DE19717160A1 - Removal of toxic gaseous components in vehicle exhausts - Google Patents

Abstract

Equipment for treating exhaust gas consists of a reaction chamber in which the gas passes around a low-tension electrode (32) which is separated from a high-tension electrode (10) by a dielectric (20). Electrode (32) is an open meshed structure. Also claimed is the process using this equipment.

Description

For the plasma-chemical conversion into gases, a gas is directly in the flowing gas Barrier discharge creates what has been state of the art for over a hundred years for example in ozone generation. This type of discharge also enables Pressure range of about one atmosphere generating a thermal Non-equilibrium state, which is responsible for the efficiency of the plasma chemical implementation is important. The discharge vessel, hereinafter called the reactor, consists of a narrow gap in which the discharge is ignited and two flat ones Electrodes that limit the gap. The gap can be closed at the side, or the electrodes are rolled up into a coaxial vessel. It's important, that at least one of the electrodes on the side of the discharge gas facing is covered by a dielectric barrier that gives the discharge its name.

Every time a high-frequency high voltage rises, arises in the discharge space an electric field that leads to the formation of streamer discharges. Due to the statistical nature of the ignition process, the discharge does not start at all points at the same time. The charge flowing through the streamer channels can be transferred via the insulating Dielectric does not flow onto the electrodes and one builds up there Surface charge on the outside in an environment around the streamer shields the created field. In this area is in the same voltage half-wave ignition no longer possible, so that the discharge from a mosaic of small channels consists. If there is sufficient distance between the voltage half-waves, ignite the Channels in the next half wave are again statistically distributed over the surface and the Discharge appears homogeneous over time.

Usually a smooth discharge is used to create a barrier discharge Electrode system used to ensure the most even statistical distribution possible of the thin discharge channels, also called filaments, over the electrode surface guarantee. However, it is known that the chemical reactions in Barrier discharges at atmospheric pressure are not homogeneous in the entire volume expire, but only be initiated in the channels. Due to the length of time Single discharges of a few nanoseconds, however, have to be reacted to excited molecules that can take up to milliseconds. Another Excitation of the same gas volume with a further single discharge during this time range can disrupt the chemical reaction or at least unnecessarily Cause energy consumption.  

There are therefore various approaches in the literature, the gas time for the after-reaction to give, or to cause additional subsequent reactions by catalysts.

Corona discharge is another form of non-thermal gas discharge, whereby in contrast to the barrier discharge, the thermalization is not carried out automatically time limit of the discharge duration, but by a narrowly limited active discharge area is reached. There is a very inhomogeneous area in this area electric field, so that the charge carriers due to diffusion the area in which inelastic collisions can take place before thermalization begins. The inhomogeneous field of sufficient field strength is shaped by inhomogeneities generated by tips and edges on the electrodes. Often there is an additional one intermittent external shutdown of the voltage source required to form prevent thermal arcing. In R. KUTZNER, J.G.H. SALGE: Generation of non-thermal transient gas discharges at atmospheric pressure for pollution control in: 12th Int. Symp. Plasma Chemistry; Minneapolis. Aug 21-25, 1995 such a reactor for the denitrification of combustion gases is described, care is taken that in areas between sharp-edged Knife electrodes are given the gas time for after-reactions. However, it is disadvantageous the high voltage required in these systems and the risk of Short circuit formation between metallic electrodes due to possible Deposits from the exhaust gas. In addition, the corona discharge has a spatial mean a significantly lower electron energy than that previously described Barrier discharge.

When generating barrier discharges, compliance has been observed a discharge gap of non-vanishing width for the function of the Discharge is not absolutely necessary. Because of the dielectric barrier is coming it does not cause short circuits, and the limitation of the discharge current Surface charges cause the gas discharge to increase by a few millimeters spreads the area of the shortest distance. This phenomenon will for example when cleaning film webs on a rotating roller (see DE-OS 35 38 541) or for ozone generators with particularly good cooling (see DE-OS 37 31 168) used.

Another method for creating spatially preferred areas in one Barrier discharge is described in DE-OS 195 18 970. Avoiding Peaks and edges that would reduce the average electron energy that can be achieved a spatial preference is given by striped wrinkles on the electrodes  the formation of discharge filaments. A running of the filaments with the gas flow is avoided and it is formed between the excitation zones Dead spaces in which after-reactions of the gas can take place.

A further development of such post-reaction zones are subsequent reaction zones in which the difference is not reacting to the plasma-excited molecules, but rather Classically chemically determined secondary and chain reactions take place. This Subsequent reactions can be promoted by catalytic effects. The DE-OS 34 14 121 and DE-OS 44 23 397 describe the series connection different types of gas discharges with catalysts. Common disadvantage Such devices is that the gas reacting after the electrical excitation must flow over comparatively long distances until it catalytically equipped subsequent reaction zones reached.

An alternative concept is in DE-OS 36 44 090 and in DE-OS 36 42 472 presented. Here is in a tubular catalyst by applying a DC voltage generates an electric field in the radial or longitudinal direction, igniting a gas discharge if possible. The reason is that the here brought directly into the area of the electric field Catalysts would be destroyed by the action of the plasma. The spatial proximity the subsequent reaction area to the excitation area is therefore rather disturbing.

It is known that in the plasma chemical aftertreatment of exhaust gases Excitation and a post-reaction phase with clearly different ones Time constants take place. A catalytic Material significantly improve the efficiency of the implementation. It is harmful however, that many catalysts through contact with the electrical discharge be damaged or destroyed.

The object of the present invention is to eliminate these disadvantages. This Object is achieved by a device according to claim 1, with the preferred Embodiments according to claims 5 and 10 and a method according to claim 12. The invention improves known devices in that the excitation and the after-reaction take place in spatially separate areas of the reactor (can), but they are very close to each other (e.g. in the millimeter range). Becomes additionally used catalytic material, so despite the close proximity to Discharge avoided a direct current flow from the discharge to the catalyst will.  

By creating a barrier discharge close to the wall in the immediate vicinity of one same electrical potential area by the inventive Design of the post-cleaning device has succeeded in that on the one hand Excitation, post-reaction and, if necessary, follow-up reaction area of the person to be treated Gas can be reached in quick succession and on the other hand a current flow over one of the Walls is avoided. The invention also relates to the assignment of this wall with a catalytic material. Any state of the art catalysts can be used here because there is no destruction by the action of electrical Discharge takes place.

In contrast to known devices, gas discharge is arranged and catalyst not in a row, but in the same reactor area side by side. The distances the gas has to travel are therefore the length of time adapted to different reaction types (excitation, post-reaction). So it's a sufficiently fast transport of excited molecules from the excitation zone to Subsequent reaction zone, caused by ambipolar diffusion or by directional Gas flow or through vortex possible.

Suitable catalysts are, for example, noble metal or metal oxide catalysts, as well as zeolite ceramics. Since the catalyst surface is preferably not on the electrical discharge is involved (the material is not from the electrical current of the Gas discharge is enforced, at least not in the predominant one Current direction), it is irrelevant whether it is an electrical insulator or a Head acts. In addition, the surface of the catalyst can be designed as desired without influencing the field course in the excitation zone. As a major advantage compared to coating smooth electrode surfaces (which of course is also possible is) this gives rise to the possibility of adding rough or porous support materials use and thus for the first time catalysts with a sufficiently large surface area in Area of a gas discharge.

So far, the correct operating temperature of catalysts has been set for example shortly after the cold start of an engine or at full load critical for the catalyst efficiency on the one hand and the service life on the other.

There are resistance or burner-heated catalysts for the cold start phase but who spent a lot of precious energy in the fight against heat capacity must become. The device according to the invention enables, by means of a Gas discharge to stimulate reactions in the exhaust gas and at the same time the waste heat of the  Use gas discharge to heat the catalyst. This is advantageous Arrangements where the electrical discharge is mainly in the inflow area of the Catalyst is generated to transport heat in the flow direction of the exhaust gas to use, or where a coaxial system gas discharge around the catalyst trains. On the one hand, the catalyst is quasi against the environment Heat radiation is isolated and on the other hand is from all directions by means of Discharge heat supplied.

Is a good supply of heat during the heating phase and the avoidance of Radiation losses to the outside are advantageous, however, with high engine loads Exhaust gas temperatures of over 400-600 ° C to damage the catalyst to lead. If such heat-sensitive catalyst materials are used, then to prefer an arrangement where the catalyst is on the outer wall of an Muffler housed coaxial reactor sits and one by the wind some cooling can be done. The poorer efficiency of such Catalytic converter after the cold start can be increased by increasing the electrical power in the Gas discharge can be compensated for by the power used in continuous operation. In addition to the faster heating due to its waste heat, the gas discharge causes in itself a conversion of pollutants, so that the high emission peaks in the heating phase, which to a large extent contributes to the total emissions in the contribute to the entire test cycle.

Plasma chemical conversion in the gas phase opens up the possibility Reactions that tend to be slow or simultaneous in thermodynamic equilibrium run with disruptive side reactions to perform more effectively. An important Application example is the aftertreatment of exhaust gases from internal combustion engines with the aim of reducing nitrogen oxides or hydrocarbons. Especially the Use in motor vehicles requires robust, compact and, above all, efficient Reactors to use as little as possible of the energy generated by the engine for to have to spend the electrical aftertreatment of the exhaust gas.

The invention can also be used in the aftertreatment of exhaust gases from rail drives, Stationary engines or power plants, as well as from toxic industrial exhaust gases will. In addition, there are also synthetic reactions in the chemical industry Using the device according to the invention is conceivable.

The structure of the invention is shown in the accompanying drawings Device explained in more detail and by way of example, but of course also others  geometric arrangements than those described here are possible.

Fig. 1 shows an unscaled section through a partial area of a discharge device, which is further extended perpendicular to the plane of the drawing, with a first electrode 10 , a dielectric barrier 20 , a counter electrode 32 , an excitation area 63 , a post-reaction area 68 , a dead space 69 , one Wall 39 and catalyst 79 located thereon. In the direction of flow 61 , 62 of the gas, the area can of course also be extended further.

The configurations as shown schematically in FIG. 1 are to be explained further below:
The generation of a barrier discharge, including the special case of a dielectric surface discharge described here, requires at least one (eponymous) dielectric barrier ( 20 ). This is usually a high voltage resistant electrical insulator, e.g. B. glass or ceramic. The dielectric barrier represents a delimitation of the reactor space through which the gas to be treated flows in the longitudinal direction from an inflow side ( 61 ) to an outflow side ( 62 ). On the side of the dielectric ( 20 ) facing away from the reactor space, that is to say the gas, there is a flat first electrode ( 10 ), which can be an unbroken or an openwork sheet. You can e.g. B. be formed by a conductive coating, a mounted film or a fine-mesh fabric. Alternatively, the mechanically load-bearing structure can then preferably be formed from this solid metallic (eg sheet metal tube) electrode ( 10 ) and the dielectric ( 20 ) can be produced by coating the electrode.

The counter electrode ( 32 ), which is connected to the other pole of the voltage source, not shown, is located on the side of the dielectric ( 20 ) facing the reactor space and thus the gas, and very close to it. It is designed as a perforated flat structure, parts of this structure being able to attach dielectric. This designation is intended to express that it must have free space between electrode structures. So it can be a grid, parallel or (not necessarily perpendicular) crossed rods, a network, woven fabric, Ge layer, knitted fabric, braid or the like. Made of conductive material, for. B. of wire or the like. Preferably, it consists of round or rounded rods, wires or from coarse-mesh wire mesh. In the figure, a section through individual wire or rod elements is to be indicated at 32 . The diameter of such an element is preferably in the range from about 0.5-3 mm, particularly preferably in the range from about 0.8 to 2 mm. The network or any other structure can e.g. B. have a clear mesh size of 1 to 4 mm. Of course, values below or above are also possible. The flat structure can also be an expanded metal mesh, that is to say a half-offset slotted sheet, the sheet being erected in the area of the slots by stretching. Like the second electrode, the first electrode ( 10 ) can be a wire mesh. In this case, the first wire network should preferably be designed so closely that the course of the electric field on the side of the dielectric ( 20 ) facing the gas is not disturbed by the meshes of the first wire network ( 10 ).

When an AC voltage is applied to the system described, a gas discharge ( 63 ) occurs in the gap area between the structures of the second electrode ( 32 ) and the dielectric ( 20 ). The gas discharge is therefore primarily generated near the surface of the dielectric. The gas ( 61 ) flowing past is swirled behind the wires ( 32 ) or the like and briefly enters the excitation area ( 63 ). Immediately afterwards, the gas swirls into the adjacent post-reaction area ( 68 ), where the plasma-induced reaction is given time to largely run off. In experiments it was observed that the essentially wedge-shaped gap under rod-shaped electrodes, which are in the (mostly hydrocarbon and particle-containing) exhaust gas stream, is "burned free" of deposits in a short time when the discharge is switched on, so that no degradation of the electrode system due to contamination occurs .

The second delimitation of the discharge space can be formed by a wall ( 39 ) which can be connected to the same pole of the voltage source as the inner, ie second electrode ( 32 ). However, this circuit arrangement is optional since the wall does not have to perform an electrode function. As a result, no discharge will form between the wall and the electrode ( 32 ), and there will be no significant current flow through the wall ( 39 ). Due to the recommended, significantly greater distance between the wall ( 39 ) and the dielectric ( 20 ) and the first electrode ( 10 ) compared to the distance between the structures of the second electrode ( 32 ), there will also be a difference between the dielectric ( 20 ) and the wall ( 39 ) do not form a gas discharge, and the after-reaction area ( 68 ) actually remains unaffected. By suitably dimensioning the dead space ( 69 ) between the electrode ( 32 ) and the wall ( 39 ) it can be achieved at the same time that practically no gas flows untreated through the reactor. For example, if the wires of the second electrode ( 32 ) have a diameter of approximately 1 to 2 mm, the efficiency of the treatment increases with a reduction in the distance between the dielectric ( 20 ) and the wall ( 39 ) or its catalyst layer ( 79 ) about 6 mm to about 2.5 to 3.5 mm clearly. Very good values are obtained for a distance between the dielectric and the wall (or catalyst support) of approximately 3 to 3.5 mm with a thickness of the electrode wires ( 32 ) of approximately 2 mm.

A particularly advantageous development of the invention results if the wall ( 39 ) is coated with a catalyst ( 79 ) or is formed by the catalyst itself. The dead space ( 69 ) can now serve as a subsequent reaction zone, where primarily catalytically triggered reactions take place. Of course, it is also possible that the catalytically ( 79 ) or electrically ( 10 or 32 ) active area of the respective "walls" in the direction of flow 61 , 62 are of different lengths, or that the "walls" on both sides are catalytic over part of their length and wear electrically active areas.

FIG. 2 shows an alternative embodiment of the reactor described, in which there is a further gas discharge space behind the dielectric 20 to increase the possible gas throughput, so that two gas discharge spaces 60 , 66 are present. The structure of the second gas discharge space is analogous to the (first) gas discharge space described above and comprises a perforated electrode, e.g. B. wire electrode 12 , and an optionally at the same potential wall 19th The electrodes 12 , 32 each connected to one pole of the voltage source (not shown) can be designed as rods which are offset from one another or, as shown, offset from one another, as crossed rods or in each case as a wire mesh. All other electrode structures mentioned above are of course also possible. The direction of the gas flows ( 61 ) through both rooms will usually be parallel, but can also be in opposite directions (not shown).

A further enlargement of the reaction space is possible if the rear side of the wall ( 39 ) of the first reaction space ( 60 ) coincides with the wall ( 19 ) of the second reaction space ( 66 ) and further reaction spaces follow. It is not necessary for this wall to be gas-tight and this can also be dispensed with entirely. FIG. 3 shows the resulting electrode structure, which preferably essentially only consists of layers of wires 12 , 32 (or any other electrode structure described above for the second electrode) and dielectrics 20 lying closely on one another.

The number of layers is not limited. Alternating layers of wire or the like ( 12 or 32 ) are each connected to different poles of the voltage source. As illustrated, the electrodes are expediently to be formed by open tissue or braid, preferably made of wire, so that there is sufficient open space ( 69 ) for the gas flow ( 61 ). A possible catalytic coating can, as shown in the upper half of FIG. 3, be applied to the wires in whole or in part ( 71 ), a possible destruction by the action of the gas discharge on the contact surface with the dielectric being accepted. Alternatively, a catalytic intermediate layer ( 73 ) can be introduced in the wire mesh, which is shown in the lower half of the figure in FIG. 3 and, like the coating 79 as shown in FIG. 1, due to the shielding effect of the electrodes made of wires or the like ( 32 ) is protected from discharge.

The resulting planar arrangement of all shown and described Embodiments can be flat or rolled up to form a coaxial system will.

Reference list

10th

first or high voltage electrode

12th

Schematic representation of a second electrode, drawn as a section through a conductive rod or wire

19th

Reactor wall, possibly at the same electrical potential as the electrode

12th

20th

,

20th

'Dielectric

32

openwork electrode, e.g. B. wire or mesh electrode

39

Reactor wall, possibly at the same electrical potential as the electrode

32

60

Gas discharge space

61

Inflow side

62

Outflow side

63

Surface discharge

66

further gas discharge space

68

Fluidization zone (post-reaction area)

69

Dead space (without gas discharge)

71

Catalyst material

73

catalytic material

79

Catalyst material

Claims (14)

1. Apparatus for the aftertreatment of exhaust gas, comprising

• - at least one first reactor chamber, through which gas to be treated can flow in the longitudinal direction from an inflow side ( 61 ) to an outflow side ( 62 ),
• - a first electrode ( 10 , 12 ),
• a flat dielectric ( 20 ) which shields the first reactor space from the first electrode ( 10 , 12 ) and is attached close to or on top of it (or vice versa),
• - A second electrode ( 32 ), which is arranged near the dielectric in the first reactor space and is designed as a perforated flat structure, the second electrode ( 32 ) being switchable against the first electrode ( 10 , 12 ).

2. Device according to claim 1, characterized in that the electrode 10 is a solid sheet. 3. Apparatus according to claim 1 or 2, wherein the reactor space is further delimited by a wall ( 39 ) opposite the dielectric, which can be interconnected with the second electrode. 4. The device according to claim 3, wherein on the wall ( 39 ) and / or on the second electrode ( 32 ) catalyst material ( 71 , 79 ) is applied, and / or in which the wall ( 39 ) consists of catalyst material. 5. The device of claim 1 comprising

• a first and a second reactor space ( 60 , 66 ) through which the gas to be treated can flow in the longitudinal direction from an inflow side ( 61 ) to an outflow side ( 62 ),
• a first electrode ( 12 ) designed as a perforated flat structure and arranged in the second reactor space ( 66 ),
• a second, perforated electrode ( 32 ) which is arranged in the first reactor space ( 60 ),
• - A flat dielectric ( 20 ) that separates the two electrode spaces from one another.

6. The device according to claim 5, wherein the two reactor spaces ( 66 , 60 ) each have the dielectric opposite walls ( 19 , 39 ) and are delimited by them, being on one or both of these walls and / or on one or both of the electrodes ( 12 , 32 ) catalyst material ( 71 , 79 ), or in which one or both of these walls consists of catalyst material. 7. The device of claim 6, wherein the walls ( 19 , 39 ) are coated with catalyst material ( 79 ). 8. The device according to claim 1, comprising at least two reactor spaces which can be flowed through in the longitudinal direction from an inflow side ( 61 ) to an outflow side ( 62 ) of the gas to be treated and in each case by dielectrics ( 20 , 20 ', 20 ''...) From each other are separated, with each reactor space alternatingly having a first ( 12 ) or a second ( 32 ) electrode designed as an openwork structure and the distance between two dielectrics ( 20 , 20 ') being selected such that structural parts of the first or the second electrode are both near the first ( 20 ) and near the second ( 20 ') dielectric. 9. The device according to claim 8, wherein catalyst material ( 71 , 73 ) is arranged on the electrode surfaces ( 12 , 32 ) or in free spaces of the electrodes. 10. The device according to one of claims 5 to 9, wherein the electrodes Rods or wires and / or lattice-shaped or reticulated or as a fabric or Braid are formed. 11. Device according to one of claims 5 to 10, wherein the catalyst material ( 71 , 73 ) is applied to the electrode surface and / or is embedded as an intermediate layer in the electrode structure. 12. A method for the aftertreatment of exhaust gas, in which the gas is subjected to an electrical barrier discharge and in the same reaction chamber first passes through an excitation region ( 63 ) and shortly thereafter through an after-reaction region ( 68 ). 13. The method according to claim 12, wherein the gas in the post-reaction area Catalyst material catalyzed subsequent reactions. 14. The method according to claim 12 or 13, wherein the gas after the after-reaction area ( 68 ) passes through a subsequent reaction area ( 69 ), in which it is subjected to classic chemical reactions caused by catalyst material, the subsequent reaction area ( 69 ) in the main flow direction of the gas ( 61 , 62 ) is arranged next to the after-reaction area ( 68 ).