EP1294469A1 - Method and device for treating by electrical discharge gaseous volatile organic effluents - Google Patents

Abstract

The invention concerns a method and a device for treating by electrical discharge gaseous volatile organic effluents. The effluents are subjected (A) to a low pressure expansion with temperature decrease to 150 DEG K </= 173 DEG K and to electrical discharges (B), using a supersonic nozzle, thereby enabling to carry out a treatment without significant NO>x <formation. The invention is useful for treating polluting gases such as styrene, toluene.> <

Description

METHOD AND DEVICE FOR THE TREATMENT BY ELECTRIC DISCHARGE OF GAS VOLATILE ORGANIC EFFLUENTS

The invention relates to a method and a device for treating electrically volatile gaseous volatile organic effluents at low pressure and at low temperature.

The industrial processes implemented in high value-added processing industries make use of very sophisticated materials processing and transformation processes which use volatile organic effluents, the effect of which can ultimately be dangerous for the environment and the species evolving in it.

This is the case, in particular, in the context of the automobile industry, aeronautical construction, and, more generally, of any industry using paint coating or surface treatment processes by means of paints, lacquers or other treatment products.

The solutions proposed at present, in order to reduce or eliminate the level of emission of such effluents, are not numerous and come up against a formidable problem which relates to the emission of large quantities of nitrogen oxide N0 _X , at emission levels between 500 and 2000 ppm, nitrogen oxide, at such concentrations, proves to be one of the major polluting gases of large urban or industrialized sites. Among the solutions envisaged, mention may be made of the technique known as the "Glidarc" technique, which consists in generating, at atmospheric pressure, sliding electric arcs, which allow, to a certain extent, the destruction of such volatile organic compounds, referred to below as VOCs. With reference to FIG. 1 a, it is recalled that this technique consists in generating an electric arc AR between two divergent electrodes, a flow of gas containing the VOCs being directed towards these electrodes. An electric arc AR starts at the neck of the electrodes A, in accordance with Paschen's law, moves by sliding towards the flared part B under the action of the gas flow. The electrical voltage drop across the arc increases with the length of the arc. When the aforementioned voltage drop reaches the value of the breakdown voltage at the neck, the displaced electric arc is short-circuited and extinguished by a new arc, the process is repeated according to an oscillatory relaxation phenomenon.

For a more detailed description of the "Glidarc" process, one can usefully refer to patent application n ° 2 639 172 published on 05/18/1990 in the name of ELECTRICITE DE FRANCE.

Other techniques implementing the creation of electric arcs, or at least a pre-ionization of the oxidizing gases in electric burners, have been implemented in order to ensure a lively combustion, at high temperature, of these last.

In this type of technique, the purpose of the preionization process is to increase the conductivity of the flame, in the absence of any addition of additive liable to present a risk of pollution. Among the aforementioned type of technique, a solution described in French patent application 2,577,304 published on August 14, 1986 in the name of ELECTRICITE DE FRANCE and STEIN-HEURTEY, implements a nozzle, the pre-ionization process being carried out by creating electric arcs between an axial electrode and one or more annular electrodes, forming a nozzle.

In the process implemented by the aforementioned technique, the arcs generated and the lively combustion produced are produced at high temperature, above 2000 ° K. In particular, the electric arcs generated are arcs in thermodynamic equilibrium at high temperature and the phenomenon of expansion, and the correlative lowering of gas caused by the nozzle effect is substantially obscured by the aforementioned significant temperature increase .

In both of the above techniques, the production of nitrogen oxide NO _x is important. Indeed, this production is essentially of thermal origin, the maximum of the level of emission of nitrogen oxide NO _x being located at a temperature close to 3500 ° K.

The present invention, on the contrary, relates to the implementation of a method and a device for the treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature making it possible to notably avoid the formation of nitrogen oxide NO _X.

In particular, the subject of the present invention is a method and a device for treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature using a phenomenon of electric discharge outside local thermodynamic equilibrium, the treatment of these effluents, therefore, being carried out at low temperature and at low pressure, which makes it possible substantially to avoid the formation of nitrogen oxide NO _x .

Another object of the present invention is also the implementation of a method and a device for treatment by electrical discharge of gaseous volatile organic effluents, containing hydrogen, the treatment leads to low pressure allowing in these effluents to release hydrogen radicals having, due to the low pressure surrounding them, a lifetime ten times greater than that of these same hydrogen radicals subjected to atmospheric pressure. The method of treatment by electrical discharge of volatile organic gaseous effluents, object of the present invention, is remarkable in that it consists at least in subjecting these effluents to an expansion at low pressure, in order to generate a lowering of temperature. the latter at a temperature of between 150 ° K and 173 ° K and subjecting the effluents at low temperature and at low pressure thus obtained to electric discharges to cause a rupture of the hydrocarbon chains constituting these gaseous organic effluents, in the absence for creating NO _x nitrogen oxide compounds.

The device for the treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, object of the present invention, is remarkable in that it comprises, at least, an intake stage for these effluents, a stage of the same effluents allowing to generate a relaxation at low pressure and a lowering of the temperature of these effluents to a temperature between 150 ° K and 173 ° K. In addition, an electrical discharge generator stage between a high voltage electrical potential and a low voltage electrical potential is provided, this electrical discharge being applied to these effluents at low pressure and at low temperature to cause rupture of the hydrocarbon chains of these latter, in the absence of creation of nitrogen oxide compound NO _x . The process and the device which are the subject of the present invention find application in the destruction of volatile organic effluents, in the production of hydrogen radicals with increased lifetime in hydrocarbon products, in particular in the cracking of hydrocarbons used or produced by industry. petroleum, in the absence of crippling production of nitrogen oxide.

The method and the device which are the subject of the present invention will be better understood on reading the description and the observation of the drawings below in which, in addition to FIG. 1 relating to the prior art:

- Figure 2a shows, by way of illustration, a functional diagram of the steps for implementing the method of treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, object of the present invention;

FIG. 2b represents, by way of illustration, a functional diagram of the stages of an alternative implementation of the method of treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, object of the present invention, in which, by repeating the steps of the process which is the subject of the invention as illustrated in FIG. 2a, a synergistic effect is however obtained;

- Figure 3a shows, by way of illustration, a sectional view, along a plane of radial symmetry, of the device for treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, object of the present invention;

- Figures 3bι, 3b ₂ , 3b ₃ and 3b _{4 show} diagrams illustrating the principle of the different flow regimes in a nozzle such as that used in the device object of the invention illustrated in Figure 3a;

- Figure 4a shows, by way of illustration, a sectional view, along a plane of radial symmetry, of an alternative embodiment of the device for treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, object of the present invention, this variant embodiment being more particularly suitable for implementing the method which is the subject of the present invention as illustrated in FIG. 2b;

- Figure 4b shows a diagram of the profile of the relative pressures recorded in the flow produced in the device object of the present invention as shown in Figure 4a;

- Figure 4c shows a diagram giving the position of the shock wave generated in a nozzle of the device object of the present invention shown in Figure 4a as a function of the gas flow; FIG. 4d represents a diagram of the values of the Paschen parameter, product of the pressure in a straight section of the flow and of the electrical distance, likely to generate an electric discharge in this section, this diagram making it possible to locate the position of the electric discharge for the minimum value of this parameter;

- Figure 5a shows, by way of illustration, a sectional view along a radial plane of symmetry of the central electrode equipping the device object of the present invention as illustrated in Figure 3a or 4a; - Figure 5b shows, by way of illustration, a preferred diagram of connection and electrical supply of the electrodes fitted to the device object of the present invention as illustrated in Figure 4a;

- Figure 6a shows a preferred non-limiting embodiment of the device object of the invention as illustrated in Figure 3a;

- Figures 6bι and 6b ₂ , sectional view VI-VI of Figure 6b _{lA show} a detail of an embodiment of a deflecting part used in the embodiment of Figure 6a;

- Figure 6c shows another preferred non-limiting embodiment of the device object of the invention as shown in Figure 3a or 6a.

A more detailed description of the process for the electrical discharge treatment of gaseous volatile organic effluents at low pressure and at low temperature in accordance with the object of the present invention will now be given in connection with FIG. 2a.

In general, it is indicated that the process which is the subject of the present invention consists in treating the abovementioned effluents when these are accompanied by a carrier gas, these effluents then being able to correspond to aerosols, vapors or other gases which are in suspension, respectively mixed with the abovementioned carrier gas. Of course, in the context of an industrial implementation, it is indicated that the carrier gas can consist of air, ambient air, which, taking into account the proportion of nitrogen contained in the ambient air above, implies, in the absence of special precautions, the risk of creating nitrogen oxide NO _x .

The process which is the subject of the present invention precisely makes it possible, in the absence of any particular precaution, in particular of a chemical nature, to avoid any risk of emission of the abovementioned nitrogen oxides at unacceptable levels, as will be described below. after.

According to the process which is the subject of the present invention, this consists, in a step A, in subjecting the volatile organic gaseous effluents to an expansion at low pressure, in order to generate a lowering of the temperature of these effluents, the latter being brought to a temperature between 150 ° K and 173 ° K.

By low pressure expansion means any expansion of organic effluents, and of course of the carrier gas, at a pressure less than or equal to 350 millibars for example.

The aforementioned gaseous volatile organic effluents having been brought to the state of low pressure and of reduced temperature are then subjected, in step B, to a treatment by electric discharges. This procedure makes it possible to treat the aforementioned volatile organic gaseous effluents by breaking the chains. constituent hydrocarbons of the latter, in the absence of creation of nitrogen oxide compounds NO _X , as will be described and illustrated later in the description. In general, it is indicated that step A must be carried out prior to step B or, where appropriate, at least concomitantly. It is understood in particular that due to the abovementioned relaxation phenomenon, the electric discharge treatment operation makes it possible to crack the carbonaceous bonds CH and to generate hydrogen radicals whose lifespan in the atmosphere at low pressure mentioned above is lengthened significantly. It has been observed during investigations that for a low pressure of the order of 300 millibars, the lifetime of the hydrogen radicals is approximately ten times greater than at atmospheric pressure.

In addition, due to the abovementioned expansion phenomenon, it is indicated that the lowering of the temperature of the gaseous volatile organic effluents makes it possible to eliminate any risk of emission of nitrogen oxide for the reasons which will be indicated below.

In general, it is indicated that the electrical discharges generated are formed by electrical discharges at low temperature, that is to say in the medium of gaseous volatile organic effluents brought to the temperature between 150 ° K and 173 ° K, these electrical discharges being generated in an environment outside of local thermodynamic equilibrium, which makes it possible to maintain the environment of the discharge atmosphere at a temperature substantially lower than the temperature room. For this reason, the level of nitrogen oxide emission from the nitrogen contained in the carrier gas is significantly reduced and, under certain experimental conditions, practically negligible, as will be described later in the description.

With regard to the electric discharges generated in an environment outside local thermodynamic equilibrium, it is indicated that these electric discharges are distinguished in a particularly characteristic manner from the electric arcs of the prior art, which on the contrary are generated in an environment in thermodynamic equilibrium and which, for this reason, reach very high temperatures.

A more detailed description of an alternative implementation of the method which is the subject of the present invention will now be given in conjunction with FIG. 2b.

In general, and in accordance with a particularly remarkable aspect of the process which is the subject of the present invention, it consists, in a step Ai, of subjecting the gaseous organic effluents to steps A and B shown in FIG. 2a, that is ie respectively at a low pressure expansion step by lowering the temperature to a temperature between 150 ° K and 173 ° K, and, concomitantly or successively, subjecting the cooled and low temperature effluents to discharges electrical under conditions similar to those explained above in the description in conjunction with Figure 2a.

Step i can then advantageously be repeated, this repetition making it possible, from gaseous organic effluents having already undergone a treatment during the implementation of step Ai, in particular from ozone 0 ₃ and oxygen radicals generated by the emanations of the electrical discharge carried out in step B of the aforementioned step Ai, to further submit the effluents gaseous organics treated with a phenomenon of enhanced oxidation by means of ozone and the oxygen radicals thus created.

By repetition of step Ai, as illustrated in FIG. 2b, is meant the repetition of steps A and B constituting it at least qualitatively, the conditions for implementing these steps can however be modified according to specific criteria related to the mode of production of the low pressure expansion processes during the repetition, as will be described in more detail later in the description. In particular, the relaxation carried out during the repetition may be less intense, the pressure and the temperature reached being able to be greater than in the case of the first relaxation, the repeated relaxation being consequently less strong, as will be described later in the description.

The process for the treatment by electrical discharge of gaseous volatile organic effluents at low pressure and low temperature, which is the subject of the present invention, can advantageously be implemented by means of a specific device particularly suitable for implementing this process, as will now be described in connection with Figures 3a and 3bι to 3b and the following figures.

As shown in FIG. 3a above, it is indicated that the device which is the subject of the present invention comprises at least one stage 1 for admitting the effluents volatile organic gases with which an expansion stage 2 for these effluents is associated, this expansion stage making it possible to generate expansion at low pressure, pressure less than 350 to 400 millibars for example, and a lowering of the temperature of these effluents to a temperature between 150 ° K and 173 ° K.

Finally, the device which is the subject of the present invention comprises an electrical discharge generator stage, marked with the reference 3, between a high voltage electrical potential and a low voltage electrical potential. The electric discharge is applied to volatile organic effluents at low pressure and at low temperature, this operating procedure making it possible to treat volatile organic effluents by breaking the hydrocarbon chains constituting them in the absence of the creation of oxide compounds. 'nitrogen NO _x .

As shown in FIG. 3a, and in a nonlimiting preferential manner, it is indicated that the expansion stage 2 is constituted by a supersonic nozzle under the operating conditions which will be described later in the description.

As shown in the above-mentioned figure, it is indicated that the device which is the subject of the invention can advantageously be made up and comprise at least, in a tube T provided with a pipe for admitting the carrier gas, this pipe bearing the reference 10, a central electrode EC of revolution, this central electrode extending substantially along the longitudinal axis of the tubing T. More specifically, it is indicated that the central electrode EC can be produced by a cylindrical rod of electrically conductive material, such as copper, stainless steel for example.

As has also been shown in FIG. 3a, the device which is the subject of the invention successively comprises, and arranged substantially in revolution around the central electrode EC so as to form the expansion stage 2, a chamber d 'inlet, denoted CA, volatile organic gaseous effluents, the inlet chamber CA constituting in fact a reservoir of the buffer tank type of the carrier gas and volatile organic gaseous effluents contained in the latter.

In addition, and with the aim of forming the expansion stage 2, the device which is the subject of the invention comprises a nozzle 20 comprising a nozzle neck and forming a duct of the Venturi duct type, for example, the intake face of the nozzle being in direct engagement in the inlet chamber CA. Of course, the nozzle 20 surrounds the central electrode EC and constitutes with the latter a supersonic flow channel, if necessary subsonic, for the gaseous volatile organic effluents and the carrier gas contained in the inlet chamber CA.

Taking into account the specific pressure conditions in the inlet chamber CA and the flow parameters imposed by the nozzle 20, in particular the nozzle neck and the central electrode EC, the assembly consisting of the carrier gas and the effluents VOC is subjected to a flow causing a shock wave accompanied by the expansion and the decrease in temperature of gaseous volatile organic effluents mentioned above, as will be described later in the description.

More specifically, it is indicated that the nozzle 20 and the nozzle neck consist of an electrically conductive material, such as copper or stainless steel, and formed by a tube of revolution having a convergent / divergent profile. The central electrode EC and the revolution tube constituting the nozzle 20 are coaxial to form the cylindrical expansion channel of the carrier gas and organic VOC effluents.

Finally, the central electrode EC and the nozzle neck as well as the complete nozzle 20 are connected to an electric voltage generator to form the electric discharge generator stage 3 between the central electrode EC and the nozzle 20. In the figure 3a, it is indicated that the electric voltage generator is not shown, only the connections to the latter, 200 and 201, of the nozzle 20, respectively of the central electrode EC being shown, so as not to unnecessarily overload the drawing .

As far as the mechanical execution of the assembly is concerned, it is indicated that the inlet chamber CA can be produced directly in the tube T formed by a cylindrical sleeve, a shutter 11 forming a shim for holding the central electrode EC and made of electrically insulating material being provided so as to maintain and center the above-mentioned central electrode EC.

In FIG. 3a, the direction of flow e of the carrier gas and of the organic VOC effluents is shown. As regards the nozzle 20, this can be held by a mechanical piece of insulating material denoted 21, fixed directly to the internal wall of the tube T. The connection of the nozzle 20 and the nozzle neck to the external electric generator by the connection 200 can then be produced in a manner known as such by a sealed electrical crossing making it possible to ensure the passage through the body of the tubing T of the conductor 200 under suitable electrical insulation conditions.

With regard to the mechanical mounting of the central electrode EC at the downstream end of the nozzle 20, the upstream end of the nozzle 20 being constituted by the opening of the nozzle converging with respect to the flow e and the downstream end being constituted by the end of termination of the nozzle with respect to this direction of flow, it is indicated that the central electrode EC can be maintained if necessary by any suitable device, not shown in the drawing, and located outside a part of the tubing T and the nozzle 20 for example. Indeed, it is indicated that, beyond the termination end of the nozzle in the direction of flow e, the VOC effluents and the carrier gas are, for example, at atmospheric pressure Patm, as shown in FIG. drawing and that will be explained in more detail later in the description.

The operating mode of the device which is the subject of the present invention as shown in FIG. 3a will now be described in conjunction with FIGS. 3bχ to 3b ₄ . In Figure 3bι, there is shown the profile of a nozzle type Laval nozzle convergent / divergent, the neck of the nozzle being noted C on the abscissa Xc. The profile is defined by the radius r of any straight section of the nozzle, each section having an area A and the section at the neck of the nozzle having an area A _c .

The inlet chamber CA is deemed to constitute a large upstream reservoir where the organic VOC effluents and the carrier gas are at a pressure P ₀ and have a specific mass po as well as an original temperature T ₀ . At the outlet of the nozzle 20, atmospheric pressure Patm prevails.

In reference to Hugoniot's theorem and for a gas or mixture of gases leaving the inlet chamber CA with zero speed, the flow is necessarily subsonic in the convergent of the nozzle.

If at the abscissa neck Xc the speed of the gas mixture is lower than the speed of sound, the flow remains subsonic in the whole of the diverging part of the nozzle.

If, on the contrary, the speed of the fluid reached at the neck of the nozzle 20 the speed of sound, two isentropic flows are then possible in the divergent: a supersonic flow with increase in speed and decrease in pressure when the area A of the nozzle section increases; a subsonic flow with decrease in speed and increase in pressure when the aforementioned area A increases. Under these flow conditions, it is then possible to establish the value of the ratio of the area A of a any section of the abscissa nozzle X at the value of the area A _c of the section of the nozzle at the abscissa neck Xc as a function of the ratio of the pressures of the gas or gas mixture in the nozzle section considered and of the ratio γ = Cp / Cv of specific heats at constant pressure and volume of the gas or gas mixture, respectively as a function of this same ratio and of the Mach M number of the flow. It is also possible to establish the ratio of the temperatures of the gas or gas mixture in a cross section of the nozzle to the temperature To of the gas in the inlet chamber CA as a function of this same ratio of the specific heats at pressure and volume. constants and the Mach M number of the flow. The relationships (1), (2) and (3) below express the value of these relationships:

(3)

I ₊ 1 = M In these relationships, the values of P and M mentioned above, relating to the pressure and Mach number, denote the values of pressure and Mach number of the flow in any section of area A from the nozzle ^' to the abscissa X considered.

The P / Po ratio of the pressure in any section of the nozzle to the generating pressure Po and the Mach number M are represented along the nozzle of the abscissa 0 corresponding to the inlet face of the nozzle 20 up to 'at' the abscissa Xs, termination of the nozzle, as mentioned previously in the description, these representations being given in FIGS. 3b ₂ and 3b ₃ .

There are in fact different possibilities of flow regime depending on the value of the external pressure P _a of the medium into which the nozzle opens. The different flow regimes as a function of the relative values of the aforementioned pressures will not be described, because they correspond to elements known from the state of the art. However, it is indicated that, in accordance with a particularly remarkable aspect of the method and of the device which are the subject of the present invention, the expansion achieved is obtained by means of the use of a supersonic nozzle thanks to which a stationary OC shock wave s 'established in the divergent part of the nozzle.

As shown in FIG. 3b ₄ , for P _a = Pu, the shock wave is located in the outlet section of the nozzle, that is to say at the terminal end of the latter; for P _a > P _N , the shock wave goes up in the divergent at the same time as it weakens, the flow in the divergent upstream of the shock wave OC being supersonic and subsonic downstream, as shown in Figure 3b above; for P _a = P _P , the shock wave reaches the throat of the nozzle and the flow is then subsonic over the whole of the divergent.

In the preceding relations, P _N and P _P denote particular values of the pressure of the external medium.

The calculation of the position of the shock wave in the divergent part of the nozzle can be carried out in the following manner.

The passage of the gas or of the gaseous mixture constituted by the carrier gas and the volatile organic effluents VOC is accompanied by an increase in entropy with a drop in the pressure generating the flow. Consequently, if po is the pressure generating the flow upstream of the shock wave, we can consider that the subsonic flow immediately after the shock wave is isentropic with a generating pressure p'o <Po- Le ratio of these pressures is given by the relation (4) below:

In this relation Mi is the Mach number immediately upstream of the shock wave considered.

However, the shutdown temperature, which is equal to the flow generating temperature, remains the same on either side of the shock wave. The flow downstream of the shock wave can be considered as an isentropic flow from the reservoir with a generating temperature T ' ₀ equal to the previous temperature T ₀ but with a generating pressure p' ₀ less than p ₀ .

It is then possible to determine the characteristic parameters p'o / po _r M and T of the subsonic flow downstream of the abovementioned shock wave OC fixed in a cross section of the diverging portion of the nozzle at the abscissa xl, and with a section ratio Aι / A _c through the use of the relationships (1), (2), (3) and (4) previously cited in the description.

The number of Mach Mi upstream of the shock wave can then be calculated for a ratio Aι / A _c given from relation (2).

The ratio of the generating pressures po / p'o ⁼ A ' _c / A _c is then obtained from relation (4) and makes it possible to obtain the area A' _c of the section of the neck corresponding to the subsonic flow. downstream of the shock wave.

For any section of area A of the nozzle and a ratio A / A ' _c of this flow, the ratio p / p'o and the number of Mach M for the area A considered are then determined from the relations ( 1) and (2) above. The temperature ratio for any section of the flow is then deduced from the Mach number from relation (3).

Taking into account these elements and the precise calculation of the position of the shock wave hung in steady state in the divergence of the nozzle 20, it is then possible, depending on the treatment chosen, that is to say say of the nature of the treated volatile organic effluents, to provide for an admission of the effluents to be treated in the vicinity of this shock wave, in order to obtain an optimization of the abovementioned treatment, as will be described later in the description.

A more detailed description of a device according to the object of the present invention, more particularly intended for the implementation of the method illustrated in FIG. 2b, will now be given in conjunction with FIG. 4a and the following figures 4b to 4d.

In FIG. 4a, the same references represent the same elements as in the case of FIG. 3a.

In particular, with reference to FIG. 4a above, it is indicated that the expansion stage 1 can be completed by another expansion stage 4 ensuring an analogous function and making it possible to repeat step Ai of the process described above in connection with the Figure 2b.

With reference to FIG. 4a above, it is indicated that the additional expansion stage 4 can be formed by another nozzle 40 of revolution around the central electrode EC and arranged in cascade with the nozzle 20.

Preferably, as shown in FIG. 4a, the nozzle 20 constituting the expansion stage 2 and the nozzle 40 constituting the additional expansion stage 4 are cascaded around the central electrode EC via a passage chamber forming a mixing chamber and denoted CB. The assembly formed by the nozzle neck and the nozzle 20, by the passage chamber CB, and by the nozzle 40 and the nozzle neck constituting the additional expansion stage 4 / 00330

22

is coaxial with the central electrode EC and of revolution around it. The passage chamber CB may comprise an intake outlet for gaseous volatile organic effluents passing through the wall of the tube T and bearing the reference D. The passage chamber CB, forming a mixing chamber, makes it possible to introduce turbulence at the level flow, which has the effect of promoting the treatment of VOCs. Preferably, the nozzles 20 and 40 can be constituted by Laval nozzles.

Given these elements, the additional indications are given below by way of example as regards the execution of a device as shown in FIG. 4a. The inlet chamber CA can be constituted by a cylindrical chamber with a diameter between 30 and 70 mm. In the inlet chamber A there reigns the pressure generating Po of the flow.

The opening angle of the nozzle 20, that is to say of the divergent thereof, can be taken equal to a value between 2 and 5 °.

The passage chamber CB, also of cylindrical shape, may have a length between 30 and 70 mm and a diameter between 30 and 80 mm.

While the neck of the first nozzle 20 may have a length of a few tens of millimeters and a constant diameter over this length of between 17 mm and 20 mm, the neck of the second nozzle 40 is preferably a neck of radius greater than that of the neck of the first nozzle 20, but of neck length, over which the radius of the neck is substantially constant, less than that of the first nozzle. The central electrode can have a diameter between 13 and 16 mm. In any event, the aforementioned dimensioning parameters can be adapted according to the industrial application chosen. Preferably, in order to eliminate any trace of humidity in the carrier gas, that is to say the compressed air supplying the device via the pipe 10, a filtering system is provided before injection. The air flow constituting the carrier gas can then be between 20 and 200 Nm ³ / h. It is recalled that 1 Nm ³ denotes a volume of 1 m ³ of gas measured under normal conditions of temperature and pressure.

Given these implementation parameters, it is thus possible, as shown in FIG. 4a, to obtain a shock wave phenomenon in the divergent part of the first, 20, respectively the divergent part of the second nozzle 40. This operating mode then allows the implementation of the method which is the subject of the present invention according to the embodiments shown in FIG. 2a and / or 2b. In addition, it is indicated that the first nozzle 20 can be a supersonic or subsonic nozzle while the second nozzle 40 can be supersonic or subsonic, conditionally to the flow regime of the first nozzle 20. The experimental results are given in FIG. 4b representing the P / Po ratio of the pressure P in a cross section of the flow, that is to say on the path of the gas between the inlet chamber CA and the outlet at atmospheric pressure of the device as shown in Figure 4a. The P / P ₀ ratio is measured for different volume flow rates D _g previously mentioned. Upon observation of FIG. 4b, it can be seen that the flow becomes supersonic at the neck of the first nozzle for a volume flow rate D _g = 35 Nm ³ / h for a generating pressure Po = 1.333 atm. The pressure measured at the neck P _c then checks the relation (5):

For higher flow rates, that is to say greater than 35 Nm ³ / h, the passage through the point corresponding to the value P / Po = 0.528 is always carried out substantially at the same abscissa, with a slight corresponding offset at the position of the actual neck of the device.

The area A _c of the cross-section of the gas mixture at the neck and the corresponding diameter φ _c obtained from the flow parameters, and not the construction dimensions of the assembly, for different flow rates D _g are given in the table below:

The values of A _c calculated are in good agreement, to within 2%, with the value φ _c retained for the corresponding construction implementation. We observe in Figure 4b the appearance of a shock wave, which is established in the divergence of the first nozzle 20 at a distance δ _x from the neck the greater the greater the gas flow rate. FIG. 4c represents the position δ _x of the shock wave with respect to the neck as a function of the volume flow rate D _g of the gas admitted into the intake chamber CA. In FIG. 4c above, the gas flow rates are expressed in Nm ³ / h and the position difference δ _x in mm. With regard to the scheme for creating electrical discharges, all of the following elements will be indicated.

The voltage required to initiate each discharge is directly related to the inter-electrode distance noted as di- Generally, it is indicated that the initiation voltage at the neck is equal to the breakdown voltage according to Paschen's law .

For the implementation of the process which is the subject of the present invention, it is indicated that it is necessary to choose a value which is sufficiently low for the generator to allow at least priming at the neck of each nozzle, but moreover sufficiently high for the discharge has time to develop properly and that the power dissipated in the plasma cord, constituting the discharge, is not too low taking into account the physicochemical reactions sought. It is indicated that an optimum operating mode of the device which is the subject of the present invention in fact corresponds to a cut-off of the discharge by at least one reboot at the neck when the power dissipated in the plasma cord corresponds substantially to the charge adaptation with the generator. It is in fact understood that under these conditions, the efficiency of the energy transfer is then optimal.

The phenomenon of extinction of each discharge, which corresponds substantially to a phenomenon of relaxation oscillation under the conditions indicated previously in the description, can be achieved in three different ways: the discharge voltage reaches the breakdown voltage at the throat of the nozzles . The priming zone is then the space where the electrodes, central electrode and corresponding nozzle, are closest and where the electric field is sufficient, taking into account the flow conditions. This new discharge short-circuits the old one which is no longer maintained and then ends up going out.

This operating mode is the operating mode when the supply by the electric generator is a continuous supply.

The length, and the resistance, of the discharge increases over time, as well as the energy necessary to maintain it. From a critical length, the power supply can no longer provide enough energy to maintain the plasma. The conductivity of the discharge decreases and eventually cancels, leading to the extinction of the above-mentioned discharge. The discharge voltage then increases suddenly to reach the no-load voltage of the generator and a new ignition occurs at the neck of the nozzle. - When the voltage generator is an alternating generator, the passage through the zero value of the current / 00330

27

discharge may cause the latter to go out. Indeed, in the case where the duration of the zero crossing is sufficiently short and does not exceed the recombination time of the charge carriers, their density does not instantly drop to zero. Under these conditions, there may be re-ignition of the discharge in or in the vicinity of the plasma cord rather than at the neck of the nozzle considered. Such a phenomenon is analogous to the back-cracking phenomenon observed during experiments.

Taking into account Paschen's law, the initiation of the discharge must be done at the point of the nozzle where the product P x deiec / that is to say the product of the pressure P of the gas or gas mixture for a section of any given nozzle, by the inter-electrode distance and the distance between the central electrode EC and the convergent / divergent body of the nozzle, is the smallest.

From the experimental results giving the pressure distribution in the nozzle given in FIG. 4b as well as the dimensions of the nozzle deduced from the construction characteristics of the latter, FIG. 4d shows the evolution of the product P x die for different gas flow rates or gas mixture. From the observation of FIG. 4d above, it appears that the breakdown must occur, in particular at the level of the shock wave OC, where the product P xd _é i _ec is the weakest.

However, for higher gas or gas mixture flow rates, the shock wave is pushed back into the divergence of the first nozzle 20 and the product P x deiec then passes through its minimum upstream of the shock. In this case, the discharge begins upstream of the above-mentioned shock wave.

In view of the above observations, it appears possible, in accordance with the method and the device which are the subject of the present invention, to adjust the relative position of the shock wave and of the discharge initiation point in order to choose the most favorable situation. depending on the volatile gaseous effluents treated, taking account of course of the nature of these.

It is understood in particular that in such conditions, the combined effects of the turbulence of the shock wave and of a discharge at very low temperature, and therefore very strongly out of thermodynamic equilibrium, are such as to optimize the treatment carried out and to reduce the emission level of nitrogen oxide NO _x .

A more detailed description of a preferred embodiment of the device, object of the present invention, will now be given in connection with FIG. 5a.

This embodiment is more particularly suitable for fine adjustment of the position of admission of the organic effluents to be treated with respect to the position of the shock wave generated in the first nozzle 20, if necessary in the second nozzle 40. The aforementioned preferred embodiment will be described in connection with FIG. 3a with respect to the first nozzle 20, all of the elements being able to be taken up similarly with respect to the second nozzle 40. As shown in FIG. 5a above, the central electrode EC is formed by an electrode hollow whose end, in the direction of flow of the gas mixture comprising the organic effluents to be treated VOC, is completely closed.

The central electrode EC thus forms an intake channel for organic effluents by means of an intake intake, denoted PRA, situated at the other end, that is to say at the upstream end of the hollow electrode EC.

Referring to FIG. 3a above, it is indicated that the hollow electrode thus constituted EC can be advantageously mounted between the sealing and holding shutter 11 and a holding device 12, located at the terminal end of the hollow electrode EC outside the tubing T and mechanically connected to the latter. The holding device 12 can be constituted, by way of nonlimiting example, by a ring holding the end of the hollow electrode EC firmly, this ring being attached by fixing lugs to the tubing T.

In addition, the hollow central electrode EC has holes passing through the side wall of the latter, these holes forming a nozzle on at least one cross section of the above-mentioned central electrode EC. In FIG. 5a, the holes forming the nozzle bear the reference 13. They can be diametrically opposite, a number of two holes being represented in FIG. 5a, or on the contrary offset by 120 ° in the cutting plane QQ represented in FIG. 5a cited above.

Thus, under these conditions, the admission of the VOC effluents to be treated can be carried out via the PRA intake socket, transmitted by the channel formed by the hollow electrode EC and distributed. at the nozzles 13 under optimal distribution conditions, as will be explained below.

The pressure of the VOC effluents to be treated at the PRA intake outlet can be between 0.35 and 5 bars. This embodiment allows, thanks to the nozzles 13, to determine the pressure in a given section of the entire device, even in the presence of electrical discharges without however requiring the introduction of a Pitot tube, which, taking into account the small cross-section of the gas mixture at each nozzle would be likely to significantly disturb the field of flow velocities. The diameter of the holes constituting the nozzles 13 can be between 0.5 and 1 mm. The nozzles 13 also make it possible to inject the VOC effluents into the flow.

The embodiment represented in FIG. 5a is particularly advantageous insofar as, taking into account the displacement δ _x of the shock wave as a function of the flow rate D _g as represented in FIG. 4c, it is advantageous to admit the organic effluents to treat VOC in a neighborhood upstream or downstream of the effective position of the above-mentioned shock wave, depending on the treatment to be applied to organic effluents introduced via the carrier gas. Under these conditions, as shown in FIG. 5a, the central electrode EC, hollow electrode, is then removable in translation G along its longitudinal axis of symmetry XX, in order to allow the straight section of the central electrode EC to be positioned provided with holes 13 forming a nozzle in the upstream or downstream neighborhood, if necessary coincidentally, of the position of shock wave, and thus optimize the treatment of volatile organic effluents according to the nature of the latter.

For this purpose, as shown in the aforementioned figure, the shutter and holding systems 11, respectively 12, can advantageously be provided with a guide system of the rolling guide type 110, respectively 120, allowing sliding mounting. G of the central electrode EC in the holding and sealing devices 11, respectively 12. As regards the sealing, this must be maintained at the level of the only holding and sealing system 11 by the intermediate seals, which are not shown in the drawing. Taking into account the range of variations of the position δ _x of the shock wave as a function of the flow rate D _g , as represented in FIG. 4c, it is indicated that the upstream end of the central electrode EC, that is that is to say in the vicinity of the holding and sealing device 11, can advantageously be provided with a system for translational movement of the central electrode EC formed, for example, by a micrometric screw VM, which, from a suitable thread of the external end of the central electrode EC, allows the displacement of the aforementioned central electrode in translation along the axis of symmetry XX of the latter. The micrometric screw device VM will not be described in detail since it corresponds to a device known from the state of the art. Given these elements, it is thus possible to carry out a precise adjustment of the plan embodying the organic VOC effluent inlet section to be treated with respect to the plane in which the OC shock wave is generated, depending on the flow conditions of the gas mixture in the first nozzle 20 for example. It is the same with regard to the second nozzle 40, the dimensioning which can then be carried out and the conditions for establishing the shock waves which can be determined, in particular their position, taking into account the measurement values of the product P x die. shown in Figure 4d.

Various indications relating to the mode of electrical supply of the central electrode EC and of each nozzle 20, respectively 40, will now be given below. When the electrodes are supplied from a direct current generator, such a generator can be produced from the supply of the three-phase network. It can then include, in cascade, an autotransformer, a step-up transformer, a rectifier assembly and a filter cell for example. The average value of the no-load voltage can be adjusted by acting on the autotransformer so as to reach a maximum value of 4.5 kV. The production of such a generator will not be described in detail since the mode of implementation thereof corresponds to the use of elements known from the state of the art.

In the presence of the electrical discharge, the generator is then subjected to the periodic transition from a short-circuit regime to an idle operating regime, with numerous instabilities due to the almost simultaneous extinction and re-ignition. On the contrary, when the supply generator is a periodic supply generator, preferably, the generator is constituted in the form of a high sinusoidal voltage supply delivering a voltage of 10 kV at a frequency of 50 Hz for example. The generator is then constituted by a high voltage transformer with magnetic leaks, the magnetic leaks of such a transformer making it possible to maintain a practically sinusoidal current whose effective value remains constant and of the order of 0.14 amperes for a primary voltage. of 220 volts.

An alternative implementation in alternative power supply may consist in using a high voltage electronic power supply delivering a periodic voltage at 25 kHz. Under these operating conditions, the maximum value of the current delivered can reach 180 mA.

Each of the aforementioned supply generators can be used to supply one or the other of the nozzles 20, respectively 40.

However, tests carried out have made it possible to demonstrate a preferred mode of feeding, as shown in FIG. 5b.

This preferred mode of supply consists in using an alternating generator at a frequency of 50 Hz making it possible to initiate the discharge in one or the other of the nozzles 20 or 40 without limitation of flow rate.

Correct centering of the central electrode EC and a good adjustment of the gas injection then makes it possible to obtain substantially uniform plasmagenic zones in each nozzle. While two generators independent can be used to supply the first 20, respectively the second nozzle 40, it is however possible to use a single supply to initiate the two discharges in the aforementioned nozzles simultaneously.

The corresponding supply mode as shown in FIG. 5b then consists in connecting the tubing T and finally the second nozzle 40, which is not electrically isolated from the body of the tubing T, at the reference voltage or ground voltage. The first nozzle 20, electrically isolated from the tubing body T by the fixing and isolation piece 21 shown diagrammatically in FIG. 5b, consists in applying the alternating voltage between the central electrode and the first nozzle 20. Under these conditions, as shown in Figure 5b above, and although a loss of power can be highlighted at the second nozzle, the supply diagram shown in this figure allows to secure the device whose tubing body T is then brought to the reference potential, that is to say to the mass.

With regard to the operation of the devices, objects of the present invention, as shown in FIGS. 3a and 4a, it is indicated that, depending on the mode of electrical supply, certain specific effects could be highlighted. This is particularly the case during a 25 kHz supply where, under the effect of the flow of gas or gas mixture, the electric discharge appears more unstable. Finally, it is indicated that the body of the second nozzle 40 may be provided with a coating 401 of material oxidation catalyst, increasing the yield of VOC treatment. In particular, the expansion channel formed by the central electrode EC and the wall of the second nozzle 40, in particular the coating 401 mentioned above, can be subdivided into elementary expansion channels CD delimited by radial and concentric walls for example comprising elements made of oxidation catalyst material. Among the oxidation catalysts which may be used, mention may be made of platinum-iridium, silica-alumina, for example.

In the case of an alternative 50 Hz supply with leakage choke, the supply current is practically sinusoidal and the discharge voltage in the first nozzle 20 fluctuates around a substantially constant mean value Uo = 1 kV under the conditions of experimentation.

A non-limiting preferred embodiment of the device for the treatment by electrical discharge of VOC effluents will now be described in conjunction with FIGS. 6a, 6bι, 6b ₂ and 6c, relating to specific elements making it possible to improve the stability of the zone of electric shock in the nozzles used.

As shown in FIG. 6a, the intake chamber CA, if necessary the passage chamber CB, can comprise a deflecting part 14 making it possible to print with a jet of gas or of gaseous mixture, coming from the intake pipe gas 10, a substantially vortex movement in a plane substantially orthogonal to the longitudinal axis XX of the inlet chamber CA or passage CB and the nozzle 20 or 40. This movement swirling stabilizes the electric discharge zone in the vicinity of the above-mentioned plane.

As shown in FIGS. 6bi and 6b ₂ , the deflecting part 14 can be made of an electrically insulating, molded material, such as PVC, substantially of revolution, comprising a central orifice 140 allowing the passage of the central electrode. EC. In addition, deflection and injection orifices formed in the thickness of the deflecting part 14 provide, with the internal side wall of the inlet chamber CA or passage CB, a profile of radial deflection towards the internal side wall of each chamber, in order to allow a tangential injection of the gas jet or gas mixture in the vicinity of the abovementioned internal side wall. The gas jet admitted from the pipe 10 upstream of the deflecting part 14 is subdivided into elementary transverse jets directed towards this internal side wall. The deflecting part 14 can be installed by means of a spacer 14a. Under these conditions, the electric discharge, initiated radially in the absence of vortex movement of the gas or gas mixture, is on the contrary driven in rotation in the original discharge plane and maintained in the latter. This allows, on the one hand, to stabilize the abscissa of the electric discharge plane and, on the other hand, to delocalize the points of electric discharge impacts on a circle, intersection of the discharge plane and the central electrode EC, respectively of the nozzle. The stabilizing effect of the electric discharge zone can also be obtained, or reinforced, thus as shown in Figure 6c, thanks to the implementation of a CO coil generating a magnetic field B collinear with the longitudinal axis XX of the inlet chamber CA or passage CB and the nozzle. The CO coil can advantageously be placed outside the tubing T. The magnetic field B is of substantially constant and stationary amplitude, over a distance corresponding to the extent of the dimension of the chamber CA or CB, increased at least the length, in this same direction, of the distance from the inlet of the nozzle to the outlet of the nozzle. The existence of this magnetic field makes it possible to cause the electric discharge in rotation, constituted by the plasma cord forming a current tube, according to a magnetron effect. The rotary drive is located substantially in a plane orthogonal to the longitudinal axis XX. When the discharge current is continuous, the superimposed magnetic field is continuous, while when the discharge current is sinusoidal, the superimposed magnetic field is sinusoidal, at the same frequency and in phase or in phase opposition with the discharge current.

The embodiments shown in Figures 6a, 6bι, 6b ₂ and 6c can be implemented separately or in combination. In the embodiment of the aforementioned figures, the pipe 10 for supplying carrier gas and effluents can be formed in the body of the tubular T and lead to the flat face of the latter by an intake orifice OR, in order to allow the installation of the CO coil on the surface of revolution of the tubing T, without major obstacle. Due to the significant improvement in the stability of the electrical discharge zone, it is then possible to provide an alternative electrical supply of the electrodes, central electrode EC and nozzle, at significant frequencies up to 25 kHz or more, and thus obtaining a reduction in the production of nitrogen oxide NO _x .

Experimental results of implementation of treatment of specific organic effluents will now be given during the implementation of the process which is the subject of the present invention, by means of the devices previously described.

The aforementioned tests were carried out using an industrial device, designated by "TESTO 33", manufactured and sold worldwide by the company TESTOTERM GmbH, D-7825 Lenzkirch / Schwarz ald, Kolumban-Kayser Strasse 17, Germany, this device putting implementing an operating principle based on an electrochemical technique. This principle of gas analysis method is based on redox reactions and involves exchange of electrons.

During the measurements carried out, the range of use of the electrochemical cell was between 0 and 2000 ppm and the measurement accuracy was plus or minus 20 ppm for concentrations below 400 ppm and 5% for higher concentrations. With regard to nitrogen oxides, it is indicated that the measurement system gave an indication of all the NO _X detected. EXPERIMENTAL RESULTS

MEASUREMENT OF THE NO _x CONCENTRATIONS AT THE OUTPUT OF THE SINGLE NOZZLE DEVICE ACCORDING TO FIGURE 3a For the various modes of electrical supply previously mentioned in the description, the following results have been obtained: in all cases, the production of NO _x decreases as the flow of gas or gas mixture increases. In particular, for an alternative supply at 25 kHz, the amount of NO _x produced and detected quickly becomes lower than the detection limit of the "TESTO 33" test device. In supersonic regime, when the nozzle 20 is fed continuously, a low production of NO _x can be demonstrated. This operating mode corresponds to a correct development of the electrical discharge in the aforementioned nozzle. However, the transition to an arc regime, that is to say an electric arc in thermodynamic equilibrium, causes a significant increase in the production of NO _x . Such a regime should therefore be avoided.

During an alternating supply at 50 Hz, the phenomenon of transition to arc regime does not appear because the passage of the supply current through zero leaves the possibility for the discharge to cut off and then to reboot at another point. nozzle. With an alternative power supply at 50 Hz, it is possible to obtain low value NO _x concentrations, that is to say between 50 and 100 ppm, under supersonic conditions. It should be noted that the production of NO _x may vary due to a change in the humidity of the air, that is to say of the carrier gas used in the reactor. Under these conditions, for an application on an industrial scale, it can be envisaged to provide the intake pipe of the carrier gas with a dehumidifying filter.

DESTRUCTION OF POLLUTANTS The destruction of pollutants is carried out optimally for a supersonic regime of the first nozzle 20. Under these conditions, the electric discharge in this nozzle is not established at the neck but at the level of the shock wave, in accordance to Paschen's law. In particular, the supersonic discharges thus obtained are of interest for the destruction of volatile VOC gas effluents such as styrene and toluene.

Destruction of styrene C6H5CH ₃ The tests consisted of an injection carried out at the level of the intermediate chamber CB downstream of the shock wave produced at the level of the first nozzle, the flow rate of the carrier gas, that is to say of air, being of the order of Dg = 60 to 70 Nm ³ / h. Under these conditions, the analyzes showed a destruction rate close to 80% with a very low formation of NO _x , the emission rate being less than 20 ppm.

Destruction of toluene C ₆ H ₅ CH ₃ The tests consisted of injecting toluene into the intake chamber CA, that is to say into an area where the temperature is close to room temperature, part of the gas charged with toluene at a lower flow rate, however, being introduced by the hollow central electrode EC. The adjustment of the nozzles, that is to say of their position with respect to the intake face of the first nozzle 20, was chosen so as to diffuse the toluene admitted by the central electrode EC into the chamber AC inlet at the same temperature close to room temperature. The measurements were carried out with a gas mixture flow rate D _g substantially equal to 40 Nm ³ / h, making it possible to perform supersonic operation in the first nozzle 20 and subsonic operation in the second nozzle 40.

The results giving the electric power applied to each nozzle, the flow rate of gas or gas mixture, the initial concentration of toluene, in ppm, and the rate of destruction of toluene, in%, as well as the production of NO _x , in ppm , are given in the table below:

A higher NO _x emission concentration is noted in the case of the second operating nozzle subsonic with respect to the first nozzle operating in supersonic regime.

PRODUCTION OF LONG LIFE HYDROGEN RADICALS The production of hydrogen radicals at low temperature is currently of great interest in the chemical industry and physical chemistry sectors such as: elimination of toxic molecules in the plasma treatment of fluoro-chlorinated industrial waste; diamond deposition from methane plasmas; healing of the dangling links of chains cut in the cracking of hydrocarbons. Investigations have made it possible to highlight the effect of pressure and in particular of low pressure on the lifetime of hydrogen atoms. It has been observed that the decrease in the number of hydrogen atoms is much slower, approximately by a factor of 10, for a pressure variation from 1 bar to 1/3 bar. In addition, for the passage from a pressure of 1 bar to 1/3 bar, the characteristic decay time θ for which the number of hydrogen atoms has been divided by 10, is itself approximately multiplied by a factor 10. The aforementioned results show how useful it can be to work under reduced pressure, in order to significantly increase the lifetime of the hydrogen radicals with the possibility of using them subsequently with improved efficiency in different applications, in particular in the cracking of hydrocarbons.

Claims

1. A method of treatment by electrical discharge of volatile organic gaseous effluents, characterized in that it consists at least: a) in subjecting said volatile organic gaseous effluents to an expansion at low pressure in order to generate a lowering of temperature of said effluents volatile organic gases at a temperature between 150 ° K and 173 ° K; b) subjecting said volatile organic gaseous effluents at low pressure and at a reduced temperature to electric discharges, which makes it possible to treat said volatile organic gaseous effluents by breaking the constituent hydrocarbon chains, in the absence of creation of compounds NO _x nitrogen oxide.

2. Method according to claim 1, characterized in that said electrical discharges are formed by electrical discharges at low temperature, said electrical discharges being generated in an environment outside local thermodynamic equilibrium, which allows the environment of the atmosphere to be maintained discharge temperature at or below room temperature.

3. Method according to one of claims 1 or 2, characterized in that it consists: al) in subjecting said organic gaseous effluents to steps a) and b); a2) repeating step a1), which makes it possible, from gaseous organic effluents having already undergone a treatment a1), and from ozone and oxygen radicals generated by the emanations of the discharge electric, to further subject said organic gaseous effluents treated to a reinforced oxidation phenomenon.

4. Device for treatment by electrical discharge of gaseous volatile organic effluents at low pressure and at low temperature, characterized in that it comprises at least: means for admitting said gaseous volatile organic effluents; - Expansion means of said volatile organic gaseous effluents making it possible to generate a low pressure expansion and a lowering of temperature of said volatile organic gaseous effluents at a temperature between 150 ° K and 173 ° K; means for generating an electrical discharge between a high voltage electrical potential and a low voltage electrical potential, said electrical discharge being applied to said gaseous volatile organic effluents at low pressure and at low temperature, which makes it possible to treat said volatile organic effluents by breaking of the constituent hydrocarbon chains, in the absence of creation of nitrogen oxide NO _x compounds.

5. Device according to claim 4, characterized in that it comprises at least, in a tube provided with a pipe for admitting a carrier gas: a central electrode of revolution, extending substantially along the longitudinal axis of this tubing, and, successively, arranged substantially revolution around this central electrode and forming said expansion means: an inlet chamber for said gaseous volatile organic effluents; - A nozzle neck, forming a Venturi duct, the intake face of which is in direct engagement in said intake chamber, which makes it possible to generate a shock wave, accompanied by said expansion and by said temperature decrease of said gaseous volatile organic effluents.

6. Device according to claims 4 and 5, characterized in that said nozzle neck is constituted by an electrically conductive material and formed by a divergent tube of revolution, said central electrode and said tube of revolution being substantially coaxial to form a cylindrical channel for expanding the carrier gas and the organic effluents, said central electrode and said nozzle throat being connected to an electric voltage generator to form said means generating electric discharge between said central electrode and said nozzle throat.

7. Device according to one of claims 5 or 6, characterized in that said central electrode is a hollow electrode, forming an inlet channel for said organic effluents, the wall of said central electrode being provided with holes forming nozzle on at least a cross section of said central electrode and allowing the admission of said organic effluents into said intake chamber.

8. Device according to claim 7, characterized in that, for a range of flow values said carrier gas and said organic effluents, said shock wave being established substantially at the level of a cross section of said nozzle neck included in a range of abscissa values, measured with respect to the intake face of said nozzle neck , said central electrode is removable in translation along its longitudinal axis, which makes it possible to position the cross section of this central electrode provided with holes forming a nozzle in the upstream vicinity of the position of the shock wave and thus to optimize the treatment of said volatile organic effluents.

9. Device according to one of claims 6 to 8, characterized in that said electrical voltage generating means are periodic generator means, which eliminates the risk of formation of electric arc at high temperature, by switching to the zero value of the periodic voltage delivered by said periodic generator means.

10. Device according to claims 1 and 2, characterized in that said expansion means comprise, in addition to said nozzle neck forming a Venturi duct, another nozzle neck, of revolution around said central electrode and arranged in cascade with said neck nozzle.

11. Device according to claim 10, characterized in that said nozzle neck and said other nozzle neck are cascaded by means of a passage chamber, forming mixing chamber, the assembly formed by said neck nozzle, the passage chamber and said other nozzle neck being coaxial and of revolution around said central electrode.

12. Device according to one of claims 10 or 11, characterized in that the wall of said other nozzle neck comprises a coating of an oxidation catalyst material, which makes it possible to increase the yield of the treatment of said volatile organic effluents.

13. Device according to claim 12, characterized in that the expansion channel formed by said central electrode and said other nozzle neck is divided into elementary expansion channels, delimited by radial walls comprising elements made of oxidation catalyst material, which increases the yield of the treatment of said volatile organic effluents.

14. Device according to one of claims 5 to 13, characterized in that said inlet chamber comprises deflector means making it possible to impart to the jet of gas or gas mixture of gaseous volatile organic effluents a substantially vortex movement in a plane substantially orthogonal to the longitudinal axis of the intake chamber and the nozzle, which makes it possible to stabilize the electric discharge zone in the vicinity of said plane.

15. Device according to one of claims 5 to 14, characterized in that the latter comprises, outside of said tube, means generating a stationary magnetic field substantially collinear with the longitudinal axis of the nozzle, which makes it possible to cause the electric discharge in a rotational movement and to substantially maintain this electric discharge in a plane orthogonal to said longitudinal axis.