The Neundorfer KnowledgeBase is an industry-leading information resource about electrostatic precipitators. The Introduction to Precipitators is a great starting point for background information, or proceed directly to specific topic areas of interest.
The downloadable manuals at the right are made available by the Environmental Protection Agency (EPA) at www.epa.gov and provide detailed information about electrostatic precipitator design, operation and maintenance.
About Electrostatic Precipitators
Introduction to Precipitators
About Precipitator Operating Theory
Design & Performance Requirements
About Precipitator Components
Power Supplies and Controls
Gas Distribution Systems
Hoppers and Dust Handling
Heaters and Purge Air Systems
About Precipitator Performance
Combustion Process Improvements (Power Plants)
Flue Gas/Fly Ash Conditioning (Power Plants)
Introduction to Precipitators (Back to top)
An electrostatic precipitator is a large, industrial emission-control unit. It is designed to trap and remove dust particles from the exhaust gas stream of an industrial process. Precipitators are used in these industries:
In many industrial plants, particulate matter created in the industrial process is carried as dust in the hot exhaust gases. These dust-laden gases pass through an electrostatic precipitator that collects most of the dust. Cleaned gas then passes out of the precipitator and through a stack to the atmosphere. Precipitators typically collect 99.9% or more of the dust from the gas stream.
Precipitators function by electrostatically charging the dust particles in the gas stream. The charged particles are then attracted to and deposited on plates or other collection devices. When enough dust has accumulated, the collectors are shaken to dislodge the dust, causing it to fall with the force of gravity to hoppers below. The dust is then removed by a conveyor system for disposal or recycling.
Depending upon dust characteristics and the gas volume to be treated, there are many different sizes, types and designs of electrostatic precipitators. Very large power plants may actually have multiple precipitators for each unit.
Basic Principles (Back to top)
Electrostatic precipitation removes particles from the exhaust gas stream of an industrial process. Often the process involves combustion, but it can be any industrial process that would otherwise emit particles to the atmosphere. Six activities typically take place:
- Ionization - Charging of particles
- Migration - Transporting the charged particles to the collecting surfaces
- Collection - Precipitation of the charged particles onto the collecting surfaces
- Charge Dissipation - Neutralizing the charged particles on the collecting surfaces
- Particle Dislodging - Removing the particles from the collecting surface to the hopper
- Particle Removal - Conveying the particles from the hopper to a disposal point
The major precipitator components that accomplish these activities are as follows:
- Discharge Electrodes
- Power Components
- Precipitator Controls
- Rapping Systems
- Purge Air Systems
- Flue Gas Conditioning
Design & Performance Requirements (Back to top)
Designing a precipitator for optimum performance requires proper sizing of the precipitator in addition to optimizing precipitator efficiency. While some users rely on the precipitator manufacturer to determine proper sizing and design parameters, others choose to either take a more active role in this process or hire outside engineering firms.
Precipitator performance depends on its size and collecting efficiency. Important parameters include the collecting area and the gas volume to be treated. Other key factors in precipitator performance include the electrical power input and dust chemistry.
- Precipitator sizing
The sizing process is complex as each precipitator manufacturer has a unique method of sizing, often involving the use of computer models and always involving a good dose of judgment. No computer model on its own can assess all the variables that affect precipitator performance.
- Collecting Efficiency
Based on specific gas volume and dust load, calculations are used to predict the required size of a precipitator to achieve a desired collecting efficiency.
- Power Input
Power input is comprised of the voltage and current in an electrical field. Increasing the power input improves precipitator collecting efficiency under normal conditions.
Process Variables (Back to top)
Gas characteristics and particle properties define how well a precipitator will work in a given application. The main process variables to consider are:
- Gas flow rate
The gas flow rate in a power plant is defined by coal quality, boiler load, excess air rate and boiler design. Where there is no combustion, the gas flow rate will have process-specific determinants.
- Particle size and size distribution
The size distribution in a power plant is defined by coal quality, the coal mill settings and burner design. Particle size for non-combustion processes will have similar determinants.
- Particle resistivity
The resistivity of fly ash or other particles is influenced by the chemical composition and the gas temperature.
- Gas temperature
Following are details of these process variables:
- Gas Flow Rate
A precipitator operates best with a gas velocity of 3.5 - 5.5 ft/sec. At higher velocity, particle re-entrainment increases rapidly. If velocity is too low, performance may suffer from poor gas flow distribution or from particle dropout in the ductwork.
- Particle Size
A precipitator collects particles most easily when the particle size is coarse. The generation of the charging corona in the inlet field may be suppressed if the gas stream has too many small particles (less than 1 µm).
Very small particles (0.2 - 0.4µm) are the most difficult to collect because the fundamental field-charging mechanism is overwhelmed by diffusion charging due to random collisions with free ions.
- Particle Resistivity
Resistivity is resistance to electrical conduction. The higher the resistivity, the harder it is for a particle to transfer its electrical charge. Resistivity is influenced by the chemical composition of the gas stream, particle temperature and gas temperature. Resistivity should be kept in the range of 108 - 1010 ohm-cm.
High resistivity can reduce precipitator performance. For example, in combustion processes, burning reduced-sulfur coal increases resistivity and reduces the collecting efficiency of the precipitator. Sodium and iron oxides in the fly ash can reduce resistivity and improve performance, especially at higher operating temperatures.
On the other hand, low resistivity can also be a problem. For example (in combustion processes), unburned carbon reduces precipitator performance because it is so conductive and loses its electrical charge so quickly that it is easily re-entrained from the collecting plate.
- Gas Temperature
The effect of gas temperature on precipitator collecting efficiency, given its influence on particle resistivity, can be significant.
- Interactions to Consider
Particle size distribution and particle resistivity affect the cohesiveness of the layer of precipitated material on the collecting plates and the ability of the rapping system to dislodge this layer for transport into the precipitator hopper without excessive re-entrainment.
About Discharge Electrodes (Back to top)
Discharge electrodes emit charging current and provide voltage that generates an electrical field between the discharge electrodes and the collecting plates. The electrical field forces dust particles in the gas stream to migrate toward the collecting plates. The particles then precipitate onto the collecting plates. Common types of discharge electrodes include:
- Straight round wires
- Twisted wire pairs
- Barbed discharge wires
- Rigid masts
- Rigid frames
- Rigid spiked pipes
- Spiral wires
Discharge electrodes are typically supported from the upper discharge frame and are held in alignment between the upper and lower discharge frames. The upper discharge frame is in turn supported from the roof of the precipitator casing. High-voltage insulators are incorporated into the support system. In weighted wire systems, the discharge electrodes are held taut by weights at the lower end of the wires.
About Collecting Plates (Back to top)
Collecting plates are designed to receive and retain the precipitated particles until they are intentionally removed into the hopper. Collecting plates are also part of the electrical power circuit of the precipitator. These collecting plate functions are incorporated into the precipitator design. Plate baffles shield the precipitated particles from the gas flow while smooth surfaces provide for high operating voltage.
Collecting plates are suspended from the precipitator casing and form the gas passages within the precipitator. While the design of the collecting plates varies by manufacturer, there are two common designs:
- Plates supported from anvil beams at either end
The anvil beam is also the point of impact for the collecting rapper
- Plates supported with hooks directly from the precipitator casing
Two or more collecting plates are connected at or near the center by rapper beams, which then serve as impact points for the rapping system
Top, center, or bottom spacer bars may be used to maintain collecting plate alignment and sustain electrical clearances to the discharge system.
About Power Supplies and Controls (Back to top)
The power supply system is designed to provide voltage to the electrical field (or bus section) at the highest possible level. The voltage must be controlled to avoid causing sustained arcing or sparking between the electrodes and the collecting plates.
Click here to view a precipitator power system animated schematic showing representative components.
Electrically, a precipitator is divided into a grid, with electrical fields in series (in the direction of the gas flow) and one or more bus sections in parallel (cross-wise to the gas flow). When electrical fields are in series, the power supply for each field can be adjusted to optimize operation of that field. Likewise, having more than one electrical bus section in parallel allows adjustments to compensate for their differences, so that power input can be optimized. The power supply system has four basic components:
- Automatic voltage control
- Step-up transformer
- High-voltage rectifier
- Sensing device
- Voltage control
Automatic voltage control varies the power to the transformer-rectifier in response to signals received from sensors in the precipitator and the transformer-rectifier itself. It monitors the electrical conditions inside the precipitator, protects the internal components from arc-over damages, and protects the transformer-rectifier and other components in the primary circuit.
The ideal automatic voltage control would produce the maximum collecting efficiency by holding the operating voltage of the precipitator at a level just below the spark-over voltage. However, this level cannot be achieved given that conditions change from moment to moment. Instead, the automatic voltage control increases output from the transformer-rectifier until a spark occurs. Then the control resets to a lower power level, and the power increases again until the next spark occurs.
Automatic Voltage Controllers (for Electrostatic Precipitators)
An electronic device used to control the application of D.C. power into a field of an electrostatic precipitator. (PIC OF MVC4 FACE PANEL AND PIC OF INTERFACE BOARD)
• Optimize power application – The primary purpose of a voltage controller is to deliver as much useful electrical power to the corresponding electrostatic precipitator field(s) as possible. This is not an easy job; electrical characteristics in the field(s) are constantly changing, which is why a voltage controller is required.
• Spark reaction – When the voltage applied to the electrostatic precipitator field is too high for the conditions at the time, a spark over (or corona discharge) will occur. Detrimentally high amounts of current can occur during a spark over if not properly controlled, which could damage the fields. A voltage controller will monitor the primary and secondary voltage and current of the circuit, and detect a spark over condition. Once detected, the power applied to the field will be immediately cut off or reduced, which will stop the spark. After a short amount of time the power will be ramped back up, and the process will start over.
• Protect system components by adhering to component limitations – The Transformer Rectifier set (TR set) can be damaged by excessive amounts of current or voltage flowing through it. Each TR set has voltage and current limits established by the manufacturer, which are labeled on an attached nameplate (PIC OF A NAMEPLATE). These nameplate limit values (typically primary and secondary current, and voltage) are programmed into the voltage controller. Through metering circuits, the voltage controller will monitor these values, and ensure these limits are not exceeded.
• Tripping – When a condition occurs that the voltage controller cannot control, often times the voltage controller will trip. A trip means the voltage controller (by way of the contactor) will shut off the individual precipitator power circuit. A short inside the electrostatic precipitator field caused by a fallen discharge electrode (wire), or a shorted out Silicone Controlled Rectifier are examples of conditions that a voltage controller cannot control. (PIC OF CLOSE-UP OF TRIP LIGHT ON MVC4 FACE PANEL)
To maximize electrostatic precipitator efficiency a voltage controller usually attempts to increase the electrical power delivered to the field. However in some conditions a voltage controller must just maintain power at a constant level. Increased electrical power into the electrostatic precipitator directly correlates with better precipitator performance, but there is a limit. If too much voltages is applied for a given condition (as mentioned in the spark reaction section), a spark over will occur. During a spark over precipitator performance in that field will drop to zero, rendering that field temporarily ineffective.
To overcome the crippling effect that spark over has to increasing the electrical power in the precipitator field, spark response algorithms have been developed that will interrupt power upon detection of a spark, then ramp power back up to a high level. These response algorithms can greatly influence overall precipitator performance.
The transformer-rectifier rating should be matched to the load imposed by the electrical field or bus section. The power supply will perform best when the transformer-rectifiers operate at 70 - 90% of the rated capacity, without excessive sparking. This reduces the maximum continuous-load voltage and corona power inputs. Practical operating voltages for transformer-rectifiers depend on:
At secondary current levels over 1500 mA, internal impedance of a transformer-rectifier is low, which makes stable automatic voltage control more difficult to achieve. The design of the transformer-rectifier should call for the highest possible impedance that is commensurate with the application and performance requirements. Often, this limits the size of the electrical field or bus section.
- Collecting plate spacing
- Gas and dust conditions
- Collecting plate and discharge electrode geometry
It is general practice to add additional impedance in the form of a current-limiting reactor in the primary circuit. This reactor will limit the primary current during arcing and also improve the wave shape of the voltage/current fed into the transformer-rectifier.
- Corona current density
Corona current density should be in the range of 10 - 100 mA/1000 ft2 of plate area. (Calculate this using secondary current divided by collecting area of the electrical field or bus section.) The actual level depends upon:
- Location of electrical field or bus section to be energized
- Collecting plate area
- Gas and dust conditions
- Collecting electrode and discharge wire geometry
About Gas Distribution Systems (Back to top)
One electrical field or bus section of an electrostatic precipitator is by itself an independent precipitator. Its operation is governed by the inlet gas and dust conditions, as well as the collecting plate and discharge electrode geometries.
Within this electrical field or bus section, one gas passage is also an independent precipitator - governed by the same factors. (Note that the gas passage shares the voltage level with the adjacent gas passages of the same electrical field or bus section, but not the corona current level, which can be different in each gas passage.)
This points to the importance of creating similar gas and dust conditions 1) at the inlet of each electrical field or bus section, and 2) further at the inlet of each gas passage of the electrical field or bus section. Ideally, uniformity is desired in:
Gas velocity distribution can be most effectively influenced by the use of gas distribution devices.
- Gas velocity
- Gas temperature
- Dust loading
The quality of gas velocity distribution can be measured in a scaled-down model of the precipitator and its ductwork, and also in the precipitator itself. Typical criteria are based on ICAC (Institute of Clean Air Companies) recommendations using average gas velocities or on a calculated RMS statistical representation of the gas velocity pattern.
In general, gas distribution devices consist of turning vanes in the inlet ductwork, and perforated gas distribution plates in the inlet and/or outlet fields of the precipitator.
About Rapping Systems (Back to top)
Rappers are time-controlled systems provided for removing dust from the collecting plates and the discharge electrodes as well as for gas distribution devices (optional) and for hopper walls (optional). Rapping systems may be actuated by electrical or pneumatic power, or by mechanical means. Tumbling hammers may also be used to dislodge ash. Rapping methods include:
- Electric vibrators
- Electric solenoid piston drop rappers
- Pneumatic vibrating rappers
- Tumbling hammers
- Sonic horns (do not require transmission assemblies)
- Discharge Electrode Rapping
In general, discharge electrodes should be kept as free as possible of accumulated particulate. The rapping system for the discharge electrodes should be operated on a continuous schedule with repeat times in the 2 - 4 minute range, depending on the size and inlet particulate loading of the precipitator.
- Collecting Plate Rapping
Collecting plate rapping must remove the bulk of the precipitated dust. The collecting plates are supported from anvil beams or directly with hooks from the precipitator casing. With anvil beam support, the impact of the rapping system is directed into the beams located at the leading and/or trailing edge of the collecting plates. For direct casing support, the impact is directed into the rapper beams located at or near the center of the top of the collecting plates.
The first electrical field generally collects about 60-80% of the inlet dust load. The first field plates should be rapped often enough so that their precipitated layer of particulate is about 3/8 - 1/2" thick. There is no advantage in rapping more often since the precipitated dust has not yet agglomerated to a sheet which requires a minimum layer thickness. Sheet formation is essential to make the dust drop into the precipitator hopper without re-entrainment into the gas stream. Rapping less frequently typically results in a deterioration of the electrical power input by adding an additional resistance into the power circuit. Once an optimum rapping cycle has been found for the first electrical field (which may vary across the face of a large precipitator), the optimum rapping cycles for the downstream electrical fields can be established.
The collecting plate rapping system of the first field has a repeat time T equal to the time it takes to build a 3/8 - 1/2"layer on the collecting plates. The plates in the second field should have a repeat time of about 5T, and the plates in the third field should have a repeat time of 25T. Ideally, these repeat times yield a deposited layer of 3/8-1/2" for the plates in all three fields. Adjustment may be required for factors such as dust resistivity, dust layer cohesiveness, gas temperature effects, electrical field height and length, and the collecting area served by one rapper.
- Gas Distribution Plate and Hopper Wall Rapping
The gas distribution plates should also be kept free of excessive particulate buildup and may require rapping on a continuous base with a cycle time in the 10-20 minute range, depending on the inlet particulate loading of the precipitator and the nature of the particulate. Gas distribution plates in the outlet of the precipitator may be rapped less often (every 30 - 60 minutes).
- Improving Rapping System Performance
All precipitator rapping systems allow adjustment of rapping frequency, normally starting with the highest frequency (the least time between raps), progressing to the lowest frequency. The times that are actually available may be limited. Rapping systems with pneumatic or electric actuators allow variations of the rapping intensity. Pneumatic or electric vibrators allow adjustments of the rapping time. State-of-the-art rapper controls allow selection of rapping sequences, selection of individual rappers, and provide anti-coincidence schemes which allow only one rapper to operate at a given time.
Rapping systems can be optimized for top precipitator performance using precipitator power input and stack opacity as criteria. Optimization of the rapping system starts with the discharge electrode rapping system operating on its own time schedule, for example with repeat times of 2 - 4 minutes. The rapping system for the gas distribution screens in the inlet and outlet of the precipitator should then be operated with repeat times of 2-3 minutes for the inlet and 2 - 3 hours for the outlet screens.
The only rapping system requiring optimization is the collecting plate rapping system. The optimization should start with the Collecting Plate Rapping Schedule determined above. Next, the rapping frequency of the inlet field should be increased or decreased until the electrical power input of the inlet field remains constant. Next, the rapping frequency of the other fields should be adjusted in sequence until their electrical power inputs remain constant. If the stack opacity trace shows rapping spikes, the rapping intensity should be reduced while observing the electrical power input of the precipitator.
The adjustment of the rapping system for optimum precipitator performance is a slow process. It requires a substantial amount of time for stabilization after each adjustment.
About Hoppers (Back to top)
Precipitator hoppers are designed to completely discharge dust load on demand. Typically, precipitator hoppers are rectangular in cross-section with sides of at least 60-degree slope. These hoppers are insulated from the neck above the discharge flange with the insulation covering the entire hopper area. In addition, the lower 1/4- 1/3 of the hopper wall may be heated. Discharge diameters are generally 8" - 12".
Insulation provides protection for facility personnel as well as working to retain as much hopper wall temperature as possible. Hopper wall temperature retention discourages condensation on the inside of the hopper. Heaters are added to ensure hot metal surfaces immediately above the fly ash discharge.
- Facilitating hopper discharge
Hopper discharge problems are caused by compaction of the fly ash in the hopper. Compaction characteristics are affected by moisture content, particle size and shape, head of material, and vibration. The flow of fly ash out of the hopper can be facilitated by the use of external vibrators. These can operate on the outside wall of the hopper or on an internal hopper baffle.
- Hopper fluidizers
Hopper fluidizers have a membrane that permits air flow to the fly ash directly above. This air flow fills the voids between the fly ash particles at a slight pressure, changes the repose angle of the particles, and promotes gravity flow.
- Ash handling system
The fly ash handling system evacuates the fly ash from the hoppers, and transports the fly ash to reprocessing or to disposal. The ash handling system should be designed and operated to remove the collected fly ash from the hoppers without causing re-entrainment into the gas flow through the precipitator. The design of the ash handling system should allow for flexibility of scheduling the hopper discharges according to the fly ash being collected in these hoppers.
Either the precipitator hopper or the feeder hopper is used for temporarily storing material prior to discharge. Three types of handling systems are in use:
- Negative pressure or vacuum system
Connects to the hopper by a simple discharge valve
- Positive pressure dilute phase system
Uses an airlock-type feeder; the feeder is separated from the hopper by an inlet gate and from the conveying line by a discharge gate
- Positive pressure dense phase system
Connects to the hopper with an airlock type feeder.
About Ductwork (Back to top)
Ductwork connects the precipitator with upstream and downstream equipment. The design of the ductwork takes into consideration the following:
- Low resistance to gas flow
Achieved by selecting a suitable cross-section for the ductwork and by installing gas flow control devices, such as turning valves and flow straighteners
- Gas velocity distribution
Gas flow control devices are used to maintain good gas velocity distribution
- Minimal fallout of fly ash
Fallout can be minimized by using a suitable transport velocity
- Minimal stratification of the fly ash
A suitable transport velocity also reduces fly ash stratification in the gas stream
- Low heat loss
The goal is to reduce the heat loss of the flue gas to a level that will prevent acid or moisture condensation in the downstream equipment, requiring the use of thermal insulation protected by external siding.
- Structural integrity
Ductwork structure supports its total load, including wind and snow loads. The design also allows for accumulated fly ash, negative/positive operating pressure, and gas temperature. Expansion joints are used to accommodate thermal growth.
About Gas Velocity Distribution (Back to top)
Efficient precipitator performance depends heavily upon having similar gas conditions at the inlet of each electrical field or bus section and at the inlet of each gas passage of the electrical field or bus section. Uniformity of gas velocity is also desirable - good gas velocity distribution through a precipitator meets these requirements:
— 85% of all measured gas velocities < 1.15 times the average gas velocity
— 99% of all measured gas velocities < 1.40 times the average gas velocity
- Improving Gas Velocity Distribution
The gas velocity distribution in a precipitator can be customized according to the design of the precipitator and the characteristics of the dust particles. Traditionally, precipitators have been designed with uniform gas velocity distribution through the electrical fields, to avoid high-velocity areas that would cause re-entrainment. While this is still a recommended practice, there is an advantage in some cases to developing a velocity profile that brings more particles closer to the hopper.
Both of these schemes have applications in site-specific conditions. Gas velocity distribution can be controlled by the following:
- Adding/improving gas flow control devices in the inlet ductwork
- Adding/improving flow control devices in the inlet of the precipitator
- Adding/improving flow control devices in the outlet of the precipitator
- Adding a rapping system to the flow control devices (where applicable)
- Adding/improving anti-sneak baffles at the peripheries of the electrical fields
- Adding/improving hopper baffles
- Eliminating air leakages into the precipitator
About Re-entrainment (Back to top)
Reducing rapping re-entrainment to an acceptable level generally requires a substantial improvement of the gas velocity distribution and the electrical power density and uniformity, as well as an extended optimization program for the collecting-plate rapping system.
Factors Affecting Re-entrainment
Re-entrainment of collected particles is the major contributor to particulate emissions of the precipitator. In some cases, re-entrainment accounts for 60 - 80% of the residual. The major causes of re-entrainment are as follows:
- Low cohesiveness
- Low adhesion to collecting plates
- Particle size
- Low resistivity
- Collecting plate design
- Discharge electrode design
- Plate spacing
- Duration (if applicable)
- Collecting plate and discharge electrode rapping
- Erosion (localized high gas velocity)
- Hopper design
- Leakage (hopper valve)
- Hopper gas flow
About Corona Power (Back to top)
Precipitator corona power is the useful electrical power applied to the flue gas stream to precipitate particles. Either precipitator collecting efficiency or outlet residual can be expressed as a function of corona power in Watts/1000 acfm of flue gas, or in Watts/1000 ft of collection area.
The separation of particles from the gas flow in an electrostatic precipitator depends on the applied corona power. Corona power is the product of corona current and voltage. Current is needed to charge the particles. Voltage is needed to support an electrical field, which in turn transports the particles to the collecting plates.
In the lower range of collecting efficiencies, relatively small increases in corona power result in substantial increases in collecting efficiency. On the other hand, in the upper ranges, even large increases in corona power will result in only small efficiency increases.
Equally, in the lower range of the corona power levels, a small increase in the corona power results in a substantial reduction in the gas stream particle content. In the upper range of the corona power level, a large increase is required to reduce the particle content.
Optimizing Corona Power
Optimum conditions depend upon the location of the field (inlet, center and outlet), fly ash characteristics (resistivity) and physical conditions (collecting plates and discharge wires). Corona power levels can be optimized by adjusting or optimizing the following:
- Particle size· Resistivity
- Spark rate setting
- Current & voltage limits
- Plate spacing
- Collecting plate & discharge electrode design
- Purge air system operation
About Performance Improvements (Back to top)
Improvement or optimization of precipitator operation can result in significant savings. Many specific situations encourage a review of precipitator operation:
- Deterioration of existing equipment
- Tightening of air pollution emission regulations
- Changes in products and/or production rates
- Frequent forced outages
- De-rating of production
To learn more about performance improvement programs, refer to the appropriate section:
Gas Velocity Distribution
Flue Gas/Fly Ash Conditioning
Equipment Improvements (Back to top)
The objectives of equipment improvements are to optimize corona power, reduce re-entrainment, and optimize gas velocity distribution inside the precipitator. Some important topics to consider when planning equipment improvements include:
For more information, see these related topics:
- Precipitator Size
When sizing the precipitator, it is important to provide a cross-section that will maintain an acceptable gas velocity. It is also important to provide for enough total discharge wire length and collecting plate area, so that the desired specific corona current and electrical field can be applied.
- Gas Velocity Distribution
Improving gas velocity distribution in the precipitator reduces particle re-entrainment and boosts precipitator efficiency. Typically, a uniform gas velocity is desired, but there are site-specific exceptions. Gas velocity distribution can be modified by using flow control devices and baffles. Refer to the special section on gas velocity distribution.
- Corona Power
The separation of dust particles from the gas flow in an electrostatic precipitator depends on the applied corona power. Corona power is the product of corona current and voltage. Current is needed to charge the particles. Voltage is needed to support an electrical field, which in turn transports the particles to the collecting plates. For additional information, refer to Corona Power.
The precipitator is divided into electrical sections that are cross-wise and parallel to the gas flow to accommodate spatial differences in gas and dust conditions. Optimization of corona power involves adjusting the corona power (secondary voltage and current) in each electrical section for optimum conditions.
- Particle Re-entrainment
Minimizing re-entrainment of dust particles is important to improvement of precipitator efficiency. Most precipitator equipment affects the re-entrainment level. For a detailed discussion, visit the special section on re-entrainment.
- Additional Equipment
Performance improvement options include the installation of a second precipitator in series with the existing precipitator; using fabric filters downstream of the precipitator; and adding a second particle collector in parallel with the existing collector. Other possibilities include sonic or electrostatic particle agglomerators upstream of the precipitator; a mechanical upstream collector; or an electrostatically-enhanced or mechanical collector, or a filter downstream of the precipitator.
- Review the General Equipment Requirements
Reviewing the Neundorfer Knowledge Base sections on equipment will provide additional insight into performance improvements.
Gas Velocity Distribution
Hoppers and Dust Handling
Combustion Process Improvements for Power Plants (Back to top)
Combustion process conditions mainly affect the corona power level. The primary contributors to combustion process conditions and their effects include:
— Flue gas flow rate Coal mills
— Flue gas moisture content
— Fly ash resistivity
— Fly ash inlet loading
— Fly ash particle size
— Fly ash particle size Furnace
— Unburned carbon (LOI)
— Base load/swing load operation Burners
— Flue gas flow rate
— Flue gas temperature Air pre-heaters
— Fly ash resistivity
— Unburned carbon (LOI)
— Gas flow pattern
— Gas temperature pattern
— SO3 distribution pattern
Bituminous coals from Eastern mines, sub-bituminous and lignite coals from Western mines, and lignites from Texas mines are substantially different from each other in the combustion process. Coal blending is now used for operational and financial benefits. This results in a wide range of boiler and precipitator operating conditions.
Precipitating fly ash from difficult coals can be improved with conditioning systems. However, the furnace and its associated equipment can still cause problems in the precipitator, particularly coal mills, burners, and air pre-heaters.
The setting of the coal mills and classifiers defines the coal particle size which in turn impacts the fly ash particle size. Larger coal particles are more difficult to combust, but larger fly ash particles are easier to collect in the precipitator.
Base-load operation of the boiler is usually better for precipitator operation than swing-load operation due to more stable operating conditions. Boiler operation at low loads may be as problematic for the precipitator as operating the boiler at its maximum load level, due to fallout of fly ash in the ductwork, low gas temperatures, and deterioration of the quality of the gas velocity distribution.
If low load operation cannot be avoided, the installation of additional gas flow control devices in the inlet and outlet of the precipitator may prove beneficial.
The operation of coal burners, together with the setting of the coal mills and their classifiers, affects the percentage of unburned carbon (LOI or UBC) in the fly ash. The use of Lo-NOx burners increases this percentage, and causes re-entrainment and increased sparking in the precipitator. Further, the UBC tends to absorb SO3, which in turn increases the fly ash resistivity. Over-fire air optimization or coal-reburn systems may reduce UBC in the fly ash.
Regenerative air pre-heaters cause temperature and SO3 stratification in the downstream gas flow. This problem is more severe in closely coupled systems, where the precipitator is located close to the air pre-heater. Depending upon site-specific conditions, flow mixing devices may be installed in the ductwork to the precipitator, or flue gas conditioning systems may be used to equalize the gas flow characteristics.
Fly Ash and Flue Gas Conditioning (Back to top)
Flue gas and fly ash characteristics at the inlet define precipitator operation. The combination of flue gas analysis, flue gas temperature and fly ash chemistry provides the base for fly ash resistivity. Typically, fly ash resistivity involves both surface and volume resistivity. As gas temperature increases, surface conductivity decreases and volume resistivity increases.
In lower gas temperature ranges, surface conductivity predominates. The current passing through the precipitated fly ash layer is conducted in a film of weak sulfuric acid on the surface of the particles. Formation of the acid film (from SO3 and H2O) is influenced by the surface chemistry of the fly ash particles.
In higher gas temperature ranges, volume conductivity predominates. Current conduction through the bodies (volume) of the precipitated fly ash particles is governed by the total chemistry of the particles.
Fly ash resistivity can be modified (generally with the intent to reduce it) by injecting one or more of the following upstream of the precipitator:
- Sulfur trioxide (SO3)
- Ammonia (NH3)
Sulfur Trioxide and Ammonia Conditioning Systems
In most cases, a sulfur trioxide conditioning system is sufficient to reduce fly ash resistivity to an acceptable level. The source of sulfur trioxide can be liquid sulfur dioxide, molten elemental sulfur, or granulated sulfur. It is also possible to convert native flue gas SO2 to SO3.
In some instances, ammonia alone has been proven a suitable conditioning agent. It forms an ammonia-based particulate to increase the space charge. The source of ammonia may be liquid anhydrous or aqueous ammonia, or solid urea.
Finally, sulfur trioxide and ammonia may be used in combination. This solution has been successful because it can lower fly ash resistivity and also form ammonia bisulfate. The latter increases the adhesion of particles, and thus reduces re-entrainment losses.
The injection of water upstream of the precipitator lowers the gas temperature and adds moisture to the flue gas. Both are beneficial in cold-side precipitator applications. However, care must be taken that all of the water is evaporated and that the walls in the ductwork or gas distribution devices do not get wet.