FR2496487A1 - DEVICE AND METHOD FOR SEPARATING FINE PARTICLES DRAINED IN A GASEOUS CURRENT - Google Patents

Abstract

DEVICE ALLOWING THE SEPARATION OF FINE PARTICLES ENTRANTED IN A GAS CURRENT AT HIGH TEMPERATURE. IT INCLUDES: -A BEDROOM 8 HAS FILTRATION BED ADAPTED TO CONTAIN A BED OF GRANULAR FILTRATION MATERIAL; -AN INPUT 32 COMMUNICATING WITH THE CHAMBER TO INTRODUCE A GAS CURRENT INCLUDING TRAINED PARTICLES INTO THE CHAMBER; -A GAS CURRENT FLOW PATH IN THIS CHAMBER BRINGING THE TRAINED PARTICLES INTO CONTACT WITH THE BED MATERIAL CONTAINED IN THE CHAMBER; - AN OUTPUT 34 TO EVACUATE THE GAS CURRENT FROM THE CHAMBER; AND AN ELECTRICAL MEANS 38 IN THE GAS CURRENT FLOW PATH BEFORE AT LEAST PART OF THE CHAMBER TO CONFER AN ELECTRICAL CHARGE FROM AN EXTERNAL ENERGY SOURCE 40 TO THE PARTICLES DRAWN IN THE GAS CURRENT FOLLOWING THE PATH OF ' FLOW. APPLICATION TO GASES PRODUCED BY A THERMAL PLANT.

Description

The present invention relates to an apparatus and methods for removing

  particles of a gas, and more particularly to such apparatus and methods

  using a granular bed filter.

  The currently proposed high efficiency thermal power plant systems that utilize combustion gases produced as a driving fluid for an associated gas turbine require the removal of the particles of the product gas at elevated temperatures above 8500C. Devices that utilize electrostatic forces for the removal of fine particles (i.e.

  about micrometers). However, their use has been

  mainly limited to applications at relatively

  low. For example, electrostatic precipitators suitable for the removal of fine particles at temperatures well below 850 ° C would prove to be of uneconomical size (if they operate) at temperatures equal to or greater than 850 ° C. Similarly, electro-fluidized bed systems in which an electric field is imposed on the bed material of the filter are not suitable at the moment for the removal of fine particles at high temperature, because it would require an input power It is disproportionate to properly clean a large volume of gas because the conductivity of the bed materials depends on the temperature. On the contrary, mechanical systems for the removal of particles, such as cyclone separators, are generally adequate for filtration.

  primary heat at high temperatures, but difficult to remove

the fine particles.

  Granular bed filtration is another technology

  available which uses a mechanical mechanism of

  separation and which seems capable of eliminating fine

  cules of a fluid stream at temperatures of the order of 8500C. Granular bed filters use granular materials such as glass or sand particles for

  to form a filter bed which is relatively inert

  against thermal and chemical attack. In operation a stream contaminated with a fluid containing small undesirable solid or liquid particles passes through a dense bed of granular material where the particles are separated from the fluid stream and settle on the bed material. The soiled material can be unloaded from the bed, cleaned, and recycled to the filter bed. We used intensively this

  for the filtration of water, however, its use

  in connection with materials transported by air

  is relatively recent. Unfortunately, the mechanical mechanism

  particulate separation scheme operated in these

  Granular bed systems are inefficient, thus requiring the use of deep beds for proper filtration. The invention therefore aims

  - to form a new and improved system of separa-

  particles that operate at relatively low temperatures

  high (eg 8501C); to provide a system capable of separating fine particles from a soiled stream; - to improve the filtration efficiency of

  granular bed filtration, and more particularly,

  improve the filtration efficiency of these systems by

  the use of electric forces.

  According to the invention, a new and improved apparatus is provided.

  and a new process for the separation of fine particles

  (that is, particles of 0.1 to 20 micrometers

  diameter approximately) of a volume of hot gas flowing.

  The apparatus includes electrical means for electrostatically charging particles into the gas prior to introduction into an associated granular bed filter. The

  electrically charged particles create a virtual charge-

  in a granule of bed material as they flow nearby. The virtual charge is of opposite sign to those of the particles and an electric force of attraction is established between the granule of the bed and these charged particles. The attractive force deviates the normal trajectory of these particles near this granule, and when the attractive force has a sufficient amplitude, these particles

24964E

  hit the surface of the granule. The particles are thus separated from the gas and collected on the surface of granules

bed.

  The following description refers to the figures

  annexed which represent, respectively:

  Figures 1,3 and 4 of the longitudinal sections of

  granular bed filtration units constructed according to

  embodiments of the present invention; and Figure 2, a cross-section of the device of Figure 1 taken along line 2-2 and looking in the

direction of the arrows.

  In Figures 1 and 2, a granular bed filter device comprises a cylindrical outer vessel. In this tank 2 a first set of annular openings 4 is arranged coaxially, internally spaced apart / to a second set of annular openings 6. The space defined between the sets of blanks 4 and 6

  by their downwardly inclined surfaces 10 and 12, respectively

  The gills of the sets 4 and 6 are vertically spaced by conventional suitable support members 14 and are arranged coaxially with each other by means of structural members 16. The outer play of 6 is in turn disposed in the outer vessel 2 by means of suitable structural members such as

  18.Structural stable granules such as, for example

  pl, sand, glass and the like are introduced into the chamber 8 of the filter by an inlet duct 20 located near the top of the outer tank 2. As the collection mechanism of the invention depends strongly on the size of the granules of the bed material used, the granule diameter of the granular material is preferably about 0.1 to 5 mm. The granular material is dispersed from the inlet and fills the chamber 8, resting on portions of the surfaces 10 and 12 of the louvers. By using this bed configuration, the cross-section of the bed material advantageously is exposed to the flow of gas directed through the

  4 and 6. The granular material moves continuously

  downwardly in the chamber 8 and is collected at the bottom of the device in a conical portion 22. The material of the collected filter is conveyed by suitable conveying means (not shown) via a connecting pipe 24 to an apparatus for conventional cleaning 26 such as

  water spray device. As a result, the particles

  separated from a gaseous flow and adhering to the granular material of the bed are removed in the apparatus 26 and are then conveyed via a conduit 28 for final rejection. The resulting clean granular material is returned via a connection duct 28 and the inlet duct 20 to the bed 8. Alternatively, spent granular material of the duct 24 can be discharged directly without recycling, clean granular material being introduced through the duct. 20. The conduit 24 comprises a valve 30 to enable control

  the removal of the used granular material.

  A stream of gas containing entrained particles is introduced through an inlet 32 into contact with the bed material contained in the chamber 8. Likewise, an outlet 34 is provided for discharging clean gas from the device. The gas flow path also includes a portion passing through means 36 for electrically charging the particles where particles entrained in the gas stream are electrically charged. In the embodiment of FIG. 1, the charging means comprises a ring-discharge electrode 38 disposed in the flow path and suitably connected to an external high voltage source 40. In operation, a volume of gas containing solid particles flows around the electrode 38. During the excitation of this electrode 38, an electric charge is imparted to the particles of the gas stream by this corona discharge device. The temperature of the circulating gas is conventionally of the order of 850.degree. C. to 1000.degree. C. and this gas enters through the inlet 32. An annular baffle 42 is provided for directing the gaseous stream through the material of the bed contained in a lower portion 44 of the chamber 8. When leaving the lower part of the chamber 8, the gas flows upwards through the central open part 46 of the set of gills 4. The gas must then pass through the bed material contained in a part upper 48 of the chamber 8 and the outlet 34. In this way, the gas stream has a second chance to have its particles removed. Furthermore,

  incoming gas is initially cleaned in the lower floor

  The upper chamber chamber 48 contains the bed material which is the dirtiest, while the upper floor 48 of the chamber removes any remaining particles from the relatively clean bed granular material Io. Thus, the embodiment of FIG. 1 provides an effective separation

fine particles.

  Figure 3 represents another embodiment of the

  present invention on which the same reference numerals

  these designate the same elements, operating in the same manner as in FIG. 1. In this case, the granular bed filtration device comprises a single passage through the granules of the bed contained in the chamber 8. As a result, the baffle 42 is absent and. the conduit of

  discharge 34 is located near the top of the vessel 2.

  Also, contrary to the embodiment of FIG. 1, the device of FIG. 3 comprises a cylindrical sieve 50

  disposed in the chamber 8 transversely to the flow path

  the gaseous current to define upstream sections

  and separate downstream 49 and 51, respectively, of the chamber 8.

  Since the material of the bed contained in the upstream section 49 is in initial contact with a contaminated gas stream, it tends to become soiled faster, thus requiring a greater discharge velocity than the bed material contained in the downstream section 51. Accordingly, means are provided for independently removing granular material from upstream and downstream sections including conical portions 52 and 54, connection conduits 56 and 58, and separate valves 60 and 62. By adjusting the valves 60 and 62, the flow rate of the bed material of the two bed sections 49 and 51 can be set independently so that the material contained

  in the upstream section can be discharged sufficiently fast-

  to prevent clogging of the filter, while the material in the downstream section 51 can be discharged more slowly to increase residence time and thereby improve

  adorn the yield of use of the bed material.

  In operation, the separation of particles

  the gas stream of the present invention both mechanically and electrically. The mechanical means comprises the collection or capture of fine particles when a particle contained in the gas stream contacts the surface of a granule of bed material. In the systems of the prior art, the likelihood was increased by increasing the number of bed granules (i.e. bed depth) with which the particle could come in contact. Particles that do not come into contact with bed material, and therefore are not captured,

  remain in the resulting effluent gas stream.

  In contrast, the present invention incorporates means for imparting an electrical charge to particles of a gaseous stream from an external power source prior to passing through the bed material to increase the amount of

  fine particles collected by granulate from the bed material.

  Thus, the collection efficiency is improved without increasing the depth of the bed. More particularly, at a temperature

  in the range of about 850 ° C. to 10,000 ° C., the granular material

  in bed 8 becomes conductive or semiconductor.

  The fine electrically preloaded particles in the gas stream induce a "virtual charge" in the

  electrically conductive bed granules. The viral load

  This is of opposite sign to that of the fine particles, which consequently results in an electric force of attraction. Fine particles flowing near the surface of the bed granules are attracted to the granules as if a charged particle of opposite sign was located near its image position in the granule of the bed. As a result, a greater proportion of fine particles are collected by each granule of the bed and thus fine particles which would normally escape capture by the granule are now collected. This mechanism is referred to

by virtual charge collection.

  The collection efficiency of electrically charged small particles of less than one micrometer size is greatly improved compared to that of the neutral fine particles in the systems of the prior art. For example, tests have shown that the fine particulate collection efficiency by use of the virtual charge mechanism according to the present invention exceeds 95% for particles of 0.1 to 20 microns and with bed granules of 0, 1 mm in diameter and a superficial gas velocity not exceeding

  0.1 m / s. This is done without it being directly necessary.

  to electrify the granular material of the filter bed as in an imposed field device. The collection efficiency is strongly dependent on the size of the bed material, the less than 0.1 mm granules being entrained in the effluent clean gas stream, and the granules greater than about 5 mm having a collection yield per virtual charge greatly diminished. The collection efficiency is also a function of the gas velocity and typically decreases as the velocity increases. Of course, particles in a given gas stream can be partially preloaded by a natural phenomenon. However, the present invention makes it possible to provide the particles with an electrical charge from an external source of energy in order to fully exploit the virtual charge collection mechanism. Referring to Figure 4, there is shown another embodiment of the present invention which successively incorporates the mechanisms of mechanical and electrical collection described above. All the elements of FIG. 4 identified by the same reference numbers as in FIGS. 1 and 3 are identical and operate in the same way. In this embodiment, a corona discharge electrode 64 is disposed coaxially in the interior region 46 of the game. of 4 to establish a corona discharge. A suitable high voltage source 66 and isolation means 68 are provided as known in the art. In this embodiment, the particles contained in a gas stream may or may not be electrically charged before entering

  in the lower floor of the room. Relative particles

  fat are eliminated in this lower tier. After passing through sections 49 and 51 of the lower floor of the chamber, the stream of gas passes through the region 46 where the

  fine particles entrained in the gas stream pass -

  around the corona discharge electrode and so are

  electrically charged. These electrically charged particles

  The resulting results are then separated using the virtual charge collection mechanism when passing through the

  bed material in the upper floor 48 of the room.

  Since the clogging problems associated with the separation of relatively large particles are limited in this embodiment at the lower stage 44, the sieve 50 and the associated sections 49 and 51 of the chamber are included only

  in this lower floor 44 of the chamber.

Claims (14)

  1. A granular bed filtration device for separating particles from a gas stream, characterized in that it comprises: - a chamber (8) with a filtration bed adapted to contain a bed of granular filtration material; - an inlet (32) communicating with the chamber for introducing into the chamber a gaseous stream including entrained particles; a flow path of the gas stream in this chamber bringing the entrained particles into contact with the material of the bed contained in the chamber; an outlet (34) for evacuating the gas stream from the chamber; and - electrical means (38) in the flow path of the gas stream before at least a portion of the chamber for imparting electric charge from an external power source (40) to particles entrained in the   gaseous flow along the flow path.   2. Device according to claim 1, characterized in that the chamber (81) is arranged to contain a material   granular bed with a diameter of between 0.1 and 5 mm.   3. Device according to claim 1, characterized in that the chamber (81) contains a granular bed material   with a diameter of between 0.1 and 5 mm.   4. Device according to any one of the claims   1 to 3, characterized in that the means (38) for charging the particles comprises an electrode disposed in the flow path of the gas stream before at least a portion of bedroom.   5. Device according to any one of the claims   1 to 4, characterized in that the chamber 8 comprises a first (44) and a second (48) stage arranged in series along the   flow path of the gas stream.   6. Device according to claim 5, characterized in that the means for electrically charging the particles   (38) has an electrode disposed in the flow path   gas flow between the first and second stages of the room (8).   7. Device according to any one of claims 1   at 6, characterized in that it comprises a gas-permeable sieve (50) disposed in the chamber transversely to the flow path of the gas stream to separate upstream (49) and downstream (51) sections of the chamber, and means (54, 58, 82, 52, 56, 60) cooperating with these upstream and downstream sections to independently remove granular materials from these sections. 8. Process for separating entrained particles from a gas stream, characterized in that it comprises: (a) supplying a granular filtration material to a chamber (8; from an external energy source (40) to particles entrained in a gas stream;   (c) the passage of gas stream comprising the particles   the entrained entrances through at least a portion of the bed material contained in the chamber (8) whereby the charged particles are electrically attracted to the bed material and are therefore separated from the gaseous stream and deposited on that bed material so as to producing a gaseous stream depleted of particles; and (d) unloading the depleted gaseous stream particles of the room.   9. Method according to claim 8, characterized in that the granular filtration material has a diameter of between 0.1 and 5 mm.   Method according to claim 8 or 9, characterized in that the average diameter of the particles entrained in the   gaseous current is between 0.1 and 20 micrometers.   11. Process according to any one of claims 8 to   10, characterized in that the temperature of the gas stream is greater than 850 ° C.   he The method of claim 8, characterized in that it further comprises passing the gas stream through a first portion of the bed material prior to step (b) to remove a portion of the entrained particles, and then establishing an electrical charge to the remaining entrained particulates of the gaseous stream in step (b) prior to passage of the gaseous stream to a second portion of the bed material in step (c).   13. The method of claim 8 characterized in that at least a portion of the bed material is supplied to   step (a) to distinct downstream (49) and upstream (51) sections   from the chamber, and the gaseous stream in step (c) passes successively through the bed material contained in the   upstream and downstream sections, respectively.   14. The method of claim 13, characterized in that the bed material in the upstream sections (49) and downstream (51) through which pass at least a portion of the gas stream is discharged separately from these upstream sections. and downstream.