CA1156937A

CA1156937A - Filter apparatus and method for collecting fly ash and fine dust

Info

Publication number: CA1156937A
Application number: CA000358541A
Authority: CA
Inventors: Karim Zahedi; Jeffrey C. Alexander
Original assignee: Efb Inc
Current assignee: Efb Inc
Priority date: 1979-08-23
Filing date: 1980-08-19
Publication date: 1983-11-15
Also published as: US4308036A; WO1981000524A1

Abstract

IN THE UNITED STATES PATENT AND TRADEMARK OFFICE

Inventors: Karim ZAHEDI and Jeffrey C. ALEXANDER

Title: FILTER APPARATUS AND METHOD FOR
COLLECTING FLY ASH AND FINE DUST

Abstract of the Disclosure Fly ash and fine dust in a gas stream from a coal combustor, for example, are efficiently collected in a 2-stage, tubular, granular bed filter. The stages of the filter are arranged one above the other so that the gran-ules may move from the upper stage to the lower stage under the influence of gravity. Larger particulate in the gas stream is first collected by the lower stage, and then the remaining particulate is electrically charged and col-lected by the upper stage. Both stages are electrified sufficiently to rigidify the granular beds and adhere the particulate to the granules, so that the granules and the collected particulate move downwardly through the filters as a plug. Granules that exit from the lower stage are cleaned and returned to the upper stage. Non-porous granules may be used to prevent fine particulate from be-coming embedded in the granules.

Description

SP~`CIFI ~ ON
Back~round o ~he Invent:ion This invention relates to the removal of partic-ulate from a gas stream, and more particularly to the removal of fly ash and unburned carbon par~icle air pol-lutants from gases resulting from the combustion of coal. t Increasing coal usage and tightening environmental standards are forcing industry to improve air pollution control systems or to install them where they were not previously required. The problem of fly ash and unburned carbon particle air pollutants exists, for exatnple, in many types of coal-fired boilers. Particulate emissions from coal-fired boilers are conventionally controlled by electrostatic precipitators, cyclones, scrubbers, ancl, more recently, baghouse filters. For varous reasons, industry is seeking new particulate emission control tech-nologies, with better economic and operational performance characteristics.

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Brief Description of the Invention .
The present invention employs granular bed filters, a class of apparatus that is generally well known, but that has been considered impractical for many industrial appl.ications becau~se of performance problems -- including low particulate collection efficiencies; clogging of for-aminous wall structures; re-entrainment of collected par-ticulate; inefficient removal, cleaning and reuse of bed ~ 3 ~'~

granules -- and bee~3t1se of irnpractical structural config urations.
Accordingly, a principal object of the present invention is to provide a solution to the foregoing and other problerns in granular bed filters.
A more general object of the inventlon is to pro-vide irnproved filter apparatus and methods.
Briefly stated, in one of the broader aspects of the invention, granules of a filter bed move under the influence of gravity and are rigidified by a strong elec-trical field so that the granules and collected partic-ulate move continuously downward as a plug. In another ~; broad aspect, a gas stream passes through first and second tubular granular filter beds that are stacked one above the other. In another broad aspect, a gas stream passes inwardly through a first tubular filter bed, and then the remaining particulate in the gas stream is charged electri-cally and the gas stream passes outwardly through a second tubular filter bed. In a further broad aspect of the invention, a foraminous cleavage member is employed in a granular filter bed to provide separate regions of fast and slow granule movement. In still another aspect of the invention, non-porous granules are used in a granular filter bed, removed, cleaned, and returned to the bed.

Brie Description of the Drawings The invention will be further described in con-junction with the accompanying drawings, which illustrate pre~erred and exemplary embodiments, and wherein.

Figure 1 is a d;agram~atic vertlcal sectional view of a Eirst eml)ocliment of the invention;
Figure 2 is an explanatory diagram;
Figure 3 is a diagramrnatic vertical sectional view of a second embodiment of the invention; and Figure 4 is a fur~her explanatory diagram '~ I

Detailed Description of the Preferred Embodiments The present invention is concerned with granular ` bed filters, more particularly electrified filter beds, al-though some features of the invention have broader applica-tions. As is generally known, an electrified filter bed I may comprise a shallow bed of insulating or semi-insulating granules. When the granules are retained between verti-cal foraminous walls, such as screens J perforated plates, etc., and a gas stream is passed through the filter bed horizontally, the filter may be termed a packed bed or pane1 bed filter. A high electric voltage is applied across the bed, and the voltage gradient electrically polarizes the granules in the bed, inducing oppositely positioned caps of positive and negative charge on the granules. With insulating granules, no steady electric current flows;
otherwise a steady electric current may be drawn, but this will not impede the polarization process so long as the ~¦
high voltage can be malntained.

Before passlng through ~he electrified filter be~, ¦
particulate in the gas s~ream is electrically charged by passing ~he gas stream through a corona discharge de- ~
vice, for example. Once in the bed, the charged partic- ¦
ulate is attracted to the oppositely charged caps on the granular material, resulting ln separation of the par-ticulate from the gas s~tream. The cleaned gas continues through the bed. Very high particulate collection ef-ficiencies are theoretically possible, by virtue of the immense collection surface available in the granular bed.
However, after a relatively short time, the accumulated particulate in the granular bed must be removed. For econ- ij omic reasons, the granules may be cleaned of the collected 3 particulate and returned to the bed for further use. This may be accomplished by feeding granules removed from the filter bed to a cleaning device, such as a screen sifter or fluidized bed. The particulate removed from the gran-ules may then be conveyed to a storage silo and the granules returned to the bed. In certain applications, granules coated with particulate (that may be at least partially in liquid form) may be used in manufacturing processes, as disclosed in U.S. Patent No, 4,144,359, as-signed to the same assignee as the present invention.
The use of electrifled filter beds to control air pollution emissions from coal combustors has not been prac-; tlcal because o~ a number of problems, some o which will ~ now be discussed.

.

t7 l. In most applications fly ash prGcluced by t:he burning of coal is a very c1ense dispersion (aerosol~
of fine partlcles (micron sized or smaller) o~ high elec-trical resisti-vlty. Such an aerosol is very difEicult to charge in the usual corona chargers, yet electrical charging of the f;ne particles ls required for efficient particulate collection. Increasing gas residence ~ime in the charger might be effective, but ~hen the charger would resemble a high cost electrostatic precipitator.
Increasing the corona current intensity is not possible because of the high resistivity of the 1y ash and the well known associated "back corona."

2, Several granular cleaning techniques have been proposed but have failed for a variety o:~ reasons.
In _tu cleaning of a bed by reverse pulses of compressed air is difficult to control properly and is not very ef-fective Batch-dumping of the bed requires intricate baffling and damper systems that are unacceptable to in-dustry. Contlnuous draining of the bed results in agi-tation in the bed that dislodges and re-entrains fly ash in the gas flow, thus reducing removal efficiencies.

3. Foraminous bed retaining walls become cloggedl necessitating frequent, dificult maintenance.
Rapid continuous draining of the bed alleviates the prob-lem but is inefficient,

4. Periodically, because of irregularities in the coal combustion process, large amounts of unburned carbon particulate are emitted into the gas stream. These ; -6-ff , ~ .

get collec~ed in ~he bed and cause excesslve current drain from ~he power supply because of the carbon's low electri-cal resistivity.
The present invention solves these and other prob- ;
S lems that have prevented the practical utilization of elec-trified filter beds in the environment clescribed. First, the invention employs a preliminary :Eilt,er stage that re-moves up to 90% of the particulate matter by mechanical filtration mechanisms, so that the remaining flne partic-ulate can be charged by conventional corona technology with-out overloading the charger. Second, the invention employs continuous gravitational removal of the bed granules in a 'I
manner that prevents re-entrainment of collected particu-late. Third, the invention employs multi-layered removal of granules at different rates to minimize the clogging of bed retaining walls while utilizing bed granules ef-ficiently. Fourth, granules with non-porous surfaces are employed to prevent carbon build-up in surface pores, so that the granules may be readily cleaned and reused.
Finally, the invention employs a unique structural con-iguration that greatly facilitates the practical employ-ment of electrified filter beds in industrial applications.
The invention will be described in its application to the control of air pollutant emissions from coal com-bustors, but the invention has broader applicability, particularly in the utilization of certain features of the lnvention alone or in different combinatiolls.

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Reerring now to ~lgure 1 of ~he drawings, the l;
preferred e~lbodirnent elnploys a unique 2-stage, tubular filter 10 including a first stage 12 and a second stage 14 arranged in series vertically. The lower st:age 12 has inner and outer foraminous walls or tubes 16 and 18, respec-tively, and the upper stage has inner and outer foraminous walls or tubes l9 and 20, respectively. As shown, the inner walls 16 and 19 of the two stages of the filter may be constituted by a single continuous member.
In the preferred embodiment, the inner and outer walls of each filter are coaxial cylinders with their axis arranged vertically. The cylinders are separated by 2 to 6 inches, for example, to define a space which receives a bed of granules 22 (which may range from 20 to 4 mesh, for example). The inner and outer cylinders may have diameters, respectively, of 28 inches and 36 inches, for example. In the preferred form, the tubular granular beds of the upper and lower stages are aligned to consti-tute portions of one continuous bed extending from an upper granule input funnel 24 to a lower granule removal funnel 26.
Walls 16, 18, 19, and 20 serve as retaining-walls for the filter beds and may be screens or perforated metal plates with a hole size nominally 1/8 inch, for example.
Retaining wall 18 of the ].ower filter is preferably a louvered structure which may comprise a stack of units, each having an upper conical funnel portion and a lower ~,' `7 cylindrical portion, wi~h the units attached to spacers or stringers to deflne openings (e.g., 1 inch high) be-~` tween the units. The louvers may slope upwardly and out-wardly at about 60 from horizontal and may be spaced about 3-4 inches apart. The length of the louvers is de termined by the an~le of repose of the granules and is suf-ficient to prevent spillage of the granules.
One of the problems normally associated with granule bed filters is the plugging or clogging of filter bed retaining members, which lose their gas permeability.
The louvered plate structure 18 provides wide spaces where densely fly ash laden gas impinges directly onto~bed granules, so as to reduce clogging. Because the granules - are continuously moving downward (as will be described later in detail), the incident surface of the bed is con-tinuously being renewed, thus further reducing clogging by the collected particulate. Perforated retaining plates 16, 19 and 20 may be employed in regions where the fly ash loading is not as great. They are preferably perfor-ated with holes of diameter such that at about 10-20%
of the granules are small enough to fit through the holes.
As an example, granules for use in the bed may be screened to be between 1/10 inch and 1/4 inch in diameter. 1/8 inch perforations allow granules be-tween 1/10 inch and 1/8 inch to pass through. This plate construction allows "bleeding" of some granules through the plates and re-duces plugging by other granules or fly ash. In order for the plates to bleed effectively, vibrator ~mits 68 and 70 may provide periodic vibration of the retaining plates.
These units need only be activated :For a f~w seconds every several hours. The vibrations cause granules which just 'isit" in the perforations to Eall out.
S The lower filter is surrounded by an input plenum or manifold 28, while the upper filter ls surrounded by an exhaust plenum or manifold 30, the manifolds belng separated by a horizontal wall 32 and constituting a housing surrounding the 2-stage filter. The gas stream supplied to inle~ 33 from a coal combustor, for example, and containing fly ash and other particulate, passes in~
wardly through the granular bed of the lower filter to the interior space 34 of the filters and then passes out-wardly through the upper filter to an outlet 36, which may be connected to a stack. Either a positive pressure blower or fan on the input side or an exhaust blower or fan on the output side of the 2-stage filter may be em-ployed to move the gas stream through the filter apparatus.
The interior space 34 of the filters contains a charger such as a corona discharge electrode 38, which may be hung from an insulator 40 and which may be provided with a conventional vibrator or ~Irapper~ ~not shown). Cor-ona charging devices are well known in the art, and many conventional types may be employed Electrode 38 may be a 1/8 inch square bar, for example, with a weight or insu-lator 42 at its lower end High voltage, e.g., 50-100 KV
(preferably negative DC, relative to ground) is applied from a power supply 44 to the charger electrode 38. The bed ~ '7 retaining walls and plenurn structures are preferably grounded. ~n i.on 1ux is thus dlrected from electrode 38 toward the grounded walls 16 and 19 and fills the r~gion 34. Fine parti.culate matter in the gas stream passing from the preliminary filter stage 12 receives an electri-cal charge by impaction of ions from th:is flux.
Voltage gradients are applied to the filter beds by a cylindrical foraminous electrode 46 (e.g., wi~h 1/4 inch to 1/2 inch holes) embedded in the filter beds and supported by insulators (not shown). A voltage of from 2 to 20 KVJ for example (preferably DC, although AC may be used), is applied to this electrode from a high vol-tage supply 47. The resultant voltage gradient between electrode 46 and the grounded retaining walls of the filters provides electrostatic aerosol collection forces -and serves another important purpose, which will be des-~ cribed shortly.
- Clean granules are fed into the filter beds through an inlet port 48. An outlet port 50 may be equipped with a device, such as a vibratory feeder, for controlling the granule removal ra~e. As granules are removed from the lower bed, clean granules feed in through inlet port 48 and move down through the beds by gravi-tational action. Characteristic propagation velocities of the granules through the beds may be from 1-10 feet per hour, ~or example~
As the granules and collected particulate are fed out o~ the lower filter 12, the granules are preferably .

33~ ~f cleaned of collected particulate and returned to the filter beds for reuse. This may be accomp]lshed by several rnechanisms, such as screen si~ters and fluidized beds.
A preferred mechanism ls depicted diagrammatically in Figure l. &ranules and collected particulate are fed into q a separator 52 through outlet port 50. A vibrator 54 pro-vides for the smooth flow of granules over a screen 56 and provides agitation to dislodge fly ash and other par-ticulate from the granules. The granule depth on the screen is preerably not more than a few granule diameters.
Air is vented from the separator through port 58. The vented air carries away dust separated from the granules.
Further, the updraft of air serves to eliminate plugging problems in the screen 56. The screen mesh size i~ de-termined by the size of granules desired in the filter beds. Typically, it may be 10 mesh. Any granules smal-ler than 10 mesh would then fall through the screen and exit the separator through port 60 to a storage container 62 having a vent 64. Cleaned granules of greater than 10 mesh pass over the screen and exit the separator through port 66 to a granule conveyor, such as a bucket !~
elevator, which carries the cleaned granules to the top of the upper filter, where they may be discharged into a collection vessel which eeds the granules to inlet port 48. The vented air, densely laden with ~ly ash and dust passes to a conventional storage silo, bin vent unit.
In the operation of the filter apparatus of Figure 1, polluted gas enters plenum 28 and passes hori-zontally through the filter bed of the lower ilter 12.

~ 3 This bed collects large pollutant particulate by mechani-cal fil~ra~ion mechanisTns (which are qui~e ef~icient for larger particulate [s~lpermicron] but not for small par-ticulate [submicron]), and retains the collected partlcu-late in the bed with the aid of electromechanical freezing forces in the bed, which will be described shortly. The gas exiting the first or preliminary filter is cleaned of approximately 90% of its particulate pollutants. The re-mainder is carried by the gas stream upward through region 34, where it is electrically charged by the corona dis-charge from electrode 38. Region 34 also acts as an elec-trostatic precipitator to remove some of the charged par-ticulate from the gas and deposit it on the perforated plate 19, but it is not a large enough region to do so with high efficiency. After being charged, the particulate and its entraining gas pass horizontally through the final filter bed of filter 10, where electrostatic collection forces clean the particulate from the gas with ultra-high efficiencies while electromechanical free~ing forces aug-ment its retention in the beds Finally, the cleaned gas exits the filter apparatus through plenum 30.
As alluded to earlier, the high voltage on elec-trode 46 has an important purpose in addition to providing electrostatic collection forces for the upper ~ilter. ~en a large electric field (Eoj is applied to a bed of granules or particles ~, as shown in Figure 2 t large inter-~arti.cle forces are electrically induced in the direction of the electric field. Such forces are a function of the contact area between particles and can be written as:

( ~) 47r E R2(R-C) E 2 ln2 (R(~) 5 where fe = elec~romechanical force R = bed particle radius RC = contact radius Eo = electric field intensity Eo = permittivity oE free space These forces can be large enough to completely ; 10 support a granular bed between sets of vertical parallel plates, for example. A large electromechanical chaining effect is caused by the concentration of electric field near the inter-particle contact points. These large elec-tric fields are the result of constrictions of the current paths at the contact points. It can be shown that the electric field around the contact area is several orders of magnitude larger than the ambient electric field ~i.e., the applied voltage divided by the distance between the electrodes~. A field intensity of about 5 x 10 volts per meter (close to the electrical breakdown strength of the bed) has been found to be effective to "freeze" or rigidify t~le bed. A field intensity of the order of lO volts per meter appears to be required. If the field intensity is too high, say 3 x 106 volts per meter, undesirable ion-! 25 ization and bed breakdown will occur.

~ 3 ~

In the present invention, the elec~romechani.cal chaining effect is used to rigidify the bed and also to achieve s~able aclhesion of the fly ash and other fine par-ticulate to ~he ~ed granules. The large electric flelds S (an order oE magnitude larger than needed for electr;.cal enhancement of collision of fly ash with bed granules) serves to irnprove the adherence of the ash to the bed 3 granules. This appli.es to both submicron and supermicron ash which may have contacted the bed granules by either inertial impaction or electrostatic attraction. The col-lected ash a beco~es part of the electromechanical chain as shown in Figure 2. As the granule bed moves downwardly between the retaining walls of the fil~ers toward the out-; let funnel, the strong electric field forces the granules to move as a substantially solid plug, the plug being formed by chains of the bed granules. As a result, the grinding effect between particles that is normally characteristic of moving beds is eliminated, and the chance of re-entrain-ment of the collected ash and other fine particulate is drastically reduced.
In summary, the s~rong electric fields employed ln the invention affect the filtration process by forming chains of collected ash and bed granules which enhance the adhesion of the ash to the granules and which cause ~he bed ko move downwardly toward the outlet funnel as a plug. The usual individual bed granule motion is replaced by the motion of chains of granules between the retaining walls. To achieve maximum retention of the ash inside the '~

bed while the bed is being moved, external mechanical disturbances should be minimiæed while the electromechani-cal chains move gently between the retaining walls.
Conventional bed granules, such as gravel, sand, limestone, e~c., have a porous surface structure. Upon repeated collection of unburned carbon particulate, the carbon becomes embedded in the granule pores. The gran-ules cannot then be effectively cleaned of the carbon, and the carbon build-up results in excessive current re-quirements for the electrified filter bed. It has been discovered that by using granules with a non-porous, prefer-ably smooth surface structure in which the carbon partic- il ulate cannot become embedded, the collected carbon can be ~!
effectively cleaned from the granules. This has been suc-cessfully demonstrated using glass granules, such as glass beads or crushed glass (cullet) in circumstances where ordinary gravel failed because of carbon build~up in the surface pores. Current requirements were 100 times more for the gravel than for the glass granules. Crushed glass ~0 is especially suitable because it is inexpensive and readily available.
In the structure illustrated in Figure l, a louvered inlet plate 18 is employed to minimize the prob-lem of plugging of oraminous retaining walls. It has been fo~md that when inlet loadings of dust are high and/or when the dust is of a sticky nature, granules in the inlet louver region can plug the louver openings and become fro-zen in the louvers. Eventually, khe permeability to gas is lost. This problem can be remedied, as is already ~;, -16~

, known, by Easter mot;on of the granules through the bed, but faster motion leads to increased re-entrainment of collected dust and to poor utilization o the granules.
In addition, the capacity of granu]e handling equipment must be increased several times.
In accordance with the invention it has been dis-covered how to employ ~ast granule movement without the disadvantages just described. More par~icularly, it has been discovered that the desired results can be achieved by employing a coarse "cleavage" screen (screen openings much larger than the granule size) a small distance behind the inlet louvers (approximately 1 inch behind, for example, for overall bed depths of about ~ inches to 6 inches~.
- Two solid feeding funnels below the cylindrical filter bed allow separate control of feed rates in front o and behind the screen, The screen creates a cleavage surface in the bed, and the granules in the front face can be moved ~-~ at a greater velocity than in the bulk of the bed. The screen allows gas passage but does not plug, because of the constant granule motion at its surface and some gran- -ule motion through it. To this end, the screen is of a mesh size greater than the granule size. For example, a screen with at least 1/2 inch openings may be e~ployed for granules of about 1/8 inch diameter, Figure 3 i.llustrates a second embodiment of the invention employing the feature just described. Parts corresponding to those shown in Figure 1 are designated by corresponding primed reference numerals. In this .

~ 3'~

ernbodiment, all of the retaining walls are louvered to minimize clogging, and, in addi~ion, a cleavage screen 72 is employed in the lower filter unit close to the inlet retaining wall 18'. If the total depth of the filter bed between retaining walls 18' and 16' is 4 inches to 6 inches, for example, screen 72 may be spacecl about 1 inch from re-taining wall 18'. Separa-~e outlet funnels 26a and 26b are provided, each controlled by a separate control valve 74 or 76. The control valves are adjusted so that the flow of granules between screen 72 and the lnlet retaining wall 18' is much faster ~at least several times faster) than the flow rate of granules in the remainder (the bulk) of the bed. For example1 the fast rate may be 8 to 10 feet per hour, as compared to about 1 foot per hour for the slow rate. Figure 4 illustrates diagrammatically the move-ment of the granules in the fast and slow zones, separated by the cleavage screen 72. Pre-filter 12' may not be electri-fied, in which event screen 72 may be grounded (or left "floating"). Screen 72 may also be used as a high voltage electrode for filter 12'.
While several preferred embodiments of the inven-tion have been shown and described, it will be apparent to those skilled in the art that changes can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims. For e.xample, a plurality of 2-stage filters in accordance with the invention may be arranged in parallel in a common housing to provide addi-tional filter capacity. Instead of employing an electrode within the filter beds, a high voltage may be applied be-tween opposite, insulated retaining walls of the bed.

Claims

CLAIMS:

1. Apparatus for removing particulate from a gas stream comprising a pair of foraminous walls separated to define a space for a bed of granules, the walls being oriented so that the granules may move from an upper region of the walls to a lower region of the walls under the influence of gravity, means for supplying granules to the upper region, means for removing granules from the lower region, means for passing a gas stream containing particulate through the walls and the bed of granules, and means for applying an electrical field to the granules of sufficient magnitude to rigidify the bed and adhere the par-ticulate to the bed granules so that the granules and the collected particulate move continuously downward as a plug.

2. Apparatus in accordance with Claim 1, wherein an electric field of the order of 105 volts per meter is applied to the bed.

3. Apparatus in accordance with Claim 1, wherein the walls comprise inner and outer tubes defining a tub-ular space therebetween containing said bed.

4. Apparatus in accordance with Claim 3, wherein the means for passing the gas stream defines a flow path through a lower portion of the bed into the outer tube and then through an upper portion of the bed and out of the outer tube.

5. Apparatus in accordance with Claim 4, wherein means for electrically charging particulate in the gas stream is located inside the inner tube.

6. Apparatus in accordance with Claim 1, further comprising means for cleaning granules removed from the bed and means for returning cleaned granules to the bed.

7. Filter apparatus comprising a pair of foraminous walls separated to define a space for a bed of granules that may move from an upper region to a lower region of the bed under the influence of gravity, means for passing through the walls and the bed a gas stream containing particulate that is to be collected by the granules, an intermediate foraminous wall between the aforementioned walls that divides said space into two portions, and means for moving granules in said portions so that the rate of movement is greater in one of said portions than in the other, wherein said one portion is adjacent to the wall through which the gas stream enters the bed.

8. Apparatus in accordance with Claim 7, wherein said other portion has a depth in the direction of gas stream flow therethrough that is several times the depth of said one portion in said direction.

9. Apparatus in accordance with Claim 7, wherein said regulating means produces granule movement in said one portion that is several times the rate of granule movement in said other portion.

10. Apparatus in accordance with Claim 7, wherein the intermediate foraminous wall has openings therethrough that are large enough to pass a substantial number of the granules of the bed.

11. A method of removing particulate from a gas stream comprising providing a bed of granules that moves continuously from a first region to a second region, sup-plying granules to the first region, removing granules from the second region, passing a gas stream containing particulate through the bed of granules, and applying an electrical field to the granules of sufficient magnitude to rigidify the bed and adhere the particulate to the bed granules so that the granules and the collected particu-late move continuously as a plug.

12. A method in accordance with Claim 11, wherein an electric field of the order of 105 volts per meter is applied to the bed.

13. A method in accordance with Claim 11, further comprising cleaning granules removed from the bed and re-turning cleaned granules to the bed.

14. A method in accordance with Claim 11, wherein the particulate comprises carbon particles, and wherein the granules have a non-porous surface in which the carbon particles cannot become embedded.

15. In a method of filtration, providing a bed of granules that may move from an upper region to a lower region of the bed under the influence of gravity, passing through the bed a gas stream containing particulate that is to be collected by the granules, providing a foraminous wall in the bed that divides the bed into two downwardly moving portions, and regulating the downward movement of granules in said portions so that the rate of movement is greater in one of said portions than in the other, said one portion being adjacent to the surface of the bed through which the gas stream enters the bed.

16. A method in accordance with Claim 15, wherein said other portion has a depth in the direction of gas stream flow therethrough that is several times the depth of said one portion in said direction.

17. A method in accordance with Claim 15, wherein said regulating produces granule movement in said one portion that is several times the rate of granule movement in said other portion.

18. A method in accordance with Claim 15, wherein a substantial number of the granules of the bed pass through said wall.