Exhaust-gas aftertreatment device with nitrogen oxide storage catalytic converter, and operating method therefor

[0002] The present invention relates to an exhaust-gas aftertreatment device for an internal combustion engine.

[0003] European Published Patent Application No. 0 560 991 describes an exhaust-gas aftertreatment device with nitrogen oxide storage catalytic converter, in which, during the lean-burn operating phases, the nitrogen oxides are removed from the exhaust gas from the associated internal combustion engine by being stored in the nitrogen oxide storage catalytic converter. In relatively short, rich-burn operating phases of the internal combustion engine, the nitrogen oxides, which are usually stored in nitrate form in the storage material of the nitrogen oxide storage catalytic converter, are released again and, at the catalytic centers of the catalytic converter, which are preferably formed by precious metals, such as for example platinum, are reacted with the reducing exhaust-gas constituents in the rich exhaust gas to form nitrogen (nitrate regeneration). The recurring alternation between lean-burn and rich-burn operating phases allows effective removal of nitrogen oxides from the exhaust gases from predominantly lean-burn internal combustion engines, such as lean-burn direct injection spark-ignition engines or diesel engines.

[0004] Nitrogen oxide storage catalytic converters are gradually deactivated by the uptake of sulphur which is present in the exhaust gas and accumulates in the storage material in the form of sulphates, thus increasingly reducing the ability of the nitrogen oxide storage catalytic converter to store nitrogen oxides over the course of time.

[0005] German Published Patent Application No. 198 27 195 describes a method which allows nitrogen oxide storage catalytic converters which have been deactivated in this manner to be regenerated. In methods of this or similar types, the sulphur which is stored in sulphate form is released from the nitrogen oxide storage catalytic converter under reducing conditions and at elevated temperatures, in the form of gaseous compounds (desulphating). It is preferable for the sulphur in sulphate form to be reduced to form sulphur dioxide (SO₂). Depending on the amount of sulphur which has built up in the nitrogen oxide storage catalytic converter and on the procedure used for desulphating, however, it is possible that some proportion of the sulphur which is in sulphate form may also be reduced to form gaseous hydrogen sulphide (H₂S). Since even the tiniest concentrations of hydrogen sulphide are experienced as an extremely unpleasant smell, the emission of H₂S into the environment must be avoided at all costs during desulphating of the nitrogen oxide storage catalytic converters which have been deactivated by sulphur. However, conditions which are similar to those which are deliberately established during a desulphating operation may also occur during particular driving states, for example full-load enrichment, and may then likewise cause undesired emission of H₂S into the environment.

[0006] German Published Patent Application No. 199 21 974 describes an exhaust-gas aftertreatment device which, in addition to a nitrogen oxide storage catalytic converter, has a particle filter, upstream of which there is an oxidation catalytic converter, the nitrogen oxide storage catalytic converter being arranged downstream of the particle filter in the exhaust-gas aftertreatment device. With the selected arrangement of oxidation catalytic converter, particle filter and nitrogen oxide storage catalytic converter, it is possible for both nitrogen oxides and particles to be effectively removed from the exhaust gas in particular from diesel engines. The oxidation catalytic converter has the function of oxidizing the nitrogen monoxide (NO) contained in the exhaust gas to form nitrogen dioxide (NO₂). The NO₂which is formed oxidizes the particles, which predominantly include carbon and have been deposited on the particle filter, even at relatively low temperatures. However, in this case too it is necessary for the nitrogen oxide storage catalytic converter to be subjected to a desulphating procedure from time to time, and consequently, under corresponding operating states or driving states, the problem of H₂S being released into the environment persists.

[0007] It is an object of the present invention to provide an exhaust-gas aftertreatment device with a nitrogen oxide storage catalytic converter and a method for operating the exhaust-gas aftertreatment device, so that the release of H₂S into the environment may be avoided.

[0008] The above and other beneficial objects of the present invention are achieved by providing a device and a method as described herein.

[0009] The exhaust-gas aftertreatment device according to the present invention is distinguished in that the exhaust gas which emerges from the nitrogen oxide storage catalytic converter may be fed to the SCR catalytic converter when the internal combustion engine is in a desulphating operating mode with a reducing exhaust-gas composition, in order for H₂S which is formed during the desulphating to be removed. In this context, a SCR catalytic converter (SCR=selective catalytic reduction) should be understood to mean a catalytic converter which is generally employed to remove nitrogen oxides under oxidizing conditions. Catalytic converters of this type catalyze a selective reduction reaction in which nitrogen oxides are reduced using a reducing agent, e.g., ammonia (NH₃), to form harmless N₂.

[0010] Corresponding tests have shown that H₂S which is fed to a SCR catalytic converter is reacted under reducing conditions to form SO₂, with the result that the release of H₂S to the environment during the desulphating of nitrogen oxide storage catalytic converters, which is carried out under reducing conditions, may be avoided. Since SO₂is far less odor-intensive than H₂S, this effectively solves the problem of H₂S being released into the environment. Moreover, the desulphating methods, which are often complicated precisely in order to avoid the release of H₂S, may be greatly simplified. In this context, as is customary, the term reducing should be understood to mean a gas composition which, irrespective of any residual oxygen content, has an excess of constituents with a reducing action, such as hydrogen, carbon monoxide and/or hydrocarbons. The overall result, therefore, is an oxygen deficit with regard to the oxidation capacity of the abovementioned constituents with a reducing action.

[0011] The abovementioned benefits of preventing the release of H₂S and/or NH₃into the environment also arise if further components for cleaning exhaust gases, such as a particle filter or a 3-way catalytic converter or an oxidation catalytic converter, are provided in the exhaust-gas aftertreatment device, provided that the SCR catalytic converter is arranged downstream of the nitrogen oxide storage catalytic converter.

[0012] In one example embodiment according to the present invention, the SCR catalytic converter contains vanadium pentoxide (V₂O₅) and/or tungsten oxide (WO₃) and/or titanium dioxide (TiO₂).

[0013] In an example embodiment of a method according to the present invention, the internal combustion engine is in a desulphating operating mode, the method including the steps of:

[0014] establishing a reducing exhaust-gas composition upstream of the nitrogen oxide storage catalytic converter,

[0015] releasing the sulphur which is bound in the nitrogen oxide storage catalytic converter, to form hydrogen sulphide (H₂S),

[0016] feeding the hydrogen sulphide to an SCR catalytic converter which is arranged downstream of the nitrogen oxide storage catalytic converter in the exhaust-gas aftertreatment device,

[0017] and reacting the hydrogen sulphide in the SCR catalytic converter to form sulphur dioxide under reducing exhaust-gas conditions.

[0018] The desulphating operating mode of the internal combustion engine may, for example, include a purely rich-burn operating mode of the internal combustion engine with an air/fuel ratio (λ) of 0.98 or less which is established for several seconds at simultaneously increased exhaust-gas temperatures of approximately 500° C. or more. However, it is also possible to establish an oscillating lean/rich cyclical operation or to establish individual, short rich-burn operating phases, which are separated a number of times by lean-burn operating phases, for desulphating purposes. According to the method of the present invention, the desulphating may be carried out such that predominantly H₂S is formed and is then converted into SO₂in the downstream SCR catalytic converter under reducing conditions, so that the odor problem during desulphating may be avoided. It is possible to eliminate the use of desulphating methods, which are particularly complicated in view of the need to avoid the release of H₂S, for example only very slight enrichment of approximately λ=0.99, optionally interrupted a number of times by lean-burn operating phases.

[0019] The likelihood of H₂S being released during the desulphating of nitrogen oxide storage catalytic converters with a relatively high sulphur load is correspondingly more pronounced. To avoid the release of H₂S as a result of considerable accumulation of sulphur, the desulphating may be carried out at relatively regular intervals, although this may be disadvantageous in terms of fuel consumption and driveability. However, the use of the method according to the present invention allows a relatively high accumulation of sulphur in the nitrogen oxide storage catalytic converter to be tolerated, and consequently the time intervals between desulphating operations may be selected to be correspondingly long, which may lead to considerable advantages with regard to operation of the internal combustion engine.

[0020] FIG. 1 is a schematic circuit diagram of an internal combustion engine with associated exhaust-gas cleaning installation.

[0021] FIG. 2 illustrates a measurement result, which is illustrated in diagram form, of a laboratory test with a periodic gas change and cyclical supply of H₂S without a SCR catalytic converter in the gas path.

[0022] FIG. 3 illustrates a diagram which is similar to that illustrated in FIG. 2, with a result of a test which is similar to that carried out in FIG. 2, but with a SCR catalytic converter in the gas path.

[0023] In the example embodiment illustrated in FIG. 1, a diesel engine is used as the internal combustion engine 1. Combustion air is fed to the internal combustion engine via the intake air line 3. The internal combustion engine 1 is assigned the exhaust-gas aftertreatment device 2, which in the exhaust pipe 4 has an oxidation catalytic converter 5, a particle filter 6, a nitrogen oxide storage catalytic converter 7 and a SCR catalytic converter 8.

[0024] In this example embodiment, the particle filter 6 is connected immediately downstream of the oxidation catalytic converter 5, and the SCR catalytic converter 8 is connected immediately downstream of the nitrogen oxide storage catalytic converter 7. An exhaust-gas turbo charger 12, which is used to compress the combustion air, which is cooled by a charge-air cooler 13 arranged in the intake air line after it has been compressed, is accommodated in the exhaust pipe 4, upstream of the oxidation catalytic converter 5. Furthermore, the internal combustion engine 1 is assigned a control unit 9 which is used, inter alia, to control combustion. A control or signal line 10 is provided for transmission of the control signals required for this purpose. Further components which are used to measure and control operating parameters, such as oxygen sensors, temperature sensors, throttle valves, further signal lines, etc., may be present as required as peripherals for the internal combustion engine 1 or for the exhaust-gas aftertreatment device 2.

[0025] The internal combustion engine 1 is operated primarily in lean-burn mode. Particles which are present in the exhaust gas are retained in the particle filter 6. When the exhaust gas passes through the oxidation catalytic converter 5, NO which is present in the exhaust gas is oxidized to form NO₂and then, in the downstream particle filter 6, may in turn oxidize carbon-containing particles which have accumulated at this filter, with the result that the particle filter 6 is continuously regenerated. The exhaust gas which flows out of the particle filter and still contains nitrogen oxides is fed to the nitrogen oxide storage catalytic converter 7 which removes the nitrogen oxides from the exhaust gas by storing them, e.g., in nitrate form. Nitrate regeneration of the storage catalytic converter 7 is performed from time to time as a function of the quantity of stored nitrogen oxides. For this purpose, the internal combustion engine 1 is briefly switched to rich-burn mode, with the result that a rich exhaust gas is generated, with an excess of reducing agents, such as carbon monoxide, hydrogen or hydrocarbons. This leads to the nitrogen oxides which are stored in the nitrogen oxide storage catalytic converter 7 being released, and these nitrogen oxides are then reduced by the reducing agents in the exhaust gas at the precious-metal centers which are present in the catalytic layer of the nitrogen oxide storage catalytic converter.

[0026] The main product formed as a result of the reduction of the nitrogen oxide is nitrogen. In addition, depending on the conditions under which the nitrate regeneration is carried out, greater or lesser quantities of the reduction product NH₃are also formed. It may be undesirable for this compound to be released into the environment. Since, typically, in particular V₂O₅-containing SCR catalytic converters have a pronounced capacity to store NH₃, however, NH₃may be collected by being taken up with the aid of the SCR catalytic converter 8 which according to the present invention is arranged downstream of the nitrogen oxide storage catalytic converter 7. In the lean-burn internal combustion engine operation which follows the nitrate regeneration, this accumulated NH₃is available as an additional reducing agent for the selective reduction of nitrogen oxides in the SCR catalytic converter 8. This additionally may increase the efficiency of the removal of nitrogen oxides in the exhaust-gas aftertreatment device 2.

[0027] When sulphur-containing fuel is used, the exhaust gas from the internal combustion engine 1 contains sulphur dioxide, which is taken up by the catalytic material of the nitrogen oxide storage catalytic converter 7 to form stable sulphates, which may increasingly reduce the ability of the catalytic converter to store nitrogen oxides over the course of time. Therefore, accumulated sulphur may have to be constantly removed from the nitrogen oxide storage catalytic converter 7 in a regeneration operation using a desulphating procedure. For this purpose, the internal combustion engine 1 is switched to a desulphating operating mode. This desulphating operating mode involves increasing the exhaust-gas temperature to over 500° C., for example by further injection of fuel, and establishing a reducing exhaust-gas composition, to air/fuel ratio (λ) values of approximately 0.95 or below, in a similar manner to that used for nitrate regeneration. Under these conditions, it is possible for the relatively stable sulphates in the nitrogen oxide storage catalytic converter 7 to be decomposed by reduction. Depending on the temperature, the degree of enrichment and the quantity of sulphur stored in the nitrogen oxide storage catalytic converter, greater or lesser quantities of H₂S are formed. However, this H₂S is reacted under the reducing conditions of the desulphating to form much less odor-intensive SO₂at the SCR catalytic converter 8, which according to the present invention is arranged downstream of the nitrogen oxide storage catalytic converter 7. As a result, the odor problem which is usually associated with the desulphating of nitrogen oxide storage catalytic converters may be avoided.

[0028] A particular benefit of using the SCR catalytic converter 8 downstream of the nitrogen oxide storage catalytic converter 7 for the purpose of converting H₂S into SO₂under reducing conditions may be that in this manner the desulphating procedure may be simplified, since it does not have to seek to minimize the levels of the undesirable desulphating product H₂S. For example, the desulphating may be shortened significantly by greater enrichment of the mix, with λ values of below 0.95, with an associated increased and quicker release of H₂S from the nitrogen oxide storage catalytic converter 7.

[0029] The possibility of using the SCR catalytic converter 8 downstream of the nitrogen oxide storage catalytic converter 7 in order to convert H₂S into SO₂under reducing conditions is not dependent on the presence of the upstream cleaning components, such as for example particle filter 6 and oxidation catalytic converter 5. Rather, the action which has been described occurs independently of these aspects, and consequently it is also possible for other or further exhaust gas cleaning components, depending on the particular requirement, to be connected upstream or downstream of the SCR catalytic converter 8.

[0030] The diagrams illustrated in FIG. 2 and FIG. 3 explain the effect of the SCR catalytic converter with regard to the conversion of H₂S into SO₂under reducing conditions. The diagrams illustrate results of laboratory tests in which synthetic test gases A, B and C with the following compositions have been used:

[0031] Test gas A reproduces an exhaust gas from an internal combustion engine which is operated in a very lean-burn mode. The composition of test gas B approximately corresponds to the composition of the exhaust gas from an internal combustion engine which is operated in rich-burn mode, similar to that which is to be expected when a desulphating operating mode is established. Test gas C, compared to test gas B, has the same composition but with the addition of H₂S in a concentration which may occur during a correspondingly executed desulphating operation.

[0032] In a blind test, the laboratory testing installation was initially exposed to the test gases in succession, cyclically, without a catalytic converter, and the concentrations of the gases on the outlet side of the installation were recorded by a metrological arrangement. In accordance with the times t1 to t9 which are plotted on the horizontal time axis of the diagram shown in FIG. 2, the laboratory testing installation was exposed to the H₂S-containing test gas C at times t1, t4 and t7. At times t2, t5 and t8, the gas was changed to test gas B, and at times t3, t6 and t9 to test gas A. The time for which test gas C was supplied (t2-t1, t5-t4, t8-t7) was in each case 20 s. Accordingly, the time for which test gas B was supplied was in each case 40 s, and the time for which test gas A was supplied was in each case 120 s. As illustrated in FIG. 2, during phases in which the H₂S-containing test gas C is supplied, in each case a correspondingly high H₂S concentration is measured, with a certain overshoot above the set value being observed immediately after test gas C starts to be supplied, which originates from the metrology. After switching to the H₂S-free rich test gas B, the H₂S concentration rapidly drops to zero. The switch to the supply of the lean test gas A may be recognized from in that the C₃H₆concentration drops to zero. When the test gas C, which likewise contains C₃H₆, is next switched on, an immediate rise in the C₃H₆concentration to the set value of 500 ppm is recorded.

[0033] In this blind test, it is of particular importance that there is no formation of SO₂, as determined by metrology. This is apparent in the diagram on account of the absence of a corresponding SO₂concentration trace.

[0034] FIG. 3 illustrates a result of a laboratory test which was carried out in a similar manner. However, in this test the test gases A, B and C were passed over a conventional SCR catalytic converter with a V₂O₅content of 3%. The space velocity set for the gas throughput was 35,000 l/h. During the periodic gas change as described above, at the same time the temperature of the test installation or of the catalytic converter was increased continuously. This is illustrated in the measurement diagram shown in FIG. 3 by measurement curves T1 and T2, which correspond to the temperatures measured upstream of the catalytic converter and downstream of the catalytic converter. Since the time scale of the diagram shown in FIG. 3 has changed from that of the diagram shown in FIG. 2, the changes in the concentration of the gas components which occur during the cyclical gas change are in this case illustrated as narrow peaks. However, it remains apparent that small quantities of H₂S may only be measured on the output side of the test installation while the test is starting up, at temperatures below 150° C. Above approximately 150° C., it is no longer possible to detect any H₂S, and H₂S is increasingly reacted to form SO₂during the test phases in which rich test gas is supplied. The ever-greater drop in the concentration of C₃H₆as the temperature rises in the test phases in which rich test gas is supplied is of no importance with regard to the reaction of H₂S to form SO₂and is attributable to oxidation or C₃H₆reaction on the basis of a steam reforming process.

[0035] Overall, the laboratory test demonstrates that even relatively high concentrations of H₂S in a desulphating process under reducing conditions are reacted to form SO₂at a conventional SCR catalytic converter at temperatures of over approximately 150° C.