IMPROVED COMPACT HYBRID PARTICULATE COLLECTOR (COHPAC)

Abstract: A method and apparatus for efficient removal of particulates from a gas is described which incorporates a barrier filter (44) (e.g. baghouse) internally of an electrostatic precipitator (34). An alternative embodiment is disclosed which incorporates an electrostatic precipitator (34) and a barrier filter (44) (i.e. baghouse) in series, with a pre-charger (40) interposed therebetween. The series arrangement enables the barrier filter (44) to operate at significantly higher filtration velocities than normal 4.06-20.32 cm/s (8-40 ft/min) versus 0.76-2.54 cm/s (1.5-5 ft/min) and reduces the size of the barrier filter (44) significantly. The pre-charger (40) adds an additional electrostatic charge to particulates exhausted from the electrostatic precipitator (34) and replaces charge lost in lengthy and poorly insulated conduits.

Description: IMPROVED COMPACT HYBRID PARTICϋLATE COLLECTOR (COHPAC) Technical Field This invention relates to pollution control, namely filtering of particulate matter, and more specifically, to filtering of flyash and other particulates from flue gas Background Art It is well known in the art how to build and use electrostatic precipitators. It is also known in the art how to build and use a barrier filter such as a baghouse. Further, it is known in the art how to charge particles and that charged particles may be collected in a barrier filter with lower pressure drop and emissions than uncharged particles collected for the same filtration velocity. For example, in U.S. Patent No. 3,915,676 which issued on October 28, 1975 to Reed et al. , an electrostatic dust collector is disclosed where the dirty gas is moved through an electrostatic precipitator to remove most of the particulate matter. The gas stream then passes through a filter having a metal screen and dielectric material wherein an electric field is applied to the filter which permits a more porous material to be used in the filter. The filter is of formacious and dielectric material to collect the charged fine particles. The filter and precipitator are designed in a concentric tubular arrangement with the dirty gas passing from the center of the tubes outward. In U.S. Patent No. 4,147,522 which issued on April 3, 1979, to Gonas et al.. the dirty gas stream passes through a tubular precipitator and then directly into a filter tube in series with the precipitator tube. The particles are electrically charged and are deposited on the fabric filter which is of neutral potential with regard to the precipitator. The major portion of the particles are however deposited in the electrostatic precipitator. No electric field is applied to the fabric filter. Precipitator and filter tube are cleaned simultaneously by a short burst of air. In U.S. Patent No. 4,354,858 which issued on October 19, 1982 to Kumar et al.. electrically charged particles in a gas stream are filtered from the stream by a filter medium which includes a porous cake composed of electrically charged particulates previously drawn from the gas stream and collected on a foraminous support structure. In U.S. Patent No. 4,357,151 which issued on November 2, 1982 to Helfritch et al.. an apparatus is disclosed which first moves dirty gas through a corona discharge electrodes located in the spaced between mechanical filters of the cartridge type having a filter medium of foraminous dielectric material such as pleated paper. The zone of corona discharge in the dirty gas upstream of the filter results in greater particle collection efficiency and lower pressure drop in the mechanical filters. In U.S. Patent No. 4,411,674 which issued on October 25, 1983 to Forσac, a cyclone separator is disclosed wherein a majority of the dust is removed from dirty air in a conventional fashion followed by a bag filter. The bottoms of the filter bags have open outlets for delivering dust into a bottom chamber. The particulates are continuously conducted out of the bag filter apparatus for recirculation back to the cyclone separator. In U.S. Patent No. 1,853,393 which issued on April 12, 1932 to Anderson. a method is disclosed in which an electrostatic field is used to agglomerate fine particulate matter in a gas stream before collecting the agglomerated particles in a downstream barrier filter. Anderson '393 teaches that charging of particulates in a gas stream will agglomerate them and improve the efficiency of a filter. Anderson *393 does not first collect a majority of the particulates (see page 3, left column, lines 28-33) . Japanese Patent No. 3,176,909 discloses a device which first precipitates flyash in an electrostatic precipitator, and then collects unburned carbon particles in a downstream baghouse. This compensates for the low resistivity of unburned carbon (which makes collection in an electrostatic precipitator very difficult) , thereby eliminating the need for a separate denitrification plant. Japan '909 does not use the residual charge imparted on particulates to improve the collection efficiency of the downstream baghouse. The Japan '909 device will not impart a residual charge (since low resistivity carbon will pass easily through a precipitator) . In all the above patents, the inventors show ways to reduce pressure drop and emissions across a barrier filter by pre-charging or mechanical pre-collection of the particles in the gas stream. In contrast, the present invention improves the collection efficiency of a conventional electrostatic precipitator by incorporating the following three refinements therein: 1. A barrier filter is used to augment the electrostatic precipitator. The precipitator serves to remove 90-99% of the particulates from the flue gas. The efficiency of the filter is increased due to the reduced particle concentration, and this increases the overall collection efficiency of the system. 2. The barrier filter is positioned as closely as possible to the electrostatic precipitator to take advantage of the residual electrostatic charge on uncollected particulates. The closer the filter, the greater the residual charge left by the active fields of the precipitator. The residual charge on the remaining particulates further increases the collection efficiency of the barrier filter. 3. The system is operated at a much higher flow rate while maintaining full regulatory compliance. This is possible as a result of the two above-described refinements, and it is accomplished by correctly sizing the barrier filter to provide the increased flow rate. In all the above patents, the inventors are looking for ways to reduce pressure drop and emissions across a barrier filter by pre-charging or mechanical pre-collection of the particles in the gas stream. U.S. Patent No. 5,024,681 issued to Chang also accomplishes the foregoing, but it does so by connecting a baghouse downstream of an electrostatic precipitator. This can be a costly proposition due to the retrofit duct work, and it is often difficult to place the baghouse in proximate to the electrostatic precipitator to capture the full residual charge on exhausted particulates. The present invention solves these problems by modifying the electrostatic precipitator itself. Alternatively, a baghouse can be connected downstream of the electrostatic precipitator, and a pre-charging unit can be interposed therebetween. Disclosure of Invention The invention is a method for retrofit filtering of particulates in a flue gas from a combustion source having an existing electrostatic precipitator connected to a smoke stack comprising the steps of removing a last field from a plurality of fields in the electrostatic precipitator, inserting a barrier filter in the electrostatic precipitator in a space vacated by the last field, the barrier filter being arranged to collect particulates at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , and the particulates being exhausted f om the electrostatic precipitator before a residual electric charge imparted by said electrostatic precipitator substantially dissipates. The invention also comprises the apparatus for carrying out the above-described steps, the apparatus comprising a multi-field electrostatic precipitator for removing 90-99% of particulates in the flue gas, and for imparting a residual electrostatic charge on remaining particulates in the flue gas, the electrostatic precipitator having a last field removed and a barrier filter installed in the space vacated by the removed field and in fluid communication with the electrostatic precipitator for filtering the flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby the barrier filter collects the remaining particulates exhausted in the flue gas before the electrostatic charge imparted by the electrostatic precipitator substantially dissipates. In the above-described method and apparatus, the initial fields of the precipitator remove the majority of particulates from the flue gas, and the barrier filter removes those which remain. Since the barrier filter is internal to the electrostatic precipitator, the particulates escaping to the barrier filter carry a peak residual charge. The preserved charge vastly increases the collection efficiency of the system, and the system can be operated at a high flow rate while maintaining full regulatory compliance. In accordance with another embodiment of the invention, a method is disclosed for removing particulates from a flue gas comprising the steps of flowing the flue gas through an electrostatic precipitator which imparts a residual electrostatic charge on remaining particulates exhausted therefrom, flowing the flue gas exhausted from the electrostatic precipitator through a pre-charger downstream of said electrostatic precipitator for imparting an additional electrostatic charge, and flowing the flue gas through a barrier filter downstream of the pre-charger at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , the barrier filter collecting the remaining particulates before the electrostatic charge imparted by said electrostatic precipitator and said pre-charger substantially dissipates. The above-described alternative embodiment also comprises an apparatus, including an electrostatic precipitator for removing 90-99% of particulates in said flue gas, and for imparting a residual electrostatic charge on remaining particulates exhausted therefrom in said flue gas, a pre-charger placed downstream of the electrostatic precipitator and in luid communication therewith, the pre-charger imparting an additional electrostatic charge on remaining particulates, and a barrier filter placed downstream of said pre-charger and in fluid communication therewith, the barrier filter filtering said flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby the barrier filter collects the remaining particulates exhausted in the flue gas before the electrostatic charge imparted by said electrostatic precipitator and said pre-charger substantially dissipates. The above-described invention makes full use of both the reduced particle concentration and the residual charge on remaining particulates. Brief Description of Drawings FIG. 1 is a block diagram of a flue gas treatment system according to one embodiment of the present invention. FIG. 2 is a graphical description of the effect of low particle concentrations and the charging of particles on barrier filter pressure drop. FIG. 3 is a graphical description of the effect of particle charging and filtration velocity on the particle penetration across a barrier filter. FIG. 4 illustrates one example of pre-charging unit 40 of FIG. 1. FIG. 5 illustrates a second embodiment of the invention having a pre-charging unit 40 interposed between the electrostatic precipitator 34 and barrier filter 44. FIGS. 6 and 7 illustrate a plan view, and a side view, respectively, of a second embodiment of the present invention in which the last field of a multi-field precipitator is replaced by a conventional baghouse. Best Modefs) for Carrying Out the Invention Referring now to the drawings, Fig. 1 shows a block diagram of a first embodiment of the invention comprising a flue gas treatment system 10 for the treatment of flue gas exiting a boiler 12 of the type used in a utility fossil-fuel-fired power plant. It should be recognized that the invention applies equally well to any process that requires gas stream particulate control. Fuel supply 18 may be, for example, coal, oil, refuse derived fuel (RDF) or municipal solid waste (MSW) . Boiler 12 also receives air 20 over inlet duct 22. Boiler 12 functions to combust the fuel 14 with air 20 to form flue gas 24 which exits boiler 12 by means of outlet duct 26. Boiler 12 also has a water inlet pipe 28 and a steam outlet pipe 30 for removing heat in the form of steam from boiler 12 generated by the combustion of fuel 14 with air 20. Flue gas 24 is comprised of components of air and the products of combustion in gaseous form which include: water vapor, carbon dioxide, halides, volatile organic compounds, trace metal vapors, and sulfur and nitrogen oxides and the components of air such as oxygen and nitrogen. Flue gas 24 also contains particulates comprising unburned and partially combusted fuel which includes inorganic oxides of the fuel (known as flyash) , carbon particles, trace metals, and agglomerates. Flue gas 24 may also contain particulates generated by the addition of removal agents 19 for sulfur oxide and other gas phase contaminates such as halides and trace metal vapors which are added into boiler 12 by way of duct 21, into duct 26, or into reactor vessel 17 by way of duct 23 upstream of the precipitator 34. Ducts 21, 26 and 23 may also convey solid materials if required for the selected removal agents 19 for the respective duct. Examples of sulfur oxide and other gas phase contaminate removal agents 19 include calcium carbonates, oxides and hydroxides, and sodium carbonates and bicarbonates. The particles or particulates in flue gas 24 can vary considerably in size, shape, concentration and chemical composition. Flue gas 24 passes through duct 26 through reactor vessel 17 and through duct 27 as flue gas 25 to an inlet of electrostatic precipitator 34 which functions to charge and collect particles on electrodes within the electrostatic precipitator 34. Reactor vessel 17 may facilitate the chemical reaction of removal agents 19 with flue gas 24 to provided treated flue gas 25. Electrostatic precipitator 34 may remove, for example, from 90-99.9% of the particles and/or particulates. Therefore, flue gas 24 exits electrostatic precipitator 34 as treated flue gas 36 entering outlet duct 38. Treated flue gas 36 has roughly from 0.1-10% of the particulates or particles contained in the original flue gas 24 and also contains a certain amount of electrostatic charge which was transferred to it from the electrostatic precipitator 34. These particles were not collected within the electrostatic precipitator but exited at outlet duct 38. The particle concentration in the flue gas 36 exiting the electrostatic precipitator 34 is reduced significantly by the precipitator and contains a residual charge imparted by the precipitator. These characteristics permit highly efficient filtering. For instance, a hypothetical situation which describes the effect of low particle concentrations and the charging of particles on barrier filter pressure drop is shown in Fig. 2. Curve 60 in Fig. 2 shows the pressure drop across a barrier filter filtering particles from flue gas directly from boiler 12 in Fig. 1 without pre-filtering by an electrostatic precipitator 34. Curve 61 shows what would happen when a significant portion of the particles in the flue gas is removed by an electrostatic precipitator 34 before entering the barrier filter 44, and assuming that the particles entering the barrier filter 44 have no electrical charge. Curve 62 shows what would happen to the pressure drop depicted by curve 61 if a residual electrical charge is carried by the particles exiting the electrostatic precipitator 34 and entering the barrier filter 44. It can be seen that for the same pressure drop across the barrier filter, indicated by points 63, 64 and 65 on curves 60-62 respectively, in Fig. 2, the condition represented by curve 62 allows significantly higher filtration velocity (also defined as air-to-cloth ratio or volumetric flow rate of flue gas per unit of effective filter area) than the other conditions represented by curves 60 and 61. A barrier filter downstream of an electrostatic precipitator and collecting particles having a residual electrical charge is capable of operation at a filtration velocity of 11.18 centimeters per second (22 ft/min) versus 2.03 centimeters per second (4 ft/min) for a barrier filter filtering flue gas without pre-cleaning and charging by an electrostatic precipitator. Fig. 3 is a hypothetical situation showing the effect of particle charging and filtration velocity on the particle penetration across a barrier filter. The particle penetration across a barrier filter increases as the filtration velocity increases as shown by curve 80 but is enhanced significantly by charging the particles as shown by curve 81. Thus, the charged particles exiting the electrostatic precipitator could be filtered at high filtration velocities without increasing emissions across the barrier filter. Because of the low particle loading and the electrical charge on the particles, a downstream barrier filter 44 can be adjusted in size to filter flue gas 36 at filtration velocities (also called air-to-cloth ratio) in the range from 4.06- 20.32 centimeters per second (8-40 feet per minute) . The above-described advantages depend on the proximity of the barrier filter 44 to electrostatic precipitator 34. Therefore, barrier filter 44 is preferably very close to electrostatic precipitator 34 so as to receive particulates retaining the maximum residual charge imparted by electrostatic precipitator 34. Unfortunately, in many instances it is not structurally feasible to place electrostatic precipitator 34 in proximity to barrier filter 44. In such cases the duct(s) connecting electrostatic precipitator 34 with barrier filter 44 may be prolonged and insufficiently insulated. Consequently, the particles or particulates previously charged in electrostatic precipitator 34 will lose their electrostatic charge prior to collection by barrier filter 44 The embodiment shown in FIG. 1 compensates for the above-described loss of charge. In FIG. l, a pre- charging unit 40 is constructed integrally with barrier filter 44. FIG. 4 illustrates one example of the pre-charging unit 40 of FIG. 1. Pre-charging unit 44 comprises a plurality of elongate discharge electrodes 100 protruding into a corresponding plurality of discharge conduits 102, the discharge conduits 102 being in fluid communication with barrier filter 44. The discharge electrodes 100 are mounted on a conductive plate 106, which is in turn held by insulated supports 108 positioned at the edges of plate 106. The discharge conduits are also mounted on a conductive plate 104. All of the above-described components are contained in pre-charging unit housing 110, which extends downwardly to a dust discharge vent 120. A voltage potential is applied between plates 104 and 106. In operation, flue gas 36 enters pre-charging unit 40 through inlet duct 42. The flue gas 36 cycles upward through conduits 102 toward barrier filter 44. While the flue gas 36 is inside conduits 102, an electrostatic charge is imposed by oppositely charged discharge electrodes 100 and discharge conduits 102. Referring back to FIG. 1, flue gas 48 exiting barrier filter 44 passes over outlet duct 50 through fan 52 and duct 54 to the inlet of smoke stack 46. Flue gas 48 exits smoke stack 46 as gas 58, which in turn mixes with the ambient air or atmosphere. Fan 52 functions to overcome the additional pressure drop required to draw flue gas 48 across the barrier filter 44 to maintain a face velocity in the range from 4.06-20.32 centimeters per second (8-40 feet per minute) across barrier filter 44. Fan 52 also functions to draw flue gases 36 and 24 from electrostatic precipitator 34 and boiler 12 respectively. Fan 52 also functions to move flue gas 48 through duct 54 and out of smoke stack 46 as flue gas 58. As a result of the above-described device the efficiency of the barrier filter 44 is maximized because the residual charge imparted by electrostatic precipitator 34 (and lost to conduit 38) is replenished by pre-charging unit 40. In alternative embodiments pre-charging unit 40 may be placed at other positions along the duct work. For example, FIG. 5 shows a second embodiment of the invention having a pre-charging unit 40 interposed between the electro-static precipitator 34 and barrier filter 44. The input of pre-charging unit 40 is connected to the electrostatic precipitator via duct 38, and the output of pre-charging unit 40 is connected to barrier filter 44 via duct 42. The operation of pre- charging unit 40 is the same as described above. Examples of acceptable barrier filters 44 include baghouses of the pulse-jet type, reverse flow, or shake- deflate type for periodically removing the dust cake accumulated on the surface of the bag filter. Since the electrostatic precipitator 34 and the barrier filter 44 are separate devices, each can be cleaned independently of the other. By operating the barrier filter 44 with a higher face velocities of 4.06-20.32 centimeters per second (8-40 feet per minute) the size of the barrier filter with respect to conventional barrier filter is greatly reduced, thereby allowing both the barrier filter 44 and pre-charging unit 40 to be retrofit into existing boiler systems between the electrostatic precipitator and smoke stack 46. This allows substantial capital and installation cost savings and requires very little real estate for installation. Another embodiment of the present invention for accomplishing the above-described and other objectives is shown in FIGs. 6 and 7. This embodiment is a simple retrofit for flue gas treatment systems having larger electrostatic precipitators (i.e more than one electrostatic field) . It has been found that the last field of the precipitator 34 can be removed and replaced by a conventional baghouse. The reduced particle concentration in the flue gas 36 exiting the electrostatic precipitator 34, coupled with the residual electrical charge imparted by the precipitator allows operation of the baghouse at very high filtration velocities. Hence, the baghouse can be made very compact. As shown in FIG. 6, a compact baghouse 44 can be retrofit into the space vacated by the eliminated field of precipitator 34, and no interconnecting ducts are necessary. FIG. 7 is a side-view of the retrofit device of FIG. 6. Having now fully set forth the preferred embodiments and certain modifications of the concept underlying the present invention, various other embodiments as well as certain variations and modifications of the embodiment herein shown and described will obviously occur to those skilled in the art upon becoming familiar with said underlying concept. It is to be understood, therefore, that within the scope of the appended claims, the invention may be practiced otherwise than as specifically set forth herein. Industrial Applicability Currently, there are approximately 1200 coal-fired utility power plants representing 330,000 MWe of generating capacity that are equipped with electrostatic precipitators. Present precipitators typically remove 90-99.9% of the flyash in the flue gas. However, existing and pending regulations to control sulfur dioxide emissions from the flue gas require utilities to switch fuel types (such as from high to low sulfur coal) , or add sulfur dioxide control upstream of the precipitators. Fuel switching and sulfur control upstream of the precipitators generally modify flyash properties, reduce precipitator collection efficiency, and increase stack particulate emissions. In addition, particulate emissions standards are getting increasingly stringent. Faced with these increasingly strict environmental requirements, utilities are looking for low cost retrofits to upgrade the performance of their precipitators. One approach would be to replace the existing under- performing precipitator with a baghouse or barrier filter or conventional design which are generally accepted as an alternative to precipitators for collecting flyash from flue gas. Conventional designs can be categorized as low-ratio baghouses (reverse-gas, sonic-assisted reverse- gas, and shake-deflate) which generally operate at filtration velocities of 0.76 to 1.27 centimeters per second (1.5 to 2.5 f /min), also defined as air-to-cloth ratio, volumetric flow rate of flue gas per unit of effective filter area, or (cubic feet of flue gas flow/min/square foot of filtering area) , and high-ratio pulse-jet baghouses which generally operate at 1.52 to 2.54 centimeters per second (3 to 5 ft/min). Baghouses generally have very high collection efficiencies (greater than 99.9%) independent of flyash properties. However, because of their low filtration velocities, they are large, require significant space, are costly to build, and are unattractive as replacements for existing precipitators. Reducing their size by increasing the filtration velocity across the filter bags will result in unacceptably high pressure drops and outlet particulate emissions. There is also potential for "blinding" the filter bags — a condition where particles are embedded deep within the filter and reduce flow drastically. It would be commercially advantageous to reduce pressure drop and emissions across a barrier filter by pre-charging coupled with mechanical pre-collection of the particles in the gas stream. The invention accomplishes both objectives by incorporating a barrier filter downstream of a conventional electrostatic precipitator with a pre- charger interposed therebetween. The invention further provides an equally effective retrofit to a conventional electrostatic in which the last field of a multi-field precipitator is replaced by a conventional baghouse.

Claims: Claims 1. A method for removing particulates from flue gas comprising: flowing said flue gas through an electrostatic precipitator for removing 90-99% of said particulates, and for imparting a residual electrostatic charge on remaining particulates exhausted from said electrostatic precipitator in said flue gas; flowing said flue gas exhausted from said electrostatic precipitator through a pre-charger downstream of said electrostatic precipitator for imparting an additional electrostatic charge to said remaining particulates; flowing said flue gas through a barrier filter downstream of said pre-charger at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , said barrier filter collecting the remaining particulates exhausted in said flue gas from said electrostatic precipitator before the electrostatic charge imparted by said electrostatic precipitator and said pre-charger substantially dissipates. 2. The method of claim 1, further comprising cleaning said barrier filter of collected particulates at times said pressure drop across said barrier filter exceeds 2.54 to 30.48 centimeters of water (1 to 12 inches of water) . 3. The method of claim 1, further comprising cleaning said barrier filter at set time intervals. 4. The method of claim 1, wherein said barrier filter is a baghouse. 5. The method of claim 1, further comprising maintaining a f ce velocity of said flue gas at said barrier filter by drawing said flue gas through said barrier filter with a fan. 6. The method of claim 1, wherein said pre-charger is integrally formed with said barrier filter. 7. A method for retrofit filtering of particulates in a lue gas from a combustion source having an existing electrostatic precipitator connected to a smoke stack comprising the steps of: connecting a first electrically insulated duct to said electrostatic precipitator; connecting a pre-charger to said duct downstream of said electrostatic precipitator for collecting particulates exhausted from said electrostatic precipitator in said flue gas, said pre-charger imparting an additional electrostatic charge to said particulates exhausted from said electrostatic precipitator; connecting a second electrically insulated duct to said pre-charger; and connecting a barrier filter to said second duct downstream of said pre-charger for collecting particulates exhausted from said pre-charger in said flue gas, said barrier filter being positioned to receive charged particulates exhausting from said pre-charger while a residual electrostatic charge imparted on said particulates by said electrostatic precipitator and said pre-charger is maintained; and maintaining a filtration velocity of flue gas through said barrier filter in the range of from 4.06- 20.32 centimeters per second (8-40 feet per minute). 8. The method of claim 7, further including cleaning particulates off said barrier filter at times said pressure drop across said barrier filter exceeds a predetermined value in the range from 2.54-30.48 centimeters of water (1-12 inches of water) . 9. The method of claim 7, further including cleaning particulates off said barrier filter at set time intervals. 10. The method of claim 7, wherein said barrier filter is a baghouse. 11. The method of claim 7, further comprising maintaining a face velocity of said flue gas at said barrier filter by drawing said flue gas through said barrier filter with a fan. 12. The method of claim 7, wherein said combustion source is a fossil-fuel-fired boiler. 13. A method for retrofit filtering of particulates in a flue gas from a combustion source having an existing electrostatic precipitator connected to a smoke stack comprising the steps of: connecting a first electrically insulated duct to said electrostatic precipitator; connecting a pre-charger to said duct downstream of said electrostatic precipitator for collecting particulates exhausted from said electrostatic precipitator in said flue gas, said pre-charger imparting an additional electrostatic charge to said particulates exhausted from said electrostatic precipitator; connecting a barrier filter to said pre-charger downstream of said pre-charger for collecting particulates exhausted from said pre-charger in said flue gas, said barrier filter being positioned integrally with said pre-charger for receiving charged particulates exhausted from said pre-charger while a residual electrostatic charge imparted on said particulates by said electrostatic precipitator and said pre-charger is maintained; and maintaining a filtration velocity of flue gas through said barrier filter in the range of from 4.06- 20.32 centimeters per second (8-40 feet per minute). 14. The method of claim 13, further including cleaning particulates off said barrier filter at times said pressure drop across said barrier filter exceeds a predetermined value in the range from 2.54-30.48 centimeters of water (1-12 inches of water) . 15. The method of claim 13, further including cleaning particulates off said barrier filter at set time intervals. 16. The method of claim 13, wherein said barrier filter is a baghouse. 17. The method of claim 13, further comprising maintaining a face velocity of said flue gas at said barrier filter by drawing said flue gas through said barrier filter with a fan. 18. The method of claim 13, wherein said combustion source is a fossil-fuel-fired boiler. 19. A method for retrofit filtering of particulates in a flue gas from a combustion source having an existing multiple field electrostatic precipitator connected to a smoke stack, the method comprising the steps of: removing a last field from a plurality of fields in said electrostatic precipitator; inserting a compact barrier filter in said electrostatic precipitator in a space vacated by said last field, said barrier filter being proportioned to filter particulates at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) ; whereby the remaining fields of said electrostatic precipitator serve to remove a majority of particulates from said flue gas and impart a residual charge on remaining particulates, and said remaining particulates flow through said barrier filter before said residual electric charge substantially dissipates. 20. The method of claim 19, wherein said barrier filter is a baghouse. 21. The method of claim 19, wherein said combustion source is a fossil-fuel-fired boiler. 22. A method for removing particulates from flue gas, comprising: a first step of flowing said flue gas through a plurality of fields in an electrostatic precipitator for removing 90-99% of said particulates, and for imparting a residual electric charge on remaining particulates; a second step of flowing said flue gas through a barrier filter located internally of said electrostatic precipitator at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby said barrier filter collects the charged remaining particulates before said residual electric charge substantially dissipates. 23. The method of claim 22, wherein said barrier filter is a baghouse. 24. The method of claim 22, wherein said combustion source is a fossil-fuel-fired boiler. 25. An apparatus for removing particulates from flue gas comprising: an electrostatic precipitator for removing 90-99% of particulates in said flue gas, and for imparting a residual electrostatic charge on remaining particulates exhausted therefrom in said flue gas; a pre-charger placed downstream of said electrostatic precipitator and in fluid communication therewith, said pre-charger for imparting an additional electrostatic charge on said remaining particulates; a barrier filter placed downstream of said pre- charger and in fluid communication therewith, said barrier filter for filtering said flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby said barrier filter collects the remaining particulates exhausted in said flue gas from said electrostatic precipitator before the electrostatic charge imparted by said electrostatic precipitator and said pre-charger substantially dissipates. 26. The apparatus according to claim 25, wherein said barrier filter is formed integrally with said pre- charger. 27. An apparatus for removing particulates from flue gas comprising: a multi-field electrostatic precipitator having a plurality of fields for removing 90-99% of particulates in said flue gas, and for imparting a residual electrostatic charge on remaining particulates in said flue gas, said electrostatic precipitator having at least one field removed; a barrier filter installed in a space vacated by said at least one removed field for filtering said flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute); said barrier filter operating to collect the remaining particulates before the electrostatic charge imparted by said plurality of fields substantially dissipates. 28. The apparatus according to claim 27, wherein said barrier filter is a baghouse. AMENDED CLAIMS [received by the International Bureau on 1 June 1992 (01.06.92); original claims 20,22-24 and 28 cancelled; original claims 19 and 27 amended; new claim 29 added; other claims unchanged (4 pages)] 16. The method of claim 13, wherein said barrier filter is a baghouse. 17. The method of claim 13, further comprising maintaining a face velocity of said flue gas at said barrier filter by drawing said flue gas through said barrier filter with a fan. 18. The method of claim 13, wherein said combustion source is a fossil-fuel-fired boiler. 19. A method for retrofit filtering of particulates in a flue gas from a combustion source having an existing conventional electrostatic precipitator connected thereto and a smoke stack connected to said precipitator, said electrostatic precipitator further comprising a plurality of electrostatic discharge electrodes and corresponding collecting electrodes enclosed within a housing, an inlet to said housing connected to said combustion source, and an outlet from said housing connected to to said smokestack, the method comprising the steps of: removing at least one discharge electrode and collecting electrode from within said housing of said electrostatic precipitator; attaching a tubesheet within said housing to subdivide said space vacated by said removed electrodes into a separate filter section downstream of the remaining discharge and collecting electrodes, and an outlet section downstream of said separate filter section; supporting a compact baghouse filter within said separate filter section by said tubesheet to filter particulates therein, said baghouse filter being proportioned to filter particulates at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-4Q feet per minute); whereby said remaining discharge electrodes and corresponding collecting electrodes in said electrostatic precipitator serve to remove a majority of particulates from said flue gas and impart a residual charge on remaining particulates discharged to said separate filter section, and said remaining particulates are collected by said baghouse filter before said residual electric charge substantially dissipates. 21. The method of claim 19, wherein said combustion source is a fossil-fuel-fired boiler. 25. An apparatus for removing particulates from flue gas comprising: an electrostatic precipitator for removing 90-99% of particulates in said flue gas, and for imparting a residual electrostatic charge on remaining particulates exhausted therefrom in said flue gas; a pre-charger placed downstream of said electrostatic precipitator and in fluid communication therewith, said pre- charger for imparting an additional electrostatic charge on said remaining particulates; a barrier filter placed downstream of said pre-charger and in fluid communication therewith, said barrier filter for filtering said flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby said barrier filter collects the remaining particulates exhausted in said flue gas from said electrostatic precipitator before the electrostatic charge imparted by said electrostatic precipitator and said pre- charger substantially dissipates. 26. The apparatus according to claim 25, wherein said barrier filter is formed integrally with said pre-charger. 27. An apparatus for removing particulates from flue gas comprising: an electrostatic precipitator having a housing enclosing separate particle removal sections, an inlet to said housing, and an outlet from said housing, said particle removal sections further comprising, an electrostatic removal section including a plurality of collecting electrodes and discharge electrodes enclosed within said housing for removing a substantial portion of particulates from said flue gas, and for imparting a residual electrostatic charge on remaining particulates in said flue gas, a separate filter section located downstream of said electrostatic removal section to filter said particulates before the electrostatic charge imparted by said electrostatic removal section substantially dissipates, said separate filter section including a compact baghouse filter having a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , and an outlet section located downstream of said separate filter section and separated therefrom by a tubesheet installed within said housing to support said baghouse filter; whereby a collection efficiency of said baghouse filter is enhanced by removing said substantial portion of particulates from said flue gas in said electrostatic removal section and imparting a residual charge on the remaining particulates discharged to said separate filter section, thereby allowing collection of the remaining particulates in said baghouse filter at said high filtration velocity. 29. In a conventional electrostatic precipitator having a housing enclosing electrostatic discharge and collecting electrodes for electrostatically filtering particulates from a flue gas, an inlet to said housing, and an outlet from said housing, a retrofit improvement comprising: a tubesheet installed within a space vacated by removing at least one discharge electrode and collecting electrode from within said housing of said electrostatic precipitator; said tubesheet subdividing said vacated space into a separate filter section and an outlet section; an electrostatic section comprising a plurality of remaining discharge electrodes and corresponding collecting electrodes for electrostatically removing a substantial portion of said particulates from said flue gas and for imparting a residual electrostatic charge on remaining particulates in said flue gas; and a compact baghouse filter installed in said separate filter section downstream of said electrostatic section for supplemental mechanical filtering of said remaining particulates from said flue gas before said residual charge substantially dissipates, said compact baghouse filter being supported on said tubesheet and having a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) ; whereby a combination of removing said substantial portion of particulates from said flue gas in said electrostatic section and imparting a residual charge on the remaining particulates discharged to said separate filter section increases an efficiency of said baghouse filter to allow effective filtering of said remaining particulates from said flue gas at said high filtration velocity. STATEMENT UNDER ARTICLE19 The changes in the claims are made to bring this application into conformance with the corresponding U.S. Application Serial No. 451,517 and to thereby overcome the Examiner's conclusion that the invention of claims 19-28 lacks an inventive step. Claims 19 and 27 as amended and new claim 29 of the present application are now identical to claims 16, 24, and 26, respectively, of the corresponding U.S. Application, which claims were allowed for the following reasons. Claims 25 and 26 are unchanged, but are believed to be novel for the same reasons. The aim of the present invention is to combine the following three concepts to improve the collection efficiency of a conventional electrostatic precipitator. 1. Incorporate a baghouse filter to augment the electrostatic precipitator. The precipitator serves to remove 90-99% of the particulates from the flue gas. The efficiency of the filter is increased due to the reduced particle concentration, and this increases the overall collection efficiency of the system. 2. Position the baghouse filter in as closely as possible to the electrostatic precipitator to take advantage of the residual electrostatic charge on uncollected particulates. The closer the filter, the greater the residual charge left by the active fields of the precipitator. Alternatively, a pre-charger may be included upstream of the filter to boost the charge on the uncollected particulates. In either case, the residual charge on the remaining particulates further increases the collection efficiency of the baghouse filter. 3. Operate the system at a much higher flow rate, but maintain full regulatory compliance. This is only possible as a result of the two above-described concepts, and it is accomplished by correctly sizing the baghouse filter to provide the increased flow rate. One embodiment of the present invention solves these problems by modifying the electrostatic precipitator itself. In general terms, this is accomplished by removing the last field from the electrostatic precipitator and installing a baghouse filter in the vacated space. In contrast to the prior art, this embodiment makes full use of both the reduced particle concentration and the residual charge on remaining particulates. The initial fields of the precipitator remove the majority of particulates from the flue gas, and the baghouse filter removes those which remain. Since the baghouse filter is internal to the electrostatic precipitator, the particulates escaping to the baghouse filter carry a peak residual charge. The preserved charge vastly increases the collection efficiency of the system, and the system can be operated at a high flow rate while maintaining full regulatory compliance. In contrast to the prior art, this is a commercially practical retrofit improvement. A second embodiment solves the same problems by incorporating a pre-charging unit prior to the baghouse filter - - and downstream of the electrostatic precipitator. The result is the same as described above. Anderson '393 merely teaches that charging of particulates in a gas stream will agglomerate them and improve the efficiency of a filter. Anderson '393 does not first collect a majority of the particulates (see page 3, left column, lines 28-33). It is even more apparent that Anderson '393 does not modify an existing electrostatic precipitator. Anderson '393 does not teach how to remove a field from an existing electrostatic precipitator, nor replacement of the field with a filter. Initial collection of a majority of the particulates in a plurality of active fields and further collection of residual particles in a baghouse filter which has been substituted for one (preferably the final) field are essential elements of the present invention. These elements are clearly embodied in claim 19 as amended herein. Likewise, claim 27 as amended recites "a separate filter section...located downstream of said electrostatic removal section to filter said particulates before the electrostatic charge imparted by said electrostatic removal section substantially dissipates, said separate filter section including a compact baghouse filter having a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) ." Similarly, claim 25 recites "a pre-charger placed downstream of said electrostatic precipitator," and "a baghouse filter placed downstream of said pre-charger...for filtering said flue gas at a high filtration velocity in the range of from 4.06-20.32 centimeters per second (8-40 feet per minute) , whereby said baghouse filter collects the remaining particulates exhausted in said flue gas from said electrostatic precipitator before the electrostatic charge imparted by said electrostatic precipitator and said pre-charger substantially dissipates." The above-described structure recited in claims 19, 25, and 27 is not taught or suggested by Anderson '393. Hence, these claims are not obvious over Anderson '393. Japan '909 compensates for the low resistivity of unburned carbon (which makes collection in an electrostatic precipitator very difficult) . Japan '909 discloses a device which first precipitates flyash in an electrostatic precipitator and then collects unburned carbon particles in a downstream baghouse. This eliminates the need for a separate denitrification plant. However, Japan '909 does not use the residual charge imparted on particulates to improve the collection efficiency of the downstream baghouse. In fact, the Japan '909 device will not even impart a residual charge (since low resistivity carbon will pass easily through a precipitator). Moreover, Japan '909 does not modify an existing electrostatic precipitator. Japan *909 does not teach how to remove a field from an existing electrostatic precipitator, nor replacement of the field with a filter. Therefore, the combination of Anderson and Japan '909 do not obviate the novelty of the invention. Claims 20, 22-24, and 28 are canceled. Claim 21 depends from claim 19 and includes the same limitations.

Inventors: CHANG RAMSAY, CHANG, RAMSAY
Applicants: ELECTRIC POWER RES INST, ELECTRIC POWER RESEARCH INSTITUTE
Application year: 1992
Main classification: B03C3
IPC codes: B03C3