Study on the Change of Dielectric Properties and Chemical-Mechanism during Coal Low-Temperature Oxidation

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 oxygencontaining 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 T 2 to T 3. Coal spontaneous combustion is one of the major disasters in collieries, and it had occurred in countries all over the world. Researchers reported that many of the coal spontaneous combustion areas were concealed 1,2. Accurate detection of the high-temperature zone in the early low-temperature oxidation stage is a major interest within the coal spontaneous combustion prevention and control field. Electromagnetic detecting technology is a conventional coal fire detection technology. Some researchers had made progress in locating the coal fire area using magnetotelluric field monitoring equipment. However, this method is greatly affected by the content of magnetic mineral components in the coal seam, especially pyrite 3. Some other researchers 4,5 had tried to locate coal fire by ground-penetrating radar technique based on voids and clefts induced by coal fires. Magnetic mineral components do not restrict this method, but it may be affected by formation fractures. The limitations on electromagnetic detection of coal fire may be reduced if we can find the mapping relationship between coal dielectric constant and temperature in low-temperature oxidation stage. The prediction time may be advanced as well. 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 also the critical parameters for the development of various geological exploration instruments (e.g., ground-penetrating radar, transient electromagnetic (TEM) instruments, etc.) 6-10. Therefore, 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. Many researchers had reported the di...

Structural Designs of Transparent Polyimide Films with Low Dielectric Properties and Low Water Absorption: A Review

“…The absorption of water and moisture evidently affects the dielectric properties of PI materials because water has a large dielectric constant of approximately 80 [ 46 , 47 ]. Consequently, water absorption by the bulk material, or the formation of a water layer on the surface of a PI film, decreases its efficiency for low-k applications.…”

Study on the thermal kinetics and microscopic characteristics of oxidized coal

“…3a). Firstly, the coal sample was evenly mixed with dry KBr at a mass ratio of 1:150, and then fully ground in an agate mortar (Zhu et al 2020b). The powder mixture was then placed in the mold and pressed by a tablet press (Fig.…”