KR102258675B1 - Boil-Off Gas Processing System and Method - Google Patents

Abstract

The present invention supplies the BOG naturally vaporized in the liquefied gas storage tank as the fuel of the engine, and the surplus BOG remaining after supplying the engine is the BOG to be supplied as the BOG naturally vaporized in the liquefied gas storage tank and the fuel of the engine. It relates to a system and method for treating boil-off gas by re-liquefying it using as a refrigerant.
The BOG treatment system according to the present invention includes: a high-pressure compressor for compressing BOG discharged from a liquefied gas storage tank to a gas fuel pressure condition required by a high-pressure engine; a reliquefaction line into which excess BOG exceeding the fuel demand amount required by the high-pressure engine is introduced from among the high-pressure BOG compressed in the high-pressure compressor; a second fuel line branched from the reliquefaction line and into which BOG corresponding to the fuel demand amount required by the low-pressure engine from among the high-pressure BOG introduced into the reliquefaction line is introduced; a fuel decompression device for decompressing the high-pressure BOG to be supplied as the fuel of the low-pressure engine to a pressure required by the low-pressure engine; and a heat exchanger for recovering and cooling the high-pressure BOG to be reliquefied from the high-pressure BOG introduced into the reliquefaction line by recovering the cooling heat of the reduced pressure BOG decompressed by the decompression device for fuel.

Description

Boil-Off Gas Processing System and Method}

The present invention supplies the BOG naturally vaporized in the liquefied gas storage tank as a fuel of the engine, and the surplus BOG remaining after supplying the engine is the BOG to be supplied as the BOG naturally vaporized in the liquefied gas storage tank and the fuel of the engine. It relates to a system and method for treating boil-off gas by re-liquefying it using as a refrigerant.

In recent years, consumption of liquefied gas such as liquefied natural gas (LNG) is rapidly increasing worldwide. The liquefied gas obtained by liquefying the gas at a low temperature has the advantage of increasing the storage and transport efficiency because the volume is very small compared to the gas. In addition, since liquefied gas including liquefied natural gas can remove or reduce air pollutants during the liquefaction process, it can be viewed as an eco-friendly fuel that emits less air pollutants during combustion.

Liquefied natural gas is a colorless and transparent liquid obtained by cooling and liquefying natural gas containing methane as a main component to about -163°C, and has a volume of about 1/600 compared to natural gas. Therefore, when the natural gas is liquefied and transported, it can be transported very efficiently.

However, since the liquefaction temperature of natural gas is a cryogenic temperature of -163 ℃ atmospheric pressure, liquefied natural gas is sensitive to temperature changes and evaporates easily. For this reason, the storage tank that stores liquefied natural gas is insulated, but external heat is continuously transferred to the storage tank. Therefore, during the transportation of liquefied natural gas, the liquefied natural gas is continuously evaporated in the storage tank and boiled gas (Boil). -Off Gas, BOG) occurs.

Boil-off gas is a kind of loss and is an important problem in transport efficiency. In addition, if the boil-off gas accumulates in the storage tank, the internal pressure of the tank may increase excessively, and there is a risk of damage to the tank in severe cases. Therefore, various methods for treating BOG generated in the storage tank are being studied. Recently, for the treatment of BOG, a method of re-liquefying BOG and returning it to a storage tank, and turning BOG into fuel such as a ship's engine, etc. A method of using it as an energy source for consumers is being used.

As a method for re-liquefying BOG, there are a method of re-liquefying BOG by heat-exchanging it with a refrigerant by having a refrigeration cycle using a separate refrigerant, and a method of re-liquefying BOG itself as a refrigerant without using a separate refrigerant. have. In particular, a system employing the latter method is called a partial re-liquefaction system (PRS).

Meanwhile, among engines generally used in ships, DF engines (Dual Fuel Diesel Electric (DFDE), Dual Fuel Diesel Generator (DFDG)) and X-DF (eXtra long stroke Dual Fuel) are engines that can use natural gas as fuel. There are gas fuel engines such as engines and ME-GI engines (MAN Electronic Gas-Injection Engine).

The DFDE is composed of 4 strokes, and adopts an Otto Cycle in which natural gas having a relatively low pressure of about 6.5 bar is injected into the combustion air inlet and compressed while the piston rises.

The X-DF engine is composed of two strokes, uses about 16 bar of natural gas as fuel, and adopts an auto cycle.

The ME-GI engine is composed of two strokes and adopts a diesel cycle in which high-pressure natural gas near 300 bar is directly injected into the combustion chamber near the top dead center of the piston.

An object of the present invention is to provide a BOG reliquefaction system and method, which can increase overall reliquefaction efficiency and reliquefaction amount by stabilizing reliquefaction performance.

According to an aspect of the present invention for achieving the above object, there is provided a high-pressure compressor for compressing boil-off gas discharged from a liquefied gas storage tank to a gas fuel pressure condition required by a high-pressure engine; a reliquefaction line into which excess BOG exceeding the fuel demand amount required by the high-pressure engine is introduced from among the high-pressure BOG compressed in the high-pressure compressor; a second fuel line branched from the reliquefaction line and into which BOG corresponding to the fuel demand amount required by the low-pressure engine from among the high-pressure BOG introduced into the reliquefaction line is introduced; a pressure reducing device for fuel for reducing the high-pressure BOG to be supplied as fuel of the low-pressure engine along the second fuel line to a pressure required by the low-pressure engine; and a heat exchanger for recovering and cooling the high-pressure BOG to be reliquefied from the high-pressure BOG introduced into the reliquefaction line by recovering the cooling heat of the reduced pressure BOG decompressed by the decompression device for fuel. system is provided.

Preferably, the heat exchanger may include a second heat exchanger configured to exchange heat between the reduced pressure BOG and the high-pressure BOG to be reliquefied, thereby recovering the cooling heat of the reduced-pressure BOG primarily to cool the high-pressure BOG. can

Preferably, the heat exchanger heat-exchanges the boil-off gas before compression supplied from the liquefied gas storage tank to the high-pressure compressor, the high-pressure boil-off gas to be re-liquefied, and the reduced pressure boil-off gas discharged after heat exchange in the second heat exchanger. , a first heat exchanger for cooling the high-pressure BOG by recovering the cooling heat of the reduced pressure BOG secondarily.

Preferably, a fuel merging line branched from the reliquefaction line downstream of the second heat exchanger and connected to the fuel pressure reducing device; may include.

Preferably, a fuel branch valve whose opening and closing is controlled so that an amount of fuel demanded by the low-pressure engine from among the high-pressure BOG supplied to the first heat exchanger through the reliquefaction line is supplied to the pressure reducing device for fuel; a fuel merging valve whose opening and closing is controlled so that the fuel demand amount required by the low-pressure engine from among the low-temperature, high-pressure boil-off gas cooled by the second heat exchanger is supplied to the pressure reducing device for fuel; and a control unit configured to control the opening and closing of the fuel branch valve and the fuel merging valve according to the flow rate of the high-pressure boil-off gas supplied to the first heat exchanger to control a branch point of the high-pressure boil-off gas to be supplied to the pressure reducing device for fuel. can

Preferably, it may further include a; fuel heater for adjusting the temperature of the reduced pressure boil-off gas recovered from the cooling heat in the heat exchanger to a temperature required by the low-pressure engine.

Preferably, a pressure reducing device for re-liquefaction for decompressing the high-pressure boil-off gas, which is first cooled in the first heat exchanger and secondly cooled in the second heat exchanger; and the reliquefied BOG reliquefied while decompressed by the depressurizing device for reliquefaction may be recovered to the liquefied gas storage tank.

Preferably, by gas-liquid separation of the gas-liquid mixture generated while decompressed by the decompression device for reliquefaction, the reliquefied BOG in a liquid state is recovered to the liquefied gas storage tank, and the non-condensed BOG in a gaseous state is the first It may further include a; gas-liquid separator to join the boil-off gas flow before compression supplied to the heat exchanger.

Preferably, the second fuel line may be branched from the reliquefaction line upstream of the first heat exchanger and connected to the pressure reducing device for fuel.

According to another aspect of the present invention for achieving the above object, a high-pressure compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to the gas fuel pressure condition required by the high-pressure engine; a high-pressure fuel supply step of supplying an amount of fuel demanded by the high-pressure engine from among the high-pressure BOG compressed in the high-pressure compression step to the high-pressure engine; a low-pressure fuel supply step of supplying the low-pressure engine by reducing the amount of fuel demand required by the low-pressure engine among the remaining high-pressure BOG exceeding the fuel demand of the high-pressure engine; and a re-liquefaction step of re-liquefying the remaining high-pressure BOG exceeding the fuel demand required by the high-pressure engine and the low-pressure engine with the cooling heat of the reduced-pressure BOG to be supplied to the low-pressure engine and recovering it to the liquefied gas storage tank. , a boil-off gas treatment method is provided.

Preferably, in the re-liquefaction step, the BOG before compression in the high-pressure compression step is cooled by exchanging heat between the BOG before compression, the high-pressure BOG to be reliquefied, and the reduced pressure BOG to cool the high-pressure BOG. first cooling step; and a secondary cooling step of further cooling the high-pressure BOG by exchanging heat between the high-pressure BOG cooled in the first cooling step and the reduced pressure BOG, wherein the cooling heat of the reduced pressure BOG is the secondary cooling It may be recovered primarily in the step, and may be recovered secondarily in the first cooling step.

Preferably, in the low-pressure fuel supply step, before being supplied to the primary cooling step, the pressure is reduced by branching the amount of fuel demand required by the low-pressure engine among the high-pressure BOG, or at a low temperature that has undergone the secondary cooling step. Further comprising; a fuel branch control step of depressurizing by branching the amount of fuel demand required by the low-pressure engine from the high-pressure BOG, and in the fuel branch control step, the fuel branching according to the fuel demand amount required by the high-pressure engine You can choose a branch.

According to the present invention, in the BOG re-liquefaction system for re-liquefying BOG as a refrigerant after compressing BOG, by compressing BOG to be supplied as fuel of a medium-pressure or low-pressure engine to a high pressure and then supplying it under reduced pressure. , since it can be used as an additional refrigerant for cooling the boil-off gas to be re-liquefied by the reduced pressure, the cooling heat of the boil-off gas can be recovered to the maximum, and thus the reliquefaction amount of the boil-off gas can be increased. .

In addition, the BOG to be supplied as fuel for the medium or low pressure engine is extracted at different locations depending on the operating conditions of the vessel, so that the system is flexibly operated according to the amount of BOG to be reliquefied and/or the amount of BOG to be supplied as fuel. can do.

1 is a schematic diagram illustrating a general BOG treatment system.
2 is a schematic diagram illustrating a BOG treatment system according to an embodiment of the present invention.

In order to fully understand the operational advantages of the present invention and the objects achieved by the embodiments of the present invention, reference should be made to the accompanying drawings illustrating preferred embodiments of the present invention and the contents described in the accompanying drawings.

Hereinafter, the configuration and operation of a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings. Here, in adding reference numerals to elements of each drawing, it should be noted that only the same elements are marked with the same numerals as possible, even if they are indicated on different drawings. In addition, the following examples may be modified in various other forms, and the scope of the present invention is not limited to the following examples.

In an embodiment of the present invention described below, the liquefied gas may be applied to various liquefied gases, for example, LNG (Liquefied Natural Gas), LEG (Liquefied Ethane Gas), LPG (Liquefied Petroleum Gas), and liquefied It may be a liquefied petrochemical gas such as liquefied ethylene gas and liquefied propylene gas. Alternatively, it may be a liquid gas such as liquefied carbon dioxide, liquefied hydrogen, or liquefied ammonia. However, in the embodiments to be described later, it will be described as an example that LNG, which is a representative liquefied gas, is applied.

LNG has methane as its main component, and includes ethane, propane, butane, and the like, and its composition may vary depending on the production area.

In addition, in an embodiment of the present invention to be described later, liquefied gas and boil-off gas of the liquefied gas may be used as a fuel for an engine, particularly a marine engine.

In addition, although the BOG treatment system and method according to an embodiment of the present invention, which will be described later, is described as an example applied to a ship, it may be applied on land.

In addition, in the embodiment to be described later, the ship is described by taking the case of a LNG carrier that transports liquefied natural gas as cargo, but the present invention is an LNG FSRU (Floating LNG FSRU) equipped with a storage tank for storing liquefied natural gas. Liquefied gas storage tanks such as Storage Regasification Unit), LNG FPSO (Floating Production Storage Offloading), and LNG RV (Regasification Vessel) are provided, and an engine supplied with liquefied gas as fuel is applied. It is applicable to all ships equipped with a reliquefaction device for reliquefying.

Therefore, the present invention can be applied to all ships in which an engine supplied with liquefied natural gas as a fuel is provided, and has propulsion by the engine or to generate electric power by driving the engine.

In addition, in an embodiment of the present invention to be described later, the fuel demander may include any two or more of a high-pressure engine, a medium-pressure engine, and a low-pressure engine. The high-pressure engine will be described as an engine using gas fuel of about 100 bar to 400 bar, preferably about 150 bar, for example, a ME-GI engine. Further, the medium pressure engine may be an engine using gaseous fuel of about 10 bar to 20 bar, preferably about 16 bar, for example an X-DF engine, and the low pressure engine is about 5 bar to 10 bar, preferably It may be an engine that uses about 6.5 bar of gaseous fuel, for example a DF engine or a DFDG engine, or a DFGE engine.

In an embodiment of the present invention to be described later, a high-pressure engine and a low-pressure engine, ie, a ME-GI engine and a DFGE, will be described as examples of fuel demanders.

First, a general BOG treatment system and method will be described with reference to FIG. 1 . A typical BOG treatment system includes an LNG storage tank (TK), a compressor (HC), a heat exchanger (HE), a pressure reducing device (EV), and a gas-liquid separator (SP). In addition, as a flow path through which boil-off gas discharged from the LNG storage tank (TK) flows, the first line ( L1), branching from the first line (L1) between the compressor (HC) and the ME-GI engine, a heat exchanger (HE), a pressure reducing device (EV), a gas-liquid separator (SP), and an LNG storage tank (TK) A second line (L2) connecting the second line (L2), the gas-liquid separator (SP) and the first line (L1), in particular, the third connecting the first line (L1) between the LNG storage tank (TK) and the heat exchanger (HE) It includes a line L3 and a fourth line L4 branched from the middle end of the compressor HC and connected to the DF engine.

Referring to FIG. 1 , in a general BOG treatment method, BOG generated in the LNG storage tank (TK) is introduced into the compressor (HC) along the first line (L1), and the ME-GI engine or the It is compressed to the pressure required by fuel demanders such as DF engines.

BOG compressed to a pressure required by the DF engine using only a partial front-end compression part of the compressor HC is branched into the fourth line L4 and supplied as fuel of the DF engine. In addition, BOG compressed to a pressure required by the ME-GI engine using all of the plurality of compression units of the compressor HC is supplied as fuel of the ME-GI engine along the first line L1.

If the amount of BOG discharged from the LNG storage tank (TK) is greater than the BOG fuel demand required by the ME-GI engine and the DF engine, the surplus BOG in the amount exceeding the fuel demand is removed from the compressor (HC). After being compressed to the pressure required by the ME-GI engine, it is branched to the second line L2 and reliquefied.

The compressed BOG branched to the second line (L2) exchanges heat with the BOG before compression flowing into the compressor (HC) from the LNG storage tank (TK) along the first line (L1) in the heat exchanger (HE). , is cooled.

The compressed BOG cooled in the heat exchanger (HE) is decompressed by the decompression device (EV). As described above, the BOG is reliquefied through the compression, cooling and decompression processes.

BOG decompressed by the decompression device (EV) is gas-liquid separated in the gas-liquid separator (SP), and the reliquefied BOG in liquid state is recovered to the LNG storage tank (TK), and the non-condensed gaseous BOG is not reliquefied. BOG is joined with BOG flowing from the LNG storage tank (TK) to the heat exchanger (HE) along the third line (L3).

Next, a BOG treatment system and method according to an embodiment of the present invention will be described with reference to FIG. 2 .

The BOG treatment system and method according to an embodiment of the present invention is the same as that of the general BOG treatment system and method described above in that BOG is used as a refrigerant to reliquefy BOG, but in this embodiment In the example, there is a difference in that the amount of reliquefaction can be maximized by additionally generating cooling heat of BOG to be supplied as fuel, and additionally recovering and using it as a reliquefaction refrigerant.

The BOG treatment system according to an embodiment of the present invention includes an LNG storage tank 100 for storing LNG; a high-pressure compressor 200 for compressing the boil-off gas discharged from the LNG storage tank 100 along the boil-off gas line BL to the pressure required by the high-pressure engine, that is, the ME-GI engine; a high-pressure engine using the high-pressure boil-off gas compressed in the high-pressure compressor 200 as a fuel; The surplus high-pressure BOG exceeding the gas fuel demand required by the high-pressure engine among the high-pressure BOG compressed in the high-pressure compressor 200 is removed from the LNG storage tank 100 along the BOG line BL. heat exchangers (300, 400) for cooling the boil-off gas before compression transferred to the evaporator and cooling heat of the reduced pressure boil-off gas to be supplied as fuel of a low-pressure engine; a pressure reducing device 500 for re-liquefaction for decompressing the cooled high-pressure boil-off gas through the heat exchangers 300 and 400; and a decompression device 700 for fuel that depressurizes the high-pressure BOG to be supplied as fuel of the low-pressure engine from among the excess high-pressure BOG and supplies it to the heat exchangers 300 and 400 .

The heat exchangers 300 and 400 of the present embodiment transfer the remaining high-pressure BOG exceeding the gas fuel demand amount required by the high-pressure engine among the high-pressure BOG compressed by the high-pressure compressor 200 along the BOG line BL. a first heat exchanger 300 for primary cooling with the cooling heat of the boil-off gas transferred from the LNG storage tank 100 to the high-pressure compressor 200 and the reduced pressure boil-off gas to be supplied as fuel of the low-pressure engine; and a second heat exchanger 400 for secondary cooling by heat-exchanging the high-pressure BOG cooled in the first heat exchanger 300 with the low-temperature decompressed BOG to be supplied as fuel of the low-pressure engine.

That is, the first heat exchanger 300 of this embodiment may be a three-stream heat exchanger, and the second heat exchanger 400 may be a two-stream heat exchanger. In addition, the high-pressure BOG is cooled and the low-pressure BOG is heated by heat exchange in the heat exchangers 300 and 400 . More specifically, in the first heat exchanger 300 , the cooling heat of the boil-off gas before compression and the reduced pressure boil-off gas is recovered, and in the second heat exchanger 400 , the cooling heat of the reduced pressure boil-off gas is recovered.

In addition, according to the present embodiment, the BOG after the compression process by the high pressure compressor 200, the cooling process by the heat exchangers 300 and 400, and the decompression process by the pressure reducing device 500 for reliquefaction is in a supercooled state or at least Some may be in a reliquefied gas-liquid mixed state.

The BOG treatment system according to the present embodiment separates the BOG decompressed in the decompression device 500 for reliquefaction, and recovers the reliquefied BOG in a liquid state to the LNG storage tank 100, which is not reliquefied. The non-condensed BOG in the gaseous state is combined with the BOG flow before compression flowing from the LNG storage tank 100 to the first heat exchanger 300 using the gas recovery line (GL); a gas-liquid separator 600; may include

The boil-off gas line BL of this embodiment connects the LNG storage tank 100 , the first heat exchanger 300 , and the high-pressure compressor 200 . BOG discharged from the LNG storage tank 100 before compression is supplied to the low-temperature flow path of the first heat exchanger 300 along the BOG line BL and is used as a refrigerant to cool the high-pressure BOG. BOG before compression, whose temperature is increased by heat exchange with compressed BOG in the first heat exchanger 300 , is along the BOG line BL, from the low-temperature flow path of the first heat exchanger 300 to the high-pressure compressor 200 . is transferred to

In addition, according to this embodiment, branched from the boil-off gas line BL between the LNG storage tank 100 and the first heat exchanger 300 to bypass the first heat exchanger 300 and It may further include a heat exchanger bypass line (BL1) connected between the high pressure compressor (200).

That is, according to the present embodiment, if necessary. All or part of the boil-off gas discharged from the LNG storage tank 100 using the heat exchanger bypass line BL1 is not used as the refrigerant of the first heat exchanger 300 , that is, the low temperature of the first heat exchanger 300 . Instead of being supplied to the flow path, the first heat exchanger 300 may be bypassed to be directly transferred to the high-pressure compressor 200 .

For example, there is no BOG to be reliquefied, or the temperature of BOG to be reliquefied can be sufficiently lowered to a target temperature only by heat exchange in the second heat exchanger 400 to be described later, or the first heat exchanger 300 is maintained. When the first heat exchanger 300 is not used for maintenance, etc., the opening/closing valve (reference numeral not given) installed in the heat exchanger bypass line BL1 is opened so that boil-off gas flows through the first heat exchanger 300 . After bypassing, it can be controlled to merge into the boil-off gas line BL from the upstream of the high-pressure compressor 200 .

The high-pressure compressor 200 of the present embodiment may compress the boil-off gas to a pressure required by the high-pressure engine, that is, about 150 bar or more, which is a pressure required by the ME-GI engine.

Also, the high-pressure compressor 200 may be a multi-stage compressor that compresses the boil-off gas in multiple stages including a plurality of compression units. In this embodiment, as shown in FIG. 2 , the high-pressure compressor 200 compresses the boil-off gas to the pressure required by the ME-GI engine through five stages including five compression units.

According to this embodiment, it includes both a high-pressure engine and a low-pressure engine using boil-off gas as fuel, similar to the general BOG treatment system described above, and compresses BOG using a compressor to supply the high-pressure engine and the low-pressure engine simultaneously. have.

As shown in FIG. 1 , in the general BOG reliquefaction system described above, BOG compressed to a low pressure using only some compression parts such as three compression parts at the front end of the compressor (HC) including a plurality of compression parts is a low pressure engine. and a fourth line L4 branched from the middle of the compressor HC to be supplied to the .

Therefore, in the general method of reliquefying BOG, BOG to be supplied to the high-pressure engine is compressed to the high pressure required by the high-pressure engine and supplied to the high-pressure engine along the first line L1, and BOG to be supplied to the low-pressure engine is supplied to the low-pressure engine in the low-pressure engine. It is compressed to the required low pressure and supplied to the low pressure engine along the fourth line L4.

On the other hand, in the BOG treatment system according to the present embodiment, as shown in FIG. 2 , a first fuel line branched from the BOG line BL at the downstream of the high pressure compressor 200 and connected to the ME-GI engine ( FL1); a reliquefaction line (RL) branched from the boil-off gas line (BL) downstream of the high pressure compressor 200 and connected to the first heat exchanger 300; and a second fuel line FL2 branched from the reliquefaction line RL upstream of the first heat exchanger 300 and connected to the fuel pressure reducing device 700 .

Accordingly, in the BOG treatment method according to the present embodiment, after the BOG is compressed at a high pressure in the high-pressure compressor 200, the BOG fuel demand amount of the ME-GI engine is branched into the first fuel line FL1 to the ME- It is supplied by the GI engine.

The remaining remaining after branching to the first fuel line FL1, that is, the high-pressure BOG to be supplied to the DF engine and the high-pressure BOG to be reliquefied, are branched to the reliquefaction line RL.

In addition, the surplus high-pressure BOG of this embodiment is branched into two flows between the high-pressure compressor 200 and the heat exchanger 300, and one flow is supplied to the heat exchanger 300 along the reliquefaction line RL. It is liquefied, and the remaining flow is supplied to the pressure reducing device 700 for fuel along a second fuel line FL2 to be described later and supplied as fuel of the low-pressure engine.

That is, the BOG to be supplied as fuel of the DF engine among the high-pressure BOG branched to the reliquefaction line RL is branched from the upstream of the first heat exchanger 300 to the second fuel line FL2, and the pressure reducing device 700 for fuel ) is supplied.

The remaining high-pressure BOG to be branched to the second fuel line FL2, that is, to be reliquefied, is supplied to the first heat exchanger 300 through the reliquefaction line RL.

The high-pressure BOG branched to the second fuel line FL2 and supplied to the fuel pressure reducing device 700 is reduced to a pressure required by the low pressure engine in the fuel pressure reducing device 700, that is, to about 6.5 bar in this embodiment. do.

The pressure reducing device 700 for fuel may be an adiabatic expansion means, for example, an expander or an expansion valve. The high-pressure BOG is expanded isentropically or isenthalpy to the pressure required by the DF engine by the decompression device 700 for fuel, and the temperature is also lowered.

According to the present embodiment, the low-temperature reduced pressure BOG whose pressure and temperature are lowered while being decompressed by the pressure reducing device 700 for fuel is supplied to the low-temperature flow path of the second heat exchanger 400 and cools the high-pressure BOG to be reliquefied. It is used as a refrigerant.

In the second heat exchanger 400 , the low-temperature reduced-pressure BOG decompressed by the fuel decompression device 700 and the high-pressure BOG discharged after being cooled by heat exchange in the first heat exchanger 300 exchange heat. The temperature of the low-temperature reduced pressure boil-off gas rises, and the temperature of the high-pressure boil-off gas decreases.

The reduced pressure BOG whose temperature is increased by heat exchange in the second heat exchanger 400 is transferred to the first heat exchanger 300 , and the second fuel line FL2 with the temperature increased by heat exchange in the first heat exchanger 300 . ) and supplied to the DF engine.

As described above, according to the present embodiment, the BOG to be supplied to the high-pressure engine and BOG to be supplied to the low-pressure engine and BOG to be reliquefied are not supplied to the low-pressure engine in the middle of the high-pressure compressor 200 . are all compressed to the high pressure required by the high-pressure engine using the high-pressure compressor 200, and then the BOG to be supplied to the low-pressure engine is decompressed by the decompression device 700 for fuel to additionally generate cooling heat, BOG to be reliquefied is cooled by cooling heat, and the reduced pressure BOG is heated while exchanging heat with BOG to be reliquefied, adjusted to the temperature required by the low pressure engine, and then supplied to the low pressure engine.

Therefore, the reliquefaction amount is increased by reducing the BOG to be supplied to the low-pressure engine and additionally generating cooling heat, and at the same time, the reduced BOG is cooled while cooling the BOG to be reliquefied and the temperature rises. can satisfy

According to this embodiment, between the first heat exchanger 300 and the low-pressure engine, a fuel heater ( 800); may be installed.

The fuel heater 800 is operated when the temperature of the reduced pressure boil-off gas transferred from the second heat exchanger 400 and the first heat exchanger 300 is lower than the gas fuel temperature condition required by the low-pressure engine to heat the reduced pressure boil-off gas. may be a means to

In addition, the low-temperature high-pressure BOG whose temperature is lowered by heat exchange with the reduced-pressure BOG in the second heat exchanger 400 is discharged from the second heat exchanger 400 along the reliquefaction line RL to the decompression device 500 for re-liquefaction. ) is transferred to The low-temperature, high-pressure BOG discharged from the second heat exchanger 400 may be in a supercritical state, and may be in a gas-liquid mixed state at least partially liquefied, or may be in a supercooled liquid state.

The pressure reducing device 500 for reliquefaction may be an adiabatic expansion means, for example, an expander or an expansion valve (Joule-Thomson valve), for adiabatically expanding the low-temperature, high-pressure boil-off gas. The low-temperature, high-pressure BOG is expanded isentropically or isenthalpy to the operating pressure of the LNG storage tank 100 by the decompression device 500 for reliquefaction, and the temperature is also lowered.

The low-temperature, high-pressure BOG may be a gas-liquid mixture such that flash gas is generated while the pressure is reduced by the decompression device 500 for reliquefaction. The gas-liquid separator 600 separates the gas-liquid mixture, the liquid reliquefied BOG is recovered to the LNG storage tank 100 and the non-reliquefied gaseous uncondensed BOG is the first heat exchanger 300 . into the low-temperature flow path of

The reliquefaction line RL through which the liquid reliquefied BOG flows from the gas-liquid separator 600 to the LNG storage tank 100 is connected from the lower end of the gas-liquid separator 600 to the LNG storage tank 100 . The gas recovery line GL, through which non-condensed BOG in gaseous state flows from the gas-liquid separator 600 to the low-temperature flow path of the first heat exchanger 300 , is connected from the upper end of the gas-liquid separator 600 to the first heat exchanger 100 . It is connected to the boil-off gas line (BL) from the upstream.

In addition, according to this embodiment, it is branched from the reliquefaction line RL between the second heat exchanger 400 and the gas-liquid separator 600, and the second fuel line FL2 upstream of the pressure reducing device 700 for fuel. It may further include a; fuel merging line (FL3) connected to.

Depending on the operating state of the ship, the boil-off gas fuel to be supplied to the low-pressure engine may be branched to the pressure reducing device 700 for fuel using the second fuel line FL2 upstream of the first heat exchanger 300 as described above. , may be branched downstream of the second heat exchanger 400 using the fuel merging line FL3.

For example, in the operating state where the amount of BOG fuel required by the ME-GI engine, which is a propulsion engine, increases, the amount of cooling heat that can be recovered from BOG before compression is required to cool the high-pressure BOG to be reliquefied. more than cold and heat with At this time, the low-temperature, high-pressure boil-off gas is branched to the fuel merging line FL3 downstream of the second heat exchanger 400 and decompressed by the fuel decompression device 700 , and then the second heat exchanger 400 and the first The heat exchanger 300 recovers the cooling heat of the reduced pressure boil-off gas.

In the second fuel line FL2, a fuel branch valve (reference numeral not assigned) installed between a branch point from the reliquefaction line RL or a branch point from the reliquefaction line RL and the pressure reducing device 700 for fuel. ); can be installed.

In addition, in the fuel merging line FL3, a fuel merging valve (reference numeral 700) provided between a branch point from the reliquefaction line RL or a branch point from the reliquefaction line RL and the fuel pressure reducing device 700 . not granted); may be installed.

By the control unit (not shown), the flow rate of BOG before compression supplied to the first heat exchanger 300 , the flow rate of BOG supplied to the first fuel line FL1 , and BOG branched to the reliquefaction line RL The opening and closing amount of the fuel branch valve and fuel merging valve can be adjusted according to the operating conditions of the vessel, such as the flow rate of the vessel.

According to this embodiment, both the boil-off gas generated in the LNG storage tank 100 and the gas discharged from the gas-liquid separator 600 are compressed at high pressure using the high pressure compressor 200 . Before sending this high-pressure boil-off gas to the first heat exchanger 300, the boil-off gas to be supplied to the low-pressure engine is branched, and the pressure is reduced to the pressure required by the low-pressure engine using the fuel decompression device 700 . At this time, the reduced pressure boil-off gas discharged from the pressure reducing device 700 for fuel is in a very low cryogenic temperature state.

Although the high-pressure BOG to be reliquefied is primarily cooled by heat exchange in the first heat exchanger 300 , it is higher than the temperature of the reduced pressure BOG expanded by the pressure reducing device 700 for fuel. Therefore, the cold heat is additionally recovered by heat-exchanging the low-temperature reduced pressure boil-off gas in the second heat exchanger 400 .

Then, the low-temperature reduced-pressure BOG from which the cooling heat is recovered in the second heat exchanger 400 is used as a refrigerant for further cooling the high-pressure BOG cooled primarily in the first heat exchanger 300 secondarily, and then again in the first The remaining cooling heat is recovered by cooling the high-pressure boil-off gas in the heat exchanger 300 . Through this process, the temperature of the high-pressure BOG can be cooled lower than the general BOG treatment method, and the amount of reliquefaction of BOG is increased by maximally recovering the cooling heat generated to generate BOG for fuel. can do it

As described above, the embodiments according to the present invention have been reviewed, and the fact that the present invention can be embodied in other specific forms without departing from the spirit or scope of the present invention in addition to the above-described embodiments is recognized by those with ordinary skill in the art. It is self-evident to Therefore, the above-described embodiments are to be regarded as illustrative rather than restrictive, and accordingly, the present invention is not limited to the above description, but may be modified within the scope of the appended claims and their equivalents.

100: LNG storage tank
200: compressor
300: first heat exchanger
400: second heat exchanger
500: pressure reducing device for reliquefaction
600: gas-liquid separator
700: pressure reducing device for fuel
800: fuel burner
BL: Boil-off gas line
BL1 : Heat exchanger bypass line
RL: Reliquefaction line
GL : gas recovery line
FL1: first fuel line
FL2: 2nd fuel line
FL3 : fuel merging line

Claims (11)

a high-pressure compressor for compressing the boil-off gas discharged from the liquefied gas storage tank to the gas fuel pressure condition required by the high-pressure engine;
a fuel decompression device for decompressing the high-pressure BOG to be supplied as fuel of the low-pressure engine from among the high-pressure BOG compressed in the high-pressure compressor to a pressure required by the low-pressure engine;
a heat exchanger for recovering and cooling the remaining high-pressure BOG from the high-pressure BOG compressed by the high-pressure compressor, which exceeds a demand amount required by the low-pressure engine, by recovering the cooling heat of the decompressed BOG decompressed by the decompression device for fuel; and
Including; a pressure reducing device for re-liquefaction by reducing the pressure of the high-pressure boil-off gas cooled in the heat exchanger to be reliquefied;
The reduced-pressure BOG recovered from cooling heat in the heat exchanger is supplied to the low-pressure engine, and the BOG re-liquefied in the decompression device for re-liquefaction after being cooled in the heat exchanger is recovered to the liquefied gas storage tank. system. The method according to claim 1,
the heat exchanger,
A second heat exchanger for cooling the high-pressure BOG by exchanging heat between the reduced pressure BOG and the high-pressure BOG to be reliquefied, and recovering the cooling heat of the reduced pressure BOG primarily to cool the high-pressure BOG. The method according to claim 2,
the heat exchanger,
By exchanging heat with BOG before compression supplied from the liquefied gas storage tank to the high-pressure compressor, the high-pressure BOG to be reliquefied, and the reduced-pressure BOG discharged after heat exchange in the second heat exchanger, the cooling heat of the reduced-pressure BOG is reduced. The second heat exchanger for cooling the high-pressure boil-off gas by recovering the second heat exchanger; further comprising a, boil-off gas treatment system. The method of claim 3,
and a fuel merging line branched downstream of the second heat exchanger and connected to the fuel pressure reducing device. The method of claim 3,
a fuel branch valve that is controlled to open and close so that the amount of fuel demanded by the low-pressure engine from among the high-pressure boil-off gas supplied to the first heat exchanger is supplied to the pressure reducing device for fuel;
a fuel merging valve whose opening and closing is controlled to supply the fuel demand amount required by the low-pressure engine from the low-temperature high-pressure boil-off gas cooled by the second heat exchanger to the pressure reducing device for fuel; and
A control unit that controls opening and closing of the fuel branch valve and the fuel merging valve according to the flow rate of the high-pressure boil-off gas supplied to the first heat exchanger to control a branch point of the high-pressure boil-off gas to be supplied to the pressure reducing device for fuel; BOG treatment system. The method according to claim 1 or 2,
Further comprising a; fuel heater for adjusting the temperature of the reduced-pressure BOG recovered from the cooling heat in the heat exchanger to a temperature required by the low-pressure engine. delete The method of claim 3,
By gas-liquid separation of the gas-liquid mixture generated while decompressed by the depressurization device for reliquefaction, the reliquefied BOG in a liquid state is recovered to the liquefied gas storage tank, and the non-condensed BOG in a gaseous state is supplied to the first heat exchanger. A gas-liquid separator that joins the boil-off gas flow before compression; further comprising, a boil-off gas treatment system. A high-pressure compression step of compressing the boil-off gas discharged from the liquefied gas storage tank to the gas fuel pressure condition required by the high-pressure engine;
a high-pressure fuel supply step of supplying an amount of fuel demanded by the high-pressure engine from among the high-pressure BOG compressed in the high-pressure compression step to the high-pressure engine;
a fuel decompression step of decompressing the fuel demand amount required by the low pressure engine from among the remaining high pressure BOG exceeding the fuel demand amount of the high pressure engine to the pressure required by the low pressure engine;
a cooling step of cooling the remaining high-pressure BOG exceeding the fuel demand amount required by the high-pressure engine and the low-pressure engine with the cooling heat of the reduced pressure BOG to be supplied to the low-pressure engine; and
Including; a re-liquefaction step of re-liquefying the high-pressure BOG by reducing the pressure of the high-pressure BOG cooled in the cooling step;
The reduced-pressure BOG from which cooling heat is recovered in the cooling step is supplied to the low-pressure engine, and the BOG re-liquefied in the re-liquefaction step is recovered to the liquefied gas storage tank. The method of claim 9,
The cooling step is
a primary cooling step of cooling the high-pressure BOG by exchanging heat between the BOG before compression in the high-pressure compression step, the high-pressure BOG to be reliquefied, and the reduced pressure BOG; and
A secondary cooling step of further cooling the high-pressure BOG by exchanging heat with the high-pressure BOG cooled in the first cooling step and the reduced pressure BOG;
The cooling heat of the reduced pressure boil-off gas is primarily recovered in the secondary cooling step, and secondarily recovered in the first cooling step, the boil-off gas treatment method. The method of claim 10,
In the fuel depressurization step,
Before being supplied to the primary cooling step, the pressure is reduced by branching the amount of fuel demanded by the low-pressure engine among the high-pressure BOG, or fuel required by the low-pressure engine from the low-temperature high-pressure BOG that has undergone the secondary cooling step. Further comprising; a fuel branch point control step of depressurizing by branching the amount of demand
In the fuel branch point control step, the BOG treatment method for selecting the fuel branch point according to the fuel demand amount required by the high-pressure engine.

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