Abstract
The study aims to investigate the variation of the coal dielectric properties during coal low-temperature oxidation and the effect of oxidation product on the coal dielectric properties. Four different types of coal were prepared under low-temperature oxidation condition, and the coal dielectric properties were measured with an impedance analyzer at frequencies ranging between 20 Hz and 30 MHz. The oxygen-containing functional groups in oxidized coal samples were semi-quantitatively evaluated using Fourier transform infrared spectroscopy. Low-temperature oxidation stage of coal spontaneous combustion could be predicted according to the change of coal dielectric properties in the process of temperature rise. It was found that the dielectric constant of coal with high water content decreased exponentially with temperature. For coal sample with low water content, the dielectric properties changed piecewise linearly with temperature. Coal dehydration was considered to be the reason for the decrease of the dielectric constant from 30 °C to 120 °C. The increase of the relative content of oxygen-containing functional groups, especially carbonyl compounds, could be the reason for the rise of the coal dielectric constant from T2 to T3.
Highlights
It is believed that the dielectric properties of coal are the basis for studying the propagation of electromagnetic waves in the coal seam
This study had established a temperature-dependent dielectric constant model of spontaneous combustion coal based on different moisture content and explained the principles of these changes from oxidation products
The following main conclusions were drawn: (1) Based on different water contents, the relationship between coal dielectric constant and temperature during spontaneous combustion was described by different mathematical models
Summary
It is believed that the dielectric properties of coal are the basis for studying the propagation of electromagnetic waves in the coal seam. The dielectric properties are the critical parameters for the development of various geological exploration instruments (e.g., ground-penetrating radar, transient electromagnetic (TEM) instruments, etc.)[6,7,8,9,10] It is of great theoretical and practical value to study the coal dielectric properties under low-temperature oxidation condition for the detection of concealed coal low-temperature oxidation areas. Current studies have not dealt with the dielectric properties and corresponding chemical mechanism of coal at the low-temperature oxidation stage. This stage is believed to be the most critical stage for preventing and controlling spontaneous coal combustion[17,18,19,20].
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