DE10324013A1 - Exhaust gas cleaning system, from internal combustion motor, has storage catalyst to hold nitrogen oxide in buffer until selective reduction catalyst has reached its working temperature - Google Patents

Abstract

The catalyst assembly to clean the exhaust gas from a motor (2), especially with spontaneous ignition and/or direct fuel injection, has a storage catalyst (111) which can store nitrogen oxide from the exhaust at least partially and especially on starting a cold motor, as long as the unit (6) for the selective catalytic reduction has not achieved its operating temperature. The nitrogen oxide is released when the selective reduction unit is at its working temperature and allows nitrogen oxide to pass through with the exhaust gas. An oxidation catalyst (4) is between the storage catalyst and the selective reduction unit. A valve (83) can deliver the reduction agent selectively into the oxidation catalyst.

Description

The The invention relates to a device for purifying the exhaust gas Internal combustion engine with an arrangement for selective catalytic Reduction. The invention further relates to a method for cleaning of exhaust gases from an internal combustion engine, in which an exhaust gas flow through directed a device for selective catalytic reduction becomes.

State of technology

To reduce the nitrogen oxide content in oxygen-rich exhaust gas, as is emitted in particular by diesel internal combustion engines and by internal combustion engines with gasoline direct injection, it is known to introduce a reducing agent into an exhaust gas tract. A suitable reducing agent is, for example, NH ₃ , which can be introduced as a gas into the exhaust gas stream. In this so-called selective catalytic reduction (SCR), the ammonia with the nitrogen oxides contained in the exhaust gas is selectively converted to molecular nitrogen and water. The problem is the insufficient activity of the known SCR system at exhaust gas temperatures below about 250 ° to view C. a prescreening of an oxidation catalyst the one hand ensures a reduction in the proportions of deactivating acting hydrocarbons and on the other hand for oxidation of NO to NO _2, which overall leads to a substantial increase in _NOx conversion at exhaust gas temperatures above about 150 ° C However, phases with even lower exhaust gas temperatures occur relatively frequently, especially when used in passenger cars, which illustrates an average catalyst temperature of less than 180 ° C in the well-known MVEG test cycle (MVEG: Motor Vehicles Emissions Expert Group; an expert group of the European Commission).

Around a good distribution of the reducing agent on the SCR catalytic converter to ensure, a mixing distance of approx. 40 cm can be provided, which may also be a mixing device is provided. Such a mixing device for one Emission control system is in the older German patent application the file number 101 31 803.0. Here, an im Exhaust pipe arranged mixing body a gas impact surface as well as a beam impact surface on, so that exhaust gas flowing from the internal combustion engine onto the gas impact surface and across feedable to the exhaust gas flow Reducing agent can hit the beam impact surface.

The DE 197 40 702 provides to further reduce the nitrogen oxide emission, especially when the internal combustion engine is warming up, by connecting an adsorption catalyst upstream of a catalyst for selective catalytic reduction than is possible with conventional systems for selective catalytic reduction. The adsorption catalyst is regenerated by supplying fuel after the engine.

Advantages of invention

The device according to the invention or the method according to the invention have the Advantage that the exhaust gas can be denitrified with simple means is guaranteed even at low exhaust gas temperatures, the Storage catalytic converter without additional Means self-regulating depending regenerated from the temperature of the exhaust gas and the released Nitrogen oxides due to the already existing arrangement for selective catalytic reduction can be eliminated. The storage catalytic converter effectively buffers nitrogen oxides as long as the arrangement not yet their working temperature for selective catalytic reduction has reached.

By those in the dependent Measures listed claims are advantageous developments and improvements in the independent claims specified devices or methods possible.

It is particularly advantageous to additionally provide an oxidation catalyst. With such a configuration of the oxidation and the so-called SCR catalytic converter (arrangement for selective catalytic reduction), a reduction of the NO _x emission limit values below an amount of emitted NO _x can be achieved, which ensures compliance with the permissible exhaust gas standards during the MVEG test cycle , Such a reduction in the NO _x emission can be achieved in that the temperature-resistant oxidation catalyst which is already present and used for nitrogen oxidation is additionally used for the NOx reduction during a cold start phase. In a close-fitting position of the oxidation catalyst has reached a temperature of more than 100 ° C after about 50 seconds, which splits off for a reduction of NO _x by NH ₃ or an NH ₃ reducing agent is sufficient. Oxidation catalysts mainly have precious metals such as platinum as their active component. This favors oxidation reactions of hydrocarbons, carbon monoxide and nitrogen monoxide even at low temperatures. If NH ₃ is injected as a reducing agent, these catalysts show a relatively strong De-NO _x activity even at temperatures below 100 ° C.

If a configuration with a changeover device is chosen instead of a separate feed device for the oxidation catalyst, this can reduce the construction effort. The invention around in the same way, however, includes an embodiment with separate and separately controllable supply devices for reducing agents.

The Switching device for the optional supply of the reducing agent in the exhaust gas flow upstream or in the oxidation catalytic converter or in the SCR catalytic converter can be used as a valve, in particular as a 3/2-way valve, be trained. In this way, the reducing agent can be optional the oxidation or the SCR catalyst, whichever Temperature levels have reached these while driving.

A embodiment The invention provides that the switching device is designed as a mixing valve is. This way, during a transition period a simultaneous application of the oxidation and the SCR catalyst done with reducing agent. With such a mixing valve can an abrupt switchover can be avoided, so that depending on the achieved Operating temperatures of the catalysts an optimal cleaning effect can be achieved.

The Switching device is preferably temperature controlled so that while a cold start phase with still low exhaust gas temperatures of the oxidation catalytic converter and after a warm-up phase, the SCR catalytic converter with reducing agent can be applied.

The feeding each preferably comprises a metering device for metering amounts and Nozzles for Distribution or atomization of the reducing agent in the exhaust gas stream.

Preferably is the at least one oxidation catalyst in the immediate vicinity of one Exhaust gas outlet of the internal combustion engine arranged so that it already after a short time relatively high temperatures and thus a high cleaning effect reached.

An ammonia-containing or ammonia-releasing substance, for example, can be used as the reducing agent, which can bring about NO _x reduction. Urea or ammonium carbamate are examples of such substances.

at a method according to the invention for cleaning exhaust gases from an internal combustion engine, in which a Exhaust gas flow through at least one oxidation catalytic converter arranged in the exhaust gas duct and a device for selective catalytic downstream Reduction (SCR catalyst) is passed, the exhaust gas flow Reducing agent added, which according to the invention the exhaust gas flow optionally before or within the at least one Oxidation catalyst is supplied. Optionally, the reducing agent becomes both catalysts at the same time or only one of the catalysts. The reducing agent is preferred by means of a nozzle distributed or atomized.

A embodiment of the method according to the invention sees a temperature-controlled supply of the reducing agent in the oxidation catalyst and / or in the device for selective catalytic Reduction before.

If NH _{3 is} applied to the oxidation catalytic converter, it shows a relatively pronounced de-NO _x activity at temperatures above 100 ° C. However, the usable temperature window for NO _x reduction is relatively narrow, since above approx. 250 ° C to 300 ° C there is no longer any nitrogen oxide reduction, but an additional nitrogen oxide production through the oxidation of ammonia is observed. It must therefore be ensured that the oxidation catalyst is only exposed to the reducing agent in a starting phase (in the MVEG test only up to about 350 s). The reducing agent is preferably fed into the oxidation catalyst at exhaust gas temperatures of less than approximately 150 ° C. to 200 ° C. in the oxidation catalyst.

To the SCR catalytic converter normally also has such a time period reaches its working temperature and the reducing agent is injected on the SCR catalytic converter switched. This can take place in the SCR catalytic converter at temperatures of approx. 150 ° C to 200 ° C. The reducing agent is metered onto the oxidation catalyst in principle at operating points with a low exhaust gas temperature not only during a cold start - possible and provides a very effective NOx reduction potential here, where with the SCR catalyst insufficient activity is achieved. With a injection in front of the oxidation catalytic converter up to a time of approximately 600 s to achieve a significant increase in sales of the exhaust gas cleaning system. A suitable, sensible changeover point of the temperature-controlled changeover valve can at 100 to 200 ° C, preferably at 130 to 180 ° C lie.

A practical version of the system can, for example, provide a 3/2-way changeover valve which is actuated as a function of the catalytic converter temperatures and the engine operating point. Such an upgrade of an existing system is relatively simple and can be accomplished with little effort. The catalyst system, the temperature sensors and the dosing system are already available, although these components do not have to be modified. Only the changeover valve and the reducing agent supply line upstream of the oxidation catalytic converter need to be retrofitted. With a suitable dosing strategy, a effective reduction of nitrogen oxides can be achieved over an entire test cycle (MVEG cycle). In the MVEG test, a NO _x sales increase of approx. 40% can be achieved, so that even with relatively low raw emissions, it is possible to meet the relatively strict US standards.

The Oxidation catalyst can be in a preferred embodiment be designed as a catalytically coated particle filter. The The catalytic coating of the particle filter has a similar effect here Coating a known oxidation catalyst. Furthermore can a separate particle filter between the oxidation catalytic converter and the SCR catalytic converter be provided, which causes a filtering of the soot particles.

Farther the oxidation catalyst and the additional one can be omitted Eindüsposition are in front of the storage catalytic converter. The platinum-containing storage catalytic converter takes over in this arrangement the function of the oxidation catalyst.

The invention is explained in more detail below with reference to the accompanying drawing. The only one shows 1 a schematic representation of an internal combustion engine with an exhaust gas aftertreatment unit in an exhaust gas duct.

In an exhaust duct 28 an internal combustion engine 2 is an oxidation catalyst 4 and one as an SCR catalyst 6 designated device for selective catalytic reduction arranged. The internal combustion engine 2 has an inlet duct 21 for the supply of fresh gas 22 as well as outlet channels 26 on that in a collector 27 to the exhaust duct 28 are merged. There is an exhaust gas turbine in the exhaust gas duct 24 of an exhaust gas turbocharger 23 arranged over a shaft 25 is coupled to a compressor, not shown here. The exhaust gas turbocharger 23 is optional and serves to improve the performance and exhaust behavior of the internal combustion engine 2 ,

The internal combustion engine 2 is preferably a diesel internal combustion engine with auto-ignition or a gasoline engine with direct fuel injection. Both types of engine each emit a relatively oxygen-rich exhaust gas. The exhaust gas flow 29 passes through a storage catalytic converter in succession 111 , the oxidation catalyst 4 and the SCR catalytic converter 6 and leaves the exhaust gas cleaning system as cleaned exhaust gas 14 , which is directed to the outside via a silencer (not shown).

The exhaust gas purification system also has a feed device 8th for supplying a reducing agent 81 in the exhaust stream 29 on. The feeder 8th includes a switching device 83 as well as one with a first nozzle 85 communicating first connection line 84 and one with a second nozzle 87 related second trunk 86 , The first nozzle 85 is in the exhaust duct 28 upstream of the oxidation catalyst 4 arranged and used for fine distribution or atomization of the reducing agent 81 in front of the oxidation catalyst 4 , The second nozzle 87 is upstream of the SCR catalyst 6 and downstream of the oxidation catalyst 4 arranged and serves to supply reducing agent 81 in the exhaust stream 29 in front of the SCR catalytic converter 6 ,

First and second connection lines 84 . 86 flow into the switching device 83 for an optional distribution of the reducing agent on the first and / or second connecting line 84 . 86 can worry. The control of the switching device 83 is preferably temperature-dependent, so that the oxidation catalyst in a cold running phase 4 and after reaching a certain temperature, the SCR catalyst 6 with reducing agent 81 can be applied.

The one between the exhaust gas turbocharger 23 and the nozzle 85 , ie in front of the catalyst arrangement 4 located storage catalytic converter can in particular be arranged close to the engine. A storage catalytic converter provided in this way stores the nitrogen oxide emissions of the engine in the low temperature range below 100 ° C. After a certain time, the storage capacity of the storage catalytic converter is exhausted and this allows the nitrogen oxide molecules to pass through. If the storage catalytic converter is heated further, its nitrogen oxide storage capacity decreases and the previously stored amount of nitrogen oxides is released again. The storage catalytic converter arranged close to the engine thus causes a delayed release of the nitrogen oxides. If the storage catalytic converter is dimensioned accordingly, the time delay is preferably at least 300 seconds, so that after the storage capacity of the storage catalytic converter has been exhausted, the downstream system for selective catalytic reduction has reached its working temperature and can reduce the nitrogen oxides to nitrogen and water. The storage catalytic converter should have the following properties:
Nitrogen oxide storage even at low temperatures; Decrease in the storage capacity already at temperatures of approx. 300 ° C in order to ensure that the storage catalytic converter has run empty at the end of a driving cycle during normal driving operation and is available for nitrogen oxide storage at the following cold start; good regenerability solely due to heating, in particular also good desulfating properties. A storage catalytic converter that contains Al2O3, CeO2 and ZrO2 and contains platinum in a concentration of approx. 90 g per can be used to meet the required properties Cubic feet.

In a cold-running phase of the exhaust gas cleaning system, only the first nozzle 85 supplied with reducing agent by appropriate setting of the switching device 83 ,

Subsequently, when the catalytic converters have already reached a predetermined operating temperature, the second connecting line 86 and the second nozzle 87 with reducing agent 81 applied.

A typical transition temperature can be approximately 100 ° C to 200 ° C, preferably approximately 130 to 180 ° C, above which a switchover to the loading of the SCR catalytic converter 6 with reducing agent 81 can be done. Switching can also be carried out in an advantageous manner by means of a mixing valve which acts on the oxidation catalyst at the same time 4 and the SCR catalyst 6 in the area of the transition temperature. The choice of the transition temperature is explained by a clever use of the different operating temperatures of the oxidation catalytic converter and the SCR catalytic converter. This fact is already in the 4 to 6 the German patent application with the file number 10162383.6 described; this description is hereby incorporated into the present patent application.

Of the invention also includes an alternative configuration, the two separate feeders for the Oxidation catalyst and for the SCR catalyst provides.

The oxidation catalyst 4 In an alternative embodiment, it can be a catalytically coated particle filter which, due to its catalytic coating, has the same effect as a known oxidation catalyst. In addition to the configuration shown, there can be between the oxidation catalyst 4 and the SCR catalyst 6 a separate particle filter can be arranged. This brings about a further improvement in the cleaning effect of the exhaust gases.

In a further alternative embodiment, the storage catalytic converter and the oxidation catalytic converter or the storage catalytic converter and the catalytically coated filter can be integrated in a single component, so that the nozzle 85 is arranged in front of this single component. This can be done by using an oxidation catalyst with nitrogen oxide storage capacity. The simultaneous integration of the particle filter takes place via catalytic coating of a particle filter.

In a simple embodiment the oxidation catalyst can also be omitted.

Claims (13)

Device for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition and / or with direct fuel injection, in which the exhaust gas flows through a storage catalytic converter in a first step and an arrangement for selective catalytic reduction in a further step, the arrangement for selective catalytic reduction means for the post-motor supply of a reducing agent, characterized in that the storage catalytic converter ( 111 ) is designed in such a way that it can at least partially store nitrogen oxides contained in the exhaust gas, in particular when the internal combustion engine is cold started, as long as the arrangement ( 6 ) for selective catalytic reduction has not yet reached its working temperature, and that the storage catalytic converter ( 111 ) at the latest when the arrangement for selective catalytic reduction has reached its working temperature, releases previously stored nitrogen oxides or allows new nitrogen oxides to flow in from the internal combustion engine, so that both the previously stored nitrogen oxides and the newly added nitrogen oxides after leaving the storage catalytic converter ( 111 ) can be reduced. Device according to claim 1, characterized in that between the storage catalyst ( 111 ) and the arrangement ( 6 ) an oxidation catalyst ( 4 ) is arranged. Apparatus according to claim 2, that the means for the post-motor reducing agent supply a switching device ( 83 ) have, so that the reducing agent can optionally be introduced into the exhaust gas stream upstream of the at least one oxidation catalytic converter. Apparatus according to claim 2, that the means for the post-motor reducing agent supply a switching device ( 83 ) have, so that the reducing agent can optionally be introduced into the at least one oxidation catalyst. Device according to claim 3 or 4, characterized in that that the switching device is designed as a valve. Apparatus according to claim 5, characterized in that the valve is a 3/2-way valve. Apparatus according to claim 5, characterized in that the valve is designed as a mixing valve. Device according to one of claims 3 to 7, characterized in that that the switching device is temperature controlled. Device according to claim 2, characterized in that the at least one oxidation catalyst as a catalytically coated Particle filter is formed. Device according to claim 2, characterized in that between the at least one oxidation catalyst and the Arrangement a particle filter is provided. Device according to one of the preceding claims, characterized characterized that the means for post-motor reductant supply have a switching device so that the reducing agent optionally introduced into the exhaust gas flow upstream of the storage catalytic converter can be. Device according to one of the preceding claims, characterized characterized that the means for post-motor reductant supply have a switching device so that the reducing agent can optionally be introduced into the storage catalytic converter. Method for cleaning the exhaust gas of an internal combustion engine, in particular an internal combustion engine with auto-ignition and / or with direct fuel injection, in which the exhaust gas is a storage catalytic converter in a first step ( 111 ) and in a further step an arrangement ( 6 ) flows through for selective catalytic reduction, the arrangement for selective catalytic reduction having means for the post-engine supply of a reducing agent, characterized in that the storage catalyst, in particular when the internal combustion engine is cold started, at least partially stores nitrogen oxides contained in the exhaust gas, as long as the arrangement ( 6 ) for selective catalytic reduction has not yet reached its working temperature, and that the storage catalytic converter releases stored nitrogen oxides by then, at the latest when the arrangement for selective catalytic reduction has reached its working temperature, or allows new nitrogen oxides to flow in from the internal combustion engine, so that both the nitrogen oxides stored up to that point as well as the newly flowing nitrogen oxides are reduced after leaving the storage catalytic converter.