JP2016080279A - Boil-off gas recovery system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently recover boil-off gas in a liquefied gas carrier and increase flexibility of setting of a target vessel speed of the liquefied gas carrier.SOLUTION: BOG (Boil-Off Gas) in a cargo tank 11 is fed as cooling gas to a first heat exchanger 14 through first piping 12 and further compressed at a multi-stage high-pressure gas compressor 15. Middle pressure BOG is extracted from an intermediate stage of the high pressure gas compressor 15, fed as gas to be cooled to the first heat exchanger 14 through second piping 16 and cooled, and is expanded through a Joule-Thomson valve 17 and partially liquefied. This gas-liquid double-phase flow is cooled by a re-liquefaction device 28, a part of the residual boil-off gas is liquefied and transmitted to a first separator 18. Gas component separated by the first separator 18 is returned back to the upstream side of the first heat exchanger 14 of the first piping 12 through a pressure adjustment valve 19 and liquid component is returned back to the cargo tank 11 using a transfer pump 21.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a system for collecting boil-off gas (BOG) generated in a cargo tank loaded with liquefied gas.

  Some liquefied natural gas carriers (LNG tankers) use boil-off gas (BOG) generated in cargo tanks as fuel for propulsion engines, power generation engines, and steam boilers. However, in recent years, the propulsion efficiency has been improved by adopting electric propulsion using a dual fuel-fired medium speed diesel power generation engine and dual fuel-fired low-speed diesel direct-propulsion. As a result, the amount of fuel gas consumed by the propulsion engine is suppressed, and the ship speed range in which all of the boil-off gas cannot be consumed as fuel is widening. Therefore, the surplus boil-off gas needs to be re-liquefied by a re-liquefaction device (Patent Documents 1 and 2) using a compressor and a refrigerant and collected in a cargo tank, or incinerated by a gas combustion device or a gas-fired boiler.

JP 2001-132899 A JP 2005-265170 A

  When a conventional reliquefaction apparatus using a compressor or a refrigerant is installed, the initial cost increases and the power consumption is large, so the operation cost is high. On the other hand, in an LNG tanker using a dual fuel-fired low-speed diesel direct drive, a high-pressure gas compressor is mounted to supply a high-pressure gas of about 30 MPa to a diesel engine. Therefore, in such a ship, it is conceivable to configure the BOG recovery device using a high-pressure gas compressor that supplies fuel gas to a dual fuel-fired low-speed diesel engine.

  An object of the present invention is to efficiently recover boil-off gas in a liquefied gas carrier ship and increase the degree of freedom in selecting a target ship speed.

  The boil-off gas recovery system of the present invention performs heat exchange between a high-pressure gas compressor that compresses boil-off gas from a cargo tank, a boil-off gas that is sent to the high-pressure gas compressor, and a boil-off gas that is compressed in the high-pressure gas compressor. 1 heat exchanger, a first liquefaction means for expanding the boil-off gas compressed in the high-pressure gas compressor and cooled through the first heat exchanger, and liquefying a part of the boil-off gas, and liquefied by the first liquefaction means A second liquefaction means for liquefying a part of the boil-off gas that has not been liquefied via a reliquefaction device; and a gas-liquid two-phase flow in which a part of the boil-off gas is liquefied by the first and second liquefaction means A first separator that separates the liquid component into a gas component separated by the first separator from the first separator. At the inlet side of the lesser, it is refluxed to the upstream side of the first heat exchanger, and the liquid component separated by the first separator is transferred to the cargo tank, from the high-pressure gas compressor to the first heat exchanger. It is characterized in that the pressure of the supplied boil-off gas is equal to or higher than the critical point.

  The first liquefaction means preferably includes an expansion valve or a Joule Thomson valve. It is preferable that a pressure adjusting valve is further provided in a path for returning the gas component separated by the first separator to the upstream side of the first heat exchanger, and the pressure in the first separator is maintained at a predetermined pressure. The gas-liquid two-phase flow generated by the first liquefaction means is transferred to the first separator via, for example, a reliquefaction device. The boil-off gas recovery system further includes, for example, a second separator, the gas-liquid two-phase flow generated by the first liquefaction means is separated into a gas component and a liquid component by the second separator, and the gas separated by the second separator The components are transferred to the first separator via the reliquefaction device. A gas-liquid two-phase flow generated by the first liquefaction means may be separated into a gas component and a liquid component by the first separator and separated by the first separator. A part of the gas component may be sent to the second liquefaction unit by the gas compressor, liquefied by the second liquefaction unit, and refluxed to the first separator.

  The boil-off gas recovery system includes a transfer pump that transfers, for example, a liquid component from the first separator to the cargo tank. The boil-off gas recovery system may further include a flow control valve that controls the flow rate of the boil-off gas that is compressed in the high-pressure gas compressor and transferred to the first heat exchanger, thereby reducing the liquefaction recovery amount of the boil-off gas recovery system. Control. The high-pressure gas compressor discharges a part of the compressed boil-off gas, for example, as a fuel for a binary fuel-fired low-speed diesel engine. The high-pressure gas compressor is, for example, a multistage compressor, and the first heat exchanger may be supplied with boil-off gas extracted from the middle stage. Alternatively, the boil-off gas may be supplied to the first heat exchanger by reducing the pressure from the discharge side of the high-pressure gas compressor.

  The liquefied gas carrier of the present invention is characterized by including any of the above boil-off gas recovery systems.

  According to the present invention, a boil-off gas can be efficiently recovered and a degree of freedom in selecting a target ship speed can be increased in a liquefied gas carrier ship.

It is a block diagram which shows the structure of the boil-off gas collection | recovery system of 1st Embodiment of this invention. (A) When the heat exchange is performed in a state where the gas to be cooled is at a pressure below the critical point, and (b) Exchange inside the heat exchanger when the heat exchange is performed at a pressure state where the gas to be cooled is above the critical point It is a graph which shows the change of calorie | heat amount and temperature. It is a pressure-specific enthalpy (ph) diagram of methane which is the main ingredient of boil-off gas. It is a graph which shows the relationship between the ship speed at the time of liquefied gas loading in this embodiment, consumption gas fuel consumption, and boil-off gas generation amount. It is a block diagram which shows the structure of the boil-off gas collection | recovery system of the modification of 1st Embodiment. It is a block diagram which shows the structure of the boil off gas collection | recovery system of 2nd Embodiment. It is a block diagram which shows the structure of the boil-off gas collection | recovery system of the modification of 2nd Embodiment. It is a block diagram which shows the structure of the boil-off gas collection | recovery system of 3rd Embodiment.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a block diagram showing a configuration of a boil-off gas recovery system according to the first embodiment of the present invention.

  The boil-off recovery system 10 of the present embodiment is applied to a ship carrying a high-pressure gas compressor and carrying a liquefied gas such as natural gas, and particularly to a liquefied gas carrying ship using dual fuel-fired low-speed diesel (direct connection) propulsion. Is suitable.

  The liquefied gas (LNG in the present embodiment) is loaded on the cargo tank 11, and the boil-off gas (about −162 ° C.) generated in the cargo tank 11 is guided to the cargo machinery room 13 through the first pipe 12, and the first It is transferred to the high-pressure gas compressor 15 via the heat exchanger 14. The high-pressure gas compressor 15 is, for example, a multistage compressor, and a high-pressure gas of about 30 MPa is discharged from the discharge side, for example, a dual fuel-fired low-speed diesel engine (main engine) or a dual-fuel-fired diesel for power generation. It is supplied as gas fuel to engines and gas-fired boilers. In addition, when the high pressure gas compressor 15 is a multistage compressor and the required gas pressure of the power generating diesel engine or boiler is lower than the high pressure gas, the fuel gas to these is extracted and supplied from the middle stage of the compressor You can also.

  On the other hand, from the middle stage of the high-pressure gas compressor 15, surplus boil-off gas is extracted into the second pipe 16 as, for example, a medium-pressure gas (about 45 ° C.) of about 10 MPa, and the first heat is passed through the second pipe 16. It is transferred to the exchanger 14. In the first heat exchanger 14, the boil-off gas passing through the first pipe 12 is used as a cooling side fluid to cool the medium pressure boil-off gas (cooled side gas) in the second pipe 16, and the boil-off gas is, for example, about − It is cooled to around 100 ° C.

  The flow rate control valve 23 provided in the second pipe 16 controls the flow rate of the medium pressure gas transferred to the first heat exchanger 14 to adjust the liquefaction amount, thereby controlling the liquefaction recovery amount of the boil-off gas recovery system. can do. In FIG. 1, the flow control valve 23 is installed on the upstream side of the heat exchanger, but may be installed on the downstream side.

  However, here, the pressure of the boil-off gas extracted from the middle stage of the high-pressure gas compressor 15 is preferably set to a pressure equal to or higher than the critical point of the same gas (in this embodiment, the critical point pressure is about 4 MPa). Further, in the high-pressure gas compressor 15, for example, an oil-free compressor is used for the stage until the excess boil-off gas is extracted, and an oil-feed compressor is used for the discharge-side high-pressure stage thereafter. preferable. In addition, when using an oil supply type compressor for all, the filter (not shown) for removing the oil component carried over to the 2nd piping 16, for example is arrange | positioned.

  A Joule Thomson (JT) valve (or expansion valve) 17 is provided on the downstream side of the first heat exchanger 14 in the second pipe 16. When the boil-off gas is depressurized to a predetermined separator set pressure via the Joule-Thompson valve 17, the temperature is lowered to the saturation temperature at the pressure after depressurization, and a part of the boil-off gas is liquefied and the gas-liquid 2 It becomes a phase flow. When the boil-off gas is reduced to, for example, atmospheric pressure by the Joule Thomson valve 17, the boil-off gas is cooled to about -160 ° C.

  In the present embodiment, the boil-off gas and the liquefied gas that have become a gas-liquid two-phase flow are then guided to the reliquefaction device 28. Although the reliquefaction device 28 and the second heat exchanger 24 are not shown, the reliquefaction device 28 is composed of a low-temperature refrigerant cycle equal to or lower than the saturation temperature of the boil-off gas, and the gas-liquid two-phase flow flowing through the second pipe 16 Heat exchange with the refrigerant further liquefies part of the saturated temperature gas that is not liquefied. Note that the amount of boil-off gas liquefied in the reliquefaction device 28 is controlled by controlling the load of the refrigerant cycle.

  The boil-off gas partially liquefied through the Joule-Thomson valve 17 and the reliquefaction device 28 is then transferred to the first separator 18 through the second pipe 16 and gas-liquid separation is performed. The boil-off gas separated by the first separator 18 is returned to the upstream side of the first heat exchanger 14 of the first pipe 12 through the third pipe 20 provided with the pressure regulating valve 19. On the other hand, the separated liquefied gas (LNG) is transferred by the transfer pump 21 from the cargo machinery room 13 to the cargo tank 11 in the cargo section through the fourth pipe 22 and collected. If the reliquefied gas can be transferred to the cargo tank 11 by the separator pressure, the transfer pump 21 can be omitted. The pressure in the first separator 18 is maintained at the set pressure by the pressure adjustment valve 19.

  In the present boil-off gas recovery system 10, the capacity of the high-pressure gas compressor 15 is determined from the amount of boil-off gas generated in the cargo tank 11, the amount of gas consumed by an engine or boiler that uses the gas as fuel consumption, and the like. The high-pressure gas compressor capacity is preferably a capacity obtained by further adding the amount of gas recirculated to the upstream side of the first heat exchanger 14 in order to increase the liquefaction amount.

  Next, the cooling effect in the first heat exchanger 14 will be described with reference to FIGS. FIG. 2A shows the heat of the cooling side gas and the cooled side gas in the first heat exchanger 14 when the pressure of the boil-off gas (cooled side gas) extracted into the second pipe 16 is below the critical point. FIG. 2B is a graph showing changes in heat exchange and temperature inside the exchanger. FIG. 2B corresponds to this embodiment, and the pressure of the boil-off gas (cooled side gas) extracted from the second pipe 16 is critical. It is a graph corresponding to Drawing 2 (a) when it is more than a point. 2 (a) and 2 (b), the left end of the horizontal axis corresponds to the inlet of the cooling side gas and the outlet of the cooled gas, and the right end corresponds to the outlet of the cooling side gas and the inlet of the cooled side gas. The axis is temperature (° C). FIG. 3 is a schematic pressure-specific enthalpy (ph) diagram of methane, which is the main component of the boil-off gas.

  As shown in FIG. 2 (a), when the pressure of the cooled side gas is below the critical point (for example, 3.5 MPa), the cooled side gas falls to the saturation temperature in the first heat exchanger 14, and then Heat exchange is performed in a partially liquid state. At this time, since there is a heat exchange region where the heat exchange is at a low saturation temperature, the pinch point becomes severe during the heat exchange in the first heat exchanger 14 with the cooling side, and the amount of heat that can be exchanged is limited ( Line L1 in FIG. 3: Cooling from 45 ° C. to −100 ° C. at a pressure below the critical point (3.5 MPa).

  On the other hand, as shown in FIG. 3, when the pressure of the cooled side gas is equal to or higher than the critical point (for example, 10 MPa), heat is exchanged in a supercritical state, and the cooled side gas is liquefied in the first heat exchanger 14. There is nothing. That is, since there is no heat exchange in a saturated state (no phase change), the pinch point is relaxed, and a heat exchanger capable of sufficient heat exchange can be designed (straight line L2 in FIG. 3: pressure above the critical point) (Cooled to 45 ° C. to −100 ° C. at 10 MPa).

  Next, the boil-off gas liquefaction amount adjustment control using the flow control valve 23 and the reliquefaction device 28 will be described with reference to FIG.

  FIG. 4 is a graph showing the relationship between the operating ship speed V and the used fuel gas consumption Q and the boil-off gas generation amount. In FIG. 4, the horizontal axis represents the ship speed V and the vertical axis represents the fuel gas consumption Q. A curve S is a curve showing the relationship between the ship speed and the fuel gas consumption (unit time), and the fuel consumption Q is approximately proportional to the cube of the ship speed V. The straight line M1 (NATURAL BOG) is an amount per unit time at which the liquefied gas (natural gas) in the cargo tank 11 spontaneously evaporates and becomes boil-off gas. That is, in FIG. 4, when only the boil-off gas and all of it is used as fuel for the engine and boiler that consume gas in the ship, a ship speed V1 corresponding to the intersection P1 of the curve S and the straight line M1 is obtained. On the other hand, on the lower speed side (V <V1 region) than the operating point P1, the difference between the straight line M1 and the curve S becomes surplus boil-off gas, and on the higher speed side (V> V1 region) than the operating point P1, the curve S and The difference between the straight lines M1 is the amount of gas fuel that needs to be added.

  When the flow control valve 23 is opened and the boil-off gas recovery device is operated while the reliquefaction device 28 is stopped, the boil-off gas is partially liquefied by the first heat exchanger 14 and the Joule Thomson valve 17. As a result, the amount of boil-off gas generated per unit time is substantially reduced. Assuming that the amount of boil-off gas generated per unit time when the recovery by the liquefaction system using the first heat exchanger 14 and the Joule Thomson valve 17 is maximized is a straight line M2, the same liquefaction system allows the boil-off gas per unit time. Can be reduced from the straight line M1 to the straight line M2, and the intersection with the curve S indicating the operating point can be moved from P1 to P2. Therefore, it is possible to perform a decelerating operation that reduces the operation speed V from the ship speed V1 at the operating point P1 to the ship speed V2 at the operating point P2 while suppressing the generation of surplus boil-off gas.

  Further, when the operation speed V is operated between V1 and V2, the amount of liquefaction is controlled by controlling the flow rate of the cooled side gas transferred to the first heat exchanger 14 by the flow rate control valve 23. Therefore, the amount of boil-off gas can be optimally controlled between M1 and M2 in accordance with the gas fuel used, and the processing of surplus boil-off gas in this operation region is not necessary.

  Since M2 is the limit of liquefied gas recovery of the liquefaction system using the first heat exchanger 14 and the Joule Thompson valve 17, in the present embodiment, the reliquefaction device 28 is used for further deceleration operation. Thus, the boil-off gas is further liquefied, and the liquefied gas recovery capability of the boil-off gas recovery system 10 is increased. A straight line M3 in FIG. 4 per unit time when the first heat exchanger 14, the Joule Thomson valve 17 and the reliquefaction device 28 are used and the liquefied gas recovery amount of the boil-off gas recovery system 10 of this embodiment is maximized. The amount of boil-off gas generated. That is, by adjusting the reliquefaction amount using the reliquefaction device 28, the amount of boil-off gas generated per unit time can be adjusted between M2 and M3. As a result, it is possible to perform decelerating operation with the operation speed V lowered to the ship speed V3 of the operating point P3, which is the intersection of the straight line M3 and the curve S, without performing surplus boil-off gas processing (incineration disposal, etc.). Become.

  Next, a modification of the boil-off gas recovery system 10 of the first embodiment will be described with reference to FIG. In the boil-off gas recovery system 10 ′ according to the modified example, a bypass passage 26 is provided for the reliquefaction device 28, and a flow rate control valve 27 is provided in the bypass passage 26. Other configurations are the same as those of the boil-off gas recovery system 10 of the first embodiment. That is, in the modified example, the amount of liquefied gas recovered using the flow rate control valve 27 can be adjusted according to the target ship speed or the like.

  As described above, according to the boil-off gas recovery system of the first embodiment and the modification thereof, the boil-off before compression is performed using the high-pressure gas compressor used for fuel gas supply of the dual fuel-fired low-speed diesel engine. Since the boil-off gas after cooling is compressed using the gas and liquefaction is performed using the Joule-Thompson valve, the boil-off gas is recovered more efficiently than when the boil-off gas is recovered using only the reliquefaction device. A reliquefaction treatment can be performed. As a result, the reliquefaction apparatus can be reduced in size, and the boil-off gas can be efficiently recovered while suppressing the initial cost and the operating cost, and the degree of freedom in selecting the target ship speed is increased.

  Further, in this embodiment, since the gas component that has not been liquefied is recirculated to the cooling side of the heat exchanger as a cooling side gas, the amount of gas circulation in the boil-off gas recovery system can be secured at a certain amount or more, The cooling amount of the system can be increased by lowering the inlet temperature of the cooling side gas of the exchanger.

  Furthermore, in this embodiment, the pressure in the separator is maintained at a set value by the pressure adjustment valve 19, so that the inlet pressure of the transfer pump 21 to the cargo tank 11 is maintained at a constant value, and the effective suction head ( Available NPSH is prevented from falling below the required net suction head (required NPSH).

  Next, the boil-off gas recovery system of the second embodiment will be described with reference to the block diagram of FIG. In addition, about the structure similar to 1st Embodiment, the description is abbreviate | omitted using the same referential mark.

  In the boil-off gas recovery system 30 of the second embodiment, the second separator 31 is provided on the upstream side of the reliquefaction device 28 in the first embodiment. That is, the gas-liquid two-phase flow from the Joule-Thompson valve 17 is guided to the second separator 31 by the second pipe 16 and gas-liquid separation is performed. The boil-off gas that has not been liquefied is sent to the reliquefaction device 28 through the fifth pipe 32 and further cooled by heat exchange with the refrigerant circulating in the refrigerant pipe 25. As a result, a part of the boil-off gas is liquefied and sent to the first separator 18 as a gas-liquid two-phase flow. On the other hand, the liquefied gas (LNG) separated by the second separator 31 is returned to the cargo tank 11 through the sixth pipe 34 by the transfer pump 33. In FIG. 6, the sixth pipe 34 is connected to the downstream side of the transfer pump 21 of the fourth pipe 22.

  In the second embodiment, only the boil-off gas that has not been liquefied in the reliquefaction process through the first heat exchanger 14 and the Joule Thomson valve 17 is cooled and liquefied by the reliquefaction device 28. Therefore, the liquefied gas recovery efficiency can be further increased.

  FIG. 7 is a block diagram showing a configuration of a boil-off gas recovery system 30 'according to a modification of the second embodiment. In this modification, a bypass passage 35 that bypasses the reliquefaction device 28 is provided in the fifth pipe 32, and a flow rate control valve 36 is provided in the bypass passage 35. That is, in the modified example, the amount of liquefied gas recovered using the flow rate control valve 36 can be adjusted according to the target ship speed or the like. Other configurations are the same as those of the boil-off gas recovery system 30 of the second embodiment.

  Next, the boil-off gas recovery system of the third embodiment will be described with reference to the block diagram of FIG. In addition, about the structure similar to 1st Embodiment, the description is abbreviate | omitted using the same referential mark.

  In the boil-off gas recovery system 40 of the third embodiment, the second pipe 16 from the Joule-Thompson valve 17 is directly connected to the first separator 18, and the reliquefaction device 28 is a boil-off gas that is gas-liquid separated by the first separator 18. Used only for cooling. That is, both ends of the seventh pipe 41 are connected to the first separator 18, and the seventh pipe 41 exchanges heat with the refrigerant in the refrigerant pipe 25 through the reliquefaction device 28. The boil-off gas in the first separator 18 supplied from one end of the seventh pipe 41 to the seventh pipe 41 is supplied to the reliquefaction device 28 by the gas compressor 42, and is partially liquefied by the reliquefaction device 28. A gas-liquid two-phase flow is formed downstream of 28 and returned to the first separator 18 again.

  In the third embodiment, by using the reliquefaction device 28 only for cooling the boil-off gas that has not been liquefied, the number of separators can be reduced to one while increasing the liquefaction efficiency, so that the equipment cost can be reduced.

  In this embodiment, the cooled side gas is extracted from the middle stage of the high-pressure gas compressor and transferred to the heat exchanger, but it may be transferred from the discharge side to the heat exchanger through the decompressor. Good. In this embodiment, only one reliquefaction device is used, but a plurality of small reliquefaction devices may be arranged in parallel.

10, 10 ', 30, 30', 40 Boil-off gas recovery system 11 Cargo tank 12 First piping 13 Cargo machinery room 14 First heat exchanger 15 High-pressure gas compressor (multistage compressor)
16 Second piping 17 Joule Thomson valve (expansion valve)
18 First separator 19 Pressure regulating valve 20 Third piping 21, 33 Transfer pump 22 Fourth piping 23, 27, 36 Flow control valve 24 Second heat exchanger 25 Refrigerant piping 26, 35 Bypass passage 28 Reliquefaction device 31 First 2 separator 32 5th piping 34 6th piping 41 7th piping 42 Reliquefaction device gas supply gas compressor

Claims (13)

A high-pressure gas compressor that compresses boil-off gas from the cargo tank;
A first heat exchanger for exchanging heat between the boil-off gas sent to the high-pressure gas compressor and the boil-off gas compressed in the high-pressure gas compressor;
First liquefaction means for expanding the boil-off gas compressed in the high-pressure gas compressor and cooled via the first heat exchanger, and liquefying a part of the boil-off gas;
A second liquefaction means for liquefying a part of the boil-off gas that has not been liquefied by the first liquefaction means, via a reliquefaction device;
A first separator that separates a gas-liquid two-phase flow in which a part of the boil-off gas is liquefied by the first and second liquefaction means into a gas component and a liquid component;
The gas component separated by the first separator is refluxed from the first separator to the upstream side of the first heat exchanger at the inlet side of the high-pressure gas compressor and separated by the first separator. Transferred liquid components to the cargo tank,
The boil-off gas recovery system for liquefied gas, wherein the pressure of the boil-off gas supplied from the high-pressure gas compressor to the first heat exchanger is equal to or higher than a critical point.   The boil-off gas recovery system according to claim 1, wherein the first liquefying means includes an expansion valve or a Joule Thomson valve.   A pressure regulating valve is further provided in a path for returning the gas component separated by the first separator to the upstream side of the first heat exchanger, and the pressure in the first separator is maintained at a predetermined pressure; The boil-off gas recovery system according to any one of claims 1 and 2.   The boil-off according to any one of claims 1 to 3, wherein the gas-liquid two-phase flow generated by the first liquefaction means is transferred to the first separator via the reliquefaction device. Gas recovery system.   A second separator, the gas-liquid two-phase flow generated by the first liquefaction means is separated into a gas component and a liquid component by the second separator, and the gas component separated by the second separator is The boil-off gas recovery system according to any one of claims 1 to 3, wherein the boil-off gas recovery system is transferred to the first separator via a reliquefaction device.   The boil-off gas recovery system according to any one of claims 4 and 5, further comprising a bypass passage that bypasses the reliquefaction device.   The gas-liquid two-phase flow generated by the first liquefaction means is directly separated into a gas component and a liquid component by the first separator, and a part of the gas component separated by the first separator is separated by a gas compressor. The boil-off gas recovery system according to any one of claims 1 to 3, wherein the boil-off gas recovery system is sent to the second liquefaction unit, liquefied by the second liquefaction unit, and refluxed to the first separator.   The boil-off gas recovery system according to any one of claims 1 to 7, further comprising a transfer pump that transfers the liquid component from the first separator to the cargo tank.   The liquefied recovery amount of the boil-off gas recovery system is further controlled by further comprising a flow rate control valve that controls the flow rate of the boil-off gas compressed in the high-pressure gas compressor and transferred to the first heat exchanger. The boil-off gas recovery system according to any one of 1 to 8.   The boil-off gas recovery system according to any one of claims 1 to 9, wherein the high-pressure gas compressor discharges a part of the compressed boil-off gas as fuel for a dual fuel-fired low-speed diesel engine.   The boil-off gas according to any one of claims 1 to 10, wherein the high-pressure gas compressor is a multi-stage compressor, and the boil-off gas extracted from an intermediate stage is supplied to the first heat exchanger. Collection system.   The boil-off gas recovery system according to any one of claims 1 to 10, wherein the boil-off gas is supplied to the first heat exchanger by reducing pressure from a discharge side of the high-pressure gas compressor.   A liquefied gas carrier ship comprising the boil-off gas recovery system according to any one of claims 1 to 12.

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