To prevent the gas over limit in the upper corner of the 215101 working face of the Yue Nan coal mine, a numerical simulation method was used to analyze the gas concentration in the upper corner of the working face at different air intake volumes and mining velocities. The research results show that the gas concentration in the upper corner is 0.78, 0.52, 0.39, and 0.32% when the wind speed of the intake airflow roadway is 1, 1.5, 2, and 2.5 m/s, respectively, and an optimal wind speed of the intake airflow roadway is selected as 2 m/s. When the wind speed of the intake airflow roadway is 2 m/s, the working face mining velocity is 1, 2, 3, and 4 m/d, and the gas concentration in the upper corner is 0.27, 0.39, 0.58, and 0.83%, respectively, and an optimal working face mining velocity of 3 m/d is selected. Under the optimal mining conditions, the working face wind leakage area is divided, with 0∼30 m of the working face as the main leakage area and 150–180 m as the wind flow compensation area. According to the wind speed in the gob, the wind flow disturbance area is divided, the gob 0–50 m is the wind flow intense disturbance area, which is the main area of the upper corner gas source; the gob 50–62 m is the wind flow medium disturbance area, which is the secondary area of the upper corner gas source; the gob 62–75 m is the slight disturbance area, which has less influence on the upper corner gas concentration; the gob after 75 m is the wind flow undisturbed area, and the upper corner gas concentration is almost unaffected by it.
The permeability of coal exhibits multiscale characteristics in space and time, which is caused by the presence of micro and nanopores in coal. Water, free gas, and adsorbed gas are common engineering fluids in coal seams during gas extraction. Thus, it is of significance to study the multiscale characteristics and mechanisms of seepage−diffusion of different fluids in coal for gas extraction engineering. Experiments of seepage−diffusion for He, CH 4 , and water are carried out using ϕ50 × 100 mm cylindrical coal. It is found that the apparent diffusion coefficient for He, CH 4 , and water are not a constant but a variable that decays dynamically with time. The phenomenon is independent of fluid properties and determined only by the multiscale characteristics of pores in coal, and then a novel model of multiscale dynamic apparent diffusion that can accurately describe the full-time process of flow in various fluids is proposed. The mechanism of dynamic seepage−diffusion for different fluids in coal is elucidated based on a new proposed geometrical model of multiscale pores. At the early stage of flow, fluids first flow in or out of the largest pores outside coal, and at the later stage of flow, fluids flow in the micro and nanopores inside the coal matrix. The pore sizes through which the fluid flows decrease with time, which leads to a dynamic attenuation of the apparent diffusion−permeability with time. The initial apparent permeability K 0 for He/CH 4 shows a "U"-shaped pattern of decreasing and then increasing with the decrease of gas pressure. When the gas pressure is lower than the turning point, the slip effect dominates, making the initial apparent permeability K 0 decrease with the increase of gas pressure. When the gas pressure is higher than the turning point, the effect of the pressure difference of the gas dominates, and K 0 increases with the increase of gas pressure. When water flows in multiscale pores, it occupies the space of large pores, making the homogeneity of pores increase, and the decay coefficient of apparent permeability becomes smaller. The multiscale permeability can explain the reason for the rapid attenuation of coalbed methane production in the late stage. It is of great significance for coalbed methane productivity prediction.
Positive pressure sampling enables the fixed-point and rapid acquisition of coal samples, but the derivation of loss volume during sampling is usually based on the law of gas desorption from granular coal at atmospheric pressure, which seriously affects the reasonableness of loss amounts under positive pressure and thus leads to errors in gas content determination. The gas loss under positive pressure is the key to the accurate determination of the gas content of coal seams. To obtain reliable loss data, under different positive pressures, we tested the gas desorption process of anthracite coal samples with different adsorption equilibrium pressures, analyzed the effect of positive pressure on gas desorption, studied the changes in the gas desorption rate caused by positive pressure, recorded the fluctuation of the amount of gas loss, and compared the values of loss under different conditions. The results show that the positive pressure is the main factor affecting gas desorption compared to the adsorption equilibrium pressure. The positive pressure has an inhibitory influence on gas desorption. Under the same positive pressure, the gas desorption rate shows a decreasing trend over time, and at the same time, the gas desorption rate gradually decreases accompanied by the increasing positive pressure. The gas loss error rate increases with increasing adsorption pressure under the same positive pressure. However, under the same adsorption pressure, the error rate of loss quantity presents a significant increase with positive pressure. The relative error of gas loss under different positive pressures can reach 63–180%, and the positive pressure has an obvious influence on gas loss. This study has experimentally confirmed that positive pressure has a greater effect on gas desorption than adsorption pressure, which will theoretically improve the method of deriving the amount of gas loss and will provide a basis for the accurate determination of gas content under positive pressure in engineering terms.
In comparison to traditional longwall mining, "roof-cutting and pressurereleasing" mining along gob-side entry retaining changes the permeability of the gob, as well as the pressure-relief characteristics and caving mode of the overlying strata. These changes are a result of the interaction of these factors, which also changes the boundary condition of the gob and the airflow movement law of the working face and the gob. In order to study the law of air flow movement in the working face and gob under the "roof-cutting and pressure-releasing" mining along gob-side entry retaining, the permeability model of gob was established under the engineering background of the 21,304 experimental working face of Chengjiao coal mine, then using fluent numerical simulation software, the movement law of air flow in working face and gob is simulated. The results show that the law of air leakage is much different from that of traditional longwall mining, and there are two main air leakage routes, First, most of the airflow will flow directly into the gob-side entry retaining under the action of inertia, and it will collide with the air flow provided by the fan at the end of the gob-side entry retaining, and the air will leak in the gob along the airflow direction; second, when the remaining airflow flows to the working face, the air leakage is serious in the air inlet corner, and most of the air flow flows into the gob. In view of the air leakage area, the air leakage prevention measures are put forward, such as setting the baffle plate, hanging the wind shield at the corner, and blocking the wall of the roadway with guniting; the simulation results show that the air leakage area is obviously reduced, and it is consistent with the measured data. The simulation results can generally describe the law of air flow movement in the face and gob with "roof-cutting and pressure-releasing".
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