Mitigation of coal spontaneous combustion and enhanced coalbed methane recovery using liquid CO₂: Mechanisms, field applications, and implications for mines

Abstract

Spontaneous gas and coal combustion represent primary disasters threatening the safety of underground coal mines. Achieving the collaborative governance of the two disasters and enhancing the ability to prevent and mitigate mine disasters are technical challenges faced by high-gas/outburst mines. CO2 has become the primary choice for collaborative disaster governance because of its efficient control of the oxidation process of residual coal in goaf, enhanced coalbed methane (ECBM) recovery, and the goal of “2030 carbon peak and 2060 carbon neutralisation”. Therefore, this study adopted summary and engineering verification methods. Firstly, the basic physical and chemical properties of CO2 were analysed, and the three mechanisms of action of liquid CO2 for preventing coal spontaneous combustion (CSC), namely, “CO2 adsorbed and hindered oxidation reactions, absorbs ambient heat and reduces ambient temperature, and reduce the oxygen concentration in the goaf and inhibiting gas explosion”, and the six mechanisms of action of liquid CO2 ECBM recovery, namely, “pressure fracturing, low-temperature frostbite, physical extraction and chemical corrosion, low-viscosity permeability, phase change pressurisation, and competitive adsorption”, were summarised. Second, the effect was verified by the field application of liquid CO2 CSC emergency prevention and control at the Qinggangping Coal Mine and the engineering test of liquid CO2 ECBM recovery in the Shuanglong Coal Mine. Finally, based on the application status of liquid CO2 in coal mines, a new model of “liquid CO2 prevention and control of CSC and enhancing coalbed methane recovery comprehensive disaster reduction technology” is proposed. The results of the emergency prevention and control of liquid CO2 CSC show that CO2 sinking drives CH4 out of the roadway, avoids the accumulation of CH4 near the fire area, and achieves explosion suppression. The concentrations of C2H2 and C2H4 in the mine decreased rapidly to 0. No open fire or severe combustion occurred in the mine, and the fire area was effectively controlled. After the ventilation of the mine was restored, the isolated and closed 42108 working face was injected with liquid CO2 again. The CO concentration of the inlet and return air along the channel gradually decreased to zero, and the fire area of the working face was further controlled. The engineering test of liquid CO2-ECBM recovery showed that the dominant seepage range was 1215 m from the injection hole, and the dominant diffusion range was 2530 m from the injection hole. The average CH4 flow rate in the field extraction test was more than three times that of the original area. Through two field cases, long-distance liquid CO2 prevention and control of CSC and an ECBM recovery technical framework were proposed, which are of great significance for further improving mine disaster prevention and mitigation.

Keywords

• Underground coal mine
• Liquid CO2
• Coal spontaneous combustion
• Coalbed methane
• CO2-ECBM