Exhaust-gas cleaning system with nitrogen oxide accumulator catalyst and sulphur oxide trap and operating method therefor

[0002] This application claims the priority of German application No. 199 60 430.4, filed Dec. 15, 1999, the disclosure of which is expressly incorporated by reference herein.

[0003] The present invention relates to a method for operating an exhaust-gas cleaning system which comprises a nitrogen oxide adsorption accumulator and a sulphur oxide trap and to an exhaust-gas cleaning system which can be operated using this method.

[0004] Exhaust-gas cleaning systems with a nitrogen oxide (NO_x) adsorption accumulator, such as an NO_xadsorber catalytic converter, are used in particular in motor vehicles with an internal-combustion engine in order to minimize the nitrogen oxide emissions. In operating phases involving conditions which are unfavourable to nitrogen oxide reduction, such as in lean-burn mode, it is known for nitrogen oxide to be temporarily stored in the NO_xaccumulator by an adsorption process. This stored nitrogen oxide can then be converted back to nitrogen in a suitable subsequent reduction operating phase, such as in rich-burn mode. The terms lean-burn and rich-burn mode are understood to mean operation with oxygen-rich and low-oxygen exhaust-gas composition, respectively, for example as a result of an engine being operated with a lean or rich engine air ratio (i.e., with an air/fuel ratio for the air/fuel mixture burnt in the engine which is above or below the stoichiometric value).

[0005] The SO_xtrap prevents sulphur poisoning of the NO_xadsorber. This is because, particularly when the engine is in lean-burn mode, sulphur which is contained in standard fuels and engine oils leads to sulphur dioxide being present in the exhaust gas. The sulphur dioxide is taken up by the NO_xadsorber as a result of sulphates being formed, thereby reducing the capacity of this adsorber to store NO_x. The SO_xtrap prevents this by trapping the sulphur oxides before they reach the NO_xadsorption accumulator. It is subjected to a desulphurization treatment at the latest when its SO_xstorage capacity is exhausted. To achieve effective desulphurization, it is known to set elevated exhaust-gas temperatures of, for example, over 600° C. and a rich exhaust air ratio, i.e. a ratio which is below the stoichiometric level.

[0006] A further special method for periodically desulphurizing a nitrogen oxide or sulphur oxide accumulator of an exhaust-gas cleaning system using secondary-air supply means is described in German patent application 199 22 962. In this method, during part of the desulphurization period, the exhaust-gas cleaning system is operated in such a way that, after a predeterminable desulphurization temperature has been reached, the accumulator air ratio (i.e., the air ratio in the exhaust gas supplied to the accumulator which is to be desulphurized), is made to oscillate between an oxidizing atmosphere, on the one hand, and a reducing atmosphere, on the other hand, by correspondingly changing the amount of secondary air supplied.

[0007] Patent DE 197 47 222 C1 describes a method for operating an exhaust-gas cleaning system in which on demand, from time to time, an NO_xaccumulator catalytic converter is desulphurized, i.e., accumulated sulphate is removed. The exhaust-gas cleaning system comprises secondary-air supply means having a secondary-air line and a secondary-air pump which can be controlled, for example, by an engine management system.

[0008] A problem with the conventional periodic regeneration of the SO_xtrap is that the NO_xaccumulator catalytic converter which follows it is exposed to the sulphur components released from the SO_xtrap, and in unfavourable circumstances this may lead to its ability to store NO_xbeing partially blocked by the accumulation of sulphates. DE 198 02 631 A1 discloses an exhaust-gas cleaning system in which this problem is combatted by providing a bypass around the NO_xaccumulator catalytic converter in the exhaust system, via which bypass the exhaust gas is guided past the NO_xaccumulator catalytic converter while the SO_xaccumulator catalytic converter is desulphurized.

[0009] The present invention is based on the technical problem of providing an exhaust-gas cleaning system and an operating method which enable the SO_xtrap to be desulphurized with the minimum possible outlay and without there being any risk of sulphur poisoning of the nitrogen oxide accumulator catalytic converter.

[0010] In the method according to the present invention, the direction of flow of the exhaust gas during the desulphurizing phases is selected to be different from during the normal operating phases. During the normal operating phases, the exhaust gas which is to be cleaned is passed firstly via the SO_xtrap and then via the NO_xadsorption accumulator. The sulphur oxide constituents of the exhaust gas, which are undesirable in the nitrogen oxide adsorption accumulator, remain trapped in the SO_xtrap, provided that the storage capacity of the SO_xtrap has not yet been exhausted and there is as yet no saturation behaviour. If the desulphurization phase is selected, the exhaust-gas flow is switched over in such a way that it passes through the two exhaust-gas cleaning components, namely the SO_xtrap and NO_xadsorption accumulator, in the reverse direction (i.e., is passed firstly via the NO_xadsorption accumulator and then via the SO_xtrap). This prevents sulphur compounds released during the desulphurization of the SO_xtrap from polluting the NO_xadsorption accumulator.

[0011] In an embodiment of the present invention, secondary air is fed into the exhaust gas downstream of the SO_xtrap during the desulphurization phase. The secondary air is used to oxidize undesirable sulphur compounds, such as H₂S and COS, which are released in the SO_xtrap, for which purpose an oxidation catalytic converter is connected downstream of the SO_xtrap.

[0012] An exhaust-gas cleaning system is equipped with an SO_xtrap and an NO_xadsorption accumulator and can be operated according to the present invention. For this purpose, it has means for reversing the direction of flow of the exhaust gas in such a manner that the exhaust-gas stream can optionally be passed firstly through the SO_xtrap and then through the NO_xadsorption accumulator or, conversely, firstly through the NO_xadsorption accumulator and then through the SO_xtrap.

[0013] A configuration of the exhaust-gas cleaning system according to the present invention provides, as further components, an oxidation catalytic converter and secondary-air supply means.

[0014] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

[0015] FIG. 1 depicts an exhaust-gas cleaning system in a normal operating phase; and

[0016] FIG. 2 depicts the exhaust-gas cleaning system from FIG. 1 in a desulphurization operating phase.

[0017] FIG. 1 and 2 diagrammatically depict a part of an exhaust-gas cleaning system which is of interest in the context of the present invention, having an SO_xtrap 1, which is also referred to as an SO_xaccumulator catalytic converter, and a catalytically active nitrogen oxide adsorber, i.e., an NO_xaccumulator catalytic converter 2. In the example shown, the SO_xtrap 1 and the NO_xaccumulator catalytic converter 2 are connected in series in a common housing 3 which is arranged in an associated section 4 of an exhaust system which carries the exhaust gas 5 which is to be cleaned. Alternatively, the two exhaust-gas cleaning components 1, 2 may be arranged in separate housings but in series in the exhaust-gas system section 4.

[0018] The exhaust-gas cleaning system shown has means for reversing the direction of flow of the exhaust gas flowing through the exhaust system section 4 in which the SO_xtrap 1 and the NO_xaccumulator catalytic converter 2 are arranged. These means include a branching junction 6, at which the two connections of the exhaust system section 4 which includes the SO_xtrap 1 and the NO_xaccumulator catalytic converter 2 meet an exhaust-gas supply line 7 and an exhaust-gas discharge line 8. In the branching junction 6 there is a controllable flap 9 which can be switched between a normal operating position shown in FIG. 1 and a desulphurization position shown in FIG. 2.

[0019] In the normal operating position shown in FIG. 1, the SE flap 9 passes the exhaust gas supplied by the supply line 7 into the left-hand part, as seen in FIG. 1, of the exhaust system section 4, so that in that section it firstly flows via the SO_xtrap 1 and then via the NO_xaccumulator catalytic converter 2, from where it returns to the branching junction 6 and, through the flow-guiding effect of the flap 9, is passed into the discharge line 8. In its position shown in FIG. 2, by contrast, the flap 9 passes the supplied exhaust gas into the right-hand part of the exhaust system section 4 which includes the two exhaust-gas cleaning components 1, 2, so that the exhaust gas flows firstly via the NO_xaccumulator catalytic converter 2 and then via the SO_xtrap 1, from where it returns to the branching junction 6 and, in turn, is passed into the discharge line 8 under the action of the flap 9.

[0020] As an alternative to the flap 9, it is possible to use any other conventional flow-guiding means which can be switched between two positions in order: in one position, to guide the exhaust gas in one direction and, in the other position, to guide the exhaust gas in the other direction through the exhaust system section 4 which includes the SO_xtrap 1 and the NO_xaccumulator catalytic converter 2 (e.g., a corresponding four-way valve or a suitable slide or opener element). Furthermore, it will be understood that, depending on the particular requirements, further exhaust-gas cleaning components, in particular further catalytic converter bodies, may be provided in addition to the exhaust-gas cleaning components shown in the present case, for example a three-way catalytic converter or a nitrogen oxide reduction catalytic converter may be provided in the exhaust-gas discharge line 8. The exhaust-gas cleaning system which has been constructed in this way and is suitable in particular for cleaning the exhaust gases from a spark-ignition or diesel engine of a motor vehicle, allows the following advantageous method of operation.

[0021] During so-called normal operating phases, the flap 9 is set into its normal operating position shown in FIG. 1. The exhaust gas 5 which is to be cleaned then firstly reaches the SO_xtrap 1, which adsorbs any sulphur compounds which are contained in the exhaust gas, after which the exhaust gas from which the sulphur has been removed is fed to the NO_xaccumulator catalytic converter 2. The function of the NO_xaccumulator catalytic converter then differs according to whether, during the normal operating phase, it is being run in adsorption mode or in description mode. In the adsorption mode, the NO_xaccumulator catalytic converter 2 adsorbs nitrogen oxides contained in the exhaust gas, while in description mode it releases the nitrogen oxides which have been adsorbed and temporarily stored again, and these nitrogen oxides are then reduced to form nitrogen, for which purpose the NO_xaccumulator catalytic converter 2 can simultaneously act as a nitrogen oxide reduction catalytic converter, or a catalytic converter of this type may be connected downstream as a separate catalytic converter body, as is standard procedure for the person skilled in the art. It is known the NO_xaccumulator catalytic converter 2 is preferably operated as long as possible in adsorption mode, during which the exhaust gas may be of lean composition, until the NO_xaccumulator catalytic converter 2 has reached a certain level of loading, after which the accumulated nitrogen oxides are released again during a short description phase. The description operation is known to generally include the setting of a rich exhaust-gas composition, for example by suitably altering the air/fuel mixture which is burnt by the combustion device generating the exhaust gas and/or by injecting a reducing agent, such as ammonia, into the exhaust gas upstream of the NO_xaccumulator catalytic converter 2.

[0022] Therefore, during normal operation outlined above, the SO_xtrap 1 is positioned upstream of the NO_xaccumulator catalytic converter 2 and prevents sulphur poisoning of the latter. If, after a prolonged period of normal operation, the SO_xtrap 1 has reached a certain degree of saturation, normal operation is interrupted by a so-called desulphurization phase. For this purpose, the flap 9 is moved into its desulphurization position shown in FIG. 2, with the result that the incoming exhaust gas is then passed firstly via the NO_xaccumulator catalytic converter 2 and only then into the SO_xtrap 1. Moreover, the standard desulphurization conditions are established, with regard, for example, to composition and temperature of the exhaust-gas stream. If, despite the SO_xtrap 1, some sulphur has been accumulated in the NO_xaccumulator catalytic converter 2, this sulphur is also removed during the desulphurization operation. The desulphurization operation releases the sulphur which has been accumulated in sulphate form in the SO_xtrap 1, so that the ability of this trap to take up sulphur is restored.

[0023] The reversal of flow of the exhaust gas in the exhaust system section 4 during the desulphurization phase with respect to the direction of flow of the exhaust gas in normal operation means that the exhaust gas which leaves the SO_xtrap 1 during the desulphurization operation and contains the released sulphur or corresponding sulphur compounds does not enter the NO_xaccumulator catalytic converter 2. This prevents the possibility of sulphur which is released in the SO_xtrap 1 during the desulphurization from being accumulated in the NO_xaccumulator catalytic converter 2.

[0024] Since emission of the sulphur compounds, such as H₂S and/or COS, which enter the exhaust gas during the desulphurization of the SO_xtrap 1 to the environment is generally undesirable, it is possible to provide an oxidation catalytic converter 10 as one possible remedy. This oxidation catalytic converter is arranged downstream of the SO_xtrap 1, as seen in the direction of flow of the exhaust gas during n desulphurization, in the same exhaust system section 4, as indicated by dashed lines in FIG. 2, or in the adjoining exhaust-gas discharge line 8. To enable the problematical sulphur compounds to be converted optimally in the oxidation catalytic converter 10 despite the fact that the composition of the exhaust gas supplied in the oxidation catalytic converter 10 is generally selected to be rich for the desulphurization of the SO_xtrap 1, for this situation secondary-air supply means 11 of conventional type are additionally provided, as diagrammatically indicated by dashed lines in FIG. 2. At a secondary-air inlet point 12 which lies between the SO_xtrap 1 and the oxidation catalytic converter 10, the secondary-air supply means 11 feed air into the exhaust system section 4 during a desulphurization operation. As a result, sufficient oxygen to convert the sulphur compounds which have been released is admixed with the exhaust gas flowing to the oxidation catalytic converter 10.

[0025] It will be understood that, to control operation of the exhaust-gas cleaning system, a suitably designed control unit (not shown) is provided, which, for example, may simultaneously be used to control the combustion device which generates the exhaust gas, a measure which is inherently common to the person skilled in the art and therefore requires no further explanation.

[0026] It is clear from the above explanation of an advantageous example that the operating method according to the present invention and the exhaust-gas cleaning system according to the present invention are suitable for cleaning exhaust gases not only from a motor vehicle engine but also from a stationary combustion device. The exhaust-gas cleaning allows periodic adsorption and description of an NO_xadsorption accumulator; protection for the NO_xadsorption accumulator from sulphur poisoning by an associated SO_xtrap; and periodic desulphurization of the SO_xtrap without the NO_xadsorption accumulator being exposed to sulphur compounds which are released.

[0027] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.