Optimization of drying of low-grade coal with high moisture content using a disc dryer

Abstract

In this study, low-grade coal with high moisture content was dried in a novel disc dryer equipped with a heating plate and rotary blades. Raw coal was fed into the center of the disc dryer and then was transferred from the center of the heating plate to the outside of the plate by the rotary blades during the drying process. According to a numerical analysis using a model of a single solid spherical particle without any porosity, the temperature of the coal particles reached that of the heating plate within 5min. The analysis also showed that at a heating plate temperature of 150°C, the moisture content of the raw coal was lowered from 34% to below 3% within 5min, and all of the raw coal was dried after 10min of drying time. Experimental studies were used to investigate the influence of the following factors on the effectiveness of the novel disc dryer: heating plate temperature, coal feed rate, rotational speed of the rotary blades, drying environment, and position of the coal on the heating plate. When the heating plate temperature was high, the moisture content of the dried coal was greatly decreased. However, on the basis of energy-efficiency considerations, it was recommended that the temperature of the heating plate be maintained at 150°C. Furthermore, a decrease in the coal feed rate could lower the moisture content, and a high rotational speed of the rotary blades could slightly reduce the moisture content as well. In addition, the reduction in moisture content could be remarkably enhanced by using a vacuum pump to remove the evaporated water vapor from the inside of the dryer. The position of the coal on the heating plate was also important. The temperature of the raw coal could be increased without evaporation up to a certain distance from the center of the dryer. However, the moisture of the raw coal began to evaporate in a region near the outside of the heating plate before discharging, where the raw-coal temperature reached about 100°C. Overall, it was concluded that the size and temperature of the heating plate should be considered in the design and operation of a disc dryer for drying low-grade coal. In addition, the dispersion of raw coal on the heating plate was important during the drying process. The disc dryer could enhance the conductive heat transfer coefficient between raw coal and a heat source and mixing of the coal to reduce drying time.