A METHOD AND APPARATUS FOR PRODUCING A TRIGGERED PULSED STREAMER DISCHARGE

Abstract: A corona/streamer discharge is produced in a reactor by applying a slowly rising voltage (v1.1, 1.2) across a pair of electrodes (1.1, 1.2) to produce an electric field less than that at which breakdown would occur. A further electrode (1.3) has edges capable of producing field enhancement. The further electrode (1.3) is supplied with a biasing voltage (V1.3, 1.2) which minimises field enhancement until a trigger pulse is superimposed, at which point field enhancement on the edges of the electrode (1.3) initiates multiple streamers in the main electric field.

Description: "A Method and Apparatus for producing a triggered pulsed streamer discharge" This invention relates to the field of the generation of pulsed corona/streamer electrical discharges. Pulsed corona discharges can be used for initiating and stimulating chemical reactions in gaseous media and also for the treatment of solid surfaces. The invention described here is primarily concerned with the treatment of gaseous media but the application to solid surfaces should not be excluded. In a pulsed corona reactor repetitive short duration high voltage pulses are applied to a pair of electrodes. The gas to be treated resides within the space between the electrodes. One or both of the electrodes has sharp edges or other method to enhance the surface electric field. On the application of the high voltage pulses, electrical discharges are initiated in the regions where the electric field is very high and propagate either partially or fully across the space. The gas under treatment passes through the reactor and the effect of the electrical discharges is to initiate or stimulate various chemical reactions. In the simplest type of corona reactor the electrodes consist of a coaxial arrangement of a single wire coaxially located in the centre of a metal cylinder. There are many other possible configurations of electrodes. Various applications or potential applications of pulsed corona systems include: the production of ozone, the oxidation of volatile hydrocarbons, the removal of odours, the destruction of various toxins emitted in incinerator flue gases and power plant flue gases, the treatment of NOx and SOx in power plant and incinerator flue gases, and the treatment of exhaust gases from i.e. engines. In the field of pulsed corona it is already known that it is possible to initiate various chemical reactions in a gaseous medium by passing a pulsed corona discharge through the gas. Repetitive high voltage pulses with a magnitude typically in the region 20 kV to 200 kV and of duration 50-200 ns are applied across a pair of electrodes. Pulse repetition rates are typically about 100-1000 Hz and pulse energies are around 1-100 Joules. The electrodes are normally designed such that the electric field is greatly enhanced on the surface of one or both electrodes when compared with the average electric field between the electrodes. When the high voltage pulse is applied transient corona discharges or more accurately streamer discharges are initiated in the regions of high electric field and/subsequently propagate across (either fully or partially) the electrode spacing. High energy electrons in the streamers cause ionisation, excitation and dissociation of the gaseous molecules to form ions, excited atoms, dissociated molecules and possibly various reactive atoms/molecules. Interaction of these species with molecules can stimulate various beneficial chemical reactions. The main disadvantage with this method is that it requires the generation of repetitive high voltage pulses with magnitudes typically in the region of 20- 200kV of a few Joules energy which have to be delivered to the load in a short duration of about 10-250 ns . The very short rise time in conjunction with the high voltage requirements make the design of the power supply technically very demanding. Various different types of fast switching modulators (eg thyratrons, MOSFETS, pulse compressors, spark gaps, magnetic switches) in conjunction with high voltage generation techniques (eg pulse transformers, transmission line transformers) drive a corona reactor. Because the pulses are very short duration -50 ns and the duration between pulses is quite long -50 ms, the ratio of peak power to average power is high and is typically of the order of -1,000,000. The absolute value of peak power required is typically -200 MW. In designing a system to deliver these peak powers a number of different types of modulators can be employed eg thyratrons, MOSFETS and spark gaps. A typical high voltage power MOSFET can deliver a peak power -20 kW, so to produce a peak power of 200 MW would require an effective series and parallel combination of about 10,000 transistors. If the peak power that is required can be reduced then this would significantly reduce the costs of a modulator based on semiconductors. If the duration of the pulse can be increased then it is also possible to use alternative modulators such as thyristors in conjunction with high voltage pulse transformers. With other types of modulators, eg spark gaps and thyratrons, the number of devices required for a given average power will similarly reduce if the peak power requirement is lowered. Further, the lifetime before failure is reduced in these types of devices when operating at lower peak powers. In order to achieve the very fast rise time requirement for the conventional reactor the self capacitance of the reactor and the inductance between the reactor and the high voltage pulser must be kept to a reasonably small value. The self capacitance requirement then limits the maximum size that the reactor can assume. If the rise time requirement can be relaxed then much larger industrial size reactors suitable for the clean up of flue gases from incinerators or power stations can be more easily realised. The key technical difficulty is in achieving the very high peak powers required. The invention described here details a method of obtaining similar electrical discharges to that obtained in a conventional pulsed corona system but with a greatly reduced peak power requirement with the average power requirements roughly similar. The invention provides a method of generating a corona discharge, the method comprising the steps of: (a) generating a strong electrostatic field between a pair of electrodes in a relatively slow time, within a given space, and (b) causing initiation and launching of streamers discharges within or into this space. Once launched into this space the streamers can then propagate and the electrical discharge develops in a similar manner to the conventional pulsed corona system. The present invention also provides a pulsed corona/streamer discharge system comprising first and second electrodes forming a main pair of electrodes, and a third electrode, means for applying a voltage across the main pair of electrodes and means for applying simultaneously a bias voltage (which may zero or DC or time-varying and synchronised with the voltage applied across the main pair of electrodes) between the third electrode and one of the other electrodes, the system also comprising means for superimposing a fast trigger pulse on top of the bias voltage between the third electrode and said one other electrode so as to modify the electric field adjacent the surface of one of the electrodes to cause one or more streamers to be initiated and launched. ,„„„„„ PCT/GB99/03792 WO 00/30227 Further sets of electrodes may be introduced to enhance the performance of the system. For some designs the bias voltage can effectively be zero. The voltage applied across the main electrodes may be a DC voltage, but more preferably will be a pulse waveform with a relatively slow rise time, for example, from 100 ms to a few hundred ns . As this voltage is applied across the main electrodes a bias voltage is also applied at the same time between the third electrode and one of the other electrodes, and a fast trigger pulse with a rise time typically in the region of l-200ns is superimposed on the bias voltage. The bias voltage can be time varying and is synchronised with the main charge voltage so as to inhibit streamer initiation as the main charge voltage is impressed. The pulse waveform applied to the main electrodes will typically have a peak value in the range 50 kV to 2 MV but may be outwith this range. Streamers may be initiated by applying trigger voltage pulses applied over bias voltages across a number of electrodes, the electrodes being designed to cause a localised enhancement of the electric field near the surface of one of the electrodes on the application of the trigger voltage pulse. (In some electrode arrangements, the bias voltage may be effectively zero) . The rise time requirement for the trigger pulse is similar to that required for the main pulse in a conventional pulsed corona reactor. The main difference is that the voltage and peak power requirement is greatly reduced. Methods of generating these trigger pulses are the same as for the conventional corona reactor and as such are readily known by persons skilled in the art. The requirement for the main pulse is for a high voltage pulse of similar magnitude to a conventional pulsed corona reactor. However, the rise time requirement is greatly relaxed. The problem of producing a slow rise time pulse is less technically demanding than for a faster pulse and solutions to providing this type of supply are well known to those skilled in the art. The precise design of the power supplies does not form part of this invention. Embodiments of the invention will now be described, by way of example, with reference to the drawings, in which: Fig. 1A illustrates schematically an embodiment of the invention using planar electrodes; Fig . IB shows the waveform of pulses applied to the electrodes of Fig . 1A . 00/30227 Figs. 2A and 2B are schematic end and side views, respectively, of a second embodiment using cylindrical electrodes; Figs. 3A and 3B are views similar to Fig. 2 showing a modification of the embodiment of Fig. 2; Fig. 4A is a side view of a further embodiment using a planar electrode system in which streamers are launched from the surface of an electrode; Fig. 4B is a plan view of part of Fig. 4A; Fig. 4C shows the waveform of pulses applied t the electrodes in this embodiment; Figs. 5A and 5B are views similar to Fig. 4 showing a modified version of the embodiment of Fig. 4; Fig. 6 shows an embodiment similar to that of Fig. 4 but in cylindrical form; Fig. 7 shows an embodiment similar to that of Fig. 5 but in cylindrical form; Fig. 8 shows a modification of the embodiment of Fig . 4 ; Fig . 9 shows another modification of the embodiment of Fig . ; 00/30227 Fig. 10A shows one form of power supply applied to the embodiment of Fig . 1 ; and Fig. 10B shows voltage waveforms of the arrangement of Fig. 10A. The fundamental technical difficulty with the conventional system is that very high peak powers are required but the average power is relatively modest. It is therefore desirable to devise schemes which produce similar corona/streamer discharges to the conventional corona reactor but with a greatly reduced peak power requiremen . The basic idea of the triggered corona/streamer system is as follows: In the conventional corona reactor there are only two sets of electrodes and the corona/streamer discharges are launched as a short duration high voltage pulse is impressed across them. The streamers propagate across the reactor during the period of the impressed high voltage pulse. For a uniform electric field, the pulsed dielectric strength of atmospheric air is around 25-50 kV/cm depending on the gap spacing and the charge pulse duration. But, it is known that streamers, once initiated, will propagate in electric fields as low as 4kV/cm. So it is possible to set up an electric field below the pulsed breakdown field and then inject or launch streamers into this electric field. Although streamers will propagate in electric fields as low as 4 kV/cm, for best performance, it is desirable to operate as close to the pulsed breakdown field strength as possible. Thus, for the triggered streamer reactor there are at least three sets of electrodes. A high voltage pulse is applied across the major pair of the reactor electrodes in a relatively slow time to set up the electric field in the main volume of the reactor. A time varying or DC bias voltage may be simultaneously applied to the third electrode with respect to one of the other electrodes. (In some designs this bias voltage will be effectively zero) . When the main pulsed voltage and bias voltage reach their peak or are near their peak a fast high voltage trigger pulse is superimposed on the bias voltage. Streamers are initiated by a process of electric field enhancement on one of these electrodes and the streamers subsequently propagate in the electric field between the main electrodes. It may be advantageous to arrange the electrodes and supplies such that the average electric field along the path the streamers traverse is increased by the application of the trigger pulse In a conventional pulsed corona discharge reactor, repetitive short duration high voltage pulses are applied across a pair of electrodes. This has the disadvantage that the power supply used to drive the reactor must be capable of producing extremely high peak powers and this requirement effects the cost, lifetime of the modulator components and the number of modulators required to achieve a given peak power requirement . For the present invention, a high voltage pulse is applied across the main reactor electrodes in a relatively slow time (perhaps from 10 ms to a few 100 ns) and then streamers are launched into the main reactor volume by applying a high voltage pulse to another electrode with respect to one of the main electrodes. The rise time and duration of the trigger pulse is as short as in the conventional reactor but because the voltage required is much smaller the peak power requirement is greatly reduced. In the invention described here two or more high voltage power supplies may be required but the peak power requirement of these supplies will be greatly reduced in comparison to that of a conventional pulsed corona system with the same average power. To increase the energy input per pulse it may be advantageous to have a Pulse Forming Network in parallel with the self capacitance of the main reactor electrodes. In its simplest form the Pulse Forming Network may be a capacitor. In the invention described there are many possible ways of configuring the electrodes and the streamers can be initiated and launched for various regions within the reactor space. The configurations can be broadly divided into three classes: 1. One where the streamers are initiated somewhere at a location intermediate a pair of electrodes in a similar manner to a field distortion spark switch. 2. The second case is where the streamers are launched at the surface of one the main electrodes in a similar manner to a trigatron spark switch. 3. A third case where the streamers are launched by recessed electrodes; again this not dissimilar to the trigatron but it is proposed here that it may be advantageous to recess the trigger electrode (s) much further than in a trigatron spark switch. In the field distortion spark gap and the trigatron spark switch, a streamer is launched which subsequently develops into a spark or arc discharge. This occurs because, if only one streamer is launched, the energy stored in the self capacitance of the spark gap is sufficient to cause spark breakdown. It is desirable, for the corona reactor, to have streamer formation without the formation of sparks. One key preferred idea in the present invention is that spark breakdown can be inhibited by launching a large number of streamers almost simultaneously. Spark breakdown is inhibited due to the electrical loading effect of a large number of discharges on charging circuit, the self capacitance of the main gap and any PFN which might be connected in parallel with the main gap. Some possible configurations for the invention will now be described by way of example. Planar system with streamers launched within the gap. Figure 1 shows a scheme with planar electrodes to launch streamers within the electrode gap. The system comprises three sets of electrodes. A relatively slow charging high voltage pulse is applied on a first electrode 1.1 with respect to either 1.2 or 1.3. Electrode 1.3 is constructed either of wires or contoured strips. At the same time as the main pulse voltage is applied a transient bias voltage is maintained between third and second electrodes 1.3 and 1.2 to keep the electric field on the sharp edges of electrode 1.3 to a minimum. To trigger the streamers the potential between electrodes 1.3 and 1.2 is dropped to zero or is partially or fully reversed in polarity. When this happens the electric field on the sharp edges of electrode 1.3 is greatly enhanced and streamers are initiated on the edges of this electrode and propagate towards electrode 1.1. In this scheme it may be unnecessary to use two power supplies. One way this may be achieved is to pulse charge electrode 1.1 with respect to 1.2 and allow electrode 1.3 to partially float. To trigger the system electrode 1.3 can be shorted to electrode 1.2. If this is done through an inductive element or a shorted transmission line then it is also possible to reverse the potential between electrode 1 .3 and electrode 1 .2 . Coaxial system with streamers launched within the gap. Figure 2 shows a variation of the scheme shown in Figure 1 modified for a coaxial geometry. Electrodes 2.1, 2.2, and 2.3 are all cylindrical. The mode of operation is identical to that shown in Figure 1. In Figure 2 the electrode 2.3 with the field enhancing edges is closer to the outer cylinder 2.2 than the inner cylinder 2.1. In Figure 3 this electrode 3.3 is moved closer to the inner cylinder 3.1. This has the advantage that electrode 3.3 is in the region of higher electric field. Again the mode of operation is the same as the scheme shown in Figure 1. The main high voltage pulse is applied between electrode 3.2 and electrode 3.1 or between 3.2 and electrode 3.3. At the same time as the main voltage pulse is applied a bias voltage is maintained between electrodes 3.1 and 3.3 so as to minimise the electric field at the sharp edges of electrode 3.3. When the main pulse voltage reaches or nears its maximum a fast high voltage trigger pulse is superimposed over the bias voltage which is applied across electrode 3.3 with respect to 3.1. Streamers are then launched from the sharp edges of electrode 3.3 towards electrode 3.2. However, it may be desirable to change which electrode is at ground potential and this could any one of the three. „„,,„„„„ PCT/GB99/03792 00/30227 15 Planar system with streamers launched from surface of electrode. Figure 4 shows one possible scheme for a triggered streamer reactor using a planar geometry. The design has similarities to the trigatron spark gap operation. The ground electrode 4.2 consists of a plane with shaped slots. The electrode 4.3 consists of linear blades which fit into the slots either flush or slightly recessed. A slowly rising high voltage pulse is applied to electrode 4.1 with respect to 4.2 and 4.3. When this pulse voltage reaches its peak or near its peak, a fast high voltage trigger pulse, of the opposite polarity to that applied to electrode 4.1, is applied to the electrode 4.3 with respect to electrode 4.2. Streamers are launched from the edges of the electrode 4.3 and these propagate towards electrode 4.1. Figure 5 shows a variation of the scheme shown in Figure 4. The blades of electrode 4.3 are replaced with small cylindrical rods with shaped ends in electrode 5.3. The ground plane 5.2 has a large number of shaped holes penetrating its surface. The mode of operation is identical to the scheme of Fig. 4. Figure 6 shows an arrangement similar to Figure 5 but in coaxial cylindrical form. Similarly, Figure 7 is a coaxial cylindrical version of the scheme of Figure 4. Using conducting and dielectric inserts for electric field shaping. It may be desirable to use conducting inserts to reduce electric field enhancement whilst the main charge voltage is applied. As an example the scheme in Figure 4 is shown modified in Figure 8. Conducting inserts 80 are placed between electrodes 8.3 and 8.2. If the resistivity of the inserts 80 is sufficiently low then when the relatively slow charge pulse is impressed across the main gap then surface charge will spread sufficiently fast over the conducting inserts 80 so as to make the lower plane appear smooth from an electrostatic view. When a pulse is subsequently applied between electrodes 8.3 and 8.2 the electric field enhancement on the tips of electrode 8.3 will be similar to the case without the conducting inserts. Using dielectrics will also reduce field enhancement of electrode 8.3 but will not make the lower plane appear totally smooth from an electrostatic view. Using more than three sets of electrodes. In designing an electrode configuration it is desirable to obtain minimum of electric field enhancement on the electrode surfaces when the relatively slow charging pulse is impressed but to maximise the field enhancement when the faster trigger pulse is applied. Improved performance, in this respect, may be achieved by using multiple sets of electrodes. An example of how this might be achieved is shown in the in Figure 9. The electrode 4.3 in Figure 4 is replaced with two sets of electrodes 9.4 and 9.3. The mode of operation is similar that as described for the system in Figure 4. That is: a slowly rising high voltage pulse is applied to electrode 9.1 with respect to 9.2, 9.3 and 9.4. When this pulse voltage reaches its peak or near, its peak, a fast high voltage trigger pulse, of the opposite polarity to that applied to 9.1, is applied to the trigger electrodes 9.3 and 9.4. The magnitude of the voltage pulse on electrode 9.3 with respect to electrode 9.2 is greater than that applied to electrode 9.4 with respect to 9.2. Streamers are launched from the top edges of the trigger electrode 9.3 and these propagate towards 9.1. Recessing the trigger electrodes. For the schemes shown in Figures 4-8 the capacitance between the trigger electrode and the ground electrode may be quite high. It may be desirable to recess the electrodes much further than a conventional trigatron spark gap to reduce this capacitance. As mentioned above, the novel approach of the invention greatly eases the problem of power supplies. Figure 10 shows a suitable arrangement, applied to the embodiment of Fig. 1. Figure 10 shows an arrangement in which the trigger pulse is derived from energy in the electrode system. A high voltage pulse transformer, Tl, is used to pulse charge the electrodes and simultaneously maintain the bias voltage. When the main charge voltage reaches its peak or near its peak the high voltage switch, SI, is closed. The switch could be a spark gap or a thyratron. On closure of the switch the self capacitance of electrodes 1.2 and 1.3 starts to discharge through inductance LI . Provided the resistance of the switch is sufficiently small then the voltage between electrodes will reverse in polarity. The inductor L2 is significantly greater than LI and acts to decouple the charge circuit from the trigger circuit for the relatively short duration of the trigger pulse.

Claims: CLAIMS 1. A method of generating an electrical discharge, the method comprising the steps of: (a) generating a strong electrostatic field between a pair of electrodes in a relatively slow time; and (b) causing initiation and launching of streamer discharges within or into said field. 2. The method of claim 1, in which said electric field is increased over time to a value below that at which electrical breakdown will occur, and said streamer discharges are initiated by providing a trigger pulse to produce a localised enhancement of the field. 3. The method of claim 1 or claim 2, in which a number of streamer discharges are initiated at corresponding locations in the electric field, said number being sufficient to load the external circuit to prevent any one of the discharges causing a spark discharge. 4. A pulsed electrical discharge apparatus for producing streamer discharges, comprising first and second electrodes forming a main pair of electrodes, and a third electrode, means for applying a voltage across the main pair of electrodes and means for applying simultaneously a bias voltage between the third electrode and one of the first and second electrodes, the system also comprising means for superimposing a fast trigger pulse on top of said bias voltage so as to modify the electric field adjacent the surface of one of the electrodes to cause one or more streamers to be initiated and launched. 5. The apparatus of claim 4, in which the bias voltage is DC or zero. 6. The apparatus of claim 4, in which the voltage applied across the main electrodes is a pulse waveform with a relatively slow rise time. 7. The apparatus of claim 6, in which the bias voltage is time varying and synchronised with the voltage applied across the main pair of electrodes. 8. The apparatus of claim 7, in which said rise time is in the range 200 ns - 100 ms, and the trigger pulse has a rise time in the range 1 - 200 ns . 9. The apparatus of any of claims 4 to 8, in which the first and second electrodes form continuous parallel surfaces and the third electrode is disposed between the first and second electrodes and is formed with discontinuities. 10. The apparatus of any of claims 4 to 8, in which the first electrode forms a continuous surface, the second electrode forms a surface parallel with the second electrode and which is provided with a plurality of openings, and the third electrode comprises portions in registry with said openings. 11. The apparatus of claim 10, in which said portions of the third electrode have ends which are substantially aligned with the plane of the second electrode. 12. The apparatus of claim 10, in which said portions of the third electrode have ends aligned with said openings of the second electrode but spaced away from the second electrode in the direction opposite to the first electrode. AMENDED CLAIMS 1 [received by the International Bureau on 21 April 2000 (21.04.00); 2 original claims 1 and 2 amended; remaining claims unchanged (2 pages)] 3 1. A method of generating an electrical discharge, 4 comprising the steps of: 5 6 (a) generating in a relatively slow time an 7 electrostatic field between a pair of electrodes 8 which is comparable to the pulsed intrinsic self- 9 breakdown electric field strength of the medium 10 between the electrodes; and 11 12 (b) causing initiation and launching of streamer 13 discharges within or into said field. 14 15 2. The method of claim 1, in which said electric lfi field is substantially free from field 17 discontinuities, and said streamer discharges are 18 initiated by providing a trigger pulse in such a 9 manner as to produce localised discontinuities 0 which enhance the electric field in one or more 1 localised areas. 2 3 3. The method of claim 1 or claim 2 , in which a 4 number of streamer discharges are initiated at 5 corresponding locations in the electric field, 6 said number being sufficient to load the external 7 circuit to prevent any one of the discharges 8 causing a spark discharge. 9 0 4. A pulsed electrical discharge apparatus for 1 producing streamer discharges, comprising first and second electrodes forming a main pair of electrodes, and a third electrode, means for applying a voltage across the main pair of elec t ro d es and means for applying simultaneously a bias voltage between the third electrode and one STATEMENT UNDER ARTICLE 19 Claims 1 and 2 have been amended to clarify the nature of the invention, especially with regard to the single document cited in the International Search Report. The remaining claims are unchanged. The cited document (Al ra) relaters to an ozoni-zer which is essentially a pulsed two-electrode corona discharge apparatus, but is provided with a pre-ionization source to seed the gas with free electrons. The plus electrode 2 of Akira is provided with projections 1 which will act to distort or concentrate the electric field whenever voltage is applied across the main electrodes. In the present invention, as discussed in the specification energy is applied to the apparatus in two ways. First, energy is applied at a relatively slow rate to produce a strong electric field, that is a field which is close to the field strength which would produce breakdown in the medium of interest. Secondly, a relatively fast pulse is applied to initiate streamer production. The invention makes use of electrode configurations which ensure that the electric field in the first stage is substantially free of discontinuities, thus allowing a high field strength to be achieved without breakdown, while the application of the fast pulse does produce local field enhancement discontinuities for the purpose of initiating streamer production. In Akira, the electrode arrangement is such that the field cannot be raised to a high level without producing breakdown, owing to the presence of projections which would produce discontinuities and concentrations of field strength. It would appear that the applied voltage in Akira would be limited to no more that about one-half of the intrinsic self-breakdown field strength if discharges are to be avoided. In fact, in Akira only a drift field is applied before the main pulse, and this drift field would be of the order only of hundreds of volts or a few kV. Therefore, Aldra discloses no more than a conventional corona discharge ozonizcr plus a pre-ionizer, whereas the present invention operates in an entirely novel manner which gives a greatly reduced peak power requirement

Inventors: SOMERVILLE IAIN, SOMERVILLE, IAIN
Applicants: KELVIN INSTR LIMITED, SOMERVILLE IAIN, KELVIN INSTRUMENTS LIMITED, SOMERVILLE, IAIN
Application year: 1999
Main classification: H01T19
IPC codes: B01J19, H01T2, H01T19