Experimental study on evolution and mechanism for dielectric response of different rank coals in terahertz band

Abstract

Terahertz technology holds the potential for applications in identifying risks and development stages of coal spontaneous combustion (CSC). However, one critical factor influencing the precision of THz technology in CSC applications is the coal rank. In light of these, this study aimed to investigate the permittivity (0.4–1.6 THz) of different rank coals. Additionally, its underlying mechanism was analyzed utilizing Fourier-transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy. Results indicate that increased coal rank leads to a decrease in both real permittivity (from 2.462 to 2.327) and imaginary permittivity (from 0.0447 to 0.0250), primarily due to diminished moisture (from 12.82 % to 1.35 %). After moisture removal, increased coal rank results in an increased real permittivity (from 2.292 to 2.315), primarily due to the carbon crystallite (interlayer spacing changed from 3.553 to 3.529, and stacking height shifted from 14.805 to 19.743) and microstructure profiles. Moreover, minerals are identified as another primary factor affecting real permittivity. For imaginary permittivity, increased coal rank leads to a decrease (from 0.0411 to 0.0245), primarily due to oxygen-containing functional groups (from 42.96 % to 31.71 %). The conclusion advances the fundamental theoretical research on coal permittivity, providing a solid theoretical foundation for THz technology in CSC applications.