WO1999028015A1

WO1999028015A1 - Method and apparatus for processing effluents using non-thermal plasma

Info

Publication number: WO1999028015A1
Application number: PCT/NO1998/000357
Authority: WO
Inventors: Torfinn Johnsen
Original assignee: Applied Plasma Physics As
Priority date: 1997-12-03
Filing date: 1998-12-02
Publication date: 1999-06-10
Also published as: AU1789199A; NO975603D0

Abstract

A method is described for achieving cost-effective and reliable purification of a contaminant (3) in a reaction chamber (2) by means of non-thermal plasma. The non-thermal plasma is created by applying to the electrodes (4, 6) in the reaction chamber a powerful direct voltage and a superimposed alternating voltage with a relatively lower amplitude. Alternative designs of the electrodes (4, 6) in the reaction chamber are also described. These designs are specially suited to the application of the method.

Description

Method and apparatus for processing effluents using non-thermal plasma

The present invention concerns a method and an apparatus for generating in a cost-effective and energy-effective manner a stable low-temperature plasma in a reaction chamber through a controlled generation of free electrons through gas discharge.

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 effluent. 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/liquid), 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 passing a gas through the electrical field between electrodes to which a high voltage is applied. This leads 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.

In a plasma (i.e. a material with a significant number of charged particles), under static conditions the charged particles will establish a space charge which attempts to equalise the voltage drop in the transition electrode/gas, thereby reducing or stopping the detachment of electrons. This is a well known problem, and this problem of a static voltage drop has traditionally been solved either by applying alternating voltage over the electrodes or by applying short (nano/microsecond) direct current pulses. The traditional solution methods are highly effective for generating non-thermal plasma on a small scale, at laboratory level, but entail practical problems when scaled up to industrial scale. In many cases traditional alternating voltage will have too low a frequency to effectively prevent the plasma by means of altered space charge distribution from compensating for a large voltage drop between electrode and surroundings, and if the frequency is sufficiently high, the power produced could be so great that the energy consumption will be unacceptably high in larger environments. In addition electromagnetic radiation will be created, with the potential health hazard (or shielding problems) which this entails.

High voltage generators which will produce sufficiently powerful pulses (20,000 - 100,000 V) of extremely short duration (nanoseconds) are difficult to build on a large scale in a cost-effective manner. In addition this method will also entail generating very intense electromagnetic radiation with the health hazard and/or shielding problems which this entails.

The object of the present invention is to achieve a cost-effective and reliable method of generating a sufficient quantity of free electrons through gas discharge for purifying large environments/effluent (several million m^/hour) without using an unacceptably large amount of energy and without generating significant electromagnetic radiation. It is also an object to design and arrange the electrodes which are employed in the creation of non-thermal plasma in a manner which permits a large number of free electrons to be detached, while at the same time the electrode's mechanical properties are not impaired unnecessarily, and so that as much as possible of the material which has to be purified passes close to the electrodes.

These objects are achieved by employing the methods according to the present patent claims.

The invention will now be described in more detail with reference to the accompanying drawings.

Figure 1 illustrates the principle design of a reaction chamber for purifying gas by means of non-thermal plasma. Figure 2 illustrates the voltage course over the electrodes in a traditional reaction chamber for purifying gas/liquid by means of non-thermal plasma, and the corresponding voltage course for a reaction chamber according to the invention.

Figure 3 illustrates a possible design of the electrodes in a reaction chamber according to the invention.

Figure 1 illustrates the principle design of a reaction chamber 2 for purification by means of non-thermal plasma, including electrodes 4, 6 and voltage source 1. It is stressed that this description only indicates the electrical connection of the elements and not their physical design. The material 3 which is to be purified is fed into the reaction chamber at one end, and after purification is emitted at the other end 5. The electrodes 4, 6 will traditionally be supplied with high voltage in the form of pulses, see figure 2a, thus causing electrons to be detached and supplied with energy, but without establishing a counter field in the form of space charge distribution. This method has the significant drawback that it is costly to produce equipment which has to be capable of working with such high voltage pulses. When supplying electrical pulses with an amplitude as high as that involved here, the electromagnetic radiation will also be substantial, resulting in additional high costs for shielding.

It has been found that by establishing an electrical direct voltage over the electrodes 4, 6 and subsequently applying an alternating voltage with considerably lower amplitude superimposed on the direct voltage, sufficient numbers of gas discharges/free electrons are generated to give a low- temperature plasma with purifying properties. This voltage variation is illustrated in figure 2b. (Please note that figure 2 is not intended to be in scale, and that the voltage variation may take a different form from that indicated there). The powerful direct voltage (10 - 100 kV) ensures a powerful electrical field and thereby good conditions for acceleration of the free electrons which are generated, while an overlaid alternating voltage

("ripple") with the correct frequency ensures the maintenance of a substantial voltage drop per length unit in the immediate vicinity of the electrode 4 (an approximate discontinuity in the voltage), thus causing electrons to actually be detached and supplied with a certain kinetic energy before migrating into surrounding gas/material. It will be possible to produce equipment which works according to this method in a far more cost-effective manner than equipment which works according to traditional methods. Since the electromagnetic radiation is a result of the alternating voltage component, such radiation will be greatly reduced in comparison with previously known methods.

The most effective frequency for the overlaid alternating voltage will depend on the properties of the material which is to be purified. According to an advantageous embodiment of the invention, therefore, it is possible to vary the frequency of the overlaid alternating voltage. Relevant frequencies may be between 10 Hz and 1 GHz.

It will also be possible to vary the design of the electrodes 4, 6 in the reaction chamber 2 in order to achieve simultaneous detachment of as many electrons as possible. The most common method of designing electrodes and reaction chambers is to design the reaction chamber as a number of tubes, where the tube wall is an electrode 6, and where along each tube's axis there is provided a wire electrode 4. Basically it is desirable to design the wire electrodes as thin as possible, since the detachment of electrons increases with the curvature of the electrode surface. This, however, results in mechanically weak electrodes, which are subject to wear during use. It is, therefore desirable to make the electrodes stronger, for example by increasing their cross sectional area, without reducing the detachment of electrons. This can be achieved by designing the wire electrodes with a cross section which is in the form of a star with from 3 to 6 arms. A design of this kind is illustrated in figure 3. A second alternative is to arrange a large number of electrode wires, from as few as in the order of 10 to as many as 10,000,000, together in a mesh mounted over the polluting material's direction of flow. The width of such a mesh may vary between about one millimetre and about one metre. It will also be possible to design the electrode meshes in such a manner that the width is made adjustable, for example by inserting several electrodes, or by adjusting the distance between the individual electrodes.

According to the invention the electrode wires will be designed in such a manner that they can be exposed to tensile forces of from 1 to 100 N. In a further embodiment according to the invention, water vapour is added to the gas 3 before it is fed into the reaction chamber, in order to increase the gas's electrical conductivity. It is also possible to add materials which have a reaction-enhancing effect. Which materials are best suited will depend on which materials one wishes to purify, but suitable candidates may be alcohols, ozone, hydrogen peroxide, etc.

Claims

PATENT CLAIMS

1. A method for purifying contaminants (gas/liquid/solid material) by passing the material which is to be purified through a reaction chamber with electrodes which are connected to a voltage source and which establish an electrical field, whereby free electrons are generated and supplied with energy through controlled discharges, thereupon colliding with molecules in said material, thus creating ions and natural purification processes through inducing chemical oxidation and reduction, characterized in that to the electrodes in the reaction chamber there is applied a direct voltage and a superimposed alternating voltage with an amplitude which is less than the value of the direct voltage, the direct voltage having a value which is sufficient to accelerate free charges in the reaction chamber, while the alternating voltage has an amplitude which is sufficient to maintain, periodically at least, a voltage drop per length unit in the vicinity of the electrodes to enable electrons to be detached.

2. A method according to claim 1, characterized in that the direct voltage can be varied between 1 OkV and 200kV, and that the overlaid alternating voltage has an amplitude which can be varied between 1% and 50% of the direct voltage.

3. A method according to claim 1 or 2, characterized in that the frequency of the overlaid alternating voltage can be varied between 10 Hz and 1GHz, depending on the properties of the material which requires to be purified.

4. A method according to one of the preceding claims, characterized in that in the reaction chamber there is employed at least one electrode designed with a cross section in the form of a star with 3 to 6 arms.

5. A method according to one of the preceding claims, characterized in that a number of electrode wires are mounted in such a manner that they form a mesh across the contaminant's direction of flow.

6. A method according to claim 5, characterized in that from 10 to 10,000,000 electrodes are employed in the mesh of electrodes.

7. A method according to claim 5 or 6, characterized in that the mesh of electrodes is designed with a width between 1 millimetre and 1 metre.

8. A method according to one of the claims 5 to 7, characterized in that the mesh of electrodes is made adjustable, either by enabling electrodes to be inserted and removed during operation, or by permitting the distance between the individual electrodes to be increased and reduced.

9. A method according to one of the preceding claims, characterized in that the electrodes are exposed to tensile forces of from IN to 100N.

10. A method according to one of the preceding claims, characterized in that the moisture in the material which i to be purified is increased by adding water vapour before the material is fed into the reaction chamber.

11. A method according to one of the preceding claims, characterized in that a reaction-promoting material is added to the material which i to be purified before it is fed into the reaction chamber.

12. A reaction chamber for purifying contaminants, designed with electrodes (4, 6) which are connected to a voltage source (1) and which establish an electrical field, whereby free electrons are generated and supplied with energy through controlled discharges, thereupon colliding with molecules in said material, thus creating ions and natural purification processes through inducing chemical oxidation and reduction, characterized in that the voltage source (1) is designed to be able to generate a direct voltage and a superimposed alternating voltage with an amplitude which is less than the value of the direct voltage.

13. A reaction chamber according to claim 12, characterized in that said direct voltage has a value of at least 1 OkV and that the overlaid alternating voltage has an amplitude which corresponds to at least 1% of the direct voltage level.

14. A reaction chamber according to claim 12 or 13, characterized in that the voltage source (1) is designed in such a manner that the superimposed alternating voltage has a frequency of at least 10 Hz, and that this may preferably be varied between 10 Hz and 1 GHz.

15. A reaction chamber according to one of the claims 12 to 14, characterized in that at least one electrode (4) in the reaction chamber is designed with a cross section in the form of a star with 3 to 6 arms.

16. A reaction chamber according to one of the claims 12 to 15, characterized in that a number of electrode wires are arranged in such a manner that they form a mesh across the contaminant's direction of flow.

17. A reaction chamber according to claim 16, characterized in that the mesh of electrodes consists of from 10 to 10,000,000 electrodes.

18. A reaction chamber according to claim 16 or 17, characterized in that the mesh of electrodes is designed with a width of between 1 millimetre and 1 metre.

19. A reaction chamber according to one of the claims 16 to 18, characterized in that the mesh of electrodes is made adjustable, either by enabling additional electrodes to be inserted and removed during operation, or by permitting the distance between the individual electrodes to be increased and reduced.