Device and Method for Producing Enameled Wires

Abstract

A device (1) and a method for producing enameled wires, comprises an application device (3) for applying at least one enamel coating, a furnace (4) for solidifying the enamel coating and an exhaust gas purification device (7) for removing at least nitrogen oxides from an exhaust gas (9) of the furnace (4). The exhaust gas purification device (7) has a unit (13) for the selective catalytic reduction of nitrogen oxides in the exhaust gas (9) of the furnace and a feeding apparatus (11) for feeding a reducing agent, preferably an ammonia-containing compound, in particular a urea solution, into the exhaust gas (9) of the furnace (4). The feeding apparatus (11) has at least one outlet opening, which is designed in such a way that the reducing agent exits from the outlet opening substantially in the flow direction of the exhaust gas (9).

Claims

1. A device (1) for producing enamelled wires, comprising: an application device (3) for applying at least one enamel coating; a furnace (4) for solidifying the enamel coating; and an exhaust gas purification device (7) for removing at least nitrogen oxides from an exhaust gas (9) of the furnace (4), wherein the exhaust gas purification device (7) has a unit (13) for selective catalytic reduction of nitrogen oxides in the exhaust gas (9) of the furnace and a feeding apparatus (11) for feeding a reducing agent into the exhaust gas (9) of the furnace (4), wherein a part of the feeding apparatus (11) which is straight is arranged in a bent pipe section (15) and the feeding apparatus (11) has at least one outlet opening which is designed in such a way that the reducing agent exits from the outlet opening substantially in the direction of flow of the exhaust gas (9).

2. The device (1) according to claim 1, wherein the bent pipe section (15) has a curvature between 60 and 150°, or between 75 and 120°, or between 80 and 100°, or substantially 90°.

3. The device (1) according to claim 2, wherein the part of the feeding apparatus (11) arranged in the bent pipe section (15) is enclosed, apart from the outlet opening, by a protective pipe (25).

4. The device (1) according to claim 1, wherein the feeding apparatus (11) has a feed line with the at least one outlet opening, wherein the outlet opening has a nozzle oriented substantially in the direction of flow of the exhaust gas (9) for injecting the reducing agent substantially in the direction of flow of the exhaust gas (9).

5. The device (1) according to claim 4, wherein the nozzle of the outlet opening is an atomising nozzle for splitting the reducing agent into fine droplets.

6. The device (1) according to claim 5, wherein the atomising nozzle is a two-substance nozzle (19) which has a nozzle inner chamber (33) and a nozzle outer chamber (34), wherein the nozzle inner chamber (33) is for feeding a first medium, wherein the first medium contains the reducing agent, wherein the nozzle outer chamber (34) is for feeding a second medium.

7. The device (1) according to claim 6, wherein the two-substance nozzle (19) is an externally mixing two-substance nozzle which has a nozzle outlet (35), wherein the nozzle inner chamber (33) and the nozzle outer chamber (34) are separately connected to the nozzle outlet (35) for the separate discharge of the first medium and the second medium from the two-substance nozzle.

8. The device (1) according to claim 6, wherein the two-substance nozzle (19) is an internally mixing two-substance nozzle which has a mixing chamber (37), wherein the nozzle inner chamber (33) and the nozzle outer chamber (34) are connected to the mixing chamber (37) for mixing the first medium with the second medium in the mixing chamber (37).

9. The device (1) according to claim 1, wherein the exhaust gas purification device (7) has, in the direction of flow of the exhaust gas (9) downstream of the feeding apparatus (11) and upstream of the unit (13) for selective catalytic reduction of nitrogen oxides, a conical pipe section (12) for the flow of the exhaust gas (9) therethrough, wherein the diameter of the conical pipe section (12) increases in the direction of flow of the exhaust gas (9), wherein the fed reducing agent is introduced into the conical pipe section (12) at least in stages.

10. The device (1) according to claim 9, wherein the conical pipe section (12) has a half opening angle of between 1 and 10°, or between 2 and 7°, or between 3 and 5°, or substantially 4°.

11. The device (1) according to claim 1, wherein the unit (13) for selective catalytic reduction has at least one denitrification catalyst (21) and at least one oxidation catalyst (22) for breaking down carbon monoxide and/or hydrocarbons and/or ammonia.

12. The device (1) according to claim 11, wherein the at least one denitrification catalyst (21) has a first coated metal support and a first washcoat and the at least one oxidation catalyst (22) has a second metal support and a second washcoat.

13. A method for producing enamelled wires, comprising: applying at least one enamel coating to a metal wire; and solidifying the at least one enamel coating in a furnace (4), wherein an exhaust gas (9) from the furnace (4) is freed at least of nitrogen oxides, wherein the exhaust gas (9) from the furnace (4) is subjected to a selective catalytic reduction of nitrogen oxides, wherein a reducing agent is fed to the exhaust gas (9) via a feeding apparatus (11), a part of the feeding apparatus (11) which is straight is arranged in a bent pipe section (15) and the reducing agent is introduced into the exhaust gas (9) substantially in the direction of flow of the exhaust gas (9).

14. The method according to claim 13, wherein the introduced reducing agent, is liquid and evaporates in a conical pipe section (12), wherein ammonia is formed.

15. The method according to claim 13, wherein the mass flow of the exhaust gas (9) and the concentrations of the nitrogen oxides in the exhaust gas (9) are determined in the direction of flow before the reducing agent is fed and in the direction of flow after the selective catalytic reduction, wherein the amount of the introduced reducing agent is determined on the basis of the determined mass flow of the exhaust gas (9) and the determined concentrations of the nitrogen oxides in the exhaust gas (9).

16. The method according to claim 13, wherein the reducing agent comprises an ammonia-containing compound or a urea solution.

17. The device according to claim 1, wherein the reducing agent comprises an ammonia-containing compound or a urea solution.

18. The device according to claim 6, wherein the second medium comprises a compressed air.

19. The device (1) according to claim 11, wherein at least two denitrification catalysts (21) are provided in succession in the direction of flow of the exhaust gas (9).

20. The device (1) according to claim 12, wherein the first washcoat comprises oxides of titanium, vanadium and/or tungsten and the second washcoat comprises platinum and/or palladium.

Description

[0025] The invention is further explained below with reference to the non-limiting exemplary embodiment shown in the drawings.

[0026] FIG. 1 schematically shows a device for producing enamelled wires;

[0027] FIG. 2 schematically shows a sectional view of an exhaust gas purification device according to the invention for the device according to FIG. 1;

[0028] FIG. 3 schematically shows a view of a feeding apparatus of the exhaust gas purification device according to FIG. 2;

[0029] FIG. 4 schematically shows a sectional view of a first embodiment of a two-substance nozzle of the feeding apparatus according to FIG. 3;

[0030] FIG. 5 schematically shows a sectional view of a second embodiment of a two-substance nozzle of the feeding apparatus according to FIG. 3.

[0031] FIG. 1 shows a device 1 for producing enamelled wires 2. The device 1 has an application device 3 for applying a plurality of enamel coatings to a metal wire. In addition, a furnace 4 is provided for drying and curing the enamel coatings on the metal wire. The furnace 4 has a drying zone 4′ and a curing zone 4″. Circulating air 5 is circulated in the furnace 4 by means of a fan 6. The circulating air 5 is enriched with various pollutants, as will be explained in detail below. In addition, a circulating-air oxidation catalyst 7′ is provided as well as, in a line carrying exhaust gases 9, an exhaust gas purification unit 7 for removing the pollutants, in particular nitrogen oxides, from the exhaust gas of the furnace. FIG. 1 also shows a cooler 8 for cooling the enamelled wire.

[0032] The wire insulation is produced by drying and curing the liquid coating layers applied to the wire in the circulating-air furnace 4 at approx. 550-700° C. Since the application of the required final coating thickness must be carried out in several passes, up to 24 covers of the same wire are passed through the drying or curing furnace. During the drying of the liquid enamel coatings on the metal wire, enamel solvents are evaporated at high temperature, causing the circulating air of the wire enamelling installation to become continuously saturated with solvent vapour along the drying zone.

[0033] During air circulation operation, the solvent-laden air constantly flows through a palladium-platinum catalyst, which serves as a circulating-air oxidation catalyst 7′, whereby the hydrocarbons (C.sub.mH.sub.n=solvent vapour) are exothermically oxidised with the oxygen O.sub.2 of the fresh air drawn into the furnace to form carbon dioxide CO.sub.2 and water vapour H.sub.2O. During this exothermic chemical reaction, thermal energy is therefore released, which heats the process air. However, the solvent nitrogen can also be increasingly oxidised there to form nitrogen oxides NO.sub.x (predominantly nitrogen monoxide NO, nitrogen dioxide NO.sub.2). The conversion rate in the circulating-air oxidation catalyst 7′ generally increases with rising temperature level. In areas outside the catalyst, the solvent-air mixture can already react chemically prematurely due to the sufficiently high temperature level (e.g. drying zone, heating zone).

[0034] The mass flows of air and solvent introduced into the system are approximately constant during production. Due to the process, a hot exhaust gas 9 is extracted directly after the circulating-air catalyst and discharged into the surrounding environment. The exhaust gas is discharged via a steel pipe with an internal diameter of 80 mm by means of a radial blower. A vertically oriented counter-current heat exchanger is usually flange-mounted directly on the pipe outlet and uses the thermal energy of the exhaust gas to generate water vapour as a protective and cleaning gas for the annealing process of the raw wire. In standard operation, the exhaust gas temperature upstream of the evaporator is between 300-500° C. and downstream of the evaporator is about 250 280° C.

[0035] Relevant pollutant components in the exhaust gas of the installation during the enamelling process are carbon monoxide CO, remaining volatile organic substances (hydrocarbons specified as total bound carbon or total C) and nitrogen oxides NO.sub.R. CO is produced by the incomplete combustion of hydrocarbons. Remaining residual amounts of hydrocarbons in the furnace exhaust gas are attributed to the fact that the solvent vapours present are insufficiently oxidised in the air circulation process, especially in the circulating-air catalyst 7′. This can be caused, for example, by an overloading of the process air with hydrocarbons (i.e. if the solvent input into the system is too high), an insufficient temperature level in the circulating-air catalyst or also by too little reactive oxygen (fresh air feed or exhaust gas amount) in the system.

[0036] NO.sub.x in wire enamelling machines is what is known as fuel NO.sub.x and not thermal NO.sub.x. The formation of NO.sub.x in the enamelling process is thus largely based on the chemical reaction of the nitrogen components bound in the fuel (enamel solvent) with atmospheric oxygen. Thermal NO.sub.x would be formed from the nitrogen in the air at temperatures of over 1000° C. Nevertheless, experience shows that higher temperatures promote the chemical conversion rate of N.sub.2 to NO.sub.x (especially in the circulating-air catalyst). A major source of NO.sub.x in wire coating is the solvent NMP (N-methyl-2-pyrrolidone), which is the basic solvent of the polyamideimide (PAI) enamel often used as an overcoat. NMP contains high amounts of nitrogen, which is oxidised to form NO.sub.x in the furnace process, especially in high-temperature areas (e.g. heating zone, circulating-air catalyst).

[0037] In horizontal installations for enamelling round wires with diameters of up to 1.6 mm, exhaust gas quantities of approx. 65-70 Nm.sup.3/h are present. As mentioned above, the exhaust gas contains residual amounts of nitrogen monoxide NO, nitrogen dioxide NO.sub.2, carbon monoxide CO and remaining residual solvent components in the form of unburned hydrocarbons C.sub.nH.sub.m, which are to be actively cleaned. The pollutant limits for industrial exhaust gas are set by a wide variety of legal standards.

[0038] FIG. 2 shows an exhaust gas purification device 7 according to the invention, which has a connection element 10 for connecting the exhaust gas purification device 7 to the steel pipe of the furnace 4 provided for discharging the exhaust gas 9, a feeding apparatus 11, a conical pipe section 12 and a unit 13 for selective catalytic reduction. In principle, the exhaust gas purification device 7 can be installed in any exhaust air pipeline. The exhaust gas cleaning device 7 can be oriented horizontally or vertically. Since vertical orientation makes it more difficult to access the exhaust gas cleaning device 7 for maintenance and repairs, horizontal orientation of the exhaust gas cleaning device 7 is preferred in horizontal installations for enamelling round wires.

[0039] In the embodiment shown, the exhaust gas 9 of the circulating-air furnace 4 is directed via a straight vertical pipe section 14 to a 90° bent pipe section 15 and subsequently into the conical pipe section 12. In the bent pipe section 15, in which the exhaust gas 9 is diverted from a vertical to a horizontal flow, the feeding apparatus 11 is arranged in part, wherein the feeding apparatus 11 has a feed line which, in the embodiment shown, is embodied as a straight nozzle lance 16. The nozzle lance 16 is horizontally guided through a hole 17 in the bent pipe section 15, so that one end 18 of the nozzle lance 16 is arranged in a circular vertical cross-sectional area at the end of the bent pipe section 15 which is adjacent to the conical pipe section 12 and in which the flow of the exhaust gas 9 is horizontally oriented. An outlet opening is arranged at the end 18 of the nozzle lance 16, which in the embodiment shown is a two-substance nozzle 19. In this case, the two-substance nozzle 19 is arranged at the centre of the circular vertical cross-sectional area at the end of the bent pipe section 15 which is adjacent to the conical pipe section 12, so that the nozzle outlet of the two-substance nozzle 19 points towards the centre of the circular opening of the conical pipe section 12 which is adjacent to the bent pipe section 12 and coincides with the circular vertical cross-sectional area of the bent pipe section 12. In the direction of flow of the exhaust gas 9, the cross-section of the conical pipe section 12 increases so that the conical pipe section 12 has a half opening angle of 4°. The increasing cross-section in the conical pipe section 12 reduces the flow velocity of the exhaust gas 9. The conical pipe section and the unit 13 for selective catalytic reduction are thermally insulated with a 50 mm thick layer of mineral wool so that the exhaust gas 9 does not cool down too much between the injection of the reducing agent and the unit 13 for selective catalytic reduction.

[0040] In the embodiment shown, a reducing agent is injected into the exhaust gas together with compressed air via the two-substance nozzle 19, wherein the spray 20 formed during injection projects into the conical pipe section 12. Since the nozzle outlet of the two-substance nozzle 19 points towards the centre of the circular opening of the conical pipe section 12, the reducing agent is injected in the form of fine droplets together with the compressed air substantially in the direction of flow of the exhaust gas 9 in the middle of the flowing exhaust gas 9 in order to achieve homogeneous mixing of the exhaust gas 9 with the reducing agent. In the embodiment shown, an aqueous urea solution with a mass fraction of urea (NH.sub.2)2CO of 32.5% is used as reducing agent. This aqueous urea solution is known from the vehicle industry as “AdBlue”. After injecting the reducing agent into the exhaust gas 9 via the two-substance nozzle 19, the urea (NH.sub.2)2CO is thermolysed to form ammonia NH.sub.3 and isocyanic acid HNCO with the help of the hot exhaust gas 9. The water H.sub.2O of the reducing agent evaporates completely and reacts with the isocyanic acid HNCO to form ammonia NH.sub.3 and carbon dioxide CO.sub.2. Due to the homogeneous mixing with the exhaust gas 9 and the longer residence time in the conical pipe section 12 due to the decreasing flow velocity of the exhaust gas 9, the urea and the isocyanic acid are completely converted into vaporous ammonia in the conical pipe section 12.

[0041] In the embodiment shown, downstream of the conical pipe section 12 in the direction of flow of the exhaust gas is the unit 13 for selective catalytic reduction, which has two denitrification catalysts 21 and one oxidation catalyst 22. The denitrification catalysts 21—also called SCR catalysts—are coated solid metal supports with a first metal support and a cell density of 300 cpsi. Metal supports, in contrast to ceramic products, are characterised by a much higher temperature resistance and mechanical resistance. This means that the SCR catalyst could also be used in enamelled wire furnace types, where much higher exhaust air temperatures of up to 650° C. are present. The mechanical resistance protects the metal catalysts from damage during transport, which occurs more easily in the case of ceramic catalysts. A first washcoat of aluminium oxide with a very high specific surface area is imprinted on the first metal support and is coated with the relevant oxides of titanium, vanadium and/or tungsten. The applied first washcoat significantly increases the reaction surface of the denitrification catalysts 21, thereby increasing the efficiency of the chemical reactions taking place. Of particular importance in the case of the coated metal catalysts is the lower ammonia storage capacity, which allows a much faster readjustment of the AdBlue dosage in the event of changing NO.sub.x loadings of the unit. This can significantly reduce the risk of stoichiometric overdosing of AdBlue, which is very relevant for the formation of urea deposits in the denitrification catalyst 21. Due to their structural design with high porosity, ceramic extrudates have a greater tendency to store ammonia, as a result of which the chemical reactions and thus also the control of the urea dosage are particularly sluggish.

[0042] In the two denitrification catalysts 21 of the unit for selective catalytic reduction, the vaporised ammonia NH.sub.3 reacts with nitrogen monoxide NO and nitrogen dioxide NO.sub.2 as well as oxygen O.sub.2 from the exhaust gas 9 and the compressed air to form nitrogen N.sub.2 and water vapour H.sub.2O.

[0043] Downstream of the two denitrification catalysts 21 connected in series, in the direction of flow of the exhaust gas 9, is the oxidation catalyst 22, which is a coated solid metal support with a second metal support and a cell density of 300 cpsi. A second washcoat of aluminium oxide with a very high specific surface area, which is coated with platinum and/or palladium, is applied to the second metal support. The applied second washcoat significantly increases the reaction area of the oxidation catalyst 22, thereby increasing the efficiency of the chemical reactions taking place. In the oxidation catalyst 22, hydrocarbons C.sub.nH.sub.m and carbon monoxide CO remaining in the exhaust gas 9 react to form carbon dioxide CO.sub.2 and water vapour H.sub.2O. Excess ammonia NH.sub.3, which did not react in the denitrification catalysts 21, reacts to form nitrogen N.sub.2 and H.sub.2O. Between the two denitrification catalysts 21 connected in series and the oxidation catalyst 22, a gap with a minimum gap width of 10 mm is provided in each case between two adjacent catalysts in order to achieve better mixing of the exhaust gas flow through generated turbulences after the exhaust gas 9 leaves the catalysts.

[0044] In the embodiment shown, a first NO.sub.x sensor 23 for measuring the concentration of the nitrogen oxides in the exhaust gas 9 before the injection of the reducing agent is arranged at the straight vertical pipe section 14. Furthermore, downstream of the oxidation catalyst 22 in the direction of flow of the exhaust gas 9 there is arranged a second NO.sub.x sensor 24 for measuring the concentration of nitrogen oxides in the exhaust gas 9 after the unit 13 for selective catalytic reduction. The nitrogen oxide concentrations measured with the aid of the first 23 and second 24 NO.sub.x sensors are used to calculate the amount of ammonia required stoichiometrically to reduce the concentration of nitrogen oxides in the exhaust gas 9 after the unit 13 for selective catalytic reduction to a legally specified target value. The NO.sub.x measurement with the first NO.sub.x sensor 23 is used to calculate the stoichiometric target injection amount of urea solution. The NO.sub.x measurement with the second NO.sub.x sensor 24 is used to check the result of the chemical process and to control the dosing amount of the urea solution accordingly. Furthermore, the mass flow and the temperature of the exhaust gas 9 are taken into account for calculating the stoichiometrically required amount of ammonia. The mass flow of the exhaust gas is determined, for example, with the aid of a dynamic pressure measured via a Prandtl tube or a differential pressure of the exhaust gas 9 measured via a Venturi tube. The amount of ammonia stoichiometrically required for the conversion of the nitrogen oxides of the exhaust gas 9 is used to determine the amount of urea or urea solution injected via the two-substance nozzle 19. Due to the parallel injection of the reducing agent into the exhaust gas 9, not only is the deposition due to wall film formation of the reducing agent minimised, but also a much finer signal of the current NO.sub.x output concentration or the current dosing amount with low oscillations and low amplitudes is achieved.

[0045] FIG. 3 shows the bent pipe section 15 and the feeding apparatus 11 of the exhaust gas purification device 7 according to the invention as shown in FIG. 2. The part of the feeding apparatus 11 arranged in the bent pipe section 15 is enclosed by a protective pipe 25 which shields the nozzle lance 16 and the two-substance nozzle 19 from the exhaust gas flowing through the bent pipe section 15. In this way, the protective pipe prevents possible deposits of enamel residues on the partly cool surface of the nozzle lance 16 and the two-substance nozzle 19 and also protects the entire two-substance nozzle 19 against the partly transversely inflowing exhaust gas 9. To protect the outlet opening of the two-substance nozzle 19, the protective pipe 25 extends in the direction of flow of the exhaust gas 9 as far as the outlet opening. This means that the outlet opening of the two-substance nozzle 19 is completely enclosed by the protective pipe 25 and is thus protected from high temperatures and flow influences of the exhaust gas 9 flowing past. The protective pipe 25 is welded to the bent pipe section and partially protrudes from it, wherein a flange 26 is attached to the end of the protective pipe 25 located outside the bent pipe section 15. The flange 26 of the protective pipe 25 is connected by means of a clamping ring 27 to a flange 28 attached to the nozzle lance 16, so that the nozzle lance 16 and the two-substance nozzle 19 are fixed in the protective pipe 25. When the clamping ring 27 is loosened, the nozzle lance 16 with the two-substance nozzle 19 can be pulled out of the protective pipe 25 for maintenance of the two-substance nozzle 19.

[0046] In the embodiment shown, the feeding apparatus 11 has a first line 29 for feeding the reducing agent and a second line 30 for feeding the compressed air at a standard pressure of 6 bar. The first line 29 guides the reducing agent from a pump (not shown) into an inner pipe 31, which is arranged in the nozzle lance 16 and guides the reducing agent to the two-substance nozzle 19. The second line 30 guides compressed air into an outer pipe 32 arranged coaxially around the inner pipe 31, which outer pipe is arranged in the nozzle lance 16 and guides the compressed air to the two-substance nozzle 19. Shortly before the second line 30 enters the outer pipe 32, there is arranged a branch 30a of the second line 30, which leads to a compressed air inlet of the pump. In the event of a leakage of the second line 30, no compressed air would then flow through the nozzle or through the pump, and this would be immediately detected by a pressure sensor of the pump. In this way, the entry of liquid reducing agent without compressed air into a forcibly highly heated nozzle and the associated urea deposits can be prevented. At the beginning of the outer pipe 32, the nozzle lance 16 has a compensator 32a, which can compensate for thermally induced different longitudinal expansions of the inner pipe 31 and outer pipe 32.

[0047] In the direction of flow of the exhaust gas 9 upstream of the unit 13 for selective catalytic reduction, the temperature of the exhaust gas 9 is measured for control purposes, as the exhaust gas temperature should be between 220 and 550° C. Exhaust gas temperatures below 220° C. inhibit the decomposition of urea into ammonia or can promote its crystallisation. Exhaust gas temperatures above 550° C. require larger amounts of urea for chemical reasons and can more easily lead to overheating of the two-substance nozzle 19. Outside the permissible temperature range between 220 and 550° C., no reducing agent is injected. Furthermore, in the device according to the invention, a wire movement is checked via a rewinder signal, wherein no reducing agent is injected in the absence of a wire movement. With the aid of a measurement of the concentration of oxygen in the exhaust gas 9 in the direction of flow of the exhaust gas 9 upstream of the unit 13 for selective catalytic reduction, it is checked whether or not enamel is being introduced into the device according to the invention. If enamel is not being introduced, no reducing agent is injected into the exhaust gas 9 via the two-substance nozzle 19. If the device according to the invention fails or is shut down, the lines carrying the reducing agent and the nozzle lance 16 are flushed and completely emptied. A check of the filling level of a reducing agent tank by means of a float switch secures the pump against dry running. Sufficiently long pre-cooling times of the two-substance nozzle 19 with compressed air before starting the injection of the reducing agent prevents overheating of the inner pipe 31 of the nozzle lance 16 and thus boiling of the aqueous urea solution and urea failure in the two-substance nozzle 19. Sufficiently long flushing of the reducing agent-carrying lines with compressed air after the end of injection eliminates remaining reducing agent in the nozzle lance 16 and in the two-substance nozzle 19.

[0048] FIG. 4 shows a first embodiment of the two-substance nozzle 19, wherein the two-substance nozzle 19 is designed as an externally mixing two-substance nozzle. The externally mixing two-substance nozzle has a nozzle inner chamber 33, a nozzle outer chamber 34 and a nozzle outlet 35, which is arranged in a nozzle cap 36. In the shown embodiment according to FIG. 3, the inner pipe 31 of the nozzle lance 16 guides a mixture of the reducing agent and another compressed air to the nozzle inner chamber 33 of the externally mixing two-substance nozzle and the outer pipe 32 of the nozzle lance 16 guides the compressed air to the nozzle outer chamber 34 arranged coaxially around the nozzle inner chamber 33. The nozzle inner chamber 33 and the nozzle outer chamber 34 are connected separately to the nozzle outlet 35 so that the mixture of the reducing agent and the further compressed air exits the externally mixing two-substance nozzle separately from the compressed air. The nozzle cap 36 has a small depth, whereby the atomisation of the reducing agent takes place directly at the nozzle outlet 35. This results in a considerably lower back-pressure in the second line 30 to the pump, so that a diaphragm pump, for example, can be used as the pump. The nozzle cap 36 of the externally mixing two-substance nozzle can be unscrewed at any time and replaced by another cap.

[0049] FIG. 5 shows a second embodiment of the two-substance nozzle 19, wherein the two-substance nozzle 19 is designed as an internally mixing two-substance nozzle. The internally mixing two-substance nozzle has a nozzle inner chamber 33, a nozzle outer chamber 34, a mixing chamber 37 and a nozzle outlet 35. In the shown embodiment according to FIG. 5, the inner pipe 31 of the nozzle lance 16 guides the reducing agent to the nozzle inner chamber 33 of the internally mixing two-substance nozzle and the outer pipe 32 of the nozzle lance 16 guides the compressed air to the nozzle outer chamber 34 arranged coaxially around the nozzle inner chamber 33. The nozzle inner chamber 33 and the nozzle outer chamber 34 are connected to the mixing chamber 37 for mixing the reducing agent with the compressed air. From the mixing chamber 37, the reducing agent mixed with the compressed air exits the internally mixing two-substance nozzle via the nozzle outlet 35. When using an internally mixing two-substance nozzle, a pure reducing agent such as the aqueous urea solution can be fed to the nozzle inner chamber 33 via the inner pipe 31 without compressed air. In this case, for example, an electric gear pump can build up the necessary pressure and the dosing rate of the reducing agent can be controlled, for example, via a differential pressure measurement at the positions before and after a proportional valve of the gear pump.