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 1215 m from the injection hole, and the dominant diffusion range was 2530 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.