Nowadays, environment protection has become major concern due to awareness among people. Authorities are now forced to set increasingly more stringent limits on all pollutant discharges. These also include emissions from the industrial plants of solid particulate and other gaseous pollutants. Commonly used air pollution control devices are shown below. In this article, an overview of Electrostatic Precipitators (ESP) is discussed.
Electrostatic Precipitators (ESP)
Electrostatic Precipitators (ESP) are widely used device in various domains to remove the pollutant particulates, especially in industrial plants. ESP removes particles from a flowing gas (such as air) using the force of an induced electrostatic charge. An ESP applies energy only to the particulate matter being collected. Hence it is very efficient in its consumption of energy (in the form of electricity). ESP are most commonly used in following applications;
- Refuse & sewerage sludge dryers and incinerators
- Coal- and oil-fired boilers, coal driers and coal mills
- Production plants for the cement, limestone, gypsum, pulp and paper industry (kilns, mills, driers and coolers)
- Electro-metallurgical, chemical, gas and detergent manufacturing plants
- SO2, SO3, acid mist and ammonia control
Advantages and Disadvantages
- ESPs are very efficient (up to 99% efficiency), even for small particles
- Generally more economical than other particulate control devices
- Designed to handle wet and dry gas compositions for a wide range of gas temperatures
- Can handle large volumes of gas flow with low pressure drop
- High initial capital costs
- Once installed, ESPs take up a lot of space and cannot be easily redesigned
- May not work properly on high electrical resistive particles
Particle Charging: With the application of voltage, the inside is divided into ionization region and drift region. The electrons escape from molecules as the electric field around negative electrode is strong. The positive ions move towards the corona, while the negative ions and electrons moves towards the collecting plates. Corona discharge is the form of electrical discharge due to the ionization of a fluid as a result of an energized conductor.
Particle Transport: In the moving way, under the influence of electric field, negative ions cohere and charge the particles, making the particles be forced towards collecting-plate (see figure below).
Particle Collection: As soon as the particles reach the plate, they will be neutralized and packed by the succeeded ones subsequently. The continuous process happens, as a result, particles are collected on the collecting plate and form a cake. Solid cakes are removed by rapping the plates at regular time intervals with a mechanical or electromagnetic rapper that strikes a vertical or horizontal blow on the edge of the plate.
Precipitator performance is very sensitive to the following parameters;
- Particle chemical composition and electrical resistivity
- Gas stream humidity
- Gas stream temperature
- Particle size distribution (Within the range of 0.01 mm to 100 mm)
- Fly ash content at the precipitator inlet
Types of ESP
- Plate or Tabular ESPs
- Single or Two Stage ESPs
- Dry or Wet ESPs
Plate or Tabular ESP: The functional design of an ESP incorporates either plate or tubular collection surfaces. Plate ESPs primarily collect dry particles and are used more often than tubular precipitators. They can have wire-plate or flat-plate electrodes.
Wire-plate ESP: Gas flows between parallel plates of sheet metal and high-voltage metal wires. It allows many flow lanes to operate in parallel, making it suitable for handling large volumes of gas. Plate-wire precipitators are among the most common types of ESPs.
Flate-Plate ESP: Small precipitators use flat plates instead of wires for high-voltage electrodes. It increase the average electric field used to collect particles and provide additional surface area for particle collection. Flat plate ESPs can be used in applications with high-resistivity particles with small (1 to 2 µm) diameters.
Tabular ESPs: It consists of parallel arrangements of tubes. The tubes may be arranged as a circular, square, or hexagonal honeycomb with gas flowing upwards or downwards. They are used in applications involving wet or sticky particulate.
Single or Two Stage ESPs:
Single Stage ESPs: Most industrial scale ESPs are single stage. They use very high voltages to charge particles and incorporate charging and collection together in the same stage. Sets of electrodes and collector surfaces (plates or tubes) operate in parallel to each other.
Two-stage ESPs operate in series rather than parallel configuration. This allows more time for particle charging and economical construction for smaller sizes. They are usually applied to sources emitting oil mists, smokes, fumes, or other liquid aerosols.
Dry or Wet ESPs:
Dry electrostatic precipitators are used to capture particles in dry product streams. They use periodic rapping to separate the accumulated dust from the collector plates and discharge electrodes. Typically, rapping will also project some of these particles (around 10-15 percent) back into the gas stream (known as re-entrainment). Dry electrostatic precipitators are often not suitable for submicron particulate applications because of particle size, resistivity, and other issues.
Wet electrostatic precipitators are used to strip wet (saturated) gas streams of particles. They use water sprays to condition/trap particles for collection and also to clean the particles off collection surfaces. WESPs collect particulate matter not suitable for dry ESPs, including sticky, moist, flammable, explosive, or high resistivity solids.