CH642870A5 - METHOD AND DEVICE FOR SEPARATING PARTICLES SUSPENDED IN A GAS. - Google Patents

Description

The present invention relates to a separation method according to the preamble of claim 1. It also relates to a separator according to the preamble of claim 6 for the implementation of the method.

The invention aims in particular to obtain the separation of solid particles suspended in a gas by circulating the gas in an enclosure where they are retained by electrostatic effect.

The electrostatic dusting methods are based on the attraction exerted on electrically charged dust by one or more electrodes brought to a potential of sign opposite to that of the dust.

Thus, the electrostatic dedusting installations comprise means intended to circulate in a chamber the gaseous fluid 40 charged with dust, a device capable of electrically charging said dust and one or more electrodes capable of attracting said dust.

According to a known technique, the dust contained in the stream of gas to be purified is electrically charged by producing a corona electric discharge in this gas. For this purpose, the latter is passed in the interval between a first electrode, constituted by a conductive tip or a taut conductive wire, and a second electrode with a relatively large surface, flat or cylindrical for example, and a difference is applied. potential of the order of more than 50 sieurs tens of kilovolts between these electrodes.

The very intense electric field in the vicinity of the first electrode causes the formation of electronic avalanches in a small area, called the active area, which generate a large quantity of ions and electrons. Electrons, very mobile, tend to leave the active area quickly, causing the formation of a high concentration of positive or negative ions at the edge of this active area, depending on whether the first electrode is positive or negative compared to the second electrode. . This concentration of ions constitutes a charge of space. The dust which circulates in the space charge zone 60 acquires by diffusion or by bombardment a charge of the same sign as the space charge. The final charge of each dust particle depends on its size, its residence time in this area and the value of the space charge, measured by the product of the quantity of ionized particles 65 per unit volume in the space considered and the charge of these particles.

In the case where the dust-laden gas is explosive, for example a wheat silo atmosphere where the very fine dust from

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gluten which accumulates in the ambient air provides a very explosive mixture, the creation of a corona discharge is to be prohibited, the slightest spark can be the cause of considerable damage.

Furthermore, the efficiency of a corona discharge decreases as the temperature of the gas in which it is produced increases, and this due to the thermal agitation of the molecules of the gaseous fluid. When one of these molecules collides with a negative ion, it causes the detachment of the electron from the latter, which generates an increase in the electronic current of the discharge, with as a consequence a lowering of the yield of charge production of space and the appearance of instabilities in the landfill.

As a result, dust removal from combustion gases from fireplaces, for example from fluidized bed fireplaces burning coal or low calorific recovery fuels, using a corona discharge, is practically impossible. In the absence of an effective dusting procedure, it has so far been impossible to associate such fireplaces directly with piston engines or gas turbines without these being rapidly deteriorated by the action of dust.

Dedusting techniques are also known by electrostatic precipitation which do not involve a corona discharge, but an association of very fine droplets with the dust or particles which it is sought to eliminate.

Thus, for example, it has been proposed to purify a gas stream by a gas-liquid contact by spraying a liquid into a supersonic nozzle supplied with compressed air, the resulting atomizer being injected, generally in counter-current , in a stream of gaseous fluid to be purified. The nozzle is brought to a high electrical potential relative to the mass of the installation, so that the water droplets which exit therefrom are charged and stick to the dusts to entrain them towards metallic parts electrically connected to the mass of the installation, thus causing their separation from the gas. The residual dust, which is entrained with the droplets in the gas stream beyond the nozzles, is in turn precipitated on an electrode brought to a suitable electrical potential.

Another technique of this type consists in producing a jet of fine water droplets at the outlet of a grounded nozzle disposed opposite an annular electrode polarized by a high voltage in order to communicate a charge of predetermined sign to said water droplets. When the particles to be removed from the atmosphere receiving the jet are themselves already electrically charged, the charged water droplets are attracted to these particles and form a mist which makes it possible to obtain their deposition.

Both of these techniques implement a washing with water of the gas to be purified and, thus, do not make it possible to dry treat a gas or any atmosphere in which the formation of sludge is to be prohibited. They are, moreover, ineffective with regard to atmospheres at temperatures for which the water droplets are vaporized before they associate with the particles to be eliminated.

The present invention relates in particular to a method of separating solid or dust particles suspended in a gas by electrostatic precipitation and which, in particular, makes it possible to solve the problems previously stated for the dedusting of explosive atmospheres or at high temperature.

The method for electrostatic separation of particles according to the invention is defined by claim 1.

By this method, electrical charges are thus produced in a first medium contained in said chamber and they are transferred to a second medium, contained in said enclosure where the gas to be purified circulates, to create a charge of space there. The first and second media are electrically independent, so that no sparks from the first media can propagate to the second medium. In addition, the specific characteristics of the first medium in which the formation of ions is generated are not influenced by those of the second medium where these ions are used to charge particles to be precipitated by electrostatic means.

It is planned to maintain the space charge on the path of the gas to be purified at a value very substantially lower than that which would be sufficient for the initiation of a corona discharge at any point of the corresponding enclosure. This completely eliminates the risk of discharge or spark in the event that the atmosphere to be purified is explosive.

It has been found that values of space charges, sufficiently low to present no danger, are quite effective in electrostatically charging the particles with a view to their precipitation.

In addition, provision can be made for such atmospheres to use a negative corona discharge in the first chamber. Good energy efficiency is obtained and a stable transfer of negative ions on the path of the stream of fluid to be purified.

In the case of an atmosphere to be purified at high temperature, provision may be made to maintain a corona discharge in a chamber whose temperature is sufficiently low to obtain a good space charge efficiency and, from this chamber, transfer the ions into the hot gases to be purified. Advantageously, provision is made to inject positive ions produced from a positive corona discharge. This avoids the appearance in the second chamber of electrons generated by collision of negative ions with the molecules of the gas excited by thermal agitation.

Provision is also made for regulating the space charge so as to limit the probability that electrons are produced by ionization in the volume of hot gas to be dedusted. This adjustment can be made by acting on the potential of the tip around which the corona effect is produced in the first chamber, which varies the current carried by the microparticles in the second chamber.

The invention also relates to an electrostatic separator for the implementation of the method. This separator is defined by claim 6.

The description which follows, given by way of example, refers to the appended drawings, in which:

fig. 1 is a schematic perspective representation with partial cutaway of an installation according to the invention;

fig. 2a and 2b show two mounting variants of injectors in the installation, seen in section through the plane I-I of FIG. 1;

fig. 3 is a schematic view in longitudinal section of an injector used for the implementation of the invention;

fig. 4 is a sectional view with partial cutaway of an embodiment for the dedusting of hot gases;

fig. 5 is a schematic view in vertical section of another embodiment of a hot gas purification installation;

fig. 6 is a cross-sectional view along the plane VI-VI of FIG. 5;

fig. 7 schematically shows another embodiment of an ion injection device in an installation according to the invention;

fig. 8, 9 and 10 show three alternative embodiments of the device of FIG. 7, and fig. 11 illustrates an example of implementation of the injector. An enclosure is constituted by a parallelepipedal corridor 10 (fig. 1) bounded by two parallel vertical walls 11 and 12, a floor 13, and an upper wall 15 (not shown in fig. 1). The enclosure 10 has an inlet 14 for the admission of gas to be purified and an outlet 16 for their evacuation after precipitation of the solid particles contained in these gases. The inlet 14 opens into a charging zone 17 followed, along the path of the gas to be purified in the enclosure 10, by an electrostatic precipitation zone 19 comprising a plurality of plates 20 parallel to the walls 11 and 12 connected alternately to sources of positive and negative potential respectively.

In the load zone 17 open a plurality of injectors 21 aligned in vertical rows 23 and 24, the injectors of row 23 passing through the wall 11 and the injectors of row 24 passing through the wall 12.

Each injector has at its anterior end a nozzle 25 (fig. 2a and 2b) opening into the passage 10, a body 26 tra5

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sant the wall 11 or 12, respectively perpendicularly thereto and a rear part 28 connected, on the one hand, to a common pipe 29 for the admission of compressed moist air and, on the other hand, to a cable high voltage supply 42 (see fig. 2a).

The injectors 24 of FIG. 2a, five in number in each row 23 and 24, are mounted in the walls 11 and 12, so that the nozzle 25 of each injector in row 23 is placed opposite the nozzle 25 of a homologous injector of row 24.

The injectors of fig. 2b are staggered, the other injectors passing through the wall 12 forming a row 24 'of injectors whose axes are offset with respect to the axes of the four injectors of the row 23' in the wall 11.

Each injector 21 (FIG. 3) comprises a tabular conductive or insulating body 30 delimiting an internal cylindrical chamber 32. A nozzle 34, defining a nozzle neck 35, is mounted in front of the tube 30 in the axis of the latter. The diverging part of the nozzle opens into a tabular nozzle 36 forming the injection nozzle 25. The rear part 28 of the tube 30 is extended by a hollow cylinder 38 closed at its rear part and having a lateral opening 39 connected to the delivery pipe. compressed air supply 29. The rear wall 40 of the hollow cylinder 38 carries a sealed insulating bushing 41 to which the electrical supply cable 42 is connected; the bushing is connected to a first tapered electrode or needle 45 fixed by a mount 44 on the internal wall of the tube 30. The mount 44 is insulating, it consists of a star structure comprising for example three radial branches. The needle 45 is metallic and disposed along the axis of the tube 30, its point ending at the neck 35 of the nozzle 34. This nozzle is made of an electrically conductive material and constitutes a second electrode connected to a voltage source continues 48 by the cable 49 and to the earth of the installation by a link 51. The needle 45 is connected by the conductor 42 to the other pole of the high voltage source 48.

In operation, when the voltage reaches a sufficiently high value, a corona discharge is established between the needle 45 and the nozzle 34 in the wet gas passing through the neck 35 of the latter.

If the central electrode or needle 45 is negative, it collects positive ions and the electrons move away from it by diffusing. In the gaseous fluid where the discharge occurs, the electrons quickly attach themselves to the molecules of the electronegative gases, generating negative ions that are less mobile than these electrons, which form the space charge. It can be shown that the electrical energy yield of the formation of a negative space charge improves all the more since the gas, seat of the discharge, facilitates the formation of negative ions. The low mobility of the negative ions also makes it possible to obtain a stable space charge around the central electrode 45. This is the case with dry or humid air. With weakly electronegative gases, there is reason to fear the instability phenomena which occur when the electrons, not attaching themselves in the form of negative ions, create ionized filaments through the gas, which degenerate into arcs. electrical devices capable of causing the central electrode to short-circuit and, consequently, adversely affecting the operation of the device.

If the central electrode 45 is positive, the electrons progress rapidly towards the latter, leaving a very large quantity of ions which create a plasma sufficiently dense to cause the formation of an ionized channel which appears as a spark initiator. This channel progresses from the central electrode towards the second electrode by pushing in front of it the active zone, seat of the avalanches. If the ionized channel progresses to the second electrode, a short circuit between these two electrodes occurs. By limiting the potential difference applied between the electrodes, it is possible to limit the progression of the active area so that the discharge is maintained without initiating a spark and producing a dangerous short circuit, the active area being surrounded by a charge of space made up of positive ions.

The air admitted into line 30 has an average hygrometric degree, for example 50% relative humidity under normal conditions of temperature and pressure. There is a fairly wide latitude in this regard and any air whose humidity is greater than 10% is suitable for the implementation of the process, which allows it to be applied without requiring special measures for humidifying the ambient air in a wide variety of places. In the case where the air is too dry, we first begin to compress it to the generating pressure necessary to obtain the supersonic expansion, then we humidify it by passage in a humidifier before admitting it in driving 30.

The supersonic expansion of the humid air in the divergent along the neck 35 of the nozzle 34 produces ice microparticles with a diameter of the order of a hundredth of a micron which trap the ions generated by the corona discharge maintained by the difference of high potential existing between the tip of the needle 45 and said nozzle body 34. The jet of microparticles at the outlet of the nozzle entrains the charges trapped inside the nozzle 25 towards the load zone 17 in the enclosure 10. These charges are released by the vaporization of the microcrystals of ice about ten centimeters from the nozzle 34. They then spread by diffusion and under the effect of their own charge of space in zone 17 before being collected by the metal walls 11, 12, 13 and 15.

The value of the space charge thus created can be controlled by acting on the parameters of formation of the corona discharge and, in particular, the potential difference applied between the electrodes, the speed and the pressure of the air, the dimension nozzle causing expansion of compressed air, etc.

The value of this space charge can be relatively low compared to that implemented by the corona discharge inside the injector 21, while providing sufficient ion density in space 17 to charge the dust transported in a gas stream at a level allowing their subsequent precipitation in the electrostatic precipitation zone 19.

The electric current transported by the charged particles in the charging zone 17 is relatively weak compared to the current injected by the injector 21. The major part of this current therefore goes in the form of ion current to the metal wall of the charging chamber 17 which is connected to ground in parallel with the body of the nozzle 34 and which plays a role similar to the auxiliary electrode of conventional dust collectors with corona discharge.

The gaseous fluid charged with dust is admitted to the inlet 11 of the enclosure 10 according to arrow 52 (FIG. 1) and passes through the charging zone 17 where the dust particles are charged by diffusion and bombardment in contact with the charge. space, so that they are then precipitated on the polarized plates 20 of the electrostatic precipitation zone 19 as they pass between them; the purified gas leaves the enclosure 10 in the direction of arrow 53.

In an exemplary embodiment, the needle 45 is brought to a negative potential of 12 kV relative to the nozzle and a current of 50 | iA is produced at the outlet of the latter when the pipe 29 supplies the injector with a flow humid air of 20 m3 / h (measured under normal pressure and temperature conditions) under a generating pressure of 6 bar, resulting in a supersonic expansion to a Mach number close to 1.5 at the neck 35 of the nozzle 34, the diameter of which is 2.3 mm.

The enclosure 10 has a height of about 100 cm and a width of 40 cm. The charging zone has an effective length of 20 cm and the injectors are arranged face to face in this zone, their nozzles being spaced 30 cm apart. Each pair of injectors, face to face, lets through a total current of 100 pA which, with the geometry considered and taking into account the mobility of the ions, makes it possible to create a charge of space in the area 17 of 1013 positive ions or negative / m3 at least, which corresponds to an electric field of 1.7-W5 V / m.

The gas admitted, mechanically purified beforehand, conveys a flow of residual dust at a speed of 2 m / s at a flow rate of 7 g / s, the average diameter of this dust being 3 n. Each dust crosses the charging area in 0.1 s. by acquiring around 300 negative charges on average, corresponding to a charge current from the injectors of 12 pA.

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The flow of charged dust enters the precipitation zone 19, the dimensions of which are as follows: height 100 cm, length 100 cm, the distance between the plates being 2.60 cm and these plates being connected to alternately positive and negative potentials 10 kV.

The entrainment speed of the gaseous fluid carrying the charged particles is 2.8 m / s in this zone and the duration of passage between the plates is 0.35 s, to obtain an almost total precipitation of the latter.

The embodiment shown in fig. 4 is a gas dust collector comprising an enclosure 110 delimited by walls 111, 112, 113, similar to the walls 11, 12, 13 of the enclosure of FIG. 1 and, in order, between its inlet 114 and its outlet 116, a first granular filter bed 115, driven by a slow downward movement, and a charging zone 117 into which open series of injectors 121 forming vertical rows 123 and 124 respectively passing through the partitions 111 and 112. The injectors 121 are similar to those of FIGS. 1 to 3. Following zone 117 is a second filter 119 comprising a vertical granular bed animated by a slow downward movement filling the space between two perforated or meshed metal plates 125 and 126 transverse to the partitions 111 and 112 and connected respectively to the positive and negative terminals of a high-voltage direct current supply, or to two terminals of an alternating supply, to effect the electrostatic precipitation of the particles of the gas stream from the chamber 117 on the grains of the filter charged by affecting.

The gas to be dedusted arrives in the direction of arrow 152 at the inlet 114; the purified gas is delivered in the direction of arrow 153 at the outlet 116. This separator differs from the previous one by its reduced volume.

The two separators described are applicable to the dedusting of gases loaded with highly insulating dust for which the known devices are ineffective.

The dust collector in fig. 5 and 6 receives gases to be dedusted at a pressure of 12 bar and at 900 ° C., such as those originating from the combustion of lean coal or combustible waste in a hearth supplied using the pressurized dry ash fluidized bed technique.

This hot gas dust collector includes filter elements of generally cylindrical shape and the circulation of gases is designed to minimize the heat losses of these gases between the inlet and the outlet of the dust collector. These gases come from a fluidized bed hearth which is supplied with combustion air that has been preheated.

The gases to be dedusted are admitted under pressure through a pipe 201 into a tank 202, internally lined with an insulating layer 203 of thermal insulation, and of generally vertical cylindrical configuration having at its upper and lower ends two hemispherical domes, respectively 205 and 206. Between the heat-insulating layer 203 and a metal wall 211 is provided a series of ventilation channels 208 which are intended to circulate fresh air, before its admission as an oxidizer in the hot gas generator hearth, for its Warming. Inside the space limited by the heating pipes 208 is housed a granular bed filter 207 of shape substantially homothetic to that of the reservoir 202 with respect to its center. This filter comprises an external partition 212 and an internal partition 214, between which is provided a space filled with small alumina beads (diameter 2 mm) forming a granular bed 210. The partition 212 has an opening at its upper connected part to a pipe 216 passing through the upper dome 205 of the pressure tank 202 so as to admit granules 218 circulating in the space 210 in the direction of the arrow 220. At its other end, the partition 212 comprises an outlet pipe 222 passing through the lower dome 206 of the reservoir 202 so as to allow the evacuation of the granules from the filtering bed 210 in the direction of the arrow 224. The mass of granules filling the space between the partitions 212 and 214 flows very slowly, by example 1 m / h, from top to bottom.

The space between the metal wall 211 separating the air heating channels 208 from the interior of the tank and the partition 212 is divided by a transverse annular partition 225 at mid-height into two chambers, the lower one 227 in which opens the hot gas inlet 201, the other upper 228 connected to an outlet 230 of purified gas.

The partitions 212 and 214 include annular screens capable of retaining the alumina balls of the bed 210 to constitute two annular filtering zones through which the gas to be purified can pass, one at 232 between the chamber 227 and the chamber 250 delimited by the partition 214, and the other at 234, between the chamber 250 and the chamber 228. Thus, the hot gases entering through the inlet 201 undergo a first mechanical purification by crossing the zone 232 of the granular bed, at the lower part from the reservoir 202, then a second purification, by crossing the granular bed in the zone 234 in the direction of the exit 230.

This second pass is accompanied by electrostatic precipitation of the particles. In fact, two upper 240 and lower 242 insulating rings separate the screen area from the rest of the internal partition 214 and two similar upper 243 and lower 244 insulating rings separate the screen area from the rest of the external partition 212 of the filter 207. The screen isolated from the partition 214 in the filter zone 234 is connected to a positive pole 320 of a continuous high voltage source while the opposite annular screen of the partition 212 is connected to a negative pole 321 of this voltage source not shown, so as to influence the alumina beads included in zone 234 by influence. Alternatively, the sieves can be subjected to a high alternating voltage.

The interior of the chamber 250 delimited by the internal partition 214 constitutes a charge zone in which the solid particles having passed through the filtration zone 232 are passed through a charge of space formed by ions from two injectors of ions 252 and 254, operating by entrainment of the ions using aerosol microparticles, and penetrating into this chamber at the center of the lower and upper domes of this chamber 250 to project two charge flows towards one of the other along the vertical axis of the tank.

The finest dust which has escaped from the filter zone 232 is loaded into the chamber 250 and is filtered and precipitated electrostatically in the zone 234 of the bed of granules. The granules of this zone are gradually renewed from the tubing 220 and, after leaving the zone 234, are then reused in the purely mechanical separation zone 232.

The purified gas from the outlet 230 of the electrostatic filter is admitted to the inlet of a gas turbine or possibly of a piston engine after having followed an intermediate stage of chemical filtration to remove the alkaline compounds or the vanadium.

In the example which has just been presented, the distance between the injectors 252 and 254 is approximately 1 m, the diameter of the cylindrical chamber 250 being 0.4 m. The dusted gas is at a pressure of 12 bar and at a temperature of 900 ° C.

The 252 and 254 supersonic nozzle injectors are supplied with pressurized moist air. The neck of the metal nozzle is grounded; it has a diameter of 1 mm. The insulated metal needle, such as 45 in fig. 3, is connected to a potential source of 20 to 25 kV. The current injected by each injector is of the order of 250 | tA for an air flow rate in line 29 of 15 m3 / h, measured under normal pressure and temperature conditions, for a generating pressure of 27 bar.

For a flow of gas to be dusted of 3600 m3 / h measured under normal conditions of temperature and pressure, which corresponds to the application of a power of the order of 1 MW at the input of a generator such that a gas turbine, with a dust load of 100 g / m3, the first dedusting, comprising a passage in a cyclone, then the crossing of the zone 232 of the granular bed, performs a purification at approximately 93%, this that is, there are 7 g of particles left to be removed every second.

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With the geometry indicated, the electric field produced in the chamber 250 is approximately 500 kV / m with a minimum density of ions of the order of 1014 per m3, which represents a sufficient space charge to allow particles of 3 ji of average diameter crossing the volume considered in 1.05 s to acquire approximately 300 elementary charges, which is sufficient to allow them to be collected by the polarized balls of the electrostatic filter with granular bed in zone 234. In these conditions, the current carried by the charged particles to the electrostatic filtering zone 234 is 12 nA. This current is low compared to the total current injected by the injectors defined above. Most of this current is therefore discharged through the metal wall of the enclosure delimited by the metal partition 214 which is grounded.

As already indicated, the particles transferred into the charge zone 250 via the injectors 252 and 254 are positive ions. The value of the space charge resulting from this transfer of positive ions is much lower than the value of the space charge in the corona discharge inside the injectors themselves. Furthermore, to prevent local elevations of the electric field produced inside the zone 250 from creating involuntary local discharges in certain parts of this zone, the internal surface of the metal partition 214 delimiting the space zone 250 is polite. This eliminates the small spikes on this surface which could give rise to electron-generating avalanches, which, given the high temperature of the gases, would risk greatly reducing the quantity of charges communicated to the dust particles and annoyingly affecting the efficiency of electrostatic precipitation.

In the case where one operates on very hot gases, for example at 900 ° C, as in the previous example, one can advantageously use an injection device slightly modified compared to that of FIG. 3 for producing injector devices such as 252 and 254. It may indeed happen, in particular with very high gas temperatures, that the melting or sublimation of microparticles carrying charge at the outlet of the injector is very quickly and therefore in the immediate vicinity of the injector. The ions thus released can then return to the injector and be captured by it, thereby reducing the charge of space available to charge the particles transported by the gas to be dedusted.

There are two types of provisions to avoid or limit this capture. According to a first arrangement, the injector is brought to a positive potential with respect to the walls of the metal enclosure in which the gases to be dusted circulate so as to create an electric field distribution having the effect of separating the ions produced from the masses of the injector.

According to another arrangement which can be used individually or in combination with the first, provision is made to cool the current of microparticles emitted by the injector. This cooling can in particular be obtained by blowing a stream of cold gas, for example air, all around the flow of microparticles injected into the enclosure. Under these conditions, the heat transfers between the enclosure and the microparticles are delayed and the sublimation of the latter with release of the charges occurs only in an area of the enclosure sufficiently distant from the injector to prevent their capture by the latter. this.

An injection device 310 (FIG. 7) comprises an injector tube 312 limiting a chamber 314 inside which the stream of pressurized moist air can circulate in the direction of the arrow 316 in the direction of an orifice the end 318 of the tube 312, the internal profile of which defines the nozzle. Coaxial with the tube 312 is mounted a conductive needle electrode 320, the end 322 of which ends in the vicinity of the neck 324 of this nozzle. The needle 320 and the tube 312 are electrically connected to a high voltage source 328. In addition, the tube 312 is maintained at a relatively high positive electrical potential, for example 20 kV, with respect to ground, by a source tension 330. The injector tube 312 is mounted coaxially inside a metal tube 332 whose walls converge towards an opening 336 at its end 334 slightly downstream of the end 318 of the tube 312 in the direction of gas flow inside the chamber 314. The tube 332 is mounted in an opening formed in the wall 340 of a metal enclosure 342 of an electrostatic hot gas deduster, as shown for example in FIG. 5. The wall 340 is grounded. The tube 332 is polarized at a potential which can be identical to, or different from that of the tube 312 thanks to a voltage source 321. It crosses the wall 340 by an insulating bushing 333.

Inside the enclosure 342, the tube 332 is surrounded by a coil 344 in which a current of a non-conductive cooling fluid can be circulated. The fluid supply pipes to the coil are made of a dielectric material so as to support the high positive voltage of the source 331. Means not shown are provided for circulating a current of air in the direction of the enclosure 342, in the direction of arrow 346,

in the annular space between the tubes 312 and 332.

In operation, a flow 350 of charged microparticles injected into the enclosure 342 is surrounded by a vein of cold air which is substantially tubular at the exit from the opening 336 of the tube 332 which delays the heating of these microparticles and their subsequent sublimation until that they have moved away from the injector tube 312. Furthermore, this is brought to a high potential with respect to the walls 340, which creates a distribution of potential inside the enclosure 342, which tends to move away from the injector tube 312 the ions released by evaporation of the microparticles.

The fact that the blowing of cold air or any other gas at the outlet 336 of the tube 332 cools the gases to be dedusted is not a drawback in applications for supplying hot gases to engines such as gas turbines from poor fuel stoves. In fact, the temperatures of the gases which can be obtained at the outlet of such homes are much higher than the maximum temperature of approximately 900 ° C. which a gas turbine can withstand when it enters the current state of technology. It is therefore sufficient to provide the temperature of the gases leaving the hearth as a function of the flow rate of cooling gas from the injectors so as to obtain, after mixing, the desired temperature at the inlet of the turbine.

The embodiment of FIG. 7 can be the subject of many variants. Thus, fig. 8 shows a construction in which the injector tube 312 is mounted directly in the wall 340 of the enclosure 342 using an insulating bushing 400. As in the case of FIG. 7, the body of the tube 312 is brought to a positive high voltage with respect to the walls 340 which are grounded. No cold air is blown around the microparticles.

In fig. 9, the outer surface of the injector tube 312, mounted as in the case of FIG. 8 is surrounded, inside the enclosure 342, by a coil 402 in which a cooling fluid circulates.

The coolant can be a dielectric fluid such as oil. The oil supplies to the coil 402 are produced using dielectric pipes of sufficiently large length to hold the high voltage applied to the injector 312. The oil can also be replaced by deionized water according to known techniques .

In the device of fig. 10, the injector tube 312 is polarized by a voltage source 409. It is surrounded by the tube 332 for blowing fresh air into the enclosure 342 around the flow of injected microparticles. The tube 332 crosses the wall 340 by an insulating bushing 406. It is maintained at a high positive potential by a continuous high voltage source 408.

In an exemplary embodiment, in FIG. 11, an injector device 412 has been shown, as described with reference to FIG. 7 above, at the end of a bent cane 410 in an enclosure 412 traversed in the direction of arrow 441, by a stream of hot gases to be dedusted at the speed of 3 m / s. The rod 410 passes through the wall 440 of the enclosure to supply the injector 412 with moist air, blowing air for the tube 332 and cooling water. The injec-

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tor 412 is oriented so that the stream of cold air blown around the stream of projected microparticles is in the same direction and in the same direction as that of the gas to be dusted, and the speed of this stream of air is at least that of gas, in this example 3 m / s. s

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The outlet diameter of the tube 332 is approximately 4 cm. The flow of cold gas is approximately 2% of the flow of hot gas to be dedusted, the temperature of the latter being slightly greater than 900 ° C. The area of action of the injector is then located within a radius of approximately 15 cm from the end of the 412 injector.

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Claims (16)

642,870 2 1. A method of separating particles in suspension in a gas, according to which the gas is circulated in an enclosure to electrically charge the particles in order to then collect them by electrostatic precipitation, characterized in that one produces in a separate chamber from this enclosure of ions trapped by ice microcrystals, obtained by supersonic expansion of a stream of wet gas in a corona discharge produced in said separate chamber, in that ions thus trapped are injected into the enclosure, which causes the ice microcrystals to change state therein, thereby releasing trapped ions to create a charge of space, through which the gas with suspended particles to be separated is passed. 2. Method according to claim 1, characterized in that the wet gas is air whose hygrometric degree measured under normal conditions of temperature and pressure is greater than 10%. 3. Method according to one of claims 1 or 2, characterized in that the flow rate of ice microcrystals injected into said enclosure is adjusted to maintain the space charge at a prescribed value. 4. Method according to one of claims 1 to 3, characterized in that the gas stream is formed of hot gas and in that positive ions are trapped to form a positive charge on the path of said stream of hot gas. 5. Method according to one of claims 1 to 3, characterized in that the gas stream is formed of air charged with gluten particles and that one operates negative ions to form a negative space charge in this stream of air. 6. electrostatic separator of particles suspended in a gas for implementing the method according to claim 1, comprising an enclosure for circulating a current of this gas, means comprising an ion generator by corona discharge for electrically charging said particles, and means on the path of the gas stream for electrostatically precipitating the charged particles, characterized in that the generator comprises means suitable for defining a chamber distinct from said enclosure, and communicating with the latter by an orifice, with in this chamber separate a conductive supersonic nozzle, the outlet of which at least partially defines said orifice, means for introducing a wet gas under pressure into the chamber in order to cause its supersonic expansion in the nozzle, a tapered electrode leading to the neck of the nozzle, and means for establishing a high DC voltage between said conductive nozzle and said electrode, af in to produce a corona discharge in the gas flowing through this nozzle, at the same time as, in the corona discharge zone, microparticles of ice suitable for trapping the ions before being injected through said orifice into said enclosure. 7. Separator according to claim 6, intended for the elimination of flammable particles in an atmosphere of grain storage silo, characterized in that the tapered central electrode is brought to a negative potential with respect to that of the nozzle. 8. Separator according to claim 7, intended for the elimination of dust conveyed in the hot combustion gases, characterized in that the tapered central electrode is brought to positive potential with respect to that of the nozzle. 9. Separator according to one of claims 6 to 8, characterized in that said orifice is formed in an injector, and in that are provided means suitable for limiting the capture of ions introduced into the enclosure in the immediate vicinity of said injector . 10. Separator according to claim 9, characterized in that said means for limiting the capture of ions comprise means suitable for creating an electric field distribution in the vicinity of the injector which tends to establish a high potential difference between this injector and the walls of the enclosure, in order to separate from the injector the ions injected into the enclosure. 11. Separator according to claim 10, characterized in that it further comprises means suitable for supporting a high voltage to supply a non-conductive cooling fluid to the injector. 12. Separator according to one of claims 6 to 11, in particular for the dedusting of hot gases, characterized in that 5 said means for limiting the capture of ions include means suitable for delaying the change of state of said microparticles in the enclosure near the injector. 13. Separator according to claim 12, characterized in that the delay state change means comprise means for blowing a stream of cold gas around the flow of microparticles injected into the enclosure. 14. Separator according to claim 13, characterized in that said blowing means comprise a tube mounted around an injector and penetrating into said enclosure to inject cold gas is in the interval between this injector and this tube. 15. Separator according to claim 14, characterized in that it comprises means for cooling said tube. 16. Separator according to one of claims 14 or 15, characterized in that this tube is insulated from the wall of said en- 20 surrounded and in that it comprises means for establishing a potential difference between this tube and this wall.