Method and apparatus for reliquefying BOG and recycling it into LNG tanks

Abstract

A method and apparatus for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, the method comprising the steps of recovering BOG (F2) from the headspace of an LNG tank (3), compressing the BOG in a first compression stage (70a) to a first pressure p1 between 0.8 MPa absolute and 1.8 MPa absolute to remove a first portion of the gas, and compressing a second portion of the gas from the process in a final compression stage (70b) to a pressure of at least 12.0 MPa absolute. cooling at least a portion of the further compressed gas to a first temperature T1 between -20°C and -100°C; expanding the gas from the process to a third pressure p3 between 0.8 MPa absolute and 2.0 MPa absolute; and separating the gas from the process into a liquid phase and a gas phase to combine the gas phase with the first portion of the gas removed from the first compression stage and to return the liquid phase into the LNG tank (3).

Description

[Technical field]

[0001] The present invention relates to the technical field of reliquefying boil-off 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, which is produced by cooling natural gas to cryogenic temperatures, has a small volume and is therefore very suitable for storage and transportation. Furthermore, liquefied gases such as LNG contain fewer pollutants and therefore comply with regulatory requirements better than, for example, heavy crude 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 rises slightly. In LNG storage tanks, LNG therefore naturally vaporizes continuously to produce boil-off gas (BOG).

[0004] The formation of BOG represents 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 of these methods 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 planned to use BOG as a refrigerant to reliquefy compressed BOG in a partial reliquefaction system (PRS).

[0006] However, this system has the drawback that nitrogen accumulates in the gas mixture during the reliquefaction cycle. Natural gas is a gas mixture with methane as the main component, but often also ethane, propane, butane and other hydrocarbons. Other minor components may include hydrogen sulfide, nitrogen and carbon dioxide. Nitrogen is typically contained in natural gas in a proportion of about 1% to 15%. The boiling point of nitrogen is -196°C, significantly lower than that of methane, which transitions to the gas phase at -161°C. Therefore, since N2 can hardly be reliquefied in a typical BOG reliquefaction system, its proportion in the mixture increases over time. The quality of the natural gas decreases. In addition, a significant part of the capacity of the compression system is used up by the increased N2 content, significantly reducing the efficiency of the system. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] US Patent Application Publication No. 2019 / 0351988 Summary of the Invention [Problem to be solved by the invention]

[0008] It is therefore an object of the present invention to overcome such shortcomings of the prior art, and in particular to provide a method or partial reliquefaction system (PRS) for partial reliquefaction of BOG in which N2 build-up is reduced or prevented over successive cycles. [Means for solving the problem]

[0009] This object is solved by a method having the features of claim 1 and by a device having the features of claim 8. In particular, the object is solved by a method for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, the method comprising: a) recovering BOG (F2) from the headspace of an LNG tank; b) compressing the BOG in a first compression stage to a first pressure p1 between 0.8 MPa absolute and 1.8 MPa absolute and withdrawing a first portion of this gas; c) further compressing a second portion of the gas from step b) in a final compression stage to a second pressure p2 of at least 12.0 MPa absolute, preferably of between 12.0 MPa and 40.0 MPa absolute, particularly preferably of between 15.0 MPa and 30.0 MPa absolute; d) cooling at least a portion of the further compressed gas from step c) to a first temperature T1 between −20° C. and −100° C.; e) expanding the gas from step d) to a third pressure p3 between 0.8 MPa absolute and 2.0 MPa absolute; f) separating the gas from step e) into a liquid phase and a gas phase, f1) combining the gas phase with the removed first portion of the gas from step c); f2) returning the liquid phase to the LNG tank.

[0010] It has been found that such a method is particularly good for removing nitrogen from the system, making it possible to use the nitrogen-enriched gas for useful purposes. In step f), the nitrogen can be almost entirely in the gas phase. If the gas phase is combined with BOG compressed to p1 in the first compression stage (step c), it can be used to reliably operate low-pressure gas injection engines.

[0011] The method has the further advantage that the flash gas is generally withdrawn from the reliquefaction system PRS instead of being compressed again, which reduces the load on the first compression stage compared to conventional systems, which can be smaller in size and / or operate more efficiently, and overall reduces energy consumption.

[0012] Further compression of the second portion of the gas from step b) in the final compression stage to a second pressure p2, followed by cooling (step d)) and isenthalpic expansion (step e)) serves to efficiently re-liquefy the gas, in part using the Joule-Thomson effect. At the second pressure p2, a relatively high pressure is targeted, so that T w After conventional water cooling to 35° C., a highly compressed gas at about 35° C.-45° C. is obtained with a correspondingly low enthalpy. By a further cooling step and isenthalpic expansion e) of the gas, it is possible to bring the compressed gas initially to a temperature T1 and with expansion to a pressure p3 to an even lower temperature, i.e. to conditions favorable for phase separation.

[0013] In the above process, the cooling in step d) is preferably performed at least in part by heat exchange (preferably indirect heat exchange) with cooling BOG (F2) from the head space of the LNG tank. It is possible to carry out one or more cooling steps of the described process using a separate cooling circuit with a corresponding refrigerant (typically N2). However, this is costly and energy intensive. In contrast, the coolant at a temperature slightly higher than the boiling point of LNG is already present in the system as BOG.

[0014] In a preferred embodiment, in substep f2), the liquid phase is cooled to a temperature T2 between -140°C and -161°C before being returned to the LNG tank, thereby reducing the formation of new BOG. ​​Preferably, this cooling is achieved by heat exchange in countercurrent flow against the BOG from the LNG tank. In this way, the existing cooling capacity is optimally utilized. Before or during the delivery of the liquid phase to the LNG tank, the reliquefied gas is finally expanded to ambient pressure of 0.1 MPa absolute.

[0015] If, as mentioned above, in both steps f2) and d) the cooling is carried out by heat exchange (preferably indirect heat exchange) with cooling BOG from the head space of the LNG tank, it is advantageous if particularly cold BOG taken directly from the LNG tank is used for the cooling in sub-step f2) and then the BOG is used for the cooling in step d) at a slightly higher temperature.

[0016] In a particularly preferred embodiment, the cooling in step d) is at least partially carried out by heat exchange with said gas phase from step f). After phase separation, the gas phase has a pressure p3 and a temperature of typically about -80°C. Since the gas phase is intended for use in low pressure gas injection engines, such low temperatures and often such high pressures are not required. It is therefore possible to use the gas phase as a refrigerant in the cooling process. By further expanding the gas phase from step f) before using it as a coolant, it is possible to further reduce the temperature using the Joule-Thomson effect.

[0017] It is particularly preferred if both the gas phase from step f) and the BOG from the headspace of the LNG tank are used as coolants in step d), where the gas phase from step f) is used to pre-cool the warmer compressed gas, while the BOG from the LNG tank is used to cool the already pre-cooled compressed gas. In such an arrangement, the gas compressed to p2 and typically present at a temperature well above 100° C. is first cooled by water cooling to about 35° C.-45° C., then cooled in heat exchange with the gas phase from step f) to an intermediate temperature of about 25° C. to -15° C., and further downstream cooled to a temperature T1 between -20° C. and -100° C. by heat exchange with the cooling BOG from the LNG tank. Through this sequence of heat exchange steps, the existing cooling capacity of the BOG and compressed gas is used to optimize the use of the cooling capacity available in the system.

[0018] In step d), a portion of the further compressed gas from step c) is preferably fed to the supply line for the high pressure gas injection engine (2). In this embodiment, the highly compressed gas at pressure p2 can be used to drive the high pressure gas injection engine or alternatively can be reliquefied. Natural gas is the fuel of choice, especially in liquefied gas tankers, to keep the emissions of air pollutants at a relatively low level. The ability to adjust the amount fed to the gas injection engine or PRS allows flexibly to take into account climatic and weather conditions as well as the fuel requirements of the high pressure gas injection engine.

[0019] In a preferred embodiment, in step f), the pressure p3 is monitored and controlled to have a value within a predefined range. This can be achieved by a pressure sensor. The measurements make it possible to optimize the conditions in the gas-liquid separator and, if necessary, to adjust the LNG delivery rate. Additionally or alternatively, in step f), the volume of the liquid phase can be monitored in order to adjust the amount returned to the LNG tank depending on said value.

[0020] A further aspect of the present invention is an apparatus for re-liquefying boil-off gas (BOG) back to a liquefied natural gas (LNG) tank, comprising: a first heat exchanger comprising a line for passing a cooling fluid (preferably BOG from an LNG tank) and a line for passing, preferably in countercurrent, the compressed gas to be cooled; a multi-stage compressor (10) comprising at least a first compression stage and a final compression stage, the first compression stage being configured to compress the BOG (F2) from the LNG tank to a first pressure p1 between 0.8 MPa absolute and 1.8 MPa absolute, and the final compression stage being configured to compress the pre-compressed BOG to a second pressure p2 of 12.0 MPa absolute or more, preferably 12.0 MPa to 40.0 MPa absolute, particularly preferably 15.0 MPa to 30.0 MPa absolute; a branch line (6) arranged in fluid communication downstream of the first compression stage and leading further downstream to a supply line for a low pressure gas injection engine, a gas combustion unit, or both; -Return line, a first expansion unit configured to expand the compressed gas from a second pressure p2 to a third pressure p3, p3 being between 0.8 MPa absolute and 2.0 MPa absolute, preferably between 1.0 MPa absolute and 1.8 MPa absolute; a gas-liquid separator configured to separate a liquefied gas portion for return to the LNG tank (3) at pressure p3 and to supply a gaseous portion to a bypass line, the bypass line communicating with the branch line; the multi-stage compressor is connected upstream in fluid communication with the head space of the LNG tank, preferably via the line of the heat exchanger, for passing BOG for cooling, the multi-stage compressor is connected downstream in fluid communication with the line of the first heat exchanger, via the return line, for passing compressed gas to be cooled, further downstream to the first expansion unit and further downstream to the gas-liquid separator, the first heat exchanger being in particular configured to cool at least a portion of the BOG further compressed to the second pressure p2 to a first temperature T1 between -20°C and -100°C.

[0021] Such an apparatus is capable of carrying out the method according to the invention. The nitrogen present in the natural gas accumulates in the gas phase of the gas-liquid separator and is removed from the system and put to a useful purpose as a fuel mixture. Via a bypass line, the gas phase can be combined with the BOG compressed to p1 in the first compression stage, thus ensuring a reliable supply of fuel to the low-pressure gas injection engine. In general, the apparatus according to the invention makes it possible to remove the flash gas from the reliquefaction cycle from the system instead of compressing it repeatedly, which reduces the load on the multi-stage compression system (especially the first compression stage) and allows a smaller design.

[0022] The first compression stage may comprise one or more piston compressors, each with water cooling. The same applies for each higher compression stage. It is also preferable to provide water cooling after the final compression stage. In this way, it is possible to provide highly compressed LNG at pressure p2 and at a temperature of about 35°C-45°C, which is well suited for processing in the reliquefaction system PRS.

[0023] Low pressure gas injection engines, fuelled through a supply line, typically use gas at a pressure of about 0.6 MPa to 1.8 MPa absolute, preferably at a pressure of about 0.6 MPa absolute. The pressure p1 in the branch line may be higher than this target pressure, so a throttle valve can be provided between the branch line and the supply line to release the gas pressure.

[0024] The expansion unit can be an expansion valve or an expander. During the expansion, the Joule-Thomson effect is used to further reduce the temperature of the gas to be reliquefied. Since the pressure p3 in the gas-liquid separator can be higher than the pressure p2 in the branch line, the bypass line can also have an expansion unit.

[0025] In a preferred embodiment, the above-mentioned device comprises a second heat exchanger having a line for passing a cooling fluid (preferably BOG from an LNG tank) and a line for passing the compressed gas to be cooled (preferably in countercurrent), in which the line for passing the compressed gas to be cooled is arranged in fluid communication between the gas-liquid separator and the LNG tank, and preferably the line for passing the cooling fluid is arranged in fluid communication between the head space of the LNG tank and the first heat exchanger. With the second heat exchanger, it is possible to use particularly cold BOG to cool the reliquefied gas immediately after exit, while the slightly warmer BOG is used as coolant in the cooling stage downstream of the water cooling system and upstream of the gas-liquid separator. The targeted use of BOG as coolant in different sections of the PRS increases the overall reliquefaction rate.

[0026] The device preferably comprises a third heat exchanger, in which the line for cooling is part of the bypass line and the line to be cooled is part of the return line. This means that the gas phase separated in the gas-liquid separator with pressure p3 and temperature of about -82°C can be used first as a refrigerant and then downstream as a fuel for a low-pressure gas injection engine. A further expansion unit can be arranged between the gas outlet of the gas-liquid separator and the third heat exchanger. By further expanding the gas phase from step f) before using it as a coolant, the temperature can be further reduced using the Joule-Thomson effect.

[0027] It is preferable that the apparatus further comprises a second expansion unit configured to expand compressed gas from a third pressure p3 to atmospheric pressure, the second expansion unit being arranged in fluid communication between a liquid outlet of the gas-liquid separator and the LNG tank, preferably between a conduit of the second heat exchanger for passing compressed gas to be cooled and the LNG tank.

[0028] After the gas-liquid separator, the liquid phase is typically at a pressure p3 and a temperature of about −110° C. After cooling to about −155° C. in a second heat exchanger, the reliquefied gas can be expanded again in a further expansion unit, for example to atmospheric pressure, to achieve low temperatures, in particular around the boiling point of natural gas.

[0029] The device may be part of a fuel gas supply system for supplying gas stored in the LNG tank to a high pressure gas injection engine, and further comprises an outlet arranged in fluid communication downstream of the second compression stage of the multi-stage compressor and further downstream leading to a supply line for the high pressure gas injection engine, and the compressed gas may be supplied to the return line (particularly from the outlet) as long as its amount exceeds the fuel requirement of the high pressure gas injection engine.

[0030] In this embodiment, the highly compressed gas at pressure p2 can be used to drive a high pressure gas injection engine or reliquefied in the PRS. It is also possible to use natural gas, for example, to drive a transport vehicle. Natural gas is the fuel of choice, especially in liquefied gas tankers, to keep air pollutant emissions at a relatively low level. The ability to adjust the amount supplied to the gas injection engine or PRS allows for a flexible response to climatic and weather conditions and the fuel requirements of the high pressure gas injection engine.

[0031] An aspect of the invention relates to an apparatus as described above, wherein the liquid-gas separator comprises a pressure sensor for measuring the pressure in the liquid-gas separator and a controller for actuating a valve arranged between the gas outlet of the liquid-gas separator and the bypass line in response to the measured pressure. The apparatus as described above can be designed such that the liquid-gas separator comprises a level sensor and a control unit for actuating a valve arranged between the liquid outlet of the liquid-gas separator and the LNG tank in response to the measured level. The pressure and the liquid level in the liquid-gas separator can be regulated via a valve by a corresponding control system.

[0032] The invention further relates to the use of the above-mentioned device in ships, in particular ships propelled by high pressure gas injection engines. Considering the limited space available on board ships, it is particularly useful if it is possible to have smaller compression stages in a multi-stage compressor for improved efficiency and continuous extraction of the N2 component from the LNG.

[0033] The invention is further illustrated by the figures, which are intended for illustrative purposes and are not to 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 a method according to the present invention; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] FIG. 1 is a schematic diagram of an apparatus for reliquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank. BOG (F2) is taken from an LNG tank 3 at about −161° C. and is first fed to a second heat exchanger 21, where it passes as cooling fluid in a line 5 in countercurrent to the reliquefied gas to be cooled. Further downstream, the BOG is fed to a first heat exchanger 20 (i.e. a line for passing a cooling fluid) in countercurrent to the compressed gas to be cooled. The BOG thus heated to a temperature of about 30° C. is then fed to a multi-stage compressor 10 and compressed in a first compression stage 70a. The first compression stage preferably comprises one or two piston compressors 71, 72 connected in parallel or in series, each with subsequent water cooling. The first compression stage 70a is set to compress the BOG to a first pressure p1, for example of 1.2 MPa absolute. After the first compression stage 70a, a branch line 6 is arranged downstream in fluid communication, which leads further downstream to a supply line for the low pressure gas injection engine 4. By means of a valve arranged on the branch line 6, the gas can be decompressed to the pressure required by the gas injection engine 4 (for example 0.6 MPa absolute). The figure shows that the multi-stage compressor arrangement 10 has a second compression stage, which is also the final compression stage 70b. The final compression stage 70b is configured to compress the pre-compressed BOG to a second pressure p2 of about 30.0 MPa absolute. This is achieved by three piston compressors 73, 74, 75, each with a subsequent water cooling. However, it is also possible to use a different number or different types of compressors, which can be connected in parallel or in series.

[0036] Downstream of the multi-stage compressor arrangement 10, on the one hand, there is arranged an outlet 7 in fluid communication with the supply line for the high-pressure gas injection engine 2, and on the other hand, there is arranged a return line 8, the contents of which are indirectly cooled further downstream using a first heat exchanger 20, in particular to a temperature of about -70°C. As a result, compressed gas can be fed to the return line 8 if its amount exceeds the fuel requirement of the high-pressure gas injection engine 2. Further downstream of the first heat exchanger 20, there is connected a first expansion unit 30, which is set to isenthalpic expand the compressed and cooled gas from pressure p2 to a third pressure p3 of about 1.5 MPa absolute, whereby the temperature is further reduced to about -110°C.

[0037] A gas-liquid separator 40 is connected downstream of the expansion unit 30 and is configured to separate a liquefied gas portion for return to the LNG tank 3 at pressure p3 and to supply a gaseous portion to a bypass line 9 which leads to the branch line 6. As can be seen from FIG. 1, the exemplary device comprises a third heat exchanger 22, the line for cooling of which is part of the bypass line 9 and the line to be cooled is part of the return line 8, which part corresponds to a section of the return line upstream of the first heat exchanger 20. According to the embodiment shown, the highly compressed and water-cooled BOG from the last compression stage 70b is first cooled in indirect heat exchange with the gas phase from the gas-liquid separator and then further cooled in indirect heat exchange with the BOG from the LNG tank before expansion and phase separation takes place.

[0038] The liquid phase leaves the gas-liquid separator 40 and is further indirectly cooled in the second heat exchanger 21 to a temperature T2 of only about -155°C using BOG fed directly from the LNG tank into the cooling fluid jacket 5. Finally, the liquid is expanded to atmospheric pressure in the expansion unit 31 and returned to the LNG tank.

[0039] The valves 80 and 31, 50 serve to control the pressure and the liquid level, respectively, in the gas-liquid separator 40. They can be actuated depending on the pressure and / or the level measured in the gas-liquid separator. Optionally, it is also possible to arrange a valve between the outlet of the gas-liquid separator 40 and the line of the second heat exchanger 21 for the passage of the fluid to be cooled, in order to control the filling level in the gas-liquid separator.

[0040] FIG. 2 shows a schematic Mollier diagram to illustrate the method according to the invention. The x-axis shows the enthalpy of the system and the y-axis the pressure of the gas. A particular temperature is indicated by the isotherm T w , T1 and T2 (dotted lines), as well as the boiling curve (Siedekurve; boiling line) and the dew line (Taulinie; dew dotted line). Method steps related to changes in enthalpy, temperature and / or pressure are shown with dashed lines.

[0041] In step a), BOG is withdrawn from the headspace of the LNG tank and cooled to a temperature T w In step b), the BOG is compressed in a first compression stage to a first pressure p1 between 0.8 MPa absolute and 1.8 MPa absolute (in this case comprising two compression operations with subsequent water cooling) and a first portion of this gas is withdrawn (not shown). In step c), the gas is further compressed in a final compression stage to a high pressure p2 (in this case consisting of three compression operations each with subsequent water cooling). Following this, in step d), at least a portion of the further compressed gas from step c) is first compressed to a pressure p2 by water cooling. w and then to a first temperature T1 between -20°C and -100°C. In step e) an isenthalpic expansion occurs up to a third pressure p3 between 0.8 MPa absolute and 2.0 MPa absolute. In step f) the gas is then separated into liquid and gas phases in order to combine the removed first portion of gas from step b) with the gas phase (substep f1) and to return the liquid phase to the LNG tank 3 (substep f2).

[0042] Figure 2 shows that in substep f2), the liquid phase is further cooled to a temperature T2 only slightly above the boiling point of natural gas before being returned to the LNG tank at ambient pressure, which corresponds to the cooling in the second heat exchanger 21 in Figure 1. It is also possible to understand that in substep f1), the gas phase can be further expanded and / or reheated, for example in indirect heat exchange with the compressed gas to be cooled, corresponding to its use as a refrigerant in heat exchanger 22 in Figure 1, before being combined with the pre-compressed BOG from the first compression stage b).

Claims

1. 1. A method for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, comprising: a) recovering BOG (F2) from the headspace of the LNG tank (3); b) compressing the BOG in a first compression stage (70a) to a first pressure p between 0.8 MPa absolute and 1.8 MPa absolute; １ and withdrawing a first portion of the gas; c) compressing a second portion of the gas from step b) in the final compression stage (70b) at a second pressure p of at least 12.0 MPa absolute, preferably between 12.0 MPa and 40.0 MPa absolute, particularly preferably between 15.0 MPa and 30.0 MPa absolute. ２ and further compressing the mixture to d) subjecting at least a portion of the further compressed gas from step c) to a first temperature T between −20° C. and −100° C. １ and cooling to e) subjecting the gas from step d) to a third pressure p between 0.8 MPa absolute and 2.0 MPa absolute. ３ and expanding the f) separating the gas from step e) into a liquid phase and a gas phase, f １ a) combining the gas phase with the removed first portion of the gas from step c); f ２ ) returning the liquid phase to the LNG tank (3).

2. Process f ２ ) before returning the liquid phase to the LNG tank (3), - Temperature T between -140°C and -161°C ２ 2. The method of claim 1, wherein the LNG is cooled to a temperature of 1000° C., preferably by heat exchange with cooling BOG (F2) from the headspace of the LNG tank (3).

3. 2. The method of claim 1, wherein the cooling in step d) is performed at least in part by heat exchange with cooling BOG (F2) from the headspace of the LNG tank (3).

4. 2. The method of claim 1, wherein the cooling in step d) is performed at least in part by heat exchange with the gas phase from step f).

5. 2. The method according to claim 1, wherein in step d) a portion of the further compressed gas from step c) is fed to a supply line for a high-pressure gas injection engine (2).

6. In step f), the pressure p ３ The method of claim 1 , wherein is monitored and controlled to have a value within a predetermined range.

7. 7. The method according to any one of claims 1 to 6, wherein in step f) the volume of the liquid phase is monitored in order to adjust the amount returned to the LNG tank depending on said value.

8. 1. An apparatus for re-liquefying boil-off gas (BOG) back into a liquefied natural gas (LNG) tank, comprising: a first heat exchanger (20) provided with a line for passing a cooling fluid, preferably BOG from the LNG tank (3), and a line for passing, preferably in countercurrent, the compressed gas to be cooled; A multi-stage compressor (10) comprising at least a first compression stage (70a) and a final compression stage (70b), said first compression stage (70a) compressing the BOG (F2) from said LNG tank (3) to a first pressure p between 0.8 MPa absolute and 1.8 MPa absolute. １ The final compression stage (70b) is configured to compress the pre-compressed BOG to a second pressure p of 12.0 MPa absolute or more, preferably 12.0 MPa to 40.0 MPa absolute, particularly preferably 15.0 MPa to 30.0 MPa absolute. ２ a multi-stage compressor (10) configured to compress the air to a branch line (6) arranged downstream in fluid communication with said first compression stage (70a) and leading further downstream to a supply line for a low-pressure gas injection engine (4), a gas combustion unit, or both; - a return line (8), - compressing the compressed gas to a second pressure p ２ to the third pressure p ３ a first inflation unit (30) configured to inflate to p ３ a first expansion unit (30) having a pressure between 0.8 MPa absolute and 2.0 MPa absolute, preferably between 1.0 MPa absolute and 1.8 MPa absolute; - pressure p ３ a gas-liquid separator (40) configured to separate a liquefied gas portion for return to the LNG tank (3) and to supply a gaseous portion to a bypass line (9), the bypass line (9) communicating with the branch line (6), the multi-stage compressor (10) is connected upstream in fluid communication with the head space of the LNG tank (3), preferably via the line of the heat exchanger (20) for passing BOG for cooling; the multi-stage compressor is connected downstream in fluid communication with the line of the first heat exchanger (20) via the return line (8) for passing compressed gas to be cooled; further downstream, the multi-stage compressor is connected to the first expansion unit (30); and further downstream, the multi-stage compressor is connected to the gas-liquid separator (40).

9. a second heat exchanger (21) having a line (5) for passing a cooling fluid, preferably BOG from the LNG tank (3), and a line for passing, preferably in countercurrent, the compressed gas to be cooled; 9. The apparatus according to claim 8, wherein in the second heat exchanger (21), the line (5) for passing compressed gas to be cooled is arranged in fluid communication between a liquid outlet of the gas-liquid separator (40) and the LNG tank (3), and preferably the line for passing a cooling fluid is arranged in fluid communication between the head space of the LNG tank (3) and the line of the first heat exchanger for passing a cooling fluid.

10. The device according to claim 8, further comprising a third heat exchanger (22), the line for cooling of which is part of said bypass line (9) and the line to be cooled of which is part of said return line (8).

11. - compressing the compressed gas to a third pressure p ３ a second expansion unit (31) configured to expand the pressure from 0.1 to atmospheric pressure; 10. The apparatus according to claim 9, wherein the second expansion unit (31) is configured to be in fluid communication between the liquid outlet of the gas-liquid separator (40) and the LNG tank (3), preferably between the line (5) of the second heat exchanger (21) for passing compressed gas to be cooled and the LNG tank (3).

12. 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); - further comprising an outlet (7) arranged in fluid communication downstream of said second compression stage (70a) of said multi-stage compressor (10) and leading further downstream to a supply line for a high-pressure gas injection engine (2); 9. The device according to claim 8, 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).

13. 9. The apparatus according to claim 8, wherein the gas-liquid separator (40) comprises a pressure sensor for measuring the pressure in the gas-liquid separator, and a controller for operating a valve (80) arranged between a gas outlet of the gas-liquid separator (40) and the bypass line (9) in response to the measured pressure.

14. 9. The apparatus of claim 8, wherein the liquid-gas separator (40) comprises a level sensor and a controller for actuating a valve (80) arranged between a liquid outlet of the liquid-gas separator (40) and the LNG tank (3) in response to the measured level.

15. Use of a device according to any one of claims 8 to 14 in a ship, in particular a ship propelled by a high pressure gas injection engine (2).

Images