Effect of coal rank on structure and dielectric properties of chars

The study was undertaken to investigate the effect of coal rank on dielectric properties, through structural differences of various coal chars. Chars from three different coals were prepared in the temperature range 850-1600 °C, and dielectric properties were determined by a vector network analyzer in the 2-18 GHz frequency range. Crystalline structure of char samples was characterized using X-ray diffraction, and the relative concentration of aromatic structure was further assessed by Fourier transform infrared spectroscopy. Results show that the relative dielectric constant and dielectric loss factor of chars largely depended on coal rank, and carbon layer stacking in combination with aromatic carbon content, related to orientation polarization, could account for dielectric properties of chars from different rank coals. Moreover, a considerable amount of silicon carbide was found in higher temperature chars, which greatly improved microwave dielectric absorption.