WO2014068172A1

WO2014068172A1 - Method for mixing reducing agent with exhaust gas and exhaust system

Info

Publication number: WO2014068172A1
Application number: PCT/FI2013/050923
Authority: WO
Inventors: Jussi UUSIPAAVALNIEMI; Ari Suominen
Original assignee: Wärtsilä Finland Oy
Priority date: 2012-10-29
Filing date: 2013-09-24
Publication date: 2014-05-08
Also published as: FI20126122L

Abstract

In the method for mixing reducing agent with exhaust gas of an internal combustion engine, a partial exhaust gas stream is separated from the main exhaust gas stream on the downstream side of the catalyst elements (3a) of a catalytic converter (3), reducing agent is mixed with the partial exhaust gas stream, and the mixture of the reducing agent and the exhaust gas is introduced into the main exhaust gas stream on the upstream side of the catalyst elements (3a). The invention also concerns an exhaust system.

Description

Method for mixing reducing agent with exhaust gas and exhaust system Technical field of the invention

The present invention relates to a method for mixing reducing agent with exhaust gas in accordance with the preamble of claim 1 . The invention also con- cerns an exhaust system for an internal combustion engine, as defined in the preamble of the other independent claim.

Background of the invention

Nitrogen oxide (NOx) emissions of internal combustion engines in ships and power plants are a growing concern and subject to continuously tightening regulations set by the International Maritime Organization (IMO) and other legislative bodies. To a certain extent, the requirements set by different regulations can be met by means that are directly related to the operation of the engine, such as exhaust gas recirculation, high fuel injection pressures in diesel engines, water injection into the air intake duct etc. However, many engine- related measures for reducing NOx emissions have a negative effect on the fuel efficiency, and they are not adequate for meeting the most stringent emission limits. For ultimate NOx reduction, selective catalytic reduction (SCR) is needed. With the SCR, NOx reductions of up to 90% or even greater can be achieved.

In an SCR system a catalyst material and a reducing agent are used to decompose the NOx that is formed during the combustion. In a typical SCR system, the catalyst is arranged on the surface of a supporting ceramic material that forms a honeycomb structure inside a reactor. On the surface of the cata- lyst, the NOx reacts with ammonia that is used as the reducing agent and nitrogen and water is formed. In practice, urea is often used instead of ammonia for safety reasons. This applies especially to the marine applications, where leaking ammonia is a serious hazard. Urea is injected into the exhaust gas flow in an aqueous solution. Due to the high temperature of the exhaust gas, water evaporates and the urea molecules break up into ammonia and carbon dioxide. For effective NOx reduction and minimized usage of urea, as well as for minimum ammonia slip through the SCR system, effective mixing of the urea with the exhaust gas flow is essential. In many prior art solutions, sufficient mixing has been ensured by arranging a relatively long exhaust duct section between the point of urea injection and the catalyst elements. Another known solution is to guide part of the exhaust gas stream into a by-pass duct that is arranged upstream from the catalytic converter, and the reducing agent is mixed with exhaust gases in the by-pass duct. In both type of constructions, there needs to be enough space before the catalytic converter for accommodating the pipes where the reducing agent is mixed with the exhaust gas. This may be a problem especially in ships, where the available space is limited. Another problem is that the increased thermal efficiency of modern medium- and slow- speed engines leads to low exhaust temperatures. If the catalytic converter is placed far from the engine, low temperature of the exhaust gases may have a negative effect on the functioning of the catalytic converter. Constructions that are used for facilitating mixing of the reducing agent can also have other negative effects. For instance, they can cause pressure losses in the exhaust system.

Summary of the invention

An object of the present invention is to provide an improved method for mixing reducing agent with exhaust gas of an internal combustion engine. The characterizing features of the method according to the invention are given in the characterizing part of claim 1 . Another object of the invention is to provide an improved exhaust system for an internal combustion engine. Characterizing features of the exhaust system according the invention are given in the characterizing part of the other independent claim.

In the method according to the invention, a partial exhaust gas stream is separated from the main exhaust gas stream on the downstream side of the cata- lyst elements of a catalytic converter, reducing agent is mixed with the partial exhaust gas stream, and the mixture of the reducing agent and the exhaust gas is introduced into the main exhaust gas stream on the upstream side of the catalyst elements. The exhaust system according to the invention comprises a catalytic converter, which is provided with one or more catalyst elements for selective catalytic reduction, exhaust duct parts for introducing exhaust gas from the engine into the catalytic converter and for guiding the main exhaust gas stream away from the catalytic converter, a flow channel having a first end and a second end, the first end being connected to the exhaust system at a point downstream from the catalyst elements and the second end being connected to the exhaust system at a point upstream from the catalyst elements. The exhaust system further comprises means for introducing reducing agent into the flow channel, and means for introducing exhaust gas from the downstream side of the catalyst elements into the flow channel and for introducing the mixture of the exhaust gas and the reducing agent further into the main exhaust gas stream between the engine and the catalyst elements.

With the method and the exhaust system according to the invention, good mix- ing of the reducing agent and the exhaust gas can be ensured. The need for a long exhaust duct section before the catalytic converter can be avoided, and the mixing space is easy to accommodate even in ships. The catalytic converter can be arranged close to the engine, which helps to keep the temperature high enough for effective reduction of NOx emissions. According to an embodiment of the invention, the reducing agent is mixed with the partial exhaust gas stream in a flow channel, which flow channel comprises a first end, which is connected to the exhaust system of the engine at a point downstream from the catalyst elements, and a second end, which is connected to the exhaust system at a point upstream from the catalyst elements. According to another embodiment of the invention, a pump is used for introducing the partial exhaust gas stream into the flow channel and for introducing the mixture of the exhaust gas and the reducing agent into the main exhaust gas stream. The reducing agent can be introduced into the partial exhaust gas stream upstream from the pump. This improves mixing of the reducing agent and the exhaust gas. On the other hand, if the reducing agent is introduced into the partial exhaust gas stream downstream from the pump, the risk that the reducing agent causes damage to the pump can be reduced.

The flow channel can be provided with a mixing chamber that is arranged between the first end and the second end of the flow channel. The mixing cham- ber further enhances mixing of the reducing agent and the exhaust gas. The reducing agent can be introduced into the mixing chamber.

Brief description of the drawings Embodiments of the invention are described below in more detail with reference to the accompanying drawing, in which

Fig. 1 shows an exhaust system according to a first embodiment of the invention.

Fig. 2 shows an exhaust system according to a second embodiment of the in- vention.

Description of embodiments of the invention

In figure 1 is shown an exhaust system of an internal combustion engine 1 . The engine 1 is a large piston engine, such as a main or an auxiliary engine of a ship or an engine that is used at a power plant for producing electricity. However, the invention is applicable also to other types of engines, in which selective catalytic reduction (SCR) is used. In the example of figure 1 , the cylinders 2 of the engine 1 are arranged in line, but also other cylinder configurations, such as V-configuration, could be used. In that case, also the exhaust system may have a configuration that differs from the configuration shown in figure 1 . The exhaust system of the engine 1 is provided with a catalytic converter 3, which is used for selective catalytic reduction. The catalytic converter 3 comprises catalyst elements 3a, through which exhaust gas is arranged to flow. Reducing agent, such as urea, is introduced into the exhaust gas stream upstream from the catalyst elements 3a, and NOx emissions from the engine 1 are decomposed on the surface of the catalyst elements 3a of the catalytic converter 3.

The exhaust system of the engine 1 is provided with an exhaust duct that comprises several parts 10a, 10b, 10c. An exhaust manifold 10a is connected to the cylinders 2 of the engine 1 . From the exhaust manifold 10a, exhaust gases of the engine 1 flow in a first exhaust duct part 10b into the catalytic converter 3. A second exhaust duct part 10c is arranged downstream from the catalytic converter 3 for guiding the main exhaust gas stream away from the catalytic converter 3 and further into a turbine 6a of a turbocharger 6. The tur- bocharger 6 also comprises a compressor 6b for pressurizing the intake air, which is introduced through a charge air cooler 9 into the cylinders 2 of the engine 1 . The engine 1 does not need to be provided with a turbocharger 6. However, it could also be provided with more than one turbocharger 6. The catalytic converter 3 can be located in any place in the exhaust system, i.e. upstream from all the turbochargers 6, downstream from the turbochargers 6, or between two turbochargers 6, if the engine 1 is provided with more than one turbocharger 6. The exhaust system can also comprise many other components, such as oxidation catalysts, particle filters, or heat exchangers.

The reducing agent is stored in a tank 5. By means of a feed pump 1 1 , reducing agent is delivered from the tank 5 through a feed duct 12 to an injection nozzle 7, which is arranged to inject the reducing agent into exhaust gas stream. Any conventional components can be used as the means 7, 1 1 , 12 for introducing the reducing agent into the exhaust gas stream, and their construction is therefore not described in more detail here. The reducing agent is not injected directly into the main exhaust gas stream between the engine 1 and the catalytic converter 3, but the reducing agent is mixed with a partial exhaust gas stream. The partial exhaust gas stream is separated from the main exhaust gas stream on the downstream side of the catalyst elements 3a of the catalytic converter 3. The mixture of the exhaust gas and the reducing agent is introduced into the main exhaust gas stream on the upstream side of the cata- lyst elements 3a of the catalytic converter 3. The terms "downstream" and "upstream" refer here to the flow direction of the main exhaust gas stream. The upstream side is thus between the engine 1 and the catalyst elements 3a. An advantage of the invention is that the catalytic converter 3 can be arranged close to the engine 1 . Since the reducing agent evaporates and mixes with ex- haust gases in a partial exhaust gas stream, a long exhaust duct section is not needed before the catalytic converter 3, and good mixing results can still be achieved. This is beneficial especially in ships, where the available space is often very limited. Pressure losses in the system can also be reduced. Still another potential benefit is that part of the NOx that has flown through the catalyt- ic converter 3 is returned to the catalytic converter 3, which may improve the NOx reducing effect. In the embodiment of figure 1 , the exhaust system of the engine 1 is provided with a flow channel 8. The flow channel 8 comprises a first end 8a, which is connected to the rest of the exhaust system downstream from the catalytic converter 3, and a second end 8b, which is connected to the exhaust system upstream from the catalytic converter 3. The flow channel 8 is thus in fluid communication with the exhaust duct parts 10b, 10c on both the upstream side and the downstream side of the catalyst elements 3a. The flow channel 8 is arranged apart from the exhaust duct parts 10a, 10b, which establish fluid communication between the engine 1 and the catalytic converter 3. Since there is always a small pressure loss in the exhaust system across the catalytic converter 3, the exhaust system needs to be provided with means 4 for introducing the exhaust gas into the flow channel 8 and for introducing the mixture of the exhaust gas and the reducing agent from the flow channel 8 into the main exhaust gas stream. This means that in the embodiment of figure 1 , the flow direction needs to be correct, i.e. from the first end 8a of the flow channel 8 towards the second end 8b. In the embodiment of figure 1 , the flow channel 8 is provided with a pump 4 for forcing the exhaust gas to flow from the first end 8a of the flow channel 8 towards the second end 8b and further into the first exhaust duct part 10b, where the mixture is mixed with the main exhaust gas stream. Instead of the pump 4, for instance a blower could be used. In the embodiment of figure 1 , the injection nozzle 7 is arranged to inject the reducing agent upstream from the pump 4, the expression "upstream" now referring to the flow direction of the partial exhaust gas stream in the flow channel 8. The injection on the upstream side of the pump 4 further facilitates mixing of the reducing agent and the exhaust gases.

The embodiment of figure 2 works mainly in the same manner as the embodiment of figure 1 . The embodiment of figure 2 differs from the first embodiment in that the injection nozzle 7 is arranged to inject the reducing agent downstream from the pump 4. The benefit of this embodiment is that the risk of hav- ing the pump 4 damaged by the reducing agent can be reduced. Another difference is that the flow channel 8 is provided with a mixing chamber 8d, which is arranged between the ends 8a, 8b of the flow channel 8. In the embodiment of figure 2, the reducing agent is injected into the mixing chamber 8d. However, the reducing agent could also be injected upstream from the mixing cham- ber 8d. The mixing chamber 8d facilitates mixing of the reducing agent with the exhaust gas. It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the flow channel could be connected directly to the catalytic converter upstream and downstream from the catalyst elements inside the catalytic converter.

Claims

1 . A method for mixing reducing agent with exhaust gas of an internal combustion engine (1 ), characterized in that a partial exhaust gas stream is separated from the main exhaust gas stream on the downstream side of the cata- lyst elements (3a) of a catalytic converter (3), reducing agent is mixed with the partial exhaust gas stream, and the mixture of the reducing agent and the exhaust gas is introduced into the main exhaust gas stream on the upstream side of the catalyst elements (3a).

2. A method according to claim 1 , characterized in that the reducing agent is mixed with the partial exhaust gas stream in a flow channel (8), which flow channel (8) comprises a first end (8a), which is connected to the exhaust system of the engine (1 ) at a point downstream from the catalyst elements (3a), and a second end (8b), which is connected to the exhaust system at a point upstream from the catalyst elements (3a).

3. A method according to claim 2, characterized in that a pump (4) is used for introducing the partial exhaust gas stream into the flow channel (8) and for introducing the mixture of the exhaust gas and the reducing agent into the main exhaust gas stream.

4. A method according to claim 3, characterized in that the reducing agent is introduced into the partial exhaust gas stream upstream from the pump (4).

5. A method according to claim 3, characterized in that the reducing agent is introduced into the partial exhaust gas stream downstream from the pump (4).

6. A method according to any of claims 2-5, characterized in that the re- ducing agent is introduced into a mixing chamber (8d) that is arranged between the first end (8a) and the second end (8b) of the flow channel (8).

7. An exhaust system for an internal combustion engine (1 ), which exhaust system comprises

- a catalytic converter (3), which is provided with one or more catalyst el- ements (3a) for selective catalytic reduction, - exhaust duct parts (10a, 10b, 10c) for introducing exhaust gas from the engine (1 ) into the catalytic converter (3) and for guiding the main exhaust gas stream away from the catalytic converter (3),

- a flow channel (8) having a first end (8a) and a second end (8b), the first end (8a) being connected to the exhaust system at a point downstream from the catalyst elements (3a) and the second end (8b) being connected to the exhaust system at a point upstream from the catalyst elements (3a),

characterized in that the exhaust system comprises

- means (7, 1 1 , 12) for introducing reducing agent into the flow channel (8), and

- means (4) for introducing exhaust gas from the downstream side of the catalyst elements (3a) into the flow channel (8) and for introducing the mixture of the exhaust gas and the reducing agent further into the main exhaust gas stream between the engine (1 ) and the catalyst elements

(3a).

8. An exhaust system according to claim 7, characterized in that the flow channel (8) is provided with a mixing chamber (8d).

9. An exhaust system according to claim 8, characterized in that the reducing agent is introduced into the mixing chamber (8d).

10. An exhaust system according to any of claims 7-9, characterized in that the exhaust system is provided with a pump (4) for introducing the exhaust gas into the flow channel (8) and for introducing the mixture of the exhaust gas and the reducing agent into the main exhaust gas stream.

1 1 . An exhaust system according to claim 10, characterized in that the means (7, 1 1 , 12) for introducing reducing agent into the mixing space (8) are arranged upstream from the pump (4).

12. An exhaust system according to claim 10, characterized in that the means (7, 1 1 , 12) for introducing reducing agent into the mixing space (8) are arranged downstream from the pump (4).