In coal mine goafs, spontaneous combustion of coal can result in methane accumulation, which raises the danger of methane explosion disasters. As an atmospheric control tool, ventilation is applied to ensure air quality for avoiding disasters in underground mines. However, during the process of the spontaneous combustion of coal in coal mine goafs, the impact of ventilation dilution on the possible methane explosions induced by coal combustion has not been well investigated. In this study, a validated gas flow model for the spontaneous coal combustion environment in goafs of coal mines is adopted to investigate the influence of ventilation dilution at the three stages of the spontaneous combustion of coal. The research conclusions suggest that (1) ventilation dilution is a quick measure to dilute methane concentration and intensify heat transfer in the vertical direction in coal mine goafs; (2) ventilation dilution can lessen the danger of methane explosions by diluting methane concentration to the lower explosive limit for methane when coal combustion takes place on the air-inlet side; (3) however, ventilation dilution increases the methane explosion risk by decreasing methane concentration, resulting in explosive methane limits, if coal combustion occurs on the air-return side. This provides a reference for the management of ventilation during a spontaneous coal combustion disaster in the goafs of coal mines with methane.

Coal permeability is related to the fracture-pore structure of coal and is a key factor in determining gas drainage efficiency. The characteristics of the methane flow in coal fractures are different from those in coal matrix pores. However, due to the difficulty of observing fast methane flow in coal fractures, the effect of gas flow in coal fractures on coal permeability has seldom been considered and investigated. In this study, a cylindrical coal sample is used for the measurement of coal permeability under different gas pressures, and an abrupt change in coal permeability evolution was observed. Then, a tandem fracture-pore permeability model was adopted to analyze these new methane flow phenomena. In this permeability model, the deformation of coal fractures was directly analyzed and modeled without the reversed derivation. With the consideration of elastic modulus of coal fractures, the deformation of coal fractures is controlled by the effective strain of coal fractures, the adsorption-induced strain and effective strain of coal matrix. The research results show that (1) coal fractures quickly and significantly influence coal permeability by resisting coal deformation; (2) a complete evolution of coal permeability consists of the fast permeability change caused by methane flow in coal fractures and the slow permeability change caused by methane flow in coal matrix; (3) the low efficiency of gas mass exchange between coal fractures and coal matrix leads to a two-stage evolution for gas desorption flow and coal permeability.

For adjacent coal seams, the downward invasion of harmful gases from an overlying coal mine goaf to the lower mining face could occur, and a high-level drainage tunnel is usually adopted for disaster prevention. Due to the high cost, instead of a high-level drainage tunnel, the high-level directional boreholes are widely adopted. In this study, the effect of a high-level drainage tunnel to prevent the downward invasion of carbon dioxide from the overlying coal mine goaf is analysed by applying a flow model in the numerical simulation. Then, the high-level directional boreholes are analysed to investigate the possibility of taking the place of the high-level drainage tunnel. The research results show that (1) for close adjacent coal seams, the downward invasion range of harmful carbon dioxide from the overlying coal mine goaf reaches one-third of the mining face, around 60 m wide; (2) a high-level drainage tunnel can effectively prevent the downward invasion of carbon dioxide from the overlying coal mine goaf by reducing carbon dioxide concentration within 0.3%; (3) the nine high-level directional boreholes with a careful layout can reduce the downward invasion of carbon dioxide from the overlying coal mine goaf to keep carbon dioxide concentration below 0.8% at the mining area, ensuring the coal mining safety.

In China, coal remains the dominant component of the energy mix and continues to underpin national economic activity (Bai et al. 2022; Zhang and Ponomarenko 2023). With increasing mining depth and ...