DE102007042448A1 - Exhaust gas cleaning system for selective catalytic reduction of nitrogen oxide, comprises catalyst with catalytically active components for selective catalytic reduction, and exhaust gas of internal combustion engine flows through system - Google Patents

Abstract

The exhaust gas cleaning system (1) comprises a catalyst (4) with catalytically active components for selective catalytic reduction (SCR). Exhaust gas of an internal combustion engine flows through the system. The exhaust gas is provided before the selective catalytic reduction catalyst of a nitrogen oxide storage catalytic converter (3), where a dosing device (6) supplies a reducing agent for nitrogen oxide to the exhaust gas. A shifting bypass is arranged in the exhaust line, such that the exhaust line is partially guided to the nitrogen oxide storage catalytic converter. An independent claim is included for a method for removing nitrogen oxide from exhaust gas of an internal combustion engine.

Description

The The invention relates to an exhaust gas purification system for the selective catalytic reduction (SCR) of nitrogen oxides according to the Preamble of claim 1 and a method for exhaust gas purification.

State of the art

to Reduction of pollutants, in particular for the reduction of nitrogen oxides Various processes have established themselves in which reducing Fluids (gases or liquids) in the exhaust system of a Internal combustion engine to be initiated.

to Reduction of nitrogen oxides has become especially the SCR technology proven in the oxygen-rich exhaust gas contained Nitrogen oxides (NOx) with the help of ammonia or a corresponding to Ammonia convertible precursor substance selectively to nitrogen and water are reduced. Preference is given here in particular resorted to aqueous urea solutions. The urea solution is made by means of hydrolysis catalysts or directly on the SCR catalyst to ammonia and carbon dioxide hydrolyzed. The urea solution is prepared by means of special Dosing systems before the hydrolysis catalyst or the SCR catalyst injected into the exhaust stream. Emission control systems with a SCR catalysts are characterized by a high degree of conversion of nitrogen oxide emissions. The disadvantage of this system is that the selective catalytic reduction with common SCR catalysts and also the hydrolysis of the urea only above 160 up to 200 ° C, that is, the light-off temperature for the SCR catalyst for the low temperature range at 160 to 200 ° C is located. At light-off temperature or Also, light-off temperature of a catalyst usually becomes understood the temperature at which 50% of pollutants, here the Nitrogen oxides are reacted. In operating conditions with exhaust gas temperatures below this temperature range can be from the engine produced nitrogen oxides for the most part unchanged Pass the exhaust system and enter the environment. This is for example in diesel vehicles after a cold start or under operating conditions with low load or idle the case. To the catalyst and it is to bring the incoming exhaust gas to light-off temperature For example, the cold catalyst with hot is known Gas or use an electric heater.

One Another concept for reducing nitrogen oxide emissions at low levels Exhaust gas and catalyst temperatures for emission control systems with an SCR catalyst system is that to reach the light-off temperature resulting nitrogen oxides with the help of another Catalyst device store and conditions that a ensure high conversion of nitrogen oxides, again to give or implement, that is to reduce.

The DE 100 54 877 describes an exhaust gas purification system containing a catalytic converter with catalytically active components for selective catalytic reduction (SCR components) and additionally a storage component for nitrogen oxides. This storage component can cache the nitrogen oxides contained at low temperatures below 160 to 200 ° C on the SCR catalyst and release again at higher temperatures, so that they can then be reduced on the SCR catalyst by means of excess ammonia.

The DE 103 08 287 B4 discloses a method for removing nitrogen oxides from the lean exhaust gas of an internal combustion engine, wherein the nitrogen oxides contained in the exhaust gas first adsorbed after a cold start or idle of an internal combustion engine on a nitrogen oxide storage catalyst and desorbed again at elevated exhaust gas temperature and at one in the exhaust line downstream SCR catalyst to be implemented wherein between the nitrogen oxide storage catalyst and the SCR catalyst, a decomposable to ammonia compound is supplied.

Both Systems have the disadvantage that the storage catalytic converter also after the Einspeichervorgang the nitrogen oxides, for example when starting the engine, too after reaching the light-off temperature of the SCR catalyst with the Exhaust gas flow is applied. The regeneration can in operation the emission control system so only under further action with exhaust gas and nitrogen oxides contained therein, so that it can come to a back pressure in the nitrogen oxide storage catalyst. In addition, the load with too high exhaust gas temperatures causes a rapid aging of the nitrogen oxide storage catalyst.

task

task The present invention is therefore an exhaust gas purification system and a method for exhaust gas purification with a nitrogen oxide storage catalyst and an SCR catalyst providing a reliable and highly effective nitrogen oxide removal from the exhaust gases of an internal combustion engine and improved regeneration and extended life allow the storage catalyst.

This is inventively with an emission control system according to the patent claim 1 and a method for Abgasreini achieved according to claim 10. In the dependent claims each preferred developments of the invention are given.

According to the invention proposed an emission control system for the selective catalytic reduction of nitrogen oxides available provide at least one catalyst with catalytically active Components for Selective Catalytic Reduction (SCR Components) contains and in the exhaust stream before the SCR catalyst at least a nitrogen oxide storage catalyst and at least one metering device for the supply of at least one reducing agent is arranged for nitrogen oxides to the exhaust gas, with a switchable Bypass is arranged in the exhaust system that the exhaust gas flow at least partially past the nitrogen oxide storage catalyst can be directed.

Of the Nitrogen storage catalyst can especially after the start of a Internal combustion engine to accumulate nitrogen oxide emissions, the due to a too low SCR catalyst temperature at this yet can not be implemented. The nitrogen oxide emission can Overall, this significantly reduced. The storage The nitrogen oxides is a reversible process and the nitrogen oxides can under conditions that require a high conversion of Ensure nitrogen oxides on the SCR catalyst, discharged again or otherwise implemented.

Under a switchable bypass is inventively a Passage understood in the exhaust system, through which the exhaust stream at least can be partially conducted without the nitrogen oxide storage catalyst to happen. The connection of the bypass is carried out according to the invention in particular after a cold start or after the engine is idling was operated when the operating conditions, in particular the Exhaust gas temperature, a high conversion at the SCR catalyst ensured. By the connection can advantageously a back pressure in the NOx storage catalyst avoided be because the exhaust gas at least partially around the NOx storage around can flow. Particularly preferably, the connection takes place such that the exhaust gas flow is substantially complete the bypass is passed. This has the advantage that the NOx storage catalyst is not flowed through with excessive exhaust gas temperatures. This can cause rapid aging of the NOx storage catalytic converter be counteracted. In addition, the regeneration can the NOx storage catalyst without further exposure to exhaust gas and the nitrogen oxides contained therein during the normal engine operation. In addition, in this way the Number of regeneration cycles for the NOx storage catalyst be reduced to a minimum, which in turn prolonged Can contribute service life of the storage catalyst. The regeneration may preferably only once, for example, at the end of the Einspeichervorgangs and / or after connecting the bypass. Especially preferred can the regeneration of the nitrogen oxide storage catalyst once done at the end of the driving cycle.

Farther the NOx storage catalytic converter can be replaced by the lower admission, for example, only in the first phase after engine start or idle operation, designed smaller and thereby advantageously heated faster to be particularly fast operational readiness for storage to reach the nitrogen oxides. For this purpose, the NOx storage catalyst also close to the cylinder outlet of the Motors are arranged.

Prefers the connection can be done by a flap mechanism, the can be easily integrated into the exhaust system. This mechanism can be done, for example, by special sensors, such as temperature sensors be triggered automatically.

According to the invention Thus, all the benefits of SCR technology and efficient use of nitrogen oxides also at temperatures below the light-off temperature of the SCR catalyst and / or under lean exhaust conditions from the exhaust gas of an internal combustion engine be removed. This allows for a cold start, idle or at transient operating conditions a significant Reduction of the resulting nitrogen emissions can be achieved.

In In another preferred embodiment, the bypass or in be designed a Canning with the storage catalyst. in this connection the bypass is so space saving in a structural unit with designed the storage catalyst. The bypass can also be designed as a separate exhaust pipe. The bypass can thus to the spatial and physical requirements of the Variable exhaust system can be adjusted.

In a further preferred embodiment, an oxidation catalytic converter can be arranged in the exhaust gas stream in the exhaust gas purification system according to the invention directly upstream of the nitrogen oxide storage catalytic converter. This can advantageously improve the conversion of NO to NO ₂ and the storage of nitrogen oxides in the storage catalyst. Furthermore, in this way, active heating measures by hydrocarbons, such as a late ignition angle or a late post-injection, are made possible. As a result, the exhaust gas flow can be brought faster to favorable temperatures.

In other preferred embodiments, an oxidation catalyst can be arranged in the exhaust gas stream before and / or after the SCR catalyst. An upstream oxidation catalyst can advantageously oxidize NO to NO ₂ , which can significantly accelerate the SCR reaction up to a ratio of NO to NO ₂ of about 1: 1. Thus, this combination of an oxidation catalyst with an SCR catalyst can lead to significantly higher reaction rates. By means of an oxidation catalyst connected downstream of the SCR catalytic converter, excess reducing agent can be advantageously reacted, if appropriate. In this way, in the case of an overdose of the reducing agent or the reducing agent, in particular of ammonia, undesired emission can be avoided.

Of the NOx storage according to the invention can take the form of a Coating be applied to an inert support. Particularly preferably, the carrier is metallic or built up ceramic material. Advantageously, can the carrier materials selected according to the invention a rapid warming, for example after a cold start guarantee.

In In another preferred embodiment, the nitrogen oxide storage catalyst at least one compound of ele ments from the group of alkali metals, Contain alkaline earth metals or rare earths and with at least one of the platinum group metals platinum, palladium, rhodium or iridium is activated.

In A further embodiment of the invention provides that the SCR catalyst on one or more Fe zeolite components based. These materials are for the invention Application particularly suitable and inexpensive.

Around can reduce the particulate emission from the exhaust gas in another preferred embodiment additionally a particle filter be arranged in the exhaust system. This can, for example, in the exhaust stream be used before the SCR catalyst.

The The invention further relates to a process for exhaust gas purification, in particular for denitrification using the SCR technology. According to the invention provided that at low exhaust gas temperatures, especially after a cold start or idle the internal combustion engine, the exhaust stream first passed over a NOx storage catalyst and the nitrogen oxides contained adsorbed by the NOx storage catalyst and only after reaching the light-off temperature of the SCR catalyst a bypass is switched on, whereby the exhaust gas flow at least partially passed around the nitrogen oxide storage catalyst around and fed to the downstream of the exhaust gas SCR catalyst can be.

According to the invention Upon reaching the light-off temperature of the SCR catalyst operating conditions understood under which the nitrogen oxides reacted on the SCR catalyst can be. As a result, the total nitrogen oxide emission be significantly reduced from the exhaust of an internal combustion engine. Advantageously, these are not active catalyst heating measures on the SCR catalyst necessary, for example, a negative impact on the fuel consumption can have.

About that In addition, after completion of the storage process, the nitrogen oxides, for example after a cold start according to the invention by the Connection of the bypass' a dynamic pressure in the nitrogen oxide storage catalytic converter be avoided because the exhaust gas at least partially around the NOx storage can flow around. Particularly preferably, the connection takes place such that the exhaust gas flow is substantially complete passed through the bypass. This has the advantage that the NOx storage catalyst is not is traversed by excessive exhaust gas temperatures. hereby can advantageously a rapid aging of the NOx storage catalyst counteracted and the service life be extended.

When Reducing agents for nitrogen oxides may be preferred according to the invention the reducing agent present in the fuel, such as hydrocarbons or Carbon monoxide, or ammonia or a precursor compound be used by ammonia. At the different catalysts in the exhaust system can be the same or different Reducing agents are used.

The Regeneration of the nitrogen oxide storage catalyst can be achieved by desorption and implementation of the nitrogen oxides at higher temperatures In a preferred embodiment of the invention Be method for the regeneration of the nitrogen oxide storage catalyst Hydrocarbons used. These hydrocarbons can be unburned come directly from the exhaust stream or by enrichment, the is called by increasing the hydrocarbon content, be introduced into the exhaust stream. This can be done, for example, by appropriate changes of the air / fuel mixture or by separate injection of fuel in the exhaust stream upstream of the storage catalytic converter.

For the selective catalytic reduction of the nitrogen oxides on the SCR catalyst, it is particularly preferable for an aqueous urea solution to be injected into the exhaust gas stream. But it can also ande Pre precursor compounds are used, which can be easily converted to ammonia. In the exhaust line, a hydrolysis catalyst can be inserted for easier decomposition of the urea in ammonia and carbon dioxide. In this case, a 32.5% strength aqueous urea solution, which is uniformly designated by the industry as Adblue and whose composition in the DIN 70070 is regulated. The inventive method can advantageously be avoided when using aqueous urea systems, the formation of isomerization, since before or on the SCR catalyst no injection of the reducing agent must be carried out at low Abgasstrangtemperaturen.

In In a preferred embodiment, the regeneration of the NOx storage catalyst only be carried out once at the end of the driving cycle. This means that only after switching off the engine in the follow-up phase a post-injection of reducing agent, preferably of hydrocarbons, is generated for the regeneration of the nitrogen oxide storage catalyst and the storage catalyst again for the next storage process is available. In this way, the number of regeneration cycles for the NOx storage catalyst can be reduced to a minimum. This can advantageously prolong the life of the Contribute storage catalytic converter.

The Regeneration can also only once, for example, at the end of Einspeichervorgangs and / or done after connecting the bypass.

The The invention will be described below by way of example with reference to drawings explained. However, the invention is not limited to those shown Embodiments limited.

In show this:

1 a schematic representation of an emission control system according to the invention

2 a Canning with a nitrogen oxide storage catalyst and a bypass for the exhaust stream in a sectional view.

1 shows a schematic representation of an embodiment of an emission control system according to the invention 1 , Just behind the two engine exhaust outlets 2 can each have an exhaust gas line with a nitrogen oxide storage catalytic converter according to the invention (NOx storage catalytic converter) 3 be arranged. Both exhaust lines are merged in the course of the emission control system to an exhaust line. This has the advantage that the two-part NOx storage can be dimensioned smaller and designed in each case by the division into two. Thus, they can reach their light-off temperature for the storage of nitrogen oxides faster. Both NOx storage catalytic converters 3 are in the exhaust stream upstream of the SCR catalyst 4 positioned. The nitrogen oxide storage catalysts 3 can store in this way, especially after the start of an internal combustion engine or idle operation resulting nitrogen oxide emissions that can not be implemented due to a low exhaust or SCR catalyst temperature at the SCR catalyst. Overall, the sales of nitrogen oxides can thus be significantly improved at low exhaust gas temperatures. The inventively provided bypass is not shown in this illustration. In the exhaust gas flow to the NOx storage catalytic converters 3 and in front of the SCR catalyst is an oxidation catalyst 5 arranged in the exhaust system. By means of these oxidation catalysts, NO can be advantageously oxidized to NO ₂ , which can markedly accelerate the SCR reaction up to a ratio of NO to NO ₂ of about 1: 1. Thus, this combination of an oxidation catalyst with an SCR catalyst can lead to significantly higher reaction rates. In the exhaust stream upstream of the SCR catalyst, a metering device 6 be arranged for the supply of at least one reducing agent for nitrogen oxides to the exhaust gas. For this purpose, a precursor compound of ammonia, particularly preferably a urea solution, is preferably fed as reducing agent for the SCR reaction. In addition, in the embodiment shown, between the metering device 6 and the SCR catalyst a Partikelfil ter 7 used, which can cause a reduction of the particle emissions.

2 shows the nitrogen oxide storage catalyst according to the invention 3 in a canning 8th with a bypass 9 for the exhaust gas flow in a sectional view. The bypass 9 is in the illustration shown by two flaps 10a and 10b completely closed. The exhaust gas flow is in this case by the NOx storage catalyst 3 guided. The direction of the exhaust gas flow is indicated by the directional arrows A. The flap 10a is preferably arranged in such a way that when bypassed, that is to say the opening, it is bypassed. 9 at the same time the NOx storage 3 can close, so that it can no longer be flowed through by the exhaust stream. The bypass 9 can preferably be connected via a flap mechanism in the exhaust stream, which can be triggered for example by one or more sensors. This can be, for example, a temperature sensor for the exhaust gas flow or for the catalyst temperature. After the connection of the bypass 9 is inventively preferred that the exhaust gas flow completely through the bypass 9 to be led.

According to the invention, a concept for exhaust gas purification with a nitrogen oxide storage catalyst and an SCR catalyst is thus provided, with a reliable and highly effective removal of nitrogen oxides from the exhaust gases of an internal combustion engine even at low exhaust gas and catalyst temperatures, especially after a cold start and idle operation, and in addition an improved regeneration and extended life of the storage catalyst are possible.

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 10054877 [0005]
• - DE 10308287 B4 [0006]

Cited non-patent literature

• - DIN 70070 [0028]

Claims (12)

Emission control system ( 1 ) for the selective catalytic reduction of nitrogen oxides containing at least one catalyst ( 4 ) with catalytically active components for the selective catalytic reduction (SCR components) and flows through the exhaust gas of an internal combustion engine and in the exhaust gas stream upstream of the SCR catalyst ( 4 ) at least one nitrogen oxide storage catalyst ( 3 ) and at least one metering device ( 6 ) is arranged for the supply of at least one reducing agent for nitrogen oxides to the exhaust gas, characterized in that a switchable bypass ( 9 ) is arranged in the exhaust line, that the exhaust gas flow at least partially to the nitrogen oxide storage catalyst ( 3 ) can be passed by. Emission control system ( 1 ) according to claim 1, characterized in that the bypass ( 9 ) in a canning ( 8th ) with the storage catalyst ( 3 ) is designed. Emission control system ( 1 ) according to claim 1 or 2, characterized in that the bypass ( 9 ) is switchable by a flap mechanism. Emission control system ( 1 ) according to one of claims 1 to 3, characterized in that in the exhaust gas stream upstream of the nitrogen oxide storage catalytic converter ( 3 ) an oxidation catalyst ( 5 ) is arranged. Emission control system ( 1 ) according to one of claims 1 to 4, characterized in that in the exhaust gas stream before and / or after the SCR catalyst ( 4 ) an oxidation catalyst ( 5 ) is arranged. Emission control system ( 1 ) according to one of claims 1 to 5, characterized in that the nitrogen oxide storage catalyst ( 3 ) is applied as a coating on an inert metallic or ceramic carrier. Emission control system ( 1 ) according to one of claims 1 to 6, characterized in that the nitrogen oxide storage catalyst ( 3 ) contains at least one compound of elements from the group of alkali metals, alkaline earth metals or rare earths and is activated with at least one of the platinum group metals platinum, palladium, rhodium or iridium. Emission control system ( 1 ) according to one of claims 1 to 7, characterized in that the SCR catalyst ( 4 ) based on one or more Fe zeolite components. Emission control system ( 1 ) according to one of claims 1 to 8, characterized in that a particle filter ( 7 ) is arranged in the exhaust system. Process for the removal of nitrogen oxides from the exhaust gas of an internal combustion engine, characterized in that the exhaust gas at low exhaust gas temperatures after a cold start or idling of the internal combustion engine first passed through a NOx storage catalyst and the nitrogen oxides contained in the NOx storage catalyst ( 3 ) are adsorbed and after reaching the light-off temperature of the SCR catalyst ( 4 ) a bypass ( 9 ), whereby the exhaust gas flow at least partially around the nitrogen oxide storage catalyst ( 3 ) and the downstream in the exhaust stream SCR catalyst ( 4 ) is supplied. A method according to claim 10, characterized in that the regeneration of the nitrogen oxide storage catalyst ( 3 ) is carried out by reduction with hydrocarbons. Method according to claim 10 or 11, characterized that the regeneration of the nitrogen oxide storage catalyst only on End of the driving cycle.