BE1006164A3 - Catalyst device for reducing oxides of nitrogen. - Google Patents

Abstract

In a catalyst device for the reduction of nitrogen oxides in exhaust gases, these occur over a wide range of temperatures. To obtain a sufficiently effective elimination of nitrogen oxides in this temperature range and to allow adaptation to each temperature range of the exhaust gases, at least two catalyst beds (7, 8) made of different catalytic materials are arranged one behind the other in the direction of the exhaust gas flow, to reduce nitrogen oxides. The various catalytic materials exhibit their most marked catalytic efficiency in the neighboring, but different, partial areas of the exhaust gas temperature range.

Description

       

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  Catalyst device for the reduction of nitrogen oxides.



   The present invention relates to a catalyst device for the reduction of nitrogen oxides in an exhaust gas which occurs over a wide range of temperatures.



   In internal combustion engines used for the production of current, the speed of rotation is generally constant, so that the volume flow rate of the exhaust gases undergoes little variation. However, the temperature of the exhaust gases varies depending on the engine load and, therefore, also the Nos content. the range of exhaust gas temperatures is, for example, between 290 C and 460 C.



   The review "Staub-Reinhaltung der Luft" 49 (1989), pages 37-43 contains a description of various catalyst materials which can be used at high exhaust gas temperatures as well as at lower temperatures of these gases.



   Other materials which can serve as catalyst are described in document DE 35 05 648 C2 and EP 0 168 811 B1.



   In the current state of the art, it has been based on the fact that a single material is used as a catalyst to cover the whole range of temperatures.

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 exhaust gases. This is difficult to achieve in practice.



   The object of the present invention is to provide a catalyst device of the aforementioned type, which has a sufficient nitrogen oxide removal efficiency over a wide range of exhaust gas temperatures and which can be adapted to each range of temperatures. exhaust gases.



   According to the present invention, this result is achieved by the fact that at least two catalyst beds made of different catalytic materials are arranged one after the other in the direction of flow of the exhaust gases in order to reduce the oxides. nitrogen and the fact that the various materials serving as catalysts exhibit their most marked catalytic effect in partial areas, similar but different from the temperature range of the exhaust gases.



   Since there are at least two different materials serving as catalysts in the exhaust gas stream, these can be chosen with a view to obtaining optimum efficiency in a partial range of the exhaust gas temperature range. engine exhaust. The catalyst device can therefore be easily adapted to the characteristic of the flow of exhaust gases from each engine. If a higher amount of nitrogen oxides is to be reduced at higher temperatures rather than at lower exhaust gas temperatures, the catalyst bed which is most effective at the higher temperatures can then be arranged. appropriately.



   The required elimination of nitrogen oxides can therefore be ensured in full over the entire range of exhaust gas temperatures without having to provide a single material for this purpose.

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 catalyst and specially designed for each range of exhaust gas temperatures. It may suffice to provide two catalyst beds, one of which is effective at lower temperatures and the other at higher temperatures in the exhaust gas temperature range. However, additional catalyst beds can also be provided for the average temperature ranges by making them using suitably chosen catalytic materials.



   Advantageous embodiments of the present invention are the subject of secondary claims and the description below of an exemplary embodiment. The figures represent respectively:
FIG. 1, a catalyst device seen in section and
Figure 2 is a diagram of exhaust gas temperatures.



   A catalyst device has an inlet 1 for an exhaust gas stream from an internal combustion engine. The housing 2 contains, arranged one behind the other in the direction of flow of the exhaust gases, a soot particle filter 3, a buffer against the temperature peaks 4, a device with a nozzle 5 for the ammonia, a flow mixer 6, a first bed of Denox catalyst 7, a second bed of Denox catalyst 8 and a bed of oxidation catalyst 9. Next there is an outlet 10.



   The soot particle filter 3 removes soot particles from the exhaust gases. The combustion of accumulated soot particles in the filter causes temperature spikes. These are equalized in the buffer for temperature peaks 4 so as not to damage the elements located downstream.



   Ammonia is injected disper-

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   sée in the exhaust gas stream filtered by means of a nozzle device 5. The exhaust gas stream containing ammonia is prepared and homogenized in the flow mixer 6.



   The first catalyst bed 7 contains a catalytic material whose optimum efficiency lies in a range of temperatures higher than those of the catalytic material of the second catalyst bed 8. The catalytic material of the first catalyst bed 7 is formed, for example, based on iron sulphate Fe (SO4) 3 or a molecular sieve (zeolite). The catalytic material of the second catalyst bed 8 functions, for example, with a mixture of titanium oxide (Ti02) and vanadium pentoxide (V20S) or with a mixture of iron oxide (FeZ03) and chrome (cor203).



   FIG. 2 shows in broken lines the NO load of the exhaust gases for a case of application at input 1 as a function of the operating temperature. It has been admitted, in this particular case, that the exhaust gas temperature range is approximately between 290 C and 460 C. The NO load of the exhaust gas stream increases from the temperature of 300 C to 'at a temperature of 450 C in a practically linear manner from 1000 ppm to more than 2000 ppm.



   The dashed line in FIG. 2 represents the temperature-dependent efficiency of the catalytic material of the first catalyst bed 7.



  It can be seen that the efficiency of this catalytic material is lower in the temperature range
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 lower between 300 C and 400 C than in the range between 400 C and 460 C. The dotted line in FIG. 2 indicates the catalytic efficiency of the catalytic material of the second catalyst bed 8. It can be seen that its efficiency decreases from about 400 C.

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   The solid line in Figure 2 indicates the NO content at the exhaust outlet in the range of exhaust gas temperatures.
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  Up to around 430 C, the NOX content is less than 100 ppm at the outlet. The NOX content then increases to about 460 C to slightly exceed 100 ppm, but without reaching, however, 200 ppm.



   At comparatively lower temperatures up to around 400 ° C., the NO of the exhaust gases is only slightly transformed in the catalyst bed 7. The transformation takes place mainly in the catalyst bed 8 according to the formula:
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At comparatively higher temperatures, the transformation also takes place according to the above equation, but mainly in the catalyst bed 7 and not mainly in the catalyst bed 8. In the catalyst bed 8 react only the amounts of NH3 and NOX which have not yet been transformed in the catalyst bed 7.



   In the oxidation catalyst bed 9, the
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 hydrocarbons are oxidized and the CO is transformed into CO.


    

Claims (7)

CLAIMS 1. Catalyst device for the reduction of nitrogen oxides in exhaust gases which occur over a wide range of exhaust gas temperatures characterized in that at least two catalyst beds (7,8) made up of different catalytic materials are arranged one after the other in the direction of flow of the exhaust gases in order to reduce nitrogen oxides and in that the different catalytic materials exhibit their most marked catalytic efficiency in different fields partial, neighboring but different, of the exhaust gas temperature range.  2. Catalyst device according to claim 1, characterized in that an ammonia supply is arranged before the front catalyst bed (7) in the direction of flow of the exhaust gases.  3. Catalyst device according to claim 1 or 2, characterized in that the catalyst bed (7) whose optimum efficiency lies in a higher temperature range is arranged before the other catalyst bed (8 ) in the flow direction.  4. Catalyst device according to one of the preceding claims, characterized in that the catalytic material for the upper partial range of the exhaust gas temperatures is based on iron sulphate.  5. Catalyst device according to one of the preceding claims, characterized in that the catalytic material for the temperature range  <Desc / Clms Page number 7>  lower consists of a mixture of titanium oxide and vanadium pentoxide or iron oxide and chromium oxide.  6. Catalyst device according to one of the preceding claims, characterized in that a soot particle filter (3) with a buffer for temperature peaks (4) is mounted before the previous catalyst bed (7) in the flow direction.  7. Catalyst device according to one of the preceding claims, characterized in that a bed of oxidation catalyst (9) is arranged behind the rear catalyst bed (8) in the direction of flow.