Electrostatic precipitator

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Electrostatic precipitator of a biomass heating plant

Electrostatic precipitators , also: EGR (Electric gas cleaning) , electrostatic dust filter , Electrostatic ( ESP from English electrostatic precipitator ) are systems for separating particles from gases based on the electrostatic principle based. Since it strictly speaking no filters in the traditional sense is, is the scientifically correct term electrostatic precipitator or electric dust collector .

principle

Schematic sketch of a plate electrostatic precipitator with a wire electrode

The separation in the electrostatic precipitator can be divided into five separate phases:

  1. Release of electrical charges , mostly electrons
  2. Charging of the dust particles in the electric field or ionizer
  3. Transport of the charged dust particles to the precipitation electrode (NE)
  4. Adhesion of dust particles to the precipitation electrode
  5. Removal of the dust layer from the collecting electrode.

The dust particles often have a natural charge , but this is by no means sufficient to accelerate the particle with sufficient force to the oppositely charged electrode. Therefore, they are strongly charged in an electric field . The field is formed between the emitting negative spray electrode with a high voltage of up to over 100 kV and the grounded collecting electrode . The mechanism of charge generation that is decisive for the conditions in the electrostatic precipitator is impact ionization . The free electrons present in the gas are strongly accelerated in the electrostatic field of the corona membrane in the vicinity of the spray electrode ( gas discharge ). When they hit gas molecules, further electrons are either split off or attached to the gas molecules. In the first case, new free electrons and positive gas ions are created , in the second case negative gas ions. The positive gas ions are neutralized by the spray grille, while the negative charges (free electrons and gas ions) migrate towards the collecting electrode.

The charging of a dust particle begins when it enters the space through which the spray current flows and is caused by the accumulation of negative charges when they collide with the dust particle. The charging process takes place by field charging or by diffusion charging . During field charging, gas ions hit dust particles due to their directional movement and charge them until they are saturated. For very small particles (d <0.1 µm), the influence of field charging disappears - the dust particles are charged by diffusion charging (in the case of collisions caused by Brownian molecular motion ).

The charged dust particles migrate through the applied electrical force ( Coulomb's law ) of the applied DC voltage field transversely to the direction of flow of the gas to the collecting electrode, where they release their charges. Since the drift speed to the collecting electrode is relatively low ( Stokes law ), the filter alley must have a certain length or the gas to be cleaned must not flow through too quickly.

After the dust particles have given up their charge, they are bound by adhesive forces that are essentially determined by the electrical field strength within the adhering dust layer. A speck of dust is considered “separated” if the adhesive forces are greater than the flow force of the gas.

The dust layer that forms on the collecting electrode must be cleaned at regular intervals. In most cases this is done by tapping with a hammer. The dust dissolves and falls into a collecting bunker. However, a certain percentage of the dust particles are carried away by the gas flow and have to be recharged and separated.

In smaller electrostatic precipitators, e.g. B. for room air purification, the particles are usually positively charged, whereby the separation mechanism works according to the Penney principle. In large electrostatic precipitators, the particles (mostly dust particles) are negatively charged (so-called Cottrell principle).

Power supply and control of the electrostatic precipitator

The degree of separation of an electrostatic precipitator depends in particular on the voltage between the spray and separation electrode. The high DC voltage required for this is generated by a so-called voltage conversion system. This usually consists of a mains transformer , which transforms the mains voltage to around 80 kV to over 100 kV, and a rectifier . A thyristor controller is connected to the primary circuit of the transformer as an actuator . A choke is connected upstream to limit the current . Systems that work with a mains transformer and thyristor controller can in principle only react within 10 ms. There are also voltage conversion systems that use a transverter . With these, pulse operation with short pulses is also possible (so-called µs pulse).

A voltage conversion system for electrostatic precipitators has the following parameters:

  • Power or output current
  • Output voltage (high voltage)

The voltage conversion system is controlled by the so-called filter control. The filter control has the following functions:

  • Limitation of the current to specified values
  • Bringing the high voltage to the maximum possible value - just below the voltage breakdown - and thus achieving a sufficient spray current
  • Determination of the breakdown limit, detection of a breakdown, differentiation of different types of breakdown and reaction to it
  • Detecting the dusts occurring during high-ohmic back-corona and response

The filter control generally works as a current regulator and, after being switched on, approaches the set current with a specified ramp. Furthermore, functions for breakdown detection and breakdown processing are subordinate: If a breakdown is detected, the run-up ramp is aborted, the high voltage for deionization may be temporarily blocked and a new run-up ramp, possibly with lower end values, is started.

Influences on the separation / effectiveness

Particle transport

The particle transport depends on the applied electric field, as well as on the properties of the gas flowing through and the dust to be separated. Both the electrical conditions and the flow dynamics are largely determined by the geometry of the separator (in particular the geometry of the separating and spraying electrodes). Another effect is the reaction of the charged particles on the electric field. Since the charging time of the particles is relatively short compared to the deposition time, a cloud of negatively charged particles is created. The negatively charged particles (particle space charge) influence each other on the way to the separation electrode (repulsion of the same polarity) and thereby limit the ion current. This is a general process that always occurs to a small extent in electrical precipitators. With a very high input concentration, in particular fine particles, this particle space charge can become so strong that the current of the corona discharge drops to per thousand values ​​of the clean gas current consumption. One then speaks of corona quenching. This problem can be largely minimized or even avoided by choosing a suitable distance between the spray and deposition electrodes (approx. 4-6 cm under ambient conditions) and the use of spray electrodes with a low corona threshold voltage (thin wires or structures with tips).

Layer of dust

The charge of the separated particles and the incoming ion current have to flow away through the dust layer of the already separated particles on the precipitation plates. If the dust layer has a high electrical resistance (depending on: composition, grain size, temperature, etc.), there is a strong voltage drop across the dust layer, which can ultimately lead to a corona discharge in the dust layer. This creates charge carriers of both polarities, which leads to an ion current, contrary to the deposition current, in the direction of the spray electrodes. In some cases, flashovers also occur within the already deposited layer of dust, which, like an explosion , hurls dust back into the gas flow. This effect is called "back-spraying" (back corona) and leads to a reduction in the particle transport speed.

Re-training

Reentrainment is the entrainment of already separated dust with the gas flow. The majority of the reentrainment occurs when knocking the precipitation plates (knocking losses). But even in normal separation operations, reentrainment losses arise from the dust layer. Here one speaks of erosion losses. In terms of design, attempts are made to counteract reentraiment by using appropriate electrode geometries (for example, trapping spaces).

application

Electrostatic precipitator with trough chain conveyor (green) for transporting the ash into the ash pan (orange)

Electrostatic precipitators are mainly used in the cleaning of flue gases, for example in coal-fired power plants , in smelting , cement production or in heating plants and thermal power plants fired with solid fuels (in addition to coal, e.g. wood, wood chips , wood pellets ) .

Overall degrees of separation of up to 99.9% are achieved, which prevents the emission of up to 10 tons of fly ash per day in a coal-fired power station . A power plant filter may be a few tens of meters high, the plate spacing is in the range of a few tens of centimeters, and up to several hundred filter lanes can be connected in parallel. Depending on the type of knocking system used, wear occurs both on the knocking parts and their drives and on the knocked precipitation or spray electrodes and their suspensions.


In the metalworking and metalworking industry, electrostatic precipitators are used in particular for the extraction and separation of aerosols , consisting of cooling lubricants (KSS) and material abrasion particles. About 50% of the separators used in metalworking and processing companies are electrostatic separators of various designs.

Electrostatic precipitators clear up the clouding of visibility caused by tobacco smoke, for example in restaurants and smoking cabins . A separation of gaseous toxins, in particular carbon monoxide , does not take place.


Designs

Electric filters differ in the shape and size of the filter alley (pipes, plates), the shape of the spray electrodes (helix, wire, mandrel electrode, sawtooth ionizer, wave ionizer, etc.), the operating voltage ( direct voltage , alternating voltage , pulsed direct voltage, pulse-superimposed direct voltage) and the type of Cleaning (tapping, rinsing, changing cassettes). There are series with and without their own fan . Special steels or even lead are used in aggressive atmospheres .

Challenges

The separation of particularly toxic dusts or dusts with particle sizes below a  micrometer ( fine dust ) poses a particular challenge to the separation rate of electrostatic precipitators. Their effects on the environment and health are greater than those of coarse, less toxic dusts and, of all things, a high separation rate large amounts of fine dust causes particular difficulties due to the space charge effects. There is a minimum separation of respirable dusts.

history

  • First recorded record of the electrical separation of smoke by William Gilbert around 1600.
  • A study by Benjamin Franklin around 1745 deals with corona discharges.
  • Experimental cleaning of a mist in a glass vessel through a hollow field in 1824.
  • Publication by Oliver Lodge in 1884 on this phenomenon.
  • First commercial attempt at electrical separation in 1885 by Walker, Hutchings and Lodge in a lead smelter, which failed, however, because lead dust is extremely difficult to separate.
  • Trials by Frederick Gardner Cottrell around 1906 led to the first successful commercial application in the separation of sulfuric acid mist in the Pinole powder factories and the Selby smelter.
  • WA Schmidt, a former Cottrell student, designed the first electrostatic precipitators in the cement industry around 1910.
  • Derivation of the exponential separation law by W. Deutsch in 1922.

See also

literature

  • Harry J. White: dedusting industrial gases with electrostatic precipitators . German publishing house for basic industry, Leipzig 1969.
  • VDI 3678 Bl.1 Electrostatic precipitator: process gas and exhaust gas cleaning . VDI-Verlag, Düsseldorf 1996.
  • Friedrich Löffler: Dust separation . Thieme, Stuttgart; New York 1988, ISBN 3-13-712201-5 .

Individual evidence

  1. ↑ https://www.highvolt.de/portaldata/1/Resources/HV/Downloads/11-1-2-3.pdf Electrical equipment for electrostatic precipitators. Company publication from Highvolt / Dresden, accessed on May 11, 2019
  2. ↑ Patent US4056372 : Electrostatic precipitator. Applied on April 15, 1976 , published November 1, 1977 , applicant: Nafco Giken, inventor: Tsutomu Hayashi. ‌
  3. ↑ Guideline VDI 3678
  4. ↑ Heinz Aigner: EUROPEAN PATENT WRITING, EP 1 033 171 B1.
  5. ↑ M. Stieß: Mechanical process engineering. Volume 2, Springer, Berlin 1997, ISBN 3-540-55852-7 , p. 40.
  6. ↑ Christian Lübbert: To characterize the quenched state in the electrostatic precipitator. online dissertation, BTU Cottbus, 2011 (PDF; 1.8 MB).
  7. ↑ BGIA report 9/2006, suction and separation of cooling lubricant emissions. Main association of commercial trade associations (HVBG), Trade Association Institute for Occupational Safety and Health (BGIA), Sankt Augustin 2006 , ISBN 3-88383-714-8 , p. 10f.
  8. ↑ Klaus Görner, Kurt Hübner: Gas cleaning and air pollution control. Springer-Verlag, 2013, p. F39.
  9. ↑ Andreas Küchler: High voltage technology: Basics - Technology - Applications. Springer Science & Business Media, 2009, p. 562.