable for the design of efficient microwave drying technologies, particularly for drying materials with high-water content and viscosity, such as typical coal slimes and sewage sludge.