The upper corner of coal mining face is close to the side of coal wall and goaf, and the airflow speed is very low, and the local area is in a vortex state. This vortex makes it difficult for the gas gushing out of the goaf to enter the main airflow, which makes the high-concentration gas circulate near the upper corner and gather in the vortex area, forming the upper corner gas overrun. If there is a lag prop-pulling in the upper corner of the coal mining face, there will be a breeze area near the top cutting row besides the vortex area in the upper corner, and the gas leaked from the goaf will accumulate here.
The pressure difference between two sides in the upper corner of coal mining face is one of the reasons for gas overrun.
Any section of roadway airflow has static pressure, latent pressure and dynamic pressure, and the sum of the three is the total pressure, and the size of the total pressure difference determines the direction and speed of airflow. Because the static pressure and latent pressure on both sides of the upper corner are the same, and the airflow speed is different, the airflow in the coal mining face turns here, which leads to the slowdown of the airflow speed in the upper corner and the decrease of the airflow speed difference between the two sides of the upper corner, where the airflow speed is greatly reduced, and there is no velocity difference in the upper corner.
Prevention and treatment of gas overrun in upper corner
In view of the situation that the gas in the upper corner exceeds the limit, there are 10 common prevention methods, namely: ① setting temporary air curtain in the upper corner; (2) Increase the air volume of the working face; (3) setting the airflow in the goaf; (4) The local ventilator is installed in the coal mining face; (5) installing air and water ejectors in the return air lane of the coal mining face; ⑥ Install special exhaust fan; ⑦ High gas drainage; (8) establishing a coal mining face tail discharge system; Pet-name ruby corner gas on three-phase foam; Attending to change the ventilation mode, etc. Now analyze them separately.
1 Set air curtain at the upper corner of coal mining face.
When the gas in the upper corner of the coal mining face exceeds the limit, an air curtain is hung near the upper corner of the coal mining face to divert the airflow in the working face, and the air curtain is used to guide more airflow through the upper corner to dilute the high-concentration gas. The air curtain can be made of soft air duct cloth, and the length is generally not less than 10m.
In the production process of a coal mine, the gas in the upper corner was abnormal, and the methane concentration reached 2%, so an air curtain was added near the upper corner. According to the field observation, after using the air curtain, the methane concentration in the upper corner quickly dropped below 1%. However, due to the existence of air curtain, the coal cutting of shearer, prop pulling near the upper corner, pedestrians at the upper exit and material transportation are all greatly affected, and the air curtain is often destroyed and ineffective, which leads to the gas concentration in the upper corner rising rapidly above the over-limit concentration. As a result of this repeated operation, the gas concentration in the upper corner will inevitably rise and fall, which is extremely unstable, forming a major hidden danger in safety production. At the same time, the existence of the wind curtain,
Therefore, this method is mainly used in places with little gas in the upper corner, and can only be used as a temporary measure. This method is actually to improve the pressure difference between the two sides in the upper corner of the coal mining face and solve the eddy current problem in the upper corner.
2. Increase the air volume of the working face.
The dispersion of the gas accumulated in the upper corner vortex area by the working face airflow mainly depends on the air convection between the working face airflow and the upper corner gas accumulation area and the diffusion of the main airflow. After long-term field observation, it is found that under the condition of normal air supply in the working face, it is impossible for the speed-limiting airflow to disperse the high-concentration gas in the vortex gathering area in the upper corner. The working face adopts the method of increasing air volume. Although the convection between the airflow in the upper corner accumulation area and the main airflow in the working face can be increased, with the increase of air volume, the negative pressure increases and the airflow speed in the goaf increases, which deepens the gas streamline in the goaf and strengthens the exchange between the airflow and the Nevas in the goaf. If there are other air leakage passages in the goaf, the air leakage will increase. In short, if the air volume of the working face increases, the amount of gas carried by the air flow will increase. At the same time, excessive air volume has the following disadvantages: (1) increasing the dust concentration in the airflow of the mining face, worsening the working environment of the working face and increasing the difficulty of dust prevention; (3) The air volume in the working face is too large, which is easy to make the wind speed in the roadway exceed the provisions of "Coal Mine Safety Regulations" and affect the standardization of mine quality.
3. Set the goaf wind.
According to the reason of gas overrun in the upper corner of coal mining face, if the air volume entering the goaf can be reduced, the gas emission in the goaf can be reduced, so as to avoid gas overrun in the upper corner. On the goaf side of the working face, an air guide cover is arranged from the outlet of the working face to the upper corner along the top cutting row, which can minimize air leakage into the goaf. Especially at the exit of the working face, the airflow directly enters the working face here. Therefore,
It can be seen that this treatment method can fundamentally reduce the gas emission from the goaf, but because the air hopper is located at the edge of the goaf, the gangue falling from the goaf is easy to destroy the air hopper, resulting in an increase in air leakage of the air hopper; At the same time, because the wind is advancing with the work, it increases the difficulty and workload of workers.
Therefore, this method is limited by many conditions and its application effect is not ideal. Only when the gas accumulation speed in the upper corner of coal mining face is not high (2 ~ 3m3/min) and the gas concentration is not too high (about 3%), the application effect is obvious.
4 install local ventilator
The working face is 5.5kw or 2×2.2kw small local ventilator installed 10 ~ 15 m away from the upper corner of the coal mining face, and the rubber duct is used to lead the wind to the upper corner of the return air, forming a positive pressure area in the upper corner of the coal mining face. Fresh airflow is introduced through local ventilator to dilute the gas in the upper corner of coal mining face, so that the gas concentration there is reduced to below the specified level. (2) It can increase the air volume in the upper corner of the coal mining face and dilute the high concentration gas here in time; (3) Due to the small volume and occupied space of the air duct, the influence brought by the construction of the working face can be greatly reduced; (4) Under the normal operation of the fan, it is a safe and reliable control technology to discharge gas in the upper corner of the coal mining face in this way.
5 install feng shui ejector
When the gas in the upper corner of the coal mining face exceeds the limit, a feng shui ejector is installed, and high-pressure water and wind are used as power, or high-pressure water or wind can be used as power respectively to form a larger negative pressure area. The main air flow in the working face will increase the air flow through the upper corner due to the pressure difference to meet the requirements of the fan. In this way, the high-concentration gas in the upper corner will be diluted by the airflow flowing through the working face here and then enter the air duct and be discharged into the return air lane. This method has the following advantages. ② Air volume can be adjusted by changing wind pressure and water pressure; (3) simple structure and convenient installation and movement. However, it is necessary to strengthen management to prevent the power source (water and wind) from suddenly stopping, resulting in the sudden accumulation of gas in the upper corner of the coal mining face.
6 Install special exhaust fan.
(1) pulsating ventilation technology. Pulsating ventilation technology is a set of reliable, economical, reasonable and practical pulsating fans designed by using the theory that the turbulent diffusion coefficient of air flow is related to the pulsating characteristics of air flow. By superimposing pulsating airflow on normal ventilation airflow, the turbulent diffusion coefficient of airflow is increased, the ability of airflow to disperse local accumulated gas is improved, and the problem of gas accumulation in the upper corner of coal mining face is fundamentally solved.
(2)GDS- 1 automatic gas discharge system. The GDS- 1 automatic gas discharge system developed by Chongqing Branch of China Coal Research Institute consists of a spark-less fan, a gas sensor, a control device, a damper, an exhaust device and several air ducts. The main structure is shown in Figure 2. High-concentration gas in the upper corner enters the hard air duct Y through the air suction device X, two-stage sensors detect the gas concentration in the air ducts at the front and rear ends of the air door K, and the microcontroller in the control device determines the opening or closing of the air door and the opening or closing angle according to the gas concentration value, thus changing the air volume mixed with fresh air, so that the Nevas concentration of the exhaust air duct does not exceed the standard.
(3) Small hydraulic fan. Hydraulic fans are divided into monitoring devices and executing devices. The monitoring device comprises a control processor and a gas sensor, and the executing device comprises a small hydraulic fan and a hydraulic power system. The working principle of the monitoring device is that the gas concentration sensor placed in the upper corner of the working face detects the gas concentration in real time, and converts the detected gas concentration signal into an analog electrical signal and transmits it to the control processor. The central processor processes and judges the detected analog signals, sends out instructions to control the opening and closing of the relay, and controls the hydraulic fan in real time. When the gas concentration exceeds the limit, the fan begins to blow away the gas accumulated in the upper corner; When the gas concentration drops to the safety limit, the fan stops running vividly.
(4) Install a compressed air fan to discharge gas. This method has the characteristics of air, water ejector and gas moving pump exhaust. Rigid air duct shall be laid along the return air lane of the working face, with the air inlet of the air duct at a distance of 0.5m from the upper corner, the air outlet installed in the air hole or return air lane of the area, and a fan installed every 200 ~ 300m in the return air lane, powered by compressed air.
(5) Install a mobile drainage pump to discharge the gas in the upper corner. L rigid air ducts are laid along the return air lane of the working face, and 1 drainage flower pipes are laid in front of the air ducts (in the goaf). The length of drainage flower tube is about 15 ~ 20m, which requires low negative pressure drainage. The pipeline is connected with the rigid air duct of the return air system, thus forming a negative pressure area at the corner, so that the gas at the corner can flow to the drainage pipe.
7 high gas drainage
(1) hole arrangement method: directly arrange the drilling field in the return air lane of the working face, drill holes from the roof to the broken zone above the working face, and extract gas from the upper adjacent layer and its nearby coal line. The drilling direction of the working face is opposite, and the drilling spacing is 15m. Three boreholes are drilled in each drilling field, and the boreholes are protected by the coal body in front of the working face. After the working face is in place, install the gas drainage pipeline in the return air lane.
(2) Hole layout parameters. Drilling design is based on two principles, one is that the final hole level of drilling is located at the upper boundary of the fracture zone, and the other is that the level where drilling enters the pressure relief zone is located at the top of caving zone and above the lower boundary of the fracture zone. According to the ground pressure theory, the roof and floor strata are caving after coal mining. After the subsidence of overlying strata is stable, the mining fracture zone of overlying strata can be divided into "vertical three zones" and "horizontal three zones", that is, the mining area is vertically divided into three zones. Along the advancing direction of the working face, the wind roadway and the machine roadway are divided into three areas: the influence area of coal wall support, the separation area and the recompression area. With the continuous advancement of the working face, the "horizontal three areas" in the direction of the working face advance alternately.
(3) The best position of the roof drain. The normal distance is 8 ~ 25m above the roof of vertical coal seam, (10 ~ 15m) (at the top of caving zone, below the fractured zone), and the horizontal distance is 8 ~ 30m (8 ~ 17m) in the return air lane. The specific mine should be comprehensively determined according to its actual situation. Drilling is over.
8 Other methods
(1) Set up the tailpipe system of the working face. Non-metallic pipes can be laid along the return air roadway (goaf) of the working face, and cement can be used. The pipeline is connected with the return air system (not connected with the return air lane of the coal mining face), and a negative pressure zone is formed in the upper corner of the coal mining face, so that the gas in the upper corner of the coal mining face flows to the return air along the pipeline.
(2) Three-phase foam is used to squeeze the gas in the upper corner of the working face. Three-phase foam technology is used to squeeze the space occupied by gas and reduce the gas concentration. The three phases are water, ash and nitrogen, and the ash can be made of materials such as yellow mud and coal-carbon power generation slag, and the water cement ratio (mass ratio) is 1: 4: 1. This method has the advantages of fast processing speed and obvious teaching effect.
(3) Change the ventilation mode. The ventilation mode of coal mines in China mostly adopts headwind. Because the gas discharged from coal mining face is lighter than air, its natural flow direction is the same as the upwind direction. Under normal wind speed (greater than 0.5 ~ 0.8m/s), gas may flow in layers and accumulate locally, which is easy to cause gas accumulation in the upper corner, and the downwind direction is opposite to the natural flow direction of gas. They are easy to mix, and the gas is not easy to flow in layers and accumulate locally, which can prevent the gas accumulation in the upper corner. However, Article 1 15 of Coal Mine Safety Regulations stipulates that downward ventilation is not allowed in coal mining faces with coal (rock) and gas (carbon dioxide) outburst hazards. Therefore, you must be careful when using downward ventilation.
After the above analysis, combined with the actual situation on site, once the gas in the upper corner of the coal mining face exceeds the limit, immediately hang the wind curtain and install the wind shield in the upper corner of the coal mining face; Increase the air intake of the working face, increase the pressure difference of the working face, and check whether all seals related to the working face are leaking. If there is air leakage, it should be stopped in time. The above methods can't solve the problem, so a special exhaust fan (feng shui ejector) should be installed as soon as possible for evacuation.
The above methods are temporary and acute measures, and the main methods to control the gas overrun in the upper corner should be: high position drainage, tail drainage and gas drainage in the upper corner. The fundamental method is to mine the liberated layer and conduct roadway drainage or pre-drainage in advance, so that the gas content in coal seam can be reduced to below 8m3/t, while other methods are uncertain and unstable. Therefore, the gas control in the upper corner should be considered in advance, constructed in advance, put into use early and get early results.