AU735231B2 - Method for cleaning dust-laden process off-gases - Google Patents

Abstract

The invention relates to a method for cleaning waste process gases containing water vapour and dust, in a wet-type electrostatic filter. According to this method, the gases which pass through said wet-type electrostatic filter are charged in a space between one or several emission electrodes and one or several collecting electrodes and the particles of dust are precipitated on the collecting electrode(s). The invention is characterized in that the speed at which the waste gases pass through the wet-type electrostatic filter is set at a value of 4 m/s to 9 m/s.

Description

-1 Method for cleaning dust-laden process off-gases The invention relates to a method for cleaning dustladen process off-gases in a wet electrostatic filter.

Wet electrostatic filters are units for dust precipitation which are increasingly being used for the precipitation of extremely fine dust. Electrostatic filters are usually set up on the basis of the Deutsch equation, according to which: Cciean =exp Cunfiltered' V For the given boundary conditions of degree of dust precipitation (Cclean/Cunfiltered) and off-gas volumetric flow rate V, the product of migration velocity and collecting electrode surface area (w.A) remains the significant parameter, i.e. the filter, in particular the collecting electrode surface area required, can be dimensioned if the migration velocity of the dust which is to be precipitated is known.

Incidentally, the migration velocity is dependent on a plurality of parameters, such as dust particle size, dust composition and off-gas velocity.

According to the theoretical model described in the literature (Weber, Brocke, Apparate und Verfahren der industriellen Gasreinigung [Methods and Apparatus of Industrial Gas Cleaning]; Vol. 1; pp. 352 353, and Parker Applied Electrostatic Precipitation; ist Edition; 1997; p. 128), the maximum migration velocity in electrostatic filters is achieved at off-gas flow velocities in the range from 2.0 m/s to m/s. Higher gas velocities increase the extent to which particles which have already been precipitated on Rthe collecting electrode are swirled up again and also reduce the migration velocity and therefore the extent 1J of precipitation.

2 In the case of wet electrostatic filters, in contrast to dry electrostatic filters, the problem of dust which has already been precipitated being swirled up again at relatively high off-gas velocities in practice does not arise. The upwards movement of the film on the wall only occurs at very high off-gas velocities, in the range of over 10 m/s. The discharge of drops as a result of them becoming detached from the film on the wall is theoretically possible, and, depending on the thickness of the film of water, even at significantly lower off-gas velocities, but, due to the high likelihood of the released drops being precipitated again, this phenomenon does not contribute significantly to the particle discharge.

Furthermore, it is known from the literature (see Weber, p. 360) that the average effective migration velocity falls as the fineness of the dust increases.

Wet electrostatic filters are usually designed for offgas flow velocities of from 1.5 m/s to 2 m/s. In the case of the dedusting of sintering installations, for example, the small particle sizes result in migration velocities on the collecting electrode surface of approximately 10 cm/s. This low off-gas velocity entails an increased outlay on flow-evening devices on the inlet and outlet side in the filter in order to make the off-gas flow more uniform. For example, offgas flow velocities of the order of magnitude of approximately 20 m/s are found within off-gas ducts upstream and downstream of the filter. This means an acceleration on the outlet side of the filter or a deceleration on the inlet side of the filter by a factor of approximately 10-13 in each case.

Therefore, the object of the invention is to provide a method for cleaning dust-laden off-gases in a wet electrostatic filter, which method is characterized in that the off-gas flow velocity is set to a value which 3 is such that the highest possible migration velocity is produced and, at the same time, the drawbacks of the prior art, in particular expensive flow-evening devices and large collecting electrode surfaces, are avoided.

In the context of the invention, wet electrostatic filters are understood to mean both those in which the off-gas is completely or almost completely saturated with steam before it enters the filter, as well as those in which the off-gas exhibits a considerable subsaturation of steam when it enters the filter.

Therefore, in the latter wet electrostatic filters, the cleaning by the collecting electrodes has to be effected by spraying in additional water.

This object is achieved by the subject matter of the invention, which is characterized in that the off-gas flow velocity is set to a value of from 4 m/s to 9 m/s.

As a result of the flow velocity being increased to this range, the migration velocity of the particles rises from approximately 10 cm/s, for the migration velocities with currently conventional off-gas flow rates of from 1.5 m/s to 2 m/s in wet electrostatic filters used in the dedusting of sintering installations, to over 40 cm/s.

It is thus possible to reduce the size of the collecting electrode surface, and therefore also the weight of the collecting electrode, in a ratio which is proportionate to the increase in the migration velocity, while maintaining the same level of efficiency. Furthermore, the outlay on flow-evening devices on the inlet and outlet sides of the filter is thus minimized (deceleration or acceleration by a factor of only about 2-5 now required). The size of the wet electrostatic filter is in this way reduced accordingly, with the result that the investment costs are also lower.

4 The invention will now be explained in more detail with reference to Figures 1 and 2.

Fig. 1 shows a diagram illustrating off-gas flow velocity against migration velocity.

Fig. 2 shows a diagram indicating grain size classes against cumulative concentrations.

Fig. 1 illustrates, using a system of coordinates, measurement points 1 which were determined in tests, the off-gas flow velocity in m/s being plotted on the abscissa and the migration velocity of the particles in cm/s being plotted on the ordinate.

As can be seen, the migration velocity increases in an approximately linear manner up to off-gas velocities of approximately 6 m/s, while at higher velocities it can be seen that the curve becomes flatter. There is scarcely any undesirable release of droplets within the range according to the invention of from 4 m/s to 9 m/s.

Fig. 2 shows, in a system of coordinates, cumulative curves 2 from particle size analyses of unfiltered gas dusts present in various tests, the particle size classes of the gas particles in pm being plotted on the abscissa, and the cumulative concentrations of the respective particle size classes in per cent being plotted on the ordinate. As can be seen from the curves, most of the particles lie in a grain size range of between 0.03 pm and 4.0 pm.

The tests which led to the measurement points shown in Fig. 1 and Fig. 2 were carried out in a wet electrostatic filter of the following design: Tube filter with water cooling STube internal diameter 200 mm Number of tubes Tube length Discharge electrode Voltage Current 6 (two-stage) 2.6 m Rod 45 kV 17 mA The dust-laden unfiltered gas, which originates from a sintering installation, is initially passed through a scrubber, where preliminary separation of relatively large particles 16 Rm) takes place and the gas is enriched or saturated with steam. The fine precipitation takes place in the wet electrostatic filter described above.

Claims (7)

1. Method for cleaning steam-containing, dust-laden process gases in a wet electrostatic filter, in which preliminary separation of relatively large dust particles, in particular of dust with a grain size of greater than 16 um, as well as enrichment or saturation of the process gases with steam, are carried out in a scrubber, and in which the gases or the dust particles contained therein, which then flow through the wet electrostatic filter, are charged in a space between one or more discharge electrodes and one or more collecting electrodes and are precipitated on the collecting electrode(s), characterized in that the off-gas flow velocity through the wet electrostatic filter is set to a value of from 4 m/s to 9 m/s, and the wet electrostatic filter effects all the remaining dust precipitation in the area of the collecting electrodes, which are of sufficient length to achieve this.

2. Method according to Claim 1, characterized in that the off-gas flow velocity through the wet electrostatic filter is set to a value of from m/s to 9 m/s.

3. Method according to Claim 2, characterized in that the off-gas flow velocity through the wet electrostatic filter is set to a value of from m/s to 9 m/s.

4. Method according to one of Claims 1 to 3, characterized in that at least 90% of the fine dust to be precipitated comprises particles of a size of up to 4 rm.

AMENDED SHEET 6a Method according to one of Claims 1 to 4, characterized in that at least 80% of the total amount of particles of dust to be precipitated lies within a grain size range of from 0.03 |im to 2.0 pm.

6. Method according to one of Claims 1 to 6, characterized in that the discharge electrode used is a rod-like electrode.

7. Method for cleaning process gases substantially as herein described with reference to the accompanying examples and/or figures. 9 9 e H:\suzanneg\Keep\Speci\67116-98-spec dated 27-4-2001.doc 27/04/01