Electrostatic precipitator
Cottrell first applied the device to the collection of sulphuric acid mist and lead oxide fumes emitted from various acid-making and smelting activities.
The intent of the organization was to bring inventions made by educators (such as Cottrell) into the commercial world for the benefit of society at large.
Research Corporation has provided vital funding to many scientific projects: Goddard's rocketry experiments, Lawrence's cyclotron, production methods for vitamins A and B1, among many others.
Research Corporation set territories for manufacturers of this technology, which included Western Precipitation (Los Angeles), Lodge-Cottrell (England), Lurgi Apparatebau-Gesellschaft (Germany), and Japanese Cottrell Corp. (Japan), and was a clearinghouse for any process improvements.
Tubular precipitators are often used for mist or fog collection or for adhesive, sticky, radioactive, or extremely toxic materials.
Since relatively low applied voltage is used and no sulfuric acid vapor is present in the test environment, the values obtained indicate the maximum ash resistivity.
In an ESP, where particle charging and discharging are key functions, resistivity is an important factor that significantly affects collection efficiency.
Dust particles with high resistance are held too strongly to the plate, making them difficult to remove and causing trapping problems.
Surface conduction on particles is closely related to surface-leakage currents occurring on electrical insulators, which have been extensively studied.
[8] An interesting practical application of surface-leakage is the determination of dew point by measurement of the current between adjacent electrodes mounted on a glass surface.
This method has been used effectively for determining the marked rise in dew point, which occurs when small amounts of sulfuric acid vapor are added to an atmosphere (commercial Dewpoint Meters are available on the market).
The dust layer breaks down electrically, producing small holes or craters from which back corona discharges occur.
Disruptions of the normal corona process greatly reduce the ESP's collection efficiency, which in severe cases, may fall below 50% .
The most marked effects of back corona on the current-voltage characteristics are: The Figure below and to the left shows the variation in resistivity with changing gas temperature for six different industrial dusts along with three coal-fired fly ashes.
The problem lies in onset of corona causing large amounts of current to surge through the (low resistivity) dust layer.
After removal of the unburned combustibles during the above-mentioned annealing procedure, the descending temperature mode curve shows the typical inverted “V” shape one might expect.
It is theorized that ammonia reacts with H2SO4 contained in the flue gas to form an ammonium sulfate compound that increases the cohesivity of the dust.
Increasing the moisture content of the gas stream by spraying water or injecting steam into the duct work preceding the ESP lowers the resistivity.
The presence of SO3 in the gas stream has been shown to favor the electrostatic precipitation process when problems with high resistivity occur.
Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda ash (sometimes as raw trona), have also been used to reduce particle resistivity.
Therefore, the chemical composition of the flue gas stream is important with regard to the resistivity of the particles to be collected in the ESP.
Mechanism is not clear, various ones proposed; If injection of ammonium sulfate occurs at a temperature greater than about 600 °F (320 °C), dissociation into ammonia and sulfur trioxide results.
More recently, it has been recognized that a major reason for loss of efficiency of the electrostatic precipitator is due to particle buildup on the charging wires in addition to the collection plates (Davidson and McKinney, 1998).
ESPs continue to be excellent devices for control of many industrial particulate emissions, including smoke from electricity-generating utilities (coal and oil fired), salt cake collection from black liquor boilers in pulp mills, and catalyst collection from fluidized bed catalytic cracker units in oil refineries to name a few.
For a coal-fired boiler the collection is usually performed downstream of the air preheater at about 160 °C (320 °F) which provides optimal resistivity of the coal-ash particles.
The original parallel plate–weighted wire design (see figure of Plate and Bar precipitator above) has evolved as more efficient (and robust) discharge electrode designs were developed, today focusing on rigid (pipe-frame) discharge electrodes to which many sharpened spikes are attached (barbed wire), maximizing corona production.
WESPs are commonly used to remove liquid droplets such as sulfuric acid mist from industrial process gas streams.
The WESP is also commonly used where the gases are high in moisture content, contain combustible particulate, or have particles that are sticky in nature.
[12] Plate precipitators are commonly marketed to the public as air purifier devices or as a permanent replacement for furnace filters, but all have the undesirable attribute of being somewhat messy to clean.
[14] However, electrostatic precipitators offer benefits over other air purifications technologies, such as HEPA filtration, which require expensive filters and can become "production sinks" for many harmful forms of bacteria.
