By Bryant Purse, Raymond Zbacnik, Mitchell Krasnopoler, Kiewit Power Engineers
Electrostatic precipitators (ESPs) have been used for over 100 years to remove entrained solid particulates or fine mists from gas flows in the power, cement, metal production, paper, and other industries. As a response to ever changing state and federal regulations for coal-fired utility power plants, existing ESPs or new air quality control systems are needed to meet lower emission limits. Since many of these ESPs have been in operation for 30 to 50 years, many are in need of upgrades to improve performance and reliability. Even if a new pulse jet fabric filter will be installed downstream of an ESP, an ESP upgrade may be economical because the enhanced ESP could reduce maintenance costs and preserve fly ash sales. Many of the modern ESP designs and controls can be installed in old ESPs. This article will address some of the general principles of dry electrostatic precipitators and discuss modern methods for upgrading ESPs.
Basics of Electrostatic Precipitators
ESPs can be designed for high volumetric gas flow rates, variable temperatures and pressures, and variable particulate loading. Precipitators have been placed in a number of locations at different power plants, including downstream of the economizer (hot side), downstream of the air preheater (cold side), or after a wet scrubber (which requires a wet ESP). This article will focus on dry ESPs and will highlight typical upgrade options. With a properly designed electrostatic precipitator, 99% collection efficiency is possible with medium and high ash coals.
History and Principles of ESPs
The first electrostatic precipitator was developed by a physical chemistry professor, Dr. Frederick Cottrell, in 1906. He was awarded a patent for his design on August 11, 1908. Dr. Cottrell successfully demonstrated in his research the precipitation of particulates from an air stream via particle charging in an electric field. The principles in this early design still apply today. Transformer-rectifiers energize discharge electrodes with a negative potential, producing an electrical field between the discharge electrodes and the positively-grounded collecting plates.
Particulate matter that enters the electrical field develops a negative charge and migrates away from the discharge electrodes and towards the collecting plates. When the particulates reach the collection plates, the negative charge is neutralized and a cake-like layer of ash accumulates. Migration and collection of the charged particles depends upon the particulate resistivity and the electrical field between the two electrodes, as well as the gas flow profile. Particulate matter that precipitates on the collection plates is periodically removed by mechanical rapping. The fly ash falls to collection hoppers from which it is then disposed.
Reasons for ESP Upgrades
A number of factors influence ESP upgrades including performance degradation, increased maintenance, poor reliability, changes in particulate resistivity, sodium ion depletion (primarily for hot-side ESPs), volumetric flow rate changes, increased inlet particulate loading (typically due to dry sorbent injection), stricter emission regulations and compliance protocols, and even normal wear and tear such as plate warping and leaks in the casing. An ESP upgrade will improve performance and reliability, which will reduce the plant’s operating costs in the long run.
One of the primary drivers for ESP upgrades is the EPA’s new Mercury and Air Toxics Standards (MATS) regulations. The MATS ruling established a filterable particulate emission limit of 0.03 lb/MMBtu based on EPA Test Method 5.
|An example of a rigid discharge electrode. Photo courtesy of www.environmentawareness.com|
A complicating factor with the MATS regulations is that they impose stricter limits on acid gas and mercury emissions. The regulations call for an HCl emission limit of 0.002 lb/MMBtu and a mercury emission limit of 1.2 lb/TBtu. At many plants, techniques to meet these new limits include activated carbon injection (ACI) for mercury removal and perhaps dry sorbent injection (DSI) or acid gas control. Plant personnel must ensure that the existing ESP can handle the additional particulate loading.
Changes in particulate loading from ACI or DSI treatment are not the only issue. Ash resistivity (the ability of the ash to accept and transfer an electrical charge) changes with the addition of carbon or scrubber particulates. Also, resistivity may be altered due to changes in such operating conditions as temperature, moisture, and chemical composition.
Sodium Ion Depletion:
Another issue that can drive ESP upgrades is sodium ion depletion. This phenomenon influences hot side ESP performance with PRB as the fuel. The nature of the PRB ash (low sodium, low chloride, high ash), increases ash resistivity and has forced some power plants to de-rate the unit periodically and shut it down for collection plate washing. Mechanical rapping is simply not sufficient. Sodium depletion occurs when positively-charged sodium ions migrate towards the negative charge of the ionizing electrodes and form a sodium-rich layer in the collected ash. This outer layer dislodges by rapping, but the inner ash layer, depleted of sodium ions, has different characteristics making it much more difficult to remove because of the resistivity increase. The layer continues to build, reducing the ability to apply power to the field. Because sodium depletion alters the chemical properties of the ash, an ESP upgrade may not completely solve the problem, but an upgrade may combat the decrease in performance.
Due 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 plants may even be a polishing fabric filter installed downstream of the existing ESP. The configuration will be heavily dependent upon the plant’s current process conditions and the desired outlet emissions. The most common 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 ESP’s effective height to the ESP’s effective length) or fly ash/flue gas conditioning (altering the chemical/physical characteristics of the fly ash) are less common. Neither of these methods will be discussed in this article.
Upgrading the Collection Electrodes
Collection electrodes (CEs) typically are of a plate design for dry ESPs. Tubulare collection electrodes have been utilized, but these are primarily for wet ESP applications. In most cases, collection plates include stiffeners that act as baffles to prevent particle re-entrainment. Properly designed collection plates eliminate excessive rapping 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 frame or 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 the unit. New collecting plates will restore DE-to-CE spacing and alignment. New CEs may also improve rapping efficiency by decreasing rapping density and allowing increased rapping acceleration. Wider spacing along with new power supplies will increase the voltage and power input to the electric fields.
Upgrading the Discharge Electrodes
Discharge electrodes receive negative, high voltage, direct current and generate the field that charges the entrained dust particles. A simple increase of applied voltage is not necessarily a good solution because of the threat of spark-over between the discharge and collection electrodes. Spark-over causes a short-term breakdown of the electric field. It is important to design an ESP where sparking does not occur too frequently. For well-designed ESPs, sparking usually occurs between 50 and 100 times per minute.
The discharge electrode should be designed to optimize the induced electrical field, and should be customized for the individual process. In the past, the weighted wire design for the discharge electrode was common, but mechanical fatigue has caused operating problems. Although most ESP vendors prefer the tube and pin electrode design, other common rigid discharge electrodes should also be considered, such as rigid masts and rigid frame electrodes.
Selection of modern discharge electrodes, per the correct requirements for the application, can greatly improve ESP performance.
Upgrading the Rapping System
Rapping is the process by which a mechanically-induced force is applied to the collections plates to dislodge the collected ash.
For maximum efficiency, it is important to allow some buildup of dust particles and not rap the plates too frequently.
Plates are typically rapped once the dust layer reaches a thickness range of 0.03 to 0.50 inches. Rapping in this range prevents re-entrainment of ash.
One method to increase rapper efficiency is improvement of the rapper controls, in part by setting proper rapping frequencies.
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.
Increasing the number of rappers will improve the rapping system by enhancing the rapping energy. Modification of rapper placement or by dedicating existing or new rappers to fewer plates increases rapper density. Such options 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 adequate power 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 the T-R set.
However, before increasing the power to the unit, the electrode design and plate spacing may be modified instead. 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 the ESP, 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
Maldistribution 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 be exposed 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. The methods can help identify what devices are needed to optimize the flue gas flow into, through, and out of the precipitator.
As previously discussed, ESP upgrades involve many different methods that can be optimized in part or in whole. Emissions testing provides the ultimate indicator that the performance of the ESP may have declined or that 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 precipitator upgrades, 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.
“Steam, Its Generation and Use,” Chapter 33, “Particulate Control,” Babcock & Wilcox Company, Edition 41, Barberton, Ohio, 2005.
US Patent No. 895,729; “Art of Separating Suspended Particles from Gaseous Bodies,” Application Filed 9 July 1907. Awarded 11 August 1908.
US EPA Technology Transfer Network, “Particulate Matter Controls,” Section 6.
Bryant Purse is an AQCS Process Engineer for Kiewit Power Engineers of Lenexa, Kansas. He develops performance testing procedures, equipment specifications, process calculations, and supports field performance testing, proposals, and client presentations.
Ray Zbacnik is an AQCS Process Specialist for Kiewit Power Engineers of Lenexa, Kansas. Zbacnik has almost 40 years of chemical engineering experience, which includes 20 years of AQCS experience in the power industry.
Mitchell Krasnopoler serves Kiewit Power Engineers of Lenexa as the Manager of Air Quality. He has almost 25 years of engineering experience in air pollution control technologies; extensive experience in flue gas desulfurization (FGD) design, operations and testing expertise and over 30 years of design experience for various fossil and nuclear power generation projects.
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