Study on the effects of oxidation temperature and ambient humidity on the terahertz spectroscopy characteristics of lignite

In this paper, the terahertz (THz) spectroscopy characteristics of lignite with different oxidation temperatures under different ambient humidity were investigated by terahertz time-domain spectroscopy (THz-TDS), and the mechanism was analyzed by combining the synchronous thermal analyzer. Results indicate that the refractive index of lignite has abnormal dispersion characteristics. THz waves have different transmission characteristics in lignite with different oxidation temperatures, which is reflected in the amplitude difference of time-domain and frequency-domain signals. The complex refractive index of lignite shows obvious oxidation temperature dependence, which is closely related to the physicochemical changes that occur during coal oxidation. Besides, ambient humidity has a significant effect on the THz spectroscopy measurement, which follows the Beer-Lambert law. As humidity increases, the amplitude of time-domain and frequency-domain signals decays linearly, while the complex refractive index increase linearly. Humidity has a similar effect on coal samples with different oxidation temperatures, which means that based on linear effects and good fit results, THz spectroscopy measurements in humid environments can be corrected by a simple linear empirical formula. The results of this study are helpful to study the spectroscopy characteristics of coal in the THz band, and provide guidance for the application of terahertz technology in coal mines.