Liquefied natural gas double-layer tank liquid nitrogen circulation cooling and fire extinguishing system and method

Abstract

The invention relates to the field of liquefied natural gas, in particular to a liquefied natural gas double-layer tank liquid nitrogen circulation cooling and fire extinguishing system and method. The system comprises a double-layer tank body, a liquid nitrogen cooling and fire extinguishing assembly and a liquid nitrogen collecting and circulating assembly, the double-layer tank body comprises an inner tank body and an outer tank body, the inner tank body is located in the outer tank body and provided with a temperature sensor, and a plurality of superconducting magnetic suspension supporting modules are arranged between the outer tank body and the inner tank body; the liquid nitrogen cooling and fire extinguishing assembly comprises a liquid nitrogen storage tank and a liquid nitrogen circulating main pipe, the liquid nitrogen circulating main pipe communicates with the liquid nitrogen storage tank and is spirally wound on the peripheral wall of the inner tank body, and a plurality of exhaust holes are formed in the part, located in the cavity, of the liquid nitrogen circulating main pipe in the extending direction; the liquid nitrogen collecting and circulating assembly comprises a loop pipe, a liquid nitrogen collecting tank and a circulating pipe, the loop pipe communicates with the cavity and the liquid nitrogen collecting tank and is further connected with an air suction pump and a cooling and pressurizing device in series, and the circulating pipe communicates with the liquid nitrogen collecting tank. The inner tank body can be rapidly and comprehensively cooled, and a fire can be rapidly extinguished when the fire occurs.

Description

Technical Field

[0001] The present invention relates to the field of liquefied natural gas, and particularly to a liquid nitrogen circulation cooling and fire extinguishing system and method for a double-layer tank of liquefied natural gas. Background Art

[0002] Liquefied natural gas is a liquid form formed by cooling natural gas to -162 °C, and its volume is only 1 / 600 to 1 / 625 of the gaseous state. Maintaining a low temperature is the key to maintaining the liquid state. Once the temperature rises, the liquefied natural gas will quickly vaporize, causing the volume to expand hundreds of times, leading to a sudden increase in the pressure of the storage tank and even causing overpressure leakage.

[0003] Although the liquefied natural gas transport tank adopts an adiabatic design (such as high-vacuum multi-layer insulation, polyurethane insulation layer), external heat may still invade through radiation, conduction or defects in the insulation layer, resulting in slow evaporation of the liquefied natural gas. For example, the heat generated by the operation of the pump during the unloading process and the heat leakage at the pipe connection will exacerbate the gasification. In addition, when the liquefied natural gas contacts a high-temperature medium (such as seawater or air), a rapid phase change may occur, instantly releasing a large amount of gas and forming a shock wave. Although it does not directly explode, it may damage the equipment or cause secondary accidents.

[0004] In addition, liquefied natural gas is prone to fire during transportation. Traditional fire extinguishing technologies such as dry powder and carbon dioxide have low fire extinguishing efficiency, are easily affected by environmental factors, and water cannot be used to extinguish liquefied natural gas fires, but will instead exacerbate the fire.

[0005] Therefore, in order to solve the safety hazards caused by high temperature during the transportation and loading / unloading of liquefied natural gas transport vehicles, it is urgent to design a liquid nitrogen circulation cooling and fire extinguishing system for the double-layer tank of liquefied natural gas transport vehicles. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the defects of the prior art and provide a liquid nitrogen circulation cooling and fire extinguishing system for a double-layer tank of liquefied natural gas, which can achieve rapid and all-round cooling of the inner tank, and can quickly extinguish the fire in case of a fire, eliminating the safety hazards such as instability of liquefied natural gas, phase change overpressure, and tank impact damage caused by high temperature during the transportation and loading / unloading of liquefied natural gas transport vehicles.

[0007] To solve the above technical problem, the technical solution of the present invention is: A liquid nitrogen circulation cooling and fire extinguishing system for a double-layer tank of liquefied natural gas, comprising:

[0008] A double-layer tank, including an inner tank and an outer tank, the inner tank is located inside the outer tank and is equipped with a temperature sensor, and a plurality of superconducting magnetic levitation support modules are arranged between the outer tank and the inner tank to suspend the inner tank in the outer tank and form a cavity between it and the outer tank;

[0009] The liquid nitrogen cooling and fire extinguishing assembly includes a liquid nitrogen storage tank and a liquid nitrogen circulation main pipe. The liquid nitrogen circulation main pipe is connected to the liquid nitrogen storage tank, spirally wound around the outer peripheral wall of the inner tank body, and is provided with a plurality of exhaust holes along the extending direction at the part located in the cavity.

[0010] The liquid nitrogen collection and circulation assembly includes a circuit pipe, a liquid nitrogen collection tank and a circulation pipe. One end of the circuit pipe is located in the cavity, and the other end is connected to the liquid nitrogen collection tank. An air suction pump and a cooling and pressurizing device are connected in series on the circuit pipe. The nitrogen gas in the cavity sequentially passes through the air suction pump and the cooling and pressurizing device and becomes liquid nitrogen and enters the liquid nitrogen collection tank. The circulation pipe is connected to the liquid nitrogen collection tank and extends into the cavity for spraying liquid nitrogen onto the top of the inner tank body.

[0011] Furthermore, in order to better prevent inflammability and explosiveness of liquefied natural gas, the liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank further includes an argon cooling and fire extinguishing assembly, including an argon storage tank and an argon pipe. The argon pipe is respectively connected to the argon storage tank and the cavity.

[0012] Furthermore, the argon pipe is connected to the bottom of the cavity.

[0013] Furthermore, in order to better spray liquid nitrogen, the circulation pipe is connected with a transverse branch pipe, and a plurality of swirl atomizing nozzles are installed below the transverse branch pipe. The swirl atomizing nozzles are located in the cavity.

[0014] Furthermore, mass flow controllers are respectively configured for the part of the liquid nitrogen circulation main pipe located outside the cavity and the part of the circulation pipe located outside the cavity.

[0015] Furthermore, in order to prevent cavitation, the winding pitch of the liquid nitrogen circulation main pipe on the inner tank body is 1 - 2 m, and the liquid nitrogen flow rate ≤ 2 m / s.

[0016] Furthermore, the diameter of the exhaust holes is 3 - 6 mm, and the spacing is 200 - 300 mm.

[0017] Furthermore, in order to reduce solar heat radiation, the liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank further includes a phase change cold storage board installed on the outer tank body. The phase change cold storage board is used to transfer the cold energy of liquid nitrogen to its phase change material to store cold energy for cooling the outside of the outer tank body.

[0018] The present invention also relates to a cooling and fire extinguishing method based on the liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank, including:

[0019] Step S1, during the transportation of the liquefied natural gas transport vehicle, if the temperature sensor detects that the temperature of the liquefied natural gas in the inner tank exceeds the threshold value, first turn on the suction pump to establish negative pressure, and then open the valves on the liquid nitrogen circulation main pipe and the circulation pipe. The liquid nitrogen storage tank releases liquid nitrogen into the cavity;

[0020] Step S2, the liquid nitrogen enters the liquid nitrogen circulation main pipe, and the liquid nitrogen circulates and absorbs heat, reducing the temperature of the inner tank. The liquid nitrogen absorbs heat and vaporizes into nitrogen gas. Under the action of the exhaust holes on the liquid nitrogen circulation main pipe, it enters the cavity, and then enters the loop pipe under the action of the suction pump. The nitrogen gas passing through the loop pipe is made into liquid nitrogen by the cooling and pressurizing device and then transported to the liquid nitrogen storage device; the liquid nitrogen in the liquid nitrogen storage device is sprayed through the circulation pipe to cool the inner tank a second time.

[0021] After adopting the above technical solution, the present invention has the following beneficial effects:

[0022] 1. In the cavity of the present invention, a spirally wound liquid nitrogen circulation main pipe is arranged, and the multi-point release of the liquid nitrogen flow is realized through the exhaust holes, greatly improving the speed and uniformity of the liquid nitrogen coverage. At the same time, it also greatly reduces the floating of nitrogen gas, so that the liquefied natural gas at the top of the inner tank maintains the same low temperature as the liquefied natural gas at the bottom, achieving the effect of cooling the inner layer of the tank in all directions.

[0023] 2. The present invention is provided with a loop pipe, which can not only recover the vaporized nitrogen gas, but also pressurize and cool it into liquid nitrogen through the cooling and pressurizing device, and spray it on the top of the inner tank to make up for the problem of insufficient cooling of the top of the inner tank caused by the inevitable floating of nitrogen gas. The cycle cooling further realizes the all-round cooling of the inner layer of the tank. At the same time, by using the cascade of cold energy, the cooling effect is improved, and the time for maintaining the low temperature of the liquefied natural gas during transportation is also greatly extended.

[0024] 3. Traditional double-layer tanks usually adopt a multi-point radial support method, achieving the support function by setting radial non-metallic elements on the support surfaces of the inner and outer tanks. This mechanical support method will cause mechanical thermal bridges, conduct external heat into the inner tank, significantly increase heat leakage, and reduce energy efficiency. The present invention sets up a superconducting magnetic levitation support module. The superconducting magnetic levitation support module maintains a superconducting state under the action of low-temperature liquid nitrogen, suspending the inner tank in the cavity. By combining the cold energy of liquid nitrogen with superconducting magnetic levitation, a non-contact effect between the inner tank and the outer tank is achieved, reducing cold energy loss, eliminating stress concentration caused by temperature difference deformation, and reducing damage to the tank structure. Moreover, the superconducting magnetic levitation support module contains electromagnetic coils, and the generated magnetism can strengthen the turbulent heat transfer of liquid nitrogen / nitrogen in the cavity. The magnetic induction intensity is coupled with the fluid velocity field, significantly increasing the heat transfer coefficient in the cavity and reducing the local temperature gradient. Finally, the superconducting magnetic levitation technology can generate a strong external magnetic field. In addition to accelerating the turbulent flow of liquid nitrogen and nitrogen as mentioned above, when a fire occurs, by adjusting the intensity and direction of the magnetic field, the combustion chain reaction can be disrupted, thereby changing the shape and combustion rate of the flame, achieving the effect of suppressing the flame and avoiding the risk of secondary reaction of traditional fire extinguishing technologies to such flammable and explosive chemicals. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a schematic structural diagram of the liquid nitrogen circulation cooling and fire extinguishing system for the double-layer liquefied natural gas tank of the present invention;

[0026] Figure 2 is a schematic structural diagram of the double-layer tank of the present invention;

[0027] In the figure, 1 is the inner tank, 2 is the outer tank, 3 is the temperature sensor, 4 is the superconducting magnetic levitation support module, 5 is the cavity, 6 is the liquid nitrogen storage tank, 7 is the liquid nitrogen circulation main pipe, 8 is the return pipe, 9 is the liquid nitrogen collection tank, 10 is the circulation pipe, 11 is the suction pump, 12 is the cooling and pressurizing device, 13 is the argon storage tank, 14 is the argon pipe, 15 is the transverse branch pipe, 16 is the swirl atomizing nozzle, 17 is the mass flow controller, 18 is the phase change cold storage plate, and 19 is the exhaust hole. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] In order to make the content of the present invention be more clearly understood, the following further describes the present invention in detail according to specific embodiments and in conjunction with the drawings.

[0029] Embodiment 1

[0030] As Figure 1 and Figure 2 shown, a liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank includes:

[0031] Double-layer tank body, including an inner tank body 1 and an outer tank body 2. The inner tank body 1 is located inside the outer tank body 2 and is equipped with a temperature sensor 3. A plurality of superconducting magnetic levitation support modules 4 are arranged between the outer tank body 2 and the inner tank body 1 to make the inner tank body 1 levitate inside the outer tank body 2 and form a cavity 5 with the outer tank body 2;

[0032] Liquid nitrogen cooling and fire extinguishing assembly, including a liquid nitrogen storage tank 6 and a liquid nitrogen circulation main pipe 7. The liquid nitrogen circulation main pipe 7 is connected to the liquid nitrogen storage tank 6 and is spirally wound around the outer peripheral wall of the inner tank body 1. A plurality of exhaust holes 19 are arranged along the extending direction of the part located in the cavity 5;

[0033] Liquid nitrogen collection and circulation assembly, including a return pipe 8, a liquid nitrogen collection tank 9 and a circulation pipe 10. One end of the return pipe 8 is located in the cavity 5, and the other end is connected to the liquid nitrogen collection tank 9. An air suction pump 11 and a cooling and pressurizing device 12 are connected in series on the return pipe 8. The nitrogen in the cavity 5 is successively changed into liquid nitrogen by the air suction pump 11 and the cooling and pressurizing device 12 and enters the liquid nitrogen collection tank 9. The circulation pipe 10 is connected to the liquid nitrogen collection tank 9 and extends into the cavity 5 for spraying liquid nitrogen onto the top of the inner tank body 1.

[0034] The cooling and fire extinguishing method includes:

[0035] Step S1, the double-layer tank body is a part of a liquefied natural gas transport vehicle. During the transportation process of the liquefied natural gas transport vehicle, if the temperature sensor 3 detects that the temperature of the liquefied natural gas in the inner tank body 1 exceeds the threshold value, first turn on the air suction pump 11 to establish negative pressure, and then open the valves on the liquid nitrogen circulation main pipe 7 and the circulation pipe 10, and the liquid nitrogen storage tank 6 releases liquid nitrogen into the cavity 5;

[0036] Step S2, the liquid nitrogen enters the liquid nitrogen circulation main pipe 7, and the liquid nitrogen circulates and absorbs heat, and the temperature of the inner tank body 1 decreases. The liquid nitrogen absorbs heat and gasifies into nitrogen. Under the action of the exhaust holes 19 on the liquid nitrogen circulation main pipe 7, it enters the cavity 5, and under the action of the air suction pump 11, it passes through the return pipe 8. The nitrogen passing through the return pipe 8 is made into liquid nitrogen by the cooling and pressurizing device 12 and then transported to the liquid nitrogen storage device; the liquid nitrogen in the liquid nitrogen storage device is sprayed through the circulation pipe 10 to cool the inner tank body 1 for the second time.

[0037] Specifically, in the closed cavity 5, the liquid nitrogen gasifies into nitrogen. There is a significant difference in density between the liquid nitrogen itself and nitrogen. Two substances in different phases will form a density difference, and there is a stratification effect for gases. The low-density gas will gradually float up to a higher density similar to it. For a general liquid nitrogen discharge pipe, since the liquid nitrogen will continuously gasify during transportation and may float before reaching the bottom, this will cause a nitrogen gas covering layer to form at the upper end of the inner cavity 5, and the lower end is the unvaporized liquid nitrogen, which will result in only the liquefied natural gas at the lower end of the inner tank body 1 being able to maintain a low temperature.

[0038] In the cavity 5 of this embodiment, a nitrogen liquid circulation main pipe 7 wound in a spiral is arranged, and the multi-point release of the nitrogen liquid flow is realized through the exhaust holes 19, which greatly improves the speed and uniformity of the nitrogen liquid coverage. At the same time, it also greatly reduces the floating of nitrogen gas, enabling the liquefied natural gas at the top inside the inner tank body 1 to maintain a low temperature like that at the bottom, achieving the effect of cooling the entire inner layer of the tank body in all directions.

[0039] In addition, in this embodiment, a return pipe 8 is provided, which can not only recover the vaporized nitrogen gas, but also pressurize and cool it into liquid nitrogen through the cooling and pressurizing device 12, and spray it on the top of the inner tank body 1 to make up for the problem of insufficient cooling of the top of the inner tank body 1 caused by the inevitable floating of nitrogen gas. It cools down in a cycle, further realizing the all-round cooling of the inner layer of the tank body. At the same time, by using the cascaded cold energy, the cooling effect is improved, and the time for maintaining the low temperature of the liquefied natural gas during transportation is also greatly extended.

[0040] In addition, traditional double-layer tank bodies usually adopt a multi-point radial support method, and the support function is realized by setting radial non-metallic elements on the support surfaces of the inner and outer tank bodies. This mechanical support method will cause mechanical thermal bridges, conduct external heat into the inner tank, significantly increase heat leakage, and reduce energy efficiency.

[0041] In this embodiment, a superconducting magnetic levitation support module 4 is also provided. The superconducting magnetic levitation support module 4 maintains a superconducting state under the action of low-temperature liquid nitrogen, enabling the inner tank body 1 to levitate in the cavity 5. By combining the cold energy of liquid nitrogen with superconducting magnetic levitation, a non-contact effect between the inner tank body 1 and the outer tank body 2 is achieved, reducing cold energy loss, eliminating stress concentration caused by thermal deformation due to temperature difference, and reducing damage to the tank body structure.

[0042] Moreover, the superconducting magnetic levitation support module 4 contains electromagnetic coils, and the generated magnetism can strengthen the turbulent heat transfer of liquid nitrogen / nitrogen gas in the cavity. The magnetic induction intensity is coupled with the fluid velocity field, significantly increasing the heat transfer coefficient in the cavity 5 and reducing the local temperature gradient.

[0043] Finally, the superconducting magnetic levitation technology can generate a strong external magnetic field. In addition to accelerating the turbulent flow of liquid nitrogen and nitrogen gas as mentioned above, it can also, in case of a fire, disrupt the combustion chain reaction by adjusting the intensity and direction of the magnetic field, thereby changing the shape and combustion rate of the flame, achieving the effect of suppressing the flame and avoiding the risk of secondary reaction of traditional fire extinguishing technologies to such flammable and explosive chemicals.

[0044] Among them, there are various types of superconductors, and yttrium barium copper oxide superconductors are preferably used. This type of superconductor allows partial magnetic flux lines to penetrate into the material interior under specific conditions, but will be "pinned" and fixed by defects or impurities in the material. This characteristic enables the superconductor to both repel the magnetic field (diamagnetism) and stably levitate the magnet through the pinning effect, forming a dynamic balance.

[0045] In this embodiment, the inner tank 1 is made of high-strength low-temperature-resistant steel. The outer tank 2 can be provided with a high-vacuum interlayer, and the interlayer is filled with expanded perlite, with a thermal conductivity ≤ 0.004 W / (m·K). The distance between the inner tank 1 and the outer tank 2 is preferably 50 - 80 mm. The liquid nitrogen storage tank 6 is used to store liquid nitrogen, maintain a low-temperature and high-pressure environment, and the daily evaporation rate ≤ 0.5%.

[0046] In this embodiment, valves and mass flow controllers 17 are respectively provided on the parts of the liquid nitrogen circulation main pipe 7 outside the double-layer tank and the parts of the circulation pipe 10 outside the double-layer tank. And the parts of the liquid nitrogen circulation main pipe 7 outside the double-layer tank and the parts of the circulation pipe 10 outside the double-layer tank are made of low-temperature-resistant stainless steel, vacuum-insulated, with a pressure resistance ≥ 1.5 MPa and a heat leakage rate ≤ 5 W / m 2 。

[0047] In this embodiment, as Figure 1 shown, the circulation pipe 10 can also be connected with a transverse branch pipe 15. A number of swirl atomizing nozzles 16 are installed below the transverse branch pipe 15, and the swirl atomizing nozzles 16 are located in the cavity 5. The material of the swirl atomizing nozzles 16 is titanium alloy, which can withstand low temperatures of -150°C and the erosion of high-speed airflows. The swirl atomizing nozzles 16 are evenly distributed along the transverse branch pipe 15, with a spacing of 300 - 500 mm. The nozzle diameter of the swirl atomizing nozzles 16 is 1 - 3 mm, the atomized liquid droplet diameter ≤ 50 μm, and the working pressure is 0.3 - 0.8 MPa.

[0048] In this embodiment, as Figure 1 shown, a pressure relief valve is installed in the cavity 5, which automatically opens to release pressure when the pressure in the cavity 5 exceeds the set threshold value to prevent equipment damage.

[0049] In this embodiment, the winding pitch of the liquid nitrogen circulation main pipe 7 on the inner tank 1 is preferably 1 - 2 m, and the liquid nitrogen flow rate ≤ 2 m / s to prevent cavitation.

[0050] In this embodiment, the end of the liquid nitrogen circulation main pipe 7 extending into the inner tank 1 is a closed end, the diameter of the exhaust hole 19 is 3 - 6 mm, and the spacing is 200 - 300 mm.

[0051] In this embodiment, the return pipe 8 is used to collect the gasified nitrogen, adopts a gas-liquid two-phase flow design, and the flow rate is controlled at 5 - 10 m / s. The suction pump 11 adopts a magnetic levitation low-temperature suction pump, with a working temperature range of -200 - 50°C and a pumping speed ≥ 100 m 3 / h, and the power consumption ≤ 3 kW.

[0052] Embodiment Two

[0053] On the basis of Embodiment One, as Figure 1As shown, the liquid nitrogen circulation cooling and fire extinguishing system for the double-layer LNG tank also includes an argon cooling and fire extinguishing component, which includes an argon storage tank 13 and an argon pipe 14. The argon pipe 14 is respectively connected to the argon storage tank 13 and the cavity 5.

[0054] In this embodiment, as Figure 1 shown, the argon pipe 14 is connected to the bottom of the cavity 5.

[0055] In this embodiment, when a fire breaks out in the LNG, argon is released from the bottom of the cavity 5 through the argon pipe 14, and liquid nitrogen is injected from the top of the tank. Argon, liquid nitrogen, and nitrogen quickly diffuse in the cavity 5 to extinguish the fire in all directions without polluting the LNG.

[0056] Specifically, using argon as a supplement, since argon belongs to high-density inert gas, it is easier to form an inert gas layer at the bottom of the enclosed space to inhibit the evaporation and diffusion of LNG; and argon is compatible with the low-temperature environment of liquid nitrogen, which can avoid the sudden drop in pressure caused by liquefaction; when a fire breaks out in the LNG, through the stratified injection design, liquid nitrogen is injected from the top of the outer layer of the inner tank 1 for cooling, and argon is released from the bottom for filling to form a vertical inert gradient. And due to the good airtightness of the double-layer tank, the liquid nitrogen, nitrogen, and argon diffuse faster and more evenly, which can effectively reduce the oxygen concentration and extinguish the fire.

[0057] Embodiment Three

[0058] Based on Embodiment One or Embodiment Two, the liquid nitrogen circulation cooling and fire extinguishing system for the double-layer LNG tank also includes a phase change cold storage plate 18 installed on the outer tank 2. The phase change cold storage plate 18 is used to transfer the cold energy of the liquid nitrogen to its phase change material to store the cold energy for cooling the outside of the outer tank 2.

[0059] Specifically, by setting the phase change cold storage plate 18, there will be cold energy transfer during the vaporization process of liquid nitrogen. Part of it is used to maintain the low temperature of the LNG, and part of it can be absorbed and stored by the phase change cold storage plate 18. The outer layer of the outer tank 2 will be exposed to sunlight during transportation, and the natural thermal radiation will affect the temperature of the inner layer of the outer tank 2. Therefore, the phase change cold storage plate 18 can transfer the stored cold energy to the outer layer of the outer tank 2 to reduce the solar thermal radiation, which can achieve the cascaded utilization of cold energy and reduce energy consumption.

[0060] Taking the above ideal embodiments based on the present invention as an inspiration, through the above description, relevant staff can completely make various changes and modifications without departing from the technical idea of this invention. The technical scope of this invention is not limited to the content in the specification, and its technical scope must be determined according to the scope of the claims.

Claims

1. A liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank, characterized in that: It includes: A double-layer tank body, including an inner tank body (1) and an outer tank body (2). The inner tank body (1) is located inside the outer tank body (2) and is equipped with a temperature sensor (3). A plurality of superconducting magnetic levitation support modules (4) are arranged between the outer tank body (2) and the inner tank body (1) to make the inner tank body (1) levitate inside the outer tank body (2) and form a cavity (5) between it and the outer tank body (2); A liquid nitrogen cooling and fire extinguishing assembly, including a liquid nitrogen storage tank (6) and a liquid nitrogen circulation main pipe (7). The liquid nitrogen circulation main pipe (7) is connected to the liquid nitrogen storage tank (6), spirally wound around the outer peripheral wall of the inner tank body (1), and a plurality of exhaust holes (19) are arranged along the extending direction of the part located in the cavity (5); A liquid nitrogen collection and circulation assembly, including a return pipe (8), a liquid nitrogen collection tank (9) and a circulation pipe (10). One end of the return pipe (8) is located in the cavity (5), and the other end is connected to the liquid nitrogen collection tank (9). An air suction pump (11) and a cooling and pressurizing device (12) are connected in series on the return pipe (8). The nitrogen in the cavity (5) sequentially passes through the air suction pump (11) and the cooling and pressurizing device (12) to become liquid nitrogen and enter the liquid nitrogen collection tank (9). The circulation pipe (10) is connected to the liquid nitrogen collection tank (9) and extends into the cavity (5) for spraying liquid nitrogen onto the top of the inner tank body (1).

2. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 1, characterized in that: It further includes an argon cooling and fire extinguishing assembly, including an argon storage tank (13) and an argon pipe (14). The argon pipe (14) is respectively connected to the argon storage tank (13) and the cavity (5).

3. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 2, characterized in that: The argon pipe (14) is connected to the bottom of the cavity (5).

4. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 1, characterized in that: The circulation pipe (10) is connected to a transverse branch pipe (15), and a number of swirl atomizing nozzles (16) are installed below the transverse branch pipe (15). The swirl atomizing nozzles (16) are located in the cavity (5).

5. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 1, characterized in that: Mass flow controllers (17) are respectively configured for the part of the liquid nitrogen circulation main pipe (7) located outside the cavity (5) and the part of the circulation pipe (10) located outside the cavity (5).

6. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 1, characterized in that: The winding pitch of the liquid nitrogen circulation main pipe (7) on the inner tank body (1) is 1 - 2 m, and the liquid nitrogen flow rate ≤ 2 m / s.

7. The liquid nitrogen circulation cooling and fire extinguishing system for a double-layer liquefied natural gas tank according to claim 1, characterized in that: The diameter of the exhaust hole (19) is 3 - 6 mm, and the spacing is 200 - 300 mm.

8. The liquefied natural gas double-layer tank body liquid nitrogen circulation cooling and fire extinguishing system according to claim 1, wherein it further includes a phase change cold storage plate (18) installed on the outer tank body (2), and the phase change cold storage plate (18) is used to transfer the cold energy of liquid nitrogen to its phase change material to store cold energy, so as to cool the outside of the outer tank body (2).

9. A cooling and fire extinguishing method based on the liquefied natural gas double-layer tank body liquid nitrogen circulation cooling and fire extinguishing system according to any one of claims 1 - 8, wherein it includes: Step S1, during the transportation process of the liquefied natural gas transport vehicle, if the temperature sensor (3) detects that the temperature of the liquefied natural gas in the inner tank body (1) exceeds the threshold value, first turn on the suction pump (11) to establish negative pressure, and then open the valves on the liquid nitrogen circulation main pipe (7) and the circulation pipe (10), and the liquid nitrogen storage tank (6) releases liquid nitrogen into the cavity (5); Step S2, the liquid nitrogen enters the liquid nitrogen circulation main pipe (7), the liquid nitrogen circulates and absorbs heat, the temperature of the inner tank body (1) decreases, the liquid nitrogen absorbs heat and vaporizes into nitrogen gas, and enters the cavity (5) under the action of the exhaust hole (19) on the liquid nitrogen circulation main pipe (7), and passes through the loop pipe (8) under the action of the suction pump (11). The nitrogen gas passing through the loop pipe (8) is made into liquid nitrogen by the cooling and pressurizing device (12) and then transported to the liquid nitrogen storage device; the liquid nitrogen in the liquid nitrogen storage device is sprayed through the circulation pipe (10) to cool the inner tank body (1) for the second time.

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