Dielectric characterization of Indonesian low-rank coal for microwave processing

- Dielectric parameters of coal fluctuate considerably with increasing temperature.
- Effective dielectric loss of coal relies on polarization and electronic conduction.
- Power absorption associates with wave periodicity restrained by coal dimension.
- Variable frequency microwave heating may promote upgrading of low-rank coals.
- Selecting target temperature between 500 and 700 °C is crucial for coal 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 °C to approximately 1000 °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 °C due to dewatering. It then has a considerable increase up to 750 °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 °C, associated with rise of microwave loss. The microwave penetration depth of the coal presents a depth peak at approximately 400 °C, indicating potential application of variable frequency microwave heating to large-scale processing. The reflection loss (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 − 42.39 dB and − 47.92 dB obtained at thicknesses of 0.16 and 0.06 m at 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 °C, in which target temperature can be selected for efficient coal processing.