JP2012122554A - Facility for preservation and re-gasification of liquefied gas, and method for re-liquefying boil-off gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a facility for preservation and re-gasification of liquefied gas, which can use a compact heat exchanger when performing heat exchange between liquefied gas before re-gasification and boil-off gas to re-liquefy the boil-off gas.SOLUTION: An apparatus 1 for re-liquefying boil-off gas, includes: the heat exchanger 10 for re-liquefying, which concentrates boil-off gas by heat exchange with liquefied natural gas (LNG) introduced to an apparatus for re-gasification; compressors 5, 6, 9 for re-liquefying, which compress the boil-off gas introduced to the heat exchanger 10 for re-liquefying; and a desuperheater 16 disposed upstream of a boil-off gas flow of the heat exchanger 10 for re-liquefying and spraying the LNG to increase the wetness of the boil-off gas.

Description

  The present invention relates to a liquefied gas storage / regasification facility provided with a reliquefaction device for reliquefying boil-off gas.

  For example, LNG storage tanks that store LNG in FSRU (Floating Storage and Regasification Unit) and FPSO (Floating Production, Storage and Offloading) that receive, store, and regasify LNG (liquefied natural gas) (Liquefied gas storage tank) is provided. In this LNG storage tank, LNG evaporates due to inevitable intrusion heat, stirring by LNG reception, etc., and boil-off gas (hereinafter referred to as “BOG”) is generated. In order to keep the internal pressure constant while avoiding an increase in the pressure in the LNG storage tank due to this BOG, the BOG is extracted from the tank and subjected to combustion treatment or open to the atmosphere, or returned to the liquid (reliquefied again). There is a method of returning to the LNG storage tank. As a method of reliquefying BOG and returning it to the LNG storage tank, a method is generally used in which BOG extracted from the LNG storage tank is pressurized by a compressor, cooled by the cold generated by the refrigerator, and condensed. (See Patent Document 1). As a refrigerator used for such applications, a Brayton cycle using nitrogen or the like as a primary refrigerant is used.

JP 2005-265170 A

  However, the conventional cooling method using the Brayton cycle requires a large-scale plant such as a compressor or an expander, and has a problem that a predetermined skill level is required for handling the plant.

  The present invention enables a compact special heat exchanger to be used when re-liquefying the boil-off gas by exchanging heat between the liquefied gas and the boil-off gas in a facility for gasifying a large amount of the liquefied gas. An object is to provide a gas storage regasification facility and a boil-off gas reliquefaction method.

In order to solve the above problems, the liquefied gas storage and regasification facility and the boil-off gas reliquefaction method of the present invention employ the following means.
That is, the liquefied gas storage and regasification facility according to the present invention includes a liquefied gas storage tank that stores the liquefied gas, and a recycle that gasifies the liquefied gas when the liquefied gas in the liquefied gas storage tank is discharged to the outside. In a liquefied gas storage and regasification facility comprising a gasifier and a boiloff gas reliquefaction device for reliquefying the boiloff gas vaporized from the liquefied gas in the liquefied gas storage tank, the boiloff gas reliquefaction device comprises: A heat exchanger for reliquefaction that condenses the boil-off gas by exchanging heat with the liquefied gas led to the regasifier, and a compression for reliquefaction that compresses the boil-off gas led to the heat exchanger for reliquefaction And slow heat that increases the wetness of the boil-off gas by spraying a part of the liquefied gas, disposed on the upstream side of the boil-off gas flow of the reliquefaction heat exchanger Characterized in that it comprises and.

Condensation by thermal contact with the liquefied gas by boosting the condensing pressure by boosting the boil-off gas led to the re-liquefaction heat exchanger that condenses the boil-off gas by another compressor such as re-liquefaction or boiler Gain ability. The gas to be introduced into the condenser is preheated with a slow heat generator that increases the wetness of the boiloff gas by spraying the liquefied gas in advance, thereby slowing down the boiloff gas before entering the heat exchanger for reliquefaction. When it reaches the lowest temperature, it becomes saturated. As a result, the temperature difference from the liquefied gas (coolant side) at the boil-off gas inlet of the reliquefaction heat exchanger can be reduced, but this is a reliquefaction heat exchanger assumed to be used in the present invention. An aluminum plate fin heat exchanger is an indispensable requirement for countermeasures against stress caused by temperature gradients.
Further, according to the present invention, the boil-off gas is reliquefied by using the sensible heat of the liquefied gas that is regasified by the regasifier when discharged outside as cold heat in the reliquefaction heat exchanger. As a result, there is no need to generate additional cooling and overall energy efficiency is improved.

  By adopting a plate-type heat exchanger as a heat exchanger for reliquefaction, it is possible to realize a more compact facility than a shell-and-tube heat exchanger, for example, and it can be placed on a narrow ship or on a floating body. To improve. As the plate heat exchanger, for example, there are stainless steel or aluminum blazed types, but according to the present plan, an inexpensive blazed joining method of aluminum plates can be applied.

  Furthermore, the liquefied gas storage and regasification facility of the present invention includes a fuel gas compressor that guides the boil-off gas in the liquefied gas storage tank to a boiler, and the fuel gas compressor has a capacity control for controlling a discharge amount. And a motor having a variable rotation speed for rotating an impeller that compresses boil-off gas, and is used in combination as the reliquefaction compressor, and the capacity control valve is configured to supply the boil-off gas to the boiler. The opening degree is controlled based on the opening degree of the boiler fuel gas valve that adjusts the flow rate, and the rotation speed of the motor is controlled so that the capacity control valve is in a predetermined opening range. It is characterized by.

The opening degree of the capacity control valve of the fuel gas compressor is controlled based on the opening degree of the boiler fuel gas valve that adjusts the flow rate of the boil-off gas supplied to the boiler. The opening of the fuel gas valve for the boiler is determined according to the demand of the boiler, and the load control of the fuel gas compressor is generally adjusted to be close to the opening with the best controllability while observing this valve opening. However, if the recirculation valve opens or the compressor load control suddenly moves during capacity control of the fuel gas compressor, a constant discharge pressure cannot be obtained, and gas combustion must be stopped urgently. There is a case. In particular, when the demand for boilers decreases and the fuel gas consumption decreases, the opening of the fuel gas valve for the boiler decreases, and accordingly, compressor capacity control means (usually guide vanes such as IGV and DGV for suction, (Or the suction adjustment valve is applied), the compressor discharge pressure will also be reduced, so that when used together as a primary compressor for reliquefaction, condensation in the reliquefaction process will occur. There is a problem that the pressure also decreases and the condensation efficiency deteriorates.
In the present invention, the rotation speed of the compressor motor is adjusted so that the opening degree of the capacity control valve falls within a predetermined opening range, so that the opening degree of the capacity control mechanism of the compressor can be maintained within a desired range. As a result, the discharge pressure can be kept high, and a high condensing capacity can be obtained in the heat exchanger for reliquefaction.
The “predetermined opening range” of the capacity control valve is an opening range where the controllability of the capacity control valve is good, for example, a range of 80 to 95%.

  Further, in the liquefied gas storage and regasification facility of the present invention, when the discharge pressure of the fuel gas compressor becomes a predetermined value or less, the reopening control target value of the boiler fuel gas valve is reduced so as to reduce the target value. It is characterized by setting.

  When the discharge pressure of the fuel gas compressor is lowered, the condensation pressure in the reliquefaction heat exchanger is lowered and the condensation capacity is reduced. Therefore, when the discharge pressure of the fuel gas compressor becomes a predetermined value or less, the differential pressure before and after the boiler gas valve is increased by reducing the target value of the opening control of the boiler fuel gas valve. Thus, the upstream pressure of the boiler gas valve, that is, the discharge pressure of the fuel gas compressor can be increased.

  The liquefied gas storage and regasification facility of the present invention further includes a fuel gas compressor that guides the boil-off gas in the liquefied gas storage tank to a boiler, and the reliquefaction compressor includes the fuel gas compressor and the fuel gas compressor. The boil-off gas upstream of the fuel gas compressor is sucked and discharged to the re-liquefaction heat exchanger side.

  By separating the reliquefaction compressor from the fuel gas compressor (i.e., not using the fuel gas compressor in combination with the reliquefaction compressor) and guiding and compressing the boil-off gas upstream of the fuel gas compressor, The reliquefaction compressor can be operated independently of the fuel gas compressor whose operation state is changed according to the demand of the boiler, and stable control becomes possible. This is particularly effective when the required capacity of the fuel gas compressor is large and the discharge pressure of the fuel gas compressor is operated in a relatively small range.

  The boil-off gas reliquefaction method of the present invention includes a liquefied gas storage tank that stores liquefied gas, and a regasification device that gasifies the liquefied gas when the liquefied gas in the liquefied gas storage tank is discharged to the outside. And a boil-off gas re-liquefaction device for re-liquefying the boil-off gas vaporized from the liquefied gas in the liquefied gas storage tank, in the boil-off gas re-liquefaction method used in the liquefied gas storage and re-gasification equipment, The reliquefaction device compresses the reliquefaction heat exchanger that exchanges heat with the liquefied gas led to the regasification device to condense the boiloff gas, and the boiloff gas led to the reliquefaction heat exchanger. The reliquefaction compressor and the boiloff gas flow upstream of the reliquefaction heat exchanger are disposed on the upstream side of the boiloff gas flow by spraying a part of the liquefied gas. And a slow heat sink to decrease the temperature of the, and performs re-liquefaction of boil-off gas by supplying the BOG to the reliquefaction heat exchanger from the slow-heater.

The boil-off gas led to the re-liquefaction heat exchanger that condenses the boil-off gas is increased in pressure by the re-liquefaction compressor and the liquefaction capacity is increased by increasing the condensation pressure. Furthermore, the boil-off gas before entering the heat exchanger for reliquefaction can be brought close to saturation by providing a slow heat generator that reduces the temperature of the boil-off gas by spraying the liquefied gas. As a result, the temperature difference from the liquefied gas (coolant) at the boil-off gas inlet of the reliquefaction heat exchanger is reduced to reduce the heat exchange load of the reliquefaction heat exchanger, and design restrictions are imposed on the same temperature difference. Various heat exchangers can be selected including the adoption of a compact heat exchanger such as an aluminum plate blazed type.
Further, according to the present invention, the boil-off gas is reliquefied by using the sensible heat of the liquefied gas that is regasified by the regasifier when discharged outside as cold heat in the reliquefaction heat exchanger. Therefore, it contributes to improving the overall energy efficiency of the liquefied gas storage maximum gasification facility.

  According to the present invention, the boil-off gas before entering the re-liquefaction heat exchanger is brought close to saturation by providing a slow heat generator that reduces the temperature of the boil-off gas by spraying the liquefied gas. The temperature difference with the liquefied gas at the boil-off gas inlet of the cooler is reduced to reduce the heat exchange load of the heat exchanger for reliquefaction, and the aluminum plate blazed type is equipped with design restrictions on the temperature difference. Various heat exchangers can be selected including the adoption of a compact heat exchanger.

It is the schematic which showed the boil off gas reliquefaction apparatus concerning 1st Embodiment of this invention. It is the longitudinal cross-sectional view which showed the specific structure of the heat sink shown in FIG. It is the graph which showed the temperature change in the heat exchanger for reliquefaction. FIG. 2 is a control block diagram showing control of the fuel gas compressor shown in FIG. 1. It is the graph which showed the relationship between the flow volume and pressure of the fuel gas compressor which performed control of FIG. It is the control block diagram which showed control of the fuel gas compressor as a comparative example. It is the graph which showed the relationship between the flow volume and pressure of the fuel gas compressor which performed control of FIG. It is the schematic which showed the boil off gas reliquefaction apparatus concerning 2nd Embodiment of this invention.

Embodiments according to the present invention will be described below with reference to the drawings.
[First Embodiment]
The first embodiment of the present invention will be described below.
LNG storage facilities (liquefied gas storage and regasification facilities) are provided in ships such as floating bodies such as FSRU (Floating Storage and Regasification Unit) and FPSO (Floating Production, Storage and Offloading) and LNG ships. FIG. 1 shows a BOG reliquefaction apparatus 1 for reliquefying boil-off gas (hereinafter referred to as “BOG”) generated in an LNG storage tank (liquefied gas storage tank) of the LNG storage facility.

  As shown in FIG. 1, the BOG reliquefaction device 1 includes two fuel gas compressors 5 and 6 that compress BOG guided from the evaporation header 3 of the LNG storage tank. Each of the fuel gas compressors 5 and 6 pumps BOG as fuel to the boiler 7. The gas discharged from each of the fuel gas compressors 5 and 6 is guided not only to the boiler 7 but also to the reliquefaction heat exchanger 10 described later. That is, the fuel gas compressors 5 and 6 are used together as a reliquefaction compressor. Each of the fuel gas compressors 5 and 6 is a turbo type compressor, and an impeller is rotated by a motor (see reference numeral 14 in FIG. 4) whose rotational speed is variable by an inverter device. Each of the fuel gas compressors 5 and 6 is provided with an IGV (Inlet Guide Vane; see reference numeral 12 in FIG. 4) as a capacity control valve, and the discharge capacity is variable. Note that a DGV (Discharge Guide Vane) may be used as the capacity control valve.

  A booster 9 is provided downstream of the fuel gas compressors 5 and 6 on the reliquefaction heat exchanger 10 side. The booster 9 is a turbo compressor, and further boosts the BOG discharged from the fuel gas compressors 5 and 6. The booster 9 and the fuel gas compressors 5 and 6 constitute a reliquefaction compressor. Note that the booster 9 can be omitted when the discharge gas pressure obtained from the fuel gas compressors 5 and 6 is sufficiently high (for example, 0.45 MPa (absolute pressure) or more).

  A slow heat generator 16 is provided on the downstream side of the booster 9. The slow heatr 16 reduces the temperature of the BOG by spraying LNG. In the slow heatr 16, it is preferable to reduce the temperature to the saturated state, but the purpose is achieved if the temperature reaches the vicinity of the saturated state. FIG. 2 shows details of the heat sink 16. The slow heatr 16 includes a hollow cylindrical container 18. A discharge port 20 is provided at the top of the container 18, and an introduction port 22 is provided at the side of the container 18. The inlet 22 is connected to the discharge side of the booster 9, and the outlet is connected to the reliquefaction heat exchanger 10. An LNG introduction tube 24 is inserted into the upper side portion of the container 18. The LNG introduction pipe 24 is connected to an LNG branch pipe 26 (see FIG. 1) that branches and guides a part of the LNG that has been subjected to heat exchange in the reliquefaction heat exchanger 10. A plurality of spray holes are formed in the lower part of the LNG introduction pipe 24, and the introduced LNG is sprayed downward. A gas-liquid contact porous body 28 is disposed in the space between the LNG introduction pipe 24 and the lower introduction port 22, and the sprayed LNG comes into contact with the introduced BOG to reduce the BOG temperature. It has come to be. A mist separator 30 is disposed in the space between the LNG introduction pipe 24 and the discharge port 20, and separates and removes the mist from the BOG before being discharged toward the reliquefaction heat exchanger 10.

As shown in FIG. 1, the BOG that has passed through the slow heatr 16 and reduced in temperature is led to the re-liquefaction heat exchanger 10 and condensed and liquefied by heat exchange with LNG.
The liquefaction heat exchanger 10 is an aluminum blazed plate heat exchanger. Stainless steel is also a candidate, but the aluminum blaze type is less expensive.
The LNG as the coolant (cold heat source) led to the reliquefaction heat exchanger 10 is LNG before being regasified and discharged to the outside. Specifically, the LNG is added from the LNG storage tank to the suction drum 32. LNG used for pressure transfer is used. The suction drum 32 is provided in a suction portion of a high-pressure liquid pump (not shown) before regasification. The LNG line 34 shown in FIG. 1 is a sidestream line that branches off from a main LNG line (not shown) led from the LNG storage tank to the high-pressure liquid pump and rejoins.
A part of the LNG after the heat exchange that has passed through the reliquefaction heat exchanger 10 is led to the slow heat exchanger 16 via the branch LNG pipe 26. In addition, as the LNG guided to the slow heat exchanger 16, an LNG branched from the LNG before entering the reliquefaction heat exchanger 10 may be used.

  The LNG from which BOG has been reliquefied by the reliquefaction heat exchanger 10 is guided to the separator 36. In the separator 36, the gas and liquid are separated, the gas component is guided to the evaporation header 3, and the liquid component is guided to the LNG storage tank.

Next, the effect of this embodiment is demonstrated.
The BOG led to the reliquefaction heat exchanger 10 that condenses the BOG is boosted by the fuel gas compressors 5 and 6 and the booster 9 which are reliquefaction compressors, and the liquefaction capacity is increased by increasing the condensation pressure. It was decided. Furthermore, by providing the slow heat generator 16 that reduces the temperature of the BOG by spraying LNG, the BOG before entering the reliquefaction heat exchanger 10 can be brought close to saturation. Thereby, the temperature difference with the LNG (coolant) at the BOG inlet of the reliquefaction heat exchanger 10 can be reduced, and the load on the reliquefaction heat exchanger 10 can be reduced.
FIG. 3 shows temperature changes of BOG and LNG (coolant) in the reliquefaction heat exchanger 10. The horizontal axis in the figure is the heat load [kW], and the vertical axis is the fluid temperature [° C.]. In the figure, the right side is the BOG upstream side, and the left side is the LNG upstream side. A black square indicates LNG, and shows a state at a flow rate of 65 kg / h and a pressure of 0.6 MPa (absolute pressure). A black rhombus indicates BOG, and shows a state at a flow rate of 5560 kg / h and a pressure of 0.45 MPa (absolute pressure). As can be seen from the figure, even in the vicinity of the BOG inlet where the temperature difference is maximum, the temperature difference can be suppressed to 20 ° C. or less. As described above, this is an effect obtained by introducing the BOG near the saturation state after reducing the temperature into the re-liquefaction heat exchanger 10. If the temperature difference between BOG and LNG is 20 ° C. or less, a compact aluminum blazed plate heat exchanger can be adopted as the reliquefaction heat exchanger 10.
In addition, since the sensible heat of LNG regasified by the regasifier when discharged to the outside can be used as cold heat in the reliquefaction heat exchanger 10, BOG can be reliquefied, so that energy efficiency Is good.

Next, a method for controlling the fuel gas compressors 5 and 6 shown in FIG. 1 will be described with reference to FIGS. In the following, one fuel gas compressor 6 will be described as a representative example for simplification of description. The same control is performed for the other fuel gas compressor 5.
As described above, the fuel gas compressor 6 includes the IGV 12 serving as a capacity control valve and the electric motor 14 having a variable rotation speed.
The BOG discharged from the fuel gas compressor 6 is guided to the booster 9 shown in FIG. The BOG guided to the boiler 7 side passes through the heater 40, is adjusted to a gas flow rate required by the boiler by the boiler fuel gas valve 42, and then is supplied to the boiler 7 and burned.

  The opening degree of the boiler fuel gas valve 42 is determined according to the demand of the boiler 7. Specifically, when the demand for the boiler 7 is large and the fuel gas consumption is increased, the opening degree of the boiler fuel gas valve 42 is increased, and the demand for the boiler 7 is small and the fuel gas consumption is decreased. Moves toward the smaller opening of the boiler fuel gas valve 42.

  When there are a plurality of boiler fuel gas valves 42, the current actual opening is input to a high selector 44 that selects a high value, for example, the larger opening of the actual opening of the two boiler fuel gas valves 42. Is selected and output, and becomes the input of the first PID controller 46. SP1 as a set value of the boiler fuel gas valve 42 set by the third PID controller 49 is input to the first PID controller 46. The first PID controller 46 compares the actual opening input from the high selector 44 with SP1, and when the actual opening is larger than SP1, it can be determined that the demand on the boiler 7 side is large, so the opening of the IGV 12 Is issued, and when the actual opening is smaller than SP1, it can be determined that the demand on the boiler 7 side is small, so the instruction is issued so that the opening of the IGV 12 is reduced.

  The actual actual opening of the IGV 12 is input to the second PID controller 48. The second PID controller 48 receives SP2 as the IGV opening setting value. The set value of SP2 is an opening range in which the controllability of the IGV 12 is good, for example, 70 to 95%, preferably 80 to 95%. The second PID controller 48 compares the actual opening of the IGV 12 with SP2, and if the actual opening is larger than SP2, issues a speed increase command to the electric motor 14 so that the actual opening of the IGV 12 becomes smaller. When the actual opening of the IGV 12 is smaller than SP2, a deceleration command is issued to the electric motor 14 so that the actual opening of the IGV 12 is increased.

A pressure sensor 50 for measuring the discharge pressure is provided downstream of the fuel gas compressor 6, and the pressure value P detected by the pressure sensor 50 is input to the third PID controller 49. The third PID controller 49 receives SP3 as a lower limit set value of the discharge pressure of the fuel gas compressor 6. The third PID controller 49 outputs SP1 input to the first PID controller 46. When the pressure value P is smaller than SP3, control is performed so that SP1 becomes smaller.
When SP1 is reduced by the command of the third PID controller 49, the opening degree of the boiler fuel gas valve 42 is reduced, and the pressure loss in the boiler gas valve 42 can be increased to increase the differential pressure. Even so, the pressure on the upstream side of the boiler gas valve 42, that is, the discharge pressure of the fuel gas compressor 6 can be increased.
The setting of SP1 may be automatically performed using the third PID controller 49, but the operator may check the pressure value P and manually change SP1.

By controlling the fuel gas compressor 6 as described above, the following effects can be obtained.
The opening degree of the IGV 12 of the fuel gas compressor 6 is controlled such that the opening degree of the boiler fuel gas valve 42 that adjusts the flow rate of the BOG supplied to the boiler 7 is a certain opening degree. Since the opening degree of the boiler fuel gas valve 42 is determined according to the demand of the boiler 7, the opening degree of the IGV 12 of the fuel gas compressor 6 is determined depending only on the opening degree of the boiler fuel gas valve 42. In some cases, a desired discharge pressure cannot be obtained. That is, when the demand for the boiler 7 is reduced and the fuel gas consumption is reduced, the opening degree of the boiler fuel gas valve 42 is reduced, and accordingly, the opening degree of the IGV is also reduced.

  For example, as shown in FIG. 6 as a comparative example, the first PID that outputs the command value based on the deviation between the actual opening of the boiler fuel gas valve 42 and SP1 that is the opening setting value of the boiler fuel gas valve 42. Consider a configuration including a controller 46 and a controller 52 that controls the IGV 12 and the motor 14 under a certain function in accordance with a command value output from the first PID controller 46. This is generally considered cascade control of the fuel gas compressor 6. In this case, as shown in FIG. 7, when the flow rate is equal to or higher than a predetermined value, the rotational speed control FQ by the motor 14 is performed. That is, as the flow rate increases from a high flow rate to a low flow rate, the rotation number is gradually decreased from 0.5 to 0.5, such as 1.0 to 0.9, 0.8. The rotation speed is a numerical value normalized with a rating of 1.0. Then, when the flow rate decreases and is equal to or less than a predetermined value, the opening degree of the IGV 12 is controlled to be reduced. That is, the IGV opening is gradually reduced so that the opening of the IGV 12 is -20% and -40%. When such control is performed, as is apparent from FIG. 7, the surge line L in the low flow region is lowered, that is, an operation in which a high discharge pressure cannot be obtained at a low flow rate. In this case, in the case of the present embodiment in which the fuel gas compressor 6 is used as a reliquefaction compressor, the pressure required for condensation in the reliquefaction heat exchanger 10 may not be obtained.

  Therefore, in this embodiment, the rotational speed of the motor 14 is further controlled so that the opening degree of the IGV 12 falls within a predetermined opening degree range. Thereby, since the opening degree of IGV12 can be maintained in the desired range, a desired discharge amount, ie, a desired discharge pressure, can be maintained. Specifically, as shown in FIG. 5, since the opening of the IGV 12 is controlled so that the IGV 12 falls within a predetermined opening range, the opening of the IGV 12 is reduced from the high flow rate side (high rotation side). The surge line can be raised as with L1 and L2. A surge line L1 in the figure indicates a surge line when the opening degree of the IGV 12 starts to be reduced when the rotational speed of the motor 14 is 1.0, and the surge line L2 indicates that the rotational speed of the motor 14 is 0.7. The surge line when the opening of the IGV 12 starts to be reduced is shown.

  Thus, according to the control method of the present embodiment, the fuel gas compressor 6 can be operated with a high discharge pressure even at a low flow rate. As a result, the pressure of the BOG supplied to the reliquefaction heat exchanger 10 can be maintained at a predetermined value or higher, and a high condensing capacity can be obtained in the reliquefaction heat exchanger 10.

[Second Embodiment]
Next, a second embodiment of the present invention will be described with reference to FIG.
This embodiment is different from the first embodiment in that the fuel gas compressor is not used as a reliquefaction compressor. Therefore, components common to the first embodiment are denoted by the same reference numerals, description thereof is omitted, and differences are described.
As shown in FIG. 8, two boosters 9a and 9b are connected in series. As a result, the BOG is compressed in two stages. The upstream side of the booster 9 b is connected to the upstream side of the fuel gas compressors 5 and 6. That is, only the BOG branched at the branch point A without being led from the evaporation header 3 to the fuel gas compressors 5 and 6 is led to the boosters 9a and 9b.
This embodiment operates the boosters 9a and 9b, which are reliquefaction compressors, independently of the fuel gas compressors 5 and 6 whose operation state is changed according to the demand of the boiler 7 by adopting the above configuration. And stable control is possible. In particular, the present invention is effective for a system that operates in a range where the capacity of the fuel gas compressors 5 and 6 is large and the discharge pressure of the fuel gas compressors 5 and 6 is relatively small.
In addition, the effect that the temperature difference in the reliquefaction heat exchanger 10 can be reduced by providing the slow heat exchanger 16 and a compact heat exchanger can be adopted is the same as that of the first embodiment.

1 Boil-off gas reliquefaction device 5, 6 Fuel gas compressor 7 Boiler 9, 9a, 9b Booster (reliquefaction compressor)
10 Heat exchanger for reliquefaction 12 IGV (capacity control valve)
14 Electric motor 16 Slow heater

Claims (6)

A liquefied gas storage tank for storing liquefied gas; and
A regasification device for gasifying the liquefied gas when the liquefied gas in the liquefied gas storage tank is discharged to the outside;
A boil-off gas reliquefaction device for reliquefying the boil-off gas vaporized from the liquefied gas in the liquefied gas storage tank;
In the liquefied gas storage and regasification facility with
The boil-off gas reliquefaction device includes a reliquefaction heat exchanger that exchanges heat with the liquefied gas guided to the regasification device to condense the boil-off gas;
A reliquefaction compressor that compresses boil-off gas that is directed to the reliquefaction heat exchanger;
A slow heat exchanger disposed upstream of the boil-off gas flow of the re-liquefaction heat exchanger, and for reducing the temperature of the boil-off gas by spraying a part of the liquefied gas;
A liquefied gas storage and regasification facility characterized by comprising:   The liquefied gas storage and regasification facility according to claim 1, wherein the reliquefaction heat exchanger is an aluminum plate-blazed heat exchanger. A fuel gas compressor for guiding the boil-off gas in the liquefied gas storage tank to a boiler;
The fuel gas compressor includes a capacity control valve for controlling the discharge amount and an electric motor having a variable rotation speed for rotating an impeller for compressing boil-off gas, and is used in combination as the reliquefaction compressor,
The opening of the capacity control valve is controlled based on the opening of a boiler fuel gas valve that adjusts the flow rate of the boil-off gas supplied to the boiler.
The liquefied gas storage and regasification facility according to claim 1 or 2, wherein the electric motor is controlled so that the rotation speed of the capacity control valve is within a predetermined opening range.   4. The liquefied gas storage and regasification according to claim 3, wherein when the discharge pressure of the fuel gas compressor becomes equal to or lower than a predetermined value, control is performed so as to reduce the opening of the fuel gas valve for the boiler. Facility. A fuel gas compressor for guiding the boil-off gas in the liquefied gas storage tank to a boiler;
The reliquefaction compressor is provided separately from the fuel gas compressor, and sucks boil-off gas upstream of the fuel gas compressor and discharges it to the reliquefaction heat exchanger side. The liquefied gas storage regasification facility according to claim 1 or 2. A liquefied gas storage tank for storing liquefied gas; and
A regasification device for gasifying the liquefied gas when the liquefied gas in the liquefied gas storage tank is discharged to the outside;
A boil-off gas reliquefaction device for reliquefying the boil-off gas vaporized from the liquefied gas in the liquefied gas storage tank;
In a boil-off gas reliquefaction method used in a liquefied gas storage regasification facility comprising:
The boil-off gas reliquefaction device includes a reliquefaction heat exchanger that exchanges heat with the liquefied gas guided to the regasification device to condense the boil-off gas;
A reliquefaction compressor that compresses boil-off gas that is directed to the reliquefaction heat exchanger;
A slow heat exchanger disposed upstream of the boil-off gas flow of the reliquefaction heat exchanger, and increasing the wetness of the boil-off gas by spraying a part of the liquefied gas;
With
A boil-off gas re-liquefaction method, wherein boil-off gas is re-liquefied by supplying boil-off gas from the slow heat exchanger to the re-liquefaction heat exchanger.

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