Process for the catalytic NOx reduction of a thermal engine, and device for said purpose

Abstract

In a process for the catalytic NOx reduction in oxygen-containing exhaust gases of a thermal engine in one or several reduction catalytic converters, hydrocarbons of the fuel of the thermal engine are used in the reduction catalytic converter(s). In order to avoid toxic or dangerous substances and gases, respectively, during said NOx reduction, the process is characterized by the use of a gas mixture as a reducing agent for the reduction catalytic converter(s), which gas mixture is produced in a controlled manner from the fuel of the thermal engine in a catalytic reformer and/or in a fuel cell.

Description

The invention relates to a process for the catalytic NO_x reduction in oxygen-containing exhaust gases of a thermal engine in one or several reduction catalytic converters, wherein hydrocarbons of the fuel of the thermal engine are used, as well as to a device for said purpose.

The worldwide increasing industrialization and the ever-growing number of motor vehicles and airplanes pollute the atmosphere to an increasing extent. In this connection, particular importance is attached to the nitrogen oxides forming during the combustion of fossil fuels which, to a significant extent, come from engines which are operated in a non-stationary manner. They are responsible for the formation of ground-level ozone and acid rain and have to be reduced drastically.

Measures with respect to engine technology (e.g. exhaust gas recirculation, common rail injection), which suppress the formation of harmful substances already during the combustion of the fuel/air mixture, are far from being able to implement the maximum emission values determined by the European Union in Euro 4 and 5. And since the three-way catalytic converter operates in an optimal manner only at an excess-air coefficient λ of nearly one, alternative techniques for downstream processes must be developed for the high-oxygen exhaust gases (λ>>1) of diesel and lean-mix Otto engines.

A proven method of reducing the NO_x emission in high-oxygen exhaust gases is the SCR process which has been used in power plant technology since the mid-70ies. In the classic variant, ammonia is used selectively, i.e. at a stoichiometric ratio, as a reducing agent. With the aid of a suitable catalyst (such as titanium dioxide-supported vanadate/tungstate catalysts or zeolites), the nitrogen oxides are converted into nitrogen and water at an exhaust-gas temperature of about 250 to 500° C.

A specific embodiment to be used with gas turbines is described in DE 198 10 275, wherein the reducing agent stored externally and separately, preferably the ammonia or urea which is commonly used for SCR, is injected together with the coolant and an additional additive in the area of the turbines.

Application in the motor vehicle sector—and also in the airplane sector—is rendered difficult since, under non-stationary conditions, a number of problems arise which are irrevelant in a stationary operation. One major problem associated with the use in motor vehicles is that a separate reducing agent tank must be carried along. Possible leakages in the tank and pipeline system must be regarded as extremely dangerous due to the toxicity of the ammonia. At present, the research and development activities concentrate on the use of NH₃-cleaving substances; in this context, urea, (NH₂)2CO and ammonium carbamate, NH₂CO₂NH₄, which are carried along as an aqueous solution or in solid form and which, if required, release ammonia via thermohydrolysis or thermolysis, respectively, can be mentioned. The use of ammonium carbamate is simple by comparison and is described, for instance, in WO-A-96/06674. The problem of a homogeneous distribution and metering as well as of a complete degradation of urea in the exhaust gas has already been solved as well; an invention concerning this matter is described, for example, in DE 199 13 462 or previously in EP 0 487 886 and EP 0 555 746.

However, SCR processes are also tested with non-nitrogenous reducing agents: alternative SCR and HC-SCR processes, respectively. In this case, hydrocarbons, among other things, are used as reducing agents, which hydrocarbons are removed from the fuel tank and are supplied to a cracking catalyst via a gas stream. The fuel is split into reactive hydrocarbons which are injected into the engine's exhaust gas and are reacted with the nitrogen oxides on catalysts containing noble metals:
NO+“CH₂”+O₂½N₂+CO₂+H₂O.

Here, it is problematic, however, that numerous side reactions occur, wherein, according to
2NO+‘CH₂’+O₂N₂O+CO₂+H₂O,
also the formation of noxious nitrous oxide in comparatively large amounts is possible (Th. Wahl: Katalysierte NO_x Entfernung mittels organischer Reduktionsmittel, Dissertation Universitat Karlsruhe, 1998). In U.S. Pat. No. 5,921,076, a mixing process is disclosed wherein the catalytic reduction of nitrogen oxides with noble metal catalysts at low temperatures is supported by the addition of hydrogen. However, according to this disclosure, hydrogen is thereby either stored separately or generated by thermal or catalytic cracking. This expansion improves the low-temperature kick-off function of the conversion of nitrogen oxide but retains a high risk of the formation of nitrous oxide due to the mixed operation and due to the reaction proceeding in some operating stages exclusively as an HC-SCR. A precise control of the composition of the reducing agent stream, which control can be compared to a catalytic reformer and is necessary for achieving high conversion rates across the entire operating area of the thermal engine, is impossible when using the method of producing hydrogen without any suitable sensor technology such as a measurement of the concentration of hydrogen, as illustrated in U.S. Pat. No. 5,921,076. In addition, Wahl emphasizes the uneconomicalness of the HC-SCR process which, for the reduction of one equivalent of NO_x, requires roughly 15-20 times the respective amount of hydrocarbons.

The reaction of nitrogen oxides with methane has turned out to be very effective (M. Jahn, Katalytische Stickoxidreduktion mit Methan, Diss., Techn. Univ. Berlin, 1999). CH₄ is able to transform the total amount of NO_x entirely into N₂, CO₂ and H₂O at temperatures of from 300 to 500° C. and residence times in the reactor of b=10.000 to 100.000 l/h. The intermediary step of an oxidation from NO to NO₂ is of particular importance, since only the far more reactive NO₂ is able to convert CH₄ into N₂. However, only zeolites are suitable catalysts for high conversion rates, which is why extra care must be taken that a water vapour content of 1.5% is not exceeded since otherwise a deactivation of the active catalyst surface might occur. This reduces the sole applicability in the field of typical internal combustion engines, since, in this case, the ideal combustion already produces water contents of approx. 6% (by weight) in the exhaust gas.

The use of hydrogen for the reduction of nitrogen oxides in the exhaust gas of stationary sources or also of mobile applications has also already been examined for several years. Experimental analyses which were conducted for a thesis relating to this topic have shown that nitrogen oxides of an oxygen-containing exhaust gas can be reduced with H₂ in the tubular honeycomb reactor which is used for purifying the exhaust gases of motor vehicles (A. Rogowski, Katalytische NO-Reduktion mit Wasserstoff in sauerstoffhaltigen Abgasen im Wabenrohrkatalysator, Dissertation, Techn. Univ. Berlin, 1992). The analyses were also carried out under technical operating conditions as customary for motor vehicles (T=400-500° C., space velocities and residence times, respectively, of b=10.000 l/h to b=100.000 l/h). Two reactions took place on the catalyst surface:
Reduction: 2NO+2H₂→N₂+2H₂O
Oxidation: 2H₂+O₂→2H₂O.
The reduction of NO occurred both in case of an ignited and in case of a quenched hydrogen-oxygen reaction. A formation of byproducts as observed in the HC-SCR, i.e., for example, a formation of nitrous oxide or nitric acid, could thereby not be observed.

B. Frank et al. describe in Applied Catalysis B: Environmental 19, p. 45-47, Elsevier Science BV, 1998, the reduction of nitrogen oxides by hydrogen and lean operating conditions on a Pt—Mo—Co/α-Al₂O₃ catalyst. In a temperature window of 140-160° C., a conversion rate of about 40 to 80% is thereby detected, with the oxygen content within a particular window playing a stimulating role for the NO/H₂ reaction.

In EP 0 881 367, a variant of monitoring the catalyst functionality is described. In this process, a hydrocarbon sensor (HC sensor) is mounted downstream of the catalytic converter which is a DeNO_x catalytic converter. The closed control circuit for this monitoring includes the HC sensor as an actual-value sensor, the metering control unit as a metering controller, the fuel metering device as an actuator and the DeNO_x catalytic converter as a controlled system. Said arrangement of the control circuit permits a precise and effective metering of the air/fuel mixture.

In a further document, namely DE 197 55 600, a control of the fuel/air ratio during the operation of an internal combustion engine comprising a catalytic converter and an exhaust-gas probe arranged behind the catalytic converter in the flow direction is described, i.e., that a lean operation and a greasy operation of the internal combustion engine are controlled alternately, wherein the extent of lubrication and/or the length of the fat phases is/are changed depending on the behaviour of the signal of said exhaust-gas sensor in a previous fat phase. In doing so, the engine is operated with a lean mixture on a temporal average.

If the exhaust-gas probe indicates an excess amount of reducing agent in a fat phase, the amount of the reducing agent to be supplied is reduced for a subsequent fat phase. If the reduction is sufficient, the exhaust-gas probe will no longer indicate an excess amount at the end of the subsequent fat phase. Thereupon, the amount of the reducing agent to be added in subsequent fat phases is increased successively until another response of the exhaust-gas sensor arranged behind the catalytic converter will occur.

DE 44 41 261 describes a process for the aftertreatment of exhaust gases. The metering device consists of a continuously conveying positive-displacement pump operated at a variable rotational speed controlled by the control means. Said positive-displacement pump has a cylindrical solid of rotation arranged in a cylinder and driven by an electric motor, which solid of rotation is provided, on its generated surfaces, with at least one flight leading into the cylinder from a reducing-agent inlet opening. The reducing-agent outlet opening is connected to a reducing-agent introduction point at the entrance of the catalytic converter.

The invention described in DE 103 00 555 relates to a deterioration detection means and to a deterioration detection process for a device for controlling the exhaust gases of internal combustion engines. The respective object consists in permitting a highly accurate detection in terms of the deterioration of the NO_x controllability of a catalytic converter, irrespective of the accuracy of the detection of an exhaust-gas concentration.

This means that, with this device and with this process, it is determined in the following way as to whether the catalytic converter has deteriorated. Simultaneously with the detection that the internal combustion engine is on the point of stopping, fuel is added to the exhaust-gas control device which is equipped with the catalytic converter. Due to the fuel to be added, the temperature of the catalytic converter, i.e., the temperature of the device for controlling exhaust gases, is raised in order to activate the catalytic converter. A period of time is measured which passes until the temperature of the catalytic converter drops to a predetermined temperature at which the activation of the catalytic converter stops. The time which passes from the addition of the fuel until the predetermined temperature has been achieved is compared to a time which passes in an equal manner from the addition of the fuel to a non-deteriorated catalytic converter until the temperature of the catalytic converter drops to a predetermined temperature after the increase caused by the addition of the fuel. On the basis of the comparison result, it is determined as to whether the catalytic converter has deteriorated.

Furthermore, it is known (DE 44 41 261 A1, EP 0 881 367 A) to provide reducing agents in the form of a fuel which is supplied to the catalytic converter and branched off from the fuel of the internal combustion engine in order to improve the performance of a catalytic converter which is provided in an exhaust pipe of an internal combustion engine. In this way, the performance of a catalytic converter can at best be improved but it is not possible to achieve an effective NO_x reduction in an exhaust gas of a thermal engine.

The invention aims at avoiding the above-described disadvantages and difficulties and has as its object to provide a process and a device for carrying out said process which enable an efficient reduction of the NO_x emission in an effective manner while avoiding the formation of toxic or at least dangerous substances and gases, respectively. In addition, the process and the device, respectively, should be easily controllable, i.e., they should be adjustable to various operating conditions of the thermal engine so that the invention can be used both for thermal engines in the stationary range and for thermal engines in the motor vehicle sector and in the airplane sector, respectively.

In a process of the initially described kind, said object is achieved according to the invention by using a gas mixture as a reducing agent for the reduction catalytic converter(s), which gas mixture is produced in a controlled manner from the fuel of the thermal engine in a catalytic reformer and/or in a fuel cell.

According to the invention, the nitrogen oxides are reduced in a downstream process of a catalytic method by means of a gas mixture used as a reducing agent, which gas mixture either is produced in a reformer from the fuel required for the internal combustion engine or accumulates as an anode exhaust gas of a fuel cell operated directly or indirectly with hydrocarbons and is introduced in the direction of the gas flow in front of or in the catalytic converter. The generation from the fuel renders superfluous the use of separate reducing agents such as, for example, ammonia or urea. According to the invention, the reformate consisting of H₂, CO, CH₄ and N₂ can be used directly as a reducing agent in the catalytic converter. Since there is no production of reactive hydrocarbons like in thermal or catalytic cracking processes, the occurrence of side reactions and the formation of byproducts is avoided. If the anode exhaust gas of fuel cells is used, an overall higher degree of efficiency is achieved as well.

Advantageous variants are characterized in subclaims 2 to 15.

A device for carrying out the process comprising at least one reduction catalytic converter provided in an exhaust pipe of a thermal engine, into which reduction catalytic converter a reducing-agent feed line runs, is characterized in that a reformer and/or a fuel cell is (are) provided, into which a fuel pipe runs which supplies fuel from a fuel tank into the thermal engine and from which reformer or from which fuel cell, respectively, the reducing-agent feed line originates.

Advantageous embodiments are characterized in subclaims 17 to 26.

The invention is explained in further detail below by way of several exemplary embodiments illustrated in the drawing in FIGS. 1 to 6.

A hydrocarbon mixture is located in a fuel tank 1 for a thermal engine designed as an internal combustion engine 2, which hydrocarbon mixture is used, on the one hand, in the internal combustion engine 2 and, on the other hand, serves, after an appropriate pretreatment for the “on-board” production of a reformate, as an exhaust-gas reducing agent for a catalytic converter 5 (FIGS. 1 to 3, 5 and 6) or as a fuel of a directly operated fuel cell 9a, 9b, respectively (FIGS. 2, 3 6), for example, in the form of an ancillary energy supply unit (APU=Auxiliary Power Unit).

According to the invention, the fuel from the fuel tank 1 is reacted in a reformer 4a, 4b by means of which long chain-like hydrocarbons can be converted into a mixture of CO and gases rich in H. The reforming plant comprises, for example, two reaction chambers. In the first chamber, the liquid fuel is mixed with combustion air and evaporated. Afterwards, the homogeneous mixture of fuel/air is catalytically reformed in the second reaction chamber directly (catalytically supported partial oxidation, 4a) and/or with the addition of water (ATR=Auto-Thermal Reactor, 4b). Since air instead of O₂ is used for the oxidation of the hydrocarbons, the resulting mixture of reformate and gas is composed of H₂, CO and, to a small extent, also of CH₄ and N₂.

The supply is effected via a metering valve 6 which is controlled by a metering control system 7 which is linked to the electronic engine management 7.2. A desulfurization means which optionally is provided in the fuel pipe from the fuel tank 1 to the reformer 4a, 4b is indicated by 3.

The reformate is fed directly into the catalytic converter 5 (FIG. 1) in order to enter there into different oxidation-reduction reactions with the NO_x of the exhaust gas, wherein, according to the prior art, the stream of reducing agent is controlled in all embodiments via an electronic management system depending on the concentration of nitrogen oxide in the exhaust gas. If a direct detection is impossible for technical measurement reasons, controlling is performed, also in accordance with the prior art, via indirect parameters such as the engine operating point or the temperature of the exhaust gas or the catalytic converter, respectively, or a combination of said variables.

In a further advantageous embodiment (FIG. 2), instead of directly utilizing the reformate, the anode exhaust gas of a fuel cell 9a, 9b operated by the fuel from the fuel tank 1, for example, of a solid oxide fuel cell 9a (SOFC) or of a molten carbonate fuel cell 9b, (MCFC), can also be used. In doing so, the composition of the reducing agents changes partially since the individual components are converted differently. Since, however, the efficiency of the fuel cell or of the stack, respectively, increases with a decreasing fuel utilization, an overall advantageous degree of efficiency can be achieved if the anode exhaust gas is utilized appropriately. In a particularly advantageous embodiment, the electric energy produced by the fuel cell 9a and/or 9b can thereby be used for the electric heating of the catalytic converters 5. A low-temperature catalytic converter 8 arranged downstream of the catalytic converter 5 designed as a high-temperature catalytic converter is supplied with reducing agent via a metering valve 6.1. The metering valve 6.1 is controlled via a metering control system 7.1 coupled to the electronic engine management.

In a further embodiment according to the invention comprising a reformer 4a, 4b as well as a fuel cell 9a, 9b, the composition of the reducing agent can, in addition, be influenced selectively by branching off a partial stream of the reformate from the reformer 4a, 4b in front of the fuel cell 9a, 9b and by mixing the same by means of a mixing valve 11.1 with the anode exhaust gas of the same fuel cell 9a, 9b in such a way that optimum characteristics will be achieved for the composition of the reducing agent stream supplied to the low-temperature catalytic converter 8 (FIG. 3). Using the mixing valve 11, anode exhaust gas of the fuel cell 9a, 9b can be admixed to the reformate of the reformer 4a, 4b and supplied to the catalytic converter 5.

According to a specific embodiment, the processing of the reducing agents by a separate reformer 4a, 4b can be omitted since the conversion of the fuel via internal reformation takes place in the fuel cell 9a, 9b and the anode exhaust gas is used accordingly as a reducing agent.

The function of a reformer is as follows:

As is known, in case of diesel and lean-mix Otto engines, the fuel which is burnt with air in the internal combustion engine 2 produces exhaust gases which comprise the harmful substances that are common for motor vehicles and, in addition, a high content of O₂. In a downstream process, the NO_x of these exhaust gases can be reduced to such an extent that they produce harmless N₂ and H₂O. For this purpose, the exhaust gases are conducted into a single-stage or multi-stage catalytic converter 5, 8 where they are reacted with the reformate containing a reducing agent.

The following partial reactions take place in the catalytic converter 5, 8 under the technical operating conditions which are common for motor vehicles:
2NO+2H₂→N₂+2H₂O  (equation 1)
½O₂+H₂→H₂O  (equation 2)
2NO+2CO→N₂+2CO₂  (equation 3)
2NO+O₂2NO₂  (equation 4)
2NO₂+2CH₄+O₂→N₂+2CO+4H₂O  (equation 5)
CH₄+ 3/2O₂→CO+2H₂O  (equation 6)
CO+½O₂→2CO₂  (equation 7)

The reactions (equation 1) and (equation 2) correspond to the oxidation-reduction reactions of hydrogen (described as mentioned above in: A. Rogowski, Katalytische NO-Reduktion mit Wasserstoff in sauerstoffhaltigen Abgasen im Wabenrohrkatalysator, Dissertation, Techn. Univ. Berlin, 1992).

The reaction (equation 3) is the long-known reduction of NO with CO, which is one of those reactions which also proceed in the catalytic exhaust gas purification in a three-way catalytic converter. In this case, it is true, however, that the CO is preferably oxidized to CO₂ if the mixture hitting the catalytic converter becomes increasingly “leaner” in fuel and thus richer in O₂, and that there no longer remains enough CO for the slower NO reduction. In the concrete case, the oxidation of the CO does not cause any disruptions since, besides the CO, other reducing agents are also available for the reduction of NO_x.

The reactions (equation 4) to (equation 7) are part of the reaction system of the reduction with methane (M. Jahn, Katalytische Stickoxidreduktion mit Methan, Diss., Techn. Univ. Berlin, 1999).

The gas mixture of the reformer thus constitutes a combination of three known reducing agents. The possibility of using the mixture directly renders superfluous any complex gas cleaning procedures and, furthermore, there is no need for tanks for intermediate storage.

The utilization of the reformate as a reducing agent can occur in an array of catalytic converter assemblies.

In an internal combustion engine 2 as common for vehicle applications, an assembly in the exhaust-gas strand will be chosen such that optimum operating temperatures of the reaction will be guaranteed. For the SCR reaction with hydrogen, an assembly close to the engine can be taken as a basis (FIG. 1), for the reduction of noble metal catalysts as described by B. Frank et al. and in U.S. Pat. No. 5,921,076, an assembly which is rather more remote from the engine might also be chosen due to the lower temperatures (FIG. 4). In a specific embodiment, a combination of both catalytic converter variants is provided in order to achieve the highest possible conversion rates of nitrogen oxide (FIGS. 2, 3 and 5).

In all cases, a thermal integration with the reformate or reformer 4a, 4b, respectively, or with the fuel cell 9a, 9b, which both typically exhibit a temperature level that goes significantly beyond the level of the exhaust gas catalyst of the main engine, which level is required for the progression of the catalytic reaction, will be important in terms of the temperature management of said catalytic converter.

In case of gas turbines 10 operated in a stationary manner and comprising a compressor 10/1, a combustion chamber 10/2 and a turbo set 10/3 (FIG. 6), as common in commercial-sized thermal plants, the introduction of the reducing agent, which, according to the invention, is produced from the fuel, can take place in analogy to the manner described in DE 198 10 275, and the assembly of the DeNO_x catalytic converter in the flue gas stream can be realized in a similar manner. Here, the advantage achieved according to the invention can be seen in the omission of the necessary external supply of reducing agent, since this involves a substantial transport effort which can be regarded as a “transport of hazardous materials” if ammonia is used as the reducing agent.

The application for mobile gas turbines as used for the purpose of airplane propulsion systems constitutes a specific embodiment according to the invention. In this application, the use of techniques for the aftertreatment of exhaust gases has so far been impossible, since the known SCR techniques require the incorporation of additional reducing agent tanks, which causes substantial application problems for this use. According to the invention, the reducing agent produced “on board” via the reformer or the fuel cell is thus introduced at an appropriate point into the turbine part, for example, via the cooling ducts. The respective catalyst surfaces are thereby located either on the unmoved parts such as guide blades and interior casing surfaces or, in case of appropriately stable catalyst surfaces, also on the turbine blades themselves. Via the overall endothermic reduction of the nitrogen oxides, an additional cooling effect can be caused in the turbo set in order to provide, together with the decrease in nitrogen oxide emissions, a basis for an increase in the turbine inlet temperature and hence in the degree of efficiency.

In turboprop engine embodiments, the additional arrangement of a catalytic converter arranged downstream in the flue gas stream constitutes a further embodiment according to the invention, since, in this case, the flue gas mass flow is not required for the actual forward movement.

The liquid and gaseous fuels used today exhibit sulfur contents of between 10 and 600 mg/kg of fuel (kerosene for airplane propulsion systems), whereby, at present, 350 mg/kg of fuel is considered as a threshold value for diesel whereas the threshold value for gasolines amounts to 150 mg/kg of fuel. A binding time schedule for the distribution of fuels with a reduced sulfur content is proposed by the European Commission in their KOM (2001) 241 as follows: area-wide supply of “low-sulfur” fuel having a maximum amount of 50 milligrams of sulfur per kilogram of fuels from Jan. 1, 2005, as well as of “sulfur-free” fuel having a maximum amount of 10 milligrams of sulfur per kilogram of fuel from Jan. 1, 2009.

Minimum sulfur contents in the fuel are relevent since, on the one hand, minor amounts of sulfur can already act as a catalytic poison and might substantially impair the service life of the catalytic converter and, in addition, might shift the balance between exothermic and endothermic reactions in a reformer in favour of the exothermic reaction. This could result in local and temporary temperature increases in the reformer which might destroy the substrate and the catalytic converter.

Thus, in a specific embodiment of the invention, it is envisaged that the fuel provided for the conversion in the reformer 4a, 4b or in the fuel cell 9a, 9b will be desulfurized. However, the desulfurization means 3 for the fuel also has positive effects on the composition of the exhaust gas and leads in particular to a reduction in the particulate emission. In a further advantageous embodiment, it is therefore possible to conduct the entire fuel of the thermal engine 2 or essential parts thereof via the desulfurization means 3, in addition to the fuel for the reformer 4a, 4b and/or the fuel cell 9a, 9b.

Claims

1. A process for catalytic NOx reduction in oxygen-containing exhaust gases of a thermal engine in one or several reduction catalytic converters, comprising using hydrocarbons of the fuel of the thermal engine in the reduction catalytic converter(s), and using a gas mixture as a reducing agent for the reduction catalytic converter(s), and producing the gas mixture in a controlled manner in a fuel cell from fuel of the thermal engine.

2. A process according to claim 1, further comprising producing the gas mixture in a controlled manner from the fuel of the thermal engine in a catalytic reformer and in a fuel cell.

3. A process according to claim 2, further comprising supplying the gas mixture produced in the catalytic reformer to the fuel cell for further processing.

4. A process according to claim 2, further comprising supplying a portion of the gas mixture produced in the catalytic reformer to the fuel cell and mixing a portion with the gas mixture processed in the fuel cell and using it as a reducing agent.

5. A process according to claim 2, wherein the catalytic reformer operates in accordance with catalytically supported partial oxidation.

6. A process according to claim 2, wherein the reformer is an Auto-Thermal Reactor (ATR).

7. A process according to claim 1, wherein the fuel cell is a solid oxide fuel cell (SOFC=Solid Oxide Fuel Cell).

8. A process according to claim 1, wherein the fuel cell is a molten carbonate fuel cell (MCFC=Molten Carbonate Fuel Cell).

9. A process according to claim 7, further comprising converting the fuel of the thermal engine directly in the fuel cell as an internal reformation and using the anode exhaust gas as a reducing agent.

10. A process according to claim 1, further comprising converting nitrogen oxides in a catalyst containing noble metals at temperatures of between 90 and 200° C. in the converters.

11. A process according to claim 1, further comprising converting in an SCR (Selective Catalytic Reduction) catalytic converter in a temperature range of from 350 to 600° C. in the converters.

12. A process according to claim 1, further comprising converting in two of the catalytic converters connected one after the other in series, wherein the converters comprise an SCR catalytic converter and a low-temperature noble metal catalyst, wherein each of the catalytic converters finds a flue gas temperature which is most suitable for it, and each converter is supplied with the reducing agent stream independently of the other one.

13. A process according to claim 1, wherein the thermal engine is a gas turbine for mobile use, the method further comprising injecting the reducing agent into the turbine, wherein the turbine includes parts selected from the group consisting of moving blades, guide blades and inner casing parts and the selected parts are shaped as catalytic surfaces.

14. A process according to claim 1, wherein the gas turbine is for stationary use, and the reducing agent is injected into the turbine, wherein turbine includes parts selected from the group consisting of moving blades, guide blades and inner casing parts and the selected parts are shaped as catalytic surfaces and the process further comprising converting by an SCR catalytic converter in a flue gas stream downstream of the turbine.

15. A process according to claim 1, wherein the gas turbine is for mobile use as a turboprop engine in an airplane propulsion system, and that the reducing agent is injected into the turbine, wherein turbine includes parts selected from the group consisting of moving blades, guide blades and inner casing parts and the selected parts are shaped as catalytic surfaces and the process further comprising converting by an SCR catalytic converter in a flue gas stream downstream of turbine.

16. A process according to claim 1, further comprising achieving a cooling effect in the turbo set via an endothermic oxidation-reduction reaction.

17. A process according to claim 13, wherein the fuel cell produces electrical power and the process further comprising using the electrical power of the fuel cell for electric heating of the catalytic converters.

18. A device for catalytic Nox reduction in oxygen-containing exhaust gases of a thermal engine, comprising at least one reduction catalytic converter provided in an exhaust pipe of a thermal engine, a reducing-agent feed line running into the reduction catalytic converter, a fuel cell into which a fuel pipe runs and operable to supply fuel from a fuel tank into the thermal engine and the reducing-agent feed line originates from the fuel cell.

19. The device according to claim 18, further comprising a reformer and the fuel cell are provided in series.

20. A device according to claim 18, further comprising a metering valve in the reducing agent pipe, which the metering valve being actuated via a metering control system which is linked to an electronic engine management.

21. A device according to claim 19, further comprising for ensuring the optimum operating temperature of the catalytic converter, at least one of the reduction catalytic converter the reformer and the fuel cell is thermally integrated in a casing via heat exchangers, or heat-conducting connections or structural integration.

22. A device according to claim 18, wherein the catalytic converter is a noble metal catalyst.

23. A device according to claim 18, wherein the converters include a high-temperature catalytic converter and a low-temperature catalytic converter connected in series, and each of the catalytic converters is flow-connected to at least one of the reformers and the fuel cell via a metering valve.

24. A device according to claim 19, further comprising mixing valves for mixing the reducing agents from the reformer and the fuel cell.

25. A device according to claim 18, further comprising an electronic control means operable for supplying the mass flow of the reduction gas mixture individually for each catalytic converter, the electronic controlling means comprising a solenoid valve and a controlling device for pulse-width control.

26. A device according to claim 19, further comprising a desulfurization system arranged upstream of at least one of the reformers and the fuel cell.

27. A device according to claim 19, further comprising a desulfurization system arranged in series in front of the branching of the fuel for at least one of the reformers and the fuel cell.

28. A device according to claim 19, wherein the reformer is an Auto-Thermal Reformer (ATR) operating according to the catalytically supported partial oxidation.

29. A device according to claim 18, wherein the fuel cell is a solid oxide fuel cell (SOFC) (9a) or a molten carbonate fuel cell (MCFC) (9b).