WO2001000301A1

WO2001000301A1 - Filtering device and method for treating a polluted air stream

Info

Publication number: WO2001000301A1
Application number: PCT/ZA2000/000118
Authority: WO
Inventors: Rian Strydom; Cornelius Andries Barnardt
Original assignee: PRETORIUS JOHANNES
Current assignee: PRETORIUS JOHANNES
Priority date: 1999-06-29
Filing date: 2000-06-29
Publication date: 2001-01-04
Anticipated expiration: 2001-12-29
Also published as: AU6122300A

Abstract

This invention relates to a filtering device (10) and method for extracting pollutants from a zone. This invention further relates to a corona discharge reactor (16) and filtration unit for treating a polluted air stream. This invention yet further relates to a method of treating pollutants in a gas stream and to a method of generating a corona discharge. The filtering device (10) includes a duct (12) defining an endless passage (13) extending through a preliminary condensing and impingement filter (14); a secondary coalescer filter (15); a corona discharge filtration unit (16); an ozone filter (17); an air advancement means in the form of a blower (18); and a pollutant collecting means generally designated by reference numeral (20).

Description

FILTERING DEVICE AND METHOD FOR TREATING A POLLUTED AIR STREAM.

INTRODUCTION

This invention relates to a filtering device and method for extracting pollutants from a zone. This invention further relates to a corona discharge reactor and filtration unit for treating a polluted air stream. This invention yet further relates to a method of treating pollutants in a gas stream and to a method of generating a corona discharge.

BACKGROUND TO THE INVENTION

Smoke and other gas effluents emanating from restaurants and other food preparation facilities present an air pollution problem and such emissions are therefore strictly regulated in urban areas. As a result, restaurants are often forced either to select locations where the emissions are not likely to be offensive and which locations are not always the best from a business point of view, or to install high chimneys to channel emissions to an elevated level.

A conventional method of treating a restaurant effluent gas stream includes the step of filtering the effluent stream through a particulate and activated charcoal filter. A disadvantage of this method is that the charcoal has to be replaced quite often owing to clogging of the pores of the charcoal by oil present in the

gas stream. Another known method for treating a restaurant effluent gas stream includes the step of passing the effluent stream through an incinerator. A disadvantage thereof is that an incinerator is relatively expensive to buy and to operate.

USA patent number 5,827,407 discloses a system for the treatment of gaseous pollutants, which utilises a corona discharge to destroy volatile organic compounds and biological matter. Other gas stream treatment systems utilising corona discharge are also known, such as the systems disclosed in USA patents 5,147,423 and 5,593,560.

The above known systems each includes a corona discharge reactor including two closely spaced electrodes between which a corona discharge is generated. A common disadvantage of the known systems referred to above is that they are not suitable for treating gas streams carrying a relatively high concentration of pollutants, oils, or other organic matter. One of the reasons for this is that the pollutants tend to accumulate on the electrodes, so that the surface of both electrodes becomes conductive and/or clogged after a while, resulting in the

corona discharge being interrupted.

Conventionally, gaseous and particulate pollutants emanating from a zone where food is prepared in restaurants, are extracted by a system comprising an extraction hood; ducting; and a fan for extracting the air away from the hood along the ducting. The hood is usually situated relatively high (approximately 1 ,5m) above the source of the pollutants. A first disadvantage of the conventional system is that excessively large volumes of air have to be extracted to maintain sufficient air velocity in the zone. Relatively large extraction fans which are energy intensive therefore have to be used. In the conventional system, the extracted air is conveyed along the ducting, which may be several meters long, and emitted into atmosphere. In cold regions, the large volume of hot air that is extracted from the restaurant cools the inside of the restaurant off, thus counteracting the heating effect of heaters and fireplaces.

OBJECTS OF THE PRESENT INVENTION

Objects of the present invention are therefore to provide a corona discharge reactor, a filtering device, a method of treating pollutants in a gas stream and a method of generating a corona discharge, with which the aforesaid disadvantages may be overcome or at least minimised.

SUMMARY OF THE INVENTION

According to a first aspect of the invention there is provided a filtering device for extracting pollutants and odours from a food preparation zone, the filtering device comprising a duct defining an endless flow passage for an air stream, the passage extending through the zone and being exposed in the zone, the arrangement being such that the duct defines an inlet for pollutants in the zone; advancement means for circulating the air stream along the passage; and a filtration means disposed in the passage for removing pollutants from the air stream.

Further according to the invention the filtration means comprises a corona discharge filtration unit.

The corona discharge filtration unit may include a corona discharge reactor defining a passage for a polluted gas stream, the reactor including an electrode disposed in the passage; and cleaning means for removing pollutants from the surface of the electrode.

The electrode may comprise a central electrode and the reactor may include a second spaced outer electrode extending around the central electrode.

The central electrode may be elongate and the outer electrode may extend helically about the central electrode.

The passage may extend transverse the longitudinal axis of the central electrode, through the space between the two electrodes.

The central electrode may comprise a tube of a material having a high dielectric strength at temperatures in the range between 20 and 200°C.

The tube may be a quartz tube. The tube may be filled with a metal powder. Preferably the tube is filled with aluminium powder.

The electrodes may be insulated from each other, with each electrode being provided with an electrical contact for connecting to a relatively high voltage alternating current supply.

The power supply may comprise a relatively high voltage transformer. Preferably the power supply comprises a first high voltage transformer having an output of a first frequency, a second high voltage transformer having a relatively higher frequency output, and a combination device, for superimposing the output of the second transformer on the output of the first transformer.

The output frequency of the first transformer may be between 45 and 55 Hz and the output frequency of the second transformer may be in the range between 5 and 25 kHz.

The cleaning means may include a first reservoir for a cleaning liquid and at least one outlet from the first reservoir for directing the cleaning liquid onto the

surface of the electrodes.

The filter device may include a plurality of parallel extending corona discharge

reactors.

The reactors may be disposed vertically below the first reservoir. A second reservoir may be disposed below the reactors.

The cleaning means may include a pump for pumping the cleaning liquid from the second reservoir to the first reservoir.

The first reservoir may be provided with a mounting plate for receiving the upper ends of the inner electrodes.

The outlet for the cleaning liquid may be defined in the mounting plate towards the upper end of each electrode for continuously dispensing cleaning liquid onto the outer surface of the central electrode and the inner surface of the outer electrode.

The mounting plate may be of an insulating material.

The filtering device may include a housing for the corona discharge reactors, the housing defining an inlet on one side and an outlet towards an opposite side, and the passage extending transverse the longitudinal axis of the reactors from the inlet to the outlet.

The respective sizes of the outlet openings in the mounting plate may decrease in size from the inlet to the outlet of the housing.

The electrical contact of each central electrode may comprise a connector stub disposed on one end of each electrode.

The connector stubs of the central electrodes may be connected to each other by means of a connector plate.

The connector plate may be connected to the high voltage alternating current supply by a high voltage connector.

The second reservoir may be of a conductive material, with the outer electrodes being connected to the second reservoir.

Further according to the invention the filtering device includes an ozone filter disposed downstream from the corona discharge filtration unit, for filtering ozone from the air stream.

The duct may be provided with an air directing means disposed on one border region of the zone for directing an air stream across the zone to collect pollutants generated in the zone and to restrict pollutants from escaping from the zone.

The inlet of the duct may be disposed on an opposite side of the zone for collecting the pollutants and for directing the pollutants along the duct.

The air directing means may comprise an elongate outlet nozzle for directing an air blade across the front of the food preparation zone.

The outlet nozzle may extend from a plenum device disposed in the passage, for distributing the air stream along the length of the nozzle.

Further according to the invention the air advancement means comprises a suitable blower for blowing air along the passage.

Yet further according to the invention the filtering device includes a condensing and impingement filter for pre-filtering pollutants from the air stream.

Even further according to the invention, the filtering device includes a coalescer filter for removing relatively larger particles, smoke and/or carbon from the air stream.

According to a second aspect of the invention there is provided a corona discharge reactor substantially as hereinbefore described for use with a filtering device according to the first aspect of the invention.

According to a third aspect of the invention there is provided a corona discharge electrode assembly substantially as hereinbefore described for a corona discharge reactor according to the second aspect of the invention. According to a fourth aspect of the invention there is provided a method of treating a polluted gas stream, including the steps of: providing a corona discharge reactor defining a passage for the polluted gas stream, the reactor including an electrode disposed in the passage; generating a corona discharge in the passage with the electrode; passing the polluted gas stream through the passage; and removing pollutants from the surface of the electrodes.

The step of removing pollutants from the surface of the electrodes may include the further step of passing a cleaning liquid over the electrode.

The liquid may be passed continuously over the electrode.

The step of generating a corona discharge in the passage with the electrode may include the further step of applying a first alternating current having a first frequency to the electrode, and superimposing a second alternating current having a relatively higher frequency on the first current.

The method may include the further step of increasing the residence time of the polluted gas stream in the corona discharge, by disposing obstacles in the pathway of the gas stream.

According to a fifth aspect of the invention there is provided a method of generating a corona discharge, including the steps of: generating an oscillating electric field by applying a first alternating current having a first frequency to an electrode; and superimposing a second alternating current having a relatively higher frequency on the first current.

According to a sixth aspect of the invention there is provided a method for extracting pollutants from a food preparation zone including the steps of: establishing an air blade on one border region the zone and maintaining the air blade across the zone to collect the pollutants in the air blade; - collecting an air stream carrying the pollutants on an opposite border region of the zone; and passing the air stream through a corona discharge filtration unit to remove the pollutants from the air stream.

The method may include the further step of circulating the air from the filtration unit back to the zone.

According to a seventh aspect of the invention there is provided a filtering device for removing pollutants from a food preparation zone, the filtering device comprising air directing means disposed on one border region of the zone for directing an air stream across the zone to collect pollutants generated in the zone; an air receiving inlet disposed on an opposite border region of the zone for receiving the collected pollutants and for conveying the pollutants in the air stream along a passage; and a corona discharge filtration unit disposed in the passage, for removing pollutants from the air stream.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described by way of a non-limiting example with reference to the accompanying drawings wherein: figure 1 is a schematical cross-sectional front view along line B-B¹ in figure 2, of a filtering device according to a preferred embodiment of the invention for removing pollutants from a food preparation zone; figure 2 is a schematical cross-sectional side view along line A-A¹ in figure 1 ; figure 2a is a detail drawing of an air outlet nozzle of the filtering device of figure 1 ; figure 3 is a cross-sectional front view of a corona discharge filtration unit of the filtering device of figure 1 ; figure 3a is a cross-sectional side view of an oil drift eliminator of the filtering unit of figure 3; figure 3b is a perspective view of the oil drift eliminator of figure 3a; figure 4 is a cross-sectional plan view of the filtering unit of figure 3; figure 5 is a side view of a corona discharge reactor of the filtering unit of figure 3; figure 6 is a cross-sectional plan view of the reactor of figure 5; figure 7 is a schematical cross-sectional side view of an upper portion of the reactor of figure 5, in position in the filtering unit of figure 3; figure 8 is a schematical representation of a power supply for the corona discharge reactor of figure 5; and figure 9 is a plot of voltage versus time for a typical sinusoidal waveform generated by the power supply of figure 8.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring to figures 1 and 2, a filtering device according to a preferred embodiment of the invention for extracting pollutants, is generally designated by reference numeral 10. The filtering device 10 is particularly suitable for extracting pollutants and odours emanating from a cooking or frying zone Z in a food preparation facility of a restaurant or the like.

The filtering device 10 includes a duct 12 defining an endless passage 13 extending through a preliminary condensing and impingement filter 14; a secondary coalescer filter 15; a corona discharge filtration unit 16 (which is described in more detail below); an ozone filter 17; an air advancement means in the form of a blower 18; and a pollutant collecting means generally designated by reference numeral 20.

The pollutant collecting means 20 comprises an outlet nozzle 22 (shown in more detail in figure 2a) extending from a plenum device 23, connected to the blower 18. The outlet nozzle 22 establishes an air blade (indicated by arrows 24) across the front of the cooking zone Z, from the bottom to the top thereof. Referring to figure 2a, the relative angle as well as the size of the outlet opening of the outlet nozzle 22 is adjustable by pivoting the nozzle 22 about the pivot 22.1 and by rotating the adjustment screw 22.2, respectively.

The pollutant collecting means 20 further includes an inlet in the form of an elongate opening 26 for collecting the air stream containing the pollutants from the zone Z. The preliminary condensing and impingement filter 14 is located in the air inlet opening 26, for condensing and removing between 85 to 94 % of the volatile fats from the air stream. The condensed fat is removed to a reservoir via an outlet pipe (not shown).

The air passing through the condensing and impingement filter 14, collects in a second plenum device 27, from which the passage 13 extends downwardly along the side of the food preparation facility, to the coalescer filter 15. The coalescer filter 15 includes a plurality of open cell polyurethane particles, for removing smoke, carbon and relatively larger particles from the air stream.

From the coalescer filter 15, the passage 13 extends through the corona discharge unit 16 and through the ozone filter 17 to the blower 18. The ozone filter 17 comprises a polyurethane matrix impregnated with activated charcoal, for removing ozone which may emanate from the corona discharge unit 16 from the air stream, for safety reasons.

Referring to figure 3, the corona discharge unit 16 comprises a plurality of elongate vertically arranged corona discharge reactors 112; and a cleaning means. The cleaning means comprises a first reservoir 114 for cleaning liquid 130, disposed above the reactors 112, and a second reservoir 116, disposed below the reactors 112. The cleaning means is provided with a pump (not shown), for pumping the cleaning liquid 130 from the second reservoir 116 back to the first reservoir 114. Referring to figures 3a and 3b, the corona discharge unit 16 further includes an oil drift eliminator 113 disposed on the outlet side of the reactors 112. The oil drift eliminator 113 comprises a plurality of parallel extending closely spaced apart corrugated stainless steel plates 113.1 , with each plate 113.1 terminating in an upturned lip formation 113.2.

Referring to figures 5 and 6, each corona discharge reactor 112 comprises a central elongate electrode 118, and a helical outer electrode 120 extending co- axially about the central electrode 118.

Referring particularly to figure 6, the central electrode 118 comprises a quartz tube 118.1 , filled with a core of aluminium powder 118.2. Referring to figures 5 and 7, an upper end of the central electrode 118 is provided with an electrical connector in the form of a stub screw 118.3.

The outer electrode 120 is closely spaced from the inner electrode 118 and comprises a helically curved metal strip which is typically 10mm wide and coiled into a helix having a 3mm opening, for example. The outer electrode 120 is of a conducting material such as stainless steel. The second reservoir 116 is of metal and the lower end of each outer electrode 120 is connected to the second reservoir 116.

Referring to figure 7, the stub screws 118.3 of the central electrodes 118 are connected to a connector plate 122 disposed inside the first reservoir 114. The connector plate 122 is connected to a power supply 117 (referred to in more detail below) by means of a high voltage connector 124.

A floor of the first reservoir 114 provides a mounting plate 126 defining a plurality of outlet openings 128 and 129, for dispensing a cleaning liquid 130, present in the first reservoir 114, onto the outer surface of the inner electrodes 118 and onto the inner surface of the outer electrode 120. The upper end of the inner electrode 118 extends through the outlet openings 129. The cleaning liquid 130 preferably comprises mineral oil, but it will be appreciated that any other suitable liquid can be used.

The corona discharge unit 16 is provided with a housing 132 (shown in figure 4) for the reactors 112. The housing 132 defines an air inlet 134, an air outlet 136, and said passage 13 extends through the discharge unit 16, transverse the longitudinal axes of the reactors 112 from the inlet 134 to the outlet. The sizes of the outlet openings 128 and 129 gradually decrease from the inlet 134 to the outlet 136, thus to decrease the volume of cleaning liquid 130 which is dispensed onto the electrodes 118 and 120, towards the outlet 136. Referring to figure 8, the corona discharge unit 16 further includes a power supply 117 comprising a first relatively high voltage transformer 140 having a frequency of 50Hz; a second transformer 142 having a relatively higher frequency in the range between 10 and 20 kHz; and a combination box 144, for superimposing the output of the second transformer 142 on the output of the first transformer 140. The reactors 112 are thus provided with a current having a waveform substantially as depicted in figure 9.

The first reservoir 114 is filled with the mineral oil 130 and the power supply 117 is switched on so that a cold plasma volume corona discharge is generated by each of the reactors 112, in the region between the inner and outer electrodes 118 and 120. The pump 115 is activated and the cleaning liquid 130 continuously flows along the outer surface of the inner electrodes 118 and the inner surface of the outer electrodes 120, to continuously clean such surfaces.

In use, the blower 18 blows high-pressure air along the passage 13 and the outlet nozzle 22 establishes an air blade across the front of the zone Z, to collect pollutants therefrom.

The air stream containing the pollutants is collected by the air inlet 26 and passes through the condensing and impingement filter 14, wherein large fat and oil particles and droplets are removed. The air stream is conveyed along the passage 13 to the coalescer filter 15, where relatively larger particles, carbon and smoke are removed. Thereafter the air stream moves through the corona discharge filtration unit 16, wherein the pollutants are removed from the air stream as set out below.

In the corona discharge filtration unit 16, pollution particles remaining in the pre- filtered air stream are removed by being deposited on the surfaces of the electrodes while odorous vapours and gasses are broken down to non-odorous compounds.

The high electric field between the outer electrode 120 and the dielectric surface of the quartz central electrode 118 causes a volume corona discharge. The corona discharge fills the air volume between the electrodes 118 and 120, forming high densities of air ions, as well as high-energy free electrons. The ions charge pollution particles and deposit them onto the oil layer on the electrodes 118 and 120. The free electrons dissociate gaseous odorous molecules to non-odorous ones, as well as oxygen to free radical species. The latter further oxidise the dissociation products of the odorous molecules to non- offending forms.

In some cases the odorous gaseous compounds are resistant to destruction by corona discharge. This is overcome by the power supply 117, which superimposes the high frequency output of the second transformer 142 over the relatively lower frequency of the first transformer 140. The high frequency output is more efficient in the destruction of the gaseous species, while the low frequency output simultaneously removes the smoke particles. The cleaning oil 130 flowing along the surfaces of the electrodes 118 and 120 continuously coats these surfaces with a moving oil layer. The oil 130 collects in the second reservoir 116, which acts as a sump from which the oil 130 is again pumped to the first reservoir 114. The oil 130 is preferably passed through a strainer (not shown) for removing particles from the oil. This process is repeated until the oil 130 is too contaminated to be used i.e. until it becomes electrically conductive, or too thick to flow. Such oil is then flushed from the system and replaced with new oil. Alternatively, a once through-flow system can be devised if a sufficient quantity of oil is available. In such an arrangement, the polluted oil is removed and discarded as waste.

The oil 130 furthermore continuously flushes deposited pollution particles and destruction products, from the surfaces of the electrodes 118 and 120, preventing the build up of such particles. This maintains the corona discharge.

It will be appreciated that the oil 130 also contributes to the treatment process inside the corona discharge units 16. The oil 130 adsorbs and absorbs many gaseous species, and forms a sticky surface that improves the adhesion probability of particles hitting the surfaces. The velocity of the air stream and the corona discharge causes a fine mist of cleaning oil particles, which follows the air stream through the filtration unit 16. The oil drift eliminator 113 collects the oil mist and restricts the mist from exiting the filtration unit 16.

It will be appreciated that a number of corona discharge units 16 can be arranged in series or parallel, or a combination of both, with the air flow split equally between the corona discharge units 16. It will be appreciated further that the number (and geometry) of discharge units 16 that are required is determined by the volume of airflow to be treated and the nature and concentration of pollutants in the gas stream.

It was found that with a filtration unit according to the invention, relatively much less air movement is required for the removal of pollutants. For example, approximately one tenth of the total volume of air movement, which is required with conventional extraction systems, will be required by the present invention. It will be appreciated that relatively small and relatively cheap corona discharge filtration units are required to treat such relatively small air streams.

The filtering device 10 according to the invention can be applied to various cooking facilities such as deep oil frying, open flame grilling, hot plate frying and the like. It is further envisaged that it will be possible to use the filtering device 10 with small frying pans or grills in indoor applications, such as where a piece of meat is fried for a client while he is seated at his table in a restaurant, next to his table, without unwanted emissions into the restaurant's atmosphere. It will be appreciated that the filtering device 10 can also be adapted for use in a household environment such as with stoves, mobile indoor barbecue units, and the like. It will be appreciated further that variations in detail are possible with a corona discharge reactor, a filtering device, a method of treating pollutants in a gas stream and a method of generating a corona discharge according to the invention without departing from the scope of the appended claims.

Claims

1. A filtering device for extracting pollutants and odours from a food preparation zone, the filtering device comprising a duct defining an endless flow passage for an air stream, the passage extending through the zone and being exposed in the zone, the arrangement being such that the duct defines an inlet for pollutants in the zone; advancement means for circulating the air stream along the passage; and a filtration means disposed in the passage for removing pollutants from the air stream.

2. A filtering device according to claim 1 wherein the filtration means comprises a corona discharge filtration unit.

3. A filtering device according to claim 2 wherein the corona discharge

filtration unit includes a corona discharge reactor defining a passage for a polluted gas stream, the reactor including an electrode disposed in the passage; and cleaning means for removing pollutants from the surface of the electrode.

A filtering device according to claim 3 wherein the electrode comprises a central electrode and wherein the reactor includes a second spaced outer electrode extending around the central electrode.

5. A filtering device according to claim 4 wherein the central electrode is elongate and wherein the outer electrode extends helically about the central electrode.

6. A filtering device according to claim 5 wherein the passage extends transverse the longitudinal axis of the central electrode, through the space between the two electrodes.

7. A filtering device according to claim 6 wherein the central electrode comprises a tube of a material having a high dielectric strength at temperatures in the range between 20 and 200°C.

8. A filtering device according to claim 7 wherein the tube is a quartz tube.

9. A filtering device according to claim 8 wherein the tube is filled with a metal powder.

10. A filtering device according to claim 9 wherein the tube is filled with aluminium powder.

11. A filtering device according to any one of claims 4 to 10 wherein the electrodes are insulated from each other, with each electrode being provided with an electrical contact for connecting to a relatively high voltage alternating current supply.

12. A filtering device according to claim 11 wherein the power supply comprises a relatively high voltage transformer.

13. A filtering device according to claim 11 or claim 12 wherein the power supply comprises a first high voltage transformer having an output of a first frequency, a second high voltage transformer having a relatively higher frequency output, and a combination device, for superimposing the output of the second transformer on the output of the first transformer.

14. A filtering device according to claim 13 wherein the output frequency of the first transformer is between 45 and 55 Hz and the output frequency of the second transformer is in the range between 5 and

25 kHz.

15. A filtering device according to any one of claims 4 to 14 wherein the cleaning means includes a first reservoir for a cleaning liquid and at least one outlet from the first reservoir for directing the cleaning liquid onto the surface of the electrodes.

16. A filtering device according to claim 15 wherein the filter device includes a plurality of parallel extending corona discharge reactors.

17. A filtering device according to claim 16 wherein the reactors are disposed vertically below the first reservoir.

18. A filtering device according to claim 17 wherein a second reservoir is disposed below the reactors.

19. A filtering device according to claim 18 wherein the cleaning means includes a pump for pumping the cleaning liquid from the second reservoir to the first reservoir.

20. A filtering device according to any one of claims 16 to 19 wherein the first reservoir is provided with a mounting plate for receiving the upper ends of the inner electrodes.

21. A filtering device according to claim 20 wherein the outlet for the cleaning liquid is defined in the mounting plate towards the upper end of each electrode for continuously dispensing cleaning liquid onto the outer surface of the central electrode and the inner surface of the outer electrode.

22. A filtering device according to claim 20 or 21 wherein the mounting plate is of an insulating material.

23. A filtering device according to any one of claims 20 to 22 which includes a housing for the corona discharge reactors, the housing defining an inlet on one side and an outlet towards an opposite side, and a passage extending transverse the longitudinal axis of the reactors from the inlet to the outlet.

24. A filtering device according to claim 23 wherein the respective sizes of the outlet openings in the mounting plate decrease in size from the inlet to the outlet of the housing.

25. A filtering device according to any one of claims 16 to 24 wherein the electrical contact of each central electrode comprises a connector stub disposed on end of the electrode.

26. A filtering device according to claim 25 wherein the connector stubs of the central electrodes are connected to each other by means of a connector plate.

27. A filtering device according to claim 26 wherein the connector plate is connected to a high voltage alternating current supply by a high voltage connector.

28. A filtering device according to claim 27 wherein the second reservoir is of a conductive material, with the outer electrodes being connected to the second reservoir.

29. A filtering device according to any one of claims 2 to 28 which includes an ozone filter disposed downstream from the corona discharge filtration unit, for filtering ozone from the air stream.

30. A filtering device according to any one of the preceding claims wherein the duct is provided with an air directing means disposed on one border region of the zone for directing an air stream across the zone to collect pollutants generated in the zone and to restrict pollutants from escaping from the zone.

31. A filtering device according to claim 30 wherein the inlet of the duct is disposed on an opposite side of the zone for collecting the pollutants and for directing the pollutants along the duct.

32. A filtering device according to claim 31 wherein the air directing means comprises an elongate outlet nozzle for directing an air blade across the front of the food preparation zone.

33. A filtering device according to claim 32 wherein the outlet nozzle extends from a plenum device disposed in the passage, for distributing the air stream along the length of the nozzle.

34. A filtering device according to any one of the preceding claims wherein the air advancement means comprises a suitable blower for blowing air along the passage.

35. A filtering device according to any one of the preceding claims which includes a condensing and impingement filter for pre-filtering pollutants from the air stream.

36. A filtering device according to any one of the preceding claims which includes a coalescer filter for removing relatively larger particles, smoke and/or carbon from the air stream.

37. A filtering device substantially as herein described and as illustrated in the accompanying drawings.

38. A corona discharge reactor for use with a filtering device for treating a polluted gas stream, the corona discharge reactor having a passage for a polluted gas stream; an electrode disposed in the passage; and cleaning means for removing pollutants from the surface of the electrode.

39. A corona discharge reactor according to claim 38 wherein the electrode is a central electrode and wherein the reactor includes a second spaced outer electrode extending around the central electrode.

40. A corona discharge reactor according to claim 39 wherein the central electrode is elongate and wherein the outer electrode extends helically about the central electrode.

41. A corona discharge reactor according to claim 40 wherein the passage for the polluted gas stream extends transverse the longitudinal axis of the central electrode, through the space between the two electrodes.

42. A corona discharge reactor according to claim 41 wherein the central

electrode comprises a tube of a material having a high dielectric strength at temperatures in the range between 20 and 200°C.

43. A corona discharge reactor according to claim 42 wherein the tube is a quartz tube.

44. A corona discharge reactor according to claim 43 wherein the tube is filled with a metal powder.

45. A corona discharge reactor according to claim 44 wherein the tube is filled with aluminium powder.

46. A corona discharge reactor according to any one of claims 39 to 45 wherein the electrodes are insulated from each other, with each electrode being provided with an electrical contact for connecting to a relatively high voltage alternating current supply.

47. A corona discharge reactor according to claim 46 wherein the power supply comprises a relatively high voltage transformer.

48. A corona discharge reactor according to claim 46 or claim 47 wherein the power supply comprises a first high voltage transformer having an output of a first frequency, a second high voltage transformer having a relatively higher frequency output, and a combination device, for superimposing the output of the second transformer on the output of the first transformer.

49. A corona discharge reactor according to claim 48 wherein the output frequency of the first transformer is between 45 and 55 Hz and the output frequency of the second transformer is in the range between 5 and 25 kHz.

50. A corona discharge reactor according to any one of claims 39 to 49 wherein the cleaning means includes a first reservoir for a cleaning liquid and at least one outlet from the first reservoir for directing the cleaning liquid onto the surface of the electrodes.

51. A corona discharge reactor according to claim 50 which includes a plurality of parallel extending corona discharge electrodes.

52. A corona discharge reactor according to claim 51 wherein the electrodes are disposed vertically below the first reservoir.

53. A corona discharge reactor according to claim 52 wherein a second reservoir is disposed below the electrodes.

54. A corona discharge reactor according to claim 53 wherein the cleaning means includes a pump for pumping the cleaning liquid from the second reservoir to the first reservoir.

55. A corona discharge reactor according to claim 54 wherein the first reservoir provides a mounting plate for receiving the upper ends of the inner electrodes.

56. A corona discharge reactor according to claim 55 wherein the outlet for the cleaning liquid is defined in the mounting plate towards the upper end of each electrode for continuously dispensing cleaning liquid onto the outer surface of the central electrode and the inner surface of the outer electrode.

57. A corona discharge reactor according to claim 56 wherein the mounting plate comprises an insulating material.

58. A corona discharge reactor according to claim 56 or claim 57 which includes a housing, for housing the corona discharge electrodes, the housing defining an inlet on one side and an outlet towards an opposite side, and the passage extending transverse the longitudinal axis of the reactors from the inlet to the outlet.

59. A corona discharge reactor according to claim 58 wherein the respective sizes of the outlet openings in the mounting plate decrease in size from the inlet to the outlet of the housing.

60. A corona discharge reactor according to any one of claims 51 to 59 wherein the electrical contact of each central electrode comprises a connector stub disposed on one end of the electrode.

61. A corona discharge reactor according to claim 60 wherein the connector stubs of the central electrodes are connected to each other by means of a connector plate.

62. A corona discharge reactor according to claim 61 wherein the connector plate is connected to a high voltage alternating current supply by a high voltage connector.

63. A corona discharge reactor according to claim 53 to 62 wherein the second reservoir is of a conductive material, with the outer electrodes being connected to the second reservoir.

64. A corona discharge reactor according to claim 50 to 63 wherein the cleaning liquid comprises mineral oil.

65. A corona discharge reactor substantially as herein described and as illustrated in the accompanying drawings.

66. A corona discharge electrode assembly for a corona discharge reactor comprising an elongate central electrode; and an outer electrode which extends helically about the central electrode.

67. A corona discharge electrode assembly according to claim 66 which defines a passage for a polluted gas stream, the passage extending transverse the longitudinal axis of the central electrode.

68. A corona discharge electrode assembly according to claim 66 or claim

67 wherein the central electrode comprises a tube of a material having a relatively high dielectric strength at a temperature in the range between 20 to 200 °C.

69. A corona discharge electrode assembly according to claim 68 wherein the tube is a quartz tube.

70. A corona discharge electrode assembly according to claim 69 wherein the tube is filled with a metal powder.

71. A corona discharge electrode assembly according to claim 70 wherein the tube is filled with aluminium powder.

72. A corona discharge electrode assembly according to any one of claims 64 to 69 wherein the electrodes are insulated from each other, with each electrode being provided with an electrical contact for connecting to a high voltage alternating current supply.

73. A corona discharge electrode assembly substantially as herein described and as illustrated in the accompanying drawings.

74. A method of treating a polluted gas stream including the steps of providing a corona discharge reactor defining a passage for the polluted gas stream, the reactor including an electrode disposed in the passage; generating a corona discharge in the passage with the electrode; passing the polluted gas stream through the passage; and removing pollutants from the surface of the electrodes.

75. A method of treating a polluted gas stream according to claim 74 wherein the step of removing pollutants from the surface of the electrodes includes the further step of passing a cleaning liquid over the electrode.

76. A method of treating a polluted gas stream according to claim 75 wherein the liquid is passed continuously over the electrode.

77. A method of treating a polluted gas stream according to any one of claims 74 to 76 wherein the step of generating a corona discharge in the passage with the electrode includes the further step of applying a first alternating current having a first frequency to the electrode, and superimposing a second alternating current having a relatively higher frequency on the first current.

78. A method of treating a polluted gas stream according to any one of claims 74 to 77 which includes the further step of increasing the residence time of the polluted gas stream in the corona discharge, by disposing obstacles in the pathway of the gas stream.

79. A method of treating a polluted gas stream substantially as herein described with reference to the accompanying drawings.

80. A method of generating a corona discharge, including the steps of generating an oscillating electric field by applying a first alternating current having a first frequency to an electrode; and superimposing a second alternating current having a relatively higher frequency on the first current.

81. A method of generating a corona discharge substantially as herein described with reference to the accompanying drawings.

82. A method for extracting pollutants from a food preparation zone including the steps of establishing an air blade on one border region the zone and maintaining the air blade across the zone to collect the pollutants in the air blade; collecting an air stream carrying the pollutants on an opposite border region of the zone; and passing the air stream through a corona discharge filtration unit to remove the pollutants from the air stream.

83. A method according to claim 82 which include the further step of circulating the air from the filtration unit back to the zone.

84. A method for extracting pollutants from a food preparation zone substantially as herein described with reference to the accompanying drawings.

85. A filtering device for removing pollutants from a food preparation zone, the filtering device comprising air directing means disposed on one border region of the zone for directing an air stream across the zone to collect pollutants generated in the zone; an air receiving inlet disposed on an opposite border region of the zone for receiving the collected pollutants and for conveying the pollutants in the air stream along a passage; and a corona discharge filtration unit disposed in the passage, for removing pollutants from the air stream.