GB2092294A

GB2092294A - Solid Fuel Burning Apparatus

Info

Publication number: GB2092294A
Application number: GB8102608A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1981-01-28
Filing date: 1981-01-28
Publication date: 1982-08-11
Also published as: GB2092294B

Abstract

A solid fuel burning apparatus incorporates a fuel supply from a store (2) over the combustion chamber (4), fuel being delivered vertically downwardly through a discharge orifice (10) to the fire bottom which comprises a number of inclined overlapping metal plates (6). Primary air (P) is supplied through gaps between the plates in a direction that assists the movement of burning fuel in the fire bed (8). The fire bed (8) slides bodily with no cascading of fresh fuel over the top of the fire bed. Clinker is scraped downwardly by the moving fire bed towards a movable end baffle (12), and is discharged to a receptacle or conveyor below. The apparatus may be incorporated into a boiler. <IMAGE>

Description

SPECIFICATION Solid Fuel Burner The invention relates to solid fuel burners, which either may be incorporated in boilers or may be designed as pre-burners for supplying their combustion gases to existing boiler installations.

It is a problem to provide a solid fuel burner which is economical to manufacture, reliable and efficient in use, and capable of operating without attendance. Many burner designs have been proposed to meet these often conflicting requirements, but no single design has met all requirements in a satisfactory manner and still been capable of operating through the vast range of burner sizes demanded in practice. For example, a burner for a domestic heating installation may be intended to operate at an output of as little as 1 5 Kw, whereas a burner for heating a large complex such as a large shop, school or office building or a sports centre may be intended to operate at an output of as much as 2400 Kw. No single principle of burner design has previously been capable of extension to the whole of this range of outputs.

Previous designs of solid fuel burner have employed grates for the fuel bed, generally with a forced draught of primary air up through the grate and a supply of secondary and possibly tertiary air over the grate. Fuel has been fed from below the grate, from above the grate or from an end of the grate, and it has been quite common in the latter two concepts of fuel feed to have fresh fuel rolling over the fuel bed and igniting as it rolls. This has always been thought to provide the best burning characteristics, since the fuel loses its volatile combustible components into the stream of secondary air before it undergoes complete combustion in the primary air stream.

All solid fuel burners have had to cope with the problem of clinker or ash removei. Whenever a cast metal grate is used, the ash must be allowed to remain as a thin layer over the grate to protect the metal. This is, however, at the expense of efficiency. A cooled grate enables the burning temperature to be increased to above the ash fusion temperature of the fuel, so that the ash fuses to form clinker. The efficiency of the burner is thus increased, but the clinker itself must be removed periodically from the burning fuel, generally to a cooling and collecting area, and must further be removed periodically from the burner housing itself.In the past this has required regular operator attendance, but there has always been a demand for a burner that simplified and if possible automated the clinker removal without adding considerably to the cost, the size or the complexity of the design.

This invention provides a solid fuel burner comprising a fuel store arranged over a combustion chamber, wherein a fire bottom in the combustion chamber comprises a number of overlapping metal plates each inclined at an angle to the horizontal so that solid fuel delivered under gravity from the fuel store downwardly to the fire bottom forms a fire bed that slides bodily down the fire bottom towards a clinker collection zone, and a primary air supply through gaps between the overlapping plates is in a direction to assist such sliding movement.

Delivery of the fuel from the fuel store to the fire bottom is preferably assisted by a superatmospheric pressure established in the fuel store, which provides a downward pressure on the fuel to augment the gravity feed of the fuel downwardly into the combustion chamber.

Advantageously the same superatmospheric pressure is used as the source of primary air (and optionally secondary and tertiary air if there are additional air supplied to the combustion chamber) so that an increase in the superatmospheric pressure in the fuel store has the dual effect of increasing the fuel feed and increasing the supply of air to support combustion.

In the burner of the invention the relatively costly moving parts of a conventional mechanical stoker are replaced by more reliable and less expensive static parts, and the burning operation is made more efficient The delivery of the fuel from above to form a fire bed that slides bodily over the dates of the fire bottom has the advantage that the clinker that forms is progressively pushed or scraped over the smooth surface of the fire bottom towards the clinker collection zone.

The clinker collection zone may be, for example, an air-cooled or water-cooled tray at an angle to the fire bed so that clinker received thereon solidifies and retards the sliding movement of the fire bed. If desired a scraping device may be provided, periodically to remove the clinker by scaping it from the tray onto a conveyor or collecting tray below, or alternatively the clinker may be removed by being pushed off the end of the tray by the moving fire bed depositing more clinker on the tray. This latter is particularly suitable when the fire bed movement is assisted by a superatmospheric pressure in the fuel store as described above, as it is then possible to design the burner so that the rate of burning, the rate of movement of the fire bed and thus the rate of clinker removal are all governed by the pressure in the fuel store.

Alternatively the clinker collection zone may be defined by a movable end baffle at the bottom of the slope of the fire bottom. The end baffle should normally upstanding in the path of movement of the fire bed to arrest the movement of the bed. If the end baffle is located just beyond the hottest part of the fire bed, then clinker will accumulate against the baffle. Momentary and intermittent removal of that baffle from the plane of movement of the fire bed causes the clinker to fall off the lower end of the fire bottom, preferably onto a conveyor or into a collecting tray below which can be removed at intervals of, for example, one week. The collecting tray may be mounted on wheels to make it easier to remove if so desired.

The movable end baffle is preferably an upstanding longitudinal wall of a clinker tray that extends along the length of the fire bottom. The clinker tray is advantageously pivotally mounted, and periodically is moved pivotally to effect the baffle movement necessary to turn the clinker into the collecting tray. It is helpful if a secondary baffle surface on the clinker tray is effective during this pivotal movement to prevent the fire bed from sliding bodily down the fire bottom during the clinker ejection process. The secondary baffle surface is also an aid to start-up, as will be explained later.

The formation of the fire bottom from overlapping metal plates has the immediate advantage of enabling the supply of primary air to be directed to assist the sliding movement of the fire bed. However it also has other advantages.

The spacing between the overlapping portions of the plates may easily be varied during manufacture, advantageously providing a greater spacing and therefore a greater supply of primary air near the bottom of the sloping fire bottom than near the top. An alternative or an additional control is possible by having each primary air conduit separately adjustable for air flow, so that the rate of supply of primary air between different pairs of adjacent plates can be varied at will to ensure that the supply of primary air is greater nearer the bottom of the sloping fire bottom than near the top. This ensures that the burning is most intense at the bottom of the slope, and provides the optimum temperature gradient for the fuel as it is heated to drive off the volatile combustibles before it enters the higher temperature lower burning zones.

Advantageously the plates provide the top surface of a series of conduits for the supplied air, so that the air is preheated before it is delivered to the combustion zone, and the plates cooled by the air flow. Alternatively and equally advantageously, the plates may be water-cooled, and the heat transmitted to the water may be utilized in a boiler installation fired by the burner. The cooling of the plates in this way enables the fire bed temperature to be increased to well above the ash fusion temperature so that the burner may be run most efficiently.

The plate at the top end of the sloping fire bottom is advantageously curved to provide a regular gradation from the vertical to the slope of the remainder of the fire bottom. Thus fuel gravity fed from above is progressively deflected to the direction of movement of the fire bed, and this helps to maintain the movement of the fire bed as a bodily sliding movement.

Cascading of fresh fuel over the top of the moving fire bed is avoided by suitable choice of the angle of slope of the fire bottom, but this avoidance can be assisted by the provision of a fuel gate in the form of an inverted weir to restrict the passage of fuel between the gate and the top plate of the fire bottom, and optionally also by the provision of a refractory slab in the combustion chamber, just downstream of the gate and generally parallel to the plane of the fire bottom.

Fuel passing beneath the gate then passes between the refractory slab and the fire bottom so that a generally uniform depth of fire bed and a generally uniform temperature gradient is established before the fuel passes out from beneath the refractory slab. Lowering of the fuel gate towards the fire bottom restricts the flow of fuel and therefore can be used in conjunction with control of the air supply through each primary air slot to control the heat output of the burner.

Advantageously the fuel gate has a stowed position adjacent a water-cooled surface, or is in the path of the secondary air supply, or both, to prevent its warping in use.

The secondary air supply to the combustion chamber, if required, is a supply of optionally preheated air introduced over the top of the fuel bed. A tertiary air supply may also be desirable for some fuels, introducing tertiary air, optionally preheated, in the zone of an exit nozzle from the combustion chamber so as to assist combustion by any tars that would otherwise be carried by the combustion gases. If the fuel store is at superatmospheric pressure as described above, then one or both of the secondary and tertiary air supplies, as well as the primary air supply, may be taken from the fuel store.

The entire burner chamber of the burner of the invention should be airtight, so that the only means of air ingress is through the air supply passages and the only means of exit is through the flue. The design makes it relatively easy to accomplish the necessary airtight construction as the only regular access to the boiler chamber is for clinker removal, which may be well below the fire bed itself. Thus there are no problems of door seals warping as there might have been if frequent access to the burner chamber above the level of the fire bottom had been required.

If the burner of the invention is incorporated into a boiler, the combustion gases should be directed in a tortuous path past heat-exchange surfaces before they reach the flue.

Advantageously the path should be so dimensioned that there is a restriction to the flow of the combustion gases in the vicinity of the heat-exchange surfaces, as this improves boiler efficiency. For example, the path to the flue may be a tortuous path between pairs of hollow waterfilled baffles which are close enough together to form a flow restriction for the combustion gases passing therebetween, thereby increasing the air speed past the baffles.

If the burner according to this invention is used as a preburner for an existing boiler installation, it may be necessary (depending on the boiler itself) to incorporate into the preburner ducts, through a water jacket, for the combustion gases, to deliver those gases to a specific part of the boiler designed to receive the hottest combustion gases.

It is, however, always a preferred feature for the combustion chamber itself to be surrounded or partially surrounded by a water jacket connected to the boiler circuit.

Whether the burner is a preburner or integral with a boiler assembly, it is a preferred feature of the invention to provide one or more forced draught fans for generating the primary (and any secondary and tertiary) air supply, and one or more induced draught fans for inducing a flue draught. Advantageously the forced draught and induced draught fans are coupled to a microprocessor that processes input information relating to the balanced draught conditions, the fire bed temperature, the rate of burning of the fuel, the air supply temperature, the kindling control, the carbon monoxide/carbon dioxide ratio, the position of the fuel gate and/or the boiler water temperatures, and governs the fan motor speeds and all air and flue gas damper settings in response to the processed information.

The burner of the invention has a high thermal efficiency. Moreover it is economical to manufacture in that it can if necessary be made wholly from steel plate, requiring no cast parts.

The fire bottom in particular is of economical construction, being relatively easy to manufacture and to repair. The life expectancy of all components of the combustion chamber can be increased by providing each as one skin surface of a water or air chamber, so that the heat of combustion is rapidly conducted away as useful heat.

The flexibility of the design to a large range of thermal outputs can be attributed, in part at least, to the fact that a standardization of components can be achieved over a range of burner sizes.

Different thermal outputs can easily be obtained by varying the length of the fire bottom, varying the number of slots for primary air, varying the rate of supply of primary air, or doubling up the number of fire beds burning in a given combustion chamber.

Drawings: Figure 1 is a schematic vertical cross-section through the combustion chamber of a burner according to the invention; Figures 2 to 4 are vertical cross-sections through three alternative burners according to the invention, showing how the burners can incorporate more than one sloping fire bottom; Figure 5 is a longitudinal section through a boiler installation utilizing the burner of Figure 3 as a preburner; Figure 6 is a vertical section through a preburner similar to that of Figure 5 but incorporating only one sloping fire bottom; Figure 7 is a perspective sectional view of a boiler integrally incorporating a burner according to the invention; and Figure 8 is a vertical cross-section through another boiler integrally incorporating a burner according to the invention.

In the various embodiments of the invention illustrated in the accompanying drawings, the same reference numerals have where possible been used to indicate similar or analogous parts.

Referring first to Figure 1, the basic concept of a solid fuel burner according to this invention is illustrated. The burner comprises a fuel store 2 which in use is filled with the solid fuel and replenished regularly. The fuel may be coal, coke, anthracite, any of the solid by-products of coal or any blending of solid fuels, sawdust, peat, timber or refuse pellets that are cable of supporting combustion. The particle size of the fuel is not critical, and burners according to the invention have been found to operate very satisfactorily even on the cheaper, very small granular sizes that would be totally unsuitable for burning in a conventional grate. The replenishment of the fuel store 2 may be manual or automatic, and if automatic may be gravity fed from a separate bunker or fed through a screw or pneumatic conveyor, depending on the installation.

The fuel store 2 is arranged generally above a combustion chamber 4 that has a sloping fire bottom formed by a number of overlapping metal plates 6 each inclined to the horizontal. The fuel is delivered vertically downwardly from the fuel store 2 onto the plates 6 where it is shown in the drawing as a fire bed 8. The topmost plate, indicated 6a in Figure 1, is curved so as to guide the fuel smoothly from the vertical to the angle of the fire bed 8, and this guiding, coupled with the size and location of the bottom mouth 10 of the fuel store 2, ensures that the fuel moves into the fire bed 8 as a single sliding mass rather than having fresh fuel cascade downwardly over the top of the fire bed.

The bottom end of the fire bed 8 is defined by a movable end baffle 12 which is pivotally mounted about an axis 14. This movable baffle 12 is utilized for clinker removal as will later be described.

A primary air supply is provided through gaps between adjacent overlapping plates 6, as indicated by the arrows P. The air supply between different pairs of plates 6 may be controlled by having individual fans for the three primary air streams shown, or a single fan may be used to provide all three streams of primary air. In the latter case a greater rate of passage of the primary air in the lowermost air stream or streams P can be achieved by ensuring that the spacing between adjacent overlapping plates 6 is greater at the bottom of the fire bed than at the top, or by controlling the primary air supplies using dampers. Figure 1 shows a regular gradation of the spacing progressively up the fire bottom.

If a single fan is used to provide all three streams of primary air, then it preferably delivers its air first to the zone H over the fuel in the fuel store 2 to establish a superatmospheric pressure urging the fuel downwardly through the mouth 1,0. From the zone 10 the air is conducted through conduits (not shown) to the air delivery slots formed between adjacent pairs of plates.

Preheating of the primary air is achieved by passing the primary air first through the box sections 16 defined in part by the plates 6. In this way each box section acts as a plenum chamber in which the primary air is heated by contact with the respective plate 6. Arrows 18 indicate the passage of the primary air out of these plenum chambers 16.

An alternative preheating of the primary air (not shown in Figure 1) may be by introducing the primary air not through the box sections 16 but through air conduits in heat exchange relationship with the combustion gases or with water in a water jacket around the combustion chamber. In such a construction the box sections 1 6 would be filled with circulating water to provide a more efficient cooling for the plates 6.

A conduit 20 arranged longitudinally of the fire bed 8 provides a source of secondary air S which is preferably preheated and is directed generally downwardly over the fire bed. A roof 22 of the combustion chamber 4 is double skinned so as to create a water jacket 24. This helps somewhat to insulate the fuel store 2 from the heat of the combustion chamber 4, and the water in the jacket 24 is utilized by being caused to circulate to a boiler that is fired by the burner. The temperature of the water jacket 24 in use is generally sufficient to dry out any moisture in the fuel, which further increases the efficiency of the burner.

A gate or inverted weir 26 is pivotally mounted at 28 and arranged so that it can be lowered in the direction of the arrow 30 to close or partially close the fuel supply opening 10 at the bottom of the fuel store 2. In this way the fuel supply to the fire bed 8 can be controlled, and in particular fresh fuel can be prevented from rolling over the top of the fire bed. The positioning of the arcuate gate 26 immediately adjacent a cooperating arcuate surface of the water jacket 24 ensures that the gate 26 is kept cool and prevented from warping in the heat of the combustion chamber.

As the fire bed burns, it slides bodily down the plates 6 forming the fire bottom, this movement being assisted by the direction of the flow of primary air P as well as by the slope of the plates 6. The baffle 12 provides a stop to this sliding movement, and continued high temperature burning of the fuel means that the incombustible elements of the fuel collect as clinker and are pushed or scraped down into the channel formed by the end baffle 12. Periodically this is tilted anticlockwise as shown by the arrows, to discharge its clinker downwardly into a collection trough below. While the end baffle 12 is thus pivoted, the fire bed is retained in position by an arcuate end wall or secondary baffle surface 32 of the baffle, which holds the fire bed in position until the baffle has been returned to the position illustrated.This arcuate end wall 32 is also used to retain fuel after shut-down, and on start-up while auto-ignition takes place.

Automatic ignition is provided by having electric heating elements 33 in the path of the primary air immediately before it issues through the slots between the plates 6. One such element or row of elements may be provided for each slot as illustrated, or only selected streams of primary air P may be so heated. The heating should be sufficiently intense to ensure that the fuel in the path of the heated primary air P is ignited.

Shut-down may be achieved simply by discontinuing the air supply, and lowering the gate 26.

The left hand side of the combustion chamber is not shown in Figure 1, and reference is made to Figures 2 to 4 to illustrate the diverse ways in which the basic concept of this invention can be applied to different fuel burners. Figure 2 shows schematically how two such combustion chambers can be arranged side by side and fed from a single fuel store. It will be appreciated that the burner of Figure 2 is symmetrical about the plane A-A, and does in fact consist of two fuel burners according to this invention, placed back to back. Similarly Figure 3 illustrates two such burners placed front to front, and Figure 4 shows how the same basic elements can be arranged in parallel to quadruple the heat output of a single burner. By using 1,2,3 or 4 burners, the varying conditions throughout a 24-hour cycle, and the varying seasons' demands, can be met.Shutdown of some burners also enables maintenance to be carried out.

A range of burner sizes is therefore possible in accordance with the constructions illustrated in Figures 1 to 4. For example, if only one fire bed is provided in a boiler incorporating its own solid fuel burner, the heat output may be as low as 15 Kw, whereas by increasing the size, number and length of fire beds the heat output can easily be raised to 2400 Kw. If the solid fuel burner is used as a pre-burner for an existing boiler installation, then the heat output can be from 15 to 1500 Kw.

Extension of these ranges is possible as will be apparent.

Figure 5 illustrates diagramatically how the burner installation of Figure 1 can be connected as a pre-burner to an existing boiler, shown generally as 50. Such an assembly would be suitable, for example, when a gas-fired or oil-fired boiler is converted to run on solid fuel. The pre burner assembly generally is indicated as 52, and it can clearly be seen that the water jacket 24 between the fuel store 2 and the combustion chamber 4 extends down the sides as well as over the top of the combustion chamber. The plates 6 and fire bed 8 are shown only generally, and it will be seen that the combustion gases issuing from the combustion chamber 4 are directed past a refractory arch 51 (as an aid to complete combustion) and through an array of parallel water-cooled tubes 54 which extend wsll into the boiler 50 so that the combustion gases are forcibly directed against an end face 56 of the boiler. The passage of the combustion gases through the boiler is shown generally by arrows, and by heat exchange heats the water in the boiler water jacket 57. A conventional arrangement of heat exchange fins (not shown) is provided in the boiler to assist the heating of the water, and the boiler water and the water in the water jacket 24 of the pre-burner 52 together provide the heat output for the installation.

The combustion gases pass finally to a flue 58, from where they are drawn by an induced draught fan 59. A microprocessor circuit (not shown) receives inputs from the fan 59, the fan or fans (not shown) for the primary and secondary air, and other variables such as the water temperatures, balanced draught conditions, the carbon monoxide/carbon dioxide ratio, the kindling control, the rate of supply of fuel, the temperature in the combustion chamber and the position of the fuel gate (not shown in Figure 5).

Outputs from this microprocessor circuit govern the speeds of rotation of the induced draught fan 59 and the fan or fans inducing the primary and/or secondary air supply. In this way the maximum burner efficiency can be attained.

It is important that the connection between the pre-burner 52 and the boiler unit 50 is quite airtight, so that the only air supply to the combustion chamber 4 is via the forced draught fan or fans, and all of the combustion gases pass to the flue and the induced draught fan or fans.

Figure 6 shows in axial section a typical preburner according to the invention, incorporating a single fire bed 8. Only two air streams P of primary air are shown, illustrating that the number of such primary air streams can be varied. The secondary airstream S is located substantially as in Figure 1, but is directed into the combustion chamber over the top of a refractory plate 66. The plate 66 establishes a temperature gradient in the fuel as it passes from the supply opening to the combustion zone of the fire bed, and acts as a coking plate for bituminous fuels. It also helps the gate 26 in preventing fresh fuel from rolling over the top of the fire bed. Also in Figure 6 there is a third supply of air which could be a further or alternative supply of secondary air S or could be, as shown, a supply of tertiary air T.The latter would be desirable when burning a fuel with a high tar content. The combustion gases from the combustion chamber 4 issue through a nozzle 60 at one end of the combustion chamber into a boiler generally as shown in Figure 5, or to any other form of boiler with a combustion chamber or furnace tube.

Above the water jacket 24 is a space which can conveniently be utilized for housing the forced draught fan or fans and/or the microprocessor circuitry necessary to obtain optimum utilization of the air supply, by controlling the fans for both the air supplies and the induced draught. This space is well shielded from the combustion chamber 4 by the water jacket 24.

The fuel store 2 in Figure 6 is not particularly high, so that only a limited head of fuel is provided above the exit 10 of the fuel store. It may therefore be preferred to incorporate into the housing a pivoted baffle such as 62 which is spring-biased downwardly against the fuel to increase the effective head of fuel, or to maintain the zone above the fuel at superatmospheric pressure as in Figure 1.

A trough 64 is provided beneath the end baffle 1 2 for collecting the clinker as it is ejected, so that the clinker in the trough 64 need be collected only irregularly for example at weekly intervals. The timing of the tilting of the baffle 12 to eject the clinker into the trough 64 may be controlled by a time clock.

Figure 7 shows how a solid fuel burner according to the invention can be incorporated into a particularly efficient boiler assembly. Figure 7 incorporates the fuel store 2, combustion chamber 4, plates 6 and movable end baffle 12 of Figure 1 , so that construction of the burner itself need not be described in detail. Combustion gases from the combustion chamber 4 pass upwardly past a refractory arch or slab 77 and through a slot 70 in a water jacket 71 to a first plenum chamber 72, then downwardly to a second plenum chamber 73, upwardly to a third plenum chamber 74 and finally downwardly to an exit orifice 75 and through an induced draught fan 76 to a flue (not shown). The number of such plenum chambers will vary according to the rated output of the boiler.Access for gases between adjacent plenum chambers is through a relatively narrow spacing between fins welded to water-filled baffles of the cooling jacket 71, and the flow restriction so produced provides an increase in the rate of gas flow and therefore particularly good heat transfer between the combustion gases and the water in the jacket. An arrow marked F indicates diagrammatically the direction of water flow from the water jacket 71, and a further arrow marked R indicates the flow return. A control unit 78 contains control circuitry governing the running times and the speeds of the inlet fan 79 and induced draught fan 76 to obtain optimum burning.

Figure 8 illustrates an alternative boiler assembly that could be more suitable for a smaller installation. The main difference is that the combustion gases pass from the combustion chamber 4 through heat exchange passages 80 in a water jacket 24' into a heat exchange chamber 82 centrally of the water jacket. Good thermal contact with the water jacket can be ensured by lining the chamber 82 with fins for increasing the effective surface area of the chamber. From the chamber 82 the combustion gases are conducted through further conduits 86 at either end of the combustion chamber 4 to a flue 88. The water jacket 24' continues around the conduits 86, ensuring that by the time the combustion gases have reached the flue 88 they have been effectively cooled and their heat of combustion efficiently utilized. An induced draught fan 90 in the flue 88 may be necessary for efficient burning in areas where the chimney draughts fluctuate and because of the convoluted path of the combustion gases which, together, may significantly restrict the over-fire draught requirement.

Details of the fuel and air supplies and the fire bed in the burner of Figure 8 are essentially the same as those of the preceding embodiments.

Claims

1. A solid fuel burner comprising a fuel store arranged over a combustion chamber, wherein a fire bottom in the combustion chamber comprises a number of overlapping metal plates each inclined at an angle to the horizontal so that solid fuel delivered under gravity from the fuel store downwardly to the fire bottom forms a fire bed that slides bodily down the fire bottom towards a clinker collection zone, and a primary air supply through gaps between the overlapping plates is in a direction to assist such sliding movement.

2. A burner according to claim 1, wherein the primary air supply is greater near the bottom of the sloping fire bottom that near the top.

3. A burner according to claim 2, wherein the gradation of the primary air supply from the bottom of the fire bottom to the top is achieved by decreasing progressively the size of the gaps between the overlapping plates in the direction from the bottom to the top of the fire bottom.

4. A burner according to claim 2, wherein the gradation of the primary air supply from the bottom of the fire bottom to the top is achieved by providing ajustable air dampers in conduits supplying the primary air to different gaps between the overlapping plates.

5. A burner according to any preceding claim, wherein the primary air is preheated before being supplied through gaps between the overlapping plates.

6. A burner according to any preceding claim, wherein electric heaters are provided in the path of the primary air supply, to heat the primary air supply to above the ignition temperature of the fuel for initial start up.

7. A burner according to any preceding claim, wherein the topmost plate of the fire bottom is arcuate guiding fuel from a vertical path as it is delivered under gravity from the fuel store to a path at the angle of the rest of the plates.

8. A burner according to any preceding claim, wherein each plate forms the top of a box section through which the primary air passes and in which it is preheated by thermal contact with the respective plate.

9. A burner according to any of claims 1 to 7, wherein each plate forms the top of a box section through which water can circulate to cool the plate.

1 0. A burner according to any preceding claim, further comprising means for establishing a zone of superatmospheric pressure above the fuel in the fuel store, to augment the gravity feed of the fuel to the combustion chamber.

11. A burner according to claim 10, wherein the zone of superatmospheric pressure is the source of combustion air for the burner.

12. A burner according to any preceding claim, wherein the clinker collection zone is provided by an air-cooled or water-cooled tray at an angle to the fire bed so that clinker received thereon solidifies and retards the sliding movement of the fire bed.

13. A burner according to any of claims 1 to 11, wherein the clinker collection zone is defined by a movable end baffle upstanding in the path of movement of the fire bed.

14. A burner according to claim 13, wherein the movable end baffle is a pivoted tray that is tiltable to tip out its contents into a clinker collection tray or conveyor below.

1 5. A burner according to claim 13 or claim 14, wherein the end baffle incorporates a secondary baffle surface so that when tilted it forms no gap between the fuel bed and the secondary baffle surface, which retains the fire bed against sliding down the fire bottom until the end baffle is returned to its rest position.

1 6. A burner according to any preceding claim, further comprising a fuel gate acting as an inverted weir to restrict the flow of fuel from the fuel store to the combustion chamber and to reduce the tendency of fresh fuel being supplied as a cascade of fuel over the top of the fuel bed.

1 7. A burner according to claim 1 6, wherein the fuel gate comprises an elongate baffle that is arcuate in section and movable pivotally about its centre of arc to restrict fuel flow.

1 8. A burner according to claim 17, wherein the baffle has a stowed position, permitting free fuel flow to the fire bed, adjacent a water-cooled roof member for the combustion chamber.

19. A burner according to any of claims 16 to 18, wherein a refractory slab is provided, immediately downstream of the fuel gate and generally parallel to the plane of the fire bottom, further to reduce the tendency of fresh fuel being supplied as a cascade of fuel over the top of the fuel bed.

20. A burner according to any preceding claim, provided with a secondary air supply above the fire bed.

21. A burner according to claim 20, wherein the secondary air supply is preheated.

22. A boiler comprising a burner according to any preceding claim having a path for combustion gases past heat exchange surfaces to a flue for the boiler.

23. A boiler according to claim 22, wherein the combustion gas path is through an array of tubes in a water jacket to a central cavity in the water jacket, and out through further tubes in the water jacket at the sides of the combustion chamber to the flue at the rear of the combustion chamber.

24. A boiler according to claim 22, wherein the combustion gas path is a tortuous path between hollow water-filled parallel finned baffles which are close enough together to form a flow restriction for the combustion gases passing therebetween.

25. A boiler according to claim 23 or claim 24, wherein the air supply is augmented by an induced draught fan in the flue.

26. A boiler according to claim 25, wherein the air supply and the induced draught fan are controlled by a microprocessor that processes input information relating to the fire bed temperature, the balanced draught conditions, the rate of burning of the fuel, the water temperatures to and from the boiler, a kindling control and the ratio of carbon monoxide to carbon dioxide in the combustion gases.

27. A boiler pre-burner comprising a burner according to any of claims 1 to 21 provided with a water-cooled housing having a discharge orifice or nozzle for discharging its combustion gases to a separate boiler.

28. A pre-burner according to claim 27, further comprising extension tubes for the discharge orifice or nozzle for directing the combustion gases substantially towards an end heat exchange surface of the boiler.

29. A pre-burner according to claim 27 or claim 28, wherein the air supply for the preburner is augmented by an induced draught fan in a flue from the boiler.

30. A pre-burner according to claim 29 wherein the air supply and the induced draught fan are controlled by a microprocessor that processes input information relating to the fire bed temperature, the balanced draught conditions, the rate of burning of the fuel, the water temperatures to and from the boiler, a kindling control and the ratio of carbon monoxide to carbon dioxide in the combustion gases.

31. A solid fuel burner substantially as described herein with reference to any of the drawings.