Fuel gas supply system

Abstract

A fuel gas supply system for an internal combustion engine (5) has at least one fuel gas pump (29) arranged outside a liquid gas storage tank (4). The pump is connected to the liquid gas in the tank via a liquid gas tank outlet line (26). A heat exchanger circuit comprises a compressor (35) and, downstream of the compressor, a first heat exchanger (30) connected to the fuel gas supply line (25) between the fuel gas pump and a final heat exchanger (31), as well as an expansion device (36) and a second heat exchanger (37).

Description

The present invention relates to a fuel gas supply system for an internal combustion engine, wherein the fuel gas supply system comprises: a liquefied gas storage tank, a fuel gas supply line, a heat exchange circuit with a working medium, wherein the fuel gas supply line comprises: a tank outlet line for liquefied gas, at least one fuel gas pump for setting the fuel gas under a fuel gas connection pressure for the internal combustion engine, and a final heat exchanger arranged behind the at least one fuel gas pump, and wherein the heat exchange circuit comprises at least one compressor and behind this a first heat exchanger and an expansion device and a second heat exchanger arranged behind the expansion vessel.Internal combustion engines are used as engines in ships such as container ships, bulk cargo loads, tankers and LNG tankers. The internal combustion engines are normally large cross-head two-stroke engines connected to the propeller shaft and using the direct injection of the fuel, and consequently the fuel gas must be under high pressure before being supplied to the internal combustion engine. In LNG tankers, it is known that the fuel gas supply system supplies fuel gas to the internal combustion engine at a pressure of about 250 bar, wherein the fuel gas is liquefied petroleum gas or re-liquefied Boil Off Gas (BOG) directly from the LNG charging tanks.A fuel gas supply system of this type, known as the Cryostar EcoRel system, is illustrated in Figure 1 where a BOG line A from the top of the tank T is connected to the inlet of a compressor B which feeds the BOG through a BOG reheater C and a BOG condenser D to a expander E. A tank discharge pipe for LNG is provided with a pump G immersed in the LNG in the tank T, and the pump G can be activated to transfer the LNG to the expander E when the proportion of the boil-off gas in the tank is insufficient to cover the gas fuel consumption in the internal combustion engine. The expander E is a liquefied gas storage tank having a smaller storage capacity, and may be configured as a day tank for the internal combustion engine so as to store an amount of LNG required for at least a few hours of engine operation.A pump immersed in the LNG in the expander E acts as a priming pump for at least one fuel gas pump I to pressurize the fuel gas to a fuel gas pressure of about 250 bar, and the pressurized fuel gas is fed 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 heat source L and heats the fuel gas to a temperature of about 45° C., so that the fuel gas is acceptable for the engine.The cooling and condensation of boil off gas is effected through a heat exchanger circuit using nitrogen as working medium. The nitrogen is compressed in three stages in a compressor N, the nitrogen being cooled after each stage, and then the nitrogen passes a heat exchanger and is passed to a cryogenic expansion turbine P and passes first through the BOG condenser D and then through the BOG desuperheater C and back to the compressor N via the heat exchanger.A similar arrangement can be found in JP 5495697 B2. However, there is no tank outlet line for liquefied gas, but for gaseous gas (boil-off gas). The gas from the tank is evaporated here. Accordingly, the known arrangement contains a back-fluid system.JP 2009-204026 A describes another BOG liquefaction system for re-liquefying BOG and returning it to the storage tank so that gas loss by evaporation is avoided. Liquefied gas from the storage tank may be pumped out with a pump immersed in the liquefied gas in the storage tank and pressurized in another pump outside the tank at a pressure of about 100 to 120 bar and supplied to an internal combustion engine.U.S. Pat. No. 7,690,365 B2 discloses a further BOG liquefaction system for supplying an internal combustion engine in an LNG tanker with combustion gas at a delivery pressure of 200 to 300 bar. A first fuel gas pump immersed in the 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. Boil off gas from the tank is compressed and passes through the heat exchanger where the LNG cools the BOG and the liquefied BOG is returned to a storage tank. In this system there is no proper heat exchange circuit with a working medium.Other systems without a heat exchange circuit are known. U.S. Pat. No. 5,884,488 describes an LNG pump that is located at a lower level than the LNG tank so that the pump can be gravity fed with LNG and BOG. The pump is of a special construction and pumps both the liquid phase and the gas phase. WO 2013 / 170964 describes a high-pressure pump which is supplied with LPG or LNG at an admission pressure of 5.4 bar by a primer pump in the tank.During inter-port traffic, weather causes differences in engine load and thus differences in the amount of consumption of the fuel gas by the internal combustion engine and differences in the heat input to the liquefied gas storage tanks, and thus differences also occur between day and night.An object of the invention is to provide a fuel gas supply system having a highly reliable fuel gas supply at high pressure.In view of this, the fuel gas supply system according to the present invention is characterized in that the at least one fuel gas pump is arranged outside the liquefied gas storage tank and is connectable to the liquefied gas in the associated storage tank via the liquefied gas tank outlet line, that the first heat exchanger of the heat exchanger circuit is connected to the fuel gas supply line between the fuel gas pump and the final heat exchanger, and that the second heat exchanger of the heat exchanger circuit is arranged in the liquefied gas storage tank or on a liquefied gas flow line which is connected to 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 because of the low temperature in the tank, all equipment located in the tank should preferably be simple in construction. A single fuel gas pump may be present or two or more fuel gas pumps may be present and this pump or pumps are located outside the liquefied gas storage tank. For the reliability of the system, in particular for operational safety, it is advantageous to arrange the fuel gas pump outside the tank, from where it is also more easily accessible.The second heat exchanger has no moving parts and is installed in the tank as a fixed structure. Alternatively, the second heat exchanger is disposed on a liquefied gas flow line that communicates with the liquefied gas in the liquefied gas storage tank. In the latter case, the liquefied gas flows through the second heat exchanger and into the liquefied gas storage tank, and in the foregoing case, the second heat exchanger acts on the liquefied gas directly inside the liquefied gas storage tank. The second heat exchanger performs cooling of the liquefied gas in the tank, and this cooling results in the liquefied gas having a temperature below the boiling point of the gas. The boiling point of the gas is dependent on the pressure such that boiling takes place at a lower temperature as the pressure is lower. Due to cooling below the boiling point at the pressure in the tank, it is possible to reduce the pressure in the liquefied gas, and yet to avoid the formation of gaseous gas due to boiling. Thus, if desired, the fuel gas pump disposed outside the tank can exert suction on the liquefied gas in the tank via the liquefied gas tank discharge pipe without causing boiling in the liquefied gas at the same time. The fuel gas pump receives only liquefied gas without gaseous gas and the pump is thus very operationally reliable.The supply of the engine with fuel gas is based on liquefied gas from the liquefied gas storage tank and is not based on boil off gas, and the fuel gas supply is independent of weather conditions and variations in the consumption amount, and this improves the reliability of the fuel gas supply system 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. In 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 to get 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 therefore expedient for the combustion gas pressure of the combustion gas pump to be in the range from 250 bar to 450 bar.In a preferred embodiment, a third heat exchanger is arranged behind the expansion vessel in the heat exchanger circuit and in front of the at least one fuel gas pump in the fuel gas supply line. At this location, the tank outlet line passes through the third heat exchanger and is cooled by the heat exchanger circuit. The working medium, such as nitrogen, has passed through the expansion vessel just before it passes through the third heat exchanger and thus has its lowest temperature in the heat exchanger circuit. The third heat exchanger effectively ensures that the gas fuel in the tank outlet line has its lowest temperature when it enters the at least one fuel gas pump.In one embodiment, the compressor is configured to compress the working medium to a maximum pressure in the range of 40 bar to 120 bar. The desired pressure in the working medium at the exit of the compressor is a balance between obtaining a high pressure reduction in the expansion vessel and the power required to operate the compressor. The pressure may be lower than 40 bar, such as 10, 25 or 35 bar. If good efficiency is desired, the pressure may preferably be higher than the pressure at the critical point of the working medium so that the working medium is in the supercritical state during its flow through the first heat exchanger.In one embodiment, the expansion vessel is configured to provide a downstream pressure in the range of 1 bar to 12 bar. The desirable pressure range depends on the working medium. The upper end of the pressure range is preferably such a pressure that the working medium at this pressure has a boiling point below the boiling point of the liquefied gas in the tank, and when nitrogen is used as the working medium, a pressure of 12 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 vessel, nitrogen has a boiling point of about -178°C and the working medium is thus significantly colder than the liquefied gas.The working medium is liquid or substantially liquid when flowing into the second heat exchanger. The boiling point of the working medium at the pressure prevailing in the second heat exchanger is lower than the temperature of the liquefied gas in the liquefied gas storage tank, and the heat input from the liquefied gas to the working medium in the second heat exchanger causes the working medium to boil. The heat of vaporization consumed by the working medium makes the second heat exchanger very efficient with respect to cooling the liquefied gas in the liquefied gas storage tank. This principle of a boiling working medium in the second heat exchanger can be applied to all embodiments of the present invention.In one embodiment, the liquefied gas storage tank has a capacity of liquefied gas volume corresponding to a maximum of three days of fuel gas consumption with continuous full load of the internal combustion engine. In this embodiment, the liquefied gas storage tank is a so-called day tank that stores a smaller amount of fuel gas in the vicinity of the internal combustion engine. The day tank is supplemented with fuel gas at intervals, for example when the liquid fuel gas quantity is below a defined quantity in the tank. The day tank is normally maintained at ambient temperature or at a slight overpressure of a few bar. In some embodiments, the day tank may have a capacity that is less than 24 hours of fuel gas consumption.In one 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 that two or more of the liquefied gas storage tanks are tanks having a large capacity. When the embodiment is installed in a liquefied gas tanker, such as an LNG tanker or an LPG tanker, the liquefied gas storage tanks may be the storage tanks or a subset of the storage tanks, such as two storage tanks, disposed in close proximity to the machine room. If the ship does not carry a liquefied gas as a load, it may be convenient to have a plurality of liquefied gas storage tanks, such as from two to 25 or more liquefied gas storage tanks.In one embodiment, the heat exchanger circuit comprises at least two second heat exchangers arranged in at least two liquefied gas storage tanks, preferably such that at least one second heat exchanger is located in each liquefied gas storage tank.In one embodiment, the liquefied gas supply system is intended for an internal combustion engine serving as a propulsion motor 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 a prime mover in a stationary power plant.In one embodiment, the ship is selected from a group comprising container ships, bulk cargoers, passenger ships, oil tankers, tankers, RoRo ships and refrigerated ships. It is a common feature of the ships in this group that they carry a charge other than liquefied gas and in this case the ship has been equipped with gas fuel tanks in the form of liquefied gas storage tanks. A RoRo ship is a ship with a ramp so that the cargo can be brought on board and off board. In another embodiment, the ship is a guest anchor.In one embodiment, a bypass line extends from the fuel gas supply line downstream of the first heat exchanger to the liquefied gas storage tank, the bypass line being provided with a bypass pump and a shut-off valve. When the engine is taken out of operation, the shut-off valve in the bypass line may be opened and the pump activated to circulate cold liquefied petroleum gas through the tank outlet line and the bypass line, such that the system is cooled before the engine is started or maintained in the cold state while the engine is temporarily stopped.Examples of embodiments of the invention are described in more detail below with reference to the very schematic drawings. The following are shown: FIG. 1 illustrates a fuel gas supply system for an internal combustion engine for driving an LNG fuel tanker in the related art, FIG. 2 illustrates an LNG fuel tanker with a fuel gas supply system according to the present invention, FIG. 3 illustrates the outline of an internal combustion engine in the LNG fuelling tank of FIG. 2 from the end, 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 of FIG. 3 as viewed from a single cylinder on the engine, FIG. 6 illustrates a schematic representation of a first embodiment of a fuel gas supply system according to the present invention, FIG. 7 illustrates a schematic illustration of a second embodiment of the fuel gas supply system, FIG. 8 illustrates a schematic illustration of a third embodiment of the fuel gas supply system, and FIG. 9 illustrates a schematic diagram of a fourth embodiment of the fuel gas supply system.An LNG fuelling machine in Fig. 2 has an internal combustion engine serving as a main drive motor in the machine room 1 located under a structure 2. The motor drives a propeller 3 for driving the ship. The LNG tanker has a plurality, in the illustrated embodiment four, LNG storage tanks, at least one of which, and preferably a plurality of which, are liquefied gas storage tanks 4 used in a fuel gas supply system according to the present invention. Although the purpose of the LNG tanker is to transport LNG from a manufacturing facility to a LNG use site, the LNG storage tanks also act as fuel stores for the internal combustion engine during transport.The ship need not be an LNG fuelling vessel, but it may also be a fuelling vessel of any other type, wherein at least one liquefied gas storage tank 4 serves only as a fuel storage vessel, irrespective of the load of the ship. Examples of these other types of vessels are RoRo vessels, container vessels, oil tankers, car transport vessels, and mass cargoers.In FIG. 3, the internal combustion engine is shown in more detail. The internal combustion engine is a piston engine and preferably a crosshead two-stroke engine generally designated 5. The engine may have from 4 to 15 cylinders. The engine may be, for example, a MAN Diesel & Turbo engine of the ME-GI or MC type, or a Utillas brand engine or a Mitsubishi brand engine. The cylinders may have a bore in the range of, for example, 25 to 120 cm, preferably 40 to 110 cm. The crosshead two-stroke internal combustion engines used as main drive motors normally have one revolution, indicated as U / min, in the range of 55 to 195 U / min. These motors are called slow running motors. The low speed is required for the transmission of the thrust force via the propeller to the water in the wake of the ship. In order to transmit the thrust to the water, the propeller requires a large area and thus a large diameter. Since cavitation on the propeller is undesirable, it is necessary to limit the speed of the drive motor to the low speed range, such as from 60 to 200 rpm / min.The internal combustion engine 5 has a plurality of cylinders, each having a reciprocating piston in the cylinder. In the crosshead two-stroke engines, the cylinders are normally DC-scavenge cylinders, with an exhaust valve 6 located at the top of the cylinder and scavenge air ports, not shown, located at the bottom of the cylinder. The exhaust gas from the cylinder passes through an exhaust receptacle 7 and further 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 can lead through an inlet air cooler 10 to a region which surrounds the scavenging air connections in the cylinders.The engine has an injection system for direct injection of pilot fuel and gaseous fuel gas, and for safety reasons, the system for injection of fuel gas is provided with an air intake system and an inert gas system. The air inlet system is provided in a pipe 15 surrounding the gas fuel pipe 16 and the annular space between the two pipes allows monitoring of gas leakage from the inner pipe. The air inlet takes place at 11 and when the injection system is operating normally the air outlet takes place at 12. A pair of hydrocarbon detectors 13 are located behind the engine in the conduit leading to the air outlet 12. A source of pressurized inert gas 14 is connected to the fuel gas pipe 16 and when the engine is shut down, the inert gas is supplied to the fuel gas pipe to purge it for gas.A first fuel bearing 17 supplies pilot fuel to the injectors 18 on each cylinder 19 of the internal combustion engine. The pilot fuel is supplied at a pressure of, for example, 300 or 400 bar and is used to initiate each fuel injection sequence in the cylinder. The ignition fuel may be fuel oil and is capable of self-igniting in the combustion chamber with the compression pressure available in the combustion chamber at the end of the power stroke. Gas injectors 20 on each cylinder 19 are provided with control oil from a control oil pump 21 when the required ignition oil pressure is detected, and the control oil pressure is required on the gas injectors 20 to inject gas. The control oil causes gas not to be injected into the cylinders when no ignition oil is injected. The gas injectors 20 are also supplied with pressurized sealing oil via the sealing oil line 22. The sealing oil prevents gas from exiting the gas injector in a different manner than through the gas injector delivering the gas into the combustion chamber.Fuel gas from the liquefied gas storage tank 4 is supplied to a fuel gas pipe 16 via a fuel gas supply line 25 in a fuel gas supply system and flows to a reservoir 23, and a control valve 24 opens for fuel gas to the injectors 20 when the fuel gas injection is to take place. A common rail pipe may be present between the fuel gas pipe 16 and the injectors 20, and in this case, it may be possible to dispense with the accumulator 23.The fuel gas supply system for an internal combustion engine is shown in more detail in Figs. 6 to 8. The fuel gas supply line 25 extends from inside the liquefied gas storage tank 4 to the internal combustion engine 5 and has a first portion formed by a tank outlet line 26. The tank outlet conduit 26 extends into the tank to a region near an interior bottom of the tank and has an end opening that permits the flow of liquified gas. There is no pump at the tank outlet line 26 within the liquefied gas storage tank 4. As alternatives to extending from top to bottom into the tank, the tank outlet line may extend under the tank from a bottom opening therein or may extend from a lower portion of an end bottom if the liquefied gas storage tank is formed as a cylindrical tank having 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 medium in a heat exchanger circuit generally designated 28. The tank outlet line is provided with a non-illustrated shut-off valve and possibly also a non-return valve outside the liquefied gas storage tank 4, so that the tank can be connected or disconnected as desired. On the side behind the third heat exchanger 27, the fuel gas supply line continues towards the inlet of a fuel gas pump 29, which is a high pressure pump that increases the fuel gas pressure to at least a pressure required at the fuel gas inlet to the internal combustion engine 5, namely a pressure in the range of 200 bar to 700 bar, normally about 300 to 400 bar. High pressure is required because the internal combustion engine performs a direct injection of the gas into the combustion chamber at the end of the compression stroke, wherein the pressure in the combustion chamber may be e.g. 180 bar and the injection pressure must be substantially higher in order to finely distribute the gas in the combustion zone.The fuel gas pump may have different stages or two or more fuel gas pumps may be connected in series or in parallel. The fuel gas pump is a cryogenic pump and examples thereof are the model TC-34 from Cryogenic Industries, CA, USA and high pressure LNG centrifugal pumps as disclosed in Hydrocarbon Processing, July 2011, pages 37-41. The fuel gas pump is preferably a piston displacement pump with a hydraulic drive acting in both directions. The at least one fuel gas pump can also be a combination of a piston pump and a 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 medium in the heat exchanger circuit 28. The 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 temperature, preferably a temperature of about 45°C, suitable for supplying the fuel gas to the internal combustion engine. The final heat exchanger is supplied with heating liquid from an available source 32 which is separate from the heat exchanger 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 on the internal combustion engine, to which the fuel gas is supplied at a fuel gas connection pressure. The fuel gas is in a supercritical state in the portion of the fuel gas supply pipe 25 that extends from the fuel gas pump 29 to the gas fuel pipe 16.The heat exchange circuit 28 is a closed circuit including a circulation line 33 in which a working medium flows. A working medium reservoir 34 is connected via a circulation line 33 to the inlet of a compressor 35. The compressor may be a single stage compressor or a multi-stage compressor. At the exit of the compressor 35, the working medium is in a supercritical stage, such as at a pressure of 100 bar, and the exit of the compressor is connected to an inlet at the first heat exchanger 30, preferably such that the working medium flows countercurrently to the working gas to which the working medium supplies heat during passage through the first heat exchanger.From the outlet of the first heat exchanger, the circulation line 33 continues to an expansion device here expediently formed by an expansion vessel 36, in which the pressure in the working medium is reduced to a low pressure, such as a pressure in the range from 1 bar to 12 bar, so that the working medium has a temperature below the boiling point of the fuel gas in the fuel gas supply line 25 upstream of the fuel gas pump 29, such as a temperature of at least 10° C. below this boiling point, and preferably at least 20° C. below this boiling point. The pressure expansion vessel may in one embodiment comprise a nozzle disposed in a closed chamber and connected to the circulation line 33 coming from the first heat exchanger so that the working medium expands into the chamber so that both the pressure and the temperature of the working medium are lowered.From the outlet of the expansion vessel 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 medium in the heat exchanger circuit 28. This cooling is very effective because the working medium in the circulation line is at its lowest temperature when it flows out of the expansion vessel. The fuel gas can thus be cooled many degrees Celsius below its boiling point and this provides room for low pressure in the liquid fuel gas without the presence of a gas phase just before the at least one fuel gas pump 29.From the outlet of the third heat exchanger 27, the circulation line 33 in the liquefied gas storage tank 4 continues to a second heat exchanger 37, which may be in the form of a pipe section immersed in the liquefied gas in the tank, such as a spiral pipe section. The circulation line and a second heat exchanger running in the liquefied gas storage tank 4 is in one embodiment formed from a single length of pipe which has been formed into a shape and for the improved heat transfer is possibly provided with external fins, and moving parts or connections between separate elements in the tank can be avoided if necessary to optimize operational safety. In another embodiment, the second heat exchanger is a plate-shaped structure with an internal flow path for the working medium and an inlet and an outlet with fixed connections to the circulation line 33 arranged in the liquefied gas storage tank 4. The dot-dashed line indicates a top 38 of the liquefied gas in the tank. Of course, the top surface moves downward when the fuel gas is consumed. From the second heat exchanger 37, the circulation line 33 continues to an inlet to the accumulator 34.The working medium may be nitrogen, which may already be used for other purposes on a ship, where nitrogen is used as inert gas. The working medium may alternatively be argon or helium. Typical properties of these working media are shown in Table 1, together with similar properties of methane normally used as fuel gas. Helium is not preferred because of its low heat of vaporization. Table 1 Table 1Boiling point, °C-195,8-185,9-268,9-161,5Critical point, °C-147,0-122,5268,0- 82,6Critical point, bar34,048,32,346Melting point, °C-210,0-189,4-271,4-182,5Heat of evaporation kJ / kg20316120-A bypass line 39 is connected to the fuel gas supply line 25 behind the first heat exchanger 30, and extends to the liquefied gas storage tank 4 via a bypass pump 40 and a non-illustrated shut-off valve. the pump 29 may include a suction chamber having a bypass line that can be opened and closed when the bypass line 39 is opened and closed, so that the pump 29 allows a bypass flow when the bypass line 39 is opened for the bypass. The pump 29 may alternatively be a pump which allows bypass through the pumping element at standstill. When the internal combustion engine is at a standstill, the shut-off valve may be opened and the bypass pump 40 may be activated so that liquefied petroleum gas is circulated through most of the fuel gas supply line 25 to cool the system and make it ready for operation when the internal combustion engine is started.An example of the operation according to the present invention is given below with reference to the embodiment of Fig. 6. The gas flow rate in the fuel gas supply pipe 25 is given as 1 kg / sec corresponding to the LNG consumption when the internal combustion engine has a capacity of about 27 MW. The internal combustion engines relevant to ship propulsion are in a power range of about 2 MW to about 90 MW, depending on the type of engine, the bore of the engine and the number of cylinders in the engine, and the fuel gas consumption amount is proportional to the power. Nitrogen N2is used as the working medium, and the flow rate of the working medium in the circulation line 33 is 1.7 kg / s in this example.The liquefied gas at the 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 for the fuel gas in the tank outlet line 26 at a position bg just before the third heat exchanger 27. At the position cg in the fuel gas supply line 25 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 at the position dg has a pressure of about 300 bar and a temperature of about -172° C., and the same pressure and temperature are found at the position, e.g. upstream of the first heat exchanger 30. Behind this heat exchanger at position fg, the fuel gas has a pressure of about 300 bar and a temperature of about -12°C, and behind 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 medium at position i in the reservoir 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 before the compressor 35. Behind the compressor 35 at position f, the working medium has a pressure of about 100 bar and a temperature of about 28° C. Behind the first heat exchanger 30 at positions e and d, the working medium has a pressure of about 100 bar and a temperature of about -152° C. At position c behind the expansion vessel 36, the working medium has a pressure of about 5 bar and a temperature of about -178° C. and a part of the working medium is in the vapor phase. In the third heat exchanger, a part of the liquid phase evaporates and behind the third heat exchanger 30 at positions b and a, the working medium has a pressure of about 5 bar and a temperature of about -178° C. In the second heat exchanger 37, the working medium boil and behind it and at position i in the reservoir 34, the working medium has a pressure of about 5 bar and a temperature of about -161° C.Under these flow conditions, the compressor 35 requires a power of 310 kW and 542 kW are transmitted from the working medium to the combustion gas in the first heat exchanger 30; 66 kW are transmitted from the combustion gas to the working medium in the third heat exchanger 30; and 165 kW are transmitted from the combustion gas to the working medium in the second heat exchanger 33.In the first heat exchanger 30, the working medium enters at the temperature of 28°C and exits at -152°C, while the fuel gas enters in countercurrent at the temperature of -171°C and exits at -12°C, and the temperature difference between the two media 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, but the working medium has a 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 those of the above-mentioned embodiment are used for the details of the same function, and only differences from the first embodiment are mentioned.In the second embodiment of Fig. 7, the expander vessel 20 is a turbine in which the working fluid expands while the turbine receives energy for its shaft. The shaft of the turbine is connected to the shaft of the fuel gas pump, possibly via a transmission, in order to save energy.In the third embodiment of FIG. 8, the liquefied gas storage tank 4 in which the fuel gas supply pipe 25 is mounted is a day tank having a relatively small capacity so as to contain an amount of fuel gas required for some hours of operation of the internal combustion engine 5 but less than for several days of operation. This day tank can be conveniently placed next to the machine room because it is small. At least one additional higher capacity liquefied gas storage tank 4 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 circuit connecting this second heat exchanger 37 in parallel with the second heat exchanger 37 in the liquefied gas storage tank 4 serving as a day tank, and the control valves 41 are used for controlling the flow of the working medium to the single second heat exchanger 37. A fuel gas supply 42 with a service pump 43 runs from the vicinity of the inner bottom of the large tank to the day tank and the day tank may have a sensor or level control device which activates the service pump when the level in the day tank is below a preset value so that an appropriate amount of liquid fuel gas is maintained in the day tank.The second heat exchanger 37 in the single 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 an ambient pressure of about 1 bar and boil off gas is avoided. The fuel gas supply system thus does not use a boil off gas and does not have equipment for re-liquefying boil off gas. The liquefied gas content in the liquefied gas storage tanks 4 can be cooled down to a lower temperature, such as a temperature above but near the melting point of the gas, and thus allows a stopped-engine period 5 while the temperature of the liquefied gas rises slowly. While a slow heat input to the tanks occurs, it is possible to stop the engine for up to several days without 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 on a liquefied gas flow line generally indicated at 50. The liquefied gas flow line passes through the second heat exchanger 37, the liquefied gas flow line communicates with the liquefied gas in the liquefied gas storage tank via a suction line 44 extending downward into the liquefied gas storage tank 4 near its inner bottom, and a pressure line 45 returns liquefied gas to the tank after a circulation pump 46 has made the liquefied gas flow through the second heat exchanger 37. The liquefied gas flow line 50 may also be a line for transferring liquefied gas from one tank to another, and may be configured as a fuel gas supply 42 with the service pump 43.In further embodiments, it is possible to connect the tank outlet line directly to the inlet at the at least one fuel gas pump 29 and to connect the outlet from the expansion vessel 36 directly to the second heat exchanger, and thus without the need for the third heat exchanger.The single liquefied gas storage tank 4 may be connected to an inert gas line connecting an inert gas source to the upper portion of the tank, and the inert gas line may be provided with a regulating valve that maintains the ambient pressure (about 1 atm or 1 bar) in the liquefied gas storage tank 4. Alternatively, the inert gas line can be connected to an inert gas source which has a flexible wall whose outer side is open to the environment, so that under all operating conditions and thus also when the liquefied gas is cooled far below its boiling point, ambient pressure is present in the inert gas source and thus also in the liquefied gas storage tank 4.In the various embodiments described above, shut-off valves and control valves are present as are common in such a system, and in particular a shut-off valve in the fuel gas supply line 26, and this shut-off valve can connect the at least one fuel gas pump to the liquefied gas in the liquefied gas storage tank when the shut-off valve is in the open position.The fuel gas supply line 26 may be provided with a guard in the form of an outer tube having a larger diameter than the outer diameter of the fuel gas supply line 26, and the annular space between the tubes may be vented and provided with gas escape detectors at the exit of the venting air. The outer tube also serves to protect the personnel from contacting surfaces which are at a very low temperature.Details of the various embodiments described may be combined with other embodiments within the scope of the claims.

Claims

A fuel gas supply system for an internal combustion engine (5), wherein the fuel gas supply system has a liquefied gas storage tank (4), a fuel gas supply line (25) and a heat exchanger circuit with a working medium, wherein the fuel gas supply line has a tank outlet line (26) for liquefied gas, at least one fuel gas pump (29) for pressurizing fuel gas to a fuel gas connection pressure for the internal combustion engine and a final heat exchanger (31) which is located behind the at least one fuel gas pump, and wherein the heat exchanger circuit has at least one compressor (35) and, behind the latter, a first heat exchanger (30) and an expansion device (36) and a second heat exchanger which is located behind the expansion vessel, characterized in that the at least one fuel gas pump (29) is arranged outside the liquefied gas storage tank (4), and connectable via the tank outlet line (26) for liquefied gas to the liquefied gas in the associated storage tank, that the first heat exchanger (30) of the heat exchanger circuit is connected to the fuel gas supply line (25) between the fuel gas pump (29) and the final heat exchanger (31), and that the second heat exchanger (37) of the heat exchanger circuit is arranged in the liquefied gas storage tank (4) or on a liquefied gas flow line (50) which is connected to the liquefied gas in the liquefied gas storage tank (4), wherein a third heat exchanger (27) is arranged behind the expansion device (36) in the heat exchanger circuit and in front of the at least one fuel gas pump (29) in the fuel gas supply line.The fuel gas supply system according to claim 1, wherein the fuel gas pressure of the fuel gas pump (29) is in the range of 200 bar to 700 bar, preferably in the range of 250 bar to 450 bar.The fuel gas supply line according to at least one of the preceding claims, wherein the compressor (35) is designed to compress the working medium to a maximum pressure in the range of 40 bar to 120 bar.The fuel gas supply line according to at least one of the preceding claims, wherein the expansion device (36) is designed such that it supplies a downstream pressure in the range from 1 bar to 12 bar.The fuel gas supply system according to at least one of the preceding claims, wherein the liquefied gas storage tank (4) has a capacity of liquefied gas volume corresponding to at most a fuel gas consumption of three days.The fuel gas supply system according to at least one of the preceding claims, wherein the fuel gas supply system has at least two liquefied gas storage tanks (4), preferably two to 25 liquefied gas storage tanks.The fuel gas supply system according to claim 6, wherein the heat exchanger circuit (28) comprises at least two second heat exchangers (37) arranged in at least two liquefied gas storage tanks (4), preferably such that at least one second heat exchanger is arranged in each liquefied gas storage tank.Fuel gas supply system according to at least one of the preceding claims, wherein the fuel gas supply system is assigned to an internal combustion engine (5) designed as a drive motor in a ship.The fuel gas supply system according to claim 8, wherein the ship is selected from a group comprising container ships, mass cargo loads, passenger ships, oil tankers, tankers, Ro-Ro ships and refrigerated ships.The fuel gas supply line according to claim 7 or 8, wherein a bypass line (39) runs from the fuel gas supply line (25) behind the first heat exchanger (30) to the liquefied gas storage tank (4), wherein the bypass line is provided with a bypass pump (40) and a shut-off valve.The fuel gas supply system according to at least one of the preceding claims, wherein the liquefied gas storage tank (4) is connected to an inert gas source at ambient pressure.

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