Dielectric characterization of Indonesian low-rank coal for microwave processing

The present study aims to provide a preliminary guide for microwave processing of low-rank coals by investigating the dielectric characteristics of an Indonesian low-rank coal based on determination of its permittivity from 21 degrees C to approximately 1000 degrees C at 915 and 2450 MHz in argon. The results show that the relative dielectric constant of the coal gradually decreases with increasing temperature below 200 degrees C due to dewatering. It then has a considerable increase up to 750 degrees C because of devolatilization and carbonization, preceded by a slight decrease due to physical-chemical and chemical processes connected with the mobility of macromolecular chains of the coal. Thereafter, it increases with a declined rate ascribed to the coal's continuous structure transformations. The dielectric loss factor shows a similar variation, except the presence of the dielectric loss peak at about 600 degrees C, associated with rise of microwave loss. The microwave penetration depth of the coal presents a depth peak at approximately 400 degrees C, indicating potential application of variable frequency microwave heating to large-scale processing. The reflectionbss (RL) patterns reveal the strong coal slab thickness dependences of magnitude and location of absorption peaks in the range 0.02-0.20 m, with minimum RL values of -4239 dB and - 47.92 dB obtained at thicknesses of 0.16 and 0.06 mat 915 and 2450 MHz, respectively. This dissimilarity is primarily attributed to the different dielectric parameters and wavelengths at these frequencies. Microwave heating functions sufficiently in a narrow temperature range from about 500 to 700 degrees C, in which target temperature can be selected for efficient coal processing. (C) 2016 Elsevier B.V. All rights reserved.