upgrade technologies for electrostatic precipitators

7/28/2019 Upgrade Technologies for Electrostatic Precipitators

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Upgrade technologies forelectrostatic precipitatorsPosted on:Tuesday, June 18, 2013

Product descriptionMany of the precipitators in operation today were sized and designed to meet performance requirements that arefar below current requirements. Time has also taken its toll on the robust machines built many years ago. Mostunits can be upgraded, repaired or rebuilt to extend their life and improve their performance. Hamon Research-Cottrell has the design and construction experience to insure that modifications meet the objectives of todaysprecipitator operators. Hundreds of precipitators have been modified to meet more stringent performancerequirements. Units built in the 1960s and 70s can often be modified to provide 15% to 20% more collectingsurface without increasing the footprint of the unit. The size of European design precipitators can beincreased 30% and more..

Other performance improvement strategies include: Increasing the width of the gas passages and replacing wires and weights with rigid discharge

electrodes will improve the reliability of the unit, as there will be no wires to break. The precipitatorperformance will also be improved. Hamon Research-Cottrell has developed and installed pipe andspike electrodes designed to provide the corona generating characteristics needed for improvedperformance with out the replacement of the existing transformer-rectifier sets.

Precipitator Collection efficiency:Gas Flow Reduction: A significant improvement in the performanceof a precipitator installed on coal fired utility steam generators by reducing the gas flow to the unit. ManyLjungstrom style air preheaters in service today experience leakage rates of from 15% to 30%. HamonResearch-Cottrell, in partnership with its sister company Hamon Rothemuhle-Cottrell, can provide apatented automatic sealing system that can reduce the leakage to less than 10% and in some cases aslow as 3.5%. The reduction in gas flow to the precipitator can result in a significant reduction inoutlet emissions.

o Ep = 1-e-(RB)o R = Performance constant

o B = Bus sections


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Increasing the number of electrical bus sections will further improve the collection efficiency of theprecipitator.

Decreasing the amount of collecting surface that is served by each rapper.

Upgrade TechnologiesDue to the above-mentioned factors and stricter environmental regulations, many existing ESPs will have to

either be upgraded or converted to fabric filter devices, aka baghouses. A potential option at some plantsmay even be a polishing fabric filter installed downstream of the existing ESP. The configuration will beheavily dependent upon the plants current process conditions and the desired outlet emissions. The mostcommon methods of improving an electrostatic precipitator are:

Upgrading the collection electrodes Upgrading the discharge electrodes Upgrading the rapping system Upgrading the transformer-rectifier assemblies Improving the flow distribution

Other methods such as adjusting the aspect ratio (the ratio of the ESPs effective height to the ESPs effective

length) or fly ash/flue gas conditioning (altering the chemical/physical characteristics of the fly ash) are lesscommon. Neither of these methods will be discussed in this article.

Upgrading the Collection ElectrodesCollection electrodes (CEs) typically are of a plate design for dry ESPs. Tubulare collection electrodes havebeen utilized, but these are primarily for wet ESP applications. In most cases, collection plates include stiffenersthat act as baffles to prevent particle re-entrainment. Properly designed collection plates eliminate excessiverapping and ensure equal distribution of the rapping force throughout the plate.

Collection electrode design should be correlated with the discharge electrode (DE) design. For ESPs with

weighted wire discharge electrodes, typical plate spacing is 6 to 12 inches. Many modern ESPs have rigid frameor plate discharge electrodes, and in these designs the typical plate spacing is 12 to 16 inches.

A common practice for upgrading an ESP is the alteration of the plate spacing to increase the efficiency of theunit. New collecting plates will restore DE-to-CE spacing and alignment. New CEs may also improve rappingefficiency by decreasing rapping density and allowing increased rapping acceleration. Wider spacing along withnew power supplies will increase the voltage and power input to the electric fields.

Upgrading the Discharge Electrodes

Discharge electrodes receive negative, high voltage, directcurrent and generate the field that charges the entrained dust particles.A simple increase of applied voltage is not necessarily a good solutionbecause of the threat of spark-over between the discharge andcollection electrodes. Spark-over causes a short-term breakdown ofthe electric field. It is important to design an ESP where sparking doesnot occur too frequently. For well-designed ESPs, sparking usuallyoccurs between 50 and 100 times per minute.


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Rapping is the process by which a mechanically-induced force is applied to the collections plates to dislodge thecollected ash.

For maximum efficiency, it is important to allow some buildup of dust particles and not rap the plates toofrequently.

Plates are typically rapped once the dust layer reaches a thickness range of 0.03 to 0.50 inches. Rapping in thisrange prevents re-entrainment of ash.

One method to increase rapper efficiency is improvement of the rapper controls, in part by setting proper rappingfrequencies.

Pneumatic Rapping Device Mechanical Rapping Device

Impact energy

The inlet collection plates need to be rapped more frequently than those in the outlet fields. Also, rapping

discharge electrodes at a proper frequency to prevent dust accumulation on these instruments is important.Fine-tuning of existing rappers and controls may avoid the issue, and cost, of installing new rappers.


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Increasing the number of rappers will improve the rapping system by enhancing the rapping energy. Modificationof rapper placement or by dedicating existing or new rappers to fewer plates increases rapper density. Suchoptions for an existing system should be evaluated before investing in completely new rappers.

Upgrading the Transformer-Rectifier Assemblies

A critical component of a precipitator is the high-voltage equipment, consisting of a step-up transformer, a high-

voltage rectifier, and control metering and protection circuitry. The system must be designed to ensure adequatepower to the discharge electrodes without causing excessive sparking.

Depending upon the required operating conditions of the ESP, an upgrade can be as simple as modernizing theT-R set.

However, before increasing the power to the unit, the electrode design and plate spacing may be modifiedinstead. The most common upgrade utilizes a three-phase, high frequency switch mode power supplies (SMPS),with control system adjustment to prevent excessive spark-over. This upgrade efficiently delivers power to theESP, maximizes the average voltage of the ESP, and reduces the frequency of sparking.

However, utilizing a switch mode power supply may not be feasible for every application.

Improving the Flow Distribution


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Mal distribution of flue gas flow can lead to degraded performance of the ESP.

Variable flue gas flow changes the particle distribution throughout the unit. Thus, some areas of the ESP may beexposed to a greater gas flow and particulate loading that exceeds local collection capabilities.

Flow distribution devices can be installed to normalize the flows to all ESP chambers. Normalizing the flow will

prevent sneakage of untreated gas around the collecting fields.

Physical and Computational Fluid Dynamic (CFD) modeling are tools for analyzing an ESP flue gas profile. Themethods can help identify what devices are needed to optimize the flue gas flow into, through, and out of theprecipitator.

ConclusionAs previously discussed, ESP upgrades involve many different methods that can be optimized in part or inwhole. Emissions testing provides the ultimate indicator that the performance of the ESP may have declined orthat the unit will not perform to meet new regulations.

However, the cause of performance degradation may not always be clear.

Simply replacing one component may not increase ESP efficiency. Due to the complicated nature of precipitatorupgrades, the existing ESP should be evaluated and studied.

This evaluation requires the following:

Understand boiler feed coal chemistry Study the physical and chemical properties of the fly ash Evaluate the original process design conditions and current operating conditions Review the flue gas profile/flow distribution

Examine the existing ESP casing and internals for corrosion Examine the structural integrity of the ESP foundation Inspect ESP casing and components for wear Evaluate the control systems Consider site layout/configuration limitations Evaluate the ESP electrical system and electrical characteristics of the ESP.

upgrade technologies for electrostatic precipitators

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