Device and method for three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group

Abstract

The invention discloses a device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group. First, the overall layout of directional boreholes in the upper fissure zone is determined and constructed, and then a three-circuit switching device, an expansion bladder plug, and an intelligent analysis and control box are installed in sequence. During normal gas drainage, gases from different boreholes are extracted for inspection and the change in the concentration of index gases is analyzed. When there is a potential risk of spontaneous combustion, the layer and depth where the risk lies are determined, a closed nitrogen injection space is constructed, and nitrogen is accurately injected. After the coal body in this area is fully inerted and cooled, gas drainage is resumed and the detection of index gases is strengthened to eliminate the risk of spontaneous combustion in the upper part of the composite goaf. Through the method of making multiple uses of a single borehole by means of "using boreholes instead of roadways", the invention can, while controlling the gas in adjacent layers and goafs, intelligently monitor the potential risk of spontaneous combustion in the upper part of the composite goaf, determine the specific location of the potential risk area in a short period of time, and accurately inject nitrogen, truly realizing three-dimensional intelligent monitoring and prevention of spontaneous combustion, providing strong support for solving problems such as the inability to monitor and prevent the potential risk of spontaneous combustion in the upper part due to the complex internal environment of the composite goaf, and ensuring the safe production of the working face for the mining of a close-distance coal seam group.

Description

Technical Field

[0001] The invention belongs to the technical field of preventing and controlling spontaneous combustion of residual coal in mined - out areas of coal mines, and particularly relates to a device and method for three - dimensional intelligent monitoring and prevention and control of spontaneous combustion in a composite mined - out area during the mining of closely - spaced coal seams. Background Technique

[0002] The depth of coal mining in China shows an increasing trend year by year. The in - situ stress increases and the geothermal gradient increases, resulting in the transformation of many coal seams that are not prone to spontaneous combustion into coal seams prone to spontaneous combustion and self - ignition. The risk of coal seam spontaneous combustion has increased sharply. In China, 56% of coal mines are affected by coal spontaneous combustion disasters. The recovery rate of top coal in fully mechanized caving mining is only about 86%, resulting in a large amount of residual coal accumulation on the floor of the mined - out area. Under the action of air leakage, the residual coal undergoes low - temperature oxidation with oxygen. Under suitable heat - storage conditions, the heat of the residual coal accumulates, eventually leading to coal spontaneous combustion. In addition, the distance between adjacent coal seams in many mining areas in China is extremely close. The upper adjacent seam is located in the caving zone of the mined - out area of the mining coal seam and does not have mining conditions. Its spontaneous combustion tendency belongs to coal seams prone to spontaneous combustion or self - ignition. After caving during coal mining, it falls into the mined - out area. Under the action of air leakage, there is a double risk of residual coal spontaneous combustion in the mined - out area, greatly increasing the risk of coal spontaneous combustion in the composite mined - out area and severely restricting the safe production of coal mines.

[0003] The technology for monitoring and preventing and controlling spontaneous combustion in mined - out areas has been continuously developing. At present, the main means for monitoring spontaneous combustion in coal mines is the use of a bundle - tube monitoring system. By analyzing the composition and concentration changes of index gases, the degree of spontaneous combustion is inferred and judged. However, due to the particularity of the layout of the bundle - tube monitoring system, it is impossible to monitor the hidden danger of spontaneous combustion in the upper part of the composite mined - out area. At present, the most widely used technology for preventing and extinguishing fire in mined - out areas is the nitrogen injection technology. Most coal mines in China have installed underground mobile nitrogen - making equipment or ground high - power nitrogen - making equipment, and inject nitrogen into the mined - out area through nitrogen injection pipelines laid in the intake airway in advance. However, due to the complex internal environment of the composite mined - out area, the spontaneous combustion risk of the residual coal in the upper part of the mined - out area is often ignored. When there is a risk of spontaneous combustion in the upper part of the mined - out area, accurate nitrogen injection cannot be achieved.

[0004] To ensure the normal production of the coal mining face, it is necessary to extract the gas from the adjacent seams above the coal mining face and the goaf. With the improvement of high-power directional drilling rig equipment and drilling construction technology at home and abroad, the "using boreholes instead of roadways" technology for directional boreholes in the high-level fracture zone has gradually matured. More and more mine working faces use "using boreholes instead of roadways" to replace the high-level gas drainage roadways, and the effect of controlling the gas in adjacent seams and goaf is outstanding. According to the differences in the occurrence characteristics of coal seams in different mines, the number and layout of "using boreholes instead of roadways" boreholes are determined through the prediction of gas emission in the coal mining face. At present, the research on the application effect of "using boreholes instead of roadways" by scholars mainly focuses on the field of mine gas extraction, lacking the research on the multi-purpose use of a single borehole of "using boreholes instead of roadways", especially in the field of mine fire prevention and control. Different final holes of "using boreholes instead of roadways" boreholes control different horizons, which provides the possibility for realizing the three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in the upper part of the composite goaf.

[0005] At present, the research on the monitoring and prevention and control technology of spontaneous combustion in goaf by scholars is relatively mature. Through the method of using a gas sampling tube monitoring system and laying pipelines to inject nitrogen into the goaf, the risk of spontaneous combustion of the remaining coal in the mined coal seam in the goaf has been greatly reduced, but the risk of spontaneous combustion of the remaining coal in the upper part of the composite goaf is often ignored. Due to the influence of mining disturbance, the air leakage in the goaf is serious. The remaining coal in the upper part of the composite goaf undergoes low-temperature oxidation under suitable oxygen concentration and heat storage conditions, posing a great potential hazard of coal spontaneous combustion. If the prevention and control measures are improper, it may lead to serious consequences. Therefore, it is very necessary to overcome the existing technical defects and propose a new device and method that can, while controlling the gas in adjacent seams and goaf during normal coal mining, intelligently monitor the hidden danger of spontaneous combustion in the upper part of the composite goaf, determine the specific location of the hidden danger area in the short term and accurately inject nitrogen, truly realizing the three-dimensional intelligent monitoring and prevention and control of spontaneous combustion, taking into account the prevention and control of gas and coal spontaneous combustion disasters in the coal mining face, and ensuring the safe production of the working face for mining close coal seam groups. Summary of the Invention

[0006] Aiming at the problems existing in the above-mentioned prior art, the present invention provides a device and method for three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in the composite goaf for mining close coal seam groups. Through the multi-purpose use of a single borehole of "using boreholes instead of roadways", it can, while controlling the gas in adjacent seams and goaf, intelligently monitor the hidden danger of spontaneous combustion in the upper part of the composite goaf, determine the specific location of the hidden danger area in the short term and accurately inject nitrogen, eliminate the hidden danger of spontaneous combustion, and ensure the safe production of the working face for mining close coal seam groups.

[0007] A method for three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group is proposed according to the purpose of the present invention. An apparatus for three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group is used. The apparatus includes a three-loop switching device (3), a gas extraction pipe (4), a gas extraction tube (5), a nitrogen injection pipe (6), a one-way valve for the nitrogen injection pipe (7), a one-way valve for the gas extraction tube (8), a one-way valve for the gas extraction pipe (9), an expansion bladder (10), a pressure valve (11), and an intelligent analysis and control box (12);

[0008] The three-loop switching device (3) is arranged at the opening of each borehole and is respectively connected to the gas extraction pipe (4), the gas extraction tube (5), and the nitrogen injection pipe (6). The gas extraction tube (5) and the nitrogen injection pipe (6) are telescopic flexible sleeves, which can synchronously enter the deep part of the borehole along the inner wall of the borehole. One-way valves for the gas extraction pipe (9), one-way valves for the gas extraction tube (8), and one-way valves for the nitrogen injection pipe (7) are respectively provided on the three pipelines;

[0009] The expansion bladder (10) is respectively arranged at the front end and the outer wall of the nitrogen injection pipe (6). During normal gas extraction, the expansion bladder (10) is in an uninflated state, and its volume can be ignored. It can enter the deep part of the borehole along with the gas extraction tube (5) and the nitrogen injection pipe (6). A pressure valve (11) is provided in the middle of the nitrogen injection pipe (6), and the pressure valve (11) opens when a certain pressure is reached;

[0010] The intelligent analysis and control box (12) is arranged inside the drill site to control the opening and closing of the one-way valve for the gas extraction pipe (9), the one-way valve for the gas extraction tube (8), and the one-way valve for the nitrogen injection pipe (7), the opening and closing of the pressure valve (11), the telescopic state of the gas extraction tube (5) and the nitrogen injection pipe (6), and the inflation and pressure relief of the expansion bladder (10);

[0011] The method includes the following four steps:

[0012] S1: Determine the overall layout of the upward fissure zone directional boreholes and construct them:

[0013] Determine the final hole control horizon of different boreholes and the distance from the inner-offset return airway, set up a drill site on the side wall of the return airway, construct according to the predetermined overall layout of the boreholes, and install the apparatus for three-dimensional intelligent monitoring and prevention and control of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group. During the mining period, the gas in the goaf is normally extracted under negative pressure;

[0014] S2: Regularly extract and detect gas during gas extraction, determine the horizon where spontaneous combustion hazards occur when they appear, and realize three-dimensional intelligent monitoring of spontaneous combustion:

[0015] Set the concentration threshold of the natural fire index gas in the intelligent analysis control box. During normal gas drainage, the one-way valves of the extraction pipes of each borehole are opened and closed in sequence at fixed time intervals every day. The gas from different boreholes is extracted for inspection and the change of the index gas concentration is analyzed to achieve three-dimensional intelligent monitoring of natural fire. When the concentration of the index gas reaches or exceeds the threshold, it indicates that there is a natural fire hazard in the layer where the end hole of the borehole is located;

[0016] S3: Determine the depth of the natural fire hazard area, construct a closed nitrogen injection space and accurately inject nitrogen to achieve three-dimensional intelligent prevention and control of natural fire:

[0017] After the one-way valve of the extraction pipe is closed and the negative pressure extraction is stopped for 1 h, the extraction pipe and the nitrogen injection pipe cooperate with the expansion bag to extend deep into the borehole. When reaching the layer where the end hole is located, every time it extends 10 m, the one-way valve of the extraction pipe is opened to inspect the concentration of the index gas until the concentration of the index gas exceeds the threshold for the first time, so as to determine the depth of the natural fire hazard area. At this time, the one-way valve of the nitrogen injection pipe is opened, and the expansion bags at both ends expand to fully fit the inner wall of the borehole to construct a closed nitrogen injection space. At the same time, the pressure valve in the middle of the nitrogen injection pipe is opened, and low-temperature nitrogen is injected for a period of time to fully inert and cool the coal body in this area;

[0018] S4: Resume gas extraction and strengthen the detection of index gas:

[0019] After the nitrogen injection is completed, the bag gradually decompresses until it is completely deflated, and then retracts to the borehole orifice along with the extraction pipe and the nitrogen injection pipe. The one-way valve of the nitrogen injection pipe is closed, and the one-way valve of the extraction pipe is opened. Resume gas extraction and strengthen the detection of the index gas of this borehole to eliminate the natural fire risk in the upper part of the combined goaf.

[0020] Furthermore, in step S1, the overall layout of the boreholes preferably arranges high-level fissure zone directional boreholes according to the prediction law of the gas emission amount of the adjacent upper layer in the working face to meet the requirements of gas control. Then, according to the layout of the borehole layers and the position of the adjacent upper layer, boreholes are reasonably added to control different layers in the upper part of the combined goaf and eliminate the blank zone of natural fire monitoring. The inner diameter Φ of the extraction pipe is 200 mm, the inner diameter Φ of the extraction gas pipe is 20 mm, and the inner diameter Φ of the nitrogen injection pipe is 30 mm.

[0021] Furthermore, in step S2, the intelligent analysis control box checks the gas threshold alarm. The index gases are specifically CO, C2H2 and C2H4, and the alarm threshold is that the CO concentration is greater than 5 ppm or C2H2 and C2H4 are detected.

[0022] Furthermore, in step S3, the distance between the two bags is 20 m, the nitrogen injection flow rate per single hole is 90 m 3 / h, and the nitrogen injection duration is 3 days.

[0023] Further, during the restoration of gas drainage in step S4, the one-way valve of the gas extraction pipe is opened, and the concentration of the natural fire index gas in the layer where the borehole is located is detected at all times. If the concentration of the index gas does not exceed the threshold within one week, the one-way valve of the gas extraction pipe is closed and returns to the normal state.

[0024] Compared with the prior art, the advantages of the device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in the composite goaf of close-distance coal seam groups disclosed in the present invention are as follows:

[0025] (1) During the normal gas drainage period of the present invention, by sequentially opening and closing the one-way valves of the gas extraction pipes of each borehole for sampling, the gases of different boreholes are extracted for inspection and the change of the concentration of the index gas is analyzed, realizing three-dimensional intelligent monitoring of spontaneous combustion.

[0026] (2) During the period of potential spontaneous combustion hazards in the present invention, by extending the gas extraction pipe and the nitrogen injection pipe deep into the borehole for gas extraction and detection, the depth of the potential spontaneous combustion hazard area is determined. By constructing a closed nitrogen injection space with an expansion bladder and accurately injecting nitrogen, three-dimensional intelligent prevention and control of spontaneous combustion are realized, greatly ensuring the safe production of the working face for the mining of close-distance coal seam groups. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to make the objectives, methods, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the drawings. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts, where:

[0028] Figure 1 is a schematic flow chart of the device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in the composite goaf of close-distance coal seam groups disclosed in the embodiments of the present invention.

[0029] Figure 2 is a three-dimensional and sectional schematic layout diagram of the device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in the composite goaf of close-distance coal seam groups disclosed in the embodiments of the present invention.

[0030] Figure 3 is a schematic internal view of a borehole in the normal gas extraction state of the device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in the composite goaf of close-distance coal seam groups disclosed in the embodiments of the present invention.

[0031] Figure 4 is a schematic diagram of daily sampling and monitoring of boreholes in the normal gas extraction state of the device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in the composite goaf of close-distance coal seam groups disclosed in the embodiments of the present invention.

[0032] Figure 5Schematic diagram of the inside of a borehole when there is a potential for spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group, which is disclosed in an embodiment of the present invention for three-dimensional intelligent monitoring and prevention of spontaneous combustion.

[0033] Figures 2 to 5 The component names represented by the numbers in

[0034] 1 - Mining coal seam; 2 - Upper adjacent seam; 3 - Three-loop switching device; 4 - Extraction pipe; 5 - Exhaust pipe; 6 - Nitrogen injection pipe; 7 - Nitrogen injection pipe one-way valve; 8 - Exhaust pipe one-way valve; 9 - Extraction pipe one-way valve; 10 - Expansion bladder; 11 - Pressure valve; 12 - Intelligent analysis and control box. Specific embodiments

[0035] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings; the preferred embodiments are only for illustrating the present invention and not for limiting the protection scope of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] Figure 1 Flow chart provided by an embodiment of the present invention; Figure 2 Schematic diagram of the overall layout and cross-section of the borehole provided by an embodiment of the present invention; Figure 3 Schematic diagram of the inside of the borehole under normal extraction state provided by an embodiment of the present invention; Figure 4 Schematic diagram of daily sampling and monitoring of the borehole under normal extraction state for a device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group, which is disclosed in an embodiment of the present invention;

[0037] Figure 5 Schematic diagram of the inside of the borehole when there is a potential for spontaneous combustion. A device and method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in a composite goaf during the mining of a close-distance coal seam group disclosed in the present invention specifically include the following 4 steps.

[0038] S1: Determine the overall layout of the high-level fracture zone directional boreholes and construct them:

[0039] Determine the number of boreholes based on the predicted gas emission volume of the specific mine working face and the gas extraction volume of a single borehole, and investigate the height of the caving zone in the composite goaf to determine the final hole control horizon of different boreholes and the distance of the inner-offset return airway. The overall layout of the boreholes is preferably arranged with high-level fissure zone directional boreholes according to the predicted law of gas emission from the adjacent upper layers of the working face to meet the requirements of gas control. Then, reasonably add boreholes according to the layout of the borehole horizons to control different horizons in the upper part of the composite goaf and eliminate the monitoring blank zone of spontaneous combustion. Set up a drill site on the inner sidewall of the return airway and construct according to the overall layout of the predetermined boreholes to complete the processes of hole mouth reaming, sealing, hole opening, directional drilling, reaming, and installing the screen pipe.

[0040] Inside the drill site, each borehole is respectively connected to the gas extraction pipe (4), the gas extraction pipe (5), and the nitrogen injection pipe (6) through a three-circuit switching device (3). The inner diameter Φ of the gas extraction pipe (4) is 200 mm, the inner diameter Φ of the gas extraction pipe (5) is 20 mm, and the inner diameter Φ of the nitrogen injection pipe (6) is 30 mm. The gas extraction pipe (5) and the nitrogen injection pipe (6) are telescopic flexible sleeves, which can enter the deep part of the borehole along the inner wall of the borehole synchronously when there is a hidden danger of spontaneous combustion. Check valves are installed on all three pipelines. Under normal conditions, the check valve (9) of the gas extraction pipe is open, and the check valve (8) of the gas extraction pipe is closed, and the two cannot be opened simultaneously.

[0041] At the front end and outer wall of the nitrogen injection pipe (6), an expansion bladder (10) is respectively laid. During normal gas extraction, the expansion bladder (10) is in an uninflated state, and its volume can be ignored. The intelligent analysis control box (12) is set inside the drill site to control the opening and closing of the check valve (9) of the gas extraction pipe, the check valve (8) of the gas extraction pipe, and the check valve (7) of the nitrogen injection pipe, the opening and closing of the pressure valve (11), the telescopic state of the gas extraction pipe (5) and the nitrogen injection pipe (6), and the inflation and pressure relief of the expansion bladder (10). The intelligent analysis control box (12) is directly connected to the ground control room to transmit and display the inspection concentration and change of each gas in the borehole in real time.

[0042] S2: Regularly extract gas for detection, determine the layer where the hidden danger of spontaneous combustion occurs, and realize three-dimensional intelligent monitoring of spontaneous combustion:

[0043] Set the concentration threshold of the index gas for spontaneous combustion in the intelligent analysis control box (12). The index gases are specifically CO, C2H2, and C2H4. The alarm threshold is that the CO concentration is greater than 5 ppm or C2H2 and C2H4 are detected. During normal gas extraction, the check valves (8) of the gas extraction pipes of each borehole are opened and closed in sequence at a fixed time every day to extract and analyze the change of the index gas concentration of the gases from different boreholes, so as to realize three-dimensional intelligent monitoring of spontaneous combustion. When the concentration of the index gas reaches or exceeds the threshold, it indicates that there is a hidden danger of spontaneous combustion in the layer where the final hole of the borehole is located.

[0044] S3: Determine the depth of the area with the hidden danger of spontaneous combustion, construct a closed nitrogen injection space and accurately inject nitrogen to realize three-dimensional intelligent prevention and control of spontaneous combustion:

[0045] The one-way valve (9) of the gas drainage pipe is closed. After stopping the negative pressure drainage for 1 hour, the gas extraction pipe (5) and the nitrogen injection pipe (6) cooperate with the expansion bladder (10) to extend deep into the borehole. When reaching the layer where the final hole is located, for every 10 m of extension, the one-way valve (8) of the gas extraction pipe is opened to test the concentration of the target gas. And so on until the concentration of the target gas exceeds the threshold for the first time, so as to determine the depth of the spontaneous combustion hazard area. At this time, the one-way valve (7) of the nitrogen injection pipe is opened, and the two ends of the expansion bladder (10) expand to completely fit the inner wall of the borehole to build a closed nitrogen injection space. At the same time, the pressure valve (11) in the middle of the nitrogen injection pipe is opened, and low-temperature nitrogen is injected for a certain period of time to fully inert and cool the coal body in this area. The distance between the two bladders is 20 m, and the nitrogen injection flow rate per single hole is 90 m 3 / h, and the nitrogen injection duration is 3 days.

[0046] S4: Resume gas drainage and strengthen the detection of target gas:

[0047] After the nitrogen injection is completed, the expansion bladder (10) gradually decompresses until it is completely deflated, and then retracts to the borehole orifice along with the gas extraction pipe (5) and the nitrogen injection pipe (6). The one-way valve (7) of the nitrogen injection pipe is closed, and the one-way valve (9) of the gas drainage pipe is opened. While resuming gas drainage, the one-way valve (8) of the gas extraction pipe is opened, and the concentration of the spontaneous combustion target gas in the layer where the borehole is located is detected all the time. If the concentration of the target gas does not exceed the threshold within one week, the one-way valve (8) of the gas extraction pipe is closed to resume the normal state and eliminate the spontaneous combustion risk in the upper part of the combined goaf.

[0048] Finally, it should be noted that the above embodiments are only used to illustrate the method of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the methods recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding methods deviate from the scope defined by the claims of the present invention.

Claims

1. A method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in composite goaf areas of close-range coal seam group mining, using a device for three-dimensional intelligent monitoring and prevention of spontaneous combustion in composite goaf areas of close-range coal seam group mining, the device comprising a three-circuit switching device (3), an extraction pipe (4), an exhaust pipe (5), a nitrogen injection pipe (6), a nitrogen injection pipe check valve (7), an exhaust pipe check valve (8), an extraction pipe check valve (9), an expansion bag (10), a pressure valve (11) and an intelligent analysis control box (12); The three-circuit switching device (3) is arranged at the opening of each borehole, and is respectively connected to the extraction pipe (4), the air extraction pipe (5) and the nitrogen injection pipe (6); the air extraction pipe (5) and the nitrogen injection pipe (6) are telescopic soft sleeves, which can synchronously enter the deep part of the borehole along the inner wall of the borehole, and the three pipelines are respectively provided with an extraction pipe check valve (9), an air extraction pipe check valve (8) and a nitrogen injection pipe check valve (7); The expansion bag (10) is respectively arranged at the front end and the outer wall of the nitrogen injection pipe (6). During normal extraction, the expansion bag (10) is in an uninflated state, and its volume can be ignored. It can enter the deep part of the borehole along with the air extraction pipe (5) and the nitrogen injection pipe (6). A pressure valve (11) is arranged in the middle of the nitrogen injection pipe (6). When a certain pressure is reached, the pressure valve (11) opens. The intelligent analysis control box (12) is arranged inside the drilling site to control the opening and closing of the extraction pipe check valve (9), the air extraction pipe check valve (8) and the nitrogen injection pipe check valve (7), the opening and closing of the pressure valve (11), the expansion and contraction state of the air extraction pipe (5) and the nitrogen injection pipe (6), and the expansion and decompression of the expansion bag (10); The method includes the following four steps: S1: Determine the overall layout of directional drilling in high-position fracture zones and carry out construction: Determine the control layer of different drilling end holes and the distance of the inner staggered return air tunnel, set up a drilling site on the side wall of the return air tunnel, construct according to the overall layout of the planned drilling holes, and install a three-dimensional intelligent monitoring and prevention device for spontaneous combustion in the composite goaf area of ​​close-range coal seam group mining. During the mining period, the gas in the goaf is normally extracted under negative pressure; S2: Regular gas extraction and testing during gas extraction, determine the layer where the potential fire hazard occurs, and realize three-dimensional intelligent monitoring of spontaneous combustion: Set the concentration threshold of the spontaneous combustion index gas in the intelligent analysis control box. During the normal gas extraction period, the one-way valves of the gas extraction pipes of each borehole are opened and closed in sequence at a fixed time every day, and the gas from different boreholes is extracted to test and analyze the changes in the concentration of the index gas, so as to realize the three-dimensional intelligent monitoring of spontaneous combustion. When the concentration of the index gas reaches or exceeds the threshold, it indicates that there is a risk of spontaneous combustion in the layer where the end hole of the borehole is located. S3: Determine the depth of the spontaneous combustion hazard area, build a closed nitrogen injection space and accurately inject nitrogen to achieve three-dimensional intelligent prevention and control of spontaneous combustion: The one-way valve of the extraction pipe is closed, and negative pressure extraction is stopped for 1 hour. The extraction pipe and nitrogen injection pipe are extended to the deep part of the borehole in coordination with the expansion bag. When the end hole is reached, the one-way valve of the extraction pipe is opened every 10m to check the index gas concentration until the index gas concentration exceeds the threshold for the first time, so as to determine the depth of the spontaneous combustion hazard area. At this time, the one-way valve of the nitrogen injection pipe is opened, and the expansion bags at both ends are expanded to completely fit the inner wall of the borehole to construct a closed nitrogen injection space. At the same time, the pressure valve in the middle of the nitrogen injection pipe is opened, and low-temperature nitrogen is injected for a period of time to fully inert and cool the coal body in this area. S4: Resume gas extraction and strengthen indicator gas detection: After the nitrogen injection is completed, the bag is gradually depressurized to complete, and as the exhaust pipe and nitrogen injection pipe shrink and withdraw to the borehole mouth, the one-way valve of the nitrogen injection pipe is closed, and the one-way valve of the extraction pipe is opened. Gas extraction is resumed and the index gas detection of the borehole is strengthened to eliminate the risk of spontaneous combustion in the upper part of the composite goaf.

2. A three-dimensional intelligent monitoring and prevention method for spontaneous combustion in composite goaf areas of close-range coal seam group mining as claimed in claim 1, characterized in that: In the step S1, the overall layout of the drilling holes is preferentially arranged according to the predicted law of the gas outburst volume of the adjacent layers on the working face, directional drilling of high-position fracture zones is arranged to meet the gas control requirements, and then additional drilling holes are reasonably added according to the arrangement of the drilling holes and the position of the upper adjacent layers to control the different layers above the composite goaf and eliminate the blank zone for spontaneous combustion monitoring. The inner diameter Φ of the extraction pipe is 200 mm, the inner diameter Φ of the exhaust pipe is 20 mm, and the inner diameter Φ of the nitrogen injection pipe is 30 mm.

3. The method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in composite goaf areas of close-range coal seam group mining as claimed in claim 1 is characterized in that: In step S2, the intelligent analysis control box checks the gas threshold alarm, the indicator gases are specifically CO, C2H2 and C2H4, and the alarm threshold is that the CO concentration is greater than 5ppm or C2H2 and C2H4 appear in the test.

4. The method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in composite goaf areas of close-range coal seam group mining as claimed in claim 1 is characterized in that: In step S3, the distance between the two bags is 20m, and the nitrogen injection flow rate of a single hole is 90m 3 / h, and the nitrogen injection duration is 3 days.

5. The method for three-dimensional intelligent monitoring and prevention of spontaneous combustion in composite goaf areas of close-range coal seam group mining as claimed in claim 1, characterized in that: When gas extraction is restored in step S4, the one-way valve of the gas extraction pipe is opened, and the concentration of the spontaneous combustion index gas in the stratum where the borehole is located is detected at all times. If the index gas concentration does not exceed the threshold within one week, the one-way valve of the gas extraction pipe is closed and restored to normal state.

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