EP0658685B1 - Device for separating soot particles from exhaust gases of internal combustion engines - Google Patents

Description

The invention relates to an electrostatic precipitator for separating soot particles from the exhaust gases of internal combustion engines according to the preamble of claim 1.

In such devices, a ceramic honeycomb body is provided on two opposite outer surfaces with electrodes, which are arranged essentially parallel to the flow direction of the exhaust gases determined by the channels of the honeycomb body, a high voltage of 10 kV to 20 kV being applied to these electrodes. This results in a series connection of 30 to 50 channels of the honeycomb body, each of which works as a small electrostatic filter and separates the negatively charged soot particles.

In order to ensure the burning off of the separated diesel soot, the electrostatic filter is operated with such a high voltage that electrons are emitted from the more negative of the channel surfaces, which accelerate to hit the deposited soot through the collecting field and initiate oxidation reactions. This process is supported by the deposition of negatively charged oxygen ions on the substrate surface, which on the one hand are more reactive than oxygen radicals and on the other hand increase the oxygen partial pressure on the substrate surface.

The disadvantages of such a solution lie in the high voltage required, the supply of which to the electrodes in the presence of soot leads to great insulation difficulties. Discharges or parasitic currents can also occur from the honeycomb bodies themselves, which would not ensure the maintenance of the operating voltage or only if the energy consumption was too high.

Also, at high temperatures, a lot of electrical power has to be provided to support the deposition field in the form of webs made of ceramic or another good insulation material running in the direction of flow, which can be rectangular, round or wedge-shaped in cross section.

Furthermore, an electrostatic filter of the type mentioned at the outset was known from WO-A1-91 / 16528, in which a ceramic honeycomb body is provided which has through-passages through which the exhaust gas can flow and which are arranged one above the other in the manner of a brick wall. In this known filter, electrodes are arranged on the outside of the filter and in the interior of a bore passing through it, through which the exhaust gases are supplied, a plurality of rows of channels being arranged between these electrodes. With such a filter, the production of the ceramic body is difficult.

The aim of the invention is to avoid these disadvantages and to propose a device of the type mentioned at the outset which is simple to manufacture and operate.

According to the invention, this is achieved in a device of the type mentioned at the outset by the characterizing features of claim 1.

The construction of the device serving as a filter by means of several layers of electrically conductive and ceramic materials can be produced very easily. Steel foils can also be used, which are provided with one, but preferably two, thin ceramic coatings, and the ceramic material can be applied by plasma coating.

The relatively small height of the channels through which the exhaust gas can flow in the area of the filter ensures that an essentially laminar flow is formed and therefore the soot particles accumulate and are oxidized by the radicals caused by the electric field.

In this context, it is advantageous if the conditions specified in claim 2 are met.

Further advantageous embodiments of the devices according to the invention result from the further subclaims.

With these arrangements according to the invention, the voltage drop in the ceramic substrates themselves must of course also be taken into account. If this is limited to 100 volts to 400 volts per substrate, the required specific resistances of the ceramic material at the desired average working temperature at which the combustion process is to take place, depending on the filter size, are 10 ⁶ ohm cm to 10 ¹⁰ ohm cm, preferably 10 ⁷ ohm cm to 10 ⁹ ohm cm.

If you want to extend the burning function of the filter over a temperature interval of at least 300 ° C (i.e. from 100 ° C to 400 ° C) required in everyday operation, this is possible according to the invention through a high proportion of open porosity in the ceramic material. According to the invention, the ceramic starting material can also be doped with vanadium or strontium, preferably with strontium titanate.

In this connection, it is found that the filter function and the storage capacity of the device are retained even if the exhaust gas temperature leaves this working interval. At temperatures below 100 ° C, the burn-off decreases at the same time as the cell current decreases, while the filter function remains fully intact; at temperatures above 400 ° C, it only depends on the electrical power available: the increasing cell current leads when the power limit is reached the mains supply to slowly regulate the voltage and thus to slowly reduce the filter efficiency. The burning function is maintained due to the proximity of the flash point of soot (at around 500 ° C, depends on the amount of hydrocarbons) and changes to the natural burning of soot at around 500 ° C. However, only that soot burns off, which is separated in the filter, since this process requires more time than is available during the transport to the end of the exhaust line.

Embodiments according to the invention are now described below:

Fig.1: An arrangement according to the invention of ceramic filter plates with series connection.

Fig. 2: An external series connection of the filter cells.

Fig.3: A parallel connection of the filter cells.

Fig. 4: An outline and plan view of an embodiment of filter cells connected in parallel according to the invention.

Fig. 5: A schematic representation of a winding electrostatic filter connected in series.

Fig. 6: Winding electrostatic precipitator in parallel.

Fig. 7: elevation and plan view of an embodiment of parallel connected filter cells.

8a, 8b and 9a, 9b: elevations and floor plans of further embodiments of filters according to the invention.

1 shows a stack, the layers of which are formed by ceramic plates 1 structured on one side, which together with the smooth rear sides of the same plates 1 form flow channels 2 through which the exhaust gas to be cleaned flows. The spacers 3 formed on one side on the plates 1 can be formed by webs running in the direction of flow or interrupted knobs which only ensure the distance to the next plate. This stack of plates 1 is contacted only on its first and last plate 1 with high-voltage electrodes 4, 5, between which there is high voltage. As a result, the individual flow channels 2 lying one below the other are electrically connected in series internally, the flow channels 2 being arranged in a brick wall manner offset from one another.

2 shows an external electrical series connection according to the invention of these flow channels through metal foils 4 'which are located between two structured plates 1 and are connected between the respective voltage sources 6, which can be stacked independently of the potential. According to the invention, these can be diodes, which together with the capacitances of the metal foils 4 'form a cascade connection, that the side of the ceramic plate 1 opposite the metal foil 4' is always more negative than the metal foil 4 '.

3 shows an electrical parallel connection of the flow channels 2 according to the invention. Around the ceramic plates 1 each having a more negative potential than the opposite electrode 4 formed by metal foils, the plates 1 structured on one side are arranged alternately with the spacers 3 upwards and downwards. The electrodes 4 are connected in parallel and connected to one pole + a DC voltage source and all electrodes 5 to the second pole - the DC voltage source, which only has to deliver a relatively low voltage, since this only requires the required field strength in a position of Flow channels 2 must be sufficient.

4 shows sections from the elevation and plan view of a section of a filter according to FIG. 3. The stack consists of layers of ceramic plates 1 with nubs on one side, with a spacer 3 designed as a steel foil between two of them with mutually directed knobs Electrode 4 is arranged. This steel foil is stiff enough to offer resistance to the spacers 3 even at higher temperatures and forms with two ceramic plates 1 two opposing flow channels 2, which are in contact with electrodes 5 made of thinner foils, each of which also delimits plates on two flow channels 2 1 is present. Ceramic plates 1 and electrodes 4 and 5 are held by corresponding steel pins 7 and contacted by means of spacer rings 8.

5 shows a winding electrostatic filter according to the invention, which is wound in one layer from a ceramic substrate and is contacted on the inside and outside with high-voltage electrodes 4 and 5. The spacers 3 are designed as webs and are approximately 4 mm high and determine the height of the flow channels 2.

6 shows a winding electrostatic filter according to the invention consisting of two electrodes 4, 5, which are insulated from one another by steel foils, each with a ceramic plasma coating 1 ′ on both sides and corresponding spacers 3 formed as ceramic balls, which have a diameter of approx. 2 mm and which are only due to the voltage or . Deformation of the steel foils forming the electrodes 4, 5 are fixed. The filter is wound in two layers from mutually insulated foils and therefore consists of two large flow channels 2 which are connected in parallel. The ceramic coated steel foils are about 2mm apart and the tension between the metal foils serving as electrodes 4, 5 is approximately 300V.

geachtet werden muß, wenn v die mittlere Geschwindigkeit durch die Strömungskanäle 2 darstellt.7a, 7b represent sections of the elevation and plan view of two flow channels 2 arranged one above the other according to the invention. The stack. consists of electrodes 4 and 5, plasma-coated on both sides, made of thin, heat-resistant steel plates with sufficient flexural strength to be able to find 3 ceramic plates 40 mm to 50 mm wide and 2 mm to 4 mm high as spacers, on the sides of the coated electrodes 4 and 5 are arranged and thus limit the flow channels 2 on both sides. These electrodes 4 and 5 are held by corresponding steel pins 7 and alternately contacted on both sides of the flow channels by electrically conductive spacer rings 8 so that steel pins with different potential are preferably opposite one another. For this purpose, the ceramic coating 1 'ends before the spacer rings 8 but only within the ceramic spacer 3. In order to sufficiently keep the electrodes 4 and 5 away from the opposite-pole spacer rings 8, there are ceramic protective rings 9 around the metallic spacer rings 8. The plasma coating 1 has at working temperatures a volume resistivity of at least 10 ⁵ ohms and cm ^2, preferably between 10 ⁶ ohms and cm ² and 10 ⁸ ohms and cm ² . Each of the two electrodes 4, 5 is clamped on both sides of the flow channel 2, so that it is possible to reduce the thickness of the electrodes to such an extent that they are stretched as thin foils through the steel pins 7 and must therefore maintain their intended mutual distances. Flow channels 2 of this type can be stacked at any height and are particularly suitable for large-volume diesel internal combustion engines, but also for large machines with small plate spacings d, but at least compliance with the dimensional formula $$\text{d [cm]}\frac{\text{500}}{\text{v [cm / sec]}}$$

must be taken into account if v represents the average speed through the flow channels 2.

FIG. 8 shows an improvement of the device shown in FIG. 7 by the integration of the ionization part in the Separation part of the diesel electrostatic filter according to the invention. For this purpose, in the case of the more negative of the two electrodes 4, the electrode edge leaned towards the gas flow is set back and at least one discharge wire 10 is welded on by spot welding 11 and the remaining lateral remnants of the electrode 4. This results in a concentration of the field on the wire surface and the associated stronger gas discharge leads to a higher charge of the soot particles flowing past. The wire has a diameter between 50 microns and 300 microns and consists of a corrosion-resistant high-temperature alloy, a superalloy or tungsten and is preferably gold-plated or platinum-plated.

FIG. 9 shows a further improvement according to the invention of the device shown in FIG. 7 by separating the part of the negative electrode 4 'carrying the discharge wires 10 from the remaining part of this electrode 4. This embodiment of the device according to the invention allows the discharge voltage to be supplied separately by at least one of the steel pins 7 'and thus a regulation of the discharge current. The discharge on the wires can preferably be increased by superimposing the direct voltage with a high-frequency alternating voltage without the risk of a sparkover. For this purpose, the second electrode 5 contacted by the steel pin 7 ″ can also be separated from the remaining plates 5 in the area of the discharge wires, so that the filter unit is only held together by the ceramic spacers 3. In another preferred embodiment of the present invention, the entire filter unit is supplied by one type of its steel pins with approximately 300 volts to 500 volts, while the steel pins of the other type are grounded. In this case, a separation of the grounded plate type in this area is also sufficient for the control of the discharge wires, by means of which the high frequency is then supplied and the discharge intensity is regulated. Ultimately, the somewhat susceptible creep rupture strength of the discharge wires can also be avoided by replacing them again with plates, preferably in strips running transversely to the flow direction, with gaps, which are then mainly operated by a high-frequency-controlled glow discharge.

Claims (20)

1. An electrofilter for separating soot particles from the exhaust gases of internal combustion engines, preferably diesel engines, in which at least one electrically insulating layer is arranged between at least two electrically conductive layers which are used as electrodes and are at different electric potentials and between at least two layers one each of superimposed flow ducts is kept free for the exhaust gas, which duct is limited on at least one side by an electrically insulating layer and the electrical field extends perpendicular to the flow direction of the exhaust gases, characterized in that the electrofilter consists of regularly stacked or wound layers (1, 4, 5) of conductive or ceramic materials which are mutually distanced by electrically insulating spacer blocks (3) and each flow duct (2) is provided between two electrically conductive layers (4,5) which are applied to different potentials, with the difference in potential of the electrical potentials, to which two mutually adjacent electrically conductive layers (4, 5) are connected, being sufficient to secure in the free cross section of the flow a field strength of at least 100 V/mm and in each flow duct (2) the more negative limitation which is penetrated by the electrical field is provided with a ceramic surface, the electrically conductive layers (4, 5) are provided with a constant cross section and the extension d of the cross section of the flow of the at least one flow duct (2) corresponds in the direction of the electrical field to the relationship: $$\text{d [cm]≤}\frac{\text{500}}{{\text{v[cmsec}}^{\text{-1}}\text{]}}$$

   

   with v meaning the mean flow speed of the exhaust gases to be cleaned in the respective cross section of the flow of the electrofilter.
2. An apparatus as claimed in claim 1, characterized in that the field strength in each flow duct is at least 300 V/mm and the extension d of the cross section of the flow of the at least one flow duct (2) corresponds in the direction of the electrical field to the relationship $$\text{d [cm]≤}\frac{\text{200}}{{\text{v[cmsec}}^{\text{-1}}\text{]}}$$

   

   with v meaning the mean flow speed of the exhaust gases to be cleaned in the respective cross section of the flow of the electrofilter.
3. An apparatus as claimed in claim 1 or 2, characterized in that at least one insulating layer (1) is provided on at least one side with spacer blocks (3) in the form of bridges or knubs extending in the direction of the flow.
4. An apparatus as claimed in claim 3, characterized in that the spacer blocks (3) are arranged mutually offset by superimposed insulating layers (1).
5. An apparatus as claimed in one of the claims 1 to 4, characterized in that the mutual distance of the spacer blocks (3) extending parallel with respect to one another on an insulating layer (1) is at least 10 mm, preferably at least 20 mm.
6. An apparatus as claimed in one of the claims 1 to 5, characterized in that two groups of electrically conductive layers (4, 5) are provided which are applied to different potentials, with every other electrically conductive layer (4, 5) being at the same electrical potential.
7. An apparatus as claimed in one of the claims 1 to 6, characterized in that there are two insulating layers (1), e.g. ceramic ones, which are provided on one side with spacer blocks (3) in form of bridges or knubs which face one another and clamp first electrically conductive layers (4) such as metal foils, and said packages on their part are electrically connected in parallel alternatingly with second electrically conductive layers (5) such as metal foils which are multiply stacked, with said second electrically conductive layers (5) being mutually connected and being at the more negative potential and all first electrically conductive layers (4) being mutually connected and being at the more positive potential.
8. An apparatus as claimed in one of the claims 1 to 6, characterized in that the electrofilter is arranged, by interposing spacer blocks (3), from ceramically coated metal foils, with every other metal foil being at the same potential.
9. An apparatus as claimed in claim 8, characterized in that the electrofilter is wound in two layers from two mutually insulated metal foils which are ceramic-coated on at least one side, preferably on two sides, and said two foils are connected with the poles of a current source, with spacer blocks (3) being arranged between the metal foils.
10. An apparatus as claimed in claim 9, characterized in that small ceramic balls are also wound up as spacer blocks (3) between the ceramic coated metal foil whose position is fixed by the tension and the elastic deformation of the two wound metal foils.
11. An apparatus as claimed in one of the claims 1 to 10, characterized in that the insulating layers (1) produced from ceramics are thinner than 3 mm and preferably thinner as 2 mm and that they contain fibre material, preferably quartz glass, for their reinforcement.
12. An apparatus as claimed in one of the claims 1 to 11, characterized in that the specific electric resistance of the insulating layers (1) and the spacer blocks (3) is different, with the specific resistance of the spacer blocks (3) being considerably higher than that of the insulating layers (1), with particularly preferable values for the specific value of the insulating layers (1) being between 10⁶ ohm cm and 10¹⁰ ohm cm, whereas the specific resistance of the spacer blocks (3) is advantageously over 10¹¹ ohm cm.
13. An apparatus as claimed in one of the claims 1 to 12, characterized in that the temperature response curve of the operating voltage corresponds to that of the resistance value of the insulating layers (1) and the spacer blocks (3).
14. An apparatus as claimed in one of the claims 1 to 13, characterized in that the insulating layers are made from a material with a positive electric temperature coefficient.
15. An apparatus as claimed in one of the claims 1 to 14, characterized in that the material of the insulating layers (1) has a specific resistance which lies between 10⁶ ohm cm and 10¹⁰ ohm cm, preferably between 10⁷ ohm cm and 10⁹ ohm cm.
16. An apparatus as claimed in one of the claims 1 to 15, characterized in that the ceramics used for the insulating layers (1) have a surface with at least 10 %, preferably at least 30 %, of open pores.
17. An apparatus as claimed in one of the claims 1 to 16, characterized in that the ceramics are doted with vanadium and/or strontium, preferably with strontium titanate.
18. An apparatus as claimed in one of the claims 1 to 17, characterized in that in the more negative of the two electrically conductive layers of every flow duct (2) the edge of the electrically conductive layer (5) which faces the exhaust flow is offset backwardly and at least one discharge wire is attached to the lateral parts of said layers (4).
19. An apparatus as claimed in one of the claims 1 to 18, characterized in that electrically conductive layers (4) which carry at least one discharge wire (10) are mechanically separated at a right angle to the direction of flow from the remaining part of said electrically conductive layers (4) and can be controlled in an electrically separate manner.
20. An apparatus as claimed in one of the claims 1 to 19, characterized in that also the more positive of the electrically conductive layers (5) in the zone of the discharge wires (10) are mechanically and electrically separated at a right angle in the direction of flow from the remaining part of said electrically conductive layers (5) and that preferably one type of the electrically conductive layers (4, 5) is controlled with a highfrequency voltage.

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