WO2008107598A1

WO2008107598A1 - Electrostatic filter device for trapping and destroying the soot particles contained in the exhaust gases of a combustion engine

Info

Publication number: WO2008107598A1
Application number: PCT/FR2008/050086
Authority: WO
Inventors: Maxime Makarov
Original assignee: Renault Sas
Priority date: 2007-01-22
Filing date: 2008-01-22
Publication date: 2008-09-12
Also published as: FR2911633A1; FR2911633B1

Abstract

The invention relates to an electrostatic filter device of the type including at least one basic filtration module defining, in the inside thereof, a housing (1) for the exhaust gases and having, upstream from the flow, a particle electrostatic charging area (6), followed, downstream from the flow, by a charged particle trapping area (7), wherein the charging area (6) includes a plurality of filament electrodes (9) supplied with a high-voltage electric current and arranged transversally in the gaseous flow, characterised in that the basic filtration module includes a plurality of collecting plates (2) arranged in parallel in the gaseous flow and extending along the charging and trapping areas (6, 7), said collecting plates (2) being spaced from the filament electrodes (9) in the charging area and interleaved in the trapping area (7) with conducting plates (11) at the same potential as the filament electrodes (9), said collecting plates (2) being connected to the ground.

Description

Electrostatic filter device for capturing and destroying soot particles contained in the exhaust gases of a combustion engine

The present invention relates to electrostatic filter devices for capturing and destroying the soot particles which are contained in the exhaust gases of internal combustion engines of a motor vehicle, in particular diesel engines. To meet antipollution standards, it is necessary to develop filters for exhaust gases in order to avoid or limit the release into the atmosphere of pollutant gases and particles. In the case in particular of diesel-type combustion engines, particulate filters associated with catalytic devices are generally placed in the exhaust line of the engine. These devices, however, are relatively expensive and have a number of disadvantages. In particular, they are relatively fragile, have low efficiency during small journeys and have a limited life. Other types of filters also used are electrostatic filters which have the advantage of simplicity since they do not require, as the catalytic filters, the use of precious metals as catalysts and specific ceramic materials. These electrostatic filters have, therefore, a much more reasonable price. Electrostatic filters for the removal of particles suspended in gases are widely used in industry for gas flows of very low or low speeds, generally between a few millimeters per second and a few centimeters per second. The adaptation of such industrial filters to uses on motor vehicles entails a number of constraints that make this adaptation difficult. Indeed, for the exhaust gases of motor vehicles, the flow velocity is very high, of the order of several meters per second. Moreover, in the exhaust line there is a very limited volume which therefore requires very compact filters. Finally, it is necessary that the operation of these filters is provided with low power consumption. Electrostatic filtration for capturing and removing soot particles from motor vehicle exhaust is generally done in three steps. First, the soot particles are electrically charged. Then the particles thus charged are deflected by electrostatic attraction to a wall. Finally, the destruction or mechanical removal of the deposit of the particles which have thus accumulated on this wall is carried out.

Various solutions have been advocated up to now. In a first solution, the three phases mentioned above run in parallel in the same compartment of a reactor mounted in the exhaust line of the motor vehicle. Such a solution has drawbacks because the formation of a deposit on the active walls rapidly leads to a degradation of the quality of the corona discharge, which reduces the efficiency of the particle loading. In addition, deposits on the active walls cause undesirable phenomena such as parasitic electric arcs. Finally, there are phenomena of release of the collected particles, that is to say, entrainment of particles collected by the flow of exhaust gas passing through the filter. According to other solutions also recommended, the various phases mentioned above are implemented separately. In this case, however, the necessary devices are complex and large and also require high power consumption.

Whatever the devices used, the electrostatic filters mounted in the exhaust line of motor vehicle combustion engines are generally cylindrical geometry and comprise a wired axial electrode carried at high voltage. The cross section of such an electrostatic filter is relatively small, so it is necessary to provide several units in parallel to reduce the speed of the gases passing through the filter. This results in a relatively large size. In addition, the high voltage electrodes mounted parallel to the flow of exhaust gas, in such a longitudinal configuration, cause an imbalance of the corona discharge around the electrodes, the discharge is not of the same quality upstream and downstream. downstream of the filter over the length of it. Such an embodiment is described, for example, in US Pat. No. 3,157,479, which further provides for mechanical removal of the collected material. US Patent 4,871,515, in the same way as the patent

No. 4,905,470 discloses electrostatic filter devices for use in the exhaust line of a motor vehicle and includes a regeneration heating element integrated into the device. In both cases, however, the coaxial longitudinal configuration filter with respect to the electrodes causes an inefficient corona discharge, because it is heterogeneous over the length of the device, and whether the supply is made of direct current or alternating current. It is the same with regard to US Pat. No. 5,603,893, which provides a feed drawn from a longitudinal coaxial geometry of an electrostatic filter.

U.S. Patent 4,778,493 illustrates a multi-stage electrostatic filtering device as may be contemplated in the industry. The filter is large, the distance between the electrodes of each filtration unit being important. The use of such a structure could not be envisaged in the automotive field.

US Pat. No. 4,364,752 attempts to solve the problems posed by the devices previously described. The electrostatic filter described therein has a planar configuration and has two zones. In the upstream zone, with respect to the flow of the exhaust gas, transverse wires supplied with high voltage electrical current are arranged so as to cause a corona discharge. Downstream of this first precipitation zone is a second zone for cleaning with ion beams by means of an ion generator disposed within the flow. Given the structure of this device, it can only be used in industrial gas treatment processes and could not easily be adapted for use in the automotive field.

The object of the present invention is to eliminate most of the disadvantages of the known devices and to allow filtering of the exhaust gases from an internal combustion engine of a motor vehicle, particularly of the Diesel type, loaded with soot particles, so that that filtration is particularly effective and remains effective over time. The present invention also relates to an electrostatic filter device which allows easy destruction of the materials collected by the filter without any mechanical intervention.

Finally, the present invention relates to an electrostatic filter device of great compactness and whose power consumption is relatively low.

In one embodiment, an electrostatic filter device for capturing and destroying soot particles contained in the exhaust gas of an internal combustion engine of a motor vehicle, comprises at least one elementary filtration module internally defining a enclosure for the exhaust gases and having, upstream of the flow, an electrostatic particle loading zone, followed, downstream of the flow, by a charged particles capture zone.

The charging zone comprises a plurality of wired electrodes supplied with high voltage electrical current and arranged transversely in the gas flow.

The elementary filtration module comprises a plurality of grounded collector plates arranged in parallel in the gas flow and extending along the loading and trapping zones. The collector plates are separated from the wire electrodes in the charging zone and inserted in the capture zone with conductive plates at the same potential as the wire electrodes. The collector plates are coated with an insulating coating in the loading area.

The electrically charged particles in the loading zone can thus be immediately collected in the capture zone on the collecting plates. The mounting of the collector plates over the entire length of the device that is to say both in the loading zone and in the capture zone makes it possible to obtain a particularly compact and efficient assembly.

Preferably, the collector plates and the wired electrodes are supplied with alternating high voltage oscillating between a minimum negative voltage greater than the threshold for triggering the corona discharge for the wired electrodes and a maximum negative voltage below the breakdown threshold between the wired electrodes and collecting plates. In a preferred embodiment, the elementary filtration module has, at the entrance of the capture zone, a connection for the entry of gas at high temperature for the combustion of the particles captured by the collector plates, the plates collectors and the conductive plates being perforated in the capture zone. In another embodiment, the collector plates have on their surface, in the capture zone, heating elements by Joule effect, for the combustion of particles captured by the collector plates.

In this way, the particles collected by the collecting plates in the capture zone can be easily destroyed and eliminated. This elimination is preferably during regeneration phases of the filter.

For this purpose, at least one collecting plate may advantageously be equipped with a fouling detector capable of emitting a trigger signal of a regeneration procedure of the device.

The fouling detector can for example measure the surface resistance of the deposit on the surface of the collector plate. The edges of the conductive plates interposed between the collecting plates in the capture zone are preferably rounded in order to avoid the triggering of electric arcs along these edges.

For the same reasons, it is advantageous to mount a ballast resistor in series in the supply circuit of the conductive plates.

The present invention will be better understood in the study of some embodiments described by way of non-limiting examples and illustrated by the accompanying drawings, in which: - Figure 1 is a schematic sectional view of an electrostatic filter device according to the invention; Figure 2 is a sectional view along II-II of Figure 1, showing the structure of the plates used; Figure 3 is a sectional view similar to Figure 2 showing an alternative embodiment.

As illustrated in Figures 1 and 2, the electrostatic filter device according to the invention comprises a hermetic enclosure 1 of generally cylindrical shape whose section may be circular or rectangular. The device comprising said enclosure 1 is mounted in the exhaust line of an internal combustion engine of a motor vehicle, as close as possible to the output of the engine, at a place where the temperature of the exhaust gas to be treated is high. . The chamber 1 may possibly be associated with other similar speakers arranged in parallel on the flow of the exhaust gas. In this case, the device thus comprises several elementary filtration modules.

Inside the enclosure 1 are mounted a plurality of collector metal plates 2 which extend parallel to the gas flow which passes through the device shown by the arrows 3 and constitute, inside the enclosure 1, a stack of plates regularly spaced apart from each other. The exhaust gas enters through the inlet pipe 4 and out through the outlet pipe 5. The collector plates 2 extend over most of the length of the enclosure 1. The enclosure 1 can be divided into two zones according to the internal operation. A first zone, referred to as the loading zone, is referenced 6 in FIG. 1. The loading zone 6 is located on the inlet side of the enclosure 1, that is to say upstream in the flow. 3. The loading zone 6 is followed by a so-called capture zone, referenced 7, intended to collect the soot particles and which were initially electrically charged in the loading zone 6.

The collector plates 2 extend into the chamber 1 in both the loading zone 6 and in the capture zone 7. In the embodiment illustrated in FIG. 1, the collector plates 2 are, in the zone of loading 6, completely coated with an insulating coating 8. Alternatively, it is possible not to use such an insulating coating and leave all the surfaces of the collector plates 2 completely naked over their entire length.

In the charging zone 6, a plurality of wire electrodes 9 are arranged transversely in the gas flow 3, as can be seen in particular in FIG. 2. In the example illustrated in FIGS. 1 and 2, six collector plates 2 are provided. In the gaps between said collector plates 2, in the charging zone 6, groups of three wire electrodes 9 are placed equidistant from each of the collector plates 2, as can be seen in FIG. 1. In the illustrated example the wired electrode groups 9 comprise three wires stretched transversely as seen in FIG. 2 and held by lateral supports

10 of insulating material, mounted inside the enclosure 1 and appropriately fixed.

In the capture zone 7, which is devoid of wire electrodes 9 only provided in the charging zone 6, conductive plates 1 1 are arranged parallel in the gas flow 3 between the different collector plates 2. The conductive plates 11 have preferably the same shape and substantially the same area as the conductive plates 2 in the capture zone 7. In FIG. conductive plate 1 1. In the example illustrated in Figure 1, the leading edges 12 of the conductive plates 1 1 and the trailing edges 13 are slightly rounded to prevent the formation of arcs along these edges. The spacing between the various wired electrodes 9 in the longitudinal direction is preferably equal to the distance between the wired electrodes 9 and the different collector plates 2 in the charging zone 6.

The spacing between the different collector plates 2, the number of plates 2 in the electrostatic filtering device as well as the number of wire electrodes 9 can easily be chosen as a function of the flow rate of the exhaust gas to be treated and the desired template for the enclosure 1.

Two side plates 21 visible in FIG. 2 make it possible to channel the flow of the exhaust gases inside the enclosure 1. The side plates 21 are mounted on each side of the stack comprising the collecting plates 2 and the conductive plates 11 and extend over the entire length of the enclosure 1 in the charging zone 9 as in the capture zone 7. The set of wired electrodes 9 is brought to a high voltage potential via the connections 14 which connect the wired electrodes 9 to a high voltage alternating electric current generator 15 which can generate in the illustrated example a high negative voltage U sinusoidal variable form between two voltage thresholds. The set of conductive plates 11 is also raised to the same high voltage potential as the wired electrodes 9 via the connections 16 and with the interposition of a ballast resistor 17, for example a value between 1 and 10 Mohms. , mounted in series in the supply circuit in order to prevent electrical breakdowns on the conductive plates 11. The set of the collector plates 2 is connected to ground via the connection 18.

By way of example, a sinusoidal negative voltage of between 10 and 100 KHz may be applied to all the wired electrodes 9 and the conductive plates 11. The voltage will oscillate between a threshold corresponding to the triggering of the corona discharge around the wired electrodes 9 (for example 6 kilovolts for a wire with a diameter of 0.2 mm). The second voltage threshold corresponds to the breakdown threshold generating an electric arc between a wire electrode 9 and a collector plate 2 (for example 20 kilovolts for a gap between a wire electrode 9 and a collector plate 2 of 1 cm). The power consumption of such an electrostatic filter can be of the order of 200 watts.

In the example illustrated in FIG. 1, one of the collector plates 2 has a fouling detector 19 which can, for example, measure the surface resistance and which is connected to a calculator 20 which is capable, as a function of the degree of fouling detected. , to initiate a phase of regeneration of the electrostatic filter by destruction of the collected particles. When the sensor 19 detects a surface resistance that exceeds a threshold stored in the computer 20, it can program a regeneration procedure of the electrostatic filter. During this regeneration procedure, the particles collected on the surfaces of the collector plates 2 in the capture zone 7 are destroyed by heating the deposition at high temperature, for example 600 ^° C. For this purpose, in the embodiment illustrated in FIGS. 1 and 2, the enclosure 1 comprises a pipe 22 which opens into the enclosure 1 substantially at the junction between the loading and capture zones 9. A flow of gas at high temperature can then be introduced into the chamber 1. the capture zone 7 of the chamber 1 during a regeneration phase controlled by the computer 20. The rise in temperature produced by these gases at high temperature causes the destruction of soot particles that have been previously collected on the surface 2. In order that the destruction of the particles collected on all the collector plates 2 is obtained, the collector plates 2 and the conductive plates these 1 1 preferably have a perforated structure or a grid structure 23 as visible in FIG. 2. In this way, the gases at high temperature introduced through the pipe 22 can pass through the stack of different collector plates 2 and conductive plates January 1.

The high temperature gases used during the regeneration may for example be exhaust gases whose temperature has been increased by modifying the operation of the combustion engine during the regeneration phase of the electrostatic filter.

In the embodiment illustrated in FIG. 3, the regeneration is obtained by a rise in temperature, this time caused by heating elements 24 implanted over the entire surface of each of the collector plates 2 in the capture zone 7. In the example illustrated, each heating element 24 comprises a metal coil integral with each of the surfaces of the collector plates 2, or possibly embedded in the thickness of these collector plates 2. The two ends 24a and 24b of the coil 24 can pass through the wall of the enclosure 1 by insulating elements 25 and be supplied with electric current for Joule heating of the surfaces of the collector plates 2, resulting in the removal of the collected soot particles. In this embodiment, the collector plates 2 and the conductive plates 11 are devoid of perforations 23.

Claims

An electrostatic filter device for capturing and destroying soot particles contained in the exhaust gas of an internal combustion engine of a motor vehicle, of the type comprising at least one elementary filtration module internally defining an enclosure (1). ) for the exhaust gases and having, upstream of the flow, an electrostatic particle charging zone (6), followed, downstream of the flow, by a charged particles capture zone (7), the charging zone (6) comprising a plurality of wire electrodes (9) supplied with high voltage electrical current and arranged transversely in the gas flow, characterized in that the said elementary filtration module comprises a plurality of collector plates (2) arranged in parallel in the gas flow and extending along the areas (6, 7) of loading and capture, said collector plates (2) being spaced apart wired electrodes (9) in the charging zone and interposed in the capture zone (7) with conductive plates (1 1) carried at the same potential as the wire electrodes (9), said collector plates (2) being connected to the mass.

2. Device according to claim 1 wherein the collector plates (2) and the wire electrodes (9) are supplied with alternating high voltage oscillating between a minimum negative voltage, greater than the trigger threshold of the corona discharge for the wire electrodes (9). ) and a maximum negative voltage below the breakdown threshold between the wire electrodes (9) and the collector plates (2).

3. Device according to one of the preceding claims wherein the collector plates (2) are coated with an insulating coating (8) in the charging zone (6).

4. Device according to one of the preceding claims wherein said elementary filtration module has, at the inlet of the capture zone (7), a connection (22) for the entry of gas at high temperature for the purpose of combustion of particles captured by collector plates (2), the collector plates and the conductive plates (11) being perforated in the capture zone.

5. Device according to one of claims 1 to 3 wherein said collector plates (2) have on their surface in the capture zone (7), heating elements (24) by Joule effect, for the purpose of combustion particles captured by the collecting plates (2).

6. Device according to one of the preceding claims wherein at least one collector plate is equipped with a fouling detector (19) capable of emitting a trigger signal of a regeneration procedure of the device.

7. Device according to claim 6 wherein the fouling detector (19) the surface resistance of the deposit on the surface of the collector plate.

8. Device according to one of the preceding claims wherein the edges (12, 13) of the conductive plates (1 1) interposed between the collector plates (2) in the capture zone (7) are rounded.

9. Device according to one of the preceding claims wherein a ballast resistor (17) is connected in series in the supply circuit of the conductive plates (11).