METHOD OF AN APPARATUS FOR TREATING BOIL-OFF GAS FOR THE PURPOSE OF SUPPLYING AT LEAST AN ENGINE

Abstract

Apparatus for supplying natural gas fuel to an ocean-going tanker for the transport of liquefied natural gas (LNG), includes a first line with a compressor (12) having an inlet communicating with an ullage space (4) of at least one LNG storage tank (2) and an outlet communicating with a conduit leading from the compressor to at least one engine (38), and a second line with a forcing vaporiser (24) having an inlet communicating with a liquid storage region (6) of the tank (2), the second line being connected to the first line downstream of the compressor (12) and upstream of the at least one engine (38).

Claims

1-12. (canceled)

13. An apparatus for supplying natural gas fuel to an ocean-going tanker for transport of liquefied natural gas (LNG), comprising: a first line including a compressor (12) having an inlet in communication with an ullage space (4) of at least one LNG storage tank (2), and an outlet in communication with a conduit leading from the compressor to at least one engine (38); and a second line including a forcing vaporiser (24) having an inlet in communication with a liquid storage region (6) of the at least one LNG storage tank (2), the second line connected to the first line at a position downstream of the compressor (12) and upstream of the engine (38).

14. The apparatus of claim 13, wherein the compressor comprises a multistage compressor.

15. The apparatus of claim 14, wherein the compressor comprises six stages.

16. The apparatus of claim 13, wherein the first line comprises at least two compressors (12) in parallel.

17. The apparatus of claim 13, wherein the second line comprises a mist separator device positioned downstream of the forcing vaporiser (24) and upstream of a connection point to the first line.

18. The apparatus of claim 13, wherein the first line comprises at least an after cooler (60) after the compressor, and the second line comprises at least a heater (64).

19. A method for treating natural gas coming from an LNG tank for supplying at least an engine, comprising: supplying a first line with natural boil-off gas; compressing the natural boil-off gas; supplying a second line with liquefied natural gas; forcing vaporising of the liquefied natural gas; mixing compressed natural boil-off gas from the compressing with forced boil-off gas from the forcing; and supplying at least an engine with the mixed gas from the mixing.

20. The method of claim 19, wherein the engine comprises a Diesel engine selected from the group consisting of low pressure 2-stroke dual fuel Diesel engine, and a 4-stroke dual fuel Diesel engine.

21. The method of claim 19, wherein the compressing the natural boil-off gas is in at least one multistage compressor.

22. The method of claim 19, further comprising cooling compressed gas in the first line in an after cooler before mixing the compressed gas with the forced boil-off gas.

23. The method of claim 19, further comprising removing liquid after vaporization of the liquefied natural gas from fluid in the second line with a mist separation device.

24. The method of claim 19, further comprising heating the liquefied natural gas in the second line after the forcing vaporising.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings in which,

[0031] FIGS. 1 to 3 are generalised, schematic flow diagrams of different natural gas supply plants according to the invention.

[0032] Like elements in FIGS. 1 to 3 are indicated by the same reference numerals and the drawings are not to scale.

DETAILED DESCRIPTION OF THE INVENTION

[0033] Referring to FIG. 1 of the drawings, an LNG storage tank 2 (which may be one of a plurality) is located on board an ocean-going tanker (not shown). The tank 2 is insulated or has another form of thermal insulation associated therewith so as to keep down in flow of heat from the ambient environment into the liquid therein. The tank 2 has an ullage space 4 and contains a volume 6 of LNG. Since LNG boils at a temperature well below ambient, notwithstanding the thermal insulation of the tank 2, there is a continuous evaporation of the LNG into the ullage space 4. The resulting vaporised gas flows out of the tank 2 though an outlet 8 and passes along a pipeline 10 to a compressor (or blower) 12. The compressor 12 may be driven by an electric motor 14. The pressure of the gas is raised by operation of the compressor 12. The heat of compression is normally sufficient to raise the temperature of the gas being compressed to suitable temperatures being required for supplying engines such as 4-strokes low pressure Diesel engines. However, the resulting compressed gas therefore can pass through a gas heater 16 in which it may be heated by steam (or other heating medium, e.g. hot water) so as to adjust its temperature to the required temperature. To avoid overheating the gas, temperature control is provided by a valved by-pass line 20 extending from upstream to downstream of the gas heater 16. As a further measure of temperature control, the gas heater has on its upstream side a flow control valve 18 which is able to set so as to select the outlet temperature therefrom.

[0034] A second flow comes from the storage tank 2. This second flow is a flow of vaporised natural gas that is formed by employing a forcing vaporiser 24 to vaporise a flow of LNG taken by a submerged pump 26 from the volume 6 of LNG within the tank 2. The outlet of the pump 26 communicates with the forcing vaporiser 24 via a riser 28. A pressure control valve 34 opens a pipe 36 to allow liquid to be returned to the storage tank 2 for different flow rates through the vaporiser 24. The forcing vaporiser 24 has an enlarged superheating section to enable an outlet temperature of plus 20 C. to be readily achievable. The forcing vaporiser 24 has an arrangement of valves similar to that associated with the gas heater 16. Thus, there is a first flow control valve 30 on the upstream side of the vaporiser 24 to set the outlet pressure of the heater 16 so that it is equal to the outlet pressure of the compressor 12, and a valve by-pass line 32 extending from upstream to downstream of the vaporiser 24 to control the outlet temperature of the vapour. The vaporiser 24 is typically of a kind which employs steam heating to raise the temperature of the fluid flowing therethrough.

[0035] The second flow of gas is supplied into the first flow of gas (coming out from the storage tank by the pipeline 10) by a conduit 22 downstream the compressor 12 and upstream the heater 18.

[0036] The gas coming out from the gas heater 16 can be used for supplying dual fuel engines 38 which are located in an engine room 40 of the tanker.

[0037] The apparatus according to the invention preferably has various safety measures to cope with any unexpected operating conditions. For example, various valves are used by a way known from a person having ordinary skill in the art. Some valves are shown on the figures but are not described in this specification.

[0038] Under conditions when the Natural Boil-Off (NBO) is relatively warm, it may be necessary to lower the temperature. The NBO enters a spray precooler 52, where a small amount of LNG is mixed with the NBO to lower the temperature. This can result in droplets being carried downstream of the spray precooler 52. To avoid that the gas in the compressor 12 contains droplets of liquid, a phase separator vessel 42 in which the liquid disengages from the gas is located on pipeline 10. The liquid is returned via a conduit to a region of the storage tank 2 preferably below the liquid surface (in volume 6). This phase separator vessel concerns the Natural Boil-Off (NBO). A second phase separator vessel 44 is also foreseen for the Forced Boil-Off (FBO). The liquid may be withdrawn from the vessel 44 through a bottom outlet and is led by a conduit to the storage tank 2 (preferably in volume 6). The resulting natural gas, freed of particles of liquid, passes out of the top of the phase separator 44 and at a low or cryogenic temperature is mixed with the natural gas from the compressor 12 at a region upstream of the gas heater 16.

[0039] FIG. 1 (and also FIGS. 2 and 3) shows only main elements. Elements such as for example start-up line, interconnection with a second compressor, back-up vaporiser, second heater, etc. are not represented).

[0040] If desired, the phase separator 42 and/or 44 may be fitted at a region near its top with a pad of absorbent material or of wire mesh which may absorb any residual droplets of LNG from the gas in the phase separator. The liquid may be withdrawn from the vessel 42 and/or 44 through a bottom outlet continuously or at regular intervals and returned to the tank 2 by appropriate operation and control of a valve (not shown).

[0041] FIG. 2 is an embodiment of the invention with several storage tanks 2. This embodiment shows that the FBO is taken from some (1, 2 or eventually more) tanks. FIG. 2 shows also a vent mast 46 connected to the outlets 8 of the tanks 2 via a control valve 48. It also shows a Gas Combustion Unit (CGU) 50 in which gas can be burned.

[0042] The structure of the apparatus shown on FIG. 2 is the same than the structure of the apparatus of FIG. 1. On the first gas flow, in order to minimise the head (and therefore power) requirements of the compressor, the NBO is usually cooled during ballast voyage by means of a spray precooler 52 upstream of the compressor 12. The phase separator vessel 42 is an NBO mist separator and is installed between the spray precooler 52 and compressor 12, to protect the compressor from possible droplet carryover. Condensate is returned to tank 2 using a pressure-assisted drain pot (not shown). An after cooler 54 is provided in each of the lines to the dual fuel engines in the engine room 40 and the GCU 50, to provide any temperature corrections required.

[0043] On the second gas flow, a forcing vaporiser is provided to generate FBO to make up the fuel gas requirements if the NBO is insufficient. The liquid feed to the forcing vaporiser 24 is either by means of dedicated fuel gas pumps 26 are installed in 2 of the tanks, or spray pumps (not shown).

[0044] As the forced gas contains all the components of the LNG, the proportion of heavy hydrocarbons will have a negative influence on the methane number of the gas entering the engine. In order to improve the methane number under these circumstances, the forcing temperature is decreased and an FBO mist separator (second phase separator vessel 44) is installed after the forcing vaporiser 24 to remove condensate. This removes a large proportion of the heavy hydrocarbons, which are returned to the tanks 2.

[0045] The NBO and FBO streams are combined after the fuel gas compressor 12. The combined stream then passes through the fuel gas (low-duty) heater 16 before being routed to the engine(s) in the engine room 40.

[0046] The GCU 50 (or thermal oxidizer 56 shown in FIG. 1) is provided to dispose of excess NBO under low engine load conditions, where consumption is lower than NBO supply. It may also be used during compressor starting to dispose of gas while the inlet lines cool down.

[0047] Additional fuel gas is supplied by dedicated high-head fuel gas pumps and fed to the forcing vaporiser 24. The forced gas is sent to the FBO mist separator (phase separator vessel 44), where the heavy hydrocarbons are removed and returned to tank under system pressure.

[0048] FIG. 3 shows another embodiment of a treatment apparatus of the disclosure. This embodiment proposes two compressors 12. Each compressor 12 has six stages and also comprises two intercoolers 58 and an after cooler 60. A first intercooler 58 is located in a loop joining the outlet of the first stage to its inlet. The second intercooler 58 is located in series between the third and the fourth stage of the considered compressor 12. The after cooler 60 is placed downstream of the sixth stage and allows adjusting the temperature of the compressed gas flow.

[0049] In this embodiment, it is possible to deliver two different pressures. This treatment apparatus can supply for example engines in an engine room 40 but also a generator 62 or another kind of consumer.

[0050] As can be seen, in this embodiment, there is no heater after the compressors 12. The FBO gas is mixed to the compressed NBO gas downstream the compressors 12 (in the represented embodiment, also downstream the after coolers 60), upstream the generator 62 and the engines of the engine room 40.

[0051] To adapt the state of the gas coming from the second flow (FBO) to the state of the compressed gas coming from the first flow (NBO), a heater 64 is also foreseen downstream of the phase separator vessel 44. The gas coming out from the heater 64 is then mixed to the NBO compressed gas. The pressure of the gas (NBO and FBO) is for example between 10 and 20 bar. If the compressors 12 deliver two different pressures, the FBO gas can be mixed to the gas coming from only one of the compressors 12 or a second line with FBO can be foreseen.

[0052] FIG. 3 shows also a reliquefaction plant 62. In case of surplus of NBO gas, the extra gas can be reliquefied and sent back to the tanks 2, thereby conserving the cargo.

[0053] The apparatus described here above fulfill the conditions required for optimum operation of the low pressure 2-stroke dual fuel engines extensively. This includes features which maximise the available efficiency of the engines, while maintaining conditions suitable for the operation of multi-stage compressors, heaters and vaporisers.

[0054] According to an advantageous embodiment of the apparatus according to this disclosure, the compressors have to be sized to a capacity suitable for natural boil-off gas. This allows a reduction in the capacity and the installed power. The proposed FBO conditioning improves the composition of the gas and allows wider operation range, which would otherwise be limited by the effects of a lower methane number.

[0055] The pressure created by the fuel gas pump is retained and used after the forcing vaporiser. This conserves the energy input from the pump, and reduces the compressor drive power considerably.

[0056] The correct control of the fuel gas supply system and ancillary equipment (spray pumps, GCU, etc.) will allow trouble-free, automatic operation, in all operating modes.

[0057] According to the disclosure, an economic Boil-Off Gas treatment system is proposed which offers considerably benefits for the complex fuel gas handling requirements of a low pressure 2-stroke dual fuel LNG carrier.

[0058] While the invention has been illustrated and described in details in the drawings and foregoing description, such illustration and description have to be considered as illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims.