DE102019200528B4 - Method for preventing the formation of urea-based deposits and exhaust system with a coating - Google Patents

Abstract

Method for preventing the formation of urea-based deposits in an exhaust system (1) of an internal combustion engine (2), comprising: - identifying areas of the exhaust system (1) in which there is a risk of the formation of urea-based deposits (S1), - determining a spatially resolved extent of the threatening Formation of urea-based deposits for the identified areas (S2) and - applying a coating (3), designed for the catalytic decomposition of aqueous urea solution and / or the urea-based deposits, to the identified areas, taking into account the determined extent (S3).

Description

The present invention relates to a method for preventing the formation of urea-based deposits in an exhaust system of an internal combustion engine and an exhaust system for an internal combustion engine with a coating.

A major challenge in the development of lean-burn engines and corresponding drive systems is the reduction of nitrogen oxide emissions in an overall oxidizing environment under various operating conditions.

The use of SCR exhaust gas aftertreatment devices (SCR, English selective catalytic reduction) is one of the main options for removing nitrogen oxides from the exhaust gas through catalytic conversion into non-toxic substances, i.e. H. Nitrogen and oxygen, and consequently reduce nitrogen oxide emissions. SCR technology is characterized by its robustness and the high conversion rates that can be achieved. In addition to pure SCR catalytic converters, particle filters with an SCR coating, hereinafter referred to as SDPF catalytic converters, are also known.

For the selective catalytic reduction, ammonia is required, which serves as a reducing agent and can be obtained, for example, from an aqueous urea solution by thermolysis of the urea to isocyanic acid and subsequent hydrolysis of the isocyanic acid to ammonia, which is usually in gaseous form under the conditions prevailing in the exhaust system. Aqueous urea solution can advantageously be stored easily and safely on board a vehicle.

For exhaust gas aftertreatment, the aqueous urea solution can be fed to the exhaust gas upstream of the SCR catalytic converter. This can be done by spraying the aqueous urea solution into an exhaust pipe of the exhaust system of the internal combustion engine. Using mixing devices, the aqueous urea solution and its decomposition products can be mixed with the exhaust gas in order to optimize the subsequent catalytic reduction.

Ideally, during the spraying process, the aqueous urea solution should not come into contact with surfaces until it has completely decomposed into ammonia, otherwise undesirable urea deposits can form. However, due to limited space and design parameters of the spray device, this cannot usually be achieved in practice. The resulting urea deposits can cause pressure drops and blockages. In any case, the corresponding urea proportion cannot be used for the catalytic reaction and therefore represents a loss and a disruption to the system and a deviation from the usable dosage amount required by the regulation.

To detect deposits in an exhaust tract of an internal combustion engine, the DE 10 2017 111 232 A1 a device with an image recording unit is known. Using the image recording unit, an image of the surface in an exhaust pipe can be captured. The image is received by an image processing unit, which processes the image to check for deposits on the surface. The presence of a deposit and/or the size of the deposit and/or a change in the deposit over time can be determined. However, it is not possible to avoid deposits.

To avoid urea deposits, the dosage rate of the aqueous urea solution can be reduced. This is how it reveals DE 10 2013 205 799 A1 a system and a method for improved fuel efficiency for vehicles with SCR catalytic converters. In some embodiments, the flow of urea to the exhaust system may be restricted to reduce the possibility of deposit formation. The beginning of the formation of deposits can be determined when the urea flow exceeds a threshold value for the flow velocity at a specified exhaust gas flow rate and temperature. However, under certain operating conditions, this can mean that nitrogen oxides contained in the exhaust gas cannot be sufficiently catalytically reduced because there is too little ammonia available.

However, in order to be able to reliably reduce nitrogen oxide emissions and comply with strict exhaust emission guidelines, it is necessary to be able to make sufficient use of the catalytic reduction at all times and therefore to be able to reliably provide the necessary ammonia to the SCR exhaust gas aftertreatment devices over their entire service life.

The DE 11 2010 000 713 T5 suggests applying a hydrolysis catalyst layer to various areas of the aftertreatment section of an exhaust system. The hydrolysis catalyst layer increases the decomposition rate of urea and urea-based deposits and reduces the formation of urea-based deposits. The hydrolysis catalyst layer is applied to surfaces that come into contact with urea, which is released into the exhaust gas using an injector holder strand is injected. In particular, the hydrolysis catalyst layer is applied to an inner surface of the injector holder, a pipe section, an outlet pipe and to the surface of a mixer.

According to DE 11 2010 000 713 T5 the hydrolysis catalyst layer is applied indiscriminately and uniformly to all surfaces downstream of the spray point, so that a lot of material is used to form the hydrolysis catalyst layer and consequently high costs are associated with the formation of the hydrolysis catalyst layer. In addition, the application of the hydrolysis catalyst layer leads to a change in the geometry in the exhaust system, since, for example, cross sections are reduced.

The DE 42 30 056 A1 describes an atomizer device that enables high-level atomization of liquid media. For this purpose, the liquid medium is thoroughly mixed with a compressed gas before it reaches the atomizer nozzle. This is achieved by the compressed gas hitting the media stream tangentially, thus causing both media to create a swirling movement, which should continue as far as possible in the line up to the atomizer nozzle. The atomizer device can be used to spray an aqueous urea solution to reduce nitrogen oxides. Here, the surface areas of the atomizer device that come into contact with the urea are coated with a catalytic layer made of platinized mixed oxide in order to avoid deposition of urea crystals and exhaust gas residues. The above with reference to DE 11 2010 000 713 T5 However, this does not eliminate the disadvantages mentioned.

The object of the present invention is to provide options with which at least some of the disadvantages mentioned can be reduced or eliminated.

This task is solved by the subjects of the main claim and the secondary claim. Advantageous developments of the invention are specified in the subclaims.

A method according to the invention for preventing the formation of urea-based deposits in the exhaust system of an internal combustion engine includes identifying areas of the exhaust system in which there is a risk of the formation of urea-based deposits, determining a spatially resolved extent of the impending formation of urea-based deposits and applying a coating catalytic decomposition of the urea-based deposits and/or aqueous urea solution. The coating is applied to the identified areas taking into account the determined extent of the threatened formation of urea-based deposits.

The exhaust system is designed to absorb exhaust gas formed by an internal combustion engine during the combustion process and to discharge it into the environment. The exhaust line is formed by one or more exhaust pipes through which the exhaust gas flows and in which z. B. catalysts, sensors, filters etc. are arranged. In the present case, an exhaust system is understood to mean the entirety of all exhaust pipes and devices arranged on or in the exhaust pipes.

For example, the exhaust system can have an SCR and/or SDPF catalytic converter and a feed device for supplying an aqueous urea solution to the exhaust gas, wherein the feed device can have a spray device for spraying the aqueous urea solution into an exhaust line upstream of the SCR and/or SDPF catalytic converter . The stated flow directions refer to the flow direction of the exhaust gas from the internal combustion engine towards the exhaust.

In a first step of the process, those areas of the exhaust system in which the formation of urea-based deposits is to be expected or are at risk are identified. This can be, for example, surfaces of the exhaust pipes or surfaces of devices, e.g. B. mixing devices, act in the exhaust system from the injection point to the SCR and / or SDPF catalytic converter. In other words, a rough estimate is first made as to which areas, e.g. B. could be affected by urea-based deposits at all due to the flow direction of the exhaust gas and the location of the injection point.

A spatially resolved extent of the impending formation of urea-based deposits is determined for these identified areas. In other words, the areas identified in the first step are subjected to a closer look and it is determined in a spatially resolved manner at which points in the areas there is actually a risk of urea-based deposits.

For this purpose, for example, a probability of the formation of urea-based deposits in the identified areas and/or an expected amount of urea-based deposits on the identified areas can be taken into account. The likelihood of urea-based deposits forming can e.g. B. on the probability of contact with an aqueous urea solution will be dependent on the identified areas. The expected amount of urea-based deposits can e.g. B. can be determined based on the expected amount of an aqueous urea solution contacting the identified areas.

In a further step, a coating designed for the catalytic decomposition of the urea-based deposits is applied to the identified areas, taking into account the determined extent of the threatened formation of urea-based deposits.

For example, the coating can be applied to those surfaces that are highly likely to come into contact with an aqueous urea solution and/or where, if contact with the aqueous urea solution occurs, a high amount of contacting aqueous urea solution is to be expected.

Consequently, the coating can, for example, be applied continuously or only in places or otherwise varied in order to take into account the extent of the impending formation of urea-based deposits at a specific position. In addition, the application of the coating can be used to create shape and/or structure in order to positively influence the flow of gas stream and liquid film.

The coating can, for example, be a washcoat, e.g. B. comprising or consisting of a metal oxide and / or zeolite, and one or more materials for catalyzing a decomposition reaction of the urea-based deposits. Such catalytically active materials can be, for example, metals or metal oxides, such as. B. transition metals and / or transition metal oxides, such as Ti, Pd, Pt, V, Cr, Mo, Ni or their oxides.

The coating increases the speed of a decomposition reaction of the urea, e.g. B. the thermolysis of urea to isocyanic acid and / or the hydrolysis of isocyanic acid to ammonia so that z. B. urea-based deposits are decomposed in a very short time to form ammonia or do not form at all. This allows more aqueous urea solution to be introduced into the exhaust gas without the formation of urea-based deposits.

In addition, the SCR and/or SDPF catalyst can be supplied directly with ammonia, which would otherwise first have to be formed in parts of the SCR and/or SDPF catalyst. Consequently, a larger volume of the SCR and/or SDPF catalyst is available for the catalytic reduction of the nitrogen oxides contained in the exhaust gas.

The overall effectiveness of exhaust aftertreatment can be increased. In addition, the service life of the exhaust system is increased, as urea-based deposits can also lead to reduced efficiency or functionality due to the build-up of deposits and even blocking of the exhaust system.

If urea-based deposits nevertheless form under certain operating conditions, they can be dissolved again using the coating at lower temperatures than without coating.

By applying the coating in a spatially resolved manner, the coating is adapted to the actual need for catalysis to prevent the formation of deposits. On the one hand, this means that coating material can be saved and the costs for the coating can be reduced, since e.g. B. Areas where no catalytic activity is required can remain uncoated. On the other hand, more coating and/or a different coating can be specifically applied to areas in which a very high catalytic activity is required, so that urea-based deposits can also be prevented in these areas.

By applying the catalytic coating in accordance with the required catalytic activity, a change in the geometry in the exhaust system due to a reduction in the flow cross sections can also be minimized and/or the geometry can be optimized in a targeted manner. For example, the coating can be applied to holes or structures, e.g. B. a mixing device

According to various embodiment variants, the determined extent of the impending formation of urea-based deposits can be taken into account by applying the coating with varying layer thickness and/or varying composition.

In other words, the layer thickness of the coating and/or its composition can be varied to take the determined extent into account. For example, a coating with a high layer thickness and/or a composition having high catalytic activity can be applied to those areas in which contact with the aqueous urea solution is very likely and/or which come into contact with large amounts of aqueous urea solution. Vice versa A small layer thickness can be selected for those areas in which contact with aqueous urea solution is unlikely.

Preferably, the use of material for the catalytic coating can be further optimized, so that on the one hand costs can be saved and on the other hand the effectiveness of the catalytic exhaust gas aftertreatment can be maintained or even improved.

According to further embodiment variants, the method can identify the areas and/or determine the extent using laboratory tests and/or using numerical flow calculation methods, also known as CFD. Known as Computational Fluid Dynamics.

These methods can provide particularly precise spatial resolution, so that the coating can be applied particularly precisely in accordance with the catalytic requirement and the advantages mentioned above can be even more effective.

Another advantage of computational fluid dynamics is that no actual components have to be manufactured for laboratory tests, so that the associated materials and the associated costs can be saved.

According to further embodiment variants, the coating can be applied using vapor deposition, plasma deposition and/or additive manufacturing methods. For example, the vapor deposition can be carried out as chemical vapor deposition (CVD) and/or physical vapor deposition (PVD), e.g. B. thermal evaporation, electron beam evaporation, laser beam evaporation and / or sputtering can be carried out, with plasma optionally being used to support both CVD and PVD processes.

Using the methods mentioned, the coating can be applied quickly and reliably. In addition, the layer thickness in particular can be easily adjusted by z. B. is coated for a longer period of time.

The use of additive manufacturing methods, e.g. B. 3D printing, for applying the coating advantageously enables a very precise and controlled construction of the coating, so that the coating can be adapted particularly well to the actual needs.

According to further embodiment variants, the layer thickness of the coating to be applied can be at least partially taken into account when designing and / or manufacturing the exhaust system in such a way that properties of the exhaust system, such as. B. flow cross sections, temperature distribution, etc., remain unchanged compared to an exhaust system without a coating.

In other words, the exhaust system, e.g. B. the exhaust pipe(s), mixing devices and/or spray devices, can be designed so that the coating is taken into account from the outset during the design or production, e.g. B. by including the flow cross sections resulting after coating in the design of the exhaust system.

In this way, for example, in the case of a coated mixing device for mixing exhaust gas and the supplied aqueous urea solution, it can be ensured that there is sufficient mixing even after coating. In other words, a negative impairment can e.g. B. the flow conditions in the exhaust system can be at least partially avoided by the coating.

According to further embodiment variants, the coating can be applied at least partially to form and/or structure.

This allows the formation and movement of the liquid film of the aqueous urea solution and/or the gas flow to be positively influenced.

According to further embodiment variants, the coating can be applied to areas of a mixing device, preferably a compact mixing device, areas of a pipeline and/or areas of a spray device designed for spraying an aqueous urea solution into the exhaust system.

If the coating is applied to a mixing device for mixing exhaust gas and the supplied aqueous urea solution, the formation of ammonia can be promoted at the time of mixing, so that downstream of the mixing device there is exhaust gas well mixed with ammonia, which corresponds to the SCR and / or SDPF. Catalyst is supplied. The catalytic reduction of the nitrogen oxides contained in the exhaust gas can thereby be improved.

In addition, the formation of urea-based deposits can be prevented, particularly in the area of the mixing device, its function would otherwise be particularly severely affected by such deposits.

A compact mixing device differs from a general mixing device in that it is designed to be more compact and therefore requires less installation space for its arrangement. However, due to its smaller dimensions, such a compact mixing device is particularly susceptible to urea-based deposits, i.e. H. Their functionality can be comparatively impaired even with small amounts of deposits. The application of the catalytically effective coating therefore has a particularly positive effect on a compact mixing device.

An exhaust system according to the invention for an internal combustion engine has a coating which is designed for the catalytic decomposition of urea-based deposits in areas of the exhaust system in which there is a risk of the formation of urea-based deposits. The coating has a varying layer thickness and/or a varying composition.

For example, the coating can be applied taking into account the extent of the threatened formation of urea-based deposits.

The exhaust system can, for example, be part of a motor vehicle, in particular a passenger car. The internal combustion engine can in particular be a diesel engine.

The exhaust system can have exhaust gas aftertreatment devices, such as. B. SCR, SDPF, LNT (lean nitroxide trap) or oxidation catalysts. The coating is preferably arranged upstream of the SCR or SDPF catalyst. For spraying an aqueous urea solution, the exhaust line can have a feed device with a spray device, designed to spray an aqueous urea solution into the exhaust gas flowing in the exhaust line.

The exhaust system according to the invention can be produced, for example, using one of the methods described above. Consequently, the advantages of the exhaust system correspond to the advantages of the method and its embodiment variants.

According to various embodiment variants, the coating can be applied to areas of a mixing device, preferably a compact mixing device, areas of a pipeline and/or areas of a spray device designed for spraying an aqueous urea solution into the exhaust gas.

According to further embodiment variants, the exhaust system can be dimensioned at least partially taking the applied coating into account.

• 1 ein Ablaufschema eines beispielhaften Verfahrens; und
• 2 einen beispielhaften Abgasstrang in einer schematischen Darstellung.

The invention is explained in more detail below with reference to the illustrations and the associated description. Show it:

• 1 a flowchart of an exemplary method; and
• 2 an exemplary exhaust system in a schematic representation.

1 illustrates the sequence of a method for preventing the formation of urea-based deposits in an exhaust system 1 of an internal combustion engine 2. In a first method step S1, those areas of the exhaust system 1 are identified in which there is a risk of the formation of urea-based deposits. This can, for example, be the area between a spray device 7 and an SCR or SDPF catalytic converter 13 arranged downstream of the spray device 7. Since the spraying device 7 serves to introduce an aqueous urea solution into the exhaust gas 8, there is a risk that urea-based deposits will form in the area of the spraying device 7 and downstream of the spraying device 7, for example. B. the water in the aqueous urea solution evaporates and urea precipitates before the urea has been decomposed. The formation of the urea-based deposits depends, among other things, on the temperature of the exhaust gas 8, the temperature of the exhaust gas line 1, i.e. z. B. a pipeline belonging to the exhaust gas line 1, the dosage amount of the aqueous urea solution and the flow conditions in the exhaust gas 8.

In other words, in the first step S1 those areas are determined in which the formation of urea-based deposits is even possible under certain conditions.

In the next process step S2, a spatially resolved extent of the impending formation of urea-based deposits is determined for these areas. For example, the probability of contact of the aqueous urea solution with the identified areas as well as an expected amount of contacting aqueous urea solution can be determined. A kind of map can therefore be created for the identified areas, showing how high the risk of urea-based deposits forming in a particular location is. Numerical flow calculation methods can be advantageous for creating the map can be used to simulate the risk. These can be done through laboratory tests, e.g. B. to validate the results obtained using simulation.

Process steps S1 and S2 can also be carried out together.

In a third step, based on the spatially resolved extent of the impending formation of urea-based deposits determined in step S2, a coating 3 is applied to the identified areas in the exhaust system, taking into account the previously determined extent. The coating 3 serves for the catalytic decomposition of the urea-based deposits, so that on the one hand existing deposits can be decomposed and on the other hand the formation of new deposits is largely prevented.

The coating 3 can, for example, be applied with varying layer thickness 4 and/or varying composition in order to take into account the different risk of the formation of urea-based deposits. For example, in areas with a high probability of the formation of urea-based deposits, a coating 3 with a large layer thickness 4 and / or increased catalytic activity can be applied, while in areas with a low probability of the formation of urea-based deposits, the coating 3 can be dispensed with entirely or with one correspondingly smaller layer thickness 4 and/or a composition having lower catalytic activity can be applied.

2 shows an exhaust system 1 connected to an internal combustion engine 2 in an exemplary embodiment, as shown, for example. B. can be produced using the method described above. The exhaust system 1 can be arranged in a motor vehicle, for example.

In the exemplary embodiment, the internal combustion engine 2 has four cylinders 11 and is designed as a diesel engine. However, the invention is not limited to a specific number of cylinders or a specific type of engine, but can be used flexibly in combination with a variety of different engines.

The internal combustion engine 2 is supplied with supply air 10 via a supply air line 9. During the combustion process, an exhaust gas 8 is formed in the internal combustion engine 2 and is released into the exhaust system 1, which discharges the exhaust gas 8 into the environment. An exhaust gas aftertreatment device 12 is initially arranged in the exhaust system 1, which is z. B. can be an LNT catalyst or a diesel oxidation catalyst. Optionally, several or no exhaust gas aftertreatment devices 12 can also be provided. An SCR catalytic converter 13 is arranged downstream of the exhaust gas aftertreatment device 12, which can optionally also be designed as an SDPF catalytic converter.

The SCR catalytic converter 13 catalytically reduces nitrogen oxides contained in the exhaust gas 8, for which the SCR catalytic converter 13 requires ammonia. The ammonia required is obtained in situ in the exhaust gas line 1 from an aqueous urea solution. The aqueous urea solution is stored in a storage tank 14 and, if necessary, supplied via a feed line 15 to a spray device 7 arranged upstream of the SCR catalyst 13. The spraying device 7 is used to spray the aqueous urea solution into the exhaust gas 8 flowing in the exhaust gas line 1.

A mixing device 5 is also arranged between the spray device 7 and the SCR catalytic converter 13, which can preferably be designed as a compact mixing device. The mixing device 5 serves to mix the exhaust gas 8 with the sprayed aqueous urea solution.

Under certain operating conditions, it can happen that the sprayed aqueous urea solution is not completely decomposed to form ammonia, but rather urea-based deposits form in the area between the spray device 7 and the SCR catalytic converter 13. In order to largely prevent this, the exhaust system 1 has a coating 3 in areas where there is a risk of the formation of urea-based deposits, which is designed for the catalytic decomposition of urea-based deposits. The coating 3 has a washcoat, e.g. B. made of aluminum oxide or zeolite, in which catalytically active components are embedded that catalyze the decomposition of urea-based deposits.

The coating 3 has a different layer thickness 4. For example, the coating 3 is applied to the wall of the pipe of the exhaust gas line 1 opposite the spraying device 7 with a high layer thickness 4, the layer thickness 4 decreasing in the direction of the spraying device 7, since less wall wetting is to be expected in the area of the spraying device 7.

The mixing device 5 also has the coating 3, with the layer thickness 4 on the upstream side and on the downstream side, where any liquid film that may form moves, being greater than at points outside the spray that forms when the sprayed urea solution hits the exhaust gas 8 gel is. Optionally, the composition of the coating 3 can also vary.

Due to the different layer thickness 4 and/or composition, the extent of the impending formation of urea-based deposits can be taken into account.

In addition, the exhaust system 1 is the 2 partly dimensioned taking the coating 3 into account. i.e. The coating 3 was already taken into account in the construction of the exhaust system 1, so that the coating 3 influences the flow cross section as little or positively as possible.

Reference symbol list

1

Exhaust system

2

Internal combustion engine

3

Coating

4

Layer thickness

5

Mixing device

6

pipeline

7

Injection device

8th

exhaust

9

supply air line

10

supply air

11

cylinder

12

Exhaust gas aftertreatment device

13

SCR catalyst or SDPF catalyst

14

storage tank

15

Feed line

S1 to S3

Procedural steps

Claims (13)

Method for preventing the formation of urea-based deposits in an exhaust system (1) of an internal combustion engine (2), comprising: - Identifying areas of the exhaust system (1) in which there is a risk of the formation of urea-based deposits (S1), - Determine a spatially resolved extent of the threatened formation of urea-based deposits for the identified areas (S2) and - Applying a coating (3), designed for the catalytic decomposition of aqueous urea solution and/or the urea-based deposits, to the identified areas, taking into account the determined extent (S3). Procedure according to Claim 1 , wherein determining the extent takes into account a probability of formation of urea-based deposits in the identified areas and/or an expected amount of the urea-based deposits on the identified areas. Method according to one of the preceding claims, wherein the determined extent is taken into account by applying the coating (3) with varying layer thickness (4) and/or varying composition. Method according to one of the preceding claims, wherein the identification of the areas and/or the determination of the extent is carried out by means of laboratory tests and/or by means of numerical flow calculation methods. Method according to one of the preceding claims, wherein the coating (3) is applied by means of vapor deposition, plasma deposition and/or additive manufacturing methods. Method according to one of the preceding claims, comprising: - at least partially taking into account the layer thickness (4) of the coating (3) to be applied when designing and/or manufacturing the exhaust system (1) in such a way that properties of the exhaust system (1) remain unchanged compared to an exhaust system without a coating (3). Method according to one of the preceding claims, wherein the coating (3) is applied at least partially to form a shape and/or structure. Method according to one of the preceding claims, wherein the coating (3) is applied to areas of a mixing device (5), preferably a compact mixing device, areas of a pipeline (6) and / or areas of a spray device (7), designed for spraying an aqueous urea solution into a Exhaust gas (8) is applied. Method according to one of the preceding claims, wherein the coating (3) has a washcoat and one or more materials for catalyzing a decomposition reaction of the urea-based deposits. Exhaust system (1) for an internal combustion engine (2) with a coating (3), designed for the catalytic decomposition of urea-based deposits in areas of the exhaust system (1) in which the formation of urea-based deposits threatens to occur, the coating (3) having a varying layer thickness (4) and/or a varying composition. Exhaust system (1). Claim 10 , wherein the coating (3) is applied taking into account the extent of the threatened formation of urea-based deposits. Exhaust system (1). Claim 10 or 11 , wherein the coating (3) is applied to areas of a mixing device (5), preferably a compact mixing device, areas of a pipeline (6) and / or areas of a spray device (7), designed for spraying an aqueous urea solution into the exhaust system (1). is. Exhaust system (1) according to one of the Claims 10 until 12 , wherein the exhaust system (1) is at least partially dimensioned taking into account the applied coating (3).

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