WO2004027228A1

WO2004027228A1 - Exhaust gas purification system and method for purifying exhaust gas

Info

Publication number: WO2004027228A1
Application number: PCT/DE2003/001932
Authority: WO
Inventors: Markus Windenmeyer
Original assignee: Robert Bosch Gmbh
Priority date: 2002-09-12
Filing date: 2003-06-11
Publication date: 2004-04-01
Also published as: DE10242303A1

Abstract

The invention relates to an exhaust gas purification system for purifying the exhaust gas of an internal combustion engine, especially an internal combustion engine with spontaneous ignition. Said system comprises an exhaust gas aftertreatment unit (4) provided with at least one particle filter and at least one NOX reservoir (44). The invention further relates to a corresponding method for purifying the exhaust gas of an internal combustion engine (2).

Description

Exhaust gas cleaning system and method for cleaning exhaust gases

The invention relates to an exhaust gas purification system with a regenerable exhaust gas aftertreatment unit. The invention further relates to a method for purifying exhaust gases, in which an exhaust gas stream is passed through a regenerable exhaust gas aftertreatment unit.

State of the art

The known design measures such as, for example, a favorable combustion chamber design are no longer sufficient to comply with future exhaust gas limit values of internal combustion engines. In particular, internal combustion engines whose exhaust gas has a high proportion of oxygen, for example diesel engines or gasoline engines with direct fuel injection, require more effort to reduce pollutants, in particular to reduce NO _x emissions. An additional problem with the diesel engine is that its particle emissions have to be reduced.

The particles contained in the exhaust gas of diesel engines consist largely of soot, which is, for example, in a particle filter can be held back and collected. Such a filter must, however, be regenerated, ie usually burned free, after reaching a certain particle load. One possibility for regeneration is to heat the exhaust gases to a temperature of approximately 450 to 650 ° C., which can be done, for example, by metering in hydrocarbons, in particular vaporized or atomized fuel, into the exhaust system upstream of an oxidation catalytic converter. The exothermic conversion to CO, C0 ₂ and H ₂ 0 releases the heat required for free burning. The soot is burned off with the help of the oxygen in the exhaust gas. This method has disadvantages due to the relatively high temperatures that arise.

In addition to oxygen, nitrogen dioxide (N0 ₂ ) is suitable as an oxidizing agent for the carbon-containing soot for the regeneration of the diesel particle filter. This process is suitable for the oxidation of soot at much lower temperatures. The required N0 ₂ is usually provided by an oxidation catalyst, which contains precious metals and in particular platinum, from the nitrogen monoxide (NO) contained in the exhaust gas.

Another way to lower the temperature required for the regeneration of the diesel particle filter is to use catalytically coated diesel particle filters. In principle, the catalyst coatings suitable for such processes can also oxidize NO to NO ₂ . A catalyst for reducing the ignition temperature of diesel soot is described in DE 31 41 713 AI. The active substance of this known catalyst consists of silver vanada, which can be applied to a filter element serving as a structural amplifier. A monolithic ceramic body is proposed in particular as a structural amplifier.

Advantages of the invention

An exhaust gas cleaning system for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition, has an exhaust gas aftertreatment unit with at least one particle filter. According to the invention, the exhaust gas aftertreatment unit also has at least one NO _κ store. This can be integrated in the at least one particle filter or upstream of this. With such a configuration, it is possible to burn off the soot accumulated on the filter again without auxiliary materials supplied from the outside and without significant additional fuel consumption of the internal combustion engine and with little additional technical effort. The desorption of NO _x takes place primarily thermally and less so that the air-fuel ratio of the combustible mixture of the internal combustion engine is enriched, ie is reduced.

An advantageous embodiment of the invention provides that the particle filter has a coating with a storage component for storing NO _x . The NO _x storage can be made from one Carrier material, a so-called washcoat, which can consist of Al ₂ 0 ₃ Si0 ₂ or other materials commonly used for such a purpose and is applied, for example, to a honeycomb body.

One embodiment variant provides that the NO _x store is located on an oxidation catalytic converter which is connected upstream of the particle filter.

The NO _x store preferably has nitrogen oxide-storing components or compounds, for example basic oxides and / or noble metals, for example platinum. Nitrogen oxides can be stored in the NO _x store, this storage preferably taking place in particular at temperatures at which N0 ₂ does not react significantly with the soot located in the particle filter. Such temperatures are typically below 300 ° C. At temperatures at which a significant conversion of N0 ₂ with soot takes place, the nitrogen oxides are released from the NO _x storage and are available for soot oxidation. The NO _x storage component can be connected upstream of the particle filter or can be integrated into the particle filter. With the invention, it is possible to use NO _x in the exhaust gas much better for the purposes of particle filter regeneration.

According to one embodiment of the invention, the NO _x storage contains basic oxides or compounds which can store nitrogen oxides as main components. Examples of these are alkali and Alkaline earth metals, the lanthanides including yttrium and scandium, as well as Ti0 ₂ , ZrO ₂ , Bi ₂ 0 ₃ , copper oxides, silver or silver oxides and lead oxides. In contrast to the NO _x storage catalytic converter, components which, in addition to N0 _{X, can} also store larger amounts of oxygen, are not a disadvantage for this application. The NO _x storage device can also contain one or more precious metal components, preferably platinum.

The NO _x storage stores under lean, ie oxygen-rich exhaust gas _NOx. It preferably has a high storage capacity at medium temperatures between approx. 150-350 ° C. At higher temperatures above 250-400 ° C, preferably 300-350 ° C, the NO _x storage begins to desorb the stored NO _x , which is then available for the regeneration of the particle filter.

According to an advantageous embodiment of the invention, the storage behavior can be influenced by a warming-up device. Such a heating device can be an electric heater, for example. In this way, the exhaust gas temperature can be brought specifically to ranges above 350 ° C, for example immediately after the internal combustion engine starts, where it still emits exhaust gas at relatively low temperatures.

The particle filter, which can be a surface filter or a depth filter, can furthermore be provided with an active component, the function of which is to support the oxidation of nitrogen monoxide and / or the trapped soot. In this way, very effective exhaust gas cleaning is possible.

According to a further embodiment of the invention, the exhaust gas aftertreatment system can furthermore have at least one oxidation catalytic converter, which can be arranged, for example, upstream of the at least one particle filter or the at least one NO _x store. Alternatively, the oxidation catalytic converter can also be connected downstream of the particle filter or the NO _x store. With such an oxidation catalytic converter, a further reduction in the nitrogen oxide content in the exhaust gas is possible, since the oxidation catalytic converter in the diesel engine brings about a significant reduction in carbon monoxide and hydrocarbon emissions. Since the hydrocarbon emission is for particle emission, this can also be reduced to a limited extent by the catalyst. Optionally, the oxidation catalyst can also contain the storage function in addition to the oxidation function.

Furthermore, the exhaust gas aftertreatment system can have an additional device for reducing NO _x , which is connected downstream of the at least one particle filter. Such a device can be, for example, a further NO _x store, for example a NO _x storage catalytic converter.

According to a further embodiment according to the invention, the exhaust gas aftertreatment system can have an additional device for selective catalytic reduction. This device can be connected downstream of the exhaust gas aftertreatment system or integrated into it. The device for selective The catalytic reduction (so-called SCR process) can preferably have a metering device for supplying reducing agents into the exhaust gas flow upstream of the device for selective catalytic reduction. This also enables a further reduction in the nitrogen oxide content in the exhaust gas. In the so-called SCR process, a reducing agent is introduced into the catalyst via a metering system. Common reducing agents are, for example, an aqueous urea solution or diesel fuel.

A method for cleaning exhaust gases from an internal combustion engine, in particular a diesel internal combustion engine, in which an exhaust gas flow through an exhaust gas aftertreatment unit. is routed, which comprises at least one particle filter, provides according to the invention that the exhaust gas stream is passed through at least one NO _x storage. The exhaust gas flow can either be passed through a NO _x accumulator integrated in the particle filter, or successively through the NO _x accumulator or the particle filter. Both configurations can have advantages, depending on whether a modular structure or maximum integration of the exhaust gas aftertreatment unit is desired in the smallest space.

The nitrogen oxides contained in the exhaust gas are preferably stored in the NO _x accumulator at low exhaust gas temperatures between approximately 150 ° C. and approximately 350 ° C. Furthermore, the nitrogen oxides contained in the exhaust gas are preferably released in the NO _x storage at higher exhaust gas temperatures above about 300-350 ° C. and react exothermically with soot particles in the particulate particulate filters. In this way, the particle filter can be continuously regenerated. The desorption of the nitrogen oxides in the NO _x store can be initiated in a targeted manner by heating the NO _x store if necessary by means of an external heater, so that exhaust gas temperatures above 350 ° C can be reached.

Oxidation of nitrogen monoxide and / or the stored soot particles can also be supported by active components applied to the particle filter.

The exhaust gases can also be passed through an additional oxidation catalyst. This can be connected upstream or downstream of the particle filter or the NO _x store. With a very compact exhaust gas cleaning device, a very extensive cleaning effect can be achieved.

The exhaust gases can also be passed through a device for selective catalytic reduction, a reducing agent being added to the exhaust gases before or in the device for selective catalytic reduction. In this way, a further reduction in the nitrogen oxide content in the exhaust gas can be achieved.

The invention is explained in more detail in a preferred embodiment with reference to the accompanying drawings. It shows: Figure 1 is a schematic representation of an internal combustion engine with an exhaust gas aftertreatment unit in an exhaust duct

Figure 2 is a schematic representation of an exemplary configuration of the exhaust gas aftertreatment system.

FIG. 1 shows a schematic illustration of an internal combustion engine 2 with an inlet channel 21, in which an adjustable throttle valve 22 for regulating a supply of fresh gas 23 can be arranged, if necessary. The internal combustion engine can be a diesel engine (with compression ignition) or, for example, a gasoline engine (with spark ignition) with direct fuel injection. Both have a relatively high proportion of NO _x in the exhaust gas. The diesel engine also emits soot particles to a significant extent.

An exhaust gas aftertreatment unit 4 is arranged in an exhaust duct 24 of the internal combustion engine in a housing 43 or in a plurality of separate housings. An exhaust gas stream 41 upstream of the exhaust gas aftertreatment unit 4 is cleaned of soot particles and nitrogen oxides in it and leaves the exhaust gas channel 24 as a cleaned exhaust gas stream 42. The exhaust gas aftertreatment unit 4 has at least one NO _x accumulator 44 and a particle filter 45. Further components can optionally be provided and are explained in more detail with reference to the schematic illustration in FIG. 2.

Like the schematic representation of an exhaust gas aftertreatment unit 4 according to the invention from FIG. 2 makes it clear that the NO _x store 44 can be arranged in front of the particle filter 45. Optionally, both modules 44, 45 can be integrated in one structural unit. For example, the particle filter 45 can optionally be coated with a component that is capable of storing or desorbing N0 _X at the temperatures mentioned.

An oxidation catalytic converter 47 connected upstream of the NO _x store is optional and can also be located behind the particle filter 45. Another optional module is a NO _x storage catalytic converter 46 or a device for the selective catalytic reduction of the exhaust gas. A supply device 6 for supplying an active substance 61 for regeneration of the catalyst can be provided in front of the NO _x storage catalytic converter 46 or in front of the mentioned device for selective catalytic reduction, so that the active substance 61 can be supplied with the desired amount by means of a metering device 62 Exhaust gas is supplied.

Nitrogen oxides are stored in the NO _x store 44. This storage takes place in particular at temperatures at which nitrogen dioxide does not react significantly with the soot located in the particle filter 45. Such temperatures are between approx. 150 and 350 ° C. At temperatures at which a significant conversion of nitrogen dioxide with soot takes place, NO _{x is released} from the NO _x store and used for the soot oxidation in the particle filter 45. This is regenerated. In this way it is possible to use N0 _X in the exhaust gas for the purpose of particle filter regeneration. The soot is burned off at a relatively low temperature level, for example 300 to 450 ° C., since soot is oxidized with the aid of N0 ₂ at significantly lower temperatures than by means of 0 ₂ .

Claims

claims

1. Exhaust gas cleaning system for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition, with an exhaust gas aftertreatment unit which comprises at least one particle filter, characterized in that the exhaust gas aftertreatment unit (4) furthermore has at least one NO _x accumulator (44) ,

2. Emission control system according to claim 1, characterized in that the at least one NO _x -

Memory (44) is integrated in the at least one particle filter (45).

3. Emission control system according to claim 2, characterized in that the particle filter (45) a

Has coating with a storage component for storing NO _x .

4. Emission control system according to claim 1, characterized in that the at least one NO _x -

Memory (44) upstream of which at least one particle filter (45).

5. Emission control system according to one of the preceding claims, characterized in that the

NO _x storage (44) has a ceramic carrier material.

6. Emission control system according to one of the preceding claims, characterized in that the NO _x storage (44) has nitrogen oxide-storing components or compounds.

7. Exhaust gas purification system according to one of the preceding claims, characterized in that the exhaust gas aftertreatment system (4) or the NO _x store (44) has a heating device.

8. Exhaust gas purification system according to claim 7, characterized by an electric heater as a heating device.

9. Exhaust gas purification system according to one of the preceding claims, characterized in that the exhaust gas aftertreatment system (4) further comprises at least one oxidation catalyst (47).

10. The exhaust gas purification system according to claim 9, characterized by an oxidation catalyst (47) arranged upstream of the at least one particle filter (45) or the at least one N0 _X store (44).

11. Exhaust gas purification system according to claim 9, characterized by an at least one particle filter (45) or the at least one NO _x storage (44) downstream oxidation catalyst (47).

12. Exhaust gas purification system according to one of the preceding claims, characterized in that the exhaust gas aftertreatment system (4) has an additional device for reducing NO _x , which the ^* \ 4 -

at least one particle filter (45) is connected downstream.

13. Exhaust gas purification system according to claim 12, characterized by one of the exhaust gas aftertreatment system

(4) downstream NO _x storage catalyst (46).

14. Exhaust gas purification system according to one of the preceding claims, characterized in that the exhaust gas aftertreatment system (4) has an additional device for selective catalytic reduction.

15. Exhaust gas purification system according to claim 14, characterized by one of the exhaust gas aftertreatment system

(4) downstream device for selective catalytic reduction.

16. A method for cleaning exhaust gases from an internal combustion engine, in particular an internal combustion engine with auto-ignition, in which an exhaust gas stream is passed through an exhaust gas aftertreatment unit which comprises at least one particle filter, characterized in that the exhaust gas stream (41) is furthermore through at least one NO _x accumulator ( 44) is conducted.

17. The method according to claim 16, characterized in that the exhaust gas flow (41) is passed through a NO _x accumulator (44) integrated in the particle filter (45).

18. The method according to claim 16, characterized in that the exhaust gas stream (41) in succession is passed through the NO _x store (44) or the particle filter (45).

19. The method according to any one of claims 16 to 18, characterized in that nitrogen oxides (NO _x ) contained in the exhaust gas are stored at low exhaust gas temperatures between about 150 ° C and about 350 ° C in the NO _x store (44).

20. The method according to any one of claims 16 to 19, characterized in that the nitrogen oxides (NO _x ) contained in the exhaust gas are released at higher exhaust gas temperatures above about 350 ° C in the N0 _X storage (44) and exothermic with soot particles in the particle filter (45) react.

21. The method according to any one of claims 16 to 20, characterized in that the NO _x store (44) is optionally heated externally.

22. The method according to claim 21, characterized in that the exhaust gas in the NO _x storage (44) is heated to temperatures of more than 350 ° C.

23. The method according to any one of claims 16 to 22, characterized in that an oxidation of NO and / or the stored soot particles is supported by active components applied to the particle filter (45).

24. The method according to any one of claims 16 to 23, characterized in that the exhaust gases are passed through an oxidation catalyst (47).

25. The method according to any one of claims 16 to 24, characterized in that the exhaust gases are passed through a device for selective catalytic reduction.