EP4596968A1

EP4596968A1 - Marine boiler with injector for sncr agent

Info

Publication number: EP4596968A1
Application number: EP24154756.1A
Authority: EP
Inventors: Søren MØLGAARD; Kasper Gram Bilde
Original assignee: Alfa Laval Corporate AB
Current assignee: Alfa Laval Corporate AB
Priority date: 2024-01-30
Filing date: 2024-01-30
Publication date: 2025-08-06
Also published as: WO2025162716A1

Abstract

A marine boiler (100) is disclosed, comprising a burner (110) configured to combust an ammonia-based fuel to generate a flame and a furnace (120) configured to be heated by the flame. The furnace comprises a burner inlet (112) allowing the burner to release the flame into an inlet zone (10) of the furnace, an exhaust outlet (114) allowing combustion gases to be exhausted from an outlet zone (20) of the furnace, and an injector outlet (116) for introducing a selective non-catalytic reduction, SNCR, agent into the outlet zone to reduce a level of nitrogen oxides in the combustion gases. The injector outlet is arranged to introduce the SNCR agent in a peripheral part of an interior of the furnace. A method for reducing the level of nitrogen oxides in combustion gases of a marine boiler is also disclosed.

Description

Technical Field

The present invention relates to marine boilers, and more particularly to a technology for reducing nitrogen oxide (NOx) emissions from such boilers.

Background

Marine boilers conventionally use carbon-based fuels to generate the heat necessary for steam production. While effective in generating the required heat, the use of these carbon-based fuels has been increasingly scrutinised due to their contribution to carbon dioxide emissions and global warming.
To mitigate the environmental impact, a shift towards ammonia-based fuels has been proposed. Ammonia is a promising alternative, as the absence of carbon helps eliminating carbon dioxide emissions during combustion. However, this shift presents a new environmental challenge in terms of increased levels of nitrogen oxides (NOx) in the exhaust gas. NOx emissions contribute to air pollution, which is harmful to the environment and poses a human health hazard.
Therefore, there is a need for an improved technology that effectively reduces NOx emissions in marine boilers operating on ammonia-based fuels.

Summary

It is an object of the present disclosure to provide a technology that addresses at least some of the above concerns.
According to a first aspect of the present invention, there is provided a marine boiler. The marine boiler comprises a burner configured to combust an ammonia-based fuel to generate a flame, and a furnace configured to be heated by the flame to transfer heat to a thermal medium in thermal contact with the furnace. The furnace comprises a burner inlet allowing the burner to release the flame into an inlet zone of the furnace, and an exhaust outlet allowing combustion gases to be exhausted from an outlet zone of the furnace. Further, an injector outlet is provided for introducing a selective non-catalytic reduction (SNCR) agent into the outlet zone to reduce a level of nitrogen oxides in the combustion gases. The injector outlet is arranged to introduce the SNCR agent in a peripheral part of an interior of the furnace.
According to a second aspect, a method for reducing a level of nitrogen oxides in combustion gases of a marine boiler is provided. The method comprises combusting an ammonia-based fuel to generate a flame and directing the flame into a furnace of the boiler, wherein the furnace comprises an inlet zone into which the flame is released and an outlet zone comprising an exhaust outlet for exhausting combustion gases from the furnace. Further, the method comprises injecting a selective non-catalytic reduction (SNCR) agent into the outlet zone, in a peripheral part of an interior of the furnace, to reduce the level of nitrogen oxides in the combustion gases.
Conventionally, selective catalytic reduction (SCR) is employed to reduce NOx emissions. In SCR, an SCR agent (such as ammonia or urea) is injected into the exhaust gas before it passes through a catalyst bed, which facilitates a chemical reaction converting NOx into nitrogen and water vapour. The SCR unit is typically installed downstream the exhaust outlet of the boiler and requires additional infrastructure such as SCR agent storage and handling systems, as well as space for the catalyst. The space requirement may be a challenge, in particular in marine applications where the available space often is limited. To address the increased levels of NOx generated when running the boiler on ammonia-based fuels, the inventors propose to employ an SNCR process to reduce the NOx levels in the combustion gases before they reach the SCR unit. By pre-treating the combustion gases to reduce the NOx levels, the SCR unit may be omitted, or a less powerful or smaller SCR unit might suffice to take care of the remaining NOx.
The SNCR process is typically effective only within a specific temperature range, which is relatively high compared to the temperature range in which the catalytically assisted SCR unit operates. The inventors have realised that the boiler can be operated in a way that makes it possible to use the temperature gradient inside the furnace to achieve the right temperature conditions for an efficient SNCR process to take place. When combusting the ammonia-based fuel, a flame with a temperature of about 1500-1800°C can be obtained in the inlet zone of the furnace. Due to the thermal radiation inside the furnace, in which heat from the flame is transferred to the thermal medium, the temperature of the combustion gases has been observed to decrease relatively quickly as they move towards the exhaust outlet, resulting in a temperature of about 1100°C or lower in the outlet zone of the furnace. By injecting the SNCR agent in this region, a relatively efficient SNCR process can be achieved, reducing the NOx content in the combustion gases as they pass towards the exhaust outlet and any downstream exhaust tube system.
Additionally, injecting the SNCR agent into the furnace makes it possible to utilise any swirl zones or recirculation zones in the furnace to achieve an improved mixing with the combustion gases and, possibly, retain the mixture in the outlet zone for a sufficiently long period of time for the SNCR process to take place before the combustion gases are exhausted through the exhaust outlet.
The SNCR process is typically an exothermic process, in which heat is released to the combustion gases in the furnace. This may be advantageous over technologies in which the heat is released downstream the furnace, as the furnace may be operated at a higher thermal efficiency and thus release more heat before the combustion gases proceed to the downstream exhaust tube system.
By introducing the SNCR agent into the peripheral part of the interior of the furnace, the injector structure, of which the outlet may form a part, may be less exposed to the heat and hence last longer before it needs to be replaced. Additionally, the placement of the injector outlet allows for the SNCR agent to be introduced in a manner that facilitates mixing, as upwards-moving recirculation zones that may be present at the interior walls of the furnace may be employed to avoid that the SNCR agent is sucked into and consumed by the flame.
In some examples, the SNCR agent comprises ammonia, which preferably may be anhydrous ammonia (i.e., pure ammonia not dissolved in water), ammonia with a negligible water content, or ammonia dissolved in water (such as e.g. urea). The absence of water may reduce the reaction time of the SNCR process, as time otherwise would be spent on waiting for the water to evaporate before the reaction can take place. Generally, it may be desirable to reduce the reaction time, as this increases the chance that the combustion gases are retained in the right temperature zone for a sufficiently long time for the SNCR process to take place. A too long reaction time risks leading to an inefficient or incomplete SNCR process.
Accordingly, it may be beneficial to inject the SNCR in gaseous phase, as no time needs to be spent on a phase change from liquid to gaseous phase. In the same way, it may be desirable to use an SNCR agent with an as low water content as possible, such as the anhydrous ammonia discussed above.
In some examples, the SNCR agent may be identical to the ammonia-based fuel. In different words, the ammonia-based fuel may be used both as fuel for the burner and as the SNCR agent. This allows for the infrastructure to be simplified, as there is no need for separate storage and handling systems for the fuel and the SNCR agent. Instead, use can be made of the fact that the SNCR agent is readily available as fuel.
In an example, the injector outlet is arranged to introduce the SNCR agent in a recirculation zone of the furnace. The recirculation zone may also be referred to as a swirl zone and may be understood as a zone in the furnace in which the flow of combustion gases does not move directly from the inlet zone to the exhaust outlet. Instead, the combustion gases may be recirculated back towards the flame to elsewhere in the furnace. Put differently, a flow of the combustion gases may be caused to rotate in the recirculation zone, thereby allowing for an improved mixing of the SNCR agent with the combustion gases. Beneficially, the injector outlet may be arranged to introduce the SNCR agent in a portion of the recirculation zone in which a flow of the combustion gases moves away from the flame, thereby preventing the SNCR agent from being sucked into and consumed by the flame. The location and nature of the recirculation zone may be determined by the geometry of the furnace, the relative position and configuration of the burner inlet and the exhaust outlet, as well as the orientation and of the flame and the flow of fuel supplied to the flame. A few illustrating examples will be discussed in the detailed description.
The injector outlet may be arranged to introduce the SNCR agent along a direction substantially normal to the interior wall of the furnace. The SNCR agent may thus be injected in a radial direction with reference to a circumference of the furnace. In other examples, the injector outlet may be arranged to introduce the SNCR agent along a direction substantially tangential to the interior wall or, in different words, along a circumferential direction of the furnace wall. The direction in which the SNCR agent is injected into the furnace may vary depending on the configuration of the furnace and the temperature profile/gradient within the furnace. The temperature within the furnace tends to be lower closer to the furnace wall due to the heat transfer to the water. In case the temperature is too high for the SNCR reaction in the centre of the furnace, it may be beneficial to inject the SNCR agent along the circumferential direction along the wall, where the temperature may be lower. In case the temperature is too low for the SNCR reaction at the furnace wall, it may be beneficial to inject the SNCR agent radially, towards the centre of the furnace where the temperature may be higher. In further examples, the injector outlet may be substantially flush with the interior wall, such that the injector structure is less exposed to the heat generated by the flame and the passing flow of combustion gases.
The burner may be configured to direct the flame in a direction facing away from the exhaust outlet. In case the boiler is a vertical boiler, with the outlet zone arranged above the inlet zone and the exhaust outlet arranged at the top of the furnace, this means that the flame may be directed downwards, towards a bottom of the furnace. By directing the flame towards the bottom of the furnace, a better utilisation of the furnace (e.g., more efficient combustion) may be achieved.
The boiler may comprise an exhaust tube system which is fluidically connected to the exhaust outlet to guide the combustion gases away from the furnace. At least a part of the exhaust tube system, arranged downstream the exhaust outlet, may be referred to as a convective part of the boiler, comprising heat transfer means for transferring heat from the combustion gases to a thermal medium, such as water, and cooling the combustion gases before they are discharged from the boiler. The exhaust tube system may comprise boiler tubes or tube bundles at least partly surrounded with, or otherwise in thermal contact with the thermal medium.
In some examples, the exhaust tube system comprises a SCR cleaning arrangement, or SCR unit, for further reducing the level of NOx in the combustion gases after they have been discharged from the furnace. The SCR unit may hence be arranged downstream of the furnace and arranged to take care of at least some of the NOx remaining after the SNCR process. It will be appreciated that the NOx levels reaching the SCR cleaning arrangement may be relatively low due to the upstream SNCR process and that a relatively small and compact SCR cleaning arrangement can be used. This is particularly beneficial in marine application, where space typically is limited.
Additional emission reduction components may be provided to further clean the combustion gases. In an example, an oxidation catalyst may be arranged downstream the SCR cleaning arrangement to remove residual ammonia in the combustion gases.
In some examples, a gas analyser, or sensor arrangement, may be provided to generate a sensor signal indicative of characteristics of the combustion gas, such as the level of nitrogen oxides remaining in the combustion gas after the SNCR process and, if applicable, after the SCR process. The sensor arrangement may be arranged in the exhaust tube system at a position allowing the sensor arrangement to measure the level of NOx in the combustion gases leaving the outlet zone of the furnace. The sensor signal can be used as a control signal, or feedback, for controlling the operation of the boiler. The sensor signal may, for example, be used for controlling the injection of SNCR into the outlet zone, or the operation of the burner. Should the sensor signal and thus the level of NOx indicate that the SNCR process is not working as efficiently as intended, the burner can be operated to adjust the temperature in the outlet zone and/or the injector caused to adjust the amount of SNCR agent supplied to the outlet zone. The controller may be configured to control the SNCR process such that at least 50%, such as 90% or even 95% of the NOx content is removed.
In some examples, a temperature sensor may be provided, which may be configured to measure a temperature in the outlet zone. The temperature may be used by the controller for controlling the operation of the boiler to maintain the temperature in the outlet zone in a range suitable for the SNCR process, which may lie in the range of 700 and 1100°C. The temperature in the outlet zone may for example be adjusted by causing the burner to adjust the temperature or intensity of the flame, or by varying the heat transfer efficiency to the thermal medium. The latter may, for example, be achieved by increasing a flow of the thermal medium, allowing more heat to be removed by the thermal medium, or by reducing a temperature of the thermal medium supplied to the boiler.
The ammonia-based fuel may include a liquid or gaseous fuel which comprises ammonia. The amount of ammonia comprised in the ammonia-based fuel may depend on the boiler type in which the gaseous ammonia-based fuel will be combusted. The ammonia may be stored as liquified ammonia, which takes less space compared to gaseous ammonia and thereby less storage space is needed. The ammonia may then be transformed into gaseous phase and mixed with oxygen or air before being ignited in the burner.
The SNCR agent is generally understood as a chemical substance capable of chemically reducing NOx to nitrogen and water vapour. The SNCR agent may be injected into the furnace in a gaseous phase or a liquid phase. Typically, the SNCR agent comprises ammonia, which may be provided in an anhydrous form or mixed with water.
The boiler according to the present disclosure is preferably configured to be arranged onboard a marine vessel, such as a ship. The terms "marine boiler" and "boiler" may thus be used interchangeably throughout the present disclosure. However, it should be noted that the boiler is not limited to be used onboard a ship. It may also be possible to use the boiler in other marine applications, such as platforms, or land-based applications.
The medium to be heated, also referred to as a thermal medium, can be any suitable medium, such as water. The medium may, or may not, change phases, such as go from liquid to gaseous phase, partly or completely, on its way through the boiler. For example, the boiler may comprise a medium inlet arranged to receive the medium at least partly in liquid phase, e.g., in the form of liquid water, and a medium outlet arranged to discharge the medium at least partly in gaseous phase, e.g., in the form of steam.
The heat may be transferred to the thermal medium via the walls of the furnace, and/or by means of a heat exchanger system. Examples of such a heat exchanger system include one or more pipes or conduits through separating the thermal medium from the combustion gases. In an example, the thermal medium is transported through the pipes, which in turn are exposed to the heat of the combustion gases. In another example, the combustion gases are transported through the pipes, which in turn are exposed to the thermal medium.
Further features and advantages of the invention will become apparent from the following description of preferred embodiments of the invention, given by way of example only, which is made with reference to the accompanying drawings.

Brief Description of the Drawings

Various aspects and examples of the present disclosure will be readily understood from the embodiments discussed in the following detailed description and the accompanying drawings, in which:

Figure 1 shows a vertical cross section of a boiler comprising a furnace and a burner according to some examples.
Figure 2a shows a horizontal cross section of a furnace according to some examples.
Figure 2b shows a perspective view of an injector structure for injecting an SNCR agent into a furnace according to some examples.
Figure 3 is a schematic illustration of a boiler and an exhaust tube system according to some examples.
Figure 4 is a flow chart outlining a method for reducing a level of NOx emitted from a boiler according to some examples.

Detailed Description

Figure 1 is a schematic cross section of a boiler 100 according to some examples. The boiler 100 comprises a burner 110 and a furnace 120, wherein the furnace 120 is configured to be heated by a flame F which is generated by the burner 110 and directed into the interior of the furnace 120 via a burner inlet 112. The heat generated by the flame F may be transferred to a thermal medium, such as liquid water 130, in thermal contact with the furnace 120. In the present example, the water 130 is at least partly surrounding the furnace 120. The boiler 100 may be operated to generate hot water or steam 132, depending on the application and specific configuration of the boiler 100.
The heating flame F is directed into the interior of the furnace 120 via the burner inlet 112 whereas the combustion gases, resulting from the combustion process generating the flame F, may leave the furnace 120 via an exhaust outlet 114. In the present example, the combustion gases may be conveyed from exhaust outlet 114 by passing through a bundle of exhaust pipes 152 passing through the water 130 for further heat transfer/cooling of the combustion gases, which also may be referred to as flue gas or exhaust gas. The bundle of pipes 152 may form part of an exhaust tube system 150 which will be discussed in greater detail with reference to figure 3.
The interior of the furnace 120 may be divided into an inlet zone 10 into which the flame F, or at least a part of the flame F, may be released and an outlet zone 20 from which the combustion gases may be exhausted from the furnace 120 via the exhaust outlet 114. The outlet zone 20 may thus be arranged close to the exhaust outlet 114, which in the vertically oriented boiler 100 illustrated in the present example is arranged at the top of the furnace 120, whereas the inlet zone 10 may be arranged below the outlet zone 20, closer to the burner inlet 112 and the bottom of the furnace 120. During operation, a temperature gradient may be observed in the furnace 120, caused by the combustion gases releasing their heat to the surrounding water 130 and thereby being gradually cooled as they move towards the exhaust outlet 114. As a result, the temperature may be lower in the outlet zone 20 than in the inlet zone 10 of the furnace.
The burner 110 is configured to burn an ammonia-based fuel to generate the flame F. The fuel may be provided in the form of liquid or gaseous ammonia, which may be mixed with an oxidiser such as air or oxygen to enable combustion. While ammonia is a potential carbon-free fuel that might help eliminating carbon dioxide emissions during combustion, it may result in increased levels of nitrogen oxides, NOx, in the exhaust gas. Therefore, a selective non-catalytic reduction (SNCR) process may be employed, which involves injecting an SNCR agent into the outlet zone 20 to reduce the level of NOx in the flue gas.
The SNCR process involves a chemical redox reaction in which nitrogen oxide, ammonia provided by the SNCR agent, and oxygen is converted into molecular nitrogen and water as described by the following, exemplifying and simplified equation: $4 NO + 4 {NH}_{3} + O_{2} \to 4 N_{2} + 6 H_{2} O .$
The reaction typically requires a sufficient reaction time within a certain temperature range to be effective. Thus, there is a temperature window and a time window to take into account when designing the boiler 100 and the operation thereof. A further complication is mixing, as the NOx needs to meet sufficient ammonia within the temperature and time windows for the SNCR process to take place.
In the ammonia-fuelled boiler 100 shown in figure 1, the flame F typically has a temperature of about 1500-1800°C, which is too high to ensure an efficient SNCR process. Due to the thermal radiation inside the furnace 120, which causes heat to be transferred to the surrounding water 130, the temperature of the combustion gases, or flue gas, may decrease relatively quickly to about 700-1100°C in the outlet zone 20. By injecting the SNCR agent in this region 20, the temperature gradient inside the furnace 120 may be used to achieve a relatively efficient SNCR process, reducing the NOx content in the combustion gases as they pass towards the exhaust outlet 114.
The SNCR agent may be injected into the outlet zone 20 by means of one or more injector outlets 116, each forming an opening allowing the SNCR agent to pass into the furnace 120. The injector outlet 116 may form part of an injector structure 140 comprising a ducting or conduit arranged to convey the SNCR agent to the furnace 120, wherein the injector outlet 116 may form a nozzle for releasing the SNCR agent in a certain direction and/or part of the outlet zone 20. In the present example, the nozzle 116 is arranged to introduce the SNCR agent in a peripheral part of the interior of the furnace 120. Specific and more detailed examples of the injector outlet 116 are discussed below in connection with figures 2a and b.
The position and orientation of the nozzle 116 may be determined not only by the temperature of the combustion gases, but also on the flow pattern with which the combustion gases move inside the furnace 120. Beneficially, the SNCR agent may be injected in a region of the outlet zone 20 where the combustion gases recirculate or swirl before exiting through the exhaust outlet 114. This recirculating or swirling flow facilitates mixing of the SNCR agent with the combustion gases and increases the time during which the combustion gases are exposed to the SNCR agent in the outlet zone 20.
The furnace 120 typically comprises a plurality of recirculation zones in which the flow of combustion gases is rotated or recirculated back towards the flame F. The size and shape of the recirculation zones may depend on the design of the burner 110 and the furnace 120, as well as on the operating conditions of the boiler 100. In the present example, in which the flame F is directed slightly downwards and the exhaust outlet 114 is arranged at the top of the furnace 120, there may be formed several recirculation zones in the upper part of the furnace 120 which may have a suitable temperature for the SNCR reaction to take place. It is however suggested that the SNCR agent is injected in the parts of the recirculation zones that exhibit an upwards-moving flow close to the wall, as a downward-moving flow would risk sucking the SNCR agent into the flame where it would be burned instead of taking part in an SNCR reaction. Thus, by arranging the injection point at a portion of the furnace wall where the flow tends to move away from the flame, the mixing between the SNCR agent and the combustion gases may be enhanced and a more efficient SNCR process may be obtained.
In the present example, in which the flame F is directed downwards and the exhaust outlet 114 is arranged at the top of the furnace 120, such upwards-moving flow of combustion gases has been observed in the outlet zone 20 at the wall above the burner inlet 112 as well as at the wall on the lateral sides of the flame F, whereas a downwards-moving flow has been observed along the wall opposite to the burner inlet 112. Thus, in the present example, one or more injector outlets 116 may be arranged in the outlet zone 20 along any part of the furnace wall but the portion arranged opposite the burner inlet 112. Beneficially, the injector outlet 116 is arranged to introduce the SNCR agent in a circumferential direction of the wall to facilitate spreading and mixing of the SNCR agent within the furnace 120.
It will however be appreciated that the above is merely an example of an exemplary configuration and that other boiler designs may exhibit other flow patterns affecting the location of the injection points in the outlet zone 20.
In the present example, liquid or gaseous ammonia may be used both as fuel for the burner 110 and as the SNCR agent (and, if applicable, any downstream SCR cleaning arrangement). Using ammonia instead of, for example, urea or another ammonia/water solution may beneficially reduce the reaction time, as the water otherwise may need to evaporate before the SNCR reaction can take place. By eliminating, or at least reducing the amount of water injected into the outlet zone 20, the necessary time duration needed for the SNCR reaction may be reduced.
Figure 2a is a cross section taken through the outlet zone 20 of a furnace 120 of a boiler 100 which may be similarly configured as the boiler 100 shown in figure 1. The cross section shows the arrangement of the injector outlets 116, in the present example four, arranged to introduce the SNCR agent in a peripheral part of the interior of the furnace 120. Each of the injector outlets 116 may form a nozzle 116 of an injector structure 140', 140" as shown in the perspective view of figure 2b, in which the nozzle 116 is arranged on a conduit 142 extending through the exterior wall of the furnace 120. Each injector structure 140 may comprise one or more nozzles 116 which may be oriented to inject the SNCR agent in a certain direction within the outlet zone 20. In figure 2a, each injector structure 140', 140" comprises a first and a second nozzle 116 oriented to inject the SNCR agent along the furnace wall, in a circumferential direction of the wall (indicated by arrows in figure 2a). The injector structures 140' closest to the burner 110 and the burner inlet may be arranged to direct the SNCR agent away from the burner 110, whereas the injector structures 140" closest to the wall portion opposite the burner 110 may be arranged to direct the SNCR agent away from that wall portion (i.e., towards the other two injectors structures 140'). As a result, the SNCR agent may be introduced in the regions of the inlet zone 20 where there is an upwards-moving flow of combustion gases allowing the SNCR agent to be mixed with the combustion gases.
As mentioned above, the injector outlet 116 may be provided in the form of a nozzle 116 of an injector structure 140 extending through the wall of the furnace 120. The injector structure 140 may comprise a conduit or pipe 142 for conveying the SNCR agent from an exterior of the furnace 120 into the interior of the furnace 120. One or more nozzles 116 may be arranged on the portion of the pipe 142 arranged on the inside of the furnace wall to allow the SNCR agent to be injected into the furnace 120. Beneficially, the injector structure 140 may be arranged relatively close to the furnace wall, as the temperature may be lower closer to the wall than closer to a centre of the furnace 120. Put differently, the injector structure 140 may protrude a relatively short distance into the furnace so as to reduce the risk of heat induced damages to the injector structure 140. In some examples, a relatively short protrusion may be understood as a few millimetres or a couple of centimetres. In some examples, the injector outlet 116 may even be flush with the interior wall of the furnace 120. In some examples, the thermal medium 130 (i.e., the water or steam) may be used to cool the injectors and thereby reduce the risk for thermally induced damages.
It will be appreciated that the wall of the furnace 120, which also may be referred to as a circumferential wall, can have any suitable shape and this is not necessarily limited to the circular cross section indicated in figure 2a. The circumferential wall may have an oval, polygonal, rectangular, or otherwise shaped cross section. The wall may further be of various types of design, such as a solid or hollow wall, and/or have a uniform or non-uniform thickness.
Figure 3 is a schematic illustration of a boiler 100 according to an example, which may be similarly configured as the boiler 100 discussed above with reference to figure 1. Figure 3 further shows an exhaust tube system 150, which may be fluidically connected to the exhaust outlet 114 to guide the combustion gases away from the furnace 120 and, eventually, into the atmosphere. The exhaust tube system 150 may comprise one or more gas treatment components for further cleaning of the combustion gases.
In the present example, a selective catalytic reduction (SCR) cleaning arrangement 154, or SCR unit, is provided to further reduce NOx emission from the combustion gases. The SCR cleaning arrangement 154 is configured to add an SCR agent, such as anhydrous ammonia, aqueous ammonia, or a urea solution to the stream of exhaust gas. The SCR agent may thus be similar to the SNCR agent. The mixture is reacted onto a catalyst bed, which typically comprises various porous ceramic materials, such as titanium oxide, supporting an active catalytic component of a base metal such as vanadium, molybdenum, and tungsten. The reaction typically has an optimal temperature range between 360°C and 450°C but can operate at lower temperatures, such as down to about 280°C, with longer residence times. Thus, to achieve a suitable temperature of the exhaust gas, the exhaust gas may be passed through a tube bundle 152 passing through the water (as shown in figure 1). As the reaction drives towards completion, molecular nitrogen and water (in case of ammonia use) or carbon dioxide (in the case of urea use) are produced.
The exhaust tube system 150 may also comprise an oxidation catalyst 156 for removing residual ammonia from the exhaust gas. The oxidation catalyst 156 may, for example, comprise a platinum group metal, including one or more of platinum, palladium, rhodium, ruthenium, iridium, and osmium. Beneficially, the platinum group metals exhibit a relatively high activity for the oxidation of ammonia and a relatively low activity of the oxidation of nitrogen oxides. Other possible catalysts include copper oxide, manganese oxide, and iron oxide.
A gas analyser 158 may be added downstream the SCR cleaning arrangement 154 for obtaining information about characteristics of the exhaust gas, including, for example, NOx levels, oxygen levels, or hydrogen levels. The gas analyser 158 may be configured to provide real time information about the characteristics of the exhaust gas. The information about the characteristics of the exhaust gas may be provided as a sensor signal, which may be used as input by a controller (not shown) arranged to control an operation of the boiler. In an example, the controller may use information about the NOx levels provided by the gas analyser 158 to control the injection of the SNCR agent into the furnace 120, and/or the injection of the SCR agent into the SCR cleaning arrangement 154. The information about the characteristics may hence be used as feedback in a loop controlling the operation of the boiler 100 and the cleaning of its exhaust gases. The controller may, for example, be configured to increase the amount of SNCR agent injected into the outlet zone 20 of the furnace 120 in response to the level of NOx in the combustion gases exceeding a predetermined threshold. In an example, the SNCR agent may be injected at a flow rate corresponding to a reduction of the NOx concentration of at least 50%, such as 90% or even 95%. In further examples, the controller may be configured to generate an alarm signal, should the level of NOx be determined to exceed the threshold.
It will be appreciated that the controller in some examples may be configured to control the injection of the SNCR agent based on predetermined values, for example as determined by a lookup table or a predetermined function depending on one or more operational parameters of the boiler 100.
Figure 4 is a flowchart illustrating a method 200 for reducing a level of NOx emitted from a boiler 100, which may be similarly configured as any of the boilers 100 shown in figures 1, 2a-b, and 3.
In an example, an ammonia-based fuel may be combusted 210 to generate a flame F. The ammonia-based fuel may, for example, be supplied in liquid form to the burner 110, where the fuel may be atomised, mixed with air, and ignited to form the flame F. The flame F may be directed 220 into the furnace 120, via the burner inlet 110 and further into the inlet zone 10. The heat from the flame F and the combustion gases may be transferred to the thermal medium 130 of the boiler 100 as the combustion gases move through the interior of the furnace 120 towards the exhaust outlet 114. The exhaust outlet 114 is arranged in, or close to, the outlet zone 20 of the furnace, in which a temperature of the combustion gases has reached a level that is suitable for an efficient SNCR reaction to take place. This temperature is typically between 700 and 1100°C, such as 760 and 1090°C. According to the present method, the SNCR agent is injected 230 into the outlet zone 20, in a peripheral part of the interior of the furnace 120, to reduce the level of NOx in the combustion gases. In some examples, the temperature in the outlet zone 20 may be maintained or controlled 240 in the range allowing the SNCR process to occur. This may, for example, be achieved by controlling the operation of the burner 110 (and thus the heat delivered by the flame), and/or the cooling of the combustion gases (e.g. by varying the flow and/or temperature of the thermal medium heated by the combustion gases). In a further example, a sensor signal 250 indicative of the level of NOx in the combustion gases downstream the outlet zone 114 may be received 250.
As mentioned above, the operation of the boiler 100 may be controlled by a controller, or control unit, which may be incorporated in the boiler 100 or arranged at another location, physically separate from the boiler 100. The controller may be communicatively connected to the gas analyser 158 as discussed in connection with figure 3. The controller may be arranged to control an operation of one or more of the burner 110 (and hence the generation of the flame), the injector structure 140 (and hence the flow rate/amount of SNCR agent added to the furnace), and the SCR cleaning arrangement 154 (and hence the amount of SCR agent injected into the SCR cleaning process).
The control unit may generally comprise one or more processors and one or more non-transitory computer-readable media storing first computer executable instructions that, when executed by the one or more processors, cause the boiler 100 to perform at least parts of the actions shown in figure 4 and described above. Generally, the control unit may comprise circuitry which is configured to implement (using one or more non-transitory computer-readable media) the functionality described herein. Suitable processors for the execution of a program of instructions include, by way of example, both general and special purpose microprocessors, and the sole processor or one of multiple processors or cores, of any kind of computer. The processors can be supplemented by, or incorporated in, ASICs (application-specific integrated circuits). Those skilled in the art will understand that the above-described exemplary embodiments may be implemented in any suitable software, hardware, or firmware configuration or combination thereof. An exemplary hardware platform for implementing the exemplary embodiments may include, for example, an Intel x86 based platform with compatible operating system, a Windows OS, a Mac platform and MAC OS, a mobile device having an operating system such as iOS, Android, etc. In a further example, the exemplary embodiments of the above-described method may be embodied as a program containing lines of code stored on a non-transitory computer readable storage medium that, when compiled, may be executed on a processor or microprocessor.
Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practising the claimed invention, from a study of the drawing, the disclosure, and the appended claims. Moreover, in the drawings and specification, there have been disclosed preferred embodiments and examples of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for the purpose of limitation. The scope of the invention is set forth in the following claims, in which the word 'comprising' does not exclude other elements or steps, and the indefinite article 'a' or 'an' does not exclude a plurality.

Claims

A marine boiler (100), comprising:
a burner (110) configured to combust an ammonia-based fuel to generate a flame; and

a furnace (120) configured to be heated by the flame to transfer heat into a thermal medium (130) in thermal contact with the furnace;

the furnace comprising:
a burner inlet (112) allowing the burner to release the flame into an inlet zone (10) of the furnace,

an exhaust outlet (114) allowing combustion gases to be exhausted from an outlet zone (20) of the furnace, and

an injector outlet (116) for introducing a selective non-catalytic reduction, SNCR, agent into the outlet zone to reduce a level of nitrogen oxides in the combustion gases;

wherein the injector outlet is arranged to introduce the SNCR agent in a peripheral part of an interior of the furnace.
The marine boiler according to claim 1, wherein the SNCR agent is the same as the ammonia-based fuel.
The marine boiler according to claim 1 or 2, wherein the injector outlet is configured to inject the SNCR agent in gaseous phase.
The marine boiler according to any of the preceding claims, wherein the injector outlet is arranged to introduce the SNCR agent in a recirculation zone of the furnace.
The marine boiler according to claim 4, wherein the injector outlet is arranged to introduce the SNCR agent in a portion of the recirculation zone in which a flow of the combustion gases moves away from the flame.
The marine boiler according to any of the preceding claims, wherein the injector outlet is arranged to introduce the SNCR agent along a direction substantially normal to an interior wall of the furnace.
The marine boiler according to any of claims 1-5, wherein the injector outlet is arranged to introduce the SNCR agent along a direction substantially tangential to an interior wall of the furnace.
The marine boiler according to any of the preceding claims, further comprising an exhaust tube system (150) fluidically connected to the exhaust outlet to guide the combustion gases away from the furnace, wherein the exhaust tube system comprises a selective catalytic reduction, SCR, cleaning arrangement (154) for further reducing the level of nitrogen oxides in the combustion gases.
The marine boiler according to claim 8, wherein the exhaust tube system further comprises an oxidation catalyst (156) downstream of the SCR cleaning arrangement for removing residual ammonia in the combustion gases.
The marine boiler according to claim 8 or 9, wherein the exhaust tube system comprises a sensor arrangement (158) operable to generate a sensor signal indicative of the level of nitrogen oxides in the combustion gases.
A method (200) for reducing a level of nitrogen oxides in combustion gases of a marine boiler, comprising:
combusting (210) an ammonia-based fuel to generate a flame;

directing (220) the flame into a furnace of the boiler, the furnace comprising an inlet zone into which the flame is released and an outlet zone comprising an exhaust outlet for exhausting combustion gases from the furnace; and

injecting (230) a selective non-catalytic reduction, SNCR, agent into the outlet zone, in a peripheral part of an interior of the furnace, to reduce the level of nitrogen oxides in the combustion gases.
The method according to claim 11, comprising controlling the combusting of the ammonia-based fuel to maintain (240) a temperature in the outlet zone in a temperature range of 700-1100°C, thereby allowing an SNCR process to occur.
The method according to claim 11 or 12, comprising injecting the SNCR agent into a portion of a recirculation zone of the furnace, wherein the portion comprises a flow of the combustion gases moving away from the flame.
The method according to any of claims 11-13, further comprising receiving (250) a sensor signal indicative of the level of nitrogen oxides in the combustion gases downstream of the outlet zone, wherein the injecting the SNCR agent into the outlet zone is performed based on the sensor signal.
The method according to any of claims 11-14, comprising injecting the ammonia-based fuel as the SNCR agent.