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ESP – DESIGN, OPERATION & MAINTENANCE

TAPASH NAG

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The Electrostatic Precipitator

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Data required for ESP

Application

Process Data

Gas Composition

Gas pressure

Gas Moisture

Dust Composition

Particle size distribution

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Basic Design Data

Gas Flow Rate

Gas temperature

Inlet dust concentration

Sulphur Content

Ash resistivity

Environmental requirements

Outlet emissions

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Design data governs

No. of ESP per boiler

No. of required fields

Specific collecting area

Maximum gas velocity

Minimum aspect ratio

Maximum area connected to one TR set

Collecting electrode spacing

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ESP - Advantages

Very high collection efficiency

Low pressure drop

Capacity to collect sub micron particles

Robust construction – Longer life

Less maintenance

Adaptability

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ESP – Working Principle

The precipitation process involves 4 main

functions :

Corona generation

Particle charging

Particle collection

Removal of particles

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ESP – Working Principle

Corona generation: Due to ionization of gas molecules +ve ions, -ve ions and free electrons are generated

Particle charging:The –ve charges of ions and free electrons move towards +ve electrodes and the +ve charges of ions move towards –ve electrodes.When –ve ions travel towards +ve electrodes, the –ve charges get attached to the dust particles and thus the dust particles are electrically charged

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ESP – Effect of ash resistivity

Ash Resistivity: The ease with ash particle acquires an electrical chargeRanges from 1 x 108 ohm-cm to 1 x 1014 ohm-cmIdeal Resistivity is 5 x 109 ohm-cm to 5 x 1010 ohm-cmLow resistivity- Collected particles loose charge and get re-entrainedHigh resistivity- Requires bigger ESP sizeHigh resistivity- Effective migration velocity is lowerHigh resistivity- Gives rise to Back Corona effect. Discharge current increase abnormally & applied voltage is reduced

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ESP – Effect of ash resistivity

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ESP – Temp v/s ash resistivityAsh resistivity is lower at lower temperaturesAt lower temperature, moisture and sulphur oxides reduce the ash resistivityAsh resistivity is higher at 140-160oCAsh resistivity reduces at higher temperatures but it is due to the semi conductor effectHigh resistivity- Requires bigger ESP sizeHigh resistivity- Effective migration velocity is lowerHigh resistivity- Gives rise to Back Corona effect. Discharge current increase abnormally & applied voltage is reduced

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ESP – Temp v/s ash resistivity

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ESP – Temp v/s Migration Velocity

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ESP – Sizing TheoryDeutsch Anderson Equation:

ESP Collection efficiency = 1-exp(-w.SCA)

SCA = Specific Collection area = A/Q

Where w = Drift or Migration velocityA = Total surface area of electrodesQ= gas volumetric flow rate through ESP

In words, amount of area required to collect ash per unit of gas volume

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ESP – Related TermsFlue gas velocity (m/s)=

Flue gas flow in m3/s Effective cross section area in m2

Aspect Ratio =Effective length of ESP in m

Height of Collecting electrode in m

Treatment time (s) =

Effective Length of ESP in mFlue gas velocity in m/s

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ESP – Gas velocity Velocity is decided by the gas flow and collection efficiency required

Higher the velocity, higher the carryover of particles without collection. Re-entrainment

Very poor velocity alters the flow distribution pattern and affects the settling of dust particles

Optimum velocity improves the performance of the ESP

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ESP – Aspect RatioDuring rapping, the dust particles fall in a trajectory path

Lower the aspect ratio, the trajectory dust travel along with gas flow

Does not fall into hoppers- Leads to Re-entrainment

Higher the ratio, better the performance

Optimum aspect ratio depends on allowable velocity, required collection efficiency and available space

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ESP – Treatment time

Time available for capturing the dust particles

More treatment time at reasonable velocity improves the collection efficiency

Probability of capturing the re-entrained particles improves with time

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ESP – Cold StartupEnergize insulator and hopper heaters at least 6 hours before startup or leave energized if shutdown is taken for a short duration

Energize rappers and ash evacuation systems 2 hours before Startup

Increase rapper force and frequency until normal operating temperature is reached to limit adhesion of damp ash on Electrodes

Energize TRs one at a time as needed to maintain opacity limit and maintain power levels below maximum to minimize sparking once the flue gas temp is above 120oC after coal firing

At normal operating temperature, switch TR controllers to automatic and rapper settings to standard settings

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ESP – Cold Startup

Point of Contention: Should the inlet or outlet electrical fields be energized first?

Energize TRs Inlet to OutletMinimizes Power ConsumptionLimits Damp Ash Collection to Outlet Fields

Energize TRs Outlet to InletLimits Collection of Damp Ash on Electrodes by allowing dropout in Non-Energized Front Electrical Fields

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ESP – Shutdown procedureThe ESP Shutdown Procedure is just as important as the Startup Procedure to the operating integrity of the ESP

De-energize the electrical fields one at a time as required to maintain opacity limit.

Again, Two Approaches:

De-energize outlet TRs first, outlet to inlet: Power savings since outlet fields tend to operate near rated power levels

De-energize Inlet TRs first, inlet to outlet: Avoids opacity spikes due to rapper re-entrainment since downstream fields are energized to recharge and recollect re-entrained ash.

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ESP – Shutdown procedure(Cont’d)

After ESP Shutdown, maintain operation of rappers, hopper evacuation system, and hopper heaters for at least 2 hours following purge procedure.

This will ensure the ESP remains in an operational ‘clean state’ to the best extent possible for subsequent startup

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ESP – Performance Monitoring

As with the operation of any piece of equipment, performance monitoring and recordkeeping are essential to establishing a good operation and maintenance program.

The key to any monitoring program is establishing an adequate baseline of acceptable ranges that is used as a reference point.

Then, by monitoring and recording key operating parameters, the operator can identify performance problems, need for maintenance, and operating trends.

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ESP – Performance Monitoring

As with the operation of any piece of equipment, performance monitoring and recordkeeping are essential to establishing a good operation and maintenance program.

The key to any monitoring program is establishing an adequate baseline of acceptable ranges that is used as a reference point.

Then, by monitoring and recording key operating parameters, the operator can identify performance problems, need for maintenance, and operating trends.

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ESP – Performance Monitoring(2)

Typical parameters that can be monitored include:

Voltage/current

Opacity

Gas temperature

Gas flow rate and distribution

Gas composition and moisture

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ESP – Typical ChecklistDaily:1.Take and record electrical readings & Opacity meter data2.Check operation of hoppers and ash removal system3.Examine control room HVAC system4.Investigate cause of abnormal arcing in T-R enclosures & bus ductsWeekly:1.Check rapper operation2.Check & clean air filter3.Inspect control panel internalsMonthly:1.Check Operation of standby top housing pressurizing fan and thermostat2.Check operation of hopper heaters3.Check hopper level alarm operation

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ESP – Typical Checklist(Cont’d)Quarterly:1.Check & clean vibrator & rapper switch contacts2.Check opacity meter calibrationHalf Yearly:1.Clean & lubricate access doors dog bolts and hinges2.Clean & lubricate interlock covers3.Clean & lubricate test connections4.Check exteriors for visual signs of deterioration, abnormal vibration, noise and leaks5.Check oil level in T-R set and surge-arrestor spark gapAnnually:1.Conduct internal inspection2.Clean top housing or insulator compartment and all electrical insulating surfaces3.Check & correct defective alignment

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ESP – Typical Checklist(Cont’d)Annually (Cont’d):1.Examine & clean all contactors and inspect tightness of all electrical connections2.Clean & inspect all gasketed connections3.Check & adjust operation of switch gear4.Check & tighten rapper insulator connections5.Observe & record areas of corrosionOpportunity based:1.Record air load and gas load readings during & after each outage2.Clean & check interior of control sets during each outage of more than 72 hours3.Clean all internal bushings in outage more than 5 days4.Inspect condition of all grounding devices during each outage over 72 hours5.Clean hopper build ups during each outage6.Inspect & record amount and location of residual dust deposits on electrodes during each outage of 72 hours or longer7.Check all alarms, interlocks and safety devices during each outage

Source: Bibbo 1982.

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ESP – Problems

Dust Accumulation

Wire Breakage

Insulator cracking

Hopper Pluggage

Air Ingression

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ESP – Dust Accumulation

Inadequate rapping system

Sticky dust

Operation at temperatures below dew point level

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ESP – Wire breakage

Wire breakage is on of the major problems for deteriorating theperformance of ESP. Much time and effort is required to determine thereasons of wire breakage. Although most new ESPs have either rigid framesor rigid electrodes, and some weighted-wire systems have been retrofittedto rigid electrodes, the most common ESP in service today is still theweighted wire.

Wires usually fail in one of three areas: at the top of the wire, at thebottom of the wire, and wherever misalignment or slack wires reduce theclearance between the wire and plate. Wire failure may be due to electricalerosion, mechanical erosion, corrosion or combination of these.

When wire failures occur, they usually short-out the field where they arelocated. In some cases, they may short-out an adjacent field as well. Thus,the failure of one wire can cause the loss of particle collection in an entirefield or bus section

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ESP – Wire breakage

Electrical erosion is caused by excessive sparking. Sparking usually occurs at points where there is close clearance within a field due to a warped plate, misaligned guidance frames, or bowed wires. The maximum operating voltage is usually limited by these close tolerance areas because the spark-over voltage depends on the distance between the wire and the plate.

Design faults and the failure to maintain alignment generally contribute to mechanical erosion (or wear) of the wire. In some designs, the lower guide frame guides the wires or their weight hooks (not the weights themselves) into alignment with the plates.

Corrosion of the wires can also lead to wire failures. Corrosion, an electrochemical reaction, can occur for several reasons, the most common being acid dew point. When the rate of corrosion is slow and generally spread throughout the ESP, it may not lead to a single wire failure for 5 to 10 years.

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ESP – Wire breakage

Wire crimping is another cause of wire failure. Crimps usually occur at the top and bottom of the wires where they attach to the upper wire frame or bottle weight; however, a crimp may occur at any point along the wire. Because a crimp creates a residual stress point, all three mechanisms (electrical erosion, mechanical erosion, and corrosion) may be at work in this situation. A crimp can:

Distort the electric field along the wire and promote sparking

Mechanically weaken the wire and make it thinner

Subject the wire to a stress corrosion failure (materials under stress tend to corrode more rapidly than those not under stress)

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ESP – Insulator CrackingMisalignment

Thermal break down

It is the external electric field that destroys energy balance & causes intrinsic & avalanche breakdownDue to the tunnel effect the electrons pass through the valance bond to conduction band causing Zener breakdownBreakdown occurs due to combined electrical & thermal action

Insulation agingLocal discharge by gas induces local coronaIonization energy of 10-11 ev is enough to break the ceramic material molecule. Oxidation at surface may cause corrosion & carburization The electric discharge by gas inside the small holes in insulator may reach up to 1000oC causing thermal stress induced cracking

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ESP – Hopper Pluggage

Perhaps no other problem (except fire or explosion) has the potential

for degrading ESP performance as much as hopper pluggage. Hopper

pluggage can permanently damage an ESP and severely affect both

short-term and long-term performance. Hopper pluggage is difficult

to diagnose because its effect is not immediately apparent on the T-R

set panel meters. Depending on its location, a hopper can usually be

filled in 4 to 24 hours. In many cases, the effect of pluggage does not

show up on the electrical readings until the hopper is nearly full.

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ESP – Hopper Pluggage

Hopper pluggage can be caused by the following:

Obstructions due to fallen wires and/or bottle weights

Inadequately sized solids-removal equipment

Use of hoppers for dust storage

Inadequate insulation and hopper heating

Air ingress through access doors

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ESP – Air IngressionAir ingress is often overlooked as an operating problem. In some instances, it can be beneficial to ESP performance, but in most cases its effect is detrimental. Air ingress may occur within the process itself or in the ESP and is caused by leaking access doors, leaking ductwork, and even open sample ports. Air ingress usually cools the gas stream, and can also introduce additional moisture. Air ingress often causes localized corrosion of the ESP shell, plates, and wires. The temperature differential also can cause electrical disturbances (sparking) in the field. Finally, the introduction of ambient air can affect the gas distribution near the point of entry.

The primary entrance paths are through the ESP access and hopper doors. Air ingress through hopper doors may re-entrain and excessively cool the dust in the hopper, which can cause both re-entrainment in the gas stream and hopper pluggage. Air ingress through the access doors is normally accompanied by an audible in-rush of air.

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ESP – Corona generation

+

+- -

-

-

Positive Ion

Free Electron

Gas Molecule

Free Electron

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ESP – Inter electrode Zone

- -+ - + -

- -+ - + -

- -+ - + -

- -+ - + -

Electron Gas Molecule Negative gas Ion

To Collecting

Plate

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ESP – Particle Charging

- + -- + -

- + -

- + -- + -

- + -

- + -

- + -

- + -

Negatively charged particle

Negative Gas Ion

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ESP – Zener & Avalanche BreakdownZener breakdownIn Zener breakdown the electrostatic attraction between the negative electrons and a large positive voltage is so great that it pulls electrons out of their covalent bonds and away from their parent atoms. i.e., Electrons are transferred from the valence to the conduction band. In this situation the current can still be limited by the limited number of free electrons produced by the applied voltage so it is possible to cause Zener breakdown without damaging the semiconductor.

Avalanche breakdownAvalanche breakdown occurs when the applied voltage is so large that electrons that are pulled from their covalent bonds are accelerated to great velocities. These electrons collide with the silicon atoms and knock off more electrons. These electrons are then also accelerated and subsequently collide with other atoms. Each collision produces more electrons which leads to more collisions etc. The current in the semiconductor rapidly increases and the material can quickly be destroyed.