KR20130062006A - Bog reliquefaction apparatus and lng bogreliquefaction method - Google Patents

Abstract

PURPOSE: A re-liquefaction apparatus of liquefied natural gas (LNG) evaporative gas and a method thereof is provided to reduce operating costs by maintaining evaporative gas in an under-cooling state and to secure reliability by being easily operated by an inexperienced driver through a simplified process. CONSTITUTION: A re-liquefaction apparatus of liquefied natural gas (LNG) (1) includes: a storage (10) in which LNG is stored; a compressor (20) compressing the evaporative gas which is generated from the storage tank and discharged; a heat exchange unit (30) which mutually heat-exchanging the evaporative gas, which is supplied from the storage tank to the compressor, and evaporative gas, which is discharged after being compressed by the compressor, and then cools the evaporative gas which is discharged by the compressor; and a mixing unit (40) liquefying the evaporative gas by mutually mixing the evaporative gas, which is discharged from the heat-exchanging unit, and the LNG which is supplied from the storage tank.

Description

BOG RELIQUEFACTION APPARATUS AND LNG BOGRELIQUEFACTION METHOD}

The present invention relates to an apparatus and a method for reliquefaction of LNG boil-off gas, and more particularly, to boil-off gas generated in a storage tank of an LNG carrier transporting cryogenic liquefied natural gas (Liquefied Natural Gas, hereinafter referred to as LNG). It relates to a reliquefaction apparatus and method of (Boil-Off Gas, BOG).

Recently, the consumption of liquefied gas such as LNG (Liquefied Natural Gas) and LPG (Liquefied Petroleum Gas) is increasing worldwide. The liquefied gas is transported in a gaseous state via land or sea gas piping, or is transported to a distant consumer site stored in a liquefied gas carrier in a liquefied state. Liquefied gas such as LNG or LPG is obtained by cooling natural gas or petroleum gas at a very low temperature (approximately -163 ° C. in the case of LNG), and its volume is significantly reduced compared to when it is in a gaseous state, .

The liquefied gas carrier is for carrying the liquefied gas to the sea by loading the liquefied gas and loading the liquefied gas to the landlord. To this end, the liquefied gas carrier carries a storage tank (often referred to as a 'cargo window') capable of withstanding cryogenic temperatures of the liquefied gas do.

Examples of offshore structures equipped with storage tanks for storing cryogenic liquefied gas are, in addition to liquefied gas carriers, vessels such as LNG Regasification Vessels, LNG Floating Storage and Regasification Units, and LNG Floating Production Storage and LNG FPSOs. Structures such as off-loading).

LNG RV is a LNG regasification facility installed on a liquefied gas carrier capable of self-sailing and floating, and LNG FSRU stores liquefied natural gas unloaded from LNG carrier in offshore sea, in storage tank, It is an offshore structure that supplies natural gas to the demand of the land. In addition, LNG FPSO is a marine structure that is used to directly purify mined natural gas from the sea and liquefy directly to store it in a storage tank, and to transfer LNG stored in the storage tank to an LNG carrier if necessary. In this specification, a marine structure is a concept including a vessel such as a liquefied gas carrier, an LNG RV, and the like, as well as structures such as LNG FPSO and LNG FSRU.

Since the liquefaction temperature of natural gas is a cryogenic temperature of about -163 ° C at normal pressure, LNG is evaporated even if its temperature is slightly higher than -163 ° C at normal pressure. For example, in the case of a conventional LNG carrier, the LNG storage tank of the LNG carrier is heat-treated, but since the external heat is continuously transferred to the LNG, LNG is transported by the LNG carrier, The LNG storage tank is constantly vaporized and boil-off gas (BOG) is generated in the LNG storage tank.

Since the generated boil-off gas increases the pressure in the storage tank and may cause structural problems by accelerating the flow of the liquefied gas according to the fluctuation of the ship, it is necessary to suppress the generation of the boil-off gas.

In order to suppress the evaporation gas in the storage tank of the liquefied gas carrier, conventionally, a method of discharging the evaporation gas to the outside of the storage tank and incinerating it, a method of discharging the evaporation gas to the outside of the storage tank, A method of returning to the storage tank again, a method of using evaporation gas as the fuel used in the propulsion engine of the ship, a method of suppressing the generation of evaporation gas by keeping the internal pressure of the storage tank high, alone or in combination .

In the case of a floating structure equipped with a boil-off gas reliquefaction device, the boil-off gas inside the storage tank is discharged to the outside of the storage tank in order to maintain an appropriate pressure in the storage tank and re-liquefied through the re-liquefaction device. Before losing, the boil-off gas is compressed to a low pressure of approximately 4 to 8 bara (absolute pressure) and fed to the reliquefaction apparatus. The compressed boil-off gas is liquefied through heat exchange with nitrogen cooled to cryogenic temperatures in a reliquefaction apparatus including a nitrogen refrigeration cycle and then returned to the storage tank.

1 is a view schematically showing an apparatus for reliquefaction of LNG boil-off gas according to an embodiment of the prior art.

According to one embodiment of the prior art, the evaporated gas generated and evaporated in a storage tank is supplied to a plurality of compressors through an evaporation gas discharge line and compressed, and the compressed evaporated gas is supplied to a reliquefaction system and liquefied into a storage tank. Is returned.

The reliquefaction system for liquefying the evaporated gas evaporated in the storage tank, as shown in Figure 1, by expanding the nitrogen gas storage tank in which nitrogen (N ₂ ) gas is stored, and the nitrogen gas supplied from the nitrogen gas storage tank Nitrogen gas is appropriately adjusted depending on the amount of the nitrogen gas compander to be cooled, the cold box to liquefy the boil-off gas supplied from the compressor by the nitrogen gas supplied from the nitrogen gas compander, and the amount of the boil-off gas supplied from the storage tank. It includes a nitrogen gas boosting skid that serves as a replenishment tank to be supplied.

The reliquefaction apparatus according to one embodiment of the prior art re-liquefies the evaporated gas generated in the storage tank using an expensive reliquefaction system and then use it as a fuel of the main propulsion system or recycle the storage tank to recycle the storage tank. The pressure is kept at or below a certain value.

The system for reliquefaction of the boil-off gas using nitrogen gas has low liquefaction efficiency and high operating cost, and it is difficult to operate the system for the low-skilled driver because the system is complicated. There is a reliability issue, and research and development on how to efficiently handle the evaporative gas continuously generated in the storage tanks need to be continued.

Korean Patent Registration Publication No. 10-0747231 (Samsung Heavy Industries Co., Ltd.) 2007. 08. 01

Therefore, the technical problem to be achieved by the present invention is to maintain the boil-off gas in the supercooled state to reduce the operating cost, the process (simplify the process) can be easily used even for novice drivers to ensure the reliability of LNG boil-off gas It is to provide an apparatus and a method for reliquefaction.

According to an aspect of the invention, there is provided a storage tank in which liquefied natural gas (LNG) is stored; A compressor for compressing the boil-off gas generated and discharged from the storage tank; A heat exchange unit for cooling the boil-off gas discharged through the compressor by heat-exchanging the boil-off gas supplied from the storage tank to the compressor and the boil-off gas discharged after being compressed through the compressor; And a mixing unit for liquefying the boil-off gas by mixing the boil-off gas discharged from the heat exchange unit and the liquefied natural gas supplied from the storage tank to each other.

It may further include an expansion valve disposed in a line connecting the storage tank and the mixing unit to lower the pressure of the liquefied natural gas discharged through the mixing unit.

It may further include an engine fuel supply unit for supplying to the engine by vaporizing the liquefied liquefied gas liquefied in the mixing unit to a high pressure.

The engine fuel supply unit, at least one high pressure pump for compressing the liquefied evaporated gas liquefied in the mixing unit; And at least one high pressure vaporizer for vaporizing the liquefied liquefied gas compressed in the high pressure pump to supply the engine.

The engine may be a ME-GI engine for ship propulsion.

The apparatus may further include a cooling unit for recooling the cooled boil-off gas supplied to the mixing unit by heat-exchanging the boil-off gas cooled through the heat exchanger and the liquefied evaporation gas compressed through the high-pressure pump.

The apparatus may further include a boil-off gas fuel supply line branched from a line connecting the compressor and the heat exchange unit to use the boil-off gas discharged through the compressor as a fuel.

It may further include a steam provided in a line connecting the storage tank and the compressor to heat the boil-off gas supplied to the compressor during the initial operation of the compressor.

In addition, according to another aspect of the invention, the step of compressing the boil-off gas generated and discharged from the storage tank; Cooling the compressed boil-off gas by exchanging the boil-off gas compressed in the compression step with the boil-off gas supplied from the storage tank; And liquefying the cooled boil-off gas by mixing the boil-off gas cooled in the cooling step with the liquefied natural gas supplied from the storage tank and supplying the liquefied liquefied evaporation gas to the storage tank. Reliquefaction methods may be provided.

The method may further include heating the boil-off gas supplied from the storage tank to the compressor at the beginning of the operation of the compressor.

A liquefied evaporation gas compression step of compressing the liquefied evaporation gas liquefied in the mixing unit; And vaporizing the liquefied evaporated gas compressed in the liquefied evaporated gas compression step and supplying the liquefied evaporated gas to an engine.

The engine may be a ME-GI engine for ship propulsion.

The method may further include recooling the cooled boil-off gas supplied to the mixing unit by heat-exchanging the boil-off gas cooled through the heat exchanger and the liquefied evaporation gas compressed through the high-pressure pump.

Embodiments of the present invention can reduce the operating cost by maintaining the evaporated gas in a supercooled state, and can be easily used even by a novice driver in a process simplification process, thereby ensuring reliability.

1 is a view schematically showing an apparatus for reliquefaction of LNG boil-off gas according to an embodiment of the prior art.
2 is a view schematically showing an apparatus for reliquefaction of LNG boil-off gas according to an embodiment of the present invention.
3 is a flowchart illustrating a reliquefaction method of LNG boil-off gas according to another embodiment of the present invention.

In order to fully understand the present invention, operational advantages of the present invention, and objects achieved by the practice of the present invention, reference should be made to the accompanying drawings and the accompanying drawings which illustrate preferred embodiments of the present invention.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the accompanying drawings. Like reference symbols in the drawings denote like elements.

2 is a view schematically showing an apparatus for reliquefaction of LNG boil-off gas according to an embodiment of the present invention.

As shown in this figure, the reliquefaction apparatus 1 of LNG boil-off gas according to the present embodiment, the storage tank 10, the liquefied natural gas (LNG) is stored, and generated in the storage tank 10 and discharged Compressor 20 for compressing the boil-off gas, the heat exchange unit 30 for cooling the boil-off gas discharged through the compressor 20, and the boil-off gas and the storage tank 10 discharged from the heat exchange unit 30 is supplied It is provided with a mixing unit 40 for liquefying the liquefied natural gas to be liquefied boil-off gas.

In the storage tank 10, liquefied natural gas (LNG) is liquefied in a cryogenic state and has an atmospheric pressure (1.013 bar), and liquefied natural gas is vaporized by continuous heat transfer from the outside during transportation of the liquefied natural gas. (BOG; Boil-Off Gas) is generated, which acts as a pressure increase factor of the storage tank (10).

Therefore, in order to maintain the storage tank 10 at a constant atmospheric pressure level, when the internal pressure of the storage tank 10 reaches a predetermined value of about 1.03 to 1.05 bar, a safety valve (not shown) is opened and the evaporated gas is stored in the storage tank 10. Is discharged to the outside of the) is delivered to the compressor 20 through the boil-off gas discharge line (L1).

As shown in FIG. 2, the compressor 20 compresses the boil-off gas discharged from the boil-off gas to a pressure of approximately 30 to 40 bara (absolute pressure) and then supplies it to a heat exchange unit 30 to be described later.

In the present embodiment, a plurality of compressors 20 may be provided, but a pair of compressors 20 may be provided at an upper side and a pair of compressors 20 may be provided at a lower side of FIG. 2. In the present embodiment, the pair of compressors 20 provided at any one of the upper part and the lower part is a main operation compressor for compressing the boil-off gas, and the pair of compressors 20 provided at the other is main operation. It may be a preliminary compressor for the inoperability of the compressor.

On the other hand, in the present embodiment, if the compressor 20 is provided in multiple stages, such as a pair of compressors 20, the compression efficiency can be increased.

As shown in FIG. 2, the heat exchange unit 30 exchanges heat between the boil-off gas supplied from the storage tank 10 to the compressor 20 and the boil-off gas discharged after being compressed through the compressor 20. 20) serves to cool the evaporated gas discharged through.

In this case, the low temperature of the boil-off gas is increased while the compressor 20 is compressed in a simple configuration, so that the low-temperature boil-off gas can be supplied to the mixing unit 40 to be described later, and the mixing in the mixing unit 40 is performed. There is an advantage to increase the efficiency.

Specifically, the cryogenic evaporated gas discharged from the storage tank 10 is supplied to the compressor 20 through the heat exchange unit 30, and the evaporated gas supplied to the compressor 20 is about 180 ° C. at a high temperature while undergoing a compression process. Is raised. If the high-pressure compressed boil-off gas is supplied to the mixing unit 40 as it is, the mixing efficiency when mixing with the liquefied natural gas supplied from the storage tank 10, that is, the liquefaction of the compressed boil-off gas is difficult, as well as the required temperature reduction I can see the difficulty.

However, as shown in the present embodiment, when the boil-off gas compressed by the compressor 20 is supplied to the heat-exchanging unit 30 and heat-exchanged with the cryogenic boil-off gas supplied from the storage tank 10 in the heat-exchanging unit 30, the heat-exchanging unit 30 It can be seen that the mixing efficiency in the mixing unit 40 is improved because the temperature of the boil-off gas passing through can be lowered to about -85 ° C.

In this embodiment, the heat exchange unit 30 may be a heat exchanger of a heater type.

The mixing unit 40 is mixed with the liquefied natural gas supplied through the liquefied natural gas supply line (L2) pumped by the pump (P) in the storage tank 10 and the evaporated gas discharged from the heat exchange unit 30 It liquefies the boil-off gas and at the same time serves to lower the temperature of the boil-off gas.

The mixing efficiency of the mixing unit 40 is greatly affected by the temperature of the boil-off gas flowing into the mixing unit 40 as described above. In this embodiment, the mixing efficiency of the boil-off gas compressed by the heat exchange unit 30 is reduced. Since the temperature can be significantly lowered and the temperature of the boil-off gas compressed by the liquefied natural gas directly supplied through the liquefied natural gas supply line (L2) from the storage tank (10) can be lowered, an embodiment of the related art and This eliminates the need for complex, expensive reliquefaction systems, reducing operating costs, and simplifies the process, making it easier for even novice drivers.

On the other hand, the present embodiment is disposed in the liquefied evaporation gas supply line (L3) for connecting the mixing unit 40 and the storage tank 10 to return the liquefied evaporated gas mixed in the mixing unit 40 to the storage tank 10. It further includes an expansion valve 50 for lowering the temperature and pressure of the liquefied evaporated gas mixed in the mixing unit 40.

In the present embodiment, the expansion valve 50 may also be used as an emergency valve for the case where the pressure of the liquefied evaporated gas mixed in the mixing unit 40 is too high.

In addition, the present embodiment further includes an engine fuel supply unit 60 to vaporize the liquefied liquefied gas discharged by mixing from the mixing unit 40 to a high pressure to be used as the main engine E fuel.

As shown in FIG. 2, the engine fuel supply unit 60 is provided in the first fuel supply line L4 branched from the liquefied evaporation gas supply line L3 to supply liquefied evaporated gas supplied from the mixing unit 40. At least one high pressure pump 61 for pumping at high pressure, and at least one high pressure evaporator 62 for vaporizing the high pressure liquefied evaporated gas pumped by the high pressure pump 61 to the engine E and supplying it to the engine E. .

In the present embodiment, the high pressure pump 61 and the high pressure evaporator 62 may be disposed in the first fuel supply line L4 in parallel, respectively, as shown in FIG. 2. As in the case of 20, when a failure occurs in any one of the high pressure pump 61 and the high pressure evaporator 62, the other high pressure pump 61 and the high pressure evaporator 62 can be used preliminarily.

Meanwhile, in the present embodiment, the engine E to which the vaporized evaporated gas is supplied through the engine fuel supply unit 60 may be a high pressure natural gas injection engine, for example, an ME-GI engine.

This ME-GI engine is an offshore structure such as an LNG carrier for storing and transporting liquefied natural gas in a cryogenic storage tank 10 (in the present specification, the offshore structure includes an LNG carrier, an LNG carrier, such as an LNG RV, and a LNG). It can be installed in a marine plant such as FPSO, LNG FSRU, etc., in which case natural gas is used as fuel, and high pressure gas of about 150 to 400 bara (absolute pressure) depending on the load. Supply pressure is required.

The ME-GI engine will use Boil Off Gas (BOG) as a fuel if additional liquefaction equipment is installed if necessary, depending on changes in gas and fuel oil prices and the degree of regulation of emissions. Alternatively, it is possible to re-liquefy the boil-off gas to the storage tank 10 and choose whether to use heavy fuel oil (HFO). In particular, LNG can be easily used as fuel by evaporating gas when passing through certain waters. As a next-generation environmentally friendly engine, its efficiency is close to 50% and can be used as the main engine of LNG carriers in the future.

In addition, the present embodiment further includes a cooling unit 70 capable of increasing the efficiency of mixing by secondarily cooling the boil-off gas primarily cooled in the heat exchange unit 30 before being supplied to the mixing unit 40.

As shown in FIG. 2, the cooling unit 70 includes a cooler 71 provided at a first fuel supply line L4 connecting between the high pressure pump 61 and the high pressure evaporator 62, and a heat exchange unit. The first cooling line 72 for supplying the cooled evaporated gas branched from the line connecting the 30 and the mixing unit 40 to the cooler 71, and the evaporated gas cooled through the cooler 71 to the heat exchange unit ( 30) and a second cooling line 73 to be connected to be supplied to the line connecting the mixing unit 40.

Hereinafter, the process of re-cooling the boil-off gas through the cooling unit 70 will be described briefly.

As described above, since the lower the temperature of the boil-off gas flowing into the mixing unit 40, the mixing efficiency increases, the first cooling line branching off the line connecting the heat exchange unit 30 and the mixing unit 40 ( 72 is supplied to the cooler 71 through the evaporated gas (hereinafter referred to as 'first evaporated gas') cooled in the heat exchange unit (30).

The first boil-off gas is mutually heat-exchanged with the boil-off liquefied gas (hereinafter referred to as 'first liquefied evaporation gas') which is pumped at a high pressure through the high-pressure pump 61, and the first boil-off gas is connected to the first liquefied gas. By the heat exchange of the lower temperature than before the heat exchange, the lower temperature, the evaporated gas is supplied to the mixing unit 40 through the second cooling line (73).

On the other hand, the re-cooling of the boil-off gas is possible by the temperature difference between the first boil-off gas and the first liquefied evaporation gas. That is, it can be seen that the temperature (approximately -85 ° C) of the first boil-off gas is lower than the temperature (approximately -122 ° C) of the liquefied evaporation gas mixed by the mixing unit 40, and the liquefied evaporation gas is a high pressure pump (61). It is pumped by but is a liquid state, so the pressure change is large, but the temperature change is small, it can be seen that the temperature of the first liquefied evaporation gas (-102 ℃) does not differ significantly compared to the liquefied evaporation gas.

And the present embodiment is provided on the steam line (L5) connecting the storage tank 10 and the compressor 20, the evaporation of the heating by heating the cryogenic evaporation gas supplied to the compressor 20 during the initial operation of the compressor 20 It further includes steam (80) capable of pre-heating the compressor 20 by the gas to prevent brittle damage of the compressor 20.

In addition, the present embodiment, as shown in Figure 2, branched in the line connecting the compressor 20 and the heat exchange unit 30, evaporation to supply the boil-off gas to the auxiliary engine (AE) or gas conversion unit (GCU) It further comprises a gas fuel supply line (90). In addition, reference numeral L6 is a second fuel supply line L6 for supplying a fuel in a liquid state to the auxiliary engine AE.

As described above, in the present embodiment, the heat exchange unit 30, the mixing unit 40, and the cooling unit 70 having a simple structure may be replaced with a reliquefaction system 130 (see FIG. 1), which is expensive and complicated in structure. By the liquefaction of the boil-off gas can be reduced the operating cost, it is easy to use even a novice driver to simplify the process (process) has the advantage of ensuring the reliability.

3 is a flowchart illustrating a reliquefaction method of LNG boil-off gas according to another embodiment of the present invention.

LNG liquefaction method according to another embodiment of the present invention, the step of compressing the boil-off gas generated and discharged in the storage tank 10 (S100), and the compressed boil-off gas compressed in the storage tank ( 10) cooling the compressed boil-off gas by exchanging heat with the boil-off gas (S200), and mixing the boil-off gas cooled in the cooling step with the liquefied natural gas supplied from the storage tank 10 to cool the boil-off gas. Liquefied to supply the liquefied liquefied gas to the storage tank 10 (S300).

First, a step of compressing the boil-off gas generated and discharged from the storage tank 10 is performed.

Then, the compressed boil-off gas is exchanged with the boil-off gas supplied from the storage tank 10 to cool the compressed boil-off gas, which is performed in the heat exchange unit 30. This step is performed to increase the mixing efficiency of the mixing unit 40, that is, the temperature drop of the mixed liquefied evaporation gas, by lowering the temperature of the boil-off gas converted to high temperature in the compression process and supplying it to the mixing unit 40.

Finally, the cooled evaporated gas and the liquefied natural gas supplied from the storage tank 10 are liquefied in the mixing unit 40 to form liquefied evaporation gas, and the liquefied evaporation gas is stored in the storage tank through the liquefied evaporation gas supply line (L3). By supplying to (10), the reliquefaction process is completed.

Meanwhile, the present embodiment may prevent brittle damage of the compressor 20 by heating the boil-off gas supplied from the storage tank 10 to the compressor 20 at the beginning of the operation of the compressor 20, and in the mixing unit 40. After the liquefied liquefied evaporation gas is compressed, the liquefied liquefied evaporation gas may be vaporized and supplied to the engine to use the liquefied liquefied gas as an engine fuel.

In this embodiment, the evaporated gas cooled through the heat exchanger and the liquefied evaporated gas compressed through the high pressure pump 61 may be heat-exchanged with each other to recool the cooled evaporated gas supplied to the mixing unit 40. This recooling process is performed in the cooling unit 70.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Accordingly, such modifications or variations are intended to fall within the scope of the appended claims.

1: Reliquefaction apparatus of LNG boil-off gas 10: Storage tank
20: compressor 30: heat exchange unit
40: mixing unit 50: expansion valve
60: engine fuel supply unit 61: high pressure pump
62: high pressure evaporator 70: cooling unit
71: cooler 72: the first cooling line
73: second cooling line 80: steam
90: boil-off gas fuel supply line AE: auxiliary engine
E: Engine GCU: Gas Conversion Unit
L1: Evaporative Gas Discharge Line L2: LNG Natural Gas Supply Line
L4: First fuel supply line L5: Steam line
L6: second fuel supply line

Claims (14)

A storage tank in which liquefied natural gas (LNG) is stored;
A compressor for compressing the boil-off gas generated and discharged from the storage tank;
A heat exchange unit for cooling the boil-off gas discharged through the compressor by heat-exchanging the boil-off gas supplied from the storage tank to the compressor and the boil-off gas discharged after being compressed through the compressor; And
And a mixing unit configured to liquefy the boil-off gas by mixing the boil-off gas discharged from the heat exchange unit and the liquefied natural gas supplied from the storage tank to each other. The method according to claim 1,
And an expansion valve disposed on a line connecting the storage tank and the mixing unit to lower the pressure of the liquefied natural gas discharged through the mixing unit. The method according to claim 1,
LNG evaporation gas re-liquefaction apparatus further comprises an engine fuel supply unit for vaporizing the liquefied liquefied gas liquefied in the mixing unit to supply to the engine. The method according to claim 3,
At least one high pressure pump for compressing the liquefied evaporation gas liquefied in the engine fuel supply unit; And
Re-liquefying LNG boil-off gas comprising at least one high-pressure evaporator for vaporizing the liquefied liquefied gas compressed in the high pressure pump to supply to the engine. The method of claim 4,
The engine is a LNG liquefied gas re-liquefaction apparatus, characterized in that the marine propulsion ME-GI engine. The method of claim 4,
Reliquefaction of the LNG boil-off gas further comprises a cooling unit for re-cooling the cooled boil-off gas supplied to the mixing unit by mutual heat exchange between the boil-off gas cooled through the heat exchanger and the liquefied evaporation gas compressed through the high pressure pump. Device. The method of claim 6,
The cooling unit,
A cooler provided in a line connecting the high pressure pump and the high pressure evaporator;
A first cooling line which supplies the boil-off gas cooled by branching from the line connecting the heat exchange unit and the mixing unit to the cooler; And
And a second cooling line for connecting the boil-off gas cooled through the cooler to be supplied to a line connecting the heat exchange unit and the mixing unit. The method according to claim 1,
And a boil-off gas fuel supply line branched from a line connecting the compressor and the heat exchange unit to use the boil-off gas discharged through the compressor as fuel. The method according to claim 1,
And a steam provided in a line connecting the storage tank and the compressor to heat the boil-off gas supplied to the compressor during initial operation of the compressor. Compressing the boil-off gas generated and discharged from the storage tank;
Cooling the compressed boil-off gas by exchanging the boil-off gas compressed in the compression step with the boil-off gas supplied from the storage tank; And
Remixing the boil-off LNG and the liquefied natural gas supplied from the storage tank in the cooling step to liquefy the cooled boil-off gas to supply the liquefied liquefied evaporation gas to the storage tank ash Liquefaction method. The method of claim 10,
And heating the boil-off gas supplied from the storage tank to the compressor at the beginning of the compressor. The method of claim 10,
A liquefied evaporation gas compression step of compressing the liquefied evaporation gas liquefied in the mixing unit by a high pressure pump provided between the mixing unit and the engine; And
Vaporizing the liquefied evaporated gas compressed in the liquefied evaporated gas and supplying the liquefied evaporated gas to the engine. The method of claim 12,
The engine is a LNG propulsion gas re-liquefaction method, characterized in that the marine propulsion ME-GI engine. The method of claim 12,
Re-liquefying the LNG boil-off gas further comprises the step of heat-exchanging the boil-off gas cooled through the heat exchanger and the liquefied evaporation gas compressed through the high-pressure pump to re-cool the cooled boil-off gas supplied to the mixing unit .

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