Methods for cleaning air from dust when emitted into the atmosphere. Methods for dust removal.
In dust collection processes, the size of dust particles, their density, charge, resistivity, adhesive properties, wettability, etc., are essential.
The following types of dust are distinguished by the size of solid particles:
More than 10 microns;
0.25-10 microns;
0.01-0.25 microns;
Less than 0.01 microns.
The dust collection efficiency of small particles is less - 50–80%, larger ones - 90–99.9%.
The effectiveness of electrostatic filters depends. 2. Reducing particulate emissions. An installation for the deposition of dust in the exhaust gas is selected. Particle diameter distribution. The resistivity of the powders depends, in particular, on the composition of the waste. Electrostatic precipitators provide a constant separation of particles regardless of their size.
Wet scrubbers can only work efficiently if particles can become wet. Rotary scrubbers have relatively low pressure losses and operate independently of fluctuations in the exhaust gases processed in the process. In dry adsorption processes. Residual gas particles are also adsorbed. This process uses the heat from the exhaust gas to evaporate the solvent and, therefore, produce solids. Large fluctuations in the composition of the residual gas depend on the composition of the waste and counteract the inevitable increase in concentration in the residual gas.
There are two types of dust collectors: dry and wet. Dust precipitation chambers, cyclones, vortex cyclones, electrostatic precipitators collect dust by the dry method. Foam apparatuses, Venturi scrubbers, etc. are used to clean dust from the wet method.
Dry dust collectors, dust precipitation chambers. These are the simplest apparatuses that use a gravitational field for dust deposition, and when installing partitions, an inertial field. The dust collection efficiency of more than 25 microns in size is 50–80%. To clean hot flue gases from dust with a size of more than 20 microns at a temperature of 450-600 ° C, louver dust separators are used. In them, the separation of dust from the main gas stream occurs due to the inertial forces arising from a sharp turn of the gas stream being cleaned when it passes through the grate blinds. The cleaning efficiency reaches 80%.
With high pressure loss. Moisten and pour out the liquid. The lack of preliminary separation leads to more complex use and removal of gases due to their composition. With one step or several steps. Compatible wet separators - Venturi or rotating scrubbers. In the process of spraying. Like dust from residual gas. These disadvantages can be avoided by using a suitable design, especially if it is designed to obtain highly efficient separation of very fine powders. The amount of adsorbent must be greater than the calculated stoichiometric amount.
Figures 14 and 15 show the schemes of a cyclone (Greek: kyklon - rotating) and a scrubber (eng. Scrub - scrub) Venturi for dry and wet dust collection methods, respectively.
Cyclones - the main type of apparatus for collecting dust, which use a centrifugal field to precipitate it. The gas stream is introduced into the cyclone through the nozzle - 1 tangentially to the inner surface of the cyclone - 2 body (Fig. 14). The flow rotates and rotates along the body to the hopper - 4. Dust particles under the action of centrifugal force form a dust layer on the cyclone wall, which crumbles and enters the hopper. The gas stream, freed from dust, forms a vortex and leaves the cyclone through the pipe - 3. When it accumulates, the hopper is periodically unloaded from dust.
Must be separated by a subsequent separation process. In accordance with the principle of operation or high pressure and significantly respond to fluctuations. reaction products are obtained in the form of dissolved salts or dried salts. In this way. The pulverized spent load is captured by a finely divided liquid. Venturi scrubber or column scrubber. These deposits should be stopped by pH correction. For example: inkjet scrubbers. From the recovery phases of gas purification. Sulfur dioxide separation is low in this acidic environment.
The excess pressure of the gases entering the cyclone should not exceed 2500 Pa, the temperature should not exceed 400 ° C. The permissible inlet concentration of weakly sticking dust is about 1000 g / m 3, average adhering - up to 250 g / m 3. The efficiency of gas cleaning from dust of more than 5 microns in cylindrical cyclones is 80–90%. Usually they are used for pre-treatment of gases in front of electrostatic precipitators and filters. When cleaning large volumes of gases, batteries are used, consisting of the required number of cyclones installed in parallel.
This leads to sulfur compounds. For technical reasons, this separation is performed in another phase of the combustion of combustion gases. Particular attention should be paid to mercury. For this reason, hazardous waste incinerators in Europe are often equipped with multi-stage gas treatment plants. Phosphorus. This risk does not occur if a solution with a higher concentration of sodium hydroxide is used and when the reaction products are soluble in water. The most recommended are sodium hydroxide scrubbers.
If it is burned simultaneously with sulfur-containing waste. Satisfactory separation can be achieved in the easy phase of washing with alkaline gas. If lime is used as a neutralizing agent in the purification of wet gas. The salt content in the wastewater can be easily reduced by settling solid particles. 5 - in which sodium hydroxide or lime is added to the washing liquid. In the case of a semi-sedative procedure. To maintain cleaning efficiency and prevent washing, part of the washing solution must be removed from the chain.
Rotary Dust Collectors - centrifugal apparatus such as fans of a special design. They are used for cleaning gases from dust with a particle size of more than 5 microns. They have great compactness. A more promising modification is countercurrent rotary dust separators. Their sizes are 3-4 times smaller than that of cyclones, and energy consumption is 20-40% less. However, the complexity of the design and operation process makes it difficult to widespread.
Such as chlorine. Combustion products of certain elements. And the maintenance costs are lower. Rotating scrubber. Separation of bromine and iodine can be improved with. Due to acids formed during the separation process. Insoluble salt increases the risk of cleaning. When using reducing agents. But the investment costs and required surfaces are very high. Due to the formation of the mixture. This is selective catalytic reduction and selective non-catalytic reduction. For this purpose, flue gas is used.
Without a significant decrease in activity in terms of effectiveness. Dissolved mercury is converted to a less soluble form with suitable chemicals. According to hazardous waste incineration studies. Contains sodium. The catalysts can be located in various sections of the exhaust gas treatment system. Generally. Due to the high operating temperature. To protect catalysts for uncontrolled reactions involving flammable gases. Superheated ceramic catalysts are used after the flue gas treatment system.
Vortex dust collectors. These are also centrifugal devices, which use inclined nozzles or blades as a swirl of gas flows. They are able to clean large volumes of gases from fine dust fractions, less than 3-5 microns. The cleaning efficiency reaches 99%. It is little dependent on the dust content in the range up to 300 g / m 3.
A lower level of operation of such a catalyst in hazardous waste incinerators may provide a run time in the circuit and may endanger the health of the scrubber cleaning and maintenance personnel. Ammonia or urea is dissolved in washing water. Decontamination remains within the normal range of the gas purification scheme. The exhaust gases must be heated after washing the gas. If the catalyst remains dry. When the equipment temperature limit is below the condensation point. This process can cause corrosion.
In the case of selective catalytic reduction. The necessary safety measures are necessary in all cases of 1000 hours. Arsenic and other compounds should be mentioned as harmful to the catalysts. Low temperature catalysts, as a rule, become the substrate material for the deposition of salts and. ammonia. In the process of non-catalytic selective reduction. Particular attention is paid to their toxicity and their carcinogenic effects. The methods for reducing nitrogen monoxide emissions described above are not alternatives or equivalents and should be determined on a case-by-case basis.
Electrostatic precipitators. They are devices with a set of tubular precipitating, positively charged electrodes (anodes), inside of which thin rods (strings) of corona-negative, negatively charged electrodes (cathodes) are located along their axial center. Between these electrodes, which are a cylindrical electric capacitor, a direct current source creates an electric field of high tension, up to 50–300 kV / m. In this strong electric field, when charged particles collide with molecules, shock ionization of the gas occurs. However, before the breakdown of the gas, the field strength is not increased, i.e. create conditions for corona discharge in gas. Aerosol particles entering the zone between the cathode and the anode adsorb forming ions, acquire an electric charge and move to the electrode with the opposite charge. Since the area of \u200b\u200bthe rod (cathode) is much smaller than the area of \u200b\u200bthe tube, the current density at the cathode will be much higher than at the anode. The corona discharge is mainly localized at the cathode. This leads to a significantly larger discharge of cations and the formation of negatively charged aerosol particles. Therefore, impurities mainly move to the anode and settle on it. Hence the names are clear: corona and precipitation electrodes.
Exhaust gas treatment with or without wastewater discharge. The catalyst is a source of heat. Natural gas burners are used to maintain the operating temperature of the catalyst. Carbon monoxide emissions from hazardous waste incinerators are low and therefore of limited importance. But who is asking. An ammonia solution or other compounds containing trivalent nitrogen are introduced into the residual gas stream. Waste from incinerators is analyzed to determine the concentration values \u200b\u200bin: Also.
During operation, there may be problems with the safety of storage of ammonia, necessary to reduce the content of nitrogen monoxide. Depending on the operating temperature and reactivity of the materials used. Polychlorinated dibenzodioxins and dibenzofurans can be formed from certain precursors after combustion. For this, absorption processes and oxidation catalysts, an air-coated process with activated carbon or zeolites are used. Accordingly, with a zeolite moving layer.
When passing gas and impurities through an electrostatic precipitator, their flow rate is usually set in the range from 0.5 to 2 m / s. The speed of movement of charged particles to the electrodes depends on their size, charge and electric field strength. At a field strength of 150 kV / m, it is from 0.01 to 0.1 m / s for particles with a diameter of 1 to 30 μm, respectively. The electrodes are well deposited and then easily removed by shaking dust with a resistivity of 104 to 1010 Ohm · cm. At lower values, dust particles are easily discharged at the electrode, recharged, and returned back to the gas stream. Dusts with a resistivity of more than 1010 Ohm · cm are slowly discharged at the electrodes, prevent the deposition of new particles and are most difficult to capture. In this case, gas humidification is used.
Furans and mercury cannot be obtained using emission control processes. Due to potential exposure. Some of the substances mentioned above have a potential carcinogen. The formation of carbon and its compounds from catalytic reactions can be controlled by a good complete combustion of the powders in suspension and their reduction. but also by the deposition of dust and aerosols. Emission concentrations of these substances should be minimized. Polycyclic aromatic hydrocarbons. Certain substances in these groups have a carcinogenic effect.
Electrofilters are used for fine cleaning of gases from dust and fog. Dry electrostatic precipitators have a capacity of 30 to 1000 m 3 / h. They are able to purify gases with an efficiency of up to 99.9% with a dust content of up to 60 g / m 3 and a gas temperature of up to 250 ° C.
Filters Their designs are different. However, for all filters, the main element is a porous septum - a filter element. By the type of material, the partitions are distinguished: granular, flexible, semi-rigid, rigid filters.
Located with vertical units. In particular. In some cases. According to the load profile. Advantages of the adsorption process in the overflow flows: high relative inlet speed. The advantages of the adsorption process in the same way are: an almost perfect distribution of the residual gases through the cross section of the adsorber, which creates a strong current in the layer and, therefore, reduces the risk of operational defects due to an increase in temperature. All residual and polluting pollutants.
Hydrofluoric acid. The exhaust gas stream alternately passes through a layer of a layer of activated material on several substrates, which allows separate assembly of activated material with different loading speeds for separate removal. as at admission. Sulfur oxide.
Granular filters made of gravel, coke, sand are used to clean gases from large dust fractions created by crushers, screens, mills, etc. The cleaning efficiency is up to 99.9%.
Flexible porous filter elements are fabrics, felts, sponge rubber, polyurethane foam. Fabrics and felts are most often made from synthetic fibers, glass threads, receiving such fabrics as nitron, lavsan, chlorin, fiberglass. They are widely used for fine purification of gases with an initial dust content of 20-50 g / m 3. The cleaning efficiency is 97–99%.
The twisted filtration process can be used in the following ways: Recirculation of a partially depleted absorbent volume reduces the amount of residue. Using this process. The usual adsorbent is a mixture of calcium coke lesion. A small portion of the exhaust adsorbent is continuously transferred from the process and replaced with fresh material. Removing external ignition sources. With a significantly higher boiler content. The fluidized bed expands until solid particles are distributed throughout the reactor. while calcium compounds are used.
Rigid filter elements are made of porous ceramics and porous metals. They are indispensable when cleaning from impurities of hot and aggressive gases.
Semi-rigid filters such as knitted metal grids, extruded spirals and shavings made of stainless steel, brass, nickel are used to clean dust with a particle size of more than 15 microns and an initial concentration of up to 50 g / m 3 from hot gases with a temperature of up to 500 ° C.
Solids are usually discharged to the top of the reactor. Over time. Furans and heavy metals. Addition of inert substances. As with the filtering process. Prevention of dust deposits. This process can be used before or after selective catalytic reduction processes. Boiler separates dioxins. The aerosol adsorbent is mixed with an upward flow of residual gas.
The results of the industrial scale show that the values \u200b\u200bof the concentrations of pollutants are given. 10 Process with a layer and eddy currents in a reactor. Fig. 112 - supply lines 114. spray channels 411. 437 - lower and upper parts 485 - channels 420 - jet pipe 431 - heat exchanger 436 - lower and upper side outlets 438 - rotor 439 - axes Horizontal rotation 480 - Hydrocyclone 484 - Recirculation flow 100 - Cleaning tower 110 - Vertical cleaning zone 120 - Drain tank 140 - Cement removal device 130 - Power separator 17.
The filtering process consists in the deposition of dispersed particles on the pore surface of the filter element. Deposition occurs as a result of the effect of contact, diffusion, inertial, gravitational process, Coulomb interaction of charged particles. The latter is characteristic of Petryanov filters made of perchlorovinyl fibers (FPP), which are now widely used. Such ultrathin fibers carry charges on their surface, which makes it possible to achieve very high efficiency of gas purification from aerosols in the initial stage of filtration, up to 99.99% at a filtration rate of 0.01 m / s and a particle diameter of 0.34 μm. These filters are used to purify air from radioactive aerosols. After neutralizing the charge, the cleaning efficiency is reduced to 90%.
Gas is discharged after contact with the sediment. open. Avoid applying tartar to the inside walls of the restroom. with a walkway. Deploy vertically. Almost. The drive cage consists of separate individual palaces-separators. Allows to achieve high speeds of flue gases. up. Which needs to be cleaned. Reduce the concentration of sulfur dioxide in combustible gases. Water sediment goes down. In countercurrent with and in contact with flue gas. It has the following advantages. Allows you to increase the speed of the gases being cleaned.
If the particle size is larger than the pore size, a sieve effect is observed with the formation of a precipitate layer. This effect, as well as the gradual clogging of pores by the settling particles, increase the resistance of the filter element and the cleaning efficiency, but reduces its productivity. Therefore, filter elements periodically regenerate.
Filter designs: bag, roll, frame.
Bag filters most widely used for dry cleaning of gas emissions. In a cylindrical case with a conical bottom, sleeves made of fabric or felt are attached to the holes of the lower partition and to the caps of the upper partition. Dusty gas, supplied from below through the openings of the lower partition, enters the sleeves, is filtered, and is removed from the apparatus through the inter-sleeve space and the openings of the upper partition. The filter is regenerated after it is disconnected from the cleaning system by shaking the sleeves with a special device (dust is collected in the conical bottom) and blowing them back with compressed gas. The permissible concentration of dust at the inlet to the bag filter is 20 g / m 3, the highest gas temperature is 130 ° C for dacron hoses and 230 ° C for fiberglass, productivity is up to 50 m 3 / h, cleaning efficiency is about 98% .
Wet dust collectors. Wet gas cleaning devices are characterized by high efficiency of fine cleaning of fine dusts (0.3–1 μm), as well as the ability to clean hot and explosive gases from dust. They work using the deposition of dust particles on the surface of droplets or liquid films. In this case, the forces of inertia, Brownian motion, diffusion act, the interaction of charged particles, condensation, evaporation, etc. An important factor is the wettability of the particles by the liquid.
By design, wet dust collectors are divided into Venturi scrubbers, nozzle and centrifugal scrubbers, inertial-shock, bubble-foam apparatuses, etc.
Scrubber venturi (fig. 15). The main part of this scrubber is the Venturi nozzle - 1, in which the dusty gas is introduced into the narrowing part, and water is sprayed through the centrifugal nozzles - 2. In this case, gas accelerates from an input velocity of 15–20 m / s to a speed of 30–200 m / s in a narrow nozzle section. For effective cleaning, the uniform distribution of water droplets over the nozzle section is very important. In the expanding part of the nozzle, the flow is inhibited to a speed of 15–20 m / s and fed to a drip trap - 3 — direct-flow cyclone. Water consumption: 0.1–6 l / m 3. Venturi scrubbers provide high efficiency of cleaning (up to 99.9%) of aerosols with an average particle size of 1–2 μm at an initial concentration of up to 100 g / m 3. The capacity of Venturi scrubbers is up to 80,000 m 3 / h.
Nozzle and centrifugal scrubbers particles more than 10–20 microns are effectively captured. In them, the gas flow is directed at an angle to a mirror of water protruding above the surface of the sludge (Fig. 16a). Large particles settle in the water, and fine dust with a gas stream rises up to meet the rain stream created by the nozzles - 2a or a film of water supplied through nozzles in a centrifugal scrubber.
The specific water consumption in the nozzle scrubbers is 3–6 l / m 3, the gas flow rate is 0.7–1.5 m / s, the efficiency of blast furnace gas purification is 60–70%. In centrifugal scrubbers with gas dust content up to 20 g / m 3, the specific water consumption is 0.09–0.18 l / m 3, the cleaning efficiency at a gas speed of 15–20 m / s is from 80 to 98%.
Bubble Foam Dust Collectors (Fig. 16b). In them, gas for cleaning enters under a horizontal grate - 2b, then passes through holes in the grate and a layer of liquid - 4 and foam - 5. At a gas speed of up to 1 m / s, a bubble mode of cleaning is observed. With an increase in velocity to 2–2.5 m / s, a foam layer appears above the liquid. This leads to an increase in cleaning efficiency, but also increases the entrainment of spray from the apparatus. The efficiency of gas purification from fine dust reaches 95–96% with a specific water flow rate of 0.4–0.5 l / m 3.
Mist eliminators. They are used to clean the air from mists of acids, alkalis, oils and other liquids. Fogs are captured by fiber filters, on the surface of the pores of which droplets are deposited and then the liquid flows off under the action of gravitational forces. The material used is fiberglass with a fiber diameter of 7 to 30 microns or polymer fibers (lavan, polypropylene) with a diameter of 12 to 40 microns. In low-speed mist eliminators with a gas velocity of less than 0.15 m / s, the mechanism of diffusion deposition of droplets prevails, while in high-speed (2–2.5 m / s) inertial forces act.
For low speed mist eliminator use tubular filter elements. They are formed (assembled) of fibrous materials in a gap of 5-15 cm wide between two mesh cylinders, the diameters of which differ by 10-30 cm. These elements, unlike bag filters, are attached vertically to the openings of the upper partition of the cylindrical one apparatus, and the lower ends through tubular hydraulic locks are immersed in glasses with condensed liquid. The fog, passing from the outer side of the cylinder into the inner cavity, retains the droplets. The liquid formed from them flows into a glass. The cleaning efficiency of particles smaller than 3 microns is 99.9%.
High-speed mist eliminators are smaller and provide a cleaning efficiency of 90–98%. To clean the air of chromium baths from fog and splashes of chromic and sulfuric acid with a temperature of up to 90 ° C, a filter design with polypropylene fibers has been developed: FVG-T. Its productivity is 3,500–80,000 m 3 / h, and the cleaning efficiency is 96–99%.
) It is made for protection against atm pollution. (especially with the release of outgoing industrial), technol. preparing and extracting valuable products from them. Dust collection is carried out using dust collectors built into the main technol. equipment, as well as remote. The efficiency of dust collection is determined, as a rule, by the ratio of the mass of particles trapped (deposited) in the dust collector to the mass of particles at its inlet.
In the dust collection technique, a large number of apparatuses are used, differing in the design and principle of suspended particles. By the method of separating them from the flow, dust collectors are usually divided into mechanical (dry and wet) and electric devices. cleaning (see also). The work of any dust collector is based on the use of one or several. mechanisms suspended in particles. The contribution of each particular mechanism to the efficiency of the dust collector can be qualitatively characterized by the corresponding dimensionless parameter.
Gravity () occurs as a result of the vertical settling of particles under the action of gravity when passing through a gas cleaning apparatus. Gravity parameter. G is expressed by the ratio:
where F t, F c are the forces of gravity and resistance of the medium (N); d h, r h - diameter (m) and density (kg / m 3) of particles; g-acceleration fall (m / s); m r, u g (Pa · s) and velocity (m / s) of the gas stream; Cp is the Cunningham-Milliken amendment, which takes into account the increase in particle mobility, the size of which is comparable to the average path length. Gravity. The principle used in dust-precipitates. cameras.
Centrifugal occurs during the curvilinear movement of the aerodispersed flow, when centrifugal forces develop, under the influence of which particles are discarded to the surface. The centrifugal parameter w is characterized by the ratio of the centrifugal force F c acting on the particle to the resistance force of the medium:
where u w, r-velocity (m / s) and radius of rotation (m) of the gas stream. Centrifugal is used in single, group and battery, vortex devices, dynamic. .
Inertial occurs if the mass of particles or their speed of movement is so significant that they cannot follow along the streamline enveloping the obstacle, but, seeking by inertia to continue the movement, collide with the obstacle and settle on it. Inertial parameter - Stokes criterion:
where u og - the velocity of the gas flow relative to the surface of the flow or obstacle (m / s); l-characteristic linear parameter (m) of the streamlined body (for a spherical drop, the diameter of the ball, for fiber, the diameter of the cylinder). Inertia determines the operation of most wet dust collectors (), and also plays an important role in.
Engagement (touch effect) is observed when the distance from a particle moving with a gas stream to a streamlined body is equal to or less than its radius. The engagement effect is characterized by the parameter R 3 - d h / l and has creatures. value at.
Diffusion occurs as a result of continuous exposure to small suspended particles in. The diffusion parameter D oc is the reciprocal of the Peclet criterion: D oc \u003d Re -1 \u003d u g l / D 4, where D 4 is the coefficient. Brownian particles (m 2 / s). If the Stokes law is valid, when the particle size is larger than the average path, we have;
where k-; T g-t-ra (K). Diffusion similar to the engagement effect is used in DOS. at