AU644310B2

AU644310B2 - Method for dry electrostatic cleaning of dust and pollutant containing exhaust gases

Info

Publication number: AU644310B2
Application number: AU26104/92A
Authority: AU
Inventors: Jan Olscher; Wigand Schaletzki; Janusz Syga
Original assignee: Metallgesellschaft AG
Current assignee: GEA Group AG
Priority date: 1991-10-03
Filing date: 1992-10-02
Publication date: 1993-12-02
Anticipated expiration: 2012-10-02
Also published as: DE4132874C1; AU2610492A

Description

P/00/011 28/51 1 Regulation 3.2(2)

AUSTRALIA

Patent Act 1990 644.310

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Application Number: Lodged: Invention Title: METHOD FOR DRY ELECTROSTATIC CLEANING OF DUST AND POLLUTANT CONTAINING EXHAUST GASES The following statement Is a full description of this Invention, including the best method of performing it known to :-US

DESCRIPTION

The invention is in respect to a method for the dry, electrostatic cleaning of dust and pollutant-containing exhaust gases in an electrostatic precipitator, in which plate-shaped collecting electrodes are arranged and which receive their power supply in the form of d.c. voltage pulses or a d.c. voltage with superimposed pulses in cycles that consist of pulse times of the length of one pulse and pause times of the duration of multiple pulses.

The DE-PS 3526009 describes a method for optimising the power supply to an electrostatic precipitator, which provides a power supply by way of d.c. voltage pulses or a d.c. voltage with superimposed pulses in cycles that consist of pulse times of the :length of one pulse and pause times of the duration of multiple pulses. In this method the longest possible decay time at the best possible efficiency of the electrostatic precipitator is determined, whereby the definition of the decay time is the time required within which the pulse voltage U has dropped to a predetermined rest voltage UR, In pulse mode, as opposed to the normal operating mode of an electrostatic precipitator, only such a number of d.c. voltage pulses are supplied to the discharge electrode that there is only a certain number of charge carriers produced that is sufficient to precipitate the dust or pollutants present in the stream of raw gas. The thyristor is turned off afterwards for a time period of 20 400 ms and the precipitator voltage drops in an exponential function until the next d.c.

voltage pulse is applied, 2 Between the individual dc, voltage pulses the precipitator voltage is kept at an optimum lower limit, to prevent the precipitator voltage from dropping too low.

It is the task of the invention to further reduce the dust and pollutant emissions in the clean gas of an electrostatic precipitator operating in pulse mode.

This task is solved in that way, that the electrostatic precipitator is operated at a current density of 0.25 to 5 mA/mz and whereby the distance between the plate-shaped collecting electrodes is between 200 and 500 mm. Dry operating electric filters may be used for electrostatic precipitators. The current :..{density is generally understood to be that current that occurs in operation per active collecting area of the collecting electrodes.

In the pulse mode, this current density should be understood as the current during operation per active collecting area of the collecting electrodes during the pulses with the pulse voltage U.

.i If the current density of 0.25 mA/m is exceeded in an electrostatic precipitator that does not operate in pulse mode and the exhaust gas to be cleaned contains dust with a high specific resistance, then a discharge current will flow through the dust coating on the collecting electrodes which creates positive and negative ions. The negative ions are absorbed by the collecting *e •electrode, however, the positive ions move back into the gas stream and reduce there the negative charge of the dust particles.

This process, which is also called 'reverse discharge', causes a deterioration of separating ability of the electrostatic precipitator.

-3- It was surprising to see, that reverse discharges could be prevented in an electrostatic precipitator that was operated in pulse mode provided that the precipitator is driven with a current density of 0. 25 to 5 mA/m 2 and the distance between the plate-shaped collecting electrodes is between 200 and 500 mm. This allows the emission of dust and pollutant in the clean gas to be reduced by 30 to 90% compared to an electrostatic precipitator that does not operate in pulse mode. The method according to the invention has the advantage that exhaust gases are cleaned from pollutants like HF, SO 2

SO

3 as well as HCl and the non-ferrous metals like Pb, Cd, Hg and As that are present in the exhaust gas in the form of steam, gas or in sublime form, Exhaust gases can also be cleaned from dusts whose specific dust resistance is larger than 2 x 1010 ohm/cm.

a A preferred arrangement of the invention is, that the distance of the plate-shaped collection electrodes from each other is 300 to 500 mm. This measure achieves a particularly high degree of separation by che electrostatic precipitator, The object of the invention is explained in more detail below by using the diagram (Fig. 1).

Fig.l shows the precipitator voltage as a function of time t, The precipitator voltage is shown in Fig.l as a function of time t as long as the exhaust gas contains dust with a high specific dust resistance. The pulse with a pulse voltage U and the pulse time tp is followed by the off-state time and then the replenishing time The sum total of these three time periods is the cycle time t. which is followed by another cycle.

4 During the off-state time t, the precipitator voltage drops in an exponential function to a rest voltage The voltage level pulses within the replenishing time t, between the predetermined rest voltage U, and the also predetermined replenishing voltage in order to prevent the precipitator voltage from dropping too low, which would cause the driving force for the migration of the charged dust particles to the collecting electrode to drop too low.

An increase of the current density causes an increase of the precipitator voltage. This makes it possible for the method according to the invention to increase the pulse voltage U which in turn results in an increased separating ability of the electrostatic precipitator. The invention is described below in detail by way of an example: The exhaust gas created during the incineration of hard coal has a temperature of 170°C, a dew-point of 35°C, a dust content of th.p 18.25 g/m 3 and is present in quantities of n&s680m3/h The collecting area of the electrostatic precipitator is 7434 m 2 whereby plate-shaped collecting electrodes are used which are arranged at a distance of 400 mm, A romaining content of dust particles of 22 mg/m 3 was measured in the clean gas at a field strength in the range of 2.3 to 2.6 KV/cm at the point in time when the pulse voltage was U and at a current density of 0.5 mA/m.

During the operation of the electrostatic precipitator, which was working in pulse mode, no 'reverse discharge' was observed in spite of the selected current density of 0,5 mA/m 2 The emission values for dusty inorganic materials according to TA Air, dated 27.2,1986, were for Class I (Cd, Hg etc.) below 0.2 mg/Nm 3 for Class II (As, Ni etc.) below 1,0 mg/Nm 3 and for Class III (Pb, F, Sn etc below 5 mg/Nm 3 The limits for steam or gaseous inorganic materials particularly for SO 2 with 500 mg/Nm 3 were not exceeded in the test,

Claims

1. Method for the dry electrostatic cleaning of dust and pollutant containing exhaust gases in an electrostatic precipitator in which plate-shaped collecting electrodes are arranged and which has a power supply in the form of d.c. voltage pulses or a d.c. voltage with superimposed pulses and cycles that consist of pulse times of the length of one pulse and pause times of the duration of multiple pulses, characterized in that, that the electrostatic precipitator is operated with a current density of 0.25 to 5 mA/ 2 whereby the distance between the plate-shaped collecting electrodes is 200 to 500 mm from each other.

2. Method according to claim 1, characterized in that, that the distance of the plate-shaped collecting electrodes from each other is 300 to 500 mm. DATED this 1st day of October 1992. METALLGESELLSCHAFT AKTIENGESELLSCHAFT WATERMARK PATENT TRADEMARK ATTORNEYS "THE ATRIUM" 290 BURWOOD ROAD HAWTHORN. VIC. 3122. 6 4 IA t ABSTRACT Method for the dry electrostatic cleaning of dust and pollutant containing exhaust gases. A method for the dry electrostatic cleaning of dust and pollutant containing exhaust gases in an electrostatic precipitator is described, in which plate-shaped collecting electrodes are arranged and where the power supply to it is in the form of d.c. voltage pulses or a d.c. voltage with superimposed pulses and cycles that consist of pulse times of the length of one pulse and pause times of the duration of multiple pulses. The method is characterized in that, that the electrostatic precipitator is operated at a current density of 0.25 to 5 mA/m 2 and whereby the distance of the plate-shaped collecting electrodes from each other is 200 to 500 mm. With this method the 'reverse discharge' in the electrostatic precipitator can be prevented and the separating capacity can be increased. Fig. 1 7