DE102007056202A1 - Exhaust after-treatment device for an internal combustion engine and method for the after-treatment of exhaust gases of an internal combustion engine - Google Patents

Abstract

The invention relates to an exhaust gas aftertreatment device (1) for an internal combustion engine (2) having an exhaust gas line (3) which has an oxidation catalyst (4), a metering device (5) arranged behind the oxidation catalytic converter (4) in the flow direction of the exhaust gas for metering in a reducing agent into the exhaust gas Exhaust gas and an in the flow direction of the exhaust gas behind the metering device (5) arranged SCR catalyst (7) and a particulate filter (8). In order to achieve faster high temperatures in the SCR catalytic converter (7) and to improve the degree of conversion of nitrogen oxides (NOx) in the SCR catalytic converter (7) until then, it is proposed according to the invention that the particle filter (8) behind in the flow direction of the exhaust gas the SCR catalyst (7) is arranged.

Description

The The invention relates to an exhaust aftertreatment device for a Internal combustion engine according to the preamble of the claim 1, and a method for the aftertreatment of exhaust gases of an internal combustion engine according to the preamble of claim 12.

exhaust aftertreatment devices and methods of the type mentioned are in particular in Motor vehicles and especially in motor vehicles with diesel engine used to pollutant emissions and in particular emissions unburned hydrocarbons, nitrogen oxides and soot particles to diminish.

From the DE 102 07 986 A1 An exhaust aftertreatment device of the aforementioned type is already known in which the exhaust gas flowing through the exhaust gas line is first passed through an oxidation catalyst and a particle filter connected downstream of the oxidation catalyst before a reducing agent, such as ammonia (NH ₃ ) or urea, downstream of the particle filter in the flow direction of the exhaust gas (NH ₂ ) ₂ CO in solid or liquid form, is added to the exhaust gas before it flows through a SCR (Selective Catalytic Reduction) catalyst in which the contained in the exhaust gas, from a mixture of nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ) existing nitrogen oxides (NOx) with the formation of molecular nitrogen (N ₂ ) and water (H ₂ O) can be reduced.

However, the arrangement of the particulate filter before the SCR catalyst has two disadvantages. On the one hand, the particulate filter acts as a heat sink, which prevents a rapid heating of the SCR catalytic converter after a start of the internal combustion engine, as a result of which the earliest possible time of addition of ammonia (NH ₃ ) or urea (NH ₂ ) ₂ CO shifts back into the exhaust gas , because due to the risk of ammonia slip, ie an incomplete reaction of ammonia (NH ₃ ) in the SCR catalyst, below an exhaust gas temperature of about 250 to 300 ° C no ammonia (NH ₃ ) or urea (NH ₂ ) ₂ CO in the exhaust gas may be added and thus until then no implementation of the nitrogen oxides contained in the exhaust gas (NOx) is possible. On the other hand, it has been shown in experiments that the degree of conversion of the nitrogen oxides (NOx) with the metered reducing agent in the SCR catalyst not only on the amount of the reducing agent contained in the exhaust gas but in particular in a lower Tem temperature range up to an exhaust gas temperature of about 270 ° C also depends on the proportion of nitrogen dioxide (NO ₂ ) in the total nitrogen oxides (NOx), which decreases when passing the exhaust gas through a catalytically coated particulate filter, because part of the nitrogen dioxide (NO ₂ ) is bound to the surface of the particulate filter.

To improve the conversion of nitrogen oxides (NOx), the EP 1 054 722 B1 an SCR catalyst system for treating combustion exhaust gas and soot particles containing nitrogen oxides (NOx), in which the exhaust gas is first passed through an oxidation catalyst effective to convert at least a portion of the nitrogen monoxide (NO) to nitrogen dioxide (NO ₂ ) and thus increase the proportion of nitrogen dioxide in the nitrogen oxides (NOx) of the exhaust gas. The SCR catalytic converter system comprises a fines filter downstream of the oxidation catalytic converter in the flow direction of the exhaust gas, behind which first an injection device for metering in a reducing fluid and then an SCR catalytic converter is arranged. With the known SCR catalyst system, however, complete conversion of the nitrogen oxides (NO x) can not be achieved, especially in low temperature ranges below 250 ° C. In addition, here too, the fines filter acts as a heat sink and prolongs the time until sufficiently high temperatures are reached for complete conversion of the reducing fluid with the nitrogen oxides (NOx) in the SCR catalyst.

outgoing This is the object of the invention, an exhaust aftertreatment device and to improve a method of the type mentioned in the introduction, that in the SCR catalyst faster high temperatures are achieved and that until then the degree of conversion of nitrogen oxides (NOx) in the SCR catalyst can be improved.

This object is achieved in the exhaust gas aftertreatment device according to the invention in that the particle filter is arranged in the flow direction of the exhaust gas behind the SCR catalyst. By this measure, on the one hand achieved that after a start of the internal combustion engine, the exhaust gas cools less until it enters the SCR catalyst, so that in the SCR catalyst faster for a complete implementation of the nitrogen oxides (NOx) necessary temperatures are reached , On the other hand, the mixing ratio of nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) in the exhaust gas before the SCR catalyst is made to not shift in favor of the nitrogen monoxide (NO) and is about 1: 1, whereby at lower temperatures, a higher degree of Implementation of the nitrogen oxides and at temperatures of about 200 ° C complete conversion can be achieved. Overall, this can be a significant decrease in nitrogen oxide emissions recorded.

According to a preferred embodiment of the invention, a urea-water mixture is metered into the exhaust gas as a reducing agent, since the so-called AdBlue technology has now prevailed in motor vehicles. To avoid ammonia slip, that is, leakage of ammonia (NH ₃ ) into the environment, which may cause odor and health problems due to the pungent odor, toxicity and caustic effect of ammonia (NH ₃ ) the urea-water mixture in known exhaust aftertreatment devices or processes are added only at exhaust gas temperatures in the SCR catalyst of more than 250 to 300 ° C, while in exhaust gas aftertreatment devices or methods according to the invention, the metered addition of urea-water mixture due to the favorable mixing ratio of Nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) in the exhaust gas is already possible at significantly lower temperatures of about 200 ° C.

Preferably, the metering device is designed to inject the liquid contained in a reservoir, serving as a reducing agent liquid urea-water mixture in the exhaust or inject it to ensure a more uniform distribution in the exhaust stream. The water of the urea-water mixture evaporates due to the high exhaust gas temperatures even before reaching the SCR catalyst, while the urea (NH ₂ ) ₂ CO after the evaporation of water in both the exhaust line and the SCR catalyst by a hydrolysis reaction in ammonia ( NH ₃ ) and water (H ₂ O) is reacted, of which then the former serves to reduce the nitrogen oxides (NOx).

Around to reduce the space requirements and the costs of exhaust aftertreatment, the exhaust gas is advantageous immediately after passing through the SCR catalyst filtered, preferably when passing through a Particulate filter, which is useful in a common Housing is arranged with the SCR catalyst.

A further preferred embodiment of the invention Method provides that the metered addition or supply of the reducing agent as a function of an exhaust gas temperature behind the oxidation catalyst and / or before the SCR catalyst and depending is controlled by the NOx content of the exhaust gases behind the particulate filter.

in the The following is the invention with reference to an illustrated in the drawing Embodiment explained in more detail. Show it:

1 a schematic representation of an exhaust aftertreatment device according to the invention in an exhaust system of a diesel engine;

2a and 2 B : Representations of the degree of nitrogen oxide conversion in an SCR catalyst of the exhaust aftertreatment device as a function of the NO ₂ content in the nitrogen oxide (NOx) at temperatures of 200 ° C and 450 ° C.

In the 1 illustrated exhaust aftertreatment device 1 is used to reduce the emission of pollutants, especially unburned hydrocarbons (HC), nitrogen oxides (NOx) and soot particles in the exhaust gas of a diesel engine 2 of a motor vehicle.

To eliminate the unburned hydrocarbons (HC), the exhaust aftertreatment device comprises 1 one directly behind the diesel engine 2 in an exhaust system 3 of the diesel engine 2 arranged, traversed by the exhaust gas oxidation catalyst 4 in which the unburned hydrocarbons (HC) contained in the exhaust gas are oxidized with oxygen (O ₂ ) to carbon dioxide (CO ₂ ) and water (H ₂ O). The combustion of diesel fuel in the cylinders of the diesel engine 2 as well as the passage of the exhaust gas through the oxidation catalyst 4 Nitrogen oxides (NOx) are generated by means of a dosing device 5 in the exhaust metered liquid urea-water mixture 6 Reactively eliminated by adding urea (NH ₂ ) ₂ CO from the urea-water mixture 6 after its supply to the exhaust system 3 by a hydrolysis reaction in ammonia (NH ₃ ) and water (H ₂ O) is reacted, of which the former contains the nitrogen oxides (NOx) contained in the exhaust gas inside a downstream SCR catalyst 7 reduced to form molecular nitrogen (N ₂ ) and water (H ₂ O). For removing the soot particles, the exhaust aftertreatment device comprises 1 continue a particle filter 8th in which the soot particles contained in the exhaust gas are separated off and periodically burned.

The oxidation catalyst 4 is near the engine behind a lambda probe 9 for measuring the combustion air ratio in the exhaust system 3 arranged to ensure that in the oxidation catalyst 4 entering exhaust gas has a high exhaust gas temperature.

The dosing device 5 for metering in the urea-water mixture 6 is by means of a feed pump 10 from an entrained in the motor vehicle, the urea-water mixture 6 containing tank 11 fed. In the process, the feed pump 10 during operation of the diesel engine 2 from a controller 12 of the motor vehicle or the diesel engine 2 controlled as a function of the exhaust gas temperature and the nitrogen oxide content of the exhaust gas.

To measure the exhaust gas temperature is a behind the oxidation catalyst 4 in the exhaust system 3 arranged first temperature sensor 13 and one behind the metering device 5 and in front of the SCR catalyst 7 in the exhaust system 3 arranged second temperature sensor 14 while measuring the total nitrogen oxide content of the exhaust with the help of one behind the particulate filter 8th and in front of an exhaust 16 arranged NOx sensor 15 is made. While the exhaust gas temperature is especially important for the start of dosing of the urea-water mixture 6 in the exhaust system 3 The decisive factor depends on the amount of urea-water mixture added to the exhaust gas 6 mainly from the nitrogen oxide content of the SCR catalyst 7 exiting exhaust gases.

The SCR catalyst 7 and the particulate filter 8th have a common housing 17 in which the SCR catalyst 7 in front of the particle filter 8th is arranged so that it is first flowed through by the exhaust gas.

In the reduction of the nitrogen oxides (NO ₂ ) and nitrogen monoxide (NO) nitrogen oxides (NO x) in the SCR catalyst 7 the two following reactions take place: 4NO + 4NH ₃ + O ₂ ⇔ 4N ₂ + 6H ₂ O (1) NO + NO ₂ + 2NH ₃ ⇔ 2N ₂ + 3H ₂ O (2)

As can be seen from the figures 2a and 2 B In these reactions, the degree of conversion of the nitrogen oxides (NOx) into molecular nitrogen (N ₂ ) and water (H ₂ O) in the SCR catalyst depends on these reactions 7 on the one hand by the exhaust gas temperature and on the other hand by the mixing ratio of nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) or by the percentage of nitrogen dioxide (NO ₂ ) in the nitrogen oxides (NOx). While the dependence on the mixing ratio or the nitrogen dioxide content at high exhaust gas temperatures is very low, so that at each proportion between 0 and 70% almost complete conversion is achieved, as the example of the exhaust gas temperature of 450 ° C in 2 B shown, the degree of reaction at temperatures below 250 to 300 ° C is much more dependent on the mixing ratio or the proportion of nitrogen dioxide (NO ₂ ) in the nitrogen oxides (NOx). As 2a shows, at an exhaust gas temperature of 200 ° C, an optimal degree of conversion is achieved only when the proportion of nitrogen dioxide (NO ₂ ) in the nitrogen oxides (NOx) is 50%, that is, at a mixing ratio of nitrogen dioxide (NO ₂ ) Nitrogen monoxide (NO) of 1: 1, while the degree of conversion on both sides of the optimum mixing ratio decreases sharply. The optimum mixing ratio of 1: 1 is achieved by the fact that the particle filter 8th in the flow direction of the exhaust gas behind the SCR catalyst 7 is arranged, so that unlike in known exhaust aftertreatment devices with a SCR catalyst upstream particulate filter 8th does not lead to an absorption of nitrogen dioxide (NO ₂ ) on the surfaces of the particulate filter catalytically coated for combustion of the soot particles and thus to a shift of the mixing ratio in favor of nitrogen monoxide (NO).

By the arrangement of the particle filter 8th behind the SCR catalyst 7 It also avoids the particulate filter 8th after a start of the diesel engine 2 acts as a heat sink, which slows down a desired as rapid as possible heating of the SCR catalyst and thus an early metered addition of the urea-water mixture 6 opposes, since the latter can be made to prevent ammonia slip after reaching a certain minimum temperature when in the SCR catalyst 7 a substantially complete implementation of the nitrogen oxides (NOx) is ensured with ammonia (NH ₃ ). This minimum temperature can be determined by the arrangement of the particulate filter 8th behind the SCR catalyst 7 be lowered to about 200 ° C, as in accordance with 2a at this temperature and at the aforementioned optimum mixing ratio of nitrogen dioxide (NO ₂ ) to nitrogen monoxide (NO) in the SCR catalyst 7 Already a substantially complete implementation of the nitrogen oxides takes place.

1

exhaust treatment device

2

diesel engine

3

exhaust gas line

4

oxidation catalyst

5

metering

6

Urea-water mixture

7

SCR catalyst

8th

particulate Filter

9

lambda probe

10

feed pump

11

tank for urea-water mixture

12

control

13

temperature sensor

14

temperature sensor

15

NOx sensor

16

Exhaust

17

casing

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10207986 A1 [0003]
• - EP 1054722 B1 [0005]

Claims (15)

Exhaust gas aftertreatment device for an internal combustion engine having an exhaust gas line, comprising an oxidation catalyst, a metering device arranged in the flow direction of the exhaust gas downstream of the oxidation catalyst for metering a reducing agent into the exhaust gas and an SCR catalyst arranged behind the metering device in the flow direction of the exhaust gas, and a particle filter, characterized that the particle filter ( 8th ) in the flow direction of the exhaust gas behind the SCR catalyst ( 7 ) is arranged. Exhaust after-treatment device according to claim 1, characterized in that the metering device ( 5 ) is used for metering a liquid reducing agent. Exhaust after-treatment device according to claim 2, characterized in that the metering device ( 5 ) for metering in a liquid urea-water mixture ( 6 ) serves. Exhaust after-treatment device according to one of the preceding claims, characterized in that the metering device ( 5 ) with a reducing agent ( 6 ) reservoir ( 11 ) connected is. Exhaust after-treatment device according to claim 4, characterized by a conveying device ( 10 ) for the supply of reducing agent ( 6 ) to the metering device ( 5 ). Exhaust after-treatment device according to claim 5, characterized by a control device ( 12 ) for controlling the conveyor ( 10 ) as a function of at least one temperature and / or a nitrogen oxide content of the exhaust gases in the exhaust gas line (US Pat. 3 ). Exhaust after-treatment device according to claim 5 or 6, characterized by a flow direction of the exhaust gas behind the oxidation catalyst ( 4 ) in the exhaust line ( 3 ) arranged temperature sensor ( 13 ). Exhaust after-treatment device according to one of claims 5 to 7, characterized by a downstream of the metering device in the flow direction of the exhaust gas ( 5 ) and before the SCR catalyst ( 7 ) in the exhaust line ( 3 ) arranged temperature sensor ( 14 ). Exhaust after-treatment device according to one of claims 5 to 8, characterized by a downstream of the particulate filter in the flow direction of the exhaust gas ( 8th ) in the exhaust line ( 3 ) arranged NOx sensor ( 15 ). Exhaust after-treatment device according to one of the preceding claims, characterized in that the particulate filter ( 8th ) and the SCR catalyst ( 7 ) a common housing ( 17 ) exhibit. Exhaust after-treatment device according to one of the preceding claims, characterized in that upstream of the SCR catalytic converter ( 7 ) no particle filter ( 8th ) in the exhaust line ( 3 ) is arranged. A method for post-treatment of exhaust gas from an internal combustion engine, wherein the exhaust gas is filtered and passed through an oxidation catalyst and an SCR catalyst, and wherein between the oxidation catalyst and the SCR catalyst, a reducing agent is metered into the exhaust gas, characterized in that the exhaust gas after passing through the SCR catalyst ( 7 ) is filtered. A method according to claim 12, characterized in that the exhaust gas immediately after passing through the SCR catalyst ( 7 ) is filtered. A method according to claim 12 or 13, characterized in that the exhaust gas before passing through the SCR catalyst ( 7 ) is not filtered. Method according to one of claims 12 to 14, characterized in that the reducing agent ( 6 ) is metered into the exhaust gas as soon as the exhaust gas temperature in the SCR catalyst exceeds 200 ° C.