Flue gas deep in-seam cyclic sequestration system and method

Abstract

The present application belongs to the technical field of CO2 geological storage, and particularly relates to a flue gas circulating storage system and method in deep unrecoverable coal seam, which comprises a flue gas storage tank, a dust remover connected with the outlet of the flue gas storage tank, a solid-liquid mixer connected with the outlet of the dust remover, a water flow pipeline arranged in the solid-liquid mixer, a one-way valve arranged in the outlet of the solid-liquid mixer, an outlet pipeline of the one-way valve communicated with the inlet of an ultrasonic stirrer, an outlet of the ultrasonic stirrer communicated with a plunger pump, a gas injection pipe arranged in the outlet of the plunger pump, the gas injection pipe comprising a vertical section and a horizontal section, and the bottom of the vertical section located in the unrecoverable coal seam; the system further comprises a pumping and circulating device and a current cathode protection device; the present application mixes the residual SiO2 and Al2O3 particles in the flue gas with water to form a nanofluid, and simultaneously makes sulfides in the flue gas react with water to generate acid, thereby improving the permeability of the coal seam, enhancing the wettability of the coal surface, greatly increasing the storage amount of CO2 and the recovery amount of coal seam gas, and improving the carbon storage efficiency of the coal seam.

Description

Technical Field

[0001] This invention belongs to the field of CO2 geological storage technology, and particularly relates to a flue gas circulation storage system and method in deep unminable coal seams. Background Technology

[0002] Approximately 50% of my country's CO2 emissions originate from flue gas from coal-fired power plants. Furthermore, my country's energy structure, characterized by abundant coal, scarce oil, and limited natural gas, dictates that fossil fuels will remain the primary energy source for a considerable period. Reducing greenhouse gas emissions from fossil fuel combustion is crucial for achieving my country's "dual carbon goals," leading to the development of carbon capture and storage (CCS) technology. CCS technology generally comprises three steps: carbon capture, carbon transport, and carbon storage. For example, patent number 20221149044.9 discloses a "carbon dioxide storage method and system." This method involves passing captured carbon dioxide through pressurization and cooling modules into supercooled water, processing it into carbon dioxide hydrates. Spherical carbon dioxide hydrates are then injected into a sealed container and placed into a carbon dioxide storage layer. However, this method is cumbersome, and the carbon capture process suffers from challenges such as low purity, low efficiency, and high investment costs. Coal seams, as a porous medium reservoir, have been proven to be an excellent formation for carbon dioxide sequestration. They also contain various elements such as carbon, sulfur, and nitrogen. Directly injecting flue gas into deep, unminable coal seams for sequestration can achieve geological carbon dioxide sequestration while absorbing harmful gases such as sulfides and nitrogen oxides from the flue gas. This greatly reduces the technological cost of carbon capture and helps my country achieve its "dual carbon goals".

[0003] High efficiency and stability in CO2 geological storage are crucial prerequisites for achieving CCS (Chemical Supercritical Storage). For example, CN202210478169.2 discloses a method for in-situ deep supercritical storage of flue gas, injecting compressed flue gas into the storage layer at a depth of 5000-7000 m below the surface for deep storage. However, this method suffers from high drilling costs, significant uncertainty in formation storage potential, low storage efficiency due to the high concentration of impurities such as nitrogen in the flue gas, short service life of injection wells, and increased risk of gas leakage due to the presence of large amounts of free gas. Summary of the Invention

[0004] The purpose of this invention is to provide a system and method for the cyclic storage of flue gas in deep, unminable coal seams. This invention selects an unminable coal seam at a depth of 800 m as the storage layer. It utilizes the strong CO2 adsorption characteristics of coal to ensure storage stability, uses micro- to nano-sized solid particles in the flue gas to prepare nanofluids to improve coal seam wettability and increase CO2 storage capacity, and leverages the abundant porosity and fractures of the coal reservoir to enhance flue gas flow and carbon sequestration efficiency. The cyclic storage method further improves the CO2 sequestration effect in the flue gas. Compared to other methods, this method has a simple process flow, can utilize micro- to nano-sized solid particles in flue gas and cyclically store residual CO2, fully utilizes the carbon sequestration potential of the coal seam, effectively improves coal seam gas recovery, and significantly reduces CO2 storage operation costs, possessing strong potential for practical engineering application.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A flue gas circulation and storage system in a deep, unminable coal seam includes a flue gas storage tank, a dust collector connected to the outlet pipe of the flue gas storage tank, an outlet pipe of the dust collector connected to the inlet pipe of a solid-liquid mixer, the solid-liquid mixer also having a water flow pipe supplied by a water tank, a one-way valve on the outlet pipe of the solid-liquid mixer, the outlet pipe being connected to the inlet of an ultrasonic stirrer, the outlet of the ultrasonic stirrer being connected to the inlet of a plunger pump, the outlet of the plunger pump having an injection pipe located in the injection well including a vertical section and a horizontal section, the bottom of the vertical section being located in the unminable coal seam; it also includes an extraction and circulation device and a current cathodic protection device.

[0006] Furthermore, the extraction and circulation device includes an extraction pipe installed in the gas well, an extraction pump installed at the outlet of the extraction pipe, and a CO2-coalbed methane separator installed at the outlet of the extraction pump. One end of the CO2-coalbed methane separator is connected to a coalbed methane storage tank, and the other end is connected to a flue gas storage tank through a CO2 circulation pipeline and a high-pressure pump.

[0007] Furthermore, the current cathodic protection device includes an external power supply installed on the wall of the injection well. The negative terminal of the external power supply is connected to the injection well, and the positive terminal of the external power supply is connected to the anode material. The power supply is connected to a DC transformer box via a wire, and a transformer rectifier is connected to the DC transformer box via a wire. A switch is installed on the wire, and the anode material at the end is inserted from the top rock layer to the unminable coal seam.

[0008] Furthermore, the flue gas outlet pipe is equipped with a first safety valve and a first pressure gauge in sequence; the water tank outlet pipe is equipped with a water injection valve; the ultrasonic stirrer outlet pipe is equipped with a first valve and a nanofluid concentration sensor in sequence; the plunger pump outlet pipe is equipped with a first CO2 concentration sensor and a second valve; the extraction pipe is equipped with a second CO2 concentration sensor and an extraction valve; the CO2 circulation pipe is equipped with a third CO2 concentration sensor and a CO2 valve; and the high-pressure pump outlet pipe is equipped with a second pressure gauge and a second safety valve in sequence.

[0009] The sealing method using the above system includes the following steps: S1. Construction Drilling: First, determine the extent and depth of the unminable coal seam. Generally, an unminable coal seam at a depth of 800 m is selected as the sealing layer. Studies have shown that when the burial depth of the coal seam generally exceeds 800 m, the temperature and pressure conditions of the coal seam above this depth can put CO2 in a supercritical state. Supercritical carbon dioxide has low viscosity and high diffusivity, allowing CO2 to enter the coal seam fractures. At the same time, it has mass and heat transfer characteristics, making the CO2 phase change fracturing effect better. Then, from the surface, through the rock strata and aquifer, gas injection wells and gas production wells are drilled into the unminable coal seam respectively. After the gas injection well is completed, a horizontal well is drilled from the bottom of the gas injection well along the unminable coal seam to position d2. After completion, drilling work is stopped. S2. Set up a flue gas injection and coalbed methane extraction and separation system: Connect one end of the injection pipe to the outlet of the plunger pump, and extend the other end into the injection well until it reaches the bottom of the horizontal well. Extend one end of the extraction pipe from the ground into the production well, and place the other end outside the production well. Connect the plunger pump to the outlet of the ultrasonic agitator. Connect the extraction port of the extraction pump to the outlet of the extraction pipe. Connect the exhaust port of the extraction pump to the inlet of the CO2-coalbed methane separator. Connect the CO2 outlet of the CO2-coalbed methane separator to the flue gas storage tank through the CO2 circulation pipeline and the high-pressure pump. Connect the coalbed methane outlet to the coalbed methane storage tank through the pipeline. S3. Set up a nanofluid preparation system: Connect the flue gas storage tank to the dust collector through a pipeline, and connect the dust collector to the solid-liquid mixer through a pipeline; connect the outlet of the solid-liquid mixer to the inlet of the ultrasonic stirrer; connect the outlet of the ultrasonic stirrer to the plunger pump; connect the solid-liquid mixer to an external water tank; install a check valve on the pipeline connecting the outlet of the solid-liquid mixer to the ultrasonic stirrer. S4. Set up a forced current cathodic protection device: Connect the gas injection well to the negative terminal of the external power supply, connect the positive terminal of the power supply to the anode material, connect the power supply to the DC transformer box through a wire, connect the transformer rectifier to the DC transformer box through a wire, install a switch on the wire, and make the anode material into a rod or a long and thin block and insert it from the top rock layer until the unminable coal seam. S5. Flue gas injection and circulation CO2 storage: Turn on the transformer rectifier to convert the nearby AC power into DC power. Turn on the power supply. The DC power forces the current to flow from the anode material through the coal seam to the steam injection well pipeline, and finally flows back to the negative terminal of the power supply through the conductor, thereby protecting the pipeline. Open the first safety valve set on the outlet pipeline of the flue gas storage tank. The flue gas enters the dust collector to remove larger particles in the flue gas. Open the water injection valve on the water tank. Water mixes with flue gas particles with a diameter of <10μm that enter the solid-liquid mixer to form a nanofluid. At the same time, the sulfides in the flue gas become acidic when they come into contact with water. Nanofluid is introduced into an ultrasonic stirrer to thoroughly mix water and nanomaterials; the plunger pump is turned on, and wet flue gas and nanofluid are continuously injected into the horizontal well through the plunger pump. The wet flue gas and nanofluid enter the coal seam for the first sealing; then the extraction pump is turned on, and gas injection and gas production are carried out simultaneously. At this time, the CO2 that is not completely adsorbed and the displaced coalbed methane enter the CO2-coalbed methane separator after passing through the extraction pipe and extraction pump. After the mixed gas is separated in the CO2-coalbed methane separator, the coalbed methane enters the coalbed methane storage tank for storage, and the remaining CO2 gas is repressurized and injected again with the flue gas for circulation sealing.

[0010] Furthermore, an injection pipe is installed inside the injection well, and the distance between the end position d1 of the horizontal section of the injection pipe and the cutoff position d2 of the horizontal well is less than 30m. The distance between the gas production well and the injection well is greater than 200m, and the horizontal distance between the cutoff position d2 of the horizontal well and the cutoff position d3 of the gas production well is less than 20m.

[0011] Furthermore, in step S2, the injection pipe and extraction pipe are coated with a corrosion-resistant material.

[0012] Furthermore, in step S5, the mass concentration of solid particles in the flue gas in the nanofluid is between 0.01% and 2%. The change in the value displayed by the CO2 concentration sensor is observed. When the CO2 concentration at the gas production well is higher than 80% of the CO2 concentration at the injection well, injection and extraction are stopped.

[0013] The advantages of this invention are: 1. This invention fully utilizes solid particles such as SiO2 and Al2O3 in flue gas to prepare nanofluids, which improves the wettability of coal seam surfaces, effectively increases the amount of CO2 sequestered in coal seams, promotes mineralization reactions, and improves geological sequestration stability; 2. This invention re-injects residual CO2 from flue gas into the coal seam for recirculation and storage, making full use of the CO2 storage potential brought about by the change in coal seam wettability by nanofluids, thereby improving CO2 storage efficiency. At the same time, the high N2 content in the flue gas can further reduce the coal matrix expansion caused by CO2 adsorption, thereby reopening the cracks that were closed due to CO2 adsorption, promoting the recovery of permeability, and further increasing the amount of CO2 stored. 3. The pipes selected in this invention have a corrosion-resistant coating, and a forced current cathodic protection system is set up, which is provided with protective current by an external power source to reduce the risk of corrosion of equipment and pipes by wet flue gas, greatly reducing the corrosion rate of the pipes and enhancing the service life of the pipes. 4. This invention utilizes the advantages of supercritical carbon dioxide sequestration. Studies have shown that when the burial depth of coal seams generally exceeds 800m, the temperature and pressure conditions above this depth can cause CO2 to be in a supercritical state. Supercritical carbon dioxide has low viscosity and high diffusivity, allowing it to enter the coal seam fractures. It also has mass and heat transfer characteristics, making the CO2 phase change-induced cracking effect better. The supercritical carbon dioxide-induced cracking of coal is closely related to existing knowledge in related fields. The relevant research results have been recorded in our team's 2022 publications "Analysis of the Fault Characteristics of Coal Body Under Short-Time Immersion by Supercritical Carbon Dioxide Jet Impact" and "Cracking characteristic induced by supercritical carbon dioxide phasechange in micro-cracks of coal". Attached Figure Description

[0014] Figure 1 This is a schematic diagram of the structure of the present invention. Detailed Implementation

[0015] As shown in the figure, a flue gas circulation and storage system in a deep, unminable coal seam includes a flue gas storage tank 1, a dust collector 4 connected to the outlet pipe of the flue gas storage tank, and the outlet pipe of the dust collector connected to the inlet pipe of a solid-liquid mixer 7. The solid-liquid mixer also has a water flow pipe supplied by a water tank 5. A one-way valve 8 is installed on the outlet pipe of the solid-liquid mixer, and the outlet pipe is connected to the inlet of an ultrasonic stirrer. The outlet of the ultrasonic stirrer is connected to the inlet of a plunger pump 12, and the outlet of the plunger pump is equipped with an injection pipe 18. The injection pipe is located in an injection well 17 and includes a vertical section and a horizontal section, with the bottom of the vertical section located in the unminable coal seam. The system also includes an extraction and circulation device and a current cathodic protection device. The extraction and circulation device includes an extraction pipe 20 located in a gas extraction well 21, an extraction pump 24 at the outlet of the extraction pipe, and a CO2-coalbed gas separator 25 at the outlet of the extraction pump. One end of the CO2-coalbed gas separator is connected to a coalbed gas storage tank 32, and the other end is connected to a CO2 circulation pipe and a high-pressure gas flow meter. The pressure pump 28 is connected to the flue gas storage tank 1; the current cathodic protection device includes an external power supply installed on the wall of the injection well, the negative terminal of the external power supply is connected to the injection well, the positive terminal of the external power supply is connected to the anode material, the power supply is connected to the DC transformer box through a wire, the transformer rectifier is connected to the DC transformer box through a wire, a switch is installed on the wire, and the anode material at the end is inserted from the top rock layer to the unminable coal seam; the flue gas outlet pipe is equipped with a first safety valve 2 and a first pressure gauge 3 in sequence, the water tank outlet pipe is equipped with a water injection valve, the ultrasonic stirrer outlet pipe is equipped with a first valve 10 and a nanofluid concentration sensor in sequence; the plunger pump outlet pipe is equipped with a first CO2 concentration sensor 13 and a second valve 14; the extraction pipe is equipped with a second CO2 concentration sensor 23 and an extraction valve; the CO2 circulation pipe is equipped with a third CO2 concentration sensor 26 and a CO2 valve 27; the high-pressure pump outlet pipe is equipped with a second pressure gauge 29 and a second safety valve 30 in sequence.

[0016] A method for storing flue gas in a deep, unminable coal seam using a circulating storage system includes the following steps: (1) Construction Drilling: First, determine the extent and depth of the unminable coal seam 19. Then, from the surface, drill gas injection well 17 and gas production well 21 through the rock strata 15 and aquifer 16 into the unminable coal seam 19. After the gas injection wells are completed, drill a horizontal well from the bottom of the gas injection well along the unminable coal seam 19 to position d2. Drilling is stopped after this is completed. The diameters of gas injection well 17 and gas production well 21 should be greater than 100m, the distance between the two vertical wells should be greater than 200m, the distance between the end positions d1 and d2 of the injection pipe 18 should be less than 30m, and the horizontal distance between the horizontal well cutoff position d2 and the gas production well cutoff position d3 should be less than 20m. Using these parameters during construction ensures that after the subsequent supercritical carbon dioxide fracturing forms a fracture network, the fracture network can connect the horizontal well and the gas production well 21, thus facilitating subsequent flue gas injection and methane extraction.

[0017] (2) Setting up a flue gas injection and coalbed methane extraction and separation system: Connect one end of the injection pipe 18 to the outlet of the plunger pump 12, and extend the other end into the injection well until it reaches the bottom of the horizontal well. Extend one end of the extraction pipe 20 from the ground into the production well 21, and place the other end outside the production well. Connect the plunger pump 12 to the outlet of the ultrasonic agitator 9. Connect the extraction port of the extraction pump 24 to the outlet of the extraction pipe 20. Connect the exhaust port of the extraction pump 24 to the inlet of the CO2-coalbed methane separator 25. The CO2 outlet of the CO2-coalbed methane separator 25 is connected to the flue gas storage tank 1 through the CO2 circulation system, and the coalbed methane outlet is connected to the coalbed methane storage tank 32 through a pipeline. The gas injection pipe 18 and the extraction pipe 20 are uniformly coated with corrosion-resistant material. An extraction valve 22 is installed on the extraction pipe 20, and a coalbed methane valve 31 is installed on the pipeline between the CO2-coalbed methane separator 25 and the coalbed methane storage tank 32, thus completing the setup of the flue gas injection and extraction system.

[0018] (3) Setting up a nanofluid preparation system: Connect the flue gas storage tank 1 to the dust collector 4 through a pipeline, and connect the dust collector 4 to the solid-liquid mixer 7 through a pipeline; connect the outlet of the solid-liquid mixer 7 to the inlet of the ultrasonic stirrer 9; connect the outlet of the ultrasonic stirrer 9 to the plunger pump 12; connect the solid-liquid mixer 7 to an external water tank 5; install a one-way valve 8 on the pipeline connecting the outlet of the solid-liquid mixer 7 to the ultrasonic stirrer 9; install a first pressure gauge 3 and a first safety valve 2 on the pipeline connecting the flue gas storage tank 1 to the dust collector 4; install a water injection valve 6 on the pipeline connecting the water tank 5 to the solid-liquid mixer 7; install a first valve 10 and a nanofluid concentration sensor 11 on the pipeline connecting the outlet of the ultrasonic stirrer 9 to the plunger pump 12, thus completing the setup of the nanofluid formation system.

[0019] (4) Setting up a CO2 circulation system: The CO2-coalbed gas separator 25 is connected to the high-pressure pump 28 through a pipeline, and the high-pressure pump 28 is connected to the coalbed gas storage tank 1 through a pipeline; a third CO2 concentration sensor 26 and a CO2 valve 27 are installed on the pipeline connecting the CO2-coalbed gas separator 25 and the high-pressure pump 28; a second pressure gauge 29 and a second safety valve 30 are installed on the pipeline connecting the flue gas storage tank 1 and the high-pressure pump 28, thus completing the setting up of the CO2 circulation system.

[0020] (5) Set up a forced current cathodic protection system: Connect the pipeline to the negative terminal of the external power supply 36, connect the positive terminal of the power supply 36 to the anode material 37, connect the power supply 36 to the DC transformer box 34 through the wire, and connect the transformer rectifier 33 to the DC transformer box 34 through the wire; install a switch 35 on the wire, and make the anode material 37 into a rod shape or a slender block shape to complete the setting of the forced current cathodic protection system.

[0021] (6) Flue gas injection and CO2 sealing: Turn on the transformer rectifier 33 to convert the nearby AC power into DC power. Turn on the power supply 36. The DC power supply forces the current from the anode material 37 through the coal seam to the pipeline, and finally flows back to the negative terminal of the power supply 36 through the wire, thereby protecting the pipeline. Open the first safety valve 2, and the flue gas enters the dust collector 4 to initially remove the larger particles in the flue gas; open the water injection valve 6, and water mixes with the flue gas particles (SiO2, Al2O3, etc.) with a particle size <10μm that enter the solid-liquid mixer 7 to form a nanofluid. At the same time, the sulfides in the flue gas become acidic when they meet the water, which dissolves the minerals in the coal, causing changes in the pore structure of the coal, widening the pores and fissures, improving the connectivity between pores, and increasing the adsorption capacity of CO2; the nanofluid enters the ultrasonic stirrer 9 to fully mix the water and nanomaterials. The ultrasonic stirrer uses the principle of ultrasonic vibration to stir without physical contact, avoiding contamination of the nanofluid. Moreover, its vibration frequency and amplitude can be adjusted to precisely control the particle size and distribution of nanoparticles. The first valve 10 and plunger pump 12 are opened, and wet flue gas and nanofluid are continuously injected into the horizontal well through the plunger pump 12. The wet flue gas and nanofluid enter the coal seam 19 for the first time for sequestration, which will enhance the wettability of the coal seam surface. The higher the wettability of the coal seam, the more active sites and the larger the surface area can be provided, which will promote the interaction between CO2 and the coal seam surface and promote the dissolution of CO2, thereby increasing the amount of CO2 adsorbed. Due to the temperature and pressure conditions of the coal seam, CO2 is in a supercritical state. Supercritical carbon dioxide has extremely strong permeability and diffusion, making it easier for CO2 to enter the coal seam fractures. High-pressure CO2 fractures the coal seam, forming a fracture network around the horizontal well, and connecting with the gas production well through the fracture network. During the flow of CO2, it displaces coalbed methane and is adsorbed into the coal seam. N2 in the flue gas can replace the desorbed CO2 and reduce the expansion of the coal matrix caused by CO2 adsorption, thereby reopening the fractures that were closed due to CO2 adsorption, promoting the recovery of permeability, and further increasing the amount of CO2 sequestration. Then, the extraction pump 24 is turned on, maintaining simultaneous gas injection and extraction. At this time, the CO2 that is not completely adsorbed and the displaced coalbed methane enter the CO2-coalbed methane separator 25 after passing through the extraction pipe and extraction pump. Since carbon dioxide exists in the pores of the coal seam in an adsorbed form, the adsorbed CO2 and the coal seam will form a strong adsorption force, resulting in higher stability and stronger adsorption capacity with the coal seam surface, thus improving the stability of the sealing and preventing the sealed CO2 from being extracted along with the coalbed methane. After separation in the CO2-coalbed methane separator 25, the coalbed methane enters the storage tank 32 for storage. The CO2 valve 27 is opened, and the remaining CO2 gas passes through the CO2 circulation system, is repressurized, and is reinjected with the flue gas. This cycle is repeated 2-3 times to allow the nanofluid to fully combine with the coal seam, further enhancing the erosion effect on the coal seam, ensuring complete CO2 adsorption, and achieving the effect of cyclic sealing.During the sealing operation, the one-way valve 8 prevents the nanofluid from flowing back into the solid-liquid mixer 7; the water injection valve 6 controls the water flow rate; the change in the value of the nanofluid sensor 11 is observed, and the mass concentration of flue gas solid particles in the nanofluid is adjusted between 0.01% and 2% according to the actual situation; the changes in the values ​​displayed by the first and second CO2 concentration sensors 13 and 23 are observed, and injection and extraction are stopped when the CO2 concentration at the gas production well is higher than 80% of the CO2 concentration at the injection well. At this time, the CO2 valve 27, plunger pump 12, coalbed methane valve 31, extraction valve 22, extraction pump 24, high-pressure pump 28 and CO2-coalbed methane separator 25 are closed to complete the wet flue gas circulation sealing CO2 process.

[0022] CO2 Sequestration: When CO2 sequestration reaches saturation, the injection pipe 18 and the production pipe 20 are removed from the injection well 17 and the production well 21 respectively, and the injection well 17 and the production well 21 are sealed to seal the large amount of CO2 adsorbed in the unminable coal seam.

[0023] In this invention, the unminable coal seam is one with a depth greater than 800m. The adsorption capacity of this coal seam for CO2 is more than twice that for CH4. CO2 injected into the coal seam is preferentially adsorbed, causing CH4 to be replaced from its adsorbed state and converted to a free state. Simultaneously, the injection of CO2 increases the pore pressure of the coal seam, significantly improving the coalbed methane recovery rate. The dust collector removes particles >10μm from the flue gas. The solid-liquid mixer and ultrasonic stirrer form a nanofluid with water and particles such as SiO2 and Al2O3, while simultaneously acidifying the sulfides in the flue gas, enhancing the adsorption capacity of the coal seam surface, improving the connectivity of the seepage channels, strengthening the erosion effect on the coal seam, and significantly increasing the CO2 sequestration capacity. Direct gas sequestration avoids the high cost of CO2 separation. The N2 in the flue gas can reduce the expansion of the coal matrix caused by CO2 adsorption, thereby reopening the cracks closed by CO2 adsorption and promoting the recovery of permeability, further increasing the amount of CO2 sequestrated. The CO2 recycling sequestration method allows the unsequestered CO2 to be reinjected into the flue gas for re-sequestration, and allows the nanofluid to fully combine with the coal seam wall, further enhancing the erosion effect on the coal seam and increasing the amount of CO2 sequestrated, achieving the effect of recycling sequestration. When the pressure of the flue gas storage tank reaches the preset safety pressure limit, the safety valve automatically opens, allowing the storage tank to depressurize rapidly, which can ensure the safety of the system and thus avoid personal injury and equipment damage.

Claims

1. A flue gas circulation and storage system in deep, unminable coal seams, characterized in that: The system includes a flue gas storage tank, a dust collector connected to the outlet pipe of the flue gas storage tank, an outlet pipe of the dust collector connected to the inlet pipe of a solid-liquid mixer, and a water flow pipe supplied by a water tank. A one-way valve is installed on the outlet pipe of the solid-liquid mixer, which is connected to the inlet of an ultrasonic stirrer. The outlet of the ultrasonic stirrer is connected to the inlet of a plunger pump, and the outlet of the plunger pump is equipped with an injection pipe located within the injection well, comprising a vertical section and a horizontal section, with the bottom of the vertical section located in an unminable coal seam. The system also includes a extraction circulation device and a current cathodic protection device. The current cathodic protection device includes an external power supply installed on the wall of the injection well. The negative terminal of the external power supply is connected to the injection well, and the positive terminal is connected to an anode material. The power supply is connected to a DC transformer box via a wire, and a transformer rectifier is connected to the DC transformer box via a wire. A switch is installed on the wire, and the anode material at the end is inserted from the top rock strata down to the unminable coal seam.

2. The flue gas circulation and storage system in deep unminable coal seams as described in claim 1, characterized in that: The extraction and circulation device includes an extraction pipe installed in the gas well, an extraction pump installed at the outlet of the extraction pipe, and a CO2-coalbed methane separator installed at the outlet of the extraction pump. One end of the CO2-coalbed methane separator is connected to a coalbed methane storage tank, and the other end is connected to a flue gas storage tank through a CO2 circulation pipeline and a high-pressure pump.

3. The flue gas circulation and storage system in deep unminable coal seams as described in claim 2, characterized in that: The flue gas outlet pipe is equipped with a first safety valve and a first pressure gauge in sequence; the water tank outlet pipe is equipped with a water injection valve; the ultrasonic stirrer outlet pipe is equipped with a first valve and a nanofluid concentration sensor in sequence; the plunger pump outlet pipe is equipped with a first CO2 concentration sensor and a second valve; the extraction pipe is equipped with a second CO2 concentration sensor and an extraction valve; the CO2 circulation pipe is equipped with a third CO2 concentration sensor and a CO2 valve; and the high-pressure pump outlet pipe is equipped with a second pressure gauge and a second safety valve in sequence.

4. The sealing method using the system according to any one of claims 1-3, characterized in that, Includes the following steps: S1. Construction Drilling: First, determine the extent and depth of the unminable coal seam. Then, drill gas injection wells and gas production wells from the surface through the rock strata and aquifers into the unminable coal seam. After the gas injection wells are completed, drill a horizontal well from the bottom of the gas injection well along the unminable coal seam to position d2. Drilling work is stopped after completion. S2. Set up a flue gas injection and coalbed methane extraction and separation system: Connect one end of the injection pipe to the outlet of the plunger pump, and extend the other end into the injection well until it reaches the bottom of the horizontal well. Extend one end of the extraction pipe from the ground into the production well, and place the other end outside the production well. Connect the plunger pump to the outlet of the ultrasonic agitator. Connect the extraction port of the extraction pump to the outlet of the extraction pipe. Connect the exhaust port of the extraction pump to the inlet of the CO2-coalbed methane separator. Connect the CO2 outlet of the CO2-coalbed methane separator to the flue gas storage tank through the CO2 circulation pipeline and the high-pressure pump. Connect the coalbed methane outlet to the coalbed methane storage tank through the pipeline. S3. Set up a nanofluid preparation system: Connect the flue gas storage tank to the dust collector through a pipeline, and connect the dust collector to the solid-liquid mixer through a pipeline; connect the outlet of the solid-liquid mixer to the inlet of the ultrasonic stirrer; connect the outlet of the ultrasonic stirrer to the plunger pump; connect the solid-liquid mixer to an external water tank; install a check valve on the pipeline connecting the outlet of the solid-liquid mixer to the ultrasonic stirrer. S4. Set up a forced current cathodic protection device: Connect the gas injection well to the negative terminal of the external power supply, connect the positive terminal of the power supply to the anode material, connect the power supply to the DC transformer box through a wire, connect the transformer rectifier to the DC transformer box through a wire, install a switch on the wire, and make the anode material into a rod or a long and thin block and insert it from the top rock layer until the unminable coal seam. S5. Flue gas injection and circulation CO2 storage: Turn on the transformer rectifier to convert the nearby AC power into DC power. Turn on the power supply. The DC power forces the current to flow from the anode material through the coal seam to the steam injection well pipeline, and finally flows back to the negative terminal of the power supply through the conductor, thereby protecting the pipeline. Open the first safety valve set on the outlet pipeline of the flue gas storage tank. The flue gas enters the dust collector to remove larger particles in the flue gas. Open the water injection valve on the water tank. Water mixes with flue gas particles with a diameter of <10μm that enter the solid-liquid mixer to form a nanofluid. At the same time, the sulfides in the flue gas become acidic when they come into contact with water. Nanofluid is introduced into an ultrasonic stirrer to thoroughly mix water and nanomaterials; the plunger pump is turned on, and wet flue gas and nanofluid are continuously injected into the horizontal well through the plunger pump. The wet flue gas and nanofluid enter the coal seam for the first sealing; then the extraction pump is turned on, and gas injection and gas production are carried out simultaneously. At this time, the CO2 that is not completely adsorbed and the displaced coalbed methane enter the CO2-coalbed methane separator after passing through the extraction pipe and extraction pump. After the mixed gas is separated in the CO2-coalbed methane separator, the coalbed methane enters the coalbed methane storage tank for storage, and the remaining CO2 gas is repressurized and injected again with the flue gas for circulation sealing.

5. The sealing method as described in claim 4, characterized in that: In step S1, an injection pipe is installed in the injection well, and the distance between the end position d1 of the horizontal section of the injection pipe and the cutoff position d2 of the horizontal well is less than 30m. The distance between the gas production well and the injection well is greater than 200m, and the horizontal distance between the cutoff position d2 of the horizontal well and the cutoff position d3 of the gas production well is less than 20m.

6. The sealing method as described in claim 4, characterized in that: In step S2, the gas injection pipe and the extraction pipe are coated with a corrosion-resistant material.

7. The sealing method as described in claim 4, characterized in that: In step S5, the mass concentration of solid particles in the flue gas in the nanofluid is between 0.01% and 2%. The change in the value displayed by the CO2 concentration sensor is observed. When the CO2 concentration at the gas production well is higher than 80% of the CO2 concentration at the injection well, injection and extraction are stopped.

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