DE10353313B4 - Device and method for the selective catalytic reduction of nitrogen oxides contained in an exhaust gas of an internal combustion engine - Google Patents

Abstract

A device for selective catalytic reduction of the nitrogen oxides contained in an exhaust gas of an internal combustion engine comprises a nitrogen monoxide production device comprising a plasma generator (34). An ammonia generating device uses the generated nitrogen monoxide, and an exhaust gas reducing device reduces the nitrogen oxides contained in the exhaust gas by means of the generated ammonia. It is proposed that the plasma generator (34) of the nitrogen monoxide generating device comprises an air or exhaust gas channel with a reaction region (38) having at least one electrode (40) on one narrow side and at least one counterelectrode (42) on an opposite narrow side, wherein the electrodes (40, 42) are formed so that upon application of a corresponding electrical voltage between the electrodes (40, 42) takes place a corona-stabilized spark discharge.

Description

State of the art

The invention relates firstly to an apparatus for the selective catalytic reduction of the nitrogen oxides contained in an exhaust gas of an internal combustion engine, with a nitrogen monoxide production device comprising a plasma generator, with an ammonia generating device which uses the nitrogen monoxide produced, and with an exhaust gas reduction device which In the exhaust gas contained nitrogen oxides reduced by means of the generated ammonia.

A device of the type mentioned is from the DE 199 22 961 A1 known. The device described therein comprises a nitrogen oxide reduction catalytic converter for the selective catalytic reduction of nitrogen oxides contained in the emitted exhaust gas of the internal combustion engine. In this case, ammonia is used as the reducing agent.

The not yet published publication DE 102 58 185 A1 also shows a method of generating ammonia on board a motor vehicle based on a plasma process for producing NOx.

In order not to have to store the ammonia or a precursor product, for example a urea-water solution, in a tank, for example, the reduction device is preceded by an ammonia production device which generates the required ammonia, inter alia, from nitrogen monoxide, which is provided by a nitrogen monoxide production device becomes. The production of the ammonia takes place by a synthesis reaction of hydrogen and nitrogen monoxide. In order to be able to provide a sufficient amount of nitrogen monoxide, the nitrogen monoxide production device comprises a plasma generator which generates the nitrogen monoxide by plasma-technical oxidation of nitrogen. This is contained in an air stream supplied to the plasma generator.

Another way of producing the ammonia required for the selective catalytic reduction is in the DE 199 03 533 A1 described. There is no need for a separate nitric oxide generating device. Instead, the ammonia generator itself comprises a plasma generator operating on the corona principle. The plasma generator is fed to a rich exhaust gas mixture of the internal combustion engine.

Also known is the article "Design considerations for corona-stabilized repetitive switches", Applied Physics 32 (1999), 790-797, J.A. Harrower, S.J. MacGregor; F. A. Tuema, in which a special embodiment of a working on the principle of coronasized spark discharge switch is described.

The present invention has the object of developing a device of the type mentioned so that the required for the selective catalytic reduction of ammonia with the highest possible efficiency, that is, with the lowest possible energy consumption, can be generated.

This object is achieved in a device of the type mentioned above in that the plasma generator of the nitrogen monoxide generating means comprises an air or exhaust duct having a reaction region having on a narrow side at least one electrode and on an opposite narrow side at least one counter electrode, wherein the electrodes are formed so that when a corresponding electrical voltage between the electrodes a corona-stabilized spark discharge takes place.

Advantages of the invention

A plasma generator operating on the principle of corona-stabilized spark discharge also generates from ambient air the nitrogen monoxide required for the synthesis reaction because of the high possible repetition rates with very high efficiency. For the efficiency of nitrogen monoxide production is essential that the air or the exhaust gas heated very quickly, but in particular that the heated air or the heated exhaust gas are again very quickly "quenched". Temperature gradients in the order of 10 ⁶ to 10 ⁸ K / s are very favorable, since in such gradients of the nitrogen monoxide decomposition is kinetically inhibited.

In the case of the corona-stabilized spark discharge provided according to the invention, an electrical energy store is discharged within a few microseconds. Above all, this short discharge duration makes it possible to achieve the temperature gradients ("quench rates") in the range of 10 ⁸ K / s required for the formation of nitrogen monoxide. In the case of corona-stabilized spark discharges, moreover, a comparatively large distance between the electrodes ("discharge gap distance") can be realized. This allows the heating and cooling of comparatively large volumes of gas, which also increases the efficiency of nitric oxide production.

The ignition of the discharge takes place by means of sharp points on the electrodes, which cause a field elevation. At these tips starts the spark discharge with the help of a corona-stabilized discharge. Due to the large discharge gap distances, a comparatively rapid reconsolidation of the discharge gap takes place. This effect allows high repetition frequencies and, with low energy input, a high conversion rate to nitric oxide.

Thus, an overall advantage over the prior art is, above all, an improvement in the efficiency of nitrogen monoxide production and thus an improvement in the efficiency of the entire ammonia production process chain. As a result, the additional fuel consumption for the production of ammonia and ultimately for the entire exhaust aftertreatment can be reduced, in particular for lean burn engines, which is required for the selective catalytic reduction.

Advantageous developments of the invention are specified in subclaims.

First, it is proposed that the reaction area has a cross section whose width is at least about five times its height. Such a narrow gap allows a large volume flow while forming a plasma in a large area of the cross section.

The efficiency is improved when the plasma generator has a plurality of reaction areas.

In this case, the plasma generator builds small when the reaction areas are arranged in a star shape.

The design of the plasma generator is comparatively simple if the radially inner electrodes are connected to a common central conductor. Analogously, the radially outer electrodes can also be connected to a common ring conductor. In principle, however, it is also conceivable to deactivate or switch on individual reaction areas so as to be able to adapt the power of the plasma generator to the current requirements.

The manufacture of the plasma generator is simplified when the reaction region is formed by a recess in a housing made of an insulator.

The efficiency of the device according to the invention is again significantly improved if the residual oxygen contained in the nitrogen monoxide-containing product gas mixture which is produced by the nitrogen monoxide generating device is reduced. Therefore, it is proposed that the apparatus of the present invention includes a reaction device disposed between the nitrogen monoxide production device and the ammonia production device, and in which the residual oxygen contained in the gas flow exiting the nitrogen monoxide generation device is burned together with hydrogen becomes.

Alternatively, however, it is also possible for the outlet of the nitrogen oxide monoxide generating device to be connected to the inlet of a combustion chamber of the internal combustion engine to which a rich fuel-air mixture is temporarily supplied. This embodiment is cheaper overall.

As stated at the outset, hydrogen is required for the nitrogen oxide reduction by the ammonia. This can be generated in the required for a high efficiency of nitrogen oxide reduction scope by a hydrogen generating device whose inlet is connected to the outlet of a combustion chamber of the internal combustion engine to which a rich fuel-air mixture is temporarily supplied. Alternatively, the hydrogen can also be prepared by means of a partial oxidation reforming (POX) and via carbon monoxide by means of shift catalysts.

The invention also relates to a method for the selective catalytic reduction of nitrogen oxides contained in an exhaust gas by means of a device of the type mentioned above. The efficiency or yield is particularly high when the nitrogen monoxide generating device is operated at a repetition frequency of at least 5 kHz becomes. This is only possible with a corona-stabilized spark discharge.

The efficiency or the yield can be increased even further when the nitrogen monoxide generating device is triggered and operated at a repetition frequency of at least 10 kHz.

Furthermore, the method according to the invention operates with particularly high efficiency when the amount of energy of a charging capacitor is adjusted such that the temperature of the gas in the reaction region is in the range from about 3300 K to 3700 K, preferably in the range from about 3450 K to 3550 K.

A simple adjustment of a desired amount of nitrogen monoxide product is possible by varying the repetition frequency and / or the number of discharge sparks.

drawing

Hereinafter, particularly preferred embodiments of the present invention will be explained in more detail with reference to the accompanying drawings. In the drawing show:

1 a schematic representation of a device for the selective catalytic reduction of nitrogen oxides contained in an exhaust gas of an internal combustion engine;

2 a schematic diagram of a device of a conventional spark discharge;

3 a schematic diagram of a nitrogen monoxide generating device, which operates on the principle of corona-stabilized spark discharge;

4 an alternative embodiment of a nitrogen monoxide production device; and

5 an alternative embodiment of a device for selective catalytic reduction.

Description of the embodiments

In 1 an internal combustion engine carries the reference numeral 10 , It comprises four cylinders with corresponding combustion chambers in the present embodiment 12a to 12d , Combustion air is the combustion chambers 12a to 12d via an intake pipe 14 fed. Fuel enters the combustion chambers 12a to 12d over the combustion chambers 12a to 12d individually assigned injectors, which are not shown in the figure. The internal combustion engine 10 works with direct fuel injection.

In 1 is also an exhaust system 16 the internal combustion engine 10 shown. It includes an exhaust pipe 18 , which releases the combustion gases from the combustion chambers 12a to 12c dissipates. The exhaust pipe 18 leads to an exhaust gas reduction device 20 , This is in the present case a nitrogen oxide reduction catalyst. Its function will be discussed in detail below.

The combustion gases in the combustion chamber 12d be generated, via a separate exhaust pipe 22 derived. This leads to a hydrogen generating device 24 , which may be, for example, a 3-way or an oxidation catalyst. It would also be conceivable that the hydrogen generating device 24 works according to the principle of partial oxidation reforming (POX) or contains shift catalysts. Also the function of the device 24 will be explained in more detail below.

The outlet of the hydrogen generator 24 is with a reaction device 26 connected to the outlet of a nitrogen monoxide generator 28 connected is. This is in turn via a line 30 Fresh air from the intake pipe 14 fed. The outlet of the reaction device 26 leads to an ammonia generating device 32 which is, for example, a common 3-way catalyst with a suitable platinum and / or rhodium catalyst material on an alumina support. The ammonia generator 32 is again with the exhaust gas reduction device 20 connected.

The nitric oxide generator 28 includes a plasma generator 34 of which a possible embodiment in 3 and another possible embodiment in 4 is shown schematically. At the in 3 In the embodiment shown, the plasma generator comprises 34 a housing 36 from an electrical insulator. In the case 36 is a perpendicular to the plane of the drawing through the housing 36 Through extending recess present at one end to the line 30 is connected and the other end to the reaction device 26 leads.

In the area of the drawing plane, the recess is a reaction area 38 formed with a rectangular cross-section. Its width is at the in 3 embodiment shown approximately fifteen times its height. On a narrow side of the reaction area 38 is a tapered electrode 40 present, on the opposite narrow side in the present case also tapered counter electrode 42 opposite. In an embodiment not shown, this electrode may also be flat. The two electrodes 40 and 42 are low-inductance to the two poles of a high-voltage capacitor 44 connected in turn to a voltage source 46 connected. Between the electrode 40 and the high voltage capacitor 44 is a switch 48 available.

Will the switch 48 closed, it comes between the two electrodes 40 and 42 to a complete "spark break". This results between the two electrodes 40 and 42 a conductive plasma channel. Due to the special, namely tapered design of the two electrodes 40 and 42 results in a "corona-stabilized spark discharge". This is an incomplete spark break in the highly inhomogeneous or divergent field near the negatively charged electrode 40 , in the At close range, the electrons attach themselves to molecules, creating a negative space charge (corona), which reduces the electric field so much that the "streamer" (this is a start plasma region with high electric field strength) no longer to the counter electrode 42 can continue. The negative ions contribute to a state stabilization.

By a drift of the ions on the counter electrode 42 the circuit closes. Such a discharge can be operated particularly well with alternating current. An arrangement in which no such corona-stabilized spark discharge, but a normal spark break is observed, is for comparison purposes in 2 shown.

The internal combustion engine 10 and the exhaust system 16 from 1 work when the nitric oxide generator 28 a plasma generator 34 corresponding 3 includes, as follows:
If ammonia is to be generated, the combustion chamber 12d supplied an increased amount of fuel, so that in this a comparatively rich fuel-air mixture with an air ratio lambda less or even significantly less 1 is supplied. The corresponding rich exhaust gas mixture is via the exhaust pipe 22 the hydrogen generator 24 supplied, is generated in the hydrogen H ₂ . At the same time, over the line 30 the nitric oxide generator 28 Intake air supplied. The fresh intake air thus enters the reaction area 38 ,

The desk 48 is controlled via a corresponding trigger signal with a frequency of 5 to 10 kHz. This results between the two electrodes 40 and 42 Corona-stabilized spark discharges with quench rates from 10 ⁶ to 10 ⁸ K / s. The high voltage capacitor 44 is designed so that the plasma in the reaction area 38 has a temperature of about 3500 K. This temperature is optimal to high efficiency from the ambient air in the reaction area 38 Nitric oxide NO to produce.

The nitrogen monoxide NO and also a residual oxygen O ₂ still present arrive from the nitrogen monoxide generating device 28 to the reaction device 26 in which the residual oxygen O _{2 is present} in the hydrogen-producing device 24 produced hydrogen H _{2 is} burned to water H ₂ O. Nitrogen monoxide NO and hydrogen H ₂ pass from the reaction device 26 to the ammonia production facility 32 in which these two substances react to ammonia NH ₃ . The ammonia NH ₃ is now the exhaust gas reduction device 20 fed, where it reduces the nitrogen oxides NO _x contained in the exhaust gas to nitrogen N ₂ and water H ₂ O.

In the in 1 embodiment shown are the facilities 20 . 24 . 26 . 28 , and 32 as each separate components shown. It is understood, however, that these components can be readily summarized in units. It is also conceivable, the ammonia generating device 32 not with the exhaust gas reduction device 20 but simply with the exhaust pipe 18 connect to.

Instead of in 3 shown plasma generator 34 may also be in the nitrogen monoxide generating device in 4 shown plasma generator 34 be used. It should be noted that in 4 such elements and areas that have equivalent functions to elements and areas of 3 have, are not explained again in detail and also bear the same reference numerals.

The in 4 shown plasma generator has a total of twelve reaction areas 38 on, of which in 4 for purposes of illustration only one is provided with a reference numeral. The reaction areas 38 are arranged in a star shape around a central axis perpendicular to the plane of the drawing. Your radially inner electrodes 40 are with a common, coaxial with the central axis extending central conductor 50 connected. The counter electrodes 42 of six reaction areas 38 are with a radially outward of the reaction areas 38 arranged ring conductor 52 connected. The counter electrodes 42 the six other reaction areas 38 are with another, also radially outside of the reaction areas 38 arranged ring conductor 54 connected. The one ring conductor 52 is over a switch 48a with a high voltage capacitor 44a , the other ring conductor 54 via a switch 48b with a high voltage capacitor 44b connectable. Both high voltage capacitors 44a and 44b are with the same voltage source 46 connected. The other two poles of high voltage capacitors 44 are with the central conductor 50 connected.

To measure the amount of nitric oxide produced by the nitric oxide generator 28 or the plasma generator 34 can be varied, optionally 50% of the reaction areas 38 of the plasma generator 34 be shut down by the corresponding switch in operation 48a respectively 48b remains open. In addition, a variation of the amount of nitrogen monoxide product but also by a change in the repetition frequency and / or the number of discharge sparks is possible.

A variant of an internal combustion engine 10 and an exhaust system 16 is in 5 shown. Again, such elements and areas that have equivalent functions to elements and areas of 1 have the same reference numerals and are not explained again in detail.

At the in 5 shown exhaust system 16 is on a reaction device 26 waived. Instead, the product gas that is produced by the nitric oxide generator 28 is generated and which still contains an undesirably high proportion of residual oxygen O ₂ , the combustion chamber 12d fed, where the residual oxygen O _{2 is} burned.

Claims (14)

Contraption ( 16 ) for the selective catalytic reduction of in an exhaust gas of an internal combustion engine ( 10 ) containing nitrogen oxides, with a nitrogen monoxide generating device ( 28 ), which is a plasma generator ( 34 ), with an ammonia generating device ( 32 ) using the nitrogen monoxide produced, and with an exhaust gas reduction device ( 20 ), which reduces nitrogen oxides contained in the exhaust gas by means of the generated ammonia, characterized in that the plasma generator ( 34 ) of the nitrogen monoxide production device ( 28 ) an air or exhaust duct having a reaction area ( 38 ) having a rectangular cross-section, which has at least one electrode on a narrow side ( 40 ) and on an opposite narrow side at least one counter electrode ( 42 ), wherein the electrodes ( 40 . 42 ) are formed so that when applying a corresponding electrical voltage between the electrodes ( 40 . 42 ) a corona-stabilized spark discharge takes place. Contraption ( 26 ) according to claim 1, characterized in that the reaction area ( 38 ) has a cross-section whose width is approximately five times its height. Contraption ( 16 ) according to one of claims 1 or 2, characterized in that the plasma generator ( 34 ) a plurality of reaction areas ( 38 ) having. Contraption ( 16 ) according to claim 3, characterized in that the reaction areas ( 38 ) are arranged in a star shape. Contraption ( 16 ) according to claim 4, characterized in that the radially inner electrodes ( 40 ) with a common central conductor ( 50 ) are connected. Contraption ( 16 ) according to one of claims 4 to 5, characterized in that the radially outer electrodes ( 42 ) with a common ring conductor ( 52 . 54 ) are connected. Contraption ( 16 ) according to one of the preceding claims, characterized in that the reaction region through a recess ( 38 ) in a housing ( 36 ) is formed, which is made of an insulator. Contraption ( 16 ) according to one of the preceding claims, characterized in that it comprises a reaction device ( 26 ) between the nitrogen monoxide production device ( 28 ) and the ammonia production facility ( 32 ) and in the residual oxygen contained in that gas stream coming from the nitrogen monoxide production device ( 28 ), is burned together with hydrogen. Contraption ( 16 ) according to one of claims 1 to 7, characterized in that the outlet of the nitrogen monoxide production device ( 28 ) with the inlet of a combustion chamber ( 12d ) of the internal combustion engine ( 10 ) is connected to the temporarily a rich fuel-air mixture is supplied. Contraption ( 16 ) according to one of the preceding claims, characterized in that it comprises a hydrogen generating device ( 24 ) whose inlet communicates with the outlet of a combustion chamber ( 12d ) of the internal combustion engine ( 10 ) is connected to the temporarily a rich fuel-air mixture is supplied. Process for the selective catalytic reduction of an exhaust gas of an internal combustion engine ( 10 ) contained nitrogen oxides by means of a device ( 16 ) according to one of claims 1 to 10, characterized in that the nitrogen monoxide production device ( 28 ) is operated at a repetition frequency of at least 5 kHz. Method according to claim 11, characterized in that the nitrogen monoxide production device ( 28 ) is triggered at a repetition frequency of at least 10 kHz. Method according to one of claims 11 to 12, characterized in that the amount of energy of a charging capacitor ( 44 ) is adjusted so that the temperature of the gas in the reaction region ( 38 ) in the range of about 3300 K to 3700 K, preferably in the range of about 3450 K to 3550 K. Method according to one of claims 11 to 13, characterized in that for setting a desired amount of nitrogen monoxide product the repetition frequency and / or the number of discharge sparks is / will be varied.

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