A method for storing LNG in abandoned mine main roadways using high pressure and low temperature

Abstract

This invention discloses a method for storing LNG in abandoned mine main roadways using high pressure and low temperature. The main roadway of the abandoned mine serves as the LNG storage tank, which is connected to the surface via a bottom shaft and underground vertical shaft. An injection pipe extends sequentially through the main shaft and bottom shaft to the storage tank; a drain pipe extends sequentially through an auxiliary shaft and bottom shaft to the storage tank. Drain pipe branches off from the surface portion of the drain pipe, and injection pipe branches off from the surface portion of the injection pipe. Both the drain pipe and injection pipe branches are connected to the inlet and outlet of a natural gas liquefaction unit via electric valves. When the temperature and pressure inside the storage tank exceed the set storage temperature and pressure ranges, the natural gas in the storage tank is sequentially transported through the drain pipe and drain pipe branches to the surface natural gas liquefaction unit for processing before flowing back into the storage tank through the injection pipe, ensuring that the temperature and pressure inside the storage tank are maintained within the specified range. This improves the safety and stability of the storage tank and reduces operating costs.

Description

Technical Field

[0001] This invention relates to the field of abandoned mine renovation technology, and in particular to a method for storing LNG in the main roadway of an abandoned mine using high pressure and low temperature. Background Technology

[0002] As the world's largest developing country, my country's domestic natural gas consumption has maintained a high growth rate for a long time. In recent years, although my country's natural gas production has continued to grow, the gap between production and consumption has widened further, and the dependence on foreign natural gas remains high. It is estimated that by 2030, my country's dependence on foreign natural gas will exceed 50%.

[0003] As my country deepens its coal mine capacity reduction efforts, the number of abandoned mines in my country will reach 12,000 by 2020; and this number will rise to 15,000 by 2030. Developing the underground space of abandoned mines into natural gas storage facilities will be an important measure to address both underground natural gas storage and the resource utilization of abandoned mines.

[0004] Currently, LNG is mainly stored internationally in above-ground storage tanks. Compared to tank storage, underground LNG storage offers significant advantages. Underground space is larger, allowing for the storage of large quantities of LNG. This method not only saves surface space but also protects against disruptions from war and surface production activities. Furthermore, LNG storage facilities in abandoned mine shafts fully utilize abandoned underground space and the good support conditions of the main tunnels, making construction more economical. Eastern regions such as Jizhong Energy, Kailuan Coal Mine, and Xuzhou Mining Area are closer to natural gas consumption areas and ports; therefore, constructing underground storage facilities in these areas facilitates the transportation and consumption of LNG by sea.

[0005] However, existing underground LNG storage technologies still face many challenges. Due to the low storage pressure, extremely low temperatures must be maintained in the storage facilities to ensure the stored natural gas remains liquid, exacerbating heat loss and increasing the cost of maintaining the cryogenic environment. Furthermore, ultra-low temperatures place stringent requirements on construction materials and equipment. In addition, LNG storage facilities in abandoned mine shafts at standard atmospheric pressure are subject to significant formation pressure, causing the surrounding rock to undergo significant convergent deformation, severely impacting the safety and stability of the storage facility. Therefore, existing LNG storage technology in abandoned mines cannot yet meet the requirements for low-cost and long-term stable LNG storage. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a method for storing LNG in abandoned mine main roadways using high pressure and low temperature. Specifically, while ensuring that the natural gas remains in a liquid state, the storage temperature and pressure of the gas storage facility in the abandoned mine main roadway are adjusted to improve the safety and stability of the gas storage facility and reduce its operating costs.

[0007] The technical solution adopted by the present invention to solve the aforementioned technical problem is as follows:

[0008] A method for storing LNG in abandoned mine main roadways using high pressure and low temperature, the process of which is as follows:

[0009] The main roadway of the waste gas mine is used as an LNG storage tank. The LNG storage tank is connected to the surface through the bottom yard and underground vertical shafts. The underground vertical shafts are divided into main shafts and auxiliary shafts. The injection pipe extends into the storage tank through the main shaft and the bottom yard in sequence. The drainage pipe extends into the storage tank through the auxiliary shaft and the bottom yard in sequence.

[0010] A branch pipe is formed at the ground level of the drainage pipe, and a branch pipe is formed at the ground level of the injection pipe. The drainage pipe branch pipe and the injection pipe branch pipe are connected to the inlet and outlet of the natural gas liquefaction unit through electric valves. The injection pipe is used to inject liquefied LNG, and the drainage pipe is used to extract natural gas.

[0011] Drill holes around the mine to drain groundwater from the surrounding rock, while simultaneously draining the water inside the main roadway. The main roadway is then reinforced and sealed. LNG is injected into the gas storage tank. The LNG evaporates rapidly and absorbs heat from the surrounding environment, cooling the rocks around the main roadway to a set temperature. Groundwater extraction is stopped. Once the groundwater seeps back into the surrounding rock layer, the cooling effect of LNG causes a dense ice ring to form around the surrounding rock layer. LNG injection is then stopped, completing the construction of the gas storage tank.

[0012] Actual LNG storage process: Storage temperature and pressure ranges are set. LNG is transported from the LNG source to the storage tank through the injection pipe, absorbing heat from the surrounding environment. As the LNG temperature rises, some of it evaporates rapidly, and the pressure of the natural gas increases with its temperature, leading to a continuous increase in the internal pressure of the storage tank. When the temperature and pressure inside the storage tank exceed the set storage temperature and pressure ranges, the electric valve on the drain pipe branch is opened. The natural gas in the storage tank is then transported sequentially through the drain pipe and its branch to the natural gas liquefaction unit on the ground. The liquefied natural gas flows along the injection pipe branch back into the injection pipe and finally flows back into the storage tank, thus ensuring that the internal temperature and pressure of the storage tank are maintained within the specified ranges. Simultaneously, the liquid level changes inside the storage tank are monitored in real time. When the liquid level reaches the critical point, the injection of LNG into the storage tank is stopped.

[0013] The storage pressure range is 3.5 to 4.0 MPa, and the storage temperature range is -90 to -100℃. The internal pressure of the gas storage tank is under high pressure, which restricts the convergence and deformation of the surrounding rock and realizes the safe and stable underground storage of LNG.

[0014] Both the injection pipe and the drainage pipe are arranged using the shortest path within the well bottom yard.

[0015] This invention also protects a system for storing LNG using abandoned mine shafts, including a drainage subsystem, a storage subsystem, an injection and production subsystem, and a measurement and control subsystem;

[0016] The drainage subsystem is used to extract groundwater around the mine. A number of feather-shaped pumping boreholes are evenly arranged around the mine, and the bottom of the feather-shaped pumping boreholes extends below the bottom of the mine.

[0017] The storage subsystem is used to store LNG and includes a steel lining, an insulation layer, a lining and a surrounding rock layer arranged from the inside out. The area enclosed by the inner steel lining is the gas storage tank.

[0018] The injection and extraction subsystem is used to inject or extract LNG into or from the gas storage facility. It includes an injection pipe, an extraction pipe, an injection pump, an extraction pump, and a valve assembly. The extraction pipe is connected at its end to a cold-powered power generation unit and extends to the upper part of the gas storage facility to discharge the sealed natural gas from the facility. The injection pipe is connected at its starting end to an LNG source and extends to the upper part of the gas storage facility. Inside the gas storage facility, it is configured as a multi-hole injection pipe to inject LNG into the facility. The injection pump and extraction pump are installed on the ground sections of the injection pipe and extraction pipe, respectively, to provide power for the LNG transportation within the pipeline. All pipelines used employ a multi-layer insulation structure.

[0019] The measurement and control subsystem includes a radar level gauge, a temperature sensor, a pressure sensor, a control center, and a temperature and pressure regulation unit. The radar level gauge is installed on the top of the gas storage tank to monitor the changes in the liquid level of the packaged LNG inside the gas storage tank. The temperature sensor and the pressure sensor are installed inside the gas storage tank to monitor the changes in temperature and pressure inside the gas storage tank.

[0020] The temperature and pressure control unit includes a drainage pipe branch, an injection pipe branch, a natural gas liquefaction device, and an electric valve. The drainage pipe branch branches off from the ground portion of the drainage pipe and is connected to the natural gas liquefaction device to transfer natural gas vapor into the device. The electric valve is installed on both the drainage pipe branch and the injection pipe branch to control the opening and closing of the pipelines. The natural gas liquefaction device is connected to both the drainage pipe branch and the injection pipe branch to liquefy the natural gas transported by the drainage pipe branch and then transport it to the gas storage facility via the injection pipe branch.

[0021] The control center is used for data monitoring, display, and control. It is electrically connected to the electric valves, radar level gauges, temperature sensors, pressure sensors, injection pumps, extraction pumps, and valve groups. It sets the storage temperature range and storage pressure range. Since LNG absorbs heat from the surrounding rock and liquefies, when the control center detects that the temperature and pressure inside the gas storage facility exceed the set storage temperature and pressure ranges, the temperature and pressure control unit extracts the vaporized natural gas to the ground and then liquefies it through a natural gas liquefaction device for reinjection. This is used to reduce the temperature and pressure inside the gas storage facility and maintain the temperature and pressure inside the gas storage facility within the set storage temperature and pressure ranges. The storage pressure range is 3.5 to 4.0 MPa, and the storage temperature range is -90 to -100°C.

[0022] The valve assembly includes a valve assembly base and a safety valve and a flow control valve assembly disposed on the valve assembly base. The flow control valve assembly includes a flow regulating valve and a flow shut-off valve connected in series.

[0023] The steel lining is used to encapsulate LNG; the insulation layer is bonded to both the steel lining and the lining to provide thermal insulation for the LNG encapsulated inside the steel lining; the lining is distributed between the surrounding rock layer and the insulation layer to improve the support strength of the storage subsystem; the surrounding rock layer is a soil and rock layer located outside the lining, which, together with the lining, resists ground stress.

[0024] The insulation layer is a porous medium insulation material with high rigidity and good compressive strength, including materials such as extruded polystyrene board. The thickness of the insulation layer is determined according to the temperature difference between the gas storage tank and the surrounding rock. Specifically, the thickness of the insulation layer 3 is calculated based on the allowable daily evaporation of LNG sealed in the steel lining 2. The lining material is selected from high-strength freeze-thaw resistant concrete. The lining thickness is determined based on the surrounding rock stress, surrounding rock strength mass, temperature gradient, local seismic intensity, and the strength of the lining material to alleviate cracking of the surrounding rock layer and ensure that the deformation of the storage subsystem is kept within the allowable range. The steel lining material is selected from 304 grade or higher stainless steel plates.

[0025] The system is also equipped with an LNG cold energy power generation unit, which is connected to the natural gas distribution station through a natural gas pipeline. The cold energy power generation unit is used to convert the cold energy released by LNG transported through the pipeline into electrical energy, which is used to supply consumption and seasonal peak shaving.

[0026] The transportation pipelines used in this invention all adopt a multi-layer insulation structure to reduce the cold energy loss of LNG during transportation.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) In the method of the present invention, the gas storage tank is under high pressure, which restricts the convergence deformation of the surrounding rock, improves the safety and stability of the gas storage tank, and also alleviates the cracking phenomenon of the surrounding rock. The internal temperature of the gas storage tank of the present invention is much higher than -164℃ of the ground LNG concrete storage tank, which reduces the temperature difference on both sides of the insulation layer and reduces the heat loss inside the gas storage tank.

[0029] The formula for calculating thermal conductivity is:

[0030] Q / t=λA(ΔT / d)

[0031] Q / t — Represents the amount of heat passing through a unit area per unit time.

[0032] λ — represents thermal conductivity

[0033] A – Indicates the heat transfer coefficient

[0034] ΔT — represents the temperature difference

[0035] d — represents the heat transfer distance

[0036] As shown in the formula for calculating heat conduction, under the condition that the thermal conductivity, heat transfer coefficient, and heat transfer distance are constant, the temperature difference is directly proportional to the amount of heat passing through a unit area per unit time. Compared with the prior art, the gas storage tank used in this invention has a higher internal temperature and a smaller temperature difference between the internal temperature and the ambient temperature, thus requiring less energy for heat conduction and maintaining the internal temperature of the gas storage tank.

[0037] (2) The LNG storage facility of the present invention has a higher storage temperature than the prior art, and has lower requirements for the materials of the insulation layer and lining. At the same time, the low temperature resistance of the equipment used is reduced, thereby reducing the construction cost.

[0038] (3) The gas storage facility of the present invention directly uses the main roadway of the abandoned mine as the gas storage facility, eliminating the need for excavation work. This not only greatly reduces the construction cost, but also reuses the underground abandoned space resources, resulting in more significant economic benefits.

[0039] (4) The present invention is equipped with a temperature and pressure control unit. Since the liquid energy injected into the gas storage tank will absorb heat and vaporize, the pressure of the gas storage tank will increase. The temperature and pressure control unit will discharge the steam and liquefy it back into the tank, thereby keeping the internal pressure of the gas storage tank within a stable range, which is beneficial to the better detection and storage of the storage tank subsystem.

[0040] (5) The present invention adopts a multi-hole injection method at the end of the injection pipe. When low-temperature liquid energy is injected into the gas storage tank, it is easy to roll or stratify due to uneven injection, which greatly affects the safety of the gas storage tank. The multi-hole injection method effectively reduces the occurrence of this phenomenon.

[0041] (6) The method of this invention is to store LNG in the main roadway of a waste gas mine. This method also requires that the natural gas be stored in a liquid state; it ensures that the density of the stored natural gas is not too low, so as not to affect the storage capacity. This application selects a temperature of -90 to -100 degrees Celsius and a pressure of 3.5 to 4 MPa. When the storage pressure is too high, the temperature of the stored LNG will also increase. When the temperature reaches a certain value, even if the pressure is increased further, the natural gas cannot be liquefied. When the storage pressure is too low, on the one hand, it will reduce the beneficial effect of high pressure in resisting formation stress, and on the other hand, the lower the pressure, the lower the density, thus affecting the size of the storage capacity. Therefore, this application selects this range as the storage conditions. Attached Figure Description

[0042] Figure 1 A schematic diagram of the system for storing LNG using abandoned mine shafts in this invention.

[0043] Figure 2 A schematic diagram of the longitudinal section of the storage subsystem in the LNG storage system utilizing abandoned mine shafts in this invention.

[0044] Figure 3 Comparison diagrams before and after the construction of the LNG storage system utilizing abandoned mine shafts in this invention.

[0045] Figure 4 Cross-sectional view of multi-layer insulated pipe in the LNG storage system utilizing abandoned mine shafts in this invention.

[0046] Figure 5 A schematic diagram of the valve group structure in the LNG storage system utilizing abandoned mine shafts in this invention.

[0047] Figure 6 3D surface color mapping of LNG under different temperature, pressure and density conditions.

[0048] Figure 7 The flowchart of the LNG storage system built in this invention, which utilizes abandoned mine shafts for LNG storage.

[0049] Figure 8 The injection and production process flow diagram of the LNG storage system utilizing abandoned mine shafts in this invention.

[0050] In the diagram: 1-Storage subsystem; 2-Steel lining; 3-Insulation layer; 4-Liner; 5-Surrounding rock layer; 6-Ice ring; 7-Feather-shaped pumping borehole; 8-Injection pipe; 9-Valve group; 91-Flow regulating valve; 92-Flow shut-off valve; 93-Safety valve; Drainage and production pipe branch; 11-Temperature sensor; 12-Pressure sensor; 13-Natural gas liquefaction unit; 14-Radar level gauge; 15-Drainage and production pipe; 16-Cold energy power generation unit; 17-Natural gas distribution station; 18-Injection pipe branch; 19-Natural gas transmission pipeline; 20-Bottom-of-well vehicle yard; 21-Electric valve; 22-Main shaft; 23-Auxiliary shaft; 24-Steel structure layer; 25-Pipeline insulation layer; 26-Outer protective layer; 27-LNG gas source; 28-Injection pump; 29-Pumping pump; 31-Sealing wall. Detailed Implementation

[0051] To further illustrate the principles and structure of the present invention, preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0052] This invention relates to a system for storing LNG in abandoned mine shafts (see...). Figure 1-5 It includes a drainage subsystem, a storage subsystem, an injection and extraction subsystem, and a measurement and control subsystem.

[0053] like Figure 3 As shown, the storage subsystem 1 includes a steel lining 2, an insulation layer 3, a lining 4, and a surrounding rock layer 5 arranged sequentially from the inside out. The lining 4 is anchored to the surrounding rock layer 5 using sprayed mesh as a support structure. The insulation layer 3 and the steel lining 2 are then sequentially sealed and connected to the lining. At the same time, the space enclosed by the steel lining 2 forms a gas storage tank.

[0054] The steel liner 2 is made of stainless steel plates of grade 304 or above, and the LNG is encapsulated in the gas storage tank through the exhaust pipe 15.

[0055] The insulation layer 3 is tightly bonded to the steel lining 2 and the lining 4 to prevent water from entering the gaps between the layers and reducing the insulation effect. The insulation layer 3 is used to insulate the LNG sealed inside the steel lining 2. Specifically, the insulation layer 3 is made of insulation material of a certain thickness, including but not limited to insulation materials with high rigidity and good compressive strength such as extruded polystyrene board. The thickness of the insulation layer 3 should be calculated and determined according to the allowable daily evaporation of the LNG sealed inside the steel lining 2.

[0056] Lining 4 is positioned between the insulation layer 3 and the surrounding rock layer 5, and together with the surrounding rock layer 5, it bears the ground stress. High-strength, freeze-thaw resistant concrete is selected as the material for lining 4. The thickness of lining 4 should be determined based on the surrounding rock stress, temperature gradient, local seismic intensity, and the allowable stress of the lining 4 material, in order to mitigate cracking of the surrounding rock layer 5 while ensuring that the deformation of the storage subsystem 1 remains within acceptable limits.

[0057] The surrounding rock layer 5 is a soil and rock layer located outside the lining 4, which together with the lining 4 bears the ground stress.

[0058] The drainage subsystem includes a number of feather-shaped pumping boreholes 7. The feather-shaped pumping boreholes 7 are evenly arranged around the mine by drilling, and the bottom of the feather-shaped pumping boreholes should extend below the bottom of the mine. The feather-shaped pumping boreholes 7 extract groundwater around the mine, so that the rock fissures around the mine are all unsaturated, thereby reducing freeze-thaw damage to the rock mass caused by water freezing.

[0059] Specifically, after a certain thickness of rock has cooled sufficiently, the feather-shaped pumping borehole 7 stops pumping groundwater, and the groundwater seeps back into the surrounding rock layer 5. Under the low-temperature cooling effect of LNG, a dense ice ring 6 will form around the surrounding rock layer 5, making the surrounding rock layer 5 a closed permafrost layer, which enhances the overall strength of the fractured rock mass and plays a certain sealing role for the fractures, thereby improving the sealing performance and safety stability of the gas storage facility.

[0060] In particular, during the long-term use of the mine, the surrounding rock layer 5 will develop fractured areas under long-term stratum stress. Due to the long-term influence of stratum stress and excavation, the strength and integrity of the surrounding rock gradually decrease. Therefore, before the dam is built, it is necessary to grout the fractured areas of the surrounding rock layer to maintain the strength and sealing of the surrounding rock.

[0061] The injection and production subsystem includes a transport pipeline, an injection pump 28, a extraction pump 29, and a valve group 9. The transport pipeline includes an injection pipe 8 and an extraction pipe 15. The extraction pipe 15 is connected at its end to a cold energy power generation device 16, and its starting end extends through the top of the steel liner into the gas storage tank to discharge the natural gas encapsulated in the steel liner 2 from the gas storage tank and transport it to the natural gas distribution station 17 via the natural gas transport pipeline 19. The starting end of the injection pipe 8 is connected to the LNG source 27, and its end extends to the upper part of the gas storage tank. In particular, the injection pipe 8 is configured as a multi-hole injection pipe inside the gas storage tank, and the multi-hole injection pipe is evenly arranged along the length of the mine shaft to achieve the purpose of uniformly injecting LNG into the interior of the steel liner 2. This not only achieves uniform cooling inside the gas storage tank but also effectively avoids LNG stratification and tumbling. The injection pump 28 and the extraction pump 29 are respectively installed on the ground parts of the injection pipe 8 and the extraction pipe 15 to provide power for the transportation of LNG in the pipeline.

[0062] A valve assembly 9 is installed on both the injection pipe 8 and the discharge pipe 15. (See valve assembly 9 for details.) Figure 5The system includes a valve assembly base and a safety valve 93 and a flow control valve assembly mounted on the base. The flow control valve assembly includes a flow regulating valve 91 and a flow shut-off valve 92 connected in series. In an emergency, the safety valve 93 immediately shuts off the pipeline to protect equipment and personnel. The flow control valve assembly controls fluid flow and prevents backflow or excessive flow from damaging the pipeline system. Specifically, if LNG is injected into a gas storage facility at a rapid pace, causing excessive pressure and rapid cooling, severely impacting the normal operation of the storage facility, the flow regulating valve can maintain the injection / discharge rate within a reasonable range, thus more effectively maintaining the storage facility pressure.

[0063] like Figure 4 As shown, the pipeline adopts a multi-layer insulation structure, including a steel structure layer 24, a pipeline insulation layer 25, and an outer protective layer 26. The steel structure layer 24 is made of steel; the pipeline insulation layer 25 is made of polyester polyurethane foam, which is used to seal and fill the outside of the pipeline; the outer protective layer 26 is made of composite materials such as epoxy resin and fiberglass, which plays a role in mitigating external forces and enhancing pipeline protection.

[0064] Specifically, this invention utilizes a well-maintained main roadway as an LNG storage tank, connected to the surface via a bottom shaft yard 20 and an underground vertical shaft. The underground vertical shaft consists of a main shaft 22 and an auxiliary shaft 23. The injection pipe 8 extends sequentially through the main shaft 22 and the bottom shaft yard 20 into the storage tank; the drainage pipe 15 extends sequentially through the auxiliary shaft 23 and the bottom shaft yard extension 20 into the storage tank. Furthermore, both the injection pipe 8 and the drainage pipe 15 are arranged using the shortest path within the bottom shaft yard 20.

[0065] Regarding temperature and pressure monitoring and control, to ensure that natural gas remains in a liquid state, this invention selects a pressure of 3.5–4.0 MPa and a temperature of -90–-100°C as the storage conditions for LNG in abandoned mine gas storage facilities. Under these conditions, the density of LNG is maintained at 323–341 kg / m³. 3 .

[0066] The formula for calculating storage capacity is as follows:

[0067] C = ρ 气 V 气 +ρ 液 V 液

[0068] C – Gas storage capacity

[0069] V 气 Natural gas volume

[0070] ρ_gas — density of natural gas

[0071] ρ_liquid — LNG density

[0072] V 液 LNG liquid volume

[0073] In this example, the liquid level inside the gas storage tank can be measured by the radar level gauge 14, and the gas-liquid volume can be directly calculated. At the same time, the temperature and pressure inside the gas storage tank can be measured by the temperature sensor 11 and the pressure sensor 12. Based on this data, the density of natural gas and LNG can be found in the NIST database.

[0074] The measurement and control subsystem includes a radar level gauge 14, a temperature sensor 11, a pressure sensor 12, and a temperature and pressure control unit. The temperature and pressure control unit includes a drainage pipe branch 10, an injection pipe branch 18, a natural gas liquefaction unit 13, and an electric valve 21. The drainage pipe branch 10 branches off from the ground portion of the drainage pipe 15 and connects to the natural gas liquefaction unit 13 to transmit natural gas into the natural gas liquefaction unit 13. One end of the injection pipe branch 18 is connected to the natural gas liquefaction unit 13, and the other end merges into the injection pipe to transport the liquefied LNG to the injection pipe 8 and then reinject it into the gas storage tank through the injection pipe 8. Electric valves 21 are installed on both the drainage pipe branch 10 and the injection pipe branch 18 to control the opening and closing of the pipelines. The detection ends of temperature sensor 11 and pressure sensor 12 are installed inside the gas storage tank to monitor the temperature and pressure changes inside the steel liner 2, so that the LNG is stored under the temperature and pressure designed in this invention. Radar level gauge 14 is installed on the top of the gas storage tank to monitor the liquid level changes of the LNG encapsulated in the steel liner 2. As the LNG liquid level increases, when the liquid level critical point of the gas storage tank is reached, the injection pipe 8 stops injecting LNG into the steel liner 2.

[0075] Specifically, this system is equipped with an LNG cold energy power generation unit 16, which is a device that converts cold energy into electrical energy. This unit maximizes the utilization of the cold energy released from the LNG transported via the gas pipeline, thereby reducing energy waste. The process of extracting LNG from the gas storage facility is used for supplying consumer goods and seasonal peak shaving.

[0076] After the signal data lines of the entire system are concentrated nearby, several optical cables are led out to the ground through the bottom of the well yard 20 and the underground shaft, and connected to the ground data acquisition unit for data monitoring, display and control.

[0077] like Figure 6 As shown, under the condition that the natural gas is in a liquid state, the present invention selects a storage pressure range of 3.5 to 4.0 MPa and a storage temperature range of -90 to -100°C as the state for storing LNG in abandoned mine gas storage facilities. This state has a higher pressure than LNG under one standard atmosphere, the temperature inside the storage facility is closer to the ambient temperature, and its density changes little and meets the storage requirements.

[0078] This invention employs a method for storing LNG under high pressure and low temperature in abandoned mine main roadways, including a storage facility construction process and an injection and production process:

[0079] like Figure 7 As shown, the construction process is as follows: After site clearing, feather-shaped pumping boreholes 7 are evenly arranged around the mine to drain groundwater from the surrounding rock and simultaneously drain water from the main roadway. Grouting is used to reinforce the fractured rock zone. Then, lining 4, insulation layer 3, and steel lining 2 are sequentially installed on the existing support layer of the mine. Sealing walls 31 are then designed on both sides of the main roadway along its length. After the monitoring and control devices and transport pipelines are installed, the airtightness of the gas storage is measured using compressed air. Once the airtightness is ensured, LNG is injected into the gas storage. The newly injected LNG cools a certain thickness of rock. After the rock reaches the required cooling thickness, groundwater extraction is stopped. After the groundwater seeps back into the surrounding rock layer 5, the cooling effect of the LNG causes a dense ice ring 6 (i.e., a frozen ring) to form around the surrounding rock layer 5, making it a closed frozen soil layer, thus achieving a sealing effect.

[0080] like Figure 8 As shown, the injection and extraction process is as follows: LNG is transported to the gas storage facility through injection pipe 8 and absorbs heat from the surrounding environment. Some of the LNG evaporates rapidly, and the pressure of the natural gas increases with its temperature, causing the internal pressure of the gas storage facility to continuously increase. When the temperature and pressure indicated by temperature sensor 11 and pressure sensor 12 reach the range designed in this invention, the electric valve 21 on the drain pipe branch 10 is opened. A large amount of natural gas from the upper part of the gas storage facility is transported sequentially through drain pipe 15 and drain pipe branch 10 to the natural gas liquefaction unit 13 on the ground. The liquefied natural gas flows along injection pipe branch 18 into the injection pipe 8 and finally flows back into the gas storage facility, thereby ensuring that the temperature and pressure inside the gas storage facility remain within the specified range. Simultaneously, when it is necessary to transport the natural gas from the gas storage facility to the natural gas distribution station 17, the valve on drain pipe 15 is opened. The natural gas flows along drain pipe 15 through the cold energy power generation unit 16 and then along the natural gas transmission pipeline 19 to the natural gas distribution station 17.

[0081] The above description is merely an embodiment of the present invention, used to illustrate the purpose, technical solution, and method of the present invention, and is not intended to limit the patent scope of the present invention. Those skilled in the art can make improvements to the embodiments of the present invention based on the content disclosed in the application documents without departing from the technical concept and patent scope of the present invention.

[0082] Any aspects not covered in this invention are applicable to existing technologies.

Claims

1. A method for storing LNG in abandoned mine main roadways using high pressure and low temperature, characterized in that, The gas storage facility directly utilizes the main roadway of an abandoned mine as its storage tank, eliminating the need for excavation. The storage pressure range is 3.5–4.0 MPa, and the storage temperature range is -90–-100℃. The internal pressure of the storage tank is high, which restricts the convergence and deformation of the surrounding rock, achieving safe and stable underground storage of LNG. The process of this method is as follows: The main roadway of the waste gas mine is used as an LNG storage tank. The LNG storage tank is connected to the surface through the bottom yard and underground vertical shafts. The underground vertical shafts are divided into main shafts and auxiliary shafts. The injection pipe extends into the storage tank through the main shaft and the bottom yard in sequence. The drainage pipe extends into the storage tank through the auxiliary shaft and the bottom yard in sequence. A branch pipe is formed at the ground level of the drainage pipe, and a branch pipe is formed at the ground level of the injection pipe. The drainage pipe branch pipe and the injection pipe branch pipe are connected to the inlet and outlet of the natural gas liquefaction unit through electric valves. The injection pipe is used to inject liquefied LNG, and the drainage pipe is used to extract natural gas. Drilling boreholes around the mine to drain groundwater from the surrounding rock and simultaneously draining the water inside the main roadway, then reinforcing and sealing the main roadway. Before constructing the storage tank, grouting is required to reinforce the fractured areas of the surrounding rock. Grouting is then used to reinforce the fractured areas of the surrounding rock. Subsequently, lining, insulation, and steel lining are sequentially installed on the basis of the existing support layer of the mine. Then, sealing walls are designed on both sides of the length of the main roadway. After the monitoring and control devices and transportation pipelines are installed, the air tightness of the gas storage tank is measured using compressed air. Once the air tightness of the gas storage tank is ensured, LNG is injected into the gas storage tank. The LNG evaporates rapidly and absorbs heat from the surrounding environment, cooling the rocks around the main roadway to the set temperature. Groundwater extraction is then stopped. After the groundwater seeps back into the surrounding rock layer, a dense ice ring forms around the surrounding rock layer due to the cooling effect of LNG. LNG injection is then stopped, and the construction of the gas storage tank is completed. Actual LNG storage process: Storage temperature and pressure ranges are set. LNG is transported from the LNG source to the storage tank through the injection pipe, absorbing heat from the surrounding environment. As the LNG temperature rises, some of it evaporates rapidly, and the pressure of the natural gas increases with its temperature, leading to a continuous increase in the internal pressure of the storage tank. When the temperature and pressure inside the storage tank exceed the set storage temperature and pressure ranges, the electric valve on the drain pipe branch is opened. The natural gas in the storage tank is then transported sequentially through the drain pipe and its branch to the natural gas liquefaction unit on the ground. The liquefied natural gas flows along the injection pipe branch back into the injection pipe and finally flows back into the storage tank, thus ensuring that the internal temperature and pressure of the storage tank are maintained within the specified ranges. Simultaneously, the liquid level changes inside the storage tank are monitored in real time. When the liquid level reaches the critical point, the injection of LNG into the storage tank is stopped. Both the injection pipe and the drainage pipe are arranged using the shortest path inside the well bottom yard. The system used in the above method for storing LNG in abandoned mine shafts includes a drainage subsystem, a storage subsystem, an injection and production subsystem, and a monitoring and control subsystem. The drainage subsystem is used to extract groundwater around the mine. A number of feather-shaped pumping boreholes are evenly arranged around the mine, and the bottom of the feather-shaped pumping boreholes extends below the bottom of the mine. By extracting groundwater around the mine, the feather-shaped pumping boreholes make the rock fissures around the mine reach an unsaturated state, thereby reducing the freeze-thaw damage to the rock mass caused by water freezing. The storage subsystem is used to store LNG and includes a steel lining, an insulation layer, a lining and a surrounding rock layer arranged from the inside out. The area enclosed by the inner steel lining is the gas storage tank. The injection and production subsystem is used to inject or extract LNG into or from the gas storage facility. It includes an injection pipe, a drainage pipe, an injection pump, a extraction pump, and a valve assembly. The drainage pipe is connected at its end to a cold-powered power generation unit and extends to the upper part of the gas storage facility to discharge the sealed natural gas from the facility. The injection pipe is connected at its starting end to an LNG source and extends to the upper part of the gas storage facility. Inside the gas storage facility, it is configured as a multi-hole injection pipe, evenly arranged along the length of the mine shaft, to inject LNG into the gas storage facility. This not only achieves uniform cooling inside the gas storage facility but also effectively prevents LNG stratification and tumbling. The injection pump and extraction pump are installed on the ground sections of the injection pipe and drainage pipe, respectively, to provide power for LNG transportation within the pipelines. All pipelines used employ multi-layer insulation structures. The multi-layer insulation structure includes a steel structure layer, a pipe insulation layer, and an outer protective layer. The steel structure layer is made of steel. The pipe insulation layer uses polyester foam to seal and fill the outside of the pipe. The outer protective layer uses epoxy resin or fiberglass to mitigate external forces and enhance pipe protection. The measurement and control subsystem includes a radar level gauge, a temperature sensor, a pressure sensor, a control center, and a temperature and pressure regulation unit. The radar level gauge is installed on the top of the gas storage tank to monitor the changes in the liquid level of the packaged LNG inside the gas storage tank. The temperature sensor and the pressure sensor are installed inside the gas storage tank to monitor the changes in temperature and pressure inside the gas storage tank. The temperature and pressure control unit includes a drainage pipe branch, an injection pipe branch, a natural gas liquefaction device, and an electric valve. The drainage pipe branch branches off from the ground portion of the drainage pipe and is connected to the natural gas liquefaction device to transfer natural gas vapor into the device. The electric valve is installed on both the drainage pipe branch and the injection pipe branch to control the opening and closing of the pipelines. The natural gas liquefaction device is connected to both the drainage pipe branch and the injection pipe branch to liquefy the natural gas transported by the drainage pipe branch and then transport it to the gas storage facility via the injection pipe branch. The control center is used for data monitoring, display, and control. It is electrically connected to the electric valves, radar level gauges, temperature sensors, pressure sensors, injection pumps, extraction pumps, and valve groups. It sets the storage temperature range and storage pressure range. Since LNG absorbs heat from the surrounding rock and vaporizes, when the control center detects that the temperature and pressure inside the gas storage facility exceed the set storage temperature and pressure ranges, the temperature and pressure control unit extracts the vaporized natural gas to the ground and then liquefies it through a natural gas liquefaction device for reinjection. This is used to reduce the temperature and pressure inside the gas storage facility and maintain the temperature and pressure inside the gas storage facility within the set storage temperature and pressure ranges.

2. The method according to claim 1, characterized in that, The valve assembly includes a valve assembly base and a safety valve and a flow control valve assembly disposed on the valve assembly base. The flow control valve assembly includes a flow regulating valve and a flow shut-off valve connected in series.

3. The method according to claim 1, characterized in that, The steel liner is used to encapsulate LNG; the insulation layer is bonded to the steel liner and the lining respectively, and is used to keep the LNG encapsulated inside the steel liner warm and insulated. The lining is distributed between the surrounding rock layer and the insulation layer to improve the support strength of the storage subsystem; the surrounding rock layer is a soil and rock layer located outside the lining, which together with the lining resists the ground stress. The insulation layer is made of extruded polystyrene board, and its thickness is determined based on the temperature difference between the gas storage tank and the surrounding rock. Specifically, the insulation layer thickness is calculated based on the allowable daily evaporation of LNG sealed within the steel lining. The lining material is made of high-strength freeze-thaw resistant concrete, and its thickness is determined based on the surrounding rock stress, surrounding rock strength, temperature gradient, local seismic intensity, and the strength of the lining material to mitigate cracking of the surrounding rock layer while ensuring that the deformation of the storage subsystem remains within the allowable range. The steel lining material is made of 304 grade or higher stainless steel plate.

4. The method according to claim 1, characterized in that, The system is also equipped with an LNG cold energy power generation unit, which is connected to the natural gas distribution station through a natural gas pipeline. The cold energy power generation unit is used to convert the cold energy released by LNG transported through the pipeline into electrical energy, which is used to supply consumption and seasonal peak shaving.

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