Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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: This invention relates to electrostatic precipitators
and, more particularly, to a new, improved and more efficient
wet electrostatic precipitator.
Electrostatic precipitators are used for air pollution
control, gas cleaning, separation, and particle removal.
The fluid, such 2S a gaseous medium, flows under pressure between
collection plates and discharge electrodes which latter are
adapted to produce a corona and an electrostatic field when a
sufficiently high voltage is applied thereto. Preferably the
voltage is negatlve to produce a negative corona effect and an
ionization.
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Both posltive and negative ions are generated in the
corona, with the positive ions remaining on the negatively
charged discharge electrode while the negative ions ps9s over
to the grounded collection plates along the lines of force of
the electrostatic field extending therebetween. The particles
to be precipitated intercept the negative ions, are charged
thereby, and are attracted to the adjacent collection plate~
The fluid leaving the electrostatic precipitator moves on to .
recovery or exhaust.
Various means are utilized to periodically remove
particles from the collection plates, among which may be mentioned
intermittent spraying of the collection plates with water and
rapping said plates with vibrators. However, although electro-
static precipitators of the mentioned type are highly efficient,
an amount of charged and neutral particles generally escapes
from the downstream side of the stack of alternating collection
plates and discharge electrodes. Such particles either never
reached the collection plates or were reentrained from the collec-
tion plates by the fluid flowing over the collection plates.
These particles move with the fluid and experience has shown that
the charged particles partially attach themselves to pipes and
walls downstream of the collection plate area. Certain tarry
precipitates adhere to the collectlon plates and cannot be removed
by intermittant sprays or by rapping. The accummulation of such
precipitates causes breakdown of the precipitator necessitating
dismantling and manual cleaning thereof. Further, a dry electro-
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static precipitator will not remove gases from the fluid medium.One way of assuring substantially complete collection and removal
of all the particles and selective removal of gaseous contaminants
from a flowlng stream of fluid medium or the like wlthout
accummulation of precipitate on the collection plates is by ~sing
a so- lled wet electrostatic precipitator (WEP).
In the application of a wet electrostatic precipitator,
it is very important that the gas to be treated is saturated
with water vapor to prevent evaporation of the washing water
inside the precipitator which causes loss of washing water and
dry zones on the internal members, where build up of particulates
will occur. The saturation of the gas can be effected in a
spray tower or scrubber upstream of the wet electrostatic pre-
cipitator, it can be effected in the inlet section thereof, or
both arrangements can be used. The initial temperature of the
gas and the saturation temperature dictate the method to be used.
In addition, it is also necessary to obtain a good
and even velocity distribution across the wet electrostatic
precipitator, and diffusion of gas from the duct velocity down
to the wet electrostatic precipitator face velocity has to be
performed in the inlet section. Furthermore, by spraying into
the inlet section, some of the coarser particles will be removed
and the gas absorption process, such as S02 removal, will be
started
As ~le e di~losed the inlet section or
diffuser is provided with several sets of baffles in the form
of transverse rows of vertically oriented channels which are
spaced apart laterally a distance greater than their width, and
which preferably are at a slight angle to the vertical, the
successive rows being staggered relative to each other. The
flanges of these channels ex~nd upstream considered in the
direction of the flow of fluid into the precipitator, and top
sprays direct water to flow down the upstream faces of the
channels. In addition, horizontal sprays of water are directed
at the upstream surfaces of these baffles. The arrangement
of the baffles results in what may be termed an "open" flow for
the fluid into the precipitator.
Following the inlet diffuser, thereare transverse
electrostatic precipitator section in which collection plates,
in the form of sets of baffles are arranged transverse to the
direction of fluid flow, the respective sets of baffles being
spaced in the direction of flow of the medium with a discharge
electrode therebetween. Top sprays direct water to flow down-
wardly over the channel baffles, and horizontal sprays of water,
directed in both directions relative to the flow of fluid are
positioned both before and behind the transverse electrostatic
precipitator section.
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The transverse electrostatic preci?itator sections are
followed by another section containing a set of channel baffles,
again providing an "open" flow, whose flanges are directed down-
stream w~th respect to the flow of gsseous medium or the like,
and these baffles are associated with extended discharge electrodeL
of the main section, which comprises spaced parsllel collection
plates with the interposed discharge electrodes. The extended
discharge electrodes project upstream from positions intermediate
the main collection plates. A similar set of extended discharge .
electrodes project downstream facing a further set of baffles and
are positioned at the downstream end of the main section. Vertical
downstream sprays of water are applied to the main collection
plate section. More than one of such main sections may be pro-
vided, with horizontal sprays and sets of baffles spaced for
Sopen'l flow between sections.
The extended discharge section is followed by a mist
eliminator section in which two sets of transverse baffles or
collection plates are spaced along the path of the medium with
discharge electrodes positioned therebetween, the flanges of the
channels extending toward the electrodes. This section is pro-
vided with means for intermittently applying a spray of water to
the baffles to clean the same. The mist eliminator section is
followed by a further set of baffles in the outlet section. All
of the baffles following the extended discharge electrodes in the
direction of fluid flow are so arranged as to overlap in succes-
sive rows. For example, the transverse spacing of the baffles ~
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may be the same as the width of each baffle. The final baffle set in the
outlet section of the precipitator minimizes the so-called sweeping effect
when the gas in the main housing converges toward the outlet duct or stack.
An ob~ect of the invention is to provide an improved electrostatic
precipitator.
Another object of the invention is to provide an improved wet
electrostatic preci~itator for highly efficient removal of particles and
selective removal of gaseous contaminants.
~ lore particularly in accordance with the invention there is provided,
a wet electrostatic precipitator comprisin~, in combination, a housing having in-
let and outlet openings; at least one electrostatic field section is said housin
including a plurality of collection plates, a plurality of discharge electrodes
in spsced relation with said collection plates and means applying an electric
potential between said collection plates and said electrodes; means in said
housing directing a gaseous medium, containing material to be precipitated,
from said inlet opening along a flow path extending through said electro-
static field section for substantial precipitation of said material by said
electrostatic field sections; means directing continuous sprays of washing
liquid against said collection plates; and an electrostatic mist eliminator
section in said flow path in said housing intermediate the last of said
electrostatic field sections and said outlet opening, said mist eliminator
section being free from continuous sprays of washing liquid and including
further collection plates, further discharge electrodes in spaced relation
with said further collection plates and means applying an electric charge
potential between said further discharge electrodes and said further
collection plates to establish an electrostatic barrier barring passage of
very small drops of liquid therethrough.
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For a fuller understanding of the invention, reference
is had to the following description taken in connection with the
accompanying drawings, in which:
Fig. 1 is an exterior perspective view, partly broken
away and partly in section, of a wet electrostatic precipitator
embodying the invention;
Fig. 2 is a horizontal sectional view through the wet
electrostatic precipitator;
Fig. 3 is a vertical longitudinal sectional view through
the wet electrostatic precipitator;
Fig. 4 is a perspective view, partly broken away, of
discharge electrodes utilized in the precipitator;
Fig. 5 is a partial horizontal sectional view of another
wet electrostatic precipitator embodying the invention;
Fig. 6 is a longitudinal vertical sectional view, partly
in elevation, corresponding to Fig. 5;
Fig. 7 is a schematic vertical sectional view illustrating
a precipitator embodying the invention as used in association with
the gaseous and particulate effluent from aluminum reduction cells ;
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Fig. 8 ls an enlarged partial sectional view of the
inlet section or inlet diffuser of a precipitator embodying the
invention;
Fig. 9 is a horizontal sectional view taken on the line
9 - 9 of Fig. 8;
Fig. 10 is a plan view illustrating the inlet section,
the transverse section, the extended discharge section and the
main parallel plate section of the wet electrostatic precipitator,
the inlet section of which is pictured in Fig. 8;
Fig. 11 is an elevation view corresponding to Fig. 10;
Fig. 12 is a plan view of the last parallel plate
section, the mist eli~inator section and the outlet section;
Fig. 13 is an elevation view corresponding to Fig. 12;
Figs. 14 and 15 are curves illustrating the velocity
profile through the precipitator using "open" baffles as compared
to "closed" baffles of larger size and closer spacing;
Fig. 16 is a plan view illustrating the precipitator
embodying the invention as constructed and arranged for flyash
and S02 removal from the discharge of a coal-fired boiler or
steam generator; and
Fig. 17 is a schematic elevation view corresponding
to Fig. 16.
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DESCRIPTION OF THE PREFERRED ~ODIMENTS
Referring first to Figs. 1, 2 and 3, the wet electro-
statlc precipitator generally illustrated at 20 comprises a
casing 21 reinforced by structural members 22 and having doors
23 ln its side walls near the bottom for the removal of particu-
late material and for inspection, access and the like. Tubular
insulators 24 on the top wall of casing 21 support electrically
conductive rods or buses 25 connected to the discharge electrodes.
Casing 21 forms an inlet section or diffuser 26 for the entering
fluid, such as a gaseous medium, a main housing 27 for the
electrostatic precipitator, and an outlet section 28 leading to
a discharge stack 29.
In the embodiment of the invention shown in Figs. 1, 2
and 3, as the gaseous medium or the like carrying the material
to be precipitated enters main housing 27, it encounters a set
of baffles 30 which are in the form of vertically oriented
channels having their legs or flanges facing upstream of the
flow of fluid entering the precipitator. The baffles 30 are
arranged in several rows and the baffles in each row are spaced
apart substantially by the width of the channels. For example,
the channels may be 4 inches wide and be spaced flpart 4 inches
laterally of the precipitator. From Fig. 2, it will be noted
that the baffles of each succeeding row are arranged opposite the
open aces in the immediately preceding row. The set of baffles
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30 are continually sprayed from upstream with water ~rom a
plurality of nozzles 31 connected to vertically spaced, hori-
zontally extending headers 32. Nozzles 31 are adapted to pro-
duce sprays in the form of droplets of water rather than in the
form of streams of water. The baffles 30, upon which the enter-
ing gaseous medium impinges and by which it is deflected, remove
large particles from the entering gaseous medium as the medium is
caused to follow a relatively tortuous path by virtue of the .
deflecting baffles 30. The water also serves to absorb certain
gases such as water-soluble gaseous fluorides.
: High potential conductive rods or buses 25 are electric~
ally and mechanically connected to a discharge electrode frame
structure 33 including upper horizontally extending tubular
frame elements 34. Said horizontally extending tubular frame
elements 34 are joined by laterallye~ending channel-shaped beams
35 at the ends thereof and sn I beam 36 in the center thereof.
Suspended from the upstream channel-shaped beam 35 is an
extended discharge electrode assembly 36. As shown in Fig. 1,
said discharge electrode assembly consists of a tubular frame
38 supporting, togetheL with beam 35, a series of vertically
extending discharge electrodes 39 adapted to produce/electro-
static field extending substantially along the path of the gaseous
medium as will be described in greater detail below. Extended
discharge electrode assembly 37 serves to impart a precharge to
the entering particulate material, effecting some initial pre-
cipitation thereof on the back side of baffles 30. T~e back sides
. . . . - .
of said baffles ~re washed by the droplets df water also pre-
cipitated on the back sidesthereof due to the electrostatic
field produced by extended discharge electrode assembly 37.
Similarly constructed extended discharge assemblles 37' and 37 "
are respectively mounted on I beam 36 and the downstream channel-
shaped beam 35. Tubular frame 38 is substantially rectangular
having horizontally extending tubular cross members vertically
spaced therealong for providing support for discharge electrodes .
39. The discharge electrodes are formed from laterally spaced,
vertically extending metal strips mounted welded on frame
38, said strips being aligned in parallel planes extending per-
pendicular to the general plane of frame 38. In the embodiment
depicted, the strips have their longitudinal edges formed with
charge concentration points defined by projecting spikes. Dis-
charge electrodes formed of other constructions, such as wires
having barbs mounted thereon, vertically extending metal electrod~
strips formed with notches in the longitudinal edges thereof, or
the like may be utilized.
The pre-charged fluid passing through the first extended
discharge electrode assembly 37 enters the first of two main,
parallel plate electrostatic precipitator sections 40 and 41
included within maln housing 27 of the precipitator. Each of
said plate sections consists of alternating discharge electrode
assemblies 42 and grounded collection plates 43. Each discharge
electrode assembly 42 consists of a tubular frame 42'supported
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on and electrically connected to horizontally extending frame
elements 34. Said frame is substantially rectangular in con-
flguration having vertically spaced horizontally extending cross
members and supporting vertically extending discharge electrodes
44. Each of said discharge electrode assemblies extends in a
plane substantially parallel to the path of the gaseous me~ium
and is substantially equally spaced from each of the adjacent
pair of collection plates 43. Said collection plates are pre-
ferably provided with a smooth surface.
Above each plate section 40, 41 there is formed a series
of compartments 45, each of which contains a nozzle 46 connected
to a header 47 and arranged to direct water, in the form of a
spray of droplets, downwardly along the collection plates 43,
this affording continuous washing of the collection plates.
Also, in advance of each section 40 and 41 there is a series of
vertically spaced horizontally extending headers 47 provided with
nozzles 48 and supplied with water through control valves 49.
Nozzles 48 spray water, in the form of sprays of droplets,
through each plate section 40 and 41 in the direction of flow.
Thus, the collection plates 43 are continually washed with water
directed in the direction of flow with the gaseous medium and
with water directed vertically downwardly therealong. In the
embodiment depicted, all of nozzles 31, 46 and 48 produces a
spray of water droplets in a hollow cone configuration but other
spra onflgurations may be used.
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¦ The fluid medium passing through the prec~pitatorJ after
passing throu~h the fir~t plate section 40, imp~nges against a
second set of baffles in the form of vertically extending and
laterally spaced channels 30'. The second set of baffles 30' is
essentially similar to the first set of baffles 30, but there may
be a lesser number of rows of baffles 30'. For example, while
the baffles 30 are arranged in six rows, the baffles 30' may
comprise only four rows. The fluid medium striking the baffles
30' are deflected thereby to flow in a tortuous path, resulting
in further separation of particulate material from the fluid
medium. The legs or flanges of channels 30' extend upstream
in the same manner as do the legs or flanges of channels 30.
After passing by the channels 30', the fluid medium
passes through se~ond extended discharge electrode assembly 37',
- esse~tially identical with first extended discharge assembly 37.
A further charge is imparted to the fluid medium by said second
extended discharge electrode assembly, after which the fluid
medium flows through the second plate section 41, resulting in
further removal of fine particulate material from the fluid
medium. The fluid medium then passes through a third extended
discharge electrode assembly 37" which imparts a charge on such
particles as may escape from the main parallel plate sections
or be reentrained therefrom.
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After passlng through the thlrd extended discharge
electrode assembly 37 " , the fluld medium impinges against a
third set of baffles comprising channels 30 " which extend
vertically and are spaced apart lsterally in the sa~e m~nner as
the channels 30, the set of channels 30'' being essentially
similar to the set of channels 30, with their flanges extending
upstream of the direction of fluid flow. The lateral and longi-
tudinal spacing of the channels of the three sets 30, 30' and 30"
are substantially identical with each other. After passing
through the channels 30" , substantially all of the particulate
material has been removed from the fluid medium, and the latter
then flows into outlet section 28 and out through discharge
stack 29. The directional charge imparted on the particles
escaping from second parallel plate section 41 causes such
particles to be captured by char.nels 30 " , which are grounded.
A more detailed showing of discharge electrode frame
structure 33 is depicted in Fig. 4. The entire structure is
electrically coupled and a large voltage, sufficient to produce
corona discharge at points 50, is applied ~hereto, the collection
plates and baffles all being grounded. The discharge electrodes
44 and 39 May take any desired form such as wires, wires with
barbs, notched bars or the like.
1 1(~41~
.' Figs. 5 snd 6 lllustrate a modiflcation of the invention
in which parts identical with those in Figs. 1, 2 and 3 have
been given the same reference numerals and parts substantlally
similar to those in Figs. 1, 2 and 3 have been given the same
reference numerals primed. Referring to Figs. 5 flnd 6, the wet
electrostatic precipitator 20' illustrated therein again includes
a casing 21' braced by structural members 22' and having insula-
tors 24 supporting conductive busses or bars 25. Casing 21' is .
divided into an inlet section 26', a main section 27' an outlet
section 28' and a stack 29.
In this embodiment of the invention, a first set of
vertic~ly extending channels or baffles 55 is arranged directly
in the inlet diffuser section 26' and comprises several rows of
transversely spaced vertically extending channels whichare
spaced apart a distance equal to or less than their widths,
i with the channels in each succeeding row overlapping the openings
in the immediately preceding row. Also, vertical spray nozzles
56 are arranged along the upper wall of inlet section 26' both
upstream and downstream of the channels 55 so as to spray water,
. in the form of droplet sprays, against both the upstream and
downstream surfaces of the channels 55. Vertically spaced
horizontal headers 57, arranged upstream of the channels 55, have
spray nozzles directing sprays of water against the upstream
surfaces of the set of channels 55 and in tlle direction of flow
of gaseous medium through the precipitator. Headers 57 are also
: arranged downstream of the channels 55 and direct horizontal
l~
sprays of water, in the direction of fluid flow, agsinst the
set of baffles 58 arranged in advance of the first pre-
cipitator section 59. Thus, the sets of channels SS and 58
have streams of water continuously sprayed downwardly therealong
and horizontally thereagainst.
In this embodiment of the inventionl there are three
precipitator sections 59, 59' and 59" arranged in series.
The three sections are essentially identical, each including a
discharge electrode frame structure 33'. The frame structure
includes laterally extending channel-shaped beams 35' which
extend laterally within main section 27' and are secured to and
electrically connected to conductive busses 25. A series of
rectangular tubular frames 51 having horizontally extending
cross bars vertically spaced therein is mounted on and electric-
ally coupled to each pair of beams 35' by means of tubular
connecting rods 54. Each of the frames 51 supports vertically
extending discharge electrodes 44' as depicted in section S9'.
Discharge electrodes 44' are structured in the same manner as
discharge electrodes 44 illustrated in Fig. 4. In registration
with discharge electrodes 44' and extending parallel to frames
51 are parallel collection plates 43'. The collection plates
are grounded and are positioned in spaced relation with one of
frames 51 positioned intermediate each adjacent pair. The
horizontally extending, tubular side portions 52 and 52' of each
frame 51 defines an extended discharge electrode assembly.
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Spikes S3 are mounted in vertically spsced relation on the outer
edge of each of supports 52 and 52', the spikes on support 52
faclng upstream, the spike~ on support 52' facing downstream.
Each of these extended discharge electrode assemblies could be
replaced by an extended discharge electrode assembly constructed
in the same manner as extended discharge assembly 37 of the
embodiment of Figs. 1, 2 and 3.
Two rows of vertical spray nozzles ~6' are arranged
above the collection plates and electrodes of each of these
sections 59, 59' and 59 " , to spray water continuously downwardly
along the collection plates. Each of said sections consists of
an upstream-facing extended discharge section, a central paral]el
plate precipitator section and a downstream-facing extended
discharge section. The extended discharge electrode assemblies
52' of the first and second sections 59 and 59' are each
followed by channel-shaped baffle sets 60, said baffle sets
being substantially identical to sets 55 and 58, except that
they comprise only two rows of channels. A further channel set
60' is positioned adjacent each upstream-facing extended discharge
assembly 52 of the second and third sections 59' and 59" .
Between each pair of sets of channels 60 and 60', intermediate
the main precipitator sections, further vertical spray nozzles
46' direct sprays of water continuously downwardly and sets of
horizontal headers 57 direct sprays of water horizontally in
the upstream direction between and along the collection plates.
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Intermediate extended discharge electrode assembly 52' of section
59 " and outlet section 28' are a final set of channel-shaped
baffles 30" positioned and structured in a manner similar to the
corresponding baffles in the embodiment of Figs. 1, 2 and 3.
Before discussing the overall plant layout shown in Fig.
7, reference will be made to Figs. 8 through 15 which illustrate
the novel features of preferred embodiments of the invention.
Figs. 8 and 9 illustrate a preferred arrangement of the internal
members or components of an inlet section 26 " receiving gaseous
medium or the like through a conduit 61 through which the gas
flows in the direction indicated by the arrows in Figs. 8 and 9.
The purpose of the baffles 62 is to distribute the flow, act as
contact zones between the unsaturated gas and the liquid, act as
contact zones for gaseous absorpiton, and provide points of
impaction for rernoval of coarser dust particles. Thereby, the
inlet loading of dust to the wet electrostatic precipitator is
reduced. As shown in Fig. 9, in each row baffles 62 have a
transverse individual spacing larger than the width of a single
baffle, or, stated another way, the baffles have been "opened"
up transversely to provide an "open" configuration; This con-
figuration provides a balanced effect of efficient gas distribu-
tion with the creation of a minimum amount of turbulence, and
provldes a certain amount of carry through of water drops for
washing of the back sides of the baffles, and further gas
saturation and gaseous contaminate absorption downstream of each
baffle section. It will be noted that legs or flanges of the
channels face upstream. Typically, with two rows of baffle ~nd
a gas space velocity of 2.5 ft/qec., if a qpray puts in 10070 of
water, 49% will be collected on the upstream faces of the baffles,
9% on the downstream faces of the baffles and 42% will be carried
through with the gas stream. The arrangement of the baffles with
their flanges pointing upstream, as shown in Figs. 8 and 9, in-
creases the particulate collection efficiency. It will be noted
that the baffles 62 are arranged in three sets, each comprising
two rows of baffles.
The baffles typically are 2 inches wide with 1/4 inch
flanges, spaced 4 inches apart trans~ersely, and with each row
spaced 2 inches apart longitudinally from the back of the first
row of baffles to the front edges of the flanges on the second
row of baffles. This provides an open area, for gas passage,
of 33% of the full cross sectional area.
The second section of two rows of baffles, as shown
in Fig. 9, is offset 1.5 lnches transversely as compared to the
first section, and similarly with the third section of two rows
of baffles, to obstruct any straight flow path through the
section of baffles which the dust particles or water drops could
take.
Figs. 8 and 9 also show the spray configuration, and
he spray intensity as dictated by the inlet grain loading.
bearing headers
The spraying is effected by a combination of nozzle /63 spraying
bearing headers
downwardly from the top and nozzle / 64 spraying in the downstream
direction directly on to the faces of the baffles 62 in each
section. The water washes the baffles efficiently, since the
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water ls constrained by the flanges of the channel shaped baffles
to flow straight down the ~affles. This minimizes channeling of
the water and provldes a continuous sheet of water covering the
baffle surfaces. As wlll be noted particularly from Fig. 8,
each baffle section of two rows of baffles 62 is installed a
few degrees offset from the vertical to provide the best drainage
of the water down the baffles.
When the inlet loading of dust is high, there is a
potential hazard of build-up of material on the internal collec-
tion members, because of inadequate washing. However, it has
been found that a transverse electrostatic precipitator section
65, shown in Flgs. 10 and 11, can be washed very efficiently and
that one or two of these sections will remove a substantial amount
of the coarser dust particles thereby to reduce the washing load
in the parallel plate sections downstream of the transverse
electrode sections. Those gaseous contaminates which can be
absorbed in the liquid used will also be removed.
Each transverse section contains discharge electrode
assembly 66' consisting of a rectangular tubular frame supporting
discharge electrodes 44 " . The frame is aligned transverse to
the path of the medium with the electrodes of the plate type
aligned essentially parallel to said path. The electrode
assembly is positioned between two sets of facing baffles or
grounded collection plates 66. Most of the washing of said
baffles takes place from the top from spray nozzles 67 pointing
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vertically downwardly However, by u9ing the "open" baffle
configuration ~r the channel-shsped baffles 66 ln transverse
section 65 the horizontal spray nozzles 68 can be placed
further down from the top, as shown in Fig. 11, with the nozzles
68 ahead of section 65 spraying downstream and those at the exit
of section 65 spraying upstream. A substantial fraction of the
water sprayed will penetrade through the "open" baffle section
and wash the baffle surfaces facing the electrodes. This results
from the fact that the liquid drops will take an electrostatic
charge, and at least a portion of the smaller drops, those less
than 80 microns, will migrate to the collection baffles and wash
all particles that also have migrated to the baffles 66, as shown
by the dashed arrows. Larger drops will hit the baffles 66 by
impaction, or fall through the section and down into the trough.
The two rows of baffles 66 upstream and downstream of
discharge electrode assembly 66~ act as gas distribution devices
and tend to remove any skewed velocity profile that the baffles
62 in the inlet section 26' could not remove. This is accomplish d
with the introduction of a minimum amount of turbulence. The
flanges of both the upstream and downstream baffles 66 point
toward the electrode assembly 66', as seen in Fig. 10. This
has been found to be the best configuration because the turbulenc~
will be lowest in the electrostatic field zone and the discharge
of ions will be the highest.
Bsffle~ 66 typic~lly are 2 inches wide with 1/4 inch
flanges, and are spaced 4 inches apart transversely. The second
row of baffles is spaced 2 inches downstream from the first row
and aligned to be centered in the space between the first row
of baffles, providing an open area of 33% of the full cross
sectional area.
The baffles utilized in the embodiments of Figs. 1-6 do
not have the "open" construction being, in one case, 4 inches
wide with 4 inch transverse spacing, with the rows spaced 2 inches
apart in the direction of gas flow. However, it has been found
that these baffles introduce considerable turbulence, so that the
downstream surfaces of the baffles can be washed only from the
top by nozzles pointing vertically downwardly. The 2 inch
channels, used in the open configuration as mentioned above, have
distinct advantages with respect to the velocity distribution
downstream thereof. The spray from nozzles directed along the
flow path either upstream or downstream can penetrate between
the baffles and migrate to the surfaces thereof facing away from
the nozzles for effective cleaning of said nozzles and improved
contact between the medium and the sprayed liquid. Figs. 14 and
15 show,by way of example, the velocity distribution downstream
of the 2 inch baffles, with an "open" configuration, and the
4 inch baffles in the 'rclosedt' configuration, respectively.
Behind each opening between the baffles of the last rows, there
is a velocity peak, and behind each baffle in the last row there
is a minimum velocity point. The relative difference in the
minimum and maximum velocity i5 an indication of the turbulence
1~ 15
i or local disturbance to the gas flow past the baffles. From
Fig. 13 it can be seen that, with a face velocity of 2.5 ft/sec.,
the velocity sp~n for the 4 inch baffles was from 1.0 ft/sec. to
10.0 ft/sec. From Fig. 14, it can be seen that the velocity
span for the 2 inch spaced baffles is from 1.6 ft/sec. to 5.0
ft/sec. Thus, the maximum velocity was reduced by 50% by using
the 2 inch baffles with a 4 inch transverse spacing. The
turbulence caused by the "closed up~r construction can cause
reentrainment of precipitant, and even prevent precipitation,
leading to inefficient precipitation.
Referring specifically to the baffles 66 of the transverse
precipitator section 65, the width and spacing of these baffles
must be such that inertial forces due to turbulence in this
section do not overcome the electrostatic force and prevent
capture of a portion of the particles which would otherwise be
precipitated. Flow in the region of baffles 66 characterized
by a Reynolds number less than 5,000 will provide the most
favorable flow conditions. As used herein, Reynolds number (Re)
refers to:
Re ~
where: U = superficial velocity (total flow of fluid medium
divided by cross sectional area of precipitator);
W = width of baffles; and
~ r - kinematic viscosity of fluid medium.
The embodiment illustrated in Fig. 14 achieves the necessary flow
characteristics. The minimum baffle width is dictated by cost
of manufacture.
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The next stages, generally lndicated at 70 in Fig. 10,
consists of one or more parallel plate electrostatic precipitator
sections, each such section having an extended discharge section
on the upstream and downstream sides thereof. The discharge
electrode portions of said sections conslst of discharge
electrode assemblies 51' essentially identlcal with the assemblies
51' of the embodiment of Figs. 5 and 6. Like collection plates
43 " are also provided as are discharge electrodes 44 " '. The .
extended discharge section includes channel baffles 71 which are
substantially identical with baffles 66 and arranged essentially
in the same "opened" configuration. The baffles 71 are arranged
so that the velocity along the plates 43' will be the minim~m~
velocity of the velocity distribution behind the baffles. This
will minimize the reentrainment, in that the velocity along the
plates will be the smallest magnitude in the distribution.
Generally, the spacing between baffles 71 exceeds the width of
said baffles to ~inimize excursions of velocity from the super-
ficial velocity as illustrated in Figs. 14 and 15. The precise
spacing,width and position of the baffles is dictated by the
spacing between parallel plates 43 " . Since minimum excursion
from the superficial velocity occurs in the region downstream
of the baffles of the row closest to the parallel plates 43 " ,
baffles 71 are positioned so that one of the baffles of said
downstream row faces the leading edge of each collection plate
43 " . For example, the baffles of Fig. 14 are particularly
suited for a collection plate spacing of 12 inches.
1~4 ~
The baffles 71 at the inlet to the extended discharge
electrode and parallel plate sections have thelr flanges pointed
downstream, while those at the exit end have their flanges pointed
upstream. As shown in Figs. lO and 11, a vertically downwardly
directed continuous spray from overhead nozzles 46 " cleans
plates 43'. The open baffle configuration has a significant
advantage in that about one half of the liquid from the horizonta]
sprays 68 pointing downstream ahead of baffle section 71 will .
penetrate through and effect washing on the back side of the
baffles, on the discharge electrodes, and on the leading portions
of plates 43 " where the amount of the dust particles collected
is at its largest. Where more than one stage is provided, baffles
of the 'lopened up~ configuration would be provided as part of the
extended discharge sections on the downstream side of each upstrea n
parallel plate section and the upstream side of each downstream
parallel plate section. Headers, similar to header 68, would
be provided for spraying said baffles from between the two
extended discharge sections. Said baffles would now be washed
from above.
A set of transverse baffles 72 is positioned at the
outlet of the parallel plate section as part of the last extended
discharge section to capture dust and water drops escaping the
parallel plate sections. Baffles 72 are relatively closed up
with 2 inch spacing so that the second row overlaps the first
for efficient collection.
- . . . - ~- -
Because of the diminishing loading of dust snd gsseous
contaminants in the gas as it moves through the wet electrostatic
precipitator, a decreasing washing intensity downstream can be
used. The nozzles 46 " direct water vertically downwardly from
their positions above the parallel pla~es, as best seen in Fig.
11. The parallel plate section is more economical than the
transverse section for removal of high loadings, those less than
0.5 grains/cubic foot, and of very small diameter, less than 1
micron particulates and condensed drops.
In any particulate and/or gaseous removal process where
a liquid is used, it is very important to eliminate the carry
over liquid drops and mists before the gas escapes through the
exit duct of the apparatus, such as the stack 29. It has beeln
found that effecting this elimination electrostatically is highly
efficient, and the general arrangement is shown in Figs. 12 al~d
13. At the exit of the last parallel plate section after the
extended discharge section, a transverse section consisting of
dischar~e electrode assembly 66 " (substantially identical with
assembly 66') and two sets of channel baffles 72 and 73 are
provided. The baffles 72 should have a transverse spacing whlch
is the same or less than the width of each baffle, so that, with
a 2 inch wide baffle, a spacing of 2 inches or less will increase
the removal efficiency. The spacing of the two rows from each
other in the direction of flow is only l inch so as to minimi~e
impact of drops against a baffle. The electrostatic field
between the extended dischsrge electrode assemblies 66 " and
baffles 73 and 74 aids the collection of escaping dust particles
~:~
.
: . ' '- ` ' : ` `
.
~V~
and liquid drops. It hss been found by mea~urements Lhat ~his
configuration, without the electrostatic field, removes 95.6%
of the liquid drops at a face velocity of 2.5 ft/sec. With the
addition of the electrostatic field, the ef~iciency becomes
substantially higher.
The transverse section, which is operated dry, establishes
an electrostatic barrier which the small liquid drops cannot
penetrate. The mist will collect on the back side of the upstream
baffles 73 whose flanges extend in the downstream direction,
while the downstream baffles 74, whose flanges extend in the
upstream direction, will be essentially dry. However, some small
dust particles can penetrate through and will collect on the
downstream baffles 74. Consequently, the surfaces o these
baffles should be washed intermittently to prevent build up o~
materials, and this is effected by intermittently operating the
overhead nozzle 75 to wash ~e baffle.
A final set of channel baffles 77 whose flanges extend
upstream, can be applied, as shown in Figs. 12 and 13 for
minimizing the so-called sweeping effect when the gas in the main
housing converges toward the outlet duct or stack.
The wet electrostatic precipitator of the invention can
be used for simultaneous removal of flyash and S02 in a flue
gas stream from a coal fired boiler, as shown in a gene~al
schematlc manner in Figs. 16 and 17. The flue gas is saturated
by spraying water into the inlet duct which has an increasing
cross section in the downstream direction of the gas flow. As
`
shown in Figs. 16 and 17, the gas from a conduit 78, leading from
the boilers, is directed into a downwardly extending duct 81, and
then upwardly into a spray section 80 where the gas i9 saturated.
Saturation is effected by spray nozzles 82 supplied from a pump
83 connected to a water main 84. Some S02 is absorbed, and the
gas is distributed over the inlet to the diffuser or inlet section
26 " which has the configuration shown in Figs. 8 and 9. The
spray nozzles 63 and 64 in inlet section 26 " are supplied with
water from a pump 85 having an inlet line 86 communicating with
a pond 87 and an outlet line 88 communicating with the nozzles
63, 64, 67 and 46 " , The saturation process continues into the
inlet diffuser which has the several rows of transverse baffles
62 which also take out some of the larger sized dust particles
and act as flow distribution devices. The baffles are heavily
washed, as indicated in Fig. 17, and S02 is being absorbed all
the way through inlet diffuser 26''. Recirculated liquid is used
for the washing, which makes more S02 absorption possible.
The gas and the flyash then enter into the first trans-
verse electrostatic section 65 which is washed intensively and
continuously. A substantial part of the flyash is removed here.
The internals of transverse electrode section 65 are shown in
Figs. 10 and 11. More than one transverse section may be provided
as dictated by the inlet loading of flyash. The flyash not taken
out in the transverse electrostatic sections 65 passes through
the baf1es of the extended discharge electrode section and then
onto the first plate section 70. Leaving the first plate section
70, the gas and the remaining particles pass through inter-plate
section extended discharge sections for further flow distribution,
flyash collection and S02 removal.
The gas then enters the next plate section 70' ~lich
may or may not be necessary,depending upon the collection area
required for complete collection. Flyash and S02 is continuously
being removed and, since the concentration of both decreases in
the downstream direction, the washing intensity, or the number .
of liquid spray nozzles, can be reduced in a downstream direction
through the wet electrostatic precipitator.
When the clean gas is exiting from the last plate
section, it again passes through an extended discharge electrode
section 75. The last transverse electrode section is run dry and
acts as a very efficient mist eliminator. The internals are
shown, for example, in Figs. 12 and 13. The cleaned and demisted
gas then passes through some final baffles 77, which act as flow
distributors, and the gas enters into the outlet duct 29.
The heavy slurry from the spray section 80, inlet section
or diffuser 26 " , the transverse discharge electrode section 65
and the upstream half of th~ first plate section 70 is supplied
to a first clarifier 90 through lines 91. The light slurry from
the latter half of the first plate section and from the following
precipitator sections is supplied to a second clarifier 92 throug~
a line 93. The water or liquid from clarifiers 90 and 92 is
supplied with neutralizing chemicals at 89 and is delivered
into pond 87 and also into line 86 through valves 94, for
, . - , -
(r'~
recycling. The discharge from clarifiers 90 and 92 is applied
to a thickener 95 from which the sludge is discharged. The
water from thickener 95 is supplied to a line 96 leading from
clarifier 90 to pond 87.
It has been found that the maximum flyash removal is
provided by a configuration including sections of "open" washed
transverse baffles in the inlet or diffuser, to distribute the
flow and to remove the largest flyash particles. These sections
are followed by one or more sections of transverse electrostatic
precipitators having transverse collecting baffles to remove a
substantial portion of the coarse particles of the heavy inlet
loading of 3-5 grlcu. ft. presented to the wet electrostatic
precipitator. Very intensive washing of these sections prevents
build-up with the overall liquid consumption being from 20-50
gpm/l,000 cfm; with the actual value being dependent upon the
dust inlet loading and the amount of S02 to be removed. ~xtended
discharge sections serve for further removal of the heavy inlet
loading and to collect dust and liquid drops that have not been
collected in the parallel plate section or sections. The
parallel plate sections, as required, are provided to remove
the substantially lower loading of the finer particles in the
flyash. The washing intensity is decreased in the downstream
direction, since less and less particles are being collected
due to the decreasing dust loading.
.
,
1~3~
The use of the multiple troughs provides that the
heavy flyash slurry in the front section of the unit and the
lighter slurry from the rest of the unit can be separated
and treated and/or recycled with varying degrees of clarifica-
tion and filtration. With a wet electrostatic precipitator
designed and operated as described above, there have been
obtained very high migration velocities, since the collecting
plates are washed heavily so that the particles are washed
away and there is, therefore, no reentrainment or limitation .
due to dust resistivity.
Additionally, the apparatus shown in Figs. 16 and 17
including the wet electrostatic precipitator embodying the
invention effects simultaneous and highly efficient removal
of flyash and S02 in one unit, with the use of continuous sprays
from low pressure nozzles exposing a very large surface area of
liquid which ensures excellent gas absorption. The use of a
cross-flow scrubber configuration in the wet electrostatic
precipitator provides fresh liquid throughout the unit for S02
absorption, combined with long residence time of the droplets
in the wet electrostatic precipitator which gives inherently
sufficient time for efficient S02 absorption. The washed and
wetted transverse baffles 62 in the inlet diffuser 26 ", and
throughout the unit, act as S02 and liquid contact zones, and
thereby enhance the absorption. The tortuous path the gas has to
travel to pass these baffles increases the residence or contact
time between S02 and liquid.
. .
The apparatus has the pot~ntial for using all known S02
removal processes currently being used or tried in scrubber~,
among which the following are exemplary:
A. The slurry neutralized externally with lime,
Ca(OH)2, filtered for flyash and precipitate,
and then recycled.
B. The slurry neutralized externally with lime,
coarse settling and recycling of light slurry.
C. The slurry filtered for flyash and calcium
sulfite, then neutralized with the resulting
CaS03 slurry recycled.
D. External neutralization with soda ash, filtration
of flyash and recycling.
E. Dissolving ammonia in the charged liquid, filtra-
tion of flyash slurry and recycling of the liquid.
Ammonia also can be injected directly into the gas.
F. Regenerative processes, such as ammonium and
magnesium phosphate processing by filtering the
slurry for flyash, regenerating the chemicals to
recover the S02 gas, and recycling the liquid to
the wet electrostatic precipitator.
- . .
~ t shoul~l be noted that tho~L' no.:zles providing hori-
zontally directed sprays may provide sprays in the form of full
cones, while those sprays directing liquid vertically downwardly
may provide sprays o~ a fan type. As an alternative, the satura-
tion atmospllere in the chamber could be provided by steam rather
than by water sprays.
The wet electrostatic precipitator embodying the in-
vention can be used for simultalleous remova] of aluminum oxides,
solid fluorides, gaseous fluorides, tar mist and S02 from aluminum
reduction cells. ~`ig. 7 schematical]y illustrates a general
layout of a typical application for this purpose.
Referring to Fig. 7, the stream o~ gaseous medium from
the aluminum reduction cells is delivered through a conduit 97
into a saturation chamber 100. The arriving gas stream contains
dust particles and mist of extremely small si~es. Due to the fact
that small particles will talce an electrostatic charge to the
same degree as larger particles, the removal efficiency is very
high. An equivalent removal efficiency in a scrubber would
require extremely lligh pressure drops. ~ater is sprayed into the
primary flue gas coming from the aluminum reduction cells by spray
nozzles 98 in conduit 97 and by spray nozzles lOl in saturation
chamber 100. Nozzles 98 and 101 are supplied with water from
a water make-up pump 102, which also supplies water to spray
nozzles 104 in a conduit 103 conducting the saturated primary
gas from saturation ctlamber 100 to t~le inlet or diffuser section
26' " of the wet electrostatic precipitator. Inlet section 26 "'
¦has a construction identical with that of the inlet sec~ion 26 "
shown in Figs. 8 and 9, so that detailed description is believed
unnecessary. The liquid accumulating in the hopper bottom of
saturation chamber 100 is supplied through a line 106 to the
inlet of a pump 107, whose outlet is connected by a line 108 to
a clarifier 105.
A recycle pump 110 serves to supply spray nozzles 111
in conduit 103 adjacent inlet or ~iffuser section 26" ', as well .
as to supply the no~zles 64 in diffuser 26" ' and nozzles 47"
in plate sections 70 and 70'. The sludge from clarifier 105 is
directed into a hopper 112 connected to a sludge removal line,
and a line 113 connected to clarifier 105 leads to a flùoride
recovery. As indicated in Fig. 7, recycle pump 110 is supplied
with fresh plant liquor with water from clarifier 105 and with
water from sludge settling hopper 112. The wet electrostatic .
precipitator includes collection plate sections 70 and 70'
followed by a transverse discharge electrode section 75.
The primary flue gas coming from the reduction cells is
saturated in a scrubber, such as the saturation chamber 100 or
in the inlet or diffuser section 26 " ' o~ the wet electrostatic
precipitator or both. All tar vapors must be condensed to a mist
before the gas enters the main parallel plate section of the wet
electrostatic precipitator, and a certain time with gas-liquid
contact is needed in order to attain this. The gas passes
through sections of "open" baffles 62, such as
shown in Figs. 8 and 9, before entering the main
4 ~
portion of the wet electrostatic precipitator. The inlet loading
is usually very low, for example less than 0.1 gr/cubic foot
and therefore a transverse electrode section is not necessary.
The gas then passes through the sections 70 and 70' having the
extended discharge sections at the inlet and outlet ends thereof,
such as shown in Figs. 11, 12 and 13, tllese sections having
baffles in an "open" configuration, except ~or the baffles of
the final extended discharge section.
After passing through a sufficient number of electro-
static fields to obtain the necessary removal efficiency, the
gas passes through an extended discharge electrode section and
a transverse discharge electrode section 75, such as shown in
Figs. 12 and 13. The latter section is provided for mist
elimination, removal of tar drops and removal of dust particles
escaping the last fie]d of parallel pla~es, and said section is
washed only intermittently.
Substantially the same results are obtained, with
respect to removal of contaminates, by the arrangement shown
in Fig. 7 as are obtained in the arrangement shown in Figs. 16
and 17. Sufficient washing to maintain the internals in a
clean state is provided with an overall liquid consumption
of 5-12 gpm/1000 cfm of gas, with the inlet loading dictating
the selection of the liquid to gas ratio. The arrangement of
Fig. 7 provides simultaneous and highly efficient removal offume
particulates, gaseous fluorides and S02. It also has the
potential for using alkaline liquids to increase the rate of
removal of gaseous fluorides and S02 , and to improve the washing
off of the collected tar.
'I llU~
The wet electrostatic precipitator embodying the
invention can be used for simultaneous removal of condensed tar
mist, coal particles and S~ coming from a carbon baking process,
coke oven batteries or the like. The schematic is essentially
the same as shown in Fig. 7 for aluminum reduction cells, except
that there are no aluminum oxides or fluorides in the gas stream
and the tar loading is much higher. Conseq~lently, full saturatio
of the gas stream with water vapor and the condensation of all
tar vapors is very important prior to the entry of the gas into
the main part of the wet electrostatic precipitator. Other
applications of the wet electrostatic precipitator embodying
the invention are possible, the examples herein given being
merely by way of example.
It will thus be seen that the objects set forth above,
among those made apparent from the preceding description, are
efficiently attained and, since certain changes may be made in
carrying out the method of operating the wet electrostatic
precipitator and in the constructions as set forth without
departing from the spirit and scope of the invention, it is
intended that all matter contained in the above description and
shown in the accompanying drawings shall be interpreted as
illustrative and not in a limiting sense.
It is also to be understood that the following claims
are intended to cover all of the generic and specific features
of the invention herein described, and all statements of the
scope of the invention which, as a matter of language, might be
said to fall therebetween.
'.
