A electrostatic precipitator ( ESP ) is a filtering device that removes fine particles, such as dust and smoke, from gas flowing through the strength of the induced electrostatic charge minimally inhibiting the gas flow through the unit.
Unlike wet scrubbers that use direct energy to flowing fluid medium, ESP only applies energy to particulate matter that is collected and therefore highly efficient in its energy consumption (in the form of electricity).
Video Electrostatic precipitator
Invention of electrostatic precipitator
The first use of corona release to remove particles from aerosols was by Hohlfeld in 1824. However, it was not commercialized until nearly a century later.
In 1907, Frederick Gardner Cottrell, a professor of chemistry at the University of California, Berkeley, applied for a patent on a device to fill the particles and then collect them through the electrostatic apparatus - the first electrostatic precipitator. Cottrell first used the device for the collection of sulfuric acid haze and lead oxide fumes emitted from various acid-making and smelting activities. Grape vineyards in northern California are being adversely affected by lead emissions.
At the time of the discovery of Cottrell, the theoretical foundation for operations was not understood. The operational theory was developed later in Germany, with the work of Walter Deutsch and the formation of the Lurgi company.
Cottrell used the results of his invention to fund scientific research through the creation of a foundation called Research Corporation in 1912, where he commissioned a patent. The purpose of the organization is to bring the invention made by educators (such as Cottrell) into the commercial world for the benefit of the wider community. Operations Research Corporation is funded by royalties paid by commercial companies after commercialization takes place. Research Corporation has provided important funding for many scientific projects: Goddard contemplation experiments, Lawrence cyclotron, production methods for vitamins A and B 1 , among many others.
With the decision of the US Supreme Court, Corporations should be divided into several entities. The Research Corporation is separated from two commercial companies that make hardware: Research-Cottrell Inc. (operating east of the Mississippi River) and Western Precipitation (operating in western countries). Research Corporation continues to be active to this day, and the two companies formed to commercialize this invention for industrial and utility applications are still in business as well.
Electrophoresis is a term used for the migration of charged charged gas particles in direct current electrostatic fields. Traditional CRT television sets tend to accumulate dust on the screen because of this phenomenon (CRT is a direct current engine operating at about 15 kilovolts).
Maps Electrostatic precipitator
Plate precipitator
The most basic precipitator contains a series of thin vertical cables, and is followed by a large vertical-oriented flat plate stack, with plates typically spaced from 1 cm to 18 cm, depending on the application. The air currents flow horizontally through the space between the wires, and then pass through the pile of plates.
Negative voltage of several thousand volt is applied between wire and plate. If the voltage is high enough, the release of the electric corona ionizes the air around the electrode, which then ionizes the particles in the airflow.
The ionized particles, due to the electrostatic forces, are diverted to the diarded plates. Particles are formed on the collecting plate and removed from the air stream.
The two-stage design (a separate filling section before the collection section) has the benefit of minimizing ozone production, which will adversely affect the health of personnel working in enclosed spaces. For the engine room of the vessel where the gearbox produces oil mist, a two-stage ESP is used to clean the air, improve the operating environment and prevent accumulation of accumulated haze of combustible oil. The accumulated oil is returned to the gear lubricant system.
The collection efficiency ( R )
The precipitator performance is very sensitive to two particulate properties: 1) Electrical resistivity; and 2) the particle size distribution. These properties can be measured economically and accurately in the laboratory, using standardized tests. Resistivity can be determined as a temperature function in accordance with IEEE Standard 548. This test is performed in an air environment containing a certain moisture concentration. The test is run as a function of rising or falling temperatures, or both. Data was obtained by using the mean ash layer [further explanation required] electric field of 4 kV/cm. Because the used voltage is relatively low in use and no sulfuric acid vapor exists in the test environment, the values ​​obtained show maximum ash resistivity.
In ESP, where particle filling and discharging are the main functions, resistivity is an important factor that significantly affects collection efficiency. While resistivity is an important phenomenon in the inter-electrode region where most of the charging takes place, it has a very important effect on the dust layer in the collecting electrode where it occurs. Particles that exhibit high resistivity are difficult to fill. But once filled, they are not ready to give up the cargo they get when they arrive at the collection electrode. On the other hand, particles with low resistivity easily become filled and ready to discharge their charge to the grounded collection plate. Both extremes in resistivity inhibit the efficient functioning of ESP. ESP works best in normal resistivity conditions.
The resistivity, which is a characteristic of particles in an electric field, is a measure of particle resistance to transfer the charge (both receiving and handing costs). Resistivity is a function of the chemical composition of particles as well as the operating conditions of exhaust gases such as temperature and humidity. Particles can have high resistivity, moderate (normal), or low.
Mass resistivity is defined using the more general version of Ohm's Law, as given in Equation ( 1 ) below:
Where:  E is electric field strength (V/cm);  j is the current density (A/cm 2 ); and  ? is Resistivity (Ohm-cm)
A better way to display this is to solve resistivity as a function of applied voltage and current, as given in Eq. ( 2 ) below:
Where: � = Resistivity (Ohm-cm)  V = DC potential applied, (Volt);  I = measurable current, (Amperes);  l = ash layer thickness, (cm); and  A = Current measurement electrode area, (cm 2 ).
Resistivity is an electrical resistance of a 1.0 cm 2 dust sample in a cross-sectional area, 1.0cm thick, and recorded in ohm-cm units. The method for measuring resistivity will be explained in this article. The table below provides a range of values ​​for low, normal, and high resistivity.
Durability of the dust layer
Resistance affects the electrical conditions in the dust layer by potential electric fields (voltage drops) formed across the layers as negatively charged particles arrive at their surface and leak their electrical charges into the collection plate. On the metal surface of the electrified collection plate, the voltage is zero, while on the outer surface of the dust layer, where new particles and ions arrive, the electrostatic voltage caused by the gas ions can be very high. The strength of this electric field depends on the resistance and thickness of the dust layer.
In a high-dust layer of dust, dust is not conductive enough, so that the electrical charge has trouble moving through the dust layer. As a result, the electrical charge accumulates in and below the surface of the dust layer, creating a strong electric field.
Voltage can be greater than 10,000 volts. High-resistance dust particles are held too strongly to the plate, making it difficult to remove and causing rap problems.
In the low resistance layer of dust, the corona flows are readily passed to the grounded collection electrode. Therefore, the relatively weak electric field, several thousand volts, is maintained throughout the dust layer. Dust particles collected with low resistance are not strong enough attached to the collection plate. They are easily flaked and stuck in the gas stream.
The electrical conductivity of the large particle layers depends on surface and volume factors. The volume conduction, or movement of electrical charge through the particle interior, depends mainly on the composition and temperature of the particles. In areas of higher temperatures, above 500 ° F (260 ° C), volume conduction controls the conduction mechanism. Volume conduction also involves supporting factors, such as particle coating compression, particle size and shape, and surface properties.
The volume conduction is represented in the figure as a straight line at a temperature above 500 ° F (260 ° C). At temperatures below about 450 ° F (230 ° C), electric charges begin to flow across the surface moisture and chemical films are adsorbed onto the particles. Surface conduction begins to decrease the resistivity value and bend the downward curve at temperatures below 500 ° F (260 ° C).
These films are usually different both physically and chemically from the interior of the particles due to the adsorption phenomenon. Theoretical calculations show that moisture films are just a few thick molecules sufficient to provide the desired surface conductivity. The surface conduction of the particles is closely related to the surface leakage occurring on the electrical insulator, which has been studied extensively. The interesting practical application of surface leakage is the determination of the dew point by measuring the current between adjacent electrodes mounted on the glass surface. The sharp rise in current signifies the formation of moisture films on the glass. This method has been used effectively to determine the rise of the dew point, which occurs when a small amount of sulfuric acid vapor is added to the atmosphere (Dewpoint Meter commercially available in the market).
The following discussion of normal, high, and low resistance applies to ESP which is operated in a dry state; resistance is not a problem in wet ESP operation due to the concentration of water vapor in ESP. The relationship between moisture content and endurance is described later in this work.
Normal resistivity
As stated above, ESP works best in normal resistivity conditions. Particles with normal resistivity do not quickly lose their charge upon arriving at the collection electrode. These particles are slowly leaking their charge to the grounded plates and stored in the collection plate with intermolecular adhesive and cohesive forces. This allows the particulate layer to be constructed and then removed from the plate by tapping. In the normal dust resistivity range (between 10 7 and 2 x 10 10 ohm-cm), fly ash is collected more easily than dust that has low or high resistivity.
High resistivity
If the voltage drops in the dust layer becomes too high, some adverse effects may occur. First, a high voltage drop reduces the voltage difference between the exhaust electrode and the collection electrode, and thereby reduces the strength of the electrostatic field used to drive the gas ion-charged particles to the collected dust layer. When the dust layer accumulates, and the electrical charge accumulates on the surface of the dust layer, the voltage difference between the discharge and the collecting electrode decreases. The rate of migration of small particles is mainly influenced by the reduced electric field strength.
Another problem that occurs with high resistivity dust layers is called back corona. This occurs when the potential for dust reduction is so great that the release of the corona begins to appear in the gas trapped within the dust layer. The dust layer is electrically damaged, producing tiny holes or craters where the corona exhaust occurs. Positive gas ions are generated within the dust layer and accelerated toward the "negative charge" electrode. The positive ion reduces some negative charge on the dust layer and neutralizes some negative ions in the "charged particles" into the collection electrode. Normal coronary process disorder greatly reduces the efficiency of ESP collection, which, in severe cases, can fall below 50%. When the corona is re-present, dust particles accumulate in the electrodes that form the insulating layer. Often this can not be fixed without bringing an offline unit.
The third, and generally the most common problem with high resistivity dust increases the electrical spark. When the trigger level exceeds "specified splash limit," the automatic controller limits the field operation voltage. This leads to reduced particle filling and reduced migration velocity to the collection electrode. High resistivity can generally be reduced by doing the following:
- Adjust the temperature;
- Increase water content;
- Adding the conditioning agent to the gas stream;
- Increase the surface area of ​​the collection; and
- Use hot-side precipitators (sometimes and by knowing in advance about sodium depletion).
The thin dust layer and high resistivity dust especially support the formation of the corona crater back. Severe back corona has been observed with a layer of dust as thick as 0.1 mm, but a layer of dust slightly more than one thick particle can reduce the trigger voltage by 50%. The most prominent effect of the rear corona on the voltage characteristics of current is:
- Sparkage reduction of up to 50% or more; â € <â € <
- The current leap or discontinuity caused by rear corona corona formation is stable; and
- A large increase in maximum corona current, which is just below the corona crack point may be several times the normal current.
The images below and to the left show variations in resistivity with changes in gas temperature for six different industrial dusts along with three fly ash fueled coal. The image on the right illustrates the measured resistivity values ​​for various chemical compounds prepared in the laboratory.
Results for Fly Ash A (in the picture to the left) are obtained in ascending mode. This data is typical for ash with moderate to high content. The data for Fly Ash B comes from the same sample, obtained during the temperature drop mode.
The difference between the rising and falling temperature modes is due to the unburned combustion in the sample. Between the two test modes, the sample was equilibrated in dry air for 14 hours (overnight) at 850 ° F (450 ° C). This overnight annealing process usually removes between 60% and 90% of any unburned combustion present in the sample. Exactly how carbon works as a charge carrier is not fully understood, but it is known to significantly reduce the resistivity of the dust.
Carbon can act, at first, like high resistivity dust on the precipitator. Higher voltage can be required for corona generation to start. This higher voltage could be a problem for the TR Set control. The problem lies in the corona onset which causes a large number of currents to rise through a layer of dust (low resistivity). The control feels this spike as a spark. As the controller is operated in split-divider mode, power is terminated and the corona generation cycle is restarted. Thus, a lower power reading (current) is recorded with a relatively high readout voltage.
The same is believed to be the case in laboratory measurements. Parallel plate geometry is used in laboratory measurements without corona generation. Stainless steel cup keeps the sample. Other stainless steel electrode weights sit above the sample (direct contact with the dust layer). When the voltage is increased from a small amount (eg 20 V), no current is measured. Then, the threshold voltage level is reached. At this level, the current surge through the sample... so much that the supply voltage unit can trip over. After removing unburned fuel during the above mentioned annealing procedure, the temperature drop-down temperature curve shows a typical inverted "V" shape that may be expected.
Low resistivity
Particles that have low resistivity are difficult to collect because they are easily charged (highly conductive) and quickly lose their charge upon arrival at the collection electrode. The particle takes the collection electrode charge, bounces the plate, and becomes entrained in the gas stream. Thus, an attractive and repulsive electrical force that normally operates at normal resistivity and higher is less, and the strength of binding to the plate is greatly reduced. Examples of dust with low resistivity are unburned carbon in fly ash and carbon black.
If these conductive particles are rough, they can be moved upstream from the precipitator by means such as a cyclone mechanical collector.
The addition of ammonia fluid (NH 3 ) into the gas stream as a conditioning agent has found much use in recent years. It is theorized that the ammonia reacts with H 2 SO 4 contained in the exhaust gas to form ammonium sulfate compounds that increase the dust cohesiveness. This additional attachment makes up for the loss of electrical attraction.
The table below summarizes the characteristics associated with low, normal and high resistivity dust.
The water content of the exhaust stream also affects the resistivity of the particles. Increase the water content of the gas stream by spraying water or injecting steam into the duct work before the ESP lowers resistivity. In both the temperature setting and humidity conditioning, one must maintain the gas condition above the dew point to prevent corrosion problems in ESP or downstream equipment. The figure on the right shows the effects of temperature and humidity on the resistivity of cement dust. As the percentage of moisture in the gas flow increases from 6 to 20%, the dust resistivity dramatically decreases. Also, raising or lowering the temperature may decrease the resistivity of cement dust for all the percentages of moisture represented.
The presence of SO 3 in the gas stream has been shown to support the electrostatic precipitation process when problems with high resistivity occur. Most of the sulfur content in burned coal for combustion sources is converted to SO 2 . However, about 1% of sulfur turns into SO 3 . The amount of SO 3 in the flue gas usually increases with increasing sulfur content of coal. The particle resistivity decreases as the coal sulfur content increases.
Other conditioning agents, such as sulfuric acid, ammonia, sodium chloride, and soda ash (sometimes as raw trace), have also been used to reduce particle resistivity. Therefore, the chemical composition of the flue gas stream is important in relation to the resistivity of the particles to be collected in ESP. The table below lists the various conditioning agents and their operating mechanisms.
If the ammonium sulfate injection occurs at a temperature greater than about 600 ° F (320 ° C), dissociate to the result of ammonia and sulfur trioxide. Depending on the ash, SO 2 may be privileged to interact with fly ash as SO 3 conditioning. The rest recombine with ammonia to increase the space charge and increase the compactness of ash.
Recently, it has been acknowledged that the main reason for the loss of electrostatic precipitator efficiency is due to the accumulation of particles in the charging cables next to the collection plate (Davidson and McKinney, 1998). This is easily fixed by ensuring that the cable itself is cleaned at the same time when the collecting plate is cleaned.
Sulfuric acid vapor (SO 3 ) increases the effect of water vapor on surface conduction. It is physically adsorbed inside the moisture layer on the surface of the particle. The effects of relatively small amounts of acid vapor can be seen in the figure below and on the right.
The resistivity attached to the sample at 300 Â ° F (150 Â ° C) is 5 Ã- 10 12 ohm-cm. The equilibrium concentration of only 1.9 ppm sulfuric acid vapor decreases that value to about 7 x 10 9 ohm-cm.
Modern industrial electrostatic precipitates
ESPs continue to be an excellent tool for controlling many industrial particulate emissions, including smoke from power-generating utilities (coal and oil fired), salt cake collection from black liquor boilers at pulp mills, and catalytic collection of catalytic cracker fluidized bed units in oil refinery to name a few. This device treats the volume of gas from several hundred thousand ACFM to 2.5 million ACFM (1,180 mÃ,³/s) in the largest coal-fired boiler application. For coal-fired boilers, collection is usually conducted downstream of the air preheater at about 160 ° C (320 ° F) which provides optimal resistivity of coal ash particles. For some difficult applications with low-sulfur fuel hot-end units have been built operating above 370 ° C (698 ° F).
Original parallel-weighted wire designs have evolved because more efficient (and strong) discharge electrode designs are developed, currently focusing on rigid discharge electrodes (pipes) where many sharpened spikes (barbed wire), maximize corona production. The transformer-rectifier system implements a voltage of 50-100 kV at relatively high current densities. Modern controls, such as automatic voltage control, minimize electric spark and prevent curvature (sparks quenched in 1/2 cycle of TR set), avoid damage to components. The automatic plate-rapping system and the hopper-evacuation system eliminate the particles collected while on-line, theoretically allowing ESP to remain in continuous operation for many years at a time.
Electrostatic examples for bioaerosol
Electrostatic precipitators can be used to sample biological airborne particles or aerosols for analysis. Sampling for bioaerosol requires optimized precipitator design with liquid counter electrodes, which can be used to sample biological particles, eg viruses, directly to small fluid volume to reduce unnecessary sample dilution. See Bioaerosols for more details.
Wet electrostatic precipitator
A wet electrostatic precipitator (WESP or wet ESP) operates with saturated water vapor flow (100% relative humidity). 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 gases are high in water content, contain flammable particles, or have particles that are sticky in nature.
The most preferred and most modern type of WESP is the tubular downflow design. This design allows moisture and collected particles to form moving slurries that help keep the collection surface clean. Plate style and WESPs upflow design is very unreliable and should not be used in applications where particulates are sticky.
Customer-oriented electrostatic air purifier
Plate precipitators are generally marketed to the public as an air purifier or as a permanent replacement for furnace filters, but all have undesirable attributes to be somewhat messy to clean. Negative side effects of electrostatic precipitation devices are the potential for toxic ozone production and NO x . However, electrostatic precipitators offer benefits compared to other air purification technologies, such as HEPA filtering, which require expensive filters and can be a "production sink" for various forms of harmful bacteria.
With an electrostatic precipitator, if the collecting plate is allowed to accumulate large amounts of particulate matter, the particles can sometimes bind very closely to metal plates that are rapidly washed and scrubbed may be necessary to completely clean the collecting plate. The close proximity of the plate can make cleaning difficult, and the pile of plates often can not be easily dismantled for cleaning. One solution, suggested by some manufacturers, is the dishwashing of collectors in dishwashers.
Some consumer precipitation filters are sold with special soak cleaners, in which all the plates are separated from the precipitator and immersed in a large oven oven, to help loosen the tightly bonded particles.
A study by Canada Mortgage and Housing Corporation that tested various forced-air furnace filters found that ESP filters provide the best and most cost-effective way to clean air using forced air systems.
The first portable electrostatic air filter system for the home was marketed in 1954 by Raytheon.
See also
- Scrubber
- Air ionizer
- Ozone generator
- Air Cleaning System
References
External links
- Applied Applied Electrostatic
Source of the article : Wikipedia
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