DE1206397B - Process for cleaning gases or vapors in electrostatic precipitators using asymmetrical alternating voltage - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

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German classes: 12e-5

M 56791ΙΠ / 12 e

May 11, 1963

December 9, 1965

Electric separators for cleaning gases or vapors are usually operated with DC voltage operated, with the negative pole being placed on the outflow electrodes. A technical one There is no need to use ideal DC voltage. So the show in question here Coming DC voltages depending on the type of generation and depending on the number of phases the primary alternating voltage has a varying degree of ripple, i.e. varying degrees of fluctuation at the height of the tension. So far, all efforts have been towards a direct voltage permanent negative polarity.

It is also known to operate electrostatic dust collectors with alternating voltage, so that the gas to be cleaned is forced to flow through alternating electric fields, as for example in the German patents 300 811, 374 223 and 421151 is described. Furthermore, they were also pulsating DC voltages and AC voltages used, these superimposed so that Phase shifts arose, as claimed in German patents 425 026 and 657 376 is, whereby one is precisely from the phase shifts advantages for the dust separation promised. In this context it has also been recognized that a pulsating DC voltage is considerably more active than a standing DC voltage with regard to the generation of ions. Compare German patent 358 377 on this.

A change in polarity is, however, as described, for example, in German patent specification 369 879 can be taken, has been regarded as disadvantageous, since reversing the polarity of the field causes a return transport the particles migrating to the collecting electrode to the spray electrode is to be expected.

In electrostatic precipitators, the particles to be separated migrate under the influence of the electrical Field to the precipitation electrode and are deposited there. The speed of migration is directly dependent on the field strength, i. H. from the voltage drop between the vent and Collecting electrodes. Constant distances are assumed here. An increase in the field strength results in an improvement in the separation efficiency of the electrostatic precipitator.

There are limits to such an increase in the dust extractor due to the dielectric strength of the Gas and the electrical properties of the medium to be separated. The dielectric strength of the gas depends on the composition and the physical conditions of the

Process for cleaning gases or vapors in electrostatic precipitators below Use of asymmetrical alternating voltage

Applicant:

Metallgesellschaft Aktiengesellschaft, Frankfurt / M., Reuterweg 14

Named as inventor:

Karl Lohrberg,

Dipl.-Ing. Helmut Schummer, Frankfurt / M.

the same and also a function of the duration of the stress exposure.

The upper limit of the electrical resistance, for example of a dust to be separated out, at which separation can be carried out successfully, is about ΙΟ ¹² Ω cm. The relationships on which the present subject matter of the invention is based at specific dust resistance values in electrostatic dust separators from 10 ¹¹ Ω cm, at which so-called "back spraying" occurs, are, for example, in the "Archive for the ironworks", 1961, p. 221 ff., As well as in the magazine " Dust «, 1962,

P. 463 ff., And described in various company publications. Namely, if the resistance is above this limit, the following phenomenon occurs:

The particles to be deposited are electrically charged, but also migrate to the precipitation electrode, but do not release their charge there or only release it slowly. This leads to training an intermediate potential.

The voltage drop between the outlet and collecting electrodes remains present, the effective voltage drop between the outlet electrode and the surface of the deposited layer but becomes less, i.e. the separation becomes significantly worse. As the surface of this deposited Layer opposite to the collecting electrode has an electrical charge opposite Has polarity, the deposited particles act as the dielectric of a capacitor. In this Capacitor breakdown occurs at times, with a sudden increase in power consumption of the electrical separator are connected, the deduster operation becomes restless, requires a down regulation the applied voltage and calls

509 757/366

due to an additional deterioration in the degree of separation.

The invention is based on the object of eliminating the aforementioned difficulties.

According to the invention, the use of an asymmetrical alternating voltage during operation becomes more electrostatic Proposed separator, the ratio of the amplitudes of two successive Half-waves by means of an adjustable resistor connected in parallel to a rectifier, preferably an ohmic resistance, depending on the specific resistance of the Precipitation electrodes deposited dust is adjusted so that the due to the dust resistance on the collecting electrodes forming, capacitively acting boundary layer through the Half-wave of the applied alternating voltage with the smaller amplitude is just being discharged.

It is considered to be surprising, new and progressive that what has hitherto been viewed as disadvantageous Crossing the zero line under certain deposition conditions of electrostatic precipitators Brings advantages. The combination of rectifier and resistor can be used both in the earth circuit and in the high-voltage circuit of the filter is switched on.

In the prior art it is known to limit the current in series between Power source and rectifier switch on an ohmic or inductive resistor, as in German Patent 487372 described, or a parallel connection of a resistor to the electrostatic To undertake dedusting to prevent the field strength from dropping, as from the German Patent 574,652 is apparent. However, these resistors serve a different purpose from the above described resistance.

When the zero line is passed, the polarity of the outflow and precipitation electrodes is briefly reversed. Due to the high resistance of the parts to be deposited on the collecting electrodes If a layer with a capacitor effect is formed, as described above, this is the case Discharge capacitor. As a result, the surface facing the outflow electrode gets the deposited Layer again earth potential, and the full field strength is available for the deposition available again in the next half-wave.

According to a further development of the invention, the amplitude ratio of two adjacent half-waves is during operation by continuously changing the resistance as a function of the The conductivity of the precipitated medium is regulated.

The following is added to the advantages of the method according to the invention already mentioned:

In the known constructions of electrostatic precipitators, when using an alternating voltage The frequency of about 50 Hertz builds up and breaks down the electric field so quickly that This does not apply to electrical voltages that lead to breakdown when using direct voltage comes to this phenomenon. Thus, when using the asymmetrical alternating voltage according to the invention, the maximum electric field strength can be increased significantly without breakdowns occur in the separator.

If the voltage is consciously increased to the point of breakdown, the resulting arc will open Reason for crossing the zero line deleted immediately. However, the mean tension does not break as at Using DC voltage together. The electrical separator therefore does not need to be de-energized to be made. It is also not even necessary to regulate the voltage down.

The invention is explained in more detail using an exemplary embodiment.

A b b. 1 shows schematically a circuit diagram for "the high voltage supply of an electrostatic Dust collector with asymmetrical alternating voltage according to the invention;

Fig. 2 represents the graphical course of the asymmetrical Alternating voltage;

Fig. 3 shows the idealized voltage drop as a function of the distance between the spray electrode and precipitation plate not exposed to dust while

Fig. 4 shows a representation of the voltage drop with a dust-laden collecting electrode.

In the secondary circuit of the high voltage transformer 1 are a half-wave rectifier 2 and parallel to this an ohmic resistor 3 for generation the asymmetrical alternating voltage switched according to the invention. By this circuit arrangement becomes the designated 4 electrostatic dust collector with asymmetrical alternating voltage provided. The arrow through the ohmic resistance indicates that this is adjustable. At An alternating current resistor can of course also be used in place of the ohmic resistance will. In A b b. 2, the graphical course of the asymmetrical alternating voltage is plotted from from which it can be seen that with amplitude ratios of neighboring half-waves of ™ in the magnitude

Order of about -j can be worked. However, this ratio depends largely on the electrical conductivity of the dust and on the level of the voltage to be applied or on the effective value of the same.

From Fig. 3 it can be seen that the voltage curve U = / (1) between the spray electrode and the collecting electrode, provided this is free of dust, is purely linear. Fig. 4 shows that when the collecting electrode is loaded with dust, this function has a kink at the beginning of the dust layer, such that the slope of the curve shown in the graph up to the dust layer is lower than in the illustration according to A b b . 3, and within the dust layer larger than according to the illustration in Fig. 3. These relationships occur if the mean value of the voltage is taken into account without the negative half-wave of the asymmetrical voltage according to the invention. Since the dust layer is discharged when using the asymmetrical alternating voltage according to the invention, the same curve U = f (1) as in Fig. 3 is shown in this case for measurements with a dust-covered plate.

Claims (3)

Patent claims: 1. Process for cleaning gases or vapors in electrostatic precipitators below Application of asymmetrical alternating voltage, characterized in that the ratio the amplitudes of two successive half-waves by means of a rectifier adjustable resistor connected in parallel, preferably an ohmic resistor, as a function the specific resistance of the medium deposited on the collecting electrodes is set so that the due to the electrical resistance of the medium to the Precipitation electrodes forming, capacitively acting boundary layer through the half-wave the applied alternating voltage with the smaller amplitude is being discharged. 2. The method according to claim 1, characterized in that the amplitude ratio two adjacent half-waves of operation is regulated. 3. The method according to claims 1 and 2, characterized in that the electrostatic Separator in the breakdown area is operated in such a way that the resulting flashovers or arc by reaching the voltage value zero between two half-waves of the asymmetrical alternating current. Considered publications: German Patent Nos. 300 811, 358 377, 879, 421151, 425 026, 487 372, 574 652, 376; "Archiv für das Hüttenwesen", 32 (1961), 4, p. 221 ff .; "Staub", 22 (1962), 11, p. 463 ff .; Lurgi publication "Electrical flue gas cleaning in steam power plants", No. BP 21 / 3.0 d. RR. 6.61, (1961), p. 7 ff. 1 sheet of drawings 509 757/366 11.65 © Bundesdruckerei Berlin