CA2138118A1

CA2138118A1 - Device for removing dust particles from exhaust gases

Info

Publication number: CA2138118A1
Application number: CA002138118A
Authority: CA
Inventors: Andreas Kwetkus; Oliver Riccius
Original assignee: Individual
Current assignee: ABB Research Ltd Switzerland
Priority date: 1993-12-16
Filing date: 1994-12-14
Publication date: 1995-06-17
Also published as: DE4342965A1; ZA949968B; EP0658380A1

Abstract

The device for removing dust particles from exhaust gases has a fabric filter (2) arranged in a filter housing (1). A precharger (4) installed in the exhaust gas line (3) upstream of the filter housing (1) serves to charge the dust particles before the latter reach the fabric filter (2). The precharger (4) is designed as an independent unit. The precharger (4) comprises at least two grid-shaped electrodes (8, 9) which extend relative to one another in an approximately plane-parallel fashion and are aligned transverse to the exhaust gas flow. The electrodes are connected to a high-voltage source (12) which is at earth potential on one side.
Owing to these measures, the dust particles are optimally precharged and because of their charge are precipitated on the fabric filter as a loose, porous and therefore gas-permeable layer, without the need for additional electric fields to be present there.
Because of the high rates of flow prevailing in the exhaust gas line upstream of the filter, the placement there renders a knocker and cleaner dis-pensible in the precharger.

Description

2i~811B
TITLE OF THE INVENTION
DEVICE FOR REMOVING DUST PARTICLES FROM EXHAUST GASES

BACKGROUND OF THE lNV~N'l'ION

FIELD OF THE lNV~N'l'lON
The invention relates to a device for removing dust particles from exhaust gases, having a fabric filter arranged in a filter housing, and in the case of which a precharger is provided for charging the dust particles before the latter reach the fabric filter, the precharger comprising high-voltage and earthing electrodes which are connected to a high-voltage source.
A device of this type is disclosed, for example, in US-A-3,gl0,779.

DISCUSSION OF R~r~5RouND

In addition to electrostatic precipitators, so-called fabric filters have been generally adopted for the purpose of precipitating dust particles from exhaust gases, for example flue gases of coal-fired incineration plants. The exhaust gas to be de-dusted passes into the filter via an exhaust gas line. Upon passing the fabric filter, which is normally designed as a bag filter, the dust particles are precipitated on the outer surface of the filter bag. They are removed at regular intervals by knockers or vibrators or by compressed air blasts and pass into funnel-shaped collectors at the bottom of the filter housing.
The dust layer built up in this case on the filter bag likewise acts as a filter. If this dust layer becomes too thick and/or too compact, this leads 3~ to flow losses which must be prevented by relatively frequent cleaning.
In the US-A-3,910, 779 mentioned at the beginning, measures are proposed for influencing the structure of this dust layer, specifically in such a 2138~1~

way that the dust particles are precipitated as a comparatively porous coating (filter cake) on the filter material. This is achieved by the combination of two measures: the dust particles are firstly directed through a precharger and charged there by means of corona discharges. Thereafter, the now charged particles are precipitated in a steady electric field which extends essentially perpendicular to the surface on which they are to be precipitated. This steady field is generated by arranging in the interior of each filter bag a grid-shaped electrode which is at earth potential and which cooperates with a high-voltage electrode situated outside the filter bag. It is Lmperative that corona discharges are not permitted to take place in this steady field, because otherwise the desired porosity of the filter cake is no longer guaranteed.
The known device is problematical for several reasons:
20 - Particle filters for coal-fired incineration plants consist of thousands of 'parallel-connected filter bags which are mostly several metres long. The outlay for feeding the high-voltage electrodes is thus considerable. Moreover, the structure must be such that it is impossible for any large relative movements between the filter bags and the high-voltage electrodes assigned to them to occur, in order to prevent corona discharges.
30 - The rate of flow of the dust laden gas through the bag filter (typically 5 cm/sec.), is several orders of magnitude lower than the rate of flow in the ~h~ t gas line to the filter, which is of the order of magnitude of 10 m/sec.. If, now, as in the case of the device according to US-A-3,910,779, the precharger is integrated into the filter housing, precipitation of dust particles already takes place there in the precharger (position 14). This requires means for 2 1 3 ~

removing and disposing of the dust in this region as well.
SUMMARY OF THE lNv~NlION

Accordingly, one object of this invention is to provide a novel device for removing dust particles from exhaust gases, which is easy to retrofit in existing filter systems and permits optimum precharging, and in which virtually no dust precipitstion occurs in the precharger.
This ob;ect is achieved according to the invention when the precharger for charging the dust particles is designed as an independent unit which is installed in the feed line to the filter housing, where the rate of flow of the exhaust gas is very high by comparison with the rate of flow through the fabric filter, and when the earthing electrode(s) of the precharger has or have a grid structure with crisscrossing grid elements and is or are aligned transverse to the exhaust gas flow, and when the high-voltage electrode(s) is or are designed as corona discharge electrodes or act as such, and extends or extend in an approximately plane-parallel fashion relative to the earthing electrode(s) and comprises or comprise grid elements exten~;ng approximately parallel. It is preferable in this case for the high-voltage electrode also to be constructed from crisscrossing grid elements, the mesh sizes of the two grid structures being different.
The invention is based here on the fin~;ng that high precipitation rates in the particle filter can be achieved without the aid of electric fields when optimally precharged particles strike the particle filter. This is achieved by the double-grid arrangement according to the invention outside the filter housing in the exhaust gas line. An additional effect results from the fact that, in accordance with the invention, the precharger is placed in the exhaust gas line just upstream of the inlet into the particle filter: in the ~ ` 2138~18 case of the high rates of flow of typically 10 m/sec.
and more prevailing here, virtually no more dust precipitation takes place in the precharger, because such particles are immediately entrained by the flow.
Conseguently, the measures otherwise to be provided for cleaning and disposing of the dust in or on the precharger are ~limin~ted, and this substantially simplifies the design. The precharger according to the invention can easily be retrofitted, as a result of which the precipitation performance of the fabric filter can be considerably increased without intervening in the filter itself. Should the precharging be insufficient, it is possible - again without intervening in the actual particle filter - to connect a plurality of double grids in series combined in one unit, or a plurality of units can be arranged in series.
Exemplary embodiments of the invention together with the advantages achievable therewith are expl~i n~
in more detail below with the aid of the drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Figure 1 shows a diagrammatic representation of a fabric filter with an upstream precharger;
Figure 2 shows a simplified longitudinal section through the precharger of Figure l;
Figure 3 shows a simplified cross-section through the precharger in accordance with Figure 2;
Figure 4 shows a first cross section through a precharger at the level of the high-voltage electrode, the electrodes being designed as wire grids which are held in a metal frame 2~3811~
~ _ 5 _ and are sur~ounded by a housing made from insulating material;
Figure 5 shows a second cross section through the precharger in accordance with Figure 4 at the level of the electrodes at earth potential;
Figure 6 shows a partial longitudinal section through the double grid arrangement with a high-voltage electrode made from barbed wire;
Figure 7 shows a partial longitll~i n~ 1 section through the double grid arrangement with a high-voltage electrode made from metal strips with pointed lugs which are bent out of the strip plane on one side and are directed towards the earthing electrode;
Figure 8 shows a partial cross section through the double grid arrangement with a high-voltage electrode made from metal strips with pointed lugs which are bent out of the strip plane on both sides and are directed towards one another;
Figure 9 shows a partial longitudinal section through the double grid arrangement with a high-voltage electrode in the form of a saw blade;
Figure 10 shows a partial cross section through the double grid arrangement with a high-voltage electrode made from metal strips bent ~in a zigzag shape.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the device represented in Figure 1 for removing dust particles from exhaust gases comprises a fabric filter, for example a bag filter 2, arranged in a filter housing 1, and a precharger 4 installed in the exhaust gas line 3 leading to the fabric filter 2. The design and mode of operation of fabric filters are known.

~3811~

The precharger ~ is designed as an indep~n~ent unit and is mounted by means of flanges 5, 6 (Figure 2) in the exhaust gas line 3 just upstream of the inlet into the filter housing 1. In accordance with Figures 2 S and 3, it has a metallic housing 7 whose free cross section corresponds to that of the exhaust gas line 3.
Provided transverse to the direction of flow of the exhaust gas in the housing 7 are metal grids 8, 9 made from metal wires, metal strips or metal rods at a spacing of typically 100 mm and tensioned in a crisscrossing fashion. Three grids denoted by 8 are at earth potential and are connected directly to the housing wall. They form the earthing electrodes. The grids 9 respectively situated between the earthing electrodes (grid 8) are held in a metal frame 9a which is mounted in an electrically insulated fashion with respect to the housing 7 by means of insulators lOa, lOb. The upper insulators lOa simultaneously serve as an electrical bushing for the high-voltage term;n~l 11 of the grid 9. The grids 9 form the spray discharge electrodes. The grids 8 and 9 are respectively connected in parallel and connected in each case to the one or the other pole o~ a voltage source 12 which supplies a DC voltage of the order or magnitude of 40-100 kVolts.
A particularly effective charging of the dust particles in the precharger 4 can be achieved when the mesh sizes of the two grids 8 and 9 are different, it being less important whether the grid 8 at earth potential or the high-voltage grid 9 has the smaller mesh size. It is important only that the spray or corona discharges forming ~etween adjacent grids extend as far as possible over the entire cross section of the housing 7 and thus of the exhaust gas line.
In the variant embodiments represented in Figures 4 and 5, the two grids 8 and 9 are respectively clamped in a metal frame 13 or 14, which for their part are mounted in a housing 7a made from insulating material. The high-voltage terminAl 15 and earthing 2~3~

t~rmin~l 16 of the grids 8 and 9, respectively, are lead to the outside through the housing wall. In the case of this embodiment, as well, the mesh sizes of the two grids 8, 9 are selected to be different for the reasons named above.
Although according to present knowledge a double grid arrangement with crisscrossing metal wires is regarded as the simplest design from the technical economic point of view, numerous modifications exist -without leaving the scope o~ the invention - with regard to the geometry of the high-voltage electrodes 9. Thus, instead of wires tensioned in a crisscrossing fashion it is also possible to use grids having metal wires 17, metal strips or metal rods which extend only in parallel and are provided at regular intervals with tips 18 in the m~nner of barbed wire (Figure 6), in order to distribute the discharge in a pinpointed fashion over the cross-sectional area of the cross section through which flow occurs. Also suitable are metal strips 19 from which pointed lugs 20 are bent out in a regular distribution, these lugs either pointing (Figure 7) in the direction of the grid 8 at earth potential or being directed (Figure 8) towards one another. Furthermore, grid elements in the form of a saw blade 21 (Figure 9) are possible, the tips - preferably pointing here, as well, in the direction of the grid 8. Grid elements 22 (Figure 10) of zigzag shape are also suitable. All these electrode geometries offer le~s flow resistance to the ~h~ t gas flowing through them. They can all be clamped in a frame. No filter dust collects on them.
When a high voltage is applied, a substantial portion of the particles carried along in the exhaust gas are charged. They hold a charge until striking against the filter material of the bag filter 2, and form there a comparatively looser air-pervious filter cake which does little to prevent the flow through the filter material. In this way, the intervals between the cle~ni ng phases can be length~ne~. The arrangement of 2~8~

the precharger in the 2xhaust gas line 3, that is to say in a region with a high rate of flow, prevents dust precipitations there, with the result that there is also no need to provide any cleaning and dust disposing devices there either.
Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

Claims

1. The invention relates to a device for removing dust particles from exhaust gases, having a fabric filter arranged in a filter housing, and in the case of which a precharger is provided for charging the dust particles before the latter reach the fabric filter, the precharger comprising high-voltage and earthing electrodes which are connected to a high-voltage source, wherein the precharger for charging the dust particles is designed as an independent unit which is installed in the feed line to the filter housing, where the rate of flow of the exhaust gas is very high by comparison with the rate of flow through the fabric filter, and when the earthing electrode(s) of the precharger has or have a grid structure with crisscrossing grid elements and is or are aligned transverse to the exhaust gas flow, and when the high-voltage electrode(s) is or are designed as corona discharge electrodes or act as such, and extends or extend in an approximately plane-parallel fashion relative to the earthing electrode(s) and comprises or comprise grid elements extending approximately parallel.

2. The device as claimed in claim 1, wherein the high-voltage electrode(s) also has or have a grid structure with crisscrossing metal wires, metal strips or metal rods.

3. The device as claimed in claim 1 or 2, wherein arranged inside a unit are a plurality of electrodes which area alternately connected to the one or the other pole of the high-voltage source.

4. The device as claimed in claim 1 or 2, wherein a plurality of prechargers designed as units are installed in the feed line to the filter housing.

5. The device as claimed in one of claims 1 to 4, wherein the electrodes consist of metal wires, metal strips or metal rods, which are preferably tensioned in a crisscrossing fashion and of which the earthing electrodes are connected directly to the metal housing of the precharger.

6. The device as claimed in claim 1, 2 or 3, wherein the electrodes consist of metal wires, metal strips or metal rods which are preferably tensioned in a crisscrossing fashion and are respectively held in a metal frame, and wherein the housing of the precharger consists of an insulating material.