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C2H4 is used as an indicator gas to evaluate the spontaneous combustion process of coal, and the adsorption of C2H4 by coal will cause the decrease of C2H4 gas volume fraction in mine air, which in turn affects the accuracy of the assessment of the spontaneous combustion process of coal. In order to reveal the microscopic mechanism of C2H4 adsorption in bituminous coal, a molecular model of Wiser bituminous coal was constructed, and the adsorption process of C2H4 in the molecular model of bituminous coal at temperature of 293.15-313.15 K and pressure of 0.1-3 MPa was simulated using Monte Carlo method, and the changes of adsorption heat, adsorption potential energy and adsorption entropy during the adsorption process were analyzed. The results showed that the adsorption of C2H4 in bituminous coal molecules first increased rapidly with increasing pressure and then leveled off; the higher the temperature, the lower the adsorption of C2H4 when the pressure was the same; Langmuir-Freundlich model has the best fitting effect on C2H4 adsorption process, and the maximum adsorption a of C2H4 decreases with increasing temperature; the average heat of adsorption of C2H4 under different temperature conditions is 35.29-37.65 kJ/mol, indicating that the adsorption of C2H4 in bituminous coal is physical adsorption; the adsorption potential energy and entropy of C2H4 decreased

Liquid CO2 fracturing in low-permeability coal seams has dual gas-enhanced drainage effects of fracture reconstruction and displacement. It is a permeability enhancement technology that is easy to be used in combination with other technologies and has the potential for large-scale application. This paper proposes a theoretical system and a complete set of technical framework for fracturing and permeability enhancement of low-permeability coal seam with liquid CO2; according to the principle of stress superposition and the maximum tensile stress criterion, a model for determining the initiation pressure of liquid CO2 fracturing was established considering the change of phase state and low viscosity and strong permeability of liquid CO2 during the injection process; taking type I fracture criterion as the discriminant condition of fracture propagation, the quantitative characterizing relationship between fracturing orifice pressure and fracture propagation distance under constant fluid injection flow was established, and the influence law of fluid injection pressure, fracturing fluid viscosity and fracture occurrence on fracture propagation was clarified. On this basis, the four key parameters and their calculation methods to determine the performance of the complete set of equipment for liquid CO2 fracturing and permeability improvement in coal seam were proposed, the first complete set of equipment for underground fracturing and permeability improvement in China was developed, and the construction technology of fracturing and permeability improvement was developed, which was applied in Huainan Mining Area and Binchang Mining Area respectively.

China's coal seam permeability is low, and the original coal seam gas extraction is difficult. CO2 displacement of coal seam CH4 technology is an effective gas extraction technology. CO2 is injected into the coal seam under pressure, and competitive adsorption occurs with CH4 in the pores. The gas composition is nonuniformly distributed, and its viscosity μ is the dynamic parameter. As the gas is compressible, the pressure drops, and migration distance does not satisfy a linear relationship. Therefore, the gas transport does not conform to Darcy's law. The mass transfer process and a multicomponent gas competitive adsorption were investigated theoretically and experimentally. The adsorption characteristics and gas compressibility determine the distribution of the gas components in pores and change the gas dynamic viscosity in different regions. The change in the gas dynamic viscosity in the channel is the direct reason for the nonlinear pressure gradient and gas flow curve. The permeability and gas component affect the degree of nonlinear deviation of the gas flow and pressure gradient curve. This affects the nonlinear deviation degree of the curve by changing the gas dynamic viscosity in the pore channel during displacement. The reasonable displacement pressure is the critical pressure (PO) through experimental and theoretical analysis. [ABSTRACT FROM AUTHOR]

Mine gas drainage is an important work in high gas outburst coal mines. How to improve the gas drainage concentration and drainage volume has always been the direction of exploration by coal mine technicians. In this paper, the experiment and field test of displacing CH4 with liquid CO2 were carried out. In the process of low temperature liquid CO2 phase transformation, a low stress and high permeability pressure relief area is formed in the coal and rock mass, so as to accelerate the free state of primary gas in coal and realize the intensified gas extraction of low-permeability coal seams. The results show that CH4 concentration and displacement efficiency in the displacement flow decrease at a greater rate and then stabilize; the cumulative CH4 displacement volume increases rapidly and stabilizes after reaching the critical time; the average concentration of gas drainage from single hole increases from 27.2% to 53.2%, and the gas drainage purity increases 2.3 times from 0.018 m3/min to 0.041 m3/min. The technology is simple to operate, does not produce high temperature and open flame, has the advantages of economy, safety, high efficiency and so on, it can be popularized and applied in gas mines. [ABSTRACT FROM AUTHOR]

Coal mine gas disasters have severely restricted production safety. Improving gas extraction efficiency can effectively reduce disasters. Scholars have confirmed that CO2 successfully displaces coal seam CH4. This study conducted displacement and in situ experiments and compared gas drainage under different injection pressures. The displacement experiments indicated that CH4 production rates increased under increased pressures while the displacement ratios decreased. The pressure had a positive effect on sweep efficiency. The in situ experiment showed that CH4 and CO2 concentration trends in the inspection hole remained consistent. Through observing the data of the original and inspection holes, the average gas drainage concentration during low- and medium-pressure injections increased by 0.61 times and 1.17 times, respectively. The low-pressure average gas drainage scalar was increased by 1.08 times. During the medium-pressure injection, the average gas drainage purity increased by 1.94 times. The diffusion ranges of CO2 under low- and medium-pressure injections were 20–25 m and 25–30 m, respectively. The sweep efficiency of medium-pressure injection was 26% better than that of the low-pressure injection, with average pressures of 2.8 MPa and 1.4 MPa, respectively, for sweep efficiency. This study proposes an effective method for improving coal mine gas drainage efficiency. [ABSTRACT FROM AUTHOR]

To investigate the actual gas explosion and propagation laws in the roadway of an underground mine, experiments were conducted in a standard 19-L tube of rectangular section to provide a comprehensive understanding of the effects of carbon monoxide (CO) addition and obstacles placed on the flame propagation of explosions from methane–air mixtures at different conditions. The flame structure, flame propagation speed, heat loss, and explosion pressure were used to reflect the explosion behaviors of the premixed flame. The experimental results indicate that the enhancement and attenuation of the explosion pressure wave are significantly influenced by an obstacle. The existence of obstacles changed the trend of classical pressure rise, thus forming a first pressure peak. In addition, the position close to the obstacle presented a higher explosion pressure. Under 7 vol.% and 9.5 vol.% CH4 conditions, the increase of CO (from 0 vol.% to 2 vol.%) promoted explosion flame propagation. However, the CO exhibited a clear negative effect on the rate of flame propagation due to exacerbated O2 depletion of 11 vol.% CH4, and the negative effect enhances with CO increased. Besides, when the CO addition increased, the flame tended to more instability and more brightly. For heat loss, with the increase of CO content in mixed gases, there was a slight promoting, constant and inhibiting effect for 11 vol.%, 9.5 vol.%, and 7 vol.% CH4, respectively. These results are of fundamental importance for the prevention and mitigation of gas explosion in underground mines and other industrial scenarios. [ABSTRACT FROM AUTHOR]

Summary: Owing to the low coal permeability coefficients and poor gas extraction efficiencies associated with the extraction of coal in the Huainan coal mining area, the aim of this study was to design, for the first time, a high‐pressure low‐temperature liquid CO2 (LCO2) pump for engineering tests with respect to the fracturing of coal seams and the improvement of CH4 recovery in China. To reasonably calculate the initiation pressure for the coal seam fracturing as well as the gas flow parameters, theoretical analysis with field testing were integrated in the design. In addition, considering the interim gas extraction standard, the gas extraction equivalent radius based on this standard was calculated. The test results revealed a maximum LCO2 pressure of 20.6 MPa, which surpasses the theoretical maximum pressure (19.5 MPa) required for the fracturing of coal seams. During the entire gas extraction process, the CO2 concentrations in boreholes K1 and K3 decrease exponentially with time, showing attenuation coefficients of 0.0205 and 0.0231, respectively. The attenuation coefficients of the three attenuation stages of the original coal seam gas flow rate were 0.0314, 0.2142, and 0.1198, which were higher than those corresponding to the test area. The CH4 concentration in boreholes K1 and K3 in the test area were both higher than that in the original coal seam, and the maximum CH4 flow rates in these boreholes were 1.31‐ and 1.72‐fold higher than that in the original coal seam, respectively. The comparative analysis of CH4 concentrations and flow rates indirectly showed an improvement in the permeability of the coal seam. Further, a quantitative equivalent gas extraction evaluation method based on LCO2 fracturing of coal seam and gas displacement was proposed. Furthermore, based on a comprehensive evaluation, the effective displacement radius caused by the LCO2 fracturing test was 9 m, with a displacement efficiency of 23.95%, and a displacement volume ratio of 0.21. This method offered the possibility of realizing the quantitative evaluation of the LCO2 injection amount and the gas extraction effect, and provides a basis for optimizing the LCO2 fracturing of coal seams as well as the gas extraction process. [ABSTRACT FROM AUTHOR]

Mining high-gas coal seams in China has the characteristics mining of deep, high storage and low permeability, and low drainage efficiency, which seriously restrict the efficient prevention and control of mine gas disasters. Based on the characteristics of low viscosity and permeability, phase change pressurization, and strong adsorption potential energy of liquid CO2, the technology system of liquid CO2 displacement for high-gas and low-permeability coal seam was developed, and field industrial of low-pressure (0.5~2.5 MPa) and medium-pressure (2.5~15.0 MPa) combined injection test was carried out. In this test, the mode of injection followed by drainage was adopted, and the gas drainage effect was investigated for 30 days. The test results show that the effective influence radius of CO2 in this test is 20 m, and the liquid seepage radius is 5 to 7 m. After the injection of liquid CO2 into coal seam, the average gas drainage concentration and drainage purity of all drainage holes were increased by 3.2 and 3.4 times, respectively, and the gas promotion effect was significant. Taking the liquid CO2 low-medium-pressure displacement gas test area as the calculation unit, from the comprehensive benefit analysis, compared with the original drainage mode, the liquid CO2-combined pressure injection process can save 34.7% of the engineering cost and shorten the gas drainage standard time by 45.9%. Therefore, the application of this technology has important technical support and reference significance for the efficient management of gas in the same type of mine. [ABSTRACT FROM AUTHOR]