DE102013203876A1 - Selective catalytic reduction system - Google Patents

Abstract

The invention relates to an exhaust gas purification system, comprising an exhaust gas path (6) in which exhaust gas aftertreatment devices (2, 3, 4, 5) such as a first catalytic converter (2) for selective catalytic reduction and a second catalytic converter (5) for selective catalytic reduction are arranged , First and second introduction elements (7, 8) for introducing a reducing agent (reducing agent) are provided upstream of the respective catalyst (2, 5) for selective catalytic reduction. It is proposed that the first catalytic converter (2) for selective catalytic reduction is arranged in a bypass (9) which bypasses a control element (10) in the exhaust gas path (6) upstream of an exhaust gas aftertreatment device (3, 4), and wherein the first introduction element (7), which is assigned to the first catalytic converter (2) for selective catalytic reduction, is arranged upstream of the first catalytic converter (2) close to the engine.

Description

The present invention relates to an exhaust gas purification system having the features of the preamble of claim 1, comprising an exhaust path in which a first catalyst for selective catalytic reduction, and a second catalyst for selective catalytic reduction are arranged, wherein Einbringelemente for introducing a reducing agent each upstream of the respective Catalyst for selective catalytic reduction are provided.

With the EP 1 458 960 B1 Improvements in selective catalytic reduction should be achieved. EP 1 458 960 B1 describes an exhaust system for an internal combustion engine. The exhaust system comprises: a first catalyst that can selectively catalyze the reduction of NOx with a nitrogen compound as a reducing agent; a first means for introducing the nitrogen compound into an exhaust gas guided by the exhaust system upstream of the first catalyst; a particulate filter comprising a second catalyst that can selectively catalyze the reduction of NOx with a nitrogen compound as the reducing agent, the particulate filter being disposed downstream of the first catalyst; a catalyst disposed downstream of the first SCR catalyst and upstream of the particulate filter that can oxidize NO to NO 2, and a second means for introducing a nitrogen compound into the exhaust gas between the NO oxidation catalyst and the particulate filter. The exhaust system of EP 1 458 960 B1 has a very large exhaust gas resistance due to the large number of exhaust aftertreatment devices, which are arranged in series one behind the other in the exhaust path, which adversely affects the available power of the internal combustion engine. Since exhaust gas aftertreatment devices, ie in particular the first SCR catalytic converter arranged close to the engine, are always flowed through by exhaust gases, it is exposed to considerable loads, so that a relatively early aging is achieved, which requires very early replacement, in particular of the first SCR catalytic converter otherwise the normatively prescribed and required exhaust emission values are not achievable. An exchange is very time-consuming and expensive, with an early replacement of exhaust aftertreatment devices also implies a negative sense of quality of the driver compared to the vehicle manufacturer.

The US 2008/0264042 A1 also discloses an exhaust gas purification system, wherein in the exhaust line sequentially an oxidation catalyst, a diesel particulate filter and a catalyst for selective catalytic reduction are arranged. A reducing agent may be introduced into the exhaust line upstream of the SCR catalyst. A heater is provided, which the exhaust gases upstream of the SCR catalyst, for. B. after a cold start of the engine to a temperature of 700 ° C to 1000 ° C can heat.

The DE 11 2006 003 231 T5 (= US 2007/0122317 A1 ) also discloses a selective catalytic reduction system comprising two sequential selective catalytic reduction (SCR) catalysts. Upstream of the respective SCR catalyst, ammonia is introduced as a reductant, the ammonia being generated in an ammonia generation system.

In the DE 103 54 803 A1 (= US 2004/0118109 A1 ) also describes an exhaust gas purification system, wherein a reducing agent upstream of an exhaust aftertreatment device, but also in the same is introduced, upstream of the exhaust gas aftertreatment device but less reducing agent is introduced than is needed for the treatment of the exhaust gas.

For the treatment of exhaust gases from z. B. Diesel internal combustion engines are currently more catalysts for selective catalytic reduction (SCR catalysts) used to reduce NOx in the exhaust gases. For this purpose, a nitrogen compound, ie z. For example, ammonia or a precursor (urea) thereof is metered into the exhaust gases so that the ammonia on the SCR catalyst can react with the NOx components of the exhaust gas produced by the (diesel) internal combustion engine to become harmless nitrogen (N2). and water (H2O) can be reduced. Such SCR systems can have a very high conversion efficiency (up to 100% NOx), although a certain operating temperature of the SCR catalysts must be achieved. Above 200 ° C, the conversion efficiency is optimal, but at a temperature of less than 150 ° C, a conversion is rather unattainable.

Therefore, after a cold start of the engine, an SCR catalyst is inactive for a certain period of time so that the NOx components can not be converted to harmless nitrogen and water during this time until the operating temperature of the SCR catalyst is reached. Since the SCR catalytic converter is usually arranged very far away from the internal combustion engine, virtually as the last exhaust aftertreatment device in the exhaust gas line, the reaching of the required operating temperature is further delayed. Is still a precursor of ammonia, ie z. B. aqueous urea introduced into the exhaust gas, a conversion will take place only above 160 ° C, so that incomplete degradation and deposition is to be feared.

Some types of SCR catalysts can store certain amounts of ammonia, which can be used for NOx reduction if an induction is not performed. In this respect, the time after a cold start can be bridged with such a measure, so that the operability of the SCR catalyst can be achieved much faster by the existing, ie stored ammonia.

Care must be taken in positioning the SCR catalyst so that no oxidation catalysts or other oxidizing exhaust aftertreatment devices are present between the ammonia introduction initiator and the SCR catalyst. Any noble metal containing device, such as. As diesel oxidation catalysts (DOC), jacketed diesel particulate filter (CDPF), or nitrogen absorber (LNT) lead to oxidation of the introduced ammonia which then can not get to the SCR catalyst. However, such exhaust aftertreatment devices must be arranged in the exhaust line in order to retain hydrocarbons, carbon monoxide and particles (eg soot), or not or at least reduce them to the environment. All of these aftertreatment devices also require a very high exhaust gas temperature to effectively perform their function.

Against this background, the invention has for its object to provide an improved exhaust system of the type mentioned, with which the NOx conversion can be improved after a cold start of the engine.

The solution to this problem is achieved by a method having the features of claim 1. Further, particularly advantageous embodiments of the invention disclose the dependent claims.

It should be noted that the features listed individually in the following description can be combined with one another in any technically meaningful manner and show further embodiments of the invention. The description additionally characterizes and specifies the invention in particular in connection with the pictures.

According to the present invention, there is provided an exhaust gas purification system including an exhaust path in which exhaust aftertreatment devices such as a first selective catalytic reduction catalyst and a second selective catalytic reduction catalyst are disposed, first and second introduction elements for introducing a reducing agent (reducing agent) respectively upstream of each Catalyst for selective catalytic reduction are provided, wherein the first catalyst for selective catalytic reduction is arranged in a bypass, which bypasses a control element in the exhaust path upstream of an exhaust gas aftertreatment device, and wherein the first introduction element, which is associated with the first catalyst for selective catalytic reduction, upstream of the first catalyst is arranged close to the engine. Downstream of the second catalyst for selective catalytic reduction, further exhaust aftertreatment devices may be provided. Between the first catalyst for selective catalytic reduction and the second catalyst for selective catalytic reduction, but upstream of the second Einbringelementes exhaust aftertreatment devices with, for example, oxidative effect such. B. an oxidation catalyst and / or a particle filter may be arranged.

In the following, the respective catalyst for selective catalytic reduction is referred to as SCR catalyst. The further exhaust aftertreatment devices are referred to as follows: oxidation catalysts, preferably diesel oxidation catalysts (DOC); Particulate Filter (DPF), preferably Sheathed Diesel Particulate Filter (CDPF); Nitrogen trap (LNT).

In the context of the invention, it is favorable if the first SCR catalytic converter is arranged close to the engine, ie downstream of the turbocharger, directly behind the turbine side. In addition, it is expediently provided that the first SCR catalytic converter is arranged upstream of every other exhaust gas after-treatment device (DOC, optionally LNT, CDPF). In this respect, it is preferably provided that at least one oxidative exhaust aftertreatment device is arranged between the two catalysts and also upstream of the second introduction element, so that both catalysts for selective catalytic reduction are separated from one another. Since the at least oxidatively acting exhaust aftertreatment device is arranged between the two catalysts for selective catalytic reduction, the second introduction element is also required, since the reducing agent is oxidized along the exhaust path. This preferred embodiment is based on the finding that, for example, a diesel internal combustion engine generates a significant amount of NO 2 at low operating temperature, NO 2 improving the performance of the SCR catalytic converter since the reaction of the ammonia with NO and NO 2 is faster than the reaction of the ammonia either only with NO or only with NO2. If now one of the exhaust aftertreatment devices (DOC, optionally LNT, CDPF) arranged in front of the SCR catalyst, in particular before the first SCR catalyst, the largest proportion would be at low Operating temperatures generated NO2 amount due to the reaction with the carbon monoxide contained in the exhaust gas (CO) and hydrocarbons (HC) converted into NO. If in the exhaust line as the first exhaust aftertreatment device but no oxidation catalyst, such as. As the DOC is arranged, due to the high thermal mass of the SCR catalyst, the conversion of the CO and HC in the cold start of the internal combustion engine is delayed. This can lead to increased exhaust gas values, so that the legally prescribed exhaust gas values could be exceeded.

This is where the invention is based, in that the first SCR catalytic converter is bypassable by exhaust gases, which is achieved by means of the bypass in which the first SCR catalytic converter is arranged. It is thus achieved that the exhaust gases can flow directly into the DOC without flowing through the first SCR catalytic converter when the control element in the exhaust gas path is open. If the DOC has reached its operating temperature so that CO and HC can be oxidized, the control closes, so that the exhaust gases can now flow back into the exhaust path via the bypass through the first SCR catalytic converter and downstream of the control element, so that first the NOx Components can be converted before the exhaust gases reach the other exhaust aftertreatment devices (DOC, optional LNT, CDPF, 2.SCR).

Another advantage of the invention is the fact that the control can be reopened, which can be the case when the second SCR catalyst has reached a sufficiently high temperature to convert NOx. Opens the control, the first SCR catalyst is no longer traversed by exhaust gases. Thus, an early aging of the first SCR catalyst is prevented, which of course delays an exchange of the same compared to a constantly flowed through the first SCR catalyst. Thus, the manufacturer of the vehicle can have a positive effect on the quality of the vehicles detected by the consumer. The fact that the first SCR catalyst is bypassed by exhaust gases, also reduces the exhaust gas resistance, so that the part of the available power of the internal combustion engine, which had to be used to overcome the greater exhaust resistance, can now be made available to the drive train, which also has a fuel economy effect.

It is also leading, if the first SCR catalyst has a smaller volume than the second SCR catalyst. In this respect, the invention combines a small SCR catalytic converter with a large SCR catalytic converter, wherein the first SCR catalytic converter can be mounted close to the engine, preferably directly behind the turbocharger, and the second SCR catalytic converter remote from the engine at a known location below the underbody of the vehicle , The different volumes of the SCR catalysts are also possible because the first SCR catalyst is only active for a certain phase of the internal combustion engine, that is, exhaust gases flow through it. If the second SCR catalytic converter has reached its operating temperature, the exhaust gases are directly supplied to the exhaust gas aftertreatment device by opening the control element in the exhaust path, without exhaust gases passing through the first SCR catalytic converter.

It is expedient, if in front of the respective SCR catalyst in each case a Einbringelement for introducing ammonia or a precursor thereof, for. B. aqueous urea is arranged. The supply of Einbringelemente is known, so it will not be discussed further. The respective introduction element can also be referred to as an injector, and is known per se in design and control.

It is possible to arrange the first introduction element upstream of the first SCR catalytic converter, but also upstream of the optional turbocharger in the exhaust manifold. It is also possible to arrange the first introduction element downstream of the turbocharger directly behind it. It is preferred if the introduction element is arranged upstream of the first SCR catalyst in the bypass. If a turbocharger is present, the introduction element is also arranged downstream of this turbocharger but also directly behind it. The second introduction element is expediently arranged downstream of the oxidative exhaust gas component and upstream of the second SCR catalytic converter.

In the invention, therefore, the first SCR catalyst will rather reach its operating temperature after a cold start. This is also because the volume of the first SCR catalyst is reduced. Thus, an introduction, ie an injection of ammonia or a precursor thereof can be carried out upstream of the first SCR catalyst. Preferably, aqueous urea is injected. The first SCR catalyst is located very close to the internal combustion engine, preferably on a diesel engine, wherein the first SCR catalyst is arranged in a bypass, so that the exhaust gases only flow through the first SCR catalytic converter when the control element is closed, then that only then an injection of aqueous urea is generated. Of course, the control can also be arranged in the bypass, wherein the first SCR catalyst is bypassed. When the control is closed, the bypass is also closed, allowing the exhaust gases to flow through the SCR catalyst. When the control is open, the exhaust gases flow via the bypass to the first SCR catalyst passing directly into the following exhaust aftertreatment device, which is preferably a DOC. A further effect of the invention is the fact that substantially more ammonia or aqueous urea can be injected than is needed to convert NOx by means of ammonia in the first SCR catalyst close to the engine, thus improving the conversion efficiency. The conversion of aqueous urea to the necessary ammonia is well known. Of course, not the entire amount of ammonia is consumed, so that the excess of ammonia in the DOC / CDPF is oxidized, or is absorbed in the second, remote engine SCR catalyst.

With the invention of the second, remote engine SCR catalyst for actual NOx conversion at high temperatures (above 200 ° C) can be used. Once this temperature is reached, the injection of ammonia or its precursor upstream of the first close-coupled SCR catalyst can be stopped or continued. One effect is to see that the injector is cooled further, since in the close-coupled arrangement in the operation of the diesel engine considerable temperature loads prevail. Of course, other cooling options can be considered. Another effect can be seen in the fact that ammonia can be stored in the first SCR catalytic converter close to the engine, so that a required amount of ammonia is already present in the first SCR catalytic converter for the next cold start of the diesel engine.

A further advantage of the invention is that the two SCR catalysts can each have a different sheathing. The close-coupled first SCR catalyst can, for. B. be a particularly temperature-resistant, with a Fe-shell is preferred. The remote engine, second SCR catalyst, however, can be designed to be particularly sensitive to temperature, wherein preferably a Cu sheath is conceivable.

Further advantageous details and effects of the invention are explained in more detail below with reference to an embodiment shown in the figures. Show it:

1 a basic view of an exhaust gas purification system,

2 a temperature diagram, and

3 a diagram for nitrogen oxide conversion.

In the different figures, the same parts are always provided with the same reference numerals, so that these are usually described only once.

1 shows an exhaust gas purification system 1 in a basic view. The emission control system 1 has exhaust aftertreatment devices 2 . 3 . 4 . 5 on which in an exhaust path 6 an internal combustion engine, preferably a diesel engine are arranged. The internal combustion engine may include a turbocharger.

The exhaust aftertreatment devices 2 . 3 . 4 . 5 are as a first catalyst 2 for selective catalytic reduction (SCR), exemplified as an oxidation catalyst (DOC) 3 , z. As a particulate filter (DPF) 4 and as a second catalyst 5 for selective catalytic reduction (SCR) sequentially in the exhaust path 6 arranged. In addition, two Einbringelemente 7 and 8th for introducing a reducing agent (reducing agent) each upstream of the respective SCR catalyst 2 . 5 intended for selective catalytic reduction. According to the invention, the first catalyst 2 for selective catalytic reduction in a bypass 9 is arranged, which is a control 10 in the exhaust path 6 upstream of one of the following exhaust aftertreatment device, preferably bypassing upstream of the DOC, and wherein the first introduction element 7 which is the first catalyst 2 assigned to the selective catalytic reduction, upstream of the first catalyst 2 is arranged close to the engine.

The second insertion element 8th is upstream of the second SCR catalyst 5 and downstream z. B. a DPF 4 intended. The Einbringelemente 7 . 8th can also be called injectors 7 . 8th be designated, which in a known manner with a memory 11 are connected. In the store 11 is preferably urea, more preferably stored aqueous urea. The urea is converted in a known manner to ammonia. The control of the introduction elements 7 . 8th takes place in a known manner. The first introduction element 7 may be located upstream of an optional turbocharger directly in the exhaust manifold. The close-coupled arrangement promotes the conversion into ammonia, since close to the engine, even after a cold start, sufficiently high exhaust gas temperatures are present very soon, which are higher than engine-distant. An arrangement directly behind the turbocharger is also possible. In 1 it can be seen that the injection element 7 in the bypass 9 is arranged, but also a very close to the engine position of the first injection element is desired.

The control 10 may be a valve which is the exhaust path 6 completely opens or closes.

Close the control 10 , the exhaust flow is through the bypass 9 through the first SCR catalyst 2 to the DOC 3 and from there the DPF 4 flowing through to the second SCR catalyst 5 directed. In this respect, the first SCR catalytic converter is arranged close to the engine, the second SCR catalytic converter being arranged remote from the engine. In this case, the first SCR catalytic converter is preferably arranged directly behind the optional turbocharger, that is behind its exhaust gas turbine wheel. If no turbocharger is provided, the first SCR catalytic converter can be mounted even closer to the exhaust manifold. The first SCR catalytic converter is thus advantageously arranged on the exhaust side as close as possible to the internal combustion engine. This is particularly favorable in terms of the quickly achievable operating temperature of the first, close-coupled SCR catalyst.

The first SCR catalyst 2 refers to the second SCR catalyst 5 lower volume, which in turn has an advantageous effect on a rapid achievement of its operating temperature. The first SCR catalyst 2 is not permanently seconded in the exhaust stream, which has an advantageous effect on its aging process.

During a cold start of the internal combustion engine, ie the diesel engine, the control element is initially opened so that the exhaust gases flow past the first SCR catalytic converter directly into the DOC. As soon as the DOC temperature is high enough to ensure CO and HC oxidation, the control is closed so that the exhaust gases now flow through the bypass through the first SCR catalyst so that NOx conversion can begin. The second SCR catalyst would not have the necessary operating temperature for the conversion of the NOx at this early stage after the cold start, but this can be achieved by means of the first SCR catalyst.

Injection of ammonia, preferably aqueous urea, is controlled accordingly.

Reached the second, engine-remote SCR catalyst 5 its operating temperature, the control can 10 be opened, so that the exhaust gases now the first, close to the engine SCR catalyst 2 immediately be directed to the DOC.

Will be the first close-coupled SCR catalytic converter 2 is not flowed through by the exhaust, it can be seen that its aging process is interrupted. This is the first, close-coupled SCR catalytic converter 2 more durable than close-coupled SCR catalysts, which are constantly flowed through by exhaust gases. It is possible that the first injection element 7 So the injector 7 however, continues to be driven, and injects urea. Thus, the injection element can be cooled, which is expedient due to the near-engine position and the associated high temperature load. When the injection element 7 remains active, and injects urea (or ammonia), the resulting ammonia may be oxidized into the DOC or CDPF or absorbed by the second SCR catalyst. In one embodiment, the resulting ammonia can also be stored in the first SCR catalytic converter, so that this ammonia stored in the first SCR catalytic converter is immediately available for a renewed cold start.

2 shows a temperature diagram in which the temperature difference of the close-coupled, first SCR catalyst 2 and the engine-remote, second SCR catalyst is recognizable. The X axis shows the time course in seconds after the cold start, with the Y axis representing the temperature in degrees Celsius [° C]. It can be seen that the temperature of the engine-related first SCR catalytic converter is sufficiently high very quickly after the cold start. The graphs of the SCR catalysts in question are given by the associated reference numeral 2 and 5 characterized.

3 shows a NOx conversion comparison between the prior art and the inventive arrangement. The X-axis again shows the time course after the cold start in seconds. The Y-axis represents the accumulated NOx amount in grams [g]. The line 12 represents the amount of NOx produced by the diesel engine, which would be discharged to the environment without exhaust aftertreatment. The line 13 shows a prior art exhaust treatment in which a DOC, a CDPF and a SCR catalyst remote from the engine are provided. A reduction of 60% NOx is achievable. The line 14 shows a NOx estimate just downstream of the first SCR catalyst 2 , where the line 15 the expected according to the invention total output NOx behind the second SCR catalyst 5 shows. A reduction of 85% can be achieved with the arrangement and circuit of the control element according to the invention.

With the invention, the NOx reduction in the lower temperature range, in particular after a cold start of the diesel engine is significantly improved.

LIST OF REFERENCE NUMBERS

1

emission Control system

2

close to the engine, first SCR catalytic converter

3

z. Eg DOC

4

optional, e.g. DPF / CDPF

5

remote from motor, second SCR catalyst

6

exhaust path

7

Injection member / injector

8th

Injection member / injector

9

bypass

10

control

11

Storage

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1458960 B1 [0002]
• US 2008/0264042 A1 [0003]
• DE 112006003231 T5 [0004]
• US 2007/0122317 A1 [0004]
• DE 10354803 A1 [0005]
• US 2004/0118109 A1 [0005]

Claims (10)

Exhaust gas purification system, comprising an exhaust gas path ( 6 ), in the exhaust aftertreatment devices ( 2 . 3 . 4 . 5 ) like a first catalyst ( 2 ) for selective catalytic reduction, and a second catalyst ( 5 ) are arranged for selective catalytic reduction, wherein first and second introduction elements ( 7 . 8th ) for introducing a reducing agent in each case upstream of the respective catalyst ( 2 . 5 ) are provided for selective catalytic reduction, characterized in that the first catalyst ( 2 ) for selective catalytic reduction in a bypass ( 9 ), which is a control ( 10 ) in the exhaust path ( 6 ) upstream of an exhaust aftertreatment device ( 3 . 4 ), and wherein the first introduction element ( 7 ), which is the first catalyst ( 2 ) is assigned to the selective catalytic reduction, upstream of the first catalyst ( 2 ) is arranged close to the engine. Exhaust gas purification system according to claim 1, characterized in that the first catalyst ( 2 ) is arranged close to the engine. Exhaust gas purification system according to claim 1 or 2, characterized in that the control element ( 10 ) is opened, so that exhaust gases immediately the first catalyst directly into an oxidation catalyst ( 3 ), which upstream of the second catalyst ( 5 ) is arranged for selective catalytic reduction. Exhaust gas purification system according to one of the preceding claims, characterized in that the control element ( 10 ) is closed, so that exhaust gases through the bypass ( 9 ) in the first catalyst ( 2 ) upstream of an oxidation catalyst ( 3 ) in the exhaust path ( 6 ) flow back. Exhaust gas purification system according to one of the preceding claims, characterized in that the control element ( 10 ) is opened when the second catalyst ( 5 ) has reached its operating temperature. Exhaust gas purification system according to one of the preceding claims, characterized in that the first catalyst ( 2 ) has a smaller volume than the second catalyst ( 5 ) Exhaust gas purification system according to one of the preceding claims, characterized in that the first introduction element ( 7 ) is arranged close to the motor, wherein the first introduction element ( 7 ) is preferably arranged in an exhaust manifold, and wherein the first introduction element ( 7 ) is preferably arranged directly behind a turbocharger. Exhaust gas purification system according to one of the preceding claims, characterized in that the first introduction element ( 7 ) remains active, even if the control ( 10 ) at a sufficient operating temperature of the second catalyst ( 5 ) is open. Exhaust gas purification system according to one of the preceding claims, characterized in that between the two catalysts for catalytic reduction ( 2 . 5 ), but upstream of a second introduction element ( 8th ) at least one exhaust aftertreatment device ( 3 . 4 ) is arranged with oxidative action. Exhaust gas purification system according to one of the preceding claims, characterized in that between the two catalysts for catalytic reduction ( 2 . 5 ), but upstream of a second introduction element ( 8th ) an oxidation catalyst ( 3 ) and a particle filter ( 4 ) is arranged.

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