GB2090773A - Electrostatically augmented granular bed filter for high temperature particulate removal - Google Patents

Abstract

Particulate matter in a gaseous stream is electrically charged prior to its introduction into a granular bed filter comprising material which may become electrically conducting at the high gas temperature used, such that the filter collection efficiency is enhanced by resulting electrical image charge forces. As shown, the particle charging means 38 is located in the gas flow path prior to the whole of the filter bed chamber 8. Alternatively, the charging means may be located between two stages of the filter bed. <IMAGE>

Description

SPECIFICATION Electrostatically augmented granular bed filter for high temperature particulate removal This invention relates to apparatus and methods for the removal of particulate matter from a gas, and more particularly to such apparatus and methods utilizing a granular bed filter.

Currently proposed high efficiency coal-fied power plant systems which employ produced combustion gases as a motive fluid for an associated gas turbine require that particulate matter be removed from the produced gas at high temperatures typically in excess of 850do. Devices are known which employ electrostatic forces for fine particulate(i.e. from about 0.1 to 20 microns) removal. However, the use thereof has been restricted primarily to relatively low temperature applications. For example, electrostatic precipitators which are suitable for fine particulate removal at temperatures well below 8500C would be uneconomically large in size (if indeed they are operable at all) at temperatures of 850 C and higher.

Similarly, the operation of electro-fluidized bed systems in which an electric field is imposed on filter bed material is not suitable at this time for high temperature fine particulate removal, since inordinant power input would be required to suitably cleanse a large volume of gas due to the temperature-dependent conductivity of the bed materials. In contrast, mechanical systems for particulate removal such a cyclone separators generally adequate for primary filtration purposes at elevated temperatures, but have difficulty in removing fine particulate matter.

Granular bed filtration is another available technology employing a mechanical separation mechanism which appears capable of removing fine particulate matter from a fluid stream at temperatures in the range of 850 C. Granular bed filters employ granular materials such as glass or sand particles to form a filter bed which is relatively inert to thermal and chemical attack. In operation a contaiminated stream of a fluid containing small undesirable solid or liquid particles is passed through a dense bed of granular material wherein the particulates are separated from the fluid stream and are deposited on the bed material. Resulting contaiminated bed material can be discharged from the bed, cleaned, and recycled to the filter bed.This technique has been used extensively in water filtration, however, its use in conjunction with airborne material is relatively new. Unfortunately, the mechanical particulate removal mechanism exploited in such granular bed devices is quite inefficient, thereby requiring the use of deep beds for suitable filtration.

It is therefore an object of the present invention to provide a new and improved particulate removal system operable at relatively elevated temperatures (e.g. > 850 C).

It is another object of the present invention to provide such a system capable of removing fine particlate matter from an influent contaiminated stream.

Yet another object of the present invention is to improve the filtration efficiency of granular bed filtration systems, and more particularly, to improve the filtration efficiency of such systems through the use of electrical forces.

In accordance with the teachings of this invention, there is provided new and improved apparatus and method for removing fine (i.e. from about 0.1 to 20 microns in diameter) particlate matter from a volume of hot flowing gases. The apparatus includes electrical means for electrostatically charging particulate matter in the gas prior to its introduction into an associated granular bed filter. The electrically charged particulate matter generates an image charge in a granule of bed material when flowing nearby. The image charge is of an opposite polarity to that of the particulate matter and an attractive electrical force is thus established between the bed granule and the charged particulate matter.The attractive force causes the particulate matter to deviate from its normal course of flow about the bed granule and, when the attractive force is of a sufficient magnitude, the particulate matter is caused to impact the surface of the granule. The particulate matter is thereby separated from the gas and is collected on the surface of the bed granule.

For a better understanding of the invention, reference may be had to the accompanying drawing wherein: Figures i, 3 and 4 are longitudinal cross-sectional views of granular bed filtration devices constructed according to different embodiments of the present invention; and Figure 2 is a transverse sectional view of the device depicted in Figure 1 taken along line 2-2 and looking in the direction of the arrows.

In Figures 1 and 2, a granular bed filtration device includes a cylindrical outer vessel 2. Within the vessel 2 a first set of annular louvers 4 is positioned coaxiallywith and spaced inwardly from a second set of annular louvers 6. A chamber 8 is defined in the space between the louver sets 4 and 6 by the downwardly tapering surfaces 10 and 12, respectively, thereof. The louvers in sets 4 and 6 are spaced vertically by suitable conventional support means 14, and are positioned coaxiallywith respect to each other through structural members 16. The outer louver set 6 is in turn positioned with respect to the outer vessel 2 by suitable structural members as at 18.Structurally stable granulal material such as, for example, sand, glass and the like are introduced into the filter bed chamber 8 through an inlet conduit 20 located near the top of the outer vessel 2. Since the collection mechanism of the subject invention is strongly dependent upon the size of bed material employed, the diameter of the granular material is preferably from about 0.1 to 5 millimeters. The granular material is dispersed from the inlet 20 and fills the bed chamber 8, resting on portions of louver surfaces 10 and 12. Through use of this bed chamber configuration, the cross-sectional area of bed material exposed to gaseous flow directed through the louvers 4 and 6 is advantageously increased. The granular material moves continually downward through the bed chamber 8 and is collected at the bottom of the device in a conical section 22.The collected filter material is transported by suitable conveying means (not shown) via connecting conduit 24 to a conventional cleaning apparatus 26 such as a water-spray device. According, particulate matter removed from an influent gaseous stream and adhering to the granular bed material is removed therefrom in the apparatus 26 and is then transported via a conduit 28 for final disposal. Resulting cleaned granular bed material is recirculated via a connecting conduit 28 and the inlet conduit 20 to the bed 8. Alternately, spent granular material can be discarded directly from the conduit 24 without recirculation, with clean granular bed material being introduced through the conduit 20. A valve 30 is included in the conduit 24 to enable control of spent granular material removal.

A gaseous stream containing entrained particulate matter is introduced into contact with bed material contained in the chamber 8 through an inlet 32.

Similarly, an outlet 34 is provided for exhausting cleaned gas from the device. The gas flow path also includes a portion through an electrical particulate charging means 36 wherein particulate matter entrained in the gaseous stream is electrically charged.

In the embodiment depicted in Figure 1, the charging means includes a corona producing electrode 38 disposed within the flow path and suitably connected to a conventional external high voltage source 40. In operation, a volume of gas containing solid particulate matter is caused to flow about the electrode 38. Upon energizing the electrode 38 an electrical charge is imparted to the particulate matter in the gaseous stream by corona discharge means.

The temperature of the flowing gas is typically of the order from 8500C to 1 0000C and is caused to flow through the inlet 32. An annular baffle 42 is provided to direct the gaseous stream through bed material contained in a lower portion 44 of the chamber 8.

After immerging from the lower portion of the chamber 8, the gas flows upwardly through the centrally disposed open portion 46 of louver set 4.

The gas is then caused to flow through the bed material contained in an upper portion 48 of the chamber 8 and through the outlet 34. In this manner, the gaseous stream has a second chance to have particulate matter removed therefrom. Moreover, the influent gas is initially cleaned in the lower stage 44 of the chamber which contains bed material which is the dirtest, while the upper stage 48 of the chamber is removing any remaining particulate matter with granular bed material which is relatively clean. Thus, the embodiment depicted in Figure 1 provides for an effective removal of fine particulate matter.

With reference to Figure 3, there is illustrated an alternative embodiment in the present invention in which illustration all features denoted by the same reference number as in the device depicted in Figure 1 are the same, and function in the same manner as previously described. In this instance, the granular bed filtration device embodies a single pass through the granular bed material contained in the chamber 8. Accordingly, the baffle 42 is absent and the exhaust outlet duct 34 is located near the top of the vessel 2.

Also, in contrast to the embodiment of Figure 1, the device depicted in Figure 3 includes a cylindrical screen 50 disposed within the bed chamber 8 substantially transverse to the flow path of an influent gaseous stream so as to define separate upstream and downstream sections 49 and 51, respectively, of the chamber 8. Since bed material contained in the upstream section 49 is in initial contact with a dirty gaseous stream, it tends to become contaminated faster, thus requiring a more rapid discharge rate than the bed material contained in the downstream section 51. Accordingly, means are provided for independently removing granular material from the upstream and downstream bed sections including separate conical section 52 and 54, connecting conduits 56 and 58, and valves 60 and 62.By adjusting valves 60 and 62, the flow rate of bed material from the two bed sections 49 and 51 can be independently adjusted so that material contained in upstream section 49 may be removed sufficiently fast to prevent clogging of the filter, while material in the downstream section 51 can be removed more slowly to increase the residence time and thereby enhance the efficient use of the bed material.

In operation the removal of particulate matter from an influent gaseous stream according to the present invention is achieved through both mechanical and electrical mechanisms.

The mechanical mechanism involves collection or capture of fine particulates when a particle contained in the gaseous stream contacts the surface of a granule of bed material. In prior systems the probability of fine particulate capture was increased by increasing the number of bed granules (i.e. bed depth) with which the particle might come into contact. Particles not contacting bed material, and hence not captured, remained in a resulting effluent gaseous stream.

In contrast, the present invention includes means for imparting an electrical charge from an external power source to particulate matter of an infludent gaseous stream priorto its passage through bed material in order to increase the quantity of fine particlates collected per granule bed material. Thus, collection efficiency is improved without an increase in bed depth. More specifically, at en elevated temperature in the order of approximately 8500C to 10000the granular material in the bed 8 becomes conducting or semiconducting. The electrically precharged fine particulates in the gaseous stream induce an "image charge" within the electrically conducting bed granules. The image charge is of the opposite polarity to that of the fine particulate, therefore, an attractive electrical force results. Fine particulates flowing near the surface of the bed granules are attracted to the granules as if an oppositely charged particle was located at its near image position within the bed granule. Consequently, a greaterfraction of the fine particulates is collected by each bed granule and, thus, fine particulates which ordinarily would escape capture by the granule are now collected. This mechanism is termed image charge collection.

The collection efficiency of electrically charged fine particles in the sub-micron range is greatly enhanced when compared to that of uncharged fine particles in prior art systems. For example, tests indicate the efficiency of the collection of fine particulates by employment of the image charge mechanism according to the present invention is in excess of 95% for particulates of from 0.1 to 20 microns size range and a granular bed particle size of 0.1 millimeter diameter and a superficial gas velocity of no greater than 0.1 meters per second. This is accomplished without directly electrifying the material of the granular bed filter as in an imposed field device.The collection efficiency is a strong function of bed material size, with granules of less than 0.1 millimeter being entrained in a resulting clean gas exhaust stream, and granules of greater than approximately 5 millimeters exhibiting a greatly decreased image charge collection efficiency. Collection efficiency is also a function of gas velocity which typically decreases with increasing velocity. Of course, particulates in a given gaseous stream may be partially pre-charged by natural phenomena.

However, the present invention provides for affirmatively imparting particulates with an electrical charge from an external power source to fully exploit the image charge collection mechanism.

Referring to Figure 4, there is depicted an alternative embodiment of the present invention which sequentially incorporates the above-described mechanical and electrical collection mechanism. All features in Figure 4 denoted by the same reference numeral as in Figures 1 and 3 are the same, and function in the same manner as previously described. In this embodiment, a corona producing electrode 64 is provided coaxially in the inner region 46 of louver set 4 to establish a corona discharge therein. A suitable high voltage source 66 and insultating means 68 are provided as known in the art. In this embodiment, particulates contained in a gaseous stream may or may not be electrically charged before entering lower chamber stage 44.

Relatively large particulates are removed in the lower stage. Upon passing through lower chamber stage sections 49 and 51 the gas stream passes through the region 46 wherein fine particulate matter entrained in the gaseous stream is passed around the corona producing electrode 64, and is thereby electrically charged. The resulting electrically charged particulates are then removed through the aforementioned image charge collection mechanism during passage through bed material contained in the upper chamber stage 48. Since clogging problems associated with the removal of relatively large particulate matter are limited in this embodiment to the lower stage 46, the screen 50 and the associated chamber sections 49 and 51 are included only in the lower chamber stage 44.

Claims (16)

1. A granular bed filtration device for removing particulate matter from a gaseous stream comprising: a filter bed chamber adapted for containing a bed of graunularfilter material wherein; a gaseous stream flow path through the bed chamber including an inlet means in flow communication with the bed chamber for introducing a gaseous stream having particulate matter entrained therein into contact with bed material contained in said chamber, and an outlet means for exhausting gas from the bed chamber; and electrical means in the gaseous stream flow path prior to at least a portion of the bed chamber for imparting an electrical charge from an external power source to particulate matter entrained in a gaseous stream being conducted through said flow path.

2. A device as in claim 1 wherein said bed chamber is adapted for containing granular bed material of a diameter between approximately 0.1 to 5 millimeters.

3. A device as in claim 1 further comprising granular material disposed in the bed chamber and having a diameter of between approximately 0.1 to 5 millimeters.

4. A device as in claims 1-3 wherein the electrical particulate charging means includes an electrode disposed in the gaseous stream flow path prior to at least a portion of the bed chamber.

5. A device as in claims 1-4 wherein the bed chamber includes first and second stages disposed in series in the gaseous stream flow path.

6. A device as in claim 5 wherein the electrical particulate charging means includes an electrode disposed in the gaseous stream flow path intermediate the first and second bed chamber stages.

7. A device as in claims 1-6, further comprising a gas-permeable screen disposed within the bed chamber substantially transverse to the gaseous stream flow path so as to separate upstream and downstream sections of the chamber, and means cooperating with the upstream and downstream bed chamber sections for removing granular material independently for said sections.

8. A method for removing entrained particulate matter from a gaseous stream comprising the steps of: (a) providing a bed of granular filter material in a bed chamber; (b) imparting an electrical charge from an external power source to particulate matter entrained in a gaseous stream; (c) passing the gaseous stream including the charged entrained particulates through at least a portion of the bed material contained in the bed chamberwherebythe charged particulates are electrically attracted to the bed material and are thereby removed from the gaseous stream and deposited on the bed material so as to form a particulate-depleted gaseous stream; and (d) removing the particulate-depleted gaseous stream from the bed chamber.

9. A method as in claim 8 wherein the granular filter material provided has an average diameter of between approximately 0.1 to 5 millimeters.

10. A method as in claims 8 - 9, wherein the average diameter of the entrained particulate matter in the gaseous stream is from approximately 0.1 microns to approximately 20 microns.

11. A method as in claims 8 - 10, wherein the temperature of the gaseous stream is greater than approximately 850 C.

12. A method as in claim 8 further comprising the step of passing the gaseous stream through a first portion of the bed material prior to step (b) so as to remove a portion of the particulate matter entrained therein, and subsequently electrically#charg- ing remaining entrained particulate matter in the gaseous stream in step (b) prior to passing the gaseous stream to a second portion of the bed material in step (c).

13. A method as in claim 8 wherein at least a portion of the bed material is provided in step (a) to separated upstream and downstream bed chamber sections, and the gaseous stream in step (c) is passed sequentially through bed material contained in the upstream and downstream chamber sections, respectively.

14. A method as in claim 13 wherein bed material in the upstream and downstream sections through which at least a portion of the gaseous stream has been passed are separately removed from said upstream and downstream sections.

15. A granular bed filtration device substantially as described herein with reference to the accompanying drawings.

16. A method for removing entrained particulate matter from a gaseous stream substantially as described herein.