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Title:
ELECTROSTATIC PRECIPITATOR
Document Type and Number:
WIPO Patent Application WO/1997/043044
Kind Code:
A1
Abstract:
The present invention relates to electrostatic precipitators which are used to remove particulate matter from gas flows, particularly from flue gas produced by electricity power generating stations. The present precipitators are bulky and relatively inefficient. The present invention proposes a precipitator which utilises the effects of a corona wind produced by precipitator conductors to assist flow of the gas towards collector plates for precipitation thereon. It also proposes providing regions within the precipitator where no corona wind or relatively little corona wind is produced, so that particulate matter remains in these regions for a relatively long time and is more likely to be precipitated.

Inventors:
LOWKE JOHN (AU)
MORROW RICHARD (AU)
Application Number:
PCT/AU1997/000298
Publication Date:
November 20, 1997
Filing Date:
May 15, 1997
Export Citation:
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Assignee:
COMMW SCIENT IND RES ORG (AU)
LOWKE JOHN (AU)
MORROW RICHARD (AU)
International Classes:
B03C3/36; (IPC1-7): B03C3/04; B03C3/45
Domestic Patent References:
WO1988004851A11988-06-30
WO1992005875A11992-04-16
WO1996011060A11996-04-18
Foreign References:
US4689056A1987-08-25
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Claims:
- - CLAIMS :
1. An electrostatic precipitator, comprising means for creating an electric field to apply an electric charge to particles in a gas, and collection means for collecting the particles, the precipitator being physically and/or electrically adapted to utilise a corona wind, created by interaction between the electric field and particles m the gas, to assist m collection of the particles.
2. An electrostatic precipitator m accordance with claim 1, wherein the means for creating an electric field includes a plurality of conductors, each configured to produce a corona and arranged physically with respect to each other such that the corona wind produced by the interaction of the corona and particles m operation assists m facilitating collection of the particles by the collection means.
3. An electrostatic precipitator in accordance with claim 2, wherein the collector means are arranged between corona producing conductors substantially m the path of the corona wind, or adjacent the path of the corona wind, or substantially in the path of a wind, or adjacent the path of a wind, which is produced as a vector result of an applied gas flow and the corona wind, so that the wind assists m pushing gas particles towards or over the collection means.
4. An electrostatic precipitator in accordance with claims 2 or 3, wherein the collection means includes a plurality of collector members arranged adjacent each other m a group of collector members, the collector members within the group having gaps between them to allow gas flow over the collector members in response to a corona wind or the resultant vector flow of corona wind and an applied gas flow.
5. An electrostatic precipitator m accordance with claim 4, each collector member being m the form of a collector plate having a substantially curved or arcuate profile, the arrangement being such that the gas flow is over a curved surface of each collector member.
6. An electrostatic precipitator m accordance with claim 5, wherein adjacent plates curve in opposite directions.
7. An electrostatic precipitator in accordance with any one of claims 4, 5 or 6, comprising a plurality of groups of collector members arranged opposite each other, opposing pairs of collector members forming a plurality of relatively still regions between the collector members in a pair, which are relatively unaffected by the gas flow applied by the corona wind or by the vector result of the corona wind and an applied gas flow.
8. An electrostatic precipitator in accordance with claim 7, further conductors extending within the relatively still region between each pair of collector members, the further conductors being configured to enhance precipitation without causing substantial corona.
9. An electrostatic precipitator in accordance with claim 7 or claim 8, wherein the relatively still region provides a region where gas particles are likely to remain for relatively long periods, whereby to facilitate precipitation onto the collector members.
10. An electrostatic precipitator m accordance with any one of claims 1 to 5, the arrangement being such that regions are produced, where there is minimal or no corona wind, these relatively still regions resulting in particles being m the regions for a relatively long time period, whereby to increase the likelihood of precipitation.
11. An electrostatic precipitator in accordance with claim 10, further electrodes being mounted proximate or within the regions of relatively still gas, the further electrodes being arranged to apply an electric field without producing, or only producing minimal, corona. .
12. An electrostatic precipitator m accordance with claims 8 or 9, the further conductors being flat and platelike, with substantially curved edges.
13. An electrostatic precipitator in accordance with any one of claims 5 to 9 and 12, the edges of the collector plates being curved or rolled over m order to reduce the possibility of voltage discharge between the collector plates and the conductors.
14. An electrostatic precipitator in accordance with claim 4, wherein the collection means comprises a plurality of angled louvers.
15. An electrostatic precipitator in accordance with claim 14, wherein groups of louvers in the collection means extend in different directions to take into account the resultant gas flow which results from an applied gas flow and the corona wind.
16. An electrostatic precipitator m accordance with any one of claims 1 to 14, being arranged to operate in an environment where there is an applied gas flow, the physical and/or electrical arrangement of the precipitator being such as to take into account the vector result of the applied gas flow and the corona wind.
17. An electrostatic precipitator m accordance with any preceding claim, being arranged to operate m flue gas flow from an electricity generating station.
18. A method of precipitating particles from a gas flow, comprising the steps of applying an electric field to the gas to create a corona wind which assists in moving particles towards a collection means for precipitation and collection.
19. A method m accordance with claim 18, applied m flue gas flow in an electrical power generating station.
20. A method m accordance with claims 18 or 19, comprising the further step of creating relatively still regions of minimal or no corona wind, whereby to facilitate precipitation on the collector means. .
21. A method m accordance with claim 20, comprising a further step of applying an electric field withm or proximate the regions of minimal or no corona wind, without the electric field producing a corona wind or only producing a minimal corona wind.
22. A method of constructing an electrostatic precipitator for removing dust particles from a gas stream, the precipitator comprising means for creating an electric field to apply an electric charge to particles in the gas stream, and collection means for collecting the particles, the method comprising the steps of physically and/or electrically adapting the means for creating the electric field and the collection means to utilise a corona wind, created by interaction between the electric field and particles m the gas stream, to assist m collection of the particles.
23. A method of constructing an arrangement for removing dust particles from a gas by precipitation on a collection member, comprising the step of arranging electrodes on either side of the collection means to take into account the effect of a corona wind created by an electric field produced by the electrodes when charged, whereby the corona wind will assist in the precipitation of the gas particles onto the collection member.
24. A method in accordance with claim 23, a collection means comprising a plurality of adjacent collection members having gaps between them to enable gas flow.
25. A method in accordance with any one of claims 22, 23, or 24, comprising a further step of vector analysing the flow pattern of gas through the precipitator from the components of gas flow and corona wind flow, and arranging the collection means taking into account the analysis, whereby dust particle deposit on the collection means is facilitated.
26. An electrostatic precipitator, comprising first and second groups of corona creating conductors arranged, respectively, on opposite sides of a collection means comprising a plurality of collection members separated by gaps to enable gas flow, the first group of corona creating conductors being staggered with respect to the second group of corona creating conductors.
27. An electrostatic precipitator, comprising means for creating an electric field to apply an electric charge to particles m a gas, and collection means for collecting the particles, the means for creating an electric field being adapted to minimise the production of corona wind so that at least regions of no corona wind or little corona wind are created in the precipitator to facilitate precipitation on the collection means.
28. A method of precipitating particles from a gas, comprising the steps of applying an electric field to the gas and creating at least regions of relatively still gas flow where little or minimal corona wind is produced by the electric field, to assist in precipitating particles on a collection means.
Description:
- - "ELECTROSTATIC PRECIPITATOR" The present invention relates to electrostatic precipitators, and, particularly, but not exclusively, to electrostatic precipitator of the type which may be used to remove dust particles from flue gas.

Large electrostatic precipitator arrangements are used m many power plants to clean dust particles from exhaust gases, to comply with environmental regulations. In one common form of precipitator, comprising groups of parallel metallic wires separated by collector plates, a high voltage, usually greater than 10 kV, is applied to the wires and the collector plates are held at a much lower potential, usually earth. The high voltage causes electric corona activity surrounding the wires and this in turn causes dust particles in the flue gas passed through the precipitator to become electrically charged. The direction of flow of the flue gas is usually parallel to the collector plates and also perpendicular to the wires. When the flue gas particles become charged they are attracted to the collector plates. On strikmq the electrically conducting collector plates, the particles become discharged and reside on the plates. To remove the particles the plates are periodically "rapped", so that they are dislodged and fall to a collecting hopper below the precipitator, from where they can later be removed.

Such precipitators are extremely bulky. A number of different configurations for eJectrical precipitators have been proposed, based on the principle of charging dust particles so that they are attracted to a conductive plate, but all have disadvantages.

Alternative methods for removing dust particles from flue gas have also been proposed. One proposal utilises "bags" which act to filter the flue gas flow. The cost of such an arrangement is high. The present applicants have made a study of the phenomenon known as "corona wind", m relation to

- - electrostatic precipitators. This "wind" is produced by charged negative ions from an electric corona surrounding a conductor hitting neutral gas molecules around the conductor and causing their movement and convective flow. For a discussion of corona wind, see Loeb, LB, "Electric Corona", University California Press, Berkeley (1965) . This corona wind effect exists in electrostatic precipitators and the applicant's propose, m the present invention, that by appropriately arranging the electrostatic precipitator the corona wind may be used to assist collection of dust particles. No prior art electrostatic precipitator arrangements have taken the effect of corona wind into account or considered arranging a precipitator to take it into account. The present invention provides an electrostatic precipitator, comprising means for creating an electric field to apply an electric charge to particles in a gas, and collection means for collecting the particles, the precipitator being physically and/or electrically adapted to utilise a corona wind, created by interaction between the electric field and particles in the gas, to assist in collection of the particles.

In a preferred embodiment, the arrangement of the precipitator is such as to take into account the vector result of the corona wind flow and a gas stream flow such that the resultant wind flow assists m directing the dust particles towards the collection means. The gas stream flow may be the applied gas flow m a flue gas such as flue gas in an electrical power generating station.

The means for creating an electric field preferably includes a plurality of conductors, each of the conductors configured to produce a corona and arranged physically with respect to each other such that a corona wind produced by the interaction of the corona and gas particles assists in facilitating collection of the

- - particles by the collection means. The conductors are preferably elongate wires shaped to produce a corona (i.e., they may be circular in cross section) .

The arrangement is preferably such that the gas flow created by the corona wind and any applied gas flow directs particles over the collection means in order to facilitate collection.

In one embodiment, the electric field is produced by groups of a plurality of parallel extending wires extending on either side of a louvred collector plate. To optimise the effect of the corona wind m directing dust particles towards the louvred collector plate, adjacent groups of wires are staggered with respect to each other so that the electric field creates a corona wind which will be directed through the collector plate towards the opposite group of electrodes. The louvred collector plate, comprising a plurality of angled louvres spaced with respect to each other, allows gas flow through the collector plate. In another embodiment, as well as utilising the corona wind to assist directing particles towards the collection means, areas of relative lack of flow of gas, or "relatively still regions" are formed m the precipitator and precipitation of particles on the collection means in these relatively still areas is facilitated. Preferably, further conductors are provided and mounted within or proximate the relatively still areas, the further conductors being arranged to produce an electric field to assist m precipitation but not to produce a corona, so as to avoid or minimise the production of any corona wind within the relatively still areas, or any corona wind which interferes with the corona wind produced by the first conductors.

In a preferred embodiment, the collection means includes a plurality of groups of collection members and there are provided a plurality of groups of conductors,

- - the respective groups of collection members and conductors being interleaved so that each group of collection members has a plurality of conductors on either side of it and vice versa. The collection members are preferably in the form of plates having a curved or arcuate profile and the conductors are arranged such that the vector result of the applied gas flow and the corona wind directs gas over the outer, or larger, curved surface of each of the collector plates. Adjacent plates preferably curve m opposite directions to each other to facilitate flow over the outside of the plates. At least some pairs of plates m adjacent groups oppose each other and are curved towards each other. In the region between these plate pairs, a "stagnant space" of relatively still gas is created. Preferably, this stagnant space actually facilitates precipitation, as the gas particles remain within the space and are subject to an applied charge, for a relatively long period of time. Further conductors are preferably mounted within this stagnant space, the further conductors being arranged not to produce a corona or at least produce minimal corona, but are charged, so as to produce regions of no corona but high electric field facilitate precipitation without producing a corona wind

(or at least minimising any corona wind) to interfere with the gas flow.

Any arrangement which takes into account the corona wind such that it assists m dust collection is within the scope of the present invention, not just the preferred structure discussed in the preceding paragraph and which will be described m the following specific description.

The present invention further provides a method of precipitating particles from a gas flow, comprising the steps of applying an electric field to the gas to create a corona wind which assists m moving particles towards a collection means for precipitation and collection.

- - The present invention yet further provides a method of constructing an electrostatic precipitator for removing dust particles from a gas, the precipitator comprising means for creating an electric field to apply an electric charge to particles m the gas, and collection means for collecting the particles, the method comprising the steps of physically and/or electrically adapting the means for creating the electric field and the collection means to utilise a corona wind, created by interaction between the electric field and particles in the gas, to assist m collection of the particles.

The present invention yet further provides a method of constructing an arrangement for removing dust particles from a gas by precipitation on a collection means, comprising the step of arranging electrodes on either side of the collection means to take into account the effect of a corona wind created by an electric field produced by the electrodes when charged, whereby the corona wind will assist in the precipitation of the gas particles onto the collection member.

The present invention yet further provides an electrostatic precipitator, comprising first and second groups of corona creating conductors arranged, respectively, on opposite sides of a collection means comprising a plurality of collection members separated by gaps to enable gas flow, the first group of corona creating conductors being staggered with respect to the second group of corona creating conductors.

In the aspects and embodiments of the invention described above, corona wind has been harnessed to assist particle precipitation. In one embodiment as discussed above, regions of no or relatively little corona wind (the relatively still areas) are produced to further facilitate precipitation, by including conductors which produce little or no corona but high electric field in the

- - relatively still areas. In some circumstances, the corona wind can interfere with collection of particles.

From yet a further aspect the present invention provides an electrostatic precipitator, comprising means for creating an electric field to apply an electric charge to particles m a gas, and collection means for collecting the particles, the means for creating an electric field being adapted to minimise the production of corona wind so that at least regions of little or no corona wind are created m the precipitator to facilitate precipitation on the collection means.

The electrical conductors may be substantially flat plates with curved edges, to minimise the production of corona. Preferably, the collection means comprises opposing collector members mounted either side of a conductor, and an electric field is applied to the conductor to enhance precipitation.

In a preferred embodiment, the electrostatic precipitator also utilises a corona wind, so that there are regions where corona wind flow is utilised to enhance precipitation, and relatively still regions of little or no corona wind where gas movement is relatively small and which also result m increased precipitation, as discussed above m relation to the preferred embodiment of the first aspect of the invention.

It is an option, however, to have an electrostatic precipitator which does not utilise the corona wind effect, but does utilise conductors which do not produce or produce at least only minimal corona wind, to enhance precipitation.

From yet a further aspect the present invention provides a method of precipitating particles from a gas flow, comprising the steps of applying an electric field to the gas and creating at least regions of relatively still gas where little or minimal corona wind is produceα,

- - to assist in precipitating particles on a collection means for collection.

The method may also include the step of applying an electric field to other regions such that a corona wind is produced m those other regions and that this corona wind also assists in the precipitation of the particles on the collections means.

Features and advantages of the present invention will become apparent from the following description of an embodiment thereof, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram illustrating the application of an electrostatic precipitator in cleaning flue gasses m a electrical power generating station; Figure 2 is a schematic cross-sectional diagram illustrating corona wind in a conventional electrostatic precipitator;

Figure 2A is a schematic perspective view of a conventional electrostatic precipitator; Figure 3 is a schematic cross-sectional diagram of an electrostatic precipitator m accordance with a first embodiment of the present invention, illustrating corona wind flow m the precipitator;

Figure 4 is a schematic cross-sectional diagram of an electrostatic precipitator in accordance with a further embodiment of the present invention;

Figure 5 is a schematic cross-sectionaJ diagram of an electrostatic precipitator in accordance with yet a further embodiment of the present invention, illustrating an actual flow of flue gas through a collection member;

Figure 6 is a schematic cross sectional diagram of yet a further embodiment of an electrostatic precipitator m accordance with the present invention, and

Figure 7 is a plot of a measure of residual particle density against a measure of applied voltage for a precipitator arrangement in accordance with the figure 6

- - embodiment of the present invention, compared with a conventional precipitator arrangement.

A major application of electrostatic precipitators is to remove particulate matter from flue gases produced from the burning of fuel for powering electrical power generating stations. Such particulate matter is a major pollutant. The majority of world governments have legislated to force generating companies to make provision for the removal of particulate matter from the exhaust gasses. The provision of arrangements to remove particulate matter is therefore one of the major considerations of an electrical power generator and the conventional electrostatic precipitators which are generally used to remove particulate matter are extremely bulky and not terribly efficient. The cost of installing and maintaining conventional, very bulky precipitators is very high.

Figure 1 is a schematic, illustrating the position and operation of a precipitator m a coal fired electrical power generating station. It will be appreciated that a precipitator may be employed in other types of power station, such as combined cycle power stations, for example.

The power stations fuel, in this case coal 50, is burned to heat a boiler 51 to produce steam 52 to power an electrical generator 53 to produce electrical power 54. The gases which are produced by burning of the coal 50 contain large amounts of particulate matter which must be removed before the gases are exhausted to the environment, m order to minimise pollution. The flue gases 55 are lead into an electrostatic precipitator 56 which operates to precipitate out particulate matter from the gases. Clean gas 57 is then exhausted via the cooling tower or stack 58.

The particulate matter is precipitated onto collection means 59, which may include a plurality of electrically conductive plates. These plates 59 are periodically rapped so that the particulate matter falls into a dust collector 60 positioned below the plates 59.

- - An electrostatic precipitator m accordance with embodiments of the present invention, to be described in the following, may be employed as the precipitator 56. The precipitator of the present invention preferably results in increased efficiency, which enables the bulk of the precipitator to be reduced over the conventional precipitators, and increases the efficiency of particle collection. It is expected that the precipitators of the present invention will be cheaper and easier to maintain. Conventional electrostatic precipitators, as discussed m the preamble, consist of spaced groups 1, 2 of electrically conducting wires 3, groups 1, 2 being spaced either side of a conductive collector plate 4. In Figure 2 the wires and collector plate are shown m section, from the side. Figure 2A illustrates the arrangement m perspective view from above and one side. A precipitator will usually be made up of a plurality of groups 1, 2 of conductors, and collector plates 4 placed in an exhaust flue to clean exhaust gases from a power plant or the like. Figure 2A illustrates two plates 4 and three groups 1, 2 of conductors. In a conventional installation, there will be many more plates and groups of conductors in a stack, but only a small number are shown m the figures, for clarity. The collector plates 4 are usually arranged so that they extend vertically in a flue (as illustrated m Figure 1), although they are shown extending horizontally in the drawings. The electrical connections to the conductors 3 are not shown, for clarity. The precipitator is usually arranged so that the direction of flow of the flue gas is such as to be parallel to the collector plates 4 and also perpendicular to the wires 3, as indicated by arrow A m Figures 2 and 2A. A typical flow velocity is in the order of 1-2 metres per second. A large negative voltage is applied to each of the wires, m the order of 50 kV, and the collector plates

- -

4 are set at earth potential. As explained in the preamble, the electrical corona surrounding the highly charged wires 3 causes particles in the flue gas to become charged and attracted to the grounded collector plate 4. When the particles reach the collector plate 4 they are discharged and stick to it. Plates 4 are periodically rapped to dislodge the particles which are collected m a hopper below the precipitator.

With such conventional precipitators, no account whatsoever is taken of the existence of a corona wind, m the structure or operation of the precipitators.

A corona wind is produced by movement of charged particles forcing movement of uncharged particles m a convection effect. The arrows B in figure 1 show the flow direction which would be imposed on particles as a corona wind if no force were applied by the flow of flue gas. Obviously, the applied flow of the flue gas together with the force of the corona wind will result m a vector resultant of actual flow of the flue gas. It can be seen from the experimentally observed flow pattern B which would be applied by the corona wind if there was no applied flue gas flow A, however, that the flows B of opposing groups 1 and 2 of wires are directly opposite to each other. Further, because the collector plate 4 is integrally formed, there is no corona wind flow between groups of conductors 3 through the collector plate. In conventional arrangements, therefore, no account is taken of the effect of the corona wind, as can be seen. The opposing flows provided by the corona wind of respective groups 1, 2 of conductors could result m cancelling out any assistance that the corona wind might have given to precipitation of particulate matter.

It has been experimentally observed that the corona wind velocities are in the order of 2 metres per second, which is the same order of magnitude as the

- - imposed flow velocity of the flue gas flow. This is significant.

Figure 3 illustrates, in cross-section, an arrangement for a precipitator in accordance with an embodiment of the invention. The arrangement includes a means for producing an electrical field, m this case electrical conductors 6 and a collection means, m this case a louvered collection plate 7. The arrangement is adapted to take into account the effect of a corona wind which is produced by the wires 8,9, the impetus direction of which is indicated by arrows C, such that the corona wind will assist m depositing particles on the collector plate 7.

As with conventional precipitators, the electric wires 6 are arranged in groups 8, 9 on opposite sides of a collector plate 7. Unlike conventional precipitators, however, opposite wires 6 in the group 8, 9, are staggered with respect to each other, so that, for example, wire 6A is not directly opposite wire 6B, but is staggered relative thereto. Further, because the collector plate 7 is louvered, it will allow gas flow therethrough. The combined effect of this arrangement is to give rise to a corona wind flow impetus direction as indicated by C. The flow pattern will cause gas to flow towards and through the louvered collector plate, as indicated by the arrows.

The actual flow of gas will not be as indicated by arrow C but will in fact be a vector result of the applied flue gas flow D and the corona wind impetus C. The vector flow direction is m fact indicated by arrows E and r in the drawing and causes flue gas to pass over the louver plates 7. This has two major advantages.

1. The magnitude of the convective flow towards or through the plates will be assisted because it is possible for gas to flow through the space between the louvers rather than being obstructed by a flat plate, and

- - account is taken of the corona wind which is harnessed to assist the flow of gas towards the louver plates.

2. The random motion of the charged particles as they pass through the louvers is such that they are likely to have a much greater chance of striking the surface, discharging and sticking to the surface, than in the conventional configuration.

As with conventional precipitators, the precipitator of this embodiment will in practice comprise a plurality of stacked louvered collector plates and groups of conductors.

In at least preferred embodiments, the precipitator arrangement described above leads to much greater collection efficiency. This means that smaller, cheaper precipitators can be used.

Figure 4 shows a further embodiment of the invention wherein the collector plate has an alternative louvered configuration to the configuration of figure 3. The same reference numerals are used in figure 4 for components corresponding to the components in figure 3. The difference between the embodiment of figure 4 and the embodiment of figure 3 is the arrangement of the louvers 7. The louvers are divided into alternating groups 20 and 21, which lean m opposite directions to each other. Each group of louvers 20 and 21 is separated by a horizontally extending louver 22. The louver arrangement takes into account the combined vector effect of the flue gas flow D and corona wind flow, which results m a flue gas flow through the louvered collector member 7 as indicated by arrows E and F. It can be seen that the louvered plates 20, 21 are essentially parallel to the resultant direction of flue gas flow, thereby presenting the largest possible surface area to gather charged dust particles.

In designing the dimensions and the angles of the precipitator for any particular arrangement, it will be appreciated that it is necessary to take into account

- - the vector resultant of the flue gas flow and corona wind. It will be appreciated that this may be done by engineers during construction of the precipitator.

A further arrangement is shown in figure 5. Again the same reference numerals are used for corresponding elements. Collector plate 7 again comprises a louvered plate made up of a plurality of groups of oppositely extending louvers, 20A and 21A. In this arrangement the louvers are not separated by horizontally extending louvers. The actual direction of flue gas flow due to the effective contributions of the corona wind and originally imposed flue gas flow D is illustrated by arrows G. Again, maximum surface area is presented to gather charged particles. High voltages are used m electrostatic precipitators, as discussed above. One possible problem with the arrangements of figures 3, 4 and 5, is that the sharp edges of the louvers could possibly provide a point for an electric discharge between a conductor and a louver. This is undesirable and would result in a decrease in efficiency. To some extent this problem could be addressed by rounding the sides of the louvers so as not to present points for discharge. Figure 6, however, shows a schematic cross-sectional diagram of an electrostatic precipitator which is designed to take into account this possible discharge problem and also, the applicants believe, results m a further improvement to the efficiency of electrostatic precipitators.

The arrangement comprises a plurality of conductors 100 which are arranged to produce a corona. These conductors 100 are m the form of elongate wires, as with the conductors of the previously described embodiments. Also as with the previously described embodiments, the conductors are arranged m groups 101, 102, 103, 104, with respective conductors m adjacent groups being staggered with respect to one another.

- -

The collection means is formed by a plurality of collection members 105 which are also formed into groups 106, 107, 108, 109, interleaved between the respective groups of conductors 100. Each collection member 105 is in the form of an elongate conducting plate having an arcuate or curved profile, as is best seen in the cross section of figure 6. As with the collector plates of the previous embodiments, each group 106, 107, 108, 109 of collector plates 105 extends the length of the conductors 100. Adjacent collector members 105 in a group are separated by gaps 110.

As with the previous embodiments, the staggered conductors 100 produce on a flue gas a corona wind impetus which effects the flow of flue gas D producing the vector result indicated by arrows E. It can be seen that the resultant flow of gas is over the outer curved surfaces of each of the collector members 105. This results in an increase in efficiency and a greater likelihood the precipitation will occur on the plates 105. Note that the arrows E are only illustrated on the lower two groups of collector plates but it will be appreciated that the flow is the same for all of the groups of collector plates.

In addition to the curved profile of the collector plates 105, which increases efriciency and assists m guiding the gas flow, it can be seen from figure 6 that the edges 111 of the plates are curved. This curved surface minimises the chance of breakdown between the wires 100 and the plates 105 and increases the efficiency. This maximises the operating voltage by minimising the possibility of electrical breakdown. The curve also acts to stiffen the plates so that lengths of six to eight meters, for example, can be supported m existing precipitator structures. The rolling of the ends further stiffens the structure. Yet another feature of the arrangement of figure

6 is the production of zones which are relatively free of

- corona wind between opposed pairs of collector plates 105 in adjacent groups 106, 107, 108, 109. These "stagnant zones" are illustrated by reference numerals 112. There may or may not be gas movement in these zones, but relative to the zones of wind flow, there will be relatively less gas movement. This relatively slow gas movement results m particles remaining in these regions and not being re-entramed m the gas flows. There is therefore more chance and likelihood that these particles will precipitate on to the collector plates. It can be seen that these relatively still zones are created between the plates 105 that are curved towards each other. It has been found experimentally that gas enters these relatively still zones 112 and, because the particulate matter remains for a long period in these regions where there is a high electric field (further conductors 113, see later) and relatively little or no corona wind, it is more likely to be effected by the charge on the plates 105 and efficiency of precipitation m the zones is increased. In this embodiment, therefore, a region is created where there is little or no corona wind, but there is still a high electric field provided by the further conductors 113 at high voltage and by the curved collector plates at earth potential, to increase dust collection efficiency. Further conductors 113 are mounted within the relatively still zones 112 between the opposed plates 105. These plates 113 are relatively thick, with rounded edges and shaped to reduce corona. If a significant corona wind were produced by these plates it would not assist precipitation. These plates supply an electric field which enhances precipitation, without causing corona and corona wind. The conductors 113 are charged to a high voltage, in fact to the same voltage, preferably, as the conductors 100, and this charge assists in precipitation on plates 105. In fact, this high voltage plate 113 also collects particles which have become positively charged

- - due to back corona and dust layers. These positively charged particles are more efficiently collected by this system which has plates as well as wires at high negative potential (m all previous designs any positively charged particles had only wires to collect them) .

The curved earthed electrodes 105, the corona wires 100 and the high voltage plate electrodes 113 should all be periodically rapped in the conventional manner to dislodge accumulated particles. The power supply is a conventional negative polarity variable pulse power supply.

It is anticipated that embodiments of the present invention could be incorporated with an existing structure as a retro-fit, and deliver significantly better efficiencies.

Figure 7 shows experimental results of a measure of residual particles against a measure of applied voltage, for an arrangement m which groups of electrodes comprising corona producing electrodes alternate with non- corona producing electrodes, and the collector elements comprise groups of flat plates having gaps between them, i.e. similar to the Figure 6 arrangement, but the collector plates being flat instead of curved. It can be seen that this arrangement is appreciably more efficient than the conventional arrangement.

An alternative configuration of electrostatic precipitator may not utilise a corona wind, but instead be designed to provide regions where there is relatively little or no corona wind, such that gas flow is minimised, and collection efficiency is improved m this way.

Many other configurations are possible and the invention is not limited to the configuration shown in Figures 3 to 6. Many configurations may be arranged to take into account the effect of the corona wind, such that it assists collection of dust particles. Different electrical arrangements may also be used to create fields

- - which cause a corona wind to assist m flow of flue gas towards a collection means.

Variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered m all respects as illustrated and not restrictive.