Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B03—SEPARATION OF SOLID MATERIALS USING LIQUIDS OR USING PNEUMATIC TABLES OR JIGS; MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C—MAGNETIC OR ELECTROSTATIC SEPARATION OF SOLID MATERIALS FROM SOLID MATERIALS OR FLUIDS; SEPARATION BY HIGH-VOLTAGE ELECTRIC FIELDS
  • B03C3/00—Separating dispersed particles from gases or vapour, e.g. air, by electrostatic effect
  • B03C3/34—Constructional details or accessories or operation thereof
  • B03C3/66—Applications of electricity supply techniques
  • B03C3/68—Control systems therefor

Definitions

• the present invention relates to a method for use in an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes between which a varying high voltage is maintained, to control a pulsating direct current supplied thereto.
• the method is especially well suited when the pulsating direct current is generated by a pulse means, such that the length, height and frequency of the pulses can be varied independently of each other.
• the method can also be used when the pulsating current has the form of a pulse train which is synchronised with the frequency of the mains voltage and in which the pulses are generated by supplying a part of a half-wave of the mains voltage by means of a phase-angle-controlled rectifier (thyristor) , after step-up transformation to the electrodes of the precipitator, whereupon a plurality of periods of the mains voltage are allowed to pass without any current being supplied to the electrodes.
• thyristor phase-angle-controlled rectifier
• Electrostatic precipitators are often the most preferred dust separator option, especially for flue gas cleaning. They have a robust design and are highly reliable in operation. Moreover, they are very efficient, degrees of separation above 99.9% are not unusual. Since, when compared with textile barrier filters, their operating costs are low and the risk of breakdown and stoppage owing to malfunction is considerably smaller, they are a natural choice in many contexts . A problem that is often difficult to master appears when dust with high resistivity is to be separated. In such operating cases one is often forced to operate with extremely unfavourable operating parameters due to the risk for partial discharges in the gradually growing dust layer on the collecting electrodes. When partial discharges occur in the dust layer the effect will i.a. be emission of charges and dust from the collecting electrodes, so-called back corona.
• the present invention relates to a method for use in an electrostatic precipitator unit, comprising discharge electrodes and collecting electrodes between which a varying high voltage is maintained, to control a pulsating direct current supplied thereto.
• the frequency, pulse height and/or pulse length of the pulsating direct current are varied, so as to obtain a plurality of frequency-height-length combinations .
• a first point of time for the end of the supplied current pulse, and a second point of time, when the voltage between the discharge electrodes and collecting electrodes has fallen to a predetermined level U ref is determined.
• the length of a time interval between the first point of time and the second point of time is determined, and the length of the time interval is used to select the frequency-height-length combination of the pulsating direct current.
• High voltage essentially also means that the current density becomes high. If the dust that is separated conducts current well, this involves no problem, but if the dust has high resistivity and the current density in the gas is high, the separated dust layer on the collecting electrodes will be charged such that the electric field strength in the layer will be sufficient to conduct the same high current density through the dust layer. This often results in electric breakdown of the dust layer, so-called back corona, if the current density is not limited.
• the charging is controlled merely by the supplied charge, i.e. the size of the current.
• the charging can be effected in less than one millisecond if the current intensity is sufficient.
• An electrostatic precipitator unit can be regarded as a capacitor with a relatively high leakage current. The higher the voltage is above the ignition voltage of the corona discharge, the greater the leakage current. This also results in that the voltage then is falling relatively quickly if no new charge is supplied.
• the corona discharge expires approximately at the ignition voltage and, thus, the voltage drop ceases.
• the layer of dust has sufficiently high resistivity, the level is controlled by the current density, discharges also take place in the layer of dust. This leads to generation of charges of the opposite polarity and reduces the extinction voltage of the corona, such that the precipitator unit can discharge to a considerably lower level.
• the discharge also proceeds more rapidly, the leakage current at a given voltage is greater.
• the supply of current in the form of pulses to a precipitator unit usually results in an increasing voltage during the pulse and a decreasing voltage between the pulses, see Figs 1 and 2.
• the rate of increase during the pulse depends on the size of the current, and the rate of discharge depends on the voltage level at issue and whether ionisation also takes place at the dust layer, so-called back corona.
• the length of the interval is used as a control parameter, in the first place such that one tries to operate close to the conditions that result in a maximum length of the interval. According to the present invention, it is therefore suggested that the length, height and frequency of the pulses be varied, such that, for instance, for each frequency the maximum possible length of the interval is established and then this procedure is repeated for other frequencies and these interval lengths are used to select parameters for the continued operation.
• an investigation is made of a sufficiently great frequency range such that a certain frequency is found, which gives the absolutely longest interval, above which frequency a shorter interval is again obtained, and the operation is continued at a frequency close or equal to this value.
• the size of the pulse is then varied at this frequency, thereby being close to the flashover limit.
• precisely the frequency is chosen that gives a maximum for the length of the interval.
• a special pulse means for supplying the current to the electrostatic precipitator unit.
• the pulse frequency can be varied, for instance, between 1 Hz and 10 kHz, preferably between 1 Hz and 1 kHz .
• mains frequency and thyristors which, via a transformer, transmit a part of a half-wave from an essentially sinusoidal voltage to the electrostatic precipitator unit
• pulse length and pulse height vary together in a fashion that is determined by the mains voltage and that the pulse frequency can only be varied as submultiples of the mains frequency.
• the preferred variation of the shape of the pulses is such that the maximum height of the pulse is chosen which is available with the existing equipment, and to obtain a varying charge, the pulse length is varied.
• One suitably begins with a relatively low frequency and determines at this frequency the pulse shape that gives a maximum interval length and then repeats this procedure at successively higher frequencies. When a further frequency increase results in a reduced maximum length, the adjustment is interrupted.
• An alternative procedure implies that during the adjustment, one keeps the frequency constant and begins with a short pulse the length and/or charge of which is/are successively increased as long as this results in an increased length of interval. For instance, it is also possible to keep the charge of the pulses constant and vary the frequency, beginning at a low value.
• the choice of the control algorithm depends to a high degree on the design of the pulse means.
• the advantages of the invention are most significant when the current pulses have a short decay time, or at least finally decrease at a high rate.
• Fig. 1 shows the fundamental time dependency of the voltage between the electrodes for an electrostatic precipitator unit to which a relatively short current pulse is supplied;
• Fig. 2 shows the same for a somewhat longer current pulse which, however, does not result in back corona from the dust layer ;
• Fig. 3 shows the fundamental time dependency of the voltage between the electrodes for an electrostatic precipitator unit to which such a great current pulse is supplied that back corona from the dust layer limits the voltage increase;
• Fig. 4 shows the time dependency of the voltage, corresponding to Fig. 3, measured on an electrostatic precipitator unit in operation;
• Fig. 5 shows a simplified wiring diagram for a device, which is suitable for carrying out the suggested method.
• Fig. 1 shows the time dependency of the voltage as a full line when an electrostatic precipitator unit is supplied with a current pulse, with a constant amplitude, indicated by a dashed line.
• the current pulse contains a relatively small amount of charge.
• the voltage increases during the entire current pulse and then again decreases down to the corona ignition voltage.
• a double arrow “d" indicates the length of an interval between the end of the current pulse and the point of time when the voltage has fallen to a selected voltage level, U ref , which in this case is the ignition voltage of the corona.
• U ref which in this case is the ignition voltage of the corona.
• the dash-dot line indicates this voltage level U ref .
• the determination of the ignition voltage of the corona can be made, for instance, by a method as described in US 5,477,464.
• Fig. 2 shows in the same way the time dependency of the voltage when the current pulse has been somewhat extended with its amplitude maintained.
• the pulse charge is not so great that back corona starts.
• the interval "d" is here considerably longer.
• Fig. 3 shows the time dependency of the voltage when the current pulse has been extended such that the charge is sufficient for the back corona to start during the pulse. This leads to a deformation by the voltage increase being interrupted and the voltage beginning to decrease despite a continued supply of charge.
• the voltage between the electrodes at the end of the current pulse is in this case considerably lower than in Fig. 2.
• the interval "d" obtained in Fig. 3 is considerably shorter than the one in Fig. 2. This corresponds in a correct fashion to the considerably impaired function of the electrostatic precipitator unit in operation according to Fig. 3. If the method as suggested in said US 4,690,694 is used, no information on this difference will be obtained.
• the time interval from voltage maximum to corona starting voltage is approximately the same in both Figures. As a result, the method according to US 4,690,694 has so pronounced drawbacks as to make it inappropriate.
• Figs 1-3 illustrate for the sake of clarity simplified relations.
• Fig. 4 shows a curve recorded by means of an oscillograph and displaying the voltage in normal operation with an electrostatic precipitator.
• the current pulse is considerably longer than the optimal one that would be obtained with the method here suggested.
• the current average at issue is more than three times as great as the one in optimal operation.
• Fig. 5 is a fundamental wiring diagram of a voltage- converting device which supplies high-voltage direct current to a precipitator unit 1.
• the device comprises a three-phase rectifier bridge 2, a pulse generator 3, a transformer 4, a one-phase full-wave rectifier bridge 5, a choke 6, and control equipment 7 with precision resistors 8, 9 and 10.
• the three-phase rectifier bridge 2 comprises six diodes 21-26 and is, via three conductors 27, 28, 29 connected to ordinary three-phase AC mains .
• the pulse generator 3 comprises four transistors 31-34 and four diodes 35-38. The transistors are controlled by their bases being connected to the control equipment 7.
• the full-wave rectifier bridge 5 consists of four diodes 51-54.
• the control equipment 7 is connected not only to the transistors 31-34, but also to a precision resistor 8 in series with the precipitator unit 1, for measuring the current to the electrodes of the precipitator, and to a voltage divider comprising two resistors 9 and 10 connected between the electrodes of the precipitator unit for measuring the voltage between them.
• the device functions as follows. Via the conductors 27-29, the rectifier bridge 2 is supplied with three-phase alternating current. This is rectified and is transferred, via conductors 11 and 12, as a direct current to the pulse generator 3.
• the control equipment 7 controls the conducting periods of the transistors 31-34 such that a pulse-width- modulated voltage, essentially formed as a square wave, is supplied, via conductors 13 and 14, to the primary side of the transformer .
• the voltage induced in the secondary winding of the transformer 4 is rectified by the rectifier bridge 5 and, via the smoothing choke 6, the obtained direct current is supplied to the electrodes of the precipitator unit 1.
• control equipment 7 controls the transistors 31-34 and moreover monitors the current and voltage of the precipitator via the resistors 8 and 10. Since the conducting periods of the transistors are controlled, the pulse width of the generated, essentially square-wave-formed current pulse can be varied and, consequently, both current and voltage in the precipitator are controlled.
• the above described device is thought to operate with an intermediate frequency of 50 kHz and a pulse frequency of 10 Hz.
• the pulse height is the maximum one for the means and the current is controlled so that the flashover limit is continuously sensed by varying the pulse length somewhat around an average value of about 1 ms .
• the pulse frequency is decreased to 5 Hz and the pulse length is increased step-by-step during registration of the length of the time interval between the end of the current pulse and the point of time when the voltage has fallen to a determined level just above the ignition voltage of the corona.
• increased pulse length gives a shorter interval the increase is interrupted.
• the maximum interval length is registered.
• the procedure is repeated so that for each frequency the maximum interval length is determined.
• the frequency for which the maximum interval length during the adjustment is registered, is used in the continued operation.
• the pulse length is then varied in a usual manner so that essentially maximum current can be supplied.
• the interval between the adjustments is to be determined based on experience, but as the adjustment does not mean a total stoppage or a crucial disturbance it can without inconvenience be performed with short intervals if the operating parameters are varied.
• the current pulse can be generated by phase-angle-controlled rectifiers transmitting at least part of a half-wave from an essentially sinusoidal mains voltage which after step-up transformation and rectification is supplied to the electrostatic precipitator unit.
• any form of pulse means can be used in which the entire voltage decay or at least a great part thereof depends on the internal current between the electrodes of the precipitator unit. Particular caution, however, must be exercised if forced discharge, externally controlled, of the precipitator takes place. It is a condition of the invention that the registered magnitude depends on internal processes.

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• Engineering & Computer Science (AREA)
• Automation & Control Theory (AREA)
• Electrostatic Separation (AREA)

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Cited By (5)

Citations (3)

• 1997
  • 1997-09-10 SE SE9703299A patent/SE510380C2/sv not_active IP Right Cessation
• 1998
  • 1998-09-07 WO PCT/SE1998/001575 patent/WO1999012649A1/en active Application Filing
  • 1998-09-07 AU AU91003/98A patent/AU9100398A/en not_active Abandoned

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