Electrostatic precipitator

Electrostatic precipitators, also: EGR ( Electric gas cleaning), electrical dust filters, electrostatic (English: ESP electrostatic precipitator ) are systems for the separation of particles from gases, which are based on the electrostatic principle. Strictly speaking, there is no filter in the classical sense, is the scientifically correct term electrostatic or electric dust collector.

Principle

The separation in the electrostatic filter can be divided into five distinct phases:

The dust particles, although often possess a natural charge, but this is far from enough not to accelerate the particles with sufficient force to the oppositely charged electrode. Therefore, they are highly charged in an electric field. The field is formed between the emitting negative discharge electrode with a high voltage of 20 kV to 80 kV and the grounded collecting electrode. The decisive for the conditions in the electrostatic precipitator mechanism of charge generation is the " impact ionization ". The free electrons are present in the gas in the electrostatic field of the corona skin in the vicinity of the discharge electrode greatly accelerated (gas discharge ). Upon impact with the gas molecules either cleaved or more electrons are attached to the gas molecules. In the first case, so, new free electrons and positive gas ions, in the second case negative gas ions. The positive gas ions are neutralized by Spray- while the negative charges migrate ( free electrons and gas ions ) towards the collecting electrode

The charging of a dust particle begins with his entry into the space through which the spray current and is caused by the accumulation of negative charges when they collide with the dust grain. Recharging is done by field charging and diffusion charging. In the field charging the gas ions strike due to their directed movement on the dust particles and upload it to the extent until saturation occurs. For very small particles ( d < 0.1 microns ) the influence of the field charging disappears. The dust particles charged by generated by the thermal motion of the gas molecules shock events.

The charged dust particles migrate through the applied electric force ( Coulomb's law ) of the applied dc field transverse to the direction of flow of gas to the collecting electrode, where they give their charges. Since the drift velocity to the collecting electrode is relatively low ( Stokes' law ), the filter lane must have a certain length and must be slowly traversed by the gas to be purified.

After the dust particles have delivered their cargo, they are bonded by adhesive forces, which are mainly determined by the electric field strength within the adherent layer of dust. A speck of dust is considered " separated " as if the adhesive forces are greater than the force of the gas flow.

The forming on the collecting electrode layer of dust must be cleaned at regular intervals. This is done in most cases by knocking blows with a hammer mill. The dust dissolves and falls into a collection hopper. However, a certain percentage of dust is re-entrained by the gas stream and must be charged and deposited again.

In smaller electrostatic precipitators, for example, for indoor air cleaning, the particles are usually positively charged, wherein the deposition mechanism works according to the principle Penney. In large electric filters, the particles are (mostly dust ) negatively charged (so-called Cottrell principle)

Power and control of the electric filter

The separation efficiency of an electrostatic precipitator is dependent in particular on the tension between spraying and deposition electrode. The necessary rectified high voltage produced from the Spannungsumsetzanlage. This usually consists of a high voltage transformer, which converts the high voltage network to about 80 kV to 100 kV (open circuit ), and a rectifier on the high voltage side. As an actuator, a thyristor is connected in the primary circuit with two anti-parallel connected thyristors. A disadvantage of this type of power supply is the ripple - a plate assembly of EGR is similar to a capacitor - so that the curve is not always the maximum voltage is present.

Lately Spannungsumsetzanlagen have come onto the market by developing cost -effective power transistors, which provide a smooth DC voltage and sufficient power, thereby providing a higher spray current in the middle can be entered into the EGR. Also, pulsed operation with much shorter reaction times (ms - Pulser, Pulser microseconds ) can thus be realized.

To control the electric filter, especially following sizes are needed, from which the required responses are derived:

• Primary current to the high voltage transformer
• Secondary current to the electric filter
• Secondary voltage (high voltage)

Task of the filter control:

• Limitation of the modulation (especially the primary current ) at given values
• Guiding the high voltage to the maximum possible value - just below the breakdown limit - and thus achieve a sufficient spray current
• Determining the dielectric boundary itself
• Precise detection of a breakdown
• Distinguish different gearteter punches
• Optimal response to the different types of breakdowns
• Detection of back-corona at high impedance dusts
• Optimal control with recognized Rücksprühbedingungen

The filter control for power frequency generally operates as a current plate and would regulate after turning in a specified ramp to the set rated current ( primary current ). At the same time but are still functions to limit power filter and filter voltage and, above all, subordinate to the breakdown detection and processing breakdown. With a recognized breakdown the acceleration ramp of the thyristor for deionization is canceled, possibly briefly locked and a new ramp, possibly with new internal limit values ​​started.

Influences on the deposition / effectiveness

Particle transport

Transport of the particles depends on the forthcoming electric field and on the properties of the flowing gas and the deposited dust. Both the electrical conditions and the flow dynamics are strongly influenced by the geometry of the trap is determined ( in particular the geometry of the deposition and emission electrodes ). Another effect is the reaction of the charged particle to the electric field. Since the charging time of the particles against the deposition time is relatively small, form a cloud of negatively charged particles. The negatively charged particles ( particle space charge ) influence each other on the way to the deposition electrode ( repulsion of like polarity), thereby limiting the flow of ions. This is a general process, which always occurs with electrostatic precipitators to a small extent. With a very high inlet concentration, in particular of fine particles, these particles, however, can be space charge so strong that the current of the corona discharge to drop to values ​​of the per mille clean gas current. This is called corona quenching. This problem can be (about 4 .. 6 cm at ambient conditions ) and the use of corona discharge with a small corona inception voltage (thin wires or structures with peaks) minimize or even largely avoided by choosing a suitable distance between spray and deposition electrode.

Dust layer

The charge of the deposited particles and the incoming ion current must flow out through the dust layer of the already deposited particles on the precipitation plates. The dust layer has a high electrical resistance ( a function of: The composition, grain size, temperature, etc. ), there is a large voltage drop over the dust layer, which may eventually lead to a corona discharge in the dust layer. This produces charge carriers of both polarities, which results in an ionic current, contrary to the deposition current in the direction of the discharge electrodes. Partly it comes to rollovers within the already deposited layer of dust, the dust spun like a blast back into the gas stream. This effect is called back corona ( back- corona ) and leads to a reduction of the particle transport speed.

Reentrainment

Under Reentrainment refers to the entrainment of already separated dust to the gas stream. The majority of the Reentrainments produced when tapping the precipitate plates ( knock losses). But even in normal deposition operation arise Reentrainment losses from the dust layer. This is called erosion losses. Structurally one tries ( for example, fishing facilities ) counteracted by appropriate electrode geometries the Reentraiment.

Application

Their main application find electric filter in the purification of flue gases, for example in electricity generation from coal in the smelting or cement production. There Gesamtabscheidegrade be achieved up to 99.9 %, which at a coal-fired power plant prevents the emission of up to 10 tonnes of dust per day. A power plant filter is a few tens of meters high under certain circumstances, the plate spacings are in the range of several tens of centimeters up to several hundred filters streets can be connected in parallel. Depending on the type of Klopfungssystems used results in a non-negligible wear on both the throbbing parts and their drives themselves and to the tapped precipitation or discharge electrodes and their suspensions and the (usually ceramic ) insulators.

About the use of electrostatic precipitators in cars was thinking, so far, this is not a reality.

In the metal processed and processing industries find electrostatic precipitators especially in the extraction and precipitation of aerosols consisting of cooling lubricants ( KSS) and fabric abrasion particles application. Approximately 50% of the separator used in practice, metal machined and processing establishments are electrostatic precipitators of different designs.

Designs

Electric filters differ in shape and size of the filter lane (tubes, plates), the shape of the discharge electrodes ( helix wire, thorn electrode Sägezahnionisator, Wellenionisator etc. ), the operating voltage ( DC voltage, AC voltage, pulsed DC voltage pulse superimposed DC voltage ) and the type of cleaning ( palpitations, flushing, cassette change ). There are series with and without private fan. In aggressive atmospheres special steels or even lead can be installed.

Challenges

The deposition of particularly toxic fine dust in the range below one micron is a particular challenge to the deposition of electrostatic precipitators; whereas this dust into the lungs (they are respirable) and therefore can not be coughed up. Set depending on the substance a significant cancer risk dar.

History

• First surviving recording of electrical deposition of smoke by W. Gilbert around 1600.
• A study by B. Franklin in 1745 deals with corona discharges.
• Experimental purification of a mist in a glass vessel by Hohlfeld in 1824.
• Posted by O. Lodge 1884 of this phenomenon.
• First commercial test the electrodeposition in 1885 by Walker, Hutchings and lodge in a lead smelter, but the failed because lead dust is extremely poorly separable.
• Attempts by F. Cottrell in 1906 led to the first successful commercial application in the separation of sulfuric acid mist in the powder mills of Pinole and the Selby Hut.
• WA Schmidt, a former student of Cottrell, designed around 1910, the first electric filters in the cement industry.
• Derivative of the exponential Abscheidegesetzes by W. German in 1922.