electrostatic precipitator knowledgebase

8/15/2019 Electrostatic Precipitator KnowledgeBase

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Electrostatic PrecipitatorKnowledgeBase

The Neundorfer KnowledgeBase is anindustry-leading information resource aboutelectrostatic precipitators. The Introduction toPrecipitators is a great starting point for

background information, or proceed directly tospecific topic areas of interest.

The downloadable manuals at the right aremade available by the nvironmentalProtection !gency "P!# at www.epa.gov andprovide detailed information aboutelectrostatic precipitator design, operationand maintenance.

About Electrostatic Precipitators

Introduction to PrecipitatorsBasic Principles

About Precipitator Operating Theory

$esign % Performance &e'uirementsProcess (ariables

About Precipitator Components

$ischarge lectrodes)ollecting PlatesPower *upplies and )ontrols+as $istribution *ystems&apping *ystemsoppers and $ust andling$uctworkeaters and Purge !ir *ystemsThermal Insulation

About Precipitator Performance

+as $istribution&e-entrainment)orona Power Performance Improvements'uipment Improvements)ombustion Process Improvements "PowerPlants#lue +asly !sh )onditioning "Power Plants#

Available ocuments

Precipitator Tutorial

/ - *P 0peration

1 - *P )omponents

2 - *P $esign Parameters

3 - *P $esign &eview

4 - Industrial !pplications for *Ps

5 - *P 0peration % 6aintenance

Product !anuals

*mart Purge Theory of 0peration

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"ntroduction to Precipitators "Back to top# !n electrostatic precipitator is a large,industrial emission-control unit. It is designedto trap and remove dust particles from the

e7haust gas stream of an industrial process.Precipitators are used in these industries8

• Powerlectric

• )ement

• )hemicals

• 6etals

• Paper

In many industrial plants, particulate mattercreated in the industrial process is carried asdust in the hot e7haust gases. These dust-laden gases pass through an electrostatic

precipitator that collects most of the dust.)leaned gas then passes out of theprecipitator and through a stack to theatmosphere. Precipitators typically collect99.9: or more of the dust from the gasstream.

Precipitators function by electrostaticallycharging the dust particles in the gas stream.The charged particles are then attracted toand deposited on plates or other collectiondevices. ;hen enough dust hasaccumulated, the collectors are shaken to

dislodge the dust, causing it to fall with theforce of gravity to hoppers below. The dust isthen removed by a conveyor system fordisposal or recycling.

$epending upon dust characteristics and thegas volume to be treated, there are manydifferent si


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for each unit.

Basic Principles "Back to top#

lectrostatic precipitation removes particlesfrom the e7haust gas stream of an industrialprocess. 0ften the process involves

combustion, but it can be any industrialprocess that would otherwise emit particles tothe atmosphere. *i7 activities typically takeplace8

• Ioni


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electrical power input and dust chemistry.

• Precipitator si


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higher velocity, particle re-entrainment increases rapidly. Ifvelocity is too low, performance maysuffer from poor gas flow distributionor from particle dropout in theductwork.

1. Particle *i


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resistivity affect the cohesiveness ofthe layer of precipitated material onthe collecting plates and the ability ofthe rapping system to dislodge thislayer for transport into the precipitator hopper without e7cessive re-entrainment.

About ischarge Electrodes "Back to top#

$ischarge electrodes emit charging currentand provide voltage that generates anelectrical field between the dischargeelectrodes and the collecting plates. Theelectrical field forces dust particles in the gasstream to migrate toward the collecting plates.The particles then precipitate onto thecollecting plates. )ommon types of dischargeelectrodes include8

• *traight round wires• Twisted wire pairs

• Barbed discharge wires

• &igid masts

• &igid frames

• &igid spiked pipes

• *piral wires

$ischarge electrodes are typically supportedfrom the upper discharge frame and are heldin alignment between the upper and lowerdischarge frames. The upper discharge frame

is in turn supported from the roof of theprecipitator casing. igh-voltage insulatorsare incorporated into the support system. Inweighted wire systems, the dischargeelectrodes are held taut by weights at thelower end of the wires.

About Collecting Plates "Back to top#

)ollecting plates are designed to receive andretain the precipitated particles until they areintentionally removed into the hopper.)ollecting plates are also part of the electricalpower circuit of the precipitator. These

collecting plate functions are incorporated intothe precipitator design. Plate baffles shieldthe precipitated particles from the gas flowwhile smooth surfaces provide for highoperating voltage.

)ollecting plates are suspended from theprecipitator casing and form the gaspassages within the precipitator. ;hile thedesign of the collecting plates varies by

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manufacturer, there are two common designs8

• Plates supported from anvil beams at

either endThe anvil beam is also the point ofimpact for the collecting rapper

• Plates supported with hooks directly

from the precipitator casingTwo or more collecting plates areconnected at or near the center byrapper beams, which then serve asimpact points for the rapping system

Top, center, or bottom spacer bars may beused to maintain collecting plate alignmentand sustain electrical clearances to thedischarge system.

About Power 'upplies and Controls "Back to top#

The power supply system is designed toprovide voltage to the electrical field "or bussection# at the highest possible level. Thevoltage must be controlled to avoid causingsustained arcing or sparking between theelectrodes and the collecting plates.

)lick here to view a precipitator power systemanimated schematic showing representativecomponents.

lectrically, a precipitator is divided into agrid, with electrical fields in series "in the

direction of the gas flow# and one or more bussections in parallel "cross-wise to the gasflow#. ;hen electrical fields are in series, thepower supply for each field can be ad>usted tooptimiustments to compensate for their differences, so that power input can beoptimi


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• • !utomatic voltage control

• *tep-up transformer

• igh-voltage rectifier

• *ensing device

/. (oltage control !utomatic voltage control varies thepower to the transformer-rectifier inresponse to signals received fromsensors in the precipitator and thetransformer-rectifier itself. It monitorsthe electrical conditions inside theprecipitator, protects the internalcomponents from arc-over damages,and protects the transformer-rectifierand other components in the primarycircuit.The ideal automatic voltage control

would produce the ma7imumcollecting efficiency by holding theoperating voltage of the precipitator ata level >ust below the spark-overvoltage. owever, this level cannot beachieved given that conditionschange from moment to moment.Instead, the automatic voltage controlincreases output from thetransformer-rectifier until a sparkoccurs. Then the control resets to alower power level, and the powerincreases again until the ne7t sparkoccurs.

Automatic &oltage Controllers "for lectrostaticPrecipitators# !n electronic device used to control the application of$.). power into a field of an electrostatic precipitator ."PI) 0 6()3 !) P!= !=$ PI) 0I=T&!) B0!&$#

Theory

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C Optimize power application D The primary purposeof a voltage controller is to deliver as much usefulelectrical power to the corresponding electrostaticprecipitator field"s# as possible. This is not an easy >obE electrical characteristics in the field"s# areconstantly changing, which is why a voltage controlleris re'uired.

C Spark reaction D ;hen the voltage applied to theelectrostatic precipitator field is too high for theconditions at the time, a spark over "or coronadischarge# will occur. $etrimentally high amounts ofcurrent can occur during a spark over if not properlycontrolled, which could damage the fields. ! voltagecontroller will monitor the primary and secondaryvoltage and current of the circuit, and detect a sparkover condition. 0nce detected, the power applied tothe field will be immediately cut off or reduced, whichwill stop the spark. !fter a short amount of time thepower will be ramped back up, and the process willstart over.

C Protect system components by adhering tocomponent limitations D The Transformer &ectifier set

"T& set# can be damaged by e7cessive amounts ofcurrent or voltage flowing through it. ach T& set hasvoltage and current limits established by themanufacturer, which are labeled on an attachednameplate "PI) 0 ! =!6P!T#. Thesenameplate limit values "typically primary andsecondary current, and voltage# are programmed intothe voltage controller. Through metering circuits, thevoltage controller will monitor these values, andensure these limits are not e7ceeded.

C Tripping D ;hen a condition occurs that the voltagecontroller cannot control, often times the voltagecontroller will trip. ! trip means the voltage controller"by way of the contactor# will shut off the individualprecipitator power circuit. ! short inside theelectrostatic precipitator field caused by a fallendischarge electrode "wire#, or a shorted out *ilicone)ontrolled &ectifier are e7amples of conditions that avoltage controller cannot control. "PI) 0 )0*-FP0 T&IP I+T 0= 6()3 !) P!=#

OperationTo ma7imiustmaintain power at a constant level. Increasedelectrical power into the electrostatic precipitatordirectly correlates with better precipitatorperformance, but there is a limit. If too much voltages

is applied for a given condition "as mentioned in thespark reaction section#, a spark over will occur. $uringa spark over precipitator performance in that field willdrop to


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performance.

/. Transformer-&ectifiersThe transformer-rectifier rating shouldbe matched to the load imposed bythe electrical field or bus section. Thepower supply will perform best when

the transformer-rectifiers operate atG@ - 9@: of the rated capacity,without e7cessive sparking. Thisreduces the ma7imum continuous-load voltage and corona powerinputs. Practical operating voltagesfor transformer-rectifiers depend on8

o )ollecting plate spacing

o +as and dust conditions

o )ollecting plate and

discharge electrode geometry

!t secondary current levels over /4@@

m!, internal impedance of atransformer-rectifier is low, whichmakes stable automatic voltagecontrol more difficult to achieve. Thedesign of the transformer-rectifiershould call for the highest possibleimpedance that is commensurate withthe application and performancere'uirements. 0ften, this limits thesi


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About (as istribution 'ystems "Back totop#

0ne electrical field or bus section of anelectrostatic precipitator is by itself anindependent precipitator. Its operation isgoverned by the inlet gas and dust conditions,

as well as the collecting plate and dischargeelectrode geometries.

;ithin this electrical field or bus section, onegas passage is also an independentprecipitator - governed by the same factors."=ote that the gas passage shares thevoltage level with the ad>acent gas passagesof the same electrical field or bus section, butnot the corona current level, which can bedifferent in each gas passage.#

This points to the importance of creatingsimilar gas and dust conditions /# at the inletof each electrical field or bus section, and 1#further at the inlet of each gas passage of theelectrical field or bus section. Ideally,uniformity is desired in8

• +as velocity

• +as temperature

• $ust loading

+as velocity distribution can be mosteffectively influenced by the use of gasdistribution devices.The 'uality of gas velocity distribution can be

measured in a scaled-down model of theprecipitator and its ductwork, and also in theprecipitator itself. Typical criteria are based onI)!) "Institute of )lean !ir )ompanies#recommendations using average gasvelocities or on a calculated &6* statisticalrepresentation of the gas velocity pattern.

In general, gas distribution devices consist ofturning vanes in the inlet ductwork, andperforated gas distribution plates in the inletandor outlet fields of the precipitator.

About $apping 'ystems "Back to top#

&appers are time-controlled systems providedfor removing dust from the collecting platesand the discharge electrodes as well as forgas distribution devices "optional# and forhopper walls "optional#. &apping systemsmay be actuated by electrical or pneumaticpower, or by mechanical means. Tumblinghammers may also be used to dislodge ash.

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&apping methods include8

• lectric vibrators

• lectric solenoid piston drop rappers

• Pneumatic vibrating rappers

• Tumbling hammers

•*onic horns "do not re'uiretransmission assemblies#

/. $ischarge lectrode &appingIn general, discharge electrodesshould be kept as free as possible ofaccumulated particulate. The rappingsystem for the discharge electrodesshould be operated on a continuousschedule with repeat times in the 1 -3 minute range, depending on thesi


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rapping cycles for the downstreamelectrical fields can be established.The collecting plate rapping system of the first field has a repeat time Te'ual to the time it takes to build a2A - /1Hlayer on the collectingplates. The plates in the second fieldshould have a repeat time of about4T, and the plates in the third fieldshould have a repeat time of 14T.Ideally, these repeat times yield adeposited layer of 2A-/1H for theplates in all three fields. !d>ustmentmay be re'uired for factors such asdust resistivity, dust layercohesiveness, gas temperatureeffects, electrical field height andlength, and the collecting area servedby one rapper.

2. +as $istribution Plate and opper

;all &appingThe gas distribution plates shouldalso be kept free of e7cessiveparticulate buildup and may re'uirerapping on a continuous base with acycle time in the /@-1@ minute range,depending on the inlet particulateloading of the precipitator and thenature of the particulate. +asdistribution plates in the outlet of theprecipitator may be rapped less often"every 2@ - 5@ minutes#.

3. Improving &apping *ystem

Performance !ll precipitator rapping systems allowad>ustment of rapping fre'uency,normally starting with the highestfre'uency "the least time betweenraps#, progressing to the lowestfre'uency. The times that are actuallyavailable may be limited. &appingsystems with pneumatic or electricactuators allow variations of therapping intensity. Pneumatic orelectric vibrators allow ad>ustments of the rapping time. *tate-of-the-art

rapper controls allow selection ofrapping se'uences, selection ofindividual rappers, and provide anti-coincidence schemes which allowonly one rapper to operate at a giventime.

&apping systems can be optimi


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0ptimiustment of the rapping system foroptimum precipitator performance is a slowprocess. It re'uires a substantial amount oftime for stabiliustment.

About )oppers "Back to top#

Precipitator hoppers are designed to

completely discharge dust load on demand.Typically, precipitator hoppers are rectangular in cross-section with sides of at least 5@-degree slope. These hoppers are insulatedfrom the neck above the discharge flange withthe insulation covering the entire hopper area.In addition, the lower /3- /2 of the hopperwall may be heated. $ischarge diameters aregenerally AH - /1H.

/. InsulationInsulation provides protection forfacility personnel as well as workingto retain as much hopper wall

temperature as possible. opper walltemperature retention discouragescondensation on the inside of thehopper. eaters are added to ensurehot metal surfaces immediately abovethe fly ash discharge.

1. acilitating hopper dischargeopper discharge problems arecaused by compaction of the fly ash

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in the hopper. )ompactioncharacteristics are affected bymoisture content, particle si


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$uctwork connects the precipitator withupstream and downstream e'uipment. Thedesign of the ductwork takes intoconsideration the following8

• ow resistance to gas flow

!chieved by selecting a suitablecross-section for the ductwork and byinstalling gas flow control devices,such as turning valves and flowstraighteners

• +as velocity distribution

+as flow control devices are used tomaintain good gas velocitydistribution

• 6inimal fallout of fly ash

allout can be minimioints are used to accommodatethermal growth.

About (as &elocity istribution "Back totop#

fficient precipitator performance dependsheavily upon having similar gas conditions atthe inlet of each electrical field or bus section

and at the inlet of each gas passage of theelectrical field or bus section. Fniformity ofgas velocity is also desirable - good gasvelocity distribution through a precipitatormeets these re'uirements8 A4: of all measured gas velocities J /./4times the average gas velocity 99: of all measured gas velocities J /.3@times the average gas velocity

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/. Improving +as (elocity $istributionThe gas velocity distribution in aprecipitator can be customi


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Particles8 • ow cohesiveness

• ow adhesion to

collecting plates

• Particle si


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applied corona power. )orona power is theproduct of corona current and voltage.)urrent is needed to charge the particles.(oltage is needed to support an electricalfield, which in turn transports the particles tothe collecting plates.

In the lower range of collecting efficiencies,relatively small increases in corona powerresult in substantial increases in collectingefficiency. 0n the other hand, in the upperranges, even large increases in corona power will result in only small efficiency increases.

'ually, in the lower range of the coronapower levels, a small increase in the coronapower results in a substantial reduction in thegas stream particle content. In the upperrange of the corona power level, a largeincrease is re'uired to reduce the particle

content.0ptimi


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to top#

Improvement or optimi


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The separation of dust particles fromthe gas flow in an electrostaticprecipitator depends on the appliedcorona power. )orona power is theproduct of corona current andvoltage. )urrent is needed to chargethe particles. (oltage is needed tosupport an electrical field, which inturn transports the particles to thecollecting plates. or additionalinformation, refer to )orona Power.

3. *ectionali


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&e-entrainment$ischarge lectrodes)ollecting PlatesPower *upplies+as $istribution&apping *ystemsoppers and $ust andling$uctwork

Combustion Process "mprovements forPower Plants "Back to top#

)ombustion process conditions mainly affectthe corona power level. The primarycontributors to combustion process conditionsand their effects include8

)oal lue gas flow rate lue gas moisture content ly ash resistivity ly ash inlet loading ly ash particle si


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The setting of the coal mills and classifiersdefines the coal particle si


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inlet define precipitator operation. Thecombination of flue gas analysis, flue gastemperature and fly ash chemistry providesthe base for fly ash resistivity. Typically, flyash resistivity involves both surface andvolume resistivity. !s gas temperatureincreases, surface conductivity decreasesand volume resistivity increases.

In lower gas temperature ranges, surfaceconductivity predominates. The currentpassing through the precipitated fly ash layeris conducted in a film of weak sulfuric acid onthe surface of the particles. ormation of theacid film "from *02 and 10# is influenced bythe surface chemistry of the fly ash particles.

In higher gas temperature ranges, volumeconductivity predominates. )urrentconduction through the bodies "volume# of the

precipitated fly ash particles is governed bythe total chemistry of the particles.

ly ash resistivity can be modified "generallywith the intent to reduce it# by in>ecting one ormore of the following upstream of theprecipitator8

• *ulfur trio7ide "*02#

• !mmonia "=2#

• ;ater

'ulfur Trio-ide and Ammonia Conditioning'ystems

In most cases, a sulfur trio7ide conditioningsystem is sufficient to reduce fly ash resistivityto an acceptable level. The source of sulfurtrio7ide can be li'uid sulfur dio7ide, moltenelemental sulfur, or granulated sulfur. It is alsopossible to convert native flue gas *01 to*02.

In some instances, ammonia alone has beenproven a suitable conditioning agent. It formsan ammonia-based particulate to increase the

space charge. The source of ammonia maybe li'uid anhydrous or a'ueous ammonia, orsolid urea.

inally, sulfur trio7ide and ammonia may beused in combination. This solution has beensuccessful because it can lower fly ashresistivity and also form ammonia bisulfate.The latter increases the adhesion of particles,


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and thus reduces re-entrainment losses.

.ater "n/ection

The in>ection of water upstream of theprecipitator lowers the gas temperature andadds moisture to the flue gas. Both are

beneficial in cold-side precipitatorapplications. owever, care must be takenthat all of the water is evaporated and that thewalls in the ductwork or gas distributiondevices do not get wet.

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