What is an electrostatic precipitator (ESP)?
An electrostatic precipitator (ESP) is a piece of equipment that is used to capture dust particles that are formed or liberated by various industrial processes. The purpose of an ESP is to avoid these particulates being expelled into the atmosphere where they can cause pollution. ESPs are installed at many types of industrial plant, but they are most easily identified at thermal power plants where they are installed as part of the flue gas cleaning system.
Electrostatic Precipitator
The diagram below shows the position of an ESP within a coal fired power station flue gas system. Another important component used for flue gas cleaning is the flue gas desulphuriser (FDG), also referred to as a ‘scrubber tower’; the scrubber tower is shown to the left of the ESP on the below diagram.
Flue Gas System With ESP Highlighted
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Coal Fired Power Station ESPs
In coal-fired power generation plants (and other thermal power plant designs), coal is burnt in the combustion chamber (boiler or furnace) in solid or pulverised form. Coal (fuel) is typically fed to the combustion chamber with a forced draught fan supplying the required combustion air. Combustion products typically consist of flue gas composed of smoke, fly ash and heavy ash. Heavy ash falls to the bottom of the furnace and is removed periodically into ash hoppers. The combination of fly ash and smoke is removed by an induced draught fan (or fans) and disposed of via the flue stack. If an ESP is installed as part of the flue gas cleaning system, it is located between the induced draught fan and the combustion chamber.
Coal-Fired Plant Process Flow Diagram
Within certain industries, the captured dust from an ESP can be sold rather than disposed of, but this depends upon many factors e.g. location, dust properties, demand etc.
History
In the past, there was no consideration given to dust emissions from industrial plants. Later, governments reacted to reports from environmental protection agencies and the medical industry regarding the harmful effects of particulates that were being released into the atmosphere from industrial plants. An example of such a type of particulate is fly ash.
Fly ash consists of oxides of silicon, iron, calcium, and aluminium; harmful substances such as sulphur are also contained by fly ash. Studies upon the effects of fly ash in humans have found that it can cause respiratory diseases, as well as cancer, heart failure, and some immunological reactions. Other dusts which are emitted from industrial processes, such as coal dust, are also known to cause lung diseases such as pneumoconiosis. But problems stemming from fly ash are not only linked to humans. Dumping fly ash on topsoil increases the topsoil’s pH and causes harm to the plants and animals in the immediate ecosystem. Large volume dumping of fly ash has been known to cause chemical leaching into the soil, with resultant harmful effects upon aquatic marine life.
Given the negative effects of uncontrolled pollution, legislation has been enacted in most countries to reduce harmful particulates entering into the atmosphere completely. The air pollution laws enacted years ago led to further development of the electrostatic precipitator and its widespread adoption. As many cultures are now prioritising protection of the environment, legislation is likelier to become even more stringent, which will in turn will lead to further advancements in particle separation and ever more efficient ESPs.
Today, typical efficiencies for dust removal from a flue gas system range from between 98% to 99.9%. In some industries, the dust being created by the plant has monetary value and ESPs can capture this valuable commodity rather than let it be expelled to atmosphere.
How Electrostatic Precipitators Work - Basic
The electrostatic precipitator functions by charging particulates within a gas stream as the gas flows through the ESP. These negatively charged particulates are attracted to positively charge large flat plates within the ESP, where they gradually accumulate upon the surfaces of the plates. Once a significant number of particulates have accumulated on the plates, a mechanical mechanism (rapping system) hits the plates, with the resultant vibration shaking the particulates off the plates; the particulates then fall due to gravity and are collected in hoppers at the base of the ESP.
ESP Assembly Animation
How Electrostatic Precipitators Work - Advanced
Electrostatic precipitators usually have a rectangular shape with dust collecting hoppers installed at their base. The main components of an ESP consist of collecting electrodes/plates, discharge electrodes, inlet and outlet perforated screens, insulators for the discharge electrodes, rappers, and one or more electrical transformers.
Typical Thermal Power Plant Electrostatic Precipitator Components
Inlet and Outlet Perforated Screens
ESPs have a gas inlet and gas outlet. The gas stream entering the ESP passes through perforated screens and is distributed evenly to the interior of the ESP; particulates entrained within the gas stream are consequently also distributed evenly within the ESP.
Discharge Electrodes
Discharge electrodes consist of a series of wires that are arranged horizontally across the ESP and installed in several rows. Each discharge electrode is connected to a high voltage supply, which is fed from an electrical system located on top of the ESP housing. Electrical transformers increase the primary supplied voltage (usually ≈380V) to several thousand volts (usually between 20 kV to 70 kV).
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Typical Thermal Power Plant Electrostatic Precipitator Parts (close-up)
Rectification Unit
The electrical system incorporates a rectification unit to transform AC voltage to DC voltage. This transformation of AC to DC voltage is necessary to achieve the required electric field that will ionise the particulates as they pass through the ESP. DC voltage is fed to the discharge electrodes, which results in a negative electric field being generated around them. The negative electric field around the discharge electrodes causes a negative charge to be imparted onto the particulates, which causes them to be attracted to the positively charged collector plates.
How Electrostatic Precipitators Work
Collector Electrodes
Collector electrodes have a long thin rectangular shape and are also referred to as collector plates. Particulate matter is attracted to the plates by electrostatic force. Once particulate has accumulated on the plates, there is a mechanism for shaking the plates, which causes the particulates to fall due to gravity into the collection hoppers at the base of the ESP.
Electrostatic Force
Rapping System
The mechanism used to shake (bang/hit) the plates is referred to as the rapping system whilst the process is known as rapping. Other rapper systems are available, wet ESPs use water to rinse the plates, whilst dry ESPs use no water (the mechanism mentioned previously is the dry type ESP).
Collector Plate Rapping System
Rappers/hammers are connected to an electric motor via a reduction gear box with a common shaft. When the system is started, the hammers rotate and collide with the collection plates. As the hammer's impact with the collection plates, the accumulated particulates on the collection plate surfaces are liberated by the resultant vibrations, and fall into the collection hoppers at the base of the ESP.
Conveying System
Particulate/dust is removed from the hoppers via a conveyance system; it may then be discharged directly to a freight truck, train wagon, barge, or ship. Another option is to discharge the gathered particulate to slurry plant hoppers, where it is mixed with water to form a slurry. If the particulate has monetary value, it can be conveyed and dry stored in a large silo; this is usually the case with fly ash because it can be sold to cement manufacturers.
How ESPs Work Summary
The process which takes place in an ESP can be summarised by the schematic below.
Electrostatic Precipitator Process Flow
Electrostatic Precipitator Maintenance
The maintenance of an electrostatic precipitator is mainly focused on the mechanical and electrical components. The following common problems can lead to a reduction in ESP efficiency:
- Loss of the electric field due to discharge wire breakage - this typically happens due to erosion of parts by dust particles, or, due to overfilling of the collection hoppers which leads to short circuiting of some of the discharge wires; an overfilling scenario is shown in the below image.
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Overfilled Electrostatic Precipitator (collector electrodes side)
- Inability of the rapping system to clear dust from the collector plates due to loss of drive on the rapping system - this usually happens when the motor shear pin shears due to a seized bearing on the common shaft.
The above defects can only be corrected when the ESP is off-load (off-line). Maintenance work involves entering inside the ESP and visually inspecting the components and parts. To enter the ESP, the hoppers may have to be emptied first. For this reason, maintenance work is typically completed over a few days. During a long-term outage, the following maintenance tasks are usually performed (depending upon the ESP design):
- Washing/rinsing of the precipitator.
- Still-air tests to check the strength of the developed electric field around the discharge electrodes.
- Straightening of bent discharge and collecting electrodes.
- Replacement of worn collector plates.
- Replacement of damaged discharge electrodes.
- Refurbishment of rapper bearing assemblies.
- Replacement of damaged rapper hammers.
Maintenance of electrical components on the ESP typically involves checking the discharge electrode insulators for damage and functionality of motors and voltage transformers. Voltage transformers are usually of the hermetic transformer design and should be maintained in accordance with the plant’s electrical machinery maintenance plan.
Tip – the maintenance of ESP electrical transformers is often neglected due to their location (on top of the ESP). Although electrical transformers are very reliable, there have been instances where ESP transformers have failed and caught fire; this is particularly a problem with hermetic transformers as they contain mineral oil. Due to an ESP transformer’s location on top of the ESP, it is difficult to extinguish the fire even when the fire brigade arrives with specialist equipment and machinery. For this reason, the fire may be left to ‘burn-out’ in a controlled manner, with significant downtime (unscheduled outage time) occurring as a result.
Additional Resources
https://en.wikipedia.org/wiki/Electrostatic_precipitator
https://www.babcock.com/resources/learning-center/basic-esp-operation
https://energyeducation.ca/encyclopedia/Electrostatic_precipitator
https://power.mhi.com/products/aqcs/lineup/dust-collector