Coal bed gas mined by injecting flue gas into abandoned mine and separation and purification method thereof

Abstract

The invention provides a coal bed gas mining method by injecting flue gas into a waste mine and a separation and purification method thereof. A proper target area is selected for coal bed gas mining; the flue gas for injection is obtained after being collected and treated from a factory which discharges the flue gas containing a large amount of CO2; after an injection system and a coalbed methane extraction system are arranged on the ground above a goaf, coalbed methane originally existing in the goaf is extracted and collected; after the primary extraction is finished, injecting flue gas to displace the coal bed gas in the goaf, extracting again after a period of time, and circulating until the coal bed gas extraction work is finished; the extracted coal bed gas is treated by a first separation and purification device, and the treated mixed gas of CH4, N2 and the like enters a pressure swing adsorption separation and purification device for CH4 concentration; and after the extraction work is finished, the goaf is closed, and the injected smoke is sealed in the goaf. According to the method, flue gas utilization and CO2 capture and storage are both considered, operation is easy, economical and feasible, and the method is a green and safe coalbed methane exploitation mode.

Description

Technical Field

[0001] The present invention belongs to the technical field of coalbed methane development and carbon emission reduction, and specifically relates to a method for injecting flue gas into an abandoned mine to mine coalbed methane and a method for separating and purifying the coalbed methane. Background Art

[0002] Coalbed methane, also known as coal mine gas, is mainly composed of methane and is an important unconventional natural gas resource. my country is rich in coalbed methane resources. According to the statistics of the Ministry of Natural Resources in 2022, the geological resources of coalbed methane with a burial depth of less than 2,000 meters are about 30.05 trillion cubic meters, ranking third in the world. The National Energy Plan requires that the annual output of natural gas reach 230 billion cubic meters in 2025. Therefore, strengthening the development of coalbed methane is of great significance to alleviating my country's energy pressure and improving energy self-sufficiency.

[0003] Abandoned mines are former coal mining areas, and they are often rich in resources that have not been fully mined. According to data, most abandoned mines are high-gas mines, which contain rich coalbed methane resources. It is estimated that the amount of residual coalbed methane resources in abandoned mines in my country is nearly 500 billion m3. 3 . With the increase in the number of abandoned mines and the lack of related coalbed methane mining projects, the total amount of coalbed methane left in these abandoned mines will increase. If these remaining coalbed methane resources cannot be effectively utilized and are left in abandoned mines, on the one hand, it will cause a waste of high-quality energy and bring safety hazards to the mining work in the adjacent layers; on the other hand, the coalbed methane in the abandoned mines with shallow burial depths may escape into the atmosphere and cause a greenhouse effect.

[0004] Thermal power generation is one of the main power generation methods at present. It mainly generates electricity by burning fossil fuels such as coal, oil or natural gas to release energy to drive generators. However, during the power generation process, a large amount of combustion byproducts are discharged into the atmosphere in the form of flue gas. In addition, CO2 is the main component of flue gas. Therefore, capturing and utilizing flue gas is conducive to promoting the realization of my country's "dual carbon" goals.

[0005] The main component of coalbed methane is CH4, but it is often mixed with other gases (such as CO2, N2, H2O, etc.) during the mining process. In order to make coalbed methane meet the standards for commercial applications, it needs to be separated and purified. Common coalbed methane separation and purification technologies include adsorption separation, membrane separation, cryogenic distillation, solution absorption, and gas hydrate method. Among them, adsorption separation technology is simple to operate, has low energy consumption, a wide range of raw material sources, low cost, and has good application prospects.

[0006] The main component of coalbed methane is CH4, but it is often mixed with other gases (such as CO2, N2, H2O, etc.) during the mining process. In order to make coalbed methane meet the standards for commercial applications, it needs to be separated and purified. Common coalbed methane separation and purification technologies include adsorption separation, membrane separation, cryogenic distillation, solution absorption, and gas hydrate method. Among them, adsorption separation technology is simple to operate, has low energy consumption, a wide range of raw material sources, low cost, and has good application prospects. Summary of the invention

[0007] In view of the problems in my country such as low natural gas production, a large number of abandoned mines, rich coalbed methane resources but lack of development measures, and large amounts of flue gas (greenhouse gas) generated by thermal power plants and other factories, the present invention provides a method for injecting flue gas into abandoned mines to mine coalbed methane and its separation and purification. According to the method, after preliminary treatment of the flue gas containing a large amount of CO2 generated by thermal power plants and the like, the flue gas is injected into the evaluated and selected abandoned mines through a flue gas injection device to displace the coalbed methane in the goaf, and then the flue gas is collected by a coalbed methane collection device, and finally the collected coalbed methane is purified by a first separation and purification device and a pressure swing adsorption separation and purification device to meet the standards for industrial use.

[0008] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0009] A method for extracting coalbed methane by injecting flue gas into abandoned mines and separating and purifying the same, comprising the following steps: after surveying and evaluating the target abandoned mines, selecting a suitable target area for coalbed methane extraction; then collecting and treating flue gas from thermal power plants, metal smelters and other factories that emit flue gas mainly composed of CO2; after arranging the injection system and coalbed methane extraction system on the ground above the goaf, firstly extract and collect the coalbed methane originally existing in the goaf; after the initial extraction is completed, injecting flue gas to displace the coalbed methane in the goaf, and then extracting again, and repeating the cycle until the coalbed methane extraction is completed; after the extracted coalbed methane is treated by the first separation and purification device, the CO2 therein is separated and transported to the injection system for recycling, and the treated CH 4、 The mixed gas such as N2 enters the pressure swing adsorption separation and purification device to concentrate CH4 to meet the standards for industrial use; after the extraction work is completed, the goaf will be closed so that the injected flue gas is sealed in the goaf.

[0010] Before flue gas injection, suitable abandoned mines should be selected and evaluated so that the flue gas can be injected smoothly; this includes determining the coalbed methane storage area, goaf conditions, abandoned shaft conditions, and hydrogeological conditions of the overlying rock formations, so that the construction of extraction wells and injection wells can be carried out.

[0011] Furthermore, the specific steps of the method are as follows:

[0012] Step 1, assessment and selection of abandoned mines: Exploration of target abandoned mines, collection of necessary coalbed methane occurrence information, distribution of tunnels, shafts and goafs, and hydrogeological conditions of overlying rock formations; comprehensive consideration of the geographical location, traffic conditions, and surrounding environment of the abandoned mines, to determine suitable mining areas;

[0013] Step 2, flue gas source and pretreatment: Collect industrial waste flue gas from nearby thermal power plants, metal smelters and other factories that emit flue gas mainly composed of CO2. After dust removal, desulfurization, denitrification and cooling, the main components of the flue gas are CO2, N2, O2 and H2O;

[0014] Step 3, layout of injection system and coalbed methane collection system: after determining the target abandoned mine, construct injection wells and extraction wells from the ground through rock strata and aquifers to the goaf respectively. After the construction of the two wells is completed, the drilling work is completed; the injection pipeline is extended from the ground to the bottom of the injection well, and an injection well packer is installed in the injection well; the ground end of the injection pipeline is connected to the gas injection pump, and an injection valve and a pressure gauge are installed on the injection pipeline; the gas injection pump is connected to the flue gas storage tank by a pipeline, and a flue gas valve is installed in between, and the flue gas storage tank stores pre-treated flue gas; the extraction pipeline is extended from the ground to the bottom of the extraction well, and an extraction well packer is installed in the extraction well ; The ground end of the extraction pipeline is connected to the extraction pump, and an extraction valve and a CH4 concentration monitor are provided on the extraction pipeline; at the same time, it is connected to the injection pipeline through a connecting pipeline, and a connecting valve is provided on the connecting pipeline; the extraction pump is connected to the first separation and purification device by a pipeline, and the first separation and purification device is also connected to the flue gas storage tank through a CO2 recovery pipeline, and a CO2 valve is provided on the CO2 recovery pipeline; in addition, the first separation and purification device is connected to the pressure swing adsorption separation and purification device through a pipeline; the pressure swing adsorption separation and purification device is connected to the CH4 storage tank by a pipeline, and a CH4 valve is provided therebetween; finally, CO2 alarm devices are installed at the wellheads of both wells;

[0015] Step 4, initial extraction: After completing all the above work, the first extraction of coalbed methane (CH4, CO2, N2, etc.) is carried out. The purpose is to collect the original coalbed methane in the goaf, increase the efficiency of flue gas displacement of coalbed methane in subsequent steps, and improve the recovery rate of coalbed methane; open the CH4 valve, CO2 valve, connecting valve and extraction valve, and keep the flue gas valve and injection valve closed; turn on the extraction pump, and at the same time, make the first separation and purification device and the pressure swing adsorption separation and purification device enter the working state. At this time, the original coalbed methane in the goaf enters the first separation and purification device through the extraction pipe, injection pipe, connecting pipe and extraction pump. The coalbed methane in the first separation and purification device First, it goes through dust removal, dehydration, deoxygenation, desulfurization and denitrification processes, and then CO2 separation is carried out. The separated CO2 enters the flue gas storage tank through the CO2 pipeline; the main components of the coalbed methane after the first separation and purification device are CH4 and N2. In the pressure swing adsorption separation and purification device, the pressure swing adsorption separation technology is used to make the CH4 concentration reach the standard for industrial use; the separated CH4 is injected into the CH4 storage tank for storage; pay attention to the reading of the CH4 concentration monitor during the initial extraction. When the CH4 concentration drops below 3%, close the extraction pump and CH4 valve, CO2 valve, connecting valve and extraction valve. At this time, the initial extraction is completed.

[0016] Step 5, inject flue gas to displace coalbed methane and then extract it: open the flue gas valve and injection valve, and start the gas injection pump; the flue gas is injected from the flue gas storage tank into the bottom of the goaf through the gas injection pump and enters the fissure network in the goaf, thereby displacing the desorption of coalbed methane; after 4 to 8 hours of injection, close the gas injection pump, flue gas valve and injection valve, and stop injection. At this time, the CO2 rich in the flue gas will displace the coalbed methane in the goaf; after four to seven days, carry out extraction work; open the extraction valve, CO2 valve and CH4 valve, start the extraction pump and open the first separation and purification device and the pressure swing adsorption separation and purification device at the same time The device is turned on again, and the extraction work is started; during this period, the extraction pump, the first separation and purification device, and the pressure swing adsorption separation and purification device work in the same way as the initial extraction in step 4; in the subsequent extraction process, the readings of the pressure gauge and the CH4 concentration monitor are observed to stabilize the pressure at 35MPa-45MPa; the flue gas injection and coalbed methane extraction work are repeated until the last extraction work, when the CH4 concentration is lower than 0.1%, the extraction work is stopped, the gas injection pump, the extraction pump, the first separation and purification device, the pressure swing adsorption separation and purification device, and all valves are turned off, and the injection and extraction work is ended;

[0017] Step 6, separation and purification of coalbed methane: During the extraction operation, whether it is the initial extraction or the subsequent extraction, the extracted coalbed methane contains dust, CO2, H2O, N2, O2, SO2 and nitrogen oxides in addition to CH4. At this time, it is necessary to separate and purify to obtain coalbed methane with a high CH4 content; when the coalbed methane passes through the first separation and purification device, it will undergo dust removal, desulfurization, denitrification, deoxygenation and dehydration processes, and then the CO2 therein will be separated and transported to the flue gas storage tank for recycling; the coalbed methane separated from the first separation and purification device has CH4 and N2 as its main components, and CH4 needs to be further concentrated; in the pressure swing adsorption separation and purification device, adsorption separation technology is used, carbon molecular sieve is used as adsorbent, and the coalbed methane is adsorbed by the molecular sieve, and the concentration of CH4 is increased to the industrial use standard after pressure swing adsorption;

[0018] Step 7, set up a CO2 leakage detection device: After all the work is completed, carry out well sealing work, use injection well packers and extraction well packers to seal the injection well and extraction well, and then keep the CO2 leakage alarm devices at the wellheads of the two wells in working condition; the injection well packers, extraction well packers and overlying aquifers and rock formations can seal the injected flue gas in the goaf, thereby achieving the purpose of reducing flue gas emissions from factories such as thermal power plants and metal smelters.

[0019] Furthermore, the abandoned mine is a coalbed methane-rich mine that has completed coal mining, has good stability of its roof and floor, and the overlying aquifers and rock formations can effectively prevent the injected flue gas from escaping, so that it can achieve a good sealing effect in the initial stage of sealing.

[0020] Furthermore, the diameters of the extraction well and the injection well are both 150mm, and the distance between the two wells is determined by the length of the goaf. When the length of the goaf is less than 150m, the two wells are arranged at both ends of the goaf; when the goaf is greater than 150m, the two wells are arranged with a distance of 150m; the bottom of the injection well is at the bottom area of ​​the goaf, and the bottom of the extraction well is at the top area of ​​the goaf. This arrangement can not only improve the efficiency of injection and extraction, but also improve the recovery rate of coalbed methane.

[0021] Furthermore, the injection well packer and the extraction well packer are both Y453 permanent suspension packers with a sealing length of 5m to 7m. This type of packer is mainly used for operations such as drainage and gas production.

[0022] Furthermore, the flue gas includes but is not limited to the flue gas emitted by thermal power plants and metal smelters. As long as the main component of the flue gas is CO2, it meets the usage standards.

[0023] Furthermore, the pressure swing adsorption separation and purification device uses an adsorption separation process, the molecular sieve is a carbon molecular sieve, and the carbon molecular sieve is used as an adsorbent. After the pressure swing adsorption cycle, the CH4 in the coalbed methane is concentrated to meet the industrial use standards.

[0024] Compared with the prior art, the present invention has the following technical advantages:

[0025] 1. Combining the flue gas injection displacement mining technology with the mixed gas separation and purification technology can form a complete abandoned mine coalbed methane mining and utilization system. First, collect and treat flue gas from factories such as thermal power plants; then inject the treated flue gas into abandoned mines to promote the desorption and output of coalbed methane; then use the mixed gas separation and purification technology to purify the produced coalbed methane; finally, use the purified CH4 in industries, residents or power generation to maximize the utilization of resources. This method can not only effectively improve the mining efficiency of coalbed methane, but also realize the storage and efficient utilization of flue gas, which is of great significance for alleviating energy pressure, improving energy self-sufficiency and protecting the environment.

[0026] 2. The flue gas used in the present invention is waste industrial flue gas collected from thermal power plants, metal smelters and other factories. These flue gases contain a large amount of CO2, N2, SO2, etc. The captured flue gas is injected into abandoned mines to mine coalbed methane, which is beneficial to alleviate the greenhouse effect and protect the environment.

[0027] 3. The present invention selects an abandoned mine with rich coalbed methane resources and has not been developed. The coalbed methane is mined by injecting flue gas into the abandoned mine. This can not only obtain coalbed methane resources from the abandoned mine, but also capture and seal the emission of industrial waste flue gas, which can be said to kill two birds with one stone.

[0028] 4. The present invention separates and purifies coalbed methane through adsorption separation technology. The selected adsorbent is a carbon molecular sieve, which has a developed pore structure, uniform pore size distribution, high thermal stability and chemical stability. In addition, its raw materials are carbon materials with the advantages of wide sources, simple acquisition, low cost, etc. The technology is mature and easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 It is the layout diagram of the injection system and the coalbed methane collection system;

[0030] The marks in the figure are as follows: 1—gas injection pump; 2—flue gas storage tank; 3—extraction pump; 4—first separation and purification device; 5—pressure swing adsorption separation and purification device; 6—CH4 storage tank; 7—flue gas valve; 8—CO2 recovery pipeline; 9—connecting pipeline; 10—connecting valve; 11—extraction valve; 12—CH4 valve; 13—injection valve; 14—injection pipeline; 15—extraction pipeline; 16—injection well packer; 17—extraction well packer; 18—injection well; 19—extraction well; 20—rock layer; 21—aquifer; 22—roof rock layer; 23—bottom rock layer; 24—goaf; 25—pressure gauge; 26—CH4 concentration monitor; 27—CO2 valve; 28—CO2 leakage alarm device. DETAILED DESCRIPTION

[0031] The present invention is further described in detail below in conjunction with specific implementation modes.

[0032] A method for injecting flue gas into abandoned mines to mine coalbed methane and separating and purifying the same, the specific steps of the method are:

[0033] Step 1, assessment and selection of abandoned mines: After exploring the target abandoned mines, collect necessary information on the occurrence of coalbed methane, the distribution of tunnels, shafts and goafs, and the hydrogeological conditions of the overlying strata; comprehensively consider the geographical location, traffic conditions and surrounding environmental conditions of the abandoned mines, determine the appropriate mining area, and prepare for the layout of the injection system and coalbed methane collection system.

[0034] Step 2, flue gas source and pretreatment: Collect industrial waste flue gas from nearby thermal power plants, metal smelters and other factories that emit flue gas mainly composed of CO2. After dust removal, desulfurization, denitrification and cooling, the main components of the flue gas are CO2, N2, O2 and H2O, which are stored in flue gas storage tanks;

[0035] Step 3, the layout of the injection system and the coalbed methane collection system, such as Figure 1As shown in the figure: after determining the target mining area, the injection well 18 and the extraction well 19 are constructed from the ground through the rock layer 20, the aquifer 21 and the roof rock layer 22 to the goaf 24. After the construction of the two wells is completed, the drilling work is completed; the injection pipeline 14 is extended from the ground to the bottom of the injection well 18, and the injection well packer 16 is arranged in the injection well 18. The ground end of the injection pipeline 14 is connected to the gas injection pump 1, and the injection pipeline 14 is provided with an injection valve 13 and a pressure gauge 25. The gas injection pump 1 is connected to the flue gas storage tank 2 by a pipeline, and a flue gas valve 7 is arranged therebetween. The flue gas storage tank 2 contains pre-treated flue gas; the extraction pipeline 15 is extended from the ground to the bottom of the extraction well 19, and the extraction well packer 17 is arranged in the extraction well 19. The ground end of the extraction pipeline 15 is connected to the extraction pump 3, and the extraction pipeline 15 is provided with an extraction valve 11 and a CH4 concentration monitor 26. At the same time, it is connected to the injection pipeline 14 through the connecting pipeline 9, and the connecting pipeline 9 is provided with a connecting valve 10. The extraction pump 3 is connected to the first separation and purification device 4 by a pipeline, and the first separation and purification device 4 is also connected to the flue gas storage tank 2 through the CO2 recovery pipeline 8, and the pipeline is provided with a CO2 valve 27. In addition, the first separation and purification device 4 is connected to the pressure swing adsorption separation and purification device 5 through a pipeline. The pressure swing adsorption separation and purification device 5 is connected to the CH4 storage tank 6 by a pipeline, and a CH4 valve 12 is provided therebetween. Finally, a CO2 leakage alarm device 28 is installed at the wellheads of both wells.

[0036] Step 4, initial extraction: After completing all the above work, the first extraction of coalbed methane (CH4, CO2, N2, etc.) is carried out, the purpose of which is to collect the original coalbed methane in the goaf 24, increase the efficiency of flue gas displacement of coalbed methane in the subsequent steps, and improve the extraction rate of coalbed methane. Open the CH4 valve 12, CO2 valve 27, connecting valve 10 and extraction valve 11, and keep the flue gas valve 7 and injection valve 13 closed. Turn on the extraction pump 3, and at the same time make the first separation and purification device 4 and the pressure swing adsorption separation and purification device 5 enter the working state. At this time, the original coalbed methane in the goaf 24 passes through the extraction pipe 15, the injection pipe 14, the connecting pipe 9 and the extraction pump 3 into the first separation and purification device 4. The coalbed methane undergoes dust removal, dehydration, deoxidation, desulfurization and denitrification processes in the first separation and purification device 4, and then CO2 separation is carried out. The separated CO2 enters the flue gas storage tank 2 through the CO2 recovery pipeline 8. After the coalbed methane passes through the first separation and purification device 4, its main components are CH4 and N2. In the pressure swing adsorption separation and purification device 5, the pressure swing adsorption separation technology is used to make the concentration of CH4 reach the standard for industrial use. Finally, the treated coalbed methane enters the CH4 storage tank 6 for storage. During the initial extraction, pay attention to the reading of the CH4 concentration monitor 26. When the CH4 concentration drops below 3%, close the extraction pump 3 and the CH4 valve 12, CO2 valve 27, connecting valve 10 and extraction valve 11. At this time, the initial extraction is completed.

[0037] Step 5, extract after injecting flue gas to displace coalbed methane: open the flue gas valve 7 and the injection valve 13, and start the gas injection pump 1. Flue gas is injected from the flue gas storage tank 2 through the gas injection pump 1 into the bottom of the goaf 24 and enters the fracture network of the goaf 24, thereby displacing the desorption of coalbed methane. After 4 to 8 hours of injection, close the gas injection pump 1, the flue gas valve 7 and the injection valve 13, and stop the injection. At this time, the CO2 rich in the flue gas will displace the coalbed methane in the goaf. Four to seven days after the flue gas is injected, the extraction work is carried out. Open the extraction valve 11, the CO2 valve 27 and the CH4 valve 12, start the extraction pump 3, and open the first separation and purification device 4 and the pressure swing adsorption separation and purification device 5 at the same time, and start the extraction work. During this period, the working status of the extraction pump 3, the first separation and purification device 4 and the pressure swing adsorption separation and purification device 5 is consistent with the initial extraction in step 4. In the subsequent extraction process, the readings of the pressure gauge 25 and the CH4 concentration monitoring meter 26 are observed to stabilize the pressure at 35MPa to 45MPa. The flue gas injection and coalbed methane extraction work are repeated until the last extraction work, when the CH4 concentration is lower than 0.1%, the injection and extraction work are stopped, the gas injection pump 1, the extraction pump 3, the first separation and purification device 4 and the pressure swing adsorption separation and purification device 5 and all valves are closed, and the injection and extraction work is ended.

[0038] Step 6, separation and purification of coalbed methane: During the extraction operation, whether it is the initial extraction or the subsequent extraction, the extracted coalbed methane contains dust, CO2, H2O, N2, O2, SO2 and nitrogen oxides in addition to CH4. At this time, it needs to be separated and purified to obtain coalbed methane with a high CH4 content. First, when the coalbed methane passes through the first separation and purification device 4, dust removal, desulfurization, denitrification, deoxygenation and dehydration processes will be carried out, and then the CO2 will be separated and transported to the flue gas storage tank 2 for recycling. The coalbed methane separated from the first separation and purification device 4 has CH4 and N2 as its main components, and the CH4 needs to be further concentrated to meet the industrial use standards. In the pressure swing adsorption separation and purification device 5, adsorption separation technology is used, and carbon molecular sieve is used as the adsorbent. After pressure swing adsorption, the concentration of CH4 is increased to the industrial use standard, and finally stored in the CH4 storage tank 6;

[0039] Step 7, set up CO2 leakage detection device: After all the work is completed, the well is sealed, and the injection well packer 16 and the extraction well packer 17 are used to seal the injection well 18 and the extraction well 19, and then the CO2 leakage alarm device 28 at the wellhead of the two wells is kept in working state. The injection well packer 16, the extraction well packer 17 and the overlying aquifer 21 and the rock formations 20 and 22 can seal the injected flue gas in the goaf 24, thereby achieving the purpose of reducing the flue gas emissions of factories such as thermal power plants and metal smelters.

[0040] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solution of the present invention can be modified or replaced by equivalents without departing from the purpose and scope of the technical solution of the present invention, which should be included in the scope of the claims of the present invention.

Claims

1. A method for injecting flue gas into abandoned mines to mine coalbed methane and separating and purifying the same, characterized in that: After surveying and evaluating the target abandoned mines, select the appropriate target area for coalbed methane mining; then collect and treat the flue gas from the factories that emit flue gas mainly composed of CO2; After the injection system and coalbed methane extraction system are arranged on the ground above the goaf, the coalbed methane originally existing in the goaf is first extracted and collected; after the initial extraction is completed, flue gas is injected to displace the coalbed methane in the goaf, and then extraction is carried out again, and this cycle is repeated until the coalbed methane extraction work is completed; the extracted coalbed methane is processed by the first separation and purification device, and the CO2 therein is separated and transported to the injection system for recycling, and the treated mixed gas enters the pressure swing adsorption separation and purification device for CH4 concentration to meet the standards for industrial use; after the extraction work is completed, the goaf is closed to seal the injected flue gas in the goaf.

2. The method for extracting coalbed methane from abandoned mines by injecting flue gas into the abandoned mines and separating and purifying the coalbed methane therefrom according to claim 1, characterized in that: The specific steps of this method are as follows: Step 1: Exploring the target abandoned mine, collecting necessary coalbed methane occurrence information, distribution of tunnels, shafts and goafs, and hydrogeological conditions of overlying rock formations; comprehensively considering the geographical location, traffic conditions and surrounding environment of the abandoned mine, and determining the appropriate mining area; Step 2, collecting industrial waste flue gas from nearby factories that mainly produce CO2 flue gas, and subjecting the flue gas to dust removal, desulfurization, denitrification and cooling treatment, the main components of the flue gas are CO2, N2, O2 and H2O; Step 3, after determining the target abandoned mine, from the ground through the rock layer and aquifer, respectively, to the goaf to construct the injection well and the extraction well, after the construction of the two wells is completed, the drilling work is completed; the injection pipeline is extended from the ground to the bottom of the injection well, and an injection well packer is installed in the injection well; the ground end of the injection pipeline is connected to the gas injection pump, and the injection pipeline is provided with an injection valve and a pressure gauge; the gas injection pump is connected to the flue gas storage tank with a pipeline, and a flue gas valve is provided therebetween, and the flue gas storage tank contains pretreated flue gas; the extraction pipeline is extended from the ground to the bottom of the extraction well, and an extraction well packer is installed in the extraction well; the ground end of the extraction pipeline The end is connected to the extraction pump, and an extraction valve and a CH4 concentration monitor are provided on the extraction pipeline; at the same time, it is connected to the injection pipeline through a connecting pipeline, and a connecting valve is provided on the connecting pipeline; the extraction pump is connected to the first separation and purification device by a pipeline, and the first separation and purification device is also connected to the flue gas storage tank through a CO2 recovery pipeline, and a CO2 valve is provided on the CO2 recovery pipeline; in addition, the first separation and purification device is connected to the pressure swing adsorption separation and purification device through a pipeline; the pressure swing adsorption separation and purification device is connected to the CH4 storage tank by a pipeline, and a CH4 valve is provided therebetween; finally, CO2 alarm devices are installed at the wellheads of the two wells; Step 4. After completing all the above work, the first extraction of coalbed methane is carried out; open the CH4 valve, CO2 valve, connecting valve and extraction valve, and keep the flue gas valve and injection valve closed; turn on the extraction pump, and at the same time put the first separation and purification device and the pressure swing adsorption separation and purification device into working state. At this time, the original coalbed methane in the goaf enters the first separation and purification device through the extraction pipe, injection pipe, connecting pipe and extraction pump. The coalbed methane first undergoes dust removal, dehydration, deoxidation, desulfurization and denitrification processes in the first separation and purification device, and then CO2 Separation, the separated CO2 enters the flue gas storage tank through the CO2 pipeline; the main components of the coalbed methane after the first separation and purification device are CH4 and N2. In the pressure swing adsorption separation and purification device, the pressure swing adsorption separation technology is used to make the CH4 concentration reach the standard for industrial use; the separated CH4 is injected into the CH4 storage tank for storage; during the initial extraction, pay attention to the reading of the CH4 concentration monitor. When the CH4 concentration drops below 3%, close the extraction pump and the CH4 valve, CO2 valve, connecting valve and extraction valve. At this time, the initial extraction is completed. Step 5, open the flue gas valve and injection valve, and start the gas injection pump; Flue gas is injected from the flue gas storage tank into the bottom of the goaf through the gas injection pump and enters the fissure network of the goaf, thereby displacing the desorption of coalbed methane; according to the size of the goaf, after 4 to 8 hours of injection, the gas injection pump, flue gas valve and injection valve are closed and the injection is stopped. At this time, the CO2 rich in the flue gas will displace the coalbed methane in the goaf; after four to seven days, extraction work is carried out; the extraction valve, CO2 valve and CH4 valve are opened, and the first separation and purification device and the pressure swing adsorption separation and purification device are opened while the extraction pump is started, and the extraction work is started again; in the meantime, The operation of the extraction pump, the first separation and purification device, and the pressure swing adsorption separation and purification device is consistent with the initial extraction in step 4; in the subsequent extraction process, observe the readings of the pressure gauge and the CH4 concentration monitor to stabilize the pressure at 35MPa~45MPa; repeat the flue gas injection and coalbed methane extraction work until the last extraction work, when the CH4 concentration is lower than 0.1%, stop the extraction work, turn off the gas injection pump, the extraction pump, the first separation and purification device, the pressure swing adsorption separation and purification device and all valves, and end the injection and extraction work; Step 6: During the extraction operation, whether it is the initial extraction or the subsequent extraction, the extracted coalbed methane contains dust, CO2, H2O, N2, O2, SO2 and nitrogen oxides in addition to CH4. At this time, it needs to be separated and purified to obtain coalbed methane with a high CH4 content; When the coalbed methane passes through the first separation and purification device, it will undergo dust removal, desulfurization, denitrification, deoxidation and dehydration processes, and then the CO2 will be separated and transported to the flue gas storage tank for recycling; the coalbed methane separated from the first separation and purification device has CH4 and N2 as its main components, and CH4 needs to be further concentrated; in the pressure swing adsorption separation and purification device, the molecular sieve adsorbs the coalbed methane to increase the concentration of CH4 to the industrial use standard; Step 7, after all the work is completed, the well is sealed. The injection well packer and the extraction well packer are used to seal the injection well and the extraction well, and then the CO2 leakage alarm devices at the wellheads of the two wells are kept in working condition; the injection well packer, the extraction well packer and the overlying aquifer and rock formation can seal the injected flue gas in the goaf, thereby achieving the purpose of reducing flue gas emissions.

3. The method for extracting coalbed methane from abandoned mines by injecting flue gas into the abandoned mines and separating and purifying the coalbed methane according to claim 1, characterized in that: The abandoned mine is a coal-bed methane-rich mine that has completed coal mining. Its roof and floor plates have good stability, and the overlying aquifers and rock formations can effectively prevent the injected flue gas from escaping, so that it achieves a good sealing effect in the initial stage of sealing.

4. The method for extracting coalbed methane from abandoned mines by injecting flue gas into the abandoned mines and separating and purifying the coalbed methane according to claim 1, characterized in that: The diameters of the extraction well and the injection well are both 150 mm, and the distance between the two wells is determined by the length of the goaf. When the length of the goaf is less than 150 m, the two wells are arranged at both ends of the goaf; when the goaf is greater than 150 m, the two wells are arranged with a distance of 150 m between them; the bottom of the injection well is in the bottom area of ​​the goaf, and the bottom of the extraction well is in the top area of ​​the goaf.

5. The method for extracting coalbed methane from abandoned mines by injecting flue gas into the abandoned mines and separating and purifying the coalbed methane according to claim 1, characterized in that: The injection well packer and the extraction well packer are both Y453 permanent suspension packers with a sealing length of 5m to 7m.

6. The method for extracting coalbed methane by injecting flue gas into abandoned mines and separating and purifying the coalbed methane according to claim 1, characterized in that: The flue gas includes but is not limited to the flue gas emitted by thermal power plants and metal smelters. As long as the main component of the flue gas is CO2, it meets the use standards.

7. The method for extracting coalbed methane from abandoned mines by injecting flue gas into the abandoned mines and separating and purifying the coalbed methane according to claim 1, characterized in that: The molecular sieve used in the pressure swing adsorption separation and purification device is a carbon molecular sieve.

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