Forced Convective Heat Extraction in Underground High-Temperature Zones of Coal Fire Area

Abstract

Coal fires exist in almost every coal-producing country and generate huge amounts of heat energy every year. In this paper, forced convective heat-extraction is presented as a method to exploit the potential heat in coal fire zones as an energy resource. A geological model of coal fire zones and a combustion model for underground coal in an O2-depleted atmosphere are established. The borehole layouts, the heat transfer medium (HTM) injection rates, and the cooling effect of the HTM on the coal and rock are analyzed using a three-dimensional (3D) simulation software (fluent). The results show that a borehole layout of multihole injection and oriented type proves to be suitable for coal fire zones. The simulation predicts that the temperature of the extracted HTM and the rate of heat extraction decrease as extraction time increases. The simulation further predicts that the temperature of the extracted HTM can be increased by reducing the rate at which the HTM injected. Additionally, the heat-extraction rate is more stable for relatively low HTM injection rates. The temperature of the coal fire zones can be reduced effectively by using forced convective heat-extraction, with the maximum temperature of the coal fire zones and the average temperature in the residual coal zone being cubic and quadratic function relationship of the heat-extraction time, respectively. This research provides a reference for waste-energy exploitation in coal fire areas.