GB2142845A - Electrostatic screen for electrostatic precipitators - Google Patents

Abstract

In a method of operating an electrostatic screen, which comprises an electrode system positioned at the gas exit end of an electrostatic precipitator, the electrode system consisting of a number of dust collecting electrodes 5 positioned substantially in a plane perpendicular to the direction of gas flow and connected to the positive pole of a voltage source 6, and a number of emission electrodes 4 positioned immediately upstream of the dust collecting electrodes and connected to the negative pole of the same voltage source, the voltage supplied across the electrodes is smaller than the corona-extinction voltage and has superimposed on it high voltage pulses which transiently cause the voltage difference between the electrodes to exceed the corona- inception voltage. <IMAGE>

Description

SPECIFICATION Electrostatic screen The invention relates to electrostatic precipitators and more particularly to such precipitators which, in addition to the ordinary electrode sets for carrying out the dedusting proper, have a further electrode set for the formation of an electrostatic screen stretching across the precipitator at its gas exit end.

Electrostatic screens may be used for collecting charged particles not collected already by the precipitation electrodes of the electrostatic precipitator. Such a screen may consist of a number of dust collecting electrodes positioned substantially in a plane perpendicular to the direction of gas flow and a number of emission electrodes positioned immediately upstream of the dust collecting electrodes, the emission electrodes being connected to the negative pole of a voltage source and the dust collecting electrodes being connected to the positive pole. The field between the electrodes will cause the charged particles to collect on the dust collecting electrodes of the screen and thus to be decharged.The particles can be shaken down into a dust collecting aggregate at the precipitator bottom by means of a shaking mechanism similar to that known for the main dedusting electrodes of the precipitator.

In commonly known electrostatic screens the field between the electrodes of the screen is static, i.e. there is no real current between the electrodes. However, it is advantageous if there is a certain current between the electrodes of the screen. By this means the electrostatic screen can collect possible noncharged particles reaching the screen, and also the current retains the discharged dust on the dust collecting electrodes until the latter are vibrated. The current between the electrodes of the screen should be maintained at a low value, partly to avoid current-provoked sparkovers between the electrodes of the screen, which is certain to destroy its effectiveness, and partly for reasons of economy.

The efficiency of the electrostatic screen increases with the field strength between the electrodes of the screen, i.e. with increasing voltage between the electrodes. However, in conventional DC operation a high voltage between the electrodes entails a large current if the emission electrodes have a small emitting radius of curvature. If a larger radius of curvature is chosen it is impossible to maintain a stable, evenly distributed, emission or "corona" across the entire cross-sectional area of the screen.

Consequently, it is an object of the invention to provide a method of operating an electrostatic screen so as to avoid these disadvantages.

According to the invention such a method comprises supplying a voltage to the electrodes of the screen which is smaller than the corona-extinction voltage, and superimposing high voltage pulses which cause the voltage to considerably exceed the corona-inception voltage.

The corona-extinction voltage is the voltage below which the voltage across the electrodes of the screen must fall in order that corona discharge be switched off and the coronainception voltage is the voltage above which the electrode voltage must be raised in order to cause corona emission across the screen.

The corona-inception voltage will generally be above that of the corona-extinction voltage, or to be more precise the corona-inception voltage will generally be a higher negative voltage than the corona-extinction voltage as is well known.

By this method incipient local emission is avoided while the high voltage during the pulses ensures that all parts of the emission electrodes of the screen exceed the local corona-inception voltage, so that an evenly distributed emission is obtained across the entire cross-section of the screen.

By use of pulse voltages the radius of curvature of the emitting part of the screen can be made large, so that the underlying DC field can also be made high. Also the maximum field, i.e. the peak value of the field strength between the electrodes of the screen as a function of time, increases significantly due to the ion movement succeeding a pulse voltage.

Preferably, the current between the electrodes of the screen can be controlled by controlling the pulse repetition frequency, which yields the possibility of accurate control of the current even at very low current values, without the field in the electrostatic screen being reduced.

Usually, the electrodes of the electrostatic screen comprise staggered opposed U-shaped profiles, the profiles constituting the emission electrodes opening towards the gas exit of the precipitator and the profiles constituting the dust collecting electrodes being positioned immediately before the gas exit of the precipitator and opening towards the openings of the emission electrodes, but being offset in relation to these profiles to the effect that the dust collecting electrodes face the gaps between the emission electrodes.

Besides functioning as electrodes in the electrostatic screen the U-shaped profiles form a gas distribution screen, the object of which is to distribute the gas flow evenly across the precipitator cross-section.

Thus, by pulse operation of the electrostatic screen it is possible to maintain a high electric field simultaneously with the current being controlled at an "arbitrarily low" value and furthermore being evenly distributed across the cross-sectional area of the screen.

The invention will now be described in more detail, reference being made to the accompanying drawing which shows diagrammatically the exit end of an electrostatic precipitator seen from above and provided with a pulse operated electrostatic screen according to the invention.

The precipitator unit proper has emission electrodes 1 and collecting electrodes 2. Between the precipitator electrodes 1 and 2 and a gas exit duct 3 a number of U-shaped profiles 4 are positioned in spaced relationship across the cross-section of the precipitator housing, their openings facing the gas exit duct 3. These profiles serve to distribute the gas flow, but simultaneously they make up one of the electrode sets of an electrostatic screen, the other electrode set of which consists of similar U-shaped profiles 5 the openings of which face the openings of the first set of profiles, the two sets of profiles being offset in relation to each other so that the gaps between the profiles of one set face the openings in the profiles of the other.The particular construction and shape of the profiles and their more exact positioning in relation to each other can be varied within wide limits.

The two sets of screen electrodes are connected to a generator 6 generating a pulsesuperimposed direct voltage, so that both direct voltage and pulse height and frequency can vary, and poled so that the screen electrode set 4 closest to the precipitator electrodes 1, 2 is negative in relation to the screen electrode set 5 closest to the gas exit 3. Thus, the electrodes of the screen electrode set 4 act as emission electrodes and those of the latter 5 as collecting electrodes.

The direct voltage is set so as to be smaller than both the corona-inception voltage and the corona-extinction voltage, and the supplied pulses are chosen so high as transiently to cause the voltage between the electrode sets 4, 5 to substantially exceed the coronainception voltage, thus ensuring an even current distribution and also that the discharge not only takes place on the edge of the collecting electrodes, but also to some extent on the bottom of the U-shaped profile of these electrodes.

The current between the two electrode sets 4, 5 is proportional to and controllable by regulation of the pulse frequency.

The direct voltage may be set to lie between the corona-inception voltage and the corona-extinction voltage. The superimposed pulses must then be followed by a momentary voltage drop which ensures that the voltage after each pulse is lowered beneath the corona-extinction voltage.

Claims (6)

1. A method of operating an electrostatic screen comprising an electrode system positioned at the gas exit end of an electrostatic precipitator, the electrode system consisting of a number of dust collecting electrodes positioned substantially in a plane perpendicular to the direction of gas flow and connected to the positive pole of a voltage source, and a number of emission electrodes positioned immediately upstream of the dust collecting electrodes and connected to the negative pole of the same voltage source, characterized in that the voltage supplied across the electrodes is smaller than the corona-extinction voltage and has superimposed on it high voltage pulses which transiently cause the voltage difference between the electrodes to exceed the coronainception voltage.

2. A method according to claim 1, characterized in that the current through the screen is controlled by controlling the repetition frequency of the superimposed pulses.

3. A method of operating an electrostatic screen, substantially as described with reference to the accompanying drawings.

4. An electrostatic precipitator for use in a method according to claim 1 or 2, the precipitator including a screen comprising an electrode system having of a number of dust collecting electrodes positioned substantially in a plane perpendicular to the direction of gas flow adjacent the gas outlet of the precipitator, and a number of emission electrodes positioned immediately upstream of the dust collecting electrodes, characterized in that the precipitator includes a power supply supplying the screen with a high-tension direct voltage with superimposed high-tension voltage pulses, the positive terminal of the power supply being connected to the collecting electrode of the electrostatic screen and the negative terminal being connected to the emission electrode of the screen.

5. An electrostatic precipitator according to claim 3, characterized in that means are provided for regulating the DC voltage, the pulse voltage, the pulse duration and the pulse repetition frequency of the power supply.

6. An electrostatic precipitator substantially as described with reference to the accompanying drawings.