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 one TR set . Collecting electrode spacing

. Recovery of material for economic reasonsPulp and paper Industries (sodium sulphate ). Removal of abrasive material in the dust to reduce 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 to collect dust in gas flow rate of 1 m3 /s.Flue gas Velocity, m/s =Flue gas flow in m3 ESP effective cross section m2

Aspect ratio = Effective Length of ESP Collecting electrode heightTreatment Time, sec =Effective Length of ESP in m Flue gas Velocity in m/s

Gas Velocity.. Velocity is decided by the gas flow and collection efficiency required. Higher the gas velocity,higher the carryover of dust particles without Collection - Re entertainment. Very poor velocity alters the flow distribution and effects settling of Dust particles. Optimum velocity depends upon the application will improve the Performance.

Aspect Ratio.. During the rapping, the falling of dust particle take a trajectory form. Lower the aspect ratio, the trajectory dust travel 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 allowable velocity, required collection Efficiency and available space.

Treatment Time.. Time available for capturing the dust particle. More treatment time at reasonable velocity improves the collection efficiency. Probability of capturing the re-entrained particles improves with time.

RECOVERY ELECTOSTATIC PRECIPITATORThe Paper mills are often located in a sensitive environment with strict requirements of emission of dust particles and gaseous pollutants to the atmosphere. The dust particles are very fine and sticky in nature. The gases are also highly corrosive. Dedusting by means of Electrostatic Precipitators are the preferred technology in Paper mills. Black liquor recovery boilers are de-dusted by a multi chamber ElectrostatiC Precipitator often with a casing made of concrete

The casing of the precipitator for recovery boiler applications are preferred to be made of REINFORCED CEMENT CONCRETE. As the gas is rich in moisture and highly corrosive due to the presence of sulphur compounds ( sodium sulphate and sodium sulphide used in the pulp digesters ), the concrete casing is preferred to withstand corrosion.

For the same reason, the collecting electrode ( the thinnest part in the electrode system ) is made of corrosion resistant steels CORTEN - A or CORTEN B or equivalent. The thickness can be 1.5 mm to provide for an enhanced life of the collecting system.The emitting electrode shall be of austenitic stainless steel having excellent corrosion resistant properties (conforming to UHB 904L or AISI 316L or equivalent)

The load of the collecting and emitting systems are transferred to the casing through load bearing members called casing inserts. These are small parts made of steel and embedded in the concrete casing at the time of casting the same. This is done in-site.The hopper system for these precipitators shall be of flat bottom. No pyramidal nor trapezoidal type of hoppers are used for such applications. The bottom floor of the casing itself serves as the hopper and the dust from the collecting / emitting and the gas distribution screens are allowed to fall on to this floor.

The collected dust on the floor is scrapped by means of SCRAPPER CONVEYOR which runs between the inlet of the precipitator and the outlet. Structural members are mounted at desired locations on two end-less chains and scrap the collected dust to bring it to the inlet end of the precipitator casing. The conveyor is electrically driven by motors mounted on the outside of the casing

In addition to the scrapper conveyor, a CHAIN CONVEYOR is also employed to transfer the dust to a ROTARY FEEDER mounted external to the precipitator casing. The chain conveyor runs across the precipitator at the inlet end of the casing and is located inside the precipitator casing. The chain conveyor is also electric driven by a motor mounted external to the precipitator casing.The dust discharged from the chain conveyor into the rotary feeder is further conveyed to the mixing chamber where it is mixed with the spent liquor and recycled

The drives of the scrap per conveyor, chain conveyor and the rotary feeder are to be interlocked in a particular sequence by monitoring their operation through speed monitoring devices mounted on the drive shafts of these conveyors. This is essential to avoid overloading of the conveyors / their drives. The operation of the scrapper conveyor shall be interlocked with the Transformer Rectifier set so that the fields are de-energized automatically when the scrapper conveyor is NOT in operation. As the dust is sticky in nature due to the high moisture content, the gas distributor screens at the inlet of the precipitator will be rapped at the same frequency as that of the emitting electrode system.

As the flue gas is highly corrosive and rich in moisture content, special care has to be taken to ensure that the flue gas temperature at the inlet of the precipitator is sufficiently above the acid / moisture dew point to avoid any condensation on the precipitator surfaces and cause corrosion. Temperature monitors are required to be installed at the inlet duct. Some customers may prefer to have a bye-pass duct when the gas temperature is NOT sufficiently above the dew points. In such cases, diverter dampers may be required at the inlet and outlet of the precipitator casing to prevent gas flow through the precipitator. This will add to the cost of the precipitator system.

. Gas tight dampers are required to be installed at the inlet and outlet of the precipitator casing for purposes of maintenance. The ingress / leakage of atmospheric air into the precipitator casing has to be completely avoided from the point of eliminating the possibility of any local corrosion. The inspection doors on the casing have to be therefore of double construction. One inspection door located very close ( on the concrete casing ) and the other one mounted over the inner door.The concrete casing also requires thermal insulation on the outside. Light Resin Bonded (LRB) mattresses of adequate thickness can be used.

CONSTRUCTION OF ELECTROSTATIC PRECIPITATORThis consists of Supporting structure and support Bearing , these are the rigid structure supporting the entire load of the ESP. The bearings are provided between the casing colume and supporting structure to act freely for thermal expansion.CASING. The casing is known as IB casing, the side walls are made of horizontal panels, it is a leak proof arrangement with roof beams of Longitudinal and Transverse to support the internals of Collecting and Emitting systems.HOPPER.Pyramidal and flat bottom hoppers are provided under the casing to collect the ashes. It should not be treated as storage bunker.

EMITTING SYSTEM.Emitting system consists of rigid emitting frame suspended from four points on the top. The four suspension points are supported on support Insulators to give electrical insulation to the emitting frame.EMITTING ELECTRODES.The Discharge electrodes consist of hard drawn spiral wires. The coil spring form emitting electrodes are self tensioning, this stabilized positioning permits the highest possible operating voltage. The self tensioning spiral discharge electrodes allow for better transmission of the rapping force. The spiral wire electrode provides a uniform current distribution and the corona discharge occurs around the entire surface of the wire.

Rapping mechanism for Discharge electrode.A Traction of the dust will be collected on the discharge electrode and the corona will be suppressed as the dust layer grows. Frequent rapping is required to keep the electrode clean always.COLLECTING SYSTEM.The collecting system is of dimensional stability. The upper edges of the collecting plate are hung on hooks provided on the roof and the bottom is fixed with the shock bar. The collecting electrodes are made of cold rolled carbon steel or corton steel material of the order of 1.5mm thickness with G profile at the end

RAPPING MECHANISM FOR GOLLECTING ELECTRODE.The system employs tumbling hammers which are mounted on a horizontal shaft in a staggered fashion with one hammer for each shock bar. The shock bar transmits the blow simultaneously to all of the collecting plates in one row because of their direct contact with the shock bar.ELECTRICS .Rectifier Transformers are provided on top of ESP, the control panels are located in ESP control room situated in the ground. Auxiliary control panels are housed in the ESP control room to control the auxiliary equipments of ESP like Heaters, Rapping motors, conveyers etc.LT distribution board also housed in control room.

OPERATION AND MAINTENANCE OF RECOVERY ESP.ESPs are constant efficient equipment, if the input parameters are maintained to the design value then the out put efficiency (emission) will be maintained, Provided the ESP fields should be healthy.We have to ensure the healthiness of each and every equipment independently.The HVR and EC controllers should be tuned to the optimum level Depending upon the load in the steam unit.Monitor the optimum operation of the boiler by periodical check 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 any Jamming.

MAINTENANCE.Check all the Heaters are in service with thermostat control in operation.Check all the rapping motors are working as per the program set.Check the conveyors are running smoothCheck the current and voltage are to the set level in the controllers.Check the boiler is operated with optimum design condition without any excess flue gas Any pluggage problem in the entire flue gas path from boiler outlet to chimney Inlet.To monitor the maximum solid content in the liquor to be fired.

Maintain history of firing proportion to emission with parameter recordingThe gas distribution to be studied for better correction.Optimization of rapping to avoid offset in the system.Repair and replacement of rapping mechanism by suitably replacing the worn-out components.Field alignment to be checked perfectly to attain max. current and voltageCorona quenching problems to be studied and attended.The ESP rapping system should impart as high acceleration to the precipitator internals as possible to increase the intensity of rapping by increasing the size.

Poor ESP power inputDust build-upsGas flow issuesOver load. Poor ESP power input : Due to mechanical alignment deficiency, that reduces the gap between +ve and ve electrodes, sparks controls the current build up and reduce the collection efficiency.Dust build- up : The formation of accumulation is due to the reaction between solid sodium sulfate and gaseous So2, which results in the formation of acid sulfate,NaHso4 and thus corona quenching.

Gas Flow : Gas distribution, if it is not even then current distribution will be uneven, In leakage of air increases the flow rate , sneakage of gases flowing in untreated levels carry the dust.Over load : Due to higher production in the mill, Poor boiler 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 uniform distribution,Gas sneakage points to be arrested for efficiency improvement,Rapping mechanism to be checked for effective dislodging of dust particles,Cleaning of internals either by air lashing or water washing.Power supply sources to the ESP to be checked.

Electrical migrationElectrical mobility Corona dischargeESP theoryCharging mechanismsAsh resistivityFlue gas conditioningPower consumption

Reading: Chap. 5

Coulombs law

Statcoulomb (stC): the charge that causes a repulsive force of 1 dyne when 2 equal charges are separated by 1 cm (3.3310-10C)Unit charge: 4.8 10-10stC (1.610-19C)(q=ne)Electric Field

(Robert Millikan, US, 1868-1953; Nobel Prize Laureate, 1923)Hinds, Aerosol Technology, 1999http://nobelprize.org/nobel_prizes/physics/laureates/1923/millikan-bio.html

Terminal velocity in an electrical field (electrical migration velocity/drift velocity)(force balance)(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 the particles!Q: Difference between cyclone and ESP in terms of forces acting on the system? Whats the effect?

Q: How can we generate charges?Ozone generation - http://www.mtcnet.net/~jdhogg/ozone/ozonation.html

123123(20)(12)(8)Turbulent Flow with Lateral Mixing ModelElectrostatic Precipitator

Deutsch-Anderson EquationAc/Q: Specific Collection Area (SCA) Turbulent flow: uniformly mixing Perfect CollectionThe fraction of the particles removed in unit time = the ratio of the area traveled by drift 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 to be 98% efficient. The effective drift velocity of the particles is 6.0 m/min. (a) What is the total collection area? (b) Assuming the plates are 6 m high and 3 m long, what is the number of plates required?6 m3 mInternal Configuration: self-review

Random collisions between ions and particlesQ: Does q depend on time? Does q depend on dp?The total number of charges on a particleThe total charges on a particleUse esu, not SI units.

Bombardment of ions in the presence of a strong fieldTotal number of charges by field chargingQ: Is the charging rate dependent on particle size? On field strength? On time? On material?Aerosol Technology, Hinds, W. C., John Wiley & Sons, 1999. Saturation charge(Zi ~ 450 cm2/stVs)

Comparison of Diffusion & Field ChargingQ: Does collection efficiency increase as particle size increase (because of a higher number of charges)?Nit = 107 s/cm3= 5.1E = 5 KV/cmT = 298 K

Typical fly ash size distributionQ: If the ESP is used to collect the fly ash, how will the particle size distribution at ESP outlet look like?

Impact of particles resistivity on ESPs performance:

Factors: temperature, compositionFlue gas conditioning109 - 1010 ohm-cm is desiredQ: How does resistivity affect an ESPs 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 by measurementUse the same Deutsch-Anderson Equation with new we.Q: Estimate the total collection area required for a 95% efficient fly-ash ESP that treats 8000 m3/min. The ash resistivity is 1.61010 ohm-cm.

Good for moderate collection efficiency (90% ~ 95%)

High Efficiency ESP (>95%)Matts-Ohnfeldt EquationUse k = 1 for fly ash k = 0.5 or 0.6 for industrial categoryRule of Thumb Below 95%, use Deutsch-Anderson Equation Above 99%, use Matts-Ohnfeldt Equation Between them, use an averageQ: In designing a high efficiency ESP, a smaller drift velocity is to be used. Why?

Power density ~ 1-2 W/ft2Corona power

Drift velocity

Efficiency vs. Corona Powerk = 0.55 for Pc/Q in W/cfs up to 98.5%

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**Positive CoronaNegative Corona+--+electronmoleculeparticleCollection PlateCollection PlateElectrodeElectrodeStep 1

**Electrostatic Precipitator (ESP)Drift velocity of particles between the ESP plates

**Types of ESPs in terms of shapeCylindrical typePlate typeTypes of ESPs in terms of flow directionVertical gas-flowHorizontal gas-flowTypes of ESPs in terms performanceOne stage or two stagesDry or wetPlate type, horizontal gas-flow, one stage and dry ESPs are the most common ESP type in industrial application.

**Electrostatic Precipitator (ESP)One-stage ESPTwo-stage ESPDischarge electrodesCollecting electrodesThe observed minimum is because of Cunningham factor in calculation of drift velocity.

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