GB2124101A - Classification and recycling of fluidised bed material - Google Patents

Abstract

In a fluidised bed material cleaning system, bed material is passed down a drop tube (22) extending from a gas-permeable support (4) into a receptacle (18) supplied with air from a line (20); means (26) conveys pneumatically material from the receptacle (18) to a vessel (30) in which a vibratory screen (40) is mounted, by which acceptable size material is separated passed into a storage receiver (54) whence it is returned pneumatically to the bed (70), and unacceptable size material is discharged. <IMAGE>

Description

SPECIFICATION Improvements in or relating to fluidised beds This invention concerns improvements in or relating to fluidised beds particularly, although not exclusively, of this type employed for combustion.

The present invention has reference to a method and apparatus for the cleaning and reclassification of material used in a fluidised bed. In particular in a fluidised combustion bed a fuel for example coal, is burned in a mass of usually inert particulate material, for example sand. During operation of the fluidised bed, sand is lost by elutriation and a build-up of oversize material, for example ash, stone, clinkers ortramp metal, occurs. In order to ensure satisfactory operation of the fluidised bed, the amount of sand, i.e. bed material, must be maintained and the oversize material removed.

There are essentially two kinds of bed cleaning system, the first and the most ideal being the in-service' system wherein cleaning is carried out while the fluidised bed is operating, namely when hot or burning coal or other fuel is present. In this system, removal, cleaning and reclassification may take place continuously or intermittently depending upon requirements.

The second kind of bed cleaning system is designated 'out-of-service', that is where removal, cleaning and reclassification occur when the fluidised bed is cool and no fuel is being burnt. In practice, the kind of system utilised will be dictated by the operating conditions of the fluidised bed apparatus. For example, a boiler serving a continuous steam load will require an 'in-service' system which, although the most useful, is difficult to achieve.

Conventionally, bed cleaning systems possess the following common features. The material is removed from the bed by draining through a tube for passage to a vibratory screen external to the plenum chamber through which fluidising air passes, in use to the bed. The screen may need to be gas tight and for this purpose may have gas tight flexible connections located at the inlet to an outlet from the screen.

The bed material is conveyed pneumatically to and/or from the vibrating screen. Such systems display inherent disadvantages. For example, in order to avoid blockage above or in the tube, it is usual to select a diameter at least twice that of the largest expected particle. When singles coal is employed as the fuel, this requires a tube diameter of this order of .075m. The flow of material down an unrestricted tube of this size would overload the conveying pipework and the screen and thus metering is necessary. However, rotary valves, double flap valves or lock hoppers suffer adversely from the effects of elevated temperatures, between 800 1000"C, and of the abrasive nature of the bed material, whereas the use of a slide valve restricting the tube would be counter-productive.Furthermore, if the vibratory screen is not made gas tight, then fumes and dust will escape into the atmosphere. The sealing of the screen and the provision of flexible connections are expensive, because of the temperature and nature ofthe material. Additionally, if, after cleaning, the bed material is pneumatically conveyed back to the fluidised bed there is a tendency for the conveying air to pass into the undersize outlet of the screen and impede the flow of material, thus resulting in blockages. When bed material is pneumatically conveyed to the screen, it is common to choose a solids to air ratio such that the material is cooled down to below 300"C. Whilst this effect allows the use of mild steel construction, the heat of the material is lost.

The disadvantages attendant upon conventional systems centre upon the way in which the various integers are put together rather than upon the integers themselves.

An object of the present invention isito provide an improved method and apparatus for the cleaning and reclassification of material in a fluidised bed which mitigates the disadvantages of current systems, but can be employed in an 'in-service' mode.

According to a first aspect of the invention, apparatus for the cleaning and reclassification of bed material for use in combination with a fluidised bed reactor having a reaction chamber, a gas-permeable support within the chamber and adapted to support a fluidised bed, a plenum defined beneath the support, an air fan connected to the plenum, a gas outlet from the chamber, and a separator connected to the gas outlet, the apparatus comprising a drop tube extending from and opening into the gas permeable support to a receptacle, an air feed line from the fan connected into the receptable, a nozzle extending into the receptacle and connected by a conveying means to a vessel having mounted therein a vibratory screen, an outlet from the vessel for acceptable material, a discharge outlet from the vessel for reject material, a storage receiver connected to the outlet for acceptable material, and further conveying means extending from the receiver and adapted for connection to the chamber of the fluidised bed reactor.

Conveniently, the receptacle is formed as part of the plenum in the base part thereof, the receptacle opening into the plenum, and the air feed line preferably entering the receptacle in its base. The conveying means is advantageously in the form of a pipe. The vessel has a gas discharge outlet which in practice is connected to an exhaust gas conduit extending from the chamber of the reactor to the separator.

The vibratory screen within the vessel is preferably cooled by air, or other gas by a further fan which is also employed to discharge air into the storage receiver. The further conveying means may comprise a nozzle extending within the receiver and communicating with pipework for transport of material back to the chamber of the fluidised bed reactor, a valve being provided in the pipework.

According to a second aspect of the invention, there is provided a method cleaning and reclassifying fluidised bed material using the apparatus of the first aspect, the method including the steps of passing material from the fluidised bed in the chamber of the reactor through the drop tube into the receptacle until the receptacle is filled to the level of the outlet of the drop tube, conveying material from the receptacle through the nozzle under the effect of pressure from the fan supplying air to the plenum, conveying the material further through the conveying means into the vessel, feeding the conveyed material onto the vibratory screen, separating acceptable material from reject material by means of the screen, removing reject material through the discharge outlet of the vessel, passing acceptable material through the outlet therefor into the storage receiver, and returning the acceptable material through the further conveying means into the cham berofthefluidised bed reactor.

Conveniently, air is passed through air feed line into the receptacle.

As material is conveyed from the receptacle, further material passes thereinto from the bed provided the valve in the conveying pipe remains open. If it is desired to interrupt the cleaning and reclassification operation, the valve in the pipework is closed whereupon material in the receptacle will build-up to the end of the outlet of the drop tube to seal the same, thus preventing any further flow of material.

The vessel conveniently has a funnel arrangement therein for directing incoming material onto the inlet end of the vibratory screen which is preferably air cooled from the further fan.

The storage receiver is pressurizzed by air from the further fan at a sufficient level to convey material held therein through the nozzle and then into the pipework for return to the chamber of the fluidised bed reactor. The storage receiver may have an inlet for the entry of additional make-up material and the method of the invention includes the step of introducing such make-up material into the storage receiver.

By way of example only, a method and apparatus for cleaning and reclassifying fluidised bed material are described with reference to the accompanying drawing which is a diagrammatic illustration of the apparatus in combination with a fluidised bed reactor.

Referring to the drawing, a fluidised bed reactor in the form of a combustor 1 comprises a chamber 2 having a gas permeable support 4 defining a combustion zone 6 thereabove and a plenum 8 therebelow. An air fan 10 feeds the plenum 8 with air via a duct 12. The chamber 2 has an exhaust gas outlet 14 connected via a conduit 15 to a separator 16, for example a cyclone separator.

The apparatus of the invention includes a receptacle 18 formed in and opening to the plenum 8, an air feed line 20 entering the base of the receptacle and being connected into the duct 12. A drop tube 22 opens into the combustion zone 6 through the support 4 and extends downwardly into the receptacle 18, the size of the drop tube 22 being any size above the minimum size of bed material. Also extending into this receptacle 18 is a nozzle 24 of known design and the nozzle is connected to a pipe 26. The pipe 26 extends from the nozzle 24 into a cylindrical vessel 30 which has an inverted conical base 32 and an upper outlet 34 connected through a line 35 to conduit 15, the line having a valve 72.A funnel arrangement 36 is disposed within the vessel 30 and has its outlet 38 directed overthe inlet end of a vibratory screen 40 located within the vessel 30.

The base 32 of the vessel 30 is divided into two discharge sections 42,44, the section 42 registering with the outlet end of the screen 40 and section 44 disposed beneath the screen 40. The section 42 has a discharge outlet 46 for reject material and the outlet is connected to a pipe 48 having a valve 50.

The section 44 leads to an outlet 52 communicating with a storage receiver 54 via a rotary valve 56.

The storage receiver 54 has extending thereinto a nozzle 58 of known design which nozzle is connected to pipework 60 communicating with the combustion zone 6 of the combustor 1, the pipework 60 including a valve 71. Afurtherfan 62 is provided and in use feeds pressurized air into the receiver 54 and into a cowl 64 surrounding the screen 40 for the purpose of cooling the same. The receiver 54 has a valved inlet 66 for make-up material.

In operation, the combustor 1 has a bed 70 of the inert particulate material, e.g. sand, held by the support 4 within the combustion zone 6 of the chamber 2. The material is fluidised by air from the fan 10 and once start-up has been achieved byway of elevating the temperature of the material 15 to the requisite level, fuel, e.g. coal, is fed to the bed 70 for combustion therein, the combustion gases discharging through outlet 14 and into the separator 16 for cleaning prior to exhaust. During the combustion process, ash is formed and can agglomerate to form particles incompatible with the acceptable size range of material for the efficient operation of the bed.

Furthermore, it is not unknown that the coal feed may contain alien matter, for example adventitious ash, stone, and possibly tramp metal, all of which have a deleterious affect upon fluidisation and efficient operation of the combustor. Accordingly it is necessary to effect removal of reject material by cleaning the bed and reclassifying material in order to return acceptable material to the bed. In the present invention, this is achieved by passing bed material down through the drop tube 22 whereupon it enters the receptacle 18. The material continues to flow until the receptacle 18 is filled to the level of the outlet of the drop tube 22. With the fluidised bed 70 in operation, the pressure in the plenum 8 is higher than that in the separator 16.The valve 72 is opened and material from the receptacle 18 passes into the nozzle 24 and thence into the pipe 26, the amount of material passing into the receptacle 18 being governed by the rate of removal therefrom by the nozzle 24. At the same time, air is injected into the receptacle 18 via feed line 20, the affect being to cool the material therein to a temperature below 300"C.

This air, however, is heated and the heat so transferred is used in the total combustion air for the bed.

The material contained in the pipe 26 is conveyed into the vessel 30, the conveying air and any fine particles passing through outlet 34 and thence to the separator 16. Material drops down into the vessel 30 onto the funnel arrangement 36 which directs the material through its outlet 38 onto the inlet end of the screen 40. The vibratory action of the screen 40, which is cooled by air from fan 62 circulating through cowl 64, causes the acceptable sized mate rial 15 to pass through the screen into the section 44 and the reject material to flow from the outlet end of the screen into section 42 for subsequent discharge through outlet 46, pipe 48 and valve 50. The acceptable sized material in section 44 flows to outlet 52 and descends through the rotary valve 56 into the storage receiver 54, wherein it can accumulate.With valve 71 open, air from fan 62 passing into the receiver 54 causes entrainment of material through the nozzle 58 and into pipework 60 for conveyance back to the combustion zone 6 of chamber 2. The material so returned to bed 70 has been cleaned of alien material and reclassified to a size compatible with that of the other bed particles. If fresh bed material is required for top-up purposes, this can be introduced into the receiver 54 via the valved inlet 66.

It will be appreciated that the flow rate of air from the receiver 54 to the vessel 30 is minimised by the rotary valve 56 and also that the cooling air for the screen 40 has a small upward velocity and will thus not adversely affect the flow of material through the screen. The disposition of the screen 40 wholly within the vessel 30 eliminates the need for expensive sealing and flexible connecting characteristic of conventional systems.

The invention can be used in an in-service mode, that is continuously or intermittently, the valves 71 and 72 being operated accordingly.

Claims (18)

1. Apparatus for cleaning and reclassification of bed material for use in combination with a fluidised bed reactor having a reaction chamber, a gas permeable support within the chamber and adapted to support a fluidised bed, a plenum defined beneath the support, an air fan connected to the plenum, a gas outlet from the chamber, and a separator connected to the gas outlet, the apparatus comprising a drop tube extending from and opening into the gas permeable support to a receptacle, an air feed line from the fan connected into the receptacle, a nozzle extending within the receptacle and connected by a conveying means to a vessel having mounted therein a vibratory screen, an outlet from the vessel for acceptable material, a discharge outlet from the vessel for reject material, a storage receiver connected to the outlet for acceptable material, and further conveying means extending from the receiver and adapted for connection to the chamber of the fluidised bed reactor.

2. Apparatus according to claim 1 in which the receptacle is formed as part of the plenum in the base part thereof, the receptacle opening into the plenum.

3. Apparatus according to claim 1 or 2 in which the air feed line enters the receptacle in the base thereof.

4. Apparatus according to any one of the preceding claims in which the conveying means is a pipe.

5. Apparatus according to any one of the preceding claims in which the vessel has a gas discharge outlet adapted for connection to an exhaust gas conduit extending from the chamber of the reactor to the separator.

6. Apparatus according to any one of the preceding claims in which a further fan is provided for supplying air to the storage receiver.

7. Apparatus according to claim 6 in which the screen is provided with a cooling cowl, and the further fan is adapted to supply air to the cowl.

8. Apparatus according to any one of the preceding claims in which the further conveying means comprises a nozzle extending into the storage receiver and communicating with pipework adapted to be connected to the chamber of the fluidised bed reactor.

9. Apparatus forthe cleaning and reclassification of bed material substantially as hereinbefore described with reference to the accompanying drawing.

10. A fluidised bed reactor in combination with apparatus as claimed in any one of the preceding claims.

11. Afluidised bed reactor according to claim 11 in which the reactor is a combustor.

12. A method of cleaning and reclassifying fluidised bed material using the apparatus claimed in claims 1 to 9 the method including the steps of passing material from a fluidised bed in the chamber of a reactor through the drop tube into the receptacle until the receptacle is filled to the level of the outlet of the drop tube, conveying material from the receptacle through the nozzle under the effect of pressure from the fan supplying air to the plenum, conveying the material further through the conveying means into the vessel, feeding the conveyed material onto the vibratory screen, separating acceptable material from reject material through the discharge outlet of the vessels, passing acceptable material through the outlet therefor into the storage receiver, and returning the acceptable material through the further conveying means into the cham berofthefluidised bed reactor.

13. A method according to claim 12 in which air is passed through the air feed line into the receptacle.

14. A method according to claim 12 or 13 in which the valve in the conveying means remains open during the cleaning and reclassification of bed material, the material continuously passing down the drop tube into the receptacle.

15. A method according to any one of claims 12 to 14 in which the storage receiver is pressurized by air from the further fan.

16. A method according to any one of the preceding claims 12 to 15 in which air from the further fan is passed to the vibratory screen to cool the screen

17. A method according to any one of the preceding claims 12 to 16 in which additional material is fed into the storage receiver and conveyed therefrom to the fluidised bed.

18. A method of cleaning and reclassifying fluidised bed material substantially as hereinbefore described with reference to the accompanying drawing.