FR2855361A1 - Cold plasma generation device for controlling atmospheric gas pollution, has central electrode and helical shaped external electrode, where device generates cold plasma by forming electric discharges between coaxial electrodes - Google Patents

Abstract

The device has a central electrode (1) and an external electrode (2) with a helical shape. The device generates cold plasma by forming electric discharges between coaxial electrodes. The electric discharges are of corona effect discharge type, potential barrier discharge type, and radio-frequency or micro-wave discharge types.

Description

DEVICE FOR GENERATING COLD ELECTRIC SHOCK Description

  The present invention relates to a device for forming cold electrical discharges for the purpose of improving the performance of plasma-chemistry processes, in particular of gas depollution or ozone production.

  The cold electrical discharge is a process known for a long time to generate ions and radicals, activators of chemical reactions in gases. The process consists in limiting the effects which tend to make the discharge evolve towards the arc regime, that is to say to control the energy instead of dissipating it in pure heat loss.

  The most well-known cold electrical discharge devices are the electrical barrier discharge (DBD), the crown discharge, and the sliding (or hobed arc) discharge.

  The electric barrier discharge, shown diagrammatically in FIG. 1, uses two metal electrodes in coaxial configuration, a central electrode (1) and an external electrode (2) separated by a dielectric (4); when high voltage is applied, many micro-discharges (3) occur in the inter-electrode space but transition to the arc is not possible due to the dielectric. The corona discharge, shown diagrammatically in FIG. 2, consists of two electrodes in coaxial configuration supplied with direct voltage; in the vicinity of the external electrode (2) the electric field is too weak to allow the transition to the arc regime of the micro20 discharges (3). In the sliding discharge, shown diagrammatically in FIG. 3, a discharge is initiated between two divergent electrodes (1) and (2), then blown by the mixture of gases to be treated; the blown discharge (3) disappears when the voltage becomes too high, a new discharge appears when the inter-electrode space is the smallest; the discharge (3) is therefore self-sustaining under continuous voltage.

  All these devices have a major drawback: the volume occupied by electric discharges represents only a small fraction of the inter-electrode space.

  The present invention overcomes this drawback. For this, it recommends giving an electrode the shape of a helix so as to obtain a plasma uniformly distributed in the inter-electrode space.

  The invention will be better understood by schematically representing devices, according to the invention, of electric discharges: FIG. 4 represents a crown discharge device with a central electrode (1) and a counter-electrode constituted by a circular helix (2) ; FIG. 5 represents a sliding discharge device with a central electrode (1) and a counter-electrode constituted by a conical propeller (2). According to the invention, the attachment points of the electric discharges are distributed or move uniformly around the central electrode; which promotes, on the one hand, the behavior of the materials with respect to the wear of the electrodes, and, on the other hand, the homogeneity of the cold plasma.

  It is understood that the devices described according to the invention are given only by way of non-limiting examples and that one can imagine other variants and use equivalent means without departing from the scope of the invention.

  As a simple embodiment of the invention, the energy performance is compared between, on the one hand, a DBD reactor comprising, according to the invention, an external electrode constituted by an aluminum wire wound in a circular helix around 'a glass tube, and, on the other hand, a conventional DBD reactor in which the external electrode consists of an aluminum sheet. With the exception of the external electrode, the conventional DBD reactor and the DBD reactor produced according to the invention have the same geometrical characteristics: the glass tube has an internal diameter of 6 cm, a length of 20 cm, and has a central electrode in 0.5 mm diameter tungsten; the electric discharges are generated by means of a pulsed electric supply with step-up transformer; the tube is swept by an air flow of 0.1 I / min containing 500 ppm of trichlorethylene (TCE). When a high voltage pulse is applied, the volume occupied by the electrical micro-discharges is small compared to the volume of the conventional DBD reactor while the micro-discharges occupy almost the entire volume of the DBD reactor produced according to the invention with a helical counterelectrode. With the conventional DBD reactor, the TCE conversion rate is less than 70%, while with a reactor equipped, according to the invention, with a helical counter-electrode, the TCE conversion rate is total.

  It is therefore easy to understand the advantage of such a device because of its simplicity of implementation in processes for the depollution of atmospheric discharges or the production of ozone.

Claims (3)

claims   1. Device for generating cold plasmas in a gas flow by forming electric discharges between one or more coaxial electrodes, characterized in that the shape of the external electrode is helical.   2. Device according to claim 1, characterized in that the electrical discharges are of the corona discharge discharges, electric barrier discharges, hindered arc discharges, radiofrequency discharges, or microwave discharges.   3. Use of the device according to claims 1 and 2 in methods of removing undesirable volatile compounds from atmospheric emissions or in processes for producing ozone.