KR102291927B1 - Boil-Off Gas Treatment System and Method for Ship - Google Patents

Abstract

A system and method for treating BOG on a ship are disclosed. The BOG treatment system for a ship of the present invention includes: a compressor for receiving BOG generated from liquefied gas stored in a storage tank of the ship and compressing the BOG; a heat exchanger receiving all or part of the BOG compressed in the compressor and cooling the BOG with heat exchange with the uncompressed BOG to be supplied to the compressor; a pressure reducing device for further cooling by reducing the pressure of the boil-off gas cooled in the heat exchanger; a first gas-liquid separator for receiving the boil-off gas decompressed in the decompression device and separating the gas-liquid; and a second gas-liquid separator for receiving the liquefied gas separated from the first gas-liquid separator and gas-liquid separation, wherein the second gas-liquid separator has a lower internal pressure than the first gas-liquid separator, It is characterized in that the liquefied gas is supplied to the second gas-liquid separator to separate the flash gas generated and supplied to the uncompressed boil-off gas flow introduced into the heat exchanger.

Description

Boil-Off Gas Treatment System and Method for Ship

The present invention relates to a system and method for treating BOG on a ship, and more particularly, to a BOG treatment system and method for a ship capable of re-liquefying BOG generated in a storage tank with the cooling heat of BOG itself. will be.

In recent years, the consumption of liquefied gas such as liquefied natural gas (LNG) is rapidly increasing worldwide. The liquefied gas obtained by liquefying the gas at a low temperature has the advantage of increasing the storage and transport efficiency because the volume is very small compared to the gas. In addition, since liquefied gas including liquefied natural gas can remove or reduce air pollutants during the liquefaction process, it can be viewed as an eco-friendly fuel that emits less air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by cooling and liquefying natural gas containing methane to about -162°C, and has a volume of about 1/600 compared to natural gas. Accordingly, when the natural gas is liquefied and transported, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -162 ℃ atmospheric pressure, liquefied natural gas is sensitive to temperature changes and evaporates easily. For this reason, the storage tank that stores the liquefied natural gas is insulated, but external heat is continuously transferred to the storage tank. -Off Gas, BOG) occurs.

BOG is a type of loss and is an important problem in transport efficiency. In addition, when the boil-off gas is accumulated in the storage tank, the internal pressure of the tank may increase excessively, and in severe cases, there is a risk of damage to the tank. Therefore, various methods for treating BOG generated in the storage tank are being studied. Recently, for the treatment of BOG, a method of re-liquefying BOG and returning it to the storage tank, and turning BOG into fuel such as engines of ships, etc. A method of using it as an energy source of a consumer is being used.

As a method for re-liquefying BOG, a method of re-liquefying BOG by heat exchange with a refrigerant by having a refrigeration cycle using a separate refrigerant, a method of re-liquefying BOG itself as a refrigerant without a separate refrigerant, etc. There is this.

On the other hand, among engines generally used in ships, there are gas fuel engines such as DF engines, X-DF engines, and ME-GI engines as engines capable of using natural gas as fuel.

The DF engine (DFDE, DFGE) is composed of 4 strokes, and adopts an Otto Cycle that injects natural gas with a relatively low pressure of about 5.5 barg into the combustion air inlet and compresses it as the piston rises. are doing

The X-DF engine is composed of two strokes, uses about 15 barg of natural gas as fuel, and adopts an auto cycle.

The ME-GI engine is composed of two strokes and adopts a diesel cycle in which high-pressure natural gas of around 300 barg is directly injected into the combustion chamber near the top dead center of the piston.

1 schematically shows a conventional BOG treatment system for ships.

As shown in FIG. 1 , in the conventional BOG treatment system for ships, when the main engine ME and the power generation engine GE are provided, the BOG discharged from the storage tank T is compressed in the compressor 10 and It is supplied as the fuel of the main engine, and when the fuel supply pressure of the power generation engine is lower than that of the main engine, the boil-off gas that has undergone some compression of the compressor 10 is branched in the middle and supplied as the fuel of the power generation engine (GE).

Of the BOG supplied to the compressor 10, the remaining BOG is supplied as fuel for the main engine and the power generation engine, and the remaining BOG is supplied to the heat exchanger 20 and cooled through heat exchange with the BOG discharged from the storage tank T. .

BOG cooled in the heat exchanger 20 is decompressed by the decompression device 30 and partially reliquefied, and the reliquefied liquefied gas and BOG remaining in a gaseous state are supplied to the gas-liquid separator 40 and phase separated. .

The liquefied gas separated in the gas-liquid separator 40 is supplied to the storage tank T and stored again, and the gaseous BOG separated by the gas-liquid separator 40 joins the BOG discharged from the storage tank T. and is introduced into the heat exchanger 20 as a refrigerant.

In this way, the BOG is reliquefied by using BOG itself as a refrigerant without a separate refrigerant. The BOG compressed by the compressor is cooled by heat exchange with BOG before being compressed by the compressor, and then expanded by the JT valve, etc. The present applicant named the system for re-liquefying a part of the boil-off gas (PRS) (Partial Re-liquefaction System).

The present invention is further intended to propose a system capable of improving the PRS to more effectively cool the boil-off gas to increase the reliquefaction performance and to process the boil-off gas.

According to an aspect of the present invention for solving the above problems, a compressor for receiving and compressing boil-off gas generated from liquefied gas stored in a storage tank of a ship;

a heat exchanger receiving all or part of the BOG compressed in the compressor and cooling the BOG with heat exchange with the uncompressed BOG to be supplied to the compressor;

a pressure reducing device for further cooling by reducing the pressure of the boil-off gas cooled in the heat exchanger;

a first gas-liquid separator for receiving the boil-off gas decompressed in the decompression device and separating the gas-liquid;

a second gas-liquid separator for receiving the liquefied gas separated from the first gas-liquid separator and separating the gas-liquid;

The second gas-liquid separator has a lower internal pressure than the first gas-liquid separator, and the liquefied gas separated in the first gas-liquid separator is supplied to the second gas-liquid separator to separate the flash gas generated and introduced into the heat exchanger. There is provided a boil-off gas treatment system for a ship, characterized in that it is supplied as a compressed boil-off gas flow.

Preferably, the flash gas separated in the first gas-liquid separator is supplied to the uncompressed boil-off gas flow introduced into the heat exchanger, and the first gas-liquid separator decompresses the flash gas to be separated from the first gas-liquid separator and introduced into the heat exchanger. Expansion means; may further include.

Preferably, it further comprises a second expansion means for depressurizing the flash gas separated from the second gas-liquid separator to be introduced into the heat exchanger, wherein the liquefied gas separated in the second gas-liquid separator is to be stored back into the storage tank. can

Preferably, the first and second expansion means may be an expander or a Joule Thompson valve for cooling by reducing the pressure of the flash gas.

Preferably, in the compressor, the boil-off gas is compressed to a fuel supply pressure of a main engine on board, the boil-off gas compressed in the compressor is supplied to the main engine, and the boil-off gas compressed through a part of the compressor is It may be supplied to a power generation engine that is supplied with a lower pressure fuel than the main engine.

Preferably, the internal pressure of the first gas-liquid separator may be 5 to 10 barg, and the internal pressure of the second gas-liquid separator may be 2 to 5 barg.

According to another aspect of the present invention, the boil-off gas generated from the storage tank in which the liquefied gas is stored in the ship is compressed with a compressor,

All or part of the BOG compressed in the compressor is cooled by heat exchange with the uncompressed BOG to be introduced into the compressor by heat exchange, and further cooled by reduced pressure,

gas-liquid separation of the depressurized boil-off gas in a first gas-liquid separator,

The liquefied gas separated in the first gas-liquid separator is supplied to a second gas-liquid separator having a lower internal pressure than the first gas-liquid separator, and the uncompressed evaporation is introduced into the heat exchanger by separating the flash gas generated from the liquefied gas. There is provided a method for treating boil-off gas of a ship, characterized in that it is supplied as a gas stream.

Preferably, the flash gas separated in the first gas-liquid separator may be reduced in pressure and supplied to the uncompressed BOG introduced into the heat exchanger.

Preferably, the flash gas separated in the second gas-liquid separator is reduced in pressure and supplied to the uncompressed BOG to be introduced into the heat exchanger, and the liquefied gas separated in the second gas-liquid separator is stored again in the storage tank. can be

Preferably, the flash gas separated in the first and second gas-liquid separators may be reduced in pressure by an expander or a Joule Thompson valve, cooled, and supplied as an uncompressed BOG to be introduced into the heat exchanger.

Preferably, in the compressor, the boil-off gas is compressed to a fuel supply pressure of a main engine on board, the boil-off gas compressed in the compressor is supplied to the main engine, and the boil-off gas compressed through a part of the compressor is It may be supplied to a power generation engine that is supplied with a lower pressure fuel than the main engine.

In the system of the present invention, the first and second gas-liquid separators are configured and the flash gas separated from the gas-liquid separators is supplied as an uncompressed BOG flow introduced into the heat exchanger, thereby lowering the temperature of the refrigerant introduced into the heat exchanger and reducing the refrigerant flow rate. It is possible to increase the cooling performance of the heat exchanger by increasing it.

In this way, the reliquefaction performance can be improved by effectively cooling the gas to be reliquefied, and the safety of the vessel can be secured by effectively treating the boil-off gas.

1 is a schematic diagram of a conventional BOG treatment system.
2 is a schematic diagram of a BOG treatment system for a ship according to a first embodiment of the present invention.
3 is a schematic diagram of a BOG treatment system for a ship according to a second embodiment of the present invention.

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

Hereinafter, the configuration and operation of the preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, it should be noted that, in adding reference numerals to the components of each drawing, the same components are marked with the same reference numerals as much as possible even though they are displayed on different drawings.

Hereinafter, the ship in the present invention includes liquefied gas and any type of engine that can use BOG generated from liquefied gas as fuel for propulsion or power generation engines, or using liquefied gas or BOG as fuel for inboard engines. ships with self-propelled capabilities such as LNG carriers, liquefied petroleum gas carriers, and LNG RVs (Regasification Vessels), as well as LNG FPSO (Floating Production Storage Offloading) and LNG FSRU (Floating Storage Regasification). Unit) may include offshore structures that do not have propulsion capabilities but are floating in the sea.

In addition, in the present invention, the liquefied gas may include all kinds of liquefied gas that can be transported by liquefying the gas at a low temperature, generate boil-off gas in a stored state, and can be used as a fuel for engines and the like. Such liquefied gas is, for example, liquefied petrochemicals such as LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), Liquefied Ethylene Gas, Liquefied Propylene Gas, etc. It may be gas. However, in the embodiments to be described later, an example in which LPG, which is one of representative liquefied gases, is applied will be described.

Meanwhile, the fluid flowing through each line of the present embodiments may be in any one of a liquid state, a gas-liquid mixed state, a gaseous state, and a supercritical fluid state, depending on the operating conditions of the system.

FIG. 2 schematically shows a BOG treatment system for a ship according to a first embodiment of the present invention, and FIG. 3 schematically shows a BOG treatment system for a ship according to a second exemplary embodiment of the present invention.

The BOG treatment system of the first and second embodiments of the present invention shown in FIGS. 2 and 3 is a compressor 100 that receives and compresses BOG generated from liquefied gas stored in a storage tank (T) of a ship; The heat exchanger 200 receives all or part of the BOG compressed from the compressor and cools it by heat exchange with the uncompressed BOG to be supplied to the compressor, and a decompression device for additional cooling by decompressing the BOG cooled in the heat exchanger ( 300), a first gas-liquid separator 400 for gas-liquid separation by receiving the boil-off gas decompressed in the decompression device, and a second gas-liquid separator 500 for gas-liquid separation by receiving the liquefied gas separated from the first gas-liquid separator .

In the system of the present embodiments, as shown in FIGS. 2 and 3 , a boil-off gas supply line GL is provided from the storage tank T to the ship's main engine ME, and the boil-off gas supply line is provided at the rear end of the compressor. The reliquefaction line (RL) is branched from and connected to the storage tank. A compressor 100 is provided in the boil-off gas supply line GL to compress the boil-off gas, and along the reliquefaction line RL, the heat exchanger 200 , the pressure reducing device 300 , the first and second gas-liquid separators 400 , 500) is provided to re-liquefy the boil-off gas that is not supplied as fuel for the main engine, etc., and re-storage it in a storage tank.

In particular, in the first and second gas-liquid separators 400 and 500, gas, that is, flash gas is separated from the reliquefied liquefied gas and supplied as an uncompressed BOG flow at the front end of the heat exchanger, so that the refrigerant flow rate of the heat exchanger using the flash gas can increase and the reliquefaction performance can be improved.

To this end, in the first gas-liquid separator 400 to the front end of the heat exchanger 200 of the boil-off gas supply line GL, the first flash gas line FL1 and the second gas-liquid separator 500 to the front end of the heat exchanger of the boil-off gas supply line The second flash gas lines FL2 are respectively connected to each other.

In addition, the second gas-liquid separator 500 maintains an internal pressure lower than that of the first gas-liquid separator 400, and the liquefied gas separated in the first gas-liquid separator has a lower internal pressure than the first gas-liquid separator. While being supplied to the separator, the flash gas generated from the liquefied gas is separated again and supplied as an uncompressed BOG to be introduced into the heat exchanger.

However, since the flash gas as a gas and the liquefied gas as a liquid may not be 100% phase-separated even through each gas-liquid separator, the separated liquefied gas may include an unseparated flash gas.

For example, the internal pressure of the first gas-liquid separator 400 is maintained at 5 to 10 barg, more preferably around 7 barg, and the internal pressure of the second gas-liquid separator 500 is 2 to 5 barg, more preferably can be maintained at around 3 barg. As such, when the pressure of the second gas-liquid separator is lower, flash gas may be generated from the liquefied gas as it is introduced from the first gas-liquid separator having a higher pressure to the second gas-liquid separator, and the second gas-liquid separator separates it and separates the second flash It is supplied to the refrigerant flow of the heat exchanger through a gas line.

The flash gas separated in the first gas-liquid separator is also supplied as an uncompressed boil-off gas flow introduced into the heat exchanger through the first flash gas line.

A first expansion means 450 for decompressing the flash gas to be separated from the first gas-liquid separator 400 and introduced into the heat exchanger is provided in the first flash gas line FL1 . The first expansion means may be an expander or a Joule Thompson valve for depressurizing the flash gas, and the flash gas is cooled and supplied to the front end of the heat exchanger through decompression.

As such, by supplying the flash gas separated in the first and second gas-liquid separators to the front end of the heat exchanger of the boil-off gas supply line, the flow rate of the uncompressed boil-off gas flow (Cold BOG) introduced into the heat exchanger can be increased, and the first expansion By supplying the decompressed and cooled flash gas through the means to the flow of uncompressed BOG to be introduced from the storage tank to the heat exchanger, the temperature of the Cold BOG can also be lowered, thereby increasing the cooling performance of the heat exchanger and the re-liquefaction performance.

Looking at the BOG treatment method in the present embodiments as a whole, BOG generated in the storage tank T is introduced into the compressor 100 and compressed. BOG in the compressor is compressed by the fuel supply pressure of the onboard main engine (ME), and the BOG compressed in the compressor is supplied to the main engine.

The compressor 100 compresses the boil-off gas to a fuel supply pressure to the main engine, for example, 5.5 barg when a DF engine is provided, 15 barg when an X-DF engine is provided, and 300 when a ME-GI engine is provided. It can be compressed in barg. The compressor 100 may be provided as a multi-stage compressor in which a plurality of compressors and an intermediate cooler are alternately disposed and sequentially pass through them to compress the boil-off gas to the fuel supply pressure of the main engine, and the number of compressors and intermediate coolers constituting the multi-stage compressor can be changed according to the fuel supply pressure of the main engine.

BOG compressed through a part of the compressor may be supplied to the power generation engine (GE) that is supplied with a lower pressure fuel than the main engine.

For example, the main engine is an ME-GI engine, and the power generation engine supplied with a lower pressure fuel is a DFGE (Dual Fuel Generator Engine), TFGE (Triple Fuel Generator Engine), or a medium pressure engine supplied with fuel at a lower pressure than that of a ME-GI engine. can be configured as

BOG compressed through some stages of the compressor may be additionally heated by the heater 600 according to the fuel supply condition of the power generation engine and supplied to the power generation engine.

According to ship regulations, the compressor that supplies fuel to the engine must be designed for redundancy in preparation for emergency situations. It is designed to be usable. For this purpose, although only one set of compressors is shown in the drawings of the present embodiments, a plurality of compressors may be provided.

Downstream of the compressor 100, the reliquefaction line (RL) is branched from the boil-off gas supply line (GL) and is connected to the storage tank (T), and BOG that is not supplied as fuel of the main engine is branched into the reliquefaction line After reliquefaction, it is stored again in a storage tank.

The reliquefaction line RL is provided with a heat exchanger 200 for cooling the compressed BOG through heat exchange with the uncompressed BOG of the BOG supply line, and a decompression device 300 for decompressing the cooled BOG through the heat exchanger. .

The pressure reducing device 300 may be configured as an expander for reducing the compressed boil-off gas or an expansion valve such as a Joule-Thompson valve. Through decompression, the boil-off gas is adiabatically expanded and further cooled.

BOG further cooled through the decompression device is introduced into the first gas-liquid separator 400, and the liquid separated in the first gas-liquid separator is introduced into the second gas-liquid separator 500 along the reliquefaction line RL, and separated The gas, that is, the flash gas, is merged into the uncompressed BOG flow of the BOG supply line along the first flash gas line FL1 through the first expansion means 450 and is introduced as a refrigerant of the heat exchanger.

The liquid separated in the first gas-liquid separator may be partially phase-changed into a gaseous state while being introduced into the second gas-liquid separator 500 having a lower internal pressure. The second gas-liquid separator separates the phase-changed flash gas into a gaseous state and supplies it as an uncompressed boil-off gas flow of the boil-off gas supply line along the second flash gas line FL2. The liquefied gas separated in the second gas-liquid separator may be supplied to a storage tank along a re-liquefaction line and stored again.

As in the system of the second embodiment, the flash gas separated in the first gas-liquid separator is decompressed and supplied to the boil-off gas supply line, and the flash gas is reduced in the second gas-liquid separator and supplied to the boil-off gas supply line, which is introduced into the heat exchanger. By lowering the temperature of the cold BOG and increasing the refrigerant flow, it was confirmed that the reliquefaction performance could be improved by about 13% compared to the existing system as a result of the simulation test.

In the system of the second embodiment shown in FIG. 3, a second expansion means 550 for decompressing the flash gas to be separated from the second gas-liquid separator and introduced into the heat exchanger is additionally provided. That is, as in the system of the first embodiment described above, the first expansion means 450 is disposed on the first flash gas line FL1 , and the second flash gas is further connected from the second gas-liquid separator to the boil-off gas supply line. The second expansion means 550 is also provided in the line FL2, and the flash gas separated in the second gas-liquid separator is also cooled under reduced pressure to be introduced into the heat exchanger.

The second expansion means 550 may be an expander or a Joule Thompson valve for cooling the flash gas by reducing the pressure.

The temperature of cold BOG introduced into the heat exchanger by cooling the flash gas separated in the first and second gas-liquid separators by reducing the pressure through the first and second expansion means, respectively, and supplying it to the front end of the heat exchanger of the boil-off gas supply line. can be further lowered, the refrigerant flow rate can be increased, and in the case of this embodiment, it was confirmed that the reliquefaction performance can be improved by about 14% compared to the existing system as a result of the simulation test.

As shown in FIG. 3 , other configurations are similar to those of the above-described first embodiment, and thus a redundant description will be omitted.

As described above, in the present embodiments, the BOG generated from the storage tank in which the liquefied gas is stored in the ship is supplied to the compressor, compressed by the fuel supply pressure of the inboard main engine, and supplied as fuel among the BOG compressed in the compressor. BOG that is not discharged is cooled by heat exchange with uncompressed BOG to be introduced into the compressor by heat exchange, further cooled by reduced pressure, and reliquefied by gas-liquid separation.

In particular, by configuring the first and second gas-liquid separators, the internal pressure of the second gas-liquid separator is maintained lower than that of the first gas-liquid separator, and the flash gas is separated from the first and second gas-liquid separators to introduce uncompressed BOG into the heat exchanger. By supplying the flow, it is possible to lower the temperature of the refrigerant introduced into the heat exchanger and increase the flow rate of the refrigerant.

In this way, the gas-liquid separator is configured in two stages, and the separated flash gas is supplied under reduced pressure, thereby increasing the amount of flash gas supplied to the uncompressed BOG stream and lowering the temperature to improve the cooling effect of the heat exchanger.

Through this, the gas to be reliquefied in the heat exchanger can be effectively cooled, and reliquefaction performance can be improved.

The present invention is not limited to the above embodiments, and it is apparent to those of ordinary skill in the art that the present invention can be implemented with various modifications or variations without departing from the technical gist of the present invention. did it

T : storage tank
ME: main engine
GE: power generation engine
GL: BOG supply line
RL: reliquefaction line
FL1: first flash gas line
FL2: second flash gas line
100: compressor
200: heat exchanger
300: pressure reducing device
400: first gas-liquid separator
450: first expansion means
500: second gas-liquid separator
550: second expansion means
600: heater

Claims (11)

a compressor for receiving and compressing boil-off gas generated from liquefied gas stored in a storage tank of a ship;
a heat exchanger receiving all or a portion of the BOG compressed in the compressor and cooling the BOG with heat exchange with the uncompressed BOG to be supplied to the compressor;
a pressure reducing device for further cooling by reducing the pressure of the boil-off gas cooled in the heat exchanger;
a first gas-liquid separator for receiving the boil-off gas decompressed in the decompression device and separating the gas-liquid;
a second gas-liquid separator for receiving the liquefied gas separated from the first gas-liquid separator and for gas-liquid separation;
a first expansion means for decompressing the flash gas separated from the first gas-liquid separator and introduced into the heat exchanger; and
and a second expansion means for decompressing the flash gas separated from the second gas-liquid separator to be introduced into the heat exchanger;
The flash gas separated in the first gas-liquid separator is supplied to the uncompressed BOG introduced into the heat exchanger, and the liquefied gas separated in the first gas-liquid separator has a lower internal pressure than the first gas-liquid separator. By separating the flash gas generated by phase change while being supplied to the gas-liquid separator and supplying it to the uncompressed boil-off gas flow introduced into the heat exchanger,
BOG treatment system of a ship, characterized in that by reducing the pressure of the flash gas separated from the gas-liquid separator composed of two stages and supplying it to the uncompressed BOG stream to lower the temperature of the refrigerant introduced into the heat exchanger and increase the flow rate of the refrigerant. delete The method of claim 1,
The liquefied gas separated in the second gas-liquid separator is re-stored in the storage tank. 4. The method of claim 3,
The first and second expansion means are an expander or Joule Thompson valve for cooling the flash gas by reducing the pressure. 4. The method of claim 3,
In the compressor, the boil-off gas is compressed to the fuel supply pressure of the inboard main engine,
The boil-off gas compressed in the compressor is supplied to the main engine, and the boil-off gas compressed through a part of the compressor is supplied to a power generation engine receiving a lower pressure fuel than the main engine. processing system. 6. The method of claim 5,
The internal pressure of the first gas-liquid separator is 5 to 10 barg, and the internal pressure of the second gas-liquid separator is 2 to 5 barg. Compresses the boil-off gas generated from the storage tank where the liquefied gas is stored in the ship with a compressor,
All or part of the BOG compressed in the compressor is cooled by heat exchange in a heat exchanger with the uncompressed BOG to be introduced into the compressor, and further cooled by reduced pressure,
gas-liquid separation of the decompressed boil-off gas in a first gas-liquid separator,
The liquefied gas separated in the first gas-liquid separator is supplied to a second gas-liquid separator having a lower internal pressure than the first gas-liquid separator, and the uncompressed evaporation is introduced into the heat exchanger by separating the flash gas generated from the liquefied gas. supplied as a gas stream,
The flash gas separated in the first gas-liquid separator is supplied to the uncompressed BOG introduced into the heat exchanger, and the liquefied gas separated in the first gas-liquid separator has a lower internal pressure than the first gas-liquid separator. By separating the flash gas generated by phase change while being supplied to the gas-liquid separator and supplying it to the uncompressed boil-off gas flow introduced into the heat exchanger,
BOG treatment method of a ship, characterized in that the pressure of the flash gas separated from the gas-liquid separator composed of two stages is reduced and supplied as the uncompressed BOG flow to lower the temperature of the refrigerant introduced into the heat exchanger and increase the flow rate of the refrigerant. delete 8. The method of claim 7,
The liquefied gas separated in the second gas-liquid separator is re-stored in the storage tank. 10. The method of claim 9,
The flash gas separated in the first and second gas-liquid separators is decompressed by an expander or a Joule Thompson valve, cooled and supplied as an uncompressed BOG to be introduced into the heat exchanger. Way. 11. The method of claim 10,
In the compressor, the boil-off gas is compressed to the fuel supply pressure of the inboard main engine,
The boil-off gas compressed in the compressor is supplied to the main engine, and the boil-off gas compressed through a part of the compressor is supplied to a power generation engine receiving a lower pressure fuel than the main engine. processing method.

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