KR102149039B1 - Boil-Off Gas Treatment System and Method for Ship - Google Patents

Abstract

Disclosed are a boil-off gas treatment system for a ship and a method thereof. The boil-off gas treatment system for a ship of the present invention comprises: a boil-off gas supply line provided on a ship and connected to a main engine of the ship from a storage tank in which liquefied gas is stored; a compressor provided in the boil-off gas supply line to compress boil-off gas generated in the storage tank; a re-liquefaction line branching from the boil-off gas supply line downstream of the compressor, connected to the storage tank, re-liquefying the boil-off gas remaining after being supplied as fuel of the main engine, and restoring the re-liquefied boil-off gas in the storage tank; a heat exchanger provided in the re-liquefaction line and cooling all or part of the compressed boil-off gas by heat exchange with uncompressed boil-off gas to be supplied from the storage tank to the compressor; and a branch line branching from the re-liquefaction line, and reducing the pressure of the compressed boil-off gas so that the same is supplied to a power generation engine receiving pressure fuel lower than that of the main engine. The boil-off gas reduced in the branch line is supplied to the power generation engine through the heat exchanger. Therefore, the present invention can increase re-liquefaction performance of the boil-off gas.

Description

Boil-Off Gas Treatment System and Method for Ship}

The present invention relates to a system and method for treating boil-off gas of a ship, and more particularly, supplying boil-off gas generated in a storage tank as fuel for the main engine and power generation engine on board, and re-liquefying the boil-off gas not supplied as fuel. It relates to a system and method for treating boil-off gas of a ship that is stored in a storage tank.

In recent years, the consumption of liquefied natural gas (LNG) and other liquefied gases is increasing rapidly around the world. The liquefied gas obtained by liquefying the gas at a low temperature has an advantage of increasing storage and transfer efficiency because the volume is very small compared to the gas. In addition, liquefied gases, including liquefied natural gas, can remove or reduce air pollutants during the liquefaction process, and thus can be regarded as eco-friendly fuels with little emission of air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by liquefying natural gas containing methane as its main component by cooling it to about -163°C, and has a volume of about 1/600 compared to natural gas. Therefore, 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 -163°C, the liquefied natural gas is sensitive to temperature changes and is easily evaporated. For this reason, the storage tank that stores liquefied natural gas is insulated, but external heat is continuously transferred to the storage tank. Therefore, during the transportation of liquefied natural gas, the liquefied natural gas is continuously evaporated in the storage tank and boiled gas (Boil). -Off Gas, BOG) occurs.

Boil-off gas is a kind of loss and is an important problem in transport efficiency. In addition, if the boil-off gas accumulates in the storage tank, the internal pressure of the tank may increase excessively. Therefore, various methods for treating the boil-off gas generated in the storage tank have been studied. Recently, for the treatment of the boil-off gas, a method of re-liquefying the boil-off gas and returning it to the storage tank, and the boil-off gas as fuel such as a ship's engine. The method of using it as an energy source of the customer is being used.

As a method for re-liquefying the boil-off gas, a method of reliquefying the boil-off gas by heat exchange with the refrigerant by providing a refrigeration cycle using a separate refrigerant, a method of re-liquefying the boil-off gas itself as a refrigerant without a separate refrigerant There is this.

Meanwhile, among engines generally used in ships, gas-fueled engines such as DF engines, X-DF engines, and ME-GI engines are used as engines that can use natural gas as fuel.

DF engine (DFDE, DFGE) consists of four strokes, and adopts the Otto Cycle, which injects natural gas with a relatively low pressure of 5.5 barg into the inlet of the combustion air and compresses it while the piston rises. Are doing.

The X-DF engine consists of two strokes, uses 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 near 300 barg is injected directly into the combustion chamber near the top dead center of the piston.

1 schematically shows a conventional boil-off gas treatment system for ships.

In the conventional boil-off gas treatment system for ships as shown in FIG. 1, when the main engine ME and the power generation engine GE are provided, the boil-off gas discharged from the storage tank T is compressed by the compressor 10 It is supplied as 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 a partial compression process of the compressor 10 is branched from the middle and supplied as fuel of the power generation engine GE.

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

The boil-off gas cooled in the heat exchanger 20 is depressurized by the decompression device 30 and partially re-liquefied, and the re-liquefied liquefied gas and the boil-off gas remaining in a gaseous state are supplied to the gas-liquid separator 40 and phase separated. .

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

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

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

According to an aspect of the present invention for solving the above-described problem, a boil-off gas supply line provided on a ship and connected to the main engine of the ship from a storage tank in which liquefied gas is stored;

A compressor provided in the boil-off gas supply line to compress boil-off gas generated in the storage tank;

A reliquefaction line branched from the boil-off gas supply line downstream of the compressor and connected to the storage tank, and re-liquefying the remaining boil-off gas supplied as fuel of the main engine and storing it in the storage tank;

A heat exchanger provided in the reliquefaction line and cooling all or part of the compressed boil-off gas by heat exchange with uncompressed boil-off gas to be supplied from the storage tank to the compressor; And

A branch line branching from the reliquefaction line and supplying the compressed boil-off gas to a power generation engine receiving a lower pressure fuel than the main engine by decompressing the compressed boil-off gas; and

The boil-off gas reduced by the branch line is supplied to the power generation engine through the heat exchanger.

Preferably, the system further includes an expansion means provided in the branch line to decompress the compressed boil-off gas, and the branch line may be branched from the reliquefaction line at a front end of the heat exchanger.

Preferably, the system comprises: an additional line branched from the reliquefaction line at a rear end of the heat exchanger and connected to a front end of the expansion means of the branch line; And a first valve provided upstream of a confluence point of the additional line in the branch line. And a second valve provided in the additional line.

Preferably, when the ship is anchored, the first valve is opened and the second valve is closed, and the boil-off gas is branched from the front end of the heat exchanger through the branch line and supplied to the power generation engine through the expansion means and heat exchanger. ,

When the ship is operated, the first valve is closed and the second valve is opened to branch the boil-off gas at the rear end of the heat exchanger through the additional line and supply it to the power generation engine.

Preferably, the system includes: a decompression device provided in the reliquefaction line and receiving the boil-off gas cooled by heat exchange in the heat exchanger and reducing the pressure to further cool it; And

A gas-liquid separator for receiving the boil-off gas cooled through the heat exchanger and the decompression device in the re-liquefaction line and separating the gas-liquid;

The liquid separated by the gas-liquid separator is re-stored in the storage tank along the reliquefaction line, and gas may be joined to the uncompressed evaporation gas introduced from the storage tank to the heat exchanger.

Preferably, a heater provided on the branch line to heat the liquefied gas to be supplied to the power generation engine; may further include.

According to another aspect of the present invention, boil-off gas generated in a storage tank provided on a ship to store liquefied gas is compressed with a compressor and supplied to the main engine of the ship, and the remaining evaporation after being supplied to the main engine of the compressed boil-off gas The gas is cooled and reliquefied in a heat exchanger with uncompressed evaporated gas supplied from the storage tank to the compressor,

A portion of the compressed boil-off gas to be reliquefied is branched and decompressed to be supplied to a power generation engine receiving a lower pressure fuel than the main engine,

The boil-off gas decompressed to supply to the power generation engine is supplied to the power generation engine through the heat exchanger,

In the heat exchanger, there is provided a method for treating boil-off gas of a ship, characterized in that the compressed boil-off gas to be reliquefied, the boil-off gas depressurized to be supplied to the power generation engine, and the uncompressed boil-off gas to be supplied to the compressor are heat-exchanged.

Preferably, the boil-off gas to be supplied to the power generation engine may be branched at a front end of the heat exchanger, decompressed through an expansion device, and supplied to the heat exchanger.

Preferably, when the ship is anchored, the boil-off gas is branched from the front end of the heat exchanger to be decompressed and supplied to the power generation engine through a heat exchanger, and when the ship is operated, the boil-off gas is supplied to the power generation engine at the rear end of the heat exchanger. After being branched and decompressed, it can be supplied to the power generation engine through a heat exchanger.

Preferably, the boil-off gas cooled by heat exchange in the heat exchanger may be further cooled under reduced pressure and reliquefied to be re-stored in the storage tank.

Preferably, the flash gas generated from the reliquefied liquefied gas may be separated and joined to the uncompressed evaporation gas introduced from the storage tank to the heat exchanger.

In the system of the present invention, a part of the boil-off gas to be reliquefied is decompressed and supplied to the power generation engine through a heat exchanger, thereby securing additional refrigerant in the heat exchanger, thereby enhancing the cooling performance of the heat exchanger, thereby increasing the re-liquid performance. The flash gas separated from the reliquefied gas is merged into a flow of uncompressed evaporative gas introduced into the heat exchanger, so that the flash gas can be additionally supplied as a refrigerant from the heat exchanger.

In addition, it is possible to select the extraction point of the gas to be supplied to the power generation engine according to the operation situation of the ship, such as when the ship is operating and at anchor, so that the system can be operated more efficiently and flexibly.

In this way, the gas to be reliquefied can be effectively cooled by using the cold heat of the boil-off gas to be supplied to the power generation engine and the cold heat of the flash gas, while the boil-off gas to be supplied to the power generation engine can be heated. Through this, it is possible to increase the energy efficiency of the system and increase the reliquefaction performance of the boil-off gas, and to secure the safety of the ship by effectively treating the boil-off gas.

1 is a schematic diagram of a conventional boil-off gas treatment system.
Figure 2 is a schematic diagram of the boil-off gas treatment system of the ship according to the first embodiment of the present invention.
3 is a schematic diagram of a system for treating boil-off gas 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 object achieved by the implementation 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 a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, in adding reference numerals to elements of each drawing, it should be noted that only the same elements are marked with the same numerals as possible, even if they are indicated on different drawings.

Hereinafter, the ship in the present invention is all types of engines that can use liquefied gas and boil-off gas generated from liquefied gas as fuel for propulsion or power generation engines, or use liquefied gas or boil-off gas as fuel for onboard engines. LNG carriers, liquid petroleum gas carriers, and LNG RV (Regasification Vessel) ships, including LNG Floating Production Storage Offloading (FPSO), and LNG Floating Storage Regasification (FSRU). It does not have propulsion capability like the unit), but may include offshore structures that are floating on the sea.

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

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

FIG. 2 schematically shows the boil-off gas treatment system of the ship according to the first embodiment of the present invention, and FIG. 3 schematically shows the boil-off gas treatment system of the ship according to the second embodiment of the present invention.

As shown in FIGS. 2 and 3, the boil-off gas treatment system of the present embodiments is provided in the ship and connected to the main engine ME of the ship from the storage tank T in which the liquefied gas is stored. ) Is provided, and a compressor 100 for compressing the boil-off gas generated in the storage tank is provided in the boil-off gas supply line.

The boil-off gas generated from the storage tank is introduced into a compressor and compressed. The compressor 100 may be provided as a multi-stage compressor in which a plurality of compressors and intermediate coolers are alternately arranged, and through these in sequence, compressing the boil-off gas to the fuel supply pressure of the main engine.

The compressor compresses the boil-off gas to the fuel supply pressure to the main engine, e.g. 5.5 barg if a DF engine is provided, 15 barg if an X-DF engine is provided, and 300 barg if a ME-GI engine is provided. can do. 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.

According to ship regulations, the compressor that supplies fuel to the engine should be designed with redundancy in case of an emergency. Redundancy design means that when one cannot be used for reasons such as failure or maintenance, the other is replaced. It means designing to be usable. To this end, although only one set of compressors is shown in the drawings of the present embodiments, a plurality of compressors may be provided.

At the downstream of the compressor, the reliquefaction line (RL) is branched from the boil-off gas supply line and is connected to the storage tank, and the boil-off gas not supplied as fuel of the main engine is branched to the reliquefaction line and is re-liquefied and then re-stored into the storage tank. .

A heat exchanger 200 is provided in the reliquefaction line to cool all or part of the boil-off gas compressed by the compressor by heat exchange with the uncompressed boil-off gas to be supplied from the storage tank to the compressor.

Meanwhile, the heat exchanger may be provided with a printed circuit heat exchanger (PCHE) or a direct contact type heat exchanger (DCHE). The boil-off gas to be introduced into the heat exchanger may be introduced into the heat exchanger after removing the lubricating oil mixed in the compression process through an oil filter (not shown).

In the system of the present embodiments, a branch line BL branching from the reliquefaction line is provided to reduce the boil-off gas compressed by the compressor and supply it to the power generation engine GE, which receives lower pressure fuel than the main engine. In particular, the evaporation gas reduced in the branch line BL is configured to be supplied to the power generation engine GE through the heat exchanger 200.

Thus, in the heat exchanger 200 in the system of the first and second embodiments, the compressed boil-off gas to be re-liquefied in the re-liquefaction line RL, and the boil-off gas of the branch line BL depressurized to supply to the power generation engine , Three flows of uncompressed boil-off gas from the boil-off gas supply line GL to be supplied to the compressor are heat-exchanged.

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

The branch line BL may be branched from the reliquefaction line at the front end of the heat exchanger. Since the boil-off gas to be supplied to the power generation engine is branched from the front of the heat exchanger, the flow rate of the boil-off gas to be cooled in the heat exchanger (the boil-off gas to be re-liquefied, Hot BOG) is reduced, so that the boil-off gas to be re-liquefied is more effectively cooled and reliquefied. You can increase the performance.

An expansion means 300 for depressurizing the compressed boil-off gas is provided in the branch line, and a first valve V1 is provided upstream of the expansion means for controlling the opening and closing of the branch line.

The expansion means 300 may be composed of an expansion valve such as an expander or a Joule-Thomson valve for decompressing the compressed evaporated gas. Through decompression, the boil-off gas expands isentropically or adiabatically and is cooled. In this embodiment, in order to supply the power generation engine with low-pressure fuel than the main engine as described above, the evaporative gas reduced through the expansion means is passed through the heat exchanger so that the cold heat of the evaporated gas cooled while being depressurized can be used for reliquefaction. To be supplied.

A heater 500 for heating the liquefied gas to be supplied to the power generation engine is additionally provided in the branch line, and the boil-off gas heated through the heat exchanger may be additionally heated according to the fuel supply condition of the power generation engine to be supplied to the power generation engine.

The amount of boil-off gas supplied to the power generation engine through the branch line may vary depending on the load of the power generation engine and the configuration of the compressor.

As described above, in the present embodiment, since the branch line is provided to use cold heat of the boil-off gas to be supplied to the power generation engine, the re-liquefaction performance can be improved by more effectively cooling the gas to be re-liquefied. In addition, the boil-off gas to be supplied to the power generation engine is heated through a heat exchanger, thereby effectively utilizing the thermal energy of the system.

A reliquefaction unit 400 is provided downstream of the heat exchanger in the reliquefaction line RL. The reliquefaction unit receives the boil-off gas cooled by heat exchange from the heat exchanger and decompresses it for additional cooling, and a gas-liquid separator that receives the boil-off gas cooled through the heat exchanger and the decompression device in the re-liquefaction line and separates the gas and liquid. Includes 420.

The pressure reducing device 410 may be configured as an expander or an expansion valve such as a Joule-Thomson valve for reducing the compressed boil-off gas.

The liquid separated by the gas-liquid separator 420, that is, liquefied gas, is re-stored in a storage tank along the re-liquefaction line RL.

A flash gas line (FL) connected from the gas-liquid separator to the boil-off gas supply line (GL) in front of the heat exchanger is provided, and the gas separated by the gas-liquid separator, that is, the flash gas, is transferred to the uncompressed boil-off gas introduced from the storage tank to the heat exchanger. Combined, it can be supplied to the heat exchanger as a refrigerant.

The system of the second embodiment shown in FIG. 3 is branched from the reliquefaction line RL at the rear end of the heat exchanger 200 in the system of the first embodiment described above, and the expansion means 300 of the branch line BL An additional line (AL) connected to the front end is provided.

The first valve (V1) is provided upstream of the confluence point of the additional line in the branch line, and the second valve (V2) is provided in the additional line to open and close the first valve and the second valve according to the operation situation of the ship. It is possible to supply fuel to the power generation engine from the front or rear end of the heat exchanger.

More specifically, when the ship is anchored, the first valve (V1) is opened and the second valve (V2) is closed to diverge the boil-off gas at the front end of the heat exchanger through the branch line (BL), and the expansion means 300 and heat exchange. It is supplied to the power generation engine (GE) through the machine 200, and when the ship is operated, the first valve (V1) is closed and the second valve (V2) is opened to collect boil-off gas from the rear end of the heat exchanger through the additional line (AL). It can be branched and supplied to the power generation engine through an expansion means and a heat exchanger.

Since there is no or little boil-off gas supplied to the fuel of the main engine in the ship's berth situation, the amount of boil-off gas to be reliquefied through the reliquefaction line increases. Therefore, when the hot BOG flow of the reliquefaction line increases relative to the uncompressed boil-off gas flow (Cold BOG) of the boil-off gas supply line used as the refrigerant of the heat exchanger, the boil-off gas to be reliquefied can be effectively cooled. Therefore, in this case, the amount of hot BOG passing through the heat exchanger can be reduced by opening the first valve to branch the boil-off gas at the front end of the heat exchanger of the reliquefaction line and supply it to the power generation engine through the branch line.

In the operating conditions of the ship, the amount of boil-off gas to be re-liquefied is relatively small because the amount of boil-off gas supplied as fuel of the main engine is relatively small, the amount of cold BOG in the heat exchanger is large, and the amount of hot BOG is small. Therefore, in this case, the hot BOG can be sufficiently cooled by the cold heat of the uncompressed evaporative gas Cold BOG, so the first valve is closed and the second valve is opened to branch the evaporated gas through an additional line at the rear end of the heat exchanger and supply it to the power generation engine. have.

As described above, the system of the second embodiment can select an extraction point of the gas to be supplied to the power generation engine according to the operating condition of the ship, so that the system can be operated more efficiently and flexibly.

As described above, in the present embodiments, in a ship equipped with a main engine and a power generation engine receiving lower pressure fuel than that, the boil-off gas generated from the storage tank in which liquefied gas is stored is compressed with a compressor and supplied to the main engine, Of the evaporated evaporated gas, the evaporated gas not supplied to the main engine is exchanged with the uncompressed evaporated gas to be supplied to the compressor, cooled, and reliquefied, branching off part of the compressed evaporated gas to be reliquefied and decompressed to supply lower pressure fuel than the main engine. It is supplied to the receiving power generation engine, but the cooled boil-off gas is supplied to a heat exchanger while decompressed to supply it to the power generation engine, and the cold heat is recovered before being supplied to the power generation engine.

Through this, the gas to be reliquefied can be more effectively cooled by using the cold heat of the boil-off gas to be supplied to the power generation engine, and the re-liquefaction performance of the boil-off gas can be improved.

It is obvious to those of ordinary skill in the art that the present invention is not limited to the above embodiments, and can be implemented with various modifications or variations within the scope of the technical gist of the present invention. I did it.

T: storage tank
ME: main engine
GE: Power generation engine
GL: Boil-off gas supply line
RL: Reliquefaction line
BL: branch line
FL: flash gas line
100: compressor
200: heat exchanger
300: expansion means
400: reliquefaction unit
410: pressure reducing device
420: gas-liquid separator
500: heater
V1: first valve
V2: second valve

Claims (11)

A boil-off gas supply line provided on the ship and connected to the main engine of the ship from the storage tank in which the liquefied gas is stored;
A compressor provided in the boil-off gas supply line to compress boil-off gas generated in the storage tank;
A reliquefaction line branched from the boil-off gas supply line downstream of the compressor and connected to the storage tank, and re-liquefying the remaining boil-off gas supplied as fuel of the main engine and storing it in the storage tank;
A heat exchanger provided in the reliquefaction line and cooling all or part of the compressed boil-off gas by heat exchange with uncompressed boil-off gas to be supplied from the storage tank to the compressor; And
A branch line branching from the reliquefaction line and supplying the compressed boil-off gas to a power generation engine receiving a lower pressure fuel than the main engine by decompressing the compressed boil-off gas; and
The boil-off gas treatment system of a ship, characterized in that the boil-off gas reduced in the branch line is supplied to the power generation engine through the heat exchanger. The method of claim 1,
Expansion means provided in the branch line to decompress the compressed boil-off gas; further comprising,
The branch line is a boil-off gas treatment system of a ship, characterized in that branching from the reliquefaction line at the front end of the heat exchanger. The method of claim 2,
An additional line branched from the reliquefaction line at a rear end of the heat exchanger and connected to a front end of the expansion means of the branch line; And
A first valve provided upstream of a confluence point of the additional line in the branch line; And
Boil-off gas treatment system of the ship further comprising a second valve provided in the additional line. The method of claim 3,
When the ship is anchored, the first valve is opened and the second valve is closed to branch the boil-off gas at the front end of the heat exchanger through the branch line, and supply the boil-off gas to the power generation engine through the expansion means and the heat exchanger,
During the operation of the ship, the first valve is closed and the second valve is opened, and the boil-off gas is branched from the rear end of the heat exchanger through the additional line and supplied to the power generation engine. The method of claim 3,
A decompression device provided in the reliquefaction line and receiving the boil-off gas cooled by heat exchange in the heat exchanger and reducing the pressure to further cool it; And
A gas-liquid separator for receiving the boil-off gas cooled through the heat exchanger and the decompression device in the re-liquefaction line and separating the gas-liquid;
The liquid separated by the gas-liquid separator is re-stored in the storage tank along the reliquefaction line, and the gas is joined to the uncompressed boil-off gas introduced from the storage tank to the heat exchanger. . The method of claim 3,
A heater provided on the branch line to heat the liquefied gas to be supplied to the power generation engine. The boil-off gas generated in the storage tank provided on the ship to store liquefied gas is compressed with a compressor and supplied to the main engine of the ship, and the remaining boil-off gas is supplied to the main engine of the compressed boil-off gas from the storage tank to the compressor. It is cooled and reliquefied in a heat exchanger with uncompressed evaporative gas supplied to
A portion of the compressed boil-off gas to be reliquefied is branched and decompressed to be supplied to a power generation engine receiving a lower pressure fuel than the main engine,
The boil-off gas decompressed to supply to the power generation engine is supplied to the power generation engine through the heat exchanger,
In the heat exchanger, the compressed boil-off gas to be reliquefied, the boil-off gas depressurized to be supplied to the power generation engine, and the uncompressed boil-off gas to be supplied to the compressor are heat-exchanged. The method of claim 7,
The boil-off gas to be supplied to the power generation engine is branched at a front end of the heat exchanger, depressurized through an expansion means, and supplied to the heat exchanger. The method of claim 7,
When the ship is anchored, the boil-off gas is branched at the front end of the heat exchanger, depressurized, and then supplied to the power generation engine through a heat exchanger,
During operation of the ship, the boil-off gas is branched at a rear end of the heat exchanger, and the boil-off gas is supplied to the power generation engine through a heat exchanger after decompression. The method according to claim 8 or 9,
The boil-off gas cooled by heat exchange in the heat exchanger is further cooled under reduced pressure and re-liquefied to be re-stored in the storage tank. The method of claim 10,
Flash gas generated from the re-liquefied liquefied gas is separated and merged with uncompressed boil-off gas introduced from the storage tank to the heat exchanger.

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