Oxidation temperature-dependency of relative dielectric constant and reflective characteristics of lignite and bituminous coal in the presence of very high frequency to ultra-high frequency bands

• We found that coal combustion is initiated when the ambient temperature exceeds the maximum mass temperature. • The variation in the very high frequency to ultra-high frequency relative dielectric constant with increasing oxidation temperature was determined for four coal types. • The reflective characteristics of underground coal fires filled with burning coal and a mixture of burning coal/air at each stage of combustion were identified. This study investigates the relative dielectric constant with very high frequency (VHF) to ultra-high frequency (UHF) as a basis for ground penetrating radar (GPR) applications in underground coal fires (UCFs) detection. In conjunction with an effective medium model, we methodically analyze the dielectric properties and reflection characteristics of four coal types under increasing oxidation temperatures using XRD, thermogravimetric and relative dielectric constant experiments. Our findings indicate that coal combustion is initiated when the ambient temperature exceeds the maximum mass temperature. According to the cracking temperature and maximum mass temperature, the combustion process of coal can be rationally divided into three distinct stages: water loss, slow oxidation, and intense oxidation. Moisture content is identified as a key factor affecting the variations of VHF-UHF relative dielectric constant among various coals. The relative dielectric constant of coal at VHF-UHF frequencies lessens with growing frequency. During combustion, water loss and rapid oxidation stages are the main factors for decreasing VHF-UHF relative dielectric constant. The UCFs’ reflection coefficient is negatively correlated with the detection frequency, demonstrating a continuous rise as the combustion progresses, and the largest increase occurs in the intense oxidation stage. When GPR detects the cavities resulting from coal combustion , the temperature associated with the maximum reflection coefficient lessens. This research not only demonstrates the feasibility of applying GPR for UCFs detection but also provides data support for the fairly precise interpretation of GPR images of UCFs.