Method and apparatus for reliquefying boil-off gas and recycling it to LNG tanks

Abstract

The present invention relates to a method for re-liquefying boil-off gas (BOG) for return to an LNG tank, comprising the steps of: liquefying BOG removed from the headspace of an LNG tank (3) at a pressure of p high and cooling the compressed gas, preferably by water cooling (50), to a temperature T1. expand , cooling the gas expanded in step d) to a temperature T4 by a heat exchanger (20), preferably in countercurrent with cooling BOG (F2) from the head space of the LNG tank 3, and returning the gas from step e) to the LNG tank (3), high is 20.0 MPa or more, preferably 25.0 MPa or more, particularly preferably 30.0 MPa or more, and the pressure p expand relates to a method in which the pressure is 8.0 MPa to 18.0 MPa, preferably 12.0 MPa to 16.0 MPa, and particularly preferably 15.0 MPa.

Description

[Technical field]

[0001] The present invention relates to the technical field of reliquefying tail gas (BOG) from liquefied natural gas (LNG) tanks. [Background technology]

[0002] In recent years, the consumption of liquefied gases such as liquefied natural gas (LNG) has increased dramatically worldwide. LNG is obtained by cooling natural gas to extremely low temperatures and has a small volume that makes it suitable for storage and transportation. Furthermore, liquefied natural gases such as LNG contain fewer pollutants and therefore comply with regulations better than, for example, heavy fuel oil.

[0003] LNG is a clear, colorless liquid obtained by cooling natural gas, which is mainly composed of methane, to approximately -163°C. However, since natural gas is liquefied at an extremely low temperature of -163°C under normal pressure, LNG can easily vaporize if the temperature is slightly increased. In LNG storage tanks, LNG therefore naturally vaporizes continuously to produce boil-off gas (BOG).

[0004] The formation of BOG means a loss of stored LNG and therefore reduces the transport efficiency, for example in LNG tankers. If BOG accumulates in storage tanks, there is also a risk that the pressure in the storage tanks will increase and the tanks may be damaged.

[0005] To solve such problems, a method of reliquefying BOG and returning it to an LNG storage tank, a method of supplying BOG to an internal combustion engine (such as a marine engine) as an energy source, and a combination thereof have been proposed. For example, Patent Document 1 proposes supplying BOG from an LNG tank to a DFDE engine, an X-DF engine, or an ME-GI marine engine. At the same time, it is provided to use BOG as a refrigerant for reliquefying compressed BOG in a partial reliquefaction system (PRS). However, this system has the drawback that only a limited amount of BOG is available as a refrigerant and therefore only insufficiently low temperatures are achieved when the demand for reliquefaction is high. This means that only a small portion of the BOG can be effectively reliquefied. It can be seen that a significant proportion of the compressed and cooled gas is returned to the reliquefaction cycle in gaseous form, reducing the efficiency of the system. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] US Patent Application Publication No. 2019 / 0351988 Summary of the Invention [Problem to be solved by the invention]

[0007] It is therefore an object of the present invention to provide a method or partial reliquefaction system (PRS) for partial reliquefaction of BOG, in which BOG is used directly as a coolant, yet a high level of efficiency is achieved. [Means for solving the problem]

[0008] This problem is solved by a method having the features of claim 1 and by a device having the features of claim 5. In particular, the problem concerns a process for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, comprising: a) removing BOG (F2) from the headspace of the LNG tank; b) The BOG is subjected to pressure p high and compressing the mixture to c) cooling the compressed gas to a temperature T 1 and cooling to d) subjecting at least a portion of the gas from step c) to a pressure p expand and expanding the e) refrigerating the gas expanded in step d) at a temperature T 4 and cooling to f) returning the gas from step e) to the LNG tank; The pressure p high is 20.0 MPa or more, preferably 25.0 MPa or more, particularly preferably 30.0 MPa or more, and the pressure p expand is solved by a method in which the pressure is 8.0 MPa to 18.0 MPa, preferably 12.0 MPa to 16.0 MPa, and particularly preferably 15.0 MPa.

[0009] Such a method is based on the idea that the cooling liquid used to reliquefy the BOG in step e) is only available to a limited extent and / or is only used sparingly. 2 In the case of a separate cooling circuit with a natural gas tank, cooling consumes energy. If BOG from an LNG tank is used instead as coolant (especially if the engine does not need to be run simultaneously with natural gas fuel or if the engine has only a (temporarily) low fuel demand), a low flow rate is desirable from the standpoint of storage / transport efficiency. In contrast, however, coolants (especially water) for cooling the compressed gas to ambient temperature are available in virtually unlimited quantities.

[0010] The inventors have recognised that it may therefore be useful to provide a gas for the cooling step e) that has a low heat content. This is achieved by subjecting the gas to a relatively high pressure phigh and cooling at this high pressure by water quenching. If the gas is then at least partially isenthalpic expanded in step d) before being fed to cooling in step e), a higher proportion of the gas can be reliquefied with the available cooling capacity than if it were reliquefied without step d) or if starting from a lower pressure (e.g. 15.0 MPa).

[0011] A method for supplying gas partially stored in an LNG tank to a high pressure gas injection engine as exhaust vapor gas (BOG, F2) and for re-liquefying and recycling the BOG, comprising the steps according to claim 1, In step d), at least a first portion of the gas from step c) is supplied to the high pressure gas injection engine through an outlet to the extent of the fuel requirement of the high pressure gas injection engine, and at least a second portion of the gas from step c) is supplied to the high pressure gas injection engine through the outlet to the extent of the fuel requirement of the high pressure gas injection engine, expand Preferably, the liquid is expanded to

[0012] In this embodiment, the pressure p high The highly compressed gas in the PRS can be used to power high pressure gas injection engines or can be reliquefied. Natural gas is the fuel of choice, especially in liquefied gas tankers, to keep air pollutant emissions at relatively low levels. The ability to adjust the amount delivered to the gas injection engine or PRS allows for flexibility in responding to climatic and weather conditions and the fuel requirements of the high pressure gas injection engine.

[0013] In the above method, step b) is carried out after step 0), and step 0) is - comprising steps b) to d), high i) an operating mode in which the - pressure p of the BOG low ii) an operating mode in which b1) The BOG is subjected to a pressure p of 10.0 MPa to 19.9 MPa, preferably 14.0 MPa to 16.0 MPa. low and compressing the mixture to d1)p low >p expand If so, at least a portion of the gas from step c) is pumped under pressure p expand and ii) an operating mode including inflating the It is further preferred that in the case of operating mode ii), steps b1) and d1) replace steps b) and d).

[0014] In such a way, it is possible to adjust the operating mode: at low reliquefaction rates, a lower pressure p is required, since the BOG provides sufficient cooling capacity to cool the gas stream to be liquefied to a sufficiently low temperature level. low It has been found that it can be efficient to compress only up to (step b1). On the other hand, it has been proven to be more efficient to compress up to high pressures with a high liquefaction rate. Because the p of the gas to be liquefied high From p low 3. The additional cooling from expansion to 4000 rpm contributes to a higher reliquefaction rate and therefore a more efficient overall system.

[0015] In step 0), the set value p in the range of 20.0 MPa to 30.0 MPa in operation mode i) is high is selected, and / or in operation mode ii) a set value p in the range of 10.0 MPa to 19.9 MPa is selected. low Particularly preferred is a process in which the pressure is selected. The resulting continuous adjustment capability between 10.0 MPa and 30.0 MPa makes it possible in some cases to obtain even greater flexibility in the reliquefaction cycle.

[0016] One aspect of the present invention is an apparatus for re-liquefying boil-off gas (BOG) for return to a liquefied natural gas (LNG) tank, comprising: a heat exchanger comprising a line for the passage of a cooling fluid, preferably BOG from an LNG tank, and a line for the passage of the compressed gas to be cooled; - BOG (F2) from the LNG tank is pressurized high a multi-stage compressor arrangement configured to compress up to - a first cooler; - A return line; - Compressed gas at pressure p high Pressure p expand a first expansion unit configured to inflate the the multi-stage compressor arrangement is fluidly connected upstream to an extraction line leading to a head space of the LNG tank, optionally via the line of the heat exchanger for the passage of the cooling fluid, in order to return the compressed and cooled gas to the LNG tank; the multi-stage compressor arrangement is fluidly connected downstream to the first cooler, further downstream to the first expansion unit via the return line, and further downstream to the line of the heat exchanger for the passage of the compressed gas to be cooled; The pressure p high is 20.0 MPa or more, preferably 25.0 MPa or more, particularly preferably 30.0 MPa or more, and the pressure p expand relates to an apparatus in which the pressure is 8.0 MPa to 18.0 MPa, preferably 12.0 MPa to 16.0 MPa, and particularly preferably 15.0 MPa.

[0017] Such an apparatus solves the above-mentioned problem: a gas with low heat is provided for the cooling process in an indirect heat exchanger. This is done by subjecting the gas to a relatively high pressure p high and cooling at this high pressure by water quenching. If the gas is then at least partially isenthalpic expanded in step d) before being fed to cooling in a heat exchanger, a higher proportion of the gas can be reliquefied with the available cooling capacity than if it were cooled and reliquefied without expansion preceding the cooling step (i.e. starting from gas simply compressed to a lower pressure (e.g. 15.0 MPa)).

[0018] The advantage associated with such a system is the increased efficiency due to the high flow rate of BOG, as mentioned above at the process level. Another design advantage is that there is no need to control the pressure of the intermediate stage before the final compression stage. For example, a portion of the gas can be extracted through a branch line after the intermediate stage (e.g., p low If the BOG is delivered to the PRS (at high If the BOG is uniformly compressed to 1000 MPa, a single PCV is sufficient, thereby reducing the cost of the unit. Finally, with improved unit efficiency, a smaller PRS is sufficient, and with lower gas flow rates, a smaller multi-stage compressor configuration is sufficient. Surprisingly, this is despite the fact that every compression stage needs to be designed to handle the entire BOG stream. A smaller unit saves valuable space, especially when the unit is used on a ship (e.g., a tanker). Less energy is also required for efficient reliquefaction.

[0019] In a preferred embodiment, the device is part of a fuel gas supply system for supplying gas stored in an LNG tank to a high pressure gas injection engine, and in addition - an outlet disposed downstream in flow communication with the multi-stage compressor arrangement and further downstream leading to a supply line for a high pressure gas injection engine; The compressed gas may be fed to the return line if the amount exceeds the fuel requirements of the high pressure gas injection engine.

[0020] The advantage of such a device as part of a fuel gas supply system is that it can deliver highly compressed gas at pressure p highThe advantage of this is that the fuel can be reliquefied at a high pressure gas injection engine or used to drive a high pressure gas injection engine. The ability to adjust the amount delivered to the gas injection engine or PRS allows for flexibility in taking into account climatic and weather conditions and the fuel requirements of the high pressure gas injection engine (e.g. operating speed).

[0021] In one embodiment, the apparatus comprises: - Compressed gas at pressure p expand Pressure p knock-out-drum a second expansion unit that is set to inflate the tank to a pressure between 0.1 MPa and 0.3 MPa (usually 0.1 MPa to 0.3 MPa higher than the tank pressure); - a second expansion unit connected fluidly downstream thereof and configured to expand a liquefied gas portion to a pressure p knock-out-drum and a gas-liquid separator configured to return the LNG tank to the LNG tank at The second expansion unit and the liquid / gas separator are disposed in fluid communication between the line of the heat exchanger for passage of compressed gas to be cooled and the LNG tank.

[0022] For a new expansion in a second expansion unit (e.g. expansion valve, expander) the gas is cooled again by Joule-Thompson expansion. The gaseous component separated by the gas-liquid separator can be combined with the BOG taken from the storage tank and fed as coolant to the heat exchanger. The liquid component is fed to the storage tank. Due to the described configuration it is possible to re-liquefy a higher relative proportion of BOG, preferably with the same amount of available coolant.

[0023] It is preferred that the first cooler is a water cooler capable of cooling the compressed gas to a temperature of 35° C.-45° C., preferably using water from the ship's cooling water system as coolant. Water is plentiful, especially when the apparatus is used on board a ship. Cooling to an achievable temperature of 35° C.-45° C., especially when starting from gas at high pressure, results in a lower enthalpy after the first cooling step compared to processes using lower pressures.

[0024] In a preferred embodiment, the multi-stage compressor arrangement comprises a first compression stage configured to compress the BOG (F2) from the LNG tank to a first pressure p1 between 0.6 MPa and 1.8 MPa, the first compression stage comprising one or more labyrinth sealed piston compressors.

[0025] If the first compression stage has one or more labyrinth-sealed piston compressors, preferably only labyrinth-sealed piston compressors, it can be operated with little or no lubricant. On the one hand, this has the advantage that the quality of the compressed gas is not compromised by contamination with the lubricant. On the other hand, it is possible to avoid the risk that the lubricant (typically oil) will solidify when the BOG is cold in the low compression stages, accelerating the wear of mechanical parts.

[0026] In a preferred embodiment, the multi-stage compressor arrangement of the apparatus includes intermediate and final compression stages, which compress the pre-compressed gas from a first pressure p1 to the pressure p high up to, preferably optionally, the pressure p high Or the pressure p low to a predetermined target pressure value p, said final compression stage having a bypass with a controllable valve to control the return flow after said final compression stage and thus the delivery pressure, said gas at said outlet being compressed to said predetermined target pressure value p high , preferably the predetermined target pressure value p high Or p low. . , and the flow can be returned through the bypass so as to have

[0027] Such an apparatus has the aforementioned advantages that the efficiency of reliquefaction can be improved due to high gas delivery rates and the size of the apparatus elements can be reduced. Now, if it is possible to variably operate a multi-stage compressor arrangement due to a bypass with a controllable valve, or as a result, p low andp high Further advantages arise if it is possible to provide gas at the outlet with a variable pressure between p and p. A multi-stage compressor configuration allows for a side stream for reliquefaction at low reliquefaction rates. low The refrigerant can be set to compress only up to 50% of the nominal pressure, which is more efficient for low reliquefaction rates. In addition, the service life of the equipment can be improved if the equipment is operated at only 50% of the nominal pressure for a significant portion of the operating time.

[0028] It is particularly preferred that the bypass with a controllable valve is adjustable so that any set pressure between 10.0 MPa and 30.0 MPa can be set, which gives the user further flexibility to take into account climatic and weather conditions as well as the fuel requirements of the engine.

[0029] In one embodiment, the final compression stage comprises one or more piston compressors sealed with piston rings, preferably two piston compressors sealed with piston rings. The piston compressors sealed with piston rings, preferably lubricated, pump the gas through a piston ring compressor. high It is possible to compress the material to pressures in the range of

[0030] The above-described device may further comprise a branch line arranged in fluid communication downstream of the first compression stage and leading further downstream to a supply line for the low pressure gas injection engine and / or a supply line for the gas combustion unit.

[0031] This allows the X-DF engine and other low pressure (p 1 ) with fuel withdrawn after the first compression stage. This embodiment is particularly advantageous if the first compression stage is sealed with little or no lubricant, as this allows high quality fuel to be supplied to low pressure gas injection engines and / or gas combustion units.

[0032] The present invention relates to the use of such a device on board a ship (e.g. a natural gas tanker), in particular a ship propelled by a high pressure gas injection engine. When used on board a ship, the advantages mentioned above (in particular the reduced size of the device) are particularly evident due to the limited space available.

[0033] The invention is further explained by the figures, which are for illustrative purposes and should not be understood as limiting. [Brief description of the drawings]

[0034] [Figure 1] 1 is a schematic diagram of an apparatus according to the present invention; [Diagram 2] 1 is a schematic Mollier diagram illustrating the method according to the invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] 1 shows a schematic diagram of a fuel gas supply system according to the present invention, having an arrangement for re-liquefying boil-off gas (BOG) for return to a liquefied natural gas (LNG) tank 3. BOG (F2) accumulating in the headspace of the LNG tank 3 is fed via an offtake line 5 to a heat exchanger 20 where it is used as a cooling fluid in indirect heat exchange and then compressed by a multi-stage compressor arrangement 10 at high pressure p, typically around 30.0 MPa. high is compressed to

[0036] In the illustrated embodiment, the multi-stage compressor arrangement 10 comprises a first compression stage with piston compressors 71 and 72 and associated coolers, an intermediate compression stage with piston compressor 73 and associated coolers, and a final compression stage with piston compressors 74 and 75 and associated coolers. The first compression stages 71, 72 are set to compress the BOG to a pressure p1, typically 0.7 MPa. A portion of the BOG thus compressed, preferably without lubricant, can be fed to the low pressure gas injection engine 4 via a branch line 6 as required. The first compression stage can be pressure controlled by a bypass with a pressure control valve (not shown).

[0037] The multi-stage compressor arrangement 10 is fluidly connected downstream to a first water cooler 50, where the compressed gas is cooled, typically to 40° C. The outlet pressure of the highest compression stage, here consisting of piston compressors 75 and 74 and associated water coolers, is regulated via a bypass 19 having a pressure control valve 9. After leaving the first water cooler 50, the compressed BOG can be supplied as fuel to a high pressure gas injection engine 2 via an outlet 7 or to a first expansion unit 60 (e.g. an expansion valve or expander) via a return line 8. Typically, excess BOG over the fuel requirement of the engine 2 is supplied to the return line 8. The gas is compressed to a pressure p of about 15.0 MPa in the first expansion unit 60. expand Due to the isenthalpic pressure reduction, the compressed natural gas undergoes new cooling and can therefore be further cooled from about 20° C. in indirect heat exchange with BOG (F2) from the LNG tank 3 in the heat exchanger 20.

[0038] The BOG compressed by the compressor arrangement, cooled by water cooling 50, expansion 60 and heat exchange 20 is then expanded in the second expansion unit 30 to a pressure p knock-out-drumand finally separated into a liquid component and a gas component by a gas-liquid separator 40. The liquid component separated by the gas-liquid separator 40 is supplied again to the LNG tank, and the gas component separated by the gas-liquid separator is combined with the BOG discharged from the LNG tank in an outlet line 5, and is then supplied to the heat exchanger 20 and used as a cooling fluid.

[0039] In a BOG reliquefaction system such as that shown in Figure 1, natural gas is reliquefied using BOG removed from the storage tank as a refrigerant without the need for a separate cycle to reliquefy the BOG. ​​It will be appreciated that the invention is not so limited and that a separate refrigeration cycle may be established to ensure reliquefaction of all BOG, if desired. Such a separate circuit may ensure reliquefaction of the BOG, but requires separate equipment and an additional energy source.

[0040] Figure 2 shows the compression and cooling cycle in a schematic Mollier diagram with dashed lines. In its initial state, the BOG is located to the right of the dew point line at atmospheric pressure of 0.1 MPa and about -160°C. When taken from the LNG tank a), the BOG is heated above ambient temperature (about T1), especially when used as a coolant in indirect heat exchange.

[0041] In step b), for example according to the compressor configuration shown in FIG. 1 using a five piston compressor, high These may be arranged as a first compression stage 101, an intermediate compression stage 102, and a final compression stage 103, each of which is compressed at a successive temperature T 1 Thus, in total, it is possible to compress the natural gas to a pressure of typically 30.0 MPa. 1 After final cooling to 1000° C., at least a portion of the gas is then pumped in step d) at a pressure p expand The gas is expanded isenthalpic- ally up to T 2(typically about 20° C.). In step e), the gas is cooled to a temperature T 4 In the system shown in Figure 1, this is done in an indirect heat exchanger 20 using BOG from the headspace of the LNG tank for cooling. Step f) is the return of the gas to the LNG tank, which typically involves further isenthalpic expansion and separation of the liquid and gas portions in a gas-liquid separator.

[0042] Figure 2 shows the gas pressure p high The advantages of compressing the mixture to a temperature of 100° C., followed by cooling c) and expansion d) are shown. expand Compared to compression to 1000 s and subsequent water cooling, the enthalpy difference of 104 is reduced by the lower temperature T 2 It is possible to obtain the compression in the form of p expand If only the gas is at the same pressure p expand At higher temperatures T 1 and the cooling capacity of the coolant available in the heat exchanger must be used to cool the warmer gas (dotted line). In the case of BOG, the coolant is not available indefinitely, so the less compressed gas in the heat exchanger is often used to cool the warmer gas (T 4 Instead of temperature T 3 105 and the subsequently expanded gas is re-liquefied to a lesser extent, corresponding to arrow 105.

[0043] Not shown in FIG. 2 is the operating mode ii) as described above, where the BOG is heated to a pressure p low It is compressed to p low >p expand In the case of p expand In this mode of operation, the intermediate and final compression stages expand the precompressed gas to a first pressure p 1 Optionally, pressure p high (e.g. 30.0MPa) or pressure p low(e.g. 15.0 MPa). The pressure can be adjusted, for example, by a bypass having a controllable valve located in the final compression stage 103. low andp high If the controllable valve is adjustable so that any target pressure between 0.25 and 0.5 can be set, then further pressure in the return line 8 of FIG. 1 or parallel below the dashed line c) in the diagram of FIG. 2 is conceivable.

Claims

1. 1. A method for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, comprising: a) removing BOG (F2) from the headspace of an LNG tank; b) The BOG is heated to a pressure p ｈｉｇｈ and compressing the mixture to c) Bringing the compressed gas to a temperature T １ and cooling to d) subjecting at least a portion of the gas from step c) to a pressure p ｅｘｐａｎｄ and expanding the e) The gas expanded in step d) is cooled to a temperature T ４ and cooling to f) returning the gas from step e) to the LNG tank; The pressure p ｈｉｇｈ is 20.0 MPa or more, preferably 25.0 MPa or more, particularly preferably 30.0 MPa or more, and the pressure p ｅｘｐａｎｄ is 8.0 MPa to 18.0 MPa, preferably 12.0 MPa to 16.0 MPa, particularly preferably 15.0 MPa.

2. 1. A method for supplying gas stored in an LNG tank (3), partly as boil-off gas (BOG, F2), to a high-pressure gas injection engine (2) and for re-liquefying the BOG back, the method comprising the steps of claim 1, In step d), at least a first portion of the gas from step c) is supplied to the high pressure gas injection engine (2) through an outlet (7) up to the fuel requirement of the high pressure gas injection engine (2), and at least a second portion of the gas from step c) is supplied to the high pressure gas injection engine (2) at the pressure p ｅｘｐａｎｄ The method is expanded until

3. Step b) is carried out after step 0), which is - comprising steps b) to d), subjecting the BOG to a pressure p ｈｉｇｈ i) an operating mode in which the - the BOG is subjected to a pressure p ｌｏｗ ii) an operating mode in which the b1) The BOG is subjected to a pressure p of 10.0 MPa to 19.9 MPa, preferably 14.0 MPa to 16.0 MPa. ｌｏｗ and compressing the mixture to d1) p ｌｏｗ >p ｅｘｐａｎｄ If so, at least a portion of the gas from step c) is pumped under pressure p ｅｘｐａｎｄ and ii) an operating mode comprising:

3. The method according to claim 1 or 2, wherein in the operating mode ii), steps b1) and d1) replace steps b) and d).

4. In step 0), in operation mode i), a set value p in the range of 20.0 MPa to 30.0 MPa ｈｉｇｈ is selected, or in operation mode ii), a set value p in the range of 10.0 MPa to 19.9 MPa ｌｏｗ The method of claim 3 , wherein either:

5. 1. An apparatus for re-liquefying boil-off gas (BOG) for return to a liquefied natural gas (LNG) tank, comprising: a heat exchanger (20) provided with a line for the passage of a cooling fluid, preferably BOG (F2) from the LNG tank (3), and a line for the passage of the compressed gas to be cooled; - BOG (F2) from the LNG tank (3) is pumped under pressure p ｈｉｇｈ a multi-stage compressor arrangement (10) configured to compress up to a first cooler (50), - a return line (8), - compressed gas to the pressure p ｈｉｇｈ Pressure p ｅｘｐａｎｄ a first inflation unit (60) configured to inflate to the multi-stage compressor arrangement (10) is fluidly connected upstream to an extraction line (5) leading to the headspace of the LNG tank (3) via the line of the heat exchanger (20) for optionally passing a cooling fluid therethrough in order to return the compressed and cooled gas to the LNG tank (3); the multi-stage compressor arrangement (10) is fluidly connected downstream to the first cooler (50), further downstream to the first expansion unit (60) via the return line (8), and further downstream to the line of the heat exchanger (20) for passing the compressed gas to be cooled; The pressure p ｈｉｇｈ is 20.0 MPa or more, preferably 25.0 MPa or more, particularly preferably 30.0 MPa or more, and the pressure p ｅｘｐａｎｄ is 8.0 MPa to 18.0 MPa, preferably 12.0 MPa to 16.0 MPa, particularly preferably 15.0 MPa.

6. A part of a fuel gas supply system for supplying gas stored in the LNG tank (3) to a high-pressure gas injection engine (2), - an outlet (7) arranged downstream in fluid communication with said multi-stage compressor arrangement (10) and leading further downstream to a supply line for said high-pressure gas injection engine (2); 6. The device according to claim 5, wherein the compressed gas can be supplied to the return line (8) as long as its quantity exceeds the fuel requirement of the high pressure gas injection engine (2).

7. - Compressed gas at pressure p ｅｘｐａｎｄ Pressure p ｋｎｏｃｋ－ｏｕｔ－ｄｒｕｍ a second inflation unit (30) configured to inflate to - fluidly connected downstream of said second expansion unit (30), which expands the liquefied gas portion to a pressure p ｋｎｏｃｋ－ｏｕｔ－ｄｒｕｍ and a gas-liquid separator (40) configured to return the gas portion to the LNG tank (3) and supply the gas portion to the withdrawal line (5), 6. The apparatus of claim 5, wherein the second expansion unit (30) and the gas-liquid separator (40) are arranged in fluid communication between the line of the heat exchanger (20) for passing compressed gas to be cooled and the LNG tank (3).

8. 6. The apparatus of claim 5, wherein the first cooler (50) is a water cooler capable of cooling the compressed gas to a temperature of between 35°C and 45°C, preferably using water at ambient temperature as a coolant.

9. 6. The apparatus of claim 5, wherein the multi-stage compressor configuration (10) comprises a first compression stage (71, 72), the first compression stage (71, 72) configured to compress the BOG (F2) from the LNG tank (3) to a first pressure p1 between 0.6 MPa and 1.8 MPa, the first compression stage (71, 72) comprising one or more labyrinth-sealed piston compressors.

10. The multi-stage compressor configuration (10) comprises an intermediate compression stage (73) and a final compression stage (74, 75), and the intermediate compression stage (73) and the final compression stage (74, 75) convert pre-compressed gas from a first pressure p1 to a pressure p ｈｉｇｈ up to, preferably optionally, the pressure p ｈｉｇｈ Or the pressure p ｌｏｗ The final compression stage (74, 75) has a bypass (19) with a controllable valve (9) for controlling the return flow after the final compression stage (74, 75) and therefore the delivery pressure, so that the gas at the outlet (7) is compressed to the predetermined target pressure value p ｈｉｇｈ , preferably the predetermined target pressure value p ｈｉｇｈ or p ｌｏｗ 6. The device according to claim 5, wherein the air is able to be returned via the bypass (19) so as to have a

11. The device described in Claim 10, wherein the bypass (19) having the controllable valve (9) is adjustable so that any set pressure between 10.0 MPa and 30.0 MPa can be set.

12. 11. Apparatus according to claim 10, wherein the final compression stage (74, 75) comprises one or more piston compressors sealed with piston rings, preferably two piston compressors each sealed with piston rings.

13. 10. The device according to claim 9, further comprising a branch line (6) arranged downstream in fluid communication with the first compression stage (71, 72) and leading further downstream to a supply line for a low-pressure gas injection engine (4), for a gas combustion unit, or for both.

14. Use of a device according to any one of claims 5 to 13 in a ship, in particular a ship propelled by a high pressure gas injection engine (2).

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