METHOD OF STABLE COMBUSTION FOR A FLUIDIZED BED INCINERATOR

Abstract: A combustion method within a fluidized bed incinerator (10) for burning and decomposing refuse (28) such as municipal wastes is disclosed. The refuse (28) is fluid­ ized together with a fluidizing medium (32) such as sand, with primary air, being burned and decomposed. The pyroly­ sis gas produced by thermal decomposition is combusted with the secondary air supplied to the incinerator (10). By controlling the temperature inside the fluidized bed (40) in the range from 520 to 650°C by spraying water, a stable combustion is carried out, despite a change in the volume of refuse (28) thrown onto the fluidized bed (40), and the unburnt pyrolysis gas and smut densities among the exhaust gas can be decreased. The combustion air ratio can be reduced because the refuse (28) can be stably combusted and the temperature of the pyrolysis gas inside the combustion chamber (20) can be maintained at a high level.

Description: This invention relates to a method of incinerating sub­ stances such as municipal wastes and industrial wastes (called "refuse" hereinafter) inside a fluidized bed in­ cinerator. More particularly, it is concerned with a method of stable combustion in a fluidized bed incinerator. The fluidized bed incinerator is known for burning and decomposing of such refuse as municipal wastes. With the method of incineration/disposition in this fluidized bed incinerator, refuse is burned while fluidized in a fluid­ ized bed incinerator. In order to improve the combustion of the refuse along with the fluidized bed,a fluidizing medium, such as sand,is fed together with the refuse into the fluidized bed. A general type of fluidized bed incinerator is equipped with a plurality of air diffuser tubes or plates (called "air diffusers" hereinafter) supplying air down to the lower part of the incinerator body, and with a refuse feeding mechanism and a fluidizing medium feeding mechanism in the upper part. The refuse is burned while both the refuse and the fluidiz­ ing medium, thrown onto the air diffuser tubes or plates inside the incinerator body, are fluidized by the primary air blown from the air diffusers. The refuse, such as municipal wastes, generally contains a variety of materials such as low calorie refuse like food discards, high calorie refuse like plastics and rubber, refuse like shredded paper or chipped furniture, refuse like fragmented metallic or vitreous containers, bottles, or cans. Of the refuse, as it is fed to the fluidized bed, the combustibles are burned, of which substances such as plastics undergo pyrolysis generating various pyrolysis gases, while the incombustibles such as metals or glasses are left over unburnt (called "combustion residue" herein­ after). As the fluidizing medium is gradually fed onto the fluid­ ized bed, a moving bed of fluidizing medium is formed, descending as the fluidizing medium is continuously supplied. Therefore, while the combustibles are being burned and de­ composed within the fluidized bed, the combustion residue comes out of the incinerator, together with the fluidizing medium, through the gaps among the air diffusers at the lower section of the fluidized bed. The fluidizing medium and the combustion residue are separated from each other, and then the fluidizing medium is again fed into the fluidized bed. The secondary air is supplied into the fluidized bed upper section of the incinerator, where the generated pyrolysis gases are burned. Because the sand, the fluidizing medium thrown onto the fluidized bed, is oscillated while it descends and is heated, it promotes the agitation and dispersion of the refuse. For this reason, the refuse thrown onto the fludized bed is dispersed uniformly under the presence of the fluidiz­ ing medium, to be dried, ignited, burned and decomposed instantaneously. Further, the ash and the dust produced there­ in are brought to the upper section of the incinerator and are collected by an electric precipitator. Consequently, the refuse thrown onto the fluidized bed is disposed of almost completely, leaving behind some metallic, vitreous, or ceramic residue. The ratio of these substances to the refuse is usually 2%, meaning a fluidized bed in­ cinerator can dispose of 98% of the refuse. An advantage of the fluidized bed incinerator is that the volume of combustion residue can be reduced to 1/3 compared with a conventional mechanical incinerator like a stoker-type combustor. However, the refuse thrown onto the fluidized bed is burned and decomposed at high speed so that the refuse cannot be stably combusted. The refuse has a different calorific value depending on the kind of refuse and it is often difficult to always supply a constant volume of the refuse onto the fluidized bed. Suppose that a large quantity of refuse is thrown onto the fluidized bed at once, the refuse is burned and decomposed instantaneously and then a large amount of pyrolysis gases with dust is generated simultaneously. In this instance, it is not only impossible to completely combust a large amount of pyrolysis gases with the secondary air inside the in­ cinerator but it is also difficult to entirely collect the large amount of smuts in the exhaust gas by means of the electric precipitator. The principal object of this invention is to provide a method to slow the burning and decomposition of the refuse in the fluidized bed for stable combustion. Further, another object is to control the speed of the fluidized bed and to be able to carry out stable combust­ ion despite the fluctuations in the volume of refuse thrown onto the fluidized bed. An additional object of this invention is to provide a method of stable combustion for the fluidized bed incinera­ tor which is capable of reducing the volume of air supplied for the refuse and of maintaining the combustion tempera­ ture of pyrolysis at high level in the combustion chamber. Another object is to provide a type of fluidized bed incinerator which is capable of stably carrying out the combustion of the refuse in the fluidized bed. The invention solves these objects by the features of claim 1. Further developments of the invention are specified in the subclaims. The invention has the following advantages. (1) By keeping the temperature of the fluidized bed in the range from 520 to 650°C, the refuse can be slowly burned and stable combustion can be carried out without being influenced by a change of volume or quality of the refuse. (2) Since the refuse is burned and decomposed slowly, pyrolysis gas or smuts do not come out in large amounts. (3) Because the air ratio for combustion can be reduced, the combustion chamber temperature inside the incinerator can be high and the secondary combustion of pyrolysis gas can be carried out favorably. The invention will be further described with reference to the accompanying drawings. Fig. 1 is a schematic sectional view of an exemplary fluidized bed incinerator according to the present invention. Fig. 2 is a fragmentary enlarged view of Fig. 1. Fig. 3 is a graph showing the relation beween the tempera­ ture of the fluidized bed and the combustion speed ratio of the refuse, according to the present invention. Fig. 4 is a graph showing the temperature of the fluidized bed and the chronological change of exhaust gas temperature inside the incinerator when the refuse is burned according to the present invention. Fig. 5 is a graph showing the condition of smuts generated within the exhaust gas after burning the refuse in accordance with the present invention. Fig. 6 is a graph showing the temperature of the fluidized bed and the chronological change of exhaust gas temperatures at several locations in the incinerator when the refuse is burned in accordance with a conventional method. Fig. 7 is a graph showing the condition of exhaust gas generated when the refuse is burned according to a conventional method. Hereinafter, a preferred embodiment of the method of stable combustion for the fluidized bed incinerator accord­ ing to the present invention is described referring to the attached drawings. In Fig. 1, the reference numeral 10 denotes an incinerator body made up of refractory walls 12 and comprising a rectangular top wall 14, side walls 16 and an inverted rectangular pyramid bottom wall 18 connected with the lower section of the said side walls 16. The side walls 16 comprise an upper wall 16a in which a combustion chamber 20, described later, is formed, a tilted wall 16b sloped inwardly from the said upper wall 16a and a vertical side wall 16c extending vertically from the lower section of said tilted wall 16b and also connected with the bottom wall 18. An exhaust port 19 is provided in the top wall 14 and a discharge port 22 is provided at the lower center of bottom wall 18. In the space enclosed by the vertical wall 16c, a large number of air diffuser tubes 24 are provided parallel to each other to supply primary air to form a fluidized bed 40 described later. The tubes 24 extend through the vertical wall 16c out of the incinerator body 10 and are connected to a fluidizing air charging tube 26. On either side of air diffuser tubes 24, nozzle holes 25 are provided along the length direction at intervals. A duct 30,through which refuse 28 is thrown onto the air diffuser tubes 24,is connected to the upper side wall 16a of incinerator body 10, to which said precipitator 31 is further connected. The precipitator 31 comprises a screw 36, and a casing 34 connected to the duct 30. The casing 34 has a hopper section 38 for the refuse 28. The refuse 28 thrown into this hopper 38 is transferred into the duct 30 by the ro­ tation of the screw 36 and fed onto the air diffuser tubes 24 via the duct 30. The upper section wall 16a of incinerator body 10 has a charging port 37 for feeding such fluidizing medium 32 as sand into the incinerator body 10. This fluidizing medium 32 is supplied onto the air diffuser tubes 24 through the charging port 37 from a circulation unit described later. The fluidizing air charging tube 26 (not shown in the figure) is connected to an air charging source to supply the air to each air diffuser tube 24, from which air comes out, as shown by the arrows in the figure, via each nozzle hole 25. The refuse 28,along with the fluidizing medium 32 fed onto the air diffuser tubes 24,is fluidized by the said air, forming the fluidized bed 40. A screw conveyor 46 is connected to the discharge port 22 of the incinerator body 10 to transfer the fluidizing medi­ um (32) and the combustion residue among the refuse 28 which descends through the gaps among the air diffuser tubes 24. A separator 44 contains a sieve 48 which separates the fluidizing medium 32 from the combustion residue 42. The combustion residue 42 remains on the sieve 48 and is dis­ charged from a discharge port 45 of the separator 44. The fluidizing medium 32, after passing through the sieve 48, is fed back to the fluidized bed 40 via the charging port 37 through a circulation line 50 equipped with a vertical conveyor which is connected to the separator 44. The secondary air is supplied into the combustion chamber (free-board) 20 in the incinerator body 10 by air intake nozzles 52 which are installed to the upper wall 16a. A water spray 54,whose tip 54a is provided above the fluidized bed 40,penetrates the upper section wall 16a of incinerator body 10. This spray 54 is connected to a water charging line 56. A temperature sensor 58 for the temperature of the fluid­ ized bed 40 is provided inside the side wall 16c. The value detected by the sensor 58 is fed to a control unit 62 via a signal line 60. A control valve 66 is connected to the water charging line 56 to regulate the volume of water 64 sprayed onto the fluidized bed 40 from the spray 54. Based on the value detected by the temperature sensor 58, the control unit 62 controls the valve 66 via a control signal line 68 so that the temperature of the fluidized bed 40 is in the range from 520 to 650°C. Next, the method for burning the refuse in the fluidized bed incinerator will be explained. The refuse 28 is fed onto the air diffuser tubes 24 inside incinerator body 10 via the duct 30 from the precipitator 31, and the fluidizing medium 32 is supplied through the discharging port 37 from the circulation unit 50. On the other hand, the fluidizing air is supplied to each air diffuser tube 24 from the fluidizing air charging tube 26, and the primary air is blown out, as shown by the arrow in Figure 1,from each nozzle 25 of the said air diffuser tubes 24. The refuse 28 and the fluidizing medium 32,which are supplied onto the air diffuser tubes 24,are fluidized by the primary air blown in through the nozzles 25. A number of start-up burners are provided inside the incinerator body 10 (not shown in the figure) and when starting the incineration operation, the refuse 28 on the fluidized bed 40 is ignited by the flames from these burners. When the refuse 28 inside the fluidized bed 40 has all combusted by the fluidizing air, the ignition by burners is ceased. Part of refuse 28 undergoes pyrolysis and generates pyrolysis gases by the combustion heat of the refuse 28 inside the fluidized bed 40. The pyrolysis gases, containing H₂, CO and hydrocarbonaceous gases, are burned with the secondary air which is blown in,as shown by the arrow 52a, through the nozzles 52 at the combustion chamber 20 on the upper part inside the incinerator body 10. The primary air,blown out of the air diffuser tubes 24, and the secondary air,blown out of nozzles 52,are adjusted with a ratio of 2:3 to 3:3, or preferably with the ratio of 1:1, and further, the total air ratio to the theoretical air volume for combusting the refuse is adjusted in the range from 1,4 to 1,7. The exhaust gas,produced by the combustion of refuse 28 and the pyrolysis gas, is exhausted out of the incinerator from the exhaust port 19. Containing a large quantity of heat, this exhaust gas is utilized to preheat the water of the boilers etc. Since smut is contained in the exhaust gas, it is removed by an electrostatic precipitator after it has been used as the heat source. The refuse 28 and the fluidizing medium 32 have to be fed timely to the fluidized bed 40 where the refuse is burned and decomposed as mentioned above. On the other hand, the fluidizing medium 32 promotes the agitation and dispersion of the refuse and also forms a moving bed descending through the fluidized bed 40. Thereafter, the fluidizing medium 32 flows down onto the bottom wall 18,along with the combustion residue 42 within the refuse 28,from the gaps among the air diffuser tubes 24 and forms a filling bed below the air diffuser tubes 24 with the fluidizing medium 32 and the combustion residue 42 contained therein, and the said filling bed helps to adjust the thickness of fluidized bed 40 which is built up above the air diffuser tubes 24. As the combustion residue increases, the filling bed becomes big, so it is discharged by the screw conveyor 46 installed in a lower position. The screw conveyor 46 transfers the fluidizing medium 32 and the combustion residue 42 to the separator 44. In the separator 44, the combustion residue 42 is separated from the fluidizing medium 32 by the sieve 48, then the combustion residue 42 is discharged from the exhaust port 45 while the fluidizing medium 42 is fed again to the fluidized bed 40 through the circulation line 50. If the refuse 28 is usually burned in the fluidized bed 40, as mentioned above, the temperature of that fluidized bed may reach a level from 700 to 800°C. However, in this temperature range, the combustion of refuse 28 occurs so quickly that the said refuse would be instantaneously dried, ignited, burned, and decomposed. For this reason, if a large quantity of refuse 28 is thrown at one time into the fluidized bed 40, a large quantity of pyrolysis gases and smuts will be generated. As a consequence, all the pyrolysis gases cannot be burned with the secondary air supplied into the combustion chamber 20, and the smut, within the exhaust gas coming out of the incinerator body 10, and the dust cannot be completely removed by the electric precipitator. In this embodiment, therefore, the temperature of the fluidized bed 40 is detected by the temperature sensor 58, and the control unit 62 functions to regulate the control valve 66 and to control the water volume sprayed from the spray 54 so that the temperature of the fluidized bed 40 is in the range from 520 to 650°C. Since the fluidized bed 40 is maintained in the temperature range from 520 to 650°C by the water spray, the refuse 28 is burned and decomposed stably. Therefore, even if the volume of refuse 28 thrown to the fluidized bed 40 fluctuates, or a large amount of refuse 28 is thrown at one time, a stable burning and decomposition will occur so that pyrolysis gas and dust are not generated in large quantities. Further, the total air ratio required for burning the refuse 28 is 1,7 to 2,0 conventionally to the theoretical air volume, which can however be lowered to 1,4 to 1,7 according to this invention, and the temperature inside the free-board section (combustion chamber) can also be maintained at a high level. Fig. 2 shows the details of the water spray 54. The water spray 54 consists of double tubes 70, which are inserted at a downward angle from an insertion hole 72 which is provided on the upper section wall 16a to the incinerator body 10 and whose tip nozzle section 71 is positioned on the upper side of the fluidized bed 40. The insertion hole 72 is a cone-shaped hole which has a smaller open end at the outer surface of the wall 16a and the said insertion hole 72 allows the nozzle 70a on the tip of the double tube 70 to move to the right and left or up and down. The basement 74 of the double tube 70 is supported, via a universal joint 78, on a supporting body 76 which is installed on the upper section wall 16a. Further­ more, this universal joint 78 is interlocked with a linkage 80, and the nozzle 70a of the double tube 70 is moved up and down or right and left via the said universal joint 78 by the movement of the said linkage 80. The double tube 70 consists of an inner tube 82 for water and an outer tube 84 for cooling air. The basement of the inner tube 82 is connected via a flexible tube 86 to the water supplying line 56. The basement of outer tube 84 is connected via another flexible tube 88 to a cooling air supplying source (not shown in the figure). These flexible tubes 86 and 88 supply water and cooling air to the double tube 70 moving with the basement of the double tube 70. In the explanation of the aforecited embodiment, water is sprayed directly onto the fluidized bed 40 to keep the fluidized bed temperature in a range from 520 to 650°C, but this invention shall not be limited only to this way of keeping the temperature. As another means for keeping the temperature of the fluidized bed 40 in the range from 520 to 650°C, it is also acceptable, as shown by a broken line in Fig. 1, to provide a water spray 90 at the hopper 38 of the precipitator 31, in order to spray the water onto the refuse 28 inside the said hopper 38. In this case the volume of the water from the spray 90 is also adjusted so that the fluidized bed temperature by the temperature sensor 58 is in the range from 520 to 650°C. As another way to control the temperature, because the temperature of the fluidizing medium 82 which is separated by a separator 44 and circulated to the fluidized bed 40 through the circulation line 50 is at a considerably high level, the fluidizing bed temperature can be controlled lower than 650°C by providing a cooling unit 92 for the fluidizing medium 32 to the circulation line 50 to lower the temperature of the fluidizing medium 32 supplied to the fluidized bed 40. There is one more way to control the temperature. The fluidized bed temperature can be controlled at 650°C by cooling the fluidizing air which passes through the air diffuser tubes 24 and the fluidizing air tube 26. It is also acceptable to combine these means together for controlling the fluidized bed temperature. Fig. 3 is a graph showing the relation between the temperature of the fluidized bed and the combustion speed where the combustion speed ratio represents the ratio of the minimum time needed for burning and decomposition in ideal conditions and the time actually spent, after the refuse is thrown. As can also be seen from this figure, when the fluidized bed temperature is in the range from 700 to 800°C, its combustion speed ratio was 0,7 to 1,0 in the past, but this speed ratio can be maintained in the range from 0,4 to 0,6 by keeping the temperature in the range A, from 520 to 650°C as in this invention, and it is possible to lower the speed ratio to about 60% of the conventional combustion speed ratio. By burning the refuse slowly on the fluidized bed in this way, the refuse can be prevented from being burned instant­ aneously and the voluminous generation of pyrolysis gas and smut can be suppressed. In this case, a temperature of the fluidized bed lower than 520°C is not favorable because the combustion of the refuse in the fluidized bed becomes unstable (difficult), and if it is higher than 650°C, it is not favorable either because the speed becomes higher and the volume to be decomposed and the volume of smut generated are instant­ aneously increased even if the change of the volume and the quality of the refuse are relatively small. Fig. 4 and Fig. 6 show the chronological changes of the fluidized bed temperature and of the combustion gas temperature for this invention and for a conventional example. For Fig. 4 and Fig. 6, the same fluidized bed incinerator is used and the temperature is measured for 6 or 7 hours, feeding the municipal wastes at 2,5 tons/h and Fig. 4 shows the result where the secondary combustion at the free board has been fully carried out according to the present invention. As shown in Fig. 6, since the temperature inside the fluidized bed is not controlled by the conventional method, it can be seen that the fluidized bed temperature reaches a level higher than 650°C, and its temperature gradually increases because inside the combustion chamber the lower section temperature b is around 750°C, the upper section temperature c is around 850°C, and the temperature d of the combusted gas leaving the incinerator and entering into the gas cooling unit is around 950°C. This means that the pyrolysis gas generated by the thermal decomposition in the fluidized bed is burned until it enters the gas cooling unit, and that the temperature inside the combustion chamber is low, around 800°C, and that the pyrolysis gas has not been perfectly burnt. In contrast, in this invention as shown in Fig. 4, it can be understood that when water is sprayed onto the fluidized bed and its fluidized bed temperature a o is kept at 600°C ± 15°C, the lower section temperature b o of the incinerator, the upper section temperature c o as well as the temperature d o at the entrance of the gas cooling unit are all main­ tained at a high level of around 900°C. This explains the fact that the generated combustible gas is completely burned by the secondary air inside the combustion chamber. Fig. 5 and Fig. 7 are graphs showing the smut concentra­ tion in the exhaust gas from the stack, after the exhaust gas passes through the cooling unit and the dust is removed by the electric precipitator, which is then measured chronologically by a Ringelman smoke density indicator. In case of the conventional example, the smoke with a Ringelman smoke indicator value of more than about 0,5, which is the critical point for vision, is discharged disorderly as shown in Fig. 7, but in this invention little smoke with an indicated value of less than 0,5 is emitted, as shown in Fig. 5. For reference, it shall be construed that this invention should not be limited to the aforementioned embodiment. For instance, when controlling the temperature of the fluidized bed, its temperature can be controlled to the desired range even by spraying water at a constant rate onto the volume of the refuse without detecting the temperature.

Claims: 1. A stable combustion method for a fluidized bed incinera­ tor (10) comprising: forming a fluidized bed (40) by fluidizing with primary air refuse (28) such as municipal wastes and a fluidizing medium (32) inside the fluidized bed incinerator (10) when feeding the refuse (28) and the fluidizing medium (32)onto the said fluidized bed (40); burning and decomposing the refuse (28) inside the fluidized bed (40); combusting the pyrolysis gas produced by thermal decomposition with secondary air supplied to the upper section of the fluidized bed in­ cinerator (10); taking and separating the combustion residue of the refuse (28) and the fluidizing medium (32) from the lower section of the fluidized bed (40); circulating the separated fluidizing medium (32) to the fluidized bed (40); and maintaining the temperature inside the fluidized bed (40) in the range from 520 to 650°C. 2. The method of claim 1 wherein the fluidizing medium (32) consists of sand. 3. The method of claim 1 or 2 wherein the fluidized bed (40) is formed by fluidizing the refuse (28) and the fluidizing medium (32) with the primary air which is blown thereinto from air diffuser tubes (24) provided at the lower part of the incinerator body (10) parallel to each other having a large number of nozzles (25) provided on either side. 4. The method of anyone of the foregoing claims, wherein the temperature of the fluidized bed (40) is controlled in a range from 520 to 650°C by spraying water (64) onto the said fluidized bed (40). 5. The method of anyone of claims 1 to 3, wherein the temperature of the fluidized bed (40) is controlled in the range from 520 to 650°C by mixing water in advance with the refuse supplied onto the said fluidized bed (40). 6. The method of anyone of claims 1 to 3, wherein the temperature of the fluidized bed(40) is controlled by cooling the fluidizing medium (32) circulated to the said fluidized bed (40). 7. The method of claim 4, wherein the temperature inside the fluidized bed (40) is detected and water (64) is sprayed onto the fluidized bed (40) to control the temperature in the range from 520 to 650°C. 8. The method of anyone of the foregoing claims, wherein the total volume of the primary air and secondary air is from 1,4 to 1,7 times as much as the theoretical air volume for the refuse (28). 9. The method of claim 8, wherein primary air and second­ ary air are approximately in the ratio 1:1. 10. An incinerator (10) made of refractory material for conducting the method of anyone of the foregoing claims, comprising: a number of diffuser tubes (24) in the lower part of the incinerator body (10) parallel to each other and having a large number of nozzles (25) on either side for forming a fluidized bed (40) of refuse (28) and a fluidizing medium (32); means (26) for supplying said refuse (28) onto the air diffuser tubes (24); means (44, 46, 50) for taking out and separating the combustion residue (42) and the fluidizing medium (32) which have fallen from the lower section of the incinerator body (10) between the said air diffuser tubes (24); fluidizing medium circulation means (50) for circulating separated fluidizing medium (32) to the fluidized bed (40); a water spray (64) which is directed toward the upper face of the fluidized bed (40) from a side wall (12) of the incinerator body (10); and secondary air charging means (52) for supplying secondary air into the incinerator body (10) in order to combust in the upper section of incinerator body (10) the pyrolysis gas which has been produced by the thermal decomposition of the refuse in the fluidized bed (40). 11. The device of claim 10, wherein a water spray unit (54) has nozzles (71) whose position is changeable so that the water (64) can be sprayed uniformly over the fluidized bed (40). 12. The device of claim 10 or 11, wherein a water spray unit (54) has a temperature detector (58) for detecting the temperature of the fluidized bed (40), and the volume of the water (64) can be controlled by the value detected by said temperature detector (58). 13. The device of anyone of claims 10 to 12, wherein a water spray unit (54) has a double tube (70) consisting of an inner tube (82) for spraying water (64) and an outer tube (84) for cooling said inner tube (82) by air, the double tube (70) being inserted in such a way that it can face the interior of the incinerator body (10) from the outside, said water spray unit (54) having a drive mechanism (80) to rotate the tip of the said double tube (70) for spraying water (64) uniformly over the fluidized bed (40).

Inventors: INOUE, SATOSHI, NARISOKO, MINORU
Applicants: ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES CO., LTD.
Application year: 1987
Main classification: F23G5
IPC codes: F23G5, F23L7