Method for reducing the concentration of nitrogen dioxide

1. A method for reducing a concentration of nitrogen dioxide, NO 2 , in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures, wherein an exhaust gas containing nitrogen dioxide and uncombusted hydrocarbons, UHCs, is passed over a catalyst for NO 2 reduction which reduces NO 2 by reaction with UHCs and in the process oxidizes UHCs, and the nitrogen dioxide present in the exhaust gas is reduced in the presence of the catalyst at least to nitrogen monoxide, NO,

wherein the catalyst:

(i) has a storage temperature, T UHCabs , above which the catalyst stores UHCs and also partial oxidation products of UHCs,

(ii) has an activation temperature, T red , above which the catalyst catalyzes the reduction of NO 2 to at least NO by oxidation of the UHCs, and

(iii) has a working temperature, T ox , at which oxidation of UHCs with oxygen, O 2 , in the exhaust gas starts,

wherein the storage temperature T UHCabs for the storage of UHCs is below the activation temperature T red for the reduction of NO 2 , as shown in equation (I):

T UHCabs <T red ,  (I)

and

wherein the working temperature T ox for the oxidation of the UHCs with O 2 is above T red , as shown in equation (II):

T ox >T red .  (II)

2. The method as claimed in claim 1 , wherein a reaction rate for the oxidation of UHCs with oxygen is less than a reaction rate for the reduction of nitrogen dioxide with UHCs, provided that the concentration of the nitrogen dioxide after the reduction in the presence of the catalyst is above an emission limiting value.

3. The method as claimed in claim 1 , wherein the catalyst additionally has a temperature, T UHCdes , above which the catalyst liberates stored UHC and also partial oxidation products of UHCs, which temperature is above T ox , as shown in equation (III):

T UHCdes >T ox .  (III)

4. The method as claimed in claim 1 , wherein the catalyst has a temperature, T NO2abs , above which the catalyst stores nitrogen dioxide and which is below T red , as shown in equation (IV):

T NO2abs <T red ,  (IV)

and has a temperature, T NO2des , above which the catalyst releases nitrogen dioxide and which is above T red , as shown in equation (V):

T red <T NO2des .  (V)

5. The method as claimed in claim 1 , wherein a time average UHC concentration X UHC is above a time average of the nitrogen dioxide concentration X NO2 by the factor γ, which is given by the equation (VI):

γ= X UHC /X NO2 >1.  (VI)

6. The method as claimed in claim 1 , wherein heat is withdrawn from the exhaust gas before it is passed over the catalyst, in order that the exhaust gas, when passed over the catalyst, has a temperature which is lower than a maximum temperature, T max , above which, in the case of the catalyst for NO 2 reduction, the storage capacity for UHCs and/or NO 2 decreases and/or above which the catalytic activity in nitrogen dioxide reduction is reduced.

7. The method as claimed in claim 1 , wherein the exhaust gas, downstream of the catalyst, is passed over a catalyst for UHC oxidation which oxidizes UHCs and also partial oxidation products of UHCs.

8. The method as claimed in claim 1 , wherein the exhaust gas, upstream of the catalyst, is passed over a catalyst for NO 2 storage, which catalyst stores nitrogen dioxide at the temperature T NO2abs and releases it at the temperature T NO2des .

9. The method as claimed in claim 7 , wherein heat is withdrawn from the exhaust gas upstream and/or downstream of the catalyst and/or optionally upstream and/or downstream of the catalyst.

10. A device for reducing the concentration of nitrogen dioxide, NO 2 , in an exhaust gas from the incomplete combustion of hydrocarbons or hydrocarbon mixtures, in which the device has a catalyst for NO 2 reduction, the catalyst:

(i) has a storage temperature, T UHCabs , above which the catalyst stores uncombusted hydrocarbons, UHCs, and also partial oxidation products of UHCs,

(ii) has an activation temperature, T red , above which the catalyst catalyzes the reduction of NO 2 to at least nitrogen monoxide NO by oxidation of the UHCs, and

(iii) has a working temperature, T ox , at which oxidation of UHCs with oxygen, O 2 , in the exhaust gas starts, in which the storage temperature, T UHCabs for the storage of UHCs is below the activation temperature, T red for the reduction of NO 2 , as shown in equation (I):

T UHCabs <T red ,  (I)

and

wherein the working temperature T ox for the oxidation of the UHCs with O 2 is above, T red , as shown in equation (II):

T ox >T red .  (II)

11. The device as claimed in claim 10 , wherein the catalyst additionally has a temperature, T UHCdes above which the catalyst liberates stored UHCs and also partial oxidation products of UHCs, which temperature is above T ox , as shown in equation (III):

T UHCdes >T ox .  (III)

12. The device as claimed in claim 10 , in which the catalyst has a temperature, T NO2abs , above which the catalyst stores nitrogen dioxide and which is below T red , as shown in equation (IV):

T NO2abs <T red ,  (IV)

and has a temperature, T NO2des , above which the catalyst releases nitrogen dioxide and which is above, T red , as shown in equation (V):

T red <T NO2des .  (V)

13. The device as claimed in claim 10 , further comprising an additional catalyst for the UHC oxidation, the additional catalyst being mounted in such a manner that it comes into contact with the exhaust gas downstream of the catalyst.

14. The device as claimed in claim 10 , further comprising an additional catalyst for NO 2 storage, which is mounted in such a manner that the additional catalyst comes into contact with the exhaust gas upstream of the catalyst.

15. The device as claimed in claim 10 , which has, in the direction of the gas stream through the catalyst, at least one heat exchanger upstream and/or downstream of the catalyst and/or optionally upstream and/or downstream of the catalyst and/or optionally upstream and/or downstream of the catalyst.

16. A system comprising a device as claimed in claim 10 .

17. The system as claimed in claim 16 , wherein the system is at least one of a gas turbine power plant, a compressor operated by a gas turbine, a gas- or oil-operated boiler, a gas engine, and a ship engine operated by the combustion of diesel or heavy oil.