WO2004026461A1

WO2004026461A1 - Non-thermal plasma reactor

Info

Publication number: WO2004026461A1
Application number: PCT/GB2003/003910
Authority: WO
Inventors: John Sydney Carlow
Original assignee: Accentus Plc
Priority date: 2002-09-21
Filing date: 2003-09-09
Publication date: 2004-04-01
Also published as: GB0501136D0; GB2408004A; AU2003264743A1; GB0221973D0

Abstract

A non-thermal plasma reactor comprising two electrodes and a gas flow path through the reactor, characterised in that at least one electrode is provided with a multiplicity of triple junctions (2) on or within a surface of the electrode that faces the other electrode. One or both electrodes are covered by pots, stripes or portions of dielectric materials.

Description

Non-Thermal Plasma Reactor

The present invention relates to reactors for the plasma-assisted treatment of gaseous media and in particular for the at least partial removal of one or more of nitrogenous oxides, particulates including carbonaceous particulates, hydrocarbons including polyaromatic hydrocarbons, carbon monoxide, sulphur oxides, dioxins, furans and other regulated or unregulated combustion products from the gaseous media. Such products are encountered in the exhausts of internal combustion engines, effluent gases from coal-fired power stations or effluent gases from incineration or other industrial processes, such as from the pharmaceutical, food-processing, paint manufacturing, dye manufacturing, textiles, printing and incineration industries. Other treatments of gaseous media include ozone generation and fuel reforming.

One of the major problems associated with the development and use of internal combustion engines is the noxious exhaust emissions from such engines. Two of the most deleterious materials, particularly in the case of diesel engines, are particulate matter (primarily carbon) and oxides of nitrogen (NO_x) . In practice, it is found that combustion control techniques which improve the situation in relation to one of the above components of internal combustion engine exhaust emissions tend to worsen the situation in relation to the other.

Plasma assisted gas processing reactors which operate in the so-called "silent discharge" mode have been proposed. Such silent discharges are produced in a gas between electrodes, between which there is at least one dielectric layer or dielectric material arranged so that there is no possibility for a direct, that is metal- to-metal, discharge through the gas. Such silent discharge reactors are also referred to as dielectric barrier reactors.

A silent discharge reactor of this type is disclosed in US 5,746,051. The reactor comprises a number of flat rectangular electrodes interleaved by flat rectangular insulating plates. The exhaust gases pass between the electrodes and are exposed to the electric discharge.

These reactors contain at least one dielectric barrier between each pair of electrodes. These barriers are typically made of an insulator such as a ceramic. The dielectric barrier shields the electrodes from each other and so prevents the plasma from breaking down by arc discharge. However, a dielectric barrier has several disadvantages. The main disadvantage is that dielectric barriers tend to crack during use due to the thermal and/or electrical stresses that can build up. The crack in the dielectric then exposes the electrode and leads to a breakdown of the plasma. The dielectric plate must then be replaced. As the electrode is usually bonded to the dielectric plate the whole electrode and dielectric component typically requires replacement. Barriers can also crack due to mechanical stresses caused, for example, by vibrations.

The dielectric barrier can also fail due to the repeated cycles of heating and cooling that it undergoes as the reactor is switched on and off.

If a silent discharge reactor can be made which does not require a dielectric barrier then the reactor can be made lighter and will have a much reduced thermal capacity. Ceramic parts are also relatively expensive and so the cost of a reactor is reduced if the dielectric barrier can be omitted. The object of the present invention is to provide a non-thermal plasma reactor which does not require a dielectric barrier forming a shield between each pair of electrodes.

Accordingly, the present invention provides a non- thermal plasma reactor comprising two electrodes and a gas flow path through the reactor, characterised in that at least one electrode is provided with a multiplicity of triple junctions on or within a surface of the electrode that faces the other electrode.

In a preferred embodiment the present invention provides a non-thermal plasma reactor comprising at least two electrodes and a gas flow path through the reactor, characterised in that each electrode is provided with a multiplicity of triple junctions on or within each surface of the electrode that faces another electrode.

The present invention also provides a non-thermal plasma reactor comprising two electrodes and a gas flow path through the reactor, and a power supply unit adapted to generate and apply a potential across the electrodes, characterised in that at least one electrode is provided with a multiplicity of triple junctions on or within a surface of the electrode that faces the other electrode, and the reactor and the power supply unit are connected electrically to form an electrical circuit, which electrical circuit further comprises a series capacitor.

The electrodes are made of a conducting material, typically metal such as stainless steel. The conducting material can be in the form of a sheet of the solid material or may take the form of a mesh, sintered fibre material, sintered powder material or combinations thereof. Thus, the electrodes are typically chosen from metals, metal meshes, sintered metal fibres and sintered metal powder material. The metal is preferably stainless steel .

Examples of sintered metal fibre material are those made by Fairey icrofiltrex (Fareham, UK) and are available in stainless steel, Monel®, Inconel®, Hastelloy® and Fecralloy™. Stainless steel discs made by sintering powder are available from Martin Kurz & Co Inc and sold under the name Dynapore TM SPMTM . Metal woven filter cloth is made by G Bopp & Co. Use of sintered metal fibre material in plasma reactors is described in WO 03/047728.

The electrodes may be shaped in any way so as to provide a suitable gas path through the reactor. The electrodes may take the form of flat plates between which the gas passes. Concentric cylindrical electrodes may also be used. A stack of more than two electrodes may also be suitable in order to provide an increased gas path.

Where the electrodes are made of porous materials such as mesh, sintered fibres or sintered powder, the gas flow path may pass through the electrodes. In that case, the gas flow path may be defined by a combination of the reactor casing and baffles placed within the reactor to increase the gas flow path. Forcing or allowing the gases to pass through the electrodes can be advantageous as it causes particulates to be trapped on or in the electrodes and thus increases their residence time in the plasma.

The power supply unit may be any unit which is capable of supplying a voltage suitable to form a plasma between the electrodes. The potential applied across the electrodes needs to be sufficient to excite an electric discharge in the gaseous medium that flows through the reactor. The power supply may be AC, pulsed or a tesla type power supply (pulsed decaying oscillations) . Various types of pulsed power supply are possible including single pulses, multiple pulses and chirp pulses (multiple pulses with increasing frequency of single pulses within any one multiple pulse) . AC or tesla type supplies are preferred.

The triple junctions provided on the electrode or electrodes act so as to replace the need for a dielectric barrier. Thus, in the simplest form of the reactor there are two electrodes and one of the electrodes is provided with a multiplicity of triple junctions on the surface facing the other electrode. In a preferred embodiment, each of the two electrodes is provided with a multiplicity of triple junctions on the surface facing the other electrode.

The reactor may contain further electrodes which are protected against arc discharge by a dielectric barrier.

In one embodiment one pair of electrodes is provided with triple junctions on one of the facing surfaces and between each pair of electrodes there is either a dielectric barrier or one of the facing surfaces of the electrodes is provided with a multiplicity of triple junctions .

A triple junction occurs where a gas, an electrical conductor and an insulator are all in contact. The presence of triple junctions on an electrode is thought to lead to enhanced plasma formation through the triple junction effect whereby electrons from the electrical conductor can tunnel through the insulator and achieve enhanced emission compared to emission from the electrode alone due to the reduced work function in the presence of the triple junction.

As the insulator in the triple junction has a lower work function than the electrical conductor, this leads to a reduced plasma strike voltage for the reactor compared to that for a conventional dielectric barrier non-thermal plasma reactor. The plasma strike voltage required for the reactor of the present invention is typically up to two-thirds of the metal-metal strike voltage. Thus, the triple junctions control the discharge in the reactor.

It is thought that each individual triple junction is current limited. It is this that is thought to prevent the formation of a continuous current arc. Thus, multiple microdischarge streamer formation is encouraged from the electrode. The formation of many streamers prevents single arc discharge between the high voltage and low (or earth) voltage electrodes and leads to a uniform discharge. If the insulator (or part thereof) sits proud of the electrode then the plasma will be concentrated in the region above the insulator due to the effect of the dielectric constant of the insulator. Thus, in one embodiment of the present invention, the insulating material or part thereof sits proud of the electrode.

The triple junctions can be formed by depositing isolated portions of an insulating material on the conductor (electrode) . Suitable insulating materials are any materials which act as insulators and can be exposed to the temperature at which the reactor operates. Dielectric materials are typically suitable insulating materials for this purpose. The dielectric material may be an organic polymer, a polymeric material such as a polysilazane, a phosphorus-nitrogen polymer such as a polyphosphazene, a silicone polymer (or silicone resin) derived for example from a silane monomer, an inorganic material or a combination thereof. Suitable inorganic materials include alumina and silica as well as other ceramics. Quartz, glass, glass-ceramic and a micaceous glass such as MICATHERM TM are also possible materials. Adhesives are also suitable insulating materials for some applications. Organic adhesives, such as Araldite™, pressure sensitive adhesives, light sensitive adhesives or ceramic cements, such as alumina cement, may be used depending on the temperature at which the reactor will operate. Adhesives may also be used in combination with a dielectric material such as alumina powder or a solid piece of dielectric to form a chemically bonded ceramic.

The insulating material may be provided on the electrode in the form of spots, thin strips or larger portions. Suitable methods for depositing the insulator on the electrode include gas-phase deposition such as chemical vapour deposition (CVD) , physical vapour deposition (PVD) such as sputtering, laser ablation, screen printing, aerosol deposition, spray pyrolysis, wet chemical methods such as sol-gel processing, hydrothermal deposition of the dielectric material, electrolytic deposition, electrophoretic deposition, or selective dissolution of a dielectric layer which has been applied to an electrode. Adhesive may by spotted onto the electrode and allowed to set. A suitable method can be chosen according to the dielectric material used.

One or more insulating materials may be provided on the electrode. For example, different portions of insulating material on the same electrode may contain different insulators. Two insulators may also be used together, for example, by attaching a dielectric portion using an adhesive. Alternatively, polymeric and ceramic insulators may be provided on one electrode.

Where the electrical conductor is a metal sheet, the dielectric is typically deposited on the surface.

Particles or fibres of dielectric material may also be incorporated into the metal sheet when it is made. In a particular embodiment, the insulator may be provided by filling holes in the metal sheet. Where the insulator is deposited in holes, the holes may go all or part of the distance through the sheet.

Where the electrical conductor is a mesh or porous material such as a sintered fibre or sintered powder material, the dielectric may be deposited on the surface of the material or may be deposited in the pores of the material or at junctions on the fibres or wires of the mesh. A mesh combining insulating and conducting fibres may also be used.

A combined conductor-insulator material may also be used as the electrode. For example conducting and insulating powder (e.g. metal and alumina) may be sintered together to form a composite material as long as the material acts as a conductor overall and can therefore function as an electrode. Suitable materials could include alumina fibre reinforced metal and alumina powder reinforced metal. A suitable composite material could be produced by directed oxidation of molten metal. For example by the oxidation of molten aluminium in a controlled way so that alumina grows through the melt.

In one embodiment of the present invention, further triple junctions may be formed by depositing a conductor, preferably a metal on the insulator. This can be done by sputtering, vacuum deposition, painting with a high temperature conducting paint or any other suitable method. Masking the electrode may be necessary. The conductor is any conductor which will not degrade significantly in the plasma environment, preferably a metal, more preferably a precious metal such as silver, gold, palladium, platinum, rhodium, titanium, tungsten or a combination thereof. The conductor may, for example, be in the form of a small piece of metal that is attached to the electrode by an insulating glue.

Where there is a metal deposited on the insulator, two sets of triple junctions are formed. There are triple junctions where the insulator meets the electrode and there are further triple junctions where the spot of metal meets the insulator. This latter triple junction is the place where the plasma is concentrated. Electrons can be emitted from either triple junction during operation of the reactor. In a preferred embodiment of the invention, the triple junction where the insulator meets the electrode is recessed into the electrode. This reduces the discharge of electrons from this triple junction. In a preferred embodiment, the deposits of insulator are shaped so as to maximise the distance between the electrode and' the metal deposited on the insulator. For example, the insulator may have a serrated edge or be shaped like a mushroom.

The triple junctions are positioned on the surface of the electrode or within the electrode such that electrons are emitted from the surface of the electrode that faces another electrode. Thus, where only two electrodes are present, typically the surface of one or each electrode that faces the other electrode is provided with triple junctions (a single-sided electrode) . Where a stack of electrodes is present, the electrodes within the stack preferably have triple junctions on both surfaces (double-sided electrodes) and the outer electrodes each have triple junctions on the inner surface. In a preferred embodiment of the present invention, each electrode is provided with a multiplicity of triple junctions on or within each surface of the electrode that faces another electrode. Where the electrodes are formed from a porous material such as a mesh, the provision of the triple junctions by means of dielectric particles or spots within the mesh may be suitable both for use as a double-sided electrode or as a single-sided electrode.

Triple junctions may also be formed by filling a hole drilled through the electrode with insulator. An electrode with triple junctions formed in this way may be used as a double or single-sided electrode.

The spots, strips or portions of insulating material may have any shape and may be arranged in any pattern on the electrode. For example, approximately circular spots may be deposited in rows or randomly.

Typical thicknesses for deposits of insulator are from a few tenths of a millimetre up to a few millimetres, for example, up to 2 mm.

The insulating material may cover or replace (depending on the construction of the electrode) up to 90% of the electrode surface as long as the electrode, be it solid or porous continues to act as a conductor. Thus, where the electrode is a solid conductor the insulating material is provided on the surface and covers it. If the insulating material is embedded into the electrode on formation or fills holes in the electrode then it can be regarded as replacing the surface of the electrode at that point. Preferably the insulating material covers or replaces up to 50%, more preferably up to 25%, more preferably up to 20%, most preferably up to 10% of the surface. At least 1% of the surface is preferable covered or replaced by insulating material.

The insulating material is preferably in the form of many small spots, strips or portions. Thus each piece of insulating material preferably has a width of less than 5 m, preferably 3 mm, more preferably from 0.2 to 2 mm, most preferably 0.5 to 1 mm. Thus a strip or spot with a diameter of from 0.5 to 1 mm is preferred.

Where further triple junctions are formed on the insulator using a conductor, the conductor typically covers 90% of the top surface of the insulator. The conductor can cover from 10 to 90 % of the surface of the insulator, preferably 20 to 80 % and more preferably about 50% of the surface of the insulator. In practice, the insulator covers at least 50% of the top surface of the insulator, more preferably 70% and most preferably about 90% of the top surface.

The edges of the electrodes may be provided with a strip of insulating material. This protects against metal-metal discharge occurring between the edges of the electrodes. Alternatively, standard high voltage precautions may be taken to prevent the electric field intensifying at corners or edges of the electrode. For example, the corners and edges may be rounded and the edges of the electrodes may be bent away from each other if the reactor arrangement allows this.

The capacitor limits the amount of charge in the electrical circuit. Any component or electrical circuit which has the same effect as a capacitor can be used instead of a capacitor and the definition of a capacitor for the purpose of the present invention includes any such alternative component or electrical circuit.

The size of the capacitor can be chosen so as to control the streamers formed when the reactor is in use. A capacitor with a capacitance of about 500pF is typically suitable for use with a reactor where the electrodes are 120mm by 60mm and spaced 1mm apart. One or more capacitors may be used. However, the size of the capacitor may be increased in order to introduce more power into the electrical circuit.

A variable capacitor may also be used. This allows the possibility of varying the size of the capacitor with the amount of power required. For example, the amount of effluent gas emitted from an automobile varies depending on the conditions under which the vehicle is operating. In general, more particulates are emitted when the vehicle is starting to move or moving in a low gear. By linking the size of the capacitor to the engine operating conditions, more power can be supplied when it is needed such as when the vehicle is operating in a low gear.

A resistor may optionally be provided in the electrical circuit in series with the reactor and capacitor. A resistor of about 3 to 5 kΩ is typically suitable. For example, a resistor of about 5kΩ may be used with a reactor having two electrodes of 120mm by 60mm spaced 1mm apart.

The gap between the or each pair of electrodes may be filled or partially filled with a filling material which may act as a filter, a catalyst, a support for a catalyst or a combination thereof. The function of the catalyst (s) may be to aid the conversion of components in the exhaust gases for example particulate material, carbon monoxide, hydrocarbon and nitrogen oxides to harmless products. Catalysts can also be present upstream and/or downstream of the reactor so that exhaust gases can pass over the catalyst before or after entering the reactor. Combinations of different types of catalysts can be used.

A catalyst may also be present in the insulator forming the triple junction, on the electrode or in the conductor deposited on the insulator (where present) . In addition, a catalyst may be present in two or more of these positions. For example, the electrode may have a catalytic coating. In addition, where a conductor is deposited on the insulator it may be or may contain a catalyst. Alternatively, where a conductor is deposited on the insulator it may be or may contain a catalyst and further particles of the catalyst may also be present in the insulator. More than one catalyst may be used.

The present invention also provides a kit comprising a non-thermal plasma reactor, a power supply and a capacitor. These components can be connected in series to form an electrical circuit.

The present invention also provides a kit comprising a non-thermal plasma reactor and a capacitor adapted for connection in series. These components can then be further connected in series with a power supply.

The present invention can be used for processes where conventional non-thermal plasma reactors containing one or more dielectric barriers are used. Thus reactors of the present invention may be used, for example, for treating effluent gases, for ozone generation or for fuel or hydrocarbon reforming. The effluent gases may be produced by a vehicle such as an automobile or by industrial plant such as a power station. Thus, the present invention provides a vehicle, industrial plant, ozone generator or fuel or hydrocarbon reformer comprising a reactor according to the present invention. The size of the series capacitor and if present the series resistor may be altered so as to achieve a reactor and electrical circuit combination that is appropriate for the particular use.

Specific constructions of a non-thermal plasma reactor embodying the invention will now be described by way of example and with reference to the drawings filed herewith, in which:

Figure 1A is shows a schematic plan view of an embodiment of the present invention.

Figure IB shows the same embodiment of the invention as shown in Figure 1A in cross-sectional side view.

Figure 2A shows a schematic longitudinal cross- section of an embodiment of the present invention.

Figure 2B shows the same embodiment of the invention as shown in Figure 2A in cross-section.

Figures 3A, 3B, 3C and 3D show four arrangements of electrodes for reactors of the present invention.

Figure 4 shows a schematic arrangement of two electrodes, one of which is provided with triple junctions according to the present invention.

Figures 5A, 5B and 5C show schematic cross-sections of further embodiments of triple junctions according to the present invention. Figure 1A shows a plan view of a reactor of the present invention. The reactor has metal electrodes (1) on which are provided many spots of an insulating material (2) to form triple junctions. The edges of the electrodes inside the reactor are protected by a strip of insulating material (3) . The high voltage electrodes are insulated from the casing (5) by insulating bushes (4) . Effluent gases flow through the reactor in the direction of the arrow.

Figure IB shows the same embodiment of the invention as seen from the side in cross-sectional view. The reactor comprises a stack of electrodes where alternate electrodes are connected to a high voltage source (6) and the intervening electrodes are earthed (7). The casing also acts as the earth electrode. Both sides of each electrode and the inside of the casing are each provided with many spots of insulating material (2). The spots are shown schematically in the Figure. Gas for treatment in the reactor flows into the plane of the paper.

It will be appreciated that variations on this embodiment are possible within the scope of the present invention. For example, the electrodes can be made of a porous material such as a mesh.

Figure 2A shows a cross-section through a reactor with concentric electrodes. The gaseous medium enters through the inlet (21) and passes into the space between the outer earth electrode (24) and the inner mesh electrode (22). Both electrodes are provided with triple junction spots (23) . The gas is constrained to pass through the high voltage mesh electrode (22) by the casing (24) and the supports (27) for the mesh electrode. The gas then passes through perforations (29) in the outlet pipe (25) so as to leave the reactor in the direction indicated by the arrows. The high voltage supply (28) to the mesh electrode (22) is shielded by the structure (26) .

Figure 2A shows an arrangement which is of general application as regards collecting the gas from the reactor and feeding it to an outlet. The collection pipe (25) has graded holes for collecting the gas which decrease in size towards the outlet from the reactor. This is designed to aid the gas in flowing into the outlet pipe at the end away from the outlet and thereby lead to more uniform flow distribution over the mesh electrode (22) along its length in the direction of flow.

Figure 2B shows the concentric design of the reactor of Figure 2A. The outer casing (24) forms the earth electrode and surrounds the inner mesh electrode (22). At the centre of the reactor is the outlet collection pipe (25). The electrodes (24,22) are provided with triple junction spots (23) .

Figure 3A shows an arrangement of electrodes for a reactor of the present invention where there are two electrodes (31,32) . One electrode (32) is provided with triple junctions (33) on the surface facing the other electrode ( 31) .

Figure 3B shows an arrangement of electrodes for a reactor of the present invention where there are two electrodes (34,35). Each electrode is provided with triple junctions (33) on the surface facing the other electrode.

Figure 3C shows an arrangement of electrodes for a reactor of the present invention where there are three electrodes. The electrode (36) is provided with triple - In ¬

junctions (33). The electrode (38) is protected by a dielectric barrier (39) and the remaining electrode (37) is bare.

Figure 3D shows an arrangement of electrodes for a reactor of the present invention where there are three electrodes (311,312,313). Each electrode is provided with triple junctions (33) on each surface that faces another electrode.

Figure 4 shows a schematic arrangement of two electrodes (41,42) in a reactor of the present invention.

One electrode (41) is provided with a plurality of triple junctions. The electrode is provided with a plurality of spots of insulator (43) deposited on the surface of the electrode. A spot of metal (44) is provided on each spot of insulator. Triple junctions (45,46) occur where the insulator meets the electrode (45) and where the metal spot meets the insulator (46) . In operation of the reactor, microdischarge streamers

(48) form between the triple junction (46) and the other electrode (42) .

Figure 5A shows a triple junction according to the present invention where the insulator (52) has a stepped edge and rests on the electrode (51). A further conducting spot (55) is positioned on the top surface of the insulator.

Figure 5B shows a triple junction where the junction between the insulator (53) and the electrode (51) is recessed into the electrode. A conducting spot (55) covers a portion of the top surface of the insulator.

Figure 5C shows a mushroom shaped insulator (54) on an electrode (51) . The shape of the insulator is designed to maximise the distance between the junction of the insulator (54) and the electrode (51) and the junction of the conducting spot (55) and the insulator.

Claims

1. A non-thermal plasma reactor comprising two electrodes and a gas flow path through the reactor, characterised in that at least one electrode is provided with a multiplicity of triple junctions on or within a surface of the electrode that faces the other electrode.

2. A non-thermal plasma reactor according to claim 1 comprising further electrodes and wherein each gap between a pair of electrodes is provided with either a dielectric barrier or one of the facing surfaces of the electrodes is provided with a multiplicity of triple junctions on or within the surface.

3. A non-thermal plasma reactor according to claim 1 or 2 wherein each electrode is provided with a multiplicity of triple junctions on or within each surface of the electrode that faces another electrode.

4. A non-thermal plasma reactor according to any one of claims 1 to 3 wherein one or more of the electrodes comprises a mesh, sintered metal fibres or sintered metal powder.

5. A non-thermal plasma reactor according to any one of claims 1 to 4 wherein the triple junctions comprise spots, thin strips or portions of one or more insulating materials.

6. A non-thermal plasma reactor according to claim 5 wherein the triple junctions further comprise spots, thin strips or portions of one or more conductors on the insulating material.

7. A non-thermal plasma reactor according to claim 6 wherein the junction of the insulating material with the electrode is recessed into the electrode.

8. A non-thermal plasma reactor according to any one of the preceding claims wherein the edges of one or more of the electrodes are provided with a strip of insulating material .

9. A non-thermal plasma reactor comprising two electrodes and a gas flow path through the reactor, and a power supply unit adapted to generate and apply a potential across the electrodes, characterised in that at least one electrode is provided with a multiplicity of triple junctions on or within a surface of the electrode that faces the other electrode, and the reactor and the power supply unit are connected electrically to form an electrical circuit, which electrical circuit further comprises a series capacitor.

10. A non-thermal plasma reactor and power supply according to claim 9 wherein the electrical circuit further comprises a series resistor.

11. A kit comprising a non-thermal plasma reactor as defined in any one of claims 1 to 8, a power supply and a capacitor as defined in claim 9 adapted for connection in series to form an electrical circuit.

12. A kit comprising a non-thermal plasma reactor as defined in any one of claims 1 to 8, and a capacitor as defined in claim 9 adapted for connection in series with a power supply to form an electrical circuit.

13. Use of a reactor as defined in any one of claims 1 to 8 for the treatment of effluent gases, ozone generation or fuel or hydrocarbon reforming.

14. A vehicle, fuel or hydrogen reformer, power station or ozone generator comprising a reactor according to any one of claims 1 to 8.

15. A reactor for processing a gaseous medium wherein the gas is collected in an outlet pipe via holes which decrease in size towards the outlet of the reactor.