WO2007087228A2

WO2007087228A2 - Polarity reversing circuit for electrostatic precipitator systems

Info

Publication number: WO2007087228A2
Application number: PCT/US2007/001449
Authority: WO
Inventors: Ralph F. Altman; Robert N. Guenther; Grady B. Nichols
Original assignee: Electric Power Research Institute
Priority date: 2006-01-24
Filing date: 2007-01-18
Publication date: 2007-08-02
Also published as: US7413593B2; WO2007087228A3; US20060130648A1; WO2007087228A8

Abstract

A gas separation apparatus using electrostatic precipitators and mechanical rappers is enhanced by the addition of an opposite polarity refreshing power supply and a switching arrangement. The switching components selectively disconnect the primary power supply and connect the refreshing power supply to the electrostatic precipitator, causing an electrical impulse in the precipitator sufficient to dislodge precipitate from the collector plates. In one embodiment, an RC filter is further provided to control the impulse and reduce the burden that would otherwise be placed upon the refreshing power supply. In a second embodiment, a pair of SCR strings serve as the switches. Cleaning power is delivered from the capacitor through one of the SCR strings using a resonant circuit, the resonance which causes the SCR to commutate off after the impulse has been delivered. The capacitor is charged to a pre-calculated potential, dependent upon a measured potential just prior to delivery of the cleaning power, to ensure the cleaning voltage stays below a corona onset voltage.

Description

POLARITY REVERSING CIRCUIT FOR ELECTROSTATIC PRECIPITATOR SYSTEMS

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION This invention pertains generally to gas separation apparatus using an electric field. More specifically, the present invention uses non-liquid cleaning techniques to maintain electrostatic precipitator electrodes. In a most specific manifestation, a new method and apparatus are provided to dislodge ash from collection plates within an electrostatic precipitator.

2. DESCRIPTION OF THE RELATED ART Industries as diverse as mills, pharmaceutical or chemical, food processing, and cement kilns must separate contaminants or particulates from an air or gaseous stream. The gases may be a product of combustion, such as present in an exhaust stack, but may also represent other gas streams and may contain such diverse materials as liquid particulates, smoke or dust from various sources, and the like. Separators that must process relatively large volumes of gas are common in power generating facilities and factories.

The techniques used for purification of gas streams have been diverse, including such techniques as filtration, washing, flocculation, centrifugation, and electrostatic precipitation. The techniques have heretofore been associated with certain advantages and disadvantages; hence have limited application. In filtration, particulates are separated through a mechanical filter which selectively traps particles of a minimum size and larger. Unfortunately, flow through a filter is limited by the surface area and cleanliness of the filter. The filter material must be both durable and simultaneously open and porous. In higher volume systems, and in corrosive or extreme environments, filters tend to clog quickly and unpredictably, and present undesirable resistance to the passage of the gas stream. During the period of filter changing or cleaning, which can be particularly tedious, the machine, equipment, or process must be stopped or diverted. This shutdown requires either a duplicate filtration pathway, which may add substantial cost, or a shutdown of the machine or process. Until recently, these limitations presented design challenges that have primarily limited this technology to low volume purification. Washing offers an advantage over dry filtration in presenting the opportunity for selective gas or liquid particulate separation and neutralization, and in reduced gas flow resistance. Unfortunately, the liquid must also be processed; and where there are high levels of particulates, the particulates must be separated from the liquid by yet another process, or the liquid and particulates must be transported to some further industrial or commercial process or disposal location. The added weight and difficulty of handling a liquid (in addition to the particulate) during transport makes liquid separation less desirable in many instances, particularly where there may be a demonstrated application for the particulate content within the gas stream.

Similar to washing, flocculation necessitates the introduction of additional materials that add bulk to the waste stream and unnecessarily complicate the handling and disposal of the contaminants. Furthermore, the flocculating materials must also be provided as raw materials, which may add substantial expense in the operation of such a device. Consequently, flocculation is normally reserved for systems and operations where other techniques have been unsuccessful, or where a particular material is to be removed from the gas stream which is susceptible to specific flocculent that may provide other benefit.

Centrifugation presents opportunity for larger particle removal, such as separation of sand or grit from an air stream. However, centrifugation becomes slower and more complex as the size of the entrained particles or liquids become smaller. Consequently, in applications such as the removal of fly ash from a combustion stream, centrifugation tends to be selective only to relatively large particles, thereby leaving an undesirably large quantity of fine fly-ash in the effluent stream. Electrostatic precipitators have demonstrated exceptional benefit for contaminants including fly ash, while avoiding the limitations of other processes. For example, unlike centrifugation, electrostatic precipitators tend to be highly effective at removing particulates of very minute size from a gas stream. The process provides little if any flow restriction, and yet substantial quantities of contaminants may be removed from the air stream. When contaminants pass through an electrostatic precipitator, they pass between discharge electrodes and collection electrodes, which transfer an electrostatic charge to the contaminants. Once charged, the contaminants will be directed by the charge force towards the oppositely charged collecting electrodes. The collecting electrodes are frequently in the form of plates having large surface area and relatively small gap between collector plates. -The dimensions of the plates and the inter-electrode spacing is a function of the composition of the gas stream, electrode potential, particulate size of contaminants, anticipated gas breakdown potential, and similar known factors. The selection of dimension and voltage will be made with the goal of gas stream purification in mind, and in gas streams where very fine particulate matter is to be removed, such as with fly ash, relatively high voltage potentials and larger plates may be provided. The proper transfer of charge to the particulates and the subsequent electrostatic attraction to collector plates is vital for proper operation.

By design, the collector plates will accumulate contaminants. As electrically non- conductive particles are deposited, the layers of accumulating particles develop an electrical potential gradient through the thickness of the deposited layer, whereby the voltage at the exposed surface decreases in electrical potential, and possibly even reverses charge. When a sufficiently thick layer of electrically non-conductive particles has accumulated to reduce the surface potential, further significant particulate capture becomes difficult or impossible. Consequently, and in spite of the many benefits, electrostatic precipitators have heretofore been limited in efficiency by the effects of the contaminants on the collection plates. m order to provide continuous efficient operation of the precipitator, a number of automatically controlled cleaning techniques are used. One almost universal technique used in dry electrostatic precipitators is the use of a mechanical rapper device. The rapper creates vibration in the collector electrodes, in turn causing the precipitate to drop off of the electrodes. Generally the precipitate drops under the influence of gravity or is carried by a special air stream into a separate container for final disposal.

Several patents are exemplary of the use of rappers, including Brandt in U.S. patent 3,274,753; Johnston et al in U.S. patent 5,173,867, Lund in U.S. patent 5,792,240; and Terai et al in U.S. patent 6,336,961, each of which is incorporated herein by reference for their teachings of rapper systems for use with electrostatic precipitators. Unfortunately, the mechanical rapper systems of the prior art have been known to require substantial cycle times, and the mechanical forces tend to move the contaminant back into the gas stream. Furthermore, rapper systems tend to be maintenance intensive, and, for high resistivity particulate, the rapper tends to be relatively ineffective, owing to the accumulation of electrical charge on the particulate surface.

Since the release of undesirable contaminants entrained within the gas stream is undesirable, other techniques besides mechanical rappers have been proposed. Gallo et al in U.S. patent 5,378,978 and Shevalenko et al in U.S. patent 4,536,698 each illustrate electronic systems to control the accumulation of precipitate upon the electrodes. In particular, the control system of Gallo et al illustrates the challenges of prior art systems, including many components and much complexity. What is desired then is a method or apparatus to overcome these limitations of the present electrostatic precipitators. SUMMARY OF THE INVENTION

The present invention overcomes the limitations of the prior art by using readily available electronic components in a novel configuration and through a novel operational method. In a first manifestation, the invention is a method of applying electrical energy to an electrostatic precipitator collector. The method enables operationally effective cleaning using electrical energy, and enhances, supplements or eliminates the operation of mechanical rappers. According to the method, electrical energy having a first electrical polarity is applied to the electrostatic precipitator collector, and the precipitate is collected. A need for cleaning is determined, and applied electrical energy is switched, using a novel combination of high- voltage SCR switches and resonant circuit, from first electrical polarity to a second, opposite electrical polarity. Rapping may or may not be done while the second electrical polarity is being applied, to remove collected precipitate from the electrostatic precipitator collector. Finally, the applied electrical energy is reset to the first electrical polarity. In a second manifestation, the invention is a polarity reversing power supply that electrically enhances precipitate removal from an electrostatic precipitator collector. A primary power source has a first electrical power terminal of first polarity connected to the electrostatic precipitator collector and a second electrical power terminal connected to a' precipitator electrode. The primary power source, electrostatic precipitator collector and electrostatic precipitator electrode are operatively interconnected to complete a primary electrical circuit through which primary electrical current flows. A first electrical switch is electrically connected within the primary electrical circuit and has a first electrically. closed state through which primary electrical current flows and a second electrically open state through which primary electrical current is blocked. A refreshing power source has a first electrical power terminal of second polarity connected to the electrostatic precipitator collector and a second electrical power terminal connected to the precipitator electrode. A capacitor is coupled between the refreshing power source first and second electrical power terminals, in parallel to the refreshing power source. An inductor is coupled in series between the refreshing power source and electrostatic precipitator. The refreshing power source, electrostatic precipitator collector and electrostatic precipitator electrode are operatively interconnected to complete a secondary electrical circuit through which secondary electrical current flows. A second electrical switch is electrically connected within the secondary electrical circuit and has a first electrically closed state through which secondary electrical current flows and a second electrically open state through which secondary electrical current is blocked. The capacitor and inductor form a resonant circuit with the electrostatic precipitator, to both rapidly and precisely switch the voltage across the electrostatic precipitator. The first and second electrical switches are operatively coupled to prevent simultaneous closure. In a third manifestation, the invention is an electrostatic precipitator having at least one discharge electrode for charging particulates within a gas stream, at least one collector for attracting the newly charged particulates, a high voltage power source operatively and selectively able to apply a high voltage potential of a first polarity between discharge electrode and collector, and a rapper for intermittently agitating the collector. A second high voltage power source is operatively and selectively able to apply a high voltage potential of a second polarity opposite to the first polarity between discharge electrode and collector. A switch is included that in a first state operatively completes an electrical circuit to apply high voltage potential from the first high voltage power source between discharge electrode and collector while maintaining said second high voltage power source isolated therefrom, and in a second state operatively completes an electrical circuit to apply high voltage potential from the second high voltage power source between discharge electrode and collector while maintaining the first high voltage power source isolated therefrom. A resonant circuit coupled with the second high voltage power source in combination with a voltage control circuit within the second high voltage power source ensures rapid and controlled voltage transitions. A means is also provided for placing the switch in the second state simultaneous with activating the rapper.

The present invention finds particular utility in a coal-burning power plant, wherein a dry electrostatic precipitation system is employed for removing fly ash, the fly ash being collected on electrostatic plates in the system. In accordance with the teachings of the present invention, a polarity reversing circuit is provided for periodically dislodging the fly ash from the electrostatic plates.

In one embodiment, a mechanical rapping system is provided for dislodging material collected on the electrostatic plates, the polarity reversing circuit supplementing the mechanical rapping system. Preferably, the intensity of the mechanical rapping system may be varied from zero to a maximum intensity.

OBJECTS OF THE INVENTION

A first object of the invention is to improve the operational effectiveness of electrostatic precipitator systems. A second object of the invention is to reduce the time required to clean collector plates. A third object of the invention is to enhance existing cleaning techniques with a complementary and non-exclusive technique. Another object of the invention is to accomplish the foregoing using readily available electronic components, including thyristor switches. An additional object of the invention is to improve the electrical performance within an electrostatic precipitator during a cleaning cycle. Yet another object of the invention is to facilitate better collection of fly ash from coal fueled electric utility plants. These and other objects are achieved in the present invention, which may be best understood by the following detailed description and drawing of the preferred embodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates a preferred electrical circuit designed in accord with the teachings of the invention by simplified schematic diagram.

Figure 2 illustrates a preferred method designed in accord with the teachings of the invention.

Figure 3 illustrates a second preferred electrical circuit, which adds additional performance circuitry to the circuit of Figure I₅ by simplified schematic diagram. Figure 4 illustrates an alternative method useful in conjunction with the second preferred electrical circuit of Figure 3, by flow chart.

DESCRIPTION OF TEE PREFERRED EMBODIMENT

With reference to Figure 1 , a preferred polarity reversing circuit 10 includes a primary power supply 12. Power supply 12 may be of any type known in the prior art, and will typically have a first negative voltage output 13 and a second positive polarity output 14 connected to a circuit common or ground connection. In the precipitation of fly ash from an exhaust stream, using an exemplary prior art electrostatic precipitator ESP, power supply 12 will typically provide an output voltage potential of between 5 kilovolts and 150 kilovolts at an operating current typically within the range of 100 to 2500 milliamperes. The negative polarity output 13 is connected to electrostatic precipitator ESP through switch Sl, which, during the standard precipitation function, remains closed.

Second refreshing power supply 15 is also preferably provided, and may use the same or similar components as found in primary power supply 12. While this selection of similar components is not necessary for the working of this invention, the use of like or similar components makes testing and maintenance somewhat simpler than working with larger varieties of devices. Refreshing power supply 15, when applied to this exemplary circuit and for use with electrostatic precipitator ESP, will most preferably be able to provide a peak current of approximately 400 milliamperes, at a voltage potential of from 5 kilovolts to approximately 30 kilovolts. Positive output 16 is most preferably connected to electrostatic precipitator ESP through switch S2 and an RC filter comprised by series resistor R and parallel capacitor C₅ as illustrated in Fig. 1..

Preferred polarity reversing circuit 10 will have switch Sl normally closed during standard gas stream precipitation, while switch S2 will remain normally open. When electrostatic precipitator ESP requires cleaning, which may be determined through time interval calculation or through electrical sensing and detection techniques known in the art, switch S 1 will be opened and switch S2 will be closed. Electrostatic power supply ESP typically presents a large capacitive load, while most high voltage power supplies of the type used in precipitators present a large inductive output. The combination of inductance and capacitance might lead to an oscillation or ringing, and occasionally a dangerous over- voltage condition or overload for the power supply. The RC filter is provided to prevent an undesirable loading, ringing or similar oscillation or surging of refreshing power supply 15 that might otherwise occur. Resistor R also acts as a current limiter to control surge or inrush current. Capacitor C may also be used to provide an energy store which will generate a more rapid voltage transition within precipitator ESP than would be attainable otherwise for a given peak current rating for refreshing power supply 15.

Most preferably, refreshing power supply 15 will be connected through switch S2 to electrostatic precipitator ESP for an interval of approximately 1 to 10 milliseconds, which is adequate in many applications to perform operationally effective cleaning. For the purposes of this disclosure, operationally effective cleaning will be understood to be the removal of sufficient precipitate from the collection elements of electrostatic precipitator ESP to maintain satisfactory performance and permit continued operation. The exact timing, and appropriate voltage and current, will be determined by those skilled in the art for a particular electrostatic precipitator and precipitate composition. At the end of the connection interval, switches Sl and S2 will be once again restored to the normal precipitation arrangement, where SI will be closed and S2 will be open.

Switches S 1 and S2 will most preferably not be simultaneously closed. Such closure would result in resistor R serving as the entire load for both power supplies 12, 15. This is a waste of substantial electrical energy and will create a potentially very dangerous overload. Control of switches Sl, S2 to maintain at least one switch open at all times is known in the switching art, and will depend upon the actual implementation of switches. For exemplary ^"purposes only, and not limited thereto, switches Sl and S2 may be electro-mechanical switches such as relay switches, in which case the switching may be achieved using a mechanical or electro-mechanical open-before-close arrangement, or the switches may be mechanically coupled to prevent simultaneous closure. Where switches Sl and S2 are thyristors, such as but not limited solely to silicon controlled rectifiers, triacs or the like, -^{' •} activation is achieved electrically or electronically, in which case suitable control circuitry will be provided. The means to control switching of switches Sl, S2 and activation of the rapper within electrostatic precipitator ESP is illustrated by dashed line 19 in Figure 1, which is the ordinary symbol for mechanical coupling of electrical devices, but, as aforementioned, such coupling may be through electronic control as well.

The preferred physical arrangement illustrated in Figure 1 is to incorporate the RC filter and switches Sl, S2 into a separate power supply switch box 18. The exact nature of this box 18 will depend upon the type of switches chosen for switches S 1 , S2, which are known in. the art to include mechanical, electro-mechanical, solid state or vacuum tube switches. Power supplies 12, 15 are each separately housed, which simplifies maintenance by permitting easy modular replacement of malfunctioning devices.

With reference to Fig. 2, the preferred method 20 of cleaning ash from an electrostatic precipitator, which will be described herein for exemplary purposes utilizing the preferred embodiment polarity reversing circuit 10 for implementation, includes at step 22 the energizing of precipitator ESP. This is accomplished in polarity reversing circuit 10 by energizing primary power supply 12 and closing switch Sl. At step 24, precipitate will be collected, generally by passing the gas stream with entrained particulate through electrostatic precipitator ESP. During this step 24, switch Sl will remain closed and switch S2 will remain open. Precipitate will normally be collected until such a time as there is a determined need for cleaning the collector plates. This determination of need for cleaning 25 may be time-based or by other known technique, the exact method which is not critical to the operation of the present invention. The method of determining will normally be selected to optimize power while holding particulate re-entrainment at a low level.

When the need for cleaning is determined in step 25, power supply polarity will be switched at step 26. This will preferably generate an impulse of opposite polarity. As may be recognized in association with the present description, a rapid impulse offers substantial benefit where high resistivity particulate is being collected. This is due to the reverse polarity phenomenon described herein above, where high resistivity particulate will gradually form an insulation layer and static charge of opposite polarity is retained or collected in the particulate. Consequently, a rapid impulse of reversed polarity will generate very consequential electrostatic force which repels the particulate from the collector plates. The time required for a reverse polarity impulse to clear the collector will be determined by the physical, chemical and electrical characteristics of the particulate as well as the plate geometry, impulse voltage and waveform, and other factors too numerous to describe in detail herein, but may be readily determined and optimized experimentally by those skilled in the art for a given application. For the application to fly ash precipitate, a time of from 1 to 10 milliseconds has been determined to be optimal.

The electrical cleaning of precipitate is very rapid, and provides a reliable approach to the maintenance of an electrostatic precipitator. The benefit over prior art mechanical rappers, which must be tested manually or visually to determine whether they are operating properly, is very significant. For some dry high resistivity precipitates, the reverse polarity impulse may be all that is required to clean the collector plates. However, the present invention further contemplates the use of the reverse polarity impulse in conjunction with mechanical rappers, as shown by parallel step 28. Most preferably, the reverse impulse of step 26 will be timed to correspond to the mechanical impulse of step 28, thereby forming a synergistic benefit which ensures complete removal of precipitate. Once the precipitate is removed from the collector plates in step 26 and optional step

28, primary power supply 12 will be reset to provide power to electrostatic precipitator ESP, and refreshing power supply 15 will be disconnected therefrom. This is identified in Fig. 2 as step 29, where the power supply is reset to normal collecting condition. Method 20 of cleaning ash may then return to step 24, where precipitate is once again collected. As will be apparent, Fig. 2 does not include various optional steps that may be further included, depending upon the design of the physical apparatus, such as the use of ash collection techniques (hoppers, bags, etc.) as known in the prior art.

Figure 3 illustrates a second preferred polarity reversing circuit 30 suitable for effecting method 20 of Figure 2. Polarity reversing circuit 30 is electrically similar to the preferred polarity reversing circuit 10 of Figure 1. As Figure 3 illustrates, primary power supply 12 and refreshing power supply 15 are still provided, for alternative and non- simultaneous application of electrical energy to electrostatic precipitator ESP. However, SCRl , which is a series Silicon Controlled Rectifier string, is specifically used as the switch which controls application of power from primary power supply 12. SCRl, as proposed herein above, is one specific electronic implementation of switch Sl, which is preferred for the present application. A similar series SCR string, SCR2, is provided as the switch which controls application of power from refreshing power supply 15 to electrostatic precipitator ESP. As with polarity reversing circuit 10 of Figure 1, mechanical rappers may be provided which are designed to activate with the application of power from refreshing power supply 15.

Additional components are provided in polarity reversing circuit 30 which are not present in polarity reversing circuit 10. More particularly, capacitor C has been replaced by 5 capacitor C_ps, which is chosen to most preferably have a capacitance that within a range of

* approximately ±25% of the capacitance of electrostatic precipitator ESP. Series resistor R has been replaced by a combination of power supply resistors R_pS] and R_p52, and also R₃₆₁.. In addition, resonant inductor L_r is provided in series between refreshing power supply 15 and electrostatic precipitator ESP. A voltage divider or other suitable means of representing the

10 voltage V_esp across electrostatic precipitator ESP₅ the representation which may take any suitable form including analog or digital signals as well as a proportional voltage such as produced by the present voltage divider, is electrically coupled to electrostatic precipitator ESP. Finally, controller 31 has been incorporated.

Operation of polarity reversing circuit 30 is controlled through controller 31, which

15 may be any suitable type of logic implementation. For exemplary purposes only, and not limited thereto, various microcontrollers, microprocessors, computer systems, or the like are preferred, since such devices permit ready application and adaptation of the operation of polarity reversing circuit 30 to a variety of different electrostatic precipitators, gas streams and flow rates. Such devices may typically include a processor, non- volatile storage such as a

20 PROM, EEPROM, NVRAM, or any of a myriad of other known non- volatile storage,

Random Access Memory (RAM), one or more user interfaces such as displays, input devices, sound generators, lights and the like, and interfacing circuitry which permits controller 31 to effectively control the operations of SCRl, SCR2, primary power supply 12, and refreshing power supply 15.

25 Most preferably, in view of the very high voltages present within polarity reversing circuit 30, the interfacing circuitry will include voltage isolation, such as may be provided by opto-isolators, specially designed relays, and other components of like function. Such voltage isolation will most preferably protect low- voltage circuitry found within controller 31, and any persons working with controller 31 , from damage or harm that might arise from

30 unintentional overloads or component failures.

Controller 31. while still implementing the method 20 of cleaning ash, will most preferably implement several additional steps in order to provide the enhanced operation which is possible with the additional components. The additional steps are illustrated in Figure 4, as a part of the method 40 for cleaning an electrostatic precipitator. In view of the similarity with method 20, like steps have retained the same numbering. Consequently, step 22, which entails energizing electrostatic precipitator ESP, is implemented in polarity reversing circuit 30 by turning on primary power supply 12 and also providing the necessary gate signal to SCRl to permit power to flow there through. Once step 22 is completed, precipitate will be collected in step 24, for an indeterminate period of time. By this, it will be understood that either at the time of design, or through monitoring various parameters within electrostatic precipitator ESP, the particular period of time or condition necessary to trigger a need for cleaning will be detected. This detection is represented by a determination of a need for cleaning in step 25. Once the need for cleaning is determined, several new steps are provided in method

40. One of the limitations heretofore in using relatively high voltage SCR switching has been the limited ability to control the ultimate output voltage across electrostatic precipitator ESP, which is designated herein as V_esp. A typical ratio of capacitance in capacitor C to the capacitance of electrostatic precipitator ESP might have been on the order of five or ten times as much capacitance in C as in the capacitance of electrostatic precipitator ESP. This higher ratio of capacitance would ensure a rapid transition of V^. However, if V_esp prior to reversal were to be relatively low, than the discharge of C could cause V_esp to shift into a reversed polarity corona discharge. Should corona discharge begin, current would also begin to flow through electrostatic precipitator ESP, owing to the onset of corona discharge. This current, which will continue at least until C_ps is substantially discharged, or indefinitely if a refreshing power supply remained feeding power to C_ps, would lead to an inability to commutate series SCR string SCR2 off. The net result is a much longer polarity reversal time than desired. .In an extreme case, unfortunate timing of the initiation of polarity reversal where the initial V_esp is unusually low could render the polarity reversal less or completely ineffective. The discharge of C_ps could simply reverse the plates upon which the dust deposits are held.

In an exemplary electrostatic precipitator, the negative voltage V_esp produced by primary power supply 12 might range between -30 kVdc and -95 kVdc. The corona onset voltage might range between approximately 15 and 30 kVdc. Given the wide range of initial values for V_esp, which covers a range of approximately 65 kVdc, it is practically impossible to hold the reversal to the most efficient voltages that only have a 15 kVdc range using teachings of the prior art.

This limitation is overcome in method 40 by the measurement of initial voltage V_csp at step 41. With knowledge of the present value of V_esp, and the values of the other components within polarity reversing circuit 30, controller 31 is then used to calculate a value for the target voltage V_ps across C_ps. As illustrated in the following table, which was calculated using a software circuit simulator sold under the tradename PSPICE, it is practical to predict a particular initial value for V_ps, based upon an initial value for V_esp, which will produce a desired final value V_esp. For the purposes of the present simulation, the capacitance of C_ps and ESP were both set to 100 nF. Ipeak is the peak current through SCR2, and Itime is the width of the half-sinewave current through SCR2.

Table I

Consequently, using appropriate programming, it is practical to calculate a desired V_ps based upon an initial value for V_esp as shown in step 42. The calculation can be made by modeling the circuit in advance, through trial and error determination, or most preferably through real- time mapping of values for V_esp and V_ps within controller 31 , the latter which permits automatic operation and real-time adaptation to changing gas streams or other operational variances. Once V_ps is calculated, controller 31 will then turn on the refreshing power supply 15 for a sufficient duration to charge capacitor C_ps to the target V_ps, as shown in step 43. This charging is, of course, conducted while at least one of SCRl and SCR2 is turned off. It is noted that, while refreshing power supply 15 is preferred for this function, any suitable arrangement may be made to charge C_ps. Once capacitor C_ps is charged to the target V_ps, both primary power supply 12 and refreshing power supply 15 will be turned off in step 44. When primary power supply 12 is turned off, SCRl will commutate off. At some brief moment thereafter, as shown by step 46, SCR2 will be gated on by controller 31. This will cause a ringing second order circuit response, which will in turn rapidly change the output voltage across electrostatic precipitator

ESP from an initial V_esp of negative polarity greater than the corona onset voltage to the target V_65P of positive polarity which will preferably remain below the corona onset voltage. However, as capacitor C_ps discharges through L_r, a magnetic field is induced within L_r. Once capacitor C_ps drops below the combined voltage dropped across L_r and the momentary value of V_esp, the magnetic field induced by L₁. will begin to collapse, thereby maintaining current flow. This flow will continue to positively charge electrostatic precipitator ESP, but will tend to generate a negative voltage V_ps across capacitor C_ps.

Simply designing a resonant circuit as described thus far is not adequate for many systems. This is because many high voltage power supplies incorporate diodes in the output which will become forward biased and conduct current when V_ps becomes negative. As a result, very large and damaging currents can be generated by the desired resonance. To prevent this resonance from damaging refreshing power supply 15, one or more resistors R_pS are provided which are sized to limit the resonant current flowing through the output diodes to a safe level. In addition, resistors R_p5 , and R_p52 also create sufficient voltage drop to allow C_ps to develop a large negative voltage.

Ultimately, V_ps will become sufficiently negative and V_esp will become sufficiently positive to stop further current flow within the resonant circuit. At this moment, series SCR string SCR2 will commutate off, as shown in step 47, blocking further current flow through the resonant circuit and simultaneously isolating refreshing power supply 15 from V_csp- Controller 31 will then turn primary power supply 12 on in step 49, and series SCR string

SCRl on in step 50, to restore the negative potential across electrostatic precipitator ESP. Precipitate will then continue to be collected as shown in step 24, until the next need for cleaning is determined again in step 25.

As may be appreciated in light of the foregoing, using polarity reversing circuit 30, which implements the sophisticated calculation and control of V_ps based upon initial and target values for V_esp as shown in method steps 41 - 50 of Figure 4, a relatively exact and repeatable target final value for V_esp may be attained. To fully enable this ability to control final V_esp, a designer will need to carefully select the initial values of C_ps and R_p5 in consideration of the remaining circuitry. In practice, the amount of current through R_p5, which in combination with the chosen resistance of R_p5 determines peak reverse voltage across C_ps, and the selection of magnitude of V_ps just prior to turning on SCR2 in step 46, are the characteristics which are primarily used to stabilize the final value of V_esp at the end of resonance, when SCR2 turns off in step 47. Having thus disclosed several preferred embodiments and alternatives to those preferred embodiments, additional possibilities and applications will become apparent to those skilled in the art without undue effort or experimentation. Therefore, while the foregoing details what is felt to be the preferred embodiment of the invention, no material limitations to the scope of the claimed invention are intended. Further, features and design alternatives that would be obvious to one of ordinary skill in the art are considered to be incorporated herein. Consequently, rather than being limited strictly to the features recited with regard to the preferred embodiment, the scope of the invention is set forth and particularly described in the claims herein below.

Claims

CLAIMSWhat is claimed is

1. A method of applying electrical energy to an electrostatic precipitator collector which enables operationally effective cleaning using electrical energy, comprising the steps of: applying electrical energy having a first electrical polarity to said electrostatic precipitator collector; collecting precipitate on said electrostatic precipitator collector responsive to said electrical energy applying step; determining a need for cleaning said electrostatic precipitator collector; measuring an instantaneous voltage across said electrostatic precipitator collector; calculating a capacitive source target voltage responsive to said instantaneous voltage measurement; charging a capacitive source to said capacitive target voltage; switching said applied electrical energy from said first electrical polarity to a second electrical polarity derived from said capacitive source and opposite in polarity from said first electrical polarity; thereby removing said collected precipitate from said electrostatic precipitator collector responsive to said switching step; and resetting said applied electrical energy to said first electrical polarity subsequent to said removing step.

2. The method of applying electrical energy to an electrostatic precipitator collector of claim 1 , further comprising the step of rapping said electrostatic precipitator collector at a time when said applied electrical energy has said second electrical polarity.

3. The method of applying electrical energy to an electrostatic precipitator collector of claim 1, wherein said step of switching further comprises discharging said capacitive source through a resonant inductor and thereby inducing a resonant oscillation, and wherein said step of resetting further comprises the step of commutating a thyristor into a non-conductive state responsive to said resonant oscillation.

4. The method of applying electrical energy to an electrostatic precipitator collector of claim I₅ wherein said switching step further comprises the steps of: disconnecting said applied electrical energy of said first electrical polarity from said electrostatic precipitator; and subsequent to said disconnecting step, applying said electrical energy of said second electrical polarity to said electrostatic precipitator collector.

5. The method of applying electrical energy to an electrostatic precipitator collector of claim 1, wherein said switching step further comprises providing a gating signal to a thyristor, to thereby turn said thyristor on.

6. The method of applying electrical energy to an electrostatic precipitator collector of claim 1 , wherein said switching step further comprises the steps of: discontinuing charging current flow from a power source responsible for said capacitive source charging to said capacitive source; disabling current flow from a first power supply having said first electrical polarity to said electrostatic precipitator and thereby commutating a first thyristor into a non-conductive state; providing a gating signal to a second thyristor, to thereby turn said second thyristor on, said second thyristor coupling said capacitive source to said electrostatic precipitator collector.

7. The method of applying electrical energy to an electrostatic precipitator collector of claim

6, further comprising the step of resonantly discharging said capacitive source and thereby generating a voltage of polarity opposed to said capacitive target voltage polarity.

8. The method of applying electrical energy to an electrostatic precipitator collector of claim

7, wherein said step of resonantly discharging said capacitive source further comprises limiting peak current through said power source responsible for said capacitive source charging during said resonant discharging.

9. The improvement of claim 2, wherein the step of rapping comprises a mechanical rapping step, and wherein the intensity of the mechanical rapping system step may be varied from zero to a maximum intensity.

10. The process of removing particulates from the exhaust gases of an industrial process or power generation using the method of claim 1.

11. The process of claim 10, wherein the industrial process or power generation comprises a coal-burning power plant.

12. A polarity reversing power supply that electrically enhances precipitate removal from an electrostatic precipitator collector, comprising: a primary power source having a first electrical power terminal of first polarity connected to said electrostatic precipitator collector and a second electrical power terminal connected to a precipitator electrode, said primary power source, said electrostatic precipitator collector and said electrostatic precipitator electrode operatively interconnected to complete a primary electrical circuit through which primary electrical current flows; a first electrical switch electrically connected within said primary electrical circuit having a first electrically closed state through which said primary electrical current flows and a second electrically open state through which said primary electrical current is blocked; a capacitive source having a first electrical power terminal of second polarity connected to said electrostatic precipitator collector and a second electrical power terminal connected to said precipitator electrode, said capacitive source, said electrostatic precipitator collector and said electrostatic precipitator electrode operatively interconnected to complete a secondary electrical circuit through which secondary electrical current flows; and a second electrical switch electrically connected within said secondary electrical circuit having a first electrically closed state through which said secondary electrical current flows and a second electrically open state through which said secondary electrical current is blocked, said first and second electrical switches operatively coupled to prevent simultaneous closure.

13. The polarity reversing power supply of claim 12, wherein said first and second electrical switches are comprised by thyristors.

14. The polarity reversing power supply of claim 13, further comprising an inductor in series between said capacitive source and said electrostatic precipitator to form a resonant circuit therewith.

15. The polarity reversing power supply of claim 12, further comprising a rapper for mechanically agitating said electrostatic precipitator collector.

16. The polarity reversing power supply of claim 15 wherein said second electrical switch is in said first electrically closed state when said rapper is mechanically agitating said electrostatic precipitator collector.

17. The polarity reversing power supply of claim 12, further comprising a controller having as an input a representation of a first electrical potential across said electrostatic precipitator, and responsive to said input controlling a voltage across said capacitive source while causing said second electrical switch to change from said second electrically open state to said first electrically closed state.

18. An electrostatic precipitator having at least one discharge electrode for charging particulates within a gas stream, at least one collector for attracting said charged particulates within said gas stream, a high voltage power source operatively and selectively able to apply a high voltage potential of a first polarity between said at least one discharge electrode and said at least one collector, and a rapper for intermittently agitating said at least one collector, wherein the improvement comprises: a second high voltage power source operatively and selectively able to apply a high voltage potential of a second polarity opposite to said first polarity between said at least one discharge electrode and said at least one collector; a switch that in a first state operatively completes an electrical circuit to apply said high voltage potential from said first high voltage power source between said at least one discharge electrode and said at least one collector while maintaining said second high voltage power source isolated therefrom, and in a second state operatively completes an electrical circuit to apply said high voltage potential from said second high voltage power source between said at least one discharge electrode and said at least one collector while maintaining said first high voltage power source isolated therefrom; a means for detecting a magnitude of said high voltage potential of said first polarity and producing a representation thereof; and a means for controlling a magnitude of said high voltage potential of said second polarity responsive to said representation of said high voltage potential of said first polarity and to activate said switch to said second state while said magnitude of said high voltage potential of said second polarity is controlled .

19. The electrostatic precipitator of claim 18, further comprising a means for placing said switch in said second state simultaneous with activating said rapper

20. The electrostatic precipitator of claim 18, wherein said second high voltage power source further comprises a charge accumulator storing said high voltage potential of said second polarity, and further comprising an inductor in series between said charge accumulator and said at least one collector.

21. The electrostatic precipitator of claim 20, wherein said switch further comprises an SCR.

22. The electrostatic precipitator of claim 21, wherein said particulates comprise fly ash.

23. The electrostatic precipitator of claim 22, wherein said switch placing means maintains said switch in said second state for one to ten milliseconds.

24. The electrostatic precipitator of claim 23, wherein said high voltage potential of said high voltage power source is between 5,000 and 150,000 volts, and said high voltage potential of said second high voltage power source is between 5,000 and 30,000 volts.

25. In a coal-burning power plant, wherein a dry electrostatic precipitation system is employed for removing fly ash, the fly ash being collected on electrostatic plates in the system, the improvement comprising a polarity reversing circuit having a reverse polarity controlled to a magnitude less than a corona discharge potential for said electrostatic precipitation system, for periodically dislodging the fly ash from the electrostatic plates.

26. The improvement of claim 25, further including a mechanical rapping system for dislodging material collected on the electrostatic plates, said polarity reversing circuit supplementing said mechanical rapping system.