Relationship between the main functional groups and complex permittivity in pre-oxidised lignite at terahertz frequencies based on grey correlation analysis

The complex permittivity of lignite at different pre-oxidation temperatures is a key criterion for its application in the identification of oxidised lignite at terahertz (THz) frequencies. This study investigated the micro-characteristics and THz frequency permittivity of lignite heated at different pre-oxidation temperatures. Thus, Fourier transform infrared spectroscopy, scanning electron microscopy, and THz time-domain spectroscopy were employed. The relationship of the main functional groups and complex permittivity of lignite with the pre-oxidation temperature was determined by grey correlation analysis. The results showed that, with an increase in the released gases, the pores developed in the cracks generated several small particles. The active and maximum mass temperatures were the critical temperatures for the relative intensities of the main functional groups. From the active temperature to the maximum mass temperature, the main functional groups affected the complex permittivity owing to the dipole moment and relative molecular mass. Moreover, wave scattering caused by the microstructure of lignite was the main reason for the imaginary permittivity changing at certain pre-oxidation temperatures. The imaginary permittivity and dielectric loss tangent decreased at the active temperature, then increased and maintained a maximum at the intermediate temperature between the maximum mass and ignition temperatures until it finally decreased.