KR20210033093A - Boil-Off Gas Treatment System and Method for Ship - Google Patents

Abstract

Disclosed are a boil-off gas treatment system for a vessel and a method thereof. The boil-off gas treatment system for a vessel comprises: a boil-off gas supply line connected from a storage tank provided in a vessel to store liquefied gas to a main engine of the vessel; a compressor provided on the boil-off gas supply line and receiving boil-off gas produced from the liquefied gas to compress the same with fuel supply pressure for the main engine; a reliquefaction line branched from the boil-off gas supply line in the downstream of the compressor to be connected to the storage tank, and cooling compressed gas, which is not supplied as fuel for the main engine, through a heat exchange with non-compressed boil-off gas to be supplied to the compressor, and reliquefying the compressed gas to restore the gas in the storage tank; a boosting compressor additionally compressing the compressed gas branched to the reliquefaction line; a heat exchanger provided on the reliquefaction line and cooling the compressed gas, which has been additionally compressed by the boosting compressor, through a heat exchange with the non-compressed boil-off gas to be supplied from the storage tank to the compressor along the boil-off gas supply line; and a refrigerant replenishment line branched from the boil-off gas supply line in the downstream of the compressor to be connected to a front or middle end of the heat exchanger of the boil-off gas supply line. The fuel supply pressure of the main engine is lower than critical pressure of methane, and the compressed gas branched to the reliquefaction line is cooled after being additionally compressed by the boosting compressor to exceed the critical pressure.

Description

Boil-Off Gas Treatment System and Method for Ship}

The present invention relates to a boil-off gas treatment system and method for re-liquefying boil-off gas (BOG) generated from liquefied gas stored in a ship with cold heat of the boil-off gas itself.

In recent years, the consumption of liquefied natural gas (LNG) and other liquefied gas 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 transport 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 -162°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 -162°C at normal pressure, liquefied natural gas is easily evaporated because it is sensitive to temperature changes. 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 transportation efficiency. In addition, if the boil-off gas accumulates in the storage tank, the internal pressure of the tank may increase excessively, and in extreme cases, there is a risk of damage to the tank. 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 to fuel the engine of a ship, etc. 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 re-liquefying 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, etc. There is this.

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

DFDE is composed of four strokes, and adopts an Otto Cycle in which natural gas with a relatively low pressure of 5.5 bar is injected into the inlet of the combustion air and compressed while the piston is raised.

The X-DF engine consists of two strokes, uses 12 to 16 bar of natural gas as fuel, and adopts an auto cycle.

The ME-GI engine is composed of two strokes and employs a diesel cycle in which high-pressure natural gas near 300 bar 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 a ship.

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 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 for phase separation. .

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 re-liquefied 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, cooled, and then expanded 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 improve 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 connected to the main engine of the ship from a storage tank provided on a ship to store liquefied gas;

A compressor provided in the boil-off gas supply line and receiving boil-off gas generated from the liquefied gas and compressing it at a fuel supply pressure of the main engine;

At the downstream of the compressor, it is branched from the boil-off gas supply line and connected to the storage tank, and the compressed gas not supplied as fuel of the main engine is cooled by heat exchange with the uncompressed boil-off gas to be supplied to the compressor and re-liquefied. A reliquefaction line for restoring into a storage tank;

A boosting compressor further compressing the compressed gas branched into the reliquefaction line;

A heat exchanger for cooling the compressed gas further compressed by the boosting compressor through heat exchange with the uncompressed boil-off gas to be supplied to the compressor from the storage tank along the boil-off gas supply line; And

A refrigerant supplement line branched from the boil-off gas supply line downstream of the compressor and connected to a front end or an intermediate end of the heat exchanger of the boil-off gas supply line; and

The fuel supply pressure of the main engine is lower than the critical pressure of methane, and the compressed gas branched to the reliquefaction line is further compressed by the boosting compressor above the critical pressure and then cooled. Is provided.

Preferably, an expansion device that is provided in the refrigerant supplement line and expands and cools the compressed gas branched to the refrigerant supplement line by decompressing; further comprising, wherein the compressed gas branched to the refrigerant supplement line is expanded and cooled by a reduced pressure. Similar to the heat exchanger may be joined to the flow of the uncompressed boil-off gas in the front end or the middle end.

Preferably, the heat exchanger cools the compressed gas compressed through the compressor and the boosting compressor by heat exchange with the uncompressed boil-off gas to be supplied to the compressor, and first and second heat exchangers provided in series in the reliquefaction line. The boil-off gas including a portion and branched to the refrigerant supplement line to be depressurized by the expansion device may be supplied to the boil-off gas supply line between the first and second heat exchange units.

Preferably, the compressor may include first and second compression units provided in parallel with the boil-off gas supply line by receiving the boil-off gas generated from the liquefied gas and compressing it at the fuel supply pressure of the main engine.

Preferably, it further comprises a compander compressor provided upstream of the expansion device in the refrigerant replenishment line and further compresses the compressed gas branched to the refrigerant replenishment line, wherein the compander compressor includes the expansion device and the shaft. It is connected, it can be driven by the expansion energy of the compressed gas.

Preferably, the reliquefaction line includes: a pressure reducing device receiving the compressed gas cooled in the heat exchanger and reducing the pressure to cool it; And a gas-liquid separator for gas-liquid separation by receiving the compressed gas reduced by the decompression device.

Preferably, the gas separated by the gas-liquid separator joins the uncompressed evaporated gas introduced from the storage tank to the heat exchanger along the boil-off gas supply line, and the separated liquid may be stored in the storage tank.

According to another aspect of the present invention, the boil-off gas generated from a storage tank storing liquefied gas in a ship is compressed by a compressor at the fuel supply pressure of the main engine in the ship,

Among the boil-off gas compressed by the compressor, the boil-off gas not supplied to the fuel of the main engine is cooled by heat exchange in a heat exchanger with the uncompressed boil-off gas to be introduced into the compressor, further cooled under reduced pressure, and reliquefied to be re-liquefied to Save it,

The fuel supply pressure of the main engine is lower than the critical pressure of methane, and the boil-off gas to be reliquefied is compressed by the compressor, and then further compressed by a boosting compressor to the critical pressure or higher, and then cooled. Is provided.

Preferably, a portion of the boil-off gas not supplied as fuel after compression in the compressor is branched and decompressed with an expansion device, and the depressurized boil-off gas is expanded and cooled by the reduced pressure. The refrigerant in the heat exchanger may be replenished by joining the compressed boil-off gas flow.

Preferably, a compander compressor axially connected to the expansion device is provided upstream of the expansion device, and the boil-off gas to be supplemented with a refrigerant may be compressed by the expansion energy of the boil-off gas in the expansion device. .

In the system of the present invention, the refrigerant of the heat exchanger is supplemented in the section where cold heat is insufficient by branching a part of the boil-off gas compressed by the compressor and expanding and cooling it under reduced pressure to join the uncompressed boil-off gas in the front or middle stage of the heat exchanger. , It is possible to sufficiently cool the boil-off gas to be re-liquefied and improve the re-liquefaction performance.

As described above, the flow rate of the refrigerant in the heat exchanger can be secured even in the operating section in which cooling and heat is insufficient, thereby enhancing the reliquefaction performance of the boil-off gas and effectively treating the boil-off gas to secure the safety of the ship.

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 boil-off gas treatment system of 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 equipped with an engine capable of using liquefied gas and boil-off gas generated from liquefied gas as fuel for propulsion or power generation engines, or all types of liquefied gas or boil-off gas as fuel for the ship's engine. These ships include LNG carriers, liquid hydrogen carriers, and ships with self-propelled capabilities such as LNG Regasification Vessels (RVs), LNG Floating Production Storage Offloading (FPSO), and LNG Floating Storage Regasification Units (FSRU). ), but may also 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, it will be described by taking as an example that LNG, one of the representative liquefied gases, is applied.

Meanwhile, the fluid flowing through each line of the present exemplary 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.

2 is a schematic diagram of a boil-off gas treatment system of a ship according to a first embodiment of the present invention, and FIG. 3 is a schematic diagram of a boil-off gas treatment system of a ship according to a second embodiment of the present invention.

As shown in Figures 2 and 3, the system of the first and second embodiment, the boil-off gas supply line connected to the main engine (ME) of the ship from the storage tank (T) provided on the ship to store liquefied gas (GL), a compressor 100 provided in the boil-off gas supply line and receiving boil-off gas generated from the liquefied gas and compressing it with the fuel supply pressure of the main engine, and a storage tank branched from the boil-off gas supply line at the downstream of the compressor. The reliquefaction line (RL) for cooling the compressed gas supplied as fuel of the main engine and remaining compressed by heat exchange with the uncompressed evaporative gas to be supplied to the compressor, re-liquefying and storing it in a storage tank, and a re-liquefaction line are provided. And a heat exchanger 200 for cooling the compressed gas through heat exchange with uncompressed boil-off gas to be supplied to the compressor along the boil-off gas supply line from the storage tank.

The system of the present embodiments provides refrigerant supplement lines CLa and CLb branched from the rear end of the compressor of the boil-off gas supply line and connected to the upstream of the compressor of the boil-off gas supply line, and the compressed refrigerant branched to the refrigerant supplement line in the refrigerant supplement line. It is characterized in that an expansion device 400 for depressurizing the gas to expand and cool is disposed so that the refrigerant can be replenished at the front end or the middle end of the heat exchanger.

In the first and second embodiments, the boil-off gas branched to the refrigerant supplement line and decompressed through an expansion device to be expanded-cooled is joined to a flow of uncompressed boil-off gas at the front end or the middle stage of the heat exchanger having a similar temperature.

To this end, in the present embodiments, the heat exchanger 200 is cooled by heat exchange with the compressed gas compressed through the compressor and the boosting compressor with uncompressed evaporation gas to be supplied to the compressor, 2 It can be configured to include the heat exchanger (210, 220).

When the temperature of the gas passing through the expansion device 400 of the refrigerant supplement lines (CLa, CLb) is higher than the temperature of the uncompressed evaporated gas naturally generated from the storage tank, the entire heat exchanger can be joined to the uncompressed evaporated gas at the front of the heat exchanger as it is. The temperature of the refrigerant may increase, which may lower the cooling and reliquefaction rate of the compressed gas. Therefore, by comparing the temperature of the boil-off gas depressurized in the expansion device with the temperature of the uncompressed boil-off gas at the front end or the middle of the heat exchanger, branching to the refrigerant supplement lines CLa, CLb, The boil-off gas supply line between the first and second heat exchange units 210 and 220 having similar temperatures, that is, may be supplied to an intermediate section of the heat exchanger or may be joined to the front end of the heat exchanger. Through this, the compressed gas can be effectively cooled and the reliquefaction rate can be increased.

As described above, in the present embodiments, a refrigerant supplement line may be provided to reduce a portion of the compressed boil-off gas to be introduced into the refrigerant of the heat exchanger. Therefore, the amount of boil-off gas generated from the storage tank is small, or the amount of boil-off gas supplied to fuel such as the main engine is small, and the amount of boil-off gas to be reliquefied is large, so sufficient cold heat is not supplied from the boil-off gas supplied from the storage tank. In the operating section where the high-pressure high-temperature energy/low-temperature low-pressure cold-heat ratio is relatively high, and the heat exchanger lacks cold heat, the refrigerant in the heat exchanger can be replenished by depressurizing a part of the compressed gas from the front or rear end of the heat exchanger of the reliquefaction line. Thus, it is possible to sufficiently cool the boil-off gas to be reliquefied. Thus, it is possible to increase the reliquefaction performance of the boil-off gas.

2 and 3, the boil-off gas generated in the storage tank T is introduced into the compressor 100 along the boil-off gas supply line GL and compressed.

The compressor compresses the boil-off gas and compresses it to a fuel supply pressure to the main engine, for example, 5.5 bar if a DF engine is provided, and 12 to 16 bar if an X-DF engine is provided. The compressor 100 may be provided as a multi-stage compressor that, if necessary, a plurality of compressors and an intermediate cooler are alternately disposed and sequentially compress the boil-off gas.

The compressed gas compressed by the compressor is supplied as fuel such as a main engine, and the remaining boil-off gas after the fuel supply can be reliquefied. The boil-off gas to be re-liquefied is cooled through heat exchange with the uncompressed boil-off gas supplied from the storage tank, and then re-liquefied through the re-liquefaction line RL and stored in the storage tank.

In particular, in the present embodiments, a boosting compressor 150 is provided upstream of the heat exchanger of the reliquefaction line, so that the reliquefaction rate can be increased by additionally compressing the boil-off gas compressed by the fuel supply pressure of the main engine in the compressor and then cooling it. It is composed.

The critical point of methane, which constitutes the main component of the boil-off gas, is about -80°C and 55 bar, and the boil-off gas becomes a supercritical state at a temperature and pressure above the critical point. Therefore, for example, if DFDE is provided as the main engine, it is 5.5 bar, and if the X-DF engine is provided, it is 12 to 16 bar. The compressor compresses the boil-off gas according to the fuel supply pressure of the main engine. The compressed gas that is not supplied as fuel to the engine but branched to the reliquefaction line is compressed by the boosting compressor 150 to a critical pressure of 55 bar or more, more preferably 100 bar or more, and sent to the heat exchanger 200. The boosting compressor 150 may be configured as a multi-stage compressor including a plurality of compressors as needed.

The present inventors believe that when the boil-off gas generated from LNG is reliquefied, the re-liquefaction rate increases as the compression pressure of the boil-off gas is increased. It was found that there was no significant change. Therefore, in order to effectively reliquefy the boil-off gas, it is preferable to compress it to 100 bar or more, preferably 150 bar to 300 bar. However, if the main engine that receives boil-off gas as fuel does not require such high-pressure fuel, compressing all boil-off gas to high pressure only for re-liquefaction consumes energy due to high power consumption, and high-pressure compressors are expensive. Installation costs are also high. Therefore, in the present embodiments, the compressor 100 compresses only up to the fuel supply pressure of the main engine, and the boil-off gas to be reliquefied is further compressed by the boosting compressor 150 to increase the reliquefaction rate.

Through this, the compressor 100 can be designed to compress the boil-off gas at the fuel supply pressure of the main engine on board, thereby reducing the size of the motor, reducing the equipment cost, and reducing the power required by the compressor, thereby reducing the operating cost.

After compression in the compressor 100, the compressed gas branched to the reliquefaction line RL and further compressed in the boosting compressor 150 is introduced into the heat exchanger 200, exchanged with uncompressed evaporation gas, and cooled.

Meanwhile, the heat exchanger 200 may be provided with a printed circuit heat exchanger (PCHE) or a direct contact type heat exchanger (DCHE).

Downstream of the heat exchanger of the reliquefaction line RL, a decompression device 310 that receives the compressed gas cooled in the heat exchanger and decompresses and cools it, and a gas-liquid separator ( A reliquefaction unit 300 including 320 may be provided.

The pressure reducing device 310 may be configured as an expander or an expansion valve such as a Joule-Thomson valve for cooling by adiabatic expansion or isentropy expansion of the compressed and cooled evaporation gas.

The compressed, cooled, and expanded-cooled boil-off gas sequentially through a compressor, a boosting compressor, a heat exchanger, and a decompression device is re-liquefied in whole or in part and introduced into a gas-liquid separator.

The gas separated by the gas-liquid separator 320, that is, the flash gas, is joined to the uncompressed evaporated gas introduced from the storage tank to the heat exchanger along the evaporated gas supply line, and is introduced into the refrigerant of the heat exchanger.

The liquid separated in the gas-liquid separator may be stored in a storage tank along the reliquefaction line.

The system of the second embodiment shown in FIG. 3 is also configured to increase the reliquefaction rate by installing a boosting compressor 150 and a refrigerant supplement line in a ship equipped with a main engine receiving low pressure fuel, similar to the above-described first embodiment. .

However, in the second embodiment, a compander compressor 450 is provided upstream of the expansion device 400 in the refrigerant supplement line CLb, and the expansion device is connected to the expansion device by a shaft, so that the expansion energy of the boil-off gas in the expansion device is It is configured to be used for the compression of boil-off gas.

The compander compressor 450 is connected to an expansion device and a shaft so that it can be driven by receiving the expansion energy of the compressed gas, thereby reducing power consumption for additional compression of the boil-off gas in the refrigerant supplement line and increasing energy efficiency. I can. Although not shown in the drawing, the compressed gas compressed by the compander compressor can be cooled by being expanded in the expansion device after passing through its own fresh water cooling.

Other configurations are similar to those of the above-described embodiment, and thus redundant descriptions will be omitted.

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 not departing from the technical gist of the present invention. I did it.

T: storage tank
ME: main engine
GL: Boil-off gas supply line
RL: Reliquefaction line
CLa, CLb: refrigerant supplement line
100: compressor
150: boosting compressor
200: heat exchanger
300: reliquefaction unit
310: pressure reducing device
320: gas-liquid separator
400: expansion device
450: Compander compressor

Claims (10)

A boil-off gas supply line connected to the main engine of the ship from a storage tank provided on the ship to store liquefied gas;
A compressor provided in the boil-off gas supply line and receiving boil-off gas generated from the liquefied gas and compressing it at a fuel supply pressure of the main engine;
At the downstream of the compressor, it is branched from the boil-off gas supply line and connected to the storage tank, and the compressed gas not supplied as fuel of the main engine is cooled by heat exchange with the uncompressed boil-off gas to be supplied to the compressor and re-liquefied. A reliquefaction line for restoring into a storage tank;
A boosting compressor further compressing the compressed gas branched into the reliquefaction line;
A heat exchanger for cooling the compressed gas further compressed by the boosting compressor through heat exchange with the uncompressed boil-off gas to be supplied to the compressor from the storage tank along the boil-off gas supply line; And
A refrigerant supplement line branched from the boil-off gas supply line downstream of the compressor and connected to a front end or an intermediate end of the heat exchanger of the boil-off gas supply line; and
The fuel supply pressure of the main engine is lower than the critical pressure of methane, and the compressed gas branched to the reliquefaction line is additionally compressed by the boosting compressor above the critical pressure and then cooled. . The method of claim 1,
An expansion device provided in the refrigerant supplement line to expand and cool the compressed gas branched to the refrigerant supplement line by decompressing; further comprising,
The compressed gas branched to the refrigerant replenishment line is combined with the uncompressed boil-off gas flow at the front end or the middle stage of the heat exchanger similar to a temperature expanded and cooled by decompression. The method of claim 2,
The heat exchanger includes first and second heat exchange units provided in series with the reliquefaction line for cooling the compressed gas compressed through the compressor and the boosting compressor by heat exchange with the uncompressed evaporative gas to be supplied to the compressor, ,
Boil-off gas processing system of a ship, characterized in that the boil-off gas branched to the refrigerant supplement line and depressurized by the expansion device is supplied to an boil-off gas supply line between the first and second heat exchange units. The method of claim 3,
The compressor includes first and second compression units provided in parallel to the boil-off gas supply line by receiving the boil-off gas generated from the liquefied gas and compressing it at the fuel supply pressure of the main engine. Boil-off gas treatment system. The method of claim 2,
A compander compressor that is provided upstream of the expansion device in the refrigerant replenishment line and further compresses the compressed gas branched to the refrigerant replenishment line; and
The compander compressor is axially connected to the expansion device, and is driven by the expansion energy of the compressed gas. The method according to any one of claims 1 to 5, wherein in the reliquefaction line,
A decompression device for receiving the compressed gas cooled by the heat exchanger and reducing the pressure to cool it; And
A gas-liquid separator for receiving the compressed gas depressurized by the decompression device and separating the gas-liquid; a boil-off gas treatment system for a ship, characterized in that it is provided. The method of claim 6,
Evaporation of a ship, characterized in that the gas separated in the gas-liquid separator merges with the uncompressed evaporation gas introduced from the storage tank to the heat exchanger along the evaporation gas supply line, and the separated liquid is stored in the storage tank. Gas treatment system. The boil-off gas generated from the storage tank storing liquefied gas in the ship is compressed with a compressor at the fuel supply pressure of the main engine in the ship,
Among the boil-off gas compressed by the compressor, the boil-off gas not supplied to the fuel of the main engine is cooled by heat exchange in a heat exchanger with the uncompressed boil-off gas to be introduced into the compressor, further cooled under reduced pressure, and reliquefied to be re-liquefied into the storage tank. Save it,
The fuel supply pressure of the main engine is lower than the critical pressure of methane, and the boil-off gas to be reliquefied is compressed by the compressor and then further compressed by a boosting compressor to the critical pressure or higher, and then cooled. . The method of claim 8,
After compression in the compressor, some of the evaporated gas not supplied as fuel is branched and decompressed with an expansion device,
The evaporation of a ship, characterized in that the depressurized boil-off gas merges into the flow of the uncompressed boil-off gas at the front or the middle stage of the heat exchanger similar to the temperature expanded and cooled by decompression, thereby replenishing the refrigerant of the heat exchanger. Gas treatment method. The method of claim 9,
Providing a compander compressor axially connected to the expansion device upstream of the expansion device,
The boil-off gas treatment method of a ship, characterized in that it is possible to compress the boil-off gas to be supplemented with a refrigerant by the expansion energy of the boil-off gas in the expansion device.

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