Laminar flow electrostatic precipitation system

Abstract: An electrostatic precipitation system (100) utilizes laminar flow of a particulate-laden gas in order to enhance the removal of sub-micron sized particulates. The system incorporates a housing (105) through which the gas flows to an outlet port (110). The gas, which may be a flue gas enters the laminar flow precipitator (102) through an inlet port (108) for passage through a charging section (104). The flue gas and charged particles then flow to a collecting section (106). The collected particulates are subsequently collected in a hopper (112) or reentrained in the gas stream as agglomerates for subsequent removal by a secondary filter (120), the gas stream then being conveyed to a stack (14) wherein the particulate-free gas can be emitted into the atmosphere.

Description: This invention directs itself to an electrostatic precipitation system wherein 100% particulate removal can practically be achieved. Conventional industrial electrostatic precipitators collect dry particulates in a parallel plate, horizontal flow, negative-polarity, single-stage system design. Collecting plate spacing generally ranges from 9 to 16 inches (approximately 225 to 400 mm), and plate height can be up to 50 feet (approximately 15m). Flow through the precipitator is always well into the turbulent range. Due to the turbulent flow, precipitator collection efficiency is predicted utilizing the Deutsch model, which assumes that the turbulence causes complete mixing of the particles in the turbulent core of the flow gas, and electrical forces are operative only across the laminar boundary layer. This model leads to an exponential equation relating collection efficiency to the product of the electrical migration velocity of the particles and the specific collecting area of the precipitator. The exponential nature of the equation means that increasing of the specific collecting area yields diminishing returns in the efficiency at the high collection efficiency levels. Therefore, the 100% collection efficiency level is approached only asymptotically in the turbulent flow case and cannot in actuality be reached, no matter how large the precipitator. US-A-2422564 discloses an electrical dust-precipitator comprising an upstream dust-charging unit and a downstream dust-precipitating means. A sequence of ring type structures form parts of the precipitator and charging sections. These ring type structures may also act to reinforce the electrical dust-precipitator structure. DE-A-3324803 discloses a gas treatment device in which a single electrode of varying cross-section is placed in a gas flow path. GB-A-913172 discloses a multistage electrostatic filter comprising an ionisation zone and at least two collecting zones. SU-A-1220195 discusses a system for removal of dust from gases using corona and precipitating electrodes with Reynolds numbers of the first zone in the turbulent range and in the second zone defining substantially laminar flow. According to the present invention, there is provided an electrostatic precipitation system for removing sub-micron sized particulates entrained in a flue gas, comprising charging means coupled to a flue for electrostatically charging particulates entrained in a flue gas by imparting a charge of predetermined polarity to said particulates, said charged particulates including sub-micron sized particulates, an agglomerator coupled in fluid communication with said charging means and downstream thereof for flow of the flue gas therethrough, said agglomerator including a plurality of longitudinally extended plate electrodes disposed in substantially parallel spaced relation, each of said plurality of plate electrodes being devoid of corona inducing type structures, said plurality of plate electrodes being configured to provide a substantially laminar flow of said flue gas therethrough, said plurality of plate electrodes being alternately coupled to opposing polarities of a first potential difference, said first potential difference being of sufficient magnitude to attract and agglomerate the particulates but insufficient to prevent agglomerated particulates from being re-entrained into said laminar flow of the flue gas, and collecting means for collecting said agglomerated particulates coupled in fluid communication with said agglomerator and downstream thereof. To achieve laminar flow, the precipitator is divided into a charging section for imparting a charge to the particulates carried in a gas stream, an agglomeration section and a collecting section having an electrode disposed at a potential that is different from that of the charged particles, for attracting the charges particles thereto. One embodiment of the electrostatic precipitation system includes a housing coupled in fluid communication with a flue. A power source is provided having a first output for supplying a reference potential and at least a second output for supplying a potential that is negative with respect to the reference potential. The system includes an assembly for electrostatically charging particulates disposed within the housing and coupled in fluid communication with the flue having flue gas passing therethrough. The charging assembly is coupled to the first and second outputs of the power supply for imparting a charge that is negative with respect to the reference potential to the particulates carried by the flue gas. The system further includes an assembly for collecting the charged particulates disposed within the housing and downstream of the charging assembly. The collecting assembly forms a laminar flow of the flue gas therethrough. Embodiments of the invention are described hereinafter, by way of example only, with reference to the accompanying drawings, in which: FIG. 1 is a system block diagram of one embodiment of the electrostatic precipitation system; FIG. 2 is a system block diagram of a second embodiment of the electrostatic precipitation system; FIG. 3 is a sectional view of the collecting section portion of the electrostatic precipitation system taken along the section line 3-3 of FIG. 1; FIG. 4 is a sectional view of an alternate embodiment of the collecting section shown in FIG. 3; FIG. 5 is a cross-sectional elevation view of the charging and collecting sections showing the electrical connection thereof; FIG. 6 is a cross-sectional elevation view of an integrated charging and collecting section; FIG. 7 is a cross-sectional elevation view of another embodiment of an integrated charging and collecting section of the present invention; FIG. 8 is a cross-sectional elevation view of yet another embodiment of an integrated charging and collecting section of the present invention; FIG. 9 is a system block diagram of another embodiment of the present invention; and, FIG. 10 is a cross-sectional view of a portion of the embodiment shown in FIG. 9. Referring to FIG. 1, there is shown, electrostatic precipitation system 100 coupled in-line between a source 10 of particulates entrained in a gas and a stack 14 for emission of the gas to the atmosphere. Although the source of particulates 10 may be any type of source, such sources include coal or oil fired furnaces or boilers, various types of incinerators, and any combustion process wherein hazardous air pollutants in the form of particulate matter are produced. As a coal fired furnace, for example, the source 10 has a flue pipe 12 which is coupled to the gas inlet 108 of the laminar flow precipitator's vertically oriented housing 105. The particulates entrained in the flue gas entering the precipitator 102 through the inlet 108 must first be charged before they can be removed by electrostatic attraction, as such is the principal upon which all electrostatic precipitators operate. Such charging can be negative or positive, however, negative charging is more widely used. Precipitator 102 is specifically designed to create a laminar flow of flue gas in order to increase the efficiency of particulate removal. The particulates are charged as they pass through a corona discharge established between one or more pairs of parallel or concentric electrodes. The corona discharge which is necessary to efficiently impart the desired charge to the particulates to be removed, creates a "corona wind" which produces a turbulent flow in the gas pattern passing through the precipitator. Therefore, precipitator 102 is designed to separate the charging zone of the precipitator from the collection zone or agglomeration zone, the collection or agglomeration zone being enhanced by laminar flow of the gas flowing therethrough. As shown in FIG. 1, the precipitator 102 is provided with a charging section 104 disposed upstream of the collecting section 106, wherein the flue gas entering the inlet 108 passes through charging section 104 and collection section 106 to then pass through the gas outlet 110. Particulates removed in collecting section 106 are subsequently dispensed to the particulate removal hopper 112. The collecting section may incorporate rappers to mechanically dislodge the collected particulates and cause them to drop into the hopper, or a wet precipitation method may be employed wherein water is supplied through a water inlet 101 to flow down through the collecting section 106 into hopper 112 and carry the collected particulates therewith. Alternately, collecting section 106 may only temporarily collect particulates, serving as a agglomerator for system 100. Particulates are attracted to the electrode surfaces and as the particulates come in contact with one another they agglomerate. The agglomerates then become reentrained into the gas stream for subsequent removal by a downstream precipitator or filter 120. This process is likewise enhanced by laminar flow of the flue gas therethrough. Where very high collector efficiencies are required, between 99.9% and 100%, and the precipitator is operated dry, reentrainment of particulates may be a design goal of the system, making the collector into an agglomerator. For such a system, the collecting section extends a sufficient distance beyond the charging section to permit collected particles to be reentrained into the gas stream. The collected particles, however, will agglomerate before being reentrained. If necessary, the gas can be conditioned with one of several known agglomeration promoters to ensure adequate agglomeration to form particulates of sufficient size to be easily removed. These now larger particles will flow with the gas stream through the outlet 110 into a conduit 122 for transport to a secondary filter 120 for removal of these larger particles. The secondary filter 120 may be a conventional electrostatic precipitator, a fabric filter such as a bag house-type filter, or other type of particulate removal device. The gas flowing from the secondary filter 120 will flow through a conduit 124 to the inlet 16 of the stack 14 to be emitted into the atmosphere free of particulates. In a system not specifically designed to reentrain particulates, filter 120 may be optionally provided to remove any agglomerated particulates which inadvertently become reentrained in the gas stream. The laminar flow through collecting section 106 of system 100 is achieved by passing the gas through a plurality of substantially parallel collecting tubes having a predetermined diameter and at a predetermined velocity, downstream of the charging section 104 to achieve a Reynolds number less than 2,000. The well established Reynolds number is a dimensionless factor represented by the equation: Re = DV V where: D is the diameter of the tubes, V is the mean velocity, v is the kinematic viscosity of the fluid. The laminar flow, RE<2,000 must be satisfied. Thus, knowing the mean velocity of the gas and its viscosity, a tube diameter can be selected to satisfy the aforesaid relationship. As shown in FIG. 3, the collecting section 106 is formed by a plurality of collecting passages 106, the collecting passages being formed by respective tubular collecting members 118. In this particular embodiment, each of the tubular members 118 has a circular cross-sectional contour, but other shapes may be utilized and still obtain laminar flow. As shown in the alternate embodiment of FIG. 4, the collecting section 106" includes a plurality of collecting passages 116" disposed within the vertical housing 105''. Each of the collecting spaces 116'' are formed by a polygonal tubular collecting member 118'' to form the honeycomb-like structure of collecting section 106''. Referring now to FIG. 2, there is shown, the electrostatic precipitation system 100'. As in the first embodiment, the outlet of a particulate source 10 is coupled to a flue 12 which brings the flue gas and entrained particulates to the precipitator inlet 108'. The flue gas and entrained particulates flow through a charging section 104' before flowing downwardly through a vertically oriented housing portion 105' of the laminar flow precipitator 102'. The vertically oriented housing 105' encloses the collecting section 106' for removing the particulates entrained in the flue gas. The particulate-free gas flows from an outlet 110 through a conduit 122' to the inlet 16 of the stack 14 for passage therethrough into the environment. The collecting section 106' includes a plurality of parallel passageways, as in the embodiment of FIG. 1, and connection of an optional system for circulating fluid through the collecting section for carrying off the particulates removed from the gas stream. A fluid such as water enters the vertical portion 105' of precipitator 102' through an inlet 101', and directed to flow through the plurality of parallel collecting passages contained therein, like those shown in FIG. 3 or FIG. 4. The particulate-laden water is collected in the hopper 112' and flows to a pump 130 through a conduit 114. Pump 130 displaces the water through a conduit 132 to a filter 140, wherein the particulates are removed from the water and clean water may then be recirculated to flow through a conduit 142 back to the inlet 101' or alternately out as waste through a conduit 141. Where the filtered water is passed through the waste conduit 141, and not recirculated, the conduit 142 will be coupled to a fresh water source to continually supply water to the inlet 101'. As in the embodiment of FIG. 1, precipitator 102' can be a dry system. As a dry system, precipitator 102' differs from precipitator 102 only in the orientation of the charging section 104', such having a horizontal flow therethrough. As shown in FIG. 5, the charging section 104 may be formed by a plurality of parallel electrodes 126, 128 which are respectively coupled to the reference voltage output line 152 and negative voltage output line 154 of the high voltage power source 150. Power source 150 may represent multiple power supplies, with different power supplies being coupled to different sections of the precipitator 102, 102'. The reference voltage output line 152 is coupled to the ground reference terminal 156 so that the high voltage potential supplied on line 154 is more negative than the ground reference level, to impart the appropriate negative charge on particulates passing between the respective electrodes 126, 128. As will be discussed in following paragraphs, other configurations of the charging section 104 may be utilized in the laminar flow precipitator 102, 102'. As previously discussed, the collecting section 106 is formed by a plurality of small tubular collecting members 118, each having a diameter or width dimension in the range of 1 to 3 inches (approximately 25 to 75 mm) and preferably in the range of 1.5 to 2.0 inches (approximately 37 to 50 mm). Each tubular member 118 defines a respective collecting passage 116 through which the gas and charged particles pass. Each of the tubular members 118 is formed of a conductive material, and electrically connected to the reference voltage output line 152a of power source 150, which is referenced to ground potential by connection to ground terminal 156. As the conductive collecting tubes are coupled to the reference potential, and the charged particulates are charged more negatively, the particles are attracted to the inner wall surfaces of the tubes 118. A non-discharging electrode 125 extends concentrically within each collecting passage 116. Each electrode 125 may have a cylindrical configuration of predetermined diameter, and each is electrically coupled to the voltage output line 154a. Electrode 125 may be in the form of a wire-like electrode or other rod-like member, devoid of sharp corners or edges which could result in high electric field concentrations. The diameter of electrode 125 and the voltage applied thereto is selected to maximize an electric field within each space 116 without creating sparking or corona discharge. This is particularly important where collecting section 106 is used as an agglomerator. Laminar flow through section 106 is achieved for gas velocities in the range of 2.0 to 7.0 feet/second (approximately 0.6 to 2.1 m/s). Referring now to FIG. 6, there is shown an alternate configuration for the two stage laminar flow precipitator. FIG. 6 shows an electrode configuration of one of the plurality of collection passages wherein the charging section 104'' is integrated with the collecting section 106'' to have one electrode 118 in common therebetween. A cylindrically-shaped electrode 128' is electrically coupled to the negative voltage output 154 of the power supply. The electrode 128' extends a predetermined distance into the collection passage 116, the electrode being centrally located within the passage 116 in concentric relationship with the tubular member 118. The tubular member 118 is electrically coupled to the power supply output line 152. The distance that the electrode 128' extends into the tubular member 118 defines the charging section 104''. The voltage applied between the electrodes 118 and 128', the spacing therebetween, and the diameter of electrode 128' being selected to establish a corona discharge between electrode 128' and a portion of the tubular member 118a for charging the particulates being carried by the flowing gas. The remainder 118b of the tubular member 118 defines the collection section 106'', the charged particles being attracted to the inner surface of the lower portion 118b of tubular member 118. An electrode 125 is concentrically disposed within the passage 116 and electrically coupled to the high voltage output line 154a. Electrode 125 has a cylindrical contour and provides a strong electrostatic field to act on the charged particulates passing through passage 116, without inducing corona discharge. In the embodiment of Fig. 7 the electrode 128'' is coupled to the negative voltage output line 154 and extends concentrically within the passage 116 defined by the tubular member 118. The upper portion 127 of electrode 128'' is of a smaller diameter than the lower portion 129, and thereby concentrates the electric field lines directed to the reference electrode portion 118a of the charging section 104''. The upper portion 127 of electrode 128'' is dimensioned so as to induce corona discharge between the tubular electrode portion 118a and the electrode portion 127 at the applied voltage level. In order to increase the electric field between the charged particles and the collection electrode portion 118b, the negative electrode 128'' is designed to extend a predetermined distance into the collection section 106''. However, as previously discussed, corona discharge creates turbulence which would inhibit laminar flow through the collection section. Thus, the lower portion 129 of electrode 128'' is dimensioned differently than that of the upper portion 127, such being dimensioned to increase the surface area of the portion 129 to reduce the concentration of electric field lines, as compared to upper portion 127, to thereby prevent the occurrence of corona discharge and increase the electric field between the charged particles and the inner surface of the tubular member portion 118b. In this configuration, the tubular member 118 is electrically coupled to the reference voltage output line 152 (ground) to provide a reference electrode 118a for the charging section and a collection electrode 118b for the collection section of the laminar flow precipitator. In the embodiment shown in FIG. 8, the reference electrode further comprises a conductive fluid layer 168 which overlays the inner surface of the tubular member 118. Thus, the upper end of each tubular member 118 of the collecting section 106, 106' of the embodiments of FIGS. 1 and 2, are provided with a fluid distributing manifold 160 for dispensing a conductive fluid to the inner surface of the tubular members 118. Although any conducting fluid may be utilized, including fluidized particulates such as a metallic powder, the most economical fluid for such application is water. The manifold 160 shown is exemplary only and many other means may be employed for distributing the fluid to the inner surfaces of the tubular members, without departing from the inventive concept disclosed herein. The water passes into an inlet 162 and flows about an annular passage 166 to flow down through an annular orifice 165, as well as through an outlet 164 for passage to other of the manifolds 160. The water flowing from orifice 165 flows over the inner surface of the tubular member 118. The water that flows down the inner surface of each tubular member forms a conductive film 168 having the potential of the reference voltage, and thereby attracts the charged particulates thereto, as both flow through the collection section 106''. The water film 168 serves two functions: (1) the water serves to carry off the attracted particulates and prevent their reentrainment into the gas stream, and (2) acts as a moving electrode, thereby aiding in the formation of a laminar flow of the gas stream. By directing both the gas and water film 168 downwardly, both can be displaced at substantially the same rate, approximately five feet per second (approximately 1.5 m/s), providing a net relative movement therebetween of zero. As the gas and electrode have no relative movement therebetween, drag is eliminated and laminar flow is thereby achieved. Referring now to FIG. 9, there is shown, a system block diagram of another embodiment of the instant invention. The laminar flow electrostatic particulate removal system 200 is provided within a horizontally disposed housing or ductwork 205, wherein a particulate laden gas enters through one end, in a direction indicated by directional arrow 202, and flows horizontally therethrough to exit through the opposing end, as a clean gas, in a direction indicated by directional arrow 222. The electrostatic system 200 includes a charging section 210 designed to produce corona discharge therein and charge the particulates entrained in the gas stream. Subsequent to flowing through charging section 210, the gas and charged particulates pass through an agglomerator section 215, having a plurality of closely spaced passages with no corona discharge in which the gas achieves laminar flow, or near-laminar flow therethrough. The charged particulates are attracted to wall surfaces in agglomerator 215, and collect thereon, agglomerate with other particles, and become re-entrained as larger agglomerated particulates to be subsequently removed by the collecting section 220. Collecting section 220 may constitute a collection structure such as that previously described, or be formed by a conventional electrostatic precipitator, or fabric type filter. System 200 may be retrofit into an existing conventional electrostatic precipitator, wherein at least a portion of the original precipitator forms the charging section 210 of system 200. The agglomerator section 215 of system 200 provides temporary collection of particulates and may closely resemble the structure of the charging section 210, however, the alternating electrodes will be much more closely spaced and will be devoid of any discharge electrodes or other bodies between adjacent electrodes. The agglomerator 215 may be constructed from flat parallel plates which are closely spaced, the electrode spacing being less than 4" (approximately 100 mm) and preferably on the order of approximately 2" (approximately 50 mm). Each of the charging and agglomerator sections should have a sufficient longitudinal dimension such that the gas residence time ranges from 0.5 to 2.0 seconds, with a preferred residence time approximating 1.0 second. Turning now to FIG. 10, the structure of the charging and agglomerator sections can be more clearly seen. Charging section 210, disposed within the horizontally disposed ductwork 205, is formed by a plurality of alternating electrodes 212 and 214 which are coupled to opposing output lines of a power supply 150. The electrodes 212 are electrically coupled to the power supply output line 152, which is coupled to the ground reference 156. The high voltage output line 154 may supply a negative DC high voltage, a negative pulsating voltage, or combination thereof. The magnitude of the voltage between the output voltage lines 154 and 152 is sufficiently high to induce a corona discharge between the electrodes 214 and 212, without shorting thereacross. Each of the electrodes 214 may include a plurality of corona discharge electrode points 216 coupled thereto to promote the generation of corona discharge in the charging section 210. Agglomerator section 215 includes a plurality of electrodes 218 and 219 coupled to respective power supply output lines 152a and 154a of the power supply 150a. Each of the electrode plates 218, 219 are closely spaced, as previously discussed, and devoid of any corona inducing type structures. The power supply 150a operates at a different voltage than that of power supply 150, supplying sufficient voltage to attract and agglomerate particulates carried in the gas stream, without producing any corona discharge. The output line 154a of power supply 150a is referenced to the output line 152a which is coupled to the ground reference 156 and therefore coupled in common with the output line 152 of power supply 150. The gas passing through agglomerator 215 with its re-entrained agglomerates then flows to the collector section 220, which may be a separate and distinct precipitator or filter. By the arrangement shown in FIG. 10, system 200 can be retrofit into a process employing a conventional horizontal flow parallel plate electrostatic precipitator, and result in a system which benefits from laminar flow of the gas through the agglomerator 215, or both the agglomerator 215 and the collector 220. Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the scope of the invention. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular locations of elements may be reversed or interposed, all without departing from the scope of the invention as defined by the claims.

Claims: An electrostatic precipitation system (200) for removing sub-micron sized particulates entrained in a flue gas, comprising: charging means (212,214) coupled to a flue for electrostatically charging particulates entrained in a flue gas by imparting a charge of predetermined polarity to said particulates, said charged particulates including sub-micron sized particulates; an agglomerator (215) coupled in fluid communication with said charging means (212,214) and downstream thereof for flow of the flue gas therethrough, said agglomerator (215) including a plurality of longitudinally extended plate electrodes (218,219) disposed in substantially parallel spaced relation, each of said plurality of plate electrodes (218,219) being devoid of corona inducing type structures, said plurality of plate electrodes (218,219) being configured to provide a substantially laminar flow of said flue gas therethrough, said plurality of plate electrodes (218,219) being alternately coupled to opposing polarities of a first potential difference, said first potential difference being of sufficient magnitude to attract and agglomerate the particulates but insufficient to prevent agglomerated particulates from being re-entrained into said laminar flow of the flue gas; and, collecting means (220) for collecting said agglomerated particulates coupled in fluid communication with said agglomerator (215) and downstream thereof. The electrostatic precipitation system (200) as recited in claim 1, further comprising: a first source (150a) having first outputs (152a,154a) coupled to said agglomerator (215) and operable to produce said first potential difference. The electrostatic precipitation system (200) as recited in claim 2, further comprising: a second source (150) of a second potential difference having second outputs (152,154), the second source being coupled via said second outputs (152,154) to said charging means (212,214) and being operable to impart said charge of said predetermined polarity to said particulates, wherein said first potential difference is of a lesser magnitude than said second potential difference. The electrostatic precipitation system (200) as recited in any preceding claim, wherein said collecting means (220) is adapted for laminar flow of the flue gas therethrough. The electrostatic precipitation system (200) as recited in any preceding claim, wherein said agglomerator (215) is dimensioned to provide a flue gas residence time within the range of 0.5 to 2.0 seconds. The electrostatic precipitation system (200) as recited in any preceding claim, wherein said plurality of longitudinally extended plate electrodes (218,219) of said agglomerator (215) have a spacing of less than about 100mm (4.0 inches). The electrostatic precipitation system (200) as recited in any preceding claim, wherein said plurality of longitudinally extended plate electrodes (218,219) of said agglomerator (215) have a spacing approximating 50mm (2.0 inches).

Inventors: FELDMAN PAUL L, KUMAR KRISHNASWAMY S, FELDMAN, PAUL L., KUMAR, KRISHNASWAMY S.
Applicants: ENVIRONMENTAL ELEMENTS CORP, ENVIRONMENTAL ELEMENTS CORPORATION
Application year: 1996
Main classification: B03C3
IPC codes: B03C3