GB1588773A - Method and apparatus for ionizing a gas - Google Patents

Description

(54) METHOD AND APPARATUS FOR IONIZING A GAS (71) We, AIR POLLUTION SYS TEMS, INC., a corporation organised under the laws of the State of Washington, United States of America, of 114 Andover Part West, Tukwila, Washington 98188, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a method and apparatus for ionizing a gas, using an electrode structure capable of producing a high intensity, radially and circumferentially uniform electrostatic field or fields through which a gas flows for ionizing the gas for charging particles entrained in the gas. This invention is an improvement of that of our Patent No. 1,542,522.

Many industrial processes discharge considerable amounts of atmospheric contaminants as particulates in the sub-micron range. This type of particulate is most difficult to control. Fine particulate emission is becoming a major source of air pollution as the larger particulate problems have been easier to bring under control.

Currently, there are three basic approaches to the problem of handling sub-micron sized particulates in contaminated gases. The first approach is the traditional electrostatic precipitator system. The application of electrostatic precipitators to fine particulate control has several inherent problems.

The second basic type of cleaning system is the wet scrubbing approach. The wet scrubbing approach as applied to the control of fine particulates generally is of the high-energy venturi type. In order to capture the sub-micron particulates in water droplets, large quantitities of water must be injected and high relative velocities em placed, Both of these factors increase the pressure drop of the system, and operating cost is directly related to this pressure drop.

The third basic type is generally referred to as the dry filter system. A problem with equipment of this type, however, is the temperature limitation of the filter elements, the related problem of the high cost of reducing this temperature, and the difficulty in handling certain types of particulates such as "sticky" dusts.

Efforts have been made to improve the efficiency of the various techniques by electrosatically precharging the contaminants upstream of the primary collecting system. These efforts have generally been unsuccessful due primarily to the lack of an effective mechanism to produce a continuous, sufficiently intense field to adequately charge and affect the sub-micron sized particles.

Ionizers for charging particles or ionizing gases have heretofore been of the wirecylinder, wire-plate or needle point type and have been limited to field intensities of about 10 kv/cm3 in the interelectrode region. As a result, the usefulness and effectiveness of such ionizers have been limited.

It is an object of this invention to provide a method and apparatus for efficiently removing sub-micron sized contaminants along with larger particles from contaminated gas such that the gas can be discharged into the atmosphere substantially without accompanying air pollution.

According to this invention a method of ionizing a gas comprises: directing the gas to flow through a tubular outer electrode; placing a generally planar inner discharge electrode concentrically within the outer electrode so as to define an electrode gap therebetween, the inner electrode being isolated from all corona current emitting structure by at least 1.25 electrode gaps; and generating an electrostatic field between the electrodes, the intensity of the field being radially constant throughout a distance from the outer electrode at least to substantially 50% of the electrode gap towards the inner electrode such that the field is substantially uniform throughout such distance and is of generally wedgeshaped configuration diverging outwardly in a direction perpendicular to the gas flow.

Also according to this invention an apparatus for ionizing a gas comprises: a tubular outer electrode to conduct the gas therethrough; a generally planar inner discharge electrode having a perimeter generally corresponding to the cross-sectional shape of the outer electrode, the inner electrode being disposed concentrically within the outer electrode so as to define an electrode gap therebetween, and the inner electrode being isolated from all corona current emitting structure by at least 1.25 electrode gaps; means for causing the gas to flow in a stream axially through the electrode gap; and means for applying a high voltage between the electrodes for creating a corona current producing high intensity electrostatic field within the electrode gap, the intensity of which field is radially constant throughout a distance from the outer electrode at least to substantially 50% of the electrode gap towards the inner electrode such that the field is substantially uniform throughout such distance and is of generally wedge-shaped configuration diverging outwardly in a direction perpendicular to the gas flow.

The invention will now be described by way of example with reference to the drawings, in which: Figure 1 is a longitudinal section of one embodiment of apparatus embodying the invention; Figures IA and 1B are schematic illustrations of contaminated particle paths in a conventional wet scrubber and in apparatus embodying the invention, respectively; Figure 2 is a fragmentary, enlarged section of a portion of the apparatus shown in Figure 1; Figure 3 is a transverse section on the line 3-3 in Figure 2; Figure 4 is a transverse section on the line 4-4 of Figure 2; Figure 5 is a transverse section on line 5-5 of Figure 2; Figure 6 is a fragmentary, diametrical section of the throat of a modified venturi wall; Figure 7 is a diagram of the electrostatic field between the electrodes; Figures 8A-XD show in section various edge radius shapes; Figure 9 is another embodiment; Figure 10 is an isometric view of another embodiment using a plurality of axially spaced discharge electrodes enclosed in a tubular outer electrode; Figure 11 is a cross-section illustrating a single precipitator unit; and Figure 12 is a cross-section of an alternative embodiment of a precipitator unit having discharged electrodes of varying transverse dimensions.

Referring to Figure 1, the gas containing the contaminants is directed through an inlet duct 1 by a blower la to the entrance of a gas contaminant-charging venturi section 2. The gases and contaminants are accelerated to an elevated velocity that will be at a maximum in the venturi throat. The invention however is applicable also to a constant velocity gas stream in which a venturi is not employed. An intense corona discharge is maintained in the venturi throat by a highly stressed planar discharge electrode such as a disc 4 (Figure 2), centered in the venturi throat, to the outer wall 5 of the venturi in a radial direction. The corona discharge is extremely thin (less than one diameter of the outer electrode 5) in the direction of the gas flow and, hence, the resident time of the contaminant particles in the electrostatic field is short. A high level of electrostatic charge is imposed on the particles, however, for several unique reasons.

In accordance with conventional practice, the term "electric field" as used herein shall designate a non-corona field while the term "electrostatic field" as used herein shall designate a corona field.

Although an electrode having the shape of a circular disc is shown and will be described in detail, a toroid, ellipsoid, polygon such as a square or other configuration may also be used as long as the shape of the outer electrode approximately conforms to the shape of the discharge electrode. Similarly, the outer edge of the electrode 4 need not be smoothly curved as viewed in Figure 2. Other designs that can be used include, for example, blunt edges, sharp edges or edges having a plurality of closely spaced projections. It is also possible to use electrodes with serrated edges. The term discharge electrode or inner electrode as used herein is intended to cover all such configurations.

While optimum performance is obtained by centering the discharge electrode 4 concentrically within the venturi throat wall 5 (outer electrode). it will be understood by one skilled in the art that the apparatus will function effectively with off-center positioning as well.

The outer electrode 5 has a radius R( (the radius of the outer electrode as shown in an axial cross section through the outer electrode) which forms the transition between the inlet cone and the throat of the venturi.

However, the outer electrode need not be in the form of a venturi since other structures such as cylindrical or square structures having straight sidewalls, may also be used.

The venturi recovers the energy imparted to the gas stream by accelerating the stream in the venturi throat, and it provides a smooth flow of the gas past the discharge electrode 4 so that a film of cleaning fluid flowing along the wall of the outer electrode 5 as explained hereinafter is not disrupted.

Although the structure of the outer electrode may vary considerably, best results are obtained with a venturi having a radius Ro which has a ratio of above 50:1 relative to the inner electrode edge radius r.

The axial location of the discharge electrode 4 within the venturi throat can be varied within limits. Shifting the location upstream increases the gap R3 to reduce the field intensity and requires higher voltage but reduces the velocity of the contaminated gas stream. Reducing velocity both aids and detracts from ionizing efficiency within limits which will be described.

All of the above variations of the preferred illustrated configuration will degrade the performance to some degree; however, in many operations or uses of the invention it will not be necessary to obtain maximum operating conditions, and more economical construction techniques may suggest the use of one or more variations which acceptably lower ionizing efficiency.

Thus far the invention has been described as an ionizer for use upstream of a contaminant cleaning apparatus, such as a scrubber or precipitator, to substantially increase the efficiency of the cleaning apparatus. The ionizer, however, has other applications as well. For example, it may be used merely to charge particles for electrical power generation, i.e., EGD (electro-gas-dynamic) generation, or ionize gas streams for gas phase reactions, for example, generating atomic oxygen for oxidizing reactions, such as ozone generation for odor removal or sulphur dioxide to sulphur trioxide reactions. In these applications, a gas stream at the velocities described herein is directed past the ionizer in the same manner as a contaminated gas stream but it may be desirable to limit the passage of the gas through the field to a specific radial location. However, surface cleaning of the outer electrode is not necessary if particle deposition does not occur.

The electrostatic field Eo sustained between the discharge electrode 4 and outer venturi throat wall 5 is comprised of two elements, an electric field Ee and a space charge influence, as shown in the chart of Figure 7. The electric field is related to the applied voltage and the electrode geometry.

The space charge influence, comprised of ions, electrons and charged particles in the interelectrode region, is created after corona discharge has been initiated. As shown in Figure 7, the space charge influence tends to amplify the field in the region closer to the outer venturi throat wall and suppresses the highly intense field closer to the electrode.

This effect causes the field to be radially uniform thereby stabilizing the corona discharge while allowing a high electrostatic field to bridge the entire interelectrode region R3. This is accomplished without spark breakdown by electrode design, maintaining a high velocity in the region and a clean surface on the outer electrode 5.

To best understand the uniqueness of the electrostatic field reference is made to electrostatic fields produced by two types of well known prior art electrode configurations. In a wire-cylinder electrode configuration a wire extends along the axis of a cylinder.

The electric field between the cylinder and the wire is entirely radial with no components (neglecting edge effects at the ends of the cylinder) extending along the axis of the cylinder. When a voltage is initially placed between the wire and cylinder a space charge is not present. The intensity of the electric field at any point between the wire and cylinder is inversely proportional to distance from the wire so that the intensity of the field continuously decreases radially outward from the wire toward the cylinder.

As the voltage between the wire and cylinder increases to a corona starting voltage, the electrons in the air adjacent the wire (where the field is the greatest) are accelerated toward the cylinder (assuming a negative voltage on the wire), impinging on gas molecules driving off additional electrons.

Since the molecules have now lost an electron, they become positive ions which, by virtue of the wire's negative potential, accumulate adjacent the wire. The space charge continues to build up by a phenomena commonly known as the "avalanche process". In the avalanche process the high energy electrons accelerated radially outward by the electrostatic field strike additional molecules. The extremely high energy of the electrons allows them to separate electrons from the nucleus of the molecule thereby creating additional free electrons eand additional positive ions. It is to be emphasized that the avalanche process occurs only near the wire since it is in this region that the field is greatest. As the electrons migrate radially toward the cylinder, the deceleration caused by striking molecules exceeds the acceleration caused by the field since the intensity of the field is redcued away from the wire. At points where the field is approximately 30 kv/cm and below, the free electrons, instead of freeing additional electrons by the avalan che process, attach to electronegative gas molecules to form negative ions such as from 2 to O2 when air is the gas between the electrods. Oxygen is the only major electronegative component of air. Thus, for air the only negative ion is the 02. However, other negative ions may be formed for other electronegative gases. Other commonly produced electronegative stack gases include SO2, water vapor and CO2. The 2 ions are accelerated toward the positive potential cylinder where, en route, they form a negative ion space charge in the inter-electrode region. In summary, in the area adjacent the wire where the field is greater than 30 kv/cm, electrons which are accelerated by the field have sufficient kinetic energy so that when they strike molecules a free electron and a positive ion is formed by the avalanche process. Away from the wire, the electrons of reduced energy attach to oxygen molecules forming O2 ions. As positive ions accumulateadja- cent the negative potential wire and O, ions accumulate between the positive space charge and the cylinder, the electric field is modified so that the intensity of the electrostatic field adjacent the wire is reduced while the intensity of the electrostatic field toward the cylinder is increased. The reduced electrostatic field adjacent the wire reduces the quantity of free electrons and positive ions produced by the avalanche process, and the increased electrostatic field adjacent the cylinder increases the migration of 2 ions toward the cylinder. The result is a stabilizing, or negative feedback, effect which maintains the space charge density relatively constant with time. Even though the O, ions increase the intensity of the field adjacent the cylinder, the increase is insufficient to prevent the intensity of the field from continuously decreasing toward the cylinder.

Electrostatic fields produced by the wirecylinder electrode configuration are relatively inefficient since the relatively large area of the cylinder walls results in a large inter-electrode current flow to maintain a given average field intensity.

The second conventional electric field to be examined is that of the wire-plate in which a potential is placed between a wire positioned in parallel between a pair of parallel plates. With this type of electrode configuration. the gas stream passes through the field along a line perpendicular to the wire and parallel to the plates. The electrostatic field generated by a wire-plate electrode configuration produces a space charge in the same manner as the wire-cylinder electrode configuration. However. since the electrostatic field adjacent the plates is more intense directly across from the wire, the space charged formed by the 2 or negative ions from other electronegative gas is more concentrated in this area. This is in contrast to the negative ion distribution of the wire-cylinder electrode configuration where the negative ion concentration is uniformly distributed around the periphery of the cylinder. The space charge amplification of the wire-plate electrostatic field is most intense opposite the wire so that the intensity of the electrostatic field between the wire and plate is greatly increased toward the plate. The high electrostatic field at the plates and high local current deposition causes sparkover unless the average field intensity is maintained at a relatively low value. Therefore, with a wire-plate electrode configuration, it is not possible to achieve relatively large average electrostatic fields. With the wire-plate electrode configuration as with the wire cylinder, the electrostatic field, (neglecting edge effects at the ends of the wire) does not vary along the axis of the wire. In other words, the components of the electrostatic field extend in only two directions of a cartesian coordinate system and result in non-uniform electrostatic field.

In the present electrode configuration the planar discharge electrode 4 concentrically placed within the cylindrical outer electrode 5 creates an electrostatic field having components which extend along the three dimensions of a cartesian coordinate system. The X and Y components of the field (or the R components in a cylindrical coordinate system) are substantially identical to the electric field of the wire-cylinder when viewed along the axis of the cylinder.

Thus, the concentration of negative ions in the inter-electrode gap in a plane transverse to the outer electrode 5 is uniform about the periphery of the discharge electrode 4 resulting in uniform space charge amplification of the electrostatic field. However, the electric field between the discharge electrode 4 and the outer electrode 5, when viewed in a plane axial of the outer electrode 5, is substantially identical to the electric field of the wire-plate electrode configuration. The concentration of negative ions in a plane passing through the axis of the outer electrode 5 is greater in the plane of the discharge electrode 4 than at points axially spaced therefrom.

The intensity of the electric field as a function of the distance from axis of the discharge electrode 4, as illustrated by the broken line Ee in Figure 7, continues to decrease from the electrode 4 toward the outer electrode 5. The electrostatic field including the space charge amplification, as illustrated by the solid line Eo in Figure 7, is substantially constant throughout a substantial distance from the outer electrode 5 toward the discharge electrode 4 so that the electrostatic field is substantially uniform within a generally wedge shaped volume diverging outwardly in a direction perpendicular to the gas flow as illustrated at D in Figure 1. Thus the field has a radial dimension approximately equal to its axial dimension at the outer electrode 5. The uniform space charge distribution occurring in a plane transverse to the discharge electrode 4 has some of the characteristics of a wirecylinder type electrostatic field in which the field continues to decrease as the outer electrode 5 is approached. The non-uniform ion concentration occurring in the axial plane of the discharge electrode 4 has some of the characteristics of a wire-plate type electrostatic field in which the intensity of the field is increased toward the outer electrode 5. The present electrostatic field, by simulating a wire-cylinder electrostatic field in one plane and a wire-plate electrostatic field in an orthogonal plane, combines the continuously decreasing electrostatic field of the wire-cylinder with the continuously increasing electrostatic field of the wire-plate to produce an electrostatic field having an intensity which is substantially uniform in a radial direction throughout a substantial distance from the outer electrode 5 toward the discharge electrode 4 within a generally wedge shaped volume diverging outwardly in a direction perpendicular to the gas flow. Thus the field between the electrodes 4, 5 has a radial dimension equal to the spacing between the electrodes 4, 5 which is approximately equal to the axial field adjacent the outer electrode 5, i.e. the dimension of the field adjacent the outer electrode 5 in a direction perpendicular to a plane passing through the electrodes 4, 5. The uniformity of the electrostatic field allows an intense average field without producing sparkover since there is no point at which the field becomes excessively intense, such as at the plate of a wire-plate system, which limits the average intensity of the field which can be applied without sparkover. The electrostatic field configuration is also less prone to sparkover because it delivers far less current per unit area at the outer electrode at a given field intensity than the wire-plate electrode configuration.

The radial symmetry of the electrode structure produces a field which is also constant along a circumferential path of constant radius.

Both the wire-plate and wire-cylinder electrode structures generate electrostatic field which are elongated systems and contact relatively large areas thus causing a relatively large current flow. In contrast, the present electrode configuration produces current flow in a relatively small area so that an intense electrostatic field is maintained utilizing a minimum of current (and hence, power) without causing sparkover between the electrodes.

Cleaning of the outer electrode surface is necessary only to maintain the surface relatively clean in order to minimize spark breakdown. Where maximum field intensity is not necessary and lower voltages can be applied, or the ionizing occurs in clean gas streams, or during other conditions not producing serious buildup on the surface, cleaning or flushing is, of course, not required. Also, intermittent cleaning may be used.

The inner electrode ^ design introduces large amounts of current (ions) by corona discharge due to the intense field close to the electrode surface. The electrode design also provides a concentrated electric field region all the way to the venturi throat wall 5. This concentrated residual field directs the space charge on this path in its migration to the wall and is responsible for the proper field amplification. The smoothly curved, generally radial periphery of the inner electrode causes the space charge to expand circumferentially in the throat, reducing the current deposition per unit area at the outer electrode to reduce potential spark breakdown.

Since the electrostatic field is relatively thin in the direction of the gas stream, higher velocities of gas flow through the field tends to diffuse the ion concentration axially from the plane of the inner electrode. This adds further stability by expanding the space charge region in the direction of flow to decrease electrostatic field between the space charge region and outer electrode 5. This "velocity enhancement" effect is maximized at gas velocities of 50/fps and above. In addition, turbulence at these high velocities may also provide stability by mechanically disrupting the mechanism which causes spark breakdown.

To maintain the corona and, hence, the performance of the charging unit from contamination and degradation, the discharge electrode 4 is isolated from other leakage paths besides the corona discharge.

As best shown in Figure 2, a probe 10 supports the electrode 4 in its proper location in the outer electrode 5 and provides high resistance to electrical leakage both internally and on its surface. Although not shown, the probe can be moved axially or laterally if desired. The resistance is provided between the electrode and the support structure 12 of the probe in the upstream duct 1. Surface resistance is improved by providing one or more clean air bleeds 14 which are continuous slots (.030") around the circumference of the probe just upstream of the electrode 4. Clean air, provided by an outside supply 15, is fed through the probe body and passes out of these slots at high velocity. This action maintains a positive high-resistance path that the surface leakage would have to "bridge" to short the electrode 4 to ground.

The probe body includes a high-voltage cable 16 supported by dielectric hubs 18 which secure the probe to.the duct 1. The upstream end of the probe body is contained in a closed shroud 20 and a hollow, corrugated cover 22. Openings 23 allow passage of the air axially to a plurality of spaced rings 26, each with corresponding slots 24 (Figure 3). The spacing forms the series of continuous slots 14 for bleeding the air as mentioned above.

Electrode 4 also has slots 24 (Figure 4) which allow air flow downstream of the electrode. The rings and electrode disc are secured to the cable 16 by a bolt 28 fitted in a nose 30. The nose and clean air from the downstream side of the electrode prevent stagnation of charged contaminants downstream of the discharge electrode 4 and prevent deposition of particles on the surface of the electrode 4.

In order to allow the field to expand axially between the discharge electrode and the outer electrode 5, it is important to isolate the discharge electrode 4 from any other structure that generates a corona current-producing electrostatic field by at least 1.25 electrode gaps. In the embodiment illustrated in Figures 1-9, this is accomplished by mounting a single discharge electrode within the outer electrode 5. However, in the embodiment illustrated in Figures 10-12, multiple discharge electrodes are used as explained hereinafter so that this axial spacing requirement becomes more critical than in the embodiment illustrated in Figures 1-9.

The outer electrode 5, because of contaminant buildup, is kept smooth and reasonably clean for a short distance of several times the corona gap R3. This assures that disturbances in the corona from the outer electrode surface, such as contaminant buildup, will be eliminated. This cleaning can be accomplished in several ways; one technique is shown in Figures 1 and 2. Water or a similar fluid is injected by an external pump 32 in a smooth layer on the surface of the converging cone section of the venturi wall 5. Where the outer electrode 5 is a venturi the angles of convergence phi of the venturi is held at about 12.5 half-angle to minimize turbulent flow effects. The venturi in use is pointed in a downward direction and the water film is accelerated as it approaches the throat, both from gravity and friction with the moving gases. The point of water injection is about 1.5 electrode gap R3 lengths line-of-sight upstream from the electrode 4 or approximately 1:12 electrode gap R3 lengths axially upstream from the discharge electrode 4 intersecting the outer electrode. The expansion of the downstream divergent cone of the venturi is less than 3.5 , again to minimize effects from flow separation. The radius Ro that forms the transition between these angles should be no smaller than about 2 inches. Water injection is accomplished by a weir arrangement including a thin (.010-.025") continuous slot 40 formed by a surface 41 on the circumference of the converging cone with a nozzle direction beta of about 12.5 half-angle to the side wall of the venturi. The action of the water on the wall of the venturi maintains a smooth, clean surface without degrading corona performance for velocites of gas flow up to about 75 fps. Water consumption varies with venturi size and ranges from .2 to 2 gpm/1000 acfm for 5" to 50" venturi diameters.

Water is prevented from migrating upstream along the outer electrode 5 by providing an inwardly directed band or deflector 42 insulated from the cooler water.

The water from pump 32 is directed under pressure tangentially into a housing 44 and leaves the housing through slot 40 in an axial direction to minimize spiralling of the water as it passes through the electrostatic field.

The stable, high intensity electrostatic field of the p 4, should be set such that the probe occupies around 10% of the cross-sectional area of the outer electrode 5. A practical minimum is 5% and a practical maximum is 40%. A probe occupying a smaller percentage of the outer electrode 5 causes an increase in discharge electrode surface power density.

More importantly, smaller values also increase the electrode gap for constant flow capacity of the unit, thereby increasing power supply voltage requirements significantly. Values greater than 40% increase size of the outer electrode 5 and probe cost and increase probe isolation air bleed requirements and, hence, operational cost.

In summary, the ratio of the electrode gap R3 to the edge radius r should be between 50:1 and 400:1, and preferably about 100:1, the fill area (the percentage of the outer electrode occupied by the discharge electrode 4) should be between 5% and 40%, and preferably about 10%, the discharge electrode 4 must have a shape corresponding to the shape of the outer electrode 5, and the discharge electrode 4 must be spaced apart from all structures which generate corona current producing electrostatic fields by at least 1.25 electrode gaps. With these electrode geometries, typical highvoltage requirements are such that an average field of about 18-20 kv/cm can be maintained across the electrode gap R3 at standard atmospheric conditions (a pressure of 29.92 inches of mercury and a temperature of 70 degrees fahrenheit) and zero velocity. With gas velocities greater than about 50 fps, the field can be increased to about 24-28 kv/cm without sparking.

Several important functions occur in the intense corona region of the charging unit.

The suspended contaminants are field charged by the strong applied fields and ion impaction in the high ion-dense region R3.

It is presumed that the diffusion charging mechanism has minor contribution here on the fine particles due to the short residence time of the particles in the corona. There will be a slight displacement of the particles outward radially as they become charged and migrate in the strong fields of the corona. The amount of this displacement will vary with the size of the particle so some mixing, impaction and possible agglomeration can occur. This is seen as a minor effect in view of the thermal agitation and flow turbulence present. In the case of liquid aerosols, however, the effects of strong applied fields (greater than 10 kv/cm), high temperatures and turbulent mixing, cause significant agglomeration to occur, and this effect has been witnessed downstream of the corona. This can be of great benefit in the collection of fine aerosols as particles agglomerate and "grow" to larger, more easily collected sizes.

Velocity of the gases through the highly charged corona area affects the charging effciency of the system. Above about 50 fps, the space charge region of the field becomes axially spread by the gases to reduce the possibility of a spark breakdown, that is, greater stability of the corona is achieved.

With the increases in velocity, however, the advantage of increased stability begins to become offset by the disadvantage of the shorter resident time of the contaminants in the field, and thus a reduction in charge on the particles, and increased disruption of the water film on the outer electrode wall if water cleaning is used. Up to about 125 fps, there is gain in stability of the corona, but with a decrease in charging efficiency. For one system tested, the maximum charge on the particulate appears to occur at about 100 fps. To a great extent, however, gas velocity must be a trade-off between the capacity needed for efficient operation of the industrial gases being cleaned, electrode voltage requirements and venturi wall cleaning capability.

A second method of venturi wall cleaning is illustrated in Figure 6. In this embodiment, a perforated or porous air bleed section 70 is provided on the outer electrode 5 adjacent the discharge electrode 4 to provide an air film over the downstream wall of the outer electrode 5 rather than water film. Downstream of the air bleed section 70 for a distance of several electrode gap R3 lengths, the outer electrode 5 wall surface is coated with a material of high electrical resistivity for providing electrical isolation of the particles deposited in this area.

Still another method is the use of gas stream erosion to limit the thickness of the deposition to permissible levels.

Still another method is to vibrate or shock the wall to intermittently or continuously dislodge the contaminants before buildup.

The suspended particulate contaminants having passed through the electrostatic field are highly charged, of like polarity and are migrating to the outer venturi wall 5 downstream of the corona. Deposition on the wall which occurs is minor and represents only those particles travelling near the wall on their original trajectories. Since the applied field in this region is primarily of the space charge element and, therefore, the migration velocities are low in comparison to stream velocities, the bulk of the particles remain in the stream for considerable distances. At least two forms of collection of these highly charged, suspended particulates can be employed.

One technique for collecting the charged particles is a conventional electrostatic precipitator. Another technique is a wet scrubber 50 to be described. The gas contaminant charging section of the outer electrode 5 is directly attached to the throat 52 of the venturi scrubber 50. In general, the design velocity of the outer electrode is consistent with the desired velocity in the scrubber venturi such that the charging section divergent cone angle is set at about 0". The charged particle-laden gases pass through the scrubber venturi with the particles collected onto water drops by impaction and interception enhanced by the electrostatic forces. Water enters the venturi scrubber in a conventional manner as through a continuous slot 54 and is atomized by the gas stream. The water droplets are oppositely charged to the particles by induction because the atomization process occurs in a residual field region. Preferably, at low venturi velocities (below about 75 fps), the injection point should be at least two gaps R3 downstream of the discharge electrode 4 to prevent premature spark breakdown. At higher velocities, greater separation distances are required due to ions drifting downstream of the corona which tend to foul the induction process by undesirably charging the water droplets with the same polarity as the charged particles. By extending bolt 28, the induction charging field is increased axially, even though the separation distance between the electrode 4 and the injection point is increased. This also provides for a cylindrical field emitting from the bolt which drives the ions toward the outer wall 5 downstream of the electrode 4.

The collection efficiency of a conventional venturi scrubber depends upon the inertial impaction of particles on water droplets.

The impaction is accomplished by high relative velocity of the contaminated air stream and water droplets injected at low velocity. The sub-micron sized particles escape impaction by following the slip stream around the water drops instead of impacting. (An example is illustrated schematically in Figure 1A). This is due to their high aerodynamic drag-to-inertia ratio. Particle bounce and rebound also become important considerations in cases of marginal impaction and interception energies. Particles with low impaction energies fail to penetrate the water droplet due to surface tension effects.

Particles containing a high ( 10kv/cm saturation charge) electrostatic charge and with induced charge on the water droplets, as in this invention, have an attractive force between the charged particles and water droplets trajectories, as shown schematically in Figure IB. This effect results in a substantial improvement in collection efficiency over the basic scrubber efficiency.

The impaction improvement effect varies with particle size and the relative velocity between the particles and water droplets.

The sensitivity to particle size is minor with a variation in effect of only + or - 20% when considering 0.1. micron to 10 micron size particles. Since the longer the electrostatic forces have time to act, the more effective they become, lower relative velocities between charged particles and water drops yield a larger improvement effect.

Since lower velocities also yield less effective atomization of the scrubber fluid and larger equipment sizes, an optimum velocity range becomes apparent.

Below about 50 fps relative velocity, atomization in the venturi scrubber degrades rapidly; therefore, liquid requirements increase substantially to maintain efficiency. About 200 fps relative velocity, pressure drop across the system due to water droplet acceleration losses becomes excessive. Therefore, the optimum improvement in collection efficiency of the gas contaminant-charging unit on a venturi scrubber collector occurs with venturi scrubber designs around 125-150 fps in the throat.

One tested embodiment of the invention employed a gap radius R3 of 1 1/2 inches, a discharge electrode edge radius r of 1/64 of an inch, a peripheral radius R, of .875 inches, an outer electrode radius R2 of 2 3/8 inches, a converging cone angle phi of 12.5 , and an outer electrode (axial cross section) radius R( of 3-4 inches. The embodiment had a 750 cfm capacity with gas flow of about 120 fps in the scrubber venturi.

Typical prior art "scrubber only" collection efficiency of this design is approximately 81% at a .5 miron particle size. Collection efficiency is increased to approximately 95% at .5 micron size when the gas contaminant-charging unit of this invention is activated. The system at this condition consumes approximately 7.5 gpm/1000 acfm of water, 150 watts/1000 acfm charging unit power and has 4 inches of water system pressure drop.

A second tested embodiment employs a gap radius R3 of 2.15 inches, a discharge electrode edge radius r of 1/64 of an inch, a peripheral radius Rl of .875 inch, an outer electrode radius R2 of 3.03 inches, a converging cone half angle of 15 , and a venturi radius R( of 2 inches. The embodiment had a 1,000 cfm capacity, with gas flow of about 150 fps in the scrubber venturi. The typical prior art "scrubber only" collection efficiency of this design is approximately 94.6% at a 1.25 micron particle size. Collection efficiency is increased to approximately 97.5% at 1.25 micron size when the gas contaminant-charging unit of this invention is activated. The system at this condition consumes about 6 gpm/100 acfm of water, 150 watts/1000 acfm charging unit power and has 6 inches of water pressure drop.

Typical corona ionizing apparatus in the prior art have generally been limited to operating field intensities of 3-6 kv/cm. With the ionizer of this invention using the optimum electrode design and fluid velocity past the electrodes, operating field intensities up to 15 kv/cm are obtainable without spark breakdown.

Figure 9 illustrates another embodiment having electrode end portions 80a of radial configuration and a central electrode portion 80b of linear configuration. Preferably, the duct 82 is again rectangular but could be curved to match the electrode. Air ports 24 are provided as shown in Figures 3-5. All of the shapes of Figures 8A-8D can, of course, be used for the edge radius r.

Another embodiment illustrated in Figure 10 utilizes several parallel precipitator units 110. A particle-entraining gas enters an inlet manifold 112 through an annular inlet duct 114. The manifold 112 is partially filled with water 116 and a plurality of planar, spaced baffles 118, 120 are positioned in the manifold 112 such that the gas passing through the manifold 112 must flow through the water 116 around the baffles 118, 120. The water 116 is provided to cool the gases entering the inlet duct 114 to promote particles formation by condensation. Additional cooling is provided by a pre-quench spray including a plurality of spray nozzles 122 communicating with a water inlet 124 through pipes 126. The cooling water is recycled by a pump (not shown) with moderate pressure.

After the particule-laden gas has been sufficiently cooled and saturated, it is directed to the portion of the manifold 112 beneath the precipitator units 110. As best illustrated in Figure 11, a sloped, collection surface 128 is placed beneath the precipitator units 110, and an annular aperture 130 having a concentric, cylindrical shield 132 is placed beneath each precipitator unit 110 with the shield 132 extending into the lower end of a tubular outer electrode 134. Each of the precipitator units 110 includes a cylindrical outer electrode 134 enclosing a planar discharge electrode 136 mounted at the lower end of an elongated support electrode 138. As explained in detail hereinafter, the particles in the gas passing through the aperture 130 are charged within the relatively thin, radially and circumferentially uniform electrostatic field extending between the discharge electrode 136 and the outer electrode 134.

The term "relatively thin" designates a field having a radial dimension which is substantially greater than its axial dimension. The charged particles are then accelerated toward the outer electrode 134 by the relatively lower intensity electric field extending between the support electrode 138 and the outer electrode 134. The particles deposited on the inside walls of the outer electrode 134 are collected by a film of water covering the inner walls of the outer electrode 134 and flow downwardly where they are deposited on a collection surface 128. The shield 132 prevents the liquid film from being entrained in the upward gas stream when the liquid falls off the end of the outer electrode 134 onto the collection surface 128. Re-entrained liquid from the liquid film covering the inside walls of the outer electrode 134 can cause electrostatical field instability when gas velocities within the outer electrode 134 exceed 10 feet per second.

The upper end of the outer electrode 134 projects into an exhaust plenum 140 where it is surrounded by a cylindrical weir 142.

Water is continuously supplied through line 143 (Figure 10) by a pump (not shown) so that a thin film of water 144 flows over the weir 142 and down the inside walls of the outer electrode 134. A cover plate 146 extending over the weir water 144 has a cylindrical flange 147 projecting into the outer electrode 134 to allow a smooth air flow from the outer electrode 134 into the exhaust plenum 140 so as not to disturb the weir head water.

The upper ends of the support electrodes 138 extend into a high voltage housing 148 where they are connected to a high voltage bus bar 150 supported on the floor of the housing 148 by insulators 152 and a feedthrough shroud 154. The bus bar 150 is connected to an external high voltage power supply or transformer-rectifier set (not shown) through a high voltage conductor 156 (Figure 10) which passes through the high voltage housing 148 through a feedthrough insulator 160. Access to the electrodes 134, 136, 138 is provided by a plurality of annular access covers 162 on the top surface of the housing 148.

The electrostatic field extending between the discharge electrode 136 and the outer electrode 134 is substantially identical to the electrostatic field produced in the embodiments of Figures 1-9. Consequently, the manner in which the electrodes 136, 134 form a corona current producing electrostatic field, as well as the characteristics of the electrostatic field, are not repeated. The major diameter of the discharge electrode 136 should be between 0.2 and 0.5, preferably about 0.35, of the inside diameter of the outer electrode 134. The diamter of the support electrode 138 is between 0.15 and 0.25, preferably about 0.2, of the inside diameter of the outer electrode 134, and between 0.4 and 0.8, preferably 0.5 to 0.6 of the major diameter of the discharge electrode 136. A larger diameter support electrode 138 can overly suppress the axial expansion of the electrostatic field between the discharge electrodes 136 and outer electrode 134 thereby reducing the stability of the field. A smaller support electrode 138 tends to produce excess corona and sparkover between the support electrode 138 and outer electrode 134.

By securing the discharge electrode 136 directly to the support electrode 138 a single electrode mounting system may be used for both the charging stage of the precipitator as well as the collecting stage of the precipitator. Further, by varying the diameters of the discharge electrode 136 and the support electrode 138 with respect to each other and the outer electrode 134, the intensities of the fields extending between the discharge electrode 136 and the outer electrode 134 and between the support electrode 138 and the outer electrode 134 can be independently selected even though both electrodes 136, 138 are powered by a common transformerrectifier unit. Consequently the cost and complexity of the precipitator compared to conventional precipitators is greatly reduced.

In order to increase the efficiency of the precipitator, particularly where various sized particles are to be removed, multiple electrode staging may be employed as illustrated in Figure 12. A plurality of planar discharge electrodes 180, 182, 184 are secured to a support electrode 186 at axially spaced points greater than 1.25 discharge electrode gaps apart from each other. A spacing between electrodes 180, 182, 184 of greater than 2 discharge electrode gaps is preferable to prevent the fields extending from adjacent discharge electrodes from interfering with each other and to provide an electric field extending from the support electrode 186 having a length sufficient to collect charged particles. Inter-electrode spacings of less than 1.25 discharge electrode gaps cause interference between adjacent fields which prevents axial expansion of the fields thereby reducing the stability of the corona discharges. As the gas passes through the outer electrode 134 the particles entrained in the gas are thus subjected to a plurality of charging fields each of which are followed by a collecting field. If desired, the diameters of the discharge electrodes 180, 182, 184 may be varied so that the average intensities of the fields extending between the discharge electrodes 180, 182, 184 and the outer electrodes 134 are different for each electrode 180, 182, 184. Large particles are generally easier to precipitate and are thus removed with greater ease at less intense applied fields. Consequently where the particles to be collected vary in size it may be desirable progressively to increase the diameter of discharge electrodes along the gas flow so that particles are subjected to increasingly more intense fields as they flow through the outer electrode 134. The largest particles are thus collected on the inner walls of the outer electrode 134 just beyond the smaller diameter discharge electrode 180, smaller particles are collected just beyond the intermediate diameter discharge electrode 182, and the smallest parti are are collected beyond the larger dia- meter discharge electrode 184.

The larger charged particles produce localized field anomalies which tend to produce sparkover in these localized regions. By removing the larger particles near the smaller diameter discharge electrode 182 the intensity of the field between the larger diameter discharge electrode 184 and the outer electrode 134 can be increased thereby removing a greater percentage of smaller particles and increasing the overall collection efficiency of the system.

If desired a low field corona discharge may be produced between the support electrode 138 and the outer electrode 134 by helically wrapping a conductor, such as a wire, around the support electrode 138. The low field corona discharge is particularly useful where the outer electrode is intermittantly cleaned since the corona current holds the particles on the inner wall of the outer electrode until they are removed by any suitable technique such as rapping.

Where the particles are continusouly removed by a water film, for example, there is no necessity to retain the particles on the outer electrode in this manner.

WHAT WE CLAIM IS: 1. A method of ionizing a gas com prising:- directing the gas to flow through a tubular outer electrode; placing a generally planar inner discharge electrode concentrically within the outer electrode so as to define an electrode gap therebetween, the inner electrode being isolated from all corona current emitting structure by at least 1.25 electrode gaps; and generating an electrostatic field between the electrodes, the intensity of the field being radially constant throughout a distance from the outer electrode at least to substantially 50% of the electrode gap towards the inner electrode such that the field is substantially uniform throughout such distance and is of generally wedgeshaped configuration diverging outwardly in a direction perpendicular to the gas flow.

2. A method according to claim 1 wherein the average intensity of the electrostatic field is equivalent to an average intensity in air greater than 12 kv/cm at standard atmospheric pressure and temperature without the occurrence of sparkover.

3. A method according to claim 1 further including the step of adjusting the ratio between the cross-sectional area of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (16)

**WARNING** start of CLMS field may overlap end of DESC **. the discharge electrodes 136 and outer electrode 134 thereby reducing the stability of the field. A smaller support electrode 138 tends to produce excess corona and sparkover between the support electrode 138 and outer electrode 134. By securing the discharge electrode 136 directly to the support electrode 138 a single electrode mounting system may be used for both the charging stage of the precipitator as well as the collecting stage of the precipitator. Further, by varying the diameters of the discharge electrode 136 and the support electrode 138 with respect to each other and the outer electrode 134, the intensities of the fields extending between the discharge electrode 136 and the outer electrode 134 and between the support electrode 138 and the outer electrode 134 can be independently selected even though both electrodes 136, 138 are powered by a common transformerrectifier unit. Consequently the cost and complexity of the precipitator compared to conventional precipitators is greatly reduced. In order to increase the efficiency of the precipitator, particularly where various sized particles are to be removed, multiple electrode staging may be employed as illustrated in Figure 12. A plurality of planar discharge electrodes 180, 182, 184 are secured to a support electrode 186 at axially spaced points greater than 1.25 discharge electrode gaps apart from each other. A spacing between electrodes 180, 182, 184 of greater than 2 discharge electrode gaps is preferable to prevent the fields extending from adjacent discharge electrodes from interfering with each other and to provide an electric field extending from the support electrode 186 having a length sufficient to collect charged particles. Inter-electrode spacings of less than 1.25 discharge electrode gaps cause interference between adjacent fields which prevents axial expansion of the fields thereby reducing the stability of the corona discharges. As the gas passes through the outer electrode 134 the particles entrained in the gas are thus subjected to a plurality of charging fields each of which are followed by a collecting field. If desired, the diameters of the discharge electrodes 180, 182, 184 may be varied so that the average intensities of the fields extending between the discharge electrodes 180, 182, 184 and the outer electrodes 134 are different for each electrode 180, 182, 184. Large particles are generally easier to precipitate and are thus removed with greater ease at less intense applied fields. Consequently where the particles to be collected vary in size it may be desirable progressively to increase the diameter of discharge electrodes along the gas flow so that particles are subjected to increasingly more intense fields as they flow through the outer electrode 134. The largest particles are thus collected on the inner walls of the outer electrode 134 just beyond the smaller diameter discharge electrode 180, smaller particles are collected just beyond the intermediate diameter discharge electrode 182, and the smallest parti are are collected beyond the larger dia- meter discharge electrode 184. The larger charged particles produce localized field anomalies which tend to produce sparkover in these localized regions. By removing the larger particles near the smaller diameter discharge electrode 182 the intensity of the field between the larger diameter discharge electrode 184 and the outer electrode 134 can be increased thereby removing a greater percentage of smaller particles and increasing the overall collection efficiency of the system. If desired a low field corona discharge may be produced between the support electrode 138 and the outer electrode 134 by helically wrapping a conductor, such as a wire, around the support electrode 138. The low field corona discharge is particularly useful where the outer electrode is intermittantly cleaned since the corona current holds the particles on the inner wall of the outer electrode until they are removed by any suitable technique such as rapping. Where the particles are continusouly removed by a water film, for example, there is no necessity to retain the particles on the outer electrode in this manner. WHAT WE CLAIM IS:

1. A method of ionizing a gas com prising:- directing the gas to flow through a tubular outer electrode; placing a generally planar inner discharge electrode concentrically within the outer electrode so as to define an electrode gap therebetween, the inner electrode being isolated from all corona current emitting structure by at least 1.25 electrode gaps; and generating an electrostatic field between the electrodes, the intensity of the field being radially constant throughout a distance from the outer electrode at least to substantially 50% of the electrode gap towards the inner electrode such that the field is substantially uniform throughout such distance and is of generally wedgeshaped configuration diverging outwardly in a direction perpendicular to the gas flow.

2. A method according to claim 1 wherein the average intensity of the electrostatic field is equivalent to an average intensity in air greater than 12 kv/cm at standard atmospheric pressure and temperature without the occurrence of sparkover.

3. A method according to claim 1 further including the step of adjusting the ratio between the cross-sectional area of the

discharge electrode and the cross-sectional area of the outer electrode such that the ratio is between 0.05 and 0.40.

4. A method according to claim 1 further including the step of increasing the voltage in accordance with the velocity of the gas flow such that ions are carried from the field by the gas thereby allowing the average applied intensity of the field to be increased without sparkover as the velocity is increased.

5. A method according to claim 1 further including the steps of supporting the inner electrode on the upstream end of an elongate support electrode and generating a non-corona current producing electric field between the support electrode and the outer electrode such that particles entrained in the gas are charged by the field extending between the inner and outer electrodes, and subsequently migrate toward the outer electrode under the influence of the field extending between the support and outer electrodes.

6. A method according to claim 5 further including the step of mounting a plurality of said planar inner discharge electrodes on the support electrode at least 1.25 electrode gaps apart from each other, such that a plurality of radially and axially expanding, radially and circumferentially uniform electrostatic fields are formed within the outer electrode axially spaced from each other, and a radially expanding electric field is generated downstream of each electrostatic field such that particles entrained in the gas are repetitively charged by the electrostatic fields and subsequently migrate toward the outer electrode under the influence of the electric fields.

7. A method according to claim 6 wherein the transverse dimensions of the planar inner discharge electrodes are progressively increased in the direction of gas flow such that the electrostatic fields are progressively increased in the direction of gas flow whereby larger particles are removed by upstream fields in advance of downstream fields where finer particles are removed thereby increasing the stability of downstream fields such that collection efficiency of the method is increased.

8. A method according to claim 6 wherein the electrostatic fields are spaced apart by at least 1.25 times the radial dimensions of the electrostatic fields thereby preventing them from interfering with each other.

9. An apparatus for ionizing a gas comprising: a tubular outer electrode to conduct the gas therethrough; a generally planar inner discharge electrode having a perimeter generally corresponding to the cross-sectional shape of the outer electrode, the inner electrode being disposed concentrically within the outer electrode so as to define an electrode gap therebetween, and the inner electrode being isolated from all corona current emitting structure by at least 1.25 electrode gaps; means for causing the gas to flow in a stream axially through the electrode gap; and means for applying a high voltage between the electrodes for creating a corona current producing high intensity electrostatic field within the electrode gap, the intensity of which field is radially constant throughout a distance from the outer electrode at least to substantially 50% of the electrode gap towards the inner electrode such that the field is substantially uniform throughout such distance and is of generally wedge-shaped configuration diverging outwardly in a direction perpendicular to the gas flow.

10. The apparatus of claim 9 wherein the inner discharge electrode occupies between 5% and 40% of the transverse area within the outer electrode.

11. The apparatus of claim 9 further including means positioned downstream of the inner discharge electrode for collecting contaminents entrained in the gas and charged by the electrostatic field.

12. The apparatus of claim 11 further including means for cleaning the surface of the outer electrode, including means for injecting a substantially continuous layer of air along the outer electrode to prevent deposition of contaminants thereon.

13. The apparatus of claim 9 wherein the means for applying a high voltage places a voltage between the inner and outer electrodes greater than 12 kv for each centimetre of the electrode gap when air at approximately standard temperature and pressure is within the gap such that the average applied field between the electrodes is greater than 12 kv/cm.

14. The apparatus of claim 9 wherein the inner electrode is mounted on the upstream end of an elongate support electrode concentric with the outer electrode, the support electrode having a diameter between 0.15 and 0.25 of the inside diameter of the outer electrode, and between 0.4 and 0.8 of the diameter of the inner electrode such that when a predetermined potential is applied between the outer electrode and the inner and support electrodes, a radially and axially expanding, radially and circumferentially uniform corona current producing electrostatic field is generated between the inner and outer electrodes, and a radial, non-corona current producing electric field is generated between the support and outer electrodes.

15. The apparatus of claim 9 further including a plurality of generally planar inner discharge electrodes mounted on a common, elongate support electrode concentric with the outer electrode, the inner electrodes being axially spaced by at least 1.25 discharge electrode gaps such that contaminants entrained in the gas passing through the outer electrode are repeatedly charged within a plurality of radially and axially expanding, radially and circumferentially uniform electrostatic fields, and then migrate towards the outer electrode under the influence of a radial electric field downstream of each electrostatic field such that the contaminants are deposited on the inside wall of the outer electrode.

16. The apparatus of claim 15 wherein the inner electrodes are of different diameters such that different applied average fields are maintained between each of the inner electrodes and the outer electrode.