Relationship between the characteristic temperature and terahertz permittivity of different rank coals considering FeS2 content

Abstract

In this study, the characteristic temperatures and terahertz permittivity of lignite with varying pyrite content were determined using a synchronous thermal analyzer and terahertz time-domain spectroscopy. A mathematical relationship between these two parameters was established by combining experimental data. The results indicate that within the range of 0–9% pyrite content, there is a trend where pyrite initially enhances and then weakens the promotion effect on coal spontaneous combustion. A quadratic polynomial can describe this relationship, with the extremum occurring at 6%. Regarding permittivity, as the pyrite content increases from 0% to 9%, the real part of permittivity exhibits irregular variations, while the imaginary part shows a well-defined linear increase. This behavior is attributed to enhanced relaxation polarization, leading to more significant dielectric losses. Furthermore, through the combination of experimental data, a robust quadratic relationship is observed between characteristic temperature and terahertz permittivity. The credibility of this relationship is validated by comparing it with experimental results. These findings contribute to the advancement of the theoretical understanding of coal's permittivity and spontaneous combustion characteristics, and provide a theoretical possibility for the rapid estimation of spontaneous combustion characteristics in pyrite-bearing coal seams using electromagnetic methods.