282 ELECTROSTATIC PRECIPITATION This 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 2345 COLLECTION EFFICIENCY, Ep, PERCENT ELECTROSTATIC PRECIPITATOR COLLECTION EFFICIENCY Ep = 1–e –RB R = 1.3* * CON EDISON 1.0% SULPHUR 300°F FIGURE 1 © 2006 by Taylor & Francis Group, LLC experience and that of other utilities is presented in Figure 2. ELECTROSTATIC PRECIPITATION 283 The fundamental efficiency formula for an electrostatic precipitator is E = 1Ϫ e Ϫw/30.48ϫAV 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 = 1Ϫ e ϪRB 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 21 3.0 2.5 2.0 1.5 1.0 0.5 43567 COLLECTION EFFICIENCY PERCENT BUS SECTIONS-EFFICIENCY 300°F % S. IN COAL FIGURE 4 12345 BUS SECTIONS PER 100,000 CFM COLLECTION EFFICIENCY PERCENT 50 60 70 80 90 95 96 97 98 99 99.5 TEST 1965 2% S. TEST 1967 1% S. FIGURE 3 100 200 300 400 50 60 70 80 90 95 96 97 98 99 99.5 COLLECTING AREA PER 1000 CFM SQUARE FEET COLLECTION EFFICIENCY PERCENT RAVENSWOOD 30 (600°F) ASTORIA 30 ASTORIA 40,50 ARTHUR KILL 20 HUDSON AVE. 100 ASTORIA 10, 20 EAST RIVER 70 74 th ST 120,121,122 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. © 2006 by Taylor & Francis Group, LLC family of curves may be drawn (see Figures 4 and 5). 284 ELECTROSTATIC PRECIPITATION 0 90 95 96 97 98 99 99.5 99.6 99.7 99.8 99.9 3.0 2.0 1.5 1.2 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 99.95 99.96 99.97 99.98 100 200 300 400 500 600 700 800 900 1000 1100 1200 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 #/10 4 CTU EMISSION GRAINS/CU FT BOILER 20 BOILER 30 2,400,000 3,580,000 1,850,000 380,000 47,500 38,000 1,350,000 99.5 99.5 .033 .033 0.01 0.01 256,000 32,000 25,600 COLLECTION EFFICIENCY PERCENT W 3048 X EFFICIENCY=1–e – Precipitator Design Chart DESIGN CRITERIA FOR ARTHUR KILL 20 & 30 PERCENT SULPHUR IN COAL* CON EDISON A V FIGURE 5 © 2006 by Taylor & Francis Group, LLC ELECTROSTATIC PRECIPITATION 285 There 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 collecting 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 2345 10 20 3040 MA PER 1000 CFM TOTAL RECTIFIER CAPACITY COLLECTION EFFICIENCY PERCENT 2.0% SULPHUR FIGURE 7 Precipitator design curve. 10 50 60 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 COLLECTION EFFICIENCY PERCENT 2.0% SULPHUR FIGURE 8 Precipitator design curve. 1 2 3 4 5 6 7 8 9 10 11 50 60 70 80 90 95 96 97 98 99 99.5 VELOCITY FEET PER SECOND COLLECTION EFFICIENCY PERCENT 1.0% SULPHUR FIGURE 6 Precipitator design curve. © 2006 by Taylor & Francis Group, LLC 286 ELECTROSTATIC PRECIPITATION hand 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 90Јϫ 243Јϫ 167Ј 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 150Ј from a public street. 4) To reduce the external sound levels around the installation. 5) To reduce future maintenance costs. FIGURE 9 © 2006 by Taylor & Francis Group, LLC the collection efficiency (see Figure 10). ELECTROSTATIC PRECIPITATION 287 Initially, 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 ϩ42ЈϪ8Љ eleva- tion (Grade elevation is ϩ15Ј Ϫ 0Љ) which allowed for a con- venient column arrangement below. The upper units, however, were to be supported from the ϩ126ЈϪ2Љ 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 90Ј−0Љ which resulted in a girder depth of 9ЈϪ1½ Љ 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 42ЈϪ8Љ 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 C O L D P R E C I P I T A T O R 9 9 . 5 % 9 9 . 7 5 % 99.5 % 99.75 %HOT PRECIPITATOR COLLECTION EFFICIENCY COLLECTING AREA - SULPHUR 300°F AND 600°F PERCENT SULPHUR IN COAL 0 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 0.5 1.0 1.5 2.0 2.5 COLLECTION AREA PER MW SQUARE FEET/MW FIGURE 10 © 2006 by Taylor & Francis Group, LLC 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 288 ELECTROSTATIC PRECIPITATION movement 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 2½Љ in the north–south direc- tion as well as in the easterly direction. 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 + 42Ј−8Љ elevation. The G5 girders on the upper units were stiffened with horizon- 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 ROCKER BEAM 4 4 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 430 36 2 " EXPANSION 1 2 SECTION "2–2"SECTION "3–3" 3" CLEAN BALL BEARING ASSEMBLY BEARING 4 " HANGER ROD 1 4 9'-1 " 1 2 FIGURE 11 © 2006 by Taylor & Francis Group, LLC tal members and bracing as indicated in Figure 10. ELECTROSTATIC PRECIPITATION 289 at Column QQ36 at the +126Ј−2Љ elevation. Detail B in bearings (50 per assembly) were 7 / 8 Љin 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 4¼Љ diameter rod was 19Ј−10Љ 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 38Ј-11Љ Figure 11). It was possible to place the lower truss just below the expansion connections for the precipitator sup- port steel at elevation +42Ј−8Љ 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 +126Ј−2Љ 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 2½Љ outward expansion of the framing at elevations +42Ј−8Љ and +126Ј−2Љ. 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 +25Ј−2Љ 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 126Ј−2Љ 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 adding 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 +42Ј and +126Ј elevations of each column mea- 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 1½Љ to the north, and the east columns had moved a maximum of 1 11 / 16 Љ to the east. Both north and east maximum movements were recorded at the +126Ј−2Љ elevation. Comparing this to the design movement of 2½Љ it appears that the assumed steel temperature rises were ample. The fact that the +126Ј−2Љ framing is actually expanding more than the +42Ј−8Љ fram- ing is not surprising. It was expected that the upper reaches of the building would be hotter by convection, and because the elevation +126Ј−2Љ 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 © 2006 by Taylor & Francis Group, LLC Figure 11 shows the ball bearing type that was selected. The and the other at elevation 114Ј−11Љ (see Section 2–2 in 290 ELECTROSTATIC PRECIPITATION and 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 entrance flues on the east side of the building above the +126Ј−2Љ 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 +126Ј−2Љ 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 COLLECTION EFFICIENCY PERCENT EFFICIENCY=1–e –1 wA V 3048 x 90 95 96 97 98 99 99.5 99.6 99.7 99.8 99.9 99.95 99.96 99.97 99.98 0 100 200 300 400 400 500 600 700 10 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 SCA RAVENSWOOD 30 PRECIPITATOR GAS TEMP. GAS TEMP °F FLY ASH RESISTIVITY OHM-CM 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. GAS FLOW, CFM 600°F GAS FLOW, CFM 300°F AVG. PRECIP. EFF. % EMISSION GRAINS/CUFT 600°F EMISSION GRAINS/CUFT 300°F EMISSION #/10 4 BTU 685,000 85,600 68,600 4,007,000 99.6 0.010 2,920,000 0.01 .033 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 H OT P R E C IP I TAT O R 0.6–1.0% s SQUARE FT. COLLECT SURFACE 1,008,000 FT 2 ADDITIONAL SECTIONS 1,318,000 FT 2 FIGURE 12A FIGURE 12B © 2006 by Taylor & Francis Group, LLC ELECTROSTATIC PRECIPITATION 291 Electrostatic 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 sizing for bidding and licensing 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 tem- 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. 0.3 0.4 0.5 1.0 2.0 3.0 4.0 5.0 10 20 30 40 50 1000.20.10 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 PRECIPITATOR COLLECTION EFFICIENCY PERCENT EFFICIENCY=1–e – W V A V A 3048 X . . . COLLECTING AREA GAS VOLUME SCA Precipitator Design Chart DESIGN CRITERIA FOR RAVENSWOOD 30 EXPECTED PARTICLE SIZE DISTRIBUTION PREDICTED PRECIP. COLLECTION EFFIC. m %%% MICRON BY MASS PREDICTED WEIGHTED 0 – 2 2 – 5 5 – 10 1 – 3 3 – 5 5 – 10 10 .08 x x x x x x x = = = = = = = .62 2.10 11.20 10.00 20.00 56.00 99.60 99.20 97.50 99.65 99.77 99.61 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 4,007,000 9.4 600 COLLECTION EFFICIENCY PARTICLE SIZE DESIGN EFFICIENCY µ µ FIGURE 13 Precipitator collection efficiency as a function of particle size. © 2006 by Taylor & Francis Group, LLC in Table 1. Greico and Wedig peratures (Figure 12b, 13). proposed precipitation upgrading (Figures 14 and 15), but [...]... Ramsdell, R.G Jr and C.F Soutar, Anti-Pollution Program of Consolidated Edison Company of New York, paper presented at ASCE Conference Environmental Engineering, Chattanooga, Tenn May 13–17, 1968 5 Consolidated Edison Company of New York, October 1981 Environmental impact statement—Ravenswood Generation Station Unit 3, Reconversion from Oil to Coal 6 Orange and Rockland Utilities, Inc., May 1981, Environmental. .. compatible with the temperature and pH of the effluent For coal-fired power plants, the present-day filtering medium for a fabric filter is glass fiber bags treated with either teflon or silicon-graphite for lubrication to avoid abrasion between the fabric fibers The coated fiber fabric is resistant to the chemical attack of flue gas constituents and is capable of withstanding operating temperatures... include Sunbury Units 1 & 2 for Pennsylvania Power and Light and Nucla Units 1, 2 and 3 for Colorado UTE For the most part, these units were small (less than 50 MW) stoker-fired units with high air heater exit temperatures, (350°F), and fired with low sulfur coal EPA assessed the data from these units in mid-1978 and concluded that this technology was capable of reducing outlet emissions to less than 0.03... the cleaning of fabric filters on coal-fired boilers has generally been accomplished by reversing gas flow against the bag surface, by shaking the bags, or by a combination of the two approaches The fly ash is then released from the bags and falls into a hopper for removal Prior to mid-1978, eight coal fired electric utility stations were equipped with fabric filters The best documented of this group... Units 4 and 5, Reconversion from Oil to Coal 7 Altman, R., March 1989, Improving Electrostatic Precipitator Performance, EPRI Jnl., p 42 8 Grieco, G., April 1988, Proper ESP Evaluation Demands Extensive Computer Modelling, Power, p 73 9 Han, H.P and E.N Ziegler, The Performance of High Efficiency Electrostatic Precipitators, Envir Progress 3, 201 (1984) 10 Harrison, W.A et al., Medium Sulfur Coal and Fly... electric utilities of June, 1979 An additional 1000–1500 MW of fabric filter capacity came on line during the 1978–1979 period and approximately 4000 MW of capacity has been installed in the 1980–1983 period, including numerous large units in the 350–750 MW size class The fabric filter commitments are generally for use on western coals that have proven very difficult to control by standard electrostatic. .. emphasis on increasing the thermal and chemical resistance of the bags and extending bag life As ash accumulates on the filtering media, the pressure loss across the fabric filter increases In general bags are cleaned automatically upon reaching a specific pressure drop across the fabric filter or by a timing cycle The compartmentalization of fabric filters on coal-fired boilers allows for one compartment... composition Another factor, which contributes to the rapid growth of fabric filter applications in the West, is that filters tend to achieve a lower stack opacity than comparable ESPs operating on western coal Plume opacity and atmospheric visibility have historically been of much greater concern in the Western states 294 ELECTROSTATIC PRECIPITATION The fabric filter has the following advantages for... 1) Replacement of the existing precipitator with a baghouse will cost $120 million (1991) but will not improve the collection efficiency of the particular control system beyond that resulting from the proposed precipitator upgrading 2) Fuel switching may cause permanent fabric blinding because of the sticky characteristics of oil ash 3) New induced draft fans will be required because of a higher system... Taylor & Francis Group, LLC duct work would be required in order to install the baghouse downstream of the air preheater, which would require more plant site area REFERENCES 1 Ramsdell, R.G Jr (1961) Prediction of Fly Ash Precipitation A.I.E.E Paper CP 61–399 2 White, Harry J., (1963) Industrial Electrostatic Precipitation, AddisonWesley Publishing Co 3 Ramsdell, R.G Jr., Design Criteria for Precipitators . +105&apos ;-6 " EL +125&apos ;-2 " EL +126&apos ;-2 " EL +133&apos ;-2 " EL +42&apos ;-4 " EL +38&apos ;-1 1" EL +25&apos ;-2 " EL +15&apos ;-0 " EL +114&apos ;-6 " TRUSS TRUSS GRADE ROOF 430 36 2 " . taking into account hot-to-cold side conversion, fuel switch- ing and sizing for bidding and licensing purposes. 8 The effects of sulfur and sodium on fly ash resistivity and performance have. 1968. 4. Ramsdell, R.G. Jr. and C.F. Soutar, Anti-Pollution Program of Consoli- dated Edison Company of New York, paper presented at ASCE Con- ference Environmental Engineering, Chattanooga, Tenn.