Technical field of the invention

The present invention relates to a fuel storage and supply system for a combus- tion device in accordance with claim 1. The invention also concerns a method of operating a fuel storage and supply system, as defined in the other independent claim. The invention further concerns a marine vessel comprising a fuel storage and supply system.

Background of the invention Power generation by combustion is still mostly based on fossil fuels. Due to the need to cut carbon dioxide emissions, there is a growing demand for fossil free power generation. The energy density of most fuels still clearly beats the energy density of batteries, and therefore it is likely that power generation by combus tion will remain as a significant form of power generation especially in cases, where an energy source for a power generating device needs to be carried to gether with the power generating device over long distances, such as in case of ocean-going vessels and many other marine vessels.

At the moment, there is not in view a single non-fossil fuel that could or would replace in the medium term all fossil fuels. Therefore, adaptability to different fuels is a desired feature of power generation systems based on combustion.

Many combustion devices, such as internal combustion engines, are nowadays operated using liquid fuels, such as light fuel oil and heavy fuel oil. Many internal combustion engines can also be operated using natural gas that is stored either as compressed gas (CNG) or liquefied gas (LNG) and supplied into the engine in gas phase. With the use of natural gas, cleaner combustion and lower carbon dioxide emissions can be achieved compared to conventional liquid fuels, such as heavy fuel oil or light fuel oil.

A promising alternative to fossil fuels is hydrogen. Depending on the combustion device, it can be used either as a stand-alone fuel or mixed with another fuel. Another alternative for future power generation is ammonia. Ammonia can be used in many combustion devices as a stand-alone fuel or it can be mixed with another fuel. The life cycles of powerplants and marine vessels are long, and during the life cycle of a powerplant or a marine vessel even other new fuels, which can be used in the combustion devices with reasonable modifications of the devices, can emerge.

Liquid fuels can usually be stored at ambient pressure and temperature. Gase ous fuels, i.e. fuels that are gaseous at ambient pressure and normal operating temperatures of the combustion devices, may require very different storing con ditions. For instance, for storing natural gas at a pressure that is close to the ambient temperature, the gas needs to be cooled down to approximately -162 °C. Ammonia can be stored at ambient pressure at about -34 °C. The boiling point of hydrogen at atmospheric pressure is approximately -253 °C and the critical temperature is approximately -240 °C. Depending on the fuel, energy may be needed for cooling the fuel down to a storage temperature. Also, energy may be needed for evaporating a fuel prior to combustion or for heating a fuel for allowing it to be mixed with another fuel. The required cooling, heating and evaporating reduces the overall efficiency of a power generation system.

As the examples above show, emerging of new fuels with different properties causes challenges to fuel storage and supply systems.

Summary of the invention

An object of the present invention is to provide an improved fuel storage and supply system for a combustion device, the combustion device being configured to be operable using at least a mixture of a first fuel and a second fuel or the first fuel and the second fuel separately. The characterizing features of the system according to the invention are given in claim 1 . Another object of the invention is to provide an improved method of operating a fuel storage and supply system. Still another object of the invention is to provide an improved marine vessel.

The fuel storage and supply system according to the invention comprises a first fuel tank for storing the first fuel at a first temperature, a second fuel tank for storing the second fuel at a second temperature, which second temperature is higher than the first temperature, a first fuel supply line for supplying the first fuel to the combustion device, and a second fuel supply line for supplying the second fuel to the combustion device. The system further comprises at least one heat exchanger, which is configured to allow heat transfer from the second fuel to the first fuel. In the method of operating a fuel storage and supply system defined above, the flow of the first fuel and/or the second fuel through the heat exchanger is con trolled to increase the temperature and/or to evaporate the first fuel before sup plying the first fuel to the combustion device and/or to maintain desired condi tions in the second fuel tank.

By transferring heat from the second fuel to the first fuel, the second fuel is cooled down, condensed and/or solidified. The first fuel is heated and/or evap orated. Depending on the types of the first fuel and the second fuel and the conditions in which the first fuel and the second fuel are stored, the heat transfer from the second fuel to the first fuel may provide different benefits. If the first fuel is stored in the first fuel tank in liquid phase, evaporation of the fuel may be needed before the fuel is supplied to the combustion device. If the first fuel is stored at very low temperatures, heating of the fuel may reduce problems caused to the components of the fuel supply system by the low temperatures. If the first fuel is mixed with the second fuel, heating of the first fuel may be needed to avoid condensing and/or solidifying of the second fuel when being mixed with the first fuel. Cooling of the second fuel may help in maintaining appropriate conditions in the second fuel tank, for instance maintaining the pressure and/or the temperature in the second fuel tank within desired ranges. Cooling of the second fuel may thus help keeping the second fuel in liquid phase or partly in solid phase. Cooling of the second fuel may also allow supplying the second fuel to the combustion device at a lower temperature. This may help lowering the peak temperatures in the combustion device and reduce NO _x emissions.

The heat transfer from the second fuel to the first fuel may reduce the need of heating and/or evaporating of the first fuel with external energy. The efficiency of the system may thus be improved. Also, less equipment for heating and/or evaporating the first fuel may be needed, and/or the heating power required from the equipment may be lowered. This may lower the operation and maintenance costs of the system. Similarly, heat transfer from the second fuel to the first fuel may reduce the need of cooling down the second fuel with external energy. The efficiency of the system may thus be improved. Also, less equipment for cooling down the second fuel may be needed, and/or the cooling power required from the equipment may be lowered. This may lower the operation and maintenance costs of the system. According to an embodiment of the invention, the system is configured to allow flow of the first fuel through the heat exchanger to the combustion device. This allows heating and/or evaporation of the first fuel before the first fuel is supplied to the combustion device. According to an embodiment of the invention, the system comprises a fuel cir culation line for allowing the second fuel to be circulated from the second fuel tank through the heat exchanger back to the second fuel tank. This allows reg ulation of the conditions in the second fuel tank.

According to an embodiment of the invention, the heat exchanger is arranged in the second fuel tank. This allows regulating the conditions in the second fuel tank.

According to an embodiment of the invention, the heat exchanger is arranged in a tank connection space of the first fuel tank or the second fuel tank. By arrang ing the heat exchanger in a tank connection space, a separate gas-tight space for the heat exchanger is not needed.

According to an embodiment of the invention, the system comprises a fuel pump for supplying the first fuel to the combustion device.

According to an embodiment of the invention, the system comprises a fuel pump for supplying the second fuel to the combustion device. According to an embodiment of the invention, the system comprises means for mixing the second fuel with the first fuel for supplying a mixture of the first fuel and the second fuel to the combustion device.

According to an embodiment of the invention, the first fuel is LNG or liquefied hydrogen. According to an embodiment of the invention, the second fuel is ammonia.

According to an embodiment of the invention, the combustion device is an inter nal combustion engine.

A marine vessel according to the invention comprises a fuel storage and supply system defined above. Brief description of the drawings

Embodiments of the invention are described below in more detail with reference to the accompanying drawings, in which

Fig. 1 shows schematically a fuel storage and supply system according to an embodiment of the invention,

Fig. 2 shows schematically a fuel storage and supply system according to an other embodiment of the invention, and

Fig. 3 shows schematically a marine vessel comprising a fuel storage and supply system according to an embodiment of the invention. Description of embodiments of the invention

Figure 1 shows schematically a fuel storage and supply system according to an embodiment of the invention for a combustion device 3. The combustion device 3 is configured to be operable using at least a mixture of a first fuel and a second fuel or the first fuel and the second fuel separately. The combustion device 3 is preferably an internal combustion engine. The inter nal combustion engine 3 may be, for instance, a piston engine or a gas turbine. In case of a piston engine, the engine 3 can be a four-stroke engine or a two- stroke engine. The engine 3 can be a main engine of a marine vessel, i.e. an engine forming part of a propulsion system of the vessel. The engine 3 could be connected mechanically, hydraulically or electrically to a propeller or other pro pulsion device. The engine 3 could also be an auxiliary engine of a marine ves sel, i.e. an engine driving a generator for producing electricity for electrical sys tems of the vessel. The engine 3 could also be a powerplant engine, i.e. an engine driving a generator for producing electricity. The fuel storage and supply system according to the invention comprises a first fuel tank 1 for storing the first fuel at a first temperature, a second fuel tank 2 for storing the second fuel at a second temperature, which second temperature is higher than the first temperature, a first fuel supply line 4 for supplying the first fuel to the combustion device 3, and a second fuel supply line 5 for supplying the second fuel to the combustion device 3. The system further comprises at least one heat exchanger 6, 7 which is configured to allow heat transfer from the second fuel to the first fuel. The terms “first temperature” and “second temperature” should be understood as target temperatures or temperature ranges. The first temperature can thus be a target temperature, below which the temperature of the first fuel is kept, or a temperature range, within which the temperature of the first fuel is kept. Simi larly, the second temperature can be a target temperature, below which the tem perature of the second fuel is kept, or a temperature range, within which the temperature of the second fuel is kept. The second temperature could also be non-regulated. The second temperature could thus be allowed to vary freely de pending on the ambient temperature and heat transfer from the second fuel to the first fuel. The second temperature could also be a minimum temperature for the second fuel. For instance, a target temperature above the freezing point of the second fuel could be set.

The first fuel tank 1 and/or the second fuel tank 2 can be insulated for facilitating keeping the first fuel at the first temperature and/or the second fuel at the second temperature. The first fuel tank 1 and/or the second fuel tank 2 could also be provided with means for cooling the fuel.

At least the first fuel is gaseous fuel. The term “gaseous fuel” refers here to a fuel that is in gas phase at a temperature of 20 °C and a pressure of 1 atm (101 .325 kPa). The first fuel does thus not need to be stored in the first fuel tank 1 in gas phase, but the first fuel can be stored in the first fuel tank 1 in liquid phase.

Also the second fuel can be gaseous fuel, but the second fuel could also be liquid fuel. The term “liquid fuel” refers here to a fuel that is in liquid phase at a temperature of 20 °C and a pressure of 1 atm (101 .325 kPa). If the second fuel is gaseous fuel, it can be stored in the second fuel tank 2 in gas phase or liquid phase. Part of the second fuel could be even in solid phase.

In practice, even if the first and/or the second fuel is stored in liquid phase, part of the fuel in the fuel tank 1 , 2 is always in gas phase. The expression “in liquid phase” thus means that major part of the fuel in the fuel tank 1 , 2 is liquid.

By transferring heat from the second fuel to the first fuel, the second fuel is cooled down, condensed and/or solidified. The first fuel is heated and/or evap orated. Depending on the types of the first fuel and the second fuel and the conditions in which the first fuel and the second fuel are stored, the heat transfer from the second fuel to the first fuel may provide different benefits. If the first fuel is stored in the first fuel tank 1 in liquid phase, evaporation of the fuel may be needed before the fuel is supplied to the combustion device 3. If the first fuel is stored at very low temperatures, heating of the fuel may reduce problems caused to the components of the fuel supply system by the low temperatures. If the first fuel is mixed with the second fuel, heating of the first fuel may be needed to avoid condensing and/or solidifying of the second fuel when being mixed with the first fuel. Cooling of the second fuel may help in maintaining appropriate conditions in the second fuel tank 2, for instance maintaining the pressure and/or the temperature within the second fuel tank 2 within desired ranges. Cooling of the second fuel may thus help keeping the second fuel in liquid phase or partly in solid phase. Cooling of the second fuel may also allow supplying the second fuel to the combustion device 3 at a lower temperature. This may help lowering the peak temperatures in the combustion device 3 and reduce NO _x emissions.

The heat transfer from the second fuel to the first fuel may reduce the need of heating and/or evaporating the first fuel with external energy. The efficiency of the system may thus be improved. Also, less equipment for heating and/or evap orating the first fuel may be needed, and/or the heating power required from the equipment may be lowered. This may lower the operation and maintenance costs of the system. Similarly, heat transfer from the second fuel to the first fuel may reduce the need of cooling down the second fuel with external energy. The efficiency of the system may thus be improved. Also, less equipment for cooling down the second fuel may be needed, and/or the cooling power required from the equipment may be lowered. This may lower the operation and maintenance costs of the system.

In the embodiment of figure 1 , the first fuel supply line 4 is configured to allow flow of the first fuel through the heat exchanger 6 to the combustion device 3. In the embodiment of figure 1 , the first fuel always flows via the heat exchanger 6 to the combustion device 3. However, the system could also be provided with a by-pass line allowing the heat exchanger 6 to be by-passed. The system of fig ure 1 further comprises a fuel circulation line 8 for allowing the second fuel to be circulated from the second fuel tank 2 through the heat exchanger 6 back to the second fuel tank 2. Flow of the second fuel through the heat exchanger 6 may be selectively allowed. When the second fuel is allowed to flow through the heat exchanger 6 simultaneously with the first fuel, heat is transferred from the sec ond fuel to the first fuel. The second fuel thus returns to the second fuel tank 2 at a lower temperature. Depending on the type and temperature of the first fuel, the first fuel may be heated and/or (partly) evaporated.

In the embodiment of figure 1 , the fuel storage and supply system comprises a first fuel pump 9 for supplying the first fuel to the combustion device 3. The sys tem further comprises a second fuel pump 10 for supplying the second fuel to the combustion device 3. The second fuel pump 10 is also used for circulating the second fuel through the heat exchanger 6. However, the system could also be provided with a separate pump for circulating the second fuel through the heat exchanger 6.

In the embodiment of figure 1 , the fuel storage and supply system comprises an evaporator 15 for evaporating the first fuel. The evaporator 15 is arranged down stream from the heat exchanger 6. The first fuel is stored in the first fuel tank as liquid. The liquid first fuel is heated and possibly partly evaporated in the heat exchanger 6. The heated first fuel is then conducted through the evaporator 15. Because of the heat transfer in the heat exchanger 6, less energy is needed in the evaporator 15 for evaporating the first fuel before it is conducted to the com bustion device 3.

In the embodiment of figure 1 , the fuel storage and supply system comprises an evaporator 16 for evaporating the second fuel. In the evaporator 16, the second fuel is evaporated before it is supplied to the combustion device 3.

In the embodiment of figure 1 , the system needs to be provided with at least one valve for selectively conducting the second fuel either into the second fuel supply line 5 or into the fuel circulation line 8. In the system of figure 1 , a first valve 17 is arranged upstream from the evaporator 16 for the second fuel for selectively opening and closing fluid communication between the second fuel tank 2 and the evaporator 16. A second valve 18 is arranged in the fuel circulation line 8 for selectively opening and closing fluid communication between the second fuel tank 2 and the heat exchanger 6. The first and the second valves 17, 18 could also be replaced by a three-way valve. The fuel circulation line 8 could be ar ranged completely separate from the second fuel supply line 5 and provided with an own pump. In that case, the first and second valves 17, 18 would not be needed. However, the fuel circulation line 8 could be provided with a shut-off valve. A main shut-off valve 19 is arranged downstream from the evaporator 16. A main shut-off valve 20 for the first fuel is arranged downstream from the evap orator 15 for the first fuel.

The system further comprises a mixing unit 13 for mixing the second fuel with the first fuel for supplying a mixture of the first fuel and the second fuel to the combustion device 3. In the embodiment of figure 1 , the system comprises a valve 21 for opening and closing fluid communication between the first fuel tank 1 and the mixing unit 13. The system further comprises a valve 22 for opening and closing direct fluid communication between the first fuel tank 1 and the com bustion device 3. With the valves 21 , 22, the first fuel can be conducted to the combustion device 3 selectively either directly or through the mixing unit 13. The valves 21 , 22 could be replaced by a single three-way valve. The valves 21 , 22 are not necessary, but the mixing device 13 could be arranged between the first fuel tank 1 and the combustion device 3 in such a way that the first fuel is always conducted to the combustion device 3 through the mixing device 13.

The mixing unit 13 is not necessary, if the first fuel and the second fuel are used in the combustion device 3 separately. In the embodiment of figure 1 , the first fuel supply line 4 and the second fuel supply line 5 are merged into a single fuel supply line before the combustion device 3. However, the first fuel supply line 4 and the second fuel supply line 5 could be completely separate from each other.

The first fuel tank 1 is provided with a tank connection space 11 . Also the second fuel tank 2 is provided with a tank connection space 12. A tank connection space is generally required for gaseous fuels that are stored in liquefied form in the fuel tank. A tank connection space is a gas-tight space accommodating tank con nections and valves associated with them. The purpose of the tank connection space is to prevent the gas that may leak from the tank connections or the valves to enter a tank hold or other place in which the tank is located. In addition to valves, also other equipment, such as pumps or evaporators may be arranged in a tank connection space. In the embodiment of figure 1 , the fuel pumps 9, 10 and evaporators 15, 16 are arranged in the tank connection spaces 11 , 12. All fuel pipes outside the tank connection spaces 11 , 12 are double-wall pipes, where the fuel flows in an inner pipe and the outer pipe collects possibly leaking fuel. The outer pipe may be filled with inert gas, such as nitrogen.

In the embodiment of figure 1 , the tank connection space 11 of the first fuel tank 1 is divided from the first fuel tank 1 by a gas-tight partition wall. The tank connection space 11 is thus partly delimited by the shell of the first fuel tank 1 . The tank connection space 12 of the second fuel tank 2 is configured in a similar way. The tank connection spaces 11 , 12 could also be arranged apart from the fuel tanks 1 , 2. In that case, double-wall pipes would be needed between the fuel tank 1 , 2 and the respective tank connection space 11 , 12.

In the embodiment of figure 1 , the heat exchanger 6 is arranged in the tank connection space 11 of the first fuel tank 1 . However, the heat exchanger 6 could also be arranged in the tank connection space 12 of the second fuel tank 2. The heat exchanger 6 could also be arranged outside the tank connection spaces 11 , 12.

In the embodiment of figure 1 , a gas valve unit 14 is arranged upstream from the combustion device 3. The gas valve unit 14 controls the supply of fuel into the combustion device 3. The gas valve unit 14 may not be necessary, but the need for a gas valve unit 14 depends on the type of the combustion device 3.

The embodiment of figure 2 is similar to the embodiment of figure 1 and only the differences between the two embodiments are thus described. In the embodi ment of figure 2, the heat exchanger 7 is arranged in the second fuel tank 2. The fuel storage and supply system is provided with a heat exchange line 23, which allows the first fuel to be conducted through the heat exchanger 7 before being supplied to the combustion device 3. The system is provided with a first valve 24 that is arranged upstream from the evaporator 15 for the first fuel for selec tively opening and closing direct fluid communication between the first fuel tank 1 and the evaporator 15. A second valve 25 is arranged in the heat exchange line 23 for selectively opening and closing fluid communication between the first fuel tank 1 and the heat exchanger 7. The first and the second valves 24, 25 could also be replaced by a three-way valve. By means of the valves 24, 25, the first fuel can be selectively conducted to the evaporator 15 either directly or through the heat exchanger 7. By conducting the first fuel through the heat ex changer 7, the first fuel is heated and/or (partly) evaporated. The second fuel in the second fuel tank 2 is cooled down. By controlling the flow of the first fuel through the heat exchanger 7, the conditions in the second fuel tank 2 can be regulated. For instance, cooling of the second fuel may help keeping the second fuel liquefied. By controlling the temperature in the second fuel tank 2, also the evaporation of the liquid phase into the gaseous phase and the condensation from gaseous phase to liquid phase are controlled, and thereby also the pres sure of the second fuel tank 2 is controlled.

The first fuel may be liquefied natural gas (LNG). The main component of natural gas is methane. Natural gas can also comprise ethane, propane and butane, as well as water and carbon dioxide. To produce liquefied natural gas, water, car bon dioxide, heavy hydrocarbons and also some other components are removed from the gas, although small amounts of impurities may be left even after re moval of the undesirable components.

The boiling point of liquefied natural gas depends on its composition, but typi cally natural gas is cooled to approximately -162 °C for liquefaction and stored at a pressure that is close to atmospheric pressure, for instance below 250 kPa of absolute pressure. The first fuel tank 1 can thus be configured to store the first fuel at a temperature of approximately -162 °C. The space holding the LNG is formed by a shell that is made of a cold resistant material. The expression “cold resistant material” refers to a material that can withstand the temperature of liquefied natural gas with a certain safety margin. The material can be, for instance, stainless steel. An insulation layer is arranged around the shell. The insulation layer can be made of, for instance, polyurethane.

The first fuel could also be biogas, which is stored in the first fuel tank 1 as liquefied gas. The term “biogas” refers here to gas having a similar composition as liquefied natural gas but not being from fossil sources. The biogas is thus gas of which main component is methane and which originates from a renewable source. The biogas could be produced, for instance, from organic waste.

Alternatively, the first fuel could be, for instance, hydrogen. The hydrogen could be liquefied hydrogen. The boiling point of hydrogen at atmospheric pressure is approximately -253 °C and the critical temperature is approximately -240 °C. For storing the hydrogen as liquid, the fuel storage and supply system should thus be provided with means for keeping the hydrogen in the first fuel tank 1 at a sufficiently low temperature.

The second fuel could be, for instance, ammonia (NFte). Ammonia could be stored in the second fuel tank 2 as liquefied gas. The boiling point of ammonia at atmospheric pressure is approximately -33.3 °C. Ammonia could thus be stored at a temperature of approximately -34 °C or below to keep it in liquid phase at a pressure that is close to atmospheric pressure. A pressure of 10 bar is needed for keeping ammonia liquid at a temperature of 25 °C. Ammonia could thus be stored as liquid even without regulating the temperature. At least in case of a piston engine, ammonia could be used as a fuel in the combustion device 3 without mixing it with the first fuel.

If the first fuel was liquefied hydrogen, the second fuel could be liquefied natural gas. The second fuel could even be compressed natural gas.

In addition to being operable using the first and the second fuel and/or a mixture of the first fuel and the second fuel, the combustion device 3 could be operable using a third fuel or even further fuels. The combustion device 3 could be oper ated using the third or further fuel alone or mixed with the first and/or the second fuel. The third and/or further fuel could be either a gaseous fuel or liquid fuel. Examples of liquid fuels are light fuel oil, marine diesel oil and heavy fuel oil. The combustion device 3 could thus be operated in a gaseous fuel mode, in which mode one of the gaseous fuels or a mixture of two gaseous fuels is used, or in a liquid fuel mode, in which mode a liquid fuel is used.

Even when the combustion device 3 is operated using a gaseous fuel, it can consume small amounts of liquid fuel. For instance, in case of a piston engine, the engine can use liquid pilot fuel for igniting the gaseous fuel, which is used as a main fuel. An expression like “operated using a first gaseous fuel” does thus not necessarily mean that the combustion device 3 is operated using solely the gaseous fuel. However, when the combustion device 3 is operated using a gaseous fuel, combustion of the gaseous fuel forms major part of the heat re lease of the combustion device 3. For instance, in case a liquid pilot fuel is used in a piston engine, the combustion of the liquid pilot fuel can form less than 5 percent of the total heat release of the engine 3.

Figure 3 shows a marine vessel comprising a fuel storage and supply system according to an embodiment of the invention. The marine vessel can be, for instance, a cruise ship, ferry, tug, container ship, bulk carrier, tanker or some other type of cargo ship. The marine vessel comprises at least one combustion device 3 that is configured to be operable using at least a first fuel and a second fuel either separately or as a mixture.

The combustion device 3 is preferably an internal combustion engine. The inter nal combustion engine 3 may be, for instance, a piston engine or a gas turbine. In case of a piston engine, the engine 3 can be a four-stroke engine or a two- stroke engine. The engine 3 can be a main engine of the vessel, i.e. an engine forming part of a propulsion system of the vessel. The engine 3 could be con nected mechanically, hydraulically or electrically to a propeller or other propul sion device. The engine 3 could also be an auxiliary engine, i.e. an engine driv- ing a generator for producing electricity for electrical systems of the vessel.

The vessel can comprise several engines that can be operated using the first fuel and the second fuel and possible further fuels. For instance, the vessel can comprise a main engine and one or more auxiliary engines that can be operated using at least the first fuel and the second fuel. Alternatively, the vessel could comprise two or more auxiliary engines that can be operated using at least the first fuel and the second fuel. The vessel can further comprise one or more en gines that can be operated using a fuel other than the first fuel and the second fuel. The vessel could thus comprise for instance an engine that can be operated only using liquid fuel. In case the vessel comprises one or more engines that can be operated using a liquid fuel either solely or in connection with the use of a gaseous fuel, the vessel can be provided with one or more liquid fuel tanks for storing liquid fuel.

It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the ap- pended claims.