# Patent

## Title
Method for reducing the concentration of nitrogen dioxide

## Abstract

A method for reducing the concentration of nitrogen oxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures using a catalyst for reducing nitrogen dioxide by oxidizing non-combusted hydrocarbons or hydrocarbon mixtures, a device for reducing the concentration of nitrogen dioxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures using a catalyst for reducing nitrogen dioxide by oxidizing non-combusted hydrocarbons or hydrocarbon mixtures, and a system including the device is provided.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority to PCT Application No. PCT/EP2013/062244, having a filing date of Jun. 13, 2013, based off of DE 102012213728.9 having a filing date of Aug. 2, 2012, the entire contents of which are hereby incorporated by reference.
FIELD OF TECHNOLOGY
The following relates to a method for reducing the concentration (reducing the content) of nitrogen dioxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures using a catalyst for reducing nitrogen dioxide by oxidizing uncombusted hydrocarbons or hydrocarbon mixtures, a device for reducing the concentration of nitrogen dioxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures using a catalyst for reducing nitrogen dioxide by oxidizing uncombusted hydrocarbons or hydrocarbon mixtures, and also a system comprising the device.
BACKGROUND
Currently, owing to the increasing worldwide energy demand, it is increasingly required to generate energy efficiently and cleanly in various systems, and this also applies to conventional systems for generating energy which are operated by combustion of hydrocarbons or hydrocarbon mixtures.
Gas-turbine operated generators in this case are a clean and efficient possibility for generating electrical energy, not only for public energy supply but also for industrial use. Thanks to the use of high-grade ceramic protective layers and sophisticated cooling concepts, gas turbines can now be operated with mean turbine intake temperatures of sometimes above 1500° C., and thus achieve net efficiencies of 40% without heat recovery and exergetic efficiencies of above 60% with heat recovery via coupling to a steam turbine. By using optimized burners, during operation with natural gas, mean nitrogen oxide emissions (NOx=total nitrogen oxide, i.e. NO and NO2 taken together) of less than 25 ppm (by volume) and carbon monoxide emissions (CO) of below 10 ppm can be guaranteed, wherein a defined test cycle is used as a basis for determination of the emissions. The national limiting values for power plant emissions may thereby be met in many industrial nations such as, e.g. Germany, still without any further exhaust gas aftertreatment.
Gas turbines are distinguished not only by low emissions and high efficiency, but also by capabilities such as operation under very differing loads (load flexibility from 100% down to values of below 30%) and rapid load changes up to rapid start (achieving the base load in less than 30 minutes). Because of the increasing fraction of energy from renewable sources (wind, solar, etc.) which are characterized by high fluctuations over time of the power fed into the electric supply grid, increasingly use is being made of the load flexibility of gas turbines. However, as a result, pollutant emissions occur which differ substantially from the average values determined in the test cycle: in the case of low burner outputs, the gas turbine burners are to be operated under equivalent conditions (i.e. fuel/air ratios based on the values required for stoichiometric combustion), which are far below the values required for the base load of the turbine.
Since these equivalent conditions are also far below the lean-burn limit for completely premixed combustion, increased emissions of uncombusted hydrocarbons and/or hydrocarbon mixtures (UHCs), of carbon monoxide (CO), and frequently also of nitrogen oxides, occur, and here there is a characteristic feature that the nitrogen oxides are emitted to an overwhelming proportion as nitrogen dioxide (NO2). The latter, from the viewpoint of discharge into the lower atmosphere, is actually not a problem, because nitrogen monoxide is also oxidized in the course of just a few minutes in the atmosphere to form nitrogen dioxide. However, NO2 has the unpleasant property of absorbing light in the blue and near-ultraviolet spectral range, in such a manner that even the operation of gas turbines, the pollutant emissions of which meet all legal provisions, owing to a yellow discoloration of the exhaust gas plume existing from the stack of the power plant in daylight, termed “yellow plume”), can lead to problems of acceptance.
In addition, in the case of gas turbines, there is frequently the possibility of using oil as an alternative fuel. In this case, compared with operation with natural gas, elevated nitrogen oxide emissions frequently occur, and the formation of NO2 in noticeable concentrations is observed over a broad range of loads.
Gas turbine power plants are long-term investments which to date—just as with coal power plants—have usually been operated at base load, in order to achieve the highest possible yields from the generation of electrical energy. The nitrogen oxide emissions in this case are composed of virtually 100% nitrogen monoxide (NO). Yellowish exhaust gas plumes have therefore not been observed except in the case of sporadically occurring start-ups of the gas turbine. Therefore, to date there have also been no problems with visible emissions which have only been initiated by the rapidly growing fraction of renewable energies in the energy market.
Likewise, nitrogen dioxide emissions and thereby yellowish discolorations can be formed in exhaust gases of further systems which, under certain operating conditions, burn hydrocarbons or hydrocarbon mixtures incompletely, such as, for example, gas-operated compressors, gas- or oil-operated boilers, gas engines or ships operated with diesel or heavy oil. The occurrence of these NO2 emissions in general and the dependence of this phenomenon on the fuel is due to the chemical gas-phase reactions between nitrogen monoxide (NO) and UHCs or CO in certain temperatures ranges which follow an incomplete combustion, as presented in MORIO HORI, NAOKI MATSUNAGA, NICK MARINOV, WILLIAM PITZ and CHARLES WESTBROOK: AN EXPERIMENTAL AND KINETIC CALCULATION OF THE PROMOTION EFFECT OF HYDROCARBONS ON THE NO—NO2 CONVERSION IN A FLOW REACTOR; Twenty-Seventh Symposium (International) on Combustion/The Combustion Institute; 1998; pages 389-396. Not only during the start-up phase of, for example, gas turbines, but also during the operation in low partial load, however, the incomplete combustion of the fuel used is accepted, in order to be able to ensure stable operating behavior of the gas turbine.
However, the formation of NO2 can also have other causes which, in some circumstances, require different measures for reduction of NO2 emissions: thus, in the case of gas turbines having a waste-heat steam generator and integrated multistage exhaust gas aftertreatment consisting of upstream CO oxidation catalyst followed by ammonia (NH3) injection and downstream SCR catalyst (SCR, “selective catalytic reduction”), the formation of noticeable NO2 emissions was observed. To solve this problem, in EP 2 223 733 A1, the placing of the CO oxidation catalyst in a suitable temperature zone was proposed. Such an exhaust gas purification, however, is complex and costly in method terms, such that it is also not retrofittable at will, or is only retrofittable with considerable expense, in existing systems.
SUMMARY
An aspect relates to providing a method and a device for reducing the concentration of nitrogen dioxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures, which method and which device can remove nitrogen dioxide in a simple and effective manner from the exhaust gas.
Another aspect relates to providing a retrofittable solution for reducing the NO2 emissions in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures for systems which do not already have a system for selective catalytic reduction (SCR) of nitrogen oxides by means of NH3 as reducing agent, and in this case advantageously to dispense with the storage, metering and injection of any reducing agent such as, e.g., ammonia. Advantages result in this case, in comparison with a solution requiring reducing agent injection, from reduced capital costs, reduced operating costs for reducing agent and maintenance, and the avoidance of the emission of unreacted or incompletely reacted reducing agents, which themselves can be pollutants.
The solution proposed here for reducing NO2 emissions in gas turbine exhaust gases is based on the surprising finding that, by a catalytic reaction of the NO2 with uncombusted hydrocarbons, with a catalytic activity selected to be sufficiently low for the oxidation of hydrocarbons with oxygen, an exhaust gas composition can be induced at which, even in the case of unchanged nitrogen oxide (NOx) total concentration, the NO2 concentration can be brought to a value which is far below that which is given by the thermodynamic equilibrium ratio between NO2 and NO for the respective catalyst temperature.



BRIEF DESCRIPTION
Some of the embodiments will be described in detail, with reference to the following figures, wherein like designations denote like members, wherein:
FIG. 1 shows a conventional turbine power plant without exhaust gas treatment;
FIG. 2 shows a gas-turbine power plant with catalytic treatment for NO2 reduction according to one embodiment of the present invention;
FIG. 3 shows a gas-turbine power plant with waste-heat steam generator and catalytic exhaust gas purification according to a further embodiment of the following invention;
FIG. 4 shows a device according to an embodiment of the present invention;
FIG. 5 shows a further device according to an embodiment of the present invention;
FIG. 6 shows the concentration course of NO and NO2 in relative thermal equilibrium in the exhaust gas of a gas turbine and also the concentrations of NO and NO2 downstream of an NO2-reduction catalyst according to an embodiment of the present invention;
FIG. 7a shows the concentrations of NO and NO2 of example 2;
FIG. 7b shows the temperature course and the space velocity SV of example 2; and
FIG. 7c shows and the concentration course of hydrocarbons of example 2, measured as Cl fraction.



DETAILED DESCRIPTION
The method according to embodiments of the invention for reducing the concentration of nitrogen dioxide in an exhaust gas from an incomplete combustion of hydrocarbons or hydrocarbon mixtures, wherein an exhaust gas containing nitrogen dioxide, NO2, and uncombusted hydrocarbons, UHCs, is passed over a catalyst 1 for NO2 reduction which reduces NO2 by reaction with UHCs and in the process oxidizes UHCs, is distinguished in that the nitrogen dioxide present in the exhaust gas is reduced in the presence of the catalyst 1 by oxidation of UHCs at least to nitrogen monoxide, NO, in which the catalyst 1
(i) has a storage temperature TUHCabs, above which it stores UHCs and also partial oxidation products of UHCs,
(ii) has an activation temperature Tred, above which it catalyzes the reduction of NO2 to at least NO by oxidation of the UHCs, and
(iii) has a working temperature Tox, at which oxidation of UHCs with oxygen, O2, in the exhaust gas starts,
in which the storage temperature TUHCabs for the storage of UHCs is below the activation temperature Tred for the reduction of NO2, as shown in equation (I):

TUHCabs<Tred,  (I)

and

the working temperature Tox for the oxidation of the UHCs with O2 is above Tred, as shown in equation (II):

Tox>Tred.  (II)

The device according to embodiments of the invention for reducing the concentration of nitrogen dioxide, in an exhaust gas from the incomplete combustion of hydrocarbons or hydrocarbon mixtures, is distinguished in that the device has a catalyst 1 for NO2 reduction, which catalyst

    
    
        (i) has a storage temperature TUHCabs, above which it stores uncombusted hydrocarbons, UHCs, and also partial oxidation products of UHCs,
        (ii) has an activation temperature Tred, above which it catalyzes the reduction of NO2 to at least nitrogen monoxide NO by oxidation of the UHCs, and
        (iii) has a working temperature Tox, at which oxidation of UHCs with oxygen, O2, in the exhaust gas starts,
        in which the storage temperature TUHCabs for the storage of UHCs is below the activation temperature Tred for the reduction of NO2, as shown in equation (I):

TUHCabs<Tred,  (I)

        and
        the working temperature Tox for the oxidation of the UHCs with O2 is above Tred, as shown in equation (II):

Tox>Tred.  (II)

    
    


As a result of the working temperature Tox of the catalyst in the method according to embodiments of the invention and the device according to embodiments of the invention being such that it is above the activation temperature Tred, it can be ensured that, at least some of the time, a reduction of the concentration of NO2 by available UHCs is ensured. In certain applications and embodiments, the concentration of nitrogen dioxide can be reduced to a value less than 100 ppm, preferably 50 ppm, and in particular 20 ppm, for example for diesel units, or 10 ppm, preferably 8 ppm, and in particular 6 ppm, for example for gas turbines.
In particular embodiments, the method according to embodiments of the invention can be operated at a temperature below Tox, in order that a reduction of NO2 concentration that is as complete as possible can be ensured. However, it is also possible that in such embodiments, the operating temperature can briefly exceed the working temperature Tox, for example for a period of less than one hour, preferably less than 30 minutes, further preferably less than 20 minutes and particularly preferably less than 10 minutes. The more rapidly the temperature in such embodiments can be brought back below the temperature Tox, the more rapidly an improved reduction of the NO2 concentration can then be achieved. In addition, it can be possible that in such embodiments, the operating temperature can exceed the working temperature Tox for a relatively long period, if, for example, the combustion generating the exhaust gas proceeds completely and therefore NO2 is not formed at all, or is only formed in concentrations so low that a reduction is not necessary.
Likewise, in certain embodiments, it can be preferred to carry out the method according to embodiments of the invention at a temperature greater than or equal to the activation temperature Tred, in order that nitrogen dioxide can be reduced as well as possible. However, it is also possible to carry out the method according to embodiments of the invention for a short time, for example for a period of less than 30 minutes, preferably less than 20 minutes, more preferably less than 10 minutes, and particularly preferably less than 5 minutes, at a temperature below Tred, but it can be preferred to bring the temperature rapidly back to a temperature greater than or equal to Tred.
In certain embodiments, it can be possible to operate the method according to embodiments of the invention principally, for example more than 95% of the time period, in a temperature range between Tred and Tox. In certain embodiments, it can be possible to operate the method according to embodiments of the invention at a time average at a temperature in a temperature range between Tred and Tox.
However, in certain embodiments, it can also be possible to carry out the method according to embodiments of the invention over a longer time period at a temperature below Tred, for example when a store for storing NO2 is present which stores NO2 at a temperature below Tred, provided that the temperature is increased to a value above or equal to Tred, when the storage capacity of the store for storing NO2 is exhausted. A store for storing NO2 can in this case be the catalyst 1 or a further catalyst, for example a catalyst 3 for storing NO2, or a different store for storing NO2, for example an absorption liquid.
In the method according to embodiments of the invention, the concentration of NO2 at the catalyst can be reduced either completely or else only in part. Also, the NO2 can be further reduced to nitrogen N2, in which, however, it is reduced at least to nitrogen monoxide NO, in order to prevent the occurrence of yellow plume due to NO2 emission. In certain embodiments, the reduction of NO2 by UHCs to NO can be preferred, since in this case a lower energy of activation is required in comparison with the complete reduction to N2. However, in certain embodiments, a complete reduction of the NO2 to N2 can be preferred from the environmental aspect, in order to avoid the back-formation of NO2 from NO in the atmosphere after release of the purified exhaust gas, provided that the necessary conditions for the catalyst 1 according to embodiments of the invention are ensured. Likewise, in certain embodiments, it can be possible that the total amount of NOR, which comprises NO and NO2, can be reduced.
In addition, the oxidation of the UHCs in the method according to embodiments of the invention can be either a partial, or else a complete, oxidation of the UHCs. In certain embodiments, the oxidation can be a complete oxidation, since subsequent oxidation of the partial oxidation products of the UHCs, such as, for example, aldehydes and carbon monoxide, CO, which can be harmful to the atmosphere and/or the environment, can be avoided thereby. In certain embodiments, therefore, preferably complete oxidation of UHCs to carbon dioxide, CO2, can take place.
As UHCs in the method according to embodiments of the invention, both un-combusted hydrocarbons alone, and mixtures of uncombusted hydrocarbons from the incomplete combustion of hydrocarbons or hydrocarbon mixtures, come into consideration. In the method according to embodiments of the invention, the hydrocarbons and hydrocarbon mixtures in the combustion are not particularly restricted, as is also the case for the uncombusted hydrocarbons and hydrocarbon mixtures.
For example, hydrocarbons and hydrocarbon mixtures which are combusted can comprise petroleum, natural gas, diesel, heavy oil, crude oil, methane, ethane, or else partially substituted hydrocarbons such as methanol or ethanol. The hydrocarbons in this case can comprise any number of carbons, for example 1 to 40 carbon atoms, and can be branched or unbranched, saturated or unsaturated, substituted or unsubstituted or aromatic or cyclic. In this case, it is not excluded that the hydrocarbon mixtures for combustion can also comprise other substances, such as, for example, carbon, sulfur, metals or other impurities, such as also substituted hydrocarbons such as, for example, thiols, alcohols, aldehydes, ketones, amines, nitro compounds, etc. Such further substances can, in certain embodiments, be found in the exhaust gas, but can also in certain embodiments be filtered upstream of the catalyst 1 and/or the catalysts 1 and/or 2 and/or 3. In certain embodiments, it is preferred to remove substances which adversely affect the catalyst 1 and/or the catalysts 1 and/or 2 and/or 3, for example as catalyst poison, upstream of the catalyst 1 and/or the catalysts 1 and/or 2 and/or 3.
The hydrocarbons and hydrocarbon mixtures provided for combustion can, in the context of embodiments of the invention, also be called fuels, provided that nothing else results from the context.
Uncombusted hydrocarbons can be, for example, hydrocarbons having 1 to 20, preferably 1 to 10, further preferably 1 to 4, carbon atoms, for example methane, ethane, propane, propene, butane, 1-butene, 2-butene, pentane, pentene, cyclopentane, hexane, benzene or toluene. Likewise, the uncombusted hydrocarbons and uncombusted hydrocarbon mixtures can be partially oxidized reaction products from the combustion of hydrocarbons and hydrocarbon mixtures which are still capable of a further oxidation, for example aldehydes and ketones. The hydrocarbons in this case can be saturated or unsaturated and branched or unbranched, or else cyclic or aromatic. Also, the hydrocarbons, in certain embodiments, can be substituted. In certain preferred embodiments, hydrocarbons are preferably fed to the catalyst 1 and/or the catalysts 1 and/or 2 and/or 3 which do not adversely affect this and/or these.
High NO2 emissions in an exhaust gas, in the method according to embodiments of the invention, in particular in the operation of gas turbines or systems with natural gas or oil in the region of low partial load (<50%) and also at the start or other changes in load, can be reduced preferably by the combination of at least two measures in such a manner that visible discoloration of the exhaust gas plume no longer occurs and at the same time all legal limiting values are met. Changes in load or low partial loads can also occur, for example, in other systems according to embodiments of the invention, for example in systems which feed electrical power into the energy grid, or in systems, the load of which need not be continuously the same, or ships, which throttle their speed. Low loads can occur, for example, in power plants based on the combustion of hydrocarbons or hydrocarbon mixtures if a high amount of energy from fluctuating renewable energies, for example wind power and/or solar power, are placed into the energy grid, and therefore less energy from such power plants is required.
In certain embodiments, an exhaust gas, for example a gas-turbine exhaust gas, can be passed over a catalyst 1 having the following properties: above an activation temperature Tred, it reacts NO2 with uncombusted hydrocarbon emissions UHCs of the gas turbine and reduces it at least to NO, in which process the UHCs are oxidized. Above a working temperature Tox>Tred, the oxidation of UHCs with unconsumed oxygen O2 in the exhaust gas begins.
In preferred embodiments, the rate of reaction for the oxidation with O2 can be below that with NO2, provided that the NO2 concentration is above the desired emission limiting value. For example, the emission limiting value for the method according to embodiments of the invention can be 100 ppm, preferably 50 ppm, more preferably 20 ppm, particularly preferably 10 ppm and in particular 8 ppm, more particularly 6 ppm. In this manner, an improved reduction in the NO2 concentration can be achieved.
In addition, the catalyst 1 in the method according to embodiments of the invention can have the property, above a low temperature TUHCabs<Tred of storing UHCs and the reaction products of the partial oxidation of UHCs, such as, e.g., aldehydes, and liberating them at a temperature TUHCdes preferably above, in particular not until markedly above, the working temperature for the oxidation Tox, as shown in equation (III).

TUHCdes>Tox  (III)

For example, in preferred embodiments, the catalyst 1 can release the UHCs and the reaction products of the partial oxidation of UHCs at a temperature which is 20° C., preferably 50° C., and further preferably 100° C., above Tex. In such an embodiment, it can, in particular, be ensured that even at low temperatures, a stock of UHCs is present in the catalyst 1 for NO2 reduction, even if in operation of a system in the method according to embodiments of the invention, a deficit of UHCs is present, in comparison with NO2. Also, in such embodiments, owing to the storage of UHCs at low temperatures, more UHCs can be available, if the temperature increases to Tred, and so then the reduced amount of NO2 can be increased.

In preferred embodiments, the catalyst 1 can in addition have the property, above a likewise low temperature TNO2abs, which is below Tred, as shown in equation (IV) of also storing NO2 and only liberating/desorbing it at a temperature TNO2des above the activation temperature for the reduction, as shown in equation (V).

TNO2abs<Tred  (IV)

Tred<TNO2des  (V)

In such exemplary embodiments, a further reduction in the NO2 concentration is possible at low temperatures such as, for example, during the start of the combustion of hydrocarbons, for example in a system according to embodiments of the invention at ambient temperatures in the range from −40° C. to +40° C., optionally in the range from 10 to 35° C., optionally in the range from 20 to 30° C. In such embodiments, it can be preferred that the temperature TNO2des is lower than Tox, in order to ensure that during the desorption/release of NO2, sufficient UHCs are present for NO2 reduction. In preferred embodiments, TNO2abs is lower than the temperature at which the exhaust gas is passed through the catalyst 1.

In certain embodiments, for example at the start of the combustion, control of the method via the temperature also may not be possible, and so it can be preferred in such embodiments to generate more UHCs at the start, which can then be stored, in order that they are then available for NO2 reduction when the temperature is elevated. After achieving a suitable NO2 reduction, the amount of generated UHCs can then be decreased again.
In addition, the combustion in the method according to embodiments of the invention can be controlled in certain embodiments in such a manner that the time average UHC concentration, for example during the start phase and phases of low partial load, is above the time average of the NO2 emission by a certain factor γ, which factor can be calculated according to equation (VI) from the concentrations of UHCs XUHC and NO2XNO2.

γ=XUHC/XNO2>1  (VI)

The value of this factor can depend in this case, for example, on the fuel used in the combustion of hydrocarbons or hydrocarbon mixtures, for example natural gas or oil, on the catalyst material in respect of, for example, the composition, crystal structure and/or specific surface area thereof, and/or on the catalyst temperature. Also, the factor γ, in certain embodiments, can result, for example, via the amount/ratios of the materials added to the combustion such as air and hydrocarbons or hydrocarbon mixtures, and/or via the feed temperature of, for example, air and/or via control of the temperature in the combustion, for example in a combustion chamber BK. In certain embodiments, however, it is also possible that the factor γ briefly, for example in the range from 1 minute to 30 minutes, preferably in the range from 5 minutes to 10 minutes, adopts a value less than 1, if it then is reset to a value greater than 1.
In certain embodiments, the exhaust gas, downstream of the catalyst 1, can be passed over a catalyst 2 for the oxidation of UHCs, which catalyst 2 can oxidize UHCs and/or oxidation products of UHCs, for example by oxygen. In this manner, the release of UHCs or oxidation products of UHCs, which can also be environmentally harmful, can be lowered or reduced. In preferred embodiments, the catalyst 2 can catalyze the oxidation of the UHCs and/or oxidation products of UHCs at a temperature which the exhaust gas has after the passage through catalyst 1. In certain embodiments, the catalyst 2 can carry out the oxidation of the UHCs and/or oxidation products of UHCs at a temperature which is below or equal to Tred. In certain embodiments, the catalyst 2 can advantageously in addition also oxidize carbon monoxide CO, at least in part, to carbon dioxide CO2. Suitable catalyst materials for such a catalyst 2 can be suitably used by a person skilled in the art.
Likewise, in certain embodiments, it can be possible to pass the exhaust gas upstream of the catalyst 1 over a catalyst 3 for NO2 storage, which catalyst 3, in preferred embodiments, stores nitrogen dioxide at the temperature TNO2abs and releases it at the temperature TNO2des. In these embodiments, an additional NO2 storage is possible, as a result of which the concentration of NO2 in the exhaust gas can be further reduced. Via the release of NO2 at a temperature TNO2des above Tred, it can be possible in certain embodiments to reduce the NO2 concentration further, since in such embodiments, the NO2 is only passed to the catalyst 1 at a temperature above Tred. In preferred embodiments, TNO2des is below Tox, in order to ensure that the NO2 is passed to the catalyst 1 at a temperature below Tox. In certain embodiments, the exhaust gas can be passed upstream of the catalyst 1 also via one or more further NO2 storage media in addition to, or instead of, catalyst 3, for example an NO2 sorption medium, or a solid storage medium, wherein the additional storage media can also in certain embodiments have the temperature properties of catalyst 3 with respect to TNO2abs and TNO2des. In preferred embodiments, TNO2abs is lower than the temperature at which the exhaust gas is passed through the catalyst 3 and/or an additional storage medium. In certain embodiments, the catalyst 3 and a further NO2 storage medium can also be provided. Suitable catalyst materials for the catalyst 3 can suitably be determined by those skilled in the art on the basis of simple experiments, for example in measuring systems. Also, furthermore, further storage catalysts and/or other catalysts for exhaust gas purification can be provided in certain embodiments.
In certain embodiments, it can be preferred to provide measures for setting the catalyst temperature to a temperature below Tmax, for example of the catalyst 1 and/or of the catalyst 2 and/or of the catalyst 3: for example, catalysts having a good storage capacity for UHCs and/or NO2 can react with a loss of storage capacity and/or catalytic activity at temperatures above a threshold Tmax which can be below the maximum exhaust gas temperatures, for example of gas turbines. In such preferred embodiments, it can therefore be possible to prevent damage to one or more catalysts which otherwise could lead to a lowering of the reduction of the NO2 concentration.
In certain embodiments it can be possible to exploit the storage capacities of the catalyst 1 and/or of the catalysts 1 and/or 3 and/or further storage catalysts and/or storage media for a certain time period when the temperature falls below a certain temperature. In certain embodiments, however, it is preferred to control the method within the defined conditions of the method via corresponding control of the temperature and/or of the pressure and/or of the materials streams and/or of the concentrations of the substances in the exhaust gas, in such a manner that the stores can be constructed so as to be smaller.
In certain embodiments, therefore, for example in gas-turbine power plants, measures can be accepted/used for heat recovery/heat exchangers such as, e.g., steam generators, upstream of the catalysts, for example catalytic reactors, in order to ensure that the exhaust gas temperature in the catalyst is below Tmax. If no measures for heat recovery can be provided, or they are not provided, in certain embodiments it can be preferred to use less sensitive catalysts which can be operated up to the maximum exhaust gas temperature downstream of the combustion of the hydrocarbons or hydrocarbon mixtures, preferably without loss of activity, in which, therefore, Tmax is higher than the maximum exhaust gas temperature.
In certain embodiments, it is also possible to take measures to increase or decrease the temperature upstream of one or more catalysts to a defined value, for example to a value between Tred and Tox in the case of the catalyst 1 and/or to a value between TNO2abs and TNO2des in the case of the catalyst 3.
The measures for temperature elevation can proceed in certain embodiments via one or more heat exchangers/heat interchangers. The type of the heat exchanger in this case is in no way restricted. For example, it is possible, in a device according to embodiments of the invention comprising a catalyst 1 and/or a catalyst 2 and/or a catalyst 3, to provide respectively one or more heat exchangers upstream and/or downstream of the catalyst (1) and/or optionally upstream and/or downstream of the catalyst (2) and/or optionally upstream and/or downstream of the catalyst (3). Thus 0 to 30, preferably 4 to 20, and particularly preferably 10 to 15 heat exchangers can be provided, for example.
In certain embodiments, the temperatures upstream and/or downstream of the catalysts can also be adjusted additionally or solely suitably by adjusting the pressure upstream and/or downstream of the catalysts.
In certain embodiments, at least one heat exchanger is present respectively upstream of each catalyst and downstream of the last catalyst over which the exhaust gas is passed.
In certain embodiments, the temperature preconditions and/or reaction rate preconditions in the method according to embodiments of the invention and/or the device according to embodiments of the invention can be adjusted by suitable material selection of the catalyst/catalysts. In certain embodiments, the requirements of temperatures and/or reaction rates can be ensured by differing material selection of the catalysts.
The storage of UHCs and optionally NO2 in the catalyst 1 and/or optionally of NO2 in the catalyst 3 for NO2 storage can proceed according to certain embodiments not only by absorption but also by adsorption on the catalyst, which in each case can be due to chemical and/or physical processes. The composition and crystal structure of the catalysts 1 and/or 3 used for the storage can preferably be adjusted in such a manner that storage and regeneration proceed reversibly by nonreactive or reactive release of the stored substances in the method according to embodiments of the invention. In particular, the materials properties can be adjusted in such a manner that the catalyst 1 or else other storage catalysts, for example the catalyst 3 for NO2 storage, in the method according to embodiments of the invention are not damaged by the storage and regeneration by nonreactive or reactive release of the stored substances.
In the context of embodiments of the invention, a reference to the catalyst 1 and/or the catalyst 2 and/or the catalyst 3 can also be described by the reference to catalysts 1 and/or 2, provided that nothing else results from the presentation.
In certain embodiments, it can be possible that one or more catalysts 1 and/or one or more catalysts 2 and/or one or more catalysts 3 and/or one or more heat exchangers are provided in the method according to embodiments of the invention and/or the device according to embodiments of the invention. The catalysts and/or heat exchangers can here be identical or different.
In certain embodiments, it is additionally possible that the catalysts 1 and/or 2 and/or 3 are combined on a support, by, for example, a catalyst 1 being impregnated on one side and/or on opposite sides with a solution and/or an emulsion and/or a suspension of the catalyst material 2 and/or 3 and then thermally treated in such a manner that the entire catalyst 1 is not covered by the catalyst material of the catalyst 2 and/or the catalyst 3. For example, in this manner, various zeolites can be applied to cordierite supports. In such embodiments, a catalyst can then have differing zones each of which corresponds to a catalyst 1 and/or a catalyst 2 and/or a catalyst 3.
The catalyst materials for the catalysts 1 and/or 2 and/or 3 in the method according to embodiments of the invention and/or of the device according to embodiments of the invention are not particularly restricted, provided that they meet the required temperature conditions. Suitable catalyst and storage materials, characterized, for example, by composition, crystal form and/or surface properties, can be determined by those skilled in the art on the basis of known experimental methods such as temperature-programmed reaction, temperature-programmed absorption and temperature-programmed desorption, stress tests and long-term studies. Such studies may be carried out, for example, in measuring systems to which gas mixtures having a controllably adjustable composition, controllably adjustable gas volumetric stream and controllably adjustable temperature can be passed through a material-sample-containing reactor with controllably adjustable temperature, and can be quantitatively analyzed for the change in composition using suitable gas analysis technology.
For example, the NO2 storage properties and/or the UHC storage properties of a catalyst can be influenced via the acidity and/or pore structure thereof. Also, the storage properties can be adjusted via the structure of crystal pores, such as, for example, in the case of zeolites.
For example, titanium dioxide TiO2 exhibits good UHC storage which, in addition, may be stabilized, for example by tungsten trioxide WO3, in such a manner that it is usable in the full exhaust gas temperature range of the gas turbine. At the same time, such a material also exhibits a storage capacity for NO2. Via a higher content of WO3, in addition, catalyst materials can be obtained for applications at relatively high temperatures, for example up to above 650° C. Further mixed oxides of titanium dioxide with, for example, zirconium dioxide or aluminum trioxide, such as TiO2/ZrO2 or TiO2/Al2O3 are known to those skilled in the art as a basis for the catalyst production and of course can be likewise used.
If such a base material is admixed, for example, with vanadium pentoxide V2O5, reduction of NO2 by UHCs and oxidation of UHCs can occur. The activation and working temperatures Tred and Tox of these reactions fall generally with increasing V2O5 content of the catalyst. If the V2O5 content is selected to be too high, then the UHC oxidation predominates over the NO2 reduction, and so in a broad temperature range, although a good reduction of the UHC emissions can be achieved, sufficient reduction of the NO2 emissions can no longer be achieved. For example, in certain embodiments, at a V2O5 content of 1.7% by weight, even at relatively high temperatures, good NO2 reduction can be ensured, whereas, at a V2O5 content of 3% by weight, at temperatures above 400° C., in certain embodiments a decrease in the NO2 reduction can already occur.
Furthermore, zeolites such as those of the ZSM5 type exhibit excellent adsorption capacities for UHCs and with an appropriate doping with catalytically active components such as, e.g. Cu, Fe, Pt, W, In, or Ag (e.g. by impregnation with corresponding salts) also exhibit a good conversion rate of NO2 to NO and sometimes even complete reduction of the nitrogen oxides to N2. H-ZSM5 is particularly advantageous as base component of the NO2 reduction catalyst, whereas the more widely distributed Na-ZSM5 is less advantageous for this application. Also, the zeolites are generally suitable for use at relatively high temperature, for example above 650° C.
Also, diverse catalyst materials can oxidize UHCs to form diverse products. Thus, for example, the Pt-NH4ZSM5 catalyst used in example 1 can oxidize UHCs to CO2, whereas the Fe-NaZSM5 catalyst used in example 1 or else the catalyst used in example 2 oxidizes UHCs to carbon monoxide.
In certain embodiments, the control of the method according to the invention and/or of a device according to the invention and/or of a system according to the invention can be in the range from 1 minute to 30 minutes, preferably in the range from 3 minutes to 10 minutes. In certain embodiments, however, a more rapid control can also take place.
A device according to the invention for reducing the concentration of nitrogen dioxide, in an exhaust gas from the incomplete combustion of hydrocarbons or hydrocarbon mixtures has at least one catalyst 1 for NO2 reduction, which catalyst

    
    
        (i) has a storage temperature TUHCabs, above which it stores UHCs and also partial oxidation products of UHCs,
        (ii) has an activation temperature Tred, above which it catalyzes the reduction of NO2 to at least NO by oxidation of the UHCs, and
        (iii) has a working temperature Tox, at which oxidation of UHCs with oxygen, O2, in the exhaust gas starts,
        in which the storage temperature TUHCabs for the storage of UHCs is below the activation temperature Tred for the reduction of NO2, as shown in equation (I):

TUHCabs<Tred,  (I)

        and
        the working temperature Tox for the oxidation of the UHCs with O2 is above Tred, as shown in equation (II):

Tox>Tred.  (II)

    
    


Furthermore, a device according to the invention, in certain embodiments, can have one or more catalysts 1 and/or one or more catalysts 2 and/or one or more catalysts 3 and/or one or more heat exchangers.
For example, a device according to the invention can, in certain embodiments, be a catalytic reactor for exhaust gas purification which is integrated into an exhaust gas line of a gas turbine, and which contains at least one catalyst 1.
In certain embodiments, the catalyst 1 and/or the catalyst 2 and/or the catalyst 3 can be constructed as honeycomb catalyst, plate catalyst, etc., in which, for example, cell density or plate separation and length of the catalyst are selected in such a manner that the exhaust gas residence time under the relevant operating conditions in certain embodiments exceeds 30 ms and the catalytic conversion of NO2 with UHCs in the catalyst 1 is neither transport-limited nor is limited significantly by the conversion frequency of the catalyst, but on the other hand the exhaust gas counterpressure under full load is below a value which can be tolerated for the efficiency of the gas turbine, typically 5 to 10 mbar.
The catalyst 1 or the catalysts 1 and/or 2 and/or 3, in certain embodiments, can be extruded solid catalysts having thin cell walls or ceramic or metal supports of low wall thickness coated with catalyst material.
The device according to the invention can be used in systems in certain embodiments. Systems comprising a device according to the invention are also disclosed.
Examples of such systems can be gas-turbine power plants, compressors operated by one or more gas turbines, gas- or oil-operated boilers, gas engines or ship engines operated by the combustion of diesel or heavy oil.
In such systems, in certain embodiments, one or more devices for heat recovery and/or for heat exchange can be provided in the exhaust gas line of the system.
Such devices for heat recovery and/or heat exchange, in certain embodiments, can contain at least one-piece heat exchangers which come into contact with the exhaust gas. For example, such a device for heat recovery can in certain embodiments be a waste-heat steam generator.
Systems according to the invention can in certain embodiments also be district heating power plants with power-heat coupling, wherein, in such embodiments, careful matching with the other catalysts and the process can be necessary.
An exemplary system in which a device according to the invention can be provided is a gas-turbine power plant, as is shown in FIG. 1, which comprises a generator G, a compressor KP for compressing air L and a combustion chamber BK for operating a gas turbine G with a fuel B, a hydrocarbon or a mixture of hydrocarbons. Exhaust gas from the gas turbine GT in such a system is passed into the stack K via a diffuser D.
In certain embodiments of the invention, a device comprising a catalytic reactor KR can be introduced into such a system downstream of the diffuser D, as shown in FIG. 2.
In preferred embodiments, as shown in FIG. 2, for example, the device having the catalytic reactor KR is not mounted in the stack K, or is at least not mounted in the stack K in such a manner that the catalyst material can be exposed to rain, since rain can damage or destroy the catalyst or the catalysts, for example in consequence of a pressure explosion by evaporating rain. Damage can also occur in certain embodiments by strong temperature variations, since catalysts are sensitive to temperature shocks.
A further embodiment according to the invention is shown in FIG. 3 in which, downstream of the diffuser, a waste-heat steam generator AD is provided, which has a catalyst 1 and four heat exchangers WT1, WT2, WT3 and WT4. In certain embodiments, however, a waste-heat steam generator can also comprise one or more catalysts, for example catalysts 1 and/or 2 and/or 3, and also one or more heat exchangers, in which case the heat exchangers can be arranged in different ways.
An exemplary embodiment of a device according to the invention is shown in FIG. 4, in which the exhaust gas flows from left to right successively through a catalyst 3, a catalyst 1 and a catalyst 2. In FIG. 5, a further exemplary embodiment of a device according to the invention is shown in which heat exchangers WTA, WTB, WTC and WTD are additionally provided upstream of, between and downstream of the catalysts 1, 2 and 3. However, in certain embodiments it is also possible to provide more catalysts, for example catalysts 1, 2 and 3, and/or more or fewer heat exchangers and/or to dispense with the catalyst 2 and/or the catalyst 3. The device shown in FIG. 5 can have advantages in certain embodiments, in particular during start-up/running-up, or during non-steady state operation of a system, since nitrogen dioxide can be stored. Also, the temperature in the catalysts can be controlled better via the heat exchangers.
The effect of the use of a method according to the invention or a device according to the invention on the NO and NO2 concentration downstream of a catalyst in certain embodiments according to the invention is shown by way of example in FIG. 6 in comparison with the equilibrium concentrations of the two substances in a gas turbine, wherein this Figure, however, does not limit the invention.
The above embodiments, configurations and developments may be combined as desired with one another, where this is meaningful. Further possible configurations, developments and implementations of the inventions also comprise combinations that are not cited explicitly of features of the invention described with respect to the exemplary embodiments hereinbefore or hereinafter. In particular, those skilled in the art will also add individual aspects as improvements or supplementations to the respective basic form of the present invention.
Exemplary Embodiments
The invention will now be described in more detail with reference to exemplary embodiments, wherein these exemplary embodiments in no way limit the invention.
Example 1
At a measuring system, a feed gas was passed as model exhaust gas having a composition of nitrogen N2/O2/H2O=82/8/10% by volume having the impurities NO, NO2 and a total hydrocarbon concentration HCtot according to tables 1 and 2 hereinafter over two different catalyst materials, Pt—NH4ZSM5 and Fe—NaZSM5, which were coated with a cell density of 400 cells per square inch (cpsi) on a cordierite support. In this case, Pt—NH4ZSM5 is a platinum-doped, ammonium-exchanged ZSM5 zeolite, and Fe-NaZSM5 is an iron-doped, sodium-ion-exchanged ZSM5 zeolite.
The space velocity in all experiments was 50,000/hour in relation to standard gas density. The total hydrocarbon concentration HCtot was measured by means of a flame-ionization detector and the concentration was reported as parts by volume in ppm Cl fraction.
The concentrations of NO and NO2 were determined by means of electrochemical sensors.
The gas composition was measured at various temperatures, as listed in tables 1 and 2. Per catalyst, in each case two series of measurements were carried out, one with temperature increasing (ramp up) uniformly from 100° C. to 600° C., and one with temperature falling (ramp down) uniformly from 600° C. to 100° C., wherein the temperature change was +/−10 K/min. The series of measurements with increasing and falling temperatures in this case also serve for determining the storage capacity of the catalyst materials for non-combusted hydrocarbons UHCs. In the tables, in addition, the fraction of nitrogen dioxide which was removed after passage through the catalyst, with respect to the feed gas, is stated deNO2 as in percent, and may be calculated as the difference between the NO2 concentration in the feed gas and the respective NO2 concentration downstream of the catalyst, which is divided by the NO2 concentration in the feed gas.











TABLE 1









Feed gas
Pt—NH4ZSM5 (incr.)
Pt—NH4ZSM5 (falling)


















T(° C.)
NO
NO2
HCtot
NO
NO2
HCtot
deNO2
NO
NO2
HCtot
deNO2





















100
98
172
1457
102
102
1238
41%
126
98
1148
43%


200
63
207
1310
111
23
882
89%
140
28
246
86%


300
61
209
915
152
49
98
77%
163
65
113
69%




















TABLE 2









Feed gas
Fe—NaZSM5 (increasing)
Fe—Na4ZSM5 (falling)


















T(° C.)
NO
NO2
HCtot
NO
NO2
HCtot
deNO2
NO
NO2
HCtot
deNO2





















100
98
172
1457
24
145
1410
16%
35
141
1430
18%


200
63
207
1310
155
61
1610
71%
133
52
1387
75%


300
61
209
915
213
7.7
1102
96%
199
3
590
99%


400
97
173
64
160
0
128
100% 
174
0
108
100% 









As can be seen from tables 1 and 2, by use of the two catalysts Pt-NH4ZSM5 and Fe-NaZSM5, the nitrogen dioxide concentration in the exhaust gas can be lowered after passage through the respective catalyst.
Comparison of the two tables shows that the Pt-NH4ZSM5 catalyst reaches its temperature optimum for nitrogen dioxide as soon as approximately 200° C., whereas the Fe-NaZSM5 catalyst does not achieve its temperature optimum until 400° C. This could be due to the fact that the platinum-containing catalyst already catalyzes the oxidation of UHCs with oxygen at lower temperatures, and at higher temperatures, therefore, chiefly UHCs are removed from the mixture by oxidation with oxygen and can no longer be used to a great extent for reduction of NO2. Also, a decrease in the total amount of NOx is observable in some cases.
In addition, it can be seen from tables 1 and 2 that in the case of the Pt-NH4ZSM5 catalyst at 100° C., and also in the case of the Fe-NaZSM5 catalyst at all temperatures, more NO2 can be removed with increasing temperature than with falling temperature, that is to say that less NO2 remains after passage through the catalyst, which may be due to the particular storage action of the catalysts for UHCs with increasing temperature (TUHCabs<Tred). In this case, the catalysts can store UHCs at a lower temperature, which UHCs are then available at the activation temperature Tred. In the case of falling temperature, however, no UHCs can be stored, since they can be first oxidized at the higher temperatures, or used by NO2 reduction, in such a manner that fewer UHCs can remain at lower temperatures.
Example 2
An exhaust gas of a gas turbine which was being operated in partial load, was subjected in side stream to a catalytic NO2 reduction with a catalyst based on TiO2/WO3N2O5, which contained 3% by weight V2O5. Ammonia, as is usual in the case of conventional SCR catalysts, was not added for the NO2 reduction. In order to study the effect of the gas residence time on NO2 reduction, the exhaust gas mass stream conducted in the side stream was varied systematically, which produced a change in the space velocity SV of the exhaust gas. The concentrations X [ppm] of NO and NO2 in the exhaust gas upstream (NO(in) and NO2(in)) and downstream (NO(out) and NO2(out)) of the catalyst and the temperature T and the space velocity SV of the exhaust gas and also the concentration of UHCs in the exhaust gas X[ppmCl], on the basis of the Cl fraction, were in this case measured or calculated from measured values for mass flow, temperature, gas composition, and catalyst volume, and are shown in FIGS. 7a, 7b and 7c. 
It can be seen from FIG. 7a that the NO2 concentration upstream of the catalyst during start-up of the gas turbine (within the first 10 minutes) reached a peak value of around 90 ppm and the NO2 was partly reduced catalytically to NO: the NO2 concentration downstream of the catalyst fell thereby to a peak value of around 30 ppm, while the NO concentration increased from around 10 ppm to 70 ppm. This corresponds to an NO2 reduction of 67%. A further reduction in the NO2 concentration could be achieved here, for example, by an additional NO2 storage catalyst. In the further course of the experiment up to the 52nd minute, degrees of decrease of up to 100% were achieved, and the NO2 concentrations downstream of the catalyst always remained below the limit of visibility. Thereafter the NO2 concentration increased downstream of the catalyst to around 15 ppm, which is due to the fact that the UHC concentration in the exhaust gas was, from around 30 minutes, already on average below the NO2 concentration upstream of the catalyst. An increase in UHC concentration in the gas-turbine exhaust gas from the 58th minute, then, with a slight time delay of only just 4 minutes, led again to an improved reduction in the NO2 emissions to below 3 ppm. This clearly provides evidence of the effect of UHC storage, in combination with a suitable control of the gas-turbine combustion, on the reduction of the NO2 concentration. As can be seen from the Figures, the NO2 concentration downstream of the catalyst, compared with the concentration upstream of the catalyst, by suitable control of the gas turbine and of the system, and also, for example, by providing a sufficient amount of UHCs in the gas-turbine exhaust gas, can be markedly decreased, and therefore also clearly the occurrence of a yellow plume of the emitted exhaust gas can be avoided. Also, the Figures show the processes of storage and release of UHCs in the catalyst which lead to the change in the amount of NO2 and NO in the exhaust gas downstream of the catalyst.
Conventional methods, as described, for example in EP 2 223 733 A1, proceed from NO2 emissions which are formed in concentrations up to the temperature-dependent thermodynamic equilibrium ratio between NO and NO2 by catalytic oxidation of NO. The solution proposed here for reducing NO2 emissions, for example in gas-turbine exhaust gases, in contrast, is based on the surprising finding that in the case of a catalytic activity for the oxidation of hydrocarbons by oxygen which is selected to be sufficiently low, an exhaust gas composition can be induced at which the NO2 concentration can be brought to a value which is far beneath that given by the thermodynamic equilibrium ratio between NO2 and NO for the respective temperature, as may be seen from FIG. 6, for example, for a gas turbine: at temperatures below 300° C., it would be expected that the nitrogen oxides are predominantly present as NO2.
The test results in FIG. 7, in contrast, provide evidence that, by catalytic treatment, NO2 may be predominantly reduced to NO.
In contrast to conventional methods, the solution presented here does not require the stocking and injection of reducing agent (ammonia or urea) because it is not based on an NH3—SCR method. In addition, in the most favorable case, for gas turbines, for example, only one catalytic section may be required in the exhaust gas line, which in addition can be significantly more compact than each of the sections in conventional methods, because the gas residence times required for NO2 reduction are only 25-50% of the gas residence times required for SCR or CO oxidation.
Therefore, the solution provided here can in principle be retrofittable, whereas a complete SCR system, for reasons of construction space, in many cases cannot be retrofitted.
Although the present invention has been disclosed in the form of preferred embodiments and variations thereon, it will be understood that numerous additional modifications and variations could be made thereto without departing from the scope of the invention.
For the sake of clarity, it is to be understood that the use of “a” or “an” throughout this application does not exclude a plurality, and “comprising” does not exclude other steps or elements. The mention of a “unit” or a “module” does not preclude the use of more than one unit or module.

## Claims

1. A method for reducing a concentration of nitrogen dioxide, NO2, 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 NO2 reduction which reduces NO2 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, TUHCabs, above which the catalyst stores UHCs and also partial oxidation products of UHCs,
(ii) has an activation temperature, Tred, above which the catalyst catalyzes the reduction of NO2 to at least NO by oxidation of the UHCs, and
(iii) has a working temperature, Tox, at which oxidation of UHCs with oxygen, O2, in the exhaust gas starts,

wherein the storage temperature TUHCabs for the storage of UHCs is below the activation temperature Tred for the reduction of NO2, as shown in equation (I):


TUHCabs<Tred,  (I)


and
wherein the working temperature Tox for the oxidation of the UHCs with O2 is above Tred, as shown in equation (II):


Tox>Tred.  (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, TUHCdes, above which the catalyst liberates stored UHC and also partial oxidation products of UHCs, which temperature is above Tox, as shown in equation (III):


TUHCdes>Tox.  (III)

4. The method as claimed in claim 1, wherein the catalyst has a temperature, TNO2abs, above which the catalyst stores nitrogen dioxide and which is below Tred, as shown in equation (IV):


TNO2abs<Tred,  (IV)

and has a temperature, TNO2des, above which the catalyst releases nitrogen dioxide and which is above Tred, as shown in equation (V):


Tred<TNO2des.  (V)

5. The method as claimed in claim 1, wherein a time average UHC concentration XUHC is above a time average of the nitrogen dioxide concentration XNO2 by the factor γ, which is given by the equation (VI):


γ=XUHC/XNO2>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, Tmax, above which, in the case of the catalyst for NO2 reduction, the storage capacity for UHCs and/or NO2 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 NO2 storage, which catalyst stores nitrogen dioxide at the temperature TNO2abs and releases it at the temperature TNO2des.

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, NO2, in an exhaust gas from the incomplete combustion of hydrocarbons or hydrocarbon mixtures, in which the device has a catalyst for NO2 reduction, the catalyst:
(i) has a storage temperature, TUHCabs, above which the catalyst stores uncombusted hydrocarbons, UHCs, and also partial oxidation products of UHCs,
(ii) has an activation temperature, Tred, above which the catalyst catalyzes the reduction of NO2 to at least nitrogen monoxide NO by oxidation of the UHCs, and
(iii) has a working temperature, Tox, at which oxidation of UHCs with oxygen, O2, in the exhaust gas starts, in which the storage temperature, TUHCabs for the storage of UHCs is below the activation temperature, Tred for the reduction of NO2, as shown in equation (I):


TUHCabs<Tred,  (I)


and
wherein the working temperature Tox for the oxidation of the UHCs with O2 is above, Tred, as shown in equation (II):


Tox>Tred.  (II)

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


TUHCdes>Tox.  (III)

12. The device as claimed in claim 10, in which the catalyst has a temperature, TNO2abs, above which the catalyst stores nitrogen dioxide and which is below Tred, as shown in equation (IV):


TNO2abs<Tred,  (IV)

and has a temperature, TNO2des, above which the catalyst releases nitrogen dioxide and which is above, Tred, as shown in equation (V):


Tred<TNO2des.  (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 NO2 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.