CA1113307A

CA1113307A - Fluidised bed combustion

Info

Publication number: CA1113307A
Application number: CA318,135A
Authority: CA
Inventors: William M. Urquhart
Original assignee: Babcock International Ltd
Current assignee: Babcock International Ltd
Priority date: 1977-12-17
Filing date: 1978-12-18
Publication date: 1981-12-01
Also published as: SE7812932L; NL7812228A; DE2854370A1

Abstract

A B S T R A C T

This invention relates to fluidised bed combustors and provides for the operation of such combustors so that the bed forms an upper fluidised layer and a lower, more static, layer into which larger particles may fall to be drained away.

Description

~33~7 rhis invention relates to fluidised bed combustors and provides a method of operating a fluidised bed combustor in a way ~hat may enable some of the ancillary equipment previously considered comcomitant to the satisfac~
tory operation of fluidised bed combustors to be dispensed with.
According to the present invention, there is provided a fluidised bed combustor operating with a fluidised layer overlying a lower, static, layer contai.ning materi.al of dimensions greater than those that are maintained in the fluidised condition in the fluidised layer and into which material from the fluidised layer may fall, the lower layer being supported from below by a floor and there being distriubted over the floor a plurality of nozzles through which combustion air is supplied to flow upwardly through the lower layer, and to be discharged into the lower layer at a level that is nearer to the top than the bottom of the lower layer.
By way of example, embodiments ofthe invention will now be ~:.
described with reference to the accompanying, somewhat schemati.c, drawings, in which: ;
Figure 1 illustrates fluidised bed combustion apparatus;
Figure 2 is a detail of Figure 1, showing the fluidised bed supported on a floor that is in the form of a flat and horizontal distributor plate from which air discharge nozzles e~tend upwardly;

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~3~f' Figure 3 illustrates a nozzle included in the apparatus illustrated in Figures 1 and 2;
Figure 4 illustrates a modification of the nozzle arrangement illustrated in Figure 3;
Figure 5 illustrates a modification of what is shown in ~igures 1 and 2 in which -the distributor plate,in effect, -forms a number of hoppers;
Figure 6 illustrates a modification of what is shown in ~igure ~ in which the distributor plate is flat but inclined;
Figure 7 illustrates a modi-Eication of what is shown in ~igure 1 i~ which the continuous distributor plate is dispensed with; and Figure 8 illustrates another modification in which the velocities at which air is discharged at different zones of the bed are differen-t.
The fluidised bed combustion apparatus shown in Figure 1 includes a chamber ~ arranged for overhead firing. Hot gases rising from the bed pass through a passage A de~ined by vertical fluid heating tubes B, and then laterally across the fluid heating tubes C to a boiler gas take-off D. In the take-off, the exhaust gases may heat means ~not sh~wn) in which air that is subsequently supplied to the fluiclised bed is heated.
The floor o-~ the chamber 2 is formed by a flat~
horizontal, distribitor plate 1. The distributor plate 1 also forms the upper boundary o~ a wind box, or air chamber, 3 and parallel nozzles 4, of uniform height, extend upwardly from the clistributor plate 1. Above the nozzles 4, there extend cooling tubes 7 through which a fluid can be passed to extract heat from the fluidised bed.
~ rom locations spaced about the distributor plate 1, there are ash pipes 8 through which material from the bed can be drained off. The bottom 9 of the wind box 3 - 35 is in the ~orm of a hopper and an ash pipe lO leads from ", .. .
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from -the lowest point of the bottom 9. The ash pipes 8 and 10 discharge through an ash feeder. Each ash pipe is valve controlled and means (not shown) may be provided for opening the ash pipes in any desired sequence.
In operation of the apparatus shown in ~igures 1 and

2 solid fuel is fed to the bed from above and the bed may contain pieces of a size larger than would normally be considered acceptable having regard to the fact that combustion itself may not be effective to prevent the formation of large ash particles and such particles may not either be reduced in size by ~rosion. Such larger particles will, in operation, settle into a lower, compara-tively static, layer 11 resting on the dis-tributor plate 1 whilst the lighter part of the material in the bed will form a fluidised combustion layer 12 above it.
Material is drained through the ash pipes 8 at such a rate as to ensure that the upper level of the lower layer lies a little above the air nozzles 4. In this way it can be ensured that tubes 7 lie continuously wholly within the fluidised layer 12 with better heat exchange than might be expected if the -tubes were in contact with the heavier, more sta-tic, material.
Since the apparatus can operate with the supply to the bed of particles of a size -that it would previously have ~een preferred to avoid, the expense of equipment previously provided to eliminate them can be avoided.
In particular, it is possible to use unwashed coal that has not been treated to prevent the admission to the bed of large particles, so that the costs, both in the capital cost of ancillary equipment and the cost of running the equipment can be reduced.

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Figure 3 shows one of the air nozzles 4. The nozzle 4 is in -the form o:E a tube 14 closed at its upper end except for circum~erentially distributed holes 15 through which a major part o~ the air used in ~ludising, and supporting combustion in, the upper layer 11 is supplied.
At the lower end of the nozzle 4, just above -the dis-tributor plate 1, are ~urther holes 16 through which air escapes at a velocity lower than that escaping through the holes 15 and serving mainly to cool the ash in the lower layer 10 with the result that the problems o~
handling the ash are reduced. A-t the same time, a degree o~ pre-hea-ting is imparted to the combustion air, and -this tends to greater ef~iciency. It will be seen that the locations ~rom which air is discharged to the ~luidised bed are all higher than the inlets -to the ash pipes from the fluidised bed.
As is illustrated in ~igure 4, an e~-fect comparable to that obtained by the nozzles shown in Figure 3 can be obtained by using nozzles 20 and 2t which each has an opening only at its upper end, but the nozzles 20 being longer than the nozzles 21. The upper ends of the ~ormer lie just below the ~luidised layer 13 whilst the upper ends of the latter lie just above the distributor plate 1.
Xn the modification shown in ~igure 5, the ~loor 25 of the fluidised bed is in the ~orm o~ a number o-E hoppers 26 such that material in the lower layer 11 must tend to drain towards the bottom o~ one or other o~ the hoppers 26. There is an ash pipe 8 leading ~rom the lower end of each o~ the hoppers 26. Air is supplied ~rom a number oi wind boxes 27. The wind bloxes may be connected to-gether so that air is supplied at the same velocity to all the nozzles 4. Alternatively t each wind box 27 may ..
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be separately controlled so that the air supply across the fluidised bed can be ~a~ied.
The embodiment illustrated in Figure 6 differs from that in Figure 2 essentially in that the flat bottom, formed by the distributor pla~te 30, slopes continuously so that the withdrawal of ash is effected solely from ~he lower end of the distributo~ plate 30. With this arrangement, ~he link of the noz%les 4a increase progressively aCTOss the width of the bed; ~hey are shown to be supplied from a series of wind boxes 31 but these could be replaced by a single wind box, as is shown in Figure 2 In the modification illustrated in Figure 7, the floor 30 of the chamber is in the shape of a series of hoppers 41 and there are no air nozzles projecting upwardly ~rom a distributor plate. Each hopper 41 opens into a duct 42 through which ash is conveyed away, and the air that passes through the fluidised bed is int~oduced through ducts 43~ each concentric within one of the ducts 42. Ihe upper end of each duct 43 is domed, at 44, leaving a gap between the dome 44 and the upper end of the duct 43 through which air can difuse into the static layer 11 through a more-or-less continuous circle.
The method illustrated ~y Figure 7 is likely to be of a special value in the firing of heavy coal particles, or when limestone containing heavy particles is delivered to form a substantial part of the fluidised bed.
Figure 8 illustrates a modification that also uses an in~ention that is separa~ely claimed in our co-pending Canadian patent application No.
318,134 filed on December 18, 1978 in the name of Babcock ~T Wilcox Limited.
In that invention, the velocity at which fluidising air is supplied to one part of the bed is different from that which air is supplied to an adjacent pa~t of the bed, with the result that material from the part of the bed to which air is supplied at one velocity will tend to move ~owards the part to -; which material is supplied at the other velocity.

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In the particular embodiment illustrated in Figure 8, the floor is in the form oi a distributor plate 51 having parts 52 at a level higher than that of other parts 53.
; The upper ends of the nozzles 54 Prom al.l par-ts of the distributor plate 51 are at the same level, having their outlets just below the top of -the lower, static, layer 11.
; The air supplied to the shorter no~zles 54b is at a lower velocity than that of the air supplied to the longer nozzles 54a. Moreover, there are outlets only at the upper ends of -the shorter nozzles whilst the longer nozzles are also provided with outlets, as are shown in Figure 3, just above the distributor plate 51. A result of the velocity difference is that mixing between different parts of the upper layer 12 is promoted so that a smaller pro-portion of grits that contain carbon are carried out oithe bed to the boiler. At the same time, it is possible -to refire grits that have been carried to the outlet of the boiler to the area of lower fluidising velocity, which results in a longer residence time and, therefore, a greater retention and combus-tion o~ grit in the bed. This results in increased combustion effiGiency of small particles in both coal fines and re~ired grits.
It will be seen thatin all the embodiments that have been described, the locations at which the, or the major par-t, of the fluidising air is supplied l.ie above the level at which ash i.s removed from the fluidised bed.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A fluidised bed combustor operating with a fluidised layer overlying a lower, static, layer containing material of dimensions greater than those that are maintained in the fluidised condition in the fluidised layer and into which material from the fluidised layer may fall, the lower layer being supported from below by a floor and there being distributed over the floor a plurality of nozzles through which combustion air is supplied to flow upwardly through the lower layer, and to be discharged into the lower layer at a level that is nearer to the top than the bottom of the lower layer.

2. A fluidised bed combustion as claimed in claim 1 in which each nozzle is provided with two outlets, one above the other and the air that flows upwardly through the lower layer is discharged through the lower outlets and the air that is discharged at a level that is nearer to the top than the bottom of the lower layer is discharged through the upper outlets.

3. A fluidised bed as claimed in claim 1 in which the air that flows upwardly through the lower layer is dis-charged from the nozzles of one group and the air that discharges at a level that is nearer to the top than the bottom of the lower layer is discharged from the nozzles of a second group.

4. A fluidised bed combustor as claimed in claim 1, 2 or 3, in which fluid heating tubes lie above the lower layer and are immersed in the fluidised layer.