Terahertz dielectric permittivity of coal with different metamorphism degrees based on grey relational analysis

As a critical parameter in electromagnetic detection for coal mine safety, the dielectric permittivity of coal is influenced by its structural characteristics. In addition, electromagnetic waves with terahertz(THz) frequencies have extensive application potential in coal mining. Therefore, in this study, the terahertz dielectric permittivity of coals with varying metamorphism degrees was measured using the terahertz time-domain spectroscopy, and its evolution mechanism was investigated through grey relational analysis based on proximate components. Experimental results indicate that the Landau-Lifshitz-Looyenga model provides the most accurate description of the dielectric behavior of coal tablets. Both the real and imaginary parts of the terahertz dielectric permittivity exhibit a decreasing trend with increasing metamorphism degrees. Due to the highest grey correlation, moisture and ash are the main factors affecting the dielectric properties of coal. Specifically, the decrease in moisture content with increasing coal rank primarily accounts for the decline in permittivity. Ash content is the key factor causing the increase in the real part of non-stick coal and imaginary part of long-flame coals due to its high dielectric permittivity and loss. These findings can enrich the theoretical understanding of coal's dielectric behavior and offer a valuable reference for the application of electromagnetic detection technologies in coal mining.