FR2704776A1 - Electrostatic filter for dust-laden gases and method of filtration - Google Patents

Abstract

Electrostatic filtration device in which the tubes (3) for collecting the ionised dust particles are lined with heaters such as electrical resistance elements (14) which melt the dust particles and cause them to flow into the bottom collector. If several similar devices are arranged in succession and are at decreasing temperatures, the various kinds of dust particles are deposited separately, in the device where they solidify, and may be treated appropriately.

Description

DUST GAS ELECTROFILTER AND METHOD OF MAKING
FILTRATION
DESCRIPTION
The invention relates to an electrostatic precipitator for dusty gases and to a gas filtration process.

 An electrofilter in cylindrical geometry is essentially composed of tubes provided with a central wire and in which the gases flow. The wires are brought to an electrical potential different from that of the tubes in order to form an electric field sufficient to ionize the dust and repel it towards the tubes, on which they accumulate. The gases thus exit from the substantially cleaner tubes.

However, it is necessary to add a dust collector located under the tubes and means for periodically detaching the dust accumulated on the tubes. As described in a prior French patent (2,655,570), these means may include brushes which slide along the tubes by scraping their walls to detach the dust accumulated by sweeping. The brushes are hoisted in extensions of the tubes, beyond the connections to the gas inlet or outlet conduits, when they do not act.

 The object obj and of the invention is to add or even replace these brushes with pistons and heating elements of the tubes, the essential effect of which is to liquefy the dust accumulated in order to detach them. These heating elements can take the form of electrical resistances. They can offer two additional advantages: the permanent maintenance, by moderate heating outside the tube cleaning periods, of dusty gases at a temperature sufficient to avoid condensations of liquid which could corrode the tubes, and the separation of certain categories of dust.

 The filtration process according to the invention consists in bringing or bringing the dusty gases to a temperature where some of the dust is vaporized, in collecting the other dust on electrostatic filters, then in cooling the gases to a temperature where the vaporized dust is solidified. and collecting the solidified dust on other electrostatic precipitators. We can see that this process is very advantageous if the electrostatic filters used include heating elements for the tubes because the temperature adjustment is very easy. An application actually envisaged concerns the incineration gases produced by combustion and in particular the pyrolysis of waste irradiated objects the dust produced then often comprises metal chlorides and radioactive oxides which it is advantageous to separate so that the chlorides do not increase the volume of the radioactive bodies to be conditioned, and so that the chlorides can be filtered and collected without particular precautions .

 The described embodiment of the invention is particularly suitable in the nuclear industry and for low gas flow rates. It is given by way of illustration and not limitation, but others are also possible: it is in particular possible to employ electrostatic precipitators devoid of a scraping piston or any other mechanical means for stain removal of accumulated dust. Dust separation installations comprising several electrostatic precipitators can moreover include electrostatic precipitators whose cleaning means are of different types (scraping by piston, melting of dust, vibrations or ultrasound, etc.). vaporized can be collected without disadvantages on electrostatic precipitators devoid of heating elements provided that the gases reach them at a temperature sufficient for the vaporization of other dusts to be accomplished.

We will now proceed to the concrete comments of the figures appended without limitation
FIGS. 1 to 3 show an embodiment of an electrostatic precipitator according to the invention, and
- Figure 4 shows a process diagram.

 The electrostatic filter of FIGS. 1 to 3 comprises a manifold 1 in the form of a funnel and the lower end 2 of which is closed by a removable pot or is connected to a discharge duct to an installation for treating impurities. Three tubes 3 have their lower ends engaged in the flared inlet of the manifold 1, and a gas supply conduit 18 opens at this point from the funnel 1, tangentially to its wall, thus constituting a cyclonic distributor in which the gas stream swirls and discharges a first part of its impurities before entering the tubes 3. The gas circulation inside the electrostatic precipitator is ensured by a pressure difference (upstream-downstream) induced by a depressive organ.

 Each of the tubes 3 contains a wire 4 in its axis and over its entire length: the wires 4 are retained at their lower end by a retaining anchor 5 housed in the center of the funnel 1 and fixed to the upper cover which closes the end flared of it. The upper end of the wires 4 is hooked to a cover 6 of a ringing installation 7 which also includes an appendage 8 of the tube 3, which it extends above a gas evacuation duct 13 associated with each tube 3. The appendages 8 are used to receive in ordinary time a piston 9 provided with external 10 and internal linings 11 which allow it to scrape respectively the internal face of the wall of the tube 3 and the associated wire 4 to rid them of the deposited dust and The displacement of the pistons 8 is ensured by a vertical bar 12 which can be slid from outside the cover 6. Electrical connection cables 19 connect the wires 4 to an electrical potential, and the tubes 3 are connected to earth .

 The tubes 3 and the collector 1 are surrounded by heating sheaths 14 and 15 which can bring them to temperatures sufficient to melt the dust accumulated on them, which then flow to the bottom of the funnel. The heating ducts 14 and 15 essentially consist of electrical resistances connected to the terminals of a voltage generator.

 We now turn to the commentary on FIG. 4 to illustrate a process according to the invention.

 An incineration unit 20 is connected to a first electrostatic precipitator 21 and sends to it an essentially gaseous residue from the pyrolysis of irradiated nuclear waste at about 800 "C. The residues are rich in uranium and plutonium oxides as well as '' made of zinc chlorides and other metals.

 The passage through the first electrostatic filter 21 is carried out at a temperature of 500 "C. at which the chlorides are vaporized. The electrostatic filtration therefore only concerns the oxides, which are deposited on the wall of the tubes. In this example, the oxides are mainly oxides are mainly radioactive oxides (uranium oxide, plutonium oxide).

 The residue then passes through a second electrostatic precipitator 22, the ambient temperature of which is 200 ° C. The gases are therefore cooled to the point that the chlorides congeal in grains of dust which are in turn ionized and deposited on the tubes. The gases leave then the second electrostatic precipitator for other treatment elements such as a very high efficiency filter 23.

 Heat exchangers 24 shown by way of example in the form of coils (other cooling systems such as those which involve dilution with air at room temperature, or a spray of water would be suitable) provide cooling to the desired temperatures. The electrostatic precipitators 21 and 22 are cleaned together or separately, the first by sweeping and the second by heating liquefying the chlorides.

 Tests were carried out on gas at a flow rate of 525 m3 / h at 200 "C (or 300 Nm3 / h) charged with sodium chloride particles with an average diameter of 0.1 μm and at a flow rate of 36 g / h. The filtered proportion, increasing with the intensity of the ionization current, reached 98.8% for 6 mA and a corresponding voltage of 26 kV Other tests, carried out on zinc chloride particles at a flow rate of 40 g / h, 80% of which had a diameter of less than 0.3 µm, transported in a gas at a flow rate of approximately 175 Nm3 / h at temperatures from 1600C to 1800C, gave purification yields of approximately 99% with an installation with three tubes of which only two were in service (at intensities of 8 mA and 10 mA), and 99.7% when the three tubes were in service at intensities of 6 mA.

Claims (6)

CLAIM  1. Dust gas electrostatic precipitator, essentially composed of tubes (3) provided with a central wire (4) for ionizing dust, a collector (1) located under the tubes (3) and means for detaching the dust filtered and accumulated on the tubes, characterized in that these means comprise elements (14) for heating the tubes.  2. Electrofilter according to claim 1, characterized in that the means for detaching the accumulated dust also include pistons (9) scrapers from the tubes (3).  3. Electrofilter according to any one of claims 1 or 2, characterized in that heating elements (15) are also provided for the collector (1).  4. Electrofilter according to any one of claims 1 to 3, characterized in that the heating elements consist of electrical resistances.  5. A method of filtering out dusty gases, consisting in bringing the gases to a temperature where some of the dust is vaporized, in collecting the other dust on electrostatic precipitators (21), in cooling the gases to a temperature where the vaporized dust is solidified and collecting solidified dust on other electrostatic precipitators (22).  6. A filtration method according to claim 5, characterized in that the vaporized and then solidified dust comprises metal chlorides and the other dust comprises radioactive oxides.