HK1001737B

HK1001737B - Fluidized bed combustion system

Info

Publication number: HK1001737B
Application number: HK98100598.9A
Authority: HK
Inventors: Fazaleabas Abdulally Iqbal
Original assignee: Foster Wheeler Energy Corporation
Priority date: 1991-05-15
Filing date: 1998-01-22
Publication date: 1998-07-03

Description

This invention relates to a fluidized bed combustion system, more particularly, to such a system in which a recycle heat exchanger is formed integrally with the furnace section of the system.

Fluidized bed combustion systems are well known and include a furnace section in which air is passed through a bed of particulate material, including a fossil fuel, such as coal, and a sorbent for the oxides of sulphur generated as a result of combustion of the coal, to fluidize the bed and to promote the combustion of the fuel at a relatively low temperature. These types of combustion systems are often used in steam generators in which water is passed in a heat exchange relationship to the fluidized bed to generate steam and permit high combustion efficiency and fuel flexibility, high sulphur adsorption and low nitrogen oxides emissions.

The most typical fluidized bed utilized in the furnace section of these type systems is commonly referred to as a "bubbling" fluidized bed in which the bed of particulate material has a relatively high density and a well-defined, or discrete, upper surface. Other types of systems utilize a "circulating" fluidized bed in which the fluidized bed density is below that of a typical bubbling fluidized bed, the fluidizing air velocity is equal to or greater than that of a bubbling bed, and the flue gases passing through the bed entrain a substantial amount of the fine particulate solids to the extent that they are substantially saturated therewith.

Circulating fluidized beds are characterized by relatively high internal and external solids recycling which makes them insensitive to fuel heat release patterns, thus minimizing temperature variations and, therefore, stabilizing the sulphur emissions at a low level. The high external solids recycling is achieved by disposing a cyclone separator at the furnace section outlet to receive the flue gases, and the solids entrained thereby, from the fluidized bed. The solids are separated from the flue gases in the separator and the flue gases are passed to a heat recovery area while the solids are recycled back to the furnace. This recycling improves the efficiency of the separator, and the resulting increase in the efficient use of sulphur adsorbent and fuel residence times reduces the adsorbent and fuel consumption.

In the operation of these types of fluidized beds, and, more particularly, those of the circulating type, there are several important considerations. For example, the flue gases and entrained solids must be maintained in the furnace section at a particular temperature (usually approximately 871°C) consistent with proper sulphur capture by the adsorbent. As a result, the maximum heat capacity (head) of the flue gases passed to the heat recovery area and the maximum heat capacity of the separated solids recycled through the cyclone and to the furnace section are limited by this temperature. In a cycle requiring only superheat duty and no reheat duty, the heat content of the flue gases at the furnace section outlet is usually sufficient to provide the necessary heat for use in the heat recovery area of the steam generator downstream of the separator. Therefore, the heat content of the recycled solids is not needed.

However, in a steam generator using a circulating fluidized bed with sulphur capture and a cycle that requires reheat duty as well as superheater duty, the existing heat available in the flue gases at the furnace section outlet is not sufficient. At the same time, heat in the furnace cyclone recycle loop is in excess of the steam generator duty requirements. For such a cycle, the design must be such that the heat in the recycled solids must be utilized before the solids are reintroduced to the furnace section.

To provide this extra heat capacity, a recycle heat exchanger is sometimes located between the separator solids outlet and the fluidized bed of the furnace section. The recycle heat exchanger includes heat exchange surfaces and receives the separated solids from the separator and functions to transfer heat from the solids to the heat exchange surfaces at relatively high heat transfer rates before the solids are reintroduced to the furnace section. The heat from the heat exchange surfaces is then transferred to cooling circuits to supply reheat and/or superheat duty.

The simplest technique for controlling the amount of heat transfer in the recycle heat exchanger is to vary the level of solids therein. However, situations exist in which a sufficient degree of freedom in choosing the recycle bed height is not available, such as for example, when a minimum fluidized bed solids depth or pressure is required for reasons unrelated to heat transfer. In this case, the heat transfer may be controlled by utilizing "plug valves" or "L valves" for diverting a portion of the recycled solids so that they do not give up their heat in the recycle heat exchanger. The solids from the diverting path and from the heat exchanger path are recombined, or each stream is directly routed to the furnace section, to complete the recycle path. In this manner, the proper transfer of heat to the heat exchanger surface is achieved for the unit load existing. However, these type arrangements require the use of moving parts within the solids system and/or need external solids flow conduits with associated aeration equipment which adds considerable cost to the system.

Also in connection with these type of steam generators, and especially those using a circulating fluidized bed, load is controlled by regulating the solids recycle rate. This normally requires the use of a metering cooler, such as a water cooled screw, to remove solids from the recycle system. This adds mechanical complexity and costs penalties in addition to requiring downstream handling equipment. In U.S. Patent No. 4,781,574 this latter problem was addressed by disposing an air source at the separated solids outlet of a cyclone separator and discharging air into the separator in a direction opposite to the direction of flow of the separated solids. The air entrained a portion of the solids and was passed back through the separator and to the heat recovery area. Although this technique enabled the solids inventory to be controlled without incurring significant additional costs, it interfered with the operation of the separator.

EP-A-413611 describes a fluidised bed reactor where the recycling of separated entrained solids is controlled by immersing the lower end of a dipleg leading from the separator in a seal vessel, and controlling fluidisation of the particles in that seal vessel to control the rate of recycle through that seal vessel. There is no provision for cooling the recycled solids.

It is therefore an object of the present invention to provide a fluidised bed combustion system which enjoys further improvements over the prior art.

According to the invention there is provided a fluidized bed combustion system comprising an enclosure, a furnace section into which combustible particulate material is introduced, fluidizing means for fluidizing the particulate material into the furnace section to promote combustion of that particulate material, separating means for receiving a mixture of flue gases and entrained particulate material from the furnace section and separating the entrained particulate material from the flue gasses, a heat recovery section for receiving the separated flue gases, a dipleg for returning the separated material directly from the separating means to an inlet compartment, means for introducing air or gas into the inlet compartment in a flow path aligned with the flow path of the separated material form the dipleg, and means for returning separated material to the furnace section, characterised in that the enclosure is divided into the said furnace section and a recycle heat exchange section, the recycle heat exchange section being divided into the said inlet compartment and a heat exchange compartment, heat exchange means disposed in at least part of the heat exchange compartment, and means for passing the separated material from the inlet compartment to the heat exchange compartment.

In a system according to the present invention the recycle heat exchanger can include heat exchanger surfaces disposed in a relatively large area between inlet and outlet compartments to ensure a uniform distribution of the separated solids through the recycle heat exchanger to increase the heat exchange efficiency and ensure a uniform discharge of solids to the furnace.

Also, the recycle heat exchanger can be isolated from pressure fluctuations in the furnace and the solids are driven from the recycle heat exchanger to the furnace by height differentials. The need for expensive J-valves and associated ducting and a metering device and downstream handling equipment is eliminated. Further, the number of cyclone separators can be varied.

The recycle rate for the separated solids can be controlled by continuously removing recycled solids from the inlet chamber of the recycle heat exchanger.

In one embodiment of the invention a bypass passage is provided through which the solids pass during start-up and low load conditions.

In another embodiment of the invention air is introduced into the inlet compartment in two paths in a direction generally opposite to that of the flow of separated solids through the vessel. One of the latter air flow paths is from a location below the separator dipleg and in alignment therewith, and the other flow path surrounds the first flow path. The air flow through each path can be separately adjusted as necessary to facilitate the recycle of solids.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic representation depicting the system of the present invention;
Fig. 2 is an enlarged cross-sectional view taken along the line 2-2 of Fig. 1;
Fig. 3 is an enlarged, cross-sectional view taken along the line 3-3 of Fig. 2; and
Fig. 4 is an enlarged partial, enlarged perspective view of a portion of a wall of the enclosure of the system of Fig. 1; and
Fig. 5 is an enlarged sectional view taken along the line 5-5 of Fig. 1.

Description of the Preferred Embodiment

The drawings depict the fluidized bed combustion system of the present invention used for the generation of steam and including an upright water-cooled enclosure, referred to in general by the reference numeral 10, having a front wall 12, a rear wall 14 and two sidewalls 16a and 16b (Figs. 2 and 3). The upper portion of the enclosure 10 is enclosed by a roof 17 and the lower portion includes a floor 18.

A plurality of air distributor nozzles 20 are mounted in corresponding openings formed in a plate 22 extending across the lower portion of the enclosure 10. The plate 22 is spaced from the floor 18 to define an air plenum 24 which is adapted to receive air from external sources (not shown) and selectively distribute the air through the plate 22 and to portions of the enclosure 10, as will be described.

A feeder system, shown in general by the reference numeral 25, is provided adjacent the front wall 12 for introducing particulate fuel into the enclosure 10. An adsorbent, such as limestone, is introduced, via a pipe 25', into the outlet pipe of the coal feeder system from which it also is introduced into the enclosure 10. The mixture of coal and adsorbent particulate material is fluidized by the air from the plenum 24 as it passes upwardly through the plate 22. This air promotes the combustion of the fuel and the limestone adsorbs the sulphur generated by the combustion of the fuel. The resulting mixture of combustion gases and the air (hereinafter termed "flue gases") rises in the enclosure by forced convection and entrains a portion of the solids to form a column of decreasing solids density in the upright enclosure 10 to a given elevation, above which the density remains substantially constant.

A cyclone separator 26 extends adjacent the enclosure 10 and is connected thereto via a duct 28 extending from an outlet provided in the rear wall 14 of the enclosure 10 to an inlet provided through the separator wall. Although reference is made to one separator 26, it is understood that one or more additional separators (not shown) may be disposed behind the separator 26. The number and size of separators used is determined by the capacity of the steam generator and economic considerations.

The separator 26 receives the flue gases and the entrained particle material from the enclosure 10 in a manner to be described and operates in a conventional manner to disengage the particulate material from the flue gases due to the centrifugal forces created in the separator. The separated flue gases, which are substantially free of solids, pass, via a duct 30 located immediately above the separator 26, into a heat recovery section shown in general by the reference numeral 32.

The heat recovery section 32 includes an enclosure 34 divided by a vertical partition 36 into a first passage which houses a reheater 38, and a second passage which houses a primary superheater 40 and an upper economizer 42, all of which are formed by a plurality of heat exchange tubes extending in the path of the flue gases as they pass through the enclosure 34. An opening 36a is provided in the upper portion of the partition 36 to permit a portion of the gases to flow into the passage containing the superheater 40 and the upper economizer 42. After passing across the reheater 38, the superheater 40 and the upper economizer 42 in the two parallel passes, the gases pass through a lower economizer 44 before existing the enclosure 34 through an outlet 46.

As shown in Figs. 1 and 5, the floor 18 and the plate 22 are extended past the rear wall 14 and a pair of vertically extending, spaced, parallel partitions 50 and 52 extend upwardly from the floor 18. The upper portion of the partition 50 is bent towards the wall 14 to form a sealed boundary, and then towards the partition 52 with its upper end extending adjacent, and slightly bent back from, the latter wall, to form another sealed boundary. Several openings are provided through the wall 14 and the partitions 50 and 52 to establish flow paths for the solids, as will be described.

The front wall 12 and the rear wall 14 define a furnace section 54 (Fig. 1), the partitions 50 and 52 define a heat exchanger enclosure 56 and the rear wall 14 and the partition 50 define an outlet chamber 58 for the enclosure 56 which chamber is sealed off at its upper portion by the bent portion of the partition 50. A vent pipe 59 connects an opening in the rear wall 14 with an opening in the partition 50 to place the furnace section 54 and the heat exchanger enclosure 56 in communication for reasons to be described. A plurality of heat exchange tubes 60 are disposed in the heat exchanger enclosure 56 and will be described in detail later.

A subenclosure 61 is mounted on the outer surface of the partition 52 to define an inlet compartment 62. The floor 18 and the plate 22, and therefore the plenum 24, extend through the chamber 58, the enclosure 56 and the compartment 62. Additional nozzles 20 are provided through the extended portions of the plate 22.

The lower portion of the separator 26 includes a hopper 26a which is connected to a dip leg 63a which extends directly into the inlet compartment 62 to transfer the separated solids from the separator 26 to the latter compartment. The reference numerals 63b and 63c (Fig. 2) refers to the diplegs associated with two additional separators disposed behind the separator 26.

As shown in Figs. 2 and 3, a pair of partitions 64 and 65 extend between, and perpendicular to, the partitions 50 and 52 to divide the heat exchanger enclosure 56 into three compartments 56a, 56b and 56c.

The heat exchange tubes 60 are shown schematically in Figs. 2 and 3, and are located in the compartments 56a and 56c where they are divided into two spaced groups as shown to permit the installation of spray attemperation units (not shown) in the space for temperature control of superheat. The partitions 64 and 65 also divide the plenum 24 into three sections 24a, 24b and 24c (Fig. 3) extending immediately below the heat exchanger compartments 56a, 56b and 56c. It is understood that dampers, or the like, (not shown) can be provided to selectively distribute air to the individual sections 24a, 24b and 24c.

As better shown in Fig. 2, three spaced cylindrical partitions 66a, 66b and 66c are disposed in the plenum section 24 extending below the compartment 62 below, and in registery with, the diplegs 63a, 63b and 63c, respectively. As shown in Fig. 5, a conduit 70 is connected to the partitions 66a and a conduit 72 is connected to the remaining portion of the latter portion of the plenum section 24 extending below the compartment 62 for introducing air or gas from a source (not shown) into the portion of the plenum section, respectively. The air or gas passes upwardly through the extended grate 22 and the nozzles 20 associated therewith, to fluidize the separated material in the compartment 62 as will be described. Two valves 74 and 76 are provided in the conduits 70 and 72, respectively, for controlling the flow rate of the air or gas passing therethrough. It is understood that conduits and valves identical to the conduits 70 and 72 and the valves 74 and 76 are associated with the partitions 66b and 66c and the corresponding portions of the plenum 24.

Four horizontally-spaced openings (Fig. 2) 52a are provided in the lower portion of the partition 52 to connect the compartment 62 to the enclosure 56 so that the particulate material from the former passes into the interior of the latter. Four spaced openings 50a (Figs. 2 and 3) are formed in an intermediate portion of those portions of the partition 50 defining the compartments 56a and 56c. An opening 50b is also formed in that portion of the partition 50 defining the compartment 56b and extends at an elevation higher than the openings 52a. Five horizontally-spaced openings 14a (Figs. 1, 2 and 5) are formed in the lower portion of the rear wall 14.

The front wall 12, the rear wall 14, the sidewalls 16a and 16b, the partitions 50 and 52, the roof 17, and the walls defining the heat recovery enclosure 34 all are formed of membrane-type walls an example of which is depicted in Fig. 4. As shown, each wall is formed by a plurality of finned tubes 78 disposed in a vertically extending, air tight relationship with adjacent finned tubes being connected along their lengths.

A steam drum 80 (Fig. 1) is located above the enclosure 10 and, although not shown in the drawings, it is understood that a plurality of headers are disposed at the ends of the various walls described above. Also, a plurality of downcomers and pipes, such as shown by the reference numerals 82 and 84, respectively, are utilized to establish a steam and water flow circuit through the tubes 78 forming the aforementioned water tube walls, along with connecting feeders, risers, headers, etc. The boundary walls of the cyclone separator 26, the heat exchanger tubes 60 and the tubes forming the reheater 38 and the superheater 40 are steam cooled while the economizers 42 and 44 receive feed water and discharges it to the drum 80. Water is passed in a predetermined sequence through this flow circuitry to convert the water to steam and heat the steam by the heat generated by combustion of the particulate fuel material in the furnace section 54.

In operation, particulate fuel material and a sorbent material (hereinafter referred to as "solids") are introduced into the furnace section 54 through the feeder system 25. Alternately, sorbent may also be introduced independently through openings in furnace walls 12, 14, 16a and 16b. Air from an external source is introduced at a sufficient pressure into that portion of the plenum 24 extending below the furnace section 54 and the air passes through the nozzles 20 disposed in the furnace section 54 at a sufficient quantity and velocity to fluidize the solids in the latter section.

A lightoff burner (not shown), or the like, is provided to ignite the fuel material in the solids, and thereafter the fuel material is self-combusted by the heat in the furnace section. The mixture of air and gaseous products of combustion (hereinafter referred to as "flue gases") passes upwardly through the furnace section 54 and entrains, or elutriates, a majority of the solids. The quantity of the air introduced, via the air plenum 24, through the nozzles 20 and into the interior of the furnace section 54 is established in accordance with the size of the solids so that a circulating fluidized bed is formed, i.e. the solids are fluidized to an extent that substantial entrainment or elutriation thereof is achieved. Thus the flue gases passing into the upper portion of the furnace section 54 are substantially saturated with the solids and the arrangement is such that the density of the bed is relatively high in the lower portion of the furnace section 54, decreases with height throughout the length of this furnace section and is substantially constant and relatively low in the upper portion of the furnace section.

The saturated flue gases in the upper portion of the furnace section 54 exit into the duct 28 and pass into the cyclone separator(s) 26. In each separator 26, the solids are separated from the flue gases and the former passes from the separator through the dipleg 63 and into the subenclosure 62. The cleaned flue gases from the separator 26 exit, via the duct 30, and pass to the heat recovery section 32 for passage through the enclosure 34 and across the reheater 38, the superheater 40, and the economizers 42 and 44, before exiting through the outlet 46 to external equipment.

The separated solids from the diplegs 63a, 63b and 63c enter the inlet compartment 62. Air or gas is injected into the compartments 66a, 66b and 66c, via the conduits 70, and air or gas is normally injected into the compartment 62a via the conduits 72. The air or gas passes through the plate 22 to fluidize the separated solids in the compartment 62. The flow of air or gas in this manner into the compartment 62 is regulated by the valves 74 and 76 to regulate the flow of separated solids from the separator 26, through the compartment 62 and into the compartments 56a and 56c respectively, via the openings 52a in the partition 52. Air is passed into the plenum sections 24a and 24c (Fig. 3) extending below the compartments 56a and 56c, respectively, and is discharged through the corresponding nozzles 20 into the latter compartments. Thus, the solids in the compartments 56a and 56c are fluidized and pass in a generally upwardly direction across the heat exchange tubes 60a and 60b before exiting, via the openings 50a associated with the latter compartments, into the chamber 58 (Figs. 1 and 2). The solids mix in the chamber 58 before they exit, via the lower openings 14a formed in the rear wall 14, back into the furnace section 54.

Thus, the solids flow in a direction shown by the flow arrow in Fig. 5, i.e., through the openings 52a in the lower portion of the wall 52 into the lower portion of the chambers 56a and 56c, then upwardly across the tubes 60 before passing into the upper portion of the chamber 58 via the openings 50a. In the chamber 58 the solids pass downwardly, then exit through the lower openings 14a in the rear wall and pass into the lower portion of the furnace section 54.

The vent pipe 59 equalizes the pressure in the heat exchange enclosure 56, and therefore the outlet chamber 58, to the relatively low pressure in the furnace section 54. Thus the fluidized solids level in the outlet chamber 58 establishes a solids head differential which drives the solids through the openings 14a to the furnace section 54.

It is understood that a drain pipe hopper or the like may be provided on the plate 22 as needed for discharging spent solids from the furnace section 54 and the heat exchanger enclosure 56 as needed.

Feed water is introduced to and circulated through the flow circuit described above in a predetermined sequence to convert the feed water to steam and to reheat and superheat the steam. To this end, the heat removed from the solids in the heat exchanger enclosure 56 can be used to provide reheat and/or full or partial superheat. In the latter context the two groups of tubes 60a and 60b in each of the heat exchanger sections 56a and 56c can function to provide intermediate and finishing superheating, respectively, while the primary superheating is performed in the heat recovery area 32.

Since, during the above operation, fluidizing air is not introduced into the air plenum section 24b associated with the heat exchanger compartment 56b and the separated solids in the latter compartment are thus defluidized. This, plus the fact that opening 50b in the partition 50 is at a greater height than the openings 50a, very little, if any, flow of solids through the heat exchanger compartment 56b occurs. However, during initial start up and low load conditions the fluidizing air to the plenum section 24b, and therefore to the compartment 56b, is turned on; while the air flow to the sections 24a and 24c, and therefore to the compartments 56a and 56 are turned off. This allows the solids in the heat exchanger compartments 56a and 56c to slump and therefore seal this volume from further flow, while the solids from the compartment 62 pass directly through the heat exchanger compartment 56b through the opening 50b in the partition 50 as shown by the dashed arrow in Fig. 5, through the outlet chamber 58 and to the furnace section 54. Since the heat exchanger compartment 56b does not contain heat exchanger tubes, it functions as a bypass so that start up and low load operation can be achieved without exposing the tubes 60 in the heat exchanger compartments 56a and 56c to the hot recirculating solids.

Several advantages result in the system of the present invention. For example, the heat exchange efficiency in the heat exchange enclosure is relatively high and a uniform discharge of solids to the furnace section is ensured due to the uniform distribution and flow of the separated solids through the subenclosure and the chamber 58. Also the separated solids from the separators are introduced directly into the subenclosure 62, thus eliminating the need for a J-valve and associated componentary. Also, the location and number of cyclone separators can be varied in accordance with particular design requirements. Further, the air or gas flow into the partitions 66a and 66c extending in line with the diplegs 63a and 63c, respectively, can be regulated by the valves 74 independently of the flow into the remaining portions of the inlet compartment 62. Also, the air or gas flow into the partitions 66a and 66c can be carefully controlled so as to improve the flow and distribution of the separated solids through the inlet compartment, yet not interfere with, or affect the operation of, the separators 26.

it is understood that several options and variations may be made in the foregoing without departing from the scope of the invention. For example, drain pipes, or the like, (not shown) can be provided that extend from the plenum 24 below the inlet compartment 62 and the heat exchanger compartment 56 for controlling the recycle rate. Also, the opening 52b and the opening 50a that communicates with the compartment 56b can be provided with a gate valve, or the like (not shown) to block these openings and therefore prevent the flow of the separated solids therethrough during normal operation. Further the heat removed from the solids in the recycle heat exchanger enclosure can be used for heating the system fluid in the furnace section or the economizer, etc. Also, other types of beds may be utilized in the furnace such as a , circulating transport mode bed with constant density through its entire height or a bubbling bed, etc. Also, a series heat recovery arrangement can be provided with superheat, reheat and/or economizer surface, or any combination thereto. Further, the number and/or location of the separators and therefore the number bypass channels in the recycle heat exchanger can be varied. Still further, the number of openings through which the solids pass in the partitions and the walls described above can vary in accordance with particular design requirements.

Claims

A fluidized bed combustion system comprising an enclosure (10), a furnace section (54) into which combustible particulate material is introduced, fluidizing means (24) for fluidizing the particulate material into the furnace section (54) to promote combustion of that particulate material, separating means (26) for receiving a mixture of flue gases and entrained particulate material from the furnace section (54) and separating the entrained particulate material from the flue gasses, a heat recovery section (32) for receiving the separated flue gases, a dipleg (63a) for returning the separated material directly from the separating means (26) to an inlet compartment (62), means (70) for introducing air or gas into the inlet compartment (62) in a flow path aligned with the flow path of the separated material form the dipleg (63a), and means (58) for returning separated material to the furnace section (54), characterised in that the enclosure (10) is divided into the said furnace section (54) and a recycle heat exchange section, the recycle heat exchange section being divided into the said inlet compartment (62) and a heat exchange compartment (56), heat exchange means (60) disposed in at least part of the heat exchange compartment (56), and means (52a) for passing the separated material from the inlet compartment (62) to the heat exchange compartment (56).
A system as claimed in Claim 1 further comprising a by-pass passage (56b) in the heat exchange compartment (56) which does not contain heat exchange means, and means (24, 24b) for selectively passing that separated material from the inlet compartment (62) through the by-pass passage (56b) or through the remaining portion of the heat exchange compartment (56) and therefore back to the furnace section (54).
A system as claimed in Claim 2 further comprising means for selectively introducing air to the said remaining portion of the heat exchange compartment (56) or to the by-pass passage (56b) to selectively fluidize separated material therein to permit flow of separated material through the said remaining portion of the heat exchange compartment (56) or through the by-pass passage (56b), respectively.
A system as claimed in any preceding claim further comprising means (72) for introducing air or gas into an area of the inlet compartment (62) surrounding the means (70) for introducing air or gas in a flow path aligned with the flow path of the separated material from the dipleg (63a).
A system as claimed in Claim 4 further comprising means (74,76) for controlling the flow of air or gas into the area of the inlet compartment (62) to control the rate of transfer of the separated material from the inlet compartment (62).
A system as claimed in any preceding claim further comprising means (24) for introducing air or gas to the recycle heat exchange section for fluidizing the separated material in that section to seal against the backflow of separated material from the furnace section (54) through the dipleg (63a) and back to the separating means (26).
A system as claimed in any preceding claim in which the furnace section (54) and the recycle heat exchange section are divided in a partition (50) disposed in the enclosure (10) and the means for returning separated material to the furnace section (54) comprises openings (50a, 50b) formed in the lower portion of the partition (50).
A system as claimed in any preceding claim in which the inlet compartment (62) and the heat exchange compartment (56) are divided by a partition (52) disposed in the enclosure (10), and the means for passing the separated material from the inlet compartment (62) to the heat exchange compartment (56) comprise openings (52a) formed in the upper portion of that partition (52).
A system as claimed in any preceding claim in which at least a portion of the walls of the enclosure (10) are formed by tubes, and fluid flow circuit means are provided for passing fluid through these tubes to transfer heat generated in the furnace section (54) to the fluid.
A system as claimed in any preceding claim in which the fluidizing means introduce fluidizing air or gas to the furnace section (54) at a velocity sufficient to form a circulating bed.
A system as claimed in any preceding claim in which an outlet compartment (58) is provided between the furnace section (54) and the heat exchange section (56) to allow mixing of the separated material before return to the furnace section (54).
A system as claimed in Claim 11 in which the outlet compartment (58) is defined by a rear wall (14) of the furnace section (54) and an opening (14a) is provided in this wall to allow passage of separated material from the outlet compartment (58) to the furnace section (54).