Study on the thermal effect of coal oxidation under different stresses

The utilization of coal resources has been improved by using the method of narrow coal pillar mining, but this leads to a stress concentration in the coal pillars, which causes differences in the oxidation of coal pillars. To study the effect of stress on the oxidation and spontaneous combustion of coal samples, programmed heating-gas chromatography coupling experiments were carried out on coal samples under different stresses, analyzing the effect rule of stress on the gas derivatives of coal samples in the process of heating and oxidation. Furthermore, the mechanism of stress influence on thermal effect parameters is explored on the basis of that analysis. The results show that the rate of oxygen consumption, CO, CO2 concentration and heat release intensity of coal samples show a changing trend, initially increasing and then decreasing with increasing stress, and these values within coal are at the maximum when the stress is 9 MPa; and with increasing stress, the activation energy shows a "V" type change and reaches the minimum of 26.89 kJ/mol at 6 MPa, which indicates that low stress promotes coal spontaneous combustion (CSC), while high stress inhibits CSC. The thermal conduction coefficient of coal samples shows a negative correlation with temperature at the low-temperature stage, while the thermal conductivity of coal samples shows a positive correlation with temperature at the high-temperature stage, and the thermal conduction coefficient of coal samples reaches a minimum at temperatures of 70 °C and 0 MPa of stress. The porosity within coal decreases, and the thermal conductivity coefficient within coal increases with increasing stress because the increase in stress makes the macromolecules within coal disassemble into small molecules, the structure becomes more compact, and the thermal conductivity increases. The study provides an important theoretical basis for better understanding the effect mechanism of stress on CSC.

Response of thermal conductivity of loess after high temperature in northern Shaanxi burnt rock area, China

Spontaneous combustion of coal seams can produce a high temperature of about 800 ℃, which greatly changes the thermal conductivity of the overlying loess layer. The thermal conductivity of loess plays an important role in ecological restoration design and the calculation of roadbed and slope stability. In this study, loess in northern Shaanxi, China was taken as the research object to measure the mass-loss rate and heat conduction parameters of loess specimens after high temperature. The test results show that, between 23 and 900 °C, with temperature increasing, the mass-loss rate is reduced. And the heat conduction coefficient (λ), specific heat capacity (c), and thermal diffusion coefficient (α) decreased by 48.9%, 23.1%, and 35.6%. This is due to the air thermal resistance effect caused by the increase of pores and cracks in loess specimens after high temperature.