COMPOSITE CIRCULATION FLUIDIZED BED BOILER

Abstract: This invention relates to a composite circulation fluidized bed boiler. A fluidized bed portion of the fluidized bed boiler is divided by a partition into a main combustion chamber and a heat recovery chamber, and at least two kinds of air chambers, i.e. an air chamber for providing a large fluidizing speed to a fluidization medium and an air chamber for providing a small fluidizing speed are provided. The combination of air having different fluidization speeds and jetted from these air chambers generates a swirling circulation flow in the fluidization medium inside the main combustion chamber and a circulation flow of the fluidization medium is generated between the main combustion chamber and the heat recovery chamber. Inside the heat recovery chamber, heat recovery of an exhaust gas is made at a free board portion or downstream of the free board portion in an internal circulation fluidized bed boiler for lowering the fluidization medium in the form of the fluidized bed, and after the temperature of the exhaust gas is lowered, the exhaust gas thus cooled is led into a cyclone and the char in a fine particle form collected by the cyclone is returned to the portion immediately above, or into, the lowering moving layer of the fluidization medium in the main combustion chamber and/or the heat recovery chamber. Since the char is returned immediately above or into the lowering moving layer of the fluidization medium, it does not immediately scatter to the free board portion. Accordingly, though it is the fine particles, it sediments and diffuses sufficiently into the fluidized bed so that NOx generated by the combustion of coal or the like inside the layer can be reduced.

Description: TECHNICAL FIELD The present invention relates to an internal recycling type fluidized bed boiler in which combustion materials such as various coals, low grade coal, dressing sludge, oil cokes and the like are burnt by a so-called whirling-flow fluidized bed and which recovers thermal energy from a recycling fluidized bed, the interior of a free board and a heat transfer portion provided downstream of the free board portion. BACKGROUND Recently, utilization of coal as an energy source in place of petroleum has become more prevalent. In order to widely utilize coal which is inferior in its physical and chemical properties as a fuel to those of petroleum, development of processing and distribution of coal and of technology for promoting the utilization of coal has been in urgent demand. Research and development of a pulverized coal incinerating boiler and the fluidized bed boiler in the field of combustion technology have been positively advanced. With respect to combustion technology such as the above, utilization is restricted to certain kinds of coals in view of the combustion efficiency, requirements of low NOx and low SOx. Also, problems such as the complexity of coal feeding systems and difficulty in controlling load fluctuations have become evident, which problems have been particularly evidenced in small and medium sized boilers. A fluidized bed boiler is classified into two types as noted below according to the difference in a system wherein arrangement of heat transfer portions and combustion of unburnt particles flown out from the fluidized bed are taken into account. (1) Non-recycling type fluidized bed boiler (which is also referred to as a conventional type fluidized bed boiler or a bubbling type fluidized bed boiler) (2) Recycling type fluidized boiler In a non-recycling type, a heat transfer tube is arranged within a fluidized bed, and heat exchange is carried out by physical contact between the burning fuel and a fluidizing medium with high heat transfer efficiency. On the other hand, in a recycling type, fine unburnt materials, ash and/or a part of the fluidized medium (recycling solid) are merged into a flow of combustion gas and guided to a heat exchanging portion arranged independently of the combustion chamber where combustion of the unburnt particles is continued and the circulating solid having been heat exchanged is returned to the combustion chamber, the aforesaid title being named since the solid is recycled. A non-recycling type fluidized bed boiler and a recycling type fluidized bed boiler will be described with reference to Figs. 3 and 4. Fig. 3 shows a non-recycling type fluidized bed boiler, in which air for fluidization fed under pressure from a blower not shown is injected from an air chamber 74 into a boiler 71 through a diffusion plate 72 to form a fluidized bed 73, and fuel, for example, granular coal is supplied to the fluidized bed 73 for combustion. Heat transfer tubes 76 and 77 are provided in the fluidized bed 73 and an exhaust gas outlet of a free board portion, respectively, to recover thermal energy. Exhaust gas cooled to a relatively low temperature is guided from an exhaust gas outlet of the free board portion to a convection heat transfer portion 78 to recover thermal energy and discharged outside the system after contained particles are recovered at a cyclone 79. Ash recovered in the convection heat transfer portion is taken out through a tube 81 and discharged outside the system via a tube 82 together with ash taken out from a tube 80, a part thereof being returned to the fluidized bed 73 for re-burning through the air chamber 74 or a fuel inlet 75. Fig. 4 shows a recycling type fluidized bed boiler, in which air for fluidization fed under pressure from a blower not shown is blown from an air chamber 104 into a furnace 101 through a diffusion plate 102 to fluidize and burn granular coal containing lime as a desulfurizing agent to be supplied into the furnace as needed. Unlike a non-recycling type fluidized bed boiler, injecting speed of fluidizing air blown through the diffusion plate 102 is higher than the terminal speed of the fluidizing particles, and therefore, mixing of particles and gas is more actively effected and the particles are blown upward together with gas so that a fluidizing layer and a jet-stream layer are formed in that order from the bottom over the whole zone of the combustion furnace. The particles and gas are guided to a cyclone 108 after a small amount of heat exchange is effected at a water cooling furnace wall 107 provided halfway. The combustion gas passed through the cyclone 108 is heat exchanged at a convection heat transfer portion 109 arranged in a flue at the rear portion. On the other hand, the particles collected at the cyclone 108 are again returned to the combustion chamber via a flowpassage 113, and a part of the particles is guided to an external heat exchanger 115 via a flowpassage 114 for the purpose of controlling the furnace temperature and after being cooled, it is again returned to the combustion chamber while partly discharged outside the system as ash. A feature lies in that the particles are recycled into the combustion chamber in a manner as just mentioned. The recycling particles are mainly limestone supplied as a desulfurizing agent, burnt ash of supplied coal and unburnt ash, etc. In these fluidized bed boilers, selection of materials to be burnt is widely available in view of characteristics of the combustion system thereof, but some demerits thereof have become noticed. As the demerits of the bubbling type fluidized bed boiler, there are problems such as those regarding load characteristics, complexity of a fuel supply system and abrasion of heat transfer tubes in the bed, etc. In order to solve the problems inherent to such matters as above, a recycling type apparatus has become noticed. However, some further factors need to be developed in order to maintain the temperature of a recycling system including a cyclone of a combustion furnace at a proper value. In addition, there still remains a problem in the handling of a recycling solid. With respect to small and medium type boilers, there is a difficulty in making them compact. EP-A-0 230 309 discloses a composite recycling type fluidized bed boiler of the type referred to in the preamble of claim 1. This fluidized bed boiler shows an "internal" type of recirculation. GB-A-2 151 503 discloses a fluidized bed combustion apparatus using an "external" type of recirculation. To return the particles of the fluidized bed into a thermal energy recovery chamber (with a descending bed), separated by a partition from a primary combustion chamber, is not disclosed. It is an object of the present invention to provide the composite recycling type fluidized bed boiler of the preamble of claim 1 such that a reduction of NOx generation in the bed occurs. Accordingly the present invention provides for the composite recycling type fluidized bed boiler of the preamble of claim 1 the features referred to in the characterizing clause of claim 1. Preferred embodiments of the invention are disclosed in the dependent claims. The present inventors have further found that it is possible to make a boiler compact, promote combustion efficiency and reduce NOx by the arrangement of the invention. That is in an internal recycling type fluidized bed boiler in which a whirling flow is produced within a fluidized bed due to different speeds in fluidizing air, and the whirling flow is utilized to form a recycling flow of a fluidizing medium relative to a thermal energy recovery chamber, a thermal energy recovery portion such as a vaporizing tube is provided in a free board portion above the fluidized bed or in a portion downstream of the free board portion and exhaust gas is, after being cooled to a low temperature by heat exchange, directed to a cyclone and particles collected at the cyclone are returned to a descending moving bed of the fluidizing medium in the fluidized bed. The inventors further found that selection of coal is not limited to a certain kind because even coal of a high fuel ratio may be completely burnt by the whirling flow, and silica sand can be used as a fluidizing medium together with limestone for reducing SOx whereby all the problems encountered in the conventional coal boilers can be solved. An internal recycling type fluidized bed boiler is provided in which a fluidized bed is generally partitioned into a primary combustion chamber and a thermal energy recovery chamber, the primary combustion chamber being accompanied by at least two kinds of air chambers disposed below the primary chambers, i.e., an air chamber for imparting a high fluidizing speed and an air chamber for imparting a low fluidizing speed, these different fluidizing speeds being combined to thereby impart a whirling flow to a fluidizing medium within the primary combustion chamber to form a thermal energy recovery recycling flow of fluidizing medium between the primary combustion chamber and the thermal energy recovery chamber. That is, in the internal recycling fluidized bed provided with an air chamber imparting a low fluidizing speed at a portion below and opposite the thermal energy recovery chamber relative to the primary combustion chamber, exhaust gas is guided into a cyclone and collected particles at the cyclone are returned to a descending moving bed of the primary combustion chamber or the thermal energy recovery chamber. The collected particles are not always from the cyclone but collected particles from a bag filter or the like can also be returned to the descending moving bed. Returning of collected particles into the descending moving bed causes unburnt composition (char) of the collected particles to be evenly scattered within the fluidized bed so that the whole portion in the bed becomes a reduced atmosphere thereby reducing NOx in a zone ranging from the fluidized bed to the free board portion. The effect of and advantages in returning the char to the descending moving bed will be discussed hereunder. In case of returning the char to the fluidized bed, the char is immediately scattered into the free board due to the fact that the char consists of fine particles so that there is little dwelling time for the char within the bed, thereby failing to satisfactorily effect combustion of the char itself and function as a catalyst for low NOx. However, in case of returning the char to the descending moving bed, it moves downward and diffuses into the bed whilst it is finely granulated, and therefore, the char is thoroughly moved to reach an area where NOx is generated due to combustion of coal or the like within the bed, whereby NOx is advantageously reduced. Following two formulas are considered in connection with the reduction of NOx: C + 2NO → CO₂ + N₂ (oxidization reaction of char) 2CO + 2NO → 2CO₂ + N₂ (catalyst reaction of char) The char is participated in any of reactions above. It is considered that the oxidization reactivity and catalyst effect of char exert an influence on the function of reducing the generation of NOx. Heat transfer tubes are arranged in a free board portion above a fluidized bed or downstream of the free board portion, and recovery of thermal energy is primarily effected by convection heat transfer. In the past, a convection heat transfer portion has been provided independently of a free board portion. However, in order to make a boiler compact, such a convection heat transfer portion is provided unitarily with a free board portion at an upper part within a free board or downstream of a free board portion while sufficient volume required for secondary combustion in a free board portion is retained. With such an arrangement as outlined above, treatment of dust and recycling of char around a boiler can be facilitated as compared to the prior art. In addition, the temperature of gas entering into a cyclone becomes 250 to 400°C, and therefore, the cyclone need not be provided with castable lining, and the cyclone can be made of steel and be light in weight and miniaturized. A convection heat transfer portion is provided at an upper part within a free board or a furnace wall is constructed to comprise water cooling tubes. In view of such a provision as above, heat insulating material such as refractory material is lined on the convection heat transfer portion and the water cooling furnace wall on the side of the combustion chamber in order to prevent the temperature of the combustion gas within the free board from being lowered due to radiation effect. With the above arrangement, the temperature of combustion gas is maintained so as to be effective in reducing CO or the like. In the case where a convection heat transfer portion is provided downstream of the free board portion, refractory heat insulating material may be lined only on a water cooling wall constituting the free board portion. As explained hereinabove, the present invention provides a composite recycling type fluidized bed boiler effecting a combination of three circulative movements, i.e., a whirling flow circulation in the primary combustion chamber, a thermal energy recovering circulative movement of a fluidizing medium recycled between a primary combustion chamber and a thermal energy recovery chamber, and an external recycling (char recycling) for returning unburnt char to a descending moving bed of a fluidizing medium within a primary combustion chamber or a thermal energy recovery chamber. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are schematic views of different types of composite recycling type fluidized bed boilers, respectively, according to the present invention, in which heat transfer tubes such as vaporization tubes are disposed at an upper part within a free board; Fig. 3 is a schematic view of a conventional fluidized bed boiler; Fig. 4 is a schematic view of a conventional recycling type fluidized bed boiler; Fig. 5 is a graph indicating the relationship between an amount of fluidizing air at a lower portion of an inclined partition wall and a recycling amount of a fluidized medium in a thermal energy recovery chamber; Fig. 6 is a graph indicating the relationship between an amount of diffusing air for a thermal energy recovery chamber and a descending rate of downwardly moving bed; Fig. 7 is a graph generally indicating a mass flow for fluidization and an overall thermal conducting coefficient; Fig. 8 is a graph indicating an amount of diffusing air for a thermal energy recovery chamber and an overall thermal conducting coefficient in an internal recycling type; Fig. 9 is a graph indicating the relationship between a fluidizing mass flow and an abrasion rate of a heat transfer tube; Fig. 10 is a sectional view in a composite recycling type fluidized bed boiler designed so that a group of heat transfer tubes such as vaporization tubes integrally provided with a free board portion are disposed downstream of the free board portion and relatively large particles collected at said group of heat transfer tubes are returned to left and right thermal energy recovery chambers disposed on opposite sides of a primary combustion chamber; and Fig. 11 is a view showing an embodiment in which particles containing fine char collected at a cyclone are returned to a carrier such as a conveyor for returning particles collected at said group of heat transfer tubes to the fluidized bed portion. BEST MODE OF PRACTICING THE INVENTION The present invention will be schematically explained referring to the drawings. In Fig. 1, a boiler body 1 is internally provided on the bottom thereof with a diffusion plate 2 for an fluidizing air which is introduced from a fluidizing air introducing tube 15 by means of a blower 16, the diffusion plate 2 having opposite edges arranged to be higher than a central portion of the plate, the bottom of the boiler body being formed as a concave surface. The fluidizing air fed by the blower 16 is injected upwardly from the air diffusion plate 2 via air chambers 12, 13 and 14. A mass flow of the fluidizing air injected out of the central air chamber 13 is arranged to be enough to form a fluidized bed of a fluidized medium within the boiler body, that is, in the range of 4 to 20 Gmf, preferably, in the range of 6 to 12 Gmf. A mass flow of the fluidizing air injected out of the air chambers 12 and 14 on the opposite sides is smaller than the former, generally, in the range of 0 to 3 Gmf. It is preferable that air is injected in a mass flow of 0 to 2 Gmf from the air chamber 12 located below the thermal energy recovery chamber 4 and provided with a heat transfer tube 5, and air is injected in a mass flow of 0.5 to 2 Gmf from the air chamber 14 which forms a lower portion of the primary combustion chamber 3. Since the mass flow of the fluidizing air injected out of the air chamber 13 within the primary combustion chamber 3 is relatively larger than that of the fluidizing air injected out of the air chambers 12 and 14, the air and the fluidizing medium are rapidly moved upward in the portion above the air chamber 13 forming a jet stream within the fluidized bed, and upon passing through the surface of the fluidized bed, they are diffused and the fluidized medium falls onto the surface of the fluidized bed at the portions above the air chambers 12 and 14. On the other hand, in the fluidized bed above the air chamber 13, fluidizing medium under gentle fluidization at the opposite sides moves to occupy a space from where the fluidized medium is moved upward. The fluidized medium in the fluidized bed above the air chambers 12 and 14 is moved to the central portion, i.e., the portion above the air chamber 13. As a result, a violent upward stream is formed in the central portion in the fluidized bed but a gentle descending moving bed is formed in the peripheral portions. The thermal energy recovery chamber 4 makes use of the aforesaid descending moving bed. Fig. 8 shows the relationship between an overall thermal conducting coefficient and a fluidizing mass flow in a bubbling system. However, according to the present invention, a large overall thermal conducting coefficient is obtained at a fluidizing mass flow of 1 to 2 Gmf as shown in Fig. 7 without effecting such severe fluidization (generally, 3 to 5 Gmf) as in the bubbling system and sufficient thermal energy recovery can be effected. A vertical partition wall 18 is provided internally of the fluidized bed at the portion above a boundary between the air chambers 12 and 13, and the heat transfer tube 5 is arranged at the portion above the air chamber 12 to form a thermal energy recovery chamber, that is, internally of the fluidized bed between the back of the partition wall 18 and the water cooling furnace wall. The height of the partition wall 18 is designed to be sufficient for allowing the fluidized medium to pass from a portion above the air chamber 13 into the thermal energy recovery chamber 4 during operation, and an opening 19 is provided between the partition wall 18 and the air diffusion plate on the bottom so that the fluidized medium within the thermal energy recovery chamber 4 may be returned into the primary combustion chamber 3. Accordingly, the fluidizing medium diffused above the surface of the fluidized bed after having been violently moved up as a jet stream within the primary combustion chamber moves beyond the partition wall 18 into the thermal energy recovery chamber, and is gradually moved down while being gently fluidized by air blown from the air chamber 12 with heat exchange being effected through the heat transfer tube 5 during its descent. The recycling amount of the descending fluidizing medium in the thermal energy recovery chamber is dependent on the amount of diffusing air fed from the air chamber 12 to the thermal energy recovery chamber 4 and the amount of fluidizing air fed from the air chamber 13 in the primary combustion chamber. That is, as shown in Fig. 6, the amount G₁ of the fluidized medium entering into the thermal energy recovery chamber 4 increases as the amount of the fluidizing air blown out of the air chamber 13 increases. Also, as shown in Fig. 7, when the amount of diffusing air fed into the thermal energy recovery chamber 4 is varied in the range of 0 to 1 Gmf, the amount of the fluidizing medium descending in the thermal energy recovery chamber substantially varies proportionally thereto, and is substantially constant if the amount of diffusing air in the thermal energy recovery chamber exceeds 1 Gmf. The aforesaid constant amount of the fluidized medium is substantially equal to the fluidized medium amount G₁ moved into the thermal energy recovery chamber 4, and the amount of fluidized medium descending in the thermal energy recovery chamber corresponds to G₁. With these two amounts of air being regulated, the descending rate of the fluidized medium in the thermal energy recovery chamber 4 is controlled. On the other hand, thermal energy is recovered from the descending fluidized medium through the heat transfer tube 5. The heat conducting coefficient changes substantially linearly as shown in Fig. 9 when the diffusing amount of air fed into the thermal energy recovery chamber 4 from the air chamber 12 is changed from 0 to 2 Gmf, and therefore, the thermal energy recovery amount and the fluidized bed temperature within the primary combustion chamber 3 can be optionally controlled by regulating the amount of diffusing air. That is, with the amount of fluidizing air from the air chamber 13 in the primary combustion chamber 3 being kept constant, the fluidized medium recycling amount increases when the amount of diffusing air within the thermal energy recovery chamber 4 is increased and at the same time the thermal conducting coefficient is increased, whereby the amount of thermal energy recovery is considerably increased as a result of synergistic effect. If an increment of the aforesaid amount of thermal energy recovery is balanced with an increment of the generated thermal energy in the primary combustion chamber, the temperature of the fluidized bed is maintained at constant. It is said that an abrasion rate of a heat transfer tube in a fluidized bed is proportional to the cube of a fluidizing flow rate. Fig. 10 shows the relationship between a fluidizing mass flow and an abrasion rate. That is, with the amount of diffusing air blown into the thermal energy recovery chamber being kept at 0 to 3 Gmf, preferably, 0 to 2 Gmf, the heat transfer tube encounters an extremely small degree of abrasion and thus durability can be enhanced. On the other hand, coal as fuel is supplied to the initiating portion of the descending moving bed within the primary combustion chamber 3. Therefore, coal supplied as above is whirled and circulated within the high temperature fluidized bed, and even coal of a high fuel ratio can be completely burnt. Since high load combustion is made available, a boiler can be miniaturized, and in addition, there is no restriction on the kind of coals which may be selected so that the use of boilers is promoted. Exhaust gas is discharged from the boiler and guided to the cyclone 7. On the other hand, particles collected at the cyclone pass through a double damper 8 disposed at a lower portion in the boiler shown in Fig. 1 and are introduced into a hopper 10 together with coal simultaneously supplied, with the both being mixed by a screw feeder 11 and fed to the descending moving bed of the primary combustion chamber thereby contributing to the incineration of unburnt substance (char) in the collected particles and to the reduction of NOx. It is noted that particles collected at the cyclone will, of course, be mixed with coal due to whirling and circulation in the primary combustion chamber even if they are not preliminarily mixed in advance with the particles and coal being independently transported to a portion before the primary combustion chamber and fed into the descending moving bed of the primary combustion chamber. On the other hand, in an upper portion of the free board, a convection heat transfer surface means 6 is provided to effect heat recovery as an economizer and a vaporizing tube. A heat insulating material 17 such as a refractory material is mounted as required on the lower portion of the convection heat transfer surface means 6 and the water cooling furnace wall on the side of the combustion chamber in order to maintain the combustion temperature in the free board at a constant temperature, preferably, 900°C. In the case of the convection heat transfer surface means, each heat transfer tube near the free board portion is mounted so as to be wound with a heat insulating material. Needless to say, a pitch of the heat transfer tube is taken into consideration so as not to impede a flow passage of the exhaust gas. Due to the provision of the heat insulating material 17 as described above, it is possible to maintain the temperature of the lower portion of the free board portion at a high temperature so that the provision is effective to reduce CO by air blown from an air blow opening 20 for a secondary combustion in the free board portion. Fig. 2 shows a further embodiment of the present invention. Basically, this embodiment is similar, with respect to its construction, to the boiler shown in Fig. 1 and performs an operation similar thereto. What is largely different in this embodiment from the previous embodiment is that a lower portion of a partition wall 38 between a primary combustion chamber 23 and a thermal energy recovery chamber 24 is inclined so as to interrupt, in the primary combustion chamber, an upward flow from an air chamber 33 under a high fluidizing rate and to turn the flow toward an air chamber 34 under a low fluidizing rate, the angle of inclination being 10 to 60 degrees relative to the horizon, preferably 25 to 45 degrees. The horizontal length ℓ of the inclined portion of the partition wall projected onto the furnace bottom is arranged to be 1/6 to 1/2, preferably 1/4 to 1/2 of the horizontal length L of the opposing furnace bottom. The fluidized bed at the bottom of a boiler body 21 is divided by the partition wall 38 into the thermal energy recovery chamber 24 and the primary combustion chamber 23, and an air diffusion plate 22 for fluidization is provided at the bottom of the primary combustion chamber 23. The central portion of the diffusion plate 22 is arranged to be low and the side opposite the thermal energy recovery chamber to be high. Two kinds of air chambers 33 and 34 are provided below the diffusion plate 22. A mass flow of fluidizing air injected out of the central air chamber 33 is arranged to be enough for causing a fluidized medium within the primary combustion chamber to form a fluidizing bed, that is, in the range of 4 to 20 Gmf, preferably, in the range of 6 to 12 Gmf, whereas a mass flow of fluidizing air injected out of the air chamber 34 is arranged to be smaller than the former, in the range of 0 to 3 Gmf so that the fluidized medium above the air chamber 34 is not accompanied by violent up-and-down movement but forms a descending moving bed in a weak fluidizing state. This moving bed is spread at the lower portion thereof to reach the upper portion of the air chamber 33 and therefore encounters an injecting flow of fluidizing air having a large mass flow from the air chamber 33 and is blown up. Thus, a part of the fluidized medium at the lower portion of the moving bed is removed, and therefore, the moving bed is moved down due to its own weight. On the other hand, the fluidized medium blown up by the injecting flow of the fluidizing air from the air chamber 33 impinges upon the inclined partition wall 38 and is reversed and deflected, a majority of which falls on the upper portion of the moving bed to supplement the fluidized medium of the moving bed moved downwardly. As the result of the continuous operation as described above, at the portion above the air chamber 34, a slowly descending moving bed is formed and as a whole, the fluidizing medium within the primary combustion chamber 23 is caused to form a whirling flow. On the other hand, a part of the fluidizing medium blown up by the fluidizing air from the air chamber 33, reversed and deflected by the inclined partition wall 38 moves beyond the inclined partition wall 38 and enters into the thermal energy recovery chamber 24. The fluidized medium moved into the thermal energy recovery chamber 24 forms a gentle descending moving bed by the air blown by an air diffuser 32. In the case where the descending rate is slow, the fluidized medium moved into the thermal energy recovery chamber forms an angle of repose at the upper portion of the thermal energy recovery chamber, and a surplus portion thereof falls from the upper portion of the inclined partition wall 38 to the primary combustion chamber. Within the thermal energy recovery chamber, the fluidized medium is subjected to heat exchange through the heat transfer tube 25 while moving down slowly, after which the medium is returned from the opening 39 into the primary combustion chamber. The descending recycling amount and the thermal energy recovery amount of the fluidized medium within the thermal energy recovery chamber are controlled by the amount of diffusing air blown into the thermal energy recovery chamber in a way similar to that of the embodiment shown in Fig. 1. In the case of the boiler shown in Fig. 2, controlling is effected by the amount of air blown from the air diffuser 32, and the mass flow thereof is arranged to be in the range of 0 to 3 Gmf, preferably 0 to 2 Gmf. Coal as fuel is supplied to the portion above the air chamber 34 wherein the descending moving bed is formed within the primary combustion chamber 23 whereby the coal is whirled and circulated within the fluidized bed of the primary combustion chamber and incinerated under excellent conditions of combustibility. On the other hand, exhaust gas is directed to a cyclone 27 after being discharged from the boiler. The particles collected at the cyclone 27 pass through a double damper 28 and are introduced into a hopper 30 together with coal parallelly supplied. They are mixed and supplied by a screw feeder 31 to the descending moving bed in the primary combustion chamber 23, that is, a portion above the air chamber 34, to contribute to the combustion of unburnt substance (char) in the collected particles and reduction in NOx. Although not particularly shown, the particles collected at the cyclone 27 may be supplied independently of coal, unlike the supply device shown in Fig. 2, and the particles and coal may be fed by an airborne means instead of the screw feeder. On the other hand, in the upper portion of the free board, a convection heat transfer surface means 26 is provided to effect thermal energy recovering. A heat insulating material 37 such as a refractory material is mounted on the lower portion of the convection heat transfer surface means 26 and side of the water cooling furnace wall opposing the combustion chamber as required in order to maintain the combustion temperature of the free board at a constant temperature, preferably 900°C, and an air inlet 40 is provided for the purpose of secondary combustion to effectively reduce CO or the like. Another embodiment will be described with reference to Fig. 10. Fig. 10 is a sectional view and 238′ designates a V-shaped bottom of the convection heat transfer portion and 239′ designates a screw conveyor. This embodiment provides for two V-shaped bottoms 238 and 238′ (W-shaped bottoms) which are provided at the lower portion of the convection heat transfer chamber, and that particles containing relatively large char collected at the V-shaped bottoms 238 and 238′ are returned by screw conveyors 239 and 239′ to the portion directly above the descending moving beds 204 and 204′ of the fluidizing medium in the thermal energy recovery chambers provided at opposite sides of the combustion chamber. Fig. 11 shows still another embodiment of the present invention. Reference numeral 241 designates a conduit. The embodiment shown in Fig. 11 discloses that fine particles containing char collected at the cyclone 207 are directed to the screw feeder 239 at the lower portion of the convection heat transfer portion 232 by the conduit 241 and then returned together with the particles containing a relatively large char collected at the convection heat transfer portion to the portion directly above the descending moving bed in the primary combustion chamber.

Claims: A composite recycling type fluidized bed boiler comprising; a fluidized bed portion divided by a partition (18, 38, 218, 218′) into a primary combustion chamber (3, 23, 203) and a thermal energy recovery chamber, a free board portion above said fluidized bed portion, an opening (19, 39, 59, 59′, 219, 219′) formed at the lower portion of said thermal energy recovery chamber to return a fluidized medium into said primary combustion chamber, at least two kinds of air chambers at a portion below said primary combustion chamber, one being an air chamber (13, 33, 212, 214) placed adjacent to said partition and for imparting a high fluidized speed to the medium, the other (14, 34, 213) being an air chamber placed remote from said partition and for imparting a low fluidized speed to the medium, and convection heat transfer means (6, 26, 236) for recovery of thermal energy in combustion gas from said fluidized bed portion, said boiler characterized by; a cyclone (7, 27, 207) arranged downstream of said convection heat transfer means, and a transport means (11, 31, 211, 239) for returning particles collected at said cyclone to said thermal energy recovery chamber (4, 24, 204, 204′). The boiler according to Claim 1, wherein said partition is inclined so as to interrupt upward stream of the fluidized medium above said air chamber placed adjacent to said partition and to reverse and deflect said fluidized medium to a portion above said air chamber placed remote from said partition. The boiler according to Claim 1 or 2, wherein said convection heat transfer means is a group of heat transfer tubes (6, 26, 46) provided at an upper portion of said free board portion. The boiler according to Claim 1 or 2, wherein said convection heat transfer means is a group of heat transfer tubes (236) provided in a convection heat transfer chamber (232) which is connected to said free board portion. The boiler according to Claim 4, wherein particles gathered to a bottom of said convection heat transfer chamber are returned to a portion directly above said the other air chamber placed remote from said partition in said primary combustion chamber. The boiler according to Claim 4, wherein particles gathered to a bottom of said convection heat transfer chamber are returned to said thermal energy recovery chamber. The boiler according to Claim 4, wherein said thermal energy recovery chamber is provided in each of opposite sides of said primary combustion chamber, and particles gathered to a bottom of said convection heat transfer chamber are returned to both of the thermal energy recovery chambers (204, 204′). The boiler according to Claim 6 or 7, wherein said particles collected at said cyclone are returned to said thermal energy recovery chamber together with said particles gathered to the bottom of said convection heat transfer chamber. The boiler according to anyone of Claims 1 to 8 further comprising air blow means for blowing secondary air in a direction to produce a whirling flow in said free board portion. The boiler according to anyone of Claims 4 to 8 wherein said vaporization tubes are provided between a steam drum (230) provided at a portion above said convection heat transfer chamber and a water drum (231) provided below said convection heat transfer chamber. The boiler according to Claim 10, wherein said bottom of said convection heat transfer chamber is V-shaped, and said transporting means is a screw conveyor arranged in said bottom. The boiler according to Claim 10, wherein said bottom of said convection heat transfer chamber is W-shaped, and said transporting means comprises two screw conveyors (239, 239′) arranged in said bottom. A method for operating the boiler according to anyone of Claims 1 to 12 characterized by including the steps of; imparting the high fluidized speed to the medium by said one of said air chamber (13, 33, 212, 214) and imparting the low fluidized speed to the medium by the other of said air chamber (14, 34, 213) thereby providing a whirling flow of the fluidized medium including an upward stream and a descending moving bed and a recycling flow of the fluidized medium between said primary combustion chamber and said thermal energy recovery chamber, injecting air into said thermal energy recovery chamber so as to form a descending moving bed in said thermal energy recovery chamber, passing said combustion gas through said convection heat transfer means (6, 26, 236), guiding cooled exhaust gas from said convection heat transfer means into said cyclone (7, 27, 207), and returning the particles collected from said cooled exhaust gas at said cyclone to said thermal energy recovery chamber. The method according to Claim 13 further including the step of supplying a desulfurizer to said descending moving bed in said primary combustion chamber. The method according to Claim 13 or 14, wherein said cooled exhaust gas is at the temperature of 250 °C to 400 °C.

Inventors: MIYOSHI NORIHISA, NAGATO SHUICHI, OHSHITA TAKAHIRO, MIYOSHI, NORIHISA, NAGATO, SHUICHI, OHSHITA, TAKAHIRO
Applicants: EBARA CORP, EBARA CORPORATION
Application year: 1989
Main classification: F23C11
IPC codes: F23C10, F22B31