GB2164868A - Improvements in or relating to fluidised bed systems - Google Patents

Abstract

In a fluidised bed system 1, material is withdrawn through outlets 6 into ducts 8 incorporating ejectors 16, the ducts feeding into a transport pipe 18 which communicates with an air classifier tube 20 of relatively larger diameter than the pipe 18. Bed material is classified according to size in the tube 20, material of acceptable size being entrained and returned to the fluidised bed 5 and oversize ash particles being discharged from the system. <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 fluidised bed combustion system incorporating means for withdrawing material from the bed during operation and for classifying the material according to size, thereby to separate the coarser from the finer particles which latter are returned to the fluidised bed, the coarser fraction being discharged.

As is well known, solid fuel, e.g. coal, can be burnt successfully and efficiently within a fluidised bed of inert particles, usually silica sand, giving relatively higher heat transfer coefficients than are attainable with more conventional combustion systems. However, one of the problems associated with the fluidised bed combustion of coal is the formation of particles of ash which are larger than the normal bed material and if not removed, can seriously inhibit fluidisation. In practice, the larger and denser ash particles tend to gravitate to the base of the fluidised bed and if a build-up is allowed to occur, at least partial slumping of the bed due to defluidisation results.

Great attention has therefore been given to the conception and development of bed material management systems which will provide an effective means of extracting material from the bed, will convey the material to a classifier for separating the coarer from the finer size fractions, will discharge the coarser fractions from the system into a collection hopper, and will return the finer size fractions to the bed for further use. Various methods of achieving these functional requirements have been proposed and examples are to be found in our co-pending U.K. Patent Applications Nos.

82/06552, 82/35953 and 84/16025 each of which involves some degree of pneumatic conveyance for the particulate material.

An object of the present invention is to provide an approved fluidised bed system incorporating a facility for bed material management.

According to the invention a fluidised bed system includes a reactor body for retaining a fluidised bed of particulate material, a plurality -of outlets for the removal of bed material, a plurality of ducts associated respectively with the outlets, an ejector situated situated in each duct, a common transport pipe with which the ducts are in flow communication, an air mover coupled to the transport pipe, a substantially vertical classifier tube having for at least part of its length a diameter greater than that of the transport pipe, the tube connecting into the reactor body at a location in use above the fluidised bed, and a discharge section associated with the classifier tube.

Conveniently, each duct is of substantially dog-leg form having two substantially vertical portions connected by a horizontal section.

The relatively upper vertical portion communicates with the base of the fluidised bed and together with the horizontal section presents and L formation, the ejector being disposed at the corner of the L and extending into the horizontal section. The relatively lower vertical portion of each duct connects into the transport pipe, to which is connected the air mover, for example a fan supplying conveying air.

The discharge section connects into the air classifier tube, in its larger diameter length at the relatively lower end thereof. A collection hopper may conveniently be provided in the discharge section.

By way of example only, one form of fluidised bed system is described below with reference to the accompanying drawing which is a diagrammatic sectional view of the system.

Referring to the drawing a fluidised bed system is shown generally at 1 (with parts omitted for clarity) and is intended to be used in a combustion mode for burning solid fuel, e.g.

coal. The system includes a reactor body 2 provided with a distributor plate 4 for introducing a fluidising medium, e.g. air, into a bed 5 of particulate solids, for example silica sand, supported by the plate. A plurality of outlets 6 is provided in the plate 4 for the egress of bed material and each outlet communicates with a duct 8 of dog-leg form having relatively upper and lower portions 10, 12 respectively interconnected by a horizontal section 14. The portion 10 and section 14 together present an L formation and at the corner intersection thereof is disposed an air ejector 16 extending into the section 14 from the exterior thereof.

The lower vertical portion 12 of ducts 8 are teed into a common transport pipe 18 connected to a positive pressure fan (not shown) at the end thereof. The pipe 18 extends into a substantially vertical classifier tube 20 which for at least part of its length has a diameter greater than that of the pipe 18. The classifier tube 20 is connected into the reactor body through a length of horizontal tube 22 (usually but not necessarily of a smaller diameter than tube 20).

A discharge section 30 communicates with the tube 20 and includes a conduit 32 terminating in an airtight collection hopper 34.

In operation, the fluidised bed system 1 is operated as a combustor, coal or other solid fuel being burnt within the bed 5 to generate heat which may be used to generate hot water or steam. During the combustion process, ash is formed within the bed 5 and this can have a particle size greater than that of the bed particles, i.e. sand, size. In the present invention, material from the bed containing a mixture of sand, carbon and oversize ash, is withdrawn either continuously or periodically through the outlets 6 into the ducts 8. The ejectors 16 initiate, control and promote material flow in the ducts 8, the material descending through the lower portions 12 of ducts 8 into the common transport pipe 18.

Conveying air supplied by the fan (not shown), at a velocity for example of 30ms l, entrains the particles and transports them along pipe 18 into the classifier 20 which is of larger diameter than the pipe 18. In one example, the pipe diameter is 75mm and the tube diameter is 135mm. Upon entering tube 20 the velocity falls to about lOms ' because of the increase in diameter and classification of the particles occurs in tube 20, the larger, i.e.

oversize, particles descending through the conduit 32 and thence into the hopper 34, and the material of acceptable size being transported pneumatically through tube 20 and tube 22 into the reactor body 2 for return to the bed 5.

In practice, the outlets 6 are arranged over the cross-sectional area of the bed and with the common transport pipe 18 present a more compact system than those proposed hitherto.

Claims (1)

1. A fluidised bed system including a reactor body for retaining a fluidised bed of particulate material, a plurality of outlets for the removal of bed material, a plurality of ducts associated respectively with the outlets, an ejector situated in each duct, a common transport pipe with which the ducts are in flow communication, an air mover coupled to the transport pipe, a substantially vertical classifier tube having for at least part of its length a diameter greater than that of the transport pipe, the tube connecting to the reactor body at a location in use above the fluidised bed, and a discharge section associated with the classifier tube.

2. A system according to claim 1 in which each duct is of substantially dog-leg form having two substantially vertical portions connected by a horizontzal section.

3. A system according to claim 2 in which the relatively upper vertical portion communicates with the fluidised bed and together with the horizontal section presents an L formation, the ejector being disposed at the corner of the L formation and extending into the horizontal section, the horizontal section then extending into the lower vertical portion of each duct which connects into the transport pipe.

5. A system according to any one of the preceding claims in which the discharge section connects into the air classifier tube at the relatively lower end thereof.

6. A system according to any one of the preceding claims in which an airtight collection hopper is provided in the discharge section.

7. A system according to any one of the preceding claims in which a collection hopper is provided in the discharge section.

8. A fluidised bed system substantially as hereinbefore described with reference to the accompanying drawing.