Electrostatic dust collector for exhaust gases containing fine particles

Abstract

An electrostatic dust collector for exhaust gases containing fine particles having a lattice-type particle aggregating means positioned upstream of a dust collecting zone formed by a charged electrode, connected to a high voltage D.C. generator, and a dust collecting electrode maintained at ground potential, the electrodes establishing an electrostatic field therebetween, as well as a weak electrostatic field between the particle aggregating means and the charged electrode. Passage of the gas containing the fine particles through the aggregating means results in the formation of coarser particles which may be deposited upon the dust collecting electrode. Humidifying means may be provided upstream of the aggregating means to aid in the aggregating process.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to apparatus for electrostatically collecting fine particles within exhaust gases and more particularly to an electrostatic dust collector wherein the particles in the exhaust gas are electrostatically collected after they are previously aggregated into coarse particles.

2. Description of the Prior Art

One type of conventional dust collector for electrostatically collecting dust which has been widely used in industrial systems is the Cottrell type dust collector which passes the exhaust gas containing the fine dust particles through an electrostatic field formed by a pair of electrodes which face each other.

However, the conventional Cottrell type dust collectors exhibit a low coefficient of collecting fine particles making it difficult to collect such fine particles having a diameter of less than 1 .mu.. The disadvantages have been especially serious when treating exhaust gas which is at a high temperature and which contains fine particles having a specific electric resistance, such as, for example, the exhaust gases which are exhausted from a rotary kiln of a cement factory or an electric furnace of a ferrosilicon factory.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide an improved electrostatic dust collector which electrostatically collects fine particles having a diameter of less than 1 .mu..

Another object of the present invention is to provide an improved electrostatic dust collector which electrostatically collects fine particles having a specific electric resistance by lowering the specific electric resistance.

Still another object of the present invention is to provide an improved electrostatic dust collector which has a relatively simple structure and has a high coefficient of dust collection.

Yet another object of the present invention is to provide an improved electrostatic dust collector which effectively removes dust deposited upon a dust collecting electrode.

The foregoing objectives are achieved according to this invention through the provision of a grid or lattice-type particle aggregating means positioned upstream of the dust collecting zone, and wherein a charged electrode connected to a high voltage D.C. generator faces a dust collecting electrode maintained to ground or earth potential so as to form an electrostatic field within the dust collecting zone of the exhaust gas passage. The particle aggregating means results in the aggregation of the fine particles contained within the exhaust gas by collision of the fine particles within a weak electrostatic field which exists between the aggregating means and the charged electrode. A humidifying means is preferably provided upstream of the particle aggregating means when the fine particles have a high specific electric resistance. The electrostatic dust collector can be provided in an exhaust gas passage connected to a rotary kiln, combustion furnace, electric furnace or other combustion apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

Various other objects, features, and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings, in which like reference characters designate like or corresponding parts throughout the several views, and wherein:

FIG. 1 is a partial sectional side elevation view of one embodiment of the electrostatic dust collector constructed according to this invention, and showing its cooperative parts;

FIG. 2 is a diagramatical view of another embodiment of the electrostatic dust collector constructed according to this invention, and showing its cooperative parts;

FIG. 3 is a perspective view, with parts broken away, of still another embodiment of the electrostatic dust collector constructed according to this invention, and showing the specific arrangement of a humidifier and an aggregating means;

FIGS. 4 to 12 respectively show several embodiments of the aggregating means which may be utilized in this invention;

FIG. 13 is a cross-sectional view of yet another embodiment of the electrostatic dust collector constructed according to this invention, looking in the direction of the longitudinal axis of the flow passage, such specifically showing conveying means employed with such apparatus;

FIG. 14 is a cross-sectional view, similar to that of FIG. 13, showing another type of conveying means which may be employed; and

FIG. 15 is a cross-sectional view, similar to that of FIG. 13, showing a further embodiment of the apparatus constructed according to this invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings, and more particularly to FIG. 1 thereof, there is shown an electrostatic dust collector into which exhaust gas, containing fine particles exhausted from a rotary kiln, an electric furnace, or the like, not shown in the drawing, is introduced through a rectangular flow duct 1. A grid or lattice-type aggregating means 2 is mounted transversely within flow duct 1, and downstream of aggregating means 2 there is positioned a pair of parallel charged wire electrodes 3 which are supported in tension by a pair of vertical support members 5, 5' which extend from and below a pair of high voltage insulators 4 and 4', respectively. A power-driven pulley 6, a pair of idler pulleys 7 and 7' and a releasing pulley 8' are mounted upon the support members 5 and 5', for receiving the charged electrodes 3 in such manner that the tension of the wire electrodes 3 may be adjusted by means of an electric drive motor 8 and a variable speed gearing system 9.

The charged electrodes are connected, through a high voltage cable 11 to a high voltage D.C. generator 10, while the inner, surrounding wall 12 of the duct 1 is grounded so as to be connected to an earth terminal of generator 10, whereby the entire, inner, surrounding wall 12 of the duct 1 acts as another charged electrode, and thus a dust collecting chamber or zone is formed between charged electrodes 3 and dust collecting electrode 12.

When the exhaust gas containing fine particles is passed through the gaps of the aggregating means 2, the fine particles are aggregated so as to form coarse particles which are collected in the dust collecting zone upon the dust collecting electrode and then exhausted outside. Such aggregation is due to the collision of the fine particles as such proceed within the divided gas flow due to the grid, net, or lattice-type aggregating means, as well as the weak electrostatic field which exists between the charged electrode and the aggregating means. The aggregating means can be located in any suitable position depending upon the potential of the electrostatic field within the dust collecting zone.

When however the aggregating means is placed too near to the charged electrode, the aggregating means acts as the dust collecting electrode so that dust tends to deposit upon the grid or lattice thereby choking the aggregating means. The deposited dust can of course be blown out by increasing the velocity of the exhaust gas passing through the aggregating means. In order to increase the velocity of the exhaust gas, containing the fine particles, passing through the aggregating means, it is preferable to set the aggregating means in a part of the exhaust passage having a sectional area narrower than that of the dust collecting zone, as will be more apparent with reference to the embodiment of FIG. 2, as disclosed hereinafter. Similarly, when the aggregating means is located too far from the charged electrode so as not to form an electrostatic field therewith, the aggregation effect is not achieved.

Referring now to FIG. 2, another embodiment of the present invention is disclosed comprising an aggregating means 2 which is mounted within a flow dict 1 having, a reduced dimension portion in the downstream direction, and a flow deflector 13 which is fixed at a position upstream of aggregating means 2 and transversely located relative to duct 1. Downstream of aggregating means 2 is located a pair of charged electrodes 3 which are fixed in opposed relation to duct collecting electrode 12 so as to form a dust collecting zone, similar to that shown in FIG. 1. The other structural components of this embodiment are likewise similar to those shown in FIG. 1.

When the dust collector is used for treating exhaust gases containing fine particles having a specific resistance, such as, for example, in excess of 10.sup.14 .OMEGA. cm, such as for example, that gas exhausted from an electric furnace of a ferrosilicon factory, it is preferable to provide a humidifier upstream of the aggregating means. The humidifier can be only one of various conventional humidifiers such as for example, a spray type humidifier or a stream-type humidifier. the humidifier, the aggregation effect can be increased for such contributes to the formation of larger coarse particles. The exhaust gas containing the fine particles having a high specific resistance is humidified before passing through the aggregating means, so that the fine particles are effectively aggregated so as to form coarse particles, by the effect of colliding upon passing through the grid or lattice and the effect of the electrostatic field, after which they are effectively electrostatically removed. Such will be more apparent upon reference to FIG. 3, discussed hereinafter.

Referring now to FIG. 3, still another embodiment of the present invention is disclosed the electrostatic dust collector of this embodiment having a humidifier mounted within flow duct 1, wherein the humidifier includes a liquid reservoir 14, a liquid supply tube 15, a plurality of valves 16 and 16' for controlling atomization of the liquid, distributing tubes 17 and 17' for conveying the liquid to atomizers 18 and 18' and atomizing nozzles 19 and 19'.

An aggregating means 2 is fixed within the flow duct 1 at a position downstream of the humidifier, while charged electrodes 3 are supported in tension downstream of aggregating means 2 and positioned so as to face inner surrounding wall 12 which acts as a dust collecting electrode, a dust collecting zone thereby being formed similar in construction to that disclosed in FIG. 1.

In any of the embodiments disclosed heretofore, the aggregating means 2 is positioned far enough upstream of the charged electrodes and the dust collecting electrode, such that any area of means 2 which is positioned within the electrostatic field is small when compared to that area of the dust collecting zone within the electrostatic field, the two electrostatic fields being of course proportional to the areas exposed.

When the fine particles contained in the exhaust gas indicate a very high specific electric resistance value, it is especially effective to form the humidified region by using any type of humidifier when can adequately humidify the surfaces of fine particles, such as for example, a spray-type humidifier. It is most preferable to employ such a humidifier such that nozzle portions of the sprayer is directed into the exhaust gas passage, and wherein the quantity of the injected mist can be automatically varied depending upon a change in the quantity of the exhaust gas. In the case where the liquid is supplied to the sprayer, an airless system, with a capacity of supplying pressure through the pump of an amount, such as for example, 2 - 5 kg/cm.sup.2, is preferably used, such a system preventing a decrease in the quantity of exhaust gas being treated. As other embodiments of humidifying means, a liquid film, a liquid stream, or steam supply means can be used in order to contact the fine particles passing through the flow duct.

It is noted however, that if the percentage of humidity that is applied to the exhaust gas is too high, insulation of the insulators as a support means of the charged electrodes, will decrease causing creeping discharge, whereby good effective dust collection cannot be attained. Accordingly, the desired percentage of humidity is determined depending upon the amount of exhaust gas to be treated, the physical properties of the fine particles within the gas, and the rate of treatment of the fine particles contained within the exhaust gas, wherein a rate of 0.5 - 1.2 liters per minute is most preferable. By humidifying the particles, the specific resistance value of the fine particles becomes less than 10.sup.10 .mu., preferably 10.sup.5 - 10.sup.10 .mu., the particles thereby exhibiting good electrification. Consequently, when the humidified fine particles are introduced into the aggregating area, they are effectively aggregated so as to form coarse particles.

In the apparatus utilized according to the present invention, aggregating means 2 may take the form of a grid or lattice, such means being transversely fixed relative to the direction of flow of the exhaust gas within duct 1. The specific exterior configuration of the aggregating means may be of any shape or type, such as, for example, a triangle, a polygon, a circle, a slit, or the like. When a circle is employed, for example, the radius of the opening of the aggregating means can be selectively determined, the radius usually being in the range of 1.5 - 10 mm and preferably in the range of 2.0 - 8 mm.

A plurality of aggregating means may be provided in series and arranged parallel to one another such that they cross the air stream at various positions along the exhaust gas passage. Such means may be selectively arranged depending upon the amount of gas to be treated, the character of the exhaust gas, and the like.

Referring now to FIGS. 4-12, several embodiments of the aggregating means which may be employed in the present invention are disclosed, such as, for example, a wire cloth, an array of vertical bar members, a porous plate, a lattice containing a plurality of X-configured portions, a shutter arrangement, a triple-thickness array of vertical bars, a box-shaped lattice, a box-shaped lattice containing wire cloth, and a triple-thickness array of wire cloth, respectively. Metal wire of the net-type configuration is preferred in view of the cost and maintenance.

The aggregating means may be stationary relative to flow duct 1, or in the alternative, may be continuously or intermittently vibrated by an electrical or mechanical system. The particular aggregating means employed depends upon the physical properties of the particles within the exhaust gas. The flow rate of the exhaust gas passing through the aggregating means is preferred to be within the range of 0.5 - 8 m/sec.

When fine particles, such as, for example, particles having a diameter less than 1 .mu., are passed through the aggregating means, they are caused to come into contact with each other, or to at least pass within a close range of each other. In addition, an electrostatic field is formed between the aggregating means and the charged electrodes. Accordingly, the particles passing through the aggregating means are electrostatically aggregated so as to form coarse particles more than several microns in diameter. As is apparent, the function of the aggregating means, relative to the charged electrodes, is different from that of the dust collecting electrode relative to the charged electrode, and is characterized in aggregating particles by providing electrostatic field.

Similarly, the charged electrode within the aggregating zone may be either stationary or movable. However, where it is utilized for collecting particles which have a specific resistance value and which are easily deposited upon the charged electrode, the movable charged electrode is preferred as shown in the FIG. 1. In order to remove the particles deposited upon the charged electrode, a plurality of guide members not shown, may be provided.

The voltage difference existing between the charged and dust collecting electrodes is at least 150 KV. Such difference can be determined according to the various operating conditions, such as, for example, the distance between the electrode and the dust collecting electrode. It is especially preferred that the charged electrode be negative while the grounded electrode be positive.

Referring now to FIG. 13, yet another embodiment of the present invention is disclosed, wherein exhaust gas, containing fine particles, issuing from a rotary kiln, combustion furnace, electric furnace or the like, and such not being shown in the drawings, is passed through a flow passage or duct 21. The charged electrodes 22 and 22', which are aligned with the longitudinal axis of the duct 21, are supported at both ends by vertical support members 24 and 24', which are in turn mounted upon insulators 23 and 23' which are suspended from the top of duct 21. An aggregating means can be provided in the duct similar to the embodiments discussed above. The charged electrodes 22 and 22' are connected to a high voltage D.C. generator, not shown, while net-type dust collecting electrodes 25 and 25', made of various materials, such as for example, metal wire, are spaced from an inner wall of the exhaust gas passage 21 and are maintained at earth potential. The net-type electrodes 25 and 25' have a length corresponding to that of charged electrodes 22 and 22' and are fixed at their upper edges to suspending members 26 and 26' upon the upper inner wall of duct 21, and are connected at their outer edges to vibrating means located exteriorly of passage 21. Accordingly, particles deposited upon the dust collecting electrodes fall, due to vibration of the vibrating means, which are vibrated periodically. Although the dust collecting electrodes 25 and 25' are shown as being provided on either side, as well as the bottom, of the passage 21, the dust collecting electrodes can also be provided so as to completely surround the charged electrodes.

The dust deposited upon the dust collecting electrode which falls by its own weight increases the coefficient of deposition. However, it is possible to increase the coefficient of deposition still further by imparting vibration to the dust collecting electrode of the wall. The mechanism imparting vibration to the dust collecting electrode can be mechanical, such as for example, knocking the wall by hand, or some automatic electrical mechanism which will vibrate the wall of the dust collecting electrode. The removal of the dust which has fallen from the wall or the dust collecting electrode can be accomplished by hand, although it is preferable to use a conveyor belt provided under the dust collecting electrode so as to transfer the dust which has fallen upon the conveyor belt to a location exterior of the dust collector. When mist is contaminated within the exhaust gas, the slurry which is deposited can be caused to flow downstream by using an inclined bed.

Still referring to FIG. 13, a conveyor 27 is provided for discharging the dust particles which have fallen outside of the passage 21. The inner wall of the passage 21 has a bottom portion which is narrowed so as to form a channel. Accordingly, the particles which have fallen are conducted to conveyor 27 which moves along under the central part of passage 21. The conveyor 27 is driven by a motor 28 and a power transmitting means 29 which are exteriorly of the passage 21. The particles are subsequently discharged through a downward opending provided at a reversing roller 30.

Referring now to FIG. 14, there is shown another embodiment of the apparatus for removing the fallen dust particles wherein a receiving member 31, of a tray-type configuration and having a width corresponding to that of the bottom portion of the exhaust gas passage 21 is slidably inserted within the bottom portion of passage 21 and extending the length of the passage 21 such being subsequently withdrawn manually for removal of the fallen particles.

Referring now to FIG. 15 there is shown a further embodiment of the apparatus constructed according to this invention, wherein an exhaust gas passage 21 is divided into several parts by electroconductive partitions 32, 33, 34 and 35 within which are housed electrodes 36, 37, 38 and 39, and dust electrodes 40, 41, 42 and 43.

The metal wire utilized in forming the dust collecting electrodes may have a mesh of 3 - 20 mm square, but more preferably has a mesh of 5 - 10 mm square. In addition, such may take any of the various configurations as are employed in the aggregating means, such as for example, those disclosed in FIGS. 4-12.

Several test runs of the various embodiments of the present invention were conducted for treating exhaust gases emitted from various industrial systems, such as, for example a rotary kiln of a cement factory, an electric furnace of a ferro-silicon factory, and the like. The results of such tests follows:

EXAMPLE 1

Exhaust gas from a rotary kiln of a cement factory was treated by utilizing the apparatus shown in FIG. 1.

The various parameters employed in this test were as follows:

a. Distance between the charged electrodes and the dust collecting electrode --60 cm

b. Length of the dust collecting zone --20 m

c, Dimensions of the aggregating means --1.2 m .times. 1.8 m

d. Aggregating means comprised metal wire having a mesh of 3 mm square

e. Distance between the aggregating means and the charged electrodes --2 m

Voltage of minus 200 KW was applied to the charged electrode. Exhaust gas containing fine particles exhausted from a rotary kiln for manufacturing cement clinker was supplied to the dust collecting zone at the rate of 10,000 Nm.sup.3 /hour and at a temperature of 200.degree.C - 300.degree.C when exhausted. The average flow rate within the dust collecting zone was 1.3 m/sec., while the average flow rate within the aggregating zone was larger than that within the dust collecting zone.

The results demonstrated that fine particles having a particle diameter of less than 1 .mu. which could not otherwise be collected without the aggregating means were completely removed.

EXAMPLE 2

Exhaust gas from an electric furnace of a ferro-silicon factory was treated by utilizing the apparatus shown in FIGS. 3 and 13.

The various parameters employed in this test were as follows:

a. Distance between the charged electrode and the dust collecting electrode --60 cm

b. Length of the dust collecting zone --30 m

c. Dimensions of the aggregating means --1.2 m .times. 3 m

d. Metal wire having a mesh of 3 mm .times. 5 mm

e. Distance between the aggregating means and the charged electrodes --2 m

f. A pair of spray means were provided, each means having ten nozzles

Voltage of minus 200 KV was applied to the charged electrodes. Exhaust gas containing fine particles exhausted from an electric furnace of a ferro-silicon factory was supplied to the dust collecting zone at the rate of 10,000 Nm.sup.3 /hour, and at a temperature of 200.degree.C - 300.degree.C when exhausted. Average flow rate through the dust collecting zone was about 0.8 m/sec., while the flow rate through the aggregating zone was larger.

The rate of mist injection from the spray means was 0.8 litter/min. Without providing both the aggregating means and the spray means, fine particles could not be collected, whereas by providing both means, fine particles of high specific resistance value and of a particle diameter less than 1 .mu. could be collected.

It is noted that the charged electrode of the present apparatus can be an electroconductive plate or a needle having a sharp edge instead of metal wire. In addition, the distance between the charged electrode and the dust collecting electrode can be selected depending upon the high voltage generator, the kind of exhaust gas, and the velocity of the exhaust gas being treated.

Similarly, although the dust collecting electrode can be the inner wall of the dust collecting zone itself, it is preferable to use a net-type dust collecting electrode which is spaced from the wall of the dust collecting zone. In turn, while the net-type dust collecting electrode can be of a slit shape and comprise a plate having many holes punched out, it is especially preferable that it be a wire net, when factors, such as, for example, cost, workability of the dust collecting electrode, and removability of the fine particles from the dust collecting electrode, are considered.

In the characteristic structure of the net-type dust collecting electrode, the mesh of the electrode should be large enough so that the mesh does not become choked with the deposited dust, such as, for example, within the range of about 1 - 6 cm. On the other hand, when the mesh is too large, the wall itself becomes an auxiliary dust collecting electrode so that the electrostatic balance between the net-type dust collecting electrode and the wall electrode is upset. However, note too that when the mesh is too small, the mesh becomes choked with the deposited dust so that the function of the wall as the auxiliary electrode is inhibited.

The synergical effect of the net-type dust collecting electrode and the wall electrode is quite remarkable. In another characteristic structure of the net-type, dust collecting electrode, the distance between the net-type dust collecting electrode and the wall is such so as not to bridge the deposited dust between them, such as, for example, the distance being in the range of about 3 - 20 cm, and being preferably 5 - 15 cm. The amount of the deposited dust upon the wall is controlled by removing it from the wall by various means, such as, for example, by vibrating the wall, whereby the synergical effect of the net-type electrode and the wall electrode is maintained.

When the distance between the net-type electrode and the wall electrode is such so as to prevent the bridging of the deposited dust, and electrostatic field can be retained between the charged electrode and the wall electrode, even though the dust deposited upon the net-type electrode provides an insulation property. Accordingly, the coefficient of deposition of fine particles does not decrease too much. Also, when the wall is made of electroconductive material, the decrease in the coefficient of deposition of fine particles can be prevented by the electrostatic field existing between the charged electrode and the wall electrode even though dust is deposited upon the net-type electrode.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood therefore, that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein .

Claims

What is claimed as new and desired to be secured by the letters patent of United States is:

1. An electrostatic dust collector for treating exhaust gas containing fine dust particles, comprising:

an exhaust gas passage means;

at least one charged electrode wire connected to a high-voltage D.C. generator;

a pair of insulator members supporting said at least one charged electrode wire in a substantially horizontally disposed relation within said exhaust passage;

said passage means including a dust collecting electrode maintained at earth potential and facing said at least one charged electrode wire so as to form an electrostatic field and dust collecting zone therewith;

particle aggregating electrode means forming an electrostatic field with the changed electrode transversely arranged within said exhaust gas passage and positioned far enough upstream of said charged electrode that the electrostatic intensity between said at least one charged electrode wire and said dust collecting electrode is greater than the electrostatic intensity between said at least one charged electrode wire and said particle aggregating means; and

a humidifying means including a liquid supply, means for spraying said liquid in the form of a mist through a plurality of nozzles connected to said supply and means for controlling the quantity of mist being sprayed connected to the nozzles, said nozzles being disposed upstream of said particle aggregating means in said passage means for humidifying the surfaces of said fine dust particles of said exhaust gas before said exhaust gas passes through said aggregating means.

2. An electrostatic dust collector for treating exhaust gas containing fine dust particles as set forth in claim 1, further comprising means for adjusting the tension in said at least one charged electrode wire.

3. An electrostatic dust collector as set forth in claim 1, wherein said dust collecting electrode comprises a net-type configuration which is spaced from a wall of said dust collecting zone thereby preventing the accumulation of deposited dust between said wall and said net-type dust collecting electrode.

4. An electrostatic dust collector as set forth in claim 3, wherein said electrostatic dust collector further comprises means for causing dust deposited upon said net-type dust collecting electrode to fall therefrom; and means for transferring said fallen dust out of said dust collecting zone.

5. An electrostatic dust collector as set forth in claim 1, wherein said particle aggregating means and said dust collecting electrode are of a net-type configuration; and wherein the mesh of said net-type dust collecting electrode is large enough so that said net-type dust collecting electrode does not become choked, and is larger than the mesh of said particle aggregating means.