JP2024052583A - Large two-stroke uniflow scavenged turbocharged internal combustion engine configured to reduce ammonia slip and method for reducing ammonia slip in such an engine - Google Patents

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

The subject matter disclosed herein relates to large, two-stroke, uniflow scavenged, turbocharged internal combustion engines, and in particular to large, two-stroke, uniflow scavenged, turbocharged internal combustion engines, which in at least one operating mode are operated using ammonia ( _NH3 ) as the primary fuel combusted within the engine.

background

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 and place this type of compression ignition engine in a unique category.

Internal combustion engines have been primarily powered by hydrocarbon fuels, such as fuel oils, such as diesel oil, and fuel gases, such as natural gas or petroleum gas. Combustion of hydrocarbon fuels involves the production of greenhouse gases, such as carbon dioxide (CO ₂ ), which can contribute to air pollution and climate change. Unlike impurities in petroleum fuels that result in by-product emissions, the production of CO ₂ is inevitable in the combustion of hydrocarbons. The energy density and CO ₂ emissions of a particular fuel depend on the length of the hydrocarbon chain and the complexity of the hydrocarbon molecule. Thus, gaseous hydrocarbon fuels have lower CO ₂ emissions than liquid hydrocarbon fuels. However, gaseous hydrocarbon fuels are difficult and expensive to handle and store. In order to reduce CO ₂ emissions, non-hydrocarbon fuels are being developed.

Ammonia ( _NH3 ) is a compound derived from petroleum, biomass, and renewable or sustainable energy sources (wind, solar, hydro, nuclear, geothermal). _NH3 produced using renewable or sustainable energy sources has virtually zero carbon emissions when burned, and no _CO2 , SOx, particulate matter, or unburned hydrocarbons.

_NH3 has been tested and used on a small scale in small internal combustion engines such as those in automobiles, but has not yet been used in large two-stroke internal combustion engines.

In large two-stroke internal combustion engines, the combustion gases generated by the combustion of ammonia ( _NH3 ) may contain both NOx and _NH3 . NOx is limited by international regulations such as the International Maritime Organization (IMO) Tier II and Tier III. There are no formal regulations yet for _NH3 , but the practical allowable levels are much lower. Allowable _NH3 slip in the exhaust gas is difficult to achieve without an _NH3 removal system (aftertreatment system) in the engine exhaust system. Without measures, exhaust gases containing unacceptable amounts of _NH3 may be emitted into the atmosphere.

Known systems for removing _NH3 from exhaust gas 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 for some reason _NH3 is present after the SCR catalyst, it is oxidized in the ASC, which removes _NH3 . The ASC processes the entire amount of gas, similar to the SCR. Therefore, when the ASC is installed in a large two-stroke internal combustion engine, the size of the ASC is similar to the SCR catalyst, since the entire amount of exhaust gas needs to be processed. The SCR catalyst is a very voluminous device. Adding another device with a significant volume is problematic. Another drawback is that nitrous oxide (N2O) can be a by-product of _NH3 oxidation in the ASC. Nitrous oxide abatement systems are known but require temperatures in excess of 400° C. to be effective, an exhaust gas temperature not easily achievable in highly efficient marine engines.

Another known technology is to use wet scrubbers to remove _NH3 from the exhaust gases, but this introduces very large and bulky components, and the wastewater on board cannot be easily disposed of.

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

Abstract

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.

The above 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.

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, the engine comprising:
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, 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.

The inventors have realized that ammonia slip can be avoided if ammonia is reliably reduced in the SCR catalyst. However, if there is not enough NOx present in the SCR catalyst, i.e. if the molar ratio of ammonia to NOx is greater than 1, ammonia will not be reduced. The ratio of ammonia to NOx in the exhaust gas leaving the combustion chamber cannot always be (precisely) controlled or (precisely) predicted. By supplying a NOx-containing gas stream to the exhaust gas, it can be ensured that the amount of NOx required for complete reduction of ammonia is always present at the SCR catalyst and that all or at least almost all of the ammonia present in the exhaust gas is reduced at the SCR catalyst. Only a relatively small gaseous NOx stream is required to obtain the desired result.

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. The process requires the addition of some heat, but 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. Thus, an engine according to the first approach is smaller in volume and less expensive to manufacture and maintain.

In one implementation of the first approach, the engine of claim 1 includes a controller configured to control the magnitude of the NOx-containing gas flow added to the exhaust flow.

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.

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

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

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.

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

In one implementation of the first aspect, the present invention includes a NOx production system for producing the NOx-containing gas stream, the NOx production system preferably having a source for providing an _NH3 stream;

To obtain the NOx-containing gas stream, ammonia from the source is preferably catalytically oxidized to NO and _H2O .

In one implementation of the first approach, the engine includes an oxidation catalyst, which is preferably a platinum-rhodium catalyst, and the engine preferably further includes 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 scavenging air from the intake system.

In one example implementation of the first approach, the NOx production system is configured to control the ratio between NO and _NO2 in a gas containing NOx.

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.

In one implementation of the first approach, downstream of the NOx generating system is a catalytic N2O removal system, preferably an iron zeolite catalyst, for removing N2O that may be generated as a result of side reactions within the NOx generating system.

In one implementation of the first approach, the system includes a container for storing NOx-containing gas, preferably under high pressure, 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.

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

In one implementation of the first approach, 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 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 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

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 SCR catalyst, and/or

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 the location where the NOx-containing gas stream is added, and/or

The at least one ammonia sensor is configured to provide a signal representative 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; and/or

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

In one implementation of the first approach, an ammonia fuel system is provided that is configured to supply pressurized ammonia to a fuel valve that is configured to inject or introduce ammonia into the combustion chamber.

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 with 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) thereafter directing said exhaust gas stream to an SCR;
including.

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

In one 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.

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.

In one implementation of the second approach, the method includes supplying a pressurized ammonia gas flow and a pressurized air flow to an inlet of an oxidation catalyst to generate a NOx-containing gas flow exiting an outlet of the oxidation catalyst;

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.

These and other aspects will become more apparent from the accompanying drawings and the embodiments described below.

Various aspects, embodiments, and implementations will be described in detail below with reference to exemplary embodiments illustrated in the drawings.
FIG. 1 shows a front view of a large two-stroke internal combustion engine according to an exemplary embodiment. 2 shows an overview of the large two-stroke engine of FIG. 1 as seen from the rear. 2 is a schematic representation of an embodiment of the large two-stroke engine of FIG. 1 having an ammonia fuel system and an ammonia slip removal system. 1 is a flow chart of an embodiment of a process for reducing ammonia slip in a large two-stroke internal combustion engine. 4 is a flow chart of another embodiment of a process for reducing ammonia slip in a large two-stroke internal combustion engine. 4 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

In the detailed description below, the internal combustion engine will be described with reference to an example embodiment of a 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 the top dead center (TDC) of the piston, or it may be a spark ignition (also low pressure or premixed) engine, in which the scavenging air is mixed with fuel before or during compression and the mixture is ignited or otherwise. In the case of a premixed engine, a pilot ignition with an additive liquid (e.g. fuel oil) is usually provided to ensure ignition.

Figures 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. Figure 3 shows a schematic representation of a turbocharged large slow-speed two-stroke diesel engine together with its intake and exhaust systems. In this embodiment the engine has six cylinders in series. A turbocharged large slow-speed two-stroke diesel engine usually has 4 to 14 cylinders arranged in series. The cylinders are carried on a cylinder frame 23. The cylinder frame 23 is carried on an engine frame 11. Such an engine can also be used, for example, as the main engine of a ship or as a stationary engine for driving a generator in a power plant. The total power of the engine can be, for example, between 1000 kW and 110000 kW.

The engine can be configured as a dual fuel engine. The engine can be a compression ignition engine or a premixed engine. In this embodiment, the engine is a two-stroke uniflow type engine, with each cylinder having a scavenging port 18 in the lower region of the cylinder liner 1 and an exhaust valve 4 at 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 runs on ammonia fuel or ammonia-based fuel. In the conventional fuel mode, the engine runs on conventional fuels, such as fuel oil (marine diesel fuel) or heavy fuel oil.

The scavenging air is directed through the scavenging receiver 2 to the scavenging ports 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. Fuel (ammonia in the ammonia mode) is injected at high pressure into the combustion chamber in the cylinder liner 1 through a number of fuel valves 50 arranged in the cylinder cover 22 at or near TDC (Diesel principle). If the engine is configured as a premixed engine, fuel is introduced at relatively low pressure through the fuel inlet valve 50' on the way of the piston to TDC (Otto principle). Typically, two or more fuel inlet valves 50' are provided for each cylinder. The fuel inlet valves 50' can be provided in the cylinder liner above the scavenging ports 18 or in the cylinder cover 22. Combustion follows the injection of the fuel and exhaust is generated. If the engine is configured as a compression ignition engine, two or more fuel valves 50 are provided in each cylinder cover 22. The fuel valve 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 fuels into the combustion chamber may also be provided (not shown in FIG. 3). The fuel valve 50 is arranged in the cylinder cover 22 around the exhaust valve 4 located in the center of the cylinder cover 22. Although not shown, in some embodiments, an additional (usually small) fuel valve may be arranged in the cylinder cover, 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, it may be other forms of ignition enhancer, such as, for example, hydrogen. Since the engine may be a dual-engine, the engine may include a conventional fuel supply system (not shown) for supplying conventional fuel to the fuel valve 50. In some embodiments, a fuel valve 50' is arranged along the cylinder liner (shown in dashed lines). The fuel valve 50' introduces fuel into the cylinder when the piston 10 is on its way from BDC to TDC and before it passes the fuel valve 50'. Thus, when the engine is configured as a premixed engine, the piston 10 compresses a mixture of scavenging air and fuel. Ignition is timed at or near TDC. Ignition can be by spark, laser, injection of ignition fluid, etc. In the embodiment with the fuel valve 50', the pressure at which fuel is introduced is significantly lower than the pressure at which fuel is injected in the embodiment with the fuel valve 50 in the cylinder cover 22. Since the fuel valve 50 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 the scavenging air (or scavenging gas if exhaust gas recirculation is provided). In other embodiments, the engine operates according to the Otto cycle (timed ignition) and compresses a mixture of fuel and scavenging gas. When operating according to the Otto principle, the pressure required for the fuel supply system 30 to supply fuel can be significantly lower than in the case of compression ignition, and the need for a pressure booster often used in fuel valves 50 for compression ignition engines can be avoided.

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

The turbine 6 of the turbocharger 5 drives the 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 to a scavenging pipe 13 that is connected to the scavenging receiver 2. The scavenging air in the scavenging pipe 13 passes through an intercooler 14 to cool the scavenging air.

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 sufficient pressure for the scavenging receiver 2, i.e. when the engine is at low or partial load. At high engine loads, the turbocharger compressor 7 can provide sufficient compressed scavenging air, so the auxiliary blower 16 is bypassed by the check valve 15 and the electric motor 17 is switched off. The turbocharging system may have two or more turbochargers 5.

In ammonia mode, the engine is operated with 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. Ammonia may be supplied to the ammonia valve 50 in liquid or gas phase. Liquid phase ammonia may be aqueous ammonia, i.e., an aqueous ammonia solution.

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 higher than 300 bar.

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

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

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

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

In _NH3 combustion, the exhaust gas from the engine may contain both NOx and _NH3 . This is in contrast to the exhaust gas from fossil fuel combustion, which does not contain _NH3 . The ratio of these two substances in the exhaust gas 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. Since _NH3 and NOx react with each other 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. α less than 1 indicates that there is an excess of NOx. In this case, all of the _NH3 can react with NOx, and as a result, the _NH3 discharged from the outlet of the SCR catalyst 40 is essentially zero. The inclusion of some NOx in the exhaust gas is also permitted under the IMO Tier 3 regulations. α greater than 1 indicates that there is an excess of _NH3 . In this case, all NOx reacts with _NH3 , and excess _NH3 is discharged from the SCR catalyst as _NH3 slip. The _NH3 slip permitted in the exhaust gas is low. An example of the limit is 10 ppm. Therefore, it is desirable to keep α below 1.

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 embodiment presented, the SCR catalyst 40 is located on the high pressure side of the turbine 6, but in some embodiments it can be located on the low pressure side of the turbine 6. However, if located on the low pressure side of the turbine, the volume of the SCR catalyst 40 may increase. In this embodiment, the inlet of the SCR catalyst 40 is connected to the outlet of the exhaust receiver 3.

The controller 60 measures or calculates the concentration of NOx and NH ₃ in the gas stream coming from or in the exhaust receiver 3. Based on this concentration, the required amount (flow rate) of NOx to be added (to the exhaust gas stream going to the SCR catalyst 40) to obtain the desired α value is calculated. In some embodiments, this additional NOx stream is generated from a side stream coming from the ammonia fuel system 30, for example, which contains NH _3. This NH ₃ is catalytically oxidized at high temperature (preferably above 500° C.) in the oxidation catalyst 43, which is fed with an air stream for this purpose, to obtain NO and H ₂ O (water). The source of the pressurized air stream is preferably a scavenging air taken from the intake system and controlled by the control valve 27, since this is an efficient way to obtain hot pressurized air, especially 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 for the production of nitric acid ( _HNO3 ). In this case, _NH3 is catalytically oxidized with a platinum-rhodium catalytic gauze of oxidation catalyst 43, resulting in the following reaction:

_4NH3 + _5O2 -> 4NO + _6H2O

In addition to NO and water, unwanted nitrous oxide (NO) is also produced via the oxidation reaction:

_4NH3 + _4O2 -> _{2N2O +} 6H2O

If N2O is produced, this relatively small gas stream containing NOx can be treated using a decomposition catalyst for N2O. This results in a side stream, consisting mainly of 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 mixed exhaust gas stream is directed to SCR catalyst 40, where NO and _NH3 react according to the following reaction, where 1 mole of NO reacts with 1 mole of _NH3 , according to the standard SCR process:

4NO + _4NH3 + _O2 -> _4N2 + _6H2O

When the gas stream exits the SCR catalyst 40, substantially all of the _NH3 has been removed and NOx has also been reduced to International Maritime Organization Tier 3 levels.

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 needed to provide the controller 60 with sufficient information to determine α. In some embodiments, a pair of ammonia and NOx sensors are configured to measure concentrations in the exhaust receiver 3.

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 the determined α downstream of the location where the NOx-containing gas stream is added to the exhaust gas stream to a desired α, and controlling the flow rate of the NOx-containing gas stream accordingly. The flow rate control 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.

In some embodiments, the engine is configured to control α by adding NH ₃ to the first exhaust conduit 19 upstream of the inlet of the SCR catalyst 40. The NH ₃ is added through a line controlled by the ammonia control valve 41. The NH ₃ is added, for example, in the form of urea, or in the form of gaseous NH ₃ as shown. When α is substantially below 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, depending on 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 (reductant) flow is added to the exhaust gas flow. This allows both ammonia slip and NOx emissions to be substantially reduced, regardless of whether the exhaust gas coming from the exhaust receiver 3 has excess ammonia or excess NOx.

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.

The gas stream containing NOx contains both NO and _NO2 . The ratio of NO and _NO2 in the exhaust gas stream may vary depending on the operating conditions. In some embodiments, not shown, the engine is provided with means 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, when 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 less than the amount of NO, a so-called fast SCR reaction can occur.

NO + _NO2 + _2NH3 -> _2N2 + _3H2O

However, if the amount of _NO2 in the gas is too much compared to NO, the efficiency of the SCR catalyst 40 decreases, and a so-called slow SCR reaction occurs, which must be suppressed.

_8NH3 + _6NO2 -> _7N2 + _12H2O

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.

Also, at least one NOx sensor 44 is configured to provide a signal representative of the NOx concentration of the exhaust gas flow in the exhaust system upstream of the location where the NOx-containing gas flow is added. The NOx sensor 46 is configured to provide a signal representative of the NOx concentration of the exhaust gas flow in the exhaust system downstream of the location where the NOx-containing gas flow is added and upstream of the SCR catalyst 40. The at least one NOx sensor 48 is configured to provide a signal representative of the NOx concentration of the exhaust gas flow in the exhaust system downstream of the SCR catalyst 40. The at least one ammonia sensor 45 is configured to provide a signal representative of the ammonia concentration of the exhaust gas flow in the exhaust system upstream of the location where the NOx-containing gas flow is added. The at least one ammonia sensor 47 is configured to provide a signal representative of the ammonia concentration of the exhaust gas in the exhaust system downstream of the location where the NOx-containing gas flow 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 flow in the exhaust system.

4 is a flow chart showing an 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 embodiment. The method comprises operating the engine with ammonia as the main fuel, thereby producing an exhaust gas stream comprising NOx and _NH3 , determining α in the exhaust gas coming from a cylinder, adjusting α of the exhaust gas stream by adding a controlled NOx flow to said exhaust gas, and then subjecting said exhaust gas stream to SCR (selective catalytic reduction), for example in an SCR catalyst 40.

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.

The method further includes determining a molar ratio between ammonia and NOx prior to 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.

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.

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 as much molar excess as _NH3 compared to NO in the engine exhaust gas. That is,

Moles of _NH3 exhausted from engine - Moles of NO exhausted from engine = Moles of additional NO required = Moles of _NH3 to be oxidized (for 100% conversion)

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%, with 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.

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.

Various aspects and implementations of the invention have been described with several examples. However, upon review of the specification, drawings, and claims of this application, one skilled in the art will understand and be able to embody many variations in addition to the described examples in carrying out the claimed invention. The words "comprise," "have," and "include" in the claims do not exclude the presence of elements or steps not recited. Even if a claim does not explicitly state that a number of elements is more than one, this does not exclude the presence of a number of such elements.

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 (21)

1. A large two-stroke uniflow scavenging 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, preferably upstream of the exhaust driven turbine;
and means for adding a NOx containing gas stream to said exhaust stream within or upstream of said SCR catalyst. The engine of claim 1, further comprising a controller configured to control the magnitude of the NOx-containing gas flow added to the exhaust stream. The engine of claim 2, further comprising means for measuring and/or estimating the molar ratio between ammonia and NOx in the exhaust stream. 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. The engine of claim 4, wherein the controller is configured to adjust the magnitude of the exhaust flow entering the SCR catalyst to a magnitude of the NOx flow 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. 2. The engine of claim 1,
The NOx-containing gas stream contains NO and _NO2 as NOx,
The engine includes a means for adjusting the ratio between NO and _NO2 in the NOx-containing gas stream.
institution. The engine of claim 2, wherein the controller has a sensor system that provides one or more signals for determining the molar ratio of ammonia to NOx in the exhaust. a NOx production system for producing said NOx-containing gas stream, said NOx production system preferably having a source for providing a _NH3 stream;
To obtain the NOx-containing gas stream, _NH3 from the source is preferably catalytically oxidized to NO, _NO2 and _H2O ;
The engine of claim 1. 9. The engine of claim 8,
an oxidation catalyst, said oxidation catalyst being preferably 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 scavenging air from the intake system.
institution. The engine of claim 8 , wherein the NOx production system is configured to control the ratio between NO and _NO2 in the NOx-containing gas. 9. The engine of claim 8, wherein 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. The engine of claim 8, further comprising a catalytic N2O removal system, preferably an iron zeolite catalyst, downstream of the NOx production system for removing N2O that may be produced as a result of side reactions in the NOx production system. An engine as described in 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 a controlled flow of said NOx-containing gas from said container into said exhaust stream. The engine of claim 1, further comprising at least one NOx sensor configured to provide a signal representative of a NOx concentration in the exhaust stream in the exhaust system, and at least one ammonia sensor configured to provide a signal representative of an ammonia concentration in the exhaust stream in the exhaust system. 2. The engine of claim 1,
the at least one NOx sensor is configured to provide a signal representative of a NOx concentration in an exhaust stream in the exhaust system upstream of a location where the NOx-containing gas stream is added; and/or
the at least one NOx sensor is configured to provide a signal representative of a NOx concentration in an exhaust stream in the exhaust system downstream of a 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 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 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
the at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration in 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 representative of an ammonia concentration in an exhaust stream in the exhaust system downstream of the SCR catalyst.
institution. The engine of claim 1, further comprising an ammonia fuel system configured to supply pressurized ammonia to a fuel valve configured to inject or introduce ammonia into the combustion chamber. 1. A method for reducing ammonia slip from a large two-stroke uniflow scavenged turbocharged internal combustion engine, comprising:
a) operating said engine with 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) thereafter directing said exhaust gas stream to an SCR;
A method comprising: 18. The method of claim 17, comprising determining a molar ratio between ammonia and NOx prior to 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 molar ratio is greater than or equal to 1. 19. The method of claim 18, comprising 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. 20. The method of claim 19, comprising 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. 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 the oxidation catalyst;
The pressurized air flow preferably originates from an intake system, preferably at a location downstream of a turbocharger compressor, preferably upstream of an intercooler of the intake system.
20. The method of claim 17.

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