The amount of useful power, and therefore the collection efficiency, is primarily determined by the number of active high tension electrical bus sections into which the precipitator is d
Trang 1This article deals with high efficiency (99.5%)
particu-late removal techniques often required of modern central
sta-tion power plants The reader is also referred to the article
“Particulate Removal” for a discussion of control methods
including those used when more moderate conditions apply.
Electric power companies are required to analyze
proposals for, and subsequently to purchase, electrostatic
precipitators based on cost and performance
The basic design factors which determine the collection
efficiency are the collecting plate area, the velocity of the gas,
the time that the gases are in contact with the discharge wires
and collecting plates, and the electrical system supplying the
useful power to the flue gas It is the differences in these
fac-tors in the manufacturers’ proposals that give the engineer the
most trouble in choosing the precipitator that will continually
produce the required efficiency The amount of useful power,
and therefore the collection efficiency, is primarily determined
by the number of active high tension electrical bus sections
into which the precipitator is divided (see Figure 1)
The collection efficiency of a precipitator is closely
related to the useful amount of electrical power than can
be supplied to the precipitator, the greater the useful power,
the higher the efficiency If we imagine a precipitator with
all the discharge wires being supplied by one power source
through a single cable, the highest voltage that could be
maintained between the wires and the collecting plates
would be limited by the first wire to spark excessively The
reason that one wire may spark excessively before another
is due to many factors including uneven distribution of the
gas and dust as they enter the precipitator, uneven build up
of ash on the wires and plates, mechanical misalignment of
the wires or plates and the fact that the collection process
produces a different amount of ash in the gas at the entrance
and discharge end of the precipitator Even if all the wires
spark, at the same voltage, there is an appreciable loss in
efficiency due to lowered voltage in the wires operating in
parallel because the excessive sparks from one wire affect
all the others
From this it is evident that the ideal precipitator would be
one in which each wire has its own stabilization control and
power source, but this, of course, would not be economically
feasible Somewhere between these extremes is the practical
number of power sources or electrical bus sections that will
continually produce the desired efficiency
Figure 1 shows the efficiency curve which may be used in
preparing specification and predicting actual operating efficiency
This relationship between efficiency and active bus sections has been referred to by White 2 as the “Ramsdell Equation.”
An active bus section refers to a separately energized precipitator section where a transient electrical disturbance
in a given section is not reflected in any other section This condition exists when either one section is energized by
a single rectifier or when two sections are energized by a double half wave rectifier
A design criterion or an equation for the physical sizing of precipitation is required A curve based on Con Edison’s own
BUS SECTIONS PER 100,000 CMF
B 1
50 60 70 80 90
95 96 97 98 99 99.5
ELECTROSTATIC PRECIPITATOR COLLECTION EFFICIENCY
* CON EDISON 1.0% SULPHUR 300°F
FIGURE 1
experience and that of other utilities is presented in Figure 2
Trang 2The fundamental efficiency formula for an electrostatic
precipitator is
E = 1 e w/30.48AV
where
E = Collection Efficiency
A = Collection Area
V = Rate of Gas Flow
w = migration velocity, factor which is related to useful
electrical power
Low sulphur coal ash is not easy to collect with electrostatic
precipitators because of its high resistivity This was inferred
from Figure 3 which shows by tests the effect of lowering
the sulphur in the coal on collection efficiency
The Ramsdell Equation based on bus sections is a variation
of the fundamental formula and is expressed as
E = 1 e RB
where
E = Collection Efficiency
B = Number of active bus sections which is related to
useful electrical power
R = Performance Constant
Using tests data with medium and low sulphur coals and infor-mation volunteered by other utilities for high sulphur coals, a
BUS SECTIONS PER 100,000 CFM
50 60 70 80 90 95 96 97 98 99 99.5
2 1
4
BUS SECTIONS-EFFICIENCY
300°F
% S IN COAL
FIGURE 4
BUS SECTIONS PER 100,000 CFM
50 60 70 80 90
95 96 97 98 99 99.5
TEST 1965 2% S.
TEST 1967 1% S.
FIGURE 3
50
60
70
80
90
9596
97
98
99
99.5
COLLECTING AREA PER 1000 CFM
SQUARE FEET
RAVENSWOOD 30 (600°F)
ASTORIA 30
ASTORIA 40, 50 ARTHUR KILL 20
HUDSON AVE 100 ASTORIA 10, 20 EAST RIVER 70
FIGURE 2 Collecting efficiency vs Collecting area per 1000
CFM Using this efficiency test data and precipitator
collect-ing area we were able to plot the equation or the dotted curve
shown in Figure 2.
family of curves may be drawn (see Figures 4 and 5)
Trang 3
0 90
95 96 97 98 99
99.5
99.6
99.7
99.8
99.9
99.95
99.96
99.97
99.98
SQUARE FEET COLLECTING ELECTRODE PER 1000 CFM
SCA 300°F FLUE GAS TEMP.
EASTERN BITUMINOUS COAL
R.G.Ramsdell Jc DEC 16, 1980
STEAM FLOW, # PER HR.
COAL FIRED, # PER HR.
ASH, 12.5% # PER HR.
FLY ASH, # PER HR.
GAS FLOW, ACFM
AVG PRECIP.EFF %
EMISSION GRAINS/CU FT
1,850,000
380,000 47,500 1,350,000
256,000 32,000
COLLECTION EFFICIENCY PERCENT
W 3048
Precipitator Design Chart DESIGN CRITERIA FOR ARTHUR KILL 20 & 30 PERCENT SULPHUR IN COAL*
CON
EDISON
FIGURE 5
© 2006 by Taylor & Francis Group, LLC
Trang 4There is another feature of precipitator design that is of
vital importance to collection efficiency, namely the velocity
of the ash laden gas flowing through the collector The lower
the velocity, the greater treatment time available to
thor-oughly charge the flue gas and the lower the velocity, the less
chance there is for reentraining or sweeping off the fly ash
accumulated on the collecting plates The lower the sulphur
content of the coal, the higher the resistivity of the ash Since,
under these conditions the collected ash has difficulty
stick-ing to the collectstick-ing plates, lower sulphur coals require lower
gas velocity Figure 6 indicates the maximum gas velocities
required to insure the required collection efficiencies when
burning 1.0% sulphur coals
It is known that the resistivity of the fly ash is related to
sulfur content of the coal burned and also to the temperature
of the flue gas
We have already seen that the lower the sulphur content,
the higher the resistivity of the fly ash but as the flue gas
temperature drops below 300°F, the high resistivity effect of
lower sulfur is substantially reduced A 30°F decrease in gas
temperature under certain conditions will offset the effect of
a 1.0% decrease in the sulfur content of the coal
All the data we have previously discussed in this paper
was at a nominal operating temperature of 300°F
In order to insure that adequate electric power is
avail-able for charging our precipitators we again reviewed the
latest units on our system Two sets of curves were developed
from this study Figure 7 shows the total rectifier capacity in
milliamps and Figure 8 shows the total transformer capacity
in kilovolt-amperes
These curves are based on 2.0% sulfur coal Having more electric capacity is of little value with the low sulfur coals with high resistivity ash because the determining factor is how much power the ash laden flue gas will absorb, not how much of a charge can be imposed on the gas On the other
1
50 60 70 80 90 95 96 97 98 99 99.5
MA PER 1000 CFM TOTAL RECTIFIER CAPACITY
2.0% SULPHUR
FIGURE 7 Precipitator design curve.
10
5060 70 80 90
95 96 97 98 99 99.5
20 30 40 50 100 200 300 KVA PER 100,000 CFM
TOTAL TRANSFORMER CAPACITY
2.0% SULPHUR
FIGURE 8 Precipitator design curve.
50
60
70
80
90
95
96
97
98
99
99.5
VELOCITY FEET PER SECOND
1.0% SULPHUR
FIGURE 6 Precipitator design curve.
Trang 5hand high sulfur coal ash does require much more electric
capacity than shown on these curves Precipitators operating
under these conditions are sometimes referred to as power
“Hogs.”
Certain basic criteria should be met in order to attain
high collection efficiency at a modern power plant
installa-tion These include:
1) Low velocity of the gas passing through the
collection zone
2) High time of contact of the gas in the precipitator
3) Reasonably large collecting surfaces
4) High corona power
The collection efficiency of a precipitator is closely related
to the useful amount of high corona power that can be
sup-plied to the precipitator: the greater the useful power, the
higher the efficiency For better efficiency this power should
be distributed among many energized collecting sections,
each having an individual control and power supply
Burning low sulfur coals requires the upgrading of dust
collection equipment This leaves a company with the
fol-lowing alternatives if physical space exists:
1) Add collecting surface and electrical sets to
exist-ing 99.75% collectors for example to maintain the
necessary efficiencies when burning low sulfur
coals
2) Add new precipitators in series with the existing
precipitators to increase the efficiency for example
from 99% to 99.8% when burning low sulfur coal
3) SO 2 conditioning and pulsed energization. 7
“HOT” PRECIPITATOR CASE STUDY
Hot gas temperature (>600°F) precipitation offers a feasible
alternative for the low sulfur eastern coal situation
This approach was used for the newest operating coal
fired unit, at Con Edison’s Ravenswood No 30 The
loca-tion of this precipitator, between the economizer outlet of
the boiler and the air heater inlet, is shown in Figure 9
The location of this “hot” precipitator was predicated on
three considerations:
1) Anticipated reduced air heater fouling by locating
the precipitator ahead of the air heater
2) Ability to burn low sulfur coals without affecting
3) The boiler was designed to burn oil as an alternate
fuel and it was desired to be able to operate the
precipitator when burning this oil After extensive
tests on a pilot installation at Ravenswood No 10
while burning oil, it was determined that the
pre-cipitated oil ash caught in a “hot” condition could
be handled It has been demonstrated that such a
precipitator is effective in collecting oil ash
The collector is made up of four separate combination units, two double-decked for the north boiler and two double-decked for the south boiler Extensive model study was required to attain the most efficient flue design that would result in proper gas and dust distribution entering each precipitator The height
of this precipitator for the 1000 MW unit is over 15 stories The latest performance test on this Ravenswood No 30 collector when burning coal has met the most optimistic expectations
As previously mentioned, the design of the precipitator units for Boiler 30 at the Ravenswood Station featured the “hot gas” concept primarily because of its more efficient characteristics in collecting the particulate matter from flue gas while firing either fuel oil or low sulfur coal Proposed air-pollution control legis-lation at the time confirmed the need for such characteristics in new precipitator equipment An interesting structural problem arose in the design of the supporting steel for these precipitator units which we feel was resolved in a rather unique fashion Because the gas to be handled at the higher temperatures
is much greater than would be required at the normal” cold precipitator” temperature levels, the equipment itself must
be bigger and, therefore, heavier The four precipitator units
at Ravenswood—Blr 30 required a building volume approxi-mately 90 243 167 high which included space to install the large associated flue sections The decision was made to enclose the building on all its exposed sides with uninsulated metal siding (and to provide a roof) for the fol-lowing reasons:
1) To eliminate the need to weatherproof the flue and equipment insulaton
2) To eliminate wind loading on the large exposed surfaces of the flues and precipitator units
3) To improve the appearance of the installation which is only 150 from a public street
4) To reduce the external sound levels around the installation
5) To reduce future maintenance costs
FIGURE 9
the collection efficiency (see Figure 10)
Trang 6Initially, there was no reason to believe that these units
would create an excessive expansion problem for the
sup-porting steel, even with the high operating temperature
expected Calculations indicated that only a 20° temperature
rise would result overall within the enclosed building with
the possibility of a few local hot spots developing similar to
what you would normally find in the boiler house
Boiler 30 started out in construction as an oil fired unit
with provisions included to convert to coal fired at some
future date It was decided during the initial construction to
convert immediately even if it would not be ready to burn
coal until two years after its initial operation burning oil
Due to the close erection schedule which the coal
con-version work on the unit was to follow, it was necessary to
start the equipment foundations and supporting steel in the
field as quickly as possible Thus the steel was designed
before the equipment design was completed
The four units were designed to be installed on two
eleva-units were supported from steel erected on the 42 8 eleva-tion (Grade elevaeleva-tion is 15 0) which allowed for a con-venient column arrangement below The upper units, however, were to be supported from the 126 2 level and presented
a more difficult column design Since the interior columns that supported the lower units could not be carried up through the equipment to the upper supports, deep girders had to be utilized to span over the lower units to provide the required support The maximum span required was 90 −0 which resulted in a girder depth of 9 1½ for a total girder weight
of approximately 110 tons A high yield strength steel was uti-lized for these girders (ASTM A440 − f y = 46 KSI) Bracing the building was also a challenge because of the equipment space requirements, and could only be provided around the periphery of the units except below the 42 8 elevation When the hopper detail drawings were received for the precipitator sections, it was noted that the upper hopper plate stiffeners were located very close to the steel support
the hopper insulation above these upper stiffeners would be almost impossible to filed install with the stiffeners in place But more important, the supporting steel member was now in
a “heat pocket” which would not afford much air movement for cooling Since the supporting members were designed for the same top elevations in both directions, this greatly reduced the possibility of air movement longitudinally along the interior members This condition existed at all hopper locations at both levels The possibility of over-heating the supporting steel resulting in an excessive outward movement
of the support columns now had to be reckoned with for the safety of the structure
Schemes were immediately proposed to provide some type of forced ventilation or cooling system which were dismissed because of lack of space as well as for economy reasons Conditions in the field at the time were such that erection of the lower steel supports was nearing completion, and the lower precipitator shell plates were being delivered
to the job site A solution had to be found which would not delay the precipitator erection, and yet result in a stable structure under significant expansion movement
The answer was to reduce the relative expansion of sup-port framing in any one direction by providing expansion points at certain key connections The centerlines for the north and south units were located on the 390 and 320 Column Lines respectively (see Figure 11) It was felt that the build-ing should move north and south of these lines symmetrically
as the building heated up during operation This could be done by stiffening the support steel on these centerlines and
by cutting loose the connections on the 36 Line and replace the fixed connections with movable ones This meant that the maximum expansions would take place by moving the 29 and
430 Column Lines outward and at the same time allowing the center of the building (36 Line) to absorb the inward expan-sions on sliding connections This approach had the advan-tages of having to cut loose only one column line instead of several, and also it reduced individual relative expansions to
LD
EC IP
R 99.5 % 99.75 % 99.5 %
99.75 % HOT PRECIPITATOR
COLLECTING AREA - SULPHUR 300°F AND 600°F
PERCENT SULPHUR IN COAL 0
100
200
300
400
500
700
600
800
900
1000
1100
1200
1300
1400
1500
1600
1700
1800
1900
2000
2100
2200
2300
2400
2500
FIGURE 10
tions in a double deck fashion as shown in Figure 9 The lower
girders (see Section 1−1 in Figure 11) It was obvious that
Trang 7movement over one quarter of the building length rather than
over one half, at least in the north–south direction
Design Phase
To accomplish the above design changes, four major
obsta-cles had to be overcome:
1) The centerline support girders for each
precipita-tor unit had to be fixed, or at least stiffened, so that
they could come as close to being a zero expansion
line (N–S) as possible
2) Expansion points had to be designed for the 36
Line girder connections
3) Column Line 36 had to be braced in order to
stabilize the building after the connections were
cut free
4) Provisions had to be made for the outward
move-ment of the peripheral columns around the building
For the normal operating temperature of the units (670°F),
it was decided to assume the support steel temperature would rise to an average of 500°F during operation, and provide for the large movements involved This temperature rise would
be applied to all steel directly below the plan dimensions of each unit with the framing beyond these dimensions only assumed to rise to 200° F Under this assumed combination
of temperature rises, the maximum differential movement expected was approximately 2½ in the north–south direc-tion as well as in the easterly direcdirec-tion The heavily braced
LL Column Line in the boiler house would act as the zero movement line in the east–west direction
The center support girders for the lower units were restrained
by shifting adjacent vertical diagonal bracing to fix the PP320, PP390, M320, and MM390 columns at the + 42 −8 elevation The G5 girders on the upper units were stiffened with
The expansion joint design for the 36 Line connections utilized ball bearing assemblies except for the G4 connections
ROCKER
BEARING PLATE
SUPPORT BRACKET
HANGER ROD
640"
G4 GIRDER
DETAIL " A"
DETAIL "B"
3" CLEAR
3" CLEAR
SECTION "4–4"
SECTION "1–1"
FACE OF COLUMN G4 GIRDER BEARING PLATE
FACE OF COLUMN SUPPORTING
BRACKET
STOP SOLE ROCKER
COLUMN 0036
SHELL PLATE
"HOT POCKET"
HOPPER INSULATION
HOPPER PLATE
HOPPER STIFFENER
SUPPORT GIRDER
EL + 105'-6"
EL + 125'-2"
EL + 126'-2"
EL + 133'-2"
EL + 42'-4"
EL + 38'-11"
EL + 25'-2"
EL + 15'-0"
EL + 114'-6"
TRUSS
TRUSS
GRADE
ROOF
2 " EXPANSION1
SECTION "2–2" SECTION "3–3"
3 " CLEAN
BALL BEARING ASSEMBLY BEARING
4 " HANGER ROD1 9'-1 "1
FIGURE 11
tal members and bracing as indicated in Figure 10
Trang 8at Column QQ36 at the +126 −2 elevation Detail B in
bearings (50 per assembly) were 7 /8 in diameter, and were
made of M-50 steel as were the top and bottom assembly
plates The ball bearing retainer was made of stainless steel
The reaction of the G4 girder at Column QQ36 was 640 kips,
which ruled out a ball bearing design as the size that would be
required was too large for the space available It was decided
to hang the huge girders in a pendulum fashion from a bracket
above, and allow them to expand by swinging The connection
finally used is shown in Detail A in Figure 11 For the hanger
rod material, a high temperature service chrome-moly steel
was selected (ASTM 193-Grade B7) The bar material was
heat treated and stress relieved Mill tests on the material used
indicated yield points of 98 KSI and higher The total required
length of only 4¼ diameter rod was 19 −10
Horizontal bracing has to be added to the 36 Column
Line to carry interior lateral loads out to the vertically
braced RR and MM column lines This was accomplished
by adding two horizontal trusses, one at elevation 38 -11
Figure 11) It was possible to place the lower truss just
below the expansion connections for the precipitator
sup-port steel at elevation +42 −8 so the “vertical” members
of the horizontal truss could also act as supports for the ball
bearing assemblies required for the lower units This was
not the case for the upper units as the existence of other
steel (including the G4 girders) made it impossible to install
the upper truss any closer to the +126 −2 elevation New
support brackets were added to the 36 line columns to
sup-port the assemblies for the upper precipitator units
The last major design obstacle to be overcome was the
exterior column movement that would result from the 2½
outward expansion of the framing at elevations +42 −8 and
+126 −2 It was accomplished by reinforcing the column
base billets (and in a few cases by extending the footings as
well) to transfer the eccentric column loads which would
be carried down predominantly by the exterior flanges
Section 3–3 in Figure 11 indicates the type movement
expected It was also necessary to cut free the support steel
of some platforms at elevation +25 −2 so as not to restrain
the exterior columns close to their bases which would
induce high moment forces in the columns This platform
steel was resupported on lubrite plate bearings before it was
cut free to expand The column splice plates were found to
be flexible enough to carry the moments through the joints
at the expanding levels
Construction Stage
Once the necessary alterations were designed and approved,
the difficult task of implementing the changes in the field
still remained to be done One major limitation in this phase
of the job was that the precipitator erector was not anxious
to have any building connections cut free until his erection
work was essentially completed He had placed a crawler
crane on a runway structure atop the 126 −2 support steel
to erect the top units, and he felt that the vibrations already
being experienced with crane movements were large enough with the original fixed connections in place, without increas-ing them by addincreas-ing expansion joints This meant that the expansion connections would have to be effected while they were carrying their full design dead loadings This loading amounted to approximately half the total design load with the remaining half consisting primarily of fly ash loadings The erection work was done in three separate stages First, the column base plates were reinforced as this material was easy to obtain and required little fabrication Second, all the remaining work except for cutting free the expan-sion connections was erected That is, the bracing changes required to fix the lower support columns, the stiffening
of the upper centerline support girders, the two horizontal trusses along the 36 Column Line, and the brackets to support the ball bearing assemblies During this stage also, the ball bearing assemblies were jacked up under the support gird-ers, and the hanger rod connections at column QQ36 were installed The assemblies were positioned by jacking up the support brackets The G4 girder connections required the hanger rods to be lowered down through the upper bracket into position After the top and bottom bearing and rocker plates were in place, the upper nuts were then turned until
a snug fit was obtained The third and final stage included the actual burning free of the original bolted connections in
a very careful manner Less than a sixteenth of an inch drop was observed in the G4 girder elevation after the rods were completely loaded at this point The entire cutting operation
at all elevations was done in less than four weeks
Operating Experience
All precipitator units have been operating successfully The tie-in to the boilers being done in two stages The two southerly units being connected first and then the northerly ones directly after This was accomplished by taking out the south and north boilers alternately, thus reducing the rate of the unit during this period to 500 MW The complete tie-in period was just less than six months
Position readings of several exterior columns were taken before any precipitator units were energized Readings were taken at the +42 and +126 elevations of each column sured These “cold” readings were taken to obtain base mea-surements from which to determine the outward movements
of the columns once in operation “Hot” readings were taken last summer with all the units energized and showed that the north columns had moved a maximum of 1½ to the north, and the east columns had moved a maximum of 111
/16
to the east Both north and east maximum movements were recorded at the +126 −2 elevation Comparing this to the design movement of 2½ it appears that the assumed steel temperature rises were ample The fact that the +126 −2 framing is actually expanding more than the +42 −8 fram-ing is not surprisfram-ing It was expected that the upper reaches
of the building would be hotter by convection, and because the elevation +126 −2 framing has an operating unit both above and below it The lower elevation steel is more readily cooled by outside air being pulled through louvers at grade
and the other at elevation 114 −11 (see Section 2–2 in
Trang 9and across the basement floor by the forced draft fans in the
boiler house
A visit to the precipitator building with all units energized
indicated that the building temperature rises gradually as you
ascend with the upper areas being as much as 50° warmer
than in the basement The “hot spots” that were originally
expected were found to exist, especially where little
clear-ance was left between the precipitator units and the entrclear-ance
flues on the east side of the building above the +126 −2
elevation Air temperatures as high as 220°F were recorded
in such areas Support steel temperatures were taken where
access was possible and found to be as high as 240°F It is
felt that the 500°F design temperature probably exists in the
middle regions of the support steel at the +126 −2
eleva-tion but only thermo couples could confirm this A
tempera-ture differential of 115°F was recorded between the top and
bottom flanges of the north G4 girder at column QQ36
In all, the precipitator building at Ravenswood Station—
Boiler 30 houses some very unique equipment which created
special temperature problems to support it and to enclose it
Operating experience indicates that the designs undertaken
to satisfy these conditions are working well
PARTICULATE CONTROL ANALYSIS OF DEIS
The precipitators proposed in 1981 will upgrade the
con-trolled particulate emissions to below 0.033 lb/million Btu. 5
This rate assumes that coal with a heating value of 12,200
Btu/lb and 12.5% ash will be burned, that 80% of the ash if fly
ash, and electrostatic precipitators have a design efficiency of 99.75% will be employed This rate is equivalent to that from burning 0.3% sulfur oil
Particulate emission control with the original precipi-tator was adequate to meet plume opacity standards After the precipitator is upgraded, plume opacity is expected to be below 10%, or less than half of the opacity standard This is comparable to the opacity when burning 0.3% sulfur oil and
is well within the State and City limit
Con Edison’s plan was to commence coal burning using the existing hotside electrostatic precipitator at Ravenswood Unit 3 and to increase the design collection efficiency from 99.0% to 99.75% This will control the total particulate emis-sion rate to less than 0.033 lb/million Btu, which is equiva-lent to that from 0.3% sulfur oil The existing Ravenswood Unit 3 precipitator was tested at 99.2% to 99.6% efficiency when burning 1% sulfur coal The upgraded precipitator design includes the following to insure that the high collec-tion efficiency is maintained:
The mechanical dust collectors will be replaced with an additional 310,000 sq ft of electrical collecting surface area This will result in a specific collection area (SCA) of 329 (hot side) Figure 12 indicates that this will provide a design collection efficiency of 99.75% while burning coal with a sulfur content of 0.6–1%
Electrical sections will be isolated so that failure of one section will not affect performance of other sections As many as 7% of the electrical sections could be out of service without degrading precipitator efficiency below 99.6%
Precipitator Design Chart
DESIGN CRITERIA FOR RAVENSWOOD 30
90
95
97
98
99
99.5
99.7
99.8
99.9
99.95
99.97
99.98
8
10 8
10 9
10 10
10 11
10 12
10 13
10 9
10 10
10 11
10 12
10 13
300 200
500 600
CRITICAL PRECIPITATION ZONE
IDEAL PRECIPITATION ZONE MARGINAL ZONE
EASTERN BITUMINOUS COAL
SQUARE FEET COLLECTING ELECTRODE PER 1000 CFM
GAS TEMP °F
R.G RAMSDELL JR DEC 16, 1980
STEAM FLOW, #PER HR.
COAL FIRED, #PER HR.
6,500,000 BOILER 30 ASH, 12.5% #PER HR.
FLY ASH, #PER HR.
AVG PRECIP EFF %
EMISSION GRAINS/CUFT 600°F
EMISSION GRAINS/CUFT 300°F
EMISSION #/10 4
BTU
685,000 85,600 4,007,000 99.6 0.010 2,920,000 0.01
RESISTIVITY–SULPHUR–TEMPERATURE
EXPECTED
AVERAGE FLY ASH RESISTIVITY Vs GAS TEMP.
2.0
TO 4.0% S
1.5
TO 2.0% S
1.0
TO 1.5% S
0.5
TO 1.0% S
SULPHUR IN COAL SCA-COLLECTION EFFICIENCY
@ 600°F FLUE GAS TEMP.
EXISTING EFFICIENCY
AVERAGE EFFICIENCY OVER PARTICLE SIZE RANGE NEW DESIGN EFFICIENCY
PR
O
0.6–1.0% s
SQ
UARE FT COLL ECT SURF
ACE
1,008,000 FT 2
ADDITIONAL SECTIONS 1,318,000 FT 2
Trang 10Electrostatic Precipitation
Existing electrostatic precipitator performance may be
improved by the use of wide plate spacing (replacing
weighted wire discharge electrodes with rigid type), by
using intermittent energization (i.e., blocking selected
half-cycles of power to the transformer–rectifier sets powering
the ESP) and by applying flue gas conditioning. 7 The
impor-tance of modeling ESP performance via 3 different types of
computer models enables utilities to optimize their upgrade
taking into account hot-to-cold side conversion, fuel
switch-ing and sizswitch-ing for biddswitch-ing and licensswitch-ing purposes. 8
The effects of sulfur and sodium on fly ash resistivity
and performance have been discussed in the literature. 9,10
Daub 11 discusses the effects of computer controlled
energization of the tranformer–rectifier sets of a precipitator
Typical results in various European situations are presented
12 discuss the performance and economics of ESP’s for removal of heavy metals in coal, oil,
and orimulsion fired units
Good design provides for a mass flow 12% above that
anticipated, and for a 5% variation in flow distribution
between the precipitator boxes
For particulate control application at Ravenswood Unit 3,
the reasons for choosing to upgrade the hotside precipitator
included
1) Only minimal modification work is required for
upgrading the existing precipitator
2) Electrostatic precipitators will meet performance
criteria on either coal or oil firing
3) The low pressure drop across the system elimi-nate the need for additional booster fans or ID fan modifications
4) Electrostatic precipitators are a comparatively low maintenance system
5) Electrostatic precipitators performance is not adversely affected by a rapidly changing gas flow
or boiler load
6) Performance of the precipitator is not affected by changing ash characteristics or coal sulfur content since ash resistivity is low due to high gas
7) Power requirements are comparable to a bag-house filter
8) Ash removal is more reliable due to the higher temperatures
9) Air heater performance and maintenance is improve because of the cleaner flue gas
Fabric Filters: Alternative to Proposed Precipitator
Fabric filters (baghouses) can be installed at Ravenswood Unit 3 if the existing precipitators are removed They offer
no advantage in collection efficiency and opacity over the
will increase the reconversion costs by about $90 million more than the proposed precipitator upgrading
Fabric filters, as applied in the utility industry, operate by drawing dust-laden flue gas through a porous fabric bag woven
of multifilament glass yarn During operation, a fly-ash cake is formed over the cloth pores (with the glass filaments forming a
FLY ASH PARTICLE SIZE IN MICRONS
m ACTUAL DIAM.
0.2 0.10
COAL CHARACTERISTICS EASTERN BITUMINUS SULFUR NOMINALLY 1.0% RANGE 0.6–1.0%
ASH RANGE 10.0–15.0%
MIN PRECIPITATOR DESIGN CRITERIA
SCA (A/V) AREA SQ FT VOLUME CFM
W (MIGRA VEL) cm/sec.
GAS TEMP °F
90.0
95.0
96.0
97.0
98.0
99.0
99.5
99.6
99.7
99.8
99.9
99.95
99.96
99.97
99.98
.
.
.
COLLECTING AREA GAS V
Precipitator Design Chart DESIGN CRITERIA FOR RAVENSWOOD 30
EXPECTED PARTICLE SIZE DISTRIBUTION
PREDICTED PRECIP.
COLLECTION EFFIC.
MICRON BY MASS PREDICTED WEIGHTED
0 – 2
2 – 5
5 – 10
1 – 3
3 – 5
5 – 10 10
x x x x x
=
=
=
=
=
=
.62 2.10 11.20 10.00 56.00
99.60 99.20 97.50 99.65 99.77 99.63
.079 615 2.048 11.161 9.977 19.962 56.906 AVERAGE PRECIP EFFICIENCY = 99.75%
BLR 30 329 1,318,000 9.4 600 COLLECTION EFFICIENCY PARTICLE SIZE
DESIGN EFFICIENCY µ
µ FIGURE 13 Precipitator collection efficiency as a function of particle size.
peratures (Figure 12b, 13)
proposed precipitation upgrading (Figures 14 and 15), but