CA1165195A - Fluidised bed boilers - Google Patents

Abstract

TITLE: "FLUIDISED BED BOILERS"
APPLICANT : WORLD ENERGY RESOURCES CONSULTANCY SERVICE
(PTY) LIMITED

ABSTRACT OF DISCLOSURE

This invention relates to boilers and in particular to fluidised bed boilers and burners, and to a method of controlling such boilers and provides a base plate which is water cooled during operation by virtue of having the combustion chamber located within the water jacket of the boiler. The invention provides a method and apparatus for reconditioning the bed, thereby reducing the necessity of topping up the bed with new sand or a similar carrier material as well as an accurate system of controls for the boiler. A vertical and a horizontal boiler incorporating the above mentioned features are described.

Description

This invention relates to boilers and in particular to fluidised bed boilers and burners, and to a method of controlling such boilers. In ~luidised bed boilers the bed is normally external to the water circuit when the fluidised bed burner is used for steam raising. At most, a shell boiler is used in which a recessed combustion chamber is formed in the wall which receives the base plate by means of high-temperature seals to prevent the flow of air around the outside edge of the base plate. The plenum is attached to the base plate by further high-temperature seals. Whilst the base plate can be shielded from the combustion temperature by a quiescent layer of the carrier to some extent, the base plate does become hot since it is only cooled by the flow of the primary combustion lS air and tends to distort putting stress on the seals.

It is an object of this invention to overcome this problem by providin~ a base plate which is cooled during operation.
It is a further object of this invention to provide a method and apparatus for reconditioning the bed, thereby reducing the necessity of topping up the bed with new sand or a similar carrier material.

~n fluidised bed burners which use solid fuels such as coal, a certain amount oE ash and other particles which are larger than the carrier material particles are left behind after combustion. The ash, of course, is largely elutriated in the air stream, but i-t is necessary, from 1~6~3~

time to time, to remove the large particles from the bed in order to improve its characteristics since too great an amount of large particles will af~ect the heat transfer characteristics of the bed~ In the past these particles have been removed by means of sieving, but it will be appreciated that the operation of sieving is accompanied by difficulties caused by the temperature of the bed itself and its propensity to flame during the sieving operation.

The use of a water-cooled base plate and the reconditioning of the bed allows the use of an accurate system of controls although it will be appreciated that the con-trol system is not dependent on the use of either the cooled base plate or the reconditioned bed and may find application in conventional fluidised bed boilers or heaters.

It is a further ob~ect of this invention to provide a vertical and a horizontal boiler incorporating the above mentioned features.

It will be appreciated that, while this invention is described with the reference to the boi.lers suitable for producing steam, it will find equal application in heaters or burners for the production of hot wa-ter or for incineration and the provision of hot gas for drying purposes.

According to one aspect of the present i.nvention, a boiler is provided with a combustion chamber base plate having i5~S

an upper surface adapted to suppor~ a fuel bed and a lower surface spaced apart from a wall of the housing to define a water jacket, and an air plenum including one or more air passages extending ~hrough the base pla~e for connecting the plenum with the interior of the combustion chamber.
The base plate can be a block containing a plurality of ducts through which the cooling water can be pumped; if the water is taken from, and returned to, the boiler no heat is wasted. However it is preferred that the surfaces are constituted by two spaced apart members so that the space between the members can form part of a water jacket completely surrounding the combustion chamber excep~ for necessary openings such as those required for the intro-duction of fuel for instance. The water can circulate under convection in such a water jacket and in practice the water jacket could be the shell of a fire-tube boiler. The combustion chamber may further be penetrated by a plural-ity of thermosyphon tubes.
In fluidised bed burners, there is usually a quiescent layer of the carrier created by introducing the primary combustion/fluidising air into the b0d above this quiescent layer through sparge pipes or stand pipes and another aspect of this invention provides a fluidised bed burner having a base plate with upstanding combus~ion air stand pipes in which at least some of the stand pipes include or have associated therewith air flow control devices, each device i5~

being individual to a stand pipe and at least some of the devices having a common operating means.

In a further aspect of the pre ent inve~tion the fluidise~
bed burner has an auxiliary fuel introduction means leading into the bed.

The auxiliary fuel introduction means may conveniently be for fuels which, because of their lightness or size would be rapidly elutriated by the combustion air flow, for example, waste straw,sawdust, or coal dust and for this .... .~ .. ~ "
purpose the auxiliary fuel introduction means may comprise a pipe extending into the fire bed preferably at the level of the upper surface of the quiescent layex, in which pipe the auxiliary fuel would be at least partially burnt. The normal fuel would be in the form of lumps of coal bu-t all lS coals contain a certain amount of coal dust ancd this is elutriated and separated out from the combustion gases in exhaust cyclones along with the elutriated ash; the elutriated ash can be recycled so that the one or two percent elutriated coal dust therein can be reclaimed. The auxiliary fuel should preferably be injected along with air. The auxiliary fuel introduction means could alternatively be desi~necl for the introduction of a liquid Euel which Ini~ht otherwise be difficult to burn.

This fuel would be in~ected into the quiescent layer and wet it (the ash compoent in particular, would soak it up and act as a wick) and on reaching the upper surEace of the j i5~35 qui.escent layer some of the oil wetted ash would break away and enter the active fire region. The quiescent layer, although not agitated by the air flow, i5 in contact with the seething mass of the active fire and this, and a bombardment by falling fuel, ensures that the ~-quiescent layer is not static. If the fuel is injected under pressure and is such as to degrade, forming a skin, the fuel pressure will break up the skin as it forms.

Quiescent layers are created in most fluidised bed burners by introducing the combustion air some way above the true base plate by means of stand pipes or sparge pipes.
It is poss:ible to in-troduce the air through a plane perforated plate but this loses the insulating effect of the quiescent carrier layer.

In yet a further aspect of the invention a fluidised bed burner comprises a combus-tion chamber and an air plenum separated by a plane perforated plate and a layer of coarse bodies resting on the plate.

The coarse bodies would be resistant to elutriation and could be dropped onto the plane plate along with an easi.ly elutriated carri.er such as sand and alumina and then separated into a lower layer o the coarse bod.ies supporting the finer carr:ier by blasting air through it.
If -the bodies are c~raded in size, possibly progressively, it should be possible to grade the layers of bodies and -~ ~l65~S

carrier b~ blasting air therethrough at an abnormally high rate so that as to get gradi~g by elutriation.

According to another aspect of the invention a method of conditioning a fluidised bed includes the steps of transferrin~ the bed to a crusher adapted to reduce the particle size of material other than carrier material (i.e. inert incombustible material) to a size approximating the particle size of the carrier material, and returning the crushed material to the bed.

The crushed material may be returned by any one of a number of known transport methods, but in a preferred form of the invention the crushed material is collected by way o:E
a venturi and pneumaticall~ transported to an inlet in the burner above tlle plate supporting the bed.

The invention includes a control method for fluidised bed heaters in which method at least a first parameter, being the pressure of the steam raised or the temperature o~
the water being heated, and a second parameter, being the bed temperature, are sensed and applied to regulate the fuel feed, the ~irst parameter being applied in steps to limit the maximum range over which fuel ma~ be fed and the second parameter being app:Lied to regulate the feed in that range.

~lso according to the invention a method of controlling combustion in a fluidised bed heater comprises the steps of:
.

~i5~

sensing the first and second parameters and a third parameter, being the free-board gas pressure in the fluidised bed heater, at chosen number of stepped ~evels of the first paramete~, allowing the fuel feed to the bed to operate over given ranges in steps with the first parameter steps;
at the chosen steps of the first parameter dampening the flow of exhaust gases in the same number of steps;
controlling the inlet for the fluidising air or gas to the bed also in steps which result from a pneumatic connection derived from the above bed combustion chamber gas pressure in response to the same number of chosen steps in the third parame-ter;
and controlling the fuel feed in the chosen range allowed at any time in response to the second parameter.

The invention also provides that on st:art-up a special start-up control circuit should override the control system outlined above until a predetermined bed temperature has been reached.

Embodiments of the invention are now described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a schematic vertical section of a horizontal boiler according to the invention;
Figure 2 is a section taken on line II-II of Figure l;

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Figure 3 is an enlarged detail showing a combined stay and primary air supply pipe;
Figure 4 is a similar detail showing a simple primary air supply pipe;
Figure 5 is a similar detail showing an air supply pipe which includes a flow control device;
Figures 6 and 7 illustrate auxiliary fuel introduction means;
Figure 8 illustrates a base plate structure;
Figure 9 is a flow diagram of an apparatus for a fluidised bed burner incorporating a crusher;
Figuré 10 is a section in side elevation of a vertical boiler;
Figure 11 is a section taken on line II-II in Figure 10;
and Figure 12 is a diagrammatic illustration of a fluidised bed heater and its associated control circuits.

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Figures 1 and 2 show a horizontal shell boiler enclosed by an outer shell 11 within which a fluidised bed reactor chamber 12 is wholly contained. The base plate 14 of the reactor chamber, on the upper surface of which rests the bed, is spaced apart from the lower part of the shell 11 by stays 15 leaving the space be-tween the plates 14, and the lower part of the shell 11, open to the water jacket surrounding the reactor chamber 12. Further stays 16 are used to locate the chamber 12 which is of consider-able height so that the carrier or bed material of sand, alumina or the like is not over-prone to elutriation (entrainment in the air stream). To avoid the large size of the chamber blanking off water circulation~ a plurality of water tubes 17 cross through the chamber at an inclination to the horizontal and at various bearings to induce convective flows of water. If the boiler is used with a chimney generating an induced draught, an exhaust gas turbine can be installed. Banks of fire or smoke tubes 18 lead off from the combustion chamber to deliver heat from the combustion gases to water in the boiler. The banks can form a single or a multiple pass as shown in the drawings.
It will be seen that the chamber is surrounded by water. The base plate 14 is shaped so that any steam bubbles form-ed on its water side will float away and will not impair the heat transfer characteristics, and can be contoured for strength or other purposesO A series of primary air tubes 20 extend between the two members to conduct primary air from a forced draught plen-um 21 attached on the outside of the shell into ~6~

the combustion chamber 12, some of these air pipes can serve as stays 15, which pipes 2Oa are shown in Figure 3.
Each air pipe has a bore 22 and receives a standpipe 23 which in Figure 3 is screwed directly into the air pipe and S in Figure 4 which a non-structural standpipe 20b is shown, - provided with a threaded collar which screws onto the air pipe. The standpipes are of heat-resistant material and varying lengths of standpipes can be fitted to suit the ~uel to be burnt and other relevant factors. Each standpipe has its upper end blanked olf and has holes in the sides.
In Figures 3 and 4 the upper ends are blanked off by an umbrella plate 25 so that when the bed .is slumped, without an air flow preventincJ the carricr entering the holes, the umbrella plate creates a clear space with the holes being in the clear space. The size of the umbrella plate will, of course, depend on the angle of repose of the particular carri.er material used so that the clear space is in fact large enough to leave the holes ree.

Fi~ure 5 shows another design o~ standpipe which does not require the top closure -to be in the form of an umbrella plate. This has a succession oE air outlet holes 27 in an outer tube 28 which hole~ can be pro~ressively blocked off by a valve device which comprises a tube 29 slidably located within the bore of the air pipe 21 associated with the standpipe. Vpward movement of the inner tube 29 will block off the holes 27 which may be staggered -to give an infinitely variable blocklng off action. ~he end o~ the tube -3~ s 29 may be cut on an incline to give the same effect. The lower end of the tube 29 is blanked off by a plate 30 above which there is a plurality of air inlet apertures 31. When the tube 29 is fully inserted, a seal on the plate 30 sealingly engages a seat on the lower end of the air pipe 28, thereby sealing off the aperture 31 within the air pipe 28. A common activating means for the tubes 29 comprises a regulator plate 32 within the plenum which can be displaced up and down by suitable means sealingly entering the plenum. This regulator plate can be perforated or otherwise adapted to balance out any unevenness of air pressure within the plenum or be connected adjustably to the various devices by flexible couplings 33 so as to allow each standpipe to ta~e equal lS amounts of air.

It will be appreciated that use of the cooled base plate results in a number of advantages. Meat is delivered through the base plate itself from the fluidised bed so increasin~ the heat transfer 5urface area and owing to the cooling, the base plate is less liable -to distort so that a relatively inexpensive material can be used. The shell is also water cooled so that the problems associa-ted with the seal between the plenum and the prior art base plate no longer arises, an~ this enables the use of a higher forced draught pressure. In fact a hiyher pressure throughout the air flow path can be used. A higher combustion pressure with the resultant greater oxygen content concentration, leads to more intense cornbustion and thus a smaller boiler. The advantages are more pronounced in the so-called shallow bed versions of fluidised bed combustion.

To allow a higher combustion pressure to be used) a fuel inlet hatch-37 in the outer wall of the shell and the combustion chamber is fitted with a fuel feed device which limits blow-back of fuel on failure of combustion as well as limiting ingreSS of air additional to that required as secondary combustion air.(see Figure 1) The use of flow controlling standpipes also gives many advantages. One advantage is the elimination of the need to have an umbrella plate on each s-tandpipe which permits closer spacing of the standpipes for intensely active shallow bed burners. However the main advantage of regulating each standpipe instead of, or even as well as, using a common damper i5 that the air flow is more evenly shared between standpipes and regulating down t~e air flow does not change the balance of the air flow between standpipes.

It is thought that the life of the base plate 14 will be extremely long not only because of :its cooling but also because the carrier or hed material will after initially polishing the upper face of the member tend to plate out giviny a wear and heat resistant surface.

In Figure 6, a series of separators 39 are provided in the exhaust circuit for separating out any solids elutriated out of the reactor chamber 12, which solids will be either ash or unburnt fuel dust. The use o~ multiple separators such as cyclones would permit the various fractions such as the heavier fuel particles from the cyclone 39a, ash from the cyclone 39b and light fuel particles from the cyclone 39c, to be individually separated or at least for some ~ractions to be richer in unburnt fuel than others. At least the fuel rich fractions are fed back into a hopper 40 into which can also or alternatively be fed any liyht fuels li.kely to be elutriated such as straw, waste and sawdust. The contents of the hopper are allowed to fall or are fed into a stream of air which can be derived from the forced draught fan (not shown) for feeding the plenum 21. A tube or retort 41 leads into the active fire region preferably just above the quiescent layer ~nd the stream of air with entrained fuel particles is directed into and through this tube which is of course hot,so that the particles are heated whilst they pass along the tube and are at leas-t partially burnt therein. This fuel return system can be incorporated into the reconditioning system described below. (shown in dotted out].ine in Figure 9).

The auxi].iary fuel i.nt.roduction means of Fi.yure 6 is suitable for burning solid liyht fuels and Fiyure 7 shows another auxi.liary fuel introduction means suitable for liquid fuels, especially those which may be difficul-t to burn because a65~5 of a high flash point or a tendency to clog normal nozzles. The fuel is introduced under pressure by means o~
a pump 42 with an outlet 43 leading into the quiescent layer of the fluidised bed that is below the level of the air out7ets from the standpipes or sparge pipes.
The ash component of the quiescent layer soaks up the liquid fuel acting similarly to a wick in distributing the liquid fuel throughout the quiescent layer. On reaching the turbulent upper surface of the quiescent layer some of the wet ash is broken ofE and burnt in the active fire region. If any skin tends to form due to degradation of the liquid fuel, the skin will be bro~en away by the fuel pressure whilst the skin is still forming.

Figure 8 shows an alternative base plate structure. In thi.s structure, a quiesc~nt layer is not formed by means of stand or sparge pipes but th~ base plate proper 4'l is a plain perEorated plate which is thermally insulated from the active fi.re re~ion 45 by a layer of coarse xefractory bodies 46 resistant to elutriation. Above this layer of bodies 46 -there is the~usual active fire region consisting of :Euel and carrier~ The carrier can be charged into the chambe.r along with the heavier bodies and the graded layers can be Eormed by elutriation hy a high pressure air stream. Alternatively, the heavi.er hodies can be introduced firs-t and then the carrier added with or without an air current.

~:~65~

The gaps between the coarser heavy bodies must be sufficiently small forthe carrier to be unable to seep down through the gaps and through the perforations in the base plate proper. This can be done by judicious grading.

As is shown in Figure 8, the base plate 44 can be spaced from the shell 11 by the air pipes 20 which would be welded to the shell and the base plate 44, or c~lternatively, a unitary, prior art hase plate may be used which relies solely on the refractory material 46 for cooling.

A bed reconditionin~ sys-tem is shown in Figure 9 to include a fuel inlet 60 to -the reaction chamber 12. When the average pa.rticl.e size or density of the bed material is such as to interf~re with tlle efficiency of the reaction, the bed material 54 is removed through the drop tube 62 into a crushing unit 64 where the final par-ticle si~.e o~ the material can be reduced to a predetermined value.
The crushed materlal is then conveyed by a conduit 66 to be drawn off and entrained in an air stream by a venturi 68 which is fed with air from -the forced draught fan in the direction of the arrow 70. The reconditione~ carrier material is fed back through the fuel inlet 60 to the reaction chamber 12. Thus, par-t~ lly burnt coal .is returned to the bed and burnt to ash which is elutriated, and ground inert, ` incombustible ma-teri,al is introduced to the bed 54, which further reduces the necessity to top the bed up with fresh sand.

This embodiment permits substantially continuous operation of the fluidised bed burner and ensures that substantially all of the ash will be collected in the cyclones in the exhaust of the system.

In Figures 10 and 11 the plenum 110 of a vertical fluidised bed boiler is fed with air under pressure in the direction of arrow 112. The air rises through the base plate 114, on which rests a fLuidised bed 113. The heat generated from the fluidised bed flows upwardly throu~h the reaction chamber which rises high enou~h to constitute a vertical flue 115, as a first pass. The side walls 116 of the reaction chamber 115 are therefore heated by direct contact.

The heated gas then flows through a series 118 of smoke tubes which constitute a second pass, and then into a smoke box 120, from which the hot gases pass upwardly through a series 122 of smoke tubes constituting a third pass, terminatincJ in a manifold 124, which is connected to suitable cyclones and chimneys ~not shown).

A series of thermic siphon tubes 126 is provided, leading from the jacket 128, throllgh the bed 113 and the vertical Elue 115 in order to produce steam. 'rhis s-team rises and is then allowed to enter the steam space 130 above the level 132 of water above the flue 115. Baffles 132 are provided to avoid instability of the s-team space 130.

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The working pressure inside the steam space in the boiler shown is 860 KPa~

Coal or other fuel is introducPd through the inlet orifice 134 and water enters through inlet 136a. An access door 138 is provided as well as a manhole ring 140~ A
æeries o~ stays 142 serve to ensure constructional strength and the smoke pipe 118 and 122 also serve as constructional units.

A microprocessor may be used which constantaly monitors the various parameters and all of some mechanical factors.
This microprocessor may be used with a numbex of boilers in conjunction with a modulator/de-modulator (MODEM) unit, which allows the microprocessor to receive and send message~ along telephone wires to a central computer and/
lr~ or to individual computers. Thus, the central cont.rol can send warnin~ signals to customers' boilers in advance of catastrophe or damage or malfunction, thereby providing a preventative maintainance feature.

In Figure 12 a heater 210 is formed with an air dis~ributor and plenum chamber 211 havirlg an air inlet controlled by a damper 212. The i.nlet leads from the forced draught fan (not shown). Above t:he chamber 211 i.s a space occupied by the fluidised bed 213 of conventional design with its associated free~board 214. The s-team space is indicated by 5~1l9~i the reference numeral 215. There is a gas outlet controlled by a damper 216. In this case a suction fan (not shown) is connected to the outlet.

The heater 210 is fitted with four transducers. There are two ther-mocouples 217 and 218 in the bed 213 serving as temperature transducers. There are two pressure transducers 219 and 220 serving to sense the steam pressure and free-board pressure respectively.

The heater 210 is also fitted with a bed preheating unit 221 of any conventional construction which is controlled by an ignition control uni-t 222 which in turn is controlled by a start up control unit 223. Once activated the Ullit 223 is controlled by a signal from the thermocouple 217.
~t a predetermined bed temperature, the unit 223 causes the ignition control unit 222 to s~itch off the preheating unit 221. At another and lower temperature the unit 223 is once more activated and causes the unit 222 to operate once more.

The other thermocouple 218 signals a temperature controller 224 to control tlle fuel feed in a manner described later on. The controller 224 functions through the unit 223 and as long as the Ullit 223 is activated, signals from the uni-t 224 are blocked and only signals from the unit 223 pass along the line 225 to a speed con-troller 226 which regulates the coal feed to the bed 213 of a coal feeder (not shown) so ~ ~ 6 ~ ~ ~t3 that only the quantity of coal required at start-up is fed to the bed 213.

The speed controller226 is arranged to be operative over one or more ranges each from zero to a predetermined maximum.
This is done by means of a three step controller 227 which enforces a maximum speed setting on the controller 226 in response to signals from the transducer 219. At a predetermined maximum pressure the coal feed is stopped.
At a predetermined high pressure the lowest speed setting becomes operative and at two lower pressures, high speed settings become operative.

The three step controller 227 operates the damper 216 in a similar manner among one or more positions. At the maximum pressure in the freeboard the damper 216 is at its lS smallest opening ancl at two lower pressures it is at an intermediate and at its fullest openincJ. The clamper 216 should never be closed.

The setting of the damper 216 affects the pressure in the freeboard 214 so that the transducer 220 senses any changes in pressure as the aperture of -the damper 216 is changed.
The transducer 220 then sicJnals a three term proportional controller 228 which controls the damper 212 causing the flu:idising and combustion air to increase or decrease thus balancing the above bed pressure to a predetermined positive or negative pressure level. This change in combustion air ~1 ~6~ ~t.;;~

also increasçs and decreases the heat transfer surfaces covered by the fluid bed, i.e. increase in fluidising and combustion air gives an increase in heat transfer surface. A decrease in~ fluidising and combustion air gives a decrease in heat transfer surface. In all uses of fluid combustion the greater the quantity of combustion air the greater the heat extraction rom the bed and the lower the combustion ~ir the lower the heat extraction becomes from the bed. This is because the air, as it passes through the bed absorbs heat while being involved in the combustion process. The air and combustion ~ases in the boiler situation remove a proportion of the heat from the hed, while in a fluidised bed incinerator and crop drier the combustion air and gases, remove nearly all of the heat from the combustion system. The rest of the heat becomes dissipated through losses in the system which, however, is a very small percentage~

The control therefore of damper 216 in relationship with damper 212 and the correct delivery of coal to the combustion system must be related correctly with the combustion air, this being done by the three step ~ontroller 227. The response time of the controller 228 and the damper 212 is shorter than the response time oE the damper 216 to prevent pressurization of the space 214.

Note that when the unit 223 is activated it overrides the control by the transducer 214 to enforce the setting of the ~6~

damper 212 to that required at start up. As soon as the unit 223 is deactivated, the transducer 214 takes over.

In addition to the above,conventional safety circuits may also be provided, but these do not affect the operation of the invention.

As an illustration take the case of a boiler designed to deliver steam at a pressure of about 960 kPa with a maximum allowable pressure of 1000 kPa. The controller 227 would then be set to select one of four boiler states:

1. Hiyh fire at 890 kPa

2. Medium fire at 930 kPa

3. Low fire at 960 kPa 9. Slump at 1000 kPa Corresponding to the boiler states the controller 226 will have one or more maximum speed settin~g:

1. High speed 2. Second speed 3. Third speed 9. Stop The damper 216 will also have ~our settings:

1. Wide open .' .

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2. Partially open 3. Partially closed

4. Almost closed In response to this the controller 228 would place the S damper 12 into four settings resulting from a pneumatic connection derived from the above bed combustion chamber gas pressure:

1. Wide open 2. Partially open 3. Partially closed 4. Closed.

For a coal fired furnace the average bed temperature should be about 950C and the maximum allowable about 1000C.
In such a case the start up control Ullit 23 should become deactivated at AC and come into operation again if the bed temperature drops to BC, which is a temperature below that of AC. In effect the temperature controller 224 will then send out signals over a range of AC to 1000C. At the latter temperature i-t would order the speed controller 226 to stop the coal eecl while at AC it would order the controller 226 to feed coal at the maximum rate permissible in terms of the setting allowed by the controller 227.

The effect is that once the steam pressure is up and steam is consumed, more air will pass through the bed tending to iS~9S

lower the temperature, so that the controller 224 will cause more fuel to be fed to the bed once more to raise the temperature and also to provide more heat ~o raise more steam. As the demand drops, air flow will first drop while the controller 226 will operate in a range which will cause the fuel feed to be diminshed in step with the decline in demand.

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A boiler having a combustion chamber located at least partially within a housing, the combustion chamber including a base plate having an upper surface adapted to support a fuel bed a water jacket defined by the space between the lower surface of the base plate and the housing, and an air plenum including one or more air passages extending through the base plate for connecting the plenum with the interior of the combustion chamber.

2. A boiler according to claim 1 in which the combusion chamber is enclosed by the housing to form a water jacket which surrounds the combustion chamber.

3. A boiler according to claim 1 in which means is provided for the introduction into the bed of an auxiliary fuel, the auxiliary fuel introduction means being adapted to introduce fuels which, because of their lightness or size would tend to be rapidly elutriated by the combustion air flow and the auxiliary fuel introduction means comprising a pipe extending into the fire bed at the level of the upper surface of a quiescent layer of the bed.

4. A boiler according to claim 3 in the gas circuit of which, means is provided at least partially to separate from one another, elutriated, unburnt fuel dust, light combustible waste material such as straw and sawdust, and elutriated ash and which includes means to return the combustible waste material and the unburnt fuel dust to the combustion chamber via the auxiliary fuel introduction means, the means to return the fuel dust and combustible material comprising a container formed with an opening arranged to feed the contents of the container into a moving air stream.

5. A boiler according to claim 3 in the gas circuit of which, means is provided at least partially to separate from one another, elutriated, un-burnt fuel dust, light combustible waste materials such as straw and sawdust, and elutriated ash and which includes means to return the combustible waste material and the unburnt fuel dust to the combustion chamber via the auxiliary fuel introduction means.

6. A boiler according to claim 1 which has a combustion chamber base plate with upstanding combustion air stand pipes in which at least some of the stand pipes include or have associated therewith air flow control devices, each device being individual to a stand pipe and at least some of the devices having a common operating means.