WO2008022751A2

WO2008022751A2 - Method for operating an exhaust-gas purification system in a lean-burn spark-ignition engine

Info

Publication number: WO2008022751A2
Application number: PCT/EP2007/007290
Authority: WO
Inventors: Susanne Philipp; Torsten Franke; Stephan Eckhoff; Wilfried Mueller; Thomas Kreuzer; Hubert Bichler; Rainer Zimmer; Christof Schoen
Original assignee: Umicore Ag & Co. Kg; Bayerische Motoren Werke Ag; Daimlerchrisler Ag
Priority date: 2006-08-19
Filing date: 2007-08-17
Publication date: 2008-02-28
Also published as: WO2008022751A3

Abstract

To remove nitrogen oxides from the exhaust gas of a lean-burn engine known methods use an exhaust-gas purification system composed of a nitrogen oxide storage catalytic converter and an SCR catalytic converter, operating the engine with alternately lean and rich air/fuel mixtures. The ammonia required for the SCR reaction is generated during the regeneration of the nitrogen oxide storage catalytic converter. One disadvantage is that, at high temperatures, sufficient ammonia is no longer generated during the regeneration. It is therefore proposed to inject ammonia upstream of the SCR catalytic converter at exhaust gas temperatures above approximately 350˚C. The injection of ammonia does not take place at exhaust gas temperatures below 350˚C. The method is particularly suitable for lean-burn engines with jet-controlled direct fuel injection which can also be operated at high load with a lean air/fuel mixture.

Description

Method for operating an exhaust gas purification system on a lean-burn gasoline engine

description

The invention relates to a method for operating an exhaust gas purification system on a lean-burn gasoline engine, which first contains a nitrogen oxide storage catalyst and then an SCR catalyst in the flow direction of the exhaust gas, wherein the exhaust gas contains nitrogen oxides, which are converted by the operation of the emission control system to harmless compounds should.

Exhaust gas purification systems, which contain a nitrogen oxide storage catalyst and an SCR catalyst in the flow direction of the exhaust gas, are known. Such a plant is described for example in US 6,182,443 for the treatment of the exhaust gas of a diesel engine. The diesel engine is operated with a constantly lean air / fuel mixture. At low exhaust gas temperatures, the nitrogen oxides contained in the diesel exhaust are absorbed by the nitrogen oxide storage catalyst. At higher exhaust gas temperatures, the stored nitrogen oxides are thermally desorbed and reduced to nitrogen in the following SCR catalyst. For this purpose, ammonia or urea is added to the exhaust gas before the SCR catalyst when the SCR catalyst has reached its light-off temperature.

The published patent application US 2006/0010857 A1 likewise discloses an exhaust gas purification system for a diesel engine comprising a nitrogen oxide storage catalytic converter and a downstream SCR catalytic converter. The diesel engine is operated with a constantly lean air / fuel mixture. To regenerate the nitrogen oxide storage catalytic converter, a reducing agent is supplied to the exhaust gas upstream of the storage catalytic converter. During the regeneration of the storage catalytic converter, ammonia is generated by the storage catalytic converter. This is stored by the SCR catalyst and used in a period that follows directly on the regeneration of the storage catalyst for the reduction of nitrogen oxides, which are not absorbed by the nitrogen oxide storage catalyst.

The publication US 2005/0129601 Al also describes an exhaust gas purification plant, which in the flow direction of the exhaust gas, a nitrogen oxide storage lysator and an SCR catalyst. The exhaust gas is periodically emaciated and enriched. During the lean period, the nitrogen oxides contained in the exhaust gas are stored by the storage catalytic converter. During the rich period, the nitrogen oxides stored by the storage catalyst are reduced to ammonia. The mixture of ammonia and unreacted nitrogen oxides is converted to nitrogen and water on the SCR catalyst.

DE 100 11 612 A1 describes an exhaust gas purification system for an internal combustion engine, which likewise consists of a nitrogen oxide storage catalytic converter and an SCR catalytic converter. At vehicle speeds below 120 km / h, the engine is alternately operated with lean and rich air / fuel mixture in order to be able to convert the nitrogen oxides contained in the exhaust gas to the nitrogen oxide storage catalytic converter. At a speed of 120 km / h or more, the engine is operated with a stoichiometric air / fuel mixture and at full load the engine is supplied with a rich air / fuel mixture.

JP 2002-188429 also describes an exhaust gas purification system for a lean-burn engine comprising a nitrogen oxide storage catalyst and an SCR catalyst. The addition of reductant before the nitrogen oxide storage catalyst is stopped when the amount of reductant exceeds a threshold and is then fed to the SCR catalyst. JP 2003-286827 describes another exhaust gas purification system comprising a nitrogen oxide storage catalytic converter and an SCR catalytic converter. The nitrogen oxide storage catalyst absorbs the nitrogen oxides contained in the exhaust gas below a predetermined temperature and desorbs them above this temperature. The desorbed nitrogen oxides are converted by the downstream SCR catalyst. Another exhaust gas purification system for a lean-burn engine is described in JP 2004-218575. It also contains a nitrogen oxide storage catalyst and an SCR catalyst. In contrast to the systems described so far, the SCR catalytic converter is connected upstream of the nitrogen oxide storage catalytic converter.

The known methods for removing the nitrogen oxides by means of a nitrogen oxide storage catalytic converter and a downstream SCR catalytic converter are not suitable for effectively cleaning the exhaust gas from lean-burn gasoline engines over wide ranges of the possible operating states. This applies in particular to modern gasoline Lean engines with spray-guided gasoline direct injection. While conventional gasoline lean-burn gasoline engines with stratified gasoline direct injection from a certain vehicle speed can no longer be operated with lean air / fuel mixture and must be switched to stoichiometric operation, the modern gasoline lean-burn engines with spray-guided gasoline direct injection capable, even at significant to work at higher speeds with lean mixture preparation. In these operating conditions, the engine generates exhaust gas at high temperatures above, for example, 500 ° C and with high exhaust gas mass flows. These operating conditions, the known from the prior art methods are not equal. Nitrogen storage catalysts typically have a temperature window for optimum operation between about 200 and 450 ° C. Above 450 ° C, nitrogen oxide storage catalysts are no longer able to store the nitrogen oxides. Therefore, for example, the nitrogen oxide storage catalyst in US 2005/0129601 A1 can no longer form ammonia at temperatures above 450 ° C. in the rich periods for the selective reduction of the nitrogen oxides on the downstream SCR catalyst.

The object of the present invention is therefore to provide a method for operating an exhaust gas purification system comprising a nitrogen oxide storage catalytic converter and an SCR catalytic converter, which over a wide range of operating conditions, especially those with high exhaust gas temperatures and high NOx mass flows, the nitrogen oxides in the exhaust of modern Can effectively convert gasoline lean-burn engines into harmless products.

This object is achieved by a method for operating an exhaust-gas purification system on a lean-burn engine of a vehicle which first contains a nitrogen oxide storage catalytic converter and then an SCR catalytic converter in the flow direction of the exhaust gas, the exhaust gas having an exhaust gas temperature which is dependent on the instantaneous operating state of the engine and including nitrogen oxides as pollutants. The method is characterized in that

(a) the engine is operated with alternately lean and rich air / fuel mixture when the operating condition of the engine produces an exhaust gas temperature below a predetermined temperature, and

b) the engine is operated with a constantly lean air / fuel mixture when the operating condition of the engine produces an exhaust gas temperature above the predetermined temperature, and during this operating condition prior to the SCR catalyst, the exhaust gas is ammonia directly or in the form of an ammonia-decomposable one Connection is supplied.

According to the invention, the lean-burn engine is operated at low exhaust gas temperatures with alternately lean and rich air / fuel mixture. During lean operation, the nitrogen oxides contained in the exhaust gas are stored by the storage catalytic converter. If the storage capacity of the storage catalytic converter is exhausted, it is regenerated by switching the engine to rich operation. In this case, a portion of the stored nitrogen oxides is reduced to ammonia in this temperature range, which is cached by the downstream SCR catalyst. During the subsequent lean-burn operation, the stored ammonia serves to reduce nitrogen oxides which are not absorbed by the storage catalytic converter.

This mode of operation of the exhaust gas purification system is advantageous only in the lower temperature range. At higher temperatures, the storage capacity of the storage catalyst is reduced and only small amounts of ammonia are formed. Above about 450 to 500 ° C, there is no significant storage of nitrogen oxides more, so that no sufficient amounts of ammonia are formed. In the known methods, therefore, at higher temperatures, a stoichiometric or substoichiometric air / fuel mixture is converted in order to ensure a further effective NOx aftertreatment. However, with stoichiometric or substoichiometric operation, the advantage of saving fuel is no longer given. Therefore, according to the invention, at operating conditions of the engine with higher exhaust gas temperatures, the engine is constantly operated with a lean air / fuel mixture. To reduce the nitrogen oxides in the exhaust gas is in front of the SCR catalyst Ammonia or a decomposable to ammonia compound is injected into the exhaust gas, whereby the nitrogen oxides are continuously converted to nitrogen. It is also advantageous in this case that the nitrogen dioxide formed on the nitrogen oxide storage catalyst of nitrogen monoxide improves the nitrogen oxide conversion on the SCR catalyst in this temperature range.

The optimum temperature for switching from one operating mode to the other depends on the type of engine and the respective operating conditions as well as the degree of aging of the NOx storage catalytic converter and is usually between 300 and 500 ° C

The method is particularly suitable for lean burn engines with spray-guided gasoline direct injection, which can be operated even at high load conditions with a lean air / Krafitstoff mixture.

The method has several advantages over the prior art methods. In the lower temperature range, it is possible by the ammonia formed on the storage catalyst during the regeneration to achieve an additional nitrogen oxide reduction on the SCR catalyst. The emission of ammonia is avoided by absorption on the SCR catalyst.

Due to ammonia formation on the storage catalyst, the reductant metering (e.g., urea as a compound decomposable to ammonia) for the SCR catalyst at low temperatures may not yet be used, thereby reducing urea consumption. Only from about 350 ° C, it is necessary to meter reducing agent.

At very high nitrogen oxide mass flows, in addition to the storage catalytic converter, the downstream SCR catalytic converter contributes to nitrogen oxide reduction.

In case of thermal damage or poisoning of the storage catalyst, for example by sulfur, or when operating with highly sulfurized fuel, the SCR catalyst can be used solely for nitrogen oxide reduction. Furthermore, there are situations in which desulfurization of the nitrogen oxide storage catalyst is not possible because the high temperatures required for desulfurization can not be achieved. as when operating the vehicle in city traffic or in a traffic jam. Again, then the SCR catalyst can be used alone for nitrogen oxide reduction.

At engine operating points that prohibit or hinder greasing for regeneration of the storage catalyst for comfort or other reasons, the SCR system with reductant dosing may continue to be used for NOx reduction.

Exhaust gas purification systems with NOx storage catalytic converter and downstream SCR catalyst, for the operation of which the inventive method is suitable, may also contain before the NOx storage catalyst, a three-way catalyst, an oxidation catalyst or another, optionally high temperature stable NOx storage catalyst. Such a supplemental catalyst can improve hydrocarbon conversion and further increase nitrogen oxide conversion rates.

For carrying out the process, the nitrogen oxide storage catalysts and SCR catalysts known to those skilled in the art can be used. Nitrogen storage catalysts contain as storage material for the nitrogen oxides basic compounds of alkali or alkaline earth metals and at least one noble metal from the group platinum, palladium and rhodium. SCR catalysts may be based on zeolites exchanged with noble metals or subgroup metals. Alternatively, SCR catalysts are known which contain a mixture of the solid acids vanadium oxide, tungsten oxide and molybdenum oxide.

The method is particularly suitable for emission control systems from a near-engine nitrogen oxide storage catalyst and an arranged in the underbody area of the vehicle SCR catalyst. Near-engine means here that the distance of the catalyst to the exhaust manifold is less than 0.8 m and the underfloor position is characterized by a distance from the catalyst to the exhaust manifold of more than 1, 0 m. The alternating operation with lean and rich air / fuel mixture leads here already shortly after the cold start and also during operating phases of the engine with low exhaust gas temperatures to good nitrogen oxide sales. Operating phases with low exhaust gas temperatures occur, for example, in idling mode at low speed / load collectivities of the engine, or generally when using large displacement engines, and especially diesel engines.

At exhaust gas temperatures above 450 ° C, however, the nitrogen oxides are stored by the near-engine nitrogen oxide storage catalyst only to a small extent. He then acts essentially as an oxidation catalyst. According to the invention, the nitrogen oxides contained in the exhaust gas are converted in the SCR catalyst in the underfloor region in these operating conditions. For this purpose, ammonia or a decomposable to ammonia compound is supplied to the exhaust gas before entering the SCR catalyst.

It is advantageous in such an emission control system that a close-coupled nitrogen oxide storage catalyst can be much easier desulfurize than in appropriate underbody position, since heating the catalyst to the desulfurization temperature of about 550 to 800 ° C in the engine near much easier and is associated with lower fuel consumption ,

The invention will be explained in more detail below with reference to Figures 1 and 2. Show it:

Figure 1: emission control system for carrying out the method

Figure 2; Schematic comparison of the nitrogen oxide conversion of various emission control systems as a function of the temperature

Figure 1 shows the structure of the exhaust gas purification system as it can be used for the inventive method. The exhaust gas coming from the lean-burn engine is first led through a nitrogen oxide storage catalytic converter (NSC). Downstream of the nitrogen oxide storage catalytic converter, the SCR catalytic converter (SCR) is inserted into the exhaust gas line. Between the nitrogen oxide storage catalytic converter and the SCR catalytic converter, a device for injecting, for example, urea into the exhaust gas flow is provided.

FIG. 2 shows four nitrogen oxide conversion curves for three different exhaust gas purification systems as a function of the exhaust gas temperature for different modes of operation. The four different sales curves are designated as follows: NSC: Nitrogen oxide conversion of a NOx storage catalyst during rich / lean operation over the entire temperature range of the diagram

SCR: Nitrogen oxide conversion of an SCR catalyst at constant lean operation over the entire temperature range with external addition of ammonia as reducing agent

NSC + SCR (without NH ₃ ):

Nitrogen oxide conversion of an exhaust gas purification system from NOx storage catalytic converter and SCR catalytic converter in rich / lean operation over the entire temperature range without external supply of ammonia as reducing agent

NSC + SCR (with NH ₃ ):

Nitrogen oxide conversion of an exhaust gas purification system from NOx storage catalyst and SCR catalyst when operating by the novel process: rich / lean operation below 350 ° C and constant lean operation with NH ₃ - addition above 350 ° C.

The turnover curve denoted by "NSC" shows a bad NOx conversion in the high temperature range, while the sales curve denoted by "SCR" has a very good NOx conversion in the high temperature range, but has disadvantages in the low temperature range. The "NSC + SCR (without NH _3)" with designated sales curve for the emission control system of the nitrogen oxide storage catalyst and SCR catalyst exhibits at rich / lean operation without addition of NH ₃ good low-temperature activity. The high-temperature activity, however, is insufficient. For the NOx After-treatment in diesel exhaust gas, such a system is sufficient in most cases, especially with an upstream diesel oxidation catalyst, since hardly any temperatures in the subfloor are reached above 450-500 ° C. However, exhaust gas aftertreatment on lean-burn gasoline engines must also be above 500 ° C This is only possible if the exhaust gas purification system comprising nitrogen oxide storage catalyst and SCR catalyst is operated according to the method according to the invention The associated conversion curve is designated "NSC + SCR (with NH ₃ )" in FIG.

Claims

claims

1. A method for operating an exhaust gas purification system on a lean-burn engine of a vehicle, which first contains a nitrogen oxide storage catalytic converter and then an SCR catalyst in the flow direction of the exhaust gas, wherein the exhaust gas has a dependent on the current operating condition of the engine exhaust gas temperature and including nitrogen oxides indicates that: (a) the engine is alternately operated with lean and rich air / fuel mixture when the engine operating condition produces an exhaust gas temperature below a specified temperature, and (b) the engine is operated with a constantly lean air / fuel mixture when the operating condition of the engine generates an exhaust gas temperature above the predetermined temperature, and during this operating state before the SCR catalyst the exhaust gas ammonia is fed directly or in the form of a decomposable to ammonia connection.

2. The method according to claim 1, characterized ge kennzei c hnet that the predetermined temperature is in the range between 300 and 550 ° C.

3. The method according to claim 1, characterized gekennzei chnet that the nitrogen oxide storage catalyst is preceded by a three-way catalyst, an oxidation catalyst or another nitrogen oxide storage catalyst.

4. The method of claim 1, characterized adnet gekennzei that the lean-burn engine has a spray-guided gasoline direct injection.

5. The method of claim 1, d adurc h ge kennze i c hn et that the nitrogen oxide storage catalyst close to the engine and the SCR catalyst is arranged in the underfloor position of the vehicle.