The method of electrical purification of gases from suspended particles is based on the phenomenon of ionization of gas molecules by an electric charge in an electric field. Gases, as dielectrics, do not conduct electricity. However, under certain conditions, the electrical conductivity of gases is observed. This is due to the fact that the atoms or molecules of the gas become electrically charged. A small amount of charged particles is always present in a gas. Their appearance is associated with the influence of ultraviolet and cosmic rays, radioactive gases, high temperature, etc. If such a gas containing a certain amount of charge carriers is placed between electrodes connected to a high voltage current source, then ions and electrons will begin to move in the gas along field lines. The direction of movement of each charge carrier will be determined by the magnitude of the charge, and the speed of movement - by the strength of the electric field. With a sufficiently high field strength (for example, about 16 kV / cm for air at atmospheric pressure and room temperature), the moving charge carrier acquires such a high speed that, colliding with a neutral gas molecule on its way, it is able to knock out one or more external electrons from it, converting the molecule into a positive ion and a free electron. The newly formed ions also begin to move under the influence of the field, producing further ionization of the gas. This ionization is called impact ionization. Number about

Rice. 12. The main systems of electrodes of electrostatic precipitators:

a - electrostatic precipitator;

b - plate electrostatic precipitator; +U, -U – voltage applied to the electrodes; R is the radius of the tubular electrode; H is the distance between the wire and the plate electrode; d is the distance between the wires; r - wire radius

of the ions and electrons generated in this case increases like an avalanche, and with a further increase in the field, they fill the entire space between the electrodes, thereby creating conditions for an electric discharge.

The most common and important for electrical gas cleaning are spark, arc and corona discharges. The first two types of discharges can occur both in a homogeneous and in a non-uniform electric field, being a hindrance in the operation of the electrostatic precipitator. A corona discharge can only occur in an inhomogeneous electric field and with certain shape and arrangement of the electrodes. Corona discharge is used for electrical cleaning.

There are two types of electrodes used in electrostatic precipitators:

a) electrodes of a tubular electrostatic precipitator (wire in a cylindrical pipe, Fig. 12 A);×

b) electrodes of a plate electrostatic precipitator (a row of wires between the plates, Fig. 12 b).

The density of field lines of the field, and hence the voltage. The field strength is much greater for a wire than for a plate or pipe wall. Due to the indicated inhomogeneity of the field, impact ionization, and then an electric discharge, can occur near the surface of the wire, when the field strength in this region is high enough, but does not propagate to another electrode. As you move away from the wire, the field strength decreases and the speed of electrons in the gas becomes insufficient to maintain the avalanche-like process of the formation of new ions. An electrical discharge of this incomplete nature is called corona discharge. as a result, new ions are formed, the external manifestation of which is a bluish-violet glow around the wire, a low crackle and the smell of nitrogen oxides and ozone. Corona discharge, depending on the sign of the charge on the wire, can be positive or negative. Outwardly, they differ in the nature of the glow. It has been established that when a negative DC polarity is applied to the corona electrode, it is possible to achieve dust capture up to 99%, and with a positive one, only up to 70%.

With negative polarity, it is possible to keep the voltage until the spark breakdown occurs higher than with positive polarity. This allows for a larger corona diameter and higher field strength, and hence better charging and dust particle deposition.

The electrode around which the corona discharge occurs is called coronating electrode, the second electrode collecting electrode.

The field strength at which a corona occurs is called critical tension. A high voltage direct current source is used. An electric current flows through the gap separating the electrodes. corona current. An increase in voltage is possible to a value at which the electrical strength of the gas gap between the electrodes will be violated by a spark or arc electric discharge, that is, until a “breakdown” of the interelectrode gap occurs.

The installation of electrostatic precipitators consists of two parts: from the actual electrostatic precipitator or settling chamber, through which the gas to be cleaned is passed, and high-voltage equipment designed to power the electrostatic precipitator with rectified high-voltage current.

The power generating unit consists of a voltage regulator, a high-voltage transformer that converts alternating current with a voltage of 220–380 V into voltage current up to 10,000 kV, and a mechanical high-voltage rectifier that converts alternating current into rectified current. The latter is fed to the electrodes of the electrostatic precipitator using a high-voltage cable.

Collecting and corona electrodes are mounted in the precipitating part of the electrostatic precipitator. Collecting electrodes can be lamellar (from corrugated steel with stamped pockets, from carbon plates, etc.) or tubular (from pipes of round or hexagonal cross section). Corona electrodes are made of round profiled wire.

Collecting electrodes are connected to the positive contact of the mechanical rectifier and grounded; discharge electrodes are isolated from the ground and connected to the negative contact of the mechanical rectifier. When passing through the interelectrode space of the electrostatic precipitator the purified gas containing solid or liquid suspended particles, the particles are charged with ions, which, under the action of an electric field, move to the electrodes and settle on them. The main mass of suspended particles is deposited on the collecting electrodes. In this case, liquid suspended particles flow down from the electrodes, dust-like particles are removed by shaking or tapping the electrodes. The trapped particles are collected in a hopper installed under the electrostatic precipitator, from where they are removed. Depending on which particles are captured, dry and wet electrostatic precipitators are distinguished.

Rice. 13. Case (A ) and gas distribution device (b) horizontal plate electrostatic precipitator:

a) 1 - prechamber; 2 – chamber for placement of electrodes; 3 and 4 - bunkers of the prechamber and electrostatic precipitator;5 - insulating box; 6 - the neck of the service hatch; b) 1 - headlightstuk fortkamera; 2 and 3 - front and rear gas distribution grilles; 4 - side gas-cutting sheets; 5 - protective sheets; 6 – texture of the bunker; 7 - transverse sheets of the bunker.

Electrostatic precipitators are also distinguished by the direction of movement of gases: vertical and horizontal. Usually, electrostatic precipitators are installed in parallel with several devices. the electrostatic precipitator can consist of several parallel sections in order to turn off part of the sections during operation (for inspection, repair, shaking) without stopping the entire gas cleaning plant. Sometimes electrostatic precipitators have several cells in series along the gas flow, or, as they are otherwise called, electric fields. According to the number of electric fields, such electrostatic precipitators are called two-field, three-field, etc. (Fig. 13).

In addition to the described single-zone electrostatic precipitators, two-zone ones are also used. If in the first case the ionization of gas with the help of a corona discharge and the deposition of charged particles occur in the same electric field (one zone), then in the second place these processes are separated. Two-zone electrostatic precipitators consist of an ionizer, which is a system of electrodes located closer to the gas inlet, and a precipitator, made of plate-type electrodes, on which charged dust is deposited.

In the ionizer, the deposition of dust must be excluded, therefore it consists of one row of electrodes and the dusty gas does not stay in this zone for long, so that the dust has time to charge, but does not have time to settle.

The speed of movement of fly ash particles in an electric field depends on their size and charge. For particles with a radius of less than 1 micron, the magnitude of the charge is proportional to the size of the dust particle and does not depend on the strength of the electric field. On the contrary, the amount of charge that particles with a radius greater than 1 micron acquire depends mainly on the magnitude of the field strength and the radius of the particle (squared).

The residence time of gases in the electrostatic precipitator greatly affects the quality of cleaning. Many years of experience has shown that the speed of gases in electrostatic precipitators is low (ranging from 0.5 to 2 m/s), and the residence time in the filter is significant (from 2 to 9 s). Therefore, electrostatic precipitators are quite bulky. But their hydraulic resistance is small (from 50 to 200 Pa). The cleaning coefficient, especially with fine dust, is high (95-99%). They are good at capturing particles smaller than 10 microns. The energy consumption for purification is insignificant and amounts to 0.10-0.15 kWh per 1000 m 3 of gas being purified. The main disadvantages of electrostatic precipitators are high cost and the need for highly qualified service personnel.

The quality of cleaning in electrostatic precipitators is influenced by temperature and humidity of gases. With an increase in gas temperature, the voltage on the corona electrodes decreases, which can be maintained without breakdown. This also reduces the degree of purification. The effect of gas humidity on the voltage in electrostatic precipitators is inverse to the effect of temperature: an increase in humidity contributes to an increase in the breakdown voltage and, in addition, favorably affects the behavior of the dust layer on the collecting electrodes. Sulfur oxides ( SO 2) are adsorbed in the dust layer on the collecting electrodes and change the behavior of the sediment layer. With a high concentration of dust in gases and with an increase in particle size, the danger of "locking the corona" increases. The concentration of dust at which the phenomenon of crown locking is observed varies depending on the dispersed composition of the dust from a few grams per 1 N×m 3 to several tens of grams per 1 N×m 3 .

The operation of dry electrostatic precipitators is significantly affected by the electrical resistivity of the collected dust. The dust contained in gases can be divided into three groups according to the specific volume electrical resistance:

1) dust with resistance up to 10 Ohm/cm;

2) dust with resistance from 10 to 2×10 Ohm/cm;

3) dust with a resistance of more than 2×10 ohm/cm. In this case, we mean the resistance of the dust layer formed on the collecting electrodes. Due to the adsorption of gases and vapors by dust particles, which fill the voids present in the dust layer, the electrical resistivity of the material from which the dust was formed changes.

The dust grains of the first group in contact with the collecting electrodes almost instantly lose their negative charge and acquire the charge of the electrodes. Having received the charge of the same name, the dust particles bounce off the electrodes and fall back into the gas stream. For reliable trapping of dust of the first group in the design of collecting electrodes, it is necessary to provide for a minimum velocity of gases near their surface. This is achieved, for example, by using wavy electrodes in horizontal electrostatic precipitators.

Dust of the second group (its majority) is captured in electrostatic precipitators without difficulty.

With the third group of dust, its layer on the collecting electrodes acts as insulation. Electric charges coming with the settling dust are not discharged to the collecting electrode, but create a voltage in the dust layer. When the voltage rises to a value when the electric field strength (gradient) becomes excessive, an electrical “breakdown” occurs in the pores of the layer filled with gas. This phenomenon, called the "reverse corona", is accompanied by the release of positive ions, which move towards the corona electrodes and partially neutralize the negative charge of the dust particles. At the same time, the positive ions released by the collecting electrodes convert the electric field between the electrodes of the electrostatic precipitator into a field similar to that formed between two tips, which easily breaks through at a low voltage.

Under these conditions, it is impossible to maintain a voltage in the electrostatic precipitator at which effective gas purification is achieved. To reduce the electrical resistance of the collected dust and increase the efficiency of electrostatic precipitators, it is recommended:

a) lowering the temperature of the purified gas;

b) humidification of the purified gas in front of the electrostatic precipitators (water vapor is sorbed by dust particles and the dust layer becomes electrically conductive even at a temperature significantly higher than the dew point);

c) introduction of sulfuric acid mist, alkaline amine compounds and other substances into the purified gas, which lower the electrical resistance of the dust layer.

The process of capturing ash entering the electrostatic precipitator with flue gases can be divided into four stages:

1) charging of ash particles with ions formed in the ion discharge zone;

2) movement of charged ash particles in the interelectrode space towards the collecting electrode under the action of electrical and aerodynamic forces;

3) deposition and retention of ash particles on the surface of collecting electrodes;

4) periodic removal of ash settled on the electrodes into the bunker. To increase the efficiency of gas purification in electrostatic precipitators, it is necessary that the first two stages proceed as completely as possible. If the charging of particles in an electrostatic precipitator with a stable corona charge is fast enough, then their movement to the collecting electrode occurs at a relatively low speed, depending on the particle charge, their size, field strength, aerodynamic characteristics of the flow, etc. It is obvious that the separation of particles ash from gases will be the more complete, the greater the settling rate (drift velocity) of particles and the residence time of the gases being cleaned in the active zone of the electrostatic precipitator. Since the possibility of increasing the drift velocity of particles is regulated by the physical characteristics of the process, the time of their residence in the electrostatic precipitator is determined by the speed of gases and the length of the active zone of the electrostatic precipitator, which leads to an increase in the volume and cost of the apparatus.

Studies have shown that when the residence time of the cleaned gases in the electrostatic precipitator is less than 8 s, one cannot expect a high (99%) degree of gas purification even under the most favorable conditions for its operation. Based on the industrial tests of multi-field electrostatic precipitators carried out by VTI and NIIOGAZ, it was found that in order to ensure a high degree of purification, the flue gas velocity should not exceed 1.5 m/s. This conclusion agrees with the data of foreign firms, which currently guarantee a high degree of purification only with a residence time of at least 8.5 s and a speed of 1.5 m/s. It is these values ​​that should be guided by when designing devices (electrostatic precipitators).

For high-power boiler units, the choice of the size and number of electrostatic precipitators is complicated by the problems of placing these devices in the block cell and arranging them with boilers and smoke exhausters. At most domestic power plants, the layout of electrostatic precipitators is used in one row along the width of the block cell, when the longitudinal axes of the electrostatic precipitators are parallel to the longitudinal axis of the block. This arrangement makes it easier to ensure a uniform distribution of gases between the individual devices. But at the same time, on units with a capacity of 300 MW and more, electrostatic precipitators of old designs with an electrode height of 7.5 m cannot meet the requirements.

For the designed units with a capacity of 300 and 500 MW with electrostatic precipitators of a new design and electrodes of 12 m, the speed and residence time of the gases correspond to the above requirements.

It is impossible to design electrostatic precipitators for the minimum excess of air and the minimum temperature of the flue gases. The usually observed deviation of these parameters from the design ones is the reason for the increase in the velocity of gases in electrostatic precipitators by 20–25% and, as a result, some deterioration in gas purification. Thus, to ensure the required cleaning of flue gases from powerful power plants, it is necessary to count electrostatic precipitators for a 1.2-fold increase in the amount of cleaned gases (except for pressurized boilers).

In recent years, electrostatic precipitators with needle corona electrodes have been supplied to power plants. The characteristic features of the discharge from the electrodes, compared with the discharge that occurs on the electrodes of the bayonet profile, are the stability of the position of the corona points and the higher value of the current loads, which is especially important for devices installed behind boilers equipped with furnaces with liquid ash removal, as well as with high resistivity ash layer or high dust content of flue gases.

When comparing the electrodes of the two indicated types, a significant difference in the intensity of the discharge at the corona points attracts attention. An increase in the field strength and current strength of a short discharge when using needle electrodes is explained by an increase in the curvature of the surface due to the curvature in two sections. In this regard, the conditions for charging ash particles are improved, which ensures an increase in the drift velocity in the direction of the collecting electrodes. The intensification of the corona discharge in electrostatic precipitators when using needle corona electrodes is also accompanied by some side effects. In the corona zone there are electrons with an energy exceeding the activation energy. This causes a process of chemical interaction: sulfurous anhydride is oxidized to sulfuric ( SO 2 –SO 3), nitrogen oxides appear. Thus, experiments in a high-frequency corona discharge increased the content of sulfuric anhydride to 20-50% and the oxidation of nitrogen by 0.2-0.3%.

Horizontal multi-field electrostatic precipitators are devices of continuous operation. Ash is removed from the electrodes by shaking them without disconnecting the electrostatic precipitator from the power source and flue gas flow. In this case, part of the ash will inevitably enter the gas flow. This process has been named re-entrainment and is the main reason for the reduced efficiency of dry electrostatic precipitators compared to wet ones, in which particles are deposited on a water or oil film and there is no re-entrainment. The amount of re-entrainment is directly dependent on the interval between shaking the collecting electrode.

In electrostatic precipitators of domestic production, each collecting electrode is shaken after 3 minutes, regardless of the dust content of gases, cleaning efficiency, gas velocity, etc. When the resistivity of the ash is high, the ash layer prevents the charges continuously flowing to its surface from flowing to the grounded electrode. However, it should be taken into account that usually there is a non-shake off layer 1–2 mm thick on the collecting electrodes. The thickness of the layer of ash settled in 3 minutes, even when high-ash fuels are burned, is 100-200 microns for the first fields of the electrostatic precipitator. Thus, a tenfold increase in the interval between shaking will slightly increase the overall thickness of the layer. Therefore, this interval can be significantly increased. When hydrotransporting ash to an ash dump under the ash collectors, continuous hydraulic seals with an open overflow are usually installed. In this case, there are no incoming ash dispensers. Therefore, with the simultaneous discharge of a large amount of ash into them, pulp or even dry ash may be ejected through the open hatches of the hydraulic seal into the ash room. To calculate the maximum allowable time interval between shaking under the conditions of the hydraulic seal, the following equation is proposed:

Here With- the maximum allowable concentration of ash in the pulp (500-800 g/l); V- the volume of the pulp in the hydraulic seal, m 3; G- water consumption for the hydraulic seal, m 3 / s; F - the calculated section of the section of the electrostatic precipitator above this bunker, m 2; h– average degree of ash collection; t– time interval between shaking, s.

In this case, the shaking period of each electrode

T =t × P,

Where n- the number of electrodes above the given bunker.

It has been proposed the use of options that allow you to change the shaking interval. Tests have shown that an increase in the interval of shaking the collecting electrodes of the first field up to 30 minutes, and the last fields up to 2 hours, with the help of a variator, reduced the amount of ash taken out of the electrostatic precipitator (secondary entrainment) by about 1/3.

The amount of ash emitted into the atmosphere depends, in addition to the efficiency of the electrostatic precipitator, also on what part of the total operating time of the power unit individual fields of electrostatic precipitators are in a non-operating state. Most often, the fields are turned off due to malfunctions inside the electrostatic precipitator housing, which can be eliminated only when the power unit is completely shut down: wire breakage of corona electrodes (most often as a result of electroerosion), breakage of insulators and rods of the shaking mechanism, breakage and jamming of shaking strips, etc.

An examination of many electrostatic precipitators at domestic power plants shows that the design of the inlet gas ducts and the perforated grate at the inlet to the electrostatic precipitators do not provide the necessary uniform distribution of gases over the apparatus and their cross section. This leads to a general decrease in the overall efficiency of ash collection even in the normal electric mode of the electrostatic precipitator.

The ability to breathe clean air is our physiological need, a guarantee of health and longevity. However, powerful modern manufacturing enterprises pollute our environment and atmosphere with industrial emissions that are dangerous to humans.

Ensuring the purity of the air environment when performing technological processes at enterprises and removing harmful impurities from it in everyday life - these are the tasks that electrostatic filters perform.

The first such design was registered in US Patent No. 895,729 in 1907. Its author, Frederick Cottrell, was engaged in research on methods for separating suspended particles from gaseous media.

To do this, he used the action of the basic laws of the electrostatic field, passing gaseous mixtures with solid fine impurities through electrodes with positive and negative potentials. Oppositely charged ions with dust particles were attracted to the electrodes, settling on them, and the same ions were repelled.

This development served as a prototype for the creation modern electrostatic filters.

Potentials of opposite signs from a direct current source are applied to plate sheet electrodes (commonly called the term "precipitating"), collected in separate sections, and metal threads-grids placed between them.

The voltage between the grid and the plates in household appliances is several kilovolts. For filters operating at industrial facilities, it can be increased by an order of magnitude.

A stream of air or gases containing mechanical impurities and bacteria is passed through these electrodes by fans through special air ducts.

Under the influence of high voltage, a strong electric field and a surface corona discharge are formed, flowing down from the filaments (corona electrodes). It leads to ionization of the air adjacent to the electrodes with the release of anions (+) and cations (-), an ion current is created.

Ions with a negative charge under the action of an electrostatic field move to the collecting electrodes, simultaneously charging counter impurities. Electrostatic forces act on these charges, creating an accumulation of dust on the collecting electrodes. In this way, the air passing through the filter is purified.

During filter operation, the dust layer on its electrodes constantly increases. It needs to be removed periodically. For household structures, this operation is performed manually. In powerful production plants, precipitation and corona electrodes are mechanically shaken to direct contaminants into a special hopper, from where they are taken for disposal.

Design features of an industrial electrostatic filter

The details of its body can be made of concrete blocks or metal structures.

Gas distribution screens are installed at the inlet of contaminated air and the outlet of purified air, which optimally direct the air masses between the electrodes.

Dust collection takes place in hoppers, which are usually built with a flat bottom and equipped with a scraper conveyor. Dust collectors are made in the form of:

trays;

inverted pyramid;

truncated cone.

Electrode shaking mechanisms work on the principle of a falling hammer. They can be located below or above the plates. The operation of these devices significantly speeds up the cleaning of the electrodes. The best results are achieved by designs in which each hammer acts on its own electrode.

To create a high-voltage corona discharge, standard transformers with rectifiers operating from an industrial frequency network or special high-frequency devices of several tens of kilohertz are used. Their work is carried out by microprocessor control systems.

Among the various types of corona electrodes, stainless steel spirals work best, creating optimal filament tension. They are less polluted than all other models.

The designs of collecting electrodes in the form of plates of a special profile are combined into sections, created for a uniform distribution of surface charges.

Industrial filters for capturing highly toxic aerosols

An example of one of the schemes of operation of such devices is shown in the picture.

These structures use a two-stage zone for cleaning air contaminated with solid impurities or aerosol vapors. The largest particles settle on the pre-filter.

As a result, a corona discharge and impurity particles are charged. The blown air mixture passes through the precipitator, in which harmful substances are concentrated on grounded plates.

The post-filter located after the precipitator captures the remnants of unsettled particles. The chemical cassette additionally purifies the air from the remaining impurities of carbon dioxide and other gases.

The aerosols deposited on the plates simply flow down the tray under the action of gravitational forces.

Applications of industrial electrostatic filters

Purification of polluted air environments is used on:

power plants with boilers burning coal;

fuel oil burning facilities;

waste incineration plants;

industrial chemical recovery boilers;

production kilns for annealing limestone;

process boilers for biomass combustion;

ferrous metallurgy enterprises;

production of non-ferrous metals;

cement industry facilities;

enterprises of processing agricultural products and other industries.

Possibilities for purification of polluted media

The operating ranges of powerful industrial electrostatic filters with various harmful substances are shown in the diagram.

Design features of filters in household appliances

Air purification in residential premises is carried out:

air conditioners;

ionizers.

The principle of operation of the air conditioner is shown in the picture.

Polluted air is driven by fans through the electrodes with a voltage of about 5 kilovolts applied to them. Microbes, mites, viruses, bacteria in the air flow die, and impurity particles, being charged, fly to the dust trapping electrodes and settle on them.

In this case, the air is ionized and ozone is released. Since it belongs to the category of the strongest natural oxidizing agents, all living organisms inside the air conditioner are destroyed.

Exceeding the normative concentration of ozone in the air is unacceptable according to sanitary and hygienic standards. This indicator is carefully monitored by the supervisory authorities of air conditioner manufacturers.

Features of a household ionizer

The prototype of modern ionizers was the development of the Soviet scientist Chizhevsky Alexander Leonidovich, which he created to restore the health of people exhausted in prison by the hardest hard labor and poor conditions of detention.

Due to the application of high voltage to the electrodes of a source suspended from the ceiling instead of a lighting chandelier, ionization occurs in the air with the release of healthy cations. They were called "aeroions" or "vitamins from the air."

Cations gave vital energy to a weakened body, and the released ozone killed pathogenic microbes and bacteria.

Modern ionizers are devoid of many of the shortcomings that were in the first designs. In particular, the concentration of ozone is now strictly limited, measures are being taken to reduce the effect of a high-voltage electromagnetic field, and bipolar ionization devices are being used.

However, it is worth noting that many people still confuse the purpose of ionizers and ozonizers (production of ozone in the maximum amount), using the latter for other purposes, which greatly harm their health.

Ionizers, by the principle of their work, do not perform all the functions of air conditioners and do not purify the air from dust.

Collection output:

Electrostatic precipitators: principle of operation and main advantages

Nikolaev Mikhail Yurievich

cand. tech. Sciences, Associate Professor, Omsk State Technical University, Russian Federation, Omsk

E- mail: munp@ yandex. en

Esimov Asset Mukhammedovich

Technical University, Russian Federation, Omsk

E- mail: esimov007@ mail. en

Leonov Vitaly Vladimirovich

3rd year student, Faculty of Energy, Omsk State

technical university, RF, G. Omsk

ELECTROSTATIC PRECIPITATORS: WORKING PRINCIPLE AND MAIN DIGNITIES

Nikolaev Michael

Candidate of Technical Sciences, Associate Professor of Omsk State Technical University, Russia, Omsk

Esimov Aset

Leonov Vitaliy

student, the Institute of Energy of Omsk State Technical University, Russia, Omsk

ANNOTATION

This article discusses the detailed principle of operation of electrostatic precipitators. Various types of electrostatic precipitators, collecting and corona electrodes are also considered. Cases are given in which the process of ionization of gases between the electrodes occurs. The advantages of modern electrostatic precipitators are described.

ABSTRACT

This article describes the detailed working principle of electrostatic precipitators. It also considered various types of electrostatic precipitators, the collecting and corona electrodes. Situations in which the process gases between the ionization electrodes. Described the dignities of modern electrostatic precipitators.

Keywords: electrostatic precipitator; electrode; ionization; corona rank.

keywords: electrostatic precipitator; electrode; ionization; corona discharge.

An electrostatic precipitator is a device in which gases are cleaned from aerosol, solid or liquid particles under the action of electrical forces. As a result of the action of the electric field, charged particles are removed from the gas stream being purified and deposited on the electrodes. The particles are charged in the field of a corona discharge. The electrostatic precipitator is a housing of a rectilinear or cylindrical shape, inside which collecting and corona electrodes of various designs are mounted (depending on the purpose and scope of the electrostatic precipitator, as well as on the specifics of the particles being trapped). The corona electrodes are connected to a high-voltage power supply with a rectified current of 50-60 kV. Electrostatic precipitators, in which trapped solid particles are removed from the electrodes by shaking, are called dry, and those in which precipitated particles are washed off the electrodes with liquid or liquid particles (fog, splashes) are trapped are wet.

According to the number of electric fields through which the purified gas sequentially passes, electrostatic precipitators are divided into single-field and multi-field. Sometimes electrostatic precipitators are divided into chambers parallel along the gas flow - sections. On this basis, they can be single- and multi-section. The gas purified in the electrostatic precipitator passes through the active zone in vertical or horizontal directions, so electrostatic precipitators are either vertical or horizontal. According to the type of collecting electrodes, electrostatic precipitators are divided into plate and tubular. The main design types of electrostatic precipitators are horizontal plate and vertical tubular.

Figure 1. Horizontal plate electrostatic precipitator

Figure 2. Tubular electrostatic precipitator

To understand the principle of operation of an electrostatic precipitator, you must first consider the electrical circuit. It consists of elements such as a current source and two metal plates parallel to each other, which are separated by air. This device is nothing more than an air condenser, however, electric current will not flow in such a circuit, because the layer of air between the plates, like other gases, is not capable of conducting electricity.

However, one has only to apply the necessary potential difference to the metal plates, as a galvanometer connected to this circuit will record the passage of an electric current due to the ionization of the air layer between these plates.

As for the ionization of gas between two electrodes, it can occur in two cases:

1. Not independently, that is, with the use of any "ionizers", for example, x-rays or other rays. After the impact of this “ionizer” is over, recombination will gradually begin, that is, the reverse process will occur: ions of different signs will again begin to combine with each other, thereby forming electrically neutral gas molecules.

2. Independently, it is carried out by increasing the voltage in the electrical network to a value that exceeds the value of the dielectric constant of the gas used.

In electrical gas cleaning, only the second ionization is used, that is, independent.

If you start to increase the potential difference between the metal plates, then at some point it will certainly reach a critical point (breakdown voltage for the air layer), the air will be “pierced” and the current will increase sharply in the circuit, and a spark will appear between the metal plates, which they called – independent gas discharge.

Air molecules under tension begin to split into positively and negatively charged ions and electrons. Under the influence of an electric field, ions move towards electrodes that are oppositely charged. With an increase in the electric field voltage, the speed, and, accordingly, the kinetic energy of ions and electrons begins to gradually increase. When their speed reaches a critical value and slightly exceeds it, they split all the neutral molecules encountered on the way. This is how the ionization of all the gas located between the two electrodes occurs.

When a fairly significant number of ions are simultaneously formed between parallel plates, the strength of the electric current begins to increase strongly and a spark discharge appears.

Due to the fact that air molecules receive impulses from ions moving in a certain direction, along with the so-called “impact” ionization, a rather intense movement of the air mass also occurs.

Self-ionization in the technique of gas electropurification is carried out by applying high voltages to the electrodes. When ionizing in this way, it is necessary that the gas layer break through only at a certain distance between the two electrodes. It is necessary that part of the gas remains unbroken and serves as a kind of insulation that would protect the parallel electrodes from a short circuit from a spark or arc (so that there is no breakdown of the dielectric).

This "isolation" is created by selecting the shape of the electrodes, as well as the distance between them in accordance with the voltage. It is worth noting that the electrodes, which are presented in the form of two parallel planes, will not work in this case, since there will always be the same voltage between them at any point in the field, that is, the field will be invariably uniform. When the potential difference between one flat electrode and the other reaches the breakdown voltage, all the air will be pierced and a spark discharge will appear, however, air ionization will not happen due to the fact that the entire field is uniform.

An inhomogeneous field can only arise between electrodes that have the form of concentric cylinders (pipes and wires), or a plane and a cylinder (plate and wires). Directly near the wire, the field voltage is so high that ions and electrons become capable of ionizing neutral molecules, but as you move away from the wire, the field voltage and ion velocity decrease so much that impact ionization simply becomes unrealistic.

The ratio between the radius of the pipe (R) and the wire (r) must always be determined in order to avoid the appearance of a spark between two cylindrical electrodes. Calculations have shown that gas ionization without a short circuit is possible at R/r greater than or equal to 2.72.

The appearance around the wire of a weak glow or the so-called "crown" is the main visible sign that an ion discharge has occurred. This phenomenon is called corona discharge. A weak glow constantly accompanies a characteristic sound - it can be crackling or hissing.

The wire (electrode) around which the glow occurs is called the corona electrode. "Crown", depending on which pole the wire is connected to, is either positive or negative. For electrical gas cleaning, only the second option is used, that is, the negative "crown". Although, in contrast to the positive one, it is less uniform, such a “crown” is still capable of allowing a higher critical potential difference.

The following requirements are imposed on the collecting electrodes: to be strong, rigid, have a smooth surface so that the trapped dust can be removed without problems, as well as sufficiently high aerodynamic characteristics.

Collecting electrodes are conditionally divided into three large groups in terms of shape and design: 1) plate; 2) box-shaped; 3) grooved.

The following requirements are imposed on corona electrodes: they must have an exact shape in order to provide an intense and sufficiently uniform corona discharge; have mechanical strength and rigidity to ensure reliable, trouble-free and durable operation under conditions of shaking and vibration; be easy to manufacture and have a low cost, since discharge electrodes can reach a length (total) of 10 kilometers; be resistant to aggressive environments.

There are two large groups of discharge electrodes: electrodes without fixed discharge points and electrodes with fixed discharge points along the entire length of the electrode. The second sources of discharge are sharp protrusions or spikes, while it is possible to control the operation of the electrode. To do this, you need to change the distance between the spikes.

The system of collecting and corona electrodes is placed, as a rule, inside a welded metal case, in rare cases in a reinforced concrete case, which is made in the form of U-shaped frames. Equipment inside the case is loaded either from above or from the side. The housing must be thermally insulated from the outside to avoid temperature deformations and the appearance of moisture condensation.

The unit for the supply and uniform distribution of dusty air, as a rule, consists of a system of gas distribution gratings, which are installed in front of the main chamber, where a system of collecting and corona electrodes is located, and is a perforated sheet installed in two tiers, their free cross section is from 35 to 50 percent.

To remove trapped dust from electrostatic precipitators, special electrode shaking systems are used. In dry electrostatic precipitators, several such systems are usually used - these are spring-cam, shock-hammer, vibration, or magnetic-pulse systems. In addition, trapped particles can simply be washed off the electrodes with water.

Advantages of electrostatic precipitators: the possibility of the highest degree of gas purification (up to 99.9%), low energy costs (up to 0.8 kW per 1000 m 3 of gas), gas purification can be carried out even at high temperatures, the purification process can be fully automated.

Bibliography:

1. GOST R 51707-2001. Electrostatic precipitators. Safety requirements and test methods. Introduction 01/29/2001. M: Publishing house of standards, 2001.

2. Rules for electrical installations. 7th ed. M.: Publishing house of NTs ENAS, 2004.

3. Sanaev Yu.I. Electrostatic precipitators: installation, adjustment, testing, operation./Overview information. XM-14 series. M., "TSINTIKHIMNEFTEMASH", 1984.

Even in an ordinary apartment, the air needs to be cleaned, and elementary ventilation cannot always cope with this task.

In this regard, modern filters are widely used, which can delay:

• animal fur,
• dust,
• plant pollen,
• tobacco smoke, unpleasant odors,
• bacteria, viruses,
• mold, fungal spores and others.

All of these contaminants can cause allergies and are potentially dangerous. One of the most popular and affordable filters on the market is electrostatic.

Electrostatic filter for ventilation is used to remove aerosol and mechanical particles from the air: soot, soot, smoke, fine dust, toxic fumes, fine dust and other hazardous household and industrial pollutants.

Such an air purifier consists of the following components:

• coarse filter with steel mesh inside,
• the first plate filter with flat electrodes,
• second plate filter with flat electrodes,
• fine filter, usually with activated carbon.

The contents of the device may vary depending on the power level and other indicators. The more expensive the equipment, the more power it has. Inexpensive filters can be used in city apartments. For manufacturing enterprises, they acquire expensive equipment that meets fairly stringent requirements.

Air flow passing through several stages of cleaning electrostatic filter devices, namely: an ionizer, a dust collector and several filters at the outlet, it turns out to be almost sterile.
The principle of operation of an electrostatic device is the attraction of electric charges with different polarities. Particles in the air, getting into the filter, acquire an electric charge and settle on conductive plates with opposite polarity.

During the operation of such an air filter, ozone is released, which many associate with the smell of a thunderstorm. During the operation of industrial plants, N2 is destroyed to nitrogen oxides, since ozone itself is a rather dangerous and toxic substance that can cause allergic reactions and burns to the respiratory system.

Electrostatic filter - what is the efficiency

Such equipment is used in medical institutions, catering establishments, in administrative and office buildings.

VIDEO REVIEW

Manufacturer rating - which electrostatic filters are the most popular

The choice of electrostatic devices in stores is quite large. Residents may have difficulty with the selection of equipment for home, office or production workshop. First of all, you need to study the technical characteristics of the device, pay attention to the price.

Too cheap devices are unlikely to be able to cope with their task at the proper level, while very expensive ones should not be purchased for an ordinary apartment, they are intended for use in large enterprises.

For the home, as well as for the car, you can purchase a compact version Super-Plus-Ion-Auto from the manufacturer "Ecology Plus". It is a small unit, consumes about 3 watts of electricity. The cost of the product is from 30 to 50 dollars.

Plymovent Group offers SFE equipment. This is already quite serious equipment worth about 200 thousand rubles. It passes through itself 2500 cubic meters of air in one hour. And this is quite enough to serve the office, the trading floor and even the small size of the assembly shop.

Catering establishments use ovens and barbecues for cooking. Pleasant smoke during frying or baking has a downside - it can be hazardous to health, so it is important for restaurant owners to protect both visitors and employees from it.

For this, electrostatic filters Smoke Yatagan are used. They absorb soot, fats, carcinogens, odors and smoke. The pre-filter of the device must be washed periodically. The equipment is unpretentious in operation, differs in high efficiency.

VIDEO INSTRUCTION

Electrostatic filter Efva Super Plus - designed for air purification in industrial environments. Detains oil, welding aerosols emitted during metal processing, production of medical drugs, in arc welding shops and others.

OMSK STATE UNIVERSITY

THEM. F.M. DOSTOYEVSKY

DEPARTMENT OF CHEMICAL TECHNOLOGY

Essay on nature protection on the topic "Electrofilters"

Completed by: student group xx‑601(eh)

Levin D.K.

Checked by: professor

Adeeva L.N.

Department of National Economy

Omsk - 2010

Introduction

Industrial production and other types of economic activities of people are accompanied by the release into the indoor air and into the atmospheric air of various substances that pollute the air. Aerosol particles (dust, smoke, fog), gases, vapors, as well as microorganisms and radioactive substances enter the air.

At the present stage, for most industrial enterprises, the purification of ventilation emissions from harmful substances is one of the main measures to protect the air basin. By cleaning emissions before they enter the atmosphere, air pollution is prevented.

Air purification has the most important sanitary-hygienic, ecological and economic importance.

The stage of dust cleaning occupies an intermediate place in the complex "labor protection - environmental protection". In principle, dust collection, with proper organization, solves the problem of ensuring the standards for maximum permissible concentrations (MPC) in the air of the working area. However, all hazards through the dust collection system in the absence of a dust cleaning system are released into the atmosphere, polluting it. Therefore, the dust cleaning step should be considered an integral part of the dust control system of an industrial enterprise.

Gas cleaning - separation of various impurities from the gas mixture when it is released into the atmosphere in order to maintain normal sanitary conditions in areas adjacent to industrial facilities, prepare gases for use as chemical raw materials or fuel, and the impurities themselves as valuable products. Gas cleaning is usually divided into cleaning from suspended particles - dust, fog, and from vaporous and gaseous impurities that are undesirable when using gases or when they are released into the atmosphere.

Industrial methods of gas purification can be reduced to three groups:

1) with the help of solid absorbers or catalysts - "dry methods" of cleaning;

2) using liquid absorbents (absorbents) - liquid cleaning;

3) purification without the use of absorbers and catalysts.

The first group includes methods based on adsorption, chemical interaction with solid absorbers, and on the catalytic conversion of impurities into harmless or easily removable compounds. Dry cleaning methods are usually carried out with a fixed bed of sorbent, scavenger or catalyst, which must be periodically regenerated or replaced. Recently, such processes are also carried out in a "fluidized" or moving bed, which allows a continuous renewal of cleaning materials. Liquid methods are based on the absorption of the extracted component by a liquid sorbent (solvent). The third group of purification methods is based on the condensation of impurities and diffusion processes (thermal diffusion, separation through a porous partition).

The particles contained in industrial gases are extremely diverse in their composition, state of aggregation, and dispersion. Purification of gases from suspended particles (aerosols) is achieved by mechanical and electrical means. Mechanical purification of gases is carried out by: centrifugal force, filtration through porous materials, washing with water or other liquid; sometimes their force of gravity is used to release large particles. Mechanical cleaning of gases is usually carried out by dry gas cleaning (cyclone apparatus), filtration and wet gas cleaning. Electrical gas cleaning is used to trap highly dispersed dust particles or mists and provides, under certain conditions, a high cleaning factor.

In my report, I will describe the principles of electrical gas purification, the operation of electrostatic precipitators, their types, the possibilities of combined use for gas purification, as well as the advantages and disadvantages of their use.

1. The principle of operation of electrostatic precipitators

In the electrostatic precipitator, the purification of gases from solid and liquid particles occurs under the action of electrical forces. The particles are given an electric charge, and they are deposited from the gas stream under the action of an electric field.

The general view of the electrostatic precipitator is shown in fig. 1.

Rice. 1. Electrostatic precipitator: 1 - collecting electrode; 2 - corona electrode; 3 - frame; 4 - high-voltage insulator; 5 - shaking device; 6 - upper chamber; 7 - dust collector.

The process of dedusting in an electrostatic precipitator consists of the following stages: dust particles, passing through an electric field with a gas flow, receive a charge; charged particles move to electrodes with the opposite sign; deposited on these electrodes; the dust deposited on the electrodes is removed.

Particle charging is the first major step in the electrostatic deposition process. Most of the particles dealt with in industrial gas cleaning carry some charge by themselves during their formation, but these charges are too small to provide effective deposition. In practice, particle charging is achieved by passing the particles through a DC corona between the electrodes of the electrostatic precipitator. Both positive and negative corona can be used, but for industrial gas cleaning, negative corona is preferable because of greater stability and the possibility of using high operating voltages and currents, but only positive corona is used in air purification, since it produces less ozone.

The main elements of the electrostatic precipitator are corona and collecting electrodes. The first electrode in its simplest form is a wire stretched in a tube or between plates, the second is the surface of a tube or plate surrounding the corona electrode (Fig. 2).

A high voltage direct current of 30…60 kV is supplied to the discharge electrodes. The corona electrode usually has a negative polarity, the collecting electrode is grounded. This is explained by the fact that the corona at this polarity is more stable, the mobility of negative ions is higher than that of positive ones. The latter circumstance is related to the acceleration of the charging of dust particles.

After the switchgears, the processed gases enter the passages formed by the corona and precipitation electrodes, called interelectrode gaps. The electrons descending from the surface of the corona electrodes are accelerated in a high-strength electric field and acquire energy sufficient to ionize gas molecules. Gas molecules colliding with electrons are ionized and begin to move rapidly in the direction of electrodes of opposite charge, upon collision with which new portions of electrons are knocked out. As a result, an electric current appears between the electrodes, and at a certain voltage value, a corona discharge is formed, which intensifies the process of gas ionization. Suspended particles, moving in the ionization zone and sorbing ions on their surface, eventually acquire a positive or negative charge and begin to move to the electrode of the opposite sign under the influence of electrical forces. Particles are strongly charged in the first 100...200 mm of the path and are displaced to grounded collecting electrodes under the influence of an intense corona field. The process as a whole proceeds very quickly, it takes only a few seconds for the complete deposition of particles. As particles accumulate on the electrodes, they are shaken off or washed off.

Rice. Fig. 2. Structural scheme of electrodes: a - electrostatic precipitator with tubular electrodes; b - electrostatic precipitator with plate electrodes; 1 - corona electrodes; 2 - collecting electrodes.

A corona discharge is characteristic of inhomogeneous electric fields. To create them in electrostatic precipitators, systems of electrodes such as point (point) - plane, line (sharp edge, thin wire) - plane or cylinder are used. In the corona field of an electrostatic precipitator, two different mechanisms of particle charging are implemented. The most important is the charging by ions, which move towards the particles under the action of an external electric field. The secondary charging process is due to the diffusion of ions, the rate of which depends on the energy of the thermal motion of the ions, but not on the electric field. Charging in the field prevails for particles with a diameter of more than 0.5 μm, and diffusion - for particles smaller than 0.2 μm; in the intermediate range (0.2…0.5 µm), both mechanisms are important.

2. Designs and types of electrostatic precipitators

Devices for gas purification by this method are called electrostatic precipitators. The main elements of electrostatic precipitators are: a gas-tight housing with corona electrodes placed in it, to which a rectified high-voltage current is supplied, and precipitating grounded electrodes, electrode insulators, devices for uniform distribution of the flow over the cross section of the electrostatic precipitator, a hopper for collecting trapped particles, an electrode regeneration system and power supply .

Structurally, electrostatic precipitators can be with a rectangular or cylindrical body. Collecting and corona electrodes are mounted inside the housings, as well as mechanisms for shaking the electrodes, insulator units, and gas distribution devices.

The part of the electrostatic precipitator in which the electrodes are placed is called the active zone (less often, the active volume). Depending on the number of active zones, single-zone and two-zone electrostatic precipitators are known. In single-zone electrostatic precipitators, the corona and collecting electrodes are spatially not separated structurally. In two-zone electrostatic precipitators there is a clear separation. For sanitary cleaning of dusty emissions, single-zone structures are used with the placement of corona and collecting electrodes in the same working volume. Two-zone electrostatic precipitators with separate zones for ionization and sedimentation of suspended particles are used mainly in the purification of supply air. This is due to the fact that ozone is released in the ionization zone, which is not allowed to enter the air supplied to the premises.