Method of and apparatus for reducing back corona effects

An apparatus and method for reducing back corona effects while charging high resistivity dust or the like in a corona field, includes a corona discharge electrode, a passive electrode and a screen electrode, the screen electrode being positioned between the discharge electrode and the passive electrode. The screen electrode is positioned more closely to the passive electrode than the discharge electrode and is supplied with an electrical potential which is of the same polarity as and a fraction of the magnitude of the discharge electrode relative to the passive electrode. Gas having entrained therein the particulate material, which is to be removed, flows between the screen and discharge electrode and thence to a particulate material collection stage. The space between the screen and corona discharge electrode remains essentially a unipolar ion field, while ions of the opposite polarity originating at passive electrode due to back corona are collected by the screen electrode.

BACKGROUND OF THE INVENTION 
This invention relates to a method of and apparatus for reducing back 
corona effects in the process of charging particulate material in an 
electrostatic precipitator. More particularly, the present invention 
relates to a method of and apparatus for reducing the effects of back 
corona so that a substantially unipolar ion field is maintained for 
charging particulate material in a charging stage upstream from a 
particulate material collecting stage. 
Many conventionally applied methods and apparatus for control of back 
corona are based principally upon reducing the resistivity of particulate 
matter collected by electrostatic precipitation. Conditioning reagents are 
sprayed into the gas stream or introduced into the boiler along with the 
coal. These methods require special systems for handling and dispensing 
chemical conditioning agents in large quantities. 
The effectiveness of the invention does not depend upon the chemical nature 
of the particulate material to be precipitated. No chemical agents are 
required. 
It is known from U.S. Pat. No. 2,142,129 entitled "Apparatus for Electrical 
Precipitation" to Hoss et al. to pass gas with entrained particulate 
material into an apparatus which includes an external housing, a centrally 
located corona discharge electrode and an apertured intermediate 
electrode, the intermediate electrode being held at a potential less than 
but of the same polarity as the discharge electrode. The gas in which is 
entrained the particulate material passes radially through the apertured 
electrode towards the internal surface of the housing. The housing acts as 
the dust collecting electrode and the gas is passed out of the apparatus 
via an opening between the housing and the apertured electrode. Apparatus 
of this general type have the disadvantage that dust which collects 
between the apertured electrode and housing tends to cause electrical 
discharges and the probability of back corona discharge is also 
considerable. 
It is known from U.S. Pat. No. 1,605,648 entitled "Art of Separating 
Suspended Matter from Gases" to M. W. Cooke to pass gas with entrained 
particulate material into an apparatus which includes an external housing, 
a centrally located corona discharge electrode and an apertured 
intermediate electrode, the intermediate electrode being held at the same 
polarity and potential as the housing with respect to the discharge 
electrode. The gas with the entrained particulate material is passed into 
the apparatus, the particulate material collecting on the outside of the 
intermediate electrode in the neutral zone between this electrode and the 
housing with the gas passing outwardly via a circumferential opening 
between the discharge and apertured electrodes. Here again, the 
disadvantages of back corona and electrical discharges exist. 
Electrostatic precipitators are known which include a charging section or 
stage having a passive electrode and a corona discharge electrode between 
which gas containing particulate material is passed, the passive electrode 
surrounding the discharge electrode. The particulate material becomes 
charged, as a result of the corona discharge, and with the gas is passed 
to a further stage provided with an electrical field which effects 
precipitation of the charged particulate material. An example of 
precipitators of this preconditioning type can be seen in U.S. Pat No. 
3,747,299 entitled "Electrostatic Precipitator" to Ta-Kuan Chiang and the 
patent to Hoss et al, supra. It is believed that the possibility of 
sparking is reduced, as compared with the apparatus disclosed in the 
patent to Cooke, supra, because dust and other particulate material are 
not collected on the passive electrode. Nevertheless, the disadvantage of 
back corona discharge is present and, when taking place, can seriously 
reduce and even virtually completely counteract the desired charging of 
the entrained particulate material. 
The techniques of charging particulate material in electrostatic 
precipitators as noted above depends upon the presence of ions, 
principally of a single polarity, in the charging region. If comparable 
quantities of both positive and negative ions are present, very poor 
particle charging results, because of the competing effects of the 
oppositely charged ions. In a conventional electrostatic precipitator a 
layer of high resistivity particulate material on the passive electrode 
may suffer electrical breakdown, resulting in a corona discharge from the 
surface of the passive electrode. This phenomenon, known as back corona, 
produces an undesirable bipolar ion field in most of the space between the 
electrodes of the corona system. 
SUMMARY OF THE INVENTION 
It is the principal object of the present invention to provide a method of 
and apparatus for reducing the undesirable bipolar ion field between 
electrodes in the charging section or stage of an electrostatic 
precipitation system. 
It is another object of the present invention to provide a method of and 
apparatus for removing from an electrostatic precipitation system ions of 
incorrect polarity produced by back corona discharge. 
It is a further object of the present invention to provide a method of and 
apparatus for allowing ions of one polarity to flow between a corona 
discharge electrode and a passive electrode, while capturing ions of 
opposite polarity which may be produced from the passive electrode in the 
particulate material charging section or stage of an electrostatic 
precipitation system. 
The present invention, in its apparatus aspect, achieves the foregoing 
objects, as well as others, which are to become clear below, by providing 
in an electrostatic precipitator a charging stage with a three-electrode 
corona system designed to suppress the effects of the ions resulting from 
back corona discharge by removing them from the region near the passive 
electrode. This function is performed by a screen electrode placed near 
the passive electrode and conforming approximately to its shape. The 
screen electrode is insulated from the passive electrode and placed at an 
electrical potential with the same polarity, but a fraction of the 
magnitude of the voltage on the corona discharge electrode. With the 
screen voltage properly adjusted, ions originating at the discharge 
electrode will be deflected away from the metal in the screen, and will 
proceed through the open areas to the passive electrode. Ions originating 
at the passive electrode due to back corona, however, will be of the 
opposite polarity, and will be attracted to the metal in the screen, to be 
trapped and removed from the system. The space between the screen and the 
corona discharge electrode thus remains essentially a unipolar ion field. 
The gas containing particulate material sought to be removed is passed 
axially through the charging stage between the corona discharge electrode 
and the screen electrode and thence on to the collecting stage where the 
charged particulate material is collected. 
The present invention, in its method aspect, achieves the foregoing 
objects, as well as others, which are to become evident below, by passing 
gas containing particulate material to be removed through a charging stage 
between a corona discharge electrode and an intermediate electrode 
positioned between the discharge electrode and a passive electrode so as 
to charge the particulate material by corona discharge of one polarity, 
while removing ions of opposite polarity, which may be produced from the 
passive electrode, by capturing such ions by the intermediate electrode. 
The intermediate electrode is maintained at the same polarity as the 
corona discharge electrode, but at a fraction of its magnitude.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Before turning to a detailed discussion of the two exemplary embodiments 
illustrated respectively in FIGS. 3 and 4, a brief discussion of the 
apparatus shown in FIG. 1 with reference to FIG. 2 is in order and serves 
to aid one in understanding the present invention. 
The basic concept of the present invention and performance of a three 
electrode corona electrode arrangement is illustrated by reference to the 
simple corona geometry shown in FIG. 1. A corona discharge electrode 10 
having a sharp point 11 is placed three centimeters from a flat plate 
passive electrode 12. An intermediate screen electrode 13, in the form of 
a wire screen with approximately 84% open area and 0.64 cm wire spacing is 
provided as the third electrode. The screen electrode 13 is placed at a 
distance of 1 cm from the passive electrode 12, two insulating members 9 
being positioned between the screen electrode and the passive electrode. 
The current of each of the electrodes 10, 12, and 13 is monitored 
separately, respective microameters 14, 15 and 16 being provided for that 
purpose. 
Since the screen electrode 13 serves as a trap for back corona ions, the 
onset of back corona should produce a simultaneous increase in current at 
the passive electrode 12 and at the screen electrode 13, with little 
effect on the current level at the discharge electrode 10. Experimental 
results demonstrating this behavior are presented in FIG. 2. An experiment 
was conducted in which the corona system of FIG. 1 was enclosed in an oven 
at 150.degree. C., and redispersed flyash was injected into the air below 
the discharge electrode 10. After approximately four minutes operating 
time a very sharp rise in current at both the screen electrode 13 and the 
passive electrode 12 occurred, signaling the onset of back corona. The 
fluctuation in current at the discharge electrode 10 was very small in 
comparison. During this stage of the experiment, the voltage at the corona 
discharge electrode 10 was 15 kV and the voltage was 8 kV at the screen 
electrode 13. The passive electrode 12 was held at ground, that is zero 
potential. By adjusting the electrode voltages it was possible to arrive 
at steady-state conditions where the screen and passive electrode current 
levels were between three and ten times as great as the discharge 
electrode current. 
The difference in the magnitude of current at the screen electrode 13 and 
passive electrode 12 is the amount of current from the discharge electrode 
10 which has passed through the screen electrode 13 and arrived at the 
passive electrode 12. Current losses to the oven walls account for the 
experimental discrepancies. 
The significance of the experimental results are best seen in the manner 
that the screen electrode current follows the fluctuations in the current 
at the passive electrode 12, while the discharge electrode current remains 
relatively constant for fixed values of applied voltage. Ions originating 
at the passive electrode as a result of back corona discharge, are 
effectively trapped successfully by the screen electrode 13. 
In FIG. 2, the vertical dashed lines denote times at which the noted 
voltage changes at the screen electrode 13 and the discharge electrode 10 
were made, these voltages being denominated respectively in FIG. 2 as 
V.sub.Screen and V.sub.disch. Initially during the experiment, the screen 
electrode voltage was 8 kV and the discharge voltage was 13 kV. 
Turning now to FIG. 3, a first specific, exemplary embodiment of an 
apparatus for reducing back corona discharge effects is shown incorporated 
into a charging stage operatively associated with an electrostatic 
precipitator or collecting stage. 
As shown in FIG. 3, the exemplary embodiment of the charging stage is 
designated generally by the numeral 16. 
A screen electrode 17 and passive electrode 18, shown partially broken away 
are respective coaxial electrodes of cylindrical construction in the form 
of a wire which is stretched along the axis of the system. The outer 
electrode 18 is grounded, and a high voltage, of either positive or 
negative polarity, is applied to the wire discharge electrode 20 from a 
D.C. power supply 21. A separate power supply 22 is used to maintain an 
electrical potential on the cylindrical screen electrode 17, with the same 
polarity, but lower magnitude than the voltage on the discharge electrode 
20. With proper adjustment of the screen voltage, ions originating at the 
passive electrode 18 will be attracted to the screen electrode 17 so that 
the space between the corona sheath surrounding the the discharge 
electrode 20 and the screen electrode 17 will contain ions of principally 
one polarity. Particulate material passing through the region enclosed by 
the screen electrode may thus become charged without the contravening 
effects of back corona. The voltages applied to the discharge electrode 20 
and the intermediate screen electrode 17 are high and can have the 
relative magnitudes mentioned above for electrodes 10 and 13 (FIG. 1) and 
be of the same or even greater absolute magnitudes. The discharge corona 
electrode 20 is supported by insulators 23 and 24, shown somewhat 
diagrammatically. 
As shown in FIG. 3, respective insulating rings 25 and 26 are respectively 
positioned between the passive electrode 18 and the screen electrode 17 
near the ends of these electrodes. The insulating rings 25 and 26 not only 
provide support for the screen electrode 17, but the lower ring 26 
prevents dust, flyash and the like from initially entering the space 
between the passive electrode 18 and the screen electrode 17. 
In operation, gas having particulate material such as dust, flyash or the 
like entrained therein, is fed under forced or natural draft into the 
charging stage 16 from one end thereof, shown near the lower portion of 
FIG. 3, as indicated by the arrow-headed lines 27. The gas with the 
entrained material is passed axially through the stage 16 and is 
consequently subjected to the electrical and ion fields between the 
discharge electrode 20, the screen electrode 17 and the passive electrode 
18. As a result, the entrained particulate material becomes charged as a 
result of the corona discharge between the discharge electrode 20 and the 
passive electrode 18. Contemporaneously back corona discharge of ions from 
the passive electrode 18 to the discharge electrode 20 is effectively 
prevented; such back corona discharge of ions, which would otherwise make 
the ion field undesirably bipolar, being prevented by capture of such ions 
by the intermediate screen electrode 17. 
The gas with the entrained particulate material, now charged, leaves the 
charging stage 16 from that end thereof shown near the upper portion of 
FIG. 3, as illustrated by the arrow-headed bold lines 28 and thence to a 
collecting stage, shown diagrammatically as numeral 30. The collecting 
stage 30 may take a number of conventional forms, such as those 
illustrated in the U.S. patent to Ta-Kuan Chiang, supra. The gas virtually 
free of the entrained particulate material leaves the collecting stage 30, 
as illustrated diagrammatically by the arrows 31. 
As shown in FIG. 4, a further exemplary embodiment of an apparatus for 
reducing back corona discharge effects is shown incorporated into a 
charging stage which is operatively associated with an electrostatic 
precipitator or collecting stage. 
The three-electrode concept can be applied to a parallel wire-plate 
geometry as illustrated in FIG. 4. As illustrated in FIG. 4, a charging 
stage 32 includes a pair of intermediate, flat, plane screen electrodes 33 
and 34 and a pair of passive electrodes 35 and 36 positioned respectively 
in close vicinity of and spaced from the screen electrodes 33, 34. The 
screen electrodes 33, 34 and passive electrodes 35, 36 lie in parallel 
planes. A plurality of corona discharge electrodes 37 and 38, shown as two 
parallel wires, are positioned in the plane bisecting the space between 
the passive electrodes 35, 36. Operation of the system is similar to that 
described for the cylindrical configuration. The passive electrodes 35, 36 
are grounded, and a high voltage is applied from a D.C. source 40 to the 
corona wire electrodes 37, 38. The screen electrodes 33, 34 are energized 
from a voltage source 41 with a voltage sufficient to trap any ions due to 
back corona discharge from the passive electrodes 35, 36. The 
substantially unipolar ion field between the screen electrodes 33, 34 
serves as a charging region for particulate material passing through the 
charging stage. The magnitude and relative magnitudes of the voltages on 
the screen and discharge electrodes 33, 34, 37 and 38 are as in the 
embodiment of FIG. 3. The discharge electrodes 37, 38 are supported by 
conventional insulators 42 positioned near their respective ends. The 
screen electrodes 33, 34 are spaced from their respective associated 
passive electrodes 35, 36 by bar-shaped insulators 43 positioned between 
these electrodes in close vicinity to their top and bottom edges. 
In operation, gas having particulate material, such as dust, flyash or the 
like entrained therein, is fed under forced or natural draft into the 
charging stage 32 from one end thereof, shown near the left-hand side of 
FIG. 4, as illustrated diagrammatically by the arrow 44. The gas with the 
entrained particulate material is passed through the stage 32 between the 
two screen electrodes 33,34 and is consequently subjected to the 
electrical and ion fields which exist as a result of the voltages applied 
to the electrodes. As a result, the entrained particulate material becomes 
charged because of the corona discharge between the discharge electrodes 
37,38 and each of the passive electrodes 35,36. As in the embodiment of 
FIG. 3, back corona discharge from the passive electrodes 35,36 to the 
discharge electrodes 37,38 is effectively prevented. In this case, it is 
the action of the screen electrodes 33,34 which collect and capture such 
ions, thereby assuring that a substantially monopolar ion field exists 
between the screen electrodes 33,34. 
The gas with the entrained particulate material, now charged, leaves the 
charging stage 32 from the right-hand side thereof, as diagrammatically 
illustrated by the arrow 45 and passes on to a conventional collection 
stage 46 having an opening in its end through which gas substantially free 
of particulate material passes out. It is to be understood that the 
charging stage 32 and the collection stage 46 can be placed within a 
housing or gas passageway and may in fact form portions of the housing so 
as to enclose the space between the ungrounded electrodes and through 
which the gas passes. 
Alternate embodiments and variants of the invention include corona 
electrode systems of various geometrical configurations in which a screen 
or perforated metal electrode is placed between the corona discharge 
electrode and the passive electrode in such a manner as to provide for the 
removal of ions arising from back corona. The corona discharge electrode 
may be a straight wire or array of wires, barbed wire, helix or other 
form. The screen electrode will normally be placed nearer to the passive 
electrode than to the discharge electrode and conform approximately to the 
shape of the passive electrode. The function of the screen electrode in 
such devices is the same as that discussed above in connection with FIGS. 
1, 3 and 4. 
It is to be understood that the foregoing description and accompanying 
figures of drawing have been set out by way of example, not by way of 
limitation. Other embodiments and variants are possible without departing 
from the spirit and scope of the invention, its scope being defined in the 
appended claims.