Patent Publication Number: US-2023158513-A1

Title: WESP With Impaction Cleaning, And Method of Cleaning A WESP

Description:
This application claims priority of U.S. Provisional Application Serial No. 63/033,375 filed Jun. 2, 2020 and U.S. Provisional Application Serial No. 63/056,940 filed Jul. 27, 2020, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     Pollution control equipment, such as wet electrostatic precipitators (WESP) are used to remove dust, acid mist and other particulates from water-saturated air and other gases by electrostatic means. For example, particulates and/or mist laden water-saturated air flows in a region of the precipitator between discharge and collecting electrodes, where the particulates and/or mist is electrically charged by corona emitted from the high voltage discharge electrodes. As the water-saturated gas flows further within the precipitator, the charged particulate matter and/or mist is electrostatically attracted to grounded collecting plates or electrodes where it is collected. The accumulated materials are continuously washed off by both an irrigating film of liquid and periodic flushing to a discharge drain or the like. 
     Such systems are typically used to remove pollutants from the gas streams exhausting from various industrial sources, such as incinerators, coke ovens, glass furnaces, non-ferrous metallurgical plants, coal-fired generation plants, forest product facilities, food drying plants, wood product manufacturing and petrochemical plants. 
     In wood product manufacturing in particular, for example, maintenance issues are problematic, particularly due to material build-up on the collectors and on electrodes. Sticky particulates, condensation products, etc. tend to adhere to and accumulate on equipment internals, resulting in poor equipment performance with requires deleterious downtime and unnecessary expense in an effort to remove them. This has been seen in not only in the manufacture of wood products such as panelboard, for example, but also in the biofuel and other markets. Manual intervention is often necessary to adequately clean the equipment internals from the build-up of contaminants, which is highly undesirable. Dirty WESP tubes and electrodes are thus a persistent industry challenge that degrades performance for all WESP styles and designs. 
     Current industrial practice has been to try to clean the build-up in the WESP with warm water (100-130° F.), caustic solution, or a weak acid solution. In almost all cases the cleaning solution is injected into the WESP through stationary nozzles that cover a broad area to cover all surfaces of the WESP using a minimum number of nozzles to reduce cost. This spreads the mass flux of the liquid across a large area (e.g., 0.05 to 0.25 lbs/(ft 2 *s)) so there is not much energy hitting the dirty surfaces. Therefore, loose material can be removed, but material that is adhered to the surfaces is not removed. Also, since the spray is typically sprayed at a wide angle (90 degrees), very little of the spray penetrates to a depth of more than a foot in the honeycomb structure. 
     Accordingly, a method of maintaining the collecting tubes and electrodes in a clean condition with minimal manual cleaning required would be highly beneficial. 
     It is therefore an object of embodiments disclosed herein to incorporate multiple components in a WESP to provide a much greater impaction energy over areas expected to collect most of the particulate and therefore expected to get the dirtiest. 
     It is a further object of embodiments disclosed herein to minimize the amount of liquid used to clean a WESP. 
     SUMMARY 
     Problems of the prior art have been addressed by embodiments disclosed herein, which provide a method and apparatus for cleaning pollution control equipment, such as particulate removal devices, including wet electrostatic precipitators, and to provide a particulate removal device including such cleaning apparatus. In certain embodiments, the WESP includes a housing having a chamber, at least one gas inlet in fluid communication with the chamber, a gas outlet spaced from the at least one gas inlet and in fluid communication with the chamber, one or more ionizing electrodes in the housing and one or more collecting electrodes or surfaces in the housing. In some embodiments, the collecting electrodes include a bundle of tubes or cells, which may be cylindrical or hexagonal in cross-section. In some embodiments, bundle of hexagonal in cross-section. In some embodiments, the bundle of tubes forms a honeycomb pattern of hexagonal collecting zones or cells. In certain embodiments, the housing may be placed in fluid communication with a washing liquid source, such as a water source. 
     In certain embodiments, a particulate removal device, such as a WESP, having movable spray nozzles is provided, wherein the movement of the nozzles is designed so that fluid expelled therefrom impacts all or substantially all of the regions in the WESP where particulate build-up deleterious to the electrostatic performance of the WESP is expected or observed. Efficient and substantially homogeneous cleaning of the collection surfaces is achieved, such as by impact of a mass flux of a washing liquid on each surface element of the particulate collection surfaces over a certain impact time. In some embodiments, the mass flux comprises a spray emitted from the nozzles, which may be a flat fan spray that concentrates a high mass of liquid moving at a moderate velocity (e.g., 30-120 ft/sec) in a small area. In certain embodiments, the particulate removal device is an upflow WESP, and one or more lower movable spray nozzles is provided in a lower plenum upstream of the particulate collection surfaces and is capable of spraying washing liquid towards the collection surfaces to cause impaction cleaning of the same. In some embodiments, one or more upper spray nozzles is provided, which may be movable, positioned downstream of the particulate collection surfaces. The primary function of the one or more upper spray nozzles is to rinse the collection surfaces, and/or to introduce cleaning agents such as sodium hydroxide or sulfuric acid to enhance cleaning. 
     Embodiments disclosed herein include a particulate removal device for removing particulate from a process gas, the device comprising: a housing comprising a lower plenum having a gas inlet for the introduction of process gas into the housing; a gas outlet for discharge of treated process gas from the housing; at least one ionizing electrode; at least one particulate collection electrode; the lower plenum being in fluid communication with the at least one ionizing electrode and the at least one particulate collection electrode; an upper support frame; a lower support frame connected to the upper support frame and comprising at least one electrode support beam supporting the at least one ionizing electrode; and at least one movable nozzle in the lower plenum for discharging washing liquid towards the at least one collection electrode to dislodge particulate matter from the at least one collection electrode. Preferably the at least one particulate collection electrode is tubular. 
     In one exemplary embodiment, the at least one movable nozzle is rotatable about a vertical axis. In some aspects the particulate removal device further comprises a support shaft in the lower plenum and having a longitudinal axis, the support shaft supporting one or more rotational arms having at least one nozzle positioned thereon, and wherein the one or more rotational arms is adapted to rotate about the longitudinal axis. In some aspects, there are plurality of nozzles positioned on the one or more rotational arms. In some aspects, one of the plurality of nozzles is angled relative to vertical to provide hydraulic motive energy to the one or more rotational arms, whereby discharging liquid through the angled nozzle causes rotation of the one or more rotational arms. 
     In another exemplary embodiment, there is an upper nozzle assembly positioned in the housing downstream, in the direction of process gas flow from the inlet to the outlet, of the at least one particulate collection electrode. 
     In some embodiments, disclosed is a method for cleaning a collection surface of a particulate separation device, in which the collection surface is sprayed with a washing liquid over a cleaning interval, wherein a partial region of the collection surface is sprayed with a minimum quantity of washing liquid for a minimum treatment period, and wherein the washing liquid acts on the partial region with a momentum which varies in time over the minimum treatment period and is effective for dislodging particulate matter adhered to the collection surface. 
     In some embodiments, the angle of action of the washing liquid relative to a surface normal to the partial region does not remain constant over the minimum treatment period; e.g., it is varied. 
     In some embodiments, the at least one nozzle is moved or is movable relative to the partial region in such a way that a distance between the at least one nozzle and the partial region varies over the minimum treatment time. In some embodiments, the at least one nozzle is moved or is movable relative to a surface normal to the partial region in such a way that a liquid jet is emitted from the at least one nozzle at an angle varying with the surface normal to the partial region during the minimum treatment period. In some embodiments, the mass flow of the washing liquid is not constant over the minimum treatment period; e.g., it is varied. In some embodiments, the washing liquid is supplied to the one or more nozzles with a varying pressure and/or volume flow. In some embodiments, the outflow from the one or more nozzles varies in time and/or location. In some embodiments, the at least one movable nozzle is mounted on a nozzle device and is movable in at least one degree of freedom with respect to the nozzle device. In some embodiments, the at least one nozzle comprises a fluidic oscillator. 
     In certain embodiments, a method of removing particulate matter from a contaminated gas supply is disclosed, the method comprising supplying washing liquid to at least one of movable nozzle in a plenum of a particulate removal device comprising one or more ionizing electrodes, one or more particulate collection electrodes or surfaces, at least one inlet for the contaminated air and at least one outlet, the plenum being in fluid communication with the one or more ionizing electrodes and the one or more particulate collection electrodes, and discharging said washing liquid from said nozzle towards said one or more collection electrodes impacting regions of said one or more particulate collection electrodes with said washing fluid emitted from said at least one or more movable nozzles to dislodge particulate from said particulate collection electrodes to clean the same. In some embodiments, the plurality of movable nozzles is upstream, in the direct of gas flow from the inlet to the outlet during operation of said particulate removal device. A source of high voltage for charging the one or more ionizing electrodes may be provided. 
     In certain embodiments, there are a plurality of electrode support beams and a plurality of ionizing electrodes each having a free end and a supported end supported on one of the plurality of electrode support beams, wherein the free end is downstream, in the direction of process gas flow from the gas inlet to the gas outlet during operation of the device, of the supported end. 
     In certain embodiments, the particulate removal device is an up-flow WESP, where gas is introduced below the one or more ionizing electrodes and flows vertically upwardly in the device. 
     In certain embodiments, the device is compartmentalized, or modularized, wherein there are two or more units  100  in a single particulate removal device such as a WESP. In certain embodiments, the WESP has three or more modules. In some embodiments, one of the plurality of modules can be isolated from the others, taken offline and subjected to a cleaning cycle, while the remaining module or modules continue to operate to remove particulate from the process gas stream. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments disclosed herein may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting. This disclosure includes the following drawings. 
         FIG.  1    is a perspective view of an exemplary particulate removal apparatus in accordance with certain embodiments; 
         FIG.  2    is an internal view of an upper region of a particulate removal apparatus in accordance with certain embodiments; 
         FIG.  3    is another internal view of an upper region of a particulate removal apparatus in accordance with certain embodiments; 
         FIG.  4    is an internal view of a lower region of a particulate removal apparatus in accordance with certain embodiments; 
         FIG.  5    is another internal view of a lower region of a particulate removal apparatus in accordance with certain embodiments; 
         FIG.  5 A  is a perspective view of an outer hub in accordance with certain embodiments; 
         FIG.  5 B  is a perspective view of an inner hub in accordance with certain embodiments; 
         FIG.  5 C  is a perspective view of a rotational arm of a movable nozzle assembly in accordance with certain embodiments; 
         FIG.  5 D  is a view, in partial cross-section, of a rotational arm of a movable nozzle assembly in accordance with certain embodiments; 
         FIG.  6 A  is a perspective view of a lower plenum region of a particulate removal apparatus in accordance with certain embodiments; 
         FIG.  6 B  is a front view of the linkage assembly shown in  FIG.  6 B  in accordance with certain embodiments; 
         FIG.  7    is a perspective view showing a preferred fan spray pattern for a movable nozzle assembly in accordance with certain embodiments; 
         FIG.  8    is a schematic diagram of a deflector bar in accordance with certain embodiments; 
         FIG.  9    is a schematic view of a surface of a deflector bar in accordance with certain embodiments; 
         FIG.  10 A  is a top view of a moveable nozzle assembly in accordance with certain embodiments; 
         FIG.  10 B  is a front view of a moveable nozzle assembly in accordance with certain embodiments; 
         FIG.  10 C  is an enlarged view of Detail A from  FIG.  10 A ; 
         FIG.  10 D  is a front view of a pivot arm of a moveable nozzle assembly at rest in accordance with certain embodiments; 
         FIG.  10 E  is a front view of a pivot arm of a moveable nozzle assembly in motion in accordance with certain embodiments; 
         FIG.  11 A  is another front view of a moveable nozzle assembly in accordance with certain embodiments; 
         FIG.  11 B  is an enlarged view of a region of the moveable nozzle assembly of  FIG.  11 A  showing a moveable sleeve in a first position; 
         FIG.  11 C  is an enlarged view of a region of the moveable nozzle assembly of  FIG.  11 A  showing a moveable sleeve in a second position; 
         FIG.  12    is a schematic view showing a working principle of cleaning surfaces by impaction in accordance with certain embodiments; 
         FIG.  13    is a schematic diagram of an assembly including a hydraulic pulse generator for introducing washing liquid into nozzles in accordance with certain embodiments; 
         FIG.  14    is a schematic diagram of a fluidic oscillator in accordance with certain embodiments; 
         FIG.  15 A  is a schematic diagram of a particulate removal device showing the use of fresh water to flush in accordance with certain embodiments; 
         FIG.  15 B  is a schematic diagram of a particulate removal device showing the use of recirculating water to flush in accordance with certain embodiments; and 
         FIG.  16    is an internal perspective view of an exemplary particulate removal apparatus in accordance with certain embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     A more complete understanding of the components, processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawing. The figures are merely schematic representations based on convenience and the ease of demonstrating the present disclosure, and is, therefore, not intended to indicate relative size and dimensions of the devices or components thereof and/or to define or limit the scope of the exemplary embodiments. 
     Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawing, and are not intended to define or limit the scope of the disclosure. In the drawing and the following description below, it is to be understood that like numeric designations refer to components of like function. 
     The singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. 
     As used in the specification, various devices and parts may be described as “comprising” other components. The terms “comprise (s),” “include (s),” “having, “ “has,” “can,” “contain (s),” and variants thereof, as used herein, are intended to be openended transitional phrases, terms, or words that do not preclude the possibility of additional components. 
     All ranges disclosed herein are inclusive of the recited endpoint and independently combinable (for example, the range of “from 2 inches to 10 inches” is inclusive of the endpoints, 2 inches and 10 inches, and all the intermediate values). 
     As used herein, approximating language may be applied to modify any quantitative representation that may vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about” and “substantially,” may not be limited to the precise value specified, in some cases. The modifier “about” should also be considered as disclosing the range defined by the absolute values of the two endpoints. For example, the expression “from about 2 to about 4” also discloses the range “from 2 to 4.” 
     It should be noted that many of the terms used herein are relative terms. For example, the terms “upper” and “lower” are relative to each other in location, i.e. an upper component is located at a higher elevation than a lower component, and should not be construed as requiring a particular orientation or location of the structure. As a further example, the terms “interior”, “exterior”, “inward”, and “outward” are relative to a center, and should not be construed as requiring a particular orientation or location of the structure. 
     The terms “top” and “bottom” are relative to an absolute reference, i.e. the surface of the earth. Put another way, a top location is always located at a higher elevation than a bottom location, toward the surface of the earth. 
     The terms “horizontal” and “vertical” are used to indicate direction relative to an absolute reference, i.e. ground level. However, these terms should not be construed to require structures to be absolutely parallel or absolutely perpendicular to each other. 
       FIG.  1    illustrates exemplary apparatus  100  for removing particulate matter from a gas stream containing particulate matter, and may include a mist-generating member that mixes a gas stream entering the apparatus with liquid droplets, one or more ionizing electrodes that electrically charge the particulate matter and the liquid droplets; one or more collection surfaces such as one or more collection electrodes or tubes that attract and remove electrically-charged particulate matter and intermixed liquid droplets from the gas stream; a source of washing liquid; and one or more movable nozzles configured to be in fluid communication with the source of washing liquid. In certain embodiments, the one or more collection surfaces includes one or more elongated tubes or cells. In some embodiments, the tubes or cells may be hexagonal in cross-section. Other geometric shapes of the tubes or cells may be suitable, including tubes or cells of circular cross-section, square cross-section, rectangular cross-section, heptagonal cross-section, octagonal cross-section, etc. In some embodiments, the unit  100  has a lower inlet  12  and an upper outlet or exhaust  14  spaced from the lower inlet  12 . The lower inlet  12  may be in fluid communication with suitable ducting or the like to direct process gas in a generally upward flow to be treated by the unit  100  towards collection surfaces that in the embodiment shown include an array of a plurality of cells. In certain embodiments, the array of cells may be formed by coupling individual plates or walls in the desired shape such as by welding. Adjacent cells share common walls. 
     In certain embodiments, an upper or downstream (in the direction of process gas flow from the inlet  12  to the exhaust  14 ) high voltage frame  40  ( FIGS.  2 ,  3   ) and a lower or upstream (in the direction of process gas flow from the inlet  12  to the exhaust  14 ) high voltage frame  41  ( FIGS.  4 ,  5   ) are suspended from the roof or top wall  46  of the unit  100  with suitable supports including one or more support rods (three shown as  45 A,  45 B and  45 C). In certain embodiments, the upper high voltage frame  40  may include four connected support members  40 A,  40 B,  40 C,  40 D that form rectangular upper high voltage frame  40  as shown. The top wall  46  of the unit  100  may be electrically insulated from the support rods  45 A,  45 B,  45 C with respective insulators, which may be housed in respective insulator compartments. In various embodiments, the lower high voltage frame  41  may be supported from the top wall  46  such as via top wall-mounted insulators, or may be supported from side wall-mounted insulators. In some embodiments, the lower high voltage frame  41  and associated supports are not needed and are eliminated. In some embodiments, the upper high voltage frame  40  and associated supports may be eliminated, in which case the lower high voltage frame  41  may be supported from the one or more side walls or the top wall  46  (with suitable insulators). 
     In some embodiments, where the lower high voltage frame  41  is supported from the upper high voltage frame  40 , it may be so supported by one or more support electrodes  37 , preferably four, and supports a plurality of rigid electrode support beams  49 , which in turn support electrodes or masts  50 . In certain embodiments, the rigid electrode support beams  49  are spaced and positioned in a parallel horizontal array, each respectively supporting a plurality of masts  50 . Each of the plurality of masts  50  may be generally elongated and rod-shaped and extends upwardly into a respective cell  30 A, and is preferably positioned in the center of each cell  30 A and is coaxial therewith. Since in this embodiment the masts  50  are supported from the bottom by the plurality of rigid electrode support beams  49 , their free ends are downstream, in the direction of process gas flow form the inlet to the outlet, of their supported ends. Preferably the masts  50  are relatively short (e.g., less than 12 feet long, e.g., 10-12 feet long) to minimize deflection. To further minimize deflection, the walls of the masts  50  may be thicker than conventional, e.g., 0.083 inches thick. Further still, cross-bracing may be used to prevent sway of the support structure, e.g., insulated rods or struts connecting the upper high voltage frame  40  and/or lower high voltage frame  41  to a wall of the WESP. In certain embodiments, the volume of each cell  30 A defined by its outer wall or walls is empty except for a mast  50 . As can be seen in  FIG.  5   , in some embodiments each of the masts  50  is attached to a rigid electrode support beam  49  with a single bolt or other fastener, and each mast  50  can be pre-aligned prior to assembly into the unit  100 . In some embodiments, suitable position adjusters can be provided on the masts  50  or support beams  49  to properly position them in the unit  100 . 
     By supporting the masts  50  from the bottom rather than the top, cleaning of the collecting surfaces is not inhibited, and better access to the unit for maintenance is provided because there are minimal high voltage members above the array  30  of cells  30 A. The masts  50 , when positioned within each cell  30 A and connected to a high voltage source, maintain the array  30  of cells  30 A at a desired voltage. In certain embodiments, the electrical potential difference between the masts  50  and the collection surfaces is sufficient to cause current flow by corona discharge, which causes charging of the particulate entrained in the process stream. 
     In other embodiments, the lower high voltage frame  41  may be supported from top wall mounted insulators, or may be supported from electrical insulators mounted in insulator compartments on the side walls of the WESP, below the at least one collection electrode. 
     As seen in  FIGS.  4  and  5   , in certain embodiments, a nozzle movement assembly  52  is provided upstream of the cells  30 A in the direction of process gas flow through the unit  100  to optimize the ability of washing liquid to dislodge particulate matter in the of the particulate removal apparatus  100  to carry out impaction cleaning. Preferably the movement of the system may be actuated or adjusted such that the washing liquid contacts a given area for sufficient time such that the energy of the washing liquid can accomplish the cleaning action desired. Contact times desired would typically be in the range of 250-1,000 milliseconds. 
       FIG.  12    illustrates a working principle of the impaction that may be achieved with the movable nozzle assembly. A cleaned surface element (CSE) of cell  30 A is shown being impacted by impact vector IV. Insome embodiments, the vector/impact angle of the washing fluid with respect to the surface normal (N s  vector) to the surface element to be cleaned may be varied. In addition or alternatively, in some embodiments the impulse of the impacting washing fluid may be varied by varying the mass flux and/or the spray velocity and/or the spraying radius. 
     In some embodiments the movement of the movement assembly  52  may be adjusted manually. In other embodiments, an automatic control scheme may be used, such as an actuator which may be selected from a hydraulic actuator, a pneumatic actuator, an electro-static actuator, an electro-magnetic actuator, a piezoelectric actuator, an electro-mechanic actuator, an electric motor, and other actuators being capable of a remote activation. In some embodiments, the actuator may be a battery operated sealed electric motor attached to the nozzle that receives a signal to rotate the nozzle to adjust the nozzle speed. Such a signal may be transmitted wirelessly. In other embodiments, a mechanical method such as a pivot arm or a spring loaded moving sleeve using the centrifugal force of the spray system to partially block the hydraulic energy and therefore self-regulate the rotational speed may be used. For example, as shown in  FIGS.  10 A,  10 B,  10 C,  10 D and  10 E , a mechanical pivot arm  400  may be used to control the hydraulic energy in the movement assembly  52 , such as by blocking (partially or completely) an orifice  401  ( FIG.  10 D ) formed in the elongated rotational arm  202 . In certain embodiments, the pivot arm  400  is pivotally coupled to the elongated rotational arm  202 , such as with a bar  402  coupled to the arm  202  such as by welding ( FIGS.  10 C,  10 D and  10 E ). The pivot arm  400  may have an aperture (not shown) that receives the bar  402 , and is prevented from releasing from the bar with a fastener  403 , e.g., a cotter pin. When the movement assembly  52  is at rest ( FIG.  10 D ), the pivot arm  400  rests vertically and does not block the orifice  401 ; that is the orifice  401  is open to the ambient. When the movement assembly is in motion ( FIG.  10 E ), the resulting centrifugal force causes the pivot arm  400  to swing away from the resting position shown in  FIG.  10 D , causing a region of the pivot arm  400  to partially block the orifice  401 , thereby partially deflecting fluid exiting the orifice  401 . This, in turn, controls the speed of the movement assembly  52 . 
       FIGS.  11 A,  11 B and  11 C  show yet a further embodiment of controlling the rotational speed of the movement assembly  52 . In this embodiment, there is also an orifice  401  ( FIGS.  11 B and  11 C ) formed in the elongated rotational arm  202 . A sleeve  420  having an inner diameter greater than the outer diameter of the arm  202  is positioned axially on the arm  202  as best seen in  FIG.  11 B , and is free to translate or slide axially on the arm  202 . A stop  405 , such as a metal plate  406  coupled to the arm  202  such as by welding, positioned to restrict the extend of travel of the sleeve  420  on The stop  405  also acts as a seat for one end of the arm  202 . biasing member or spring  408 . The opposite end of biasing member  408  abuts against the sleeve  420 . When, the movement assembly  52  is at rest as shown in  FIG.  11 B , the orifice  401  is not blocked by the sleeve  420 . When the movement assembly  52  is in motion ( FIG.  11 C ), the resulting centrifugal force causes the sleeve  420  to slide axially on the arm  202 , compress the biasing member  408 , and partially blocking the orifice  401 , thereby partially deflecting fluid exiting the orifice  401 . This, in turn, controls the speed of the movement assembly  52 . 
     In some embodiments, the nozzle movement assembly  52  is designed for operation in a particulate laden environment, without fouling of the bearings or other components of the movement system. In certain embodiments, large clearances in the movement assembly  52  are designed to allow for this. These clearances take advantage of the fact that minor leakage of the cleaning liquid is not an issue in the design. The nozzle movement assembly  52  also should be capable of operation within a temperature range of about 40 to 200° F. 
     In certain embodiments, the nozzle movement assembly  52  includes a support shaft  201  and one or more elongated rotational arms  202  supported by the support shaft  201 . Suitable bearings are provided so that the elongated rotational arm  202  can rotate about central hub  203  of the support shaft  201 . As seen in  FIGS.  5 A and  5 B , in the embodiment shown, the central hub  203  includes an outer hub  204  and an inner hub  205 . The outer hub  204  may be generally cylindrical, and include a central aperture  206  configured to receive the inner hub  205 . The outer hub  204  includes two opposite through-holes  207 A,  207 B in its side wall  208  as shown, for receiving the elongated rotational arm  202 . The inner hub  205  includes a lower disk-shaped flange  218  and a cylindrical member  211  extending upwardly from the flange  218 . The cylindrical member is configured to be received in the central aperture  206  of the outer hub  204 , and includes opposite through-holes  212  (only one shown) in its side wall as shown, for receiving the elongated arm  202 . One or more thrust washers  215 , hub bushings  216  and gaskets may be provided, and a retainer ring  217  may be used to assemble the central hub  203  ( FIG.  5   ). In certain embodiments, flat seals  214  (e.g., ultra-high molecule weight plastic or TEFLON seals) ( FIG.  5 D ) may be provided between the inner hub  205  and outer hub  204  as shown, although fluid leakage between the inner hub  205  and outer hub  204  may be tolerated. Because the seals are over a relatively large surface area, fouling by small particulates does not inhibit the hub movement. The seals  214  allow the outer hub  204  to rotate freely. 
     Accordingly, in certain embodiments, the bearings may be designed with loose tolerances to allow movement in a dirty environment, minimizing friction losses and taking advantage of the fact that leaks through bearing seals are tolerated and not an issue to the operation of the nozzle movement assembly  52 . 
     In certain embodiments, one or more spray nozzles  305  are provided on each of the rotational arms  202  such that spray discharged from the spray nozzles  305  impacts the cells  30 A or collecting surfaces at an impaction angle. Preferably substantially all surfaces of the collection electrode below the maximum height that can be reached by the washing liquid discharged from the spray nozzle(s)  305  (based on the angle the washing liquid is discharged from the nozzle(s) are directly impacted by the washing liquid. In certain embodiments, this angle is between about 12 a  and about 30° relative to vertical. Although A 90° impact angle (i.e. perpendicular) provides the greatest cleaning energy, such an angle is not achievable since spray must be introduced above or below the collecting surfaces or cells  30 A. A further consideration on impact angle is the distance into each cell  30 A the spray can reach. The shallower (closer to 0°) the angle of impact, the further the spray can reach, but the lower the energy that impacts the cell walls. Accordingly, an angle of 12° to 30° to vertical has been found to be preferred to provide as much energy as possible while retaining impaction energy a reasonable distance into the collection tube array  30 . The distance that can be reached into a collection tube is a function of the diameter/width of the collection tube. It is preferable, therefore, to use wider and shorter tubes to maximize the cleanability of the tubes. In one preferred embodiment, 16 inch wide by 10 feet long hexagonal tubes are used with 23° impact angle of the spray system, which allows impaction cleaning approximately 3′ or approximately ⅓ of the way into the tube. 
     In certain embodiments, the spray nozzles  305  are spaced along the elongated rotational arms  202  to cover all of the collection surfaces in the array  30  as the rotational arms  202  rotate about the longitudinal axis of the support shaft  201 . In certain embodiments, both the support shaft  201  and the one or more rotational arms  202  include an internal passage and are in fluid communication with each other, so that washing liquid from a washing liquid source introduced into the support shaft  201  with a driving force such as a pump, can flow from the support shaft  201 , to the one or more rotational arms  202 , and into each nozzle  305 , from which the washing liquid is ultimately discharged. Preferably two rotational arms  202  extend coaxially radially outwardly from the hub  203  on each nozzle movement assembly  52 , and the energy of the cleaning sprays are balanced opposite each other on the two rotational arms  202 . 
     In various embodiments, a hydraulic pulse generator  450  ( FIG.  13   ) may be used upstream of the nozzle  305  to aid in impaction. For example, liquid pump  455  may introduce washing fluid to the pulse generator  450 , which causes the fluid to pulse as it flows to the nozzle  305 . In some embodiments, the pulse generator  450  may be partially bypassed via bypass line  460  to generate an oscillating liquid pressure with a positive base pressure. The bypass may have a controllable orifice  465  to vary the base line pressure manually, it may be varied in a stochastic automated manner. 
     In some embodiments as shown in  FIG.  14   , a fluidic oscillator may be used as or as part of the nozzle  305 . Fluidic oscillators typically have no moving parts and spray fluid from side-to-side, generating alternate bursts of fluid. 
     In some embodiments, rotation of the nozzle movement assembly  52 , and of the rotational arms  202  in particular, may be effectuated by positioning one or more angled nozzles  210  on one or more of the rotational arms  202 , so that hydraulic energy is used to drive the rotation of the rotational arms  202 . Preferably the angled nozzle  210  is positioned at or near the free end of a rotational arm  202 , and is positioned at an angle of 35 to 65 degrees relative to vertical. In some embodiments, there are plurality of spray nozzles  305  that are positioned at the same angle relative to vertical (e.g., 0°), and a single angled nozzle  210  that is positioned at the aforementioned angle of 35 to 65 degrees, and therefore is also angled with respect to the plurality of spray nozzles  305 . Discharging washing liquid through the one or more angled nozzles  210  causes rotation of the rotational arm  202 . In certain embodiments, the angle of the one or more angled nozzles  210  may be adjustable, so as to adjust the speed of rotation of the rotational arms  202 . In embodiments where a spray nozzle  305  is threaded onto the rotational arm  202 , the adjustment can be made by loosening or tightening the spray nozzle  305  via relative rotation of the nozzle and the rotational arm  202 . Rotational speeds up to about 10 rpm are suitable. Higher speeds could be used, but do not offer any advantage and require more energy to achieve. A fluid pressure range of about 40-100 psig is suitable to achieve the objectives discussed herein. 
     In certain embodiments, more than one such nozzle movement assembly  52  can be positioned upstream of the cells  30 A, as needed, so as to ensure spray coverage of a module effective to clean all desired surfaces. 
     In certain embodiments, multiple nozzle assemblies may be installed at different elevations (relative to horizonal) to allow for an overlapping spray pattern for improved cleaning without the assemblies potentially interfering with each other.  FIG.  16    illustrates such an embodiment. Where multiple rows of nozzle assemblies are present, preferably the nozzle assemblies that are diagonally positioned from each other are at the same elevation. For example, in the embodiment shown in  FIG.  16   , nozzle assembly  600  is at a first, lower elevation, while nozzle assembly  602  is at a second, higher elevation relative to nozzle  600 ; nozzle assembly  604  is at a high elevation, preferably the same higher elevation as the higher elevation of nozzle assembly  602 . Similarly, nozzle assembly  606  is at a low elevation, preferably the same lower elevation as nozzle assembly  600 . This pattern continues with assemblies  608 ,  610 ,  612  and  614 , so that in the embodiment shown, there are two rows of four nozzle assemblies, one row having an elevational arrangement of high-low-high-low nozzle assemblies (as viewed from left to right in the Figure), and the other a corresponding elevational arrangement of low-high-low-high nozzle assemblies (as viewed from left to right in the Figure). In this way, an overlapping spray pattern is achieved while avoiding physical contact or interference between adjacent nozzle assemblies as they rotate or otherwise move, since the rotational arms  202  of each assembly are at different elevations. 
     In certain embodiments the support shaft  201  may be angled up to 15° from vertical such that assembly  52  is angled up to 15° from horizontal. The purpose of this embodiment would be to allow a different angle of impaction within the tube to improve cleaning. Each of the multiple assemblies  52  may be installed at the same or different angle as necessary to achieve desired cleaning. Suitable angles include from about 3° to about 15°, more preferably from about 5° to about 10°. Thus, angles from about 3°, 4°, 5°, 6°, 7°, 8°, 9°, 10°, 11°, 12°, 13°, 14°, and 15° may be suitable. 
     In certain embodiments, one or more downstream nozzle assemblies  520  may be provided downstream of (e.g., above) the cells  30 A or collection surfaces, as seen in  FIG.  2   . The one or more downstream nozzle assemblies  520  provided downstream of the cells  30 A or collection surfaces may be movable, like the one or more movable nozzle assemblies  52  provided upstream of the cells  30 A or collection surfaces. The primary function of the one or more downstream nozzle assemblies  520  is to provide a rinse feature, e.g., to rinse loose material off of components such the collection surfaces, and they also optionally may be used to introduce a cleaning agent such as sodium hydroxide or sulfuric acid. Since unlike the one or more upstream nozzle assemblies  52 , impaction cleaning is not their primary function, the speed of movement of the one or more downstream nozzle assemblies  520  is not critical, and angled nozzles need not be provided to create hydraulic motive energy for rotation. Its speed of rotation may be dictated instead by the amount of water flow; the more water flow to the nozzles  520 , the faster they rotate. Alternatively, an electric motor can be used to move the one or more downstream nozzle assemblies  520 . Similarly, a motor may be used to move the one or more upstream nozzle assemblies  52 . 
     In certain embodiments, it may be desirable to optimize the spray pattern of the washing liquid discharged from the nozzles  305 . The use of fan nozzles that emit flat fan sprays  300  that concentrate a high mass of liquid at moderate velocities in a small area may be used, as shown in  FIG.  7   . In some embodiments, the mass flux may be a 10 -2,000 lbs/(ft 2 *s) with a velocity range of 30 to 120 feet per second. The lower end of 10 lbs/ (ft 2 *s) is approximately 50 to 200 times greater than conventionally used. This provides a potential impaction energy flux in the range of 140 to  450 ,000 (lb f *ft) / (ft 2 *s). The actual impaction energy flux is affected by a number of variables such as angle of impaction, surface roughness, and properties of any material that is built up on the surface. As discussed previously this high energy flux is achieved by focusing the liquid flow in a small area and requires the nozzles to be moved to adequately clean the majority of the WESP surfaces where buildup occurs. These high impaction energies could also be achieved with low volume high velocity sprays as are commonly used in a standard pressure washer. However, these systems require very small passages to achieve the high velocities necessary and these passages are very prone to fouling in the dirty environment of a WESP. Therefore the high volume, moderate velocity liquid cleaning system described herein, which allows larger flow passages that are much less susceptible to plugging, is preferred. In one preferred embodiment, mass fluxes are 100 to 500 lbs/(ft 2 *s) with a velocity range of 75 to 95 feet per second. This provides a potential impaction energy flux in the range of approximately 10,000 to 70,000 (lb f *ft) / (ft 2 *s) . 
     In an alternative embodiment, an electric motor may be used as the driving energy to drive the rotation of the rotational arms  202 . Multiple pipes may be used with the spray nozzles inserted along the length of the pipe, as shown in  FIG.  6 A . The pipes are oscillated together by a single motor  301  with a linkage assembly  303  including a lower header connecting rod  303 A and a crank arm connecting rod  303 B connected to a pivot arm  304  ( FIG.  6 B ). The oscillating movement moves the nozzles  305  so that they impact the tubes at different angles in different locations, to provide cleaning. This embodiment requires substantially more nozzles to clean the same area as the rotary spinning system embodiment, but keeps the bearing surfaces from being exposed to the process liquid. When motor  301  rotates the assembly, rod  303 B will push arm  303 C clockwise as viewed in  FIG.  6 B  during the rotation from 0 to 90 degrees. This will rotate shaft  302  clockwise. When rotating from 90 to 270 degrees, the rod  303 B will rotate arm  303 C and shaft  302  counterclockwise. The rotation will proceed back to clockwise from 270 to 0 degrees. Rod  303 A connects the two arms  303 C so that they move together. 
     In an alternative embodiment, with reference to  FIGS.  8  and  9   , one or more spray nozzles  2005  in fluid communication with a manifold  2006  or the like may be mounted on or near the side wall  101  of the WESP housing to spray washing liquid in a solid stream in a roughly horizontal plane. Preferably the nozzles  2005  are located on either side of the WESP in order to be able to clean all collecting surfaces. In certain embodiments, the one or more nozzles  2005  spray a tight column of liquid in open air, and may be smooth bore nozzles which exhibit such a tight column. A deflection bar  2010  may be provided to move horizontally along the bottom of the WESP, such as on one or more rails  2008  in conjunction with a linear drive  2009 . The deflection bar may include surfaces  2011  that when impacted by the liquid column emitted from the one or more nozzles  2005 , deflect the solid column spray at an angle, such as a 65° to 75° angle, creating a fan spray pattern into the collection tubes for cleaning the same. Thus, in this embodiment, the energy of the water hitting the appropriately configured and dimensioned surfaces  2011  of the deflection bar  2010  form the fan spray. As seen in  FIG.  9   , an adjuster  2012  may be provided to adjust the angle of the surfaces  2011  where the water column contacts for deflection. Suitable sources of drive energy to move the deflection bar  2010  may be hydraulic, such as by using a portion of the washing liquid supply used for cleaning, or electric, such as with an electric motor and a linear drive. This embodiment has the advantage of using large spray nozzles that are less prone to plugging and facilitate maintaining them while the WESP is operating. The manifold  2006  and a portion of the nozzle bodies may be located outside the WESP housing, e.g., external to side wall  101 , further facilitating maintenance thereof. 
     In certain embodiments, recirculating liquid may be used in place of fresh water or other clean liquid. As shown in  FIG.  15 A , recirculating liquid may be used continuously to quench the process gas to saturation temperature which is required for proper operation of the WESP. The embodiment of  FIG.  15 A  uses fresh water from a suitable source (e.g., city water  500 ) to supply washing fluid to the upper and/or lower spray nozzles as shown. Thus, a WESP recirculation tank  502  and a suitable driving force such as one or more pumps  505  are provided to supply the quench sprays  510  for quenching the process gas as it is introduced into the WESP, and a flush tank  503  and a suitable driving force such as one or more pumps  506  are provided to supply fresh water to the upper and/or lower spray nozzles. The flush tank  503  can be located inside of the recirculation tank  502  as shown in  FIG.  15 A  to heat the flush water using the heat from the recirculating water, which is typically 10 to 15 F less than the saturated air temperature. In practice this heats the flush water to approximately 40 to 60 F less than the recirculating water. In certain embodiments, the WESP has a fluid drain  512  in fluid communication with the recirculation tank  502  through suitable ductwork or the like. The use of fresh water limits the amount of water that can be used during the flush to less than or equal to the amount of water that is evaporated by saturating the gas and the amount of water that is removed through the system blowdown  507 . Otherwise water will accumulate in the system. 
     In some embodiments such as that shown in  FIG.  15 B , recirculating liquid also may be used as a source of washing fluid supply. Using this liquid for cleaning collection surfaces allows a much larger volume of liquid to be used for cleaning without impacting the accumulation of water in the system. The recirculating water typically has a substantial amount of solids in it (between 2-4% by weight). Accordingly, the liquid may be filtered or screened to remove larger solids (typically greater than ⅛″). Therefore, as discussed above the spraying components may be designed to function while flowing the particulate laden water. As shown in  FIG.  15 B , Water from the recirculation tank  502 ′ is used as the source of washing fluid to the upper and/or lower spray nozzles and to the quench sprays  510 ′ as shown. A suitable driving force such as one or more pumps  505 ′ are provided to supply the quench sprays  510 ′ for quenching the process gas as it is introduced into the WESP, and to supply recirculating water to the upper and/or lower spray nozzles. In certain embodiments, the WESP has a fluid drain  512 ′ in fluid communication with the recirculation tank  502 ′ through suitable ductwork or the like. In this case, fresh water from a suitable source (e.g. city water  500 ′ ) is only used as make-up water as needed to balance the system from evaporation losses and system blowdown  507 ′. 
     In certain embodiments, hotter liquid, such as recirculating water, may be used in the spraying system for improved cleaning. Higher temperatures increase the solubility of nearly all solids. By using higher temperature cleaning liquid, the effectiveness of the cleaning can be enhanced substantially. Typical temperature ranges of from 150 to 180° F. are suitable. 
     In certain embodiments, the cleaning may be performed when the process flow through the WESP module is offline. If the process is online through the WESP during a cleaning cycle, essentially no particulate is being removed because the power must be shut off during a cleaning cycle. Therefore, the cleaning cycle time must be relatively short (&lt;5 minutes) because of regulatory or downstream process requirements. Cleaning the module offline allows the system to take extended time for cleaning while minimizing the downstream impact by maintaining the particular removal of the gas in other WESP modules in parallel with the module being cleaned. The extended offline cleaning can enhance the use of common cleaning chemicals such sodium hydroxide or sulfuric acid by allowing these chemicals time to react with the buildup before being rinsed off, which can greatly improve the removal efficiency. Another benefit of this embodiment is that none of the mist created during the washing cycle is carried downstream of the equipment, since there is no airflow during the cleaning cycle. 
     If the WESP is an upflow design, another embodiment is to include a rinsing flow from the top of the WESP either during or at the conclusion of the impaction cleaning performed at the bottom of the WESP. This rinsing flow can either be stationary or moving as described for the impaction cleaning sprays. The rinsing sprays provide a method of rinsing off any solids loosened and pushed up in the WESP by the lower impaction sprays. 
     A final rinse of the WESP with fresh water after the cleaning cycle is finished may be carried out. This final cleaning cycle serves to remove residual solids left when the recirculating water is turned off as well as to flush any residual solids out of the wash piping. 
     EXAMPLE 
     Consider a 3 module upflow WESP system treating 150,000 ACFM of polluted air. Timers in the control system initiate the cleaning cycle for one of the modules. The following steps may be performed.
     The module to be cleaned is isolated from the process gas either by closing a damper or other means of stopping the process gas flow to that module.   The process gas flow is forced to flow through the two modules remaining online where it is still cleaned at a modest loss in efficiency because of the higher flow rate.   The power to the electrostatic system is turned off after the flow is stopped.   After the power is turned off, one or more lower (i.e., upstream of the collection surfaces) rotary spin systems is activated, spraying approximately 900 GPM (gallons per minute) of hot recirculating water. The spinners remain on for approximately 30 seconds, rotating at approximately 2 RPM to remove any loose deposits.   A cleaning solution of sodium hydroxide (or other cleaning agent) and water can then be applied through the upper sprays (i.e., downstream of the collection surfaces) for a short period (e.g., 15 to 30 seconds).   One or more lower rotary spin systems is then turned on again, spraying approximately 900 GPM of recirculating water. The spinners remain on for 3 to 5 minutes, rotating at approximately 2 RPM, for primary cleaning.   Once complete, an upper rinse spray, running at 450 GPM of recirculating water, is turned on for 1 to 2 minutes to wash down material dislodged by the primary cleaning cycle.   During this time, 100 GPM of fresh water may be flushed through the lower sprays for 30 to 60 seconds to flush the recirculating water out of the piping.   A final rinse of either fresh water or cleaning solution through the top sprays is carried out to clean the upper spray lance and any residue left by the recirculating water. A flow rate of approximately 100 GPM for 15 to 30 seconds may be used.   A delay of approximately 2 minutes may be employed for excess water to drain before power is turned back on to the electrodes and air flow is reestablished through the module.