Location of electrical field or bus section to be energizedCollecting plate areaGas and dust conditionsCollecting 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 the collectingplate 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 the adjacentgas passages of the same electrical field or bus section, but not the corona current level, which canbe different in each gas passage.)

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

Gas velocityGas temperatureDust loading

Gas velocity distribution can be most effectively influenced by the use of gas distribution devices.The quality of gas velocity distribution can be measured in a scaled-down model of the precipitatorand its ductwork, and also in the precipitator itself. Typical criteria are based on ICAC (Institute ofClean Air Companies) recommendations using average gas velocities or on a calculated RMSstatistical representation of the gas velocity pattern.

In general, gas distribution devices consist of turning vanes in the inlet ductwork, and perforated gasdistribution 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 and thedischarge 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 by mechanical means.Tumbling hammers may also be used to dislodge ash. Rapping methods include:

Electric vibratorsElectric solenoid piston drop rappersPneumatic vibrating rappersTumbling hammersSonic horns (do not require transmission assemblies)Discharge Electrode RappingIn general, discharge electrodes should be kept as free as possible of accumulated particulate.The rapping system for the discharge electrodes should be operated on a continuous schedulewith repeat times in the 2 - 4 minute range, depending on the size and inlet particulate loading ofthe precipitator.Collecting Plate RappingCollecting plate rapping must remove the bulk of the precipitated dust. The collecting plates aresupported from anvil beams or directly with hooks from the precipitator casing. With anvil beamsupport, the impact of the rapping system is directed into the beams located at the leading and/ortrailing edge of the collecting plates. For direct casing support, the impact is directed into therapper beams located at or near the center of the top of the collecting plates.The first electrical field generally collects about 60-80% of the inlet dust load. The first field platesshould be rapped often enough so that their precipitated layer of particulate is about 3/8 - 1/2"thick. There is no advantage in rapping more often since the precipitated dust has not yetagglomerated to a sheet which requires a minimum layer thickness. Sheet formation is essential tomake the dust drop into the precipitator hopper without re-entrainment into the gas stream.Rapping less frequently typically results in a deterioration of the electrical power input by adding anadditional resistance into the power circuit. Once an optimum rapping cycle has been found for thefirst electrical field (which may vary across the face of a large precipitator), the optimum rappingcycles for the downstream electrical fields can be established.The collecting plate rapping system of the first field has a repeat time T equal to the time it takesto build a 3/8 - 1/2"layer on the collecting plates. The plates in the second field should have arepeat time of about 5T, and the plates in the third field should have a repeat time of 25T. Ideally,these repeat times yield a deposited layer of 3/8-1/2" for the plates in all three fields. Adjustmentmay be required for factors such as dust resistivity, dust layer cohesiveness, gas temperatureeffects, electrical field height and length, and the collecting area served by one rapper.Gas Distribution Plate and Hopper Wall RappingThe gas distribution plates should also be kept free of excessive particulate buildup and mayrequire rapping on a continuous base with a cycle time in the 10-20 minute range, depending onthe inlet particulate loading of the precipitator and the nature of the particulate. Gas distributionplates in the outlet of the precipitator may be rapped less often (every 30 - 60 minutes).Improving Rapping System PerformanceAll precipitator rapping systems allow adjustment of rapping frequency, normally starting with thehighest frequency (the least time between raps), progressing to the lowest frequency. The timesthat are actually available may be limited. Rapping systems with pneumatic or electric actuatorsallow variations of the rapping intensity. Pneumatic or electric vibrators allow adjustments of therapping time. State-of-the-art rapper controls allow selection of rapping sequences, selection ofindividual rappers, and provide anti-coincidence schemes which allow only one rapper to operateat a given time.

Rapping systems can be optimized for top precipitator performance using precipitator power input andstack opacity as criteria. Optimization of the rapping system starts with the discharge electroderapping system operating on its own time schedule, for example with repeat times of 2 - 4 minutes.The rapping system for the gas distribution screens in the inlet and outlet of the precipitator shouldthen be operated with repeat times of 2-3 minutes for the inlet and 2 - 3 hours for the outlet screens.

The only rapping system requiring optimization is the collecting plate rapping system. The optimizationshould start with the Collecting Plate Rapping Schedule determined above. Next, the rappingfrequency of the inlet field should be increased or decreased until the electrical power input of the inletfield remains constant. Next, the rapping frequency of the other fields should be adjusted in sequenceuntil their electrical power inputs remain constant. If the stack opacity trace shows rapping spikes, therapping intensity should be reduced while observing the electrical power input of the precipitator.

The adjustment of the rapping system for optimum precipitator performance is a slow process. Itrequires 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. Thesehoppers are insulated from the neck above the discharge flange with the insulation covering the entirehopper area. In addition, the lower 1/4- 1/3 of the hopper wall may be heated. Discharge diametersare generally 8" - 12".

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InsulationInsulation provides protection for facility personnel as well as working to retain as much hopperwall temperature as possible. Hopper wall temperature retention discourages condensation on theinside of the hopper. Heaters are added to ensure hot metal surfaces immediately above the flyash discharge.Facilitating hopper dischargeHopper discharge problems are caused by compaction of the fly ash in the hopper. Compactioncharacteristics are affected by moisture content, particle size and shape, head of material, andvibration. 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 an internal hopper baffle.Hopper fluidizersHopper fluidizers have a membrane that permits air flow to the fly ash directly above. This air flowfills the voids between the fly ash particles at a slight pressure, changes the repose angle of theparticles, and promotes gravity flow.Ash handling systemThe fly ash handling system evacuates the fly ash from the hoppers, and transports the fly ash toreprocessing or to disposal. The ash handling system should be designed and operated to removethe collected fly ash from the hoppers without causing re-entrainment into the gas flow through theprecipitator. The design of the ash handling system should allow for flexibility of scheduling thehopper discharges according to the fly ash being collected in these hoppers.

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

Negative pressure or vacuum systemConnects to the hopper by a simple discharge valvePositive pressure dilute phase systemUses an airlock-type feeder; the feeder is separated from the hopper by an inlet gate and from theconveying line by a discharge gatePositive pressure dense phase systemConnects to the hopper with an airlock type feeder.

About Ductwork (Back to top)

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

Low resistance to gas flowAchieved by selecting a suitable cross-section for the ductwork and by installing gas flow controldevices, such as turning valves and flow straightenersGas velocity distributionGas flow control devices are used to maintain good gas velocity distributionMinimal fallout of fly ashFallout can be minimized by using a suitable transport velocityMinimal stratification of the fly ashA suitable transport velocity also reduces fly ash stratification in the gas streamLow heat lossThe goal is to reduce the heat loss of the flue gas to a level that will prevent acid or moisturecondensation in the downstream equipment, requiring the use of thermal insulation protected byexternal siding.Structural integrityDuctwork structure supports its total load, including wind and snow loads. The design also allowsfor accumulated fly ash, negative/positive operating pressure, and gas temperature. Expansionjoints are used to accommodate thermal growth.

About Gas Velocity Distribution (Back to top)

Efficient precipitator performance depends heavily upon having similar gas conditions at the inlet ofeach electrical field or bus section and at the inlet of each gas passage of the electrical field or bussection. Uniformity of gas velocity is also desirable - good gas velocity distribution through aprecipitator 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 of theprecipitator and the characteristics of the dust particles. Traditionally, precipitators have beendesigned with uniform gas velocity distribution through the electrical fields, to avoid high-velocityareas that would cause re-entrainment. While this is still a recommended practice, there is anadvantage in some cases to developing a velocity profile that brings more particles closer to thehopper.

Both of these schemes have applications in site-specific conditions. Gas velocity distribution can becontrolled by the following:

Adding/improving gas flow control devices in the inlet ductworkAdding/improving flow control devices in the inlet of the precipitatorAdding/improving flow control devices in the outlet of the precipitatorAdding a rapping system to the flow control devices (where applicable)Adding/improving anti-sneak baffles at the peripheries of the electrical fieldsAdding/improving hopper bafflesEliminating air leakages into the precipitator

About Re-entrainment (Back to top)

Reducing rapping re-entrainment to an acceptable level generally requires a substantial improvementof the gas velocity distribution and the electrical power density and uniformity, as well as an extendedoptimization 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 major causesof re-entrainment are as follows:

Particles: Low cohesivenessLow adhesion to collecting platesParticle sizeLow resistivity

Voltage Controls: Spark rate settingDesign: Collecting plate design

Discharge electrode designPlate spacing

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Rapping System: FrequencyIntensityDuration (if applicable)

Electrical Field: Collecting plate and discharge electrode rappingSparkingSaltationErosion (localized high gas velocity)Sneakage

Hopper: Hopper designLeakage (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 to precipitateparticles. Either precipitator collecting efficiency or outlet residual can be expressed as a function ofcorona power in Watts/1000 acfm of flue gas, or in Watts/1000 ft of collection area.

The separation of particles from the gas flow in an electrostatic precipitator depends on the appliedcorona power. Corona power is the product of corona current and voltage. Current is needed tocharge the particles. Voltage is needed to support an electrical field, which in turn transports theparticles 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, even largeincreases 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 power results ina substantial reduction in the gas stream particle content. In the upper range of the corona powerlevel, 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). Coronapower levels can be optimized by adjusting or optimizing the following:

Gas velocity: Uniformity

Fly Ash: Particle size· ResistivityVoltage Controls: Spark rate setting

Current & voltage limitsDesign: Plate spacing

Collecting plate & discharge electrode designRapping System: Frequency & intensitySupport Insulator: Purge air system operation

About Performance Improvements (Back to top)

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

Deterioration of existing equipmentTightening of air pollution emission regulationsChanges in products and/or production ratesFrequent forced outagesDe-rating of production

To 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, andoptimize gas velocity distribution inside the precipitator. Some important topics to consider whenplanning 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 wire length andcollecting plate area, so that the desired specific corona current and electrical field can be applied.Gas Velocity DistributionImproving gas velocity distribution in the precipitator reduces particle re-entrainment and boostsprecipitator efficiency. Typically, a uniform gas velocity is desired, but there are site-specificexceptions. 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 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. For additional information, refer to Corona Power.SectionalizationThe precipitator is divided into electrical sections that are cross-wise and parallel to the gas flowto accommodate spatial differences in gas and dust conditions. Optimization of corona powerinvolves adjusting the corona power (secondary voltage and current) in each electrical section foroptimum conditions.Particle Re-entrainmentMinimizing re-entrainment of dust particles is important to improvement of precipitator efficiency.Most precipitator equipment affects the re-entrainment level. For a detailed discussion, visit thespecial section on re-entrainment.Additional EquipmentPerformance improvement options include the installation of a second precipitator in series with theexisting precipitator; using fabric filters downstream of the precipitator; and adding a secondparticle collector in parallel with the existing collector. Other possibilities include sonic orelectrostatic particle agglomerators upstream of the precipitator; a mechanical upstream collector;or an electrostatically-enhanced or mechanical collector, or a filter downstream of the precipitator.Review the General Equipment RequirementsReviewing the Neundorfer Knowledge Base sections on equipment will provide additional insight

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