GB2165464A

GB2165464A - Pressurised fluidised bed reactor

Info

Publication number: GB2165464A
Application number: GB08525393A
Authority: GB
Inventors: Michael Robin Judd
Original assignee: South African Inventions Development Corp
Current assignee: South African Inventions Development Corp
Priority date: 1984-10-16
Filing date: 1985-10-15
Publication date: 1986-04-16
Also published as: GB8525393D0; AU4859985A; GB2165464B; DE3536871A1

Abstract

A method of operating a circulating fluidized bed having a draught tube and a circulating fluidized bed apparatus (10) including a housing (12), a draught tube (14) and separate feeds (17, 34) for two different fluidising fluids involves feeding different fluids into the apparatus via the feeds (17, 34) when the housing (12) contains a charge of particulate material, causes fluidizing of the material and circulation of the fluidized material up the interior of the tube (14) and downwardly in the housing outside the tube; and causes two essentially different processes to take place in two essentially separate process zones in the bed, respectively inside and outside the tube (14). In accordance with the method the distribution of the two different fluidizing fluids is such that they are kept essentially separate at the inlet (14.1) to the tube (14), so that one of the fluids passes essentially only up the interior of the draught tube, and not upwards through the fluidized bed outside the draught tube, and the other fluid passes upwards essentially only up through the fluidized bed outside the draught tube and not up the interior of the draught tube. <IMAGE>

Description

SPECIFICATION Pressurised Fluidized Bed Reactor This invention relates to the operation of a circulating fluidized bed having a draught tube wherein fluidized particulate material circulates in the fluidized bed by moving upwardly in the interior of the draught tube and downwardly in the fluidized bed outside the draught tube. More particularly the invention relates to a method of operation of such fluidized bed wherein a process takes place; and to a circulating fluidized bed apparatus having a draught tube and suitable for use in said method.

According to one aspect of the invention, in the operation of a circulating fluidized bed having a draught tube wherein fluidized particulate material circulates in the fluidized bed by moving upwardly in the interior of the draught tube and downwardly in the fluidized bed outside the draught tube, and a process takes place in the fluidized bed, there is provided a method of operation ofthefluidized bed which comprises using two essentially different fluidizing fluids simultaneously to fluidize the particulate material, respectively inside and outside the draught tube, and to circulate the particulate material, thereby causing two essentially different processes to take place in two separate process zones in the fluidized bed, respectively inside and outside the draught tube, the method including the step of distributing said fluids in the bed in a fashion to keep them separate from each other at the inlet to the draught tube such that one of the fluids passes essentially only up the interior of the draught tube, and not upwards through the fluidized bed outside the draught tube, and the other fluid passes upwards essentially only up through the fluidized bed outside the draught tube and not up the interior of the draught tu be.

At least one of the processes may be a physical process, whereby the particulate material and/orthe fluidizing fluid which fluidizes it, is modified physically; or at least one of the processes may be a chemical process whereby the particulate material and/orthefluidizing material which fluidizes it is modified chemically. Typically, both of the processes will be chemical processes.Naturally, some mixing of the fluidizing fluids may take place, whereby some of the fluid used to fluidize the particulate material inside the draught tube finds its way into the fluidized bed outside the draught tube, and/or some of the fluid used to fluidize the particulate material outside the draught tube finds its way into the fluidized bed inside the draught tube, but this mixing, although possibly undesirable, can in accordance with the invention be restricted to acceptable limits, so that the two processes taking place in the fluidized bed remain essentially different from each other.

This distribution of the fluids will accordingly be such as to resist, at the inlet to the draught tube, mixing of the fluid used to fluidize the particulate material inside the draught tube with the fluid used to fluidize the particulate material outside the draught tube, and such as to promote separation of said fluids at the inlet to the draught tube. This may be achieved by employing apparatus for the fluidization, which apparatus has its geometry selected and/or constructed so that such separation is promoted, and by introducing the fluidizing fluids at such positions, pressures and flow rates into the apparatus so that they are distributed in a fashion such that any component of fluid flow in a horizontal direction under the draught tube across the periphery of the inlet to the draught tube, is resisted.

For a draught tube of circular cross-section concentrically located in a fluidized bed of circular cross-section, this will involve resisting any radial component of fluid flow under the circular periphery of the inlet to the draught tube, either radially inward flow of the fluid used to fluidize the bed outside the draught tube, or radially outward flow of the fluid used to fluidize the bed inside the draught tube. In other words, a pressure balance should be sought and promoted at the inlet to the draught tube and thereunder, so that any horizontal component of nett flow under the periphery of the draught tube inlet and across said periphery is resisted. Routine experimentation will be required to determine the best or at least adequate apparatus construction and geometry and fluid feed parameters, to combat said nett flow or cross flow under the periphery of the draught tube inlet.

When a circulating fluidized bed having a draught tube is operated, eg with a gaseous fluidizing fluid and with a solids inventory such that the depth of solids outside the tube above the level of the tube inlet is essentially the same as the height of the tube, the average pressure drop in the fluidizing fluid outside the tube in an upward direction across the depth of the bed from the level of said inlet to the top of the bed, is greater than the pressure drop in the fluidized bed inside the tube in an upward direction across the depth of the bed from the tube inlet to the tube outlet, because of the common or single freeboard above the bed into which the fluidizing fluids discharge from the bed, and because the density of the bed outside the tube is greater than the density of the bed inside the tube.

The fluid pressure in the bed outside the tube around and at the level of the tube inlet will in this case thus be greater than the fluid pressure in the bed immediately under and at the level of the tube inlet, and this will tend to establish a nett horizontal component of fluid flow under and across the periphery of the tube inlet in an inward direction, and thence up and into the tube, so that a proportion of the fluidizing fluid used to fluidize the bed outside the tube will find its way under the tube inlet and thence upwardly into the tube. To combat this, said distributing of the fluids may include the step of operating the bed with a particulate solids inventory selected so that the depth of the bed above the level of the tube inlet outside the tube is less than the height of the tube.This will, for a bed having a fixed geometry, reduce the pressure drop in the bed outside the tube from the level of the tube inlet to the top of the bed, and will counteract said pressure difference and resist said nett inward flow under and across the tube inlet periphery (or even reverse it if desired), thereby promoting pressure balancing and resisting mixing in the tube of the fluidizing fluids.

Instead or in addition, for a fixed bed geometry, adjusted feed rates of the fluidizing fluids used respectivelyforfluidizing the bed inside and outside the tube can be used for pressure balancing. For example, the feed rate to the bed outside the tube may be adjusted, eg decreased, thereby leading to a reduction in the pressure drop outside the tube upwardly over the bed and a reduced pressure in the bed at the level of the tube inlet outside the tube, which in turn can lead to a reduction in nett inward flow under and across the tube inlet periphery and to pressure balancing.Adjusting the feed rate of the fluid outside the tube is preferred to adjusting the feed rate of the fluid used to fluidize the bed inside the tube, as the pressure drop outside the tube is typically more sensitive to fluid food rate changes, bearing in mind the solids-conveying high-voltage slugging-type flow typically encountered in the draught tube, which is relatively insensitive as regards pressure drop to flow rate changes.

Furthermore, and similarly for a fixed geometry, increasing the pressure drop in the bed inside the draught tube, for example by imposing a restriction at its top, i.e. by throttling its outlet, can combat said radially inward flow under and across the periphery of the draught tube inlet Distributing the fluids may thus include feeding the fluids into the bed at respective rates selected to resist any horizontal component of flow of fluid across the periphery of the draught tube inlet, under said inlet.

It is contemplated that, in use, for a fixed geometry, any one or more of the aforementioned adjustments to inventory, fluidizing fluid feed rates and indeed fluidizing fluid inlet pressures can be used to combat said horizontal flow and for pressure balancing.

Turning to the geometry of the apparatus, there will be separate inlets for the two different fluidizing fluids. Adjusting the heights of one or both of these inlets relative to the level of the inlet to the draught tube, eg by raising them to above the level of said inlet, can resist (or even reverse) inward flow under the tube inlet periphery.

Thus, in accordance with the method of the present invention, said distributing the fluids may include feeding at least one of the fluids into the fluidized bed above the level of the draught tube inlet.

Indeed, the method may include feeding both of the fluids into the bed above the level of the draught tube inlet. If there is in this case any danger of the bed's slumping below the level of the draught tube inlet, additional amounts of either or both of the fluids may also be fed into the bed below the level of the draught tube inlet.

Fluid feeding whereby both fluid feeds are above the level of draught tube inlet physically separates the fluid feeds from each other by means of the draught tube wall, but there can still be a tendency, eg beacuse of a greater pressure drop outside the tube, for fluidizing fluid outside the tube to flow downwardly in the bed outside the tube and into the bottom of the tube. Forthis reason the fluidizing fluid feed to the bed outside of the tube may be at a higher level than the fluidizing fluid feed to the draught tube, eg so that the fluid feed to the bed outside the tube is above the level of the feed to the draught tube and is also above the level of the fluid feed to the draught tube, which latter feed can be inside the draught tube as described above, (or if desired at or even below the level of the inlet to the draught tube).

In particular embodiments of the invention the elevations of the draught tube and/or the fluid inlets can thus be selected either alone or in conjunction with such fluid flow rates and pressures and inventory selected, to obtain the desired lack of flow under and across the draught tube periphery inlet, and to obtain separation of said fluids in the tube and outside the tube, while at the same time still preserving the circulation of solid particles up the draught tube and down the bed outside the tube.

Where for example fluid leaving the bed is a desired product, it may be desirable to keep the two fluidizing fluids leaving the bed separate from each other. The method may thus include the step of keeping said fluids separate from each other when they issue from the top of the bed.

Keeping the fluids separate from each other when the issue from the top of the bed may include locating a hood having a skirt over the outlet of the draught tube, so that the skirt extends downwardly around the outlet of the draught tube to below the level of said outlet, and extracting from said hood the fluid issuing from the interior of the draught tube into the hood, the rate of extraction from the hood being substantially the same as the rate at which said fluid issues from the tube into the hood.

Fluid from the tube will be removed from the hood and fluid from the bed outside the tube will be removed from the freeboard outside the hood.

Pressure balancing can also be effected here, for example by adjusting the relative fluid withdrawal rates from the hood and said freeboard, so that there is as little as practicable flow at the top of the tube from said freeboard into the hood or vice versa.

Thus, when the hood has a skirt over the top of the tube, flow through the peripheral space between the shirt and the tube is confined to solids flow and possibly such fluid as is entrained therein, with at little as possible nett fluid flow in either direction.

A circulating fluidized bed of the type in question may have a circular cylindrical draught tube coaxially located in a circular cylindrical housing, to provide an annular zone outside the draught tube; in operation there may be bubble-free fluidization of downwardly circulating particulate solid material in the annulus, and fluidization with upwardly moving bubbles or slugs of fluid in the draught tube which circulates particulate material upwardly in the tube; and the fluidizing fluids may be gases or vapours.

In a particular embodiment of the invention, the two processes may respectively be a synthesis gas production process in a synthesis gas production process zone and a combustion process in a combustion process zone, the particulate material being derived from carbon-containing feedstock, the fluidizing fluid which is fed into the particulate material in the synthesis gas production process zone containing steam, and the fluidizing fluid which is fed into the particulate material in the combustion process zone containing oxygen.More particularly, the carbon-containing feedstock may contain carbon in the form of at least one member of the group comprising coal, eg duff coal or coal discards, wood, eg wood chips, and fly-ash, the fluidizing fluid fed into the synthesis gas production process zone consisting essentially of steam, and the fluidizing fluid fed into the combustion process zone consisting essentially of air. The synthesis gas production process zone may be in the interior of the draught tube, the combustion process zone being in the bed outside the draught tube.

According to another aspect of the invention there is provided a circulating fluidized bed apparatus which comprises a housing, a draught tube within the housing and separate fluid feeds for feeding two essentially different fluidizing fluids simultaneously into the apparatus, respectively for fluidizing particulate material inside the tube and for fluidizing particulate material in the housing outside the tube, said feeds being constructed and located to permit, with an appropriate inventory of particulate material in the apparatus, simultaneous feeding of said different fluids into the apparatus in a fashion whereby the particulate material in the apparatus is fluidized both inside and outside the tube and is caused to circulate in the apparatus upwardly in the tube and downwardly outside the tube, and whereby the fluidizing fluids are distributed in a fashion to keep them separate from each other at the inlet to the draught tube such that one of the fluids passes essentially only up the interior of the draught tube, and not upwards through the fluidized bed outside the draught tube, and the other fluid passes upwards essentially only up through the fluidized bed outside the draught tube and not up the interior of the draught tube.

The apparatus may include a fluid feed for the fluidizing fluid forfluidizing particulate material outside the tube which feed is located above the level of the inlet to the draught tube; and the apparatus may include a fluid feed for the fluidizing fluid for fluidizing particulate material outside the tube which is located above the level of the feed for the fluidizing fluid for fluidizing particulate material inside the tube. The apparatus may include a fluid feed for fluidizing fluid for fluidizing particulate material inside the tube which is located under the inlet to the tube below the level of the inlet to the tube.

The apparatus may include a hood over the outlet of the tube and having a skirt which extends downwardly around the outlet of the tube to a level below the outlet of the tube, the apparatus having a fluid outlet from the hood above the level of the outlet of the tube and a fluid outlet from the freeboard of the housing, outside the hood and above the level of the outlet of the tube.

In use the apparatus will be charged with a suitable charge or inventory of the particulate material to be fluidize, such that the skirt of the hood will dip into a fluidized bed of the particulate material created in the housing outside the tube, to provide a fluid (hydraulic or pneumatic) seal between the outlet of the tube and the bed outside the outlet of the tube.

In principle there may be a plurality of fluid feeds for the fluidizing fluid forfluidizing the particulate material outside the tube, optionally at different positions and elevations. There may also be a plurality of fluid feeds for the fluidizing fluid for fluidizing the particulate material inside the tube, also optionally at different positions and elevations, e.g. one inside the tube above the tube inlet and one below the tube under the tube inlet. These feeds may comprise fluid spargers, fluid distributor plates (which may be conical in shape) and/or perforated tubes. Furthermore the apparatus in principle comprise several draught tubes, each with its own hood and its own fluid feed.However, in a particular embodiment of the invention the apparatus may comprise a single circular-cylindrical draught tube, concentrically located in a circular-cylindricai housing, defining an annular zone in the housing outside the tube. In this embodiment, the apparatus may have a single feed forfluidizing fluid for fluidizing particulate material outside the tube in the form of an annular or ring-like fluid sparger, e.g. a gas sparger, concentrically surrounding the tube at a level above the level of the inlet to the tube, the apparatus including a single feed forfluidizing fluid for fluidizing particulate material inside the tube in the form of an inverted conical fluid distributor plate which forms a floor for the fluidized bed in the housing spaced below the draught tube, the apex of said plate pointing downwardly.Optionally there may be an additional feed for fluidizing fluid for fluidizing particulate material inside the tube, in the form of a fluid sparger inside the tube above the level of the inlet to the tube.

In the apparatus of the invention the relative elevation between the draught tube and at least one member of the group comprising the hood and the fluid feeds may be adjustable, at least one of the fluid feeds being provided with adjustable supply means. In other words the elevation of one or more of the fluid feeds, the draught tube and the hood in the housing may be adjustable, and the fluid feeds may be provided with fluid supply means, such as pumps, compressors or blowers, of variable capacity and/or output pressure. Similarly, variable capacity withdrawal means may be provided for withdrawing fluids from the hood and freeboard.

The method and apparatus of the invention are particularly useful for circulating fluidized bed operation where two different processes are carried out simultaneously in two separate process zones, one in the draught tube and one in the fluidized bed outside the draught tube. Thus, for example, in the gasification of coal, coal discards, duff coals, wood such as wood chips, fly ashes and other carbon-rich feedstocks, synthesis quality gas can be produced using air as an oxidizing and fluidizing fluid in the fluidized bed outside the draught tube, and a steam as the fluidizing fluid inside the draught tube.By adjusting the geometry of the system and the flow rates of the air and steam as described above, sufficient separation of the steam and air can take place, particularly at the inlet to the draught tube, to create two separate reaction zones in the apparatus, namely a combustion process zone in the annulus outside the draught tube, and a synthesis gas production process zone inside the draught tube, (or, conversely, combustion in the draught tube and gasification in the annulus). In the former case, combustion products are withdrawn from the freeboard of the apparatus outside the hood, and synthesis gas is withdrawn from the hood.

The invention will now be described, by way of example, with reference to the accompanying diagrammatic drawing, in which the single Figure shows a schematic sectional side elevation of a fluidized bed apparatus in accordance with the invention.

In the drawing the fluidized bed apparatus is designated 10 and comprises a circular-cylindrical housing 12 within which a circular-cylindrical draught tube 14 is concentrically located. The tube 14 has a lower inlet 14.1 and an upper outlet 14.2, constituted respectively by its open lower and upper ends. The apparatus is shown with a fluid feed in the form of an inverted conical distributor plate 16 below the draught tube 14, and with a hood 18 above the outlet of draught tube 14. A steam supply is designated 20 and a combustion air supply is designated 22, with a synthesis gas outlet being designated 24 and a combustion gas outlet designated 26. The supplies 20 and 22 are respectively provided with adjustable supply means eg in the form of a steam generator and air compressor respectively designated 23 and 25.

In the drawing, the steam supply 20 is shown entering the bed via the distributor 16 at a steam feed 17 which is at a central position, more or less adjacent its lowermost inverted apex, directly under and spaced below the inlet 14.1 to the tube 14. The synthesis gas outlet 24 is from the hood 18 at a level above the level of the outlet 14.2 of the tube 14 and the combustion gas outlet 26 is from the freeboard 27 of the housing 12 at a level above the level of outlet 14.2 of the tube 14. The housing 12 and tube 14 are shown containing a fluidized bed 28, i.e.

charged with an inventory of particulate material 28 derived from coal feedstock. Bubbles 30 are shown diagrammatically in the drawing, moving up the interior of the draught tube 14, to designate slug flow.

It should be noted particularly that the hood 18 has a skirt 32 which extends downwardly around the top of the tube 14, and which skirt 32 is spaced radially from the top of the tube 14 by an annular space 33. It should be noted that the air supply is to a feed in the form of an annular or ring-like sparger 34 located concentrically around the tube 14 above the level of the steam feed 17 through the distributor 16 and above the level of the inlet 14.1 to the tube 14.

In use steam 20 and air 22 are fed to the fluidized bed, causing circulation as shown by the arrows of the particulate material, including the coal to be gasified, in the fluidized bed, by slug flow an pneumatic elevation of particulate coal up the interior 14.0 draught tube 14, from which the particulate material overflows via the interior of the hood 18 and via the annular space 33 between the skirt 32 and the top of the tube 14, into the annular region 35 in the housing 12 around the draught tube 14. In this annular region 35 between the tube 14 and housing 12 the particulate coal isfluidized, but insufficiently to prevent it from circulating downwardly in the annular region 35, and thence again into the bottom of the draught tube 14.

In operation there is a danger that combustion air from the air supply 22 can find its way into the interior of the draught tube 14 where it can cause undesirable combustion, and combustion gases from this air, and combustion gas which can find their way into the hood 18 above the fluidized bed, can lead to contamination of the synthesis gas product in the hood 18 or outlet 24, with the danger of recombination, which can be catastrophic and explosive.

Accordingly, the apparatus in accordance with the invention has the combustion air feed via the sparger 34 a bove the level of the inlet 14.1 to the draught tube 14, and the flow rates and pressures of the air supply 22 and steam supply 20 are adjusted by means of their respective supply means 25 and 23 such that there is pressure balancing at the inlet 14.1 to the draught tube 14, so that little if any combustion air enters the draught tube at its inlet and also so that little if any steam passes up the annulus 35 in the housing 12 outside the draught tube 14.Furthermore, the rate of withdrawal of synthesis gas product via the outlet 24 from the wood 18 and combustion gas via the outlet 26 from the freeboard 27 of the housing 12, are adjusted such that there is pressure balancing at the top or outlet 14.1 of the tube 14, and so that very little crossflow, if any, in either direction from the interior of the hood 18 to the freeboard 27 outside the hood, or vice versa. The dipping of the shirt 32 of the hood 18 around the top or outlet 14.1 of the tube 14to a level below the upper surface 36 of the particulate material 28 outside the tube 14, also assists in providing a pneumatic seal to combat such crossflow.In this regard it will be noted that the inventory of material 28 shown in the drawing has been selected, further to assist in pressure balancing, and the level of upper surface 36 of the particulate material 28 in the annular region 35 is accordingly below that of the outlet 14.2 of the draughttube.

The result is that good separation of the combustion process zone in the annular space 35 in the material outside the tube 14 from the synthesis process zone in the interior 14.0 of the tube 14 is achieved. Increased safety is thus promoted, together with good gas quality.

Using the fluidized bed apparatus shown has the further advantages that elutriation of fine fuel particles is reduced, and particulate feed material such as coal can be dispersed into comparatively inert bed material containing ash, which reduces the undesirable likelihood of agglomeration of feed coal and sintering thereof at feed points. Tars and oils in carbonaceous feedstocks are cracked and heavy hydrocarbon products have therefore not been observed in either the product synthesis gas stream or the combustion gas waste stream.

After-burning and synthesis gas reversion are effectively eliminated by the invention, and the draught tube design lends itself to long, thin-vessel geometry, which is economically favourable for pressurised operation, while reducing the likelihood of undesirable bubble growth phenomena. Use of a conical distributor below the draught tube resists buckling, and cyclone dip legs from cyclones (not shown) for combustion gas and synthesis gas removal in the freeboard or hood (or outside the housing) can be essentially straight, terminating in regions suitable for discharge without foot valves.

Returned particulate fines from said cyclone dip legs, can be returned into the downwardly moving bed in the zone 35 outside the draught tube so that they are unlikely to be re-elutriated.

It is contemplated that the system described with reference to the drawings can be used to provide a low energy combustion gas, or a synthesis quality gas, using duff coal which need not be graded. Size distribution can vary within wide limits without materially affecting gasifier operation, and a small proportion of larger particles up to 25 mm in size can be acceptable. Coal quality is not critical and grades of coal or char containing up to 50% by mass of ash can be used. Clearly, the higher the ash content of the coal, the less favourable will be the heat balance.An important requirementforthe ash is that its softening point should be greater than the contemplated reaction temperature of about 950"C, and its softening point should preferably be higher than 1200"C. Drying of coal feed may be necessary but only to ensure satisfactory transport properties.

The only other requirements for the process are water, used primarily for steam generation, and air for combustion. Diesel fuel or fuel gas can be used for initial heating of the process during start-up.

Coal can be fed into the combustion region outside the draught tube, below the level of the fluidized bed therein, by means of screw feeders, rotary valves or the like.

The process involves the autothermic gasification of the coal in a fluidized bed with steam in the draught tube, air being used for combustion in the fluidized bed outside the draught tube to provide the heat required for the gasification. It is contemplated that one version of the process suitable for fuel gas or power gas production may take place at marginally above atmospheric pressure, and another version of the process, suitable for synthesis gas production, may take place at a more elevated pressure. The housing 12 is typically a steel vessel lined with ceramic insulation. The outer shell of the vessel may be air cooled.

It is contemplated that the particulate fluidized bed material will comprise about 10% by mass of coal and about 90% by mass of inert material, initially refractory sand (used for start-up) but predominantly ash after the process has operated for some time. The operating temperature can be in the region of about 900"C to 1 2000C, and will be substantially constant throughout the fluidized bed, this temperature being selected so that substantially no tars are formed during the gasification.

Typically the combustion gas products from the annulus outside the draught tube will comprise essentially carbon dioxide and nitrogen, and have no substantial value other than sensible heat. Fine material, ie that of less than 100 microns particle size, which is mainly ash and is carried through the cyclones used to return elutriated material to the bed, can be removed in secondary cyclones, quenched and discarded. The clean combustion gas can then be passed through heat exchangers to recover its sensible heat, eg for steam production, and then discarded.

The synthesis or power gas withdrawn separately through the hood is dedusted in a similar fashion, cooled in heat exchangers, and can be wet scrubbed in order to remove essentially all the particulate matter and to reduce the dew point thereof.

Conventional desulphurizing may be effected thereon, depending on the gas quality required.

Coarse ash removal and bed inventory management can be achieved either by removing a proportion of the bed material from the annular gasification zone, quenching and discarding it, and/ or by adding fine refractory sand, depending on the particular characteristics of the coal being gasified.

Steam for the process can be obtained from a waste heat boiler using waste heat from the combustion gases, and a variable capacity Roots-type blower can be used as the supply means 25 for feeding the combustion air into the process, preferably preheated to about 500"C by waste gas from the process.

Claims

1. In the operation of a circulating fluidized bed having a draught tube wherein fluidized particulate material circulates in the fluidized bed by moving upwardly in the interior of the draught tube and downwardly in the fluidized bed outside the draught tube, and a process takes place in the fluidized bed, a method of operation of the fluidized bed which comprises using two essentially different fluidizing fluids simultaneously to fluidize the particulate material, respectively inside and outside the draught tube, and to circulate the particulate material, thereby causing two essentially different processes to take place in two essentially separate process zone in the fluidized bed, respectively inside and outside the draught tube, the method including the step of distributing said fluids in the bed in a fashion to keep them separate from each other at the inlet to the draught tube such that one of the fluids passes essentially only up the interior of the draught tube, and not upwards through the fluidized bed outside the draught tube, and the other fluid passes upwards essentially only up through the fluidized bed outside the draught tube and not up the interior of the draught tube.

2. A method as claimed in claim 1, in which said distributing of the fluids includes the step of operating the bed with a particulate solids inventory selected so that the depth of the bed above the level of the tube inlet outside the tube is less than the height of the tube.

3. A method as claimed in claim 1 or claim 2, in which distributing the fluids includes feeding the fluids into the bed at respective rates selected to resist any horizontal component of flow of fluid across the periphery of the draught tube inlet under said inlet.

4. A method as claimed in any one of claims 1 to 3 inclusive, in which said distributing the fluids includes feeding at least one of the fluids into the fluidized bed above the level of the draughttube inlet.

5. A method as claimed in claim 4, which includes feeding both of said fluids into the bed above the level of the draught tu be inlet

6. A method as claimed in any one of the preceding claims, which includes the step of keeping said fluids separate from each other when they issue from the top of the bed.

7. A method as claimed in claim 6, in which keeping the fluids separate from each other when they issue from the top of the bed includes locating a hood having a skirt over the outlet of the draught tube, so that the hood extends downwardly around the outlet of the draught tube to below the level of said outlet, and extracting from said hood the fluid issuing from the interior of the draught tube into the hood, the rate of extraction from the hood being substantially the same as the rate at which said fluid issues from the tube into the hood.

8. A method as claimed in any one of the preceding claims, in which the two processes are respectively a synthesis gas production process in a synthesis gas production process zone and a combustion process in a combustion process zone, the particulate material being derived from carboncontaining feedstock, the fluidizing fluid which is fed into the particulate material in the synthesis gas production process zone containing steam, and the fluidizing fluid which is fed into the particulate material in the combustion process zone containing oxygen.

9. A method as claimed in claim 8, in which the carbon-containing feedstock contains carbon in the form of at least one member of the group comprising coal, wood and fly-ash, the fluidizing fluid fed into the synthesis gas production process zone consisting essentially of steam, and the fluidizing fluid fed into the combustion process zone consisting essentially of air.

10. A method as claimed in claim 8 or claim 9, in which the synthesis gas production process zone is in the interior of the draught tube and the combustion process zone is in the bed outside the draught tube.

11. A circulating fluidized bed apparatus which comprises a housing, a draught tube within the housing and separate fluid feeds for feeding two essentially different fluidizing fluids simultaneously into the apparatus, respectively for fluidizing particulate material inside the tube and for fluidizing particulate material in the housing outside the tube, said feeds being constructed and located to permit, with an appropriate inventory of particulate material in the apparatus, simuitaneous feeding of said different fluids into the apparatus in a fashion whereby the particulate material in the apparatus is fluidized both inside and outside the tube and is caused to circulate in the apparatus upwardly in the tube and downwardly outside the tube, and whereby the fluidizing fluids are distributed in a fashion to keep them separate from each other at the inlet to the draught tube such that one of the fluids passes essentially only up the interior of the draught tu be, and not upwards through the fluidized bed outside the draught tube, and the other fluid passes upwards essentially only up through the fluidized bed outside the draught tube and not up the interior of the draught tube.

12. An apparatus as claimed in claim 11, which includes a fluid feed for the fluidizing fluid for fluidizing particulate material outside the tube which feed is located above the level of the inlet to the draught tube.

13. An apparatus as claimed in claim 11 or claim 12, which includes a fluid feed for fluidizing fluid for fluidizing particulate material outside the tube which is located above the level of the feed for fluidizing fluid for fluidizing particulate material inside the tube.

14. An apparatus as claimed in any one of claims 11 to 13 inclusive, which includes a fluid feed for fluidizing fluid forfluidizing particulate material inside the tube which feed is located underthe inlet to the tube, below the level of the inlet to the tube.

15. An apparatus as claimed in any one of claims 11 to 14 inclusive, which includes a hood over the outlet of the tube and having a shirt which extends downwardly around the outlet of the tube to a level below the outlet of the tube, the apparatus having a fluid outlet from the hood above the level of the outlet of the tube and a fluid outlet from the freeboard of the housing, outside the hood and above the level of the outlet of the tube.

16. An apparatus as claimed in any one of claims 11 to 15 inclusive, which comprises a single circularcylindrical draught tube, concentrically located in a circular-cyiindrical housing, defining an annular zone in the housing outside the tube.

17. An apparatus as claimed in claim 16, which has a single feed for fluidizing fluid for fluidizing particulate material outside the tube in the form of an annular fluid sparger, concentrically surrounding the tube at a level above the level of the inlet to the tube, the apparatus including a single feed for fluidizing fluid for fluidizing particulate material inside the tube in the form of an inverted conical fluid distributor plate which forms a floor for the fluidized bed in the housing spaced below the draught tube, the apex of said plate pointing downwardly.

18. An apparatus as claimed in any one of claims 11 to 17 inclusive, in which the relative elevation between the draught tube and at least one member of the group comprising the hood and the fluid feeds is adjustable, at least one of the fluid feeds being provided with adjustable supply means.

19. In the operation of a circulating fluidized bed having a draught tube wherein fluidized particulate material circulates in the fluidized bed by moving upwardly in the interior of the draught tube and downwardly in the fluidized bed outside the draught tube, and a process takes place in the fluidized bed, a method of operation substantially as described herein.

20. A circulating fluidized bed apparatus substantially as described and as illustrated herein.