DK201570078A1

DK201570078A1 - A fuel gas supply system for an internal combustion engine

Info

Publication number: DK201570078A1
Application number: DKPA201570078A
Authority: DK
Inventors: Niels Kjemtrup
Original assignee: Man Diesel & Turbo Filial Af Man Diesel & Turbo Se Tyskland
Priority date: 2015-02-10
Filing date: 2015-02-10
Publication date: 2016-09-12
Also published as: DE102016001343A1; JP6262268B2; KR101851745B1; CN105863888A; JP2016173101A; DK178668B1; CN105863888B; KR20160098080A

Abstract

A fuel gas supply system for an internal combustion engine has at least one fuel gas pump located outside a liquefied gas storage tank and the pump is connected to liquefied gas in the tank via a tank outlet line for liquefied gas. A heat exchange circuit comprises a compressor and downstream thereof a first heat exchanger, which is connected with the fuel gas supply line in between the fuel gas pump and a final heat exchanger , and an expansion device and a second heat exchanger

Description

The present invention relates to a fuel gas supply system for an internal combustion engine, the fuel gas supply system comprising a liquefied gas storage tank, a fuel gas supply line and a heat exchange circuit with a working fluid, which fuel gas supply line comprises a tank outlet line for liquefied gas, at least one fuel gas pump for pressurizing fuel gas to a fuel gas supply pressure for the internal combustion engine and a final heat exchanger located downstream of the at least one fuel gas pump, and which heat exchange circuit at least comprises a compressor and downstream thereof a first heat exchanger and an expansion device and a second heat exchanger located downstream of the expansion device.

Internal combustion engines are used as propulsion engines in vessels like container ships, bulk carriers, tankers and LNG carriers. The internal combustion engines are typically large two-stroke crosshead engines coupled to the propeller shaft and using direct injection of the fuel, and consequently fuel gas has to be highly pressurized before delivery to the internal combustion engine. For LNG carriers it is known to make a fuel gas supply system delivering fuel gas to the internal combustion engine at a pressure of about 250 bar where the fuel gas is liquefied fuel gas or re-liquefied boil off gas (BOG) directly from the LNG cargo storage tanks. A fuel gas supply system of this type, known as the Cryostar EcoRel system, is illustrated in Fig. 1 where a BOG line A from the top of the tank T is connected to the inlet of a compressor B delivering the BOG through a BOG de-superheater C and a BOG condenser D to a flash tank E. A tank outlet line for LNG is provided with a pump G submerged in the LNG within the tank T, and pump G can be activated to transfer LNG to flash tank E when the rate of boiling off gas in the tank is insufficient to cover the gas fuel consumption in the internal combustion engine. Flash tank E is a liquefied gas storage tank with a minor storage capacity and may be sized as a day tank for the internal combustion engine so that it stores an amount of LNG required for at least a few hours of operation of the engine. A pump H submerged in the LNG in flash tank E acts as a pri- mer pump for at least one fuel gas pump I for pressurizing fuel gas to a fuel gas pressure of about 250 bar and the pressurized fuel gas is delivered via a final heat exchanger J to the gas fuel inlet on the internal combustion engine K. The final heat exchanger J is supplied with a warm fluid from a heating source L and heats the fuel gas to a temperature of about 45°C so that the fuel gas is acceptable in the engine.

The cooling and condensing of boil off gas are effected by a heat exchange circuit using nitrogen as working fluid. The nitrogen is compressed in three stages in a compressor N having cooling of the nitrogen after each stage, and then the nitrogen passes a heat exchanger and is delivered to a cryogenic expansion turbine P and passed through first the BOG condenser D and then the BOG de-superheater C and via the heat exchanger back to the compressor N. The compressor and the expansion turbine are arranged on a common single gearbox.

From US patent 7,690,365 B2 there is known another BOG liquefaction system for supplying an internal combustion engine in an LNG carrier with fuel gas at a delivery pressure of 200 to 300 bar. A first fuel gas pump submerged in LNG in the liquefied gas storage tank supplies LNG at a pressure of about 30 bar to a high-pressure fuel gas pump via a heat exchanger. Boiled off gas from the tank is compressed and passes through the heat exchanger where LNG cools the BOG, and the liquefied BOG is returned to a storage tank. There is not a proper heat exchange circuit with a working fluid in this system.

Other systems without a heat exchange circuit are known. US patent 5,884,488 describes an LNG pump located a lower level than the LNG tank allowing the pump to be supplied with LNG due to gravity and by BOG. The pump is of a special design pumping both liquid phase and gas phase. WO 2013/170964 describes a high pressure pump which is supplied with LFO or LNG at a pre-pressure of 5.4 bar from a primer pump in the tank.

During sailing between ports the weather causes differences in the engine loading and thus differences in the consumption rate of fuel gas by the internal combustion engine, and differences in the heat input to the liquefied gas storage tanks, and such differences also occur be- tween day and night.

An object of the present invention is to provide a fuel gas supply system having a very reliable fuel gas supply at high pressure.

With a view to this, the fuel gas supply system according to the present invention is characterized in that the at least one fuel gas pump is located outside the liquefied gas storage tank and is connectable to liquefied gas therein via the tank outlet line for liquefied gas, that the first heat exchanger in the heat exchange circuit is connected with the fuel gas supply line in between the fuel gas pump and the final heat exchanger, and that the second heat exchanger in the heat exchange circuit is located in the liquefied gas storage tank or at a liquid gas flow line communicating with the liquefied gas in the liquefied gas storage tank.

The liquefied gas storage tank is a construction which is difficult to access when filled with liquefied gas, and due to the low temperature in the tank all equipment located in the tank should preferably be of simple design. To the reliability of the system, in particular reliability of operation, it is an advantage to locate the fuel gas pump outside of the tank where it is also easier to access.

The second heat exchanger has no moving parts and is installed in the tank as a fixed construction. Alternatively, the second heat exchanger is located on a liquid gas flow line communicating with the liquefied gas in the liquefied gas storage tank. In the latter case, the liquid gas flows through the second heat exchanger and into the liquefied gas storage tank, and in the former case the second heat exchanger acts on the liquid gas directly within the liquefied gas storage tank. The second heat exchanger performs cooling of the liquid gas in the tank, and this cooling causes the liquid gas to have a temperature below the boiling point of the gas. The boiling point of the gas is pressure dependent in the manner that boiling occurs at lower temperature when the pressure is lower. Due to the cooling below the boiling point at the pressure in the tank, it becomes possible to reduce the pressure in the liquid gas and yet avoid formation of gaseous gas due to boiling. If desired, the fuel gas pump located outside the tank may thus exert suction on the liquid gas in the tank via the tank outlet line for liquefied gas without at the same time causing boiling in the liquid gas. The fuel gas pump receives only liquid gas without gaseous gas and the pump is thus very reliable in operation.

The supply of fuel gas to the engine is based on liquid gas from the liquefied gas storage tank and not based on boil off gas, and the fuel gas supply is independent of weather conditions and variations in consumption rate, and this improves the reliability of the fuel gas supply at high pressure to the internal combustion engine.

It is preferred that the fuel gas pressure of the fuel gas pump is in the range of 200 bar to 700 bar. For certain embodiments it may be possible to use a higher fuel gas pressure, such as a pressure of 750 bar. However, the maximum pressure of 700 bar limits the energy consumption used for obtaining the pressure. The fuel gas is injected directly into the combustion chamber of the internal combustion engine, and this normally requires a pressure higher than 200 bar. It is consequently suitable that the fuel gas pressure of the fuel gas pump is in the range from 250 bar to 450 bar.

In a preferred embodiment a third heat exchanger is located downstream of the expansion device in the heat exchange circuit and upstream of the at least one fuel gas pump in the fuel gas supply line. With this location the tank outlet line passes through the third heat exchanger and is cooled by the heat exchange circuit. The working fluid, such as nitrogen, has passed the expansion device just before it passes the third heat exchanger and is thus at its lowest temperature in the heat exchange circuit. The third heat exchanger effectively ensures that the gas fuel in the tank outlet line will be at the lowest temperature when entering the inlet of the at least one fuel gas pump.

In an embodiment the compressor is sized to compress the working fluid to a maximum pressure in the range of 40 bar to 120 bar. The desired pressure in the working fluid at the outlet from the compressor is a balance between obtaining a high level of pressure reduction in the expansion device and the power required to operate the compressor. The pressure can be lower than 40 bar, such as 10, 25 or 35 bar. When a good efficiency is desired, the pressure may preferably be higher than the pressure at the critical point of the working fluid so that the working fluid is in the supercritical state during its flow through the first heat exchanger.

In an embodiment the expansion device is adapted to provide a downstream pressure in the range of 1 bar to 12 bar. The desirable pressure range depends on the working fluid. The upper end of the pressure range preferably has such a pressure that the working fluid at that pressure has a boiling point below the boiling point of the liquid gas in the tank, and if nitrogen is used as working fluid a pressure of 1 2 bar corresponds to a boiling point of about -165°C, and a pressure of 1 bar corresponds to a boiling point of about -196°C. At a preferred pressure of about 5 bar at the outlet of the expansion device, nitrogen has a boiling point of about -178°C, and the working fluid is thus markedly colder than the liquid gas.

The working fluid is liquid, or primarily liquid, when it flows into the second heat exchanger. The boiling point of the working fluid at the pressure prevailing in the second heat exchanger is lower than the temperature of the liquid gas in the liquefied gas storage tank, and the heat influx from the liquid gas to the working fluid in the second heat exchanger causes the working fluid to boil. The heat of vaporization consumed by the working fluid makes the second heat exchanger very efficient in cooling the liquid gas in the liquefied gas storage tank. This principle of a boiling working fluid in the second heat exchanger may be applied to all embodiments of the present invention.

In an embodiment the liquefied gas storage tank has a capacity in volume of liquid gas corresponding to at the most three days of fuel gas consumption at continuous full load of the internal combustion engine. In this embodiment the liquefied gas storage tank is a so-called day tank holding a minor amount of fuel gas in vicinity of the internal combustion engine. The day tank is supplemented with fuel gas at intervals, such as when the level of liquid fuel gas is below a predetermined level in the tank. The day tank is typically held at ambient pressure or at a slight overpressure of a few bars. In some embodiments the day tank may have a capacity corresponding to less than 24 hours of fuel gas consumption.

In an embodiment the fuel gas supply system comprises at least two liquefied gas storage tanks. One of the liquefied gas storage tanks may be a day tank, but it is also possible two or more of the liquefied gas storage tanks are tanks of large capacity. In case the embodiment is installed in a liquefied gas carrier, such as a LNG carrier or a LPG carrier, the liquefied gas storage tanks can be the cargo tanks, or a subset of the cargo tanks, such as two cargo tanks located closest to the engine room. In case the vessel is not carrying liquefied gas as cargo it can be suitable to have a plurality of liquefied gas storage tanks, such as from two to 25 or more liquefied gas storage tanks.

In an embodiment the heat exchange circuit comprises at least two second heat exchangers located in at least two liquefied gas storage tanks, preferably so that at least one second heat exchanger is located in each liquefied gas storage tank.

In an embodiment the fuel gas supply system is for an internal combustion engine serving as propulsion engine in a ship. The fuel gas supply system may alternatively be for an auxiliary engine in a ship, or for an internal combustion engine serving as prime mover in a stationary power plant.

In an embodiment the ship is one from the group comprising container ships, bulk carriers, passenger ships, oil tankers, RoRo-vessels and reefers. It is a common feature of the ships in this group that they carry a cargo that is different from liquefied gas, and in this case the ship has been equipped with gas fuel tanks in form of liquefied gas storage tanks.

In an embodiment a by-pass line extends from the fuel gas supply line downstream of the first heat exchanger to the liquefied gas storage tank, which by-pass line is provided with a pump and a stop valve. When the internal combustion engine is taken out of operation the stop valve in the by-pass line may be opened and the pump activated to circulate cold liquid gas through the tank outlet line and the bypass line so that the system is cooled down prior to starting the internal combustion engine or maintained in cold condition while the engine is temporarily stopped.

Examples of embodiments of the invention are described in further detail in the following with reference to the highly schematic drawings, on which

Fig. 1 illustrates a prior art fuel gas supply system for an internal combustion engine for propelling an LNG carrier,

Fig. 2 illustrates an LNG carrier with a fuel gas supply system according to the present invention,

Fig. 3 illustrates an end outline of an internal combustion engine in the LNG carrier of Fig. 2,

Fig. 4 illustrates a fuel gas system of the internal combustion engine in Fig. 3,

Fig. 5 illustrates in more detail the fuel gas system in Fig. 3 viewed for a single cylinder on the engine,

Figs. 6 illustrates a diagrammatic view of a first embodiment of a fuel gas supply system according to the present invention,

Fig. 7 illustrates a diagrammatic view of a second embodiment of the fuel gas supply system,

Fig. 8 illustrates a diagrammatic view of a third embodiment of the fuel gas supply system, and

Fig. 9 illustrates a diagrammatic view of a fourth embodiment of the fuel gas supply system.

An LNG carrier in Fig. 2 has an internal combustion engine serving as main propulsion engine in the engine room 1 located below a superstructure 2. The engine drives a propeller 3 for propulsion of the vessel. The LNG carrier has a plurality, in the illustrated embodiment four, LNG storage tanks, of which at least one and preferably more are liquefied gas storage tanks 4 used in a fuel gas supply system according to the present invention. Although the purpose of the LNG carrier is to transport LNG from a production site to a utilization site for LNG, the LNG storage tanks also acts as fuel storage for the internal combustion engine during the transport.

The vessel need not be an LNG carrier, but can also be a vessel of any another type, where at least one liquefied gas storage tank 4 is serving only as a fuel storage, independent from the cargo of the vessel. Examples of such other types of vessels are RoRo-vessels, container ships, oil tankers, car carriers and bulk carriers.

In Fig. 3 the internal combustion engine is shown in more detail. The internal combustion engine is piston engine, and preferably a two-stroke crosshead internal combustion engine, generally designated 5. The engine can have from 4 to 15 cylinders. The engine can e.g. be of the make MAN Diesel & Turbo and the type ΜΕ-GI or MC, or of the make Wårtsilå, or of the make Mitsubishi. The cylinders can have a bore in the range of e.g. 25 to 120 cm, preferably from 40 to 110 cm. The two-stroke crosshead internal combustion engines used as main propulsion engines typically have a speed indicated as rpm in the range from 55 to 195 rpm. These engines are named low speed engines. The low speed is required for transferring the propulsion thrust via the propeller to the water in the wake of the vessel. In order to transfer the thrust to the water the propeller needs a large area, and thus a large diameter. As cavitation at the propeller is undesired it is necessary to limit the speed of the propulsion engine to the low speed range, such as from 60 to 200 rpm.

The internal combustion engine 5 has a plurality of cylinders, each having a reciprocating piston in the cylinder. In the two-stroke crosshead internal combustion engines the cylinders are typically of the uniflow scavenging type where an exhaust valve 6 is located at the top of the cylinder and scavenge air ports (not shown) are located at the lower end of the cylinder. The exhaust gas from the cylinder is passed to an exhaust gas receiver 7 and onwards to the turbine part of a turbocharger 8, the compressor part of which supplies compressed inlet air to an inlet air chamber 9. From this chamber the inlet air may pass through an inlet air cooler 10 to an area surrounding the scavenge air ports in the cylinders.

The engine has an injection system for direct injection of pilot fuel oil and of gaseous fuel gas, and for safety reasons the system for injecting fuel gas is provided with an air intake system and an inert gas system. The air intake system is provided in a pipe 15 surrounding the gas fuel pipe 16, and the annular space between the two pipes allows for monitoring for gas leaks from the inner pipe. The air intake takes place at 11, and if the injection system is operating normally the air outlet takes place at 12. A pair of hydrocarbon detectors 13 is placed downstream of the engine in the conduit leading to air outlet 12. A source of pressurized inert gas 14 is connected to fuel gas pipe 16, and at shutdown of the engine the inert gas is supplied to the fuel gas pipe for purging the same for gas. A first fuel storage 17 supplies pilot fuel to fuel injectors 18 on each cylinder 19 of the internal combustion engine. The pilot fuel is supplied at a pressure of e.g. 300 or 400 bar and is used to initiate each fuel injection sequence in the cylinder. The pilot fuel can be fuel oil and is able to self-ignite in the combustion chamber at the compression pressure available in the combustion chamber at the end of the combustion stroke. Gas injectors 20 on each cylinder 19 are provided with control oil from a pump 21 when the required pilot oil pressure is detected, and the control oil pressure is required at the gas injectors 20 in order to inject gas. The control oil ensures that gas is not injected into the cylinder if the pilot oil fails to inject. Gas injectors 20 are also supplied with pressurized sealing oil via sealing oil line 22. The sealing oil prevents gas from escaping from the gas injector otherwise than through the gas injection nozzle delivering the gas into the combustion chamber.

Fuel gas from the liquefied gas storage tank 4 is supplied via a fuel gas supply line 25 in a fuel gas supply system to fuel gas pipe 16 and flows to an accumulator 23 and a control valve 24 opens for fuel gas to the injectors 20 when fuel gas injection is to take place. There may be a common rail pipe between the fuel gas pipe 16 and the injectors 20 and in that case it may be possible to dispense with accumulators 23.

The fuel gas supply system for an internal combustion engine is shown in more detail in figures 6 to 8. The fuel gas supply line 25 extends from within liquefied gas storage tank 4 to the internal combustion engine 5 and has an initial section formed by a tank outlet line 26.

Tank outlet line 26 extends into the tank to an area near an inside bottom of the tank and has an end opening allowing inflow of liquid gas. There is no pump on tank outlet line 26 within the liquefied gas storage tank 4. As alternatives to extending upwards down into the tank, the tank outlet line may extend below the tank from a bottom opening therein or it may extend from a lower portion of an end bottom in case the liquefied gas storage tank is shaped as a cylindrical tank with a horizontal central axis and end bottoms.

The tank outlet line is connected to a third heat exchanger 27 in which fuel gas in the fuel gas supply line is cooled by a working fluid in a heat exchange circuit generally designated 28. The tank outlet line is provided with a stop valve (not shown) and possibly also a non-return valve outside the liquefied gas storage tank 4 so that the tank can be disconnected or connected at choice. On the downstream side of third heat exchanger 27 the fuel gas supply line continues to the inlet of a fuel gas pump 29, which is a high-pressure pump increasing the fuel gas pressure to be at least at a pressure required at the fuel gas inlet to the internal combustion engine 5, viz. a pressure in the range from 200 bar to 700 bar, typically about 300 to 400 bar. A high pressure is required because the internal combustion engine performs direct injection of the gas into the combustion chamber at the end of the compression stroke where the pressure in the combustion chamber may be e.g. 180 bar, and the injection pressure needs to be considerably higher in order to finely distribute the gas to the combustion zone.

The fuel gas pump may have several stages or there can be two or more fuel gas pumps connected in series or in parallel. The fuel gas pump is a cryogenic pump, and examples are model TC-34 from Cryogenic Industries, CA, USA, and high pressure centrifugal LNG pumps as disclosed in Hydrocarbon Processing, July 2011, pages 37-41. The fuel gas pump is preferably a piston displacement pump having an hydraulic actuator acting in both directions. The at least one fuel gas pump may also be a combination of one piston pump and one centrifugal pump.

From the outlet of the fuel gas pump the fuel gas supply line 25 is connected to a first heat exchanger 30 in which fuel gas in the fuel gas supply line is heated by the working fluid in heat exchange circuit 28. Fuel gas supply line 25 continues from the outlet of the first heat exchanger 30 to a final heat exchanger 31 in which fuel gas is heated to a temperature above ambient, preferably a temperature of about 45°C, suitable for delivery of the fuel gas to the internal combustion engine. The final heat exchanger is supplied with a heating fluid from an available source 32 which is separate from the heat exchange circuit 28.

From the outlet of the final heat exchanger 31 the fuel gas supply line 25 is connected to the gas fuel pipe 16 at the internal combustion engine to which the fuel gas is delivered at a fuel gas supply pressure. The fuel gas is in the supercritical state in the section of the fuel gas supply line 25 extending from the fuel gas pump 29 to the gas fuel pipe 16.

The heat exchange circuit 28 is a closed circuit with a circulation line 33 in which a working fluid flows. A store 34 for working fluid is connected via circulation line 33 to the inlet of a compressor 35. The compressor can be a one stage compressor or a compressor with several stages. At the outlet of compressor 35 the working fluid is in supercritical stage, such as at a pressure of 100 bar, and the outlet of the compressor is connected to an inlet on the first heat exchanger 30, preferably so that the working fluid is flowing in counter-flow with the fuel gas to which the working fluid delivers heat during passage of the first heat exchanger.

From the outlet of the first heat exchanger the circulation line 33 continues to an expansion device 36 in which the pressure in the working fluid is reduced to a low pressure, such as a pressure in the range of 1 bar to 12 bar, so that the working fluid is at a temperature below the boiling point of the fuel gas in the fuel gas supply line 25 upstream of fuel gas pump 29, such as at a temperature at least 10°C below this boiling point, and preferably at least 20°C below this boiling point. The pressure expansion device can in one embodiment comprise an orifice nozzle located in a closed chamber and connected with the circulation line 33 coming from the first heat exchanger so that the working fluid expands into the chamber, thus lowering both the pres- sure and the temperature of the working fluid.

From the outlet of expansion device 36 the circulation line 33 is connected to the third heat exchanger 27 in which fuel gas in the fuel gas supply line is cooled by the working fluid in heat exchange circuit 28. This cooling is very effective because the working fluid is at its lowest temperature in the circulation line when flowing out from the expansion device. The fuel gas may thus be cooled many degrees Celcius below its boiling point, and this leaves room for low pressures in the liquid fuel gas without any presence of gaseous phase just upstream of the at least one fuel gas pump 29.

From the outlet of the third heat exchanger 27 the circulation line 33 continues into the liquefied gas storage tank 4 to a second heat exchanger 37 which may be in form of a pipe section submerged in the liquid gas in the tank, such as a spiral shaped pipe section. The circulation line and second heat exchanger extending inside the liquefied gas storage tank 4 is in an embodiment formed of a single length of pipe set in a shape and possibly provided with external fins for enhanced heat transfer, and moving parts or connections between separate elements inside the tank may be avoided, if desired in order to optimise reliability in operation. In another embodiment the second heat exchanger is a plate-shaped structure with an internal flow path for the working fluid, and an inlet opening and an outlet opening with fixed connections to the circulation line 33 located within the liquefied gas storage tank 4. The dot-and-dash line indicates an upper surface 38 of liquid gas within the tank. Naturally, the level of the upper surface moves downwards as the fuel gas is consumed. From the second heat exchanger 37 the circulation line 33 continues to an inlet on the store 34.

The working fluid can be nitrogen which may already be used for other purposes on a ship where nitrogen is used as an inert gas. The working fluid can alternatively be of argon or helium. Typical properties of these working fluids are shown in Table 1 together with similar properties of methane, which is typically used as the fuel gas. Helium is not preferred due to its low heat of vaporization.

Table 1

A by-pass line 39 is connected to the fuel gas supply line 25 downstream of the first heat exchanger 30 and extends via a pump 40 and a stop valve (not shown) to the liquefied gas storage tank 4. Pump 29 may be embodied with a suction chamber with a by-pass conduit to be opened and closed when by-pass line 39 is opened and closed so that pump 29 allows by-pass flow when by-pass line 39 is open for by-pass. Pump 29 may alternatively be of a type allowing by-pass through the pumping member at standstill. When the internal combustion engine is at stand-still the stop valve can be opened and the pump 40 activated so that liquid gas is circulated though the majority of the fuel gas supply line 25 in order to cool the system and keep it ready for operation when the internal combustion engine is started.

An example of the operation of the present invention is given in the following with reference to the embodiment of Fig. 6. The gas flow rate in the fuel gas supply line 25 is taken as 1 kg/s, corresponding to the LNG consumption when the internal combustion engine has a power of about 27 MW. The internal combustion engines relevant to ship propulsion range in powers from about 2 MW to about 90 MW, in dependency of the type of engine, the bore of the engine, and the number cylinders in the engine, and the fuel gas consumption rate is proportional with the power. Nitrogen N2 is used as working fluid, and the flow rate of working fluid in circulation line 33 is 1.7 kg/s in this example.

The liquid gas at position ag in the liquefied gas storage tank 4 has a pressure of about 1 bar and a temperature of about -161°C, and about the same temperature and pressure are relevant to the fuel gas in the fuel gas supply line 25 at position bg just upstream of the third heat exchanger 27. At position eg downstream of this heat exchanger the fuel gas has a pressure in the range of about 0.7 to 1 bar and a temperature of about -176°C. After pressurizing in the at least one fuel gas pump 29 the fuel gas has at position dg a pressure of about 300 bar and a temperature of about -172°C, and the same pressure and temperature are found at position e.g. just upstream of first heat exchanger 30. Downstream of this heat exchanger at position fg the fuel gas has a pressure of about 300 bar and a temperature of about -12°C, and downstream of the final heat exchanger 31 at position gg the fuel gas has a pressure of about 300 bar and a temperature of about 45°C.

The working fluid at position i in the store 34 has a pressure of about 5 bar and a temperature of about -161°C, and about the same temperature and pressure are relevant at position h just upstream of the compressor 35. Downstream of compressor 35 at position f the working fluid has a pressure of about 100 bar and a temperature of about 28°C. Downstream of the first heat exchanger 30 at positions e and d the working fluid has a pressure of about 100 bar and a temperature of about -152°C. At position c downstream of the expansion device 36 the working fluid has a pressure of about 5 bar and a temperature of about -178°C and a portion of the working fluid is in vapour phase. In the third heat exchanger the vapour phase condenses and downstream of the third heat exchanger 30 at positions b and a the working fluid has a pressure of about 5 bar and a temperature of about -178°C. In the second heat exchanger 37 the working fluid boils and downstream thereof and at the position i in store 34 the working fluid has a pressure of about 5 bar and a temperature of about -161°C.

With these flow conditions the compressor 35 requires a power of 310 kW, and 542 kW is transferred from the working fluid to the fuel gas in the first heat exchanger 30; 66 kW is transferred from the fuel gas to the working fluid in the third heat exchanger 30; and 165 kW is transferred from the fuel gas to the working fluid in the second heat exchanger 33.

In the first heat exchanger 30 the working fluid enters at the temperature of 28°C and exits at -152°C, whereas the fuel gas enters in counter-flow at the temperature of -171°C and exits at -12°C, and temperature difference between the two fluids is thus 40°C at one end of the heat exchanger and 19°C at the other end, and within the heat exchanger the temperature difference is smaller, however the working fluid has higher temperature than the fuel gas at all positions within the heat exchanger.

In the following description of other embodiments the same reference numerals as in the above-mentioned embodiment are used for details of the same function, and only differences with respect to the first embodiment are mentioned.

In the second embodiment of Fig. 7 the expansion device 20 is a turbine in which the working fluid expands while the turbine receives power to its shaft. The shaft of the turbine is coupled to the shaft of the fuel gas pump, possibly via a gear box, in order to save energy.

In the third embodiment of Fig. 8 the liquefied gas storage tank 4, in which the fuel gas supply line 25 is mounted, is a day tank of a comparatively small volume so that it contains a volume of fuel gas required for some hours of operation of the internal combustion engine 5, but less than some days of operation. This day tank is convenient to install near the engine room because it is of small size. At least one additional liquefied gas storage tank 4 of larger volume is also installed, and a second heat exchanger 37 is also installed in this tank, and the circulation line 33 is provided with a separate loop connecting this second heat exchanger 37 in parallel with the second heat exchanger 37 in the liquefied gas storage tank 4 serving as day tank, and control valves 41 are used to control the flow of working fluid to the individual second heat exchanger 37. A fuel gas feed line 42 with a service pump 43 extend from near the inner bottom of the large tank to the day tank, and the day tank may have a sensor or level control device that activates the service pump when the liquid level in the day tank is below a preset value, so that a suitable amount of liquid fuel gas the day tank is maintained in the day tank.

The second heat exchanger 37 in the individual tank maintains the gas content in the tank at a lower temperature than the boiling temperature of the gas. The pressure level in the tanks can thus be maintained at ambient pressure of about 1 bar, and boil-off of gas is avoided. The fuel gas supply system is thus not using boil-off gas and has no equipment for boil off gas re-liquefaction. The liquid gas contents in the liquefied gas storage tanks 4 can be cooled to a lower temperature, such as a temperature above, but near the melting point of the gas, and thus allow a period with stopped internal combustion engine 5 while the temperature of the liquid gas slowly rises. As heat influx to the tanks occurs slowly it is possible to stop the engine for up to several days without having any boiling in the tanks.

In the fourth embodiment of Fig. 9 the second heat exchanger 37 is located outside the liquefied gas storage tank 4 at a liquid gas flow line generally designated 50. The liquid gas flow line passes through the second heat exchanger 37. The liquid gas flow line is communicating with the liquefied gas in the liquefied gas storage tank via a suction line 44 extending down into the liquefied gas storage tank 4 near the inner bottom thereof, and a delivery line 45 returning liquefied gas to the tank after a circulation pump 46 has caused the liquefied gas to flow through the second heat exchanger 37. Liquid gas flow line 50 may also be a line for transferring liquid gas from one tank to another and may be embodied as fuel gas feed line 42 with the service pump 43.

In other embodiments it is possible to connect the tank outlet line directly to the inlet on the at least one fuel gas pump 29 and to connect the outlet from expansion device 36 directly to the second heat exchanger, and thus dispense with the third heat exchanger.

The individual liquefied gas storage tank 4 may be provided with an inert gas line connecting an inert gas source with the upper portion of the tank, and the inert gas line may be provided with a regulator valve maintaining ambient pressure (about 1 atm or 1 bar) within the liquefied gas storage tank 4. Alternatively the inert gas line may be connected to an inert gas source having a flexible wall, the outside of which is open to the ambient, so that ambient pressure is present in the inert gas source and thus also in the liquefied gas storage tank 4 during all operating conditions, and thus also when the liquefied gas is cooled well below its boiling point.

The various embodiments described in the above there are stop valves and control valve as usual in such a system, and in particular a stop valve in fuel gas supply line 26 and this stop valve can connect the at least one fuel gas pump to liquefied gas in the liquefied gas storage tank, when the stop valve is in open position.

The fuel gas supply line 26 can be made with a protection in form of an outer pipe of larger diameter than the outer diameter of the fuel gas supply line 26, and the annular space between the pipes may be ventilated and provided with gas leakage detectors at the outlet of the ventilation air. The outer pipe also serves to protect personnel from getting in contact with surfaces having a very low temperature.

Details of the various embodiments described can be combined into further embodiments within the scope of the patent claims.

Claims

1. A fuel gas supply system for an internal combustion engine, the fuel gas supply system comprising a liquefied gas storage tank, a fuel gas supply line and a heat exchange circuit with a working fluid, which fuel gas supply line comprises a tank outlet line for liquefied gas, at least one fuel gas pump for pressurizing fuel gas to a fuel gas supply pressure for the internal combustion engine and a final heat exchanger located downstream of the at least one fuel gas pump, and which heat exchange circuit at least comprises a compressor and downstream thereof a first heat exchanger and an expansion device and a second heat exchanger located downstream of the expansion device, characterized in that the at least one fuel gas pump is located outside the liquefied gas storage tank and is connectable to liquefied gas therein via the tank outlet line for liquefied gas, that the first heat exchanger in the heat exchange circuit is connected with the fuel gas supply line in between the fuel gas pump and the final heat exchanger, and that the second heat exchanger in the heat exchange circuit is located in the liquefied gas storage tank or at a liquid gas flow line communicating with the liquefied gas in the liquefied gas storage tank.

2. A fuel gas supply system according to claim 1, wherein the fuel gas pressure of the fuel gas pump is in the range of 200 bar to 700 bar, preferably in the range from 250 bar to 450 bar.

3. A fuel gas supply system according to claim 1 or 2, wherein a third heat exchanger is located downstream of the expansion device in the heat exchange circuit and upstream of the at least one fuel gas pump in the fuel gas supply line.

4. A fuel gas supply system according to one or more of claims 1 to 3, wherein the compressor is sized to compress the working fluid to a maximum pressure in the range of 40 bar to 120 bar.

5. A fuel gas supply system according to one or more of claims 1 to 4, wherein the expansion device is adapted to provide a downstream pressure in the range of 1 bar to 12 bar.

6. A fuel gas supply system according to one or more of claims 1 to 5, wherein the liquefied gas storage tank has a capacity in volume of liquid gas corresponding to at the most three days of fuel gas consumption at continuous full load of the internal combustion engine.

7. A fuel gas supply system according to one or more of claims 1 to 6, wherein the fuel gas supply system comprises at least two liquefied gas storage tanks, such as from two to 25 liquefied gas storage tanks.

8. A fuel gas supply system according to claim 7, wherein the heat exchange circuit comprises at least two second heat exchangers located in at least two liquefied gas storage tanks, preferably so that at least one second heat exchanger is located in each liquefied gas storage tank.

9. A fuel gas supply system according to one or more of claims 1 to 7, wherein fuel gas supply system is for an internal combustion engine serving as propulsion engine in a ship.

10. A fuel gas supply system according to claim 9, wherein the ship is one from the group comprising container ships, bulk carriers, passenger ships, oil tankers and reefers.

11. A fuel gas supply system according to claim 8 or 9, wherein a by-pass line extends from the fuel gas supply line downstream of the first heat exchanger to the liquefied gas storage tank, which by-pass line is provided with a pump and a stop valve.

12. A fuel gas supply system according to one or more of claims 1 to 11, wherein the liquefied gas storage tank is connected with an inert gas source at ambient pressure.