History of Electrical Precipitation
Briefly, here below we can see a summary of the main steps in Electrostatic Precipitation:
- Date Significance
1600 William Gilbert, English court physician, publishes De Magnete
1745 Benjamin Franklin describes the effects of points “in drawing and throwing off the electric fire.”
1824 M. Hohlfeld, German mathematician, describes the precipitation of fog in a jar containing an electrified point.
1878 R. Nahrwold notes that the discharge from an electrified sewing needle surrounded by a tin cylinder greatly increases the collection of atmospheric dust. Nahrwold repeats the experiment with a glycerin coating to help particles adhere.
1885 Sir Oliver Lodge attempts, unsuccessfully, to remove lead fume from a smelting works in North Wales
• Frederick Cottrell 1877-1948
– Incorporated more reliable rectifier transformer circuits in ESP design – able to sustain higher voltages
– Successfully collected sulphuric acid mist in Berkeley, CA laboratory in 1906
– First successful commercial precipitator used to collect H2SO4 in Pinole, CA 200 cfm capacity
– 1912, large scale ESP used to collect cement kiln dust at 1,000,000 cfm in Riverside CA
The most basic precipitator consists in a row of thin vertical wires followed by large flat metal collection plates oriented vertically, spaced about 1 cm, or more depending on the application. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the collection plates.
A negative or positive voltage of several thousand volts is applied between wire and plate. If the applied voltage is high enough an electric (corona) discharge ionizes the gas flowing around the electrodes. Ions created by corona effect, charge the gas-flow particles. The ionized particles, move to the grounded plates.
Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure.
Precipitator performance is very sensitive due to two particulate properties: 1) Resistivity; and 2) Particle size distribution. These properties can be determined economically and accurately in the laboratory. A widely taught concept to calculate the collection efficiency is the Deutsch model, which assumes infinite remixing of the particles perpendicular to the gas stream.
Resistivity can be determined as a function of temperature in accordance with IEEE Standard 548. This test is conducted in an air environment containing a specified moisture concentration. The test is run as a function of ascending or descending temperature or both. Data are acquired using an average ash layer electric field of 4 kV/cm. Since relatively low applied voltage is used and no sulfuric acid vapor is present in the environment, the values obtained indicate the maximum ash resistivity.
Usually the descending temperature test is suggested when no unusual circumstances are involved. Before the test, the ash is thermally equilibrated in dry air at 454 °C (850°F) for about 14 hours. It is believed that this procedure anneals the ash and restores the surface to pre-collection condition.
If there is a concern about the effect of combustibles, the residual effect of a conditioning agent other than sulfuric acid vapor, or the effect of some other agent that inhibits the reaction of the ash with water vapor, the combination of the ascending and descending test mode is recommended. The thermal treatment that occurs between the two test modes is capable of eliminating the foregoing effects. This results in ascending and descending temperature resistivity curves that show a hysteresis related to the presence and removal of some effect such as a significant level of combustibles.
With particles of high resistivity (cement dust for example) Sulfur trioxide is sometimes injected into a flue gas stream to lower the resistivity of the particles in order to improve the collection efficiency of the electrostatic precipitator .
- Advantages of Electrical Precipitation
Electrostatic Precipitators (ESPs):
– collect particles from 0.01 µm to 100 µm with 99% efficiency
– operate at high temperatures, up to 1200° F (650° C)
– operate at high gas pressures, up to 150 psi (10 atm)
– operate at high flow rates, up to 3,000,000 cfm (1400 m3/s)
– operate at high particle loadings, 500 grams/m3
– have low energy costs, 16 – 100 Watts/1000 m3/h
– have very low pressure drop
ESPs can be used when:
– large volumes of particulate air pollutants are produced
– no explosion hazard exists
– high efficiency needed
– continuous processes (expensive to build but inexpensive to operate)
Electrostatic precipitation has been in use mainly in industrial plants and application:
Industries and their pollutants where ESPs are commonly used:
Process Principal / Material Collected
Electrical Utility / Fly Ash (SiO2, Al2O, Fe2O3)
Industrial Boiler Houses / Fly Ash
Steelmaking Furnaces / Iron Oxide (Fe2O3)
Cement Kilns / Calcium Oxide, Silicon Oxide
Pulp and Paper / Sodium Sulfate
Metal Machining / Oil Mist
Recently, the advantages of electrostatic precipitation have been discovered also in the field of air conditioning due to the fact that there is a better control of ozone production that was the only disadvantage of the electrostatic precipitation.