Simulation of dynamic temperature evolution in an underground coal fire area based on an optimised Thermal-Hydraulic-Chemical model

Underground coal fires (UCFs) exist in almost all coal mining countries. In this paper, an optimised Thermal-Hydraulic-Chemical model, which includes variable reaction kinetics of coal and permeability variation in UCF zones, was developed for the simulation of dynamic temperature evolution of an actual UCF in Xinjiang, China. The model was also adopted in the analysis of the effect of surface coverage permeability on the temperature field, providing a theoretical reference for UCFs control. The results demonstrate that the temperature evolution included three distinct stages which were slow-heating stage, rapid-heating stage and stable-development stage. The slow-heating stage was occupied by the low-temperature oxidation of residual coal. During the rapid-heating stage, a hotspot (combustion centre) formed in residual coal zone and moved toward the air inlet side when the temperature reached a certain threshold. And before moving back into the deeper coal seam, the hotspot developed the temperature to the maximum when it got to the air inlet side. In the stable-development stage, the hotspot spread along the coal seam with the maximum temperature staying between about 800 degrees C and 1000 degrees C, creating elliptical temperature anomalies around the combustion centre. The simulation proved to be in good agreement with the in situ measurements. Surface covering will extinguish the UCF when the permeability of the coverage layer is lower than 10(-10) m(2), otherwise the fire will persist and continue to spread.