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Electrostatic Precipitator KnowledgeBase

The Neundorfer KnowledgeBase is an industry-leading information resource aboutelectrostatic precipitators. The Introduction to Precipitators is a great starting point forbackground information, or proceed directly to specific topic areas of interest.

The downloadable manuals at the right are made available by the Environmental ProtectionAgency (EPA) at www.epa.gov and provide detailed information about electrostatic precipitatordesign, operation and maintenance.

About Electrostatic Precipitators

Introduction to PrecipitatorsBasic Principles

About Precipitator Operating Theory

Design & Performance Requirements Process Variables

About Precipitator Components

Discharge ElectrodesCollecting PlatesPower Supplies and ControlsGas Distribution SystemsRapping SystemsHoppers and Dust HandlingDuctwork Heaters and Purge Air SystemsThermal Insulation

About Precipitator Performance

Gas DistributionRe-entrainmentCorona PowerPerformance ImprovementsEquipment ImprovementsCombustion Process Improvements (Power Plants)Flue Gas/Fly Ash Conditioning (Power Plants)

Introduction to Precipitators (Back to top) An electrostatic precipitator is a large, industrialemission-control unit. It is designed to trap andremove dust particles from the exhaust gas streamof an industrial process. Precipitators are used inthese industries:

Power/Electric

Cement

Chemicals

Metals

Paper

In many industrial plants, particulate matter created in the industrial process is carried asdust in the hot exhaust gases. These dust-laden gases pass through an electrostaticprecipitator that collects most of the dust. Cleaned gas then passes out of the precipitatorand through a stack to the atmosphere. Precipitators typically collect 99.9% or more of thedust from the gas stream.

Precipitators function by electrostatically chargingthe dust particles in the gas stream. The chargedparticles are then attracted to and deposited onplates or other collection devices. When enoughdust has accumulated, the collectors are shaken todislodge the dust, causing it to fall with the force ofgravity to hoppers below. The dust is then removedby a conveyor system for disposal or recycling.

Depending upon dust characteristics and the gasvolume to be treated, there are many different sizes, types and designs of electrostaticprecipitators. Very large power plants may actually have multiple precipitators for each unit.

Basic Principles (Back to top)

Electrostatic precipitation removes particles from the exhaust gas stream of an industrialprocess. Often the process involves combustion, but it can be any industrial process thatwould otherwise emit particles to the atmosphere. Six activities typically take place:

Ionization - Charging of particles

Migration - Transporting the charged particles to the collecting surfaces

Troubleshooting Checklists

Available Documents

Precipitator Tutorial

1 - ESP Operation

2 - ESP Components

3 - ESP Design Parameters

4 - ESP Design Review

5 - Industrial Applications for ESPs

6 - ESP Operation & Maintenance

Product Manuals

Smart Purge Theory of Operation

Collection - Precipitation of the charged particles onto the collecting surfaces

Charge Dissipation - Neutralizing the charged particles on the collecting surfaces

Particle Dislodging - Removing the particles from the collecting surface to the hopper

Particle Removal - Conveying the particles from the hopper to a disposal point The major precipitator components that accomplish these activities are as follows:

Discharge Electrodes

Power Components

Precipitator Controls

Rapping Systems

Purge Air Systems

Flue Gas Conditioning

Design & Performance Requirements (Back to top)

Designing a precipitator for optimum performance requires proper sizing of the precipitator inaddition to optimizing precipitator efficiency. While some users rely on the precipitatormanufacturer to determine proper sizing and design parameters, others choose to either takea more active role in this process or hire outside engineering firms.

Precipitator performance depends on its size and collecting efficiency. Important parametersinclude the collecting area and the gas volume to be treated. Other key factors in precipitatorperformance include the electrical power input and dust chemistry.

Precipitator sizingThe sizing process is complex as each precipitator manufacturer has a unique method ofsizing, often involving the use of computer models and always involving a good dose ofjudgment. No computer model on its own can assess all the variables that affectprecipitator performance.

Collecting EfficiencyBased on specific gas volume and dust load, calculations are used to predict the requiredsize of a precipitator to achieve a desired collecting efficiency.

Power InputPower input is comprised of the voltage and current in an electrical field. Increasing thepower input improves precipitator collecting efficiency under normal conditions.

Process Variables (Back to top)

Gas characteristics and particle properties define how well a precipitator will work in a givenapplication. The main process variables to consider are:

Gas flow rateThe gas flow rate in a power plant is defined by coal quality, boiler load, excess air rateand boiler design. Where there is no combustion, the gas flow rate will have process-specific determinants.

Particle size and size distributionThe size distribution in a power plant is defined by coal quality, the coal mill settings andburner design. Particle size for non-combustion processes will have similar determinants.

Particle resistivityThe resistivity of fly ash or other particles is influenced by the chemical composition andthe gas temperature.

Gas temperature

Following are details of these process variables:

Gas Flow RateA precipitator operates best with a gas velocity of 3.5 - 5.5 ft/sec. At higher velocity,particle re-entrainment increases rapidly. If velocity is too low, performance may sufferfrom poor gas flow distribution or from particle dropout in the ductwork.

Particle SizeA precipitator collects particles most easily when the particle size is coarse. Thegeneration of the charging corona in the inlet field may be suppressed if the gas streamhas too many small particles (less than 1 m). Very small particles (0.2 - 0.4m) are the most difficult to collect because the fundamentalfield-charging mechanism is overwhelmed by diffusion charging due to random collisionswith free ions.

Particle ResistivityResistivity is resistance to electrical conduction. The higher the resistivity, the harder it isfor a particle to transfer its electrical charge. Resistivity is influenced by the chemicalcomposition of the gas stream, particle temperature and gas temperature. Resistivityshould be kept in the range of 108 - 1010 ohm-cm. High resistivity can reduce precipitator performance. For example, in combustionprocesses, burning reduced-sulfur coal increases resistivity and reduces the collectingefficiency of the precipitator. Sodium and iron oxides in the fly ash can reduce resistivityand improve performance, especially at higher operating temperatures. On the other hand, low resistivity can also be a problem. For example (in combustionprocesses), unburned carbon reduces precipitator performance because it is soconductive and loses its electrical charge so quickly that it is easily re-entrained from thecollecting plate.

Gas TemperatureThe effect of gas temperature on precipitator collecting efficiency, given its influence onparticle resistivity, can be significant.

Interactions to Consider Particle size distribution and particle resistivity affect the cohesiveness of the layer ofprecipitated material on the collecting plates and the ability of the rapping system todislodge this layer for transport into the precipitator hopper without excessive re-entrainment.

About Discharge Electrodes (Back to top)

Discharge electrodes emit charging current and provide voltage that generates an electricalfield between the discharge electrodes and the collecting plates. The electrical field forcesdust particles in the gas stream to migrate toward the collecting plates. The particles thenprecipitate onto the collecting plates. Common types of discharge electrodes include:

Straight round wires

Twisted wire pairs

Barbed discharge wires

Rigid masts

Rigid frames

Rigid spiked pipes

Spiral wires

Discharge electrodes are typically supported from the upper discharge frame and are held inalignment between the upper and lower discharge frames. The upper discharge frame is inturn supported from the roof of the precipitator casing. High-voltage insulators are incorporatedinto the support system. In weighted wire systems, the discharge electrodes are held taut byweights at the lower end of the wires.

About Collecting Plates (Back to top)

Collecting plates are designed to receive and retain the precipitated particles until they areintentionally removed into the hopper. Collecting plates are also part of the electrical powercircuit of the precipitator. These collecting plate functions are incorporated into the precipitatordesign. Plate baffles shield the precipitated particles from the gas flow while smooth surfacesprovide for high operating voltage.

Collecting plates are suspended from the precipitator casing and form the gas passageswithin the precipitator. While the design of the collecting plates varies by manufacturer, thereare two common designs:

Plates supported from anvil beams at either end The anvil beam is also the point of impact for the collecting rapper

Plates supported with hooks directly from the precipitator casing Two or more collecting plates are connected at or near the center by rapper beams, whichthen serve as impact points for the rapping system

Top, center, or bottom spacer bars may be used to maintain collecting plate alignment andsustain electrical clearances to the discharge system.

About Power Supplies and Controls (Back to top)

The power supply system is designed to provide voltage to the electrical field (or bus section)at the highest possible level. The voltage must be controlled to avoid causing sustained arcingor sparking between the electrodes and the collecting plates.

Click here to view a precipitator power system animated schematic showing representativecomponents.

Electrically, a precipitator is divided into a grid, with electrical fields in series (in the directionof the gas flow) and one or more bus sections in parallel (cross-wise to the gas flow). Whenelectrical fields are in series, the power supply for each field can be adjusted to optimizeoperation of that field. Likewise, having more than one electrical bus section in parallel allowsadjustments to compensate for their differences, so that power input can be optimized. Thepower supply system has four basic components:

Automatic voltage control

Step-up transformer

High-voltage rectifier

Sensing device

Voltage controlAutomatic voltage control varies the power tothe transformer-rectifier in response tosignals received from sensors in theprecipitator and the transformer-rectifier itself.It monitors the electrical conditions inside theprecipitator, protects the internal componentsfrom arc-over damages, and protects thetransformer-rectifier and other components inthe primary circuit. The ideal automatic voltage control would produce the maximum collecting efficiency byholding the operating voltage of the precipitator at a level just below the spark-over voltage.However, this level cannot be achieved given that conditions change from moment tomoment. Instead, the automatic voltage control increases output from the transformer-rectifier until a spark occurs. Then the control resets to a lower power level, and the powerincreases again until the next spark occurs.

Automatic Voltage Controllers (for Electrostatic Precipitators)An electronic device used to control the application of D.C. power into a field of anelectrostatic precipitator. (PIC OF MVC4 FACE PANEL AND PIC OF INTERFACE BOARD)

Theory

Optimize power application The primary purpose of a voltage controller is to deliver asmuch useful electrical power to the corresponding electrostatic precipitator field(s) aspossible. This is not an easy job; electrical characteristics in the field(s) are constantlychanging, which is why a voltage controller is required.

Spark reaction When the voltage applied to the electrostatic precipitator field is too high

for the conditions at the time, a spark over (or corona discharge) will occur. Detrimentally highamounts of current can occur during a spark over if not properly controlled, which coulddamage the fields. A voltage controller will monitor the primary and secondary voltage andcurrent of the circuit, and detect a spark over condition. Once detected, the power applied tothe field will be immediately cut off or reduced, which will stop the spark. After a short amountof time the power will be ramped back up, and the process will start over.

Protect system components by adhering to component limitations The TransformerRectifier set (TR set) can be damaged by excessive amounts of current or voltage flowingthrough it. Each TR set has voltage and current limits established by the manufacturer, whichare labeled on an attached nameplate (PIC OF A NAMEPLATE). These nameplate limitvalues (typically primary and secondary current, and voltage) are programmed into the voltagecontroller. Through metering circuits, the voltage controller will monitor these values, andensure these limits are not exceeded.

Tripping When a condition occurs that the voltage controller cannot control, often timesthe voltage controller will trip. A trip means the voltage controller (by way of the contactor) willshut off the individual precipitator power circuit. A short inside the electrostatic precipitatorfield caused by a fallen discharge electrode (wire), or a shorted out Silicone ControlledRectifier are examples of conditions that a voltage controller cannot control. (PIC OF CLOSE-UP OF TRIP LIGHT ON MVC4 FACE PANEL)

Operation

To maximize electrostatic precipitator efficiency a voltage controller usually attempts toincrease the electrical power delivered to the field. However in some conditions a voltagecontroller must just maintain power at a constant level. Increased electrical power into theelectrostatic precipitator directly correlates with better precipitator performance, but there is alimit. If too much voltages is applied for a given condition (as mentioned in the spark reactionsection), a spark over will occur. During a spark over precipitator performance in that field willdrop to zero, rendering that field temporarily ineffective.

To overcome the crippling effect that spark over has to increasing the electrical power in theprecipitator field, spark response algorithms have been developed that will interrupt powerupon detection of a spark, then ramp power back up to a high level. These responsealgorithms can greatly influence overall precipitator performance.

Transformer-RectifiersThe transformer-rectifier rating should be matched to the load imposed by the electricalfield or bus section. The power supply will perform best when the transformer-rectifiersoperate at 70 - 90% of the rated capacity, without excessive sparking. This reduces themaximum continuous-load voltage and corona power inputs. Practical operating voltagesfor transformer-rectifiers depend on:

Collecting plate spacing

Gas and dust conditions

Collecting plate and discharge electrode geometry

At secondary current levels over 1500 mA, internal impedance of a transformer-rectifier islow, which makes stable automatic voltage control more difficult to achieve. The design ofthe transformer-rectifier should call for the highest possible impedance that iscommensurate with the application and performance requirements. Often, this limits thesize of the electrical field or bus section. It is general practice to add additional impedance in the form of a current-limiting reactor inthe primary circuit. This reactor will limit the primary current during arcing and alsoimprove the wave shape of the voltage/current fed into the transformer-rectifier.

Corona current densityCorona current density should be in the range of 10 - 100 mA/1000 ft2 of plate area.(Calculate this using secondary current divided by collecting area of the electrical field orbus section.) The actual level depends upon:

Location of electrical field or bus section to be energized

Collecting plate area

Gas and dust conditions

Collecting electrode and discharge wire geometry

About Gas Distribution Systems (Back to top)

One electrical field or bus section of an electrostatic precipitator is by itself an independentprecipitator. Its operation is governed by the inlet gas and dust conditions, as well as thecollecting plate and discharge electrode geometries.

Within this electrical field or bus section, one gas passage is also an independent precipitator- governed by the same factors. (Note that the gas passage shares the voltage level with theadjacent gas passages of the same electrical field or bus section, but not the corona currentlevel, which can be different in each gas passage.)

This points to the importance of creating similar gas and dust conditions 1) at the inlet ofeach electrical field or bus section, and 2) further at the inlet of each gas passage of theelectrical field or bus section. Ideally, uniformity is desired in:

Gas velocity

Gas temperature

Dust loadingGas velocity distribution can be most effectively influenced by the use of gas distributiondevices.The quality of gas velocity distribution can be measured in a scaled-down model of theprecipitator and its ductwork, and also in the precipitator itself. Typical criteria are based onICAC (Institute of Clean Air Companies) recommendations using average gas velocities or ona calculated RMS statistical representation of the gas velocity pattern.

In general, gas distribution devices consist of turning vanes in the inlet ductwork, andperforated gas distribution plates in the inlet and/or outlet fields of the precipitator.

About Rapping Systems (Back to top)

Rappers are time-controlled systems provided for removing dust from the collecting plates andthe discharge electrodes as well as for gas distribution devices (optional) and for hopper walls(optional). Rapping systems may be actuated by electrical or pneumatic power, or bymechanical means. Tumbling hammers may also be used to dislodge ash. Rapping methodsinclude:

Electric vibrators

Electric solenoid piston drop rappers

Pneumatic vibrating rappers

Tumbling hammers

Sonic horns (do not require transmission assemblies)

Discharge Electrode RappingIn general, discharge electrodes should be kept as free as possible of accumulatedparticulate. The rapping system for the discharge electrodes should be operated on acontinuous schedule with repeat times in the 2 - 4 minute range, depending on the sizeand inlet particulate loading of the precipitator.

Collecting Plate RappingCollecting plate rapping must remove the bulk of the precipitated dust. The collectingplates are supported from anvil beams or directly with hooks from the precipitator casing.With anvil beam support, the impact of the rapping system is directed into the beamslocated at the leading and/or trailing edge of the collecting plates. For direct casingsupport, the impact is directed into the rapper beams located at or near the center of thetop of the collecting plates.The first electrical field generally collects about 60-80% of the inlet dust load. The first fieldplates should be rapped often enough so that their precipitated layer of particulate is about3/8 - 1/2" thick. There is no advantage in rapping more often since the precipitated dusthas not yet agglomerated to a sheet which requires a minimum layer thickness. Sheetformation is essential to make the dust drop into the precipitator hopper without re-entrainment into the gas stream. Rapping less frequently typically results in adeterioration of the electrical power input by adding an additional resistance into the powercircuit. Once an optimum rapping cycle has been found for the first electrical field (whichmay vary across the face of a large precipitator), the optimum rapping cycles for thedownstream electrical fields can be established.The collecting plate rapping system of the first field has a repeat time T equal to the time ittakes to build a 3/8 - 1/2"layer on the collecting plates. The plates in the second fieldshould have a repeat time of about 5T, and the plates in the third field should have a repeattime of 25T. Ideally, these repeat times yield a deposited layer of 3/8-1/2" for the plates inall three fields. Adjustment may be required for factors such as dust resistivity, dust layercohesiveness, gas temperature effects, electrical field height and length, and thecollecting area served by one rapper.

Gas Distribution Plate and Hopper Wall RappingThe gas distribution plates should also be kept free of excessive particulate buildup andmay require rapping on a continuous base with a cycle time in the 10-20 minute range,depending on the inlet particulate loading of the precipitator and the nature of theparticulate. Gas distribution plates in the outlet of the precipitator may be rapped lessoften (every 30 - 60 minutes).

Improving Rapping System PerformanceAll precipitator rapping systems allow adjustment of rapping frequency, normally startingwith the highest frequency (the least time between raps), progressing to the lowestfrequency. The times that are actually available may be limited. Rapping systems withpneumatic or electric actuators allow variations of the rapping intensity. Pneumatic orelectric vibrators allow adjustments of the rapping time. State-of-the-art rapper controlsallow selection of rapping sequences, selection of individual rappers, and provide anti-coincidence schemes which allow only one rapper to operate at a given time.

Rapping systems can be optimized for top precipitator performance using precipitator powerinput and stack opacity as criteria. Optimization of the rapping system starts with thedischarge electrode rapping system operating on its own time schedule, for example withrepeat times of 2 - 4 minutes. The rapping system for the gas distribution screens in the inletand outlet of the precipitator should then be operated with repeat times of 2-3 minutes for theinlet and 2 - 3 hours for the outlet screens.

The only rapping system requiring optimization is the collecting plate rapping system. Theoptimization should start with the Collecting Plate Rapping Schedule determined above. Next,the rapping frequency of the inlet field should be increased or decreased until the electricalpower input of the inlet field remains constant. Next, the rapping frequency of the other fieldsshould be adjusted in sequence until their electrical power inputs remain constant. If thestack opacity trace shows rapping spikes, the rapping intensity should be reduced whileobserving the electrical power input of the precipitator.

The adjustment of the rapping system for optimum precipitator performance is a slowprocess. It requires a substantial amount of time for stabilization after each adjustment.

About Hoppers (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 60-degree slope.These hoppers are insulated from the neck above the discharge flange with the insulationcovering the entire hopper area. In addition, the lower 1/4- 1/3 of the hopper wall may beheated. Discharge diameters are generally 8" - 12".

InsulationInsulation provides protection for facility personnel as well as working to retain as muchhopper wall temperature as possible. Hopper wall temperature retention discouragescondensation on the inside of the hopper. Heaters are added to ensure hot metal surfacesimmediately above the fly ash discharge.

Facilitating hopper dischargeHopper discharge problems are caused by compaction of the fly ash in the hopper.Compaction characteristics are affected by moisture content, particle size and shape,head of material, and vibration. The flow of fly ash out of the hopper can be facilitated by

the use of external vibrators. These can operate on the outside wall of the hopper or on aninternal hopper baffle.

Hopper fluidizersHopper fluidizers have a membrane that permits air flow to the fly ash directly above. Thisair flow fills the voids between the fly ash particles at a slight pressure, changes therepose angle of the particles, and promotes gravity flow.

Ash handling systemThe fly ash handling system evacuates the fly ash from the hoppers, and transports the flyash to reprocessing or to disposal. The ash handling system should be designed andoperated to remove the collected fly ash from the hoppers without causing re-entrainmentinto the gas flow through the precipitator. The design of the ash handling system shouldallow for flexibility of scheduling the hopper discharges according to the fly ash beingcollected in these hoppers.

Either the precipitator hopper or the feeder hopper is used for temporarily storing material priorto discharge. Three types of handling systems are in use:

Negative pressure or vacuum system Connects to the hopper by a simple discharge valve

Positive pressure dilute phase system Uses an airlock-type feeder; the feeder is separated from the hopper by an inlet gate andfrom the conveying line by a discharge gate

Positive pressure dense phase system Connects to the hopper with an airlock type feeder.

About Ductwork (Back to top)

Ductwork connects the precipitator with upstream and downstream equipment. The design ofthe ductwork takes into consideration the following:

Low resistance to gas flowAchieved by selecting a suitable cross-section for the ductwork and by installing gas flowcontrol devices, such as turning valves and flow straighteners

Gas velocity distributionGas flow control devices are used to maintain good gas velocity distribution

Minimal fallout of fly ashFallout can be minimized by using a suitable transport velocity

Minimal stratification of the fly ashA suitable transport velocity also reduces fly ash stratification in the gas stream

Low heat lossThe goal is to reduce the heat loss of the flue gas to a level that will prevent acid ormoisture condensation in the downstream equipment, requiring the use of thermalinsulation protected by external siding.

Structural integrityDuctwork structure supports its total load, including wind and snow loads. The design alsoallows for accumulated fly ash, negative/positive operating pressure, and gas temperature.Expansion joints are used to accommodate thermal growth.

About Gas Velocity Distribution (Back to top)

Efficient precipitator performance depends heavily upon having similar gas conditions at theinlet of each electrical field or bus section and at the inlet of each gas passage of theelectrical field or bus section. Uniformity of gas velocity is also desirable - good gas velocitydistribution through a precipitator meets these requirements: 85% of all measured gas velocities < 1.15 times the average gas velocity 99% of all measured gas velocities < 1.40 times the average gas velocity

Improving Gas Velocity DistributionThe gas velocity distribution in a precipitator can be customized according to the design ofthe precipitator and the characteristics of the dust particles. Traditionally, precipitatorshave been designed with uniform gas velocity distribution through the electrical fields, toavoid high-velocity areas that would cause re-entrainment. While this is still arecommended practice, there is an advantage in some cases to developing a velocityprofile that brings more particles closer to the hopper.

Both of these schemes have applications in site-specific conditions. Gas velocity distributioncan be controlled by the following:

Adding/improving gas flow control devices in the inlet ductwork

Adding/improving flow control devices in the inlet of the precipitator

Adding/improving flow control devices in the outlet of the precipitator

Adding a rapping system to the flow control devices (where applicable)

Adding/improving anti-sneak baffles at the peripheries of the electrical fields

Adding/improving hopper baffles

Eliminating air leakages into the precipitator

About Re-entrainment (Back to top)

Reducing rapping re-entrainment to an acceptable level generally requires a substantialimprovement of the gas velocity distribution and the electrical power density and uniformity,as well as an extended optimization program for the collecting-plate rapping system.

Factors Affecting Re-entrainmentRe-entrainment of collected particles is the major contributor to particulate emissions of theprecipitator. In some cases, re-entrainment accounts for 60 - 80% of the residual. The majorcauses of re-entrainment are as follows:

Particles: Low cohesiveness

Low adhesion to collecting plates

Particle size

Low resistivity

Voltage Controls: Spark rate setting

Design: Collecting plate design

Discharge electrode design

Plate spacing

Rapping System: Frequency

Intensity

Duration (if applicable)

Electrical Field: Collecting plate and discharge electrode rapping

Sparking

Saltation

Erosion (localized high gas velocity)

Sneakage

Hopper: Hopper design

Leakage (hopper valve)

Hopper gas flow

About Corona Power (Back to top)

Precipitator corona power is the useful electrical power applied to the flue gas stream toprecipitate particles. Either precipitator collecting efficiency or outlet residual can beexpressed as a function of corona power in Watts/1000 acfm of flue gas, or in Watts/1000 ftof collection area.

The separation of particles from the gas flow in an electrostatic precipitator depends on theapplied corona power. Corona power is the product of corona current and voltage. Current isneeded to charge the particles. Voltage is needed to support an electrical field, which in turntransports the particles to the collecting plates.

In the lower range of collecting efficiencies, relatively small increases in corona power result insubstantial increases in collecting efficiency. On the other hand, in the upper ranges, evenlarge increases in corona power will result in only small efficiency increases.

Equally, in the lower range of the corona power levels, a small increase in the corona powerresults in a substantial reduction in the gas stream particle content. In the upper range of thecorona power level, a large increase is required to reduce the particle content.

Optimizing Corona PowerOptimum conditions depend upon the location of the field (inlet, center and outlet), fly ashcharacteristics (resistivity) and physical conditions (collecting plates and discharge wires).Corona power levels can be optimized by adjusting or optimizing the following:

Gas velocity: Uniformity

Fly Ash: Particle size Resistivity

Voltage Controls: Spark rate setting

Current & voltage limits

Design: Plate spacing

Collecting plate & discharge electrode design

Rapping System: Frequency & intensity

Support Insulator: Purge air system operation

About Performance Improvements (Back to top)

Improvement or optimization of precipitator operation can result in significant savings. Manyspecific situations encourage a review of precipitator operation:

Deterioration of existing equipment

Tightening of air pollution emission regulations

Changes in products and/or production rates

Frequent forced outages

De-rating of productionTo learn more about performance improvement programs, refer to the appropriate section:

Gas Velocity DistributionCorona PowerRe-entrainmentProcess ImprovementsFlue Gas/Fly Ash ConditioningEquipment Improvements

Equipment Improvements (Back to top)

The objectives of equipment improvements are to optimize corona power, reduce re-entrainment, and optimize gas velocity distribution inside the precipitator. Some importanttopics to consider when planning equipment improvements include:

Precipitator SizeWhen sizing the precipitator, it is important to provide a cross-section that will maintain anacceptable gas velocity. It is also important to provide for enough total discharge wirelength and collecting plate area, so that the desired specific corona current and electricalfield can be applied.

Gas Velocity DistributionImproving gas velocity distribution in the precipitator reduces particle re-entrainment andboosts precipitator efficiency. Typically, a uniform gas velocity is desired, but there aresite-specific exceptions. Gas velocity distribution can be modified by using flow control

devices and baffles. Refer to the special section on gas velocity distribution.

Corona PowerThe separation of dust particles from the gas flow in an electrostatic precipitator dependson the applied corona power. Corona power is the product of corona current and voltage.Current is needed to charge the particles. Voltage is needed to support an electrical field,which in turn transports the particles to the collecting plates. For additional information,refer to Corona Power.

SectionalizationThe precipitator is divided into electrical sections that are cross-wise and parallel to thegas flow to accommodate spatial differences in gas and dust conditions. Optimization ofcorona power involves adjusting the corona power (secondary voltage and current) in eachelectrical section for optimum conditions.

Particle Re-entrainmentMinimizing re-entrainment of dust particles is important to improvement of precipitatorefficiency. Most precipitator equipment affects the re-entrainment level. For a detaileddiscussion, visit the special section on re-entrainment.

Additional EquipmentPerformance improvement options include the installation of a second precipitator in serieswith the existing precipitator; using fabric filters downstream of the precipitator; and addinga second particle collector in parallel with the existing collector. Other possibilities includesonic or electrostatic particle agglomerators upstream of the precipitator; a mechanicalupstream collector; or an electrostatically-enhanced or mechanical collector, or a filterdownstream of the precipitator.

Review the General Equipment RequirementsReviewing the Neundorfer Knowledge Base sections on equipment will provide additionalinsight into performance improvements.

For more information, see these related topics:

Gas Velocity DistributionCorona PowerRe-entrainmentDischarge ElectrodesCollecting PlatesPower SuppliesGas DistributionRapping SystemsHoppers and Dust HandlingDuctwork

Combustion Process Improvements for Power Plants (Back to top)

Combustion process conditions mainly affect the corona power level. The primary contributorsto combustion process conditions and their effects include:

Coal Flue gas flow rate Flue gas moisture content Fly ash resistivity Fly ash inlet loading Fly ash particle sizeCoal mills Fly ash particle size Unburned carbon (LOI)Furnace Base load/swing load operation Flue gas flow rateBurners Flue gas temperature Fly ash resistivity Unburned carbon (LOI)Air pre-heaters Rotation Gas flow pattern Gas temperature pattern SO3 distribution pattern

Coal

Bituminous coals from Eastern mines, sub-bituminous and lignite coals from Western mines,and lignites from Texas mines are substantially different from each other in the combustionprocess. Coal blending is now used for operational and financial benefits. This results in awide range of boiler and precipitator operating conditions.Precipitating fly ash from difficult coals can be improved with conditioning systems. However,the furnace and its associated equipment can still cause problems in the precipitator,particularly coal mills, burners, and air pre-heaters.

Coal Mills

The setting of the coal mills and classifiers defines the coal particle size which in turnimpacts the fly ash particle size. Larger coal particles are more difficult to combust, but largerfly ash particles are easier to collect in the precipitator.

Furnace

Base-load operation of the boiler is usually better for precipitator operation than swing-loadoperation due to more stable operating conditions. Boiler operation at low loads may be asproblematic for the precipitator as operating the boiler at its maximum load level, due to falloutof fly ash in the ductwork, low gas temperatures, and deterioration of the quality of the gasvelocity distribution. If low load operation cannot be avoided, the installation of additional gas flow control devicesin the inlet and outlet of the precipitator may prove beneficial.

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Coal Burner

The operation of coal burners, together with the setting of the coal mills and their classifiers,affects the percentage of unburned carbon (LOI or UBC) in the fly ash. The use of Lo-NOxburners increases this percentage, and causes re-entrainment and increased sparking in theprecipitator. Further, the UBC tends to absorb SO3, which in turn increases the fly ash

resistivity. Over-fire air optimization or coal-reburn systems may reduce UBC in the fly ash.

Air Pre-heater

Regenerative air pre-heaters cause temperature and SO3 stratification in the downstream gas

flow. This problem is more severe in closely coupled systems, where the precipitator islocated close to the air pre-heater. Depending upon site-specific conditions, flow mixingdevices may be installed in the ductwork to the precipitator, or flue gas conditioning systemsmay be used to equalize the gas flow characteristics.

Fly Ash and Flue Gas Conditioning (Back to top)

Flue gas and fly ash characteristics at the inlet define precipitator operation. The combinationof flue gas analysis, flue gas temperature and fly ash chemistry provides the base for fly ashresistivity. Typically, fly ash resistivity involves both surface and volume resistivity. As gastemperature increases, surface conductivity decreases and volume resistivity increases.

In lower gas temperature ranges, surface conductivity predominates. The current passingthrough the precipitated fly ash layer is conducted in a film of weak sulfuric acid on thesurface of the particles. Formation of the acid film (from SO3 and H2O) is influenced by the

surface chemistry of the fly ash particles.

In higher gas temperature ranges, volume conductivity predominates. Current conductionthrough the bodies (volume) of the precipitated fly ash particles is governed by the totalchemistry of the particles.

Fly ash resistivity can be modified (generally with the intent to reduce it) by injecting one ormore of the following upstream of the precipitator:

Sulfur trioxide (SO3)

Ammonia (NH3)

Water

Sulfur Trioxide and Ammonia Conditioning Systems

In most cases, a sulfur trioxide conditioning system is sufficient to reduce fly ash resistivity toan acceptable level. The source of sulfur trioxide can be liquid sulfur dioxide, molten elementalsulfur, or granulated sulfur. It is also possible to convert native flue gas SO2 to SO3.

In some instances, ammonia alone has been proven a suitable conditioning agent. It forms anammonia-based particulate to increase the space charge. The source of ammonia may beliquid anhydrous or aqueous ammonia, or solid urea.

Finally, sulfur trioxide and ammonia may be used in combination. This solution has beensuccessful because it can lower fly ash resistivity and also form ammonia bisulfate. The latterincreases the adhesion of particles, and thus reduces re-entrainment losses.

Water Injection

The injection of water upstream of the precipitator lowers the gas temperature and addsmoisture to the flue gas. Both are beneficial in cold-side precipitator applications. However,care must be taken that all of the water is evaporated and that the walls in the ductwork orgas distribution devices do not get wet.