In the deep mining process of coal seams, the mechanical environment of the coal body is complex and in the state of cyclic loading and unloading. The change in the stress state leads to the change in the pore characteristics and the permeability. To investigate the effects of cyclic loading and unloading on the pore characteristics and the permeability of coal, the seepage experiment was carried out for the coal samples using the self-developed triaxial permeation instrument. By pressure confining and continuous cyclic loading and unloading, the evolution of the porosity and the permeability of the coal samples was investigated. Under the condition of the experiment, the influences of the initial value of the confining pressure and the cyclic load amplitude on the evolution of the permeability and the pore structure characteristics of the coal samples were clarified. The experimental results showed that the porosity and the permeability decreased exponentially with an increase in the number of loading and unloading cycles, and this decreasing trend gradually weakened until the porosity and the permeability became relatively stable. When the cyclic load amplitude was the same and the confining pressure increased, the effective porosity of the coal body and the bound porosity decreased. When the confining pressure was the same and the cyclic load amplitude increased, the effective porosity of the coal body decreased and the bound porosity increased. The loss rate of the permeability of the coal samples increased gradually with an increase in the cyclic load amplitude. The same tendency was observed when the cyclic load amplitude was the same, and the confining pressure was different, while the increasing trend was not so obvious. By analyzing the relationship between the porosity and the permeability of the coal samples under different cyclic loading and unloading paths, it was found that the effective porosity and the permeability of the coal samples conformed to the power–law relationship in the process of cyclic loading and unloading, and the change in the cyclic load amplitude had a significant effect on this relationship. The influences of the cyclic load amplitude and the confining pressure on the stress sensitivity of the coal samples were considered, and the change factor of the stress sensitivity was introduced into the relationship between the porosity and the permeability. This relationship was established considering cyclic loading and unloading.

The objective of this study was to evaluate the effect of injecting flue gas (CO₂, N₂, and O₂) originating from coal-fired power plants into a coal seam on CH₄ extraction and CO₂ geological storage. To this end, a multifield thermal-fluid-solid-coupled mathematical model of flue gas injection extraction was established. The results showed that with the increase in time increase, the volume concentration of CH₄ decreased, but the CO₂, N₂, and O₂ increased. Compared with single extraction, the gas injection extraction brought about a significant reduction in the pressure and content of CH₄, an increase in the CH₄ extraction rate, and an increase in the effective radius of CH₄ extraction. In the single extraction, the temperature of the reservoir decreased, and its permeability increased. In the gas injection extraction, the temperature near the gas injection hole increased, whereas the temperature near the extraction hole decreased, and the permeability decreased overall. A method of measuring the effective radius of gas extraction by temperature is presented. The storage and extraction times of CO₂ exhibited a linear relationship, and the CO₂ escape rate increased gradually. The longer the gas injection extraction time, the greater the risk of coal and gas (CO₂) outbursts.

In view of the problem that excessive CO in underground coal mine space can easily lead to a large number of casualties, Cu-Mn-Sn water-resistant eliminators with different Sn contents were prepared by a co-precipitation method. The activity of the eliminators was analyzed by using an independently developed activity testing platform, N-2 adsorption and desorption, XRD, SEM, XPS, and FTIR to characterize the activity factors and water resistance. The results showed that Cu-Mn-Sn-20 with 20% Sn content had the highest activity, which was 3.23 times that of Cu-Mn. The main reason for the increased activity is that Cu-Mn-Sn-20 doped with 20% Sn provides a larger specific surface area and more active sites and reduces the pore size, so that the crystallization degree of Cu1.4Mn1.5O4 is lower. The doping of 20% Sn reduces the absorption of lattice water and coordination water and improves the water resistance of Cu-Mn-Sn-type eliminators. The Cu-Mn-Sn-20 water-resistant eliminator is used to quickly eliminate CO in underground coal mines, which is of great significance for the rescue workers in underground coal mines after disasters.

This paper investigated the effectiveness of a vent burst door in isolating the shock waves of gas explosions in coal mines and to reveal the mechanisms of gas explosion overpressure. A 17.5 m × φ 0.63 m columnar explosion test duct with a similar shape to the roadways in coal mines was designed. Explosion overpressure propagation characteristics and the effects of methane concentration at different locations along the duct with and without a vent burst door were experimentally investigated. The results demonstrate that the vent burst door plays a significant role in sealing the explosion area and isolating the explosion propagation. A properly designed vent burst door is a promising technique for suppressing and isolating multiple and continuous gas explosions and is poised to have a significant positive impact on improving the safety of coal mines in China and all over the world.

To investigate the compressive strength and permeability of deep mudstone under stress disturbance, a triaxial rheometer is used to conduct seepage experiments on mudstone specimens with different buried depths under triaxial compression and unloading conditions. The experimental results show that the compressive strength of mudstone specimen with a depth of 1000 m is much lower than that of specimen with a depth of 200 m, and the compressive strength of mudstone increases with the increase in confining pressure. Under constant axial pressure and unloading of the confining pressure, the mudstone with a depth of 200 m exhibits brittle failure, and the strain fluctuates in a pointwise manner with the increase in axial stress. In this case, the mudstone with a depth of 1000 m exhibits a transition from brittle failure to ductile failure, and the strain fluctuates linearly with the axial stress. Further, when the volumetric strain change reaches 0.01, it shows an oblique “Z” fluctuation. During the initial stage of unloading of confining pressure, the permeabilities of both the mudstone specimens (with depths of 200 and 1000 m) decrease gradually. As the confining pressure is unloaded, the permeability of mudstone with a depth of 1000 m increases. Until the specimen is completely destroyed, the permeability of mudstone increases rapidly. Overall, this study can serve as a useful reference for analyzing the engineering disasters associated with deep rock mass, tunnel ventilation, and gas storage.