WO2000001469A1

WO2000001469A1 - Electrode and reaction chamber for use in generation of non-thermal plasma

Info

Publication number: WO2000001469A1
Application number: PCT/NO1999/000225
Authority: WO
Inventors: Torfinn Johnsen; Kjetil Naesje
Original assignee: Applied Plasma Physics As
Priority date: 1998-07-03
Filing date: 1999-07-05
Publication date: 2000-01-13
Also published as: NO983106D0; AU4534299A

Abstract

An electrode for use in reaction chambers (1) where purification is performed by the effluent which has to be purified being conducted through one or more cells formed by electrically conductive tubes (2) and where at least one electrode (3) is mounted inside the tube or tubes. Non-thermal plasma is created by means of electrical discharges between electrode and tube, thus provoking natural purification processes through chemical oxidation and reduction. The electrode (3) consists of an electrically conductive core (5) surrounded by a resistive material (6), where at least on or near the surface of the resistive material there is provided at least one electrically conductive body (7; 7b).

Description

Electrode and reaction chamber for use in generation of non-thermal plasma

The present invention relates to an electrode and a reaction chamber wherein polluted effluents are purified by means of non-thermal plasma.

Emissions of gases and liquids from a number of production and combustion processes contain unacceptably high concentrations of contaminants, such as, for example, organic compounds and nitrogen oxides (No_x). The requirements for reducing emissions of such materials are becoming increasingly stringent, thus necessitating the production of new methods for purification of effluents. Research performed in recent years has shown that so-called "non-thermal plasma" (NTP), i.e. a state in which free electrons are generated, supplied with energy and emitted in a material/effluent (gas/fluid), has environment-purifying properties. This is due to the fact that the unstable ions formed by the electrons in collisions react with polluting/harmful elements in the material/effluent.

Non-thermal plasma, or cold-plasma, is generated by letting a gas or fluid pass through the electrical field between electrodes to which a high voltage is applied. This leads to discharges which generate free electrons with relatively high energy. These electrons will have a high probability of colliding with molecules, thereby creating excited molecules/atoms which are highly reactive. These reactive molecules will then collide with other molecules, reacting with them and thereby creating less harmful materials or materials which at least are easier to handle. In the formation of non-thermal plasma, a sufficient quantity of charged particles will be created to give the gas the properties of a plasma, but the temperature in the gas as a whole does not increase significantly.

It has been found advantageous to design such reaction chambers in the form of one or more electrically conductive tubes, all of which contain a centre electrode. All the tubes are preferably electrically connected to earth, while the centre electrodes are connected to a voltage generator which generates high DC voltage.

Since this technology is new, most of the reaction chambers have been developed for laboratory use. The object has been to achieve the highest possible degree of efficiency but without giving sufficient consideration to production costs, maintenance costs and reliability. Existing chambers of this kind are therefore expensive to produce, with a low level of reliability in industrial environments, and they are expensive to maintain.

A problem with prior art reaction chambers is that since an electrical discharge occurs between the electrodes, an electrically conductive channel of ionised gas may be created between them. As long as this channel exists the electrodes are short-circuited. This leads to power loss and reduced efficiency since no further free electrons are generated until the channel has ceased to exist.

Traditionally this problem has been solved by means of costly and sophisticated adaptations of the actual voltage generator. If the generator generates extremely short voltage pulses, the ionised channels will be suppressed at the end of each voltage pulse. Furthermore, the generator may be designed so as to detect the short circuit resulting from the ionised channel and interrupt the voltage supply, thus suppressing the channel. Such solutions, however, involve a number of drawbacks in the form of costs and complexity in production and operation.

The object of the present invention, therefore, is to provide a reaction chamber of the type mentioned in the introduction together with an electrode for use in such a reaction chamber which is so designed that ionised channels are suppressed as soon as they have arisen. By controlling the resistive, capacitive and inductive connection internally in the electrode's mass, the form of the supplied energy can be optimised for the desired gas reactions. The individual flashover obtains the characteristic of a pulse where correct rise time, voltage and duration determine the system's efficiency.

This principle is an essential feature of the invention and is achieved by means of the features which are indicated in the claims.

A more detailed description will now be given of the invention with reference to the enclosed drawings.

Figure 1 illustrates a reaction chamber for purifying polluted effluent.

Figures 2a-d illustrate an electrode according to the invention. Figure 3 illustrates an electrical equivalence diagram for a segment in the reaction chamber or for a segment on an electrode according to the invention.

Figure 4 illustrates an alternative embodiment of an electrode according to the invention.

Figure 1 illustrates the principal features of a reaction chamber of the type which is mentioned in the introduction. The chamber comprises a plurality of tubes 2 which are electrically conductive and connected to ground, each tube containing a centre electrode 3 which is connected to a voltage generator (not shown). The centre electrodes are attached to the reaction chamber via an electrical insulator 4. The polluted effluent which is to be purified is conducted through the reaction chamber, thereby passing through the electric fields which are generated in the tubes between the centre electrodes and the tube walls.

In order to achieve the desired ionisation, it is necessary to apply a very high voltage to the centre electrodes 3. This high voltage gives rise to gas discharges which generate free electrons with relatively high energy. These electrons will have a high probability of colliding with molecules, thereby creating excited molecules/atoms which are highly reactive. These reactive molecules will then collide with other molecules, reacting with them and thereby creating less harmful materials or materials which at least are easier to handle. In the formation of non-thermal plasma, a sufficient quantity of charged particles will be created to give the gas the properties of a plasma, but the temperature in the gas as a whole does not increase significantly.

As already mentioned, since an electrical discharge occurs between the centre electrode 3 and the grounded tube 2, an electrically conductive channel of ionised gas may be created between them. As long as this channel exists the electrodes are short-circuited. This leads to power loss and no further free electrodes are generated until the channel has ceased to exist.

According to the invention this problem is solved as illustrated in figure 2, where the centre electrode consists of a core 5 which is coated with a resistive material 6. Embedded in the outer part of this material are small particles of conductive material 7. A special binding material 8 may be employed to give the conductive particles a better grip on the base and one another as well as protecting the resistive material. Free electrons will now be emitted from the conductive particles, but if a channel of ionised gas is created between such a particle and the earthed tube, the resistive material which is located between the actual centre electrode and the conductive particle will ensure that the electrodes are not short-circuited and that the conductive ionised channel will be suppressed. The concentration of conductive particles may be uniform over the length of the electrodes, or it can be varied with the result that the concentration of free charged particles varies in the reaction chamber's longitudinal direction.

Figure 3 illustrates an equivalence diagram for the system's basic electrical mode of operation. A conductive particle 7 is placed on the resistive coating 6, forming an electrical circuit in combination with the chamber wall 5 and the electrode 2. The object of the circuit is to give each discharge a defined duration and a defined energy. For those skilled in the art it is easy to see that this can be achieved by adapting the resistivity in the coating while also adapting the size of the conductive particle, with the result that the equivalent capacitance between the particle 7 and the chamber wall 5 in combination with the resistance 6 determines the pulse's duration and characteristic. Figure 4 illustrates an alternative to the embodiment which is described with reference to figure 2. In this case it is not conductive particles embedded in the resistive material which surround the electrode. Instead rings 7b of conductive material are arranged on the outside of the resistive material. These rings may be arranged at regular intervals, or the distance between them may vary along the electrode. These rings will advantageously be designed with "barbs" which increase the probability of electron detachment. Between the barbs 7b and the electrode 5 an equivalent resistance and capacitance are created over the resistive coating 6, giving the same equivalence diagram as that shown in figure 3.

A possible modification which lies within the scope of the invention is to have the electrodes decrease in diameter in the effluent's direction of flow. This may be combined with how the concentration of conductive particles is changed over the electrodes' longitudinal direction or how the conductive rings surrounding the electrodes are placed and designed, thus ensuring that the concentration of free charged particles increases in the effluent's direction of flow, with the result that the effluent is never located in stable surroundings, and that when the concentration of contaminants is reduced, the concentration of free charged particles increases.

In this embodiment a reaction chamber is described consisting of a plurality of tubes and centre electrodes. It will be possible, however, to design the reaction chamber in alternative ways within the scope of the invention. For example, the electrodes may be designed as two parallel plates where one or both of the plates are coated with a resistive material as already described. A purification chamber may consist of one or more such electrode pairs.

Claims

PATENT CLAIMS

1. A reaction chamber for purifying effluents, wherein the purification is performed by conducting the effluent through one or more cells comprising two or more electrodes and where non-thermal plasma is created by means of adapted electrical discharges between the electrodes, thus provoking natural purification processes through chemical oxidation and reduction, characterized in that at least one of the electrodes (2, 3) consists of an electrically conductive core (5) surrounded by or coated with a resistive material (6), where at least on or near the surface of the resistive material there is provided conductive material (7; 7b), thus creating an electrical system (figure 3).

2. A reaction chamber according to claim 1, characterized in that the electrodes are composed of centre electrodes and cell walls, where the resistive material (6) with the conductive particles (7) may be coated on the surface of the cell wall and/or on the surface of the centre electrode.

3. An electrode according to claim 1, characterized in that the electrodes are composed of conductive plates (D) which are arranged in parallel planes with the result that the effluent is conveyed between the plates, the resistive material (6) with the conductive particles (7) being coated on the surface of one or both plates.

4. An electrode for use in reaction chambers (1) where purification is performed by conducting an effluent which has to be purified through one or more cells formed by electrically conductive tubes (2) and where at least one electrode (3) is mounted inside the tube or tubes, and where non-thermal plasma is created by means of electrical discharges between electrode and tube, thus provoking natural purification processes through chemical oxidation and reduction, characterized in that the electrode (3) consists of an electrically conductive core (5) surrounded by a resistive material (6), where at least on or near the surface of the resistive material there is provided conductive material (7; 7b).

5. An electrode according to claim 4, characterized in that the conductive material (7; 7b) on or near the surface of the resistive material is electrically conductive particles (7) embedded in the whole or parts of the resistive material (6) and/or that the electrically conductive particles (7) are bound to the surface by means of a binding material (8).

6. An electrode according to claim 4, characterized in that the conductive material (7; 7b) on or near the surface of the resistive material is electrically conductive rings (7b) which surround the resistive material (6).

7. An electrode according to claim 6, characterized in that the electrically conductive rings (7b) are designed with barbs, are star-shaped or in some other way have an irregular surface.

8. An electrode according to one of the claims 4-7, characterized in that the conductive material (7; 7b) is distributed in the electrode's longitudinal direction, thus increasing the density of the conductive material in the effluent's direction of flow.

9. An electrode according to one of the claims 4-8, characterized in that the electrode's diameter decreases in the effluent's direction of flow.