FR3029966A1 - NOX NITROGEN OXIDE REDUCTION DEVICE FOR A THERMAL ENGINE EXHAUST LINE - Google Patents

Abstract

The invention relates to a device for reducing NOx nitrogen oxides for an exhaust line of a heat engine, particularly a motor vehicle, said device comprising a source of liquid reducing agent precursor (8) and a decomposition module (6) the precursor of said precursor as a reducing agent, the module comprising a heating means and a catalyst of a reaction participating in the decomposition of the precursor into a reducing agent, the decomposition module (6) comprising a decomposition chamber containing said catalyst and heated by said heating means to a temperature of at least one of the reactions participating in the decomposition of the precursor to a reducing agent, such that the decomposition chamber comprises at least one decomposition line in which the precursor of The liquid reducing agent is at least partially decomposed into a reducing agent in liquid form as well.

Description

The invention relates to devices for reducing NOx nitrogen oxide emissions in the exhaust gases of combustion engines. The invention relates to devices for reducing emissions of NOx nitrogen oxides in the exhaust gases of thermal engines. especially vehicles of the motor vehicle type. [0002] Nitrogen oxides are the subject of active research aimed at their strong reduction or even their elimination. Legislation imposes increasing constraints on the acceptable levels of nitrogen oxides emitted. To achieve these levels, one method is to use an ammonia precursor source such as aqueous urea, the ammonia being introduced into the exhaust gas stream and reacting with the nitrogen oxides on a catalyst for reducing ammonia. oxides of nitrogen to form inert nitrogen and water. Such a nitrogen oxide reduction catalyst is known as SCR for Selective Catalytic Reduction in English or Selective Catalytic Reduction. Such a catalyst becomes active for temperatures as low as temperatures between 120 ° C and 140 ° C. To inject the aqueous urea with a complete decomposition thereof ammonia NH3, the exhaust gas must be at a temperature above 180 ° C. This is a strong constraint, especially in the case where the temperatures of the exhaust gases are regularly below 180 ° C., whereas the ammonia present is itself continuously consumed by the reduction reaction of the nitrogen oxides on the SCR catalyst. . For this reason, NH3 is pre-stocked in a so-called NOX coating for "nitrogen oxide reduction" to overcome periods when aqueous urea injection is no longer possible due to low temperatures. typically for temperatures below 180 ° C. However, when the reduction catalyst of the nitrogen oxides has no or almost no ammonia due to prolonged gas temperatures below 180 ° C, then the nitrogen oxides are no longer treated, which is a problem in terms of exhaust emissions and compliance with legal requirements. Some technologies consist, as proposed in US2008 / 0112872, in injecting aqueous urea, also known under the trade name Adblue - trademark - in a miniature hydrolysis catalyst to directly decompose Adblue ammonia before the ammonia enters the exhaust line. However, this type of device suffers from the problem that urea 3029966 2 used at low temperature polymerizes and clogs the exhaust line. In addition, the zone situated between the injection head and the heated surface has a low flow, which creates a low penetration of the fluid onto the catalyst, and thus a weak decomposition and therefore a polymerization phenomenon. The liquid evaporates, allowing the urea to concentrate as it falls on the hot surface. The concentrated urea tends to react rapidly to form a polyamide material such as biuret, the triuret, which mechanically deactivates the catalytic surface and the outlet of the injector. The injected spray falls on the hot surface and polymerizes to form a very stable polyamide, which does not decompose until it is exposed to a temperature above 450 ° C for a prolonged period. In addition, introduction of a specific hydrolysis brick into the exhaust system generates additional cost and bulk. Another proposal, which allows to partially overcome the barrier of 180 ° C, is to inject successively small amounts of urea with a temperature falling below 180 ° C. This proposal appears in FR2008 / 050397. This technology works but it does not reduce the minimum injection temperature, initially at 180 ° C, up to 170 ° C. Thus this method, although allowing greater flexibility, further limits the injection temperature of urea. [0007] There are documents describing injectors containing catalytic materials within the aqueous urea injector structures. However, these documents propose a heating member associated with the catalytic material. Integrating a reheater renders such injectors considerably more expensive. Such configurations of injectors appear in the documents FR2936958, W005025725. In addition, the fact of adopting a coating material in a structure as delicate as that of an injector proves impractical because of the fragility of the coating and the imprecision of the coating deposit, particularly in terms of the amount of material , thick, etc. The object of the invention is to provide a device for decomposing a reducing agent precursor of NOx reducing agent that overcomes the aforementioned drawbacks, and which can, in particular, prevent clogging of the device, in particular in the injector of the product before introduction into the exhaust line of the engine, which may also allow to inject the reducing agent into the exhaust line at low temperature. The object of the invention is also to achieve these objectives with a device which remains compact, compatible with the size of the environment of a heat engine and its exhaust line. The subject of the invention is a device for reducing NOx nitrogen oxides for an exhaust line of a heat engine, especially a motor vehicle, said device comprising a source of liquid reducing agent precursor and a modulus for decomposing the precursor of said precursor into a reducing agent, the module comprising a heating means and a catalyst of a reaction participating in the decomposition of the precursor into a reducing agent, the decomposition module comprising a decomposition chamber containing said catalyst and heated by said heating means to a temperature of at least one of the reactions participating in the decomposition of the precursor into a reducing agent, such that the decomposition chamber 10 comprises at least one decomposition line in which the precursor of The liquid reducing agent is at least partially decomposed into a reducing agent in liquid form as well. We therefore remain in a liquid phase, and it is therefore ammonia in the liquid phase that is injected into the exhaust line, and not ammonia gas if it is this reducing agent. who is chosen. The preferred embodiment of the invention consists in the fact that the reducing agent precursor is urea in the liquid phase (aqueous), and the reducing agent is liquid phase ammonia ( aqueous). In the remainder of the text, for the sake of brevity, mention may be made of urea for any reducing agent precursor agent and ammonia for any reducing agent resulting from the decomposition of the precursor. The invention can be implemented by means of a nitrogen oxide reduction device in a motor vehicle exhaust line, comprising a reducing agent source and a reducing agent decomposition module. in ammonia, the module comprising a heating means and a catalyst of a reaction participating in the decomposition into ammonia, the decomposition module comprising a decomposition chamber in which circulates the reducing agent which decomposition chamber contains said catalyst and which decomposition chamber is heated by said heating means to a temperature of completion of said reaction participating in ammonia decomposition, such that the decomposition chamber is a decomposition line having a sufficiently high length and a sufficient section narrow so that the precursor of the reducing agent remains in the liquid phase during its circulation ulation in the decomposition line at the temperature of completion of said reaction participating in decomposition into ammonia and decomposes, still in the liquid phase, into ammonia. Advantageously, the decomposition pipe has a length of between 1 cm and 6 cm. Advantageously, the decomposition pipe has an internal section of between 0.008 mm 2 and 0.8 cm 2. [0015] Advantageously, the decomposition pipe has a length of between 3.5 cm and 4.5 cm. Advantageously, the decomposition pipe has an internal section of between 0.38 mm 2 and 1.33 mm 2. Advantageously, the decomposition line has a ratio between its internal diameter and its length, which is between a ratio of 1 to 10 and a ratio of 1 to 1000. Advantageously, the decomposition line has a ratio between its internal diameter and its length which is located between a ratio of 1 to 40 and a ratio of 1 to 60. Advantageously, the decomposition pipe has a ratio between its internal diameter and its length which is a ratio of Advantageously, the decomposition line has an internal volume devoid of catalyst which is preferably between 5% and 80% of its total internal volume comprising the catalyst. Advantageously, the decomposition line has an internal volume 20 devoid of catalyst which is preferably between 20% and 50% of its total internal volume comprising the catalyst. Advantageously, the device comprises an exhaust gas guiding wall and the heating means is constituted by the exhaust gas guiding wall. Advantageously, according to one embodiment, the device comprises an injector body of the liquid phase reducing agent (ammonia) arranged to inject said agent (ammonia) into an exhaust line and the decomposition line forms with the injector body a member configured to be integrally mounted in a vehicle. [0024] Advantageously, the temperature of carrying out said reaction participating in the decomposition of the precursor (urea) into a reducing agent (in ammonia) is at least 100 degrees Celsius. Advantageously, the decomposition pipe describes a helical shape. Advantageously, according to another embodiment, the device comprises an injector body of the reducing agent in the liquid phase (ammonia) arranged in such a way as to inject the reducing agent (ammonia) in a line of exhaust and the decomposition line is disposed upstream of the injector body with respect to a reducing agent precursor flow direction and reducing agent (ammonia) from the reducing agent precursor source to the exhaust line . Advantageously, according to another embodiment, the device comprises an injector body of the precursor in aqueous phase (urea) and the decomposition line is disposed downstream of the injector body with respect to a direction of flow. reducing agent precursor and reducing agent from the reducing agent precursor source to the exhaust line. [0028] Advantageously, the catalyst is a stable catalyst in aqueous solution. Advantageously, the catalyst has an acidic surface in contact with aqueous urea. According to a variant, the walls of the decomposition pipe are at least partly covered with the catalyst. According to another variant compatible with the preceding one, the catalyst may be in the form of beads (or covering balls) in the pipe. For example, the decomposition line has an internal volume devoid of catalyst which is preferably between 5% and 80% of its total internal volume comprising the catalyst, and in particular between 20 and 50% of said internal volume. Other features, objects and advantages of the invention will appear on reading the description which follows, with reference to the appended figures, in which: FIG. 1 represents an exhaust line according to a mode of Embodiment of the Invention, FIG. 2 shows a decomposed urea injector according to a first embodiment of the invention, FIG. 3 represents a decomposed urea injector according to a second embodiment of the invention. - Figure 4 shows a decomposed urea injector according to a third embodiment of the invention. In Figure 1, there is shown an exhaust line extending from a motor block 30 which is referenced 1. The exhaust line comprises successively a diesel oxidation catalyst 2, a selective catalytic reduction catalyst or SCR 3, and a soot type particle filter 4. An onboard computer is represented under the reference 5, which here controls the various members of the exhaust line. A urea injection device 6 with catalytic decomposition of the urea is disposed between the diesel oxidation catalyst 2 and the selective catalytic reduction catalyst 3. The exhaust line is further equipped with a thermocouple or temperature acquisition device 7 arranged directly adjacent to the urea injection device 6. A urea reservoir 8 supplies the urea injection device 6 and two nitrogen oxide detectors or ammonia 9 and 10 are also positioned before and after the catalyst SCR 3. The urea injection device 6 comprises in particular a urea injector which will now be described an embodiment with reference to FIG. Figure 2. [0036] In Figure 2, there is shown under the reference 21 an injector body which is configured as an aqueous urea injector body. The injector body 21 is served by a urea supply line 22 and an aqueous urea pump 23. An exhaust line 24 extends in a horizontal direction at the bottom of the figure, which line Exhaust body is delimited by a wall 25. The injector body 21 thus opens into the exhaust line through an opening in the wall 25 and here injects ammonia, thereby forming an ammonia injector body . Around the injector body is disposed a fine line 26 hereinafter called a decomposition line in which the aqueous urea circulates from line 22. The aqueous urea, after decomposition, is injected into the exhaust line 24 by an injection head, not shown, which injection head is here formed in the lower part of the injector body 21. The injector body 21 can be an atomizer, an evaporator or a simple valve to control the flow of aqueous urea in the decomposition line 26. Thus the injector body 21 can be replaced by simple valves upstream and downstream of the decomposition line 26. The injector body 21 and / or the pipe Decomposition 26 may be heated externally by a dedicated heater or may be heated by conduction through the exhaust line. The injector body is placed upstream of the nitrogen oxide reduction catalyst 3, which is the seat of the reaction between the nitrogen oxides and the ammonia. A probe is advantageously placed in the device to test the quality of the decomposition of urea. The injection of the urea solution can be carried out at atmospheric pressure or at high pressure. The duct 26 here has a helical shape wound around the injector body 21, and forms a monoblock member with the latter so that the duct 26 is an integral part of the injector comprising the injector body 21. and the conduit 26. The conduit 26 may alternatively be contained in the injector body 21. [0038] The conduit 26 here describes a helical path of 4 cm in length and has an internal diameter of 1 mm. Any form of pipe can be used, such as a linear tube, a U-pipe, etc. With such dimensions, it turns out that the aqueous urea runs through line 26 without evaporation of the water in which the urea is solvated, and thus no two-phase separation occurs, a vapor phase for water and a solid phase for the urea crystals, which could then occur polymerize when subjected to high temperatures. In other words, such a dimensioning of the pipe 26 makes it possible to avoid evaporation of the aqueous urea which is known to generate a clogging by polymerization. There is therefore passage of the reducing fluid between the injector and the catalytic materials without breaking the liquid phase, thus eliminating the risk of the separation of the solvent and urea phases, the solvent being water and the urea being the reducer, during the injection at low temperature which would cause the polymerization of urea and irreversible fouling of the injector, the exhaust line and catalyst. This passage may be catalytic material to the injector or the injector to the catalytic material. More generally, such a decomposition line is found to maintain the urea in the liquid phase in a proportion satisfactory for the elimination of the clogging effect. This liquid phase maintenance effect is obtained when the decomposition line has an internal diameter of between 100 microns and one centimeter, corresponding to a section of between 0.008 mm 2 and 0.8 cm 2, for a length of between 1 cm and 6 cm. cm in this range of sections. A particularly effective effect for a vapor phase elimination combined with an adequate circulation of the aqueous urea stream is obtained for an internal diameter of between 0.7 mm and 1.3 mm, corresponding to an internal section of between 0.38 mm 2 and 1.33 mm 2, and for a pipe length of between 3.5 and 4.5 cm in this range of sections. Such conduits can be found from suppliers of chromatographic analysis equipment, especially under the name of "capillary tube" or "capillary column". These selections are considered independently of the filling of the catalytic material pipe, the holding effect in the aqueous phase proving to be due to the dimensions of the tube itself and not to the nature of its catalytic material or the quantity filling the tube in such a catalytic material. The tube 26 is here filled with catalytic particles, more precisely here catalytic beads, which here fill 60% of the volume of the tube 26. More generally, such particles can fill from 10% to 90% of the volume of the tube. Alternatively, the tube 26 may be provided with a catalytic coating on the inside wall. The porous catalytic material 3029966 8 in the decomposition line is preferably a material generating a low return pressure with good liquid flow characteristics, the aqueous urea arriving on one side and the aqueous ammonia coming out on the other side. the zone of presence of this porous material. Although the tube 26 is here described in the form of a capillary tube, a given volume is more generally adopted, independent of its shape, and preferably a thin tubular shape to maximize heat exchange, for example with the exhaust line of hot air, etc. Such a volume can have multiple shapes, such as a helix, a U-shape, a monotube, a multi-tube shape, and so on. Particularly in the case of a monotube, it preferably has a ratio between the internal diameter and the length which lies between a ratio of 1 to 10 and a ratio of 1 to 1000, with a ration of preferably between 1 to 40 and 1 to 60, preferably 1 to 50. Also, the internal volume of the tube devoid of catalytic material is preferably between 5% and 80% of its internal volume and preferably between 20 and 50%. The tube 26 is here placed in contact with the wall 25 of the exhaust line 24 so that the tube 26 is maintained at a temperature adequate to promote the decomposition reaction of aqueous urea ammonia. Alternatively, the tube 26 is equipped with an electric heater. As soon as the thermal conduction caused by the exhaust gas warms the decomposition line 26 to a temperature above 100 ° C, the reaction rate of the urea hydroyse is sufficiently high to ensure a decomposition of urea in satisfactory NH3. The decomposition reactions are, in a manner known per se, the following reactions: The thermolysis reaction: 2HN-CO-NH 2 (urea) NH 3 + HCNO The hydrolysis reaction: HCNO + H 2 O NH 3 + CO 2 The reaction catalytic decomposition or direct hydrolysis: 2HN-CO-NH 2 + H 2 O 2 × NH 3 + CO 2 According to the embodiment of FIG. 3, which uses the same reference numerals for the same elements, the decomposition conduit, referenced here 27, 30 is disposed downstream of the injector body 21 of aqueous urea. In this case, the decomposition line 27, the dimensions of which are in the aforementioned ranges with a length shorter than the decomposition line of FIG. 2, constitutes itself the injector head. Alternatively, the decomposition line 27 has a return section through which the decomposed urea rises upstream of an injection head 35 formed by the injector body 21. In this embodiment also, the conduit 3029966 9 is placed in contact with the exhaust line 24 so that the pipe is kept at a high temperature to allow efficient decomposition of the aqueous urea, and this from the initial phase of starting the vehicle and also in idle operation the vehicle, while avoiding evaporation of aqueous urea 5 through the dimensions of the pipe, Alternatively, the pipe 27 may be equipped with an electric heater. According to the embodiment shown in Figure 4, the decomposition line, here referenced 28 is also configured in the range of dimensions described above with reference to Figure 2. The decomposition pipe is here placed upstream. of the injector body 21. The injector body 21 is a conventional injector body, undergoing no particular modification. In this embodiment, the pipe 28 is equipped with an electric heater to carry it quickly and maintain it in the range of temperatures suitable for the decomposition of urea. Any type of heating can be used: Induction, heat exchanger etc. Also, the catalytic material, here deposited by deposition, can be placed inside the exhaust line. In these various embodiments, the aqueous urea is therefore circulated, alternatively any other nitrogen oxide reducing agent such as an amine-based reducing agent, for example ammonium formate, guanidine formate, biuret, etc., through a pipe of selected dimensions, which is charged with catalyst material, in particular for the hydrolysis reaction, which allows a direct decomposition of the urea in NH 3 while remaining in the liquid phase. The urea thus decomposed into NH 3 is then injected into the exhaust gas. The advantages of such a device are as follows. The liquid phase is maintained and any possibility of polymerization of the urea is avoided because there is no evaporation prior to the formation of ammonia. The long thin tube, or capillary column, is an elongate flow path for aqueous urea which provides sufficient time for the urea to undergo the hydrolysis reaction. Once ammonia is formed, it can be introduced into the exhaust line upstream of the SCR nitrogen oxide reduction catalyst at temperatures below 180 ° C. The decomposition line may be made of metal or any other heat resistant material such that the conduit may be advantageously brought into contact with the heated exhaust line or may be externally heated. Such pipes are inexpensive to produce and inexpensive to impregnate catalytic material, whatever the latter. The warmer the catalytic material, the faster the decomposition of the urea in the liquid phase. Such a device can be placed in the form of an organ ready to be placed on an existing aqueous urea injector system. The decomposition catalyst of urea may have any formulation. It is advantageously chosen to be stable in aqueous solution and it preferably has an acid-type surface. Indeed catalytic decomposition of urea occurs preferentially on acid surfaces. But any active surface can be used. The catalytic support may be made of a material that is ceramic, organic, organometallic, or metallic. Non-limiting examples are B 4 C, Si 3 N 4, BN, AlN, Al 2 O 3, ZrO 2, mullite, AIT 1, ZrB 2, TiO 2, CeO 2, Y 2 O 3, SiO 2, Fe, Sialon, carbon, organic polymers, cordierite, etc. If a catalytic coating is adopted, it can typically be composed of SiO 2, Al 2 O 3, TiO 2, ZrO 2, CeO 2, Y 2 O 3 with typical catalytic elements such as PM, Pt, Pd, Rh, Ru, Re, Ir, Au, Ag, transition metal oxides such as Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, and elements of the actinium series and their oxides, such as Th, Pa, U, Np, Pu, Am, Cm, Bk, Cf, Es, Fm, Md, No, Lr, perovskites and Cu, Fe, or zeolites or clays doped with Mn or zeolites or undoped clays. The catalytic surface may be covered with an acidifying compound in order to increase the decomposition of the urea by adding, for example, 5030H sulfuric acid groups, HNO3 nitric acid groups and H000 organic acid groups. Such examples may include HNO3-HCOOH-Pt / SiO2 or H504-HCOOH-Al2O3. The method of impregnating catalytic material for the hydrolysis of aqueous urea is known as such and may consist of a dry column packing, a chemical vapor deposition or CVD for Chemical Vapor Deposition, in a incipient wetting method or incipient wetting in English, in a suspension or slurry method in English, etc. The support material can be produced in a first phase and then the active catalytic material can be introduced in a second phase. The zoning method can also be used in the type of conduct proposed here. Such injectors can be set up so as to inject the urea thus decomposed in the main exhaust line or in a bypass thereof, that is to say in a gas stream. exhaust taken on the line and reintroduced into it. The decomposition tube may consist of a single volume or several pipes, in particular between 1 to 300 pipes in the same catalyzed structure or monolith.

Claims (10)

REVENDICATIONS1. Device for reducing NOx nitrogen oxides for an exhaust line (24) of a heat engine, particularly a motor vehicle, said device comprising a source of liquid reducing agent precursor (8) and a decomposition module (6) ) of the precursor of said reducing agent precursor, the module comprising a heating means (24,25) and a catalyst of a reaction participating in the decomposition of the precursor into a reducing agent, the decomposition module (6) comprising a decomposition chamber (26,27,28) containing said catalyst and heated by said heating means (24,25) to a production temperature of at least one of the reactions participating in the decomposition of the reducing agent precursor, characterized in that the decomposition chamber (26,27,28) comprises at least one decomposition line (26,27,28) in which the liquid reducing agent precursor is at least partially decomposed into a reducing agent in liquid form as well. 2. Device according to the preceding claim, characterized in that the reducing agent precursor is urea in the liquid phase, and in that the reducing agent is ammonia in the liquid phase. 3. Device according to one of the preceding claims, characterized in that the decomposition pipe (26,27,28) has a length of between 1 cm and 6 cm, in particular between 3.5 cm and 4.5 cm .. 4. Device according to one of the preceding claim, characterized in that the decomposition line (26,27,28) has an internal section of between 0.008 mm 2 and 0.8 cm 2 'and in particular between 0.38 mm 2 and 1 , 33 mm2. 5. Device according to one of the preceding claims, characterized in that the decomposition pipe (26,27,28) has a ratio between its internal diameter and its length which is between a ratio of 1 to 10 and a ratio of 1 to 1000, and especially between a ratio of 1 to 40 and a ratio of 1 to 60, and preferably has a ratio between its internal diameter and its length which is a ratio of about 1 to 50. 6. Device according to one of the preceding claims, characterized in that the walls of the decomposition pipe are at least partly covered with the catalyst. 7. Device according to any one of the preceding claims, characterized in that it comprises an exhaust gas guiding wall (25) and the heating means is constituted by the guiding wall (25) of the exhaust gases. exhaust. 3029966 12 8. Device according to any one of the preceding claims, characterized in that it comprises a reducing agent precursor injector body, such as urea, (21) arranged to inject the reducing agent. , such as ammonia, in an exhaust line (24) and the decomposition line (26,27,28) forms with the ammonia injector body (21) a member configured to be mounted monobloc in a vehicle. 9. Device according to one of the preceding claims, characterized in that it comprises a reducing agent injector body, such as ammonia, in the liquid phase (21) arranged to inject said agent in a line exhaust (24) and the decomposition line (28) is disposed upstream of the injector body (21) with respect to a flow direction of reducing agent and ammonia from the reducing agent source to 'to the exhaust line. 10. Device according to one of the preceding claims 1 to 8, characterized in that it comprises a reducing agent precursor injector body (21) and the decomposition pipe (27) is disposed downstream of the body. injector (21) with respect to a flow direction of the reducing agent precursor, such as urea, and reducing agent, such as ammonia from the reducing agent precursor source to the exhaust line.