Microwave drying performance of single-particle coal slime and energy consumption analyses

The drying of coal slime with high moisture is an essential step for the majority of its industrial applications. Microwave radiation could serve as an alternative way of efficient drying, owing to its unique heating properties. In this study, the weight loss and temperature distribution of spherical single-particle coal slime were measured by using a microwave thermogravimetric analysis (MTGA) device, which was designed for this study, equipped with an accurate electronic balance and fiber optic thermometers. In addition, the effects of microwave power (320, 480, 640, and 800 W) and particle size (30, 40, 50, and 60 mm) on the drying performances were studied. The microwave drying process of coal slime comprises three stages: an incubation period, whereby a rapid increase in temperature occurs, a constant-rate drying stage, during which a rapid moisture loss occurs at about 100 degrees C, and a falling-rate drying period, during which a significant increase in temperature is observed again. Weight loss rates increased with increasing power output of the microwave oven or with decreasing particle size. The energy consumption and the energy efficiency during the drying process were also analyzed. The results demonstrated that the specific energy consumption declined with the overall increase in power and particle size, while the efficiency of microwave output increased with increasing particle size. Furthermore, the diffusion kinetics of moisture in coal slime were examined. The effective diffusion coefficients of moisture increased with both increasing particle size and microwave power, whereas the activation energy of the diffusion process decreased with increasing particle size. (C) 2015 Elsevier B.V. All rights reserved.