Analysis of influencing factors in permittivity of oxidized lignite by FTIR, XRD, and THz-TDS based on orthogonal experiment

As an organic-rich and mineral-rich mixture, the permittivity of oxidized lignite is dependent on several factors in the terahertz (THz) band. In this study, thermogravimetric experiments were conducted to determine the characteristic temperatures of three types of lignite. The microstructural characteristics of lignite after treatment at 150, 300, and 450.C were investigated by Fourier transform infrared spectroscopy and X-ray diffraction. The results show that the changes in the relative contents of C-O and Si-O are contrary to those of -OH and -CH3/-CH2- with temperature variation. The relative content of C=O is unpredictable at 300 degrees C. The aromaticity and crystallite diameter show a regular variation with temperature. The microcrystalline structure of coal tends to undergo graphitization with temperature. The variation of crystallite height is random at 450 degrees C. The uniform variation in microstructural characteristics of different types of lignite with oxidation temperature proves the feasibility of identifying oxidized lignite by THz spectroscopy. Based on orthogonal experiment results, the influence of the coal type, particle diameter, oxidation temperature, and moisture content on the permittivity of oxidized lignite in the THz band was arranged in an order. The order of the factors' sensitivity for real part of permittivity is oxidation temperature > moisture content > coal type > particle diameter. Similarly, the order of the factors' sensitivity for imaginary part of permittivity is oxidation temperature > moisture content > particle diameter > coal type. The results reveal the capability of THz technology to characterize the microstructure of oxidized lignite and provide guidance for minimizing error in THz technology.