Dynamic evolution of terahertz permittivity of lignite during oxidation: Microstructural insights

Terahertz technology holds promise for monitoring coal fires and assessing associated risks, relying on the dielectric response of coal during the oxidation process. Besides, unraveling the response mechanism is crucial for enhancing the application's reliability. In light of these, this study focused on investigating the terahertz complex permittivity of lignite with different oxidation temperatures through microstructure analysis. Results indicated that oxidation temperature dependencies in the real and imaginary parts of lignite's permittivity, exhibiting a pattern of initial decrease followed by an increase. The decrease in permittivity during the dehydration and weight loss stage is primarily attributed to moisture content. In the oxygen absorption and weight gain stage, pore development and scattering effects contribute to the continuous decrease in real permittivity and rise in imaginary permittivity. During the oxidative decomposition stage, the hydroxyl group initiates an increased real permittivity. Upon reaching the combustion temperature, there is a sudden surge in permittivity, driven by the dominance of minerals in coal dielectric properties. The conclusions offer theoretical support for terahertz technology in monitoring coal fires, which can contribute to the sustainable development of the coal industry, mineral processing, and safety production.