DK181545B1

DK181545B1 - A large two-stroke uniflow scavenged turbocharged internal combustion engine configured for reducing ammonia slip and a method for reducing ammonia slip of such an engine

Info

Publication number: DK181545B1
Application number: DKPA202270475A
Authority: DK
Inventors: Zarah Friedberg Anja
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2022-09-30
Filing date: 2022-09-30
Publication date: 2024-04-24
Also published as: DK202270475A1; KR20240046056A; JP2024052583A; CN117803468A

Abstract

A large two-stroke uniflow scavenged turbocharged internal combustion engine configured for reducing ammonia slip and a method for reducing ammonia slip such engine by: a) operating the aid engine with ammonia as the main fuel thereby producing a stream of exhaust gas containing NOx and NH3, - b) adjusting the ratio between ammonia and NOx in said stream of exhaust gas by adding a controlled stream of NOx to the exhaust gas, and - c) subsequently, submitting the stream of exhaust gas to SCR.

Description

DK 181545 B1 1

A LARGE TWO-STROKE UNIFLOW SCAVENGED TURBOCHARGED INTERNAL

COMBUSTION ENGINE CONFIGURED FOR REDUCING AMMONIA SLIP AND A

METHOD FOR REDUCING AMMONIA SLIP OF SUCH AN ENGINE

TECHNICAL FIELD

This disclosure relates to a large two-stroke internal combustion engine, in particular a large two-stroke uniflow scavenged turbocharged internal combustion engine, that in at least one mode of operation is operated with ammonia (NH3) as the main fuel for combustion in the engine.

BACKGROUND

Large two-stroke uniflow scavenged turbocharged compression- ignited internal combustion crosshead engines are typically used in propulsion systems of large ships or as a prime mover in power plants. The sheer size, weight, and power output render them completely different from common combustion engines and place large two-stroke turbocharged compression- ignited internal combustion engines in a class for themselves.

Internal combustion engines have in the past mainly been operated with hydrocarbon fuels, such as fuel oil, e.g. diesel oil, or fuel gas, e.g. natural gas or petroleum gas. The combustion of hydrocarbon fuels releases carbon dioxide (C02), as well as other greenhouse gases that contribute to atmospheric pollution and climate change. Unlike fossil fuel impurities that result in byproduct emissions, CO2 is an unavoidable result of hydrocarbon combustion. The energy density and CO2 footprint of a specific fuel depend on the hydrocarbon chain length and the complexity of its hydrocarbon molecules. Hence, gaseous hydrocarbon fuels have a lower

DK 181545 B1 2 footprint than liquid hydrocarbon fuels, with the drawback that gaseous hydrocarbon fuels are more challenging and costly to handle and store. In order to reduce the C02 footprint, non-hydrocarbon fuels are being developed.

Ammonia (NH3) is a synthetic product obtained from fossil fuels, biomass, or renewable or sustainable sources (wind, solar, hydro, nuclear, or thermal), and when generated by renewable/sustainable sources, NH3 will have virtually no carbon footprint nor emit any CO2, SOX, particulate matter, or unburned hydrocarbons when combusted.

NH3 has been tested and used at a minor scale in small internal combustion engines, e.g. used in automobiles, but has not yet been used to power large two-stoke internal combustion engines.

The combustion gases generated by combustion ammonia (NH3) in a large two-stroke internal combustion engine can contain both NOx and NH3. NOx is limited by international regulations, e.g. IMO Tier II and III, while the realistically acceptable level for NH3 is quite low although currently not formally limited by regulation. In particular, the NH3 slip that can be tolerated in the exhaust gas is difficult to achieve without an NH3 abatement system (post-treatment system) in the exhaust system of the engine, i.e. without countermeasures, exhaust gas containing unacceptable amounts of NH3 could end up in the atmosphere.

Known systems for removing NH3 from exhaust gas use an Ammonia slip catalyst (ASC or AMOX). NOx is reduced using a Selective

DK 181545 B1 3

Catalytic Reduction (SCR) catalyst. NH3 slip in the exhaust gas 1s controlled using the ammonia slip catalyst (ASC). The

ASC catalyst is placed downstream of the SCR catalyst where the NOx and NH3 already have reacted to remove NOx. If for some reason, NH3 is present after the SCR catalyst, this will be oxidized over an ASC removing the NH3. The ASC treats the entire gas amount as the SCR does. Hence if an ASC was to be fitted to a large two-stroke internal combustion engine, the size of the ASC would be similar to the SCR catalyst, since all of the exhaust gas would need to be treated. Since the

SCR catalyst is a very voluminous piece of equipment, adding another very voluminous piece of equipment is problematic.

Another disadvantage 1s that nitrous oxide (N20) can be a byproduct of the NH3 oxidation on an ASC. Known abatement systems for nitrous oxides exist, but need temperatures in excess of 400C to be effective, an exhaust gas temperature that can not be achieved easily with a high-efficiency marine engine.

Another known technology is scrubbing the NH3 out of the exhaust gas, using a wet scrubber, introducing a very substantial and bulky component on board a ship and waste water stream on board not easily disposible.

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

SUMMARY

It is an object to provide a large two-stroke internal combustion engine that overcomes or at least reduces the problems mentioned above. It is another object to provide a

DK 181545 B1 4 method for reducing ammonia slip from a large 2-stroke internal combustion engine.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided a large two- stroke uniflow scavenged turbocharged internal combustion engine, which has at least one mode of operation in which the main fuel is ammonia, the engine comprising: - at least one cylinder with a cylinder liner and a reciprocating piston therein and a cylinder cover covering the cylinder, - a combustion chamber formed inside 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 a stream of exhaust gas generated by combustion of ammonia in the combustion chamber, - a turbocharging system comprising at least one compressor in the intake system for compressing scavenging air and at least one exhaust gas driven turbine in the exhaust system for driving the compressor, - a SCR catalyst in the exhaust system, preferably upstream of the exhaust gas driven turbine, - means to add a stream of gas containing NOx to the stream exhaust gas in or upstreams of the SCR catalyst.

DK 181545 B1

The inventor realized that ammonia slip can be avoided if it is ensured that the ammonia is reduced in the SCR catalyst.

However, ammonia is not reduced when insufficient NOx is present in the SCR catalyst, i.e. when the molar ratio between 5 ammonia and NOx is above 1. The ratio between ammoniaand NOx in the exhaust leaving the combustion chambers cannot always be controlled (accurately) or predicted (accurately). By providing a stream of gas containing NOx to the exhaust gas it can be ensured that the required amount of NOx for complete reduction of ammonia is always present in the SCR catalyst to ensure that all or at least nearly all of the ammonia that is present in the exhaust gas is reduced in the SCR catalyst.

Only a relatively small stream of gaseous NOx is needed to obtain the desired result.

In a possible implementation form of the first aspect, the stream of gas containing NOx is created by treating a stream of ammonia and air over an oxidation catalyst to convert ammonia to NO. Even though some heat needs to be applied to the process, the amount is smaller when compared to the before mentioned known solutions, and the size of the oxidation catalyst is small compared to the known ammonia slip catalyst installation. Thus, the engine according to the first aspect will be less voluminous and less expensive to construct and maintain.

In a possible implementation form of the first aspect, the engine comprises a controller configured to control the magnitude of the stream of gas containing NOx added to the stream of exhaust gas.

DK 181545 B1 6

In a possible implementation form of the first aspect, the engine comprises means to measure and/or estimate the molar ratio between ammonia and NOx in the stream of exhaust gas.

In a possible implementation form of the first aspect, the controller is configured to adjust the magnitude of the stream of gas containing NOx as a function of measured and/or estimated molar ratio between ammonia and NOx in the stream of exhaust gas.

In a possible implementation form of the first aspect, the controller is configured to adjust the magnitude of the stream of NOx to a magnitude that results in the stream of exhaust gas entering the SCR catalyst having a molar ratio between ammonia and NOx equal to or below 1, preferably slightly below 1.

In a possible implementation form of the first aspect, the stream of gas comprising NOx comprises NO and NO2, and wherein the engine comprises means for adjusting the ratio between NO and NO2 in the stream of gas comprising NOx.

In a possible implementation form of the first aspect, the engine comprises a sensor system that provides one or more signals that allow the controller to determine the molar ratio between ammonia and NOx in the stream of exhaust gas.

In a possible implementation form of the first aspect, the engine comprises a NOx generating system for generating the stream of gas containing NOx, the NOx generating system preferably comprising a source for providing a stream of

DK 181545 B1 7 ammonia that is, preferably catalytically, oxidized into NO and H20 to obtain the steam of gas containing NOx.

In a possible implementation form of the first aspect, the engine comprises an oxidation «catalyst, preferably a platinum-rhodium catalyst, which engine preferably comprises a supply of pressurized gaseous ammonia and a supply of pressurized air to the oxidation catalyst, the source of the pressurized air preferably being scavenging air from the intake system.

In a possible implementation form of the first aspect, the

NOx generating system is configured to control the ratio of

NO and NO2 in the gas comprising NOx.

In a possible implementation form of the first aspect, the controller is configured to determine the optimal ratio between NO and NO2 in the gas comprising NOx and configured to adjust the ratio between NO and NO2 in the gas comprising

NOx accordingly.

In a possible implementation form of the first aspect, the engine comprises downstream of the NOx generating system a catalytic N20 abatement system, preferrably an iron zeolite catalyst, for removing N20 that can be produced as result of a side reaction inside the NOx generating unit

In a possible implementation form of the first aspect, the engine comprises a container for storing the gas containing

NOx, preferably for storing the gas containing NOx under high pressure, the container preferably being connected to the

DK 181545 B1 8 exhaust gas system via a control valve to thereby allow a controlled stream of the gas containing NOx from the container into the stream of exhaust gas.

In a possible implementation form of the first aspect, the engine comprises at least one NOx sensor configured for providing a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system, and at least one ammonia sensor configured for providing a signal representative of an ammonia concentration in the stream of exhaust gas in the exhaust system.

In a possible implementation form of the first aspect, the at least one NOx sensor is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system upstream of a position where the stream of gas containing NOx is added, and/or wherein the at least one NOx sensor is configured to provide a signal representative of a NOx concentration of the stream exhaust gas in the exhaust system downstream of a position where the stream of gas containing NOx is added and upstream of the SCR catalyst, and/or wherein the at least one NOx sensor is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system downstream of the SCR catalyst, and/or wherein the at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration of the stream of exhaust gas in the exhaust system upstream of a position where the stream of gas containing NOx is added, and/or

DK 181545 B1 9 wherein the at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration of the stream of exhaust gas in the exhaust system downstream of a position where the stream of gas containing NOx is added and upstream of the SCR catalyst, and/or wherein the at least one ammonia sensor is configured to provide a signal representative of an ammonia concentration of the stream of exhaust gas in the exhaust system downstream of the SCR catalyst.

In a possible implementation form of the first aspect, the engine comprises an ammonia fuel system 30 configured for supplying pressurized ammonia to fuel valves 50,50’ that are configured to inject or admit ammonia to 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 the engine with ammonia as the main fuel thereby producing a stream of exhaust gas containing NOx and

NH3, - b) adjusting the ratio between ammonia and NOx in the stream of exhaust gas by adding a controlled stream of NOx to the exhaust gas, and - c) subsequently, submitting the stream of exhaust gas to a

SCR.

In a possible implementation form of the second aspect, the method comprises determining the molar ratio between ammonia

DK 181545 B1 10 and NOx in the stream of exhaust gas prior to adding the controlled stream of NOx to the exhaust gas, and adding the controlled stream of NOx to the stream of exhaust gas, when, and preferably only when, the molar ratio is equal or above 1.

In a possible implementation form of the second aspect, the method comprises determining the magnitude of the stream of

NOx required to lower the determined molar ratio to a level below 1, preferably slightly below 1, and adjusting the magnitude of the stream of gas containing NOx to the determined magnitude.

In a possible implementation form of the second aspect, the method comprises determining a desired ratio between NO to

NO2 in the stream of gas containing NOx and adjusting the ratio between NO to NO2 in the stream of gas containing NOx.

In a possible implementation form of the second aspect, the method comprises supplying a stream of pressurized gaseous ammonia and a stream of pressurized air to an inlet of an oxidation catalyst to create a stream of gas containing NOx leaving an outlet of the oxidation catalyst, the stream of pressurized air preferably originating from the intake system, preferably originating from the intake system at a position downstream of the compressor and preferably upstream of an intercooler.

These and other aspects will be apparent from the drawings and the embodiment (s) described below.

DK 181545 B1 11

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

Fig. 1 is an elevated front view of a large two-stroke internal combustion engine according to an example embodiment,

Fig. 2 is an elevated side view of the large two-stroke engine of Fig. 1, and

Fig. 3 is a diagrammatic representation of an embodiment of the large two-stroke engine of Fig. 1 with an ammonia fuel system and an ammonia slip abatement system,

Fig. 4 is a flowchart of an embodiment of a process for reducing ammonia slip of a large two-stroke internal combustion engine,

Fig. 5 is a flowchart of another embodiment of a process for reducing ammonia slip of a large two-stroke internal combustion engine, and

Fig. 6 is a flowchart of yet another embodiment of a process for reducing ammonia slip of a large two-stroke internal combustion engine,

DETAILED DESCRIPTION

In the following detailed description, an internal combustion engine will be described with reference to a large two-stroke low-speed uniflow scavenged turbocharged internal combustion engine with crossheads in the example embodiments, but it is understood that the internal combustion engine could be of another type. The large two-stroke low-speed uniflow

DK 181545 B1 12 scavenged turbocharged internal combustion engine can be of the (high-pressure) type in which fuel is injected at or near top dead center (TDC) of the pistons that is compression- ignited or of the (low pressure) type in which fuel is mixed with the scavenging air before or during compression (pre-mix engine) and the mixture of air and fuel is spark ignited or the like. In the pre-mix engine, there will typically be a “pilot” ignition with an ignition fluid, e.g. fuel oil, for ensuring reliable ignition.

Figs. 1, 2, and 3 show a large low-speed turbocharged two- stroke engine with a crankshaft 8 and crossheads 9 that is configured to operate according to the Diesel principle, i.e. it is a compression-ignition engine. Fig. 3 shows a diagrammatic representation of a large low-speed turbocharged two-stroke diesel engine with its intake and exhaust systems.

In this example embodiment, the engine has six cylinders in line. Large low-speed turbocharged two-stroke diesel engines have typically between four and fourteen cylinders in line, carried by a cylinder frame 23 that is carried by an engine frame 11. The engine may e.g. be used as the main engine in a marine vessel or as a stationary engine for operating a generator in a power station. The total output of the engine may, for example, range from 1,000 to 110, 000 kW.

The engine can be configured as a dual-fuel engine. The engine can be a compression-ignited engine or a premix engine. The engine according to the present embodiment is of the two- stroke uniflow type with scavenging ports 18 in the lower region of the cylinder liners 1 and a central exhaust valve

DK 181545 B1 13 4 at the top of each cylinder liner 1. The engine has at least one ammonia mode in which the engine is operated on ammonia fuel or an ammonia-based fuel and at least one conventional fuel mode in which the engine is operated on conventional fuel, e.g. fuel oil (marine diesel), or heavy fuel oil.

The scavenge air is passed from the scavenge air receiver 2 to the scavenge ports 18 of the individual cylinders 1. A piston 10 that reciprocates in the cylinder liner 1 between the bottom dead center (BDC) and top dead center (TDC) compresses the scavenge air. Fuel (ammonia in the ammonia mode) is injected through fuel valves 50 that are arranged in the cylinder cover 22 into the combustion chamber at high pressure when the piston is at or near TDC (Diesel principle).

When the engine is configured as a pre-mix engine, the fuel is admitted at a relatively low pressure when the piston is on its way towards TDC (Otto principle) from fuel admission valves 50’ (there will typically be 2 or more fuel admission valves 50’ for each cylinder). The fuel admission valves 507 can be arranged in the cylinder liner at a position above the scavenge ports 18, or in the cylinder cover 22. Combustion follows, and exhaust gas is generated. If the engine is configured for compression-ignition, each cylinder cover 22 is provided with two or more fuel valves 50. The fuel valves 50 are either configured to inject only one specific type of fuel, e.g. ammonia, and in this case, there will also be two or more fuel valves (not shown in Fig. 3) for injecting conventional fuel into the combustion chamber. The fuel valves 50 are arranged in the cylinder cover 22 around the central exhaust valve 4. Further, additional, typically smaller fuel valves (not shown) are in an embodiment provided in the

DK 181545 B1 14 cylinder cover for injecting ignition fluid, for ensuring reliable ignition of the ammonia fuel.

The ignition fluid is e.g. dimethyl ether (DME) or fuel oil, but can also be another form of ignition enhancer, such as hydrogen.

Since the engine can be a dual-fuel engine it can also be provided with a conventional fuel supply system (not shown) for supplying the conventional fuel to the fuel valves 50. In an embodiment,

the fuel valves 50’ are arranged along the cylinder liner (shown by the interrupted lines) and admit the fuel into the cylinder before the piston 10 passes the fuel valves 50’ on its way from BDC to TDC.

Thus, when the engine is configured for pre-mix operation, the piston 10 compresses a mixture of scavenging air and fuel.

Timed ignition at or near TDC is triggered by spark, laser, ignition fluid injection, or the like.

In the embodiment with the fuel valves 50’, the pressure at which the fuel is admitted is substantially lower than the pressure at which the fuel is injected in the embodiment with the fuel valves 50 in the cylinder cover 22, which injects when the piston is at or near top dead center (TDC) and the pressure at which the fuel 1s injected needs to be significantly higher than the compression pressure.

Thus, in an embodiment, the engine operates according to the Diesel principle (compression-ignition) and compresses only scavenging air (scavenging gas if exhaust gas recirculation is used), and in other embodiments, the engine operates according to the Otto cycle (timed ignition) and compresses a mixture of fuel and scavenging gas.

The pressure at which the fuel supply system 30 needs to deliver fuel can be significantly lower when operating according to the Otto principle, and pressure boosters, that are often used in the

DK 181545 B1 15 fuel valves 50 for compression-ignition engines can be avoided.

When an exhaust valve 4 is opened, the exhaust gas flows through an exhaust duct associated with the cylinders into the exhaust gas receiver 3 and onwards through a first exhaust conduit 19 via selective catalytic reaction (SCR) catalyst 40 to a turbine 6 of the turbocharger 5, from which the exhaust gas flows away to the atmosphere through a second exhaust conduit 28.

Through a shaft, the turbine 6 of the turbocharger 5 drives a compressor 7 supplied with fresh air via an air inlet 12.

The compressor 7 delivers pressurized scavenge air to a scavenge air conduit 13 leading to the scavenge air receiver 2. The scavenge air in the scavenge air conduit 13 passes an intercooler 14 for cooling the scavenge air.

The cooled scavenge air passes via an auxiliary blower 16 driven by an electric motor 17 that pressurizes the scavenge air flow when the compressor 7 of the turbocharger 5 does not deliver sufficient pressure for the scavenge air receiver 2, i.e. in low or partial load conditions of the engine. At higher engine loads the turbocharger compressor 7 delivers sufficient compressed scavenge air and then the auxiliary blower 16 is bypassed via a non-return valve 15 and the electric motor 17 is deactivated. The turbocharging system may comprise more than one turbocharger 5.

The engine is in the ammonia mode operated with ammonia as the main fuel which is supplied to the fuel valves 50 or 50'

DK 181545 B1 16 by the ammonia fuel system 30 at a substantially stable pressure and temperature. The ammonia can be supplied to the ammonia valves 50 in the liquid phase or the gaseous phase.

The ammonia liquid phase can be aqueous ammonia (ammonia- water blend).

The conventional fuel system is well known and not shown and described in further detail. The ammonia fuel system 30 supplies the fuel valves 50 or fuel admission valves 50’ with liquid phase ammonia at a medium supply pressure (e.g. 30 to 80 bar pressure). Alternatively, the ammonia fuel is supplied at a relatively low supply pressure (e.g. 30 to 80 bar pressure) to the ammonia valves 50 in the gaseous phase. If the engine of the compression-igniting type, the fuel valves 50 comprise a pressure booster that significantly raises the pressure of the ammonia fuel from the medium pressure to a high pressure to allow the ammonia fuel to be injected at a pressure well above the compression pressure of the engine.

Typically, the injection pressure for an ignition-compressing engine is above 300 bar.

In an embodiment, the engine is provided with an exhaust gas recirculation system for reintroducing a portion of the exhaust gas to the combustion chambers, together with the scavenging air, e.g. to reduce NOx generation

In the ammonia fuel system 30, ammonia is stored in the liquid phase in a pressurized storage tank at approximately 17 bar.

Ammonia can be stored in the liquid phase at a pressure above 8.6 bar and an ambient temperature of 20°C in an ammonia storage tank. However, ammonia 1s preferably stored at

DK 181545 B1 17 approximately 17 bar or higher to keep it in the liquid phase when the ambient temperature increases.

A low-pressure ammonia supply line connects an outlet of the ammonia storage tank (not shown) to the inlet of a medium- pressure feed pump (not shown). A low-pressure feed pump forces the liguid phase ammonia from the ammonia storage tank to an inlet of the medium pressure feed pump. The medium pressure feed pump forces the liquid ammonia through a medium pressure ammonia supply line (not shown) to the fuel valves 50,507.

An electronic control unit (controller) 50 is connected via signal lines or wirelessly to various components and sensors of the engine.

In NH3 combustion, the exhaust from the engine can contain both NOx and NH3 (opposite to combustion of fossil fuel that does not result in exhaust gas containing NH3). The ratio between these two substances in the exhaust gas cannot always be accurately controlled or predicted. However, the SCR catalyst 40, does not only function as a NOx removal catalyst but also as an NH3 removal catalyst. The molar ratio in the exhaust gas between NH3 and NOx, referred to as alpha, will determine how much NOx and NH3 that can be removed, since the two species react one to one. If alpha is below 1, then NOx is in excess and all the NH3 can react with the NOx, resulting in substantially zero NH3 leaving the outlet of the SCR catalyst 40 and some NOx, which is allowed by IMO Tier III.

If alpha is above 1, then NH3 is in excess and all the NOx will react with the NH3 present, and the excess of NH3 will

DK 181545 B1 18 exit the SCR catalyst as an NH3 slip. The NH3 slip that can be tolerated in the exhaust is low (an example of a limit could be 10 ppm), and therefore it is desirable to keep the alpha below 1.

The SCR catalyst 40 serves to remove both NOx components, NO and NO2, from the exhaust gas. The SCR catalyst 40 is in an embodiment vanadium based. The SCR catalyst 40 is in the present embodiment arranged on the high pressure side of the turbine 6, but could in other embodiments be placed on the low pressure side of the turbine 6, although this would increase the volume of the SCR catalyst 40. In the present embodiment, the inlet of the SCR catalyst 40 is connected to the outlet of the exhaust gas receiver 3.

Based on the concentration of NOx and NH3 in the gas stream coming from or in the exhaust gas receiver 3, which is measured or calculated by the controller 50, the required amount (magnitude of the stream) of NOx to be added to the stream of exhaust gas flowing to the SCR catalyst 40 is calculated to reach the desired alpha. This additional stream of NOx is on an embodiment produced from a side stream containing NH3, for example, coming from the ammonia fuel system 30. This NH3 is catalytically oxidized at elevated temperatures (preferably above 500 °C) together with a stream of air that is supplied to obtain NO and H20 (water) in an oxidizing catalyst 43. The source of the stream of pressurized air is preferably scavenging air taken from the intake system and controlled by a control valve 27, since this is an effective way to obtain high-temperature pressurized air, especially if the scavenging air is taken from the intake

DK 181545 B1 19 system upstream of the intercooler 14 (and downstream of the compressor 7). This side stream can be a separate feed of NH3 as in the embodiment of Fig. 3, or it can be taken from the total exhaust. In both cases, the size of the sidestream is controlled e.g. by a control valve 42 that is adjusted by the controller 50 in accordance with the above-mentioned calculation. The catalyst for the NH3 oxidation could be of a similar type as the one used in nitric acid production (HNO3) where NH3 is catalytically oxidized over platinum- rhodium catalyst gauzes of the oxidizing catalyst 43 and the following reaction takes place: 4NH3 + 502 -> 4NO + 6 H20

Besides NO and water, the oxidation can also give unwanted nitrous oxide (N20) according to the following reaction: 4NH3 + 402 -> 2N20 + 6H20

If any N20 is generated this relatively small stream of gas containing NOx can be treated using a decomposition catalyst for N20. The side stream, which now contains primarily air with NO and water is then mixed with the exhaust gas stream from the exhaust gas receiver 3. In this way, the molar based concentration of NO in the stream of exhaust gas is increased above the molar based concentration of NH3 , preferably slightly above. This mixed stream of exhaust gas is lead to the SCR catalyst 40 where the NO and NH3 will react according to the following reaction where 1 mole of NO reacts with 1 mole of NH3 according to the standard SCR process:

DK 181545 B1 20

ANO + 4 NH3 + 02 -> 4N2 + 6H20

When the stream of gas exits the SCR catalyst 40, substantially all the NH3 will have been removed, and the NOx will have been decreased to reach IMO Tier III level.

A sensor system 44,45,46,47,48,49 provides one or more signals that allow the controller 50 to determine the molar ratio (alpha) between ammonia and NOx in the stream of exhaust gas.

Preferably, the sensors comprise at least one ammonia sensor 45,477,499 configured to provide a signal representative of the concentration of ammonia in the stream of exhaust gas and at least one NOx sensor 44,46,48 configured to provide a signal representative of the concentration of NOx in the stream of exhaust gas. Three ammonia sensors 45, 47, 49, and three NOx sensors 44, 46, and 48 are shown in Fig. 3. However, it is understood that only one pair of sensors is needed to provide the controller 50 with sufficient information to determine alpha. In an embodiment, the pair of ammonia and NOx sensors are configured to measure the concentration in the exhaust gas receiver 3.

In an embodiment, the controller 50 is configured to control the magnitude of the stream of gas containing NOx in a closed loop manner, by comparing the determined alpha downstream of the position where the stream of gas containing NOx is added to the stream of exhaust gas with a desired alpha and controlling the magnitude of the stream of gas containing NOx accordingly, for example by adjusting the position of the control valve 42. Alternatively, the controller 50 is

DK 181545 B1 21 configured for feed-forward control of the magnitude of the stream of gas containing NOx.

The engine is optionally configured to control alpha by the addition of NH3, e.g. in the form of urea, or as shown in the form of gaseous NH3 that is added to the first exhaust conduit 19 upstream of the inlet of the SCR catalyst 40, through a line controlled by ammonia control valve 41. Thus, if alpha is substantially below 1, NOx emissions can be controlled by adding ammonia to the exhaust gas by opening the control valve 41. Hereto, the electronic control unit 50 is configured to adjust the amount of ammonia added, i.e. the conduit of the stream of ammonia into the first exhaust line 19, in accordance with the alpha that has been determined by the controller 50. Thus, regardless of whether the exhaust gas coming from the exhaust gas receiver 3 has an excess of ammonia or has an excess NOx, both ammonia slip and NOx emissions can be substantially reduced by adding a stream of gas containing NOx of a controlled magnitude to the stream of exhaust when alpha is 1 or there above or by adding a stream of ammonia or urea (reductant) of a controlled magnitude to the stream of exhaust gas when alpha is substantially below 1.

The controller 50 is configured to adjust the magnitude of the stream of NOx to a magnitude that results in the stream of exhaust gas entering the SCR catalyst 40 having a molar ratio between ammonia and NOx equal to or below 1, preferably slightly below 1, i.e. a molar concentration of ammonia that is equal or below the molar concentration of NOx, preferably slightly below.

DK 181545 B1 22

The stream of gas comprising NOx comprises both NO and NO2.

The ratio between NO and NO2 in the stream of exhaust gas may be different for different operating conditions. In an embodiment (not shown), the engine comprises means for adjusting the ratio between NO and NO2 in the stream of gas comprising NOx. The NOx generating system is in an embodiment configured to control the ratio between NO and NO2 in the gas comprising NOx and the controller 50 1s configured to determine the optimal ratio between NO and NO2 in the gas comprising NOx and configured to adjust the ratio between NO and NO2 in the gas comprising NOx accordingly. The amount of

NO2 versus NO can e.g. be controlled if the stream of gas containing NOx after the oxidation catalyst is cooled. This can be done in order to control the ratio between NO2 and NO and this is important for the efficiency of the SCR reactor 40. If NO2 is present in the stream of gas containing NOx but still less than the amount of NO, then the so-called fast SCR reaction can take place:

NO + NO2 + 2NH3 -> 2N2 + 3H20

However, this needs to be controlled because too much NO2 present in gas compared to NO, will decrease the efficiency of the SCR catalyst 40, and the so-called slow SCR reaction will occur: 8NH3 + 6NO2 -> 7N2 + 12H20

In an embodiment (not shown), the engine comprises a container for storing the gas containing NOx, preferably for storing

DK 181545 B1 23 the gas containing NOx under high pressure, e.g. a high pressure gas bottle containing NOx. The container is preferably connected to the exhaust gas system via a control valve to thereby allow a controlled stream of gas containing

NOx from the container into the stream of exhaust gas.

The at least one NOx sensor 44 is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system upstream of a position where the stream of gas containing NOx is added. The NOx sensor 46 is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system downstream of a position where the stream of gas containing NOx is added and upstream of the SCR catalyst 40.

The at least one NOx sensor 48 is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas 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 an ammonia concentration of the stream of exhaust gas in the exhaust system upstream of a position where the stream of gas containing NOx is added.

The at least one ammonia sensor 47 is configured to provide a signal representative of an ammonia concentration of the exhaust gas in the exhaust system downstream of a position where the stream of gas containing NOx is added and upstream of the SCR catalyst 40. The at least one ammonia sensor 49 is configured to provide a signal representative of an ammonia concentration of the stream of exhaust gas in the exhaust system downstream of the SCR catalyst 40.

DK 181545 B1 24

Fig. 4 is a flow chart illustrating 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 embodiments above. The method comprises operating the engine with ammonia as the main fuel thereby producing a stream of exhaust gas containing

NOx and NH3, determining alpha in the exhaust gas coming from the cylinders, adjusting the alpha of the stream of exhaust gas by adding a controlled stream of NOx to the exhaust gas, and subsequently, submitting the stream of exhaust gas to SCR (selective catalytic reduction), for example, in the SCR catalyst 40.

Alpha of the exhaust gas coming from the cylinders or entering the SCR catalyst 40 is determined and if alpha is equal to or above 1, a stream of NOx is added to the stream of exhaust gas, upstream of the SCR catalyst 40.

The method further comprises determining the molar ratio between ammonia and NOx prior to adding the controlled stream of NOx to the stream of exhaust gas, and adding the controlled stream of NOx to the stream of exhaust gas, when, and preferably only when, the molar ratio is equal or above 1.

In the embodiment of the method according to Fig. 5, the method comprises determining the magnitude of the stream of

DK 181545 B1 25

The amount of NO that is required (the magnitude of the stream of gas containing NOx) to be added to the stream of exhaust gas to reach the desired alpha will determine the magnitude of the flow of ammonia over the oxidation catalyst 43. The amount of NH3 that should be oxidized is at least the same amount of moles as the NH3 excess compared to NO in the engine out gas- that is: moles NH3 out of the engine - moles NO out of engine = moles NO that is needed extra = moles NH3 that should be oxidized (if 100% conversion) and this is to reach an alpha of 1. Typically, the SCR catalyst 40 is sized for an alpha between 0.8 and 0.95 and the controller is configured to adjust the process to obtain an alpha value accordingly. In the case where NH3 is added as a side stream to the oxidation catalyst 43, the concentration of NH3 in the airflow is typically around 9.5-11.5% with a yield of NO between 90-98%. The amount of airflow to the

Oxidation catalyst 43 will depend on the concentration and demand but could be in the range of 0.06-0.3 kg/kWh air corresponding to 4-20 g/kWh NH3.

In the embodiment of the method according to Fig. 6, the method comprises determining a desired ratio between NO to

The various aspects and implementations have been described in conjunction with various embodiments herein. However,

DK 181545 B1 26 other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.

The reference signs used in the claims shall not be construed as limiting the scope. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this disclosure.

Claims

DK 181545 B1 27 PATENT CLAIM

1. Large, turbocharged, longitudinally scavenged two-stroke internal combustion engine having at least one mode of operation in which the main fuel is ammonia, which engine comprises: - at least one cylinder with a cylinder liner (1) and a reciprocating piston (10) therein and a cylinder cover ( 22) covering the cylinder, - a combustion chamber formed inside the cylinder (1) between the reciprocating piston (10) and the cylinder cover (22), - an intake system for supplying scavenging air to the combustion chamber, - an exhaust system for exhausting a stream of exhaust gas generated by the combustion of ammonia 1 the combustion chamber, - a turbocharging system (5) comprising at least one compressor (7) 1 the intake system for compressing scavenging air and at least one exhaust gas driven turbine (6) in the exhaust system for operating the compressor ( 7), - an SCR catalyst (40) in the exhaust system, preferably upstream of the exhaust gas driven turbine (6), characterized by - a means for adding a gas flow containing NOx to the flow of exhaust gas in or upstream of the SCR catalyst (40) .

Engine according to claim 1, comprising a control unit (50) configured to control the amount of the flow of gas containing NOx added to the flow of exhaust gas.

DK 181545 B1 28

3. Engine according to claim 2, comprising means for measuring and/or estimating the molar ratio between ammonia and NOx in the stream of exhaust gas.

4. The engine of claim 3, wherein the control unit (50) is configured to adjust the magnitude of the flow of gas containing NOx as a function of measured and/or estimated molar ratio of ammonia to NOx in the flow of exhaust gas.

5. The engine of claim 4, wherein the control unit (50) is configured to adjust the magnitude of the NOx flow to a magnitude that results in the flow of exhaust gas entering the SCR catalyst (40) having a molar ratio of ammonia and NOx at or below 1, preferably slightly below 1,

i.e. a molar concentration of ammonia that is at or below the molar concentration of NOx, preferably slightly below.

6. An engine according to any one of claims 1 to 5, wherein the flow of NOx-containing gas comprises the NOx components NO and NO2, and wherein the engine comprises means for then adjusting the ratio between NO and NO2 1 the gas flow comprising NOx.

7. Motor according to any one of claims 2 to 6 and comprising a sensor system (44, 45, 46, 47, 48, 49) providing one or more signals which enable the control unit (50) to determine the molar ratio of ammonia to NOx in the exhaust gas stream.

An engine according to any one of claims 1 to 7, comprising a NOx generation system for generating the gas stream containing NOx, which NOx generation system

DK 181545 B1 29 preferably comprises a source for providing a stream of NH3 which, preferably catalytically, is oxidized to NO, NO2 and H20 to obtain the gas stream containing NOx.

9. Engine according to claim 8, and which comprises an oxidation catalyst (43), preferably a platinum-rhodium catalyst, preferably comprising a supply of gaseous ammonia under pressure and a supply of compressed air to the oxidation catalyst (43), where the source of compressed air is preferably scavenging air from the intake system.

10. An engine according to claim 8 or 9, wherein the NOx generation system is configured to control the ratio between NO and NO2 in the gas comprising NOx.

Engine according to claim 8, 9 or 10, wherein the control unit (50) is configured to determine the optimum ratio between NO and NO2 in the gas comprising NOx and configured to adjust the ratio between NO and NO2 in the gas comprising NOx accordingly.

12. An engine according to any one of claims 8 to 11, and which downstream of the NOx generation system comprises a catalytic N20 reduction system, preferably an iron zeolite catalyst, for removing N20 that may be produced as a result of a side reaction within the NOx generation unit .

13. Engine according to any one of claims 1 to 12, and which comprises a container for storing the gas containing NOx, preferably for storing gas containing NOx under

DK 181545 B1 30 high pressure, which container is preferably connected to the exhaust gas system via a control valve to thereby enable a controlled flow of the gas containing NOx from the container into the flow of exhaust gas.

An engine according to any one of claims 1 to 13, comprising at least one NOx sensor (44, 46, 48) configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system, and at least one ammonia sensor (45, 47, 49) configured to provide a signal representative of an ammonia concentration in the stream of exhaust gas in the exhaust system,

An engine according to any one of claims 1 to 14, wherein the at least one NOx sensor (44) is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in the exhaust system upstream of a position , wherein the flow of gas containing NOx is added, and/or wherein the at least one NOx sensor (46) is configured to provide a signal representative of a NOx concentration of the flow of exhaust gas in the exhaust system downstream of a position where the stream of gas containing NOx is added and upstream of the SCR catalyst (40), and/or wherein the at least one NOx sensor (48) is configured to provide a signal representative of a NOx concentration of the stream of exhaust gas in

DK 181545 B1 31 the exhaust system downstream of the SCR catalyst (40), and/or where the at least one ammonia sensor (45) is configured to provide a signal representative of an ammonia concentration of the flow of exhaust gas in the exhaust system upstream of a position, wherein the flow of gas containing NOx is added, and/or wherein the at least one ammonia sensor (47) is configured to provide a signal representative of an ammonia concentration of the exhaust gas in the exhaust system downstream of a position where the flow of gas containing NOx is added and upstream of the SCR catalyst (40), and/or wherein the at least one ammonia sensor (49) is configured to provide a signal representative of an ammonia concentration of the stream of exhaust gas in the exhaust system downstream of the SCR catalyst (40).

An engine according to any one of claims 1 to 15, comprising an ammonia fuel system (30) configured to supply ammonia under pressure to fuel valves (50, 507) configured to inject ammonia into or charge feed ammonia into the combustion chamber.

17. A method for reducing ammonia emissions from a large, turbocharged, longitudinally scavenged two-stroke internal combustion engine, which method comprises: - a) operating the engine with ammonia as the main fuel, thereby producing a stream of exhaust gas containing NOx and NH3,

DK 181545 B1 32 - b) adjustment of the ratio between ammonia and NOx in the flow of exhaust gas by adding a controlled flow of NOx to the exhaust gas and - c) subsequent exposure of the flow of exhaust gas to SCR.

18. A method according to claim 17, and which comprises determining the molar ratio between ammonia and NOx before addition to the controlled stream of NOx to the stream of exhaust gas, and adding the controlled stream of NOx to the stream of exhaust gas when, and preferably only when, the molar ratio is at or above 1.

19. A method according to claim 18, which comprises determining the amount of the NOx flow required to lower the determined molar ratio to a level below 1, preferably slightly below 1, and adjusting the amount of the flow of gas containing NOx to the certain size.

20. Method according to claim 19, and which comprises determining a desired ratio between NO and NO2 in the flow of gas containing NOx and adjusting the ratio between NN and NO2 in the flow of gas containing NOx.

A method according to any one of claims 17 to 20, comprising supplying a stream of gaseous ammonia under pressure and a stream of compressed air to an inlet of an oxidation catalyst (43) to produce a gas stream containing NOx which flows out of an outlet in the oxidation catalyst (43), which flow of compressed air preferably originates from the intake system, which preferably

DK 181545 B1 33 originates from the intake system at a position downstream of a compressor (7) 1 the turbocharger (5) and preferably upstream of an intermediate coil (14) in the intake system.