An exhaust gas after-treatment system and an apparatus comprising the same

Abstract

The disclosure relates to an exhaust gas after-treatment system for an ammonia-powered internal combustion engine as well as to an apparatus comprising the same. Further, the disclosure relates to use of the exhaust gas after-treatment system for purifying an exhaust gas produced by an ammonia-powered internal combustion engine. The exhaust gas after-treatment system comprises means for separating (103, 203) an exhaust gas side stream (102, 202) from an exhaust gas main stream (101, 201) produced by an ammonia-powered internal combustion engine, a first SCR catalyst unit (104, 204) for treating the separated exhaust gas side stream (102, 202), an NOx measurement device (105, 205) arranged downstream of the first SCR catalyst unit (104, 204) for determining a content of residual NOx in the separated exhaust gas side stream after the treatment in the first SCR catalyst unit, an injector system (106, 206) for injecting an SCR reagent into the exhaust gas main stream (101, 201) based on the content of residual NOx determined by the NOx measurement device, and a second SCR catalyst unit (107, 207) arranged downstream of the injector system (106, 206) for treating the exhaust gas main stream.

Description

[0001] An exhaust gas after-treatment system and an apparatus comprising the same

[0002] Technical field

[0003] This specification relates to an exhaust gas after-treatment system for an ammonia-powered internal combustion engine as well as to an apparatus comprising the same. Further, the specification relates to use of the exhaust gas after-treatment system for purifying an exhaust gas produced by an ammonia-powered internal combustion engine.

[0004] Background

[0005] Ammonia-powered engines have both NH3 and NOx emissions in the exhaust gas. A selective catalytic reduction (SCR) unit is required for treating the exhaust gas. The SCR utilizes NH3 for decomposing NOx. The concentration of NH3 in the exhaust gas may not be enough for decomposing NOx such that NOx emission limits can be met. Therefore, additional NH3 typically must be fed to the exhaust stream prior to treating the stream by the SCR. The problem is to control the amount of the fed NH3 precisely, such that adequate amount of NH3 is added. Addition of any excess NH3 consumes the chemical without the benefit of NOx reduction, thereby increasing the operating costs. Moreover, the excess NH3 may be slipped from the system, causing undesired emissions, odours and / or irritation. The levels of NOx and NH3 in the exhaust gas stream exiting the SCR can be measured and the amount of NH3 added to exhaust stream prior to the SCR may be adjusted accordingly. However, such a feedback control is slow. Further, the feedforward systems currently available require complex sensing systems and are costly. Thus further developments are needed.

[0006] Summary

[0007] The exhaust gas after-treatment system provided herein enables precise controlling of the SCR reagent feed in a cost-efficient manner. The feedforward system disclosed provides a faster and more accurate determination of the amount of the SCR reagent needed when compared to a system purely relying on a feedback mechanism. The exhaust gas after-treatment system disclosed herein ensures better emission control and helps to avoid the risk of too high NH3 emissions.

[0008] According to an embodiment, an exhaust gas after-treatment system for an ammonia-powered internal combustion engine is provided. The exhaust gas after-treatment system comprises means for separating an exhaust gas side stream from an exhaust gas main stream produced by an ammonia-powered internal combustion engine and a first SCR catalyst unit for treating the separated exhaust gas side stream. The system further comprises an NOx measurement device arranged downstream of the first SCR catalyst unit for determining a content of residual NOx in the separated exhaust gas side stream after the treatment in the first SCR catalyst unit and an injector system for injecting an SCR reagent into the exhaust gas main stream based on the content of residual NOx determined by the NOx measurement device, as well as a second SCR catalyst unit arranged downstream of the injector system for treating the exhaust gas main stream.

[0009] According to another embodiment, an apparatus is provided. The apparatus comprises an ammonia-powered internal combustion engine and an exhaust gas after-treatment system as described herein.

[0010] Finally, use of the exhaust gas after-treatment system described herein for purifying an exhaust gas produced by an ammonia-powered internal combustion engine is provided.

[0011] Brief description of the drawings

[0012] Fig. 1 illustrates, by way of an example, an exhaust gas after-treatment system according to an embodiment, and

[0013] Fig. 2 illustrates, by way of another example, an exhaust gas after- treatment system according to an embodiment.

[0014] The figures are schematic. The figures are not in any particular scale. Detailed description

[0015] The solution is described in the following in more detail with reference to some embodiments, which shall not be regarded as limiting.

[0016] Unit of temperature expressed as degrees C corresponds to °C. The following reference numbers and denotations are used in this application:

[0017] 100, 200 exhaust gas after-treatment system

[0018] 101 , 201 exhaust gas main stream

[0019] 102, 202 exhaust gas side stream

[0020] 103, 203 means for separating an exhaust has stream from an exhaust gas side stream

[0021] 104, 204 first SCR catalyst unit

[0022] 105, 205 NOx measurement device

[0023] 106, 206 injector system

[0024] 107, 207 second SCR catalyst unit

[0025] 110 ammonia-powered internal combustion engine

[0026] 208 exhaust gas duct

[0027] Ammonia (NH3) is a versatile chemical, composed of nitrogen and hydrogen. Ammonia can be directly used as a fuel in combustion systems and, in particular, in internal combustion engines. Ammonia is a high-density and safely transportable hydrogen carrier to use in energy production and transportation systems. The NH3 oxidation produces nitrogen oxides (NOx), and hence exhaust gas after-treatment techniques, such as selective catalytic reduction (SCR), are needed for removing the NOx from the exhaust gas prior to its release to the surroundings. The SCR process works by reacting NOx with NH3 or other SCR reagent, the reaction resulting in formation of nitrogen (N2) and water (H2O), which both are harmless.

[0028] The exhaust gas stream produced by the ammonia-powered engine always contains some NH3 that is under SCR conditions capable of decomposing NOx. However, the levels of NH3 in the exhaust gas stream may vary, thereby typically causing the need for additional NH3 or other SCR reagent to be fed to the exhaust stream so as to minimize the amount of NOx exiting the exhaust gas duct. As already mentioned, the amount of the SCR reagent needed may be determined by determining the levels of NOx and NH3 in the exhaust gas stream exiting the SCR, i.e., by using a feedback control. However, such a feedback control is slow. The described feedback control does not enable realtime adjustment of the SCR reagent supply as a response to the fluctuating NH3 content present in the exhaust gas stream produced by the ammonia- powered engine. Hence, also the NOx emissions may fluctuate. Thus, the ammonia-powered internal combustion engines in this respect differ for example from diesel engines producing exhaust gas side streams with nonfluctuating contents in terms of the components, such as NOx, therein. Currently available feedforward systems utilize the so-called continuous main exhaust gas stream -principle, wherein the exhaust gas main stream is treated by several SCR catalyst units. Such a system is also a slow one, since the entire exhaust gas main stream is treated by the SCR catalyst units. In such systems all of the SCR catalyst units are of the same or similar size, which increases the costs and the space needed for the equipment.

[0029] An exhaust gas after-treatment system for an ammonia-powered internal combustion engine is provided. The exhaust gas after-treatment system comprises a cost-efficient and simple feedforward system for controlling the SCR reagent supply.

[0030] Within context of this disclosure, the ammonia-powered internal combustion engine refers to an internal combustion engine powered with a fuel comprising at least 50 vol-%, such as from 50 to 100 vol-% of ammonia. Particularly the solution disclosed herein is useable for an internal combustion engine powered with a fuel essentially consisting of ammonia. Ammonia used for powering the internal combustion engine can be in liquid or gaseous form. Preferably the ammonia is in liquid form.

[0031] The exhaust gas after-treatment system disclosed herein comprises means for separating an exhaust gas side stream from an exhaust gas main stream, the exhaust gas main stream being produced by an ammonia-powered internal combustion engine. The exhaust gas main stream comprises NOx (nitrogen oxides) and NH3 (ammonia). The means for separating the exhaust gas side stream from the exhaust gas main stream may refer to any suitable means capable of dividing a gas stream into two gas streams. Such means include for example diverter plates.

[0032] According to an embodiment, the means for separating the exhaust gas side stream from the exhaust gas main stream is comprised by an exhaust gas duct, the exhaust gas duct being arranged to transport the exhaust gas from the engine for after-treatment. Alternatively, the means for separating the exhaust gas side stream from the exhaust gas main stream may refer to an y- pipe configuration. In an example, the exhaust gas side stream separated by the separating means represents at most 1 vol-% of the exhaust gas main stream. Volume ratio of the exhaust gas side stream separated by the separating means to the exhaust gas main stream may be for example from 1 :1000 to 1 :100. This means that the exhaust gas side stream separated by the separating means may represent from 0.1 to 1 vol-% of the exhaust gas main stream.

[0033] The exhaust gas after-treatment system further comprises a first SCR catalyst unit. The first SCR catalyst unit is suitable for treating the separated exhaust gas side stream. The means for separating the exhaust gas side stream from the exhaust gas main stream may be arranged to direct the exhaust gas side stream into the first SCR catalyst unit.

[0034] In the first SCR catalyst unit the NOx and NH3 of the exhaust gas side stream are configured to react. This means that no separate SCR reagent is injected into the exhaust gas side stream prior to or within the first SCR catalyst unit, but the NH3 present in the exhaust gas side stream originates from NH3 used as the fuel for powering the internal combustion engine. The first SCR catalyst unit is arranged to convert NOx into N2 and H2O. As the exhaust gas main stream (and thus also the side stream separated from it) contain markedly less NH3 than NOx, the reaction is sub-stoichiometric and the NH3 is consumed first, and the unreacted NOx remains in the exhaust gas side stream exiting the first SCR catalyst unit. The first SCR catalyst unit may also be called a diagnostic SCR catalyst unit, as the purpose of the first SCR catalyst unit is to enable determining of the remaining NOx content, i.e., the surplus NOx present in the exhaust gas. The operation of the first SCR catalyst unit is fast due to the fact that there is no need to allow the purification process to go to the end, but only to the point where substantially all of the NH3 contained by the exhaust gas is consumed.

[0035] The exhaust gas after-treatment system may further comprise a pre-heater for pre-heating the first SCR catalyst unit. Heating of the first SCR catalyst unit expedites the reaction between the NOx and NH3. Pre-heating together with the relatively small size of the first SCR catalyst unit expedites the process of determining of the content of the surplus NOx present in the exhaust gas stream.

[0036] The exhaust gas after-treatment system further comprises an NOx measurement device arranged downstream of the first SCR catalyst unit. The NOx measurement device is suitable for determining a content of residual or leftover NOx in the separated exhaust gas side stream after the treatment in the first SCR catalyst unit. The NOx measurement device comprises a sensor capable of sensing NOx in the separated exhaust gas side stream exiting the first SCR catalyst unit. Instead of measuring the contents of NH3 and NOx from the exhaust gas main stream, the system allows measuring the surplus NOx followed by the amount of NH3 subtracted from the amount of NOx present in the exhaust gas side stream. Measuring the residual NOx from the exhaust gas side stream rather than from the exhaust gas main stream enables a faster and more accurate manner for controlling the SCR reagent feed.

[0037] Further, the exhaust gas after-treatment system comprises an injector system. The injector system is suitable for injecting an SCR reagent into the exhaust gas main stream based on the content of residual NOx determined by the NOx measurement device. In an example, the SCR reagent is at least one of ammonia, ammonium hydroxide (NH4OH), or urea (CO(NH2)2). When the content of residual NOx in the exhaust gas side stream exiting the first SCR catalyst unit is high, the injector system is arranged to supply a higher dose of the SCR reagent to the exhaust gas main stream. Respectively, when the amount of the residual NOx is determined to be lower, lower amount of SCR reagent is supplied by the injector system into the exhaust gas main stream.

[0038] The exhaust gas after-treatment system preferably comprises a controller for controlling the injector system based on the content of the residual NOx in the exhaust gas side stream exiting the first SCR catalyst unit, the residual NOx determined by the NOx measurement device.

[0039] Finally, the exhaust gas after-treatment system comprises a second SCR catalyst unit arranged downstream of the -injector system for treating the exhaust gas main stream. The second SCR catalyst unit may also called a main SCR catalyst unit. The second SCR catalyst unit is capable of decomposing NOx. Particularly, the second SCR catalyst unit is capable of decomposing NOx by the SCR reagent injected to the exhaust gas main stream prior to feeding the stream into the second SCR catalyst unit, as well as by the ammonia present in the exhaust gas main stream as originating from the ammonia used as the fuel by the internal combustion engine. The second SCR catalyst unit is arranged to convert NOx into N2 and H2O. The second SCR catalyst unit is responsible for purifying the exhaust gas by decomposing NOx, such that the exhaust gas exiting the second SCR catalyst unit meets the emission limits placed for exhaust gas and can be released into the surroundings.

[0040] Size ratio, i.e., scale, of the first SCR catalyst unit to the second SCR catalyst unit may be at most 1 :100. In an example, the size ratio of the first SCR catalyst unit to the second SCR catalyst unit is of from 1 :1000 to 1 :100. Thus, size of the first SCR catalyst unit may be from 0.1 to 1 % of the size of the second SCR catalyst unit. Usage of a smaller scale SCR catalyst unit as the first SCR catalyst unit enables faster feedforward process when compared to systems wherein the entire exhaust gas stream or majority of it must be treated by an SCR catalyst unit prior to adjusting the SCR reagent feed. Further, smaller first SCR catalyst unit also saves space.

[0041] A simplified view of the exhaust gas after-treatment system 100 is shown in Fig. 1. The ammonia-powered internal combustion engine 110 is responsible for producing an exhaust gas main stream 101. The exhaust gas after- treatment system comprises means for separating 103 an exhaust gas side stream 102 from the exhaust gas main stream 101. The system comprises a first SCR catalyst unit 104, the first SCR catalyst unit 104 being arranged to treat the exhaust gas side stream 102. The system further comprises a NOx measurement device 105 that is configured to determine a content of the residual NOx present in the exhaust gas side stream 102 after exiting the first SCR catalyst unit 104. An injector system 106 is configured to inject an SCR reagent into the exhaust gas main stream 101 based on the content of the residual NOx in the exhaust gas side stream 102 determined by the NOx measurement device 105. Finally, the exhaust gas after-treatment system 100 further comprises a second SCR catalyst unit 107. The injector system 106 is arranged upstream of the second SCR catalyst unit 107.

[0042] An exemplary illustration of the exhaust gas after-treatment system 200 is shown in Fig. 2. The exhaust gas main stream 201 is arranged to be forwarded from an internal combustion engine into an exhaust gas duct 208. The exhaust gas after-treatment system comprises means for separating 203 an exhaust gas side stream 202 from the exhaust gas main stream 201 . In Fig. 2 diverter plates function as separating means. The diverter plates are arranged to forward the exhaust gas side stream 202 into a first SCR catalyst unit 204. The first SCR catalyst unit 204 is arranged to treat the exhaust gas side stream 202. After treatment in the first SCR catalyst unit 204 the exhaust gas side stream 202 is arranged to be forwarded into a NOx measurement device 205 arranged downstream of the first SCR catalyst unit 204. The NOx measurement device 205 is configured to determine a content of the residual NOx present in the exhaust gas side stream 202 after exiting the first SCR catalyst unit 204, i.e., after being exposed to the catalytic conditions of the first SCR catalyst unit. An injector system 206 is configured to inject an SCR reagent into the exhaust gas main stream 201 . The injector system 206 is configured to inject the SCR reagent based on the content of the residual NOx in the exhaust gas side stream 202 determined by the NOx measurement device 205. In Fig. 2 this is illustrated by the dashed line connecting the NOx measurement device 205 and the injector system 206. Finally, the exhaust gas after-treatment system 200 comprises a second SCR catalyst unit 207 arranged downstream of the injector system 206. Fig. 2 illustrates a system wherein the means for separating 203 an exhaust gas side stream, the first SCR catalyst unit 204, and the NOx measurement device are comprised by the exhaust gas duct 208. The exhaust gas side stream exiting the NOx measurement device may be forwarded to the second SCR catalyst unit, as illustrated in Fig. 2. Alternatively, it may be vented from the system by any conventional means.

[0043] Besides the so-called feedforward system as described herein, the exhaust gas after-treatment system may also comprise a conventional feedback system for controlling the SCR reagent supply. The exhaust gas after- treatment system may comprise a second NOx measurement device arranged downstream of the second SCR catalyst unit. Hence, NOx contained by the exhaust gas exiting the second SCR catalyst unit may be determined by the second NOx measurement device and the information may be used to finetune the SCR reagent supply, if needed.

[0044] An apparatus is provided, which apparatus comprises an ammonia-powered internal combustion engine and an exhaust gas after-treatment system as disclosed herein. According to an embodiment, the apparatus is a stationary power plant. In that case, the internal combustion engine is a stationary engine. The stationary engine may be able to be coupled or may be coupled to a generator for generating electrical power. According to another embodiment the apparatus is a marine vessel.

[0045] As already mentioned, the ammonia-powered internal combustion engine may be powered by liquid or gaseous ammonia. Preferably the ammonia-powered internal combustion engine is powered by liquid ammonia. The internal combustion engine of the apparatus disclosed herein may have an engine speed ranging from 200 to 1500 rpm. With respect to the cylinder configuration the engine can be an inline engine or a V engine. The engine may have a cylinder number of from 4 to 20. The cylinder output, i.e., the output generated by an individual cylinder may vary from 180 to 1300 kW / cyl. The rated power, i.e., the nominal power of the internal combustion engine may range from 1100 to 26 000 kW. Mean effective pressure of the engine may vary from 20 to 31 bar. Compression ratio of the engine may vary from 11 :17 to 14:15.

Claims

Claims:

1. An exhaust gas after-treatment system (100, 200) for an ammonia- powered internal combustion engine (110), the system comprising- means for separating (103, 203) an exhaust gas side stream (102, 202) from an exhaust gas main stream (101 , 201 ) produced by an ammonia-powered internal combustion engine,- a first SCR catalyst unit (104, 204) for treating the separated exhaust gas side stream (102, 202),- an NOx measurement device (105, 205) arranged downstream of the first SCR catalyst unit (104, 204) for determining a content of residual NOx in the separated exhaust gas side stream after the treatment in the first SCR catalyst unit,- an injector system (106, 206) for injecting an SCR reagent into the exhaust gas main stream (101 , 201 ) based on the content of residual NOx determined by the NOx measurement device, and- a second SCR catalyst unit (107, 207) arranged downstream of the injector system (106, 206) for treating the exhaust gas main stream.

2. The exhaust gas after-treatment system (100, 200) according to claim1 , wherein the exhaust gas main stream (101 , 201 ) comprises NOx and NH3.

3. The exhaust gas after-treatment system (100, 200) according to claim 1 or 2, wherein the second SCR catalyst unit (107, 207) is capable of decomposing NOx.

4. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein the means for separating (103, 203) the exhaust gas stream from the exhaust gas main stream (101 , 201 ) is comprised by an exhaust gas duct (208).

5. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein a size ratio of the first SCR catalyst unit (104, 204) to the second SCR catalyst unit (107, 207) is at most 1 :100.

6. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein a size ratio of the first SCR catalyst unit (104, 204) to the second SCR catalyst unit (107, 207) is from 1 :1000 to 1 :100.

7. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein the exhaust gas after-treatment system (100, 200) further comprises a pre-heater for pre-heating the first SCR catalyst unit (104, 204).

8. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein the SCR reagent is at least one of ammonia (NH3), ammonium hydroxide (NH4OH), or urea (CO(NH2)2).

9. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein the exhaust gas after-treatment system (100, 200) further comprises a controller for controlling the injector system (106, 206) based on the content of residual NOx determined by the NOx measurement device (105, 205).

10. The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, wherein the ammonia used for powering the internal combustion engine is liquid ammonia.11 .The exhaust gas after-treatment system (100, 200) according to any of the preceding claims, further comprising a second NOx measurement device arranged downstream of the second SCR catalyst unit (107, 207).

12. An apparatus comprising an ammonia-powered internal combustion engine and an exhaust gas after-treatment system (100, 200) according to any of the claims 1 -11.

13. The apparatus according to claim 12, wherein the apparatus is a stationary power plant.

14. The apparatus according to claim 12, wherein the apparatus is a marine vessel.

15. The apparatus according to any of the claims 12-14, wherein the ammonia-powered internal combustion engine has- engine speed of from 200 to 1500 rpm,- cylinder configuration: inline or V,- cylinder number from 4 to 20,- cylinder output from 180 to 1300 kW / cyl,- engine nominal power of from 1100 to 26 000 kW,- mean effective pressure from 20 to 31 bar, and / or- compression ratio from 11 :17 to 14:15.

16. Use of the exhaust gas after-treatment system (100, 200) according to any of the claims 1-11 for purifying an exhaust gas produced by an ammonia-powered internal combustion engine.

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