Electrosttic Precipitor (ESP)

 

Electrostatic Precipitator

What Is an Electrostatic Precipitator?

            An electrostatic precipitator (ESP) is defined as a filtration device that is used to remove fine particles like smoke and fine dust from the flowing gas. 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.

            In 1907, chemistry professor Frederick Gardner Cottrell patented the first electrostatic precipitator used to collect sulphuric acid mist and lead oxide fumes emitted from various acid-making and smelting activities. 

Electrostatic Precipitator

 

            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.

            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.

Working Principle of Electrostatic Precipitator.

            The electrostatic precipitator functions by charging particulates within a gas stream as the gas flows through the ESP. It consists of two sets of electrodes: positive and negative. The negative electrodes are in the form of a wire mesh, and the positive electrodes are plates. These electrodes are vertically placed and are alternate to each other. 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.

            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.

 

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).

Discharge and Collector Electrodes.

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.

            The medium that is used between the two electrodes is air. There might be corona discharge around the electrode rods or the wire mesh due to the high negativity of negative charges. The entire system is enclosed in a metallic container containing an inlet for flue gases and an outlet for filtered gases. There are plenty of free electrons as the electrodes are ionized, which interact with the dust particles of the gas, making them negatively charged. These particles move towards positive electrodes and fall off due to gravitational force. The flue gas is free from the dust particles as it flows through the electrostatic precipitator and is discharged to the atmosphere through the chimney.

 

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.

GD Screen

 

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.

 

Rapping System

The mechanism used to shake 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).

Rapping System

 

Efficiency of Electrostatic Precipitator

where,

𝜼 is the fractional collection efficiency

W is the terminal drift velocity in ms^-1

A is the total collection area in m²

Q is the volumetric air flow rate in m³s^-1

Types of Electrostatic Precipitator

There are different electrostatic types, and here, we will study each one of them in detail. Following are the three types of ESPs:

• Plate precipitator: This is the most basic precipitator type that consists of rows of thin vertical wires and stack of vertically arranged large flat metal plates that are placed at a distance of 1cm to 18cm apart. The air stream is passed horizontally through the vertical plates and then through the large stack of plates. In order to ionize the particles, a negative voltage is applied between the wire and the plate. These ionized particles are then diverted towards the grounded plates using electrostatic force. As the particles get collected on the collection plate, they are removed from the air stream.
• Dry electrostatic precipitator: This precipitator is used to collect pollutants like ash or cement in a dry state. It consists of electrodes through which the ionized particles are made to flow through and a hopper through which the collected particles are extracted out. The dust particles are collected from a stream of air by hammering the electrodes.
• Wet electrostatic precipitator: This precipitator is used to remove resin, oil, tar, paint that are wet in nature. It consists of collectors that are continuously sprayed with water making the collection of ionized particles from the sludge. They are more efficient than dry ESPs.
• Tubular precipitator: This precipitator is a single-stage unit consisting of tubes with high voltage electrodes that are arranged parallel to each other such that they are running on their axis. The arrangement of the tubes could either be circular or square or hexagonal honeycomb with gas either flowing upwards or downwards. The gas is made to pass through all the tubes. They find applications where sticky particles are to be removed.

 

 

Advantages of electrostatic precipitator:

• The durability of the ESP is high.
• It can be used for the collection of both dry and wet impurities.
• It has low operating costs.
• The collection efficiency of the device is high even for small particles.
• It can handle large gas volumes and heavy dust loads at low pressures.

 

Disadvantages of electrostatic precipitator:

• Can’t be used for gaseous emissions.
• Space requirement is more.
• Capital investment is high.
• Not adaptable to change in operating conditions.

Electrostatic Precipitator Applications

A few noteworthy electrostatic precipitator applications are listed below:

• Two-stage plate ESPs are used in the engine rooms of shipboard as the gearbox produces explosive oil mist. The collected oil is reused in a gear lubricating system.
• Dry ESPs are used in thermal plants to clean the air in ventilation and air conditioning systems.
• They find applications in the medical field for the removal of bacteria and fungus.
• They are used in zirconium sand for detaching the rutile in plants.
• They are used in metallurgical industries to clean the blast.

 

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