Coal slime hot air/microwave combined drying characteristics and energy analysis

•The hot air/microwave combined drying process was initially applied to coal slime drying.•The moisture contents of hot air drying switching into microwave drying were 15%.•The effects of different factors on drying performance and energy consumption were studied.

The drying of coal slime with high moisture content is indispensable for the majority of its industrial applications. In view of the low drying rate at the falling-rate drying period during the conventional drying process, hot-air drying followed by microwave drying, a featured deep drying method, can fundamentally solve this problem and achieve fast and efficient dewatering. In this study, the combination of hot-air and microwave drying to dry coal slime was presented, the general behavior of combined drying method was investigated at a combined fixed bed reactor with hot-air in the temperature range of 120-200 °C and a microwave oven with a maximum power of 800 W. The critical point of hot-air drying, also termed transition point at which hot air drying switched into microwave drying, were experimentally determined and the results showed that the moisture contents of the critical points were about 15% for the samples used. Furthermore, the effects of different target moisture contents and microwave powers on the drying performance and energy consumption of combined dewatering were studied. The results showed that microwave drying could significantly elevate the drying rate when the coal slime reached the critical moisture contents during the hot-air drying method, which indicated that combined drying was an effective method to achieve deep drying of coal slime with a significant reduction in total drying time.