Method and apparatus for reliquefying boil-off gas from liquefied natural gas storage facilities

Abstract

To provide a method and apparatus for efficiently reliquefying boil-off gas in liquefied natural gas storage tanks. [Solution] A method for reliquefying boil-off gas in a liquefied natural gas storage facility, comprising: a liquefied natural gas storage tank for storing liquefied natural gas; a cryogenic liquefied gas storage tank for storing cryogenic liquefied gas; and a heat exchanger that uses the cold energy of the cryogenic liquefied gas to cool the boil-off gas and reliquefy the boil-off gas, the method comprising: a liquefied natural gas storage tank pressure adjustment step for adjusting the pressure of the liquefied natural gas storage tank; a cryogenic liquefied gas storage tank pressure adjustment step for adjusting the pressure of the cryogenic liquefied gas storage tank; and a flow rate adjustment step for adjusting the flow rate of the cryogenic liquefied gas.

Description

Technical Field

[0001] The present disclosure relates to a method for reliquefying boil-off gas of a liquefied natural gas storage facility and an apparatus therefor.

Background Art

[0002] In recent years, liquefied natural gas (LNG) has attracted attention as an alternative energy and clean fuel to fossil fuels. Also, LNG is used as a fuel in factories and the like, and the introduction of satellite facilities is progressing not only in large-scale facilities but also in small-scale facilities.

[0003] LNG is stored in a storage tank installed in a facility. Inside the storage tank, a part of the LNG is constantly vaporized by heat input from the outside, generating boil-off gas (BOG), which is a factor causing the pressure inside the storage tank to rise. Therefore, various methods for treating BOG have been studied to prevent abnormal pressure increase in the storage tank due to the generation of BOG.

[0004] In Patent Documents 1 and 2, methods for using BOG as a fuel are disclosed. Also, in Patent Documents 3 to 5, methods for reliquefying BOG and returning it to the storage tank are disclosed.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Summary of the Invention

[0006] In Patent Documents 1 and 2, BOG generated by heat input is consumed in boilers or cogeneration systems, but any surplus BOG that cannot be used as fuel is released into the atmosphere or treated by combustion. The majority of BOG is methane, and from the perspective of global warming, such treatment should be avoided. Furthermore, the majority of LNG is also methane, and treating BOG in this manner reduces the proportion of methane in the storage tank, causing a change in the composition of the LNG. A change in the composition of LNG can alter its physical properties such as calorific value and density, potentially leading to malfunctions in the facility.

[0007] Patent documents 3 and 4 describe the reliquefaction of BOG (Burnout of Old Gas) using the cold energy of LNG. However, facilities that are not in continuous operation cannot secure the necessary cold energy source to reliquefy the continuously generated BOG, and therefore cannot reliquefy the BOG. ​​Furthermore, small-scale facilities have a larger surface area relative to the storage tank capacity compared to large-scale facilities. Since the amount of heat input is proportional to the surface area, the amount of heat input per unit volume of LNG is larger in small-scale facilities. Moreover, small-scale facilities have a small amount of LNG in the storage tank, and the available cold energy is limited. As a result, there is a high possibility that the amount of BOG generated will exceed the amount of BOG that can be reliquefied. Thus, the methods disclosed in patent documents 3 and 4 have limitations on the facilities in which they can be used.

[0008] In Patent Document 5, BOG is reliquefied by heat exchange with liquid nitrogen. However, it is generally known that LNG stored in storage tanks contains nitrogen. If nitrogen is reliquefied along with BOG, the liquid density of the LNG decreases. Since the formation of liquid layers with different liquid densities can cause rollover, measures are needed to prevent nitrogen from liquefying.

[0009] The purpose of this disclosure is to address the above-mentioned problems and to provide a method and apparatus for efficiently reliquefying boil-off gas in a liquefied natural gas storage tank. [Means for solving the problem]

[0010] [1] A liquefied natural gas storage tank for storing liquefied natural gas, A cryogenic liquefied gas storage tank for storing cryogenic liquefied gas, A method for reliquefying boil-off gas in a liquefied natural gas storage facility, comprising using a heat exchanger for cooling the boil-off gas and reliquefying the boil-off gas, A reliquefaction step is performed in which the boil-off gas is cooled and reliquefied using the cold energy of the aforementioned low-temperature liquefied gas, A liquefied natural gas storage tank pressure adjustment process for adjusting the pressure of the liquefied natural gas storage tank, A cryogenic liquefied gas storage tank pressure adjustment process for adjusting the pressure of the cryogenic liquefied gas storage tank, A method for reliquefying boil-off gas from a liquefied natural gas storage facility, comprising a flow rate adjustment step for adjusting the flow rate of the aforementioned cryogenic liquefied gas.

[0011] [2] A method for reliquefying boil-off gas of a liquefied natural gas storage facility according to [1], comprising a heating step of heating the low-temperature liquefied gas discharged from the heat exchanger.

[0012] [3] A method for reliquefying boil-off gas of a liquefied natural gas storage facility according to [1] or [2], comprising a pressurization step of pressurizing the low-temperature liquefied gas discharged from the heat exchanger.

[0013] [4] A method for reliquefying boil-off gas in a liquefied natural gas storage facility according to any one of [1] to [3], comprising a circulation pump for circulating the liquefied natural gas in the liquefied natural gas storage tank.

[0014] [5] The method for reliquefying boil-off gas of a liquefied natural gas storage facility according to any one of items [1] to [4], wherein the cryogenic liquefied gas is liquid nitrogen.

[0015] [6] A liquefied natural gas storage tank for storing liquefied natural gas, A cryogenic liquefied gas storage tank for storing cryogenic liquefied gas, A boil-off gas re-liquefaction device for a liquefied natural gas storage facility that re-liquefies the boil-off gas using a heat exchanger for cooling the boil-off gas. The heat exchanger is located above the liquid level of the liquefied natural gas storage tank. A liquefied natural gas storage tank pressure adjustment device for adjusting the pressure of the liquefied natural gas storage tank. A cryogenic liquefied gas storage tank pressure adjustment device for adjusting the pressure of the cryogenic liquefied gas storage tank. A flow rate adjustment device for adjusting the flow rate of the cryogenic liquefied gas. A boil-off gas re-liquefaction device for a liquefied natural gas storage facility is provided.

[0016] 〔7〕 The boil-off gas re-liquefaction device for a liquefied natural gas storage facility according to 〔6〕, wherein the heat exchanger is inside the liquefied natural gas storage tank.

[0017] 〔8〕 The boil-off gas re-liquefaction device for a liquefied natural gas storage facility according to 〔6〕 or 〔7〕, comprising a heater for heating the cryogenic liquefied gas discharged from the heat exchanger.

[0018] 〔9〕 The boil-off gas re-liquefaction device for a liquefied natural gas storage facility according to any one of 〔6〕 to 〔8〕, comprising a pressurizer for pressurizing the cryogenic liquefied gas discharged from the heat exchanger.

[0019] 〔10〕 The boil-off gas re-liquefaction device for a liquefied natural gas storage facility according to any one of 〔6〕 to 〔9〕, comprising a circulation pump for circulating the liquefied natural gas inside the liquefied natural gas storage tank.

[0020] 〔11〕 The boil-off gas re-liquefaction device for a liquefied natural gas storage facility according to any one of 〔6〕 to 〔10〕, wherein the cryogenic liquefied gas is liquid nitrogen.

Advantages of the Invention

[0021] According to the present disclosure, a method and an apparatus for efficiently re-liquefying boil-off gas in a liquefied natural gas storage tank can be provided. [Brief explanation of the drawing]

[0022] [Figure 1] Figure 1 is a schematic diagram showing an example of the configuration of a boil-off gas reliquefaction unit for a liquefied natural gas storage facility in this embodiment. [Figure 2] Figure 2 is a schematic diagram showing another example of the configuration of the boil-off gas reliquefaction unit of the liquefied natural gas storage facility in this embodiment. [Figure 3] Figure 3 is a schematic diagram showing another example of the configuration of the boil-off gas reliquefaction unit of the liquefied natural gas storage facility in this embodiment. [Figure 4] Figure 4 is a schematic diagram showing another example of the configuration of the boil-off gas reliquefaction unit of the liquefied natural gas storage facility in this embodiment. [Figure 5] Figure 5 is a schematic diagram showing another example of the configuration of the boil-off gas reliquefaction unit of the liquefied natural gas storage facility in this embodiment. [Figure 6] Figure 6 is a schematic diagram showing an example of the configuration of a liquefied natural gas storage tank in this embodiment. [Modes for carrying out the invention]

[0023] Embodiments of the present disclosure are described below, but this description is not intended to limit the scope of the claims.

[0024] Figure 1 is a schematic diagram showing an example of the configuration of a boil-off gas reliquefaction device for a liquefied natural gas storage facility in this embodiment. The boil-off gas reliquefaction device 100 for a liquefied natural gas storage facility in this embodiment includes a liquefied natural gas storage tank 1 for storing liquefied natural gas, a cryogenic liquefied gas storage tank 3 for storing cryogenic liquefied gas, a heat exchanger 2 for cooling the boil-off gas, a liquefied natural gas storage tank pressure regulator for adjusting the pressure of the liquefied natural gas storage tank 1, a cryogenic liquefied gas storage tank pressure regulator for adjusting the pressure of the cryogenic liquefied gas storage tank 3, and a flow rate regulator 6 for adjusting the flow rate of the cryogenic liquefied gas. The heat exchanger 2 is located above the liquid level of the liquefied natural gas storage tank 1.

[0025] In this embodiment, liquefied natural gas is referred to as "LNG," a liquefied natural gas storage tank as "LNG storage tank," cryogenic liquefied gas as "CLG," a cryogenic liquefied gas storage tank as "CLG storage tank," boil-off gas as "BOG," and a boil-off gas reliquefaction device in a liquefied natural gas storage facility as "BOG reliquefaction device."

[0026] The LNG storage tank 1 stores LNG. In this embodiment, BOG is generated when the LNG inside the LNG storage tank 1 vaporizes. The LNG storage tank 1 has an insulating function to keep the LNG as isolated as possible from the outside temperature. Inside the LNG storage tank 1, there is a liquid phase portion which is the lower part of the LNG storage tank 1, and a gas phase portion which is the upper part of the LNG storage tank 1. The liquid phase portion is the LNG stored inside the LNG storage tank 1, and the gas phase portion is a gas containing BOG, which is the vaporized LNG.

[0027] CLG storage tank 3 stores CLG. CLG is not particularly limited as long as it is a liquefied gas at a lower temperature than LNG, such as liquid nitrogen, liquid oxygen, or liquid air. Liquid nitrogen is preferred as CLG from the viewpoint of reusing waste cooling heat from factories and other facilities.

[0028] Heat exchanger 2 uses the cold energy of CLG to cool and reliquefy BOG. ​​As described above, BOG generated in LNG storage tank 1 is introduced into heat exchanger 2. Heat exchanger 2 is connected to CLG storage tank 3 by CLG introduction piping 7, and CLG is introduced into heat exchanger 2 from CLG introduction piping 7. The BOG introduced into heat exchanger 2 is cooled by the cold energy of CLG and reliquefied. The reliquefied LNG returns to the liquid phase portion of LNG storage tank 1. In addition, the CLG that has passed through heat exchanger 2 is discharged from heat exchanger 2 through CLG discharge piping 8.

[0029] The heat exchanger 2 is not particularly limited and examples include plate heat exchangers, plate fin heat exchangers, coil heat exchangers, etc. Preferably, the heat exchanger 2 has a BOG inlet at the top and a re-liquefied LNG outlet at the bottom. Using such a heat exchanger 2 allows for more efficient re-liquefaction of BOG.

[0030] The heat exchanger 2 is installed above the liquid level of the LNG storage tank 1, that is, above the liquid phase. This installation allows the reliquefied LNG to flow into the LNG storage tank by its own weight.

[0031] Specifically, the heat exchanger 2 can be installed inside the LNG storage tank 1, i.e., in the gas phase portion of the LNG storage tank 1, as shown in Figures 1 to 3, or installed in a BOG storage tank 13 located above the LNG storage tank 1, as shown in Figure 4. Alternatively, as shown in Figure 5, a CLG storage tank 3 can be provided above the liquid level of the LNG storage tank 1, and the heat exchanger 2 can be installed in at least a portion of the liquid phase portion of the CLG storage tank 3. In this embodiment, it is preferable to install the heat exchanger 2 inside the LNG storage tank 1. This allows for a compact housing of the heat exchanger 2, and by installing it inside the existing LNG storage tank 1, heat input from the outside air is reduced, enabling efficient heat exchange.

[0032] Furthermore, it is preferable that the heat exchanger 2 is provided with a return channel 9 for returning the reliquefied LNG to the liquid phase portion of the LNG storage tank 1. Moreover, it is preferable that the outlet of the return channel 9 is located in the liquid phase portion. This allows for more efficient reliquefaction of BOG.

[0033] The LNG pressure regulator adjusts the pressure in LNG storage tank 1. This is because the pressure in LNG storage tank 1 fluctuates depending on the amount of BOG generated and the amount of reliquefaction, which makes the LNG supply conditions unstable. Specifically, if the pressure in LNG storage tank 1 rises, the temperature of the LNG may rise, so the pressure is reduced by reliquefaction of BOG. ​​If the pressure in LNG storage tank 1 falls, the pressure necessary for LNG supply may not be secured, so the pressure is increased by the pressurizer 5.

[0034] The LNG pressure regulator may include a pressurizer 5 and a pressure gauge. Note that since the pressure reduction of the LNG storage tank 1 is performed by the heat exchanger 2, it is not necessary to provide a pressure reducing valve 4, as is the case with the CLG pressure regulator described later.

[0035] The CLG pressure regulator adjusts the pressure in the CLG storage tank 3. The majority of LNG and BOG is methane, and if the methane freezes and solidifies on the surface of the heat exchanger 2, the heat transfer area decreases, which can reduce the efficiency of reliquefaction, or it can block the BOG flow path, making it impossible to reliquefy the BOG. ​​In addition, the solidification of methane can change the composition of LNG, altering its physical properties such as calorific value and density, which may cause LNG consuming facilities that receive the LNG to become inoperable. To prevent such methane solidification, it is necessary to adjust the CLG pressure.

[0036] Specifically, the pressure in the CLG storage tank 3 is adjusted so that the temperature of the CLG supplied from the CLG storage tank 3 can be controlled within the temperature range of -182°C to -162°C, which is the melting and boiling point of methane. That is, if the temperature of the CLG storage tank 3 falls below -182°C, it is pressurized, and if it exceeds -162°C, it is depressurized. Note that the melting and boiling points of methane fluctuate depending on the pressure in the LNG storage tank 1.

[0037] Furthermore, BOG contains trace amounts of nitrogen, and when BOG is reliquefied, the nitrogen is also liquefied. The liquefaction of nitrogen reduces the liquid density of the LNG in LNG storage tank 1. As a result, LNG with different liquid densities exists in the same LNG storage tank 1, forming multiple liquid layers with different densities within LNG storage tank 1, which can cause a rapid gasification of LNG known as rollover within LNG storage tank 1. A rapid increase in pressure within LNG storage tank 1 due to rollover poses a risk of damaging LNG storage tank 1. To prevent such rollover due to nitrogen liquefaction, the pressure in CLG storage tank 3 is adjusted so that the temperature of the CLG supplied from CLG storage tank 3 is above the boiling point of nitrogen, -196°C.

[0038] The CLG pressure regulating device may include a pressure reducing valve 4, a pressurizer 5, and a pressure gauge. The flow rate adjustment device 6 adjusts the flow rate of CLG. By adjusting the flow rate of CLG, the pressure in the LNG storage tank 1 can be kept constant, and abnormal pressure increases in the LNG storage tank 1 can be prevented. Specifically, if the pressure in the LNG storage tank 1 rises, the flow rate of CLG is increased, and if the pressure in the LNG storage tank 1 falls, the flow rate of CLG is decreased. In addition, in order to prevent the solidification of methane components in BOG and the reliquefaction of nitrogen components, it is necessary to adjust the flow rate so that the CLG pressure does not drop too low.

[0039] The flow rate adjustment device 6 is installed in the CLG discharge piping 8. Preferably, the flow rate adjustment device 6 is installed outside the LNG storage tank 1 of the CLG discharge piping 8. The flow rate adjustment device 6 includes a flow rate adjustment valve or a pressure adjustment valve. If the flow rate adjustment device 6 includes a flow rate adjustment valve, the flow rate of the CLG is adjusted directly by the flow rate adjustment valve. If the flow rate adjustment device 6 includes a pressure adjustment valve, the flow rate of the CLG is adjusted indirectly by adjusting the pressure in the LNG storage tank 1. The flow rate adjustment device 6 may also include a flow meter.

[0040] Furthermore, the LNG storage tank 1 is provided with LNG receiving piping (not shown) for receiving LNG. Preferably, the LNG receiving piping is connected to both the liquid phase and the gas phase of the LNG storage tank 1.

[0041] LNG contains methane, ethane, propane, etc., and the composition of these contaminants and the liquid density vary depending on the import source. If LNG with different liquid densities is mixed and stored in LNG storage tank 1, there is a risk of rollover. To prevent rollover, it is preferable to connect the LNG receiving piping to both the liquid phase and gas phase portions of LNG storage tank 1, as described above.

[0042] Specifically, LNG is received from both the gas phase and the liquid phase of the LNG storage tank 1 while adjusting the pressure within the tank. This facilitates mixing of the received LNG with the remaining LNG in the LNG storage tank 1, even if there is a difference in the liquid density of the LNG.

[0043] According to this embodiment, the following effects and advantages are achieved. By adjusting the flow rate of CLG and reliquefying BOG, the pressure inside LNG storage tank 1 can be kept constant, thus preventing abnormal pressure increases and temperature increases in LNG storage tank 1 even when LNG is not being used continuously.

[0044] Furthermore, by adjusting the pressure in LNG storage tank 1, the LNG supply conditions can be stabilized.

[0045] Furthermore, by adjusting the pressure in the CLG storage tank 3 and preventing the solidification of methane and liquefaction of nitrogen, concerns about malfunctions such as the rollover of the LNG storage tank 1 or the inability of LNG consuming facilities that receive the LNG to operate can be mitigated.

[0046] Furthermore, because BOG (Burn-Off Gas) is not released into the atmosphere, it can reduce greenhouse gases and contribute to some of the Sustainable Development Goals (SDGs).

[0047] Furthermore, the reliquefaction of BOG (Boiled Oil Grain) can reuse CLG (Collision Grain), which is waste cooling heat from factories and other facilities. In addition, because unused cooling heat is reused as energy, there is no need to use refrigeration equipment, thus saving energy. From this perspective, it can contribute to some of the activities of the Sustainable Development Goals (SDGs).

[0048] Furthermore, in existing facilities, as shown in Figure 4, a BOG storage tank 13 can be installed above the LNG storage tank 1 and the heat exchanger 2 installed inside it, or as shown in Figure 5, a CLG storage tank 3 can be installed above the LNG storage tank 1 and the heat exchanger 2 installed in the liquid phase portion inside it. This can be applied to, for example, automobiles and ships.

[0049] Figure 2 is a schematic diagram showing another example of the configuration of the BOG reliquefaction apparatus 100 in this embodiment. In this example, the apparatus further includes a heater 10 for heating the CLG discharged from the heat exchanger 2 and a pressurizer 11 for pressurizing the CLG discharged from the heat exchanger 2.

[0050] After heat exchange, the CLG is in a gaseous state, and although its temperature has risen, it remains at a low temperature. Therefore, by further heating it in the heater 10, it can be adjusted to approximately room temperature and reused as a product within the factory. Furthermore, the CLG after heat exchange can be pressurized and reused as needed.

[0051] Furthermore, the heater 10 and the pressurizer 11 do not necessarily need to be provided simultaneously; either one may be provided. Also, the heater 10 and the flow rate adjustment device 6, and the pressurizer 11 and the flow rate adjustment device 6 do not necessarily have to be in the positional relationship shown in Figure 2.

[0052] Figure 3 is a schematic diagram showing another example of the configuration of the BOG reliquefaction apparatus 100 in this embodiment. In this example, a pressurizer 12 is provided to pressurize the CLG discharged from the CLG storage tank 3. Also, a heater 10 may be further provided, similar to the above.

[0053] The CLG discharged from the CLG storage tank 3 is pressurized in pressurizer 12 instead of pressurizer 11 and used for reliquefaction of BOG, after which it can be reused as a product.

[0054] The CLG storage tank 3 may also be a low-pressure tank. Furthermore, although not shown in the diagram, it may be equipped with a CLG pressure regulating device.

[0055] Figure 4 is a schematic diagram showing another example of the configuration of the BOG reliquefaction unit 100 in this embodiment. As described above, in this example, the BOG storage tank 13 is provided above the LNG storage tank 1 and the heat exchanger 2 is installed inside it.

[0056] Furthermore, if a BOG storage tank 13 is to be installed, it is necessary to connect the gas phase portion of the LNG storage tank 1 with the BOG storage tank 13 and to install a BOG introduction pipe 14 to introduce the BOG in the upper space of the LNG storage tank 1 into the BOG storage tank 13.

[0057] Figure 5 is a schematic diagram showing another example of the configuration of the BOG reliquefaction apparatus 100 in this embodiment. As described above, in this example, the CLG storage tank 3 is provided above the LNG storage tank 1, and the heat exchanger 2 is installed so that it is located in at least a part of the liquid phase portion of the CLG storage tank 3. From the viewpoint of heat exchange, it is preferable to install the entire heat exchanger 2 so that it is located in the liquid phase portion of the CLG storage tank 3.

[0058] In this configuration, it is necessary to provide a CLG flow rate control valve 16 to adjust the amount of CLG in the CLG storage tank 3. In Figure 5, the CLG flow rate control valve 16 corresponds to the flow rate adjustment device 6.

[0059] Furthermore, a BOG introduction pipe 14 for introducing BOG from the upper space of the LNG storage tank 1 to the heat exchanger 2, and a BOG control valve 15 for adjusting the amount of BOG introduced, must also be provided. BOG is introduced to the heat exchanger 2 only when the pressure inside the LNG storage tank 1 rises. Although not shown in the figures, a BOG storage tank may also be provided.

[0060] Figure 6 is a schematic diagram showing an example of the configuration of the LNG storage tank 1 in this embodiment. In this example, the LNG storage tank 1 is further equipped with a circulation pump 18 that connects the liquid phase portion and the gas phase portion.

[0061] By extracting LNG from the liquid phase portion using the circulation pump 18 and circulating it to the gas phase portion via the circulation path 17, it is possible to mitigate the formation of multiple liquid layers with different liquid densities within the LNG storage tank 1 when LNG with different liquid densities is present in the same LNG storage tank 1.

[0062] The embodiments disclosed herein should be considered in all respects to be illustrative and not restrictive. The scope of the present invention is indicated by the claims rather than by the foregoing description, and all modifications within the meaning and scope equivalent to the claims are intended to be included. [Explanation of Symbols]

[0063] 1. Liquefied natural gas storage tank, 2. Heat exchanger, 3. Cryogenic liquefied gas storage tank, 4. Pressure reducing valve, 5. Pressurizer, 6. Flow rate adjustment device, 7. Cryogenic liquefied gas introduction piping, 8. Cryogenic liquefied gas discharge piping, 9. Return channel, 10. Heater, 11. Pressurizer, 12. Pressurizer, 13. Boil-off gas storage tank, 14. Boil-off gas introduction piping, 15. Boil-off gas supply valve, 16. Cryogenic liquefied gas flow rate adjustment valve, 17. Circulation path, 18. Circulation pump, 100. Boil-off gas reliquefaction device for liquefied natural gas storage facility.

Claims

1. Liquefied natural gas storage tanks for storing liquefied natural gas, A cryogenic liquefied gas storage tank for storing cryogenic liquefied gas, A method for reliquefying boil-off gas in a liquefied natural gas storage facility, comprising using a heat exchanger for cooling the boil-off gas and reliquefying the boil-off gas, A reliquefaction step is performed in which the boil-off gas is cooled and reliquefied using the cold energy of the aforementioned low-temperature liquefied gas, A liquefied natural gas storage tank pressure adjustment process for adjusting the pressure of the liquefied natural gas storage tank, A cryogenic liquefied gas storage tank pressure adjustment process for adjusting the pressure of the cryogenic liquefied gas storage tank, A method for reliquefying boil-off gas from a liquefied natural gas storage facility, comprising a flow rate adjustment step for adjusting the flow rate of the aforementioned cryogenic liquefied gas.

2. A method for reliquefying boil-off gas in a liquefied natural gas storage facility according to claim 1, further comprising a heating step of heating the low-temperature liquefied gas discharged from the heat exchanger.

3. A method for reliquefying boil-off gas in a liquefied natural gas storage facility according to claim 1 or claim 2, comprising a pressurization step of pressurizing the low-temperature liquefied gas discharged from the heat exchanger.

4. A method for reliquefying boil-off gas in a liquefied natural gas storage facility according to any one of claims 1 to 3, comprising a circulation pump for circulating the liquefied natural gas in the liquefied natural gas storage tank.

5. The method for reliquefying boil-off gas from a liquefied natural gas storage facility according to any one of claims 1 to 4, wherein the cryogenic liquefied gas is liquid nitrogen.

6. Liquefied natural gas storage tanks for storing liquefied natural gas, A cryogenic liquefied gas storage tank for storing cryogenic liquefied gas, A boil-off gas reliquefaction apparatus for a liquefied natural gas storage facility, comprising a heat exchanger for cooling the boil-off gas and a device for reliquefying the boil-off gas, The heat exchanger is located above the liquid level of the liquefied natural gas storage tank. A liquefied natural gas storage tank pressure regulator for adjusting the pressure of the aforementioned liquefied natural gas storage tank, A cryogenic liquefied gas storage tank pressure regulating device for adjusting the pressure of the cryogenic liquefied gas storage tank, A boil-off gas reliquefaction device for a liquefied natural gas storage facility, comprising a flow rate adjustment device for adjusting the flow rate of the aforementioned cryogenic liquefied gas.

7. The heat exchanger is located inside the liquefied natural gas storage tank, and the boil-off gas reliquefaction device for the liquefied natural gas storage facility according to claim 6.

8. A boil-off gas reliquefaction apparatus for a liquefied natural gas storage facility according to claim 6 or claim 7, comprising a heater for heating the low-temperature liquefied gas discharged from the heat exchanger.

9. A boil-off gas reliquefaction apparatus for a liquefied natural gas storage facility according to any one of claims 6 to 8, comprising a pressurizer for pressurizing the low-temperature liquefied gas discharged from the heat exchanger.

10. A boil-off gas reliquefaction device for a liquefied natural gas storage facility according to any one of claims 6 to 9, comprising a circulation pump for circulating the liquefied natural gas in the liquefied natural gas storage tank.

11. The boil-off gas reliquefaction device for a liquefied natural gas storage facility according to any one of claims 6 to 10, wherein the cryogenic liquefied gas is liquid nitrogen.

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