GB2182578A

GB2182578A - Improvements in or relating to fluidised beds

Info

Publication number: GB2182578A
Application number: GB08526966A
Authority: GB
Inventors: Maurice John Fisher; Michael Alan Vickers; Michael Robert Stayte
Original assignee: University of Surrey
Current assignee: University of Surrey
Priority date: 1985-11-01
Filing date: 1985-11-01
Publication date: 1987-05-20
Also published as: GB8526966D0

Abstract

A fluidised bed distributor plate 2 is of membrane construction incorporating heat exchange tubes 4 connected into the working fluid circuit of a boiler. The tubes 4 may be positioned alternatively above or below the membrane. <IMAGE>

Description

SPECIFICATION Improvements in or relation to fluidised beds This invention concerns improvements in or relating tofluidised beds.

In particular, the present invention has reference to an improved fluidised bed distributor for use, for example, in a fluidised bed combustor.

Many different types of distributors are known in the fluidised bed art but currently, and for some time past, the most widely used distributorfor combustors has included a plate provided with stand pipes perforated at or neartheirfree ends for the egress, in use, of a fluidising medium which in the case of a combustor, will be air. It is also known to incorporate in the distributor a means for cooling and accordingly heat exchange tubing is provided. In this latter respect, a membrane construction has been adopted whereby heat exchange tubes are interconnected by membranes suitably perforated forthe reception of standpipes. Sandwich constructions are also known where the coolant passes between upperand lower plates ofthe distributor.

When fluidised bed combustion is used forthe generation of heat within a boiler, it is of course necessary to provide a means of extracting heat fro the bed to heat the working fluid which might be water or a thermal fluid. For this purpose it is conventional practice to incorporate within the bed or the expanded bed area an appropriate array of heat exchange tubes which communicate with the working circuit of the boiler. However, one problem attendant upon the operation of fluidised beds is that of particleimpingementuponthetubesurfacewhich occasions erosion. In view of the usual density of tubing arrays, it is thought that the close proximity of juxtaposed surfaces enhances the incidence of erosion by increasing inter tube gas and particle velocities.

An object of the present invention is to provide an improved fluidised bed apparatus and a distributor therefor.

According to the invention a fluidised bed apparatus includes a body for containing a fludised bed of particulate material, a working fluid circuit within the body and a distributor plate located in the relatively lower part of the body, the distributor plate being of membrane construction having a plurality of horizontai tubes interconnected along their length by membranes provided with apertures there through for the passage offluidising medium, the tubes being connected into the working fluid circuit whereby in use at least some heat generated in the fluidised bed is extracted by working fluid passing through said tubes.

The distributor plate may be formed such that a substantially flat surface is presented to the fluidised bed and this is achieved by attaching the membranes tothetubesintangential manneratthetopboundary curvatures of the tubes. With this form of plate, circulation or migration of particles across the distributor plate is enhanced and this is of particular value in the case of a fluidised bed combustorfor oversize ash particles as an aid to bed cleaning.

In an alternative construction of the distributor plate, the tubes are interconnected bythe membrane disposed and secured diametrally ofthetubes.

In a still further alternative, the membranes are secured in tangential manner at the lowermost boundary curvature of the tubes thereby exposing the greatest surface area for heat transfer.

The amount of heat extracted by working fluid passing through the tubes will be determined bythe designed rating of the fluidised bed apparatus, but ideally a substantial proportion, for example 40%, of all the heat generated in the fluidised bed would be so extracted. In this way the problem of in-bed tube erosion would be minimised. However, it is envisaged that some in-bed tubing might be required.

The membranes are provided with a plurality of apertures aligned in a row or rows length-wise thereof. Where more than one row of apertures is provided between the tube rows the same or separate sources forthe fluidising medium may be provided in the apparatus. If adjacent rows of apertures are supplied separately with fluidising air then reduced load can be obtained by removing the air supply to selected rows of apertures such that fluidisation and heat transfer for the cooling tubes are reduced. For example, 50% output can be obtained by supplying air only to alternate rows of apertures. By performing this strategy of load control instead of the use of continuous modulation of the air flow rate throug h all the apertu res, it is possible to design the distributor plate with lower pressure drop characteristics.

By way of example only, fluidised bed apparatus according to the invention incorporating three forms of distributor plate is described below with reference to the accompanying diagrammatic drawings in which: Figure lisa sectional end view of a firstform of distributor plate; Figure2 is a sectional end view of a second form of distributor plate; Figure 3 is a sectional end view of a third form of distributor plate.

Referring to Figure 1, there is shown a distributor plate 2for installation within a body offludised bed apparatus (not shown), the plate supporting in use a fluidised bed of particulate material and defining beneath it a plenum chamberforthe supply of fluidising medium, e.g. air.

The distributor plate 2 comprises a plurality of heat exchange tubes 4 interconnected by membranes 6 which are perforated as at 8 along their length to provide aperturesforthe passagethere-through of fluidising medium. A single row of apertures 8 is aligned along the length of each membrane which is secured along its marginal edges to the tubes by means for example of welding . In the first embodiment the membranes 6 are secured atthe diametral level ofthetubes4suchthat approximately half the heat exchange surface is above and half is below the plane of the membranes.

In Figure 2, the membranes 6 are secured tangentially at the top boundary curvatures of the tubes 4thus giving asubstantiallyflatsurfaceon which in usethe bed is supported. In practice, for example in the combustion process, the flat surface, in the absence of conventional obstructive standpipes, promotes cross migration of particles which is particularly advantageous for the removal of large ash particles from the bed.

In Figure 3, the membranes 6 are secured tangentially at the lower most boundary curvature of the tubes 4whereby substantially the whole circumferential surfaces of the tubes 4 are presented at the base of the fluidised bed for heattransfer.

In alternatives of all these embodiments, more than one row of apertures may be provided in the membranes 6 between the tubes 4.

With all these embodiments, the diameter of the tubes 4 is selected such that a substantial proportion ofthe heat generated in a fluidised combustion bed supported bythe distributor plate 2 is extracted through heattransferto a working fluid, for example water, flowing through the tubes 4.

The arrangement of the present invention thus enables a significant reduction, if not complete removal, of the usual in-bed tubes thus reducing the incidence of erosion.

Claims

1. Afluidised bed apparatus including a body for containing a fluidised bed of particulate material, a working fluid circuit within the body and a distributor plate located in the relatively lower part ofthe body, the distributor plate being a membrane construction having a plurality of horizontal tubes interconnected along their length by membranes provided with apertures therethrough for the passage offluidising medium, the tubes being connected into the working fluid circuit whereby in use at least some heat generated inthefluidised bed is extracted by working fluid passing through said tubes.

2. An apparatus according to claim 1 in which the distributor plate is formed such that a substantially flat surface is presented.

3. An apparatus according to claim 2 in which the membranes are attached to the tubes in tangential manner at the top boundary curvatures ofthe tubes.

4. An apparatus according to claim 1 in which the tubes are interconnected by the membranes disposed and secured diametrially ofthetubes.

5. An apparatus according to claim 1 in which the membranes are secured tangential manner at the lowermost boundary curvature of the tubes.

6. An apparatus according to any one of the preceding ciaims in which a plurality of apertures is provided in the membranes and aligned in a row or rows.

7. An apparatus according to claim 6 in which more than one rowofapertures is provided in each membrane and the same or a separate source of fluidising medium is provided for each row.

8. Afluidised bed apparatus substantially as hereinbefore described with reference to Figure 1,2 or3 of accompanying drawing.