1 gas source and concentration distribution in working face
There are two sources of gas in coal mining face, one is from coal wall and coal block in coal mining face; Second, from the goaf. Among them, the goaf gas also comes from two aspects, one is from the residual coal in the goaf, and the other is from the adjacent coal seams and surrounding rocks affected by mining. The distribution law of gas concentration in coal mining face is as follows: the gas concentration gradually increases from air intake to return air along the inclined direction of working face, the gradient in the middle and lower part is small and slow, and the gradient in the upper part of working face is large and fast; On the section perpendicular to the coal wall, the general change trend of gas concentration from the coal wall to the goaf is "high-low-high", but under different production processes, the change trend is different on different sections; The gas concentration in the upper corner of the working face is higher than that in other parts of the working face, and the gas concentration in the upper corner and coal wall is the highest.
2. The reason of gas accumulation in the upper corner
In the U-shaped ventilation system of the working face, under the action of the air pressure difference between the air inlet roadway and the air return roadway, the air flow of the working face is divided into two parts, one part directly flows out of the working face, the other part flows into the goaf from the middle and lower part of the working face, and then returns to the upper and upper corners of the working face through the goaf. This will inevitably cause gas accumulation in the upper corner of the working face. As shown in figure 1, it is the air leakage line diagram, wind speed isoline diagram and gas concentration diagram of goaf in U-shaped working face. The figure clearly shows that the upper corner is the centralized gas emission channel of the goaf. In addition, the air density of gas in goaf is lighter than air. When there is a height difference, it can produce a natural floating "gas force", which will inevitably make the high concentration gas in the goaf migrate to the upper corner of the coal mining face. The gas emission in the upper corner increases.
Through theoretical analysis and experimental research, the airflow movement state of coal mining face is also an important reason for gas accumulation. Near the upper corner, due to the change of the main airflow direction and the limitation of geometric conditions, the airflow takes the form of vortex. Near the boundary of goaf, the time-average velocity and pulsating velocity of eddy current are equal to zero, but a large number of high-concentration gas gushes from the upper corner of goaf, forming a potential migration mode of gas gushing; When the upper corner is close to the coal wall, the time-average velocity and pulsating velocity of the vortex are zero. If there is no gas source, there will be a wall-attached effect under the action of "gas pressure", but the thickness of the wall-attached is not large, and the gas migration mode is mainly divided and diffused. Although the average velocity and pulsating velocity are relatively large in the vortex area, due to the vortex motion form of the airflow, the gas-containing airflow keeps rotating, and it is difficult to enter the main airflow, so that the gas circulates in the upper corner and the return air lane and accumulates in the vortex area, forming gas accumulation in the upper corner.
3 measures to control gas accumulation in the upper corner of the working face
According to the causes and laws of gas accumulation in the upper corner. At present, there are two methods to prevent gas accumulation in the upper corner of working face. The first is "defense"; The second is "governance". At present, the methods to "prevent" the gas accumulation in the upper corner of the working face from exceeding the limit include open area pressure equalization method, drainage method and changing the ventilation mode of the working face; The methods to control the gas accumulation in the upper corner of the working face include wind wall method, fan drainage method, jet drainage method, pulsating ventilation control and so on.
3. The fact that1open area pressure sharing method is used to deal with the gas over-limit accumulation in the upper corner of the working face is that an air door is set on the return air side to reduce the pressure difference between the air inlet lane and the return air lane, reduce the air leakage in the goaf and suppress the gas emission. This method is only suitable for small gas emission or as an auxiliary measure.
3.2 gas drainage method to control the upper corner of the working face gas drainage method to control the gas accumulation in the upper corner. The methods include gas drainage in this coal seam and its adjacent layers, gas drainage in goaf by high-position drilling and gas drainage in upper corner by mobile gas drainage pump. Among them, the gas drainage of this coal seam and its adjacent layers and the gas drainage of goaf by high-level drilling play a fundamental role in controlling the gas accumulation in the upper corner. But its disadvantages are large construction amount, long drainage time and high cost. This paper only introduces the treatment of upper corner gas by mobile gas drainage pump.
3.3 Variable ventilation system controls the gas accumulation in the upper corner of the working face. According to the roadway layout of the working face, the ventilation system of U L, W, Y, Z and H can be used to change the air leakage direction of the goaf. This can radically cure the gas gushing from the upper corner of the goaf, but it has a large amount of work and is not suitable for the working face which is prone to spontaneous combustion.
3.4 wind barrier method to deal with the gas accumulation in the upper corner of the working face. The wind deflector is set at the end of the working face to increase the air volume in the upper corner and dilute the gas in the upper corner. This method is simple and effective, but the emission concentration can not be controlled, and the wind deflector is easy to be damaged and the reliability is poor. At the same time, it also affects the normal operation of the working face, which is not suitable for the working face of high-yield colleges and universities, and is often used as a temporary emergency measure.
3.5 The gas accumulation in the upper corner of the working face is controlled by a draw-out spark-free fan, which is called "GDS- 1 gas drainage system".
The upper corner air inlet X enters the hard ram Y two-stage sensors T 1 and T2. Detect the gas concentration value in the "fresh air mixing" air duct regulated by the damper K. After the controller D receives the sensor concentration signal, the microcontroller in the controller D determines the opening or closing of the damper and the angle of the switch according to the change value of the gas concentration. So as to change the amount of "fresh air mixing |" and ensure that the Nevas concentration of the exhaust rod does not exceed the limit. The system has the characteristics of intelligent detection, adjustment and control and convenient installation.
T 1, T2- sensor in air duct; K- damper; X- air suction device; Y- hard ramd-driver；; F—— Pull-out Sparkless Fan
3.6 Pulsating ventilation technology to control the gas over-limit accumulation in the upper corner of the working face Pulsating ventilation technology is a set of technically reliable, economically reasonable and practical pulsating ventilators developed by China University of Mining and Technology, which is based on the theory of turbulent diffusion coefficient of airflow and the pulsating characteristics of airflow, adding pulsating airflow to the normal airflow to increase the turbulent diffusion coefficient of airflow. Improve the ability of airflow to disperse local gas accumulation, and fundamentally solve the problem of gas accumulation exceeding the limit in the upper corner.
According to the special requirements of the operating environment of the pulsating fan, its power is pneumatic or hydraulic. Its characteristics are advanced, reasonable, simple, effective and economical.
There are two ways to install the pulsating fan in the upper corner of the working face: ① a cantilever beam is fixed on the side guard plate of the hydraulic support closest to the upper corner of the working face, so that the pulsating fan can be hung below the cantilever beam and move forward with the advancement of the hydraulic support; (2) Install a pulsating ventilation device on the floor of the upper corner of the working face.
4 conclusion
Treatment of gas accumulation in upper corner of coal mining face. First of all, according to the roadway layout, gas emission and support situation near the upper corner of the working face, a suitable method or a combination of several methods should be selected according to local conditions for comprehensive control. At the same time, it is necessary to strengthen monitoring and control, and deal with it immediately when the gas concentration exceeds the standard.
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