Experimental study on microwave drying of Chinese and Indonesian low-rank coals

Drying characteristics of low rank coals in a local microwave oven operating at 2.45 GHz were investigated. Effects of coal particle size, microwave power level, and coal sample size on drying characteristics were studied. Weight losses and temperature history of the samples were measured during drying. Drying rates were then obtained vs. drying time and moisture content. It was found that drying rate was increased with increasing coal particle size showing different trends from conventional drying methods reported in the literature. It was also observed that drying rate increased with increasing power output of the microwave oven or with decreasing coal sample weight. Effects of coal ash content and coal type were also investigated by comparing Chinese and Indonesian low rank coals. Ash content seems to be the governing factor in determining the drying characteristics for coal samples with small particle sizes. Drying mechanism of coals under microwave drying conditions was also discussed.

Research highlights
► Loss of moisture caused the decrease in microwave absorption and drying rate.
► Drying rate increased and the drying time decreased with increasing coal particle size.
► At small particle sizes, drying rate increased with increasing ash content of coal.