Electrostatic Precipitator

Abstract

An electrical or electrostatic precipitator is provided with a grounded collector plate upstream of the electrically charged wires. The plate is juxtaposed adjacent to the wires and provides greater overall removal of entrained discrete particles from a gas stream.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to the removal of entrained discrete particles from a gas stream by electrical or electrostatic precipitation. The entrained discrete particles may consist of liquid droplets or solid particles such as fume, dust, soot, fly ash or the like, chemical particles from a chemical process gas stream such as the off-gas from a fluid bed reactor or ore roaster, etc. The invention is generally applicable to the treating of diverse gas streams, such as flue gas, a chemical process gas stream, or the tail gas from a chemical process, and is especially useful in the prevention of air pollution due to the discharge of waste gas streams containing entrained discrete particles into the atmosphere.

2. Description of the Prior Art

Numerous arrangements of electrical or electrostatic precipitators have been suggested in the prior art, including types in which the electrically charged wires are followed by wire mesh or fiber mass filters or the like. Other types provide grids, wire mesh or rods downstream of the charged wires. Typical patents showing various configurations include U. S. Pats. Nos. 3,049,848; 2,990,912; 2,989,146; 2,973,054; 2,847,082; 2,822,058; 2,764,254; 2,715,944, 2,672,947, 2,593,377; 2,582,133 and 2,556,982 and British Pats. Nos. 962,773 and 444,333.

SUMMARY OF THE INVENTION

In the present invention, a gas or vapor stream laden with entrained discrete particles consisting of components such as those described supra or the like, is passed through a duct or similar gas passage means such as a conduit or elongated container. A first perforated plate is transversely disposed in the duct, so that the gas stream initially passes through the openings in the first plate, which is succeeded by alternate juxtaposed transverse banks of parallel spaced apart wires and secondary perforated plates. The plates are grounded and the wires are provided with an electrical or electrostatic charge, so that a high voltage potential is maintained between the ionizing wires and the grounded plates, and the entrained discrete particles are deposited from the gas stream onto the plates, due to an electrostatic precipitation mechanism in which the particles receive a charge from the wires and are discharged by and onto the plates. It has been determined that the provision of the first plate prior to the initial wire or bank of parallel wires is highly advantageous, since a considerable proportion of particles will deposit on the first plate upstream of the initial bank of wires, and thus the overall particle collection efficiency and removal of particles from the gas stream are greatly improved.

It is an object of the present invention to remove entrained discrete particles from a gas stream in an improved manner, by the provision of improved apparatus.

Another object is to provide an improved electrical or electrostatic precipitator.

A further object is to increase the particles removal efficiency of an electrostatic precipitator.

An additional object is to remove entrained discrete particles from a gas stream in an electrostatic precipitator by the provision of a collector plate upstream of the first bank of ionizing wires.

These and other objects and advantages of the present invention will become evident from the description which follows.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS

Referring now to the drawings,

FIG. 1 is a sectional elevation view of one embodiment of the invention,

FIG. 2 is a sectional plan view of FIG. 1, taken on section 2--2,

FIG. 3 is an isometric view of the preferred embodiment of the collector plates shown in FIGS. 1 and 2, and

FIG. 4 is an isometric view of an alternative embodiment and species of collector plate, showing associated ionizing wires.

Referring now to FIG. 1, a gaseous or vaporous stream 1 containing entrained discrete particles is passed via fan or blower 2 as stream 3 into the duct 4, which in this embodiment of the invention is generally horizontally oriented and of a rectangular or square cross-section. The duct 4 is connected to ground via ground wire or connection rod 5. The gas stream within duct 4 passes initially through the spaced openings in the foraminous collector plate 6, which is disposed upstream of and adjacent to the primary ionizer wires or rods 7. The elements 7 are provided with an electrical charge or potential or an electrostatic charge by means of charge lead wire 8 which connects via an insulated connection through duct 4 to wire 7. Electrical potential is furnished to wire 8 via the main high potential wire or busbar line 9, which extends from a suitable high potential source 10, which may be a transformer and rectifier combination or the like. Unit 10 furnishes a high voltage potential, either negative or positive, to busbar 9, and the opposite polarity of unit 10 may be grounded, either via duct 4 or directly to ground. The influence of the electrical charge imparted to wire 7 causes the entrained discrete particles in the gas stream, which may be either liquid droplets or solid particles such as soot, dust, fly ash, fume, fog or the like, to attain an electrical charge relative to ground, and a portion of the discrete particles are discharged by and deposited onto the primary collector plate 6.

The gas stream within duct 4 next passes in series through the openings in the plurality of downstream foraminous secondary collector plates 11, between which are disposed the secondary potential or ionizing wires 12, so that an alternative series of ionizing wires or rods 12 and secondary collector plates 11 are provided. An electrical charge or potential is provided on wires 12 via secondary lead wires 13, which extend from busbar 9 through duct 4 via an insulated connection to the wires 12. Further amounts of discrete particles are deposited from the gas stream onto the collector plates 11. A preferred configuration of duct 4 is shown in FIG. 1, in which the downstream portion 14 of duct 4 increases progressively in cross-sectional area in the direction of gas flow, so that the linear velocity of gas flow decreases in section 14. The decrease in gas flow in section 14 aids in removal of discrete particles by preventing re-entrainment of deposited material from plates 11 into the gas stream due to turbulence or high gas velocity. The final purified or cleansed gas stream, now having a depleted or negligible content of entrained discrete particles, flows into outlet duct or conduit 15, which extends from section 14 and passes the gas stream 16 to suitable utilization or atmospheric discharge via a stack or the like, not shown.

Referring now to FIG. 2, a sectional plan view of the apparatus is shown, which further illustrates the arrangement of the perforated or foraminous primary collector plate 6, the ancillary or secondary collector plates 11, and the linear banks of parallel ionizing wires or rods 7 and 12 disposed between collector plates.

FIG. 3 shows a typical collector plate provided with circular openings 17 for gas flow.

FIG. 4 shows an alternative embodiment of a collector plate, in which vertical slits 18 are provided for gas flow, with the slits 18 preferably being parallel with the ionizing wires or rods 7, which are juxtaposed and spaced from plate 6.

Numerous alternatives within the scope of the present invention will occur to those skilled in the art. The duct 4 and appurtenances such as section 14 may be horizontally or vertically oriented, or mounted in an inclined position. In all such cases the collector plates 6 and 11 and associated ionizing wires 7 and 12 will extend substantially transversely within the duct. A single downstream collector plate 11 disposed adjacent to wire 7 may be provided in some cases, in which case the banks of wires 12 and associated downstream collector plates may be omitted. Suitable rapping or shaking devices or means will usually be provided in practice, to intermittently or continuously agitate or shake the collector plates to remove deposited material. Rapping devices may also be provided for wires 7 and 12. In other instances, the plates and wires may be intermittently washed down with a suitable wash liquid such as water or a solvent for the deposited discrete particles. In instances when the entrained discrete particles in stream 1 consist of liquid droplets, continuous drainage of deposited liquid from the collector plates 6 and 11 under the influence of gravity will usually suffice to remove deposited material. The foraminous plates 6 and 11 may be provided with openings which are circular, elliptical, square, triangular or the like, or slits such as shown in FIG. 4 may be provided in practice. In some cases the collector plates may even consist of a series of flat parallel juxtaposed strips, preferably with each strip oriented opposite to an ionizing wire. Equal distribution of voltage to the various wire banks 6 and 12 may be assured in practice by the provision of various electrical chokes or the like. Half-wave or full wave electrical potential may be provided in suitable instances. In many cases, the fan or blower 2 will be provided downstream of the apparatus so that the blower draws the impurity-laden gas stream through the unit and thus handles only cleaned gas or vapor. In power plants, this arrangement is required by insurers, with the fan or blower 2 handling only flue gas free of fly ash.

An example of laboratory tests of the apparatus of the present invention will now be described.

EXAMPLE

A test apparatus was provided with five parallel banks of ionizer wires, with each bank of wires being followed by a grounded perforated collector plate. A voltage of 40 kv. was impressed on the wires and the apparatus was operated with an additional primary collector plate prior to the first bank of wires. An air stream laden with entrained particles of fly ash dust was passed through the apparatus and the collection efficiency was measured. The apparatus was then operated in a second test without the primary collector plate and the collection efficiency was measured. Following are the test results. --------------------------------------------------------------------------- Fly Ash Collected Per Hour

Plate Number Test No. 1 Test No. 2 With primary Without primary Plate Plate Grams % of total Grams % of total __________________________________________________________________________ 1 106 15.1 -- -- 2 244 35.0 237 38.0 3 149 21.3 163 26.2 4 118 16.9 104 16.7 5 61 8.8 97 15.6 6 20 2.9 22 3.5 Total 698 100.0 623 100.0 Feed Rate, Grams/Hr: 754 744 % Collection Efficiency 92.5% 84.0% __________________________________________________________________________

It is evident that greater efficiency of collection and greater total amounts of fly ash removal were attained due to the provision of the primary plate No. 1 prior to the first bank of ionizer wires. The unit was operated with foraminous collector plates having perforations consisting of 0.5 inch diameter holes in 22 gauge carbon steel plate. The holes were in staggered rows with a total open area of 48 percent. Air velocity was 3 feet/second and the dust loading was 3 grs./SCF.

Claims

We claim:

1. An apparatus for the electrostatic removal of entrained particles from a gas stream which comprises a duct, means to pass a feed gas stream containing entrained discrete particles into said duct, said duct being of constant cross-sectional area in the direction of gas flow over the initial portion of the length of said duct and of progressively increased cross-sectional area in the direction of gas flow over the terminal portion of the length of said duct, whereby the linear velocity of flow of said gas stream is substantially constant through said initial duct portion and decreases through said terminal duct portion, a first plurality of parallel spaced apart linear banks of wires, each of said first banks containing a plurality of parallel spaced apart electrically charged wires and extending transversely into said initial portion of said duct, means to continuously maintain an electrical potential relative to ground on each of the wires in each of said first banks of wires, a primary foraminous plate, said primary plate being transversely disposed within said initial portion of said duct upstream of said first banks of wires and juxtaposed adjacent to the first bank of said banks of wires, whereby said feed gas stream containing entrained discrete particles initially passes through the plurality of openings in said primary plate, a first plurality of secondary foraminous plates, each of said first plurality of secondary plates being transversely disposed within said initial portion of said duct downstream of and spaced from one of said first banks of wires, a second plurality of parallel spaced apart linear banks of wires, each of said second banks containing a plurality of parallel spaced apart electrically charged wires and extending transversely into said terminal portion of said duct, means to continuously maintain an electrical potential relative to ground on each of the wires in each of said second banks of wires, a second plurality of secondary foraminous plates, each of said second plurality of secondary plates being transversely disposed within said terminal portion of said duct downstream of and spaced from one of said second banks of wires, said primary plate and said first and second plurality of secondary plates being connected to ground, the entire periphery of each of said plates being attached to said duct by a fluid-impervious connection, said primary and secondary plates being provided with substantially linear slits for gas flow, said slits being parallel with the wires in said banks, means to remove separated particles from said primary plate and said secondary plates, said separated particles being derived from said feed gas stream, and means to remove a product gas stream of depleted entrained particles content from said duct downstream of the last of said second plurality of secondary plates.

2. The apparatus of claim 1, in which said duct is horizontally oriented, and said primary and secondary plates and banks of wires are vertically oriented.

3. The apparatus of claim 1, in which said entrained discrete particles comprise liquid droplets.

4. The apparatus of claim 1, in which said entrained discrete particles comprise solid particles.

5. The apparatus of claim 1, in which said duct is of rectangular cross-section.