Large turbocharged 2-stroke uniflow crosshead internal combustion engine

Abstract

A large turbocharged two-stroke uniflow crosshead internal combustion engine is disclosed having an operating mode mainly fuelled by a low flash point fuel such as ammonia, methanol, ethanol, DME, methane, ethane or LPG. The engine comprises a low flash point fuel system (30) having a fuel supply line (36) and a fuel return line (38) and configured to supply pressurized fuel to a fuel valve (50) located in the cylinder cover or cylinder liner, and a purge system for removing the low flash point fuel from the fuel supply line and the fuel return line, the purge system comprising means (40, 75, 41) for passing a liquid such as water or diesel through the fuel supply line and the fuel return line (36, 38) of the low flash point fuel system. This arrangement allows the low flash point fuel to be purged from the fuel system in a safe and economical manner. The low flash point fuel may be collected in a tank for reuse as fuel. The complexity and cost of handling nitrogen and fuel mixtures is eliminated by not using an inert gas such as nitrogen for purging.

Description

Technical Field

[0001] The present invention relates to a large turbocharged two-stroke uniflow crosshead internal combustion engine having at least one operating mode with a low flash point fuel such as ammonia, methanol, ethanol, DME, methane, ethane or LPG as the main fuel. This engine has at least one cylinder having a cylinder liner, a reciprocating piston within the cylinder liner, and a cylinder cover covering itself, a combustion chamber formed between the reciprocating piston within the cylinder and the cylinder cover, a fuel supply line and a fuel return line, and at least one fuel valve arranged in the cylinder cover or the cylinder liner, and a low flash point fuel system configured to supply pressurized fuel, and a purge system for removing the low flash point fuel from the fuel supply line and the fuel return line. Background of the Invention

[0002] Large turbocharged two-stroke uniflow crosshead internal combustion engines are commonly used as prime movers for large ocean-going vessels such as container ships and power plants. This type of engine is very often operated with heavy oil or fuel oil such as diesel.

[0003] In recent years, in large two-stroke diesel engines, there has been a demand to be able to handle other types of fuels such as ammonia, methanol, ethanol, LPG, methane, ethane, DME, etc. An engine that can operate in both a fuel oil mode operating only with fuel oil and an alternative fuel mode operating with an alternative fuel and pilot fuel oil is often called a dual fuel engine.

[0004] Dual fuel engines for low flash point fuels have a dedicated fuel system because of the high volatility of the fuel. In any case, for engine shutdown, fuel conversion to fuel oil, and maintenance, a device for completely discharging fuel from the fuel system such as fuel pipes, valves, pumps, etc. is required.

[0005] Known dual-fuel engines are equipped with a purge system to remove low-flashpoint fuels from the fuel system. Such a purge system first ensures that the fuel system is depressurized, and then purges with nitrogen. Nitrogen purging has the disadvantage of requiring the treatment and disposal of a mixture of nitrogen and fuel gas. Also, nitrogen must be produced on board, which increases costs in terms of OPEX and CAPEX.

[0006] EP3203053B1 describes a gas turbine, and US9732713B2 describes a dual-fuel engine, in which a gaseous fuel, such as a low-flashpoint fuel like ammonia, is introduced or injected into the combustion zone during gas-mode operation. In both documents, the fuel lines are purged by a vacuum pump system. While vacuum pumps for ammonia exist, they are neither simple nor inexpensive to use in connection with large turbocharged two-stroke uniflow crosshead internal combustion engines. Furthermore, small amounts of liquid, in the form of NH3 and a small amount of water, may remain in the pipes and must be removed by boiling, which is expected to make purging time-consuming. Additionally, repeated exposure to negative pressure during purging and excessive pressure during gas-mode operation can cause problems in keeping the piping system airtight for low-flashpoint fuel systems.

[0007] The present invention also relates to a method for operating a large turbocharged two-stroke uniflow crosshead internal combustion engine, as described above and in the appended claims. [Overview of the project]

[0008] The object of the present invention is to provide a large turbocharged two-stroke uniflow crosshead internal combustion engine of the type described at the beginning, in which the above-mentioned problems relating to purging low-flash-point fuel from the fuel system are reduced at least significantly.

[0009] The aforementioned and other issues are resolved by the features described in the independent claims. More specific implementations will become apparent from the dependent claims, specification, and drawings.

[0010] According to the first aspect, a large turbocharged two-stroke uniflow crosshead internal combustion engine is provided having at least one operating mode with a low flash point fuel as the primary fuel, such as ammonia, methanol, ethanol, DME, methane, ethane, or LPG. This engine is, A cylinder comprising a cylinder liner, a reciprocating piston within the cylinder liner, and a cylinder cover that covers itself, A combustion chamber formed between the reciprocating piston and the cylinder cover in the cylinder, A low flash point fuel system comprising a fuel supply line and a fuel return line, and configured to supply pressurized fuel to at least one fuel valve located in the cylinder cover or the cylinder liner; A purge system for removing low-flashpoint fuel from the fuel supply line and the fuel return line; Equipped with, The purge system is characterized by providing means for passing a liquid such as water or diesel through at least the fuel supply line and the return line.

[0011] Therefore, low flash point fuels can be safely and economically discharged from the fuel system. Low flash point fuels may also be recovered in a tank for reuse as fuel. Handling mixtures of nitrogen and fuel is complex and costly, but this is not necessary because inert gases such as nitrogen are not used for purging. In the case of ammonia as the low flash point fuel, the absence of an inert purge gas such as nitrogen is particularly advantageous because subsequent cleaning (scrubbing) becomes much easier. This is because the dimensions of the scrubber (cleaning machine) do not need to be determined for nitrogen ventilation, allowing it to be made smaller and less expensive.

[0012] Low flash point fuels such as ammonia, methanol, ethanol, and DME are soluble in water, while low flash point fuels such as methane, ethane, and LPG are not. Therefore, if the liquid used in the purge system is water, the water introduced through the low flash point fuel system will either mix with the former or simply push out the latter, and there will be no contamination by the purge gas.

[0013] If the low flash point fuel is insoluble in water, such as methane, ethane, or LPG, the liquid used in the purge system may be diesel or a similar liquid.

[0014] The means for guiding a liquid such as water or diesel through at least the fuel supply line and return line of the low flash point fuel system may comprise any suitable means. Preferably, however, the means for guiding the liquid comprises a liquid reservoir, a liquid conduit connecting the liquid reservoir to the low flash point fuel system, and a valve for opening and closing the liquid conduit, the valve being an on / off valve, a butterfly valve, or a similar valve.

[0015] In one embodiment of the present invention, the liquid reservoir may be positioned at a height that imparts sufficient potential energy to the liquid to guide it through the low flash point fuel system. In such an embodiment, the means for guiding the liquid through the low flash point fuel system may further include a liquid pump located in the liquid pipeline.

[0016] In one alternative embodiment, the liquid reservoir may be positioned at any suitable height, and the means for guiding the liquid through the low flash point fuel may include a liquid pump positioned to connect to the liquid pipeline.

[0017] In one embodiment of the present invention, particularly when the fuel is liquid and / or under constant positive pressure, it may be advantageous to purge a portion of the fuel from at least the fuel supply line and return line of the low flash point fuel system before introducing the liquid through these lines. In such an embodiment, the purge system may have means for depressurizing and discharging the liquid fuel from the low flash point fuel system into a tank. Such means for depressurizing and discharging the liquid fuel from the low flash point fuel system into a tank can consist of any suitable means, but preferably comprises a valve such as an on / off valve or butterfly valve opening into a pipeline connecting the low flash point fuel system to the tank.

[0018] Each of the purge systems may include first and second pipelines connected to the low flash point fuel system. The first pipeline is used to guide liquid low flash point fuel to a tank. The second pipeline may be connected to a low flash point fuel absorption system if the gaseous low flash point fuel is toxic, such as gaseous ammonia, and to an exhaust port if the gaseous low flash point fuel is not toxic.

[0019] When low flash point fuel is a gas under atmospheric conditions, the discharge of low flash point fuel from the fuel system may be carried out in a two-stage routine. The first stage of discharge is partially carried out by gravity, due to the inclination of the fuel line, and partly by the partial pressure of the fuel with an absolute pressure of 1 bar or more. Typically, the pressure in a low flash point fuel system is about 30 to 80 bar during operation of an engine using that fuel.

[0020] Once this depressurization and discharge step is complete, the fuel system is often filled with a two-phase blend of fuel. The inner surface of the pipes is wet, and there are pockets of liquid fuel in places that may not be drained by gravity. Once some of the low flash point fuel has been discharged from the fuel system by discharge to a tank, the remainder is forcibly discharged from the system in a second discharge step by passing a liquid such as water or diesel through the low flash point fuel system and sending it to a tank. If the low flash point fuel is liquid at atmospheric pressure and normal ambient temperature, such as methanol or ethanol, it can be simply discharged to a tank in the first step, which is done by gravity.

[0021] To completely empty the fuel system from low-flash-point fuels, the purge system may further include a ventilation system for forcing air into the fuel lines. This ventilation system shall include at least a valve to an air inlet or a valve to a source of pressurized air or inert gas. Before forcing air into the fuel lines, it may be advantageous to drain as much water as possible into a drain tank by gravity.

[0022] In one embodiment, the purge system may include a knockout drum configured to separate the liquid portion from the gaseous portion of the expanded low flash point fuel. Such embodiments are particularly relevant to low flash point fuels such as ammonia, LPG, and DME. In such embodiments, the purge system may also include means for guiding portions of both low flash point fuels into the same tank or separate tanks. In such embodiments, the means for guiding portions of the gaseous low flash point fuel may include liquefaction means, for example, in the form of pressurizing means and / or cooling means.

[0023] However, in such an embodiment having a knockout drum, the purge system may have means for guiding a portion of the low flash point fuel gas to a low flash point fuel absorption system. This is particularly relevant when the low flash point fuel is ammonia. Thus, when the low flash point fuel is ammonia, the low flash point fuel absorption system may have at least one pressure vessel that is at least partially filled with water during use to absorb ammonia into water to form aqueous ammonia. Aqueous ammonia is an aqueous solution of ammonia. The low flash point fuel absorption system may have at least one absorption tank. When the low flash point fuel absorption system has a plurality of absorption tanks, it is preferable that they are arranged in series.

Brief Description of the Drawings

[0024] Hereinafter, the present invention will be described in more detail while referring to exemplary embodiments shown in the drawings. [Figure 1] It is a view showing an overview of a large two-stroke diesel engine according to an exemplary embodiment as seen from the front direction. [Figure 2] It shows an overview of the large two-stroke engine of FIG. 1 as seen from the back direction. [Figure 3] It is a schematic representation of the large two-stroke engine of FIG. 1. [Figure 4] It is a diagrammatic representation of an engine equipped with a low flash point fuel system and a low flash point fuel purge system. In this engine, the low flash point fuel is ammonia. [Figure 5] It is a diagrammatic representation of an engine equipped with a low flash point fuel system and a low flash point fuel purge system. However, in this engine, the low flash point fuel is ammonia. Detailed Description

[0025] In the following detailed description, the present invention will be described in relation to a large turbocharged two-stroke crosshead internal combustion engine. However, it should be noted that in some embodiments, the internal combustion engine may be of a different type. The large turbocharged two-stroke uniflow crosshead internal combustion engine may be of a high-pressure type or a low-pressure type. In the high-pressure type, fuel is injected at or near top dead center of the piston and ignited by compression. However, typically, pilot ignition using an igniter, such as fuel oil, is also used to ensure reliable ignition. In the low-pressure type, fuel is introduced at a relatively low pressure as the piston moves towards top dead center. In this detailed description, in the example referring to Figure 4, the low flash point fuel is ammonia, and in the example referring to Figure 5, the low flash point fuel is methanol or ethanol.

[0026] Figures 1-3 depict a turbocharged large low-speed two-stroke diesel engine. This engine has a crankshaft 8 and a crosshead 9. Figure 3 is a schematic representation of the turbocharged large low-speed two-stroke diesel engine, along with its intake and exhaust systems. In this embodiment, the engine has six cylinders in series. A turbocharged large low-speed two-stroke diesel engine typically has 4 to 14 cylinders arranged in series. These cylinders are supported on a cylinder frame 23. The cylinder frame 23 is supported on an engine frame 11. Such engines can be used, for example, as the main engine of a ship or as a stationary engine to power a generator in a power plant. The total output of the engine can be in the range of, for example, 1,000 to 110,000 kW.

[0027] The engine of this embodiment is a two-stroke uniflow compression-ignition dual-fuel engine. Each cylinder liner 1 is provided with a scavenging port 18 in its lower region and an exhaust valve in the center of its top. The engine has at least one low-flash-point fuel mode, which is operated with a low-flash-point fuel such as ammonia, methanol, ethanol, DME, methane, ethane, or LPG, and at least one conventional fuel mode, which is operated with a conventional fuel such as fuel oil (marine diesel) or heavy oil.

[0028] Scavenging air is introduced through the scavenging receiver 2 to the scavenging port 18 of each cylinder 1. The piston 10 reciprocates between bottom dead center (BDC) and top dead center (TDC) in the cylinder liner 1, compressing the scavenging air. In the illustrated embodiment, in low flash point fuel mode, fuel in the form of a low flash point fuel such as ammonia, methanol, ethanol, DME, methane, ethane, or LPG is injected into the combustion chamber in the cylinder liner 1 at or near TDC through a high-pressure fuel valve 50 located in the cylinder cover 22. In another embodiment, the low flash point fuel may be introduced somewhere between the scavenging port 18 and TDC through a fuel valve 50' located in the cylinder liner. Combustion occurs, and exhaust gases are produced. Each cylinder cover 22 is provided with two or more fuel valves 50. The fuel valves 50 may be configured to inject only one specific type of fuel (e.g., ammonia). In that case, one or more fuel valves 50 for injecting conventional fuel into the combustion chamber would also be provided. Therefore, the engine will have two or more fuel valves. If the fuel valves 50 are configured to inject both a low flash point fuel such as ammonia and conventional fuel, there may be one or more fuel valves 50 in each cylinder. The fuel valves 50 are located in the cylinder cover 22, around the exhaust valve 4 which is located in the center of the cylinder cover 22. Although not shown, in some embodiments, additional (usually small) fuel valves may be located in the cylinder cover, configured to inject an igniter to ensure ignition of the low flash point fuel. The igniter is conventional diesel fuel, but may be other forms of ignition accelerators such as dimethyl ether (DME) or hydrogen. Since the engine may be a dual-fuel engine, the engine may also have a conventional fuel supply system (not shown) for supplying conventional fuel to the fuel valves 50. In some embodiments, this main fuel injection system may also be used for the purpose of ensuring ignition of the ammonia fuel.

[0029] When the exhaust valve 4 opens, the exhaust flows through the exhaust duct provided in the cylinder 1 to the exhaust receiver 3, then through the selective catalytic reduction reactor (SCR reactor) 28, through the first exhaust pipe 19, and to the turbine 6 of the turbocharger 5. From there, the exhaust flows through the second exhaust pipe 25 to the economizer 20, and is then released into the atmosphere from the outlet 21. The SCR reactor reduces emissions of exhaust gases, especially NOx.

[0030] The turbine 6 drives the compressor 7 via a shaft. The compressor 9 is supplied with outside air through the air intake 12. The compressor 7 sends the compressed scavenging air to the scavenging pipe 13, which is connected to the scavenging receiver 2. The scavenging air in the scavenging pipe 13 passes through the intercooler 14 to cool the scavenging air.

[0031] The cooled scavenging air passes through an auxiliary blower 16 driven by an electric motor 17. The auxiliary blower 16 compresses the scavenging airflow when the compressor 7 of the turbocharger 5 cannot provide sufficient pressure for the scavenging tank 2, i.e., when the engine is under low or partial load. When the engine is under high load, the compressor 7 of the turbocharger can supply sufficiently compressed scavenging air, so the auxiliary blower 16 is bypassed by a check valve 15 and the electric motor 17 is stopped.

[0032] In Figure 4, the low flash point fuel is ammonia. Therefore, in low flash point fuel mode, the engine operates using ammonia as the primary fuel. Ammonia is supplied to the ammonia valve 50 at a nearly constant pressure and temperature. The ammonia is supplied to the ammonia valve 50 in the liquid phase. This ammonia may be ammonia water (a mixture of ammonia and water).

[0033] Conventional fuel systems are well known and therefore are not illustrated or described in detail. The ammonia fuel system 30 supplies liquid-phase ammonia to the ammonia valve 50 at an intermediate supply pressure (e.g., 30-80 bar). The engine is compression-ignited, and the fuel valve 50 is equipped with a pressure booster that significantly increases the pressure of the ammonia fuel. The pressure booster increases the pressure of the ammonia fuel from intermediate to high, thereby allowing the ammonia fuel to be injected at a pressure higher than the engine's compression pressure. Typically, the injection pressure of a compression-ignited engine is higher than 300 bar.

[0034] The ammonia fuel system 30 is disclosed in more detail in Figure 4. This ammonia fuel system has a purge system. Ammonia is stored in liquid phase at approximately 17 bar in a pressurized storage tank 31. Ammonia can be stored in liquid phase in the ammonia storage tank 31 at 8.6 bar or higher at an ambient temperature of 20°C. However, in order to maintain the liquid phase even when the ambient temperature rises, it is preferable to store ammonia at 17 bar or higher.

[0035] A low-pressure ammonia supply line 32 connects the outlet of the ammonia storage tank 31 to the inlet of the intermediate-pressure supply pump 35. The low-pressure supply pump 33 pressurizes the liquid ammonia from the tank 31 so that it passes through the filter device 34 to the inlet of the intermediate-pressure supply pump 35. The intermediate-pressure supply pump 35 pumps liquid ammonia from the intermediate-pressure ammonia supply line 36 to the fuel valve 50. Some of the liquid ammonia supplied to the fuel valve 50 is injected into the combustion chamber of the engine, while another portion is returned to the ammonia return line 38. The ammonia return line 38 connects the return port of the fuel valve 50 to the low-pressure supply line 32. Thus, some of the liquid ammonia fuel is recirculated to the inlet of the intermediate-pressure supply pump 35.

[0036] For example, if operation using ammonia fuel is stopped due to a malfunction of the ammonia fuel system 30 or other reasons such as switching to a conventional fuel, the ammonia fuel system 30 is purged to remove ammonia from the system. In this case, a purge system, as further described below, is used.

[0037] A first purge line 42, including a first purge valve 43, connects a medium-pressure ammonia supply line 36 to a knockout drum 46. In some embodiments, a return line is provided in the system, as shown in Figure 4. In this case, a second purge line 44, including a second purge valve 45, connects an ammonia return line 38 to the knockout drum 46. During the purging operation, the first and second purge valves 43 and 45 are opened, and residual ammonia fuel expands into the knockout drum 46 from the ammonia fuel supply line 36 and the ammonia return line 38. This expansion is first caused by excess pressure and gravity in the ammonia fuel supply system 30 in a first removal step, and then by water introduced from the water reservoir 40 through the ammonia fuel supply line 36 and the ammonia return line 38 in a second removal step. The knockout drum 46 is configured to separate liquid ammonia from gaseous ammonia. A liquid ammonia outlet is provided in the lower region of the knockout drum 46, which is connected to a recovery tank 57 via a pipeline 72 and a valve 59. A gas-phase ammonia exhaust line 48, including a valve 49, connects the inside and outside of the knockout drum 46, thereby discharging gas-phase ammonia from the recovery tank 57 and returning it to the knockout drum 46. In some embodiments not shown, liquid-phase ammonia in the recovery tank 57 is transported to an ammonia storage tank 31 for use as ammonia fuel. The gas-phase ammonia outlet of the knockout drum 46 is connected to an ammonia absorption system 60 through a third purge line 47.

[0038] During the first possible discharge step, the fuel line is inclined, so low flash point fuels, such as ammonia, move at least partially by gravity. The low flash point fuel is also partially pushed out by the partial pressure of the fuel, which is greater than 1 bar in absolute pressure. During the first possible discharge step, the valve 74 of the third purge line 47 is opened. As a result, the gaseous ammonia in the knockout drum 46 is sent to the ammonia absorption system 60 by the excess pressure.

[0039] After as much ammonia as possible has been discharged from the ammonia fuel system by gravity and excess pressure, valve 41 is opened, and water flows from the water reservoir 40 through the water line 75 into the ammonia fuel system. Thus, the remaining ammonia fuel from the ammonia fuel system and the purge system, which have ammonia fuel lines 36, 38 and purge lines 42, 44, is led to the knockout drum 46. A water pump 73 can also be placed connected to the water line 75 to add additional energy to the water. The gaseous ammonia in the knockout drum 46 is led to the ammonia absorption system 60 through the purge line 47.

[0040] To completely empty the ammonia fuel system and force any remaining ammonia into the knockout drum 46 and possibly the ammonia absorption system 60, the purge system may be provided with a ventilation system for forcing atmospheric air through the fuel lines 36, 38, purge lines 42, 44, 47 and the knockout drum 46. This ventilation system comprises at least a pressurized air source 81 and a valve 82 that opens into the ammonia fuel system.

[0041] Before forcing the atmosphere through the fuel line, it may be advantageous to drain as much water as possible (including dissolved residual low-flashpoint fuel) into the drain tank 79 by gravity. For this purpose, as shown in Figure 4, the purge system may include a water discharge line 77 and valve 58 for draining water from the purge line 47 into the drain tank 79, and a valve 78 for draining water from the fuel lines 36, 38 and the purge line 42 into the drain tank 79.

[0042] In the illustrated embodiment, the ammonia absorption system 60 comprises a cascade of three absorption tanks 61, 63, and 65 arranged in series. This cascade is at least partially filled with water during use to absorb ammonia into water and form aqueous ammonia.

[0043] Ammonia water is an aqueous solution of ammonia.

[0044] During the purging operation, gaseous ammonia is sent via the purge line 47 to a cascade of three absorption tanks arranged in series. This cascade includes a first absorption tank 61, an intermediate absorption tank 63, and a final absorption tank 65.

[0045] A fourth vent 66 is provided in the final absorption tank 65. The fourth vent 66 connects the final absorption tank 65 to the outside. In some embodiments, there are more than three absorption tanks. This is to lower the ammonia concentration above the water in the final absorption tank, and thus lower the ammonia concentration in the gas discharged from the fourth vent 66.

[0046] The absorption efficiency of the cascade of multiple absorption tanks is maintained by periodically replacing the water in the final absorption tank 65. This water is supplied from a pressurized water (freshwater) source 71. Water containing some ammonia is reused in the upstream tanks. Thus, the water in the final tank 65 that has absorbed some ammonia is replaced with water from the water source 71, but the replaced water in the final tank 65 is sent to the intermediate absorption tank 63 through a first return line 67 controlled by a first return valve 65. Similarly, water from the intermediate absorption tank 63 is sent to the first absorption tank 61 through a second return line 69 controlled by a second return valve 70. The system is configured to compensate for water evaporating from absorption tanks 61, 63, and 65, and for ammonia water removed from the first absorption tank 61. That is, the water levels in absorption tanks 61, 63, and 65 are maintained between the minimum and maximum heights shown by the dashed lines in Figure 4.

[0047] Ammonia vapor above the water in the first absorption tank 61 flows through the first ammonia discharge line 62 to the intermediate absorption tank 63. Ammonia vapor above the water in the intermediate absorption tank 63 flows through the second ammonia discharge line 64 to the final absorption tank 65. This process is preferably carried out by the pressure of the purging process.

[0048] The ammonia concentration in the fourth vent 66 is low enough to be permitted for release into the surrounding environment. However, if necessary to comply with regulations, the amount of ammonia released from the vent tower can be further reduced by using additional absorption columns. The absorption medium used in such absorption columns is an acid. The acid adds a proton to ammonia in aqueous solution, forming ammonium hydroxide. Thus, the amount of ammonia released into the environment is reduced.

[0049] During operation, the ammonia concentration of the water in the first absorption tank 61 is higher than the ammonia concentration of the water in the intermediate absorption tank 63, and the ammonia concentration of the water in the intermediate absorption tank 63 is higher than the ammonia concentration of the water in the final absorption tank 65.

[0050] Ammonia water from the first absorption tank 61 is removed from the first absorption tank 61 through a first ammonia water return line 51 having a return pump 52. A second ammonia water return line 52, having a return pump 52' and a first return valve 54, connects the first ammonia water return line 51 to a low-pressure ammonia supply line 32. When the first return valve 54 opens, the relatively high-concentration ammonia water from the first absorption tank 61 mixes with the fuel from the ammonia storage tank 31. Thus, the ammonia absorbed by the ammonia absorption system 60 is reused as fuel in the engine. A third ammonia water return line 55, having a second return valve 56, connects the first ammonia water return line 51 to a reducing agent inlet associated with the SCR reactor 28. This reducing agent inlet may be part of the SCR reactor 28 or may be located in the exhaust path upstream of the SCR reactor 28. When the second return valve 56 opens, the ammonia absorbed by the ammonia absorption system 60 is reused as a reducing agent in the SCR reactor 28.

[0051] The cascade of water tanks 61, 63, and 65 is entirely passive. That is, there are no pumps, nor are there any auxiliary systems available to stop ammonia absorption if necessary. Therefore, the system is inherently reliable and available only when needed.

[0052] In some embodiments, the low-pressure ammonia supply line 32, the medium-pressure ammonia supply line 36, and the ammonia return line 38 are configured as double-walled pipes with a space between the inner and outer pipes, either completely or partially. It is preferable that a detection system is provided to detect the presence of ammonia in the space between the inner and outer tubes. When ammonia is detected in the space, it is possible to stop the ammonia-fueled operation of the engine, subsequently purging the ammonia fuel system, and absorbing any remaining ammonia by the ammonia absorption system 60.

[0053] The pumps and valves of the fuel system 30, the purge system, and the ammonia absorption system 60 are connected to the electronic control unit 100 by wire or wireless means. The electronic control unit 100 is configured to control these elements, for example, by adjusting the pump speed or controlling the opening and closing of valves. This enables the fuel system, the purge system, and the ammonia absorption system to operate as described above.

[0054] In Figure 5, the low flash point fuel is methanol or ethanol. Therefore, in low flash point fuel mode, the engine operates using methanol or ethanol as the primary fuel. Methanol or ethanol is supplied to the fuel valve 50 at a nearly constant pressure and temperature.

[0055] The methanol or ethanol fuel system 30 shown in Figure 5 corresponds to the ammonia fuel system 30 described in relation to Figure 4, and comprises a fuel storage tank 31, a supply line 32, a low-pressure supply pump 33, a filter mechanism 34, and a medium-pressure supply pump 35 that delivers fuel to the fuel supply line 36.

[0056] When operation with methanol or ethanol fuel is discontinued, the methanol or ethanol fuel system 30 is purged to remove methanol or ethanol from the fuel system. A purging system, as further described below, is used for this purpose.

[0057] The purging process is performed in three steps. In the first purging step, purge valves 92, 93 and 86 are opened, and residual methanol or ethanol fuel is sent from methanol or ethanol fuel supply and return lines 36 and 38 to the first drain tank 87 by gravity and, optionally by a pressurized pump 35 (if not bypassed). Then, purge valve 86 is closed. In the second purging step, vent valves 85 and 91 of vent lines 84 and 90 are opened, respectively, and water is introduced from the water reservoir 40 to the methanol or ethanol fuel supply and return lines 36 and 38, optionally assisted by a water pump 73 located in water line 75, until the methanol or ethanol fuel supply and return lines 36 and 38 are completely filled with water. Subsequently, in the third purging step, the purge valve 88 is opened, and a mixture of water and methanol or water and ethanol is discharged from the methanol or ethanol fuel supply and return lines 36, 38 into the second drain tank 89 by gravity and, optionally, by the pressurized pump 35 (if not bypassed). Once the discharge of the fuel supply and return lines 36, 38 is complete, the purge valve 88 is closed. Purge steps 2 and 3 can be repeated as necessary to achieve the required purging of the fuel supply and return lines 36, 38.

[0058] A first drain tank 87 contains nearly clean methanol or ethanol fuel, and a second drain tank 89 contains water with a low concentration of methanol or ethanol. The clean methanol or ethanol fuel in the first drain tank can, advantageously, be returned to the fuel storage tank 31 for use as fuel. The mixture of water and methanol or ethanol in the second drain tank 89 can also be returned to the fuel storage tank 31. In one embodiment of the present invention, the system may have only one drain tank.

[0059] When the low flash point fuel is water-insoluble, such as methane, ethane, and LPG, the liquid used in the purge system may be diesel or a similar liquid, and the purge system can essentially correspond to the system shown in Figure 4, except that tank 40 instead contains diesel or a similar liquid. Furthermore, the need for an absorption system is limited and can be omitted.

Claims

1. A large turbocharged two-stroke uniflow crosshead internal combustion engine having at least one operating mode that primarily uses a low flash point fuel such as ammonia, methanol, ethanol, DME, methane, ethane, or LPG as the main fuel, and at least one operating mode that primarily uses a conventional fuel such as marine diesel oil or heavy oil, wherein the engine is A cylinder comprising a cylinder liner, a reciprocating piston within the cylinder liner, and a cylinder cover that covers itself, A combustion chamber formed between the reciprocating piston and the cylinder cover in the cylinder, A low flash point fuel system comprising a fuel supply line and a fuel return line, wherein pressurized low flash point fuel is supplied to at least one fuel valve disposed in the cylinder cover or the cylinder liner through the fuel supply line, and the low flash point fuel recirculated from the at least one fuel valve is received through the fuel return line connected to the return port of the at least one fuel valve; A purge system for removing low-flashpoint fuel from the fuel supply line and the fuel return line; An apparatus comprising the purging system, wherein the purging system is provided with means for passing a liquid such as water or diesel through at least the fuel supply line and the fuel return line of the low flash point fuel system.

2. The engine according to claim 1, wherein the means comprises a liquid reservoir, a liquid pipeline connecting the liquid reservoir to the low flash point fuel system, and a valve for opening and closing the liquid pipeline, the valve being an on / off valve, a butterfly valve, or a similar valve.

3. The engine according to claim 2, wherein the liquid reservoir is positioned at a height that imparts sufficient potential energy to the liquid to pass through at least the fuel supply line and the fuel return line of the low flash point fuel system.

4. The engine according to claim 3, wherein the means comprises a liquid pump arranged to be connected to the liquid pipeline.

5. The engine according to claim 4, wherein the liquid reservoir is positioned at an appropriate height, and the means includes a water pump positioned to be connected to the liquid pipeline.

6. The engine according to claim 1, wherein the purge system comprises means for depressurizing and discharging liquid fuel from the low flash point fuel system to the tank, and the depressurizing and discharging means has a valve such as an on / off valve or a butterfly valve that opens into a pipeline connecting the low flash point fuel system to the tank.

7. The engine according to any one of claims 1 to 6, wherein the purge system comprises a knockout drum configured to separate the liquid portion from the gaseous portion of the expanded low-flashpoint fuel.

8. The engine according to claim 7, wherein the purging system comprises means for directing a portion of a gaseous low-flashpoint fuel, such as ammonia, to an ammonia absorption system having at least one absorption tank.

9. The engine according to any one of claims 1 to 6, wherein the purge system further comprises a ventilation system for forcibly passing air through the fuel supply line and the fuel return line, the purge line and the knockout drum, the ventilation system comprising at least a pressurized air source and a valve opening to the low flash point fuel system.

10. The engine according to any one of claims 1 to 6, wherein the purge system comprises a drain tank, a water discharge line and valve for discharging water from the purge line to the drain tank, and a valve for discharging water from the fuel line and purge line to the drain tank.

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