GB2133716A

GB2133716A - Classifying material withdrawn from fluidised bed

Info

Publication number: GB2133716A
Application number: GB08300663A
Authority: GB
Inventors: Brian Anthony Napier
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1983-01-11
Filing date: 1983-01-11
Publication date: 1984-08-01
Also published as: GB8300663D0

Abstract

A method of classifying fluidised bed material into fine and coarse fractions involves the withdrawal of material from the bed (60) into a classifier (16) to which is fed a heated classifying fluid medium via a feed line (24), so that entrained material is returned to the bed and unentrained material discharged. The classifying fluid may be compressed air or steam, and may be heated within the bed. <IMAGE>

Description

SPECIFICATION Improvements in or relating to fluidised bed systems This invention concerns improvements in or relating to fluidised bed systems.

In particular, although not exclusively, the invention has reference to such systems employed for the combustion of fuel. In practice, when burning for example coal in a fluidised bed, ash particles are formed and can agglomerate to a size which is undesirable for the efficient operation of the bed.

Large or dense inert material may also be present in the fuel supply. Coarse ash particles tend to gravitate to the base of the bed and can impedefluidisation. It is thus necessary to remove these particles from the base of the bed and this may be effected continuously or periodically. It is however, the case that relatively fine particles of material which may include both bed material and unburntfuel will also be removed when extracting the coarse ash. It is, therefore, desirable, if not essential, to provide a means whereby the material removed from the main fluidised bed can be classified as to size such that material of an acceptable size grading can be returned to the bed and that unwanted material can be discharged.

Various methods of achieving size grading of this kind have already been proposed. In particular, one method involves the extraction of bed material into a pipe to which classifying air is fed, the finer particles being entrained in this air for return to the bed and the coarser particles gravitating to the bottom of the pipe for discharge. The classifying air is supplied in this earlier method at ambient temperature. One of the problems associated with this method is that upon entry into the fluidised bed the classifying air is rapidly heated thus causing an increase in velocity.

Indeed the classifying air velocity in a fluidised bed maintained at 900" can be higher than when at ambient temperature by a factor of four and this constitutes a disadvantage. For example, erosion of heattransfertubing in the bed could occurdueto abrasion by bed material moving at high velocity.

Whilst it may be possible to overcome this problem in part, by shielding the tubing or careful siting of the inlet from the classifier to the bed, extra cost could well be incurred as a result. Furthermore, the high air velocity could also increase elutriation from the fluidised bed.

An object of the present invention is therefore to provide an improved method and apparatus for classifying material extracted from a fluidised bed and for returning thereto material of acceptable size grading.

Accordingly a first aspect of the invention provides a method for classifying fluidised bed material comprising the steps of withdrawing material from the bed, feeding the withdrawn material to a classifier, introducing a heated classifying fluid medium into the classifier whereby material of an acceptable size grading is entrained in said medium, returning the entrained material to the fluidised bed, and discharging unentrained material from the classifier.

The heated classifying fluid medium may be pre-heated gas, for example air, or may alternatively be steam.

In the case of preheated air, the expansion thereof and consequent velocity increase is reduced considerably in comparison with that experienced when using air at ambient temperature, thereby avoiding erosion problems. For convenience, the air may be preheated in the fluidised bed in which combustion, gasification or other heating process may be taking place. In addition to the avoidance of fhe erosion problem, particle separation using preheated air is improved, the reason being that the terminal velocity of large particles increases with temperature rise, whereas that of small particles, for example about 1 mm diameter, is unchanged at temperatures in the region of 900 .

As the alternative, steam atsuperatmospheric pressure, for example of the order of 80 psi, is employed either as a continuous classifying medium or as a high pressure starting medium if compressed air were not available. The steam may be preheated in the fluidised bed in which combustion, gasification or other heating process may be taking place.

According to a second aspect of the invention, apparatus for carrying out the method includes a vessel adapted to contain a fluidised bed of particulate material in a base region thereof, a discharge pipe communicating with the base region and leading to a classifier, a source of a heated classifying fluid medium, and a discharge means for unentrained material coupled to the classifier.

The discharge pipe is preferably connected to the centre of this base region adapted to contain the fluidised bed.

The classifier may comprise a perforate inner tube communicating with the discharge pipe and an outer jacket defining an annual chamber to which is connected a conduit extending to the source of the heated classifying fluid medium. Several classifiers may be required for large fluidised beds depending upon the fuel quality.

The source may be of steam or preheated air. In the latter case, compressed air may be supplied via a heating circuit disposed within the vessel.

By way of example only, a method of and apparatus for classifying fluidised bed material are described below with reference to the accompanying drawing which is a diagrammatic sectional view of the apparatus.

Referring to the drawing, a vessel 1 is provided with a gas outlet 3 and in a lower region 2 with a distributor plate 4 beneath which is defined a plenum 6 having an inlet 8for a fluidising medium, for example air. The plate 4 is provided with a plurality of perforated standpipes 10 and in its centre has an opening 12 to a surrounding jacket 20 defining an annular chamber 22, a feed pipe 24 for a heated classifying fluid medium being connected to the chamber 22. An extension 26 of the discharge pipe 14 leads to a rotary valve 28 above an airtight lock hopper 30 which has a further rotary valve 32 in its outlet.

A heating circuit 40 is disposed within the vessel 1 and air is fed thereto via a valve 42 from a compressed air supply (not shown) through a pipe 44. The heating circuit 40 is connected to the feed pipe 24 of the classifier through a line 46. A steam pipe 48 connects into the feed pipe 24 and has control valves 50 disposed therein either side of a cross pipe 52 between the pipe 44 and pipe 48.

In use, a bed 60 of particulate material is estab lished in the lower region 2 of the vessel 1 above the plate 4 and is fluidised by air supplied to the plenum 6 through inlet 8, the air issuing into the bed through the perforations in the standpipes 10. Fuel, e.g. coal, is fed to the bed for combustion therein. During the course of combustion, ash particles are formed and may tend to agglomerate to create coarse particles which if left in the bed can occasion defluidisation.

Inert particles present in the fuel may accummulate, also causing fluidisation problems. In the present invention, particles from the bed are continuously removed by descending through the opening 12 and discharge pipe 14. These particles flow into the classifier 16 wherein they are subjected to the passage of heated air or steam which enters the annular chamber 22 and passes through the tube 18 into the particle flow. When heated air is used, compressed air passes through the pipe 44 and valve 42 into the heating circuity 40 wherein it is heated to a temperature at or near that of the fluidised bed, for example about 900 . The heated air then flows into the line 46 and into the feed pipe 24 of the classifier.The effect of the heated air on the particles is to classify them whereby coarser ash continues to descent through the classifier and thence through the extension 26 of the discharge pipe 14 to the lock hopper 30 via the rotary valve 28.

The coarse ash may be removed from the lock hopper 30 when desired by operation of valves 28 and 32. The finer particles enter the classifier 16 and are entrained in the hot air flow which passes upwardly through the discharge pipe 14 into the fluidised bed 60. The finer particles are thus trans ported back to the bed, but by the fact that the carrier air is not, the velocity, which is usually increased when ambient air enters the bed, remains substantially constant or subject only to a minimal increase, thus avoiding erosion problems associated with high particle velocities impinging upon heating surfaces within the bed. Since the velocity is not excessive as with the use of ambient air, particles already within the bed are less likely to be elutriated.

Additionally because of the eleveted temperature of the air, particle separation is enhanced since terminal velocities of large particles, i.e. larger than 3 mm diameter increase with temperature rise whereas that of small particles is unchanged at temperatures of about 9000.

Steam may be employed as an alternative to heated air and is supplied through the pipe 48 and valves 50, the valve 42 being closed.

Whilst the classifier 16 has been shown as being disposed outside the vessel 1, it may be located within the plenum or when deep fluidised beds are used within the bed itself.

Claims

1. A method for classifying fluidised bed material comprising the steps of withdrawing material from the bed, feeding the withdrawn material to a classifier, introducing a heated classifying fluid medium into the classifier whereby material of an acceptable size grading is entrained in said medium, returning the entrained material to the fluidised bed, and discharging unentrained material from the classifier.

2. A method according to claim 1 in which the heated classifying fluid medium is a preheated gas.

3. A method according to claim 2 wherein the gas is compressed air.

4. A method according to claim 1 wherein the heated classifying fluid medium is steam.

5. A method according to claim 4 in which the steam is at superatmospheric pressure.

6. A method according to claim 2 or 3 or 4 in which the gas or steam is preheated in the fluidised bed.

7. A method according to claim 1 in which the temperature of the classifying fluid medium lies in the range 850" to 950".

8. . A method according to any one of the preceding claims in which material withdrawn from the fluidised bed descends under gravity into the classifier.

9. A method for classifying fluidised bed material substantially as hereinbefore described with refer encetothe accompanying drawing.

10. Apparatus for carrying out the method according to any one of the preceding claims includes a vessel adapted to contain a fluidised bed of particulate material in a base region thereof, a discharge pipe communicating with the base region and leading to a classifier, a source of a heated classifying fluid medium, and a discharge means for unentrained material coupled to the classifier.

11. Apparatus according to claim 10 in which the discharge pipe is connected to the centre of the base region.

12. Apparatus according to claim 10 or 11 in which the classifier comprises a perforate inner tube communicating with the discharge pipe, and an outer jacket defining an annular chamber to which is connected a conduit extending to the source of the heated classifying fluid medium.

13. Apparatus according to claim 10,11, or 12 in which the source of the heated classifying fluid medium is compressed air preheated in a heating circuit disposed within the vessel.

14. Apparatus according to claims 10,11 or 12 in which the source of the heated classifying fluid medium is steam at a superatmospheric pressure.

15. Apparatus for classifying fluidised bed material substantially as hereinbefore described with reference to the accompanying drawing.