Investigation on the microwave drying kinetics and pumping phenomenon of lignite spheres

The influence of microwave power levels (231-700 W) on microwave drying behavior of compressed lignite spheres (10 mm and 20 mm in diameter) in a modified microwave drying setup was evaluated. The microwave energy distribution in the sample was obtained through numerical simulation based on the Maxwell equation. The temperature distribution in the samples was detected through an infrared camera. The average drying rates for the 10-mm and 20-mm samples increased by 73% and 56% as the microwave power level rose from 231 to 700 W. The microwave energy mainly concentrated on the lower part of the geometric center of the samples and the uniformity index increased by 32% as the sample size rose from 10 to 20 mm, while the uniformity index of temperature distribution increased by about 2.5%. The values of activation energy for the 10-mm and 20-mm samples were determined as 134 and 41.93 W g−1 based on a modified Arrhenius equation. The pumping phenomenon was only detected on the surface of 20-mm raw lignite sphere block in the warm-up period at power levels of 539 and 700 W.