JP2019055732A - Fuel gas supply system, vessel, and fuel gas supply method - Google Patents

Abstract

To stably supply boil-off gas to an engine even when supplying the boil-off gas by compression to an engine including use conditions such as having composition ratio of the boil-off gas varied by use of a liquefier for the boil-off gas.SOLUTION: A fuel gas supply system for supplying fuel gas to an engine comprises: a tank for retaining liquefied gas; a pressurizing mechanism for compressing and feeding out the boil-off gas vaporized from the liquefied gas for supplying the same to the engine; and a liquefier for liquefying one portion of the fed out boil-off gas and recovering the same to the tank. The liquefier includes: a liquid recovery control valve for controlling the amount of the liquefied gas flowing through a liquid recovery piping for recovering the liquefied gas liquefied by the liquifier to the tank; and a gas-liquid recovery control valve for controlling the amount of the gas-liquid mixed fluid flowing through a gas-liquid recovery piping for returning the gas-liquid mixed fluid generated by liquefying of the boil-off gas into a liquid phase of the liquefied gas in the tank.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a fuel gas supply system that supplies a boil-off gas vaporized from a liquefied gas to an engine as a fuel gas, a ship using the system, and a fuel gas supply method.

  In the LNG carrier, boil-off gas is vaporized from the liquefied gas stored in the tank. In order to process this boil-off gas effectively, it is performed as fuel gas to a ship engine.

A low-pressure fluid such as boil-off gas needs to be a high-pressure fluid in order to be adapted as an engine fuel gas. For this reason, in a fuel gas supply system that supplies boil-off gas as engine fuel gas, a fuel gas such as boil-off gas is compressed using a compression device, for example, a multistage compression device. The compressed fuel gas is sent out to the engine.
On the other hand, the consumption of fuel gas in the engine may change due to load fluctuations. In this case, in order to effectively recover the excess compressed boil-off gas, the boil-off gas is liquefied and returned to the tank.

  For example, as a technique for liquefying compressed boil-off gas and returning it to the tank, after compressing the boil-off gas, a part of the boil-off gas is supplied to the engine, and the remaining boil-off gas is removed from the tank with the low-temperature boil-off gas and heat. A fuel gas supply system is known in which liquefied gas is returned to a tank by liquefaction after heat exchange with an exchanger (Patent Document 1).

JP2015-505941A

  In the fuel gas supply system, when the boil-off gas is liquefied, there is a boil-off gas that is not liquefied and maintains the gas in addition to the liquefied boil-off gas. This boil-off gas is combined with a flow of fresh boil-off gas (hereinafter also simply referred to as fresh boil-off gas) that has been vaporized from the liquefied gas in the tank to be compressed and supplied to the engine. At this time, the boil-off gas is not a single component but a mixed gas, and includes a component having a high boiling point and a component having a low boiling point. This component having a low boiling point is likely to remain as a gas in the boil-off gas that is not liquefied and maintains the gas. For this reason, the boil-off gas in which the gas is not liquefied and maintains the gas contains more components having a lower boiling point than the boil-off gas before liquefaction. That is, the composition of the boil-off gas that has been subjected to liquefaction treatment but has not been liquefied and maintained gas has more components with lower boiling points than fresh boil-off gas. For this reason, when the boil-off gas that has not been liquefied and maintains the gas is merged with the fresh boil-off gas, the composition ratio of the boil-off gas supplied to the compressor changes. For example, natural gas may contain several percent of nitrogen having a lower boiling point than methane in addition to methane. In addition, liquefied ethane may contain several percent of methane ga, which has a lower boiling point than ethane. In this case, since the boil-off gas that circulates between the compression device and the liquefaction treatment device is repeatedly subjected to liquefaction treatment, the content of low-boiling components in the boil-off gas increases with time. In this case, the content of nitrogen contained in the boil-off gas exceeds 10%, and in the case of liquefied ethane, the content of methane contained in the boil-off gas may exceed 50%. Such a change in the composition ratio of the boil-off gas is obtained by supplying a stable amount (mass) of fuel gas to the engine in a fuel gas supply system including a compression device that determines the amount of boil-off gas delivered by controlling the pressure. Becomes difficult.

  When the fuel gas supply system is applied to a ship, when the ship waits for cargo handling, waiting for a canal, or the like, the propulsion engine is stopped and the liquefaction device is driven to the maximum to liquefy and collect the boil-off gas. If this state continues for a long time, the composition ratio of the boil-off gas will change significantly. As a result, the boil-off gas cannot be used effectively as a fuel gas due to a decrease in the amount of heat as fuel gas, changes in ignition conditions, changes in combustion, etc. . Further, in the design of the compression apparatus, it becomes difficult to cope with a change in flow rate and a temperature change during compression of the boil-off gas due to a change in the composition ratio of the boil-off gas. In addition, a change in the composition ratio of the boil-off gas caused by the liquefaction device similarly reduces the liquefaction efficiency even on land bases storing natural gas. Equipment for removing nitrogen from natural gas (for example, selectively removing nitrogen gas by using a difference in boiling point) is also being studied. However, such equipment is complex and not practical for use on ships. In addition, since the load of the ship's propulsion engine changes frequently and is not constant, it is difficult to use the above equipment, and it is not easy to start the equipment, and operation maintenance of the equipment is required.

  Therefore, in the present invention, when supplying pressurized boil-off gas to the engine as a fuel gas, the boil-off gas is supplied under such a condition that the composition ratio of the boil-off gas changes depending on the use of the boil-off gas liquefaction device. However, an object of the present invention is to provide a fuel gas supply system, a fuel gas supply method, and a ship capable of stably supplying a voice-off gas pressurized to a predetermined pressure to an engine.

One embodiment of the present invention is a fuel gas supply system that supplies fuel gas to an engine. The fuel gas supply system is
A tank for storing liquefied gas;
A pressurizing mechanism for pressurizing and sending out the boil-off gas vaporized from the liquefied gas in order to supply the boil-off gas vaporized from the liquefied gas as a fuel gas to the engine;
A liquefying device that liquefies a part of the boil-off gas sent out by being pressurized by the pressurizing mechanism and collects it in the tank.
The liquefaction device includes a liquid recovery pipe that recovers the liquefied gas generated by liquefaction by the liquefying device in the tank, and a liquid recovery that is provided in the liquid recovery pipe and controls an amount of the liquefied gas flowing through the liquid recovery pipe A control valve, a gas-liquid recovery pipe for returning the gas-liquid mixed fluid of the liquefied gas generated by liquefaction by the liquefying device and the boil-off gas not liquefied into the liquid phase of the liquefied gas in the tank, and the gas-liquid recovery A gas-liquid recovery control valve that is provided in the pipe and controls the amount of the gas-liquid mixed fluid flowing through the gas-liquid recovery pipe.

The fuel gas supply system further includes a control device for controlling the pressurizing mechanism and the liquefying device,
It is preferable that the control device performs control to switch from a control state 1 in which the gas-liquid recovery control valve is closed to a control state 2 in which the gas-liquid recovery control valve is opened.

The liquefying device includes a merging control valve that controls an amount of boil-off gas that is maintained without being liquefied in the boil-off gas and merged with a flow of the boil-off gas that has been vaporized from the liquefied gas and before being pressurized,
In the control state 1, the merging control valve is further open.
In the control state 2, it is preferable that the merging control valve is closed.

  Further, the fuel gas supply system is provided in a branch pipe connected to a gas processing apparatus that consumes a part of the boil-off gas pressurized by the pressurizing mechanism as fuel, and the branch pipe is directed toward the gas processing apparatus. A gas processing control valve for controlling the amount of boil-off gas flowing is provided. In this case, it is preferable that the control device increases the opening degree of the gas processing control valve in the control state 2 as compared with the control state 1.

  Furthermore, the fuel gas supply system includes a determination device that determines whether or not the composition ratio of the boil-off gas pressurized by the pressurization mechanism has changed. In this case, it is preferable that the control device switches from the control state 1 to the control state 2 based on a determination result of the determination device.

The pressurizing mechanism includes a pressurizing device that pressurizes the boil-off gas, a bypass pipe that bypasses the pressurizing device and connects the pipe through which the boil-off gas before and after pressurization by the pressurizing device flows. An adjustment valve that controls the amount of boil-off gas flowing through the bypass pipe so that the pressure of the boil-off gas after being pressurized by the pressure device is within a predetermined range;
It is preferable that the determination device determines a change in the composition ratio of the boil-off gas based on information on the opening degree of the adjustment valve in the control state 1.

  In this case, it is preferable that the determination device determines that the composition ratio of the boil-off gas has changed when the opening degree changes with time and falls outside a predetermined range.

  It is also preferable that the determination device determines a change in the composition ratio of the boil-off gas based on energy consumption per unit time that the pressurization mechanism consumes in the control state 1 in order to pressurize the boil-off gas.

  In this case, it is preferable that the determination device determines that the composition ratio of the boil-off gas has changed when the energy consumption changes with the elapsed time and deviates from a predetermined range.

Another aspect of the present invention is the fuel gas supply system,
And a propulsion engine that is driven using fuel pressurized by the pressurizing mechanism.

Yet another embodiment of the present invention is a fuel gas supply method for supplying fuel gas to an engine. The fuel gas supply method is
Pressurizing and sending the boil-off gas vaporized from the liquefied gas in order to supply the boil-off gas vaporized from a tank storing the liquefied gas as a fuel gas to the engine;
Liquefying a portion of the pressurized boil-off gas and collecting it in the tank.
The step of collecting in the tank includes a control state 1 in which the liquefied gas generated by liquefying the boil-off gas is separated from the boil-off gas that has not been liquefied and collected in the tank, and the liquefied gas obtained by liquefying the boil-off gas, This is performed by selecting one of the states between the control states 2 in which the gas-liquid mixed fluid containing the boil-off gas that has not been liquefied is returned to the liquid phase of the liquefied gas in the tank.

  In the control state 1, it is preferable that the boil-off gas, which is maintained in the boil-off gas without being liquefied, is joined to the flow of the boil-off gas before being pressurized and vaporized from the liquefied gas.

  In the control state 2, it is preferable that the amount of boil-off gas flowing in the gas processing apparatus that consumes part of the boil-off gas pressurized by the pressurizing mechanism as fuel is larger than that in the control state 1.

Further, the fuel gas supply method includes a step of determining whether or not there is a change in the composition ratio of the boil-off gas to be pressurized,
It is preferable to switch from the control state 1 to the control state 2 based on the determination result of the composition ratio.

  The pressurizing mechanism for pressurizing the boil-off gas is a bypass pipe that bypasses the pressurizer and connects between a pressurizer that pressurizes the boil-off gas and a pipe through which the boil-off gas before and after pressurization by the pressurizer flows. And an adjustment valve that controls the amount of boil-off gas that flows through the bypass pipe so that the pressure of the boil-off gas after being pressurized by the pressurizer is within a predetermined range. In this case, in the determining step, it is preferable to determine a change in the composition ratio of the boil-off gas based on information on the opening degree of the adjusting valve in the control state 1.

  In this case, it is preferable that in the determining step, it is determined that the composition ratio of the boil-off gas has changed when the opening degree changes with time and is out of a predetermined range.

  Further, in the determining step, determining a change in the composition ratio of the boil-off gas based on energy consumption per unit time consumed by the pressurizing mechanism for pressurizing the boil-off gas in the control state 1. Is also preferable.

  In this case, it is preferable that in the determining step, it is determined that the composition ratio of the boil-off gas has changed when the consumed energy changes with the elapsed time and falls outside a predetermined range.

  According to the fuel gas supply system, the fuel gas supply method, and the ship of the above aspect, when supplying pressurized boil-off gas to the engine as fuel gas, the composition ratio of the boil-off gas changes depending on the use of the boil-off gas liquefaction device. Even when the boil-off gas is supplied including such use conditions, the voice-off gas pressurized to a predetermined pressure can be stably supplied to the engine.

It is a figure which shows an example of a structure of the fuel gas supply system of this embodiment. It is a figure which shows an example of the compression characteristic of boil-off gas which changes with the change of the composition ratio of boil-off gas. It is a figure which shows an example of the preferable modification of the fuel gas supply system of this embodiment.

  Hereinafter, the fuel gas supply system, ship, and fuel gas supply method of the present invention will be described in detail.

  FIG. 1 is a diagram illustrating an example of a configuration of a fuel gas supply system 10 that supplies a boil-off gas of a liquefied gas as a fuel gas to a propulsion engine of a ship according to the present embodiment. Although liquefied natural gas is used as the liquefied gas of the fuel gas supply system 10, the liquefied natural gas is not limited to liquefied natural gas, and liquefied gas such as pure methane or ethane can be used. The boil-off gas includes, in addition to the gas vaporized by natural heat input in the tank, gas that is forcibly vaporized by intentionally heating LNG. In the present embodiment, description will be made using a gas vaporized by natural heat input in a tank. In the case of using a gas that is forcibly vaporized, a forced vaporizer that vaporizes the boil-off gas from the liquefied gas is provided.

The fuel gas supply system 10 is used to supply boil-off gas evaporated in a tank 20 storing liquefied natural gas to the propulsion engine 40 as fuel gas in an LNG ship that transports liquefied natural gas. In the present embodiment, one propulsion engine 40 is provided, but a plurality of propulsion engines may be connected to branch pipes branched from the main pipe, respectively.
In this specification, the direction in which the boil-off gas is supplied from the tank 20 to the propulsion engine 40 is referred to as the downstream direction, the opposite direction is referred to as the upstream direction, and the downstream side is referred to as the downstream side from a certain reference position. The upstream side from the position is referred to as the upstream side.

  In the fuel gas supply system of the present embodiment, as will be described below, when the composition ratio of the boil-off gas in the pressurization mechanism and the liquefaction device changes, the boil-off gas in the pressurization mechanism and the liquefaction device is liquefied, and the tank It collects in the liquefied gas stored in. The boil-off gas is a gas-liquid mixed fluid including a boil-off gas that is maintained as a gas without being partially liquefied even if the liquefaction is performed and a liquefied gas generated by the liquefaction. In the present embodiment, all of the mixed fluid is intermittently returned to the liquid phase of the liquefied gas in the tank 20 in the tank 20. At this time, the liquefied gas in the mixed fluid returned to the liquefied gas is stored as the liquefied gas in the tank 20, while the boil-off gas in the mixed fluid is returned to the liquid phase, so that many boil-off gases are liquefied. The amount that dissolves in the gas and floats as boil-off gas in the gas phase of the tank 20 is small. Therefore, a large amount of boil-off gas is dissolved in the liquefied gas in the tank 20 and recovered into the liquefied gas. The gas-liquid mixed fluid generated from the boil-off gas whose composition ratio is returned to the tank 20 may also have a composition ratio different from that of the liquefied gas in the tank 20. However, since the volume of the liquefied gas in the tank 20 is overwhelmingly larger than the amount of boil-off gas to be recovered, even if the gas-liquid mixed fluid whose composition ratio has changed is recovered in the tank 20, the liquefaction in the tank 20 is increased. The composition of the gas hardly changes. As described above, many boil-off gases having different composition ratios are dissolved in the liquefied gas and recovered into the liquefied gas. For this reason, even when the boil-off gas is supplied under the use conditions in which the composition ratio of the boil-off gas changes depending on the use of the boil-off gas liquefaction device, the boil-off gas having a constant composition ratio can be pressurized by the pressurizing mechanism. As a result, the voice-off gas pressurized to a predetermined pressure can be stably supplied to the engine.

The fuel gas supply system 10 of this embodiment mainly includes a tank 20, a pressurizing mechanism 30, a liquefaction device 50, and a control device 60. A pressurizing mechanism 30 is provided on a main pipe 31 through which boil-off gas extending from the tank 20 to the propulsion engine 40 flows.
The pressurizing mechanism 30 is a device that pressurizes and sends out the boil-off gas vaporized from the liquefied gas in order to supply a part of the boil-off gas to the propulsion engine 40 as a fuel gas.
The liquefying device 50 is a device that liquefies a portion of the boil-off gas that has been pressurized and sent out by the pressurizing mechanism 30 and collects it in the tank 20.
The control device 60 is a device that controls the flow of the boil-off gas in the fuel gas supply system 10, and is a device that adjusts at least the adjustment valve provided in the pressurizing mechanism 30 and the liquefying device 50, and the opening of the control valve. is there.

(Pressure mechanism)
The pressurizing mechanism 30 includes gas compressors (pressurizing devices) 32a to 32e, bypass pipes 33a, 33c and 33e, adjustment valves 34a, 34c and 34e, suction snubbers 35a to 35e, discharge snubbers 36a to 36e, The heat exchangers 37a to 37e are mainly provided.
The suction snubbers 35a to 35e are containers that are provided on the upstream side of the gas compressors 32a to 32e, temporarily store the boil-off gas, and have a space configured so that the boil-off gas is smoothly sucked into the gas compressors 32a to 32e. . The discharge snubbers 36a to 36e are containers that are provided on the downstream side of the gas compressors 32a to 32e, and have a space configured to temporarily store the boil-off gas and to smoothly deliver the boil-off gas. The heat exchangers 37a to 37e are provided on the downstream side of the discharge snubbers 36a to 36e, and cool the boil-off gas that has become a high temperature by being pressurized.

The gas compressors 32a to 32e are multistage pressurizing devices connected in series that pressurize (compress) the boil-off gas and send it out. The gas compressors 32a to 32e are portions that suck the boil-off gas in the suction snubber and pressurize it to a predetermined pressure. The gas compressors 32a to 32e use, for example, a reciprocating compressor that sucks boil-off gas from the suction snubbers 35a to 35e when a movable part (plunger or piston) in the gas compressors 32a to 32e reciprocates linearly, and then pressurizes it. be able to. Of the gas compressors 32a to 32e, the oil compressors are used as the gas compressors 32a to 32d, and the oil compressor is used as the gas compressor 32e that pressurizes the boil-off gas at a high pressure. The movable parts of the gas compressors 32a to 32e are driven in conjunction with each other via a rotating shaft (not shown) that is rotated by the power of a driving source (not shown) whose driving is controlled by the control device 60. In the gas compressors 32a to 32e, the boil-off gas is compressed stepwise at the same compression rate, so that the boil-off gas is compressed to the fifth power of the compression rate. For example, by compressing the three to four times in each of the gas compressors 32 a to 32 e, BOG is compressed in ³ 5-4 ⁵ times. For example, if the pressure of the boil-off gas on the suction side of the gas compressor 32a is 0.1 MPa, the pressure on the discharge (delivery) side of the gas compressor 32a is about 0.33 MPa, and the pressure on the discharge side of the gas compressor 32b is about 1. The pressure on the discharge side of the gas compressor 32c is about 3.64 MPa, and the pressure on the discharge side of the gas compressor 32d is about 12.06 MPa. Then, the pressure on the discharge side of the gas compressor 32e is increased to a set target pressure, for example, 39.9 Mpa.

The bypass pipe 33a bypasses the gas compressors 32a and 32b and connects the suction snubber 35a and the output end of the heat exchanger 37b. That is, the part of the pipe through which the boil-off gas before being pressurized by the gas compressor 32a flows and the gas compressor This is a pipe through which boil-off gas flows, connected between pipes through which boil-off gas after pressurization by 32b flows.
The bypass pipe 33c bypasses the gas compressors 32c and 32d and connects the suction snubber 35c and the output end of the heat exchanger 37d, that is, a part of a pipe through which boil-off gas before being pressurized by the gas compressor 32c flows. This is a pipe through which boil-off gas flows, connecting between pipes through which the boil-off gas pressurized by the gas compressor 32d flows.
The bypass pipe 33e bypasses the gas compressor 32e and connects the suction snubber 35e and the output end of the heat exchanger 37e. That is, the bypass pipe 33e is connected to the pipe portion through which the boil-off gas before being pressurized by the gas compressor 32e flows and the gas compressor 32e. It is a pipe through which boil-off gas flows, connected between pipes through which the boil-off gas after pressurization flows.

  The bypass pipes 33a, 33c, 33e are provided with adjustment valves 34a, 34c, 34e for adjusting the opening degree. In addition, each of the bypass pipes 33a, 33c, 33e is provided with pressure gauges 38a, 38c, 38e for measuring the pressure of the boil-off gas pressurized by the gas compressors 32b, 32d, 32e. The pressure gauges 38a, 38c, and 38e need to be provided on the bypass pipes 33a, 33c, and 33e if the pressure of the boil-off gas at the branch point where the bypass pipes 33a, 33c, and 33e branch from the main pipe 31 can be measured. Absent. Each of the adjusting valves 34a, 34c, 34e is provided to be adjusted by pressure information measured by the pressure gauges 38a, 38c, 38e. In FIG. 1, the pressure gauges 38a, 38c, and 38e and the adjustment valves 34a, 34a, 38e, and 38e are shown in order to show that the opening degree of the adjustment valves 34a, 34c, and 34e is controlled based on the pressures measured by the pressure gauges 38a, 38c, and 38e. A dotted line connecting 34c and 34e is drawn. Specifically, information on the measured pressure measured by the pressure gauges 38 a, 38 c, and 38 e is sent to the control device 60. The control device 60 generates a control signal for feedback control that controls the amount of boil-off gas flowing through the bypass pipes 33a, 33c, and 33e based on the difference between the sent measurement pressure and the set pressure, and outputs the control signal. It sends to the adjustment valves 34a, 34c, 33e.

For example, when 1500 kg of boil-off gas is supplied from the upstream side per hour, and the gas compressors 32a and 32b or the gas compressors 32c and 32d or the gas compressor 32e pressurize 2000 kg of boil-off gas per hour and send it downstream. 500 kg of boil-off gas per hour is allowed to flow (reverse flow) through the bypass pipe 33a, the bypass pipe 33c, or the bypass pipe 33e, and is returned to the upstream side of the gas compressors 32a, 32b, the gas compressors 32c, 32d, or the gas compressor 32e. Thus, the opening degree of the adjustment valves 34a, 34c, and 34e provided in the bypass pipes 33a, 33c, and 33e is controlled so that 500 kg of boil-off gas flows per hour. Thereby, the pressure on the downstream side of the gas compressors 32a and 32b, the gas compressors 32c and 32d, or the gas compressor 32e can be adjusted to a predetermined pressure. By adjusting to a predetermined pressure, the boil-off gas flowing from the pressurizing mechanism 30 toward the propulsion engine 40 can be set to the fuel gas pressure and the fuel supply amount required by the propulsion engine 40.
In this embodiment, the bypass pipe 33a is provided so as to bypass the gas compressors 32a, 32b at the same time, and the bypass pipe 33c is provided so as to bypass the gas compressors 32c, 32d at the same time. A bypass pipe that bypasses the path may be provided.

In the pressurizing mechanism 30 of the present embodiment, a check valve 31a is provided between the fourth-stage gas compressor 32d and the fifth-stage gas compressor 32e. Oil that flows out of the oil supply type compressor included in the boil-off gas sent from the gas compressor 32e that is an oil supply type compressor to the gas compressor 32d that is an oilless type compressor and the gas compressors 32b and 32c on the upstream side thereof. The check valve 31a is provided in order to prevent impurities such as gas from flowing in and contaminating the gas compressor 32d and the devices such as the gas compressors 32b and 32c located upstream thereof.
A check valve may be provided between the first-stage gas compressor 32a and the second-stage gas compressor 32b, counting from the upstream side.
Further, a check valve 31b is provided on the downstream side of the bypass pipe 33e so that the boil-off gas once supplied to the engine 40 does not flow backward toward the upstream side.

A branch pipe 39 branches from a branch point where the bypass pipe 33 a branches from the main pipe 31 and a junction point where the bypass pipe 33 c joins the main pipe 31 and is connected to the gas processing device 70. The gas processing device 70 is a device that processes boil-off gas, and may be connected to steam utilization equipment or power generation equipment, or may be a device that simply incinerates boil-off gas. As will be described later, it is preferable that the boil-off gas can be processed even if the composition ratio of the boil-off gas, for example, the ratio of components having a low boiling point in the boil-off gas changes. For example, when the gas processing device 70 is a four-cycle internal combustion engine used for a generator, the air-fuel ratio and the ignition timing can be adjusted to cope with a change in the composition ratio of the boil-off gas. Is preferred. In addition, the gas processing device 70 is preferably configured to have a capacity capable of consuming the entire amount of boil-off gas generated from the tank 20.
The branch pipe 39 extending to the gas processing device 70 is provided with a check valve 31c so that the boil-off gas flowing to the gas processing device 70 side does not flow back to the pressurizing mechanism 30 side. Further, the branch pipe 39 is provided with a gas processing control valve 71 a and is configured to adjust the amount of boil-off gas processed by the gas processing device 70. In the gas processing control valve 71a, the opening degree of the gas processing control valve 71a is controlled by a control signal sent from the control device 60 to the contact 71b.

(Propulsion engine)
The ship used in the present embodiment is provided with a propulsion engine 40. As the propulsion engine 40, for example, a dual fuel engine that can selectively use oil such as heavy oil as fuel gas while using boil-off gas of liquefied gas as fuel gas is used.
The propulsion engine 40 burns in the combustion chamber using the supplied boil-off gas as fuel gas to extract power, and rotates a main shaft (not shown) and a propeller of the vessel (not shown) that connect the propulsion engine 40 and a propeller of the vessel (not shown). As the propulsion engine 40, for example, a two-stroke cycle low-speed diesel engine can be used.

The propulsion engine 40 is connected to an engine control unit (hereinafter referred to as ECU) 62, and the drive is controlled by the ECU 62. The ECU 62 supplies the fuel gas to the propulsion engine 40 so that the main shaft rotation speed measured by the tachometer 42 provided to measure the rotation of the main shaft connecting the propulsion engine 40 and the propeller becomes the target rotation speed. The drive of the propulsion engine 40 is controlled by controlling the opening degree of the pressure control valve 44 provided in the supply line. That is, the ECU 62 determines the load of the propulsion engine 40 so that the main shaft rotation speed of the main shaft connecting the propulsion engine 40 and the propeller for propulsion becomes the target rotation speed, and controls the pressure of the fuel gas based on this. It is. The ECU 62 determines the load of the propulsion engine 40 so that the main shaft rotational speed that changes due to changes in natural conditions such as weather, sea state wind, and wave height is maintained at the target rotational speed, as well as operator deceleration, acceleration, turning, etc. The load of the propulsion engine 40 can also be determined according to the operation command value of the propeller rotational speed provided by the instruction. In FIG. 1, in order to represent the above-described control, a dotted line connecting the tachometer 42 and the pressure control valve 44 is simply drawn.
The ECU 62 is configured to set a target pressure on the delivery side of the gas compressor 32e located on the most downstream side based on the determined load and to send the target pressure to the pressure control device 60. This target pressure is the fuel supply pressure that the propulsion engine 40 requires from the fuel gas supply system 30. The control device 62 controls the amount of boil-off gas delivered by the pressurizing mechanism 30 using the target pressure.

(Liquefaction device)
The liquefying device 50 is connected to the pressurizing mechanism 30 through the branch pipe 51. The branch pipe 51 is connected to the main pipe 31 between the branch point from the main pipe 31 of the bypass pipe 33c and the check valve 31a.
The liquefying device 50 is a device that collects the boil-off gas that has become unnecessary due to fluctuations in the load of the propulsion engine 40 into the tank 20 as liquefied gas. A part of the boil-off gas is not liquefied in spite of the liquefaction treatment of the liquefying device 50 and maintains the gas. The boil-off gas is vaporized from the liquefied gas in the tank 20 as necessary, merges with the fresh boil-off gas flowing through the main pipe 31 for pressurization, and is pressurized by the pressurizer 30 again. .

The liquefying apparatus 50 mainly includes a heat exchanger 53, an expansion valve 54, a gas-liquid separator 56, a gas pipe 57, a liquid recovery pipe 58, and a gas-liquid recovery pipe 59.
The heat exchanger 53 cools the boil-off gas before liquefying the boil-off gas.
The expansion valve 54 expands and liquefies the cooled boil-off gas. The expansion valve 54 may be a valve that changes in an isenthalpy process such as a JT (Joule Thomson) valve, but is preferably a valve that changes in an isentropy process such as an expander.
The gas-liquid separator 56 separates the cooled boil-off gas from the liquefied gas.
The gas pipe 57 vaporizes the boil-off gas separated from the liquefied gas from the liquefied gas in the tank 20 and joins the fresh boil-off gas taken out from the tank 20 and flowing.
The liquid recovery pipe 58 returns the liquefied gas separated by the gas-liquid separator 56 into the tank 20.
The gas-liquid recovery pipe 59 integrates the boil-off gas and the liquefied gas in the gas-liquid separator 56 and returns the gas-liquid mixed fluid from the gas-liquid separator 56 to the liquid phase of the liquefied gas in the tank 20.

  A control valve 55a for controlling the amount of boil-off gas flowing in the liquefying device 50 is provided at a portion of the branch pipe 51 between the heat exchanger 53 and the expansion valve 54. The gas-liquid separator 56 has a gas-liquid separation. A pressure gauge 56 a for measuring the pressure of the boil-off gas in the vessel 56 is provided. The opening degree of the control valve 55a is adjusted based on the measurement result of the pressure gauge 56a. Specifically, the measurement result of the pressure gauge 56a is sent to the control device 60, and the control device 60 generates a control signal for controlling the opening degree of the control valve 55a based on the measurement result of the pressure gauge 56a. A signal is sent to the control valve 55a.

  The gas pipe 57 is provided with a confluence control valve 57a and a pressure gauge 57b for measuring the pressure of the boil-off gas. The opening degree of the merging control valve 57a is adjusted based on a control signal generated based on the measurement result of the pressure gauge 56b or a control signal sent from the pressure control device 60 to the contact 57d. At this time, a control signal having a low target opening is selected from among a control signal indicating the target opening generated based on the measurement result of the pressure gauge 56b and a control signal indicating the target opening sent from the contact 57d. The selector 57c is used to send to the merging control valve 57a. In FIG. 1, in order to show that the opening degree of the merging control valve 57a is controlled based on the measurement result of the pressure gauge 56b, a dotted line connecting the pressure gauge 56b and the selector 57c is drawn. Specifically, the measurement result of the pressure gauge 56b is sent to the control device 60, and the control device 60 generates a control signal for the opening degree of the merging control valve 57a based on the sent measurement result, and this control signal is sent to the contact 57d. Send to. The gas-liquid separator 56 is provided with a liquid level meter 56b for measuring the liquid level of the liquefied gas.

The liquid recovery pipe 58 is provided with a liquid recovery control valve 58a. The amount of liquefied gas flowing through the liquid recovery pipe 58 is controlled by the opening degree of the liquid recovery control valve 58a. The opening degree of the liquid recovery control valve 58a is controlled so that the liquid level falls within a predetermined range based on the measurement result of the liquid level by the liquid level meter 56b.
The gas / liquid recovery piping 59 is provided with a gas / liquid recovery control valve 59a. The amount of the gas-liquid mixed fluid flowing through the gas-liquid recovery pipe 59 is controlled by the opening degree of the gas-liquid recovery control valve 59a. The opening degree of the gas-liquid recovery control valve 59a is controlled based on a control signal sent from the control device 60 to the contact 59b.
The gas-liquid recovery pipe 59 has a larger channel cross section than the liquid recovery pipe 58 and can flow a larger amount of gas-liquid mixed fluid than the liquid recovery pipe 58. Specifically, all of the boil-off gas and the liquefied gas in the gas-liquid separator 56 can be flowed in a short time.
Therefore, the gas-liquid recovery control valve 59a provided in the gas-liquid recovery pipe 59 is a valve that is larger in size than the liquid recovery control valve 58a so that a large amount of gas-liquid mixed fluid can flow.

(Change in composition ratio of boil-off gas and its response)
In such a fuel gas supply system 10, the amount of boil-off gas supplied by the pressurizing mechanism 30, the consumption of the propulsion engine 40, Various states occur between the amount of boil-off gas consumed by the gas processing device 70 and the amount recovered in the tank 20 of the liquefied gas generated by the liquefying device 50. These states include at least the following control states 1A and 1B.

(Control state 1A) Boil-off gas supply amount = propulsion engine consumption amount X1 + gas processing device consumption amount Y1, recovery amount Z1 = 0
(Control state 1B) Boil-off gas supply amount = propulsion engine consumption amount X2 (<X1) + gas treatment device consumption amount Y2 + recovery amount Z2 (> 0)
In both the control state 1A and the control state 1B, the supply amount of the boil-off gas of the pressurizing mechanism 30 is equal to the total amount of the consumption amount of the propulsion engine 50, the consumption amount of the gas processing device 70, and the recovery amount to the liquefaction tank 20. Control state (steady state). The control state 1A and the control state 1B are referred to as the control state 1 when collectively described.

  In the control state 1A, since the boil-off gas supply amount matches the boil-off gas consumption amount X1 of the propulsion engine 40 and the total amount of boil-off gas consumption amount Y1 of the gas processing device 70, the boil-off gas is supplied to the liquefaction device 50. The control valves 55a, 57a, 58a are closed so that no flow-in occurs, that is, a control state in which the recovery amount Z1 is zero is shown.

In the control state 1B, the supply amount of the boil-off gas is larger than the total amount of the boil-off gas consumption X2 of the propulsion engine 40 and the boil-off gas consumption Y2 of the gas processing device 70, and the difference is recovered in the tank 20. That is, a control state in which the control valve 55a is opened so that the liquefied gas generated by the liquefaction of the boil-off gas is recovered in the tank 20 is shown. In this control state, in addition to the liquefied gas, boil-off gas that has not been liquefied by the function of the expansion valve 54 flows into the gas-liquid separator 66. At this time, the liquid recovery control valve 58a is opened to bring the liquefied gas into the tank 20. The boil-off gas is vaporized from the liquefied gas in the tank 20 and merged with the fresh boil-off gas before pressurization flowing through the main pipe 31 while opening the merging control valve 57a. In this control state 1B, the consumption Y2 of the gas processing device 70 is adjusted so that the pressure of the boil-off gas in the tank 20 is in a predetermined range, that is, the opening degree of the gas processing gas processing control valve 71a is adjusted. Adjusted.
Thus, in the control state 1B, the supply amount of the boil-off gas of the pressurizing mechanism 30 is larger than the total amount of the consumption amount X2 of the propulsion engine 40 and the consumption amount Y2 of the gas processing device 70, and this difference is stored in the tank 20. The steady state is obtained by setting the recovery amount Z2 of the liquefied gas to be recovered.

Table 1 below shows an example of the flow of the boil-off gas and the change in the state of the control valve regarding the transition from the control state 1A to the control state 1B.
In the transition state between the control states, the supply amount of the boil-off gas, the consumption amount of the propulsion engine, the consumption amount of the gas processing device, and the total amount of the recovered amount of the liquefied gas are in a non-equilibrium state that does not necessarily match. Table 1 shows an example of changes in each flow rate and changes in the opening of the control valve. In the table, “⇒” indicates a state transition, “↑” indicates that the flow rate increases or the opening of the control valve increases as the state proceeds in the direction of “⇒”. “↓” indicates that the flow rate decreases or the opening of the control valve decreases as the state proceeds in the direction of “⇒”. “˜ →” in the table means that the opening degree slightly fluctuates during the transition of the state, but does not change greatly in the end and is substantially maintained. The flow rate in the table indicates the flow rate per unit time, and is indexed with the amount of boil-off gas generated from the tank 20 as 100. In Table 1, the transition from the control state 1A to the control state 1B is shown, but the transition from the control state 1B to the control state 1A may be reversed from each state shown in Table 1.

  In the control state 1B, the liquefied gas liquefied from the boil-off gas out of the boil-off gas is collected in the tank 20 and the boil-off gas that has not been liquefied circulates in the pressurizing mechanism 30 and the liquefying device 50. Is continued for a long time, the composition ratio of the boil-off gas pressurized by the pressurizing mechanism 30 changes with time. That is, the boil-off gas joined from the liquefier 50 through the merging control valve 57a to the fresh boil-off gas before pressurization flowing through the main pipe 31 drawn from the tank 20 through the merging control valve 57a was not liquefied by the liquefier 50 separated from the liquefied gas. The ratio of the low boiling point component of the boil-off gas is higher than that of the boil-off gas in the tank 20. For this reason, the composition ratio of the boil-off gas pressurized changes with time. That is, the composition ratio in the boil-off gas of the component having a lower boiling point than the main component of the liquefied gas increases. When the liquefied gas in the tank 20 is natural gas, the main component is methane, ethane or the like, but the natural gas contains a small amount of nitrogen having a lower boiling point than methane or ethane. On the other hand, the ratio of the nitrogen component in the boil-off gas that has passed through the liquefier 50 increases. For example, even when 1% nitrogen is mixed in liquefied natural gas, the nitrogen ratio of the boil-off gas that has passed through the liquefier 50 increases to 10% or more if the control state 1B is maintained for a long time. As the nitrogen ratio increases, the liquefied gas recovered into the tank 20 through the liquid recovery control valve 58a decreases, and the pressure of the boil-off gas by the gas compressors 32a to 32e in the pressurizing mechanism 30 increases as will be described later. . As a result, the amount of boil-off gas flowing through the bypass pipes 33a and 33c gradually increases by adjusting the opening of the adjustment valves 34a and 34c, and finally, it becomes difficult to control the amount of boil-off gas delivered by the pressurizing mechanism 30. .

FIG. 2 is a diagram showing an example of the relationship between the pressure and volume of boil-off gas received in the compression process by the gas compressor. Of the boil-off gas of the liquefied gas, the boil-off gas containing only the main component not including the component having a low boiling point rises from the pressure P1 to the pressure P2 along the path A when being compressed from the volume V1 to the volume V2. . On the other hand, the boil-off gas containing a component having a low boiling point rises from the pressure P1 to the pressure P3 (P3> P2) according to the path B even if the boil-off gas is subjected to the same volume change compression process. For this reason, the energy consumed by the gas compressor in the same compression treatment with the boil-off gas containing a large amount of components having a boiling point is larger than that of the boil-off gas not containing a component having a low boiling point.
In this way, by increasing the pressure of the boil-off gas pressurized by the same compression process, the amount (mass) of boil-off gas delivered by the pressurizing mechanism 30 is controlled (bypass pipes 33a, 33c, 33e by the adjusting valves 34a, 34c, 34e). Pressure based control of the amount of boil-off gas flowing through is difficult.

For this reason, in this embodiment, the gas-liquid mixed fluid containing the boil-off gas in which the composition ratio in the pressurizing mechanism 30 and the liquefier 50 is changed with the passage of time in the control state 1B and the liquefied gas liquefied from the boil-off gas is used. Then, the gas-liquid separator 56 is pumped through the gas-liquid recovery pipe 59 into the liquid phase of the liquefied gas in the tank 20. Since the pressure in the gas-liquid separator 56 is controlled by the control valve 55a so as to be higher than the pressure in the tank 20, the gas-liquid separator 56 is opened by opening the gas-liquid recovery control valve 59a. The boil-off gas and the liquefied gas are integrally fed to the tank 20 through the gas-liquid recovery pipe 59. This state is the control state 2 in which the control valve 59b is opened.
Therefore, the control device 60 performs control for switching the flow of the boil-off gas from the control state 1 in which the gas-liquid recovery control valve 59a is closed to the control state 2 in which the gas-liquid recovery control valve 59a is opened. Specifically, the control device 60 generates a control signal in which the opening degree of the gas-liquid recovery control valve 59a is set to 0 and sends the control signal to the gas-liquid recovery control valve 59a, and opens the gas-liquid recovery control valve 59a. The control state 2 is switched to the control state 2 in which the control signal is generated and sent to the gas-liquid recovery control valve 59a. Thereby, the boil-off gas in which the composition ratio in the pressurizing mechanism 30 and the liquefaction apparatus 50 is changed and the liquefied gas liquefied from the boil-off gas can be efficiently recovered in the tank 20.

  Specifically, when the control state 2 is started, since a gas-liquid mixed fluid having a temperature higher than that of the liquefied gas in the tank 20 flows into the liquefied gas, heat is temporarily generated in the liquefied gas in the tank 20. Flow into. For this reason, vaporization of the liquefied gas increases in the tank 20, and the pressure in the tank 20 temporarily increases. In the present embodiment, it is preferable that the control device 60 increases the opening degree of the gas processing control valve 71a as compared with the control state 1 and increases the amount of boil-off gas flowing to the gas processing device 70. Thereafter, the pressure in the tank 20 decreases. Thereby, the boil-off gas accumulated in the pressurizing mechanism 30 and the liquefying device 50 can be efficiently replaced with fresh boil-off gas evaporated from the liquefied gas. In this case, the duration of the control state 2 includes information on the amount of the gas-liquid mixed fluid recovered per unit time in the tank 20 and the amount of boil-off gas accumulated in the pressurizing mechanism 30 and the liquefying device 50 (for example, Further, it is preferable to determine the volume based on the volume of the volume occupied by the boil-off gas and the liquefied gas in the pressurizing mechanism 30 and the liquefying device 50).

  The liquefied gas in the mixed fluid returned to the liquefied gas in the tank 20 is stored as the liquefied gas in the tank 20. On the other hand, since the boil-off gas in the mixed fluid is returned to the liquid phase of the liquefied gas, a large amount of the boil-off gas is dissolved in the liquefied gas and the amount that floats as boil-off gas in the gas phase of the tank 20 is small. Therefore, a large amount of boil-off gas is dissolved in the liquefied gas in the tank 20 and recovered into the liquefied gas. The gas-liquid mixed fluid generated from the boil-off gas whose composition ratio is returned to the tank 20 may also have a composition ratio different from that of the liquefied gas in the tank 20. However, since the volume of the liquefied gas in the tank 20 is overwhelmingly larger than the amount of boil-off gas to be recovered, even if the gas-liquid mixed fluid whose composition ratio has changed is recovered in the tank 20, the liquefaction in the tank 20 is increased. The composition of the gas hardly changes.

  It should be noted that the control state 1 is that the merging control valve 57a is open, and the control state 2 is that the merging control valve 57a is closed. It is preferable because it can be recovered in a short time. Of course, the opening / closing control of the merging control valve 57a is based on the premise that the pressure measured by the pressure gauge 57b falls within a predetermined range. When this assumption is broken, the merging control valve 57a may be closed in the control state 1, and the merging control valve 57a may be opened in the control state 2.

  In the present embodiment, as shown in FIG. 3, a determination device 80 that determines whether or not the composition ratio of the boil-off gas pressurized by the pressurizing mechanism 30 has changed is provided, and the determination device 80 changes the composition ratio of the boil-off gas. Is determined, the determination result is sent to the control device 60, and the control device 60 preferably controls the opening degrees of the various control valves so as to switch from the control state 1B to the control state 2. FIG. 3 is a view showing an example of a preferred modification of the fuel gas supply system 10.

  For example, in the control state 1, the determination device 80 determines a change in the composition ratio of the boil-off gas based on information on the opening degree of the adjustment valves 34a, 34c, and 34e, for example, a value of a control signal that controls the opening degree. preferable. This determination is based on the change in the compression characteristics of the boil-off gas in which the pressure changes as the composition ratio changes even when compression is performed at the same volume shown in FIG. In this case, the determination device 80 determines that the composition ratio of the boil-off gas has changed when the information on the opening, for example, the value of the control signal of the opening changes with the elapsed time and deviates from the predetermined range. It is preferable from the point of realizing accuracy. When the ratio of the component having a low boiling point in the boil-off gas increases, the pressure of the boil-off gas pressurized by the gas compressors 32a to 32e increases as shown in FIG. In order to maintain the pressure within a predetermined range, it is necessary to increase the amount of boil-off gas flowing through the bypass pipes 33a, 33c, and 33e by increasing the opening degree of the adjusting valves 34a, 34c, and 34e. For this reason, the determination apparatus 80 can determine the change in the composition ratio of the boil-off gas based on the information on the opening amounts of the adjustment valves 34a, 34c, and 34e in the control state 1 that is a steady state.

  For example, the determination device 80 preferably determines the change in the composition ratio of the boil-off gas based on the energy consumption per unit time that the pressurization device consumes in the control state 1 in order to pressurize the boil-off gas. This determination also utilizes the change in the compression characteristics of the boil-off gas in which the pressure changes as the composition ratio changes even when compression is performed at the same volume shown in FIG. In this case, it is also preferable from the viewpoint of realizing high determination accuracy that the determination device 80 determines that the composition ratio of the boil-off gas has changed when the energy consumption changes with the elapsed time and deviates from the predetermined range. When the ratio of the component having a low boiling point in the boil-off gas increases, the pressure of the boil-off gas pressurized by the gas compressors 32a to 32e increases, so that the energy consumption for driving the gas compressors 32a to 32e increases. For this reason, the determination apparatus 80 can determine the change in the composition ratio of the boil-off gas based on the energy consumption for driving the gas compressors 32a to 32e in the control state 1 which is a steady state.

In such a fuel gas supply system 10, the following fuel gas supply method can be performed.
That is, the fuel gas supply method is
(1) pressurizing and sending out the boil-off gas vaporized from the liquefied gas in order to supply the boil-off gas vaporized from the tank 20 storing the liquefied gas as the fuel gas to the propulsion engine 40;
(2) liquefying a part of the pressurized boil-off gas and collecting it in the tank 20.
(3) The step of collecting in the tank 20 was obtained by controlling the liquefied gas generated by liquefying the boil-off gas from the boil-off gas that was not liquefied and collecting it in the tank 20 and by liquefying the boil-off gas. This is performed by selecting one of the states between the control states 2 in which the gas-liquid mixed fluid containing the liquefied gas and the boil-off gas that has not been liquefied is returned to the liquid phase of the liquefied gas in the tank 20.

  At this time, under the control of the control device 60, the boil-off gas that is maintained in the boil-off gas without being liquefied in the control state 1 is merged with the flow of the boil-off gas before being pressurized and vaporized from the liquefied gas. preferable.

  In the control state 2, it is preferable that the amount of boil-off gas flowing to the gas processing device 70 that consumes part of the boil-off gas pressurized by the pressurizing mechanism 30 as fuel is larger than that in the control state 1.

Furthermore, the determination device 80 has a step of determining whether or not there is a change in the composition ratio of the boil-off gas to be pressurized,
It is preferable that the control device 60 switches from the control state 1 to the control state 2 based on the determination result of the composition ratio.

The pressurizing mechanism 30 that pressurizes the boil-off gas includes a gas compressor 32a between a gas compressor (pressurizer) 32a to 32e that pressurizes the boil-off gas and a pipe through which the boil-off gas before and after pressurization by the gas compressors 32a to 32e flows. The bypass pipes 33a, 33c, 33e that bypass the -32e and the boil-off gas pressurized by the gas compressors 32a-32e flow through the bypass pipes 33a, 33c, 33e so that the pressure is within a predetermined range. Adjustment valves 34a, 34c, 34e for controlling the amount of boil-off gas,
In the determining step, it is preferable that the determining device 80 determines the change in the composition ratio of the boil-off gas based on the information on the opening degree of the adjusting valves 34a, 34c, 34e in the control state 1.

  At this time, in the determination step, it is preferable that the determination device 80 determines that the composition ratio of the boil-off gas has changed when the opening degree of the adjustment valves 34a, 34c, 34e changes with the elapsed time and deviates from the predetermined range. .

  In the determining step, the determination device 80 changes the composition ratio of the boil-off gas based on the energy consumption per unit time that the pressurizing mechanism 30 that pressurizes the boil-off gas consumes for pressurization in the control state 1. It is also preferable to determine.

  At this time, in the determination step, it is preferable that the determination device 80 determines that the composition ratio of the boil-off gas has changed when the energy consumption changes with the elapsed time and falls outside the predetermined range.

  The fuel gas supply system, the fuel gas supply method, and the ship according to the present invention have been described in detail above. However, the present invention is not limited to the above-described embodiment, and various improvements and modifications can be made without departing from the gist of the present invention. Of course, you may do it.

DESCRIPTION OF SYMBOLS 10 Fuel gas supply system 20 Tank 30 Pressurization mechanism 31 Main piping 31a-31d Check valve 32a-32e Gas compressor 33a, 33c, 33e Bypass pipe 34a, 34c, 34e, 55a, 57a Adjustment valve 35a-35e Suction snubber 36a- 36e Discharge snubbers 37a to 37e Heat exchangers 38a, 38c, 38e, 65a, 65b Pressure gauge 39 Branch pipe 40 Propulsion engine 42 Tachometer 44 Flow control valve 50 Liquefaction device 51 Branch pipe 53 Heat exchanger 54 Expansion valve 55a Control valve 56 Gas-liquid separator 56a Pressure gauge 56b Liquid level gauge 57 Gas pipe 55a Control valve 57a Merge control valve 57b Pressure gauge 57c Selector 57d Contact 58 Liquid recovery pipe 58a Liquid recovery control valve 59 Gas-liquid recovery pipe 59a Gas-liquid recovery control valve 60 Controller 62 d Gin control unit 70 gas processing device 71 gas treatment control valve 71b contacts 80 determining device

Claims (18)

A fuel gas supply system for supplying fuel gas to an engine,
A tank for storing liquefied gas;
A pressurizing mechanism for pressurizing and sending out the boil-off gas vaporized from the liquefied gas in order to supply the boil-off gas vaporized from the liquefied gas as a fuel gas to the engine;
A liquefying device that liquefies a portion of the boil-off gas that has been pressurized and delivered by the pressurizing mechanism and collects it in the tank;
The liquefaction device includes a liquid recovery pipe that recovers the liquefied gas generated by liquefaction by the liquefying device in the tank, and a liquid recovery that is provided in the liquid recovery pipe and controls an amount of the liquefied gas flowing through the liquid recovery pipe A control valve, a gas-liquid recovery pipe for returning the gas-liquid mixed fluid of the liquefied gas generated by liquefaction by the liquefying device and the boil-off gas not liquefied into the liquid phase of the liquefied gas in the tank, and the gas-liquid recovery A fuel gas supply system comprising: a gas-liquid recovery control valve that is provided in the pipe and controls an amount of the gas-liquid mixed fluid flowing through the gas-liquid recovery pipe. The fuel gas supply system further includes a control device for controlling the pressurizing mechanism and the liquefying device,
2. The fuel gas supply system according to claim 1, wherein the control device performs control to switch from a control state 1 in which the gas-liquid recovery control valve is closed to a control state 2 in which the gas-liquid recovery control valve is opened. The liquefying device includes a merging control valve that controls an amount of boil-off gas that is maintained without being liquefied in the boil-off gas and merged with a flow of the boil-off gas that has been vaporized from the liquefied gas and before being pressurized,
In the control state 1, the merging control valve is further open.
The fuel gas supply system according to claim 2, wherein the control state 2 is a state in which the merging control valve is closed. Furthermore, it is provided in a branch pipe connected to a gas processing device that consumes a part of the boil-off gas pressurized by the pressurizing mechanism as fuel, and controls the amount of boil-off gas flowing through the branch pipe toward the gas processing device. A gas processing control valve for
4. The fuel gas supply system according to claim 2, wherein the control device increases an opening degree of the gas processing control valve in the control state 2 compared to the control state 1. Furthermore, the fuel gas supply system includes a determination device that determines whether or not there is a change in the composition ratio of the boil-off gas pressurized by the pressurizing mechanism,
The fuel gas supply system according to any one of claims 2 to 4, wherein the control device switches from the control state 1 to the control state 2 based on a determination result of the determination device. The pressurizing mechanism includes a pressurizing device that pressurizes the boil-off gas, a bypass pipe that bypasses the pressurizing device and connects the pipe through which the boil-off gas before and after pressurization by the pressurizing device flows. An adjustment valve that controls the amount of boil-off gas flowing through the bypass pipe so that the pressure of the boil-off gas after being pressurized by the pressure device is within a predetermined range;
The fuel gas supply system according to claim 5, wherein the determination device determines a change in a composition ratio of the boil-off gas based on information on an opening degree of the adjustment valve in the control state 1.   The fuel gas supply system according to claim 6, wherein the determination device determines that the composition ratio of the boil-off gas has changed when the opening degree changes with time and falls outside a predetermined range.   The said determination apparatus determines the change of the composition ratio of boil-off gas based on the energy consumption per unit time which the said pressurization mechanism consumes in the said control state 1 in order to pressurize the said boil-off gas. Fuel gas supply system.   9. The fuel gas supply system according to claim 8, wherein the determination device determines that the composition ratio of the boil-off gas has changed when the consumed energy changes with time and falls outside a predetermined range. The fuel gas supply system according to any one of claims 1 to 9,
And a propulsion engine that is driven using fuel pressurized by the pressurizing mechanism. A fuel gas supply method for supplying fuel gas to an engine,
Pressurizing and sending the boil-off gas vaporized from the liquefied gas in order to supply the boil-off gas vaporized from a tank storing the liquefied gas as a fuel gas to the engine;
Liquefying a portion of the pressurized boil-off gas and collecting it in the tank,
The step of collecting in the tank includes a control state 1 in which the liquefied gas generated by liquefying the boil-off gas is separated from the boil-off gas that has not been liquefied and collected in the tank, and the liquefied gas obtained by liquefying the boil-off gas, The fuel is characterized in that it is performed by selecting one of the states between the control states 2 in which the gas-liquid mixed fluid containing the boil-off gas that has not been liquefied is returned to the liquid phase of the liquefied gas in the tank. Gas supply method.   The fuel gas supply according to claim 11, wherein in the control state 1, the boil-off gas that is maintained in the boil-off gas without being liquefied is merged with the flow of the boil-off gas that has been vaporized from the liquefied gas and before being pressurized. Method.   The amount of boil-off gas flowing in a gas processing device that consumes part of the boil-off gas pressurized by the pressurizing mechanism as fuel in the control state 2 is larger than that in the control state 1. Fuel gas supply method. Furthermore, the step of determining the presence or absence of a change in the composition ratio of the boil-off gas to be pressurized,
The fuel gas supply method according to any one of claims 11 to 13, wherein the control state 1 is switched to the control state 2 based on the determination result of the composition ratio. The pressurizing mechanism that pressurizes the boil-off gas is a bypass pipe that bypasses and connects the pressurizer that pressurizes the boil-off gas and a pipe through which the boil-off gas before and after pressurization by the pressurizer flows. And an adjustment valve that controls the amount of boil-off gas flowing through the bypass pipe so that the pressure of the boil-off gas after being pressurized by the pressurizer is within a predetermined range,
The fuel gas supply method according to claim 14, wherein in the determining step, a change in the composition ratio of the boil-off gas is determined based on information on an opening degree of the adjustment valve in the control state 1.   The fuel gas supply method according to claim 15, wherein in the determining step, it is determined that the composition ratio of the boil-off gas has changed when the opening degree changes with time and is out of a predetermined range.   The determination step determines a change in the composition ratio of the boil-off gas based on energy consumption per unit time consumed for pressurization by the pressurizing mechanism that pressurizes the boil-off gas in the control state 1. The fuel gas supply method described in 1.   18. The fuel gas supply method according to claim 17, wherein in the determination step, it is determined that the composition ratio of the boil-off gas has changed when the consumed energy changes with the elapsed time and deviates from a predetermined range.

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