Ash resistivity. Particle size distribution. Number of ESP per boiler. Minimum No. of fields required. Minimum specific collecting area. Maximum gas velocity. Minimum aspect ratio. Maximum area connected to oneTR set. Collecting electrode spacing
. Recovery of material for economic reasonsPulp and paper Industries (sodium sulphate ). Removal of abrasive material in the dust toreduce wear and tear of the Fan components. Removal of objectionable matter in the dust-NO2 and SO2
Specific Collecting AreaAmount of collecting area required to be provided tocollect dust in gas flow rate of 1 m3 /s.Flue gas Velocity, m/s = Flue gas flow inm3ESP effective cross section m2Aspect ratio = Effective Length ofESPCollecting electrode heightTreatment Time, sec =Effective Length of ESP inmFlue gas Velocity in m/s
Gas Velocity.. Velocity is decided by the gas flow andcollection efficiency required. Higher the gas velocity,higher the carryover ofdust particles without Collection - Re –entertainment. Very poor velocity alters the flow distributionand effects settling of Dust particles. Optimum velocity depends upon theapplication will improve the Performance.
Aspect Ratio.. During the rapping, the falling of dust particletake a trajectory form. Lower the aspect ratio, the trajectory dusttravel along with gas flow Without falling in to hoppers – Leads to re-entrainment loss.. Higher the ratio, performance will be good. Optimum aspect ratio depends on allowablevelocity, required collection Efficiency and available space.
Treatment Time.. Time available for capturing the dust particle. More treatment time at reasonable velocityimproves the collection efficiency. Probability of capturing the re-entrainedparticles improves with time.
RECOVERY ELECTOSTATICPRECIPITATORThe Paper mills are often located in a sensitiveenvironment with strict requirements of emission ofdust particles and gaseous pollutants to theatmosphere. The dust particles are very fine andsticky in nature. The gases are also highly corrosive.Dedusting by means of Electrostatic Precipitators arethe preferred technology in Paper mills. Black liquorrecovery boilers are de-dusted by a multi chamberElectrostatiC Precipitator often with a casing made ofconcrete
The casing of the precipitator for recovery boilerapplications are preferred to be made ofREINFORCED CEMENT CONCRETE. As the gas isrich in moisture and highly corrosive due to thepresence of sulphur compounds ( sodium sulphateand sodium sulphide used in the pulp digesters ), theconcrete casing is preferred to withstand corrosion.
For the same reason, the collecting electrode( the thinnest part in the electrode system ) is made ofcorrosion resistant steels – CORTEN - A or CORTEN –B or equivalent. The thickness can be 1.5 mm toprovide for an enhanced life of the collecting system.The emitting electrode shall be of austeniticstainless steel having excellent corrosion resistantproperties (conforming to UHB 904L or AISI 316L orequivalent)
The load of the collecting and emitting systems aretransferred to the casing through load bearingmembers called ‘casing inserts’. These are small partsmade of steel and embedded in the concrete casing atthe time of casting the same. This is done in-site.The hopper system for these precipitators shall be offlat bottom. No pyramidal nor trapezoidal type ofhoppers are used for such applications. The bottomfloor of the casing itself serves as the hopper and thedust from the collecting / emitting and the gasdistribution screens are allowed to fall on to this floor.
The collected dust on the floor is scrapped by meansof ‘SCRAPPER CONVEYOR’ which runs between theinlet of the precipitator and the outlet. Structuralmembers are mounted at desired locations on twoend-less chains and scrap the collected dust to bringit to the inlet end of the precipitator casing. Theconveyor is electrically driven by motors mounted onthe outside of the casing
In addition to the scrapper conveyor, a CHAINCONVEYOR is also employed to transfer the dust to aROTARY FEEDER mounted external to theprecipitator casing. The chain conveyor runs acrossthe precipitator at the inlet end of the casing and islocated inside the precipitator casing. The chainconveyor is also electric driven by a motor mountedexternal to the precipitator casing.The dust discharged from the chain conveyorinto the rotary feeder is further conveyed to themixing chamber where it is mixed with the spentliquor and recycled
The drives of the scrap per conveyor, chain conveyor andthe rotary feeder are to be interlocked in a particularsequence by monitoring their operation through speedmonitoring devices mounted on the drive shaftsof these conveyors. This is essential to avoidoverloading of the conveyors / their drives. Theoperation of the scrapper conveyor shall beinterlocked with the Transformer – Rectifier setso that the fields are de-energized automaticallywhen the scrapper conveyor is NOT inoperation.As the dust is sticky in nature due to thehigh moisture content, the gas distributorscreens at the inlet of the precipitator will berapped at the same frequency as that of the
As the flue gas is highly corrosive and rich inmoisture content, special care has to be taken toensure that the flue gas temperature at the inlet ofthe precipitator is sufficiently above the acid /moisture dew point to avoid any condensation onthe precipitator surfaces and cause corrosion.Temperature monitors are required to be installedat the inlet duct. Some customers may prefer tohave a bye-pass duct when the gas temperature isNOT sufficiently above the dew points. In suchcases, diverter dampers may be required at theinlet and outlet of the precipitator casing toprevent gas flow through the precipitator. Thiswill add to the cost of the precipitator system.
. Gas tight dampers are required to be installed atthe inlet and outlet of the precipitator casing forpurposes of maintenance.The ingress / leakage of atmospheric air into theprecipitator casing has to be completely avoidedfrom the point of eliminating the possibility of anylocal corrosion. The inspection doors on the casinghave to be therefore of double construction. Oneinspection door located very close ( on the concretecasing ) and the other one mounted over the innerdoor.The concrete casing also requires thermalinsulation on the outside. Light Resin Bonded (LRB)mattresses of adequate thickness can be used.
CONSTRUCTION OF ELECTROSTATICPRECIPITATORThis consists of Supporting structure and supportBearing , these are the rigid structure supporting theentire load of the ESP. The bearings are providedbetween the casing colume and supporting structure toact freely for thermal expansion.CASING. The casing is known as IB casing, the side walls aremade of horizontal panels, it is a leak proof arrangementwith roof beams of Longitudinal and Transverse tosupport the internals of Collecting and Emittingsystems.HOPPER.Pyramidal and flat bottom hoppers are provided underthe casing to collect the ashes. It should not be treatedas storage bunker.
EMITTING SYSTEM.Emitting system consists of rigid emitting frame suspendedfrom four points on the top. The four suspension points aresupported on support Insulators to give electricalinsulation to the emitting frame.EMITTING ELECTRODES.The Discharge electrodes consist of hard drawn spiralwires. The coil spring form emitting electrodes are selftensioning, this stabilizedpositioning permits the highest possible operating voltage.The self tensioning spiral discharge electrodes allow forbetter transmission of the rapping force. The spiral wireelectrode provides a uniform current distribution and thecorona discharge occurs around the entire surface of thewire.
Rapping mechanism for Discharge electrode.A Traction of the dust will be collected on thedischarge electrode and the corona will be suppressedas the dust layer grows. Frequent rapping is requiredto keep the electrode clean always.COLLECTING SYSTEM.The collecting system is of dimensional stability. Theupper edges of the collecting plate are hung on hooksprovided on the roof and the bottom is fixed with theshock bar. The collecting electrodes are made of coldrolled carbon steel or corton steel material of theorder of 1.5mm thickness with G profile at the end
RAPPING MECHANISMFOR GOLLECTINGELECTRODE.The system employs tumbling hammers which are mounted ona horizontal shaft in a staggered fashion with one hammer foreach shock bar. The shock bar transmits the blowsimultaneously to all of the collecting plates in one row becauseof their direct contact with the shock bar.ELECTRICS .Rectifier Transformers are provided on top of ESP, the controlpanels are located in ESP control room situated in the ground.Auxiliary control panels are housed in the ESP control room tocontrol the auxiliary equipments of ESP like Heaters, Rappingmotors, conveyers etc.LT distribution board also housed incontrol room.
OPERATION AND MAINTENANCE OFRECOVERY ESP.ESP’s are constant efficient equipment, if the inputparameters are maintained to the design value then the output efficiency (emission) will be maintained, Provided theESP fields should be healthy.We have to ensure the healthiness of each and everyequipment independently.The HVR and EC controllers should be tuned to theoptimum levelDepending upon the load in the steam unit.Monitor the optimum operation of the boiler by periodicalcheck of O2 levels in different point in the flue gas circuit.The controllers should be kept at just below spark levelAlways monitor the conveyors to work smooth to avoid anyJamming.
MAINTENANCE.Check all the Heaters are in service withthermostat control in operation.Check all the rapping motors are working as perthe program set.Check the conveyors are running smoothCheck the current and voltage are to the set levelin the controllers.Check the boiler is operated with optimum designcondition without any excess flue gasAny pluggage problem in the entire flue gas pathfrom boiler outlet to chimney Inlet.To monitor the maximum solid content in theliquor to be fired.
Maintain history of firing proportion to emissionwith parameter recordingThe gas distribution to be studied for bettercorrection. Optimization of rapping to avoid offset in the system.Repair and replacement of rapping mechanism bysuitably replacing the worn-out components. Field alignment to be checked perfectly to attainmax. current and voltageCorona quenching problems to be studied andattended.The ESP rapping system should impart as highacceleration to the precipitator internals as possibleto increase the intensity of rapping by increasing thesize.
Poor ESP power inputDust build-upsGas flow issuesOver load. Poor ESP power input : Due to mechanical alignmentdeficiency, that reduces the gap between +ve and –veelectrodes, sparks controls the current build up andreduce the collection efficiency.Dust build- up : The formation of accumulation is due tothe reaction between solid sodium sulfate and gaseousSo2, which results in the formation of acid sulfate,NaHso4and thus corona quenching.
Gas Flow : Gas distribution, if it is not even thencurrent distribution will be uneven, In leakage of airincreases the flow rate , sneakage of gases flowing inuntreated levels carry the dust.Over load : Due to higher production in the mill, Poorboiler operation with high amount of excess air,leakages in the flue gas path.
Leakages in the flue gas path to be controlledInternal alignment to be checked and corrected,Gas distribution to be checked for uniformdistribution,Gas sneakage points to be arrested for efficiencyimprovement,Rapping mechanism to be checked for effectivedislodging of dust particles,Cleaning of internals either by air lashing or waterwashing.Power supply sources to the ESP to be checked.
Coulomb’s lawStatcoulomb (stC): the charge that causes a repulsive force of 1dyne when 2 equal charges are separated by 1 cm (3.33×10-10C)Unit charge: 4.8 ×10-10stC (1.6×10-19C)221rqqKF EE = EFqE= (q=ne)Electric Field
Terminal velocity in an electrical field(electrical migration velocity/drift velocity)cTEpCVdqEπµ3=( ) qEBdqECwVpcTE ===⇒πµ3qBdqCEVZpcTE===⇒πµ3(force balance)DE FF =(for Re < 1)Q: What is the physical meaning of electrical mobility?Q: When does a particle have a higher mobility?May the force be with theparticles!Q: Difference betweencyclone and ESP in termsof forces acting on thesystem? What’s theeffect?
Positive Corona Negative Corona+ -+ -++ -+++--+- +- +---++Corona DischargeStep 1Step 2Step 3Step 4Collection Plate Collection PlateElectronMoleculeParticleElectrode ElectrodeQ: How can we generate charges?Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html
Deutsch-AndersonEquationRdtVRdtRVNdN TETE 222−=−=ππ)2exp()(0 RtVNtN TE−=⇒−−=−=η⇒QAVP cTEexp11 Ac/Q: Specific Collection Area (SCA)• Turbulent flow: uniformly mixing• Perfect Collection•The fraction of the particles removed inunit time = the ratio of the area traveled bydrift velocity in unit time to the total cross-sectionQ: How to increase the efficiency?
Q: An ESP that treats 10,000 m3/min of air is expected tobe 98% efficient. The effective drift velocity of theparticles is 6.0 m/min. (a) What is the total collectionarea? (b) Assuming the plates are 6 m high and 3 mlong, what is the number of plates required?6 m3 mInternal Configuration: self-review
Random collisions between ionsand particles+=kTtNecdekTdniipp21ln222πQ: Does q depend on time?Does q depend on dp?The total number of charges on a particle(ci ~ 2.4×104cm/s)neq =The total charges on a particleUse esu, not SI units.
Bombardment of ions in the presence of a strongfieldeZ1eZ423ii2++=tNtNeEdniipππεεTotal number of charges by field chargingQ: Is the charging rate dependent onparticle size? On field strength? Ontime? On material?Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999.+=eEdnps4232εεSaturation charge (Zi ~ 450 cm2/stV•s)
Typical fly ashsize distributionQ: If the ESP is used to collect the fly ash,how will the particle size distribution atESP outlet look like?
Impact of particles’ resistivity on ESP’sperformance:Factors: temperature, compositionFlue gas conditioning109- 1010ohm-cm is desiredQ: How does resistivity affect an ESP’s performance?
Effects of sulfur content and temperature on resistivityQ: Is S in coal good or bad?
Water spray for cement kiln dustFlue Gas Conditioning
Effective drift velocity as a function of resistivity bymeasurementUse the same Deutsch-Anderson Equation with new we.Q: Estimate the total collection area required for a 95% efficient fly-ashESP that treats 8000 m3/min. The ash resistivity is 1.6×1010ohm-cm.
Good for moderate collectionefficiency (90% ~ 95%)
High Efficiency ESP (>95%)Matts-Ohnfeldt Equation−−=keCwQAexp1ηUse k = 1 for fly ashk = 0.5 or 0.6 forindustrial categoryRule of Thumb• Below 95%, use Deutsch-Anderson Equation• Above 99%, use Matts-Ohnfeldt Equation• Between them, use an averageQ: In designing a highefficiency ESP, a smallerdrift velocity is to beused. Why?
avgCC VIP =CCeAkPw =Power density ~ 1-2 W/ft2 −−=QkPCexp1ηCorona powerDrift velocityEfficiency vs. Corona Powerk = 0.55 for Pc/Q in W/cfs up to 98.5%
20/04/13 59Electrostatic Precipitator (ESP)҉ Drift velocity of particles between the ESPplates
20/04/13 60Types of ESPs in terms of shape҉ Cylindrical type҉ Plate typeTypes of ESPs in terms of flowdirection҉ Vertical gas-flow҉ Horizontal gas-flowTypes of ESPs in termsperformance҉ One stage or two stages҉ Dry or wetPlate type, horizontal gas-flow, one stageand dry ESPs are the most common ESPtype in industrial application.
20/04/13 61Electrostatic Precipitator (ESP)One-stage ESP Two-stage ESPDischargeelectrodesCollectingelectrodesThe observedminimum is becauseof Cunninghamfactor in calculationof drift velocity.