GB2086754A - Fluidised bed furnaces - Google Patents

Abstract

In a fluidised bed furnace 10, the bed material is supported by a bed support construction 17 of hollow box form having upper and lower bounding elements 32, 33, through which a plurality of pipes, tubes, or ducts 27 extend to a desired height for discharge of a combustion promoting gas from a plenum chamber 31 below the bed support to fluidise the bed material at and above that height; a coolant, advantageously the combustion promoting gas before or after pressurisation to supply the plenum chamber, passes through the bed support 17 and about the pipes, tubes, or ducts 27. <IMAGE>

Description

SPECIFICATION Fluid bed furnaces The invention relates to fluid-bed furnaces in which a bed of incombustible particulate material is rendered fluidised by passing through it pressurised combustion-promoting gas in order. to burn fuel added to the bed material.

In our design and development of such furnaces we have encountered the problems attendant on providing a support for the bed material where, as is desirable, the fluidising combustion-promoting gas is supplied from below such a support via a distribution and gas release system passing through from below to above that support. One advantage of such furnaces is that initial heating of the bed material to achieve fuel combustion temperatures can be by gases heating by an auxiliary oil or gas burner system also below the bed support. However problems arise from that and the very high temperatures of operation of the fluidised bed as it burns the fuel.

Specifically, any such bed support must cope with quite high temperatures, perhaps particularly at initial preheating and on slumping of the bed material when the furnace is taken out of commission by stopping the flow of combustion promoting gas. Often, say on emergency shut-down, such slumping bed material will still contain fuel. The damage that can then occur both to the bed support and upstanding pipes or conduits to gas outlet heads is considerable, unless the height of the latter above the bed support is substantially more than is really necessary during furnace operation. Minimising that height and the total quantity of incombustible bed material has obvious advantages.

It is an object of this invention to provide a bed support structure that offers the opportunity to use metal therefor without an undue penalty by way of gas release height.

According to this invention there is provided a fluid-bed furnace in which a bed of incombustible particulate material is rendered fluidised by passing through it pressurised combustion-promoting gas in order to burn fuel added to the bed material, wherein the bed material is supported by a bed support construction of hollow box form through which a plurality of pipes, tubes or ducts extend through upper and lower bounding elements of the bed support to a desired height for discharge of the combustion promoting gas from a plenum chamber below the bed support into the particulate bed material which is fluidised at and above that said desired height, such hollow box form also serving for the passage therethrough-and about said pipes, tubes or ducts of a coolant.

Advantageously the coolant is the combustion promoting gas before or after pressurisation to supply said plenum chamber.

At least for relatively small area beds, for example up to about 2 X 2m, such bed supports offer particular advantages as a single box structure with top and bottom plates secured together at two opposite edges, preferably within furnace walling, and with a minimum of further bracing between them, preferably apertured plates one at each other side of the bed proper, say also within furnace walling. Then, virtually the whole interior will be swept and cooled by combustion promoting gas with a minimum of obstruction presented by the discharge pipes, tubes or ducts extending between those top and bottom plates, which is also found to aid the minimisation of temperature variations of the fluidising gas across the area of the bed.

Preferred discharge pipes can be sealingly secured to the bottom plate by way of coupling elements on the pipes that also serve in holding in place a packing or packings, preferably insulation material extending over the bottom plate to protect the latter during preheating using a burner therebelow. Where such insulating material is of sheet form not all of the discharge pipes need to have coupling elements.

Upper discharge ends of those pipes within the combustion chamber obviously risk being subjected to higher temperatures in operation of the furnace, and will be particularly heated if the furnace has to be shut down quickly by slumping the bed material, for example if a back pressure occurs due to a fault in the apparatus utilising the hot exhaust of the furnace. At least to a small extent that may be mitigated by continued cool-air-sweeping of the interior of the bed support box structure, if desired with reduced or discontinued feeding of the underlying plenum chamber. Nonetheless, differential expansion effects have to be expected between the discharge pipes and the top plate of the bed support and also the bottom plate during preheating.

As it is undesirable to have significant leakage from the bed support past the discharge pipes, sealing or minimum leakage joints are to be preferred between them and the top and bottom plates of the bed which joints permit relative movement to accommodate differential expansion effects. This is especially important at the top plate to eliminate or reduce to acceptable levels unwanted discharge of combustion promoted gas directly from the bed support into the desirably substantially static layer of particulate bed material immediately above the bed support and below upper discharge positions of the pipes.

Satisfactory sealing to both top and bottom plates is achieved herein by the cooperation of flange elements on the pipes with deformable packings between them and the upper surface of the top plate, and the use of resilient packing between the lower surface of the bottom plate and the coupling element or insulation material, preferably as part of the latter, advantageously an inner one of two or more layers of material extending over the bottom plate.

One specific embodiment of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which: Figure 1 is one section view of a fluid-bed furnace; and Figure 2 is another sectional view of the same furnace taken at right angles to Fig. 1.

In the drawings, a fluid-bed furnace 10 has a combustion chamber 11 bounded by refractory lined side walls 12, 13, end walls 14, 1 5 and arched top 16, and a bed support 1 7. An outer casing of metal plate is indicated generally at 18. One end wall 14 has an upper exhaust gas discharge duct 19, and the other end wall 1 5 affords entry 20 for solid fuel such as coal from a suitable sealed feed such as hopper 21 with rotary bottom feed 22 driven by motor 23. An access door 24 is shown in one side wall 1 2 and another may be provided, if desired, in the other side wall 12.

In operation of the furnace 10, the bed support 1 7 will carry incombustible particulate material 25, such as sand that will be fluidised to a level 26 at or just below the fuel feed inlet 20.

Fluidisation of the bed material 25 is by combustion-promoting gas such as air, from discharge pipes 27 from orifices therein, or discharge heads 27 indicated in outline only, at and usually just above the level 26 below which the bed material is substantially static even on operation of the furnace, as indicated by different shading in the drawings. Nonetheless, incombustible constituents of the fuel, having ash and other solid accretions will usually sink through that static layer and provision is shown by way of downwardly inclined channels 29, 30 through the furnace side walls 12, 1 3 for taking off bed material, preferably to be screened and the lines returned to the bed, say via the fuel feed 20 to 23.Naturally, except when drawn off, the bed material in the channels 29, 30 will be essentially static, but it is found advantageous for their entries to span the level 26 for collection of unwanted inclusions to be screened out.

The bed support 1 7 is shown as effectively dividing the furnace 10 to provide the upper combustion chamber 11 and a lower chamber 31 serving as a plenum chamber for supplying combustion-promoting gas to the discharge pipes 27.

The bed support is shown as being of hollow box form with top and bottom plates 32, 33 apertured to accommodate the discharge pipes 26 as a sliding fit therein regardless of differential expansion that may take place in operation of the furnace 1 0. The plates 32, 33 are of a strong metal of high melting point, usually steel, and the top plate 32 is protected and insulated from combustion temperatures by the static layer of bed material.

The plenum chamber 31, which is lined with refractories, has an auxiliary or preheating burner 35 at one end and an adjustable combustion gas inlet arrangement 36 at its other end.

The top and bottom plates 32, 33 of the bed support 1 7 are spaced and the interior space 37 closed off along its sides by beams 38, 39 in the side walls 12, 1 3 adjacent inner sides thereof. The beams 38, 39 actually present channels facing the interior space 37 and thus effectively extend that space somewhat into the thickness of the side walls 12, 1 3. Other beams 40, 41 adjacent outer sides of the side walls 12, 1 3 also provide structural support for those walls and the top and bottom bed support plates 32, 33. The top and bottom plates 32, 33 are also interconnected within the thickness of end walls 14, 1 5 by plates 42, 43 with apertures 44, 45 to permit through-flow of combustion promoting gas from an end space 47 between the plate 43 and closure 49.End space 47 is fed by a pressurising fan unit 48 communicating therewith at 50.

Another end space 51 between apertured plate 42 and closure plate 49 communicates at 52 with a preheating burner and plenum feed chamber 53. The burner 35 may be oil or gas fired, normally the former by fuel at 54 and is mounted to direct its flame into the plenum chamber 31. An air-feed adjuster therefore is shown at 55 and further provision for feeding combustion promoting gas from the chamber 53 into the plenum chamber 31 is shown at 56 about the burner 35, and may also be adjustable.

The gas feed arrangement 36 at the other end of the plenum chamber 31 is fed by a conduit 58 coupled by a flexible connection 59 to a communication 60 with the bed support end space 47, and contains an adjustable damper 61. Also, a barrier wall 62 is shown in front of exit space 63 of the feed system 36 so as to prevent damage by the flame of the barrier 35 and to set a desired release height for combustion promoting gas.

At preheating after initial fluidisation of the bed material, the burner 35 will be operating.

At all times, an adjustable proportion of combustion-promoting gas will be fed via the interior 37 of the bed support to keep the latter cooled to a desired or satisfactory extent.

In an alternative arrangement, traversal of the interior of the bed support is by combustion promoting gas on its way to a suitable pressurising fan. That would be achieved conveniently by dispensing with the inlet arrangement 36 to the plenum chamber 31 and the communication 52 from the end space 51 to the burner and plenum feed chamber 53.

Then, end of the plenum chamber 31 at which inlet 36 is shown could be blanked off, end space 47 of the bed support be in direct communication with atmosphere over most if not all of its width and the fan 48 shifted to the other side of the furnace for input thereto from end-space 51 of the bed support and output therefrom to the chamber 53.

The bottom plate 33 of the bed support is shown protected and insulated from the burner 35 by insulation 65. That insulation 65 is shown as having two layers 66, 67. The outer layer 66 will be of stiff insulating board and the inner layer 67 will be of soft material capable of deformation and recovery, i.e. resilient. Coupling elements 68 in the form of chips, split pins, nuts and washers, etc. serve to secure the outer layer 66 on the discharge pipes 27, preferably with some compression of the inner layer 67. Not all of the discharge pipes 27 need such coupling elements 68, as shown. That compression is achievable due to flanges 70 fixed on the discharge pipes above the bed support normally themselves with soft packings by which they bear on the top plate 32.Effectively, the discharge pipes are secured with a clamping action on the outer surfaces of the top and bottom bed support plates 32 and 33.

So long as resistance to gas flow through the insulation 65 is still high, it is not, of course, strictly necessary to have compression of the inner insulation layer 67, at least not in all operating conditions of the furnace. It may even be that some outflow of combustion promoting gas through the bottom plate 33 can be tolerated. However, the best possible seal is desired to the top plate 32 and we prefer to clamp the flanges 70 by way of plates, washers or the like 72 that are bolted or screwed onto the top plate 32. Each plate or washer 72 preferably engages over four surrounding flanges 70.

As mentioned in our patent application cofiled herewith (reference P849), bed support structures hereof may be cooled by passage therethrough of some coolant other than the combustion-promoting-gas. Where a furnace hereof is used in boiler applications with its hot exhaust gases applied to a heat-exchanger, that coolant may conveniently comprise the heat-exchange medium of the heatexchanger, for example oil, water or even steam, or a coolant common to the boiler and the furnace. Then, of course, the combustionpromoting-gas will not be fed via the bed support structure, but, instead directly to the underlying plenum chamber.

Claims (11)

1. A fluid-bed furnace in which a bed of incombustible particulate material is rendered fluidised by passing through it pressurised combustion-promoting gas in order to burn fuel added to the bed material, wherein the bed material is supported by a bed support construction of hollow box form through which a plurality of pipes, tubes or ducts extend through upper and lower bounding elements of the bed support to a desired height for discharge of the combustion promoting gas from a plenum chamber below the bed support into the particulate bed material which is fluidised at and above that said desired height, such hollow box form also serving for the passage therethrough and about said pipes, tubes or ducts of a coolant.

2. A fluid-bed furnace as claimed in claim 1, wherein the coolant is said combustion promoting gas before or after pressurisation to supply said plenum chamber.

3. A fluid-bed furnace as claimed in claim 1 or 2, wherein the bed support is a single box structure with top and bottom plates secured together at two opposite edges and with a minimum of further bracing between them.

4. A fluid-bed furnace as claimed in claim 3 wherein the bracing is apertured plates one at each other side of the bed proper.

5. A fluid-bed furnace as claimed in claim 3 or 4 wherein the bed support is within furnace walling.

6. A fluid-bed furnace as claimed in claim 3, 4, or 5 wherein the discharge pipes, tubes or ducts are sealingly secured to the bottom plate by way of coupling elements on the pipes, tubes or ducts which also serve in holding in place a packing or packings.

7. A fluid-bed furnace as claimed in claim 6, wherein the packing or packings is or are insulation material extending over the bottom plate to protect the latter during preheating by a burner therebelow.

8. A fluid-bed furnace as claimed in claim 6 or 7 wherein sealing or minimum leakage joints are provided between the discharge pipes, tubes or ducts and the top and bottom plates, which joints permit relative movement to accommodate differential expansion effects.

9. A fluid-bed furnace as claimed in claim 8 wherein sealing of the discharge pipes, tubes or ducts to the top and bottom plates is achieved by the cooperation of flange elements on the pipes, tubes or ducts with deformable packings between them and the upper surface of the top plate, and the use of resilient packing between the lower surface of the bottom plate and the coupling element or insulation material.

10. A fluid-bed furnace as claimed in claim 9 wherein the insulation material is an inner one of two or more layers of material extending over the bottom plate.

11. A fluid-bed furnace substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.