WO2000008314A1

WO2000008314A1 - Exhaust system comprising a device for catalytically reducing nox and a catalyst support body made of fibers

Info

Publication number: WO2000008314A1
Application number: PCT/DE1999/002319
Authority: WO
Inventors: Alfred Buck; Axel Hartenstein
Original assignee: Alfred Buck; Axel Hartenstein
Priority date: 1998-07-31
Filing date: 1999-07-30
Publication date: 2000-02-17
Also published as: DE19834541C2; DE19834541A1; AU6187699A

Abstract

The invention relates to an exhaust system (1) which functions according to the SCR catalyst principle. The exhaust system comprises an injection device (5) for a chemical compound which contains ammonia or which splits ammonia in the exhaust gas environment. A catalyst arrangement (9) is located downstream from the injection device (5) in which the substrate for the catalyst is formed by a knitted fabric. The threads of this knitted fabric are coated with a catalytic material, preferably vanadium pentoxide.

Description

EXHAUST SYSTEM WITH A DEVICE FOR CATALYTIC NOX REDUCΗON AND A FIBERAL CATALYST SUPPORT BODY

As is known, undesirable by-products are produced in closed combustion, as is found in combustion mocors, and nitrogen oxides at elevated temperatures. Unfortunately, the proportion of nitrogen oxides increases when the engine is adjusted particularly economically in terms of fuel consumption.

So-called SCR catalysts, as described in EP 0 558 452, are used to largely remove the nitrogen oxide from the exhaust gas stream. SCR catalysts are particularly suitable for practically completely eliminating very high levels of nitrogen oxide. At high nitrogen oxide loads in the exhaust gas, they work more effectively than catalysts in which the catalyst material is firmly seated on a substrate. SCR catalysts have also advantages in internal combustion engines, which have a considerable amount of carbon black because the Rußanceil to rial down strike in the catalyst ^bodies' with applied also in the surface of catalyst material on the Katalysacormate- and make it ineffective. That's why SCR catalytic converter devices are particularly advantageous for diesel engines.

In the system as described in EP 0 558 452, a solution of 40% urea and 60% water is injected into the exhaust gas stream.

The exhaust gas stream loaded with the urea reaches a swirling device in which a particularly good mixing of the exhaust gas stream with the solution is to be achieved. At the same time, the droplet size of the lost solution is reduced in the swirling device in order to obtain a corresponding increase in surface area. At this point, part of the NO _{: <} already reacts with the ammonia. A reduction catalytic converter connects to the outlet of the swirling device, the subscrat of which is coated with vanadium pentoxide or platinum. The substrate of this catalyst is a monolith with a variety of gas channels.

Despite the considerable spatial dimensions of this reduction catalytic converter, the dwell time of the exhaust gas flow in the oxidation catalytic converter is too short to be able to remove all of the soot and carbon onoxide contained in the exhaust gas. For this reason, in the known arrangement, the reduction catalyst is followed by an oxide acion catalyst, the substrate of which is also a perforated brick which is coated with platinum or palladium.

The large number of catalysts is also required to ensure that no free ammonia exits the exhaust, ie all of the ammonia has reacted in the desired manner with the nitrogen oxide in the gas stream. Proceeding from this, it is an object of the invention to provide an exhaust system for internal combustion engines which is more economical in terms of ammonia consumption and has smaller dimensions.

This object is achieved according to the invention by the exhaust gas system with the features of claim 1.

The use of a catalytic converter, the substrate of which is made of fibers, leads to a substantial increase in surface area and thereby two major advantages are achieved:

Due to the larger surface area, the carbon onoxide can be converted more efficiently into the non-hazardous carbon dioxide by catalysis. The escaping exhaust gas stream is practically completely free of carbon monoxide.

In addition, the fiber-based catalyst device also acts as a particle or soot filter in which the soot is trapped and is also oxidized to carbon dioxide with the aid of the catalytic coating. Finally, the fibers with the catalytic coating act as an equal carrier for the previously unused amount of ammonia-containing solution, which is deposited extremely finely on the fibers and thus forms a very large reaction surface for nitrogen oxide that is still present.

Since there is only one catalytic converter, the volume of the exhaust system is very small.

In order to prevent the fibers from coming out, the fibers are preferably processed into a textile fabric in which they are firmly anchored. It has turned demonstrated that a knitted fabric is a textile fabric that is particularly suitable for catalytic converters, that holds the fibers very well and does not dissolve even when the thread breaks. A knitted fabric can be laid very well even with stubborn fibers, without forming unwanted small folds that create large-volume gas channels through which the exhaust gas can flow uncleaned.

A particularly good filter and catalyst effect is achieved if the knitted fabric forms layers which lie directly on top of one another and are preferably connected to one another along one edge, if appropriate.

Such layers are obtained when the knitted fabric is manufactured as a continuous tubular material, which is then laid flat to form a band. The double-layer tape obtained is folded in a zigzag shape in order to obtain the desired layers stacked one on top of the other. The stack of knitted layers obtained in this way is arranged in the catalyst device in such a way that the gas flow is forced to flow through between the layers of the knitted fabric.

Another possibility of obtaining the desired layers of knitted fabric is to compress the knitted tube in the longitudinal direction of the tube, which creates annular layers which are also one above the other and are connected to one another on the inside and outside edge.

Both configurations lead to a relatively large wall thickness in the flow direction and accordingly to a good filter and catalyst effect. On the other hand, the flow resistance and thus the backflow to the internal combustion engine is low. The knitted fabric can be made of metallic and / or mineral fibers which are coated accordingly with catalyst material, for example vanadium pentoxide, or contain this material.

The better the distribution of the substance giving off the ammonia in the exhaust gas stream, the better the cleaning of the exhaust gas from the polluting nitrogen oxide. It has therefore proven to be expedient to produce droplets which are as small as possible when spraying. This can be achieved if the injection is carried out together with air, while at the same time the necessary air requirement can be generated in the exhaust gas stream. A nozzle which has very favorable properties has slit-shaped outlets for the air and essentially circular outlets for the ammonia-donating agent, the slit-shaped nozzle outlets for the ammonia-donating agent surrounding it.

Since ammonia itself is toxic, it is advantageous not to keep the ammonia in the immediate form in the vehicle or in the vicinity of the internal combustion engine. Even the smallest leakage could lead to a hazard. It is therefore cheaper to use urea or urea carbamate or other compounds instead of ammonia, in which no free ammonia is present and which give rise to the ammonia when heated. This makes handling much less dangerous.

In addition, further developments of the invention are the subject of dependent claims.

Exemplary embodiments of the subject matter of the invention are shown in the drawing. Show it:

Fig. 1 is an exhaust system according to the invention in one schematic longitudinal section

2a, b a first embodiment for the construction of the catalyst

3a, b a second embodiment for the construction of the catalyst

Fig. 4 shows the nozzle, injection device in a schematic longitudinal section and

Fig. 5 shows the nozzle of Fig. 4 in a plan view of the outlet side.

Fig. 1 shows a schematic representation of an exhaust system 1 for an internal combustion engine 2, which is preferably a diesel engine. The exhaust system I includes an exhaust duct 3, which at its upstream end merges into an exhaust manifold 4, which is connected to the outlets of the diesel engine 2. Downstream of the exhaust manifold 4, an injection device 5 opens into the gas duct 3.

The injection device 5 is used to inject a reaction medium, which contains ammonia or splits into ammonia, into the exhaust gas duct 3.

Downstream, the exhaust duct 3 contains a plurality of swirling devices 6, 7 and 8 arranged one behind the other.

The gas stream emerging from the last intermingling device 8 finally arrives in a catalytic converter device 9. After flowing through the catalytic converter device 9, the cleaned exhaust gas stream is blown out into the open via an opening 11.

A speed sensor 12 is directly or indirectly coupled to the crankshaft of the diesel engine 2 and outputs a signal proportional to the engine speed via a signal line 13. The signal reaches a converter circuit 14, which emits a control signal to a control circuit 16 on an electrical line 15. The control circuit 16 serves to control a liquid pump 17 and a compressor 18. For this purpose, the liquid pump 17 is connected to the control circuit 16 via an electrical line 18 and the compressor 18 via an electrical line 21. At least the liquid pump 17 is of a type which is continuously Petite adjustment of the volume flow permitted.

The liquid pump 17 has two connections 22 and 23, the connection 22 being on the suction side and the connection 23 on the pressure side. The suction connection 22 is connected to a storage container 25 via a pipeline 24. In the reservoir 25 there is an aqueous solution, e.g. composed of 40% urea and 60% water. The aqueous urea solution can be refilled into the storage container 25 with the aid of a filling line 26 which continues to open into the storage container and is to be shut off via a shut-off valve 27.

The pressure connection 23 is connected in terms of flow to a nebulizer nozzle 29 via a pipeline 28. The atomizing nozzle 25 is set up to atomize the aqueous urea solution as finely as possible, and it also has the task of introducing additional air into the exhaust gas duct 3, which should also be mixed well with the exhaust gas stream. The atomizer nozzle 29 therefore contains a larger set of nozzle openings which are connected via a line 31 to a pressure connection 32 of the compressor 18. The compressor 18 draws in circulating air and presses it into the line 31.

The swirling devices 6, 7 and 8 arranged downstream of the atomizing nozzle 29 are essentially baffle plates arranged at an angle, the purpose of which is to improve the distribution of the atomized urea solution, ie to mix the exhaust gas stream evenly with the atomized aqueous solution and at the same time, if necessary, to increase the droplet size reduce. This is intended to create a large surface area on which the nitrogen oxides of the exhaust gas stream can react with the ammonia. The catalyst device 9 consists of a mineral fiber knit whose fibers are coated with vanadium pentoxide.

The structure of the catalyst device 9, which is only indicated very schematically in FIG. 1, is shown in more detail in FIGS. 2a and 2b.

The substrate for the catalyst material vanadium pentoxide, as can be seen in FIG. 2a, consists of a circular knitted tube 33 in which loops 34 are indicated. The stitches 34 form rows of stitches 35 which run in the circumferential direction of the knitted tube 33 and so-called wales 36 which lie in the longitudinal direction of the knitted tube 33.

The threads from which the hose 33 is knitted are monofilaments or fibers or a mixture of the two. The material of the fibers are minerals, such as glass and quartz or metals, which are sufficiently heat-resistant.

In a previous process, the catalyst material was applied to these fibers of the knitted fabric.

The knitted tube produced in this way on a circular knitting machine is compressed in the longitudinal direction to form the catalyst device, as shown in the lower part of FIG. 2a. Seen from the outside, this gives it the shape of a tree cake or an accordion, forming circular layers 37 which lie one on top of the other in the finished catalytic converter. For the purpose of better illustration only, they are shown in FIG. 2a as spaced layers 37. vividly in order to better recognize the orientation of these layers 37.

As can further be seen from FIG. 2a, these annular layers 37 are integrally connected to one another on the outer and on the inner edge of each ring. Apart from the beginning and the end of the hose, there are no free edges in the stack formed by the layers 37 on which the knitted fabric could trickle.

A so-called candle 38 is produced from this knitted tube 33, as is shown in longitudinal section in FIG. 2b.

Since the knitted and folded hose 33 is not inherently sufficiently dimensionally stable, the catalyst candle contains two perforated tubes 39 and 40 which are inserted into one another. The tubes 39 and 40 have the same length and are coaxial with one another. They delimit a cylindrical annular space 41 between them. Since they have no other function apart from the support function, the openings contained in the two tubes are as large as possible.

Both tubes 39 and 40 are gas-tightly attached at one end to a disk-shaped cover 42. The cover 42 is, for example, welded to the outer tube 40 and to the inner tube 39 and thus closes both the annular space 41 and the interior space formed by the tube 38 in a gas-tight manner. In the annular space 41 is the leporello-like knitted tube 33 folded in the longitudinal direction. Its individual layers can be seen in the longitudinal section of FIG. 2b as wavy lines. Here too, the individual layers of the layers 37 are shown spaced apart from one another merely for the sake of illustration. In truth they are on top of each other, so with the stratification on average would no longer be visible to the naked eye.

The degree of filling of the annular space 41 depends on the required or permissible back pressure that may arise at the catalyst device 9.

At its end remote from the disk 42, the annular space 41 is closed by an annular disk 43. This disc 43 is welded gas-tight to both the outer tube 40 and the inner tube 39. An opening 44 contained in the disk 43 corresponds in diameter to the inside width of the tube 39.

The mode of operation of the catalyst candle 38 shown in FIG. 2b is as follows:

The exhaust gas flow passes through the opening 44 into the interior which is kept open by the pipe 38. From here, the exhaust gas stream flows radially through the filter and catalyst body formed by the layers 37 and it emerges on the outside of the tube 40. Since the two pipes are connected to one another both via the ring 43 and via the disk 42, the exhaust gas flow only passes through in this way. It flows approximately parallel to the superimposed layers 37, i.e. approximately in the direction parallel to the alignment of the wales in the individual layers 37.

In the simplest case, the catalyst device 9 in FIG. 1 is formed by only one catalyst candle 38 according to FIG. 2b.

The catalyst candle 38 according to FIG. 2b is used in the exhaust gas duct 3 in such a way that the opening 40

u forms the far end.

So that no exhaust gas can flow past the catalyst candle 38, it is held in the gas channel 3 in a sealed manner by means of a disk 46. The opposite end of the catalyst candle 38 can be supported in the exhaust duct 3 by means of struts 47 indicated by dashed lines.

Another way of arranging the knitted fabric is shown in FIGS. 3a and 3b. Here, too, a knitted tube 33 is assumed, which, however, after knitting, is laid flat to form a band 51 which has two layers 52 and 53 connected to one another on the edge. The tape 51 obtained in this way is folded like a leporello, according to FIG. 3b, and a stack 54 is formed which is rectangular in plan and consists of the endless tape 51. It in turn contains several layers 37 lying on top of one another, which are rectangular in plan view. They are shown spaced apart from one another in FIG. 3b only for reasons of illustration. In truth, they are in direct contact.

3b is inserted into a rectangular cage, not shown, which has the task of keeping the stack 54 in shape. In this respect, the cage corresponds functionally to the two tubes 38 and 39.

It also has the task of preventing the gas flow from escaping at the wrong places, for example flowing past the stack 54 or prematurely flowing out laterally between the layers 37. The cage can therefore be constructed, for example, in such a way that it has two walls of expanded metal which are parallel and spaced apart from one another and which are adjacent to the folded edges of the stack 54. All other walls, however, are closed. The gas flow would then flow through the stack 54 in the direction of an arrow 55.

4 and 5 finally show, in section and in a top view, a highly schematic view of the atomizing nozzle 39. It contains two channels 56 and 57 which are coaxial with one another and which are each connected individually to the line 28 or 31. At their downstream end, they pass into a nozzle plate 58 in a sealed manner. The nozzle plate 58 contains three bores 59 which are designed in such a way that they converge towards the outlet side. The bores 59 are connected in terms of flow to the channel 57. In this way, three liquid jets are generated which would meet on the central axis of the nozzle plate 58.

Arranged around the bores 59 are a plurality of, in total 6, slot-shaped openings 61 which are connected in terms of flow to the channel 46, which coaxially surrounds the inner channel 57 at least in the vicinity of the nozzle plate 58. As shown, the slit-shaped openings 61 are arranged in a scale-like manner in the circumferential direction.

The mode of operation of the exhaust system 1 shown is as follows:

An exhaust gas stream emerges from the cylinders of the diesel engine 1, which contains carbon monoxide, unburned hydrocarbons, soot and nitrogen oxides. All of these components are undesirable. In addition, the exhaust gas stream contains unburned air because the diesel engine works with an excess of air corresponding to a λ value between 1.5 and 2.

The carbon monoxide and the unburned hydrocarbons react with one another in the catalyst device 9 and they react under the catalytic action of the vanadium pentoxide to form H, O, carbon dioxide and molecular oxygen.

Without special precautions, however, the nitrogen oxide would escape from the exhaust system. In order to prevent this, the urea solution is injected into the exhaust gas stream by means of the liquid pump 17 with the aid of the injection device 5 in the correct mixing ratio and is then atomized with the aid of the compressed air from the slot-shaped nozzles 61.

With the aid of the injection device 5, a mixture of air and aqueous urea solution in a quantity proportion reaches the exhaust gas channel in such a way that the nitrogen oxide is completely broken down and, on the other hand, no free ammonia or no free urea leaves the exhaust system. For this reason, the pump 17 and the compressor 18 are controlled as a function of the speed.

The injected urea solution decomposes to ammonia and carbon dioxide at the high exhaust gas temperature. This ammonia (NH ₃ ) reacts under the catalytic action of the vanadium pentoxide to form molecular nitrogen and water.

The vanadium pentoxide in the catalyst device 9 also ensures that soot contained in the exhaust gas stream burns flamelessly with the acidic oxygen portion of the air present to form carbon dioxide.

With the arrangement according to the invention, with the aid of a single catalytic converter which is coated exclusively with vanadium pentoxide, all harmful exhaust gas components can be removed be eliminated. The knitted catalyst has the advantage that no urea solution can escape into the open. The "pore structure" of the layered knitted fabric creates such strong eddies that no droplets can get through the catalyst body. At the same time, the catalyst acts as a trap for the urea solution, which may spread extremely thinly on the surface of the fibers and thus provide a large reaction area for reaction with the harmful nitrogen oxide.

Instead of urea, urea carbamate as well as other chemical compositions can be used which produce ammonia when heated in the exhaust gas stream. Because of its toxicity, ammonia itself would only be conceivable for stationary systems.

An exhaust system that works according to the SCR catalyst principle contains an injection device for a chemical compound that contains ammonia or releases ammonia in the exhaust gas environment. A catalyst arrangement is located downstream of the injection device, in which the substrate for the catalyst is formed by a knitted fabric. The threads of the knitted fabric are coated with the catalyst material, which is preferably vanadium pentoxide.

Claims

Claims:

1. exhaust system () for internal combustion engines (2), with a gas channel (3) coming from the internal combustion engine (2),

with an injection device (5) opening into the gas channel (3) for an agent which releases ammonia when heated and

with a catalyst device (9) arranged downstream of the injection device (5) in the gas channel (3), the substrate (33) of which carries the catalyst material has fibers.

2. Exhaust system according to claim 1, characterized in that a swirling device (6, 7, 8) is arranged downstream of the injection device (5) in the gas channel (3).

3. Exhaust system according to claim 1, characterized in that only one catalyst device (9) or more catalyst devices (9) connected in parallel is or are present.

4. Exhaust system according to claim 1, characterized in that the substrate (3) has a textile fabric.

5. Exhaust system according to claim 1, characterized in that the substrate is a knitted fabric (33).

6. Exhaust system according to claim 1, characterized in that the knitted fabric (33) forms a plurality of layers (37) lying one on top of the other and that the gas flow is guided in such a way that a flow component flows through the knitted fabric (33) parallel to the layers (37).

7. Exhaust system according to claim 1, characterized in that the knitted fabric (33) layered in layers (37) is arranged such that the gas flow flows parallel to the wales (36).

8. Exhaust system according to claim 1, characterized in that the knitted fabric (33) consists of an endless hose.

9. Exhaust system according to claim 1, characterized in that the endless hose (33) is laid flat to form a band (51) and that the band (51) thus obtained is folded like a leporello to form the layers (37) lying one on top of the other.

10. Exhaust system according to claim 1, characterized in that the endless hose (33) is compressed in such a way that annular layers (37) are formed, which lie one on top of the other.

11. Exhaust system according to claim 1, characterized in that the fibers have mineral or metallic fibers.

12. Exhaust system according to claim 1, characterized in that the injection device (5) works with air.

13. Exhaust system according to claim 1, characterized in that the injection device (5) contains a channel (56) for the air and a further channel (57) for the ammonia-releasing agent.

π -

14. Exhaust system according to claim 1, characterized in that the injection device (5) at least one outlet

(61) for the air and at least one further outlet (59) for the ammonia-releasing agent.

15. Exhaust system according to claim 1, characterized in that the outlet (61) for the air is slot-shaped.

16. Exhaust system according to claim 1, characterized in that a plurality of outlets (61) for the air are present, which surround the one or more outlets (59) for the agent giving off the ammonia.

17. Exhaust system according to claim 1, characterized in that the or the outlets (59) for the agent giving off the ammonia are substantially round in cross section.

18. Exhaust system according to claim 1, characterized in that the ammonia-donating material is urea or urea carbamate.