Gas separation: processes – With control responsive to sensed condition – Electric or electrostatic field
Reexamination Certificate
2001-03-19
2002-10-08
Chiesa, Richard L. (Department: 1724)
Gas separation: processes
With control responsive to sensed condition
Electric or electrostatic field
C095S081000, C096S024000, C096S025000, C096S082000, C323S903000
Reexamination Certificate
active
06461405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of operating an electrostatic precipitator.
An electrostatic precipitator (abbreviated ESP) is a system for collecting solid particles, which operates by virtue of the movement of charges immersed in an electric field. An electrostatic precipitator has particular utility towards cleaning of flue gasses, smokes, etc. in order to remove particles of dust, ashes, soot, and the like. The gasses are made to pass through a zone wherein an electric field is directed transversely to the flow. The electric field is operated at a high voltage where a corona of free electrons is emitted from the negative electrode. The electrodes charge the particles and the charged particles will migrate under the effect of the electric field towards the positive electrode, usually designed in the form of collecting plates on which the particles deposit. On electric discharging of the particles at the positive electrodes and possibly aided by shaking the plates, the collected dust particles fall into a hopper located below the plates.
The collecting plates are usually grounded whereas the negative electrodes are constituted of thin metallic wires maintained at a high negative potential with respect to the plates.
By virtue of electrode geometry the electric field has a higher intensity adjacent the wire electrodes, which causes the ionization of the gas in the immediate surroundings and the creation of a corona. Towards the collecting plates the electric field is distributed over a larger area with a corresponding decrease of intensity. This lower intensity electric field may not be sufficient for the ionization of the gas but serves the purpose of advancing the charged particles of dust towards the collecting plates.
In a first approximation of electrical properties, the electrostatic precipitator may be represented by a capacitor with a shunt resistance that represents the leakage by the transport of charged particles between the electrodes. In order to produce ionization of the particles the electric voltage must surpass a certain minimum threshold referred to as the corona onset voltage. Upwardly the voltage will be limited by various factors depending on the mode of operation. One of these factors may be the formation of a sparkover between the electrodes, which may take the form of a short discharge or the form of a prolonged arc. Another factor recognized in the field is the formation of corona from points on the positive electrode referred to as back-corona. Back-corona represents an increase in the leak current and impairs the particle collection efficiency.
2. Description of the Prior Art
EP patent 0 286 467 suggests a power supply wherein the power fed from the mains grid into a step-up transformer is controlled through phase angle controlled thyristors, thus producing on the high voltage side pulses at double the mains frequency. The pulses charge the electrostatic precipitator to a varying voltage. According to this publication, a detection procedure is carried out at preselected time intervals wherein the power supply is blocked for a selected interval, such as from 0.1 to 5 seconds, and then resumed. The minimum values of the pulsed precipitator voltage is observed and the presence of back-corona is established if the minimum values observed after the blocked interval exceed the minimum value observed prior to the blocking interval by a detection sensitivity factor.
U.S. Pat. No. 5 311 420 suggests a power unit comprising mains powered silicon controlled rectifiers feeding into a step-up transformer. The power supply may run in intermittent energization mode wherein the precipitator is energized by a half cycle voltage pulse followed by a predetermined number of off cycles, the ratio of on to off half cycles being optimized to prevent back-corona. The back-corona condition is detected by detecting a lack of increase of the minimum peak values of output voltage of the high voltage rectifier coincident with an increase in an output current value.
U.S. Pat. No. 4 779 182 provides an inverter power supply with switches which may be operated to output a high frequency alternating current, alternating at a frequency from 1 to 3 kHz. The feed voltage may be specified and also the voltage ripple, i.e. the voltage fluctuation between an upper and a lower limit may be specified. The direct current taken from the high voltage rectifier can be interrupted by periodic blocking in order to enforce voltage ripple on the electrostatic precipitator.
EP patent 066 950 suggests a power supply effectively comprising two complete sets of thyristor controlled high voltage power units. The first set outputs a stable base voltage whereas the second set fires single pulses to be superimposed on the back ground level provided from the first set. The electrostatic precipitator voltage takes the form of a stable back ground level superimposed with pronounced spikes. The pulse duration is within the range 50 to 200 microseconds. WO-A1-9011132 discloses a method of operating an electrostatic precipitator, wherein the power fed to the high-voltage transformer primary is controlled by thyristors fed from the mains and variation of the pulse frequency is implemented by igniting the thyristors for every third, every fifth, every seventh etc. half-cycle. Thus this method only permits varying the OFF-time intervals. The precipitator voltage values measured are the voltages at the peak, at the end of the current pulse and at 1.6 ms after the end of the current pulse.
WO-A1-9310902 discloses a method where the power fed to the high-voltage transformer primary is controlled by thyristors fed from the mains. The voltage is measured 1-3 times per ms. A “figure of merit” is established using a formula involving the time integral of the square of the voltage. Variation of the pulse frequency is implemented by igniting the thyristors for only part of the half-cycles and by controlling the firing angle.
In operating conditions of high resistivity dust, the dust deposited on the plate electrode will resist discharging of the ionized particles. The voltage tends to increase across the dust layer, and to correspondingly decrease across the gas. If the voltage across the dust layer continues to build up, a point is reached where a dielectric break down through the dust layer occurs. This point is known as the onset point of the back corona discharge. The dielectric break down of the dust layer produces positive ions, which decrease particle charging, and result in a reduction of the collection efficiency.
The formation of back-corona takes some time, and this is related to the relaxation time of the dust layer.
As the dust layer can be considered as a leaky capacitor, it will tend to smooth out the current pulses delivered to the electrostatic precipitator. This effect may be put to advantage as short pulses may be applied to the electrodes without prompting the formation of back-corona on the dust layer. Rather the initiation of a back-corona situation seems to be governed by the time average value (mean value) of the precipitator current.
Therefore, in order to avoid or reduce the back-corona discharges, the mean current delivered to the precipitator has to be decreased. The problem is to do this without losing too much voltage level.
The basic control problem is then to determine the current that has to be delivered to the precipitator in accordance with the existing operating conditions. For some industrial processes, the dust resistivity can sometimes be low and sometimes be high, causing back-corona. In the first case the current has to be as high as possible, and in the second case the current has to be reduced.
The traditional power supply for ESP's used until now is a transformer rectifier set, consisting of a high voltage transformer and a bridge rectifier. A pair of antiparallel thyristors using phase angle control controls the primary voltage applied to the HV transformer.
A non-linear resistance in parallel with a capacitan
Chiesa Richard L.
F.L. Smidth Airtech A/S
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