Large two-stroke uniflow-scavenged turbocharged internal combustion engine configured to reduce ammonia slip, and method for reducing ammonia slip in such an engine

Abstract

To provide a large two-stroke uniflow scavenged turbocharged internal combustion engine, and a method for reducing ammonia slips in such an engine.SOLUTION: The method includes a) operating an engine with ammonia as a primary fuel, thereby producing an exhaust gas stream containing NOx and NH3, b) adjusting the ratio between ammonia and NOx in the exhaust gas stream by adding a controlled NOx stream to the exhaust gas, and c) then sending the exhaust gas stream to SCR.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The subject matter disclosed herein relates to large two-stroke internal combustion engines, and in particular to large two-stroke uniflow scavenged turbocharged internal combustion engines, which in at least one mode of operation are operated using ammonia (NH3) as the primary fuel combusted within the engine.

[0002] Large two-stroke uniflow scavenged turbocharged internal combustion engines are typically used in the propulsion systems of large ships and as prime movers in power plants. Their size, weight, and power output set them apart from other combustion engines, placing this type of compression ignition engine in a unique category.

[0003] Internal combustion engines have traditionally been powered primarily by hydrocarbon fuels, such as fuel oils like diesel and fuel gases like natural gas or petroleum gas. Combustion of hydrocarbon fuels results in the production of greenhouse gases, including carbon dioxide (CO2), which can contribute to air pollution and climate change. Unlike impurities in petroleum fuels, which result in by-product emissions, the production of CO2 is inevitable when hydrocarbons are burned. The energy density and CO2 emissions of a particular fuel depend on the length of the hydrocarbon chain and the complexity of the hydrocarbon molecule. Therefore, gaseous hydrocarbon fuels emit less CO2 than liquid hydrocarbon fuels. However, gaseous hydrocarbon fuels are difficult and expensive to handle and store. To reduce CO2 emissions, non-hydrocarbon fuels are being developed.

[0004] Ammonia (NH3) is a compound derived from petroleum, biomass, and renewable or sustainable energy sources (wind, solar, hydroelectric, nuclear, geothermal). NH3 produced using renewable or sustainable energy sources has virtually zero carbon emissions when burned, emitting no CO2, SOx, particulate matter, or unburned hydrocarbons.

[0005] NH3 has been tested and used on a small scale in small internal combustion engines such as automobiles, but has not yet been used in large two-stroke internal combustion engines.

[0006] In large two-stroke internal combustion engines, combustion gases produced by the combustion of ammonia (NH3) can contain both NOx and NH3. NOx is limited by international regulations such as the International Maritime Organization (IMO) Tier II and Tier III. While no formal regulations exist for NH3, realistic tolerance levels are significantly lower. Achieving acceptable NH3 slip in exhaust gases is difficult without an NH3 removal system (aftertreatment system) in the engine's exhaust system. Without countermeasures, exhaust gases containing unacceptable amounts of NH3 may be emitted into the atmosphere.

[0007] Known systems for removing NH3 from exhaust gases use an ammonia slip catalyst (ASC or AMOX). NOx is reduced using a selective catalytic reduction (SCR) catalyst. NH3 slip in the exhaust gas is controlled using an ammonia slip catalyst (ASC). The ASC catalyst is placed downstream of the SCR catalyst. In the SCR catalyst, NOx and NH3 react to remove NOx. If NH3 remains after the SCR catalyst for some reason, it is oxidized in the ASC, which removes NH3. The ASC, like the SCR, processes the entire exhaust gas. Therefore, when an ASC is installed in a large two-stroke internal combustion engine, the size of the ASC is similar to that of an SCR catalyst because it must process the entire exhaust gas. SCR catalysts are very voluminous devices. Adding another significantly bulky device is problematic. Another drawback is that nitrous oxide (N2O) can be a by-product of NH3 oxidation in the ASC. Nitrous oxide removal systems are known, but require temperatures exceeding 400°C to be effective. This is an exhaust gas temperature that is not easily achieved in highly efficient marine engines.

[0008] Another known technology is to use wet scrubbers to remove NH3 from the exhaust gases, but this introduces a very large and bulky component, and the wastewater cannot be easily disposed of on board.

[0009] DK202170273 discloses a large two-stroke internal combustion engine according to the preamble of claim 1.

[0010] One object is to provide a large two-stroke internal combustion engine that solves or at least mitigates the above-mentioned problems. Another object is to provide a method for reducing ammonia slip from a large two-stroke internal combustion engine.

[0011] These and other problems are solved by the features of the independent claims. More specific implementations will become apparent from the dependent claims, the description and the drawings.

[0012] According to a first aspect, there is provided a large two-stroke uniflow scavenged turbocharged internal combustion engine having at least one operating mode in which the primary fuel is ammonia. at least one cylinder having a cylinder liner and a reciprocating piston within the cylinder liner, and a cylinder cover covering the cylinder; a combustion chamber defined within the cylinder between the reciprocating piston and the cylinder cover; an intake system for supplying scavenging air to said combustion chamber; an exhaust system for exhausting an exhaust stream produced by combustion of ammonia in the combustion chamber; a turbocharging system having at least one compressor for compressing scavenged air, the compressor being present in the intake system, and at least one exhaust-driven turbine for driving the compressor, the exhaust-driven turbine being present in the exhaust system; an SCR catalyst disposed in the exhaust system, preferably upstream of the exhaust-driven turbine; means for adding a NOx-containing gas stream to the exhaust stream within or upstream of the SCR catalyst; Equipped with.

[0013] The inventors have realized that ammonia slip can be avoided if ammonia is reliably reduced in the SCR catalyst. However, if there is insufficient NOx present in the SCR catalyst, i.e., if the molar ratio of ammonia to NOx exceeds 1, ammonia will not be reduced. The ratio of ammonia to NOx in the exhaust gas discharged from the combustion chamber cannot always be (accurately) controlled or (accurately) predicted. By supplying a NOx-containing gas stream to the exhaust gas, the amount of NOx required for complete ammonia reduction can always be present in the SCR catalyst, and all or at least nearly all of the ammonia present in the exhaust gas can be reduced in the SCR catalyst. Only a relatively small gaseous NOx stream is required to achieve the desired results.

[0014] In one implementation of the first approach, the NOx-containing gas stream is produced by treating a stream of ammonia and air with an oxidation catalyst to convert the ammonia to NO. While this process requires the addition of some heat, the amount is small compared to the existing solutions discussed above, and the size of the oxidation catalyst is small compared to known ammonia slip catalyst devices. Consequently, engines according to the first approach are smaller in volume and less expensive to manufacture and maintain.

[0015] 10. The engine of claim 1, further comprising: a controller configured to control a magnitude of the NOx-containing gas flow added to the exhaust flow.

[0016] In one implementation of the first approach, a means is provided for measuring and / or estimating the molar ratio between ammonia and NOx in the exhaust stream.

[0017] In one example implementation of the first approach, the controller is configured to adjust the magnitude of the NOx-containing gas stream as a function of the measured and / or estimated molar ratio.

[0018] In one example implementation of the first approach, the controller 60 is configured to adjust the flow rate of the exhaust gas flow entering the SCR catalyst 40 so that the molar ratio between ammonia and NOx is less than 1, preferably slightly less than 1.

[0019] In one example of an implementation of the first approach, the NOx-containing gas stream contains NOx including NO and NO2, and the engine is provided with a means for adjusting the ratio between NO and NO2 in the NOx-containing gas stream.

[0020] In one example implementation of the first approach, the controller includes a sensor system that provides one or more signals for determining the molar ratio of ammonia to NOx in the exhaust.

[0021] In one implementation of the first aspect, the present invention includes a NOx generation system for generating the NOx-containing gas stream, the NOx generation system preferably having a source for providing an NH3 stream;

[0022] To obtain the NOx-containing gas stream, ammonia from the source is preferably catalytically oxidized to NO and H2O.

[0023] In one example of an implementation of the first approach, the engine is provided with an oxidation catalyst, which is preferably a platinum-rhodium catalyst, and the engine preferably further comprises a source of pressurized ammonia gas to the oxidation catalyst and a source of pressurized air to the oxidation catalyst, which source of pressurized air is preferably scavenged air from the intake system.

[0024] In one example of an implementation of the first aspect, the NOx production system is configured to control the ratio between NO and NO2 in the NOx-containing gas.

[0025] In one example implementation of the first approach, the controller is configured to determine an optimal ratio between NO and NO2 in the gas containing NOx and adjust the ratio between NO and NO2 in the gas containing NOx accordingly.

[0026] In one example of an implementation of the first approach, a catalytic N2O removal system, preferably an iron zeolite catalyst, is provided downstream of the NOx production system to remove N2O that may be produced as a result of side reactions within the NOx production system.

[0027] In one example of an implementation of the first approach, a container for storing NOx-containing gas, preferably for storing the NOx-containing gas under high pressure, is provided, the container preferably being connected to the exhaust system via a control valve, thereby allowing controlled flow of the NOx-containing gas from the container into the exhaust stream.

[0028] In one example implementation of the first approach, the exhaust system includes at least one NOx sensor configured to provide a signal representative of the NOx concentration of the exhaust flow in the exhaust system, and at least one ammonia sensor configured to provide a signal representative of the ammonia concentration of the exhaust flow in the exhaust system.

[0029] In one example implementation of the first approach, the at least one NOx sensor is configured to provide a signal representative of a NOx concentration in the exhaust stream in the exhaust system upstream of a location where the NOx-containing gas stream is added; and / or

[0030] the at least one NOx sensor is configured to provide a signal representative of the NOx concentration in the exhaust stream in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst; and / or

[0031] the at least one NOx sensor is configured to provide a signal indicative of the NOx concentration in the exhaust stream in the exhaust system downstream of the SCR catalyst; and / or

[0032] the at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration in the exhaust stream in the exhaust system upstream of a location where the NOx-containing gas stream is added; and / or

[0033] the at least one ammonia sensor is configured to provide a signal indicative of the ammonia concentration of exhaust gas in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst; and / or

[0034] The at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration in the exhaust stream within the exhaust system downstream of the SCR catalyst.

[0035] In one example implementation of the first aspect, there is provided an ammonia fuel system configured to supply pressurized ammonia to a fuel valve configured to inject or introduce ammonia into the combustion chamber.

[0036] According to a second aspect, there is provided a method for reducing ammonia slip from a large two-stroke, uniflow scavenged, turbocharged internal combustion engine, the method comprising: a) operating an engine using ammonia as a primary fuel, thereby producing an exhaust gas stream comprising NOx and NH3; b) adjusting the ratio between ammonia and NOx in said exhaust gas stream by adding a controlled NOx flow to said exhaust gas; c) then directing said exhaust gas stream to an SCR; Includes.

[0037] In one example implementation of the second approach, the method includes determining a molar ratio between ammonia and NOx in the exhaust gas before adding a controlled NOx flow to the exhaust gas, and adding a controlled NOx flow to the exhaust gas flow if (and preferably only if) the determined molar ratio is greater than or equal to 1.

[0038] In one example implementation of the second approach, the method includes determining the magnitude of the NOx flow required to reduce the determined molar ratio to a level less than 1, preferably slightly less than 1, and adjusting the magnitude of the NOx-containing gas flow to the determined magnitude.

[0039] In one example implementation of the second approach, the method includes determining a desired ratio between NO and NO2 in the NOx-containing gas stream and adjusting the ratio between NO and NO2 in the NOx-containing gas stream.

[0040] In one example implementation of the second aspect, the method includes supplying a pressurized ammonia gas stream and a pressurized air stream to an inlet of an oxidation catalyst to generate a NOx-containing gas stream exiting an outlet of the oxidation catalyst;

[0041] 18. The method of claim 17, wherein the pressurized air flow preferably originates from an intake system, preferably originates from the intake system at a location downstream of a compressor, preferably originates from a location upstream of an intercooler.

[0042] These and other aspects will become more apparent from the accompanying drawings and the examples described below. [Brief explanation of the drawings]

[0043] Various aspects, embodiments, and implementations will be described in detail below with reference to exemplary embodiments shown in the drawings. [Figure 1] 1 shows a front view of a large two-stroke internal combustion engine according to an exemplary embodiment; FIG. [Figure 2] Figure 1 shows an overview of the large two-stroke engine as seen from the rear. [Figure 3] 2 is a diagrammatic representation of an embodiment of the large two-stroke engine of FIG. 1 having an ammonia fuel system and an ammonia slip removal system. [Figure 4] 1 is a flow chart of an embodiment of a process for reducing ammonia slip in a large two-stroke internal combustion engine. [Figure 5] 1 is a flow chart of another embodiment of a process for reducing ammonia slip in a large two-stroke internal combustion engine. [Figure 6] 10 is a flow chart of yet another embodiment of a process for reducing ammonia slip in a large two-stroke internal combustion engine. Detailed explanation

[0044] In the following detailed description, the internal combustion engine will be described with reference to an exemplary crosshead-type, large, slow-speed, two-stroke, uniflow-scavenged, turbocharged internal combustion engine. Note that in some cases, the internal combustion engine may be of another type. A large, two-stroke, slow-speed, uniflow-scavenged, turbocharged internal combustion engine may be a compression-ignition (i.e., high-pressure) engine, in which fuel is injected at or near top dead center (TDC) of the piston. Alternatively, the large, two-stroke, slow-speed, uniflow-scavenged, turbocharged internal combustion engine may be a spark-ignition (also low-pressure, or premixed) engine, in which scavenging air is mixed with fuel before or during compression, and the mixture is ignited or otherwise mixed. Premixed engines typically use a pilot ignition with an additive (e.g., fuel oil) to ensure ignition.

[0045] 1-3 show a turbocharged, large, slow-speed, two-stroke diesel engine. The engine has a crankshaft 8 and a crosshead 9 and operates on the diesel principle, i.e., it is a compression ignition engine. FIG. 3 shows a schematic representation of a turbocharged, large, slow-speed, two-stroke diesel engine, along with its intake and exhaust systems. In this embodiment, the engine has six cylinders arranged in series. Turbocharged, large, slow-speed, two-stroke diesel engines typically have four to fourteen cylinders arranged in series. These cylinders are supported on a cylinder frame 23, which is supported on an engine frame 11. Such an engine can also be used, for example, as a main engine on a ship or as a stationary engine for driving a generator in a power plant. The total power output of the engine can be, for example, from 1,000 kW to 110,000 kW.

[0046] The engine can be configured as a dual-fuel engine. The engine may be a compression ignition engine or a premixed engine. In this embodiment, the engine is a two-stroke uniflow engine, and for each cylinder, a scavenging port 18 is provided in the lower region of the cylinder liner 1, and an exhaust valve 4 is arranged in the top center of the cylinder liner 1. The engine has at least one ammonia mode and at least one conventional fuel mode. In the ammonia mode, the engine is operated on ammonia fuel or an ammonia-based fuel. In the conventional fuel mode, the engine is operated on conventional fuels, such as fuel oil (marine diesel fuel) or heavy oil.

[0047] Scavenging air is introduced into the scavenging ports 18 of each cylinder 1 through the scavenging receiver 2. The piston 10 reciprocates between bottom dead center (BDC) and top dead center (TDC) in the cylinder liner 1, compressing the scavenging air. Fuel (ammonia in the ammonia mode) is injected at high pressure into the combustion chamber in the cylinder liner 1 at or near TDC through multiple fuel valves 50 located in the cylinder cover 22 (Diesel principle). If the engine is configured as a premixed engine, fuel is introduced at relatively low pressure through a fuel inlet valve 50' as the piston moves toward TDC (Otto principle). Typically, two or more fuel inlet valves 50' are provided for each cylinder. The fuel inlet valves 50' may be provided in the cylinder liner above the scavenging ports 18 or in the cylinder cover 22. Following fuel injection, combustion occurs and exhaust is produced. If the engine is configured as a compression ignition engine, 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, two or more fuel valves for injecting conventional fuel into the combustion chamber may also be provided (not shown in FIG. 3 ). The fuel valves 50 are arranged 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, the cylinder cover may be provided with an additional (usually small) fuel valve configured to inject ignition fluid to ensure ignition of the ammonia fuel. The ignition fluid may be, for example, dimethyl ether (DME) or fuel oil. However, other forms of ignition accelerators, such as hydrogen, may also be used. Since the engine may be a dual-cylinder engine, the engine may also include a conventional fuel supply system (not shown) for supplying conventional fuel to the fuel valves 50. In some embodiments, a fuel valve 50′ is arranged along the cylinder liner (shown in dashed lines). The fuel valve 50' admits fuel into the cylinder as the piston 10 moves from BDC to TDC but before it passes the fuel valve 50'. Thus, if the engine is configured for premixed combustion, the piston 10 compresses the scavenging air and fuel mixture. Ignition is timed to occur at or near TDC.Ignition can be achieved by spark, laser, injection of ignition fluid, etc. In embodiments with the fuel valve 50', the pressure at which fuel is introduced is significantly lower than the pressure at which fuel is injected in embodiments with the fuel valve 50 in the cylinder cover 22. Because the fuel valve 50 located in the cylinder cover 22 injects fuel when the piston is at or near TDC, the fuel injection pressure must be significantly higher than the compression pressure. Thus, in some embodiments, the engine operates according to the Diesel principle (compression ignition) and compresses only scavenging air (or scavenging gas, if equipped with exhaust gas recirculation). In other embodiments, the engine operates according to the Otto cycle (timed ignition) and compresses the fuel and scavenging gas mixture. When operating according to the Otto principle, the pressure required for the fuel delivery system 30 to deliver fuel can be significantly lower than in compression ignition cases, avoiding the need for a pressure booster often used with fuel valves 50 for compression ignition engines.

[0048] When the exhaust valve 4 opens, the exhaust flows through an exhaust duct in the cylinder 1 to an exhaust receiver 3, then through a selective catalytic reduction (SCR) catalyst 40, through a first exhaust pipe 19 and on to the turbine 6 of the turbocharger 5. From there, the exhaust is released into the atmosphere through a second exhaust pipe 28.

[0049] A turbine 6 of the turbocharger 5 drives a compressor 7 via a shaft. Outside air is supplied to the compressor 9 through an air intake 12. The compressor 7 sends compressed scavenging air into a scavenging pipe 13 connected to the scavenging air receiver 2. The scavenging air in the scavenging pipe 13 passes through an intercooler 14 for cooling the scavenging air.

[0050] The cooled scavenging air passes through an auxiliary blower 16 driven by an electric motor 17. The auxiliary blower 16 compresses the scavenging air flow when the compressor 7 of the turbocharger 5 cannot provide enough pressure for the scavenging air receiver 2, i.e. when the engine is at low or partial load. When the engine load is high, the turbocharger compressor 7 can provide sufficiently compressed scavenging air, so the auxiliary blower 16 is bypassed by the check valve 15 and the electric motor 17 is shut off. The turbocharging system may have two or more turbochargers 5.

[0051] In ammonia mode, the engine is operated using ammonia as the primary fuel. Ammonia is supplied to the fuel valve 50 or 50' by the ammonia fuel system 30 at a substantially constant pressure and temperature. The ammonia may be supplied to the ammonia valve 50 in either the liquid or gas phase. The liquid phase ammonia may be aqueous ammonia, i.e., an aqueous ammonia solution.

[0052] Conventional fuel systems are well known and are not shown or described in detail. The ammonia fuel system 30 supplies ammonia in liquid phase at an intermediate supply pressure (e.g., 30-80 bar) to the fuel valve 50 or fuel inlet valve 50'. Alternatively, the ammonia fuel is supplied to the ammonia valve 50 in gas phase at a relatively low supply pressure (e.g., 8-30 bar). In the case of a compression ignition engine, the fuel valve 50 includes a pressure booster that significantly increases the pressure of the ammonia fuel. The pressure booster increases the pressure of the ammonia fuel from intermediate pressure to high pressure, thereby allowing the ammonia fuel to be injected at a pressure higher than the engine compression pressure. Typically, the injection pressure of a compression ignition engine is greater than 300 bar.

[0053] In some embodiments, the engine is equipped with an exhaust gas recirculation system for reintroducing a portion of the exhaust gases into the combustion chamber together with the scavenging gas, for example to reduce NOx emissions.

[0054] In ammonia fuel system 30, ammonia is stored in a pressurized storage tank in liquid phase at approximately 17 bar. Ammonia can be stored in liquid phase in the ammonia storage tank at a pressure of 8.6 bar or higher at an ambient temperature of 20° C. However, it is preferable to store the ammonia at a pressure of 17 bar or higher to maintain the liquid phase even as the ambient temperature increases.

[0055] A low-pressure ammonia supply line 32 connects the outlet of an ammonia storage tank (not shown) to the inlet of a medium-pressure supply pump (not shown). The low-pressure supply pump pressurizes liquid ammonia from the ammonia storage tank to the inlet of medium-pressure supply pump 35. The medium-pressure supply pump pumps liquid ammonia from the medium-pressure ammonia supply line (not shown) to fuel valves 50, 50′.

[0056] An electronic control unit (controller) 60 is connected via signal lines or wirelessly to the various components and sensors of the engine.

[0057] In NH3 combustion, engine exhaust gases can contain both NOx and NH3. This contrasts with the absence of NH3 in exhaust gases from fossil fuel combustion. The ratio of these two substances in exhaust gases cannot always be precisely controlled or predicted. However, the SCR catalyst 40 functions not only as a NOx removal catalyst but also as an NH3 removal catalyst. Because NH3 and NOx react in a one-to-one ratio, the molar ratio of NH3 to NOx in the exhaust gas determines the amount of NOx and NH3 that can be removed. In the following, the molar ratio of NH3 to NOx in the exhaust gas is referred to as α. An α value less than 1 indicates excess NOx. In this case, all of the NH3 can react with NOx, resulting in essentially zero NH3 being emitted from the SCR catalyst 40 outlet. IMO Tier III regulations also allow for some NOx in the exhaust gas. An α value greater than 1 indicates excess NH3. In this case, all of the NOx reacts with NH3, and the excess NH3 is emitted from the SCR catalyst as NH3 slip. The allowable NH3 slip in the exhaust gas is low, e.g., 10 ppm, so it is desirable to keep α below 1.

[0058] The SCR catalyst 40 serves to remove both NOx components, NO and NO2, from the exhaust gas. In some embodiments, the SCR catalyst 40 is vanadium-based. In the illustrated embodiment, the SCR catalyst 40 is located on the high-pressure side of the turbine 6, although in some embodiments it could be located on the low-pressure side of the turbine 6. However, locating the SCR catalyst 40 on the low-pressure side of the turbine may increase the volume of the SCR catalyst 40. In this embodiment, the inlet of the SCR catalyst 40 is connected to the outlet of the exhaust receiver 3.

[0059] The controller 60 measures or calculates the concentrations of NOx and NH3 in the gas stream entering or within the exhaust receiver 3. Based on these concentrations, the required amount (flow rate) of NOx to be added (to the exhaust gas stream entering the SCR catalyst 40) to achieve the desired α value is calculated. In some embodiments, this additional NOx stream is generated from a secondary stream, such as that delivered from the ammonia fuel system 30, that contains NH3. This NH3 is catalytically oxidized at high temperatures (preferably above 500°C) in the oxidation catalyst 43. To this end, an air stream is supplied to the oxidation catalyst 43, producing NO and HO (water). The source of the compressed air stream is preferably scavenging air taken from the intake system and controlled by the control valve 27. This is because it is an efficient way to obtain high-temperature compressed air, particularly if this scavenging air is taken from the intake system upstream of the intercooler 14 (and downstream of the compressor 7). This side stream can be a separate supply of NH3, as in the embodiment of Figure 3, or it can be taken from the total exhaust. In either case, the size of the side stream is controlled, for example, by control valve 42, which is adjusted by controller 60 according to the calculations described above. The catalyst for NH3 oxidation can be of the same type as that used in the production of nitric acid (HNO3). In this case, NH3 is catalytically oxidized by the platinum-rhodium catalytic gauze of oxidation catalyst 43, resulting in the following reaction: 4NH3 + 5O2 -> 4NO + 6H2O

[0060] In addition to NO and water, unwanted nitrous oxide (NO) is also produced by the following oxidation reaction: 4NH3 + 4O2 -> 2N2O + 6H2O

[0061] If NO is produced, this relatively small gas stream containing NOx can be treated using an NO decomposition catalyst. This results in a side stream, primarily air, containing NO and water. This side stream is mixed with the exhaust gas stream from exhaust receiver 3. In this way, the molar concentration of NO in the exhaust gas stream is higher (preferably slightly higher) than the molar concentration of NH3. This combined exhaust gas stream is directed to SCR catalyst 40, where NO and NH3 react according to the following reaction, in which one mole of NO reacts with one mole of NH3, in accordance with the standard SCR process: 4NO + 4NH3 + O2 -> 4N2 + 6H2O

[0062] When the gas stream exits the SCR catalyst 40, substantially all of the NH3 has been removed and NOx has also been reduced to meet International Maritime Organization Tier 3 levels.

[0063] The sensor system 44, 45, 46, 47, 48, 49 provides one or more signals that enable the controller 60 to determine the molar ratio (α) between ammonia and NOx in the exhaust gas stream. Preferably, the sensors include at least one ammonia sensor 45, 47, 49 configured to provide a signal representative of the concentration of ammonia in the exhaust gas stream and at least one NOx sensor 44, 46, 48 configured to provide a signal representative of the concentration of NOx in the exhaust gas stream. Three ammonia sensors 45, 47, 49 and three NOx sensors 44, 46, 48 are shown in FIG. 3 . However, only one pair of sensors may be required to provide the controller 60 with sufficient information to determine α. In some embodiments, a pair of ammonia and NOx sensors is configured to measure concentrations in the exhaust receiver 3.

[0064] In some embodiments, the controller 60 is configured to control the flow rate of the NOx-containing gas stream in a closed-loop manner. This can be done by comparing a determined downstream of where the NOx-containing gas stream is added to the exhaust gas stream with a desired a and controlling the flow rate of the NOx-containing gas stream accordingly. Control of the flow rate of the NOx-containing gas stream can be done, for example, by adjusting the position of the control valve 42. Alternatively, the controller 60 is configured to feed-forward control the flow rate of the NOx-containing gas stream.

[0065] In some embodiments, the engine is configured to control α by adding NH3 to the first exhaust conduit 19 upstream of the inlet of the SCR catalyst 40. NH3 is added through a line controlled by the ammonia control valve 41. NH3 is added, for example, in the form of urea, or, as shown, in the form of gaseous NH3. When α is substantially less than 1, NOx emissions can be controlled by adding ammonia to the exhaust gas by opening the control valve 41. Here, the electronic control unit 60 is configured to adjust the amount of ammonia added, i.e., the conduit of the ammonia flow to the first exhaust line 19, in response to α determined by the electronic control unit 60. When α is 1 or greater, a controlled amount of a gas flow containing NOx is added to the exhaust gas flow. When α is substantially less than 1, a controlled amount of ammonia or urea (reducing agent) flow is added to the exhaust gas flow. This can substantially reduce both ammonia slip and NOx emissions, regardless of whether the exhaust gas from the exhaust receiver 3 has excess ammonia or excess NOx.

[0066] The controller 60 is configured to adjust the flow rate of the NOx stream such that the flow rate of the exhaust gas stream entering the SCR catalyst 40 is such that the molar ratio between ammonia and NOx is less than 1, preferably slightly less than 1. That is, the controller 60 is configured to adjust the flow rate of the NOx stream such that the molar concentration of ammonia is equal to or less than, preferably slightly less than, the molar concentration of NOx.

[0067] The NOx-containing gas stream contains both NO and NO2. The ratio of NO to NO2 in the exhaust gas stream can vary depending on operating conditions. In some embodiments, the engine includes means (not shown) for adjusting the ratio between NO and NO2 in the NOx-containing gas stream. In some embodiments, the NOx-producing system is configured to control the ratio between NO and NO2 in the NOx-containing gas. The controller 60 is configured to determine the optimal ratio between NO and NO2 in the NOx-containing gas and adjust the ratio between NO and NO2 in the NOx-containing gas accordingly. The amount of NO2 to NO can be controlled, for example, if the NOx-containing gas stream after the oxidation catalyst is cooled. This can be done to control the ratio between NO2 and NO, which is important for the efficiency of the SCR reactor 40. If NO2 is present in the NOx-containing gas stream but is less than the amount of NO, a so-called fast SCR reaction can occur. NO + NO2 + 2NH3 -> 2N2 + 3H2O

[0068] However, if the amount of NO2 in the gas is too much compared to NO, the efficiency of the SCR catalyst 40 decreases, and so-called slow SCR reaction occurs, which must be suppressed. 8NH3 + 6NO2 -> 7N2 + 12H2O

[0069] In some embodiments (not shown), the engine includes a container for storing NOx-containing gas, preferably a container for storing NOx-containing gas under high pressure, such as a high-pressure gas bottle containing NOx. The container is preferably connected to the exhaust system via a control valve, thereby providing a controlled flow of NOx-containing gas from the container to the exhaust gas stream.

[0070] At least one NOx sensor 44 is configured to provide a signal indicative of the NOx concentration of the exhaust gas flow in the exhaust system upstream of the location where the NOx-containing gas stream is added. NOx sensor 46 is configured to provide a signal indicative of the NOx concentration of the exhaust gas flow in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst 40. At least one NOx sensor 48 is configured to provide a signal indicative of the NOx concentration of the exhaust gas flow in the exhaust system downstream of the SCR catalyst 40. At least one ammonia sensor 45 is configured to provide a signal indicative of the ammonia concentration of the exhaust gas flow in the exhaust system upstream of the location where the NOx-containing gas stream is added. At least one ammonia sensor 47 is configured to provide a signal indicative of the ammonia concentration of the exhaust gas in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst 40. At least one ammonia sensor 49 is configured downstream of the SCR catalyst 40 to provide a signal representative of the ammonia concentration in the exhaust gas stream within the exhaust system.

[0071] 4 is a flow chart illustrating one embodiment of a method for reducing ammonia slip from the exhaust gas of a large two-stroke uniflow scavenged turbocharged internal combustion engine having a turbocharger 5, such as the internal combustion engine according to the above embodiments. The method includes operating the engine with ammonia as the primary fuel, thereby producing an exhaust gas stream comprising NOx and NH3, determining α in the exhaust gas coming from a cylinder, adjusting the α of the exhaust gas stream by adding a controlled NOx flow to the exhaust gas, and then subjecting the exhaust gas stream to SCR (selective catalytic reduction), for example in an SCR catalyst 40.

[0072] The α of the exhaust gas coming from the cylinder or the α of the exhaust gas entering the SCR catalyst 40 is determined, and if α is greater than or equal to 1, a NOx flow is added to the exhaust gas flow upstream of the SCR catalyst 40 .

[0073] The method further includes determining a molar ratio between ammonia and NOx before adding the controlled NOx stream to the exhaust gas stream, and adding the controlled NOx stream to the exhaust gas stream if (and preferably only if) the determined molar ratio is greater than or equal to 1.

[0074] In an embodiment of the method of FIG. 5, the method includes determining the magnitude of the NOx flow required to reduce the determined molar ratio to a level less than 1, preferably slightly less than 1, and adjusting the magnitude of the NOx-containing gas flow to the determined magnitude.

[0075] The amount of NO that needs to be added to the exhaust gas stream to achieve the desired α (the magnitude of the NOx-containing gas stream) determines the magnitude of the ammonia stream flowing through the oxidation catalyst 43. The amount of NH3 to be oxidized is at least the same molar amount as the NH3 in excess compared to NO in the engine exhaust gas. That is, Moles of NH3 emitted from the engine - Moles of NO emitted from the engine = additional moles of NO needed = moles of NH3 to be oxidized (for 100% conversion)

[0076] This is necessary to achieve α=1. Typically, the SCR catalyst 40 is sized for an α value between 0.8 and 0.95, and the controller is configured to adjust the process accordingly to obtain an α value. When NH3 is added as a side stream to the oxidation catalyst 43, the NH3 concentration in the airflow is typically about 9.5-11.5%, resulting in a NO yield of 90-98%. The amount of airflow to the oxidation catalyst 43 depends on concentration and demand, but can range from 0.06-0.3 kg / kWh of air, corresponding to 4-20 g / kWh of ammonia.

[0077] In an embodiment of the method according to FIG. 6, the method includes determining a desired ratio between NO and NO2 in the NOx-containing gas stream and adjusting the ratio between NO and NO2 in the NOx-containing gas stream.

[0078] Various aspects and implementations of the invention have been described with reference to several examples. However, upon reviewing the specification, drawings, and claims of this application, those skilled in the art will understand and be able to embody many variations in addition to the described examples in implementing the claimed invention. The words "comprise," "have," and "include" in the claims do not exclude the presence of unrecited elements or steps. The absence of an explicit reference to a plurality of elements in a claim does not exclude the presence of a plurality of such elements.

[0079] Any reference signs used in the claims should not be construed as limiting the scope of the invention. Unless otherwise noted, the drawings are intended to be read together with the specification, and are an integral part of this disclosure.

Claims

1. 1. A large two-stroke uniflow scavenged turbocharged internal combustion engine having at least one operating mode in which the primary fuel is ammonia, at least one cylinder having a cylinder liner and a reciprocating piston within said cylinder liner, and a cylinder cover covering said cylinder; a combustion chamber defined within the cylinder between the reciprocating piston and the cylinder cover; an intake system for supplying scavenging air to the combustion chamber; an exhaust system for exhausting an exhaust stream produced by the combustion of ammonia in said combustion chamber; a turbocharging system having at least one compressor for compressing scavenging air, the compressor being present in the intake system, and at least one exhaust-driven turbine for driving the compressor, the exhaust-driven turbine being present in the exhaust system; an SCR catalyst disposed in the exhaust system; and means for adding a NOx-containing gas stream to said exhaust stream within or upstream of said SCR catalyst.

2. The engine of claim 1 , further comprising a controller configured to control a magnitude of the NOx-containing gas flow added to the exhaust stream.

3. 3. The engine of claim 2, comprising means for measuring and / or estimating the molar ratio between ammonia and NOx in the exhaust stream.

4. 4. The engine of claim 3, wherein the controller is configured to adjust the magnitude of the NOx-containing gas flow as a function of the measured and / or estimated molar ratio.

5. 5. An engine as claimed in claim 4, wherein the controller is configured to adjust the magnitude of the NOx flow entering the SCR catalyst such that the molar ratio between ammonia and NOx is less than 1, preferably slightly less than 1, i.e. the molar concentration of ammonia is equal to or less than, preferably slightly less than, the molar concentration of NOx.

6. 10. The engine of claim 1, The NOx-containing gas stream is divided into NO and NO ２ and The engine detects NO and NO in the NOx-containing gas stream. ２ and a means for adjusting the ratio between institution.

7. The engine of claim 2 , wherein the controller includes a sensor system that provides one or more signals for determining the molar ratio of ammonia to NOx in the exhaust stream.

8. a NOx production system for producing said NOx-containing gas stream, said NOx production system preferably comprising NH ３ a source for supplying the flow; to obtain the NOx-containing gas stream, ３ is preferably catalytically oxidized to NO, NO ２ and H ２ Become O.

3. The engine of claim 2.

9. 9. The engine of claim 8, an oxidation catalyst, the oxidation catalyst preferably being a platinum-rhodium catalyst; The engine preferably further comprises a source of pressurized ammonia gas to the oxidation catalyst, and a source of pressurized air to the oxidation catalyst, the source of pressurized air being preferably scavenged air from the intake system. institution.

10. The NOx generating system is configured to generate NO and NO in a gas containing NOx. ２ 9. The engine of claim 8, configured to control the ratio between

11. The controller detects NO and NO in the gas containing NOx. ２ and accordingly determine the optimum ratio between NO and NO in the NOx-containing gas. ２ 9. The engine of claim 8, configured to adjust the ratio between

12. 9. An engine according to claim 8, comprising a catalytic N2O removal system, preferably an iron zeolite catalyst, downstream of said NOx producing system for removing N2O that may be produced as a result of side reactions within said NOx producing system.

13. 2. An engine according to claim 1, comprising a container for storing NOx-containing gas, preferably for storing said NOx-containing gas under high pressure, said container preferably connected to said exhaust system via a control valve thereby allowing controlled flow of said NOx-containing gas from said container into said exhaust stream.

14. 10. The engine of claim 1, comprising at least one NOx sensor configured to provide a signal representative of a NOx concentration of the exhaust stream in the exhaust system, and at least one ammonia sensor configured to provide a signal representative of an ammonia concentration of the exhaust stream in the exhaust system.

15. the at least one NOx sensor is configured to provide a signal indicative of the NOx concentration in the exhaust stream in the exhaust system upstream of the location where the NOx-containing gas stream is added; and / or the at least one NOx sensor is configured to provide a signal indicative of the NOx concentration in the exhaust stream in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst; and / or the at least one NOx sensor is configured to provide a signal indicative of the NOx concentration in the exhaust stream in the exhaust system downstream of the SCR catalyst; and / or the at least one ammonia sensor is configured to provide a signal indicative of an ammonia concentration in the exhaust stream in the exhaust system upstream of a location where the NOx-containing gas stream is added; and / or the at least one ammonia sensor is configured to provide a signal indicative of the ammonia concentration of exhaust gas in the exhaust system downstream of the location where the NOx-containing gas stream is added and upstream of the SCR catalyst; and / or the at least one ammonia sensor is configured to provide a signal indicative of an ammonia concentration in the exhaust stream in the exhaust system downstream of the SCR catalyst; 15. The engine of claim 14.

16. 10. The engine of claim 1, comprising an ammonia fuel system configured to supply pressurized ammonia to a fuel valve configured to inject or introduce ammonia into the combustion chamber.

17. 1. A method for reducing ammonia slip from a large two-stroke uniflow scavenged turbocharged internal combustion engine, comprising: a) Operating the engine using ammonia as the primary fuel, thereby reducing NOx and NH ３ generating an exhaust stream comprising: b) adjusting the ratio between ammonia and NOx in said exhaust stream by adding a NOx-containing gas stream to said exhaust stream within or upstream of an SCR catalyst; A method comprising:

18. The method of claim 17, comprising determining the molar ratio between ammonia and NOx before adding the NOx-containing gas stream to the exhaust stream, and adding the NOx-containing gas stream to the exhaust stream if the molar ratio is greater than or equal to 1.

19. 20. The method of claim 18, comprising determining a magnitude of the NOx-containing gas stream required to reduce the determined molar ratio to less than 1, and adjusting the magnitude of the NOx-containing gas stream added to the exhaust stream to the determined magnitude.

20. NO and NO in the NOx-containing gas stream ２ and determining a desired ratio between NO and NO in the NOx-containing gas stream accordingly. ２ and adjusting the ratio between

21. supplying a pressurized ammonia gas stream and a pressurized air stream to an inlet of an oxidation catalyst to produce a NOx-containing gas stream exiting an outlet of said oxidation catalyst; the compressed air flow preferably originates from an intake system, preferably originates from said intake system at a location downstream of a turbocharger compressor, preferably originates from said intake system upstream of an intercooler; 18. The method of claim 17.

22. The engine of claim 1, wherein the SCR catalyst is positioned in the exhaust system upstream of the exhaust-driven turbine.

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