DE10342003A1 - Apparatus for processing a reducing agent precursor solution for exhaust aftertreatment - Google Patents

Abstract

The invention relates to a device (10, 50) for the treatment of a reducing agent precursor solution for the reduction of nitrogen oxides in the exhaust gas of an internal combustion engine (12). The apparatus (10; 50) is characterized by having a controllable heating element (32; 64) and a catalytic coating (38) which contacts the reducing agent precursor.

Description

The The invention relates to a device for processing a reducing agent precursor solution for Reduction of nitrogen oxides in the exhaust gas of an internal combustion engine.

A such device is known per se.

modern, Diesel technology developments are aimed at reducing the tailpipe emissions of diesel vehicles continue to cut drastically. These improvements can be made with Help motor measures, changes the consumables (e.g., by additive addition) but also with the help an exhaust aftertreatment taking place behind the internal combustion engine be achieved.

Generally known SCR systems (SCR = Selective Catalytic Reduction) provide an example of taking place behind the internal combustion engine exhaust aftertreatment In SCR systems, the proportion of nitrogen oxide in the exhaust gas of an internal combustion engine reduced by a selective catalytic reduction (SCR). To the exhaust gas is an immediate reducing substance such as Ammonia or a precursor supplied, the first reducing in the exhaust gas Releases substances. As a precursor, for example, a urea-water solution (HWL) be used. In a reaction of urea with the water (combined thermolysis and hydrolysis) is formed in addition to carbon dioxide (CO2) Ammonia (NH3), which is used in the selective catalytic reduction with nitric oxide (NO) and nitrogen dioxide (NO2) to molecular Nitrogen (N2) and water is converted. The selective catalytic Reduction takes place in an SCR catalyst.

A weakness Current SCR systems have limited low temperature activity. On the SCR catalyst The ammonia acts as a highly selective reducing agent for the reduction of Nitrogen oxides. This for the nitrogen oxide reduction necessary ammonia is, for example, from a urea-water solution as reducing agent precursor thermolytically or catalytically in Won SCR catalyst. The liquid HWL is initially in the exhaust gas atmosphere evaporated. This results from the urea used [CO (NH 2) 2] Ammonia (NH3) and isocyanic acid (HNCO). The isocyanic acid must for the subsequent one However, nitrogen oxide reduction can also be converted into ammonia. This chemical reaction is ongoing in the water-containing exhaust gas from temperatures of about 200 ° C from.

dosed If you put the HWL at lower temperatures in the exhaust gas, the Danger that the isocyanic acid not completely is converted into ammonia and unconverted isocyanic acid or from isocyanic acid resulting inactivating the SCR catalyst. The NOx reduction performance of the SCR system drops drastically in this mode of operation of the SCR system.

current known SCR systems using a urea-water solution as a reducing agent precursor work, their effect only at exhaust gas temperatures above from about 200 ° C.

These lower limit temperature is, however, in efficiency-optimized internal combustion engines, in particular in diesel engines with direct injection and turbocharger in the Range of low and medium loads not always achieved, so that the reduction of nitrogen oxides in this low temperature range can not be guaranteed with the known systems.

In front In this background, the object of the invention in the specification a device with improved efficiency of an SCR system lower exhaust gas temperatures than before.

These Task is characterized in a device of the type mentioned by solved, that the device is a controllable heating element and a catalytic Coating with the reducing agent precursor in contact comes.

Advantages of invention

The controllable heating element allowed in conjunction with the catalytic Coating an independent of the exhaust gas temperature treatment of an in Dissolved water Reduktionsmittelvorproduktes, usually a urea-water solution, by Thermolysis and hydrolysis in the heating element. An evaporation of the reducing agent precursor provides an elevated Gas share above out for a more uniform reductant distribution in the exhaust gas as in a metering in liquid form, because the vaporized reducing agent mixed better with the exhaust gas than a Liquid. Thereby can the effectiveness the exhaust gas cleaning increased in the range of low exhaust gas temperatures become. This is especially for the exhaust gas cleaning after a cold start and warm-up of the internal combustion engine advantageous.

It it is preferred that the catalytic coating at least one Part of the heating element covered.

By this measure, the genann th advantages further reinforced because a heated catalytic coating favors the preparation of the reducing agent precursor.

Further is preferred that the heating element at a Zumessquerschnitt an exhaust-side end of the device is arranged.

By This measure become temperature-sensitive areas of the metering valve such as seals and coils in a sense from the hot temperature zone pulled out.

The cooling the directly mounted on the exhaust gas valve is not critical, as a electrically controllable heating element in the range of critical temperatures the metering valve is arranged. An undesirable heating of such areas is due to the between these areas and the possibly hot exhaust gas arranged heating element reduced. The heating element itself is against high temperature insensitive. A heat decoupling of dosing valve and heating element can have low cost insulators be achieved. Overall, there is the possibility of an elaborate, costly valve fluid cooling too dispense and realize a simpler air cooling.

The Heating element can over it Be used to the deposit sensitive parts the device after stopping the engine or an interruption of the metering to clean the exhaust gas. Such Parts are, for example, the valve tip and evaporator channels that protrude into the exhaust tract. There you can next to residues of the Reactor Vorproduktes also deposit soot particles. Pressing the Heating element ensures a self-cleaning of these parts. The heating element puts it sure that through the self-cleaning mechanism the functionality of the system. Encrustations and blockages are prevented or at least reduced.

Prefers is also that the device has a flash evaporator with an evaporation zone and having a arranged behind the flash evaporator reaction zone.

Of the Flash evaporator ensures a sudden evaporation of the urea-water solution. Thereby become unwanted chemical reactions that occur with a slow increase in temperature can, prevented. This results from the urea contained in the HWL [CO (NH2) 2] ammonia (NH3) and isocyanic acid (HNCO) with a high Yield. The flash evaporator is at the end of the evaporation zone gaseous Reducing agent at a temperature of about 100 to 120 ° C (the temperature of the supersaturated Steam) before. Rapid evaporation of the reducing agent also contributes This helps to keep the installation space of the flash evaporator as compact as possible.

Further it is preferred that the reaction zone is a hydrolysis catalyst having.

In The reaction zone of the flash evaporator is overheated the already gaseous reducing agent to temperatures of about 150 to 250 ° C instead. The one in this zone contained hydrolysis catalyst initiates and accelerates the reaction of in gaseous form Reducing agent contained isocyanic acid to the desired product ammonia. Out one mole of isocyanic acid can be recovered up to one mole of ammonia.

On this way you can even at exhaust gas temperatures from about 130 to 150 ° C in the SCR catalyst Nitrogen oxides are reduced. This will change the temperature range of SCR systems with Urea-water solution as reducing agent precursor so extended that this also in a driving profile with low exhaust gas temperatures, for example can be used in city traffic cycles of passenger cars. With this measure can the well-known, for heavy commercial vehicles certain SCR technology even in light Commercial vehicles or used in passenger cars become. These vehicle classes clearly show typical driving profiles lower exhaust gas temperatures than heavy commercial vehicles.

A Another preferred embodiment is characterized in that the evaporation zone has an inlet for the supply of an aerosol a urea / water solution and having air and that the reaction zone has an outlet for delivering a Ammonia / water vapor / air mixture has to the exhaust gas.

These Embodiment unfolds the above advantages in particular in light commercial vehicles, which already have a compressed air system.

alternative it is preferred that the evaporation zone has an inlet for supply of liquid urea / water solution and the reaction zone has an outlet for delivery of an ammonia / water vapor mixture to the exhaust gas.

By This embodiment, the advantages of the invention in passenger cars achieved, which are not equipped with a compressed air system.

A particularly preferred embodiment is characterized in that the device a the flash evaporator upstream 2-way valve via which the urea / water solution is alternatively discharged, bypassing the flash evaporator to the exhaust gas.

These Design allows a demand-dependent use of the flash evaporator and a bypass of the flash evaporator in operating states, where a conventional, directly to the exhaust gas metering is possible.

Prefers is also that the evaporation zone electrically or with the help of Heat storage, the of engine waste heat or waste heat be heated with heat is supplied.

A Electric heating allows activation of the flash evaporator already shortly after a cold start of the internal combustion engine. The exploitation the said heat storage allows energy savings in operating conditions with low exhaust gas temperatures with already warm combustion engine.

Prefers is further that the device comprises a controller which has a flow through the 2-way valve at low exhaust gas temperatures to the flash evaporator releases and at higher levels Exhaust gas temperatures bypassing the flash evaporator releases to the exhaust gas.

These Design allows an exhaust temperature selective use of Invention at low exhaust gas temperatures and conventional metering at higher Exhaust gas temperatures. By this configuration, the burden of the flash evaporator over the usually years of operation of motor vehicles on phases be limited with low exhaust gas temperatures. As a result results an increased Life of the flash evaporator and lower energy consumption.

Prefers is further that the control unit a flow through the 2-way valve, bypassing the flash evaporator to the exhaust gas releases when an exhaust gas temperature between one in the flow direction the exhaust arranged SCR catalyst and a mouth of a Bypassing the flash evaporator greater than 200 ° C is.

It has been shown to take place above this temperature threshold Dosing into the exhaust already without further measures to a good treatment a urea-water solution leads.

Prefers is also that the discharge to the exhaust gas through the flash evaporator through a nozzle and / or in the flow direction the exhaust gas before a static mixer in the exhaust gas, and / or a particulate filter, and / or a turbocharger and / or an oxidation catalyst.

These activities care for each a further improved distribution of the gaseous reducing agent in Exhaust.

Furthermore enable these measures a reduction in power to the flash evaporator. If the Flash evaporator in the exhaust gas in the flow direction of the exhaust gas before the metering point for the liquid urea-water solution so is placed between the flash evaporator and the dosing point for liquid HWL an exhaust gas temperature difference of about 40 to 80 K ((180 to 220) ° C - 140 ° C = 40 to 80 K), can rely on a separate heat input into the flash evaporator be waived. So you can, for example, in certain applications the flash evaporator in the exhaust manifold insert the combustion engine in front of the turbine of the exhaust gas turbocharger.

at a metering before an oxidation catalyst can, with the help a suitable dosing strategy, already the SCR activity of the oxidation catalyst be used.

A Another preferred embodiment is characterized by a flow direction the exhaust gases arranged before delivery from the oxidation catalyst.

In this case becomes exclusive the activity of the SCR catalyst to temperature ranges below current temperature limits for urea-water-solution SCR systems extended.

Prefers is further that the catalytic coating and / or the reaction zone Having alumina.

This Material has proven itself as a hydrolysis catalyst. In addition to ceramic and / or metallic honeycomb bodies can for example, with aluminum oxide coated wire knits, steel wool or metal foam can be used as a catalyst support.

Further Advantages will be apparent from the description and the attached figures.

It it is understood that the above and the following yet to be explained features not only in the specified combination, but also in other combinations or alone, without to leave the scope of the present invention.

drawings

embodiments The invention are illustrated in the drawings and in the following description explained. Show it:

1 schematically an internal combustion engine with an arranged behind the engine exhaust gas purification system and a first embodiment of the invention;

2 , schematically a second embodiment in the form of a flash evaporator;

3 an embodiment of an exhaust gas purification system with flash evaporator and compressed air assisted reducing agent metering;

4 an embodiment of an exhaust gas purification system with flash evaporator without compressed air support;

5 a possible connection location of a flash evaporator in an exhaust system with turbocharger; and

6 Operating areas of an internal combustion engine, in which a metering via a flash evaporator takes place and operating areas, in which a metering can take place directly to the exhaust gas.

description the embodiments

In the 1 is an embodiment of a device 10 for the preparation of a reducing agent precursor solution for the reduction of nitrogen oxides in the exhaust gas of an internal combustion engine 12 shown. Exhaust gases of the internal combustion engine 12 be over an exhaust pipe 14 in an SCR catalyst 16 directed. Nitrogen oxides contained in the exhaust gas are in the SCR catalytic converter 16 selectively reduced to molecular nitrogen in the presence of a reducing agent. The device 10 gets out of a reservoir 18 supplied with a reductant precursor solution. The supply of reducing agent precursor solution via a feed pump 20 and a metering valve 22 that from a controller 24 is controlled. At the control unit 24 it is preferably that for controlling the internal combustion engine 12 Any existing control unit that controls, for example, the injection of fuel into combustion chambers of the engine.

Because different operating ranges of the internal combustion engine 12 characterized by a different need for reducing agent, the control of the metering valve takes place 22 depending on operating parameters of the internal combustion engine 12 that the control unit 24 via a sensor 26 be made available and / or from maps in the engine control unit available. For the signals of the sensors 26 These are, for example, signals about temperatures of the internal combustion engine 12 and the rotational speed of the crankshaft of the internal combustion engine 12 , In addition, the controller processes 24 preferably one in the control unit 24 self-formed injection pulse width for metering fuel to the combustion chambers of the internal combustion engine 12 , By evaluation of the injection pulse widths and the speed of the internal combustion engine 12 can be, for example, operating ranges of the internal combustion engine 12 with high combustion temperatures and thus high NOx emissions of the internal combustion engine 12 identify. It is understood, however, that the operating parameters of the internal combustion engine given here as examples 12 do not represent an exhaustive list of operating parameters provided by the controller 24 for controlling the metering valve 22 are processed.

The device 10 is in a breakthrough of a wall 28 the exhaust pipe 14 arranged and secured for example by a flange connection. The device 10 has at its exhaust side end a heating element 32 on. The heating element 32 is preferably designed as an electrical resistance heater and is powered by a power source 34 supplied with a heating current. The heating current is from the control unit 24 via a switch 36 controlled. The heating element 32 is at least partially with a catalytic coating 38 provided a wall 40 a flow channel 42 at least partially lined. When opening the dosing valve 22 promotes the pump 20 Reducing agent from the reservoir 18 to the device 10 and drives the reducing agent, or a reducing agent precursor solution through the flow channel 40 , As the reducing agent precursor solution, a urea-water solution is preferably used.

The exhaust end 30 the device 10 flowing urea-water solution is flowing through the flow channel 42 through the heating element 32 heated. The heating is preferably carried out up to temperatures at which at least part of the urea-water solution is still in the flow channel 42 evaporated. As the urea-water solution flows through the flow channel 42 with the heated catalytic coating 38 comes in contact, she experiences in the flow channel 42 a combined thermolysis and hydrolysis. This means that the urea dissolved in the urea-water solution is at least partially decomposed into the reducing agent ammonia and the residual product CO ₂ . Which is in the interior the exhaust pipe 14 spreading injection jet 44 therefore already contains a proportion of ammonia. As a result, even at low exhaust gas temperatures below about 200 ° C already a selective catalytic reduction of nitrogen oxides in the subsequent SCR catalyst 16 are triggered, although the lower exhaust gas temperature alone is not sufficient for a release of the reducing agent ammonia from the urea-water solution.

The ammonia thus provided reacts selectively in the SCR catalyst 16 with the nitrogen oxides contained in the exhaust gas to molecular nitrogen (N ₂ ) and water (H ₂ O).

In principle, in devices for metering a urea-water solution to the exhaust gas, there is the risk that the metering cross-section narrows or becomes completely clogged with deposits of crystallized urea and / or deposits of soot particles from the exhaust gas as the operating time of the device increases. To prevent this, the heating element 32 the device 10 also be activated in operating phases without dosage of reducing agent to the exhaust and the wall surface 40 of the flow channel 32 heat so strongly that reducing agent residues are melted and released into the exhaust gas. This will self-clean the device 10 achieved. The controllable metering valve 22 forms preferably with the device 10 an integral unit. Therefore, there is a risk that a coil insulation of the metering valve 22 and / or other seals of the device 10 and the metering valve 22 by overheating the device 10 to be damaged.

To prevent this, is in the case 46 the device 10 an insulating element 48 arranged, which may be the hot exhaust side 30 with the heating element 42 from the remaining parts of the device 10 thermally insulated. Of course, the insulating piece 46 as a connector between the exhaust side end 30 with the heating element 42 and the rest of the device 10 be formed so that it is not in the case 46 must be arranged, but with its outer wall itself a part of the outer contour of the device 10 forms. With such a configuration, a further improved thermal insulation is achieved because metallic bridging the insulating piece 48 through parts of the metallic housing 46 be avoided.

2 shows a device 50 as a second embodiment of an apparatus for processing a reducing agent-precursor solution for the reduction of nitrogen oxides in the exhaust gas of the internal combustion engine 12 , The device 50 has a flash evaporator 52 with an evaporation zone 54 and a reaction zone 56 on. The reaction zone 56 characterized by a hydrolysis catalyst 58 which preferably has alumina surfaces. In addition to ceramic and / or metallic honeycomb bodies, it is also possible, for example, to use wire meshes coated with aluminum oxide, steel wool or metal foam as the catalyst support. The evaporation zone 54 has an entrance 60 on, over the flash evaporator 52 either an aerosol from a urea-water solution and air or a liquid urea-water solution is supplied. Aerosol delivery systems are also referred to as compressed air assisted systems. They require a source of compressed air, as is the case with heavy commercial vehicles for the operation of an air brake anyway. Compressed air-assisted systems for reducing agent supply are therefore preferably implemented in the commercial vehicle sector, while systems without compressed air assistance are preferably realized in the passenger car sector or in the light commercial vehicle sector.

In the evaporation zone 54 There is a rapid evaporation of the supplied urea-water solution. To realize a rapid evaporation, a heating element is preferred 64 used that with an external heat source 66 via a control 68 can be connected. The control 68 is from the engine control unit 24 actuated. At the heat source 66 again it can be a power supply, as it relates to the 1 has already been explained. The heating element 64 is realized in this case as resistance heating and the control 68 in this case represents a circuit breaker. Alternatively or additionally, the heat source 66 Also have heat storage, by the waste heat of the engine 12 and / or be heated by the waste heat of the exhaust gas. The control 68 For example, it can be configured as a valve, with which a stream of hot engine oil to the heating element 64 is directed.

A rapid evaporation of the reducing agent is desirable to the space of the flash evaporator 52 keep as compact as possible. In addition, a sudden evaporation ("flash evaporation") of the urea-water solution is desired in order to prevent possible chemical reactions that may occur with a slow increase in temperature. Solution dissolved urea [CO (NH2) 2] with a high yield of ammonia (NH3) and isocyanic acid (HNCO). In flash evaporator 52 lies at the end of the evaporation zone 54 gaseous reducing agent with a temperature of the supersaturated water vapor of about 100 to 120 ° C before.

In the reduction zone 56 of the flash evaporator 52 Overheating of the already gaseous reducing agent to temperatures of about 150 to 250 ° C instead. The one in this evaporation zone 56 arranged hydrolysis catalyst 58 initiates and accelerates the reaction of the Isocyanansäue contained in the gaseous reducing agent to the desired end product ammonia. From one mole of isocyanic acid can be recovered up to one mole of ammonia. For high conversion rates of isocyanic acid in ammonia, a water vapor excess is necessary. However, this water vapor excess is already present in the urea-water solution used as reducing agent.

At about 200 ° C results in adjusted residence times of the reducing agent in the reaction zone 56 a complete conversion of the reducing agent into ammonia. From the exit 62 the reduction zone 56 then flows either an ammonia / air / water vapor mixture (in systems supported by compressed air) or an ammonia / water vapor mixture (in systems not supported by air pressure). The energy used for the evaporation and overheating of the reducing agent in the flash evaporator 52 can be kept low because of the flash evaporator 52 preferably at exhaust gas temperatures upstream of the SCR catalyst 16 is flowed through from below about 200 ° C by reducing agents. The reductant mass flows necessary for the NOx reduction at these exhaust gas temperatures are generally extremely low.

In the 3 is an embodiment of an exhaust gas purification system with flash evaporator 52 and compressed air assisted reductant metering. Urea water solution is taken from a storage tank 18 taken and with the help of a pump 20 via a metering valve 22 in a mixing chamber 70 promoted. The mixing chamber 70 also gets air from a compressed air source 72 fed. The compressed air source may be, for example, a compressed air compressor and / or a compressed air reservoir, as they are already available in commercial vehicles for the operation of a compressed air brake system. In the mixing chamber 70 An aerosol of urea-water solution and air is formed. With a two-way valve 74 is the previously generated aerosol optionally in a first Aerosolleitung to flash evaporator 52 or passed into a second aerosol line, with which the aerosol bypassing the flash evaporator 52 directly at a discharge point 79 can be attributed to the exhaust gas.

When the exhaust temperature is just before the SCR catalyst 16 is higher than about 200 ° C, the direct introduction can be selected because at these temperatures, the urea-water solution is evaporated in the hot exhaust gas and converted into ammonia. The temperature can be adjusted with a temperature sensor 81 be detected or modeled in the control unit from operating variables of the internal combustion engine. At lower exhaust gas temperatures, the two-way valve 74 switched so that the aerosol out of the mixing chamber 70 in the flash evaporator 52 is passed, evaporated there and converted into a gaseous ammonia / water vapor / air mixture. This mixture is then added to the exhaust gas. For a better distribution of the gaseous reducing agent in the exhaust gas, a treatment element 80 be arranged in the exhaust system in the flow direction of the exhaust gases behind the introduction. As a treatment element 80 is a nozzle and / or a static mixer and / or a turbocharger and / or a particulate filter and / or an oxidation catalyst in question.

Both the gaseous reductant from the flash evaporator 52 as well as the aerosol from urea-water solution and air, bypassing the flash evaporator 52 via the discharge point 79 in the exhaust pipe 14 passes, distributed in the exhaust gas of the internal combustion engine 12 and gets into the SCR catalyst 16 transported. Optionally, in the system after the 3 another oxidation catalyst 82 in the flow direction of the exhaust gases before the introduction of gaseous reducing agent from the flash evaporator 52 be arranged. Next shows 3 still a heat source 66 , the heat from internal combustion engine 12 stores and via a control 68 to the flash evaporator 52 supplies. For this heat source 66 apply in connection with the 2 made statements.

4 shows a similar exhaust aftertreatment system, such as 3 , The difference of the exhaust aftertreatment system after the 4 to that of 3 is that the exhaust aftertreatment system after 4 has no air support. At the subject of 4 The urea-water solution from a reservoir 18 via a pump 20 to the two-way valve 74 guided. The control of the dosage takes place at the subject of 4 deviating from the subject of the 3 not by a single metering valve 22 between pump 20 and two-way valve 74 but takes place for both Dosierzweige 76 and 78 separated by a first metering valve 84 between the two-way valve 74 and the flash evaporator 52 and through a second metering valve 86 in the connection of the two-way valve 74 with the discharge point 79 , Both valves 84 . 86 are also from the controller 28 controlled. For the other components, the comments on 3 with the only difference being that the wires 76 and 78 who are the subject of the 3 lead an aerosol, in the subject of the 3 liquid urea-water solution lead.

In the 5 is a modification of a non-compressed air assisted dosing system shown in which the flash evaporator 52 inside the exhaust pipe 14 in front of a turbocharger 88 is arranged. The flash evaporator 52 is from the exhaust stream or part of the exhaust stream of the engine 12 flows through and in the flash evaporator 52 Resulting ammonia / exhaust gas mixture exits the flash evaporator 52 and becomes the SCR catalyst with the exhaust gas 16 transported by reacting with the nitrogen oxides in the exhaust gas to molecular nitrogen and water.

That in the 5 shown system can of course also be realized with a compressed air-assisted metering of a urea-water solution. Arranging the flash evaporator 52 in the exhaust pipe 14 in front of a turbocharger 88 has the advantage that may be due to a separate heat input to the flash evaporator 52 can be waived. The turbocharger 88 removes kinetic energy from the exhaust gas and therefore cools the exhaust gas. As pure ammonia in modern SCR catalysts 16 Already from about 140 ° C selectively reduces nitrogen oxides and evaporation of the liquid urea-water solution with subsequent conversion to ammonia in a hydrolysis from about 180 to 220 ° C occurs completely and because the cooling of the exhaust gas through the turbine of the exhaust gas turbocharger 88 is in a comparable order of magnitude, usually prevail in front of the exhaust gas turbocharger 88 sufficiently high temperatures for evaporation of the urea-water solution or an aerosol of air and urea. Therefore, in many applications, a supply of additional heat to the flash evaporator 52 be waived if the flash evaporator 52 in the exhaust gas in front of the turbocharger 88 is arranged.

In the 6 are operating areas 90 and 92 of a diesel engine in a characteristic map typically used in engine development, which is defined by axes for the mean effective pressure p and the speed n. The borderline 94 is defined as having an exhaust gas temperature of about 200 ° C behind the turbocharger for your medium pressure / speed pairs 88 established. Above the line 94 prevail higher exhaust gas temperatures and below the line 94 prevail lower exhaust gas temperatures. Accordingly, in medium pressure / speed points, which are above the line 94 lie, a dosage over the mouth 79 directly into the exhaust gas. Below the line 94 on the other hand, a dosage via the flash evaporator 52 prefers.

Claims (14)

Contraption ( 10 ; 50 ) for the treatment of a reducing agent precursor solution for the reduction of nitrogen oxides in the exhaust gas of an internal combustion engine ( 12 ), characterized in that the device ( 10 ; 50 ) a controllable heating element ( 32 ; 64 ) and a catalytic coating ( 38 ) which comes into contact with the reducing agent precursor. Contraption ( 10 ) according to claim 1, characterized in that the catalytic coating ( 38 ) at least a part of the heating element ( 32 ) covered. Contraption ( 10 ) according to claim 1 or 2, characterized in that the heating element ( 32 ) at a Zumessquerschnitt at an exhaust side end ( 30 ) of the device ( 10 ) is arranged. Contraption ( 50 ) according to claim 1, characterized in that the device ( 50 ) a flash evaporator ( 52 ) with an evaporation zone ( 54 ) and one behind the evaporation zone ( 54 ) arranged reaction zone ( 56 ) having. Contraption ( 50 ) according to claim 4, characterized in that the reaction zone ( 56 ) a hydrolysis catalyst ( 58 ) having. Contraption ( 50 ) according to claim 4 or 5, characterized in that the evaporation zone ( 54 ) an entrance ( 60 ) for supplying an aerosol from a urea / water solution and air and the reaction zone ( 56 ) an output ( 62 ) for delivering an ammonia / water vapor / air mixture to the exhaust gas. Contraption ( 50 ) according to claim 4 or 5, characterized in that the evaporation zone ( 54 ) an entrance ( 60 ) for the supply of liquid urea / water solution and the reaction zone ( 56 ) has an output for delivering an ammonia / water vapor mixture to the exhaust gas. Contraption ( 50 ) according to at least one of claims 4 to 7, characterized in that the device ( 50 ) to the flash evaporator ( 52 ) upstream 2-way valve ( 74 via the urea / water solution alternatively, bypassing the flash evaporator ( 52 ) is discharged to the exhaust gas. Contraption ( 50 ) according to one of claims 4 to 8, characterized in that the evaporation zone ( 54 ) electrically or by means of heat accumulators ( 66 ), the waste heat of the internal combustion engine ( 12 ) or exhaust heat to be heated, is supplied with heat. Contraption ( 50 ) according to claim 8 or 9, characterized by a control device ( 28 ), the one Flow through the 2-way valve ( 74 ) at low exhaust gas temperatures to the flash evaporator ( 52 ) and at higher exhaust gas temperatures, bypassing the flash evaporator ( 52 ) releases to the exhaust. Contraption ( 50 ) according to claim 10, characterized in that the control unit ( 28 ) a flow through the 2-way valve ( 74 ) bypassing the flash evaporator ( 52 ) releases to the exhaust gas when an exhaust gas temperature between an arranged in the flow direction of the exhaust gases SCR catalyst ( 16 ) and a mouth ( 79 ) a bypass of the flash evaporator ( 52 ) is greater than 200 ° C. Contraption ( 50 ) according to one of claims 4 to 11, characterized in that the discharge to the exhaust gas via the flash evaporator ( 52 ) through a nozzle and / or in the flow direction of the exhaust gas before a static mixer in the exhaust gas, and / or a particulate filter, and / or a turbocharger ( 88 ) and / or an oxidation catalyst. Contraption ( 50 ) according to at least one of the preceding claims, characterized by an oxidation catalytic converter arranged in the flow direction of the exhaust gases before discharge. Contraption ( 50 ) according to at least one of claims 1 to 13, characterized in that the catalytic coating ( 38 ) and / or the reaction zone ( 56 ) Comprises alumina.