GB2116064A

GB2116064A - Improvements in or relating to particle sizing systems for fluidised beds

Info

Publication number: GB2116064A
Application number: GB08206552A
Authority: GB
Inventors: Christopher Foster; Russell Llewelyn Dando
Original assignee: Coal Industry Patents Ltd
Current assignee: Coal Industry Patents Ltd
Priority date: 1982-03-05
Filing date: 1982-03-05
Publication date: 1983-09-21
Also published as: GB2116064B

Abstract

An enclosed particle size regrading system for a fluidised bed (1) includes a positive pressure entrainment vessel (10) to which material from the bed (1) is fed. Material is pneumatically conveyed from vessel (10) to a solids separator (24) wherein solids are separated from the entraining air. The solids are passed over a sizing means e.g. screen (30) which separates the particles into coarse and fine fractions. The fine particles are recycled to the bed (1) by air along line (37), and the coarse particles pass to a container (38). <IMAGE>

Description

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

In such systems employed for combustion purposes, it is necessary periodically to remove particulate material from the fluidised bed in order to effect size regradings, since agglomeration of particles can occur and result in adverse fluidising conditions. The fine particles are recycled to the bed, whereas the larger, oversize particles are removed from the system.

One method of size regarding which has found favour and indeed operated satisfactorily consists in the use of a vibrating screen which separates the coarse from the fine particles which are held in a freely venting hopper feeding a pneumatic transport system for conveying the fine particles back to the bed. It will be appreciated that in a fluidised bed combustion system,the material conveyed during the size regrading operation is at an elevated temperature and accordingly it has proved difficult to provide gas tight seals between the various elements used to carry out the method described above. Where the combustion process takes place under super atmospheric conditions, the problem of gas tight sealing becomes particularly relevant, and cannot easily be resolved because of the existence of relatively moving elements.

An object of the present invention is to provide a particle size regrading system for a fluidised bed which can boast substantially no moving parts and which can operate in a closed loop.

According to a first aspect, the invention provides a particle size regrading system for a fluidised bed, the system including a positive pressure entrainment vessel having an inlet connectible to a fluidised bed and an outlet connected to a solids separator having a gas outlet and a solid materials outlet, sizing means located within a duct, and connected to the solid material outlet for separating coarse from fine material, the sizing means having a coarse material discharge outlet and a separate fine material discharge outlet, conveying means connected with the fine material discharge outlet, and a conduit extending from the fine material discharge outlet and being connectible to a fluidised bed.

The positive pressure entrainment vessel is conve nientlyofthetype disclosed in our co-pending patent application no 81/17663 and incorporates a nozzle arrangement in which, in use, air passes into the vessel via an annular passage circumscribing a nozzle through which material within the vessel is pneumatically conveyed. In particular, the pressurisation of the container to effect conveyance of particles from the vessel is initiated and discontinued by means of a suitable control valve in a gas line, for example an air line, connected to the vessel.

The sizing means may advantageously be in the form of a screen which slopes downwardly from the solid material outlet of the cyclone separator. Conve nientlythesizing means is contained within a duct.

The conveying means is preferably a gas duct to which a source of conveying gas is connected. The source of conveying gas may be the entraining air used to transport material from the pressure vessel to the solids separator, e.g. a cyclone, and accordtingly the gas outlet of the separator is connected to the gas duct of the conveying means.

The coarse material discharge outlet may be arranged to discharge into a container, and an isolating valve may be associated with the outlet.

According to a second aspect of the invention a method of regrading the particle size of a fluidised bed comprises the steps of passing material from the bed in a closed system to a positive pressure entrainment vessel, pneumatically transporting material from the vessel to a cyclone separator, feeding material separated in the separator to a sizing means, grading the material into coarse and fine sizes on the sizing means, and pneumatically conveying the fine sized material to the fluidised bed.

The coarse sized material is conveniently conveyed to a container.

By way of example, one form of size grading system for a fluidised bed according to the invention is described below with reference to the accompanying drawings which diagrammatically illustrates the system.

Referring to the drawing, a fluidised bed 1 is shown contained within a combustor 2 which is provided with a suitable part 4 for the discharge of material from the bed 1 and with a recycle inlet part 6.

A size grading system for the fluidised bed 1 is indicated generally at 8 and includes a positive pressure entrainment vessel 10 having a material inlet 12 shown connected to the port 4 of combustor 2. A nozzle 14 is disposed within the vessel 10 and comprises an outer tube 16 surrounding an inner discharge tube 18, the annular space defined between the tubes 16, 18 being connected to an air supply fed through pipe 20 in which is located a solenoid operated control valve 22. The valve may alternatively be of any kind which is remotely operable. The inner discharge tube 18 is connected to a cyclone separator 24 which has a gas outlet 26 and a solid material outlet 28.A sloping screen 30 is disposed within a duct 32 connected to the outlet 28, the duct 32 having a coarse material discharge outlet 34 and a fine material discharge outlet 36; which latter is connected to the recycle inlet port 6 of the combustor 2 by line 37. The outlet 34 is connected via a line 33 to a container 38, an isolating valve 39 being incorporated in line 33, and the gas outlet 26 of separator 24 is connected to the line 37.

In operation, the fluidised bed 1 is formed in combustor 2 and combustion takes place therein.

During the coarse of combustion the size of the bed material changes due to agglomeration and it is necessary to maintain an average particle size in the bed for optimum combustion performance.

The present invention enables sizing of the bed material to be achieved in a system without any moving parts. Bed material flows out of port 4 of combustor 2 and into the vessel 10 under gravity.

When it is desired to commence the sizing of bed material, the solenoid operated control valve 22 is activated to an open position thereby allowing air to flow through pipe 20. The air flows through the annular space between tubes 16, 18 and into the vessel 10. The air issuing from the nozzle 14 entrains material in vessel 10 and transports it along the tube 18 and thence to the cyclone separator 24, wherein the particles of the material are separated from the air flow. The particles so separated exit through outlet 28 and pass into the duct 32 over the screen 30, preferably of wedge wire form. Fine particles pass through the screen 30 into the discharge outlet 36.Air exhausted through outlet 26 of the separator 24 may be used as the means for conveying the fine particles which have been separated by screen 30 and fall into the line 37, and accordingly the outlet 26 is connected to line 37 by a duct 35 as shown, the pressure drop in the line 37 being less than that across the screen 30 and the outlet 36 for this source of conveying airto be used. The fine particles transported by the air in line 37 are injected through the recycle outlet port 6 into the combustor 2. The coarse particles separated by the screen 30 pass over the end of the screen into the line 33 and thence into container 38. When it is desired to empty container 38, the isolating valve 39 is closed and the container is removed.

Bed material can be sized continually or continuously, since the vessel 10 refills from the fluidised bed 1 by gravity. When it is desired to discontinue the sizing operation, the valve 22 is closed to stop transport air reaching the nozzle 14.

The regrading system of the present invention has no operative moving parts except for the valves, is self contained, and is effectively a closed loop which can easily be sealed with the combustortherebyto enable sizing to take place even where superatmospheric conditions apply in the combustor.

Claims

1. A particle size regrading system for a fluidised bed, the system including a positive pressure entrainment vessel having an inlet connectible to a fluidised bed and an outlet connected to a solids separator havng a gas outlet and a solid material outlet, sizing means located within a duct and connected to the solid material outlet for separating coarse from fine material, the sizing means having a coarse material discharge outlet and a separate fine material discharge outlet, conveying means connected with the fine material discharge outlet, and a conduit extended from the fine material discharge outlet and being connectible to a fluidised bed.

2. A system according to claim 1 in which a gas supply line is connected to the vessel and a valve is located in the line.

3. A system according to claim 1 or 2 in which this sizing means is in the form of a screen.

4. A system according to claim 3 in which the screen of this sizing means is a wedge-wire screen.

5. A system according to claim 3 or 4 in which the screen slopes downwardly from the solid material outlet of the solids separator.

6. A system according to claim 3,4 or 5 in which the sizing means is a duct.

7. A system according to any one of the preceding claims in which the conveying means is a gas duct.

8. A system according to claim 7 in which the conveying means is connected to the gas outlet of the solids separator.

9. A system according to any one of the preceding claims in which the coarse material discharge outlet is arranged to discharge into a container.

10. A system according to claim 9 in which a line connects the coarse material discharge outlet to the container, and an isolating valve is located in the line.

11. A particle size regrading system for a fluidised bed substantially as hereinbefore decribed with reference to the accompanying drawing.

12. A method of regrading the particle size of a fluidised bed comprisng the steps of passing material from the bed in a closed system to a positive pressure entrainment vessel, pneumatically transporting material from the vessel to a solids separator, feeding material separated in the separator to a sizing means, grading the material into coarse and fine sizes on the sizing means, and pneumatically conveying the fine sized materal to the fluidised bed.

13. A method according to claim 12 in which the coarse sized material is conveyed to a container.

14. A method of regrading the particle size of a fluidised bed substantially as hereinbefore described with reference to the accompanying drawing.