Presented at the Environment & Innovation in Mining and Mineral Technology Conferenece
Santiago, Chile - May 1998

ABSTRACT

Electrostatic Precipitators (ESPs) have been in use for particle emission control for over 90 years and are utilized in many different industrial applications. With new regulations and the high cost of new pollution control equipment, many end users are looking for ways to upgrade their existing equipment. Unfortunately, there are many equipment suppliers and end users who do not understand all that is required to properly design and operate electrostatic precipitators to achieve maximum efficiency and life expectancy. There have also been many instances where the interests of the supplier do not represent the best interests of the end-user. This has resulted in a worldwide trend of replacing old equipment with oversized new equipment at great expense to the end user. A solid understanding of how ESPs operate, and what is required to maintain the proper electrical input, is critical to cost effective upgrading and rebuilding of an ESP.

INTRODUCTION

With increasing concerns about global warming, air quality, and acid rain, more pressure than ever before is being placed on reducing the emissions from industrial processes. Often, it is government agencies imposing more stringent regulations on industry that require further reduction of emissions. Sometimes, process or fuel changes result in increased emissions that must be reduced to maintain compliance with current regulations. Other times, industry takes it upon themselves to reduce emissions to be "good neighbors" and reduce public scrutiny. Over the past ten years, on a worldwide basis, electrostatic precipitator (ESP) rebuilds with performance upgrades have become a popular alternative to ESP replacement as a way of reducing particulate emissions. Where conditions permit, rebuilding and upgrading an ESP can result in significant cost savings over complete replacement with totally new systems. This paper describes some of the rebuild and upgrade options available to the ESP user to enhance precipitator performance and maintain compliance with all pollution control regulations.

BASIC CONCEPTS OF ELECTROSTATIC PRECIPITATION

To begin, it is useful to gain an understanding of the electrostatic precipitation process by studying the relationship known as the Deutsch-Anderson equation. The Deutsch-Anderson equation describes the factors involved in the collection efficiency of a precipitator. Although the basic formula neglects a number of practical and empirical considerations that should be introduced for good design, it is useful to illustrate the effect of rebuild modifications on precipitator efficiency. In its simplest form the Deutsch-Anderson equation establishes a relationship between gas volume, collecting plate area and migration velocity:

Collection Efficiency (%) N = 1 - e(-A/V)W x 100

Where:
A = effective collecting surface area of the precipitator
V = gas flow rate through the precipitator
e = base of natural logarithm = 2.718
W = migration velocity

The exponent W represents the migration velocity; the speed of movement of a particle toward the collector surface under the influence of an electrical field.

Migration velocity W = aEoEp / 2 p q

Where:
a = particle radius, microns
E_O = strength of field in which particles are charged (represented by peak voltage)
E_P = strength of field in which particles are collected (normally the field close to the collecting plates)
q = viscosity, or frictional resistance coefficient of the gas

This equation clearly indicates the squaring effect voltage (V = E x d) has on the collection efficiency of a precipitator. It also emphasizes the effect particle size has on the overall ESP efficiency; as particles size becomes finer, higher voltages are required to maintain the same collection efficiency.

When evaluating the upgrade options for any ESP system it is best to have a clear picture of how these options effect the relationships defined by the Deutsch-Anderson equation. The following sections discuss the standard methods available to upgrade ESP systems and their effect as they pertain to the Deutsch- Anderson equation.

REBUILD AND UPGRADE OPTIONS

There are many options available to improve ESP efficiency and reduce emissions. Some combination of the following methods have been used to reduce emissions on many different process applications.

• Modification of boiler / process operation
• Modification of gas distribution
• Reduce system air infiltration
• Upgrade of automatic voltage controls
• Improve system grounding
• Matching of power supplies to loads
• Additional electrical sectionalization
• Gas conditioning (SO₃, ammonia, proprietary chemical)
• Humidification (moisture conditioning)
• Additional collecting area within existing casing using compact design
• Increasing plate height to increase collecting area
• Modification of rappers and rapping control
• Modification of discharge electrode geometry and spacing
• Upgrade of auxiliary equipment
• Control particle size in Wet ESPs
• Minimize heat losses

Each of the sixteen items listed above will be discussed briefly in the sections below.

Modification of Boiler / Process Operation

Before considering a major capital expense of improving the ESP, a close look at the process operation is important. Sometimes, a minor change in process operation can result in a change of particulate chemistry, size and/or resistivity to significantly improve ESP efficiency. As an example, keeping the boiler tubes clean can improve heat transfer and result in reduced flue gas temperatures. The reduced temperatures will alter dust resistivity and allow efficiency to increase. Modification of soot blowing equipment and/or schedule may be a cost-effective technique to achieve this. Reducing gas volume by firing a furnace with O₂ rather than ambient air can improve efficiency by effectively making the ESP bigger. The reduced gas flow will decrease the gas flow rate through the ESP, reducing the "V" component of the Deutsch equation. Modifying the dust resistivity can effect the voltage input, effecting the migration velocity "W" component of the Deutsch equation.

Modification of Gas Distribution

In traditional theory, ESP efficiency is considered maximized if gas distribution is uniform; that is, the same velocity across the face of the ESP from inlet to outlet. In reality, perfectly uniform gas distribution is not possible in industrial ESPs. In the United States, gas distribution is generally required to meet the standards of the Institute of Clean Air Companies (ICAC). This standard requires 85% of all velocity readings be not more than 1.15 times the average, and that 99% of all readings be not more than 1.40 times the average. European ESP suppliers providing equipment outside of the US have not had standardized gas distribution requirements. These suppliers generally have met gas distribution criteria that is much less stringent than that required in the US. European suppliers typically place low priority on gas distribution as long as the ESP meets its efficiency or emission guarantee. Usually, the gas distribution of European supplied ESPs can be improved, and doing so can enhance ESP performance. Reduction of localized zones of high velocity and gas sneakage around the active collection zones is vital to ESP efficiency.

Recently, the traditional theory of maximizing the uniformity of gas distribution has been questioned. The concept of skewing gas distribution from top to bottom on the ESP and changing the skew from inlet to outlet is being offered. There have been several cases of documented ESP performance enhancement with skewed distribution. ESKOM, the largest electric utility in South Africa, is doing extensive investigation of this concept and has met with some good results. However, worldwide, the uniform distribution philosophy still dominates.

Improving gas distribution will affect the gas flow rate through the ESP and migration velocity of the particulate, the V and W components of the Deutsch Equation, and improve ESP collection efficiency.

Reduce System Air Infiltration

Air infiltration is an obvious but often overlooked source of precipitator performance problems. Although not specifically an upgrade method, identifying and eliminating sources of air inleakage on negative draft systems can have a profound effect on ESP performance. Air inleakage not only promotes corrosion of the precipitator and internal components, it can adversely affect ESP performance by increasing gas volume and velocity, distorting gas distribution, reducing treatment time, affecting gas temperature (particle resistivity) and causing reentrainment of particulate. Eliminating air inleakage can be the difference between acceptable and unacceptable performance on a marginally sized unit. Identifying and eliminating air inleakage should not only be a part of any unit upgrade, it should be part of routine maintenance to maintain optimum performance of an ESP.

Upgrade of Automatic Voltage Controls

Modern microprocessor based controls should replace old analog controls and modern silicon controlled rectifiers should replace old saturable core reactor power control elements. Microprocessor based controls offer faster response to sparking and can operate effectively over broader operating conditions than analog controls. They also have energization and performance features such as intermittent energization and back-corona detection that can improve ESP performance. Their self diagnostic features make them easier to troubleshoot and maintain.

Future upgrades of new technology can be made with a simple microprocessor chip change. Microprocessor controls can also be linked to a central computer control for remote data acquisition and control, and the data can be fed directly to the plant process control computers. Comparing the process trending to the ESP electrical data is a useful tool to optimize the rapper programs and identify process conditions that effect the ESP performance. Proper design and field adjustment of the AVC controller should increase voltage in the ESP fields while protecting the equipment from damage. This will improve the migration velocity (W) of the particles.

Improve System Grounding

The majority of microprocessor automatic voltage controls (AVC) available are much more sensitive to electrical noise than their predecessors. Any extraneous noise may corrupt the signal to the controller, and cause it to react erroneously to the conditions that actually exist in the collection zone of the precipitator. The result is reduced performance. Upgrading the precipitator structure, transformer rectifier (TR) and auxiliary equipment grounding may be in order. Proper grounding helps to dissipate sparking and eliminate noise interference in the voltage control signal. Individual ground cables should be run from each TR set to the ground grid at grade. Two grounds should be used in the AVC control cabinets, one for the AVC controls and another for the low voltage components and physical cabinet. The AVC control cabinets can be daisy chained together and taken to the ground grid with one of the grounds. The low voltage components and cabinets can be daisy chained together and taken to the ground grid with the other ground. The rapper controls, rappers and other auxiliary equipment can be tied to a common ground at grade. Grounding of the precipitator structure should also be reviewed. There should be an intact ground connecting the support structure to the ground grid at grade. There should also be ground jumpers across the slide pads between the support steel and precipitator columns; at the four corners (minimum). Good grounding is essential to optimum ESP performance and should be a part of any upgrade. It is important to use 4/0 size cable or larger for all runs to grade to help dissipate the high frequency energy released during a spark in the ESP.

Matching of Power Supplies to Loads

ESP performance can be enhanced with matching of power supplies to loads. Mismatching may be a result of improper application initially, or from changes in operation since initial operation. Power supply mismatch occurs when operating and rated current and voltage levels are significantly different. When operating at low voltage levels the automatic voltage control keeps the firing angle of the silicon controlled rectifiers at a low level, allowing more time for the KV signal to degrade. This reduces the average electric field (KV) applied to the particle. On the other hand, low current levels cause the current conduction time to be reduced, decreasing the time ions are being discharged into the gas stream and through the dust layer. This may also reduce particle charging and reduce the bonding force of the dust layer, dramatically effecting ESP performance. The maximum current conduction time should be 85% of the normal AC cycle. It is typically determined by the Transformer Rectifier (TR) design and the CLR sizing. A mismatch can sometimes be corrected by adding more impedance by changing the current linear reactors (CLRs), or by changing to different electrode geometries.

Changing the Transformer Rectifier (TR) size or switching the primary taps on the TR are other methods used to optimize the voltage and current waveforms. Sometimes power supplies should be replaced because of age or because they contain PCBs that are known to be carcinogens. This is a good time to investigate matching the power supplies with the existing loads. Purchasing new Transformer Rectifiers (TRs) with taps on the primary winding can allow flexibility to match the power supply to different load conditions. Typically, a +/- 20% spread at the systems feed voltage provides the maximum flexibility.

Additional Electrical Sectionalization

Many different studies and evaluations have shown that increased sectionalization of the electrical fields enhances ESP performance. Intuitively, the smaller the electrical section, the less effect any localized disturbance, such as sparking or heavy dust load, has on the remainder of the ESP. European suppliers of rigid frame ESP tend to utilize large electrical sections because their design requires it. Sectionalization across and in the direction of gas flow is limited. The use of RDE technology in the rebuild and upgrade of ESPs, will result in increased sectionalization by the very nature of its design and sectionalization can be specified. Additionally the improved sectionalization helps provide more reliable ESP performance.

Although it has no direct effect on the Deutsch-Anderson equation, increased sectionalization of an ESP improves reliability and reduces the overall effect on performance of any anomaly that exists in a particular electrical section of the precipitator allowing voltage to be maximized.

Gas Conditioning

Although gas conditioning is not a performance enhancement option normally used by the copper industry, it has been effectively used in other industries. Its most widespread use is in the power industry on coal fired boilers burning low sulfur coal. SO₃ conditioning is most commonly used world wide to reduce particle resistivity and improve ESP efficiency. For coal ash applications, the combination of SO₃ and ammonia conditioning has been even more effective than S0₃ alone. Ammonia has been typically used to improve particle agglomeration and reduce reentrainment on high carbon ash or high face velocity applications. Ammonia has also proven itself for resistivity modification on highly acidic particulate such as fluidized cat cracking applications. There are also several proprietary chemical conditioners that offer resistivity modification and agglomeration properties that can be cost competitive with the more traditional treatment technologies.

Humidification (Moisture Conditioning)

Humidification of flue gas to reduce resistivity has been common in the cement industry for many years. It has been used effectively for years in the copper industry and more recently in coal fired boiler applications. Humidification is accomplished by water injection that results in a drop in gas temperature caused by the evaporation of the injected water. The reduction in gas temperature can improve ESP efficiency by modifying resistivity and reducing gas volume, the "V" exponent in the Deutsch-Anderson equation.

Additional Collecting Area in Existing Casing

Although increasing the existing casing by increasing its width, length, or height is always an option, it is not the most cost-effective means of achieving improved ESP performance. Converting an internally rapped rigid frame European design ESP to a top rapped American Rigid Discharge Electrode (RDE) design can produce a larger specific collecting area. European rigid frame designs typically have large spaces between fields to accommodate internal rapping and to maintain electrical clearances between high voltage and grounded components. These spaces can often be used to add collecting area to an existing ESP casing when rebuilding with the RDE design and external rapping.

Adding collecting area will increase the effective collecting surface area of the precipitator, the "A" exponent in the Deutsch-Anderson equation, and improve the collection efficiency capability of a precipitator system.

Increasing Plate Height

Sometimes the field height is also increased with retrofit of the rigid frame to RDE technology. Increasing field height is usually the most economical way of increasing casing size, compared to increasing its width or length. Frequently, other plant equipment prevents enlargement of the casing in length or width, but often, an increase in the height is not restricted. Modifications to the precipitator inlet and outlet and the gas distribution may be required to effectively utilize this option. A drawback to this option is that it reduces the aspect ratio of the precipitator (ratio of total length to the height of the collector surface), a criteria used to evaluate the performance margin of an ESP.

Adding plate height will increase the effective collecting surface area of the precipitator, the "A" exponent in the Deutsch-Anderson equation, and improve the collection efficiency capability of a precipitator system.

Modification of Rapping and Rapping Controls

Rigid frame ESPs traditionally use mechanical tumbling and falling hammer rapping. These types of mechanical rapping systems offer little to no flexibility in rapping frequency and intensity to respond to changing operating conditions. Often, the entire electrical section of discharge electrodes is rapped at one time. This, combined with large electrical sections, can result in significant particle reentrainment. Tumbling hammer rapping of the collecting electrodes also can result in "over rapping" of a low resistivity particulate and produce excessive particle reentrainment. Rapping reentrainment can account for over two-thirds of the emissions from a high efficiency ESP. On the other hand, with a high resistivity particulate, "under rapping" may occur. Tumbling hammer rapping intensity cannot be adjusted, except by the costly and time consuming task of changing the hammers during an outage. Drop or falling hammer rapping systems suffer from similar limitations. They are usually more adjustable, but each mechanism must be individually adjusted.

With the retrofit of European design ESPs to the American design RDE technology rappers are located external to the gas stream and are accessible for maintenance. Use of magnetic impulse gravity impact rapping systems allow for considerable operating flexibility and easy adjustability. Gravity impact rappers use a cylindrical slug of metal to impart the rapping forces to the electrodes. The 10-20 pound (4.5-9 kg) slug is lifted by magnetic force from an electrically energized coil surrounding it. When the coil is deenergized, the slug falls by gravity to impact the rapping rods. The length of energization time of the coil determines the lift height of the slug, and thus, the rapping intensity. Since each rapper coil is independently energized by a control panel, each rapper can be individually adjusted simply and easily. Intensity, frequency and number of impacts per rap can be adjusted, making this a flexible system that minimizes rapping reentrainment.

Minimizing the effects of abnormal levels of material buildup on the collecting surfaces and discharge electrodes will influence the voltage characteristics of the circuit, which improves the migration velocity (W) for improved collection efficiency. Reducing reentrainment, although not accounted for in the basic Deutsch equation, will improve efficiency. Reentrainment is a factor in a modified version of the Deutsch equation.

Modification of Plate Spacing

Traditional Deutsch ESP theory says that ESP efficiency is maximized with collecting area. This theoretically means that for a given size casing volume, the efficiency can be maximized with close plate spacing. However, for many years now, the theory of wide plate spacing has been accepted, and use of spacing up to 16 inches (400mm) is common whereas in past, spacing of 9-10 inches (225-250 mm) was the norm. It has been shown that migration velocity increases at least proportionally with plate spacing, keeping the efficiency nearly the same for any plate spacing between these values. With wider spacing, less internal components are required, making it an attractive rebuild option. Additionally, with wider plate spacing, alignment and electrode straightness is relatively less critical. Larger transformer rectifiers may be required with the widened plate spacing, and other considerations such as insulator properties should not be overlooked.

There are applications where wide plate spacing is not recommended. For high particle concentrations and for fine particle size, wide spacing is not recommended. A phenomenon known as "space charge effect" inhibits the charging and collection of particulate under these conditions. Plate spacing may be varied in the ESP with narrower spacing in the inlet and wider spacing in the outlet. Also, more "aggressive" discharge electrodes may be used.

Manipulating the plate spacing influences components of the migration velocity (W), and can improve ESP collection efficiency.

Modification of Discharge Electrode Geometry

As mentioned in the section above on plate spacing, electrode geometry can be varied from inlet to outlet. More aggressive electrodes that have the ability to produce large amounts of corona are necessary in the inlet fields to adequately charge the particulate. An aggressive electrode is characterized by having spiked emitters, a larger quantity of spikes, more pronounced spikes, and/or spikes oriented toward the collecting plate. This is particularly important in applications with very fine particles or large quantities of particles. These conditions tend to make it difficult for less aggressive, smoother electrodes to generate adequate corona. In the case of large particle concentrations that are removed in the first few fields, a less aggressive corona producing electrode is preferred to enhance operating voltages and electric field strengths.

Manipulating the plate spacing influences components of the migration velocity (W), and can improve ESP collection efficiency.

Upgrade of Auxiliary Equipment

Although auxiliaries do not have a major effect on ESP performance, their failure or improper operation can cause component failure or create problems that will affect performance. When rebuilding and upgrading an ESP, some consideration should be given to the auxiliary equipment. Upgrade of hopper heaters, hopper level indicators, hopper vibrators, and hopper evacuation systems can minimize ash buildup problems that could lead to loss operation of electrical sections. Door seals, expansions joints and dampers should also be considered for upgrade. If a purge system is used for the high voltage support insulators, it should be sized appropriately for the rebuilt ESP to deliver sufficient air volume to properly purge the insulator and housing.

Control Particle Size in Wet ESPs

Particle size has a significant affect on ESP performance. Fine particles are difficult to collect and can charge and discharge several times as they past through the ESP. If possible process modifications should be done to increase particle size distribution entering the ESP while maintaining good process control and combustion. In Wet ESPs this typically can be done by installing misting sprays in the inlet of the ESP, or in the ductwork immediately before the ESP. The mist droplet sizes should be controlled to produces 10 - 20 micron particles. Although this increases the load to the ESP, the overall efficiency of the ESP is increased because the smaller particles agglomerate to the larger particles which are more easily collected. The misting sprays also can help to keep the collecting tubes in a Wet ESP conductive which helps reduce sparking for higher average power levels.

Minimize Heat Losses

REBUILD AND UPGRADE EXAMPLE

The best rebuild and upgrade options are site specific. Any combination of the above modifications can be made based on technical and economic considerations. The following is an example of a hypothetical but somewhat typical case. Consider a three field ESP of European rigid frame design operating on a flash smelter. The flue gas temperature is 430 degrees C. The following actions might be recommended.

It is not hard to visualize that the above modifications could improve ESP efficiency to 99.5% from its original 97%. The increase could be even greater if the ESP to be rebuilt is in poor condition.

SUMMARY

Rebuilding and upgrading an existing ESP can be a cost effective alternative to building a completely new ESP. Savings can arise from the reuse of foundations, support steel, nozzles, ductwork, casing, hoppers, ash handling equipment, access, and insulation and lagging. The above rebuild and upgrade techniques are proven technologies that have already been successfully used worldwide.

A key to rebuild and upgrade technology is the ability to predict the performance of the modified equipment with a high degree of confidence. This is not a task for the inexperienced. With extensive theoretical understanding of ESPs, a good database of experience, and specialized modeling capabilities, the performance improvement can be predicted with the high degree of confidence necessary to make this a viable alternative to more costly new equipment.

REFERENCES

1.) White, H.J. "Industrial Electrostatic Precipitation."
2.) Katz, J. "The Art of Electrostatic Precipitation."
3.) Katz, J. "Operation and Maintenance Seminar Manual", Precipitator Seminars