Note: Descriptions are shown in the official language in which they were submitted.
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31 The present invention relates to high-intensit~ io-
4l nizers which pre-char~e particulate matter entrain~ in a con-
taminated cJas stream prior to removal o the charqed particles
from the stre~n ~y electrostatic precipitation~ More specifi-
7 ¦ cally, the invention is directed to an improved ele~trode con-
8 figuration for a co-axial venturi ionizer ~Iherein focusing elec-
9¦ trodes narrow the width of the curren~ flux band unstream and
¦ downstream of the ionizer discharge plane.
11 1
12 Stanards for emissions of particulate in flue gases
~3 ! issuing fror~ coal fired electrical power station stac~.s are
14 becorning incr~asingly mor~e stringentO Current air quality stan-
151 daras require that more than 99~ of the fly ash produced by
161 burning cOal be rernoved prior to discharge o~ the co~bustior.
~7-¦ gases rom the stack. ~hus~ the efficienc~ of particulate col-
18 lection ~ust increase in propor~ion ~o the ash con~ent of the
19 co~lO ~n adfiition, in an effort ~o reduce the e~ission of cer-
tain gaseous pollutants, particularly ~he sulfur o~;ides, it has
21 beccme increasingly necessarv to use low sulfur co.~7 in elec-
22 trical power generatir.g plantsO
23 Tha electrostatic precipitator is the ~ost co~nonly
24 used device for ~he removal of particulate matter from po~lex
station stac3. gascS~ Because the size of an electrostat~c pre-
26 cipitator is ~etermined by ~he e~ficiencv vf fly ash removal
2? re~uired, an i~cr~ase in required f.ly ash collec~ion efflcienc~
28 reauires a corxesponding increase in equip~ent si~e and costO
29 Moreover, becauc;~ fly ash resi~tivity tencls to be ir.versely
relatcd to ~he l~vel of cornbustible sulfux in th~ coal burned,
31 the use of lo~ sulfur coals ~o direc~ly r~duce iJaseous suleur
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1 o:cide emissions~ produces highly resistive dusts. It has been
2 demonstra~ed that the size of the electros~atic pr~cipitator
3 necessary to achieve a given level of collection efficienc~
4 increases with increasing electrical resistivity of the fly
6 ash. -~he use of low sulfur coals therefore ~urther increases
6 the size and cost of t~e precipitatorO
7 - ~ecently, high-intensity ioniæers have been developed
8 in which a unique electrode seometry produces a stable high-
9 intensity corona discharge ~hrough which t~e part~culate-laden
10 sas is passea. The ionized flue gases producèd charge the
11 particulate matter ~o a much higher level than is achievable .
12 ~ith a con~entional electrostatic precipita~or. When the ionizex
13 is followed with an electrostatic precipitator, the higher pax-
14 ticle charse results in a higher collection efficiency in the
15 precipita~or due to h~gher migration or particle drift velocity~ .
16 In sucn a two-stage arrangement, the ionizer ac~s as the charging
17 stage and ~he precipi~at~r serves as the collectina stageO
18 Such high-intensity ioni~ers utilize a co-axial pair :-
1q1 or electrodes ~o generate a high-intensity field e:~panaing ra~ially
201 and axially parallel to the direction o~ gas flo~ The anode in
21 1 such an arrangemen~ typicallv takes the orm of a venturi di~fuser
22¦ thxoua,h which the StAC~ gases f1ow im~ediatelv prior to entering . .
231 the precipitator stage. The cathode is a ~isl~ co-a~ially mounted
24¦ within the venturi throat and is formed with a curved peripileral
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25 ¦ edge h~ina a radiusi much s~,aller than the i~ner radius o ~he
26 ¦venturiO When a high v~ltage po~er supply isi connected between
27 ¦~he anode ana ca~hode " a high-intensity coxona discharge is
~81 establi~hed in an annular region he~ween the arcua~e periphery
29 of 'che ca~hode dis3c and lthe surrounding c:yl1ndrical anode sur-
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face near the disc. Because the field is relatively narrow
in the direction of gas flow, a high intensity fieLd is
achievable without prohibitive electrical power require-
ments. The combination of high gas stream velocity through
the venturi and the high intensity transverse electric field
through which the gas stream passes produces intense ioniza-
tion and very high levels of charge on the particles and
results in increased collection efficiency notwithstanding
the high resistivity of the particulate as in the case of
fly ash from low sulfur coal.
One of the problems which has been encountered in
connection with co-axial high intensity ionizers of the type
described above results from the detrimental ~uild-up of
charged particles on the cylindrical anode waIl near the
corona discharge plane. Deposition of high resistivity
particulate matter in this region results in the phenomena
of back corona and excessive sparking with a resulting
deterioration in the applied electric field and attendant
degradation in particle charging efficiency~ Prior attempts
to o~ercome this problem have involved "cleaning" the anode
surface in the affected region to eliminate disturbances in
the corona due to contaminant build-up on the outer elect-
rode. One form of anode cleaning involves the injection of
clean gas into the venturi in the corona discharge region to
form a protective barrier between the anode wall and the
charged particles 1n the gas stream. One particularl~
effect1ve clean gas injection system is described in U.S.
patent 4,108,615 issued August 22, 1978.
According to the present invention, anode cleaning
requirements referred to above are reduced by substantially
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narrowing the width of the current flux band in the annular
region between the cathode and anode.
More specifically the invention is a high-in-
: tensity gas ionizer comprising: a venturi through which
particulate laden gas may flow; a single discharge electrode
mounted within the throat of the venturi and having a peri-
pheral edge defining a maximum dimension transverse to -the
direction of the gas flow; voltage means interconnecte~
between the discharge electrode and the venturi to establish
a high-intensity electric field within the ~enturi across
the gas flow, the peripheral edge of the discharge electrode
having at least one profile-of sufficient curvature that the
electric field establishes a corona current in a region
between the discharge electrode and the venturi; and first
and second cylindrical focusing electrodes mounted along the
direction of the gas flow within the venturi, extending
coaxially in the first and second directions away from the
discharge electrode, being electrically coupled to the
discharge electrode and maintained at approximately the same
electrical potential as the discharge electrode, having
respective diameters less than the maximum transverse
dimension of the discharge electrode, and being sized so
that corona discharge from the focusing electrodes is
substantially non-existent; the first and second focusing
electrodes being sized to increase the electric field
strength at the fringes of the region of corona current to
~axially limit the region of corona current and thus reduce
the surface area of the venturi subjected to collection of
particulates in the gas.
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Desirably the discharge electrode comprises a
cathode disc having an arcuate periphery and being co-
axially mounted within said venturi and wherein said fo-
cusing electrodes comprise cylindrical members on either
side of said disc and co-axial therewith. The focusing
electrodes on either side of the cathode disc create higher
electric fields at the fringes of the current flux band
upstream and downstream of the ionizer discharge plane and
thus narrow the width of the current flux band in the above
annular region. The higher electric fields drive the ions
to the anode with higher velocity with a consequent reduc-
tion of ion migration upstream and downstream by a mutual
repulsion. The net effect is that the width of the band on
the anode surface which is subject to particle deposition is
reduc~d substantially along with cleaning gas requirements.
The focusing electrodes are sized to provide a
high electric field near the anode surface at the fringes of
the current flux band by extending circular electrodes on
either side o~ the cathode disc a distance approximately
equal to the inter-electrode gap between the cathode per-
iphery and surrounding anode wall. The downstream focusing
electrode cylinder can be terminated beyond that distance by
a hemispherical cap. The diameter of the focusing electrode
cylinders is preferably between 20% and 40% of the anode
inside diameter but not larger than the cathode diameter.
In a further aspect the invention is a method of
narrowing the current flux band in a coaxial electrode
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high-intensity gas ionizer including a central cathode
element surrounded by a cylindrical anode surface comprising
\ the step of generating an increased electrical field having
~ ~ a strength less than the strength required to initiate
`~ 5 corona discharge, in a region on both sides of the cathode
element extending coaxially away from the cathode element by
a distance at least equal to the distance between the
cathode element periphery and the cylindrical anode surface,
to increase the velocity of ions travelling toward the anode
surface~ thereby reducing the amount of ion migration in an
axial direction.
The invention is illustrated, by way of example,
in the drawings, in which:
Figure 1 is a schematic side elevational view
; 15 illustrating a multi-stage electrostatic precipitator
incorporating a high-intensity ionizer according to the
present invention;
Figure 2 is an enlarged side view of one ionizer
: stage of the apparatus of Figure 1 partially broken away to
show the ionizer array;
~ Figure 3 is an end elevational view of the ionizer
stage of Figure 2 with the inlet partially broken away to
show the ionizer array; .
Figure 4, on the first page of the drawings, is an
enlarged partial sectional view of a single ionizer venturi
illustrating the electrode arrangement.
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Turning now to the drawings, Figure 1 shows in
schematic side elevational view an electrostatic preci-
pitator system incorporating the invention. As seen in this
Figure, the precipitator system includes a gas inlet 11 into
which gases to be cleaned are directed as sugyested by arrow
12, a gas outlet 13 from which cleaned gases are supplied to
appropriate downstream apparatus, e.g. an atmospheric
discharge duct, as suggested by arrow 14, and typically a
cascaded pair of ionizer-precipitator units generally
designated by reference numerals 15, 15'. ~ach ionizer-
precipitator unit 15, 15' includes an ionizer stage 16 (16')
and typically a pair of conventional electrostatic precipi-
tators 17, 18 (17', 18'). Each ionizer stage 16, 16' and
precipitator stage 17, 17', 18, 18' is provided with a high
voltage input connector 19 coupled to a suitable source of
high voltage as described more full~ below and a collecting
bin portion 20 for collecting particulate matter precipi-
tated from the gas as the latter flows through units 15,
15'.
In operation, gases containing particulate matter
enter the Figure 1 apparatus via inlet 11 and pass through
the first ionizer stage 16 in which the particles in the gas
are electrostatically charged. The gas bearing the elec-
trostatically charged particles next flows into successive
precipitatox stages 17, 18 in each of which the charged
particles are deflected out of the flow path of the gas
under the influence of an electrical field established
across the flow path, the particles being deposited in the
bin portions 20 of the precipitator stages 17, 18. The gas
exiting from precipitator 18 is passed through ioni~er stage
16', and precipitator stages 17', 18', to provide additional
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cleaning therefor, and the cleaned gases emerging from
precipitator stage 18' are conducted via gas outlet 13 to
appropriate downstream apparatus.
Figures 2 and 3 typically illustrate the gas inlet
11 and the first ionizer stage 16 with more detail~ As seen
in these Figures, gas inlet 11 comprises a hollow conduit of
trapezoidal or other suitable geometric configuration which
is coupled at the downstream side to a gas distributor
portion 22. Distributor portion 22 is coupled to an entry
chamber 23 formed within the housing of ionlzing unit 16 by
the side and bottom walls thereof and a vertically arranged
bulkhead 24.
Positioned within the ionizer stage 16 in a regular
array are a plurality of venturi diffusers 27 and associated
central electrode support members 28 each projecting into
either end of the associated venturi 27 (shown here up-
stream) and substantially coaxially therewith. Each member
28 is coupled to a bus bar network generally designated by
reference numeral 29 and consisting of vertically arranged
parallel bus bars (three shown here) interconnected at the
upper ends thereof by a common bus bar element 31, the
element 31 being connected to a single bus bar element 32
extending from the interior of ionizer stage 16 to an exter-
nal conventional high voltage connector shroud 33 to which a
high voltage is supplied from a suitable power source (not
shown) via high voltage connector 34. The downstream end or
outlet of each venturi 27 .is coupled to an exit chamber 36
which is in turn coupled to the inlet of electrostatic
precipitator stage 17.
Storage bin 20 is provided with a removable door
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40 for purposes of inspection and cleaning, and a vibrator
bracket 41 for permitting the use of an optional conven-
tional vibxator to assist in settling any particulate matter
collecting in bin 20 towards the bottom edge 42 thereof.
Bottom edge 42 is provided with suita~le apertures (not
shown) for enabling the particulate matter to be removed
~rom the bin 20 in a conventional manner. Bins 20 o the
remaining system elements 16', 17, 17', 18, and 18' are
con~igured in a substantially identical manner.
Each venturi element 27 and associated coaxial
member 28 generally comprises an electrode pair for generating
a high intensity electric field across the path of gas ~low
through the ionizer stage 16. For this purpose, an elec-
trode (described below) is carried by each member 28 and is
coupled to a source of relatively high negative potential,
via bus bar network 29 while each venturi conduit 27 is
coupled via the framework o~ the structure to ground poten-
tial. Thus each venturi 27 serves as an anode and each
- member 28 serves as a cathode support.
In operation, with high voltage applied between
` the cathode and anode, particles suspended in any gas
flowing through the ioinzer stage 16 are electrostatically
; charged when passing through the throat of venturi 27. In
order to ensure that substantially all charged particles
remain suspended in the flowing gas until arriving at the
; downstream precipitator 17 or 18, and do not adhere to the
ground potential anode su~face, the novel electrode con~i ;
guration shown in Figure 4 is employed.
With reference to Figure 4, each venturi element
27 is ~ormed with an inwardly tapering conical inlek section
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45, a generally cylindrical central section or throat 46 and
an outwardly tapering conical outlet portion 47. The
cathode includes a conducting disc 50 having a curved
peripheral edge which projects outwardly from the outer
surface of member 28. Disc 50 is mounted substantially
coaxially in the throat of venturi 27 and provides a highly
constricted high-intensity electric ~ield in the form of a
corona discharge between the curved periphery of disc 50 and
the surrounding anode surface 52 when a high poten~ial is
applied.
Mounted on either side of cathode disc 50 are
focusing electrodes 53 and 54. These electrodes are pre-
ferably of cylindrical cross-section and are co-axial with
cathode disc 50. Electrodes 53 and 54 are mounted to be in
electrical contact with cathode disc 50 and thus are at
cathode potential. Upstream focusing electrode 53 may be
formed by appropriately sizing electrode support member 28
and maintaining electrical continuity between cathode disc
50, member 28, and bus bar 29. In this case, the downstream
end of support member 28 functions as the upstream focusing
electrode 53. Downstream focusing electrode 54 may be
formed as an extension of support member 28 which extends a
sufficient distance downstream of cathode disc 50 and is
preferably terminated by a hemispherically shaped end
25 surface 54a. Electrode 54 may be attached to electrode 53
by a threaded stud projecting Erom electrode 54 and passing
through the center of cathode disc 50 into support member
28.
Focusing electrodes 53 and 54 increase the strength
of the electric field at the fringes of the discharge current
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flux band (indicated at 56) upstream and downstream of the
ionizer discharge plane. The increased electrical field in
these areas drives the ions to the anode with higher velocity.
Therefore, the ions migrate less upstream and downstream in
their expansion by mutual replusion. The net effect is that
the width of the current flux band impinging on the anode
surface is reduced substantially and the amount of anode
surface which must be cleaned is similarly reduced. This in
turn reduces cleaning gas requirements and results in an
overall increase in particulate collection efficiency.
Focusing electrodes 53 and 54 extend upstream and
downstream from cathode disc 50 a distance approximately
equal to the inter-electrode separation between tha periphery
of cathode disc 50 and the surrounding anode wall. For
purposes of the invention, the focusing electrode cylinders
can be terminated beyond tha~ distance if desired such as by
hemispherically capping the downstream cylinder 54 as described
above to prevent corona leakage there, however, extension of
the electrode beyond that distance will still provide
satisfactory results as indicated by the fact that the
physical structure of eIectrode 53 is formed in the depicted
embodiment by cathode support element 28 which extends out
of the inlet side of venturi 27 terminating at bus bar 2g.
The diameter of the focusing electrode cylinders
is preferably between 0.2 and 0.~ of the inside diameter of
the anode surface surrounding cathode disc 50. If the
diameter is larger or smaller the electric field at the
focusing electrode surface is increased promoting surface
corona leakage. Eowever, even outside the preferred range,
focusing electrodes of the present invention provide improve-
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d performance as compared with prior art devices not employ-
ing such electrodes.
While a pre~erred embodiment of the present
invention has been shown and described above, it will be
readily apparent to those skilled in the art that numerous
modifications and adaptations thereof may be made without
departing from the spirit and scope of the invention as
defined by the following claims.
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