Dedusting

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Dust removal is the separation of finely divided solid foreign components from a gas .

Basics

Wilhelm Beth from Lübeck , who patented the suction hose filter with cleaning by backwashing in 1886, is considered a pioneer in industrial dedusting .

In large incineration and industrial plants in particular, considerable amounts of dust are released, which can only be released into the exhaust air after passing through a dedusting system. A dedusting system is a technical device to clean dusty exhaust gas or exhaust air flows. Such systems are mainly used in cement works, power plants, foundries, iron and steel works, lime works and the chemical industry. The industrial dusts are very diverse and usually have a grain size between 0.1 and 1000 µm.

The use of different types of separators and filters depends on the composition of the dust, its concentration, the available space and the dimensions. The degree of separation is a measure of the quality of a dedusting process . The suction speed and the correct placement of the collection and suction devices are decisive for a good collection of the pollutants at the source. The closer the suction openings are in direct proximity to the dust source, the more efficient the dust capture, since the suction speed decreases with increasing distance from the dust source. In industrial exhaust gas purification and particle separation, filtering separators are increasingly being used.

The success of the dedusting can be checked by measuring the dust .

Types of dedusting

A part of the company released for demolition with a dominant dust extractor

Mass force separation

The dedusting of gases using mass force separators (or mass force dedusters) is based on the fact that dust particles are transported by gravity, inertia or centrifugal force into zones of the separator from which they can no longer be removed by the flowing gas. Due to their direct proportionality to the mass, inertial forces increase with increasing particle diameters with their third power. The characteristic of mass force separators are low investment and operating costs. They are preferred for coarse dedusting.

Gravity separators: Gravity separators such as settling chambers work on the principle of gravity. They are mainly used for the dedusting of larger particles (0.02 mm). The principle of gravity says that particles sink down due to gravity. The rate of descent varies depending on the size, density and geometry of the dust particles. The dust-laden air enters the settling chamber from below or from the side. By increasing the volume in the settling chamber, the flow speed is reduced while the residence time is increased and coarse dust settles downwards due to gravity. The cleaned air flows out of the settling chamber to the side or upwards.

Inertial separators : Inertial or deflection separators work on the principle of inertia. The dust-laden air enters the separator and is diverted. The flow is often deflected in such a way that the particle separation is supported by gravity. Coarse particles cannot follow the deflection of the air flow and are deposited. Certain mist eliminators work according to this principle .

Centrifugal separators: Centrifugal separators, often also called cyclones, separate particles from a gas flow using centrifugal forces. Their separation space is constructed rotationally symmetrical and they work according to the centrifugal force principle, which means that particles strive to maintain their direction of movement. The centrifugal force presses the particles against the outer wall and can thus be separated out in the lower area of ​​the cyclone due to the conical shape. The downward displacement increases the speed of the air flow and an upwardly rising air vortex is created in the middle of the cyclone. The cleaned air flows out of the top of the cyclone. The necessary rotary flow is generated by a suitable design of the inlet part, for example by a tangential inlet. Centrifugal separators usually have a diameter between 0.02 meters and 5 meters and can also be used at temperatures of up to 1000 ° C. They are often operated in parallel in order to achieve better separation due to their smaller diameter and in order not to exceed certain sizes with larger volume flows.

Wet dedusting

With wet dedusting, the dust particles are brought into contact with a washing liquid or bound in it and removed from the gas flow. There are different variants of wet separators such as spray washers , jet washers , vortex washers , venturi washers or rotary washers . These can be differentiated according to whether or not they have built-in components. Another distinguishing feature is whether the energy to generate an increased specific surface (usually in the form of water droplets) is brought in by the gas to be cleaned, the washing liquid or by moving fixtures. The energy requirement of the wet scrubber depends on the separating grain size and the equipment used: While the separating grain diameter in spray washers is in the range of 1 µm with a specific energy consumption of 1 kWh / 1000 m 3 , Venturi washers with a specific energy consumption of 4 kWh / 1000 m 3 a separation grain size of <0.1 µm can be achieved.

Wet separators can be used for dedusting and separating harmful gases at the same time. As a rule, a droplet separator has to be connected downstream.

Filtering separators

The filtration is one of the oldest methods of separating foreign objects from a fluid. In the case of filtering separators, the dust-laden gas hits a permeable, porous layer ( filter medium ), which holds back the dust and lets the gas flow through. Depending on the cleaning task, filtering separators can also be used as final separators , as they can achieve separation rates of over 98%. Filtering separators can be divided into two groups of depth filters and surface filters according to their mode of operation .

Surface filters (cleanable fabric filters, cleaning filters): When the exhaust gas flows through the filter fabric of a surface filter , most of the dust particles stick to the inflow side. This creates a filter cake that takes on the main cleaning performance. The filter cake is removed at regular intervals. Surface filters are used for emission reduction and process gas cleaning as well as for product recovery. Forms of packaging for surface filters include bag filters , pocket filters and filter cartridges.

Depth filter (storage filter): Depth filters mainly consist of fibers with a large cavity volume in which the particles can be deposited. The main separation mechanisms are the blocking effect , sedimentation , diffusion and inertia effects . Depth filters are mainly used to clean room and process air. They can consist of fiber materials, metallic structures, flexible foams or loose fillings. The most common form of packaging for depth filters is pocket filters. Further packaging forms can be filter mats, filter cells or filter cartridges.

Electrostatic precipitators

The principle of Elektroentstaubung in an electrostatic precipitator ( electrostatic precipitator ) is based on the attraction and retention of electrically charged particles in an electric field to oppositely charged electrodes . Since the natural charge of the particles in the gas flow is usually not sufficient for their efficient separation, a high charge of the particles must be ensured. By corona discharge gas ions are formed on a discharge electrode, which release their charge to the particles. The charged particles are deposited on the collecting electrode. In the case of dry electrostatic precipitators, the electrodes are tapped periodically, in the case of wet electrostatic precipitators the dust layer that forms is rinsed off with a liquid. The use of wet electrostatic precipitators is particularly recommended for water-soluble dusts.

Electrostatic precipitators can be used at exhaust gas temperatures of up to 500 ° C. In addition to industrial applications, they are also used for cleaning indoor air. Their separation efficiency is largely independent of the particle size.

Fire and Explosion Risks

If the dust to be separated is flammable, dedusting systems pose a fire and explosion risk. If the dust concentrations in the supply lines are normally below the lower explosion limit , the dust is concentrated in the dust extraction system so that explosive mixtures can form due to turbulence. The presence or formation of flammable gases and the resulting formation of hybrid mixtures can lead to a shift in the lower explosion limit. In addition, certain units are sources of ignition .

Individual evidence

  1. ^ Walter Barth: Development lines of dedusting technology. In: Dust: magazine for dust hygiene, dust technology, keeping the air clean, radioactive suspended matter . 21, No. 9, 1961, pp. 382-390.
  2. ↑ VDI 2066 sheet 1: 2006-11 measurement of particles; Dust measurements in flowing gases; Gravimetric determination of dust load (Particulate matter measurement; Dust measurement in flowing gases; Gravimetric determination of dust load). Beuth Verlag, Berlin, p. 19.
  3. ^ A b Günter Baumbach: Air pollution control . Springer-Verlag Berlin, Heidelberg, New York, 2nd edition 1992, ISBN 3-540-55078-X , p. 314.
  4. ^ A b c Franz Joseph Dreyhaupt (editor): VDI-Lexikon Umwelttechnik . VDI-Verlag Düsseldorf, 1994, ISBN 3-18-400891-6 , p. 792.
  5. ↑ VDI 3676: 1999-09 Inertial Separators. Beuth Verlag, Berlin. P. 9.
  6. ↑ VDI 3676: 1999-09 Inertial Separators. Beuth Verlag, Berlin. P. 23.
  7. ↑ a b VDI 3676: 1999-09 Inertial Separators. Beuth Verlag, Berlin. P. 24.
  8. ^ A b c Franz Joseph Dreyhaupt (editor): VDI-Lexikon Umwelttechnik . VDI-Verlag Düsseldorf, 1994, ISBN 3-18-400891-6 , p. 497.
  9. ↑ VDI 3676: 1999-09 Inertial Separators. Beuth Verlag, Berlin. P. 25.
  10. ↑ VDI 3676: 1999-09 Inertial Separators. Beuth Verlag, Berlin. P. 34.
  11. ↑ Karl Georg Schmidt: Naßwaschgeräte from the perspective of the operating man. In: Dust: magazine for dust hygiene, dust technology, keeping the air clean, radioactive suspended matter . 24, No. 11, 1964, pp. 485-491.
  12. ↑ VDI 3679 sheet 1: 2014-07 wet separator; Basics, waste gas cleaning of particulate matter (wet separators; Fundamentals, waste gas cleaning of particle collections). Beuth Verlag, Berlin, p. 15.
  13. ↑ VDI 3679 sheet 1: 2014-07 wet separator; Basics, waste gas cleaning of particulate matter (wet separators; Fundamentals, waste gas cleaning of particle collections). Beuth Verlag, Berlin, pp. 19-21.
  14. ^ Günter Baumbach: Air pollution control . Springer-Verlag Berlin, Heidelberg, New York, 2nd edition 1992, ISBN 3-540-55078-X , p. 336.
  15. ↑ a b Matthias Stieß: Mechanical Process Engineering 2 . Springer-Verlag Berlin, Heidelberg, New York 1994, ISBN 3-540-55852-7 , p. 17.
  16. ↑ a b VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 5.
  17. ↑ VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 2.
  18. ↑ VDI 3677 sheet 1: 2010-11 Filtering separators; Surface filters (Filtering separators; Surface filters) . Beuth Verlag, Berlin. P. 23.
  19. ↑ a b VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin. P. 7.
  20. ↑ Matthias Stieß: Mechanical Process Engineering 2 . Springer-Verlag Berlin, Heidelberg, New York 1994, ISBN 3-540-55852-7 , p. 139.
  21. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin. P. 27.
  22. ↑ VDI 3677 sheet 2: 2004-02 Filtering separators; Depth fiber filters (filtering separators) . Beuth Verlag, Berlin. P. 42.
  23. ↑ a b VDI 3678 sheet 1: 2011-09 electrostatic precipitator ; Process gas and waste gas cleaning (Electrostatic precipitators; Process and waste gas cleaning). Beuth Verlag, Berlin, pp. 8–9.
  24. ↑ VDI 3678 sheet 1: 2011-09 electrostatic precipitator ; Process gas and waste gas cleaning (Electrostatic precipitators; Process and waste gas cleaning). Beuth Verlag, Berlin, p. 16.
  25. ↑ Klaus Holzer: Aerosol separation from flue gases from plants for the incineration of hazardous waste. In: Dust - cleanliness. Air . 48, No. 5, 1988, ISSN  0949-8036 , pp. 203-208.
  26. ↑ a b VDI 3678 sheet 1: 2011-09 electrostatic precipitator ; Process gas and waste gas cleaning (Electrostatic precipitators; Process and waste gas cleaning). Beuth Verlag, Berlin, p. 3.
  27. ↑ VDI 3678 sheet 2: 2010-12 electrostatic precipitator; Process air and indoor air cleaning (Electrostatic precipitators; Process air and indoor air cleaning) . Beuth Verlag, Berlin, p. 3.
  28. ↑ VDI 2263 sheet 6: 2017-08 Dust fires and dust explosions; Hazards - assessment - protective measures; Fire and explosion protection in dust extraction systems (Dust fires and dust explosions; Hazards - assessment - protective measures; Dust fires and explosion protection in dust extracting installations). Beuth Verlag, Berlin, pp. 10-14.