FLUIDIZED BED INCINERATOR

Abstract: A method of stable combustion is disclosed for a fluidized bed incinerator (10) for burning and decomposing refuse (28) such as municipal wastes while fluidizing them. A plurality of air diffuser tubes (24) are provided inside the incinera­ tor body (10) for fluidizing the refuse (28) and a fluid­ izing medium (32). Fluidizing air is supplied at high speed or at low speed from each of the air diffuser tubes (24). By alternately forming strongly and weakly fluidized areas inside the fluidized bed (40), the refuse (28) is stably burned. Because of stable combustion of the refuse (28), the combustion air ratio can be reduced and the combustion chamber temperature inside the incinerator (10) can be maintained at a high level.

Description: This invention relates to a method for incinerating such sub­ stances as municipal wastes and industrial wastes (called "refuse" hereinafter) while fluidizing them inside a fluid­ ized bed incinerator. More particularly, it is concerned with a method for stable combustion in a fluidized bed in­ cinerator. The fluidized bed incinerator is known for disposing of refuse such as municipal wastes. The method for incinerat­ ing and disposing the refuse in this fluidized bed incinera­ tor is to burn the refuse while fluidizing them in a fluid­ ized bed incinerator with air. For improving the combustion, along with the fluidizing of refuse, a fluidizing medium, such as sand, is fed together with the refuse onto the fluid­ ized bed. A general type of fluidized bed incinerator is equipped with a plurality of air diffuser tubes or plates in the lower section of the incinerator body, and the upper section of the incinerator body is equipped with a refuse feeding unit and a fluidizing medium feeding unit. The refuse is burned while both the refuse and the fluidiz­ ing medium, thrown onto the air diffusers inside the inciner­ ator body, are fluidized by primary air blown out from the air diffusers. The refuse represented by municipal wastes generally con­ tains a variety of materials such as low calorie refuse like food discards, high calorie refuse like plastics and rubber, refuse like shredded paper and chipped furniture or refuse like fragmented metallic or vitreous containers, bottles or cans. The combustibles of the refuse fed onto the fluidized bed are burned, the plastics and similar substances of which are melted by heat to generate pyrolysis gases and the incom­ bustibles, like glass, are left unburned (called "combustion residue" hereinafter). The fluidizing medium is gradually fed within the fluidized bed and descends. Therefore, while the combustibles are burned and decomposed within the fluidized bed, the incom­ bustibles are brought down through the incinerator passing through the gaps among the air diffuser tubes at the lower section of the fluidized bed together with the fluidizing medium. The fluidizing medium is separated from the com­ bustion residue and fed again to the fluidized bed. The secondary air is fed into the upper section of the in­ cinerator above the fluidized bed and the generated pyrolysis gases are burned. Because the sand, the fluidizing medium thrown onto the fluidizing 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 fluidized bed is dispersed uniformly under the presence of the fluidizing medium to be dried, ignited, decomposed and burned instant­ ly. Further,ashes and dust are carried by the fluidizing air out of the upper section of the incinerator and collected by an electric precipitator. Consequently, the refuse thrown onto the fluidized bed is disposed almost completely, leaving behind some metallic, vitreous or ceramic residue. The ratio of these substances to the refuse is usually 2%, and therefore, incinceration by a fluidized bed incinerator can dispose 98% of the re­ fuse. 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 incinerator. However, the refuse thrown onto the fluidized bed is burned and decomposed at high speed so that the refuse cannot be stably burned. The refuse has a different calorific value depending on the kind of refuse and it is often difficult to always supply a constant volume onto the fluidized bed. Suppose that a large quantity of the refuse is thrown onto the fluidized bed at once, then a large quantity of pyrolysis gases and smut are also generated simultaneously and the refuse undergoes pyrolysis instantaneously. In this instance, it is not only impossible to completely combust a large quantity of pyrolysis gas with the secondary air inside the incinerator, but it is also difficult to entirely collect the large quantity of smut in the exhaust gas by means of an electric precipitator. The principal object of this invention is to provide a method for slowly burning and decomposing the refuse in the fluidized bed for the execution of a stable incineration. Another object is to provide a method of stable combustion for the fluidized bed incinerator by which fluidizing between the refuse and the fluidizing medium inside the fluidized bed can be controlled and a stable and slow com­ bustion of the refuse is performed. An additionional object is to provide a stable combustion method for the fluidized bed incinerator which is capable of reducing the volume of air supplied and of maintaining the temperature of the pyrolysis gas in the combustion chamber at a high level. The invention solves these objects by the features of claim 1. Further developments of the invention are specified in the subclaims. According to the invention, fluidizing air is blown out from the diffuser tubes provided in parallel inside the incinera­ tor, and refuse, such as municipal waste, is fluidized with a fluidizing medium and fluidizing air and then burned. High speed fluidizing air and low speed fluidizing air are blown from the diffuser tubes alternately and the air speed is changed cyclically from high to low, or low to high. It will be appreciated in the following disclosure that this invention will develop the following excellent effects. (1) The refuse can be burned slowly because a number of air diffuser tubes are installed in parallel in the incinerator body, and the fluidizing air is supplied out of these air diffuser tubes at different speeds, high speed or low speed, and the speed is changed alternately at every other diffusion tube. (2) Because the fluidized bed is fluidized to a greater or lesser extent, depending on the air diffuser tubes installed, and because the speed of air is changeable, the refuse can be mixed uniformly, and the circulation of fluidizing medium inside the fluidized bed can be promoted. Thus, drying, thermal decomposition and combustion of the refuse inside the fluidized bed can be carried out slowly and stably. (3) Since the air ratio for combustion can be reduced, the combustion chamber temperature can be kept high and the secondary combustion of pyrolysis gas can be done in a favorable manner. The invention will be further described with reference to the accompanying drawings. Fig. 1 is a schematic sectional view of an exemplary incinerator according to the present invention; Fig. 2 is a sectional view of line II-II of Fig. 1; Fig. 3 and Fig. 4 are sectional views showing the move­ ment of the refuse and of the fluidizing medium within the fluidized bed. Hereinafter, a preferred embodiment of the method for stable combustion according to this invention is described refer­ ring to the attached drawings. In Fig. 1, reference numeral 10 denotes an incinerator body made up of refractory walls 12, comprising a rectangular top wall 14, side walls 16 and an inverted rectangular pyramid bottom wall 18 connected to the lower section of said side walls 16. The side walls 16 comprise an upper wall 16a in which a combustion chamber 20, described later, is formed, a wall 16b, which is inclined inwardly from said upper wall 16a, and a vertical wall 16c extending vertically from the lower section of said inclined wall 16b and 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 section 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 blow out the primary air in order to form a fluidized bed 40 described later. The air diffuser tubes 24 extend through the vertical wall 16c and out of the incinerator body 10, and the fluidizing air is supplied from a fluidizing air charging source 26. Nozzle holes 25 are provided at intervals on either side of air diffuser tubes 24 along the length thereof. A duct 30 is connected to the upper section wall 16a of in­ cinerator body 10 to feed in refuse 28, such as municipal waste, and a precipitator 31 is connected to the said duct 30. The precipitator 31 comprises a casing 34 connected with the duct 30 and a screw 36. The casing 34 has a hopper section 38 for the refuse 28. The refuse 28, thrown into this hopper section 38, is transferred into the duct 30 by the rotation of the screw 36, and fed onto the air diffuser tubes 24 via the duct 30. The upper section of wall 16a of the incinerator body 10 has a charging port 36 for feeding a fluidizing medium such as sand, into the incinerator body 10. This fluidizing medium 32 is fed onto the air diffuser tubes 24 through the charging port 37 from a circulation unit described later. The refuse 28 and the fluidizing medium 32, which are fed onto the air diffuser tubes 24, are fluidized by the fluid­ izing air, blown out of the nozzles 25 of the air diffuser tubes 24, to form the fluidized bed 40. A screw conveyor 46 for transferring to a separator 44, the combustion residue within the fluidizing medium 32 and the refuse which is streamed down from the gaps among the air diffuser tubes 24 is connected to the discharge port 22 of the incinerator body 10. The 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 and a circulation line 50, equipped with a vertical conveyor, etc. which is connected to the separator 44. The secondary air is introduced into the combustion chamber 20 in the upper interior of the incinerator body 10 by air intake nozzles 52 which are installed in the upper wall 16a. As shown in Fig. 2, a first header 60a and a second header 60b are installed on both sides of the incinerator body 10 for supplying the fluidizing air at low speed and high speed. To these headers 60a and 60b, the air diffuser tubes 24a and 24b are connected alternately, i.e. every other tube to one header 60a, 60b, respectively. The first header 60a and the second header 60b are connected to the air charging source 26 shown in Fig. 1. This air charging source 26 has a valve (not shown) for cyclically supplying the fluidizing air at high and low pressure al­ ternately from the first header 60a and from the second header 60b. Therefore, when the high speed fluidizing air is blown out from each nozzle 25 of the air diffuser tubes 24a, the low speed fluidizing air is blown out from each nozzle 25 of the air diffuser tubes 24b at the same time, and conversely, when the low speed fluidizing air is blown out of each nozzle 25 of the air diffuser tubes 24a, the high speed fluid­ izing air is blown out of each nozzle 25 of the air diffuser tubes 24b. The interval of blowing, changing high speed to low speed, or reversely should preferably be set at 10 seconds to 3 minutes. The speed um of fluidizing air blown out of the nozzles 25 of the air diffuser tubes 24a and 24b is set at um/umf=1,0 to 2,5, at the high speed and at um/umf=0,5 to 1,5 at the low speed, where umf is the starting speed for fluidizing. The primary air, blown out of the air diffuser tubes 24 and the secondary air, blown out of nozzles 25, are adjusted with a ratio of 2:3 to 3:2, or prefereably with the ratio of 1:1, and further, the total air ratio is adjusted in the range from 1,4 to 1,7 times the theoretical air volume for burning the refuse. Next, the method for burning the refuse in the aforesaid fluidized bed incinerator is described. The refuse 28 is fed onto the air diffuser tubes 24 inside the incinerator body 10 from the duct 30, while the fluid­ izing medium 32 is fed from the charging port 37 through the circulation line 50. The fluidizing air is supplied to the air diffuser tubes 24a and 24b from the air charging source 26 for fluidizing and, from each nozzle 25 of the air diffuser tubes 24a and 24b, the fluidizing air is blown out alternately at high speed and low speed. Both the refuse 28 and the fluidizing medium 32 fed to the air diffuser tubes 24 are fluidized by the primary fluidiz­ ing air from the nozzles 25. A number of start-up burners are provided inside the inciner­ ator body 10 (not shown in the figure) and the refuse 28 in­ side the fluidized bed 40 is burned by flames from the burn­ ers when the operation is started. Once the refuse 28 inside the fluidized bed 40 starts burn­ ing with the fluidizing air, the ignition by burners is ceased. The refuse 28 is partly decomposed to a pyrolysis gas by the combustion heat in the fluidized bed 40. This pyrolysis gas contains such combustible gases as H₂, CO and hydrocarbona­ ceous gases which are burned with the secondary air blown, as shown by an arrow 52a, through the nozzles 52 into the combustion chamber 20 in the upper inside section of the incinerator body 10. Further, the total air ratio required for combusting the refuse 28 is conventionally 1,7 to 2,0 against the theoret­ ical air volume. It can, however, be lowered to 1,4 to 1,7 according to this invention, and the temperature inside the free-board area (combustion chamber) can also be maintained at a high level. The exhaust gas generated by the combustion of refuse 28 and the combustion of pyrolysis gas is exhausted to the outside of the incinerator from the exhaust port 19. Since this ex­ haust gas has a high calorific value, it is used as a heat source for heating the water of a boiler and the like. In addition, since smut is contained in the exhaust gas, dust is removed from the gas after it has been used as a heat source by an electric precipitator. The refuse 28 and the fluidizing medium 32 are fed sequent­ ially to the fluidized bed 40 and the refuse 28 is burned and decomposed as mentioned earlier. On the other hand, the fluidizing medium 32 promotes the agitation and dispersion of the refuse 28 and also forms a moving bed descending inside the fluidized bed 40. There­ after, the fluidizing medium 32 streams down, together with the combustion residue 42, through the gaps among the air diffuser tubes 24, remains on the bottom wall 18 and forms a filling bed just below the air diffuser tubes 24 by the fluidizing medium 32 and the combustion residue 42 contained therein. The said filling bed serves to adjust the thickness of the fluidized bed 40 which is formed above the air diffuser tubes 24. The filling bed, increased by the increment of the combustion residue, is discharged by the screw conveyor 46 which is installed below the said filling bed. The screw con­ veyor 46 transfers the fluidizing medium 32 and the combust­ ion residue 42 to the separator 44. In the separator 44, the combustion residue 42 and the fluidizing medium 32 are separated by the sieve 48, where the combustion residue 42 is discharged from the discharge port 45 and the fluidizing medium 42 is fed back to the fluidized bed 40 through the circulation line 50. The fluidizing air blown into the fluidized bed 40, as shown in Fig. 3, is blown at high speed from the air diffuser tubes 24a, respectively, as shown by arrows A, while the low speed fluidizing air is blown from the air diffuser tubes 24b, as shown by arrows b. In the fluidized bed 40 above each of the air diffuser tubes 24a and 24b, strongly and weakly fluidized areas are formed alternately above air diffuser tubes 24a and 24b, and some refuse 28 above areas A moves toward the lesser fluidized areas, as shown by broken arrows 61. In the lesser fluidized areas, the amount of air supplied toward the refuse 28 and the fluidizing medium 32 is small, so that the refuse 28 is burned slowly. However, soon after the situation in Fig. 3, if the speed of the fluidizing air from the air diffuser tubes 24a is slowed, as shown by arrows a in Fig. 4, and the speed of the fluidizing air from the air diffuser tubes 24b is increased, as shown by arrows B, the strong and weak situations reverse, so that the slowly burnt refuse 28 starts moving toward the more fluidized areas, as shown by the broken arrows 62 in Fig. 4. By changing the situation of fluidizing with high and low speed air from every air diffuser tube in this way alter­ nately and continuously, the refuse 28, within the fluidized bed 40, cannot only be agitated uniformly but can also be stably burned. It is not favorable if the maximum speed of air blown out of the air diffuser tubes 24a and 24b is slower than the fluidizing starting speed, because then the fluidizing does not happen, and if the speed of the air is more than 2,5 times (um/umf > 2,5) that of the fluidizing starting speed, the void ratio of the refuse in that fluidized area becomes greater and accelerates the combustion too much, which is not favorable either. On the other hand, when slowing down the speed of the air, it is preferable to keep the speed slower than 1,5 times that of the fluidizing starting speed umf or than about half of this speed. In case of um/umf being lower than 0,5, the stream-down speed of the refuse 28 becomes greater than desired, generating too much combustion residue, which is not favorable. In the aforementioned embodiment, an explanation is attempt­ ed by way of the number of air diffuser tubes 24 as 5 to 7 pcs. The number of tubes can be more or less than the aforementioned quantity if necessary.

Claims: 1. A method of stable combustion in a fluidized bed in­ cinerator for burning refuse while fluidizing it with fluidizing air and with a fluidizing medium over a number of air diffuser tubes for supplying the fluidizing air which are provided in parallel at intervals inside the fluidized bed invinerator, characterized in that the fluidizing air is alternately supplied at high speed and low speed from every other air diffuser tube and that the speed of the fluidizing air from each of the air diffuser tubes is periodically alternated at high and low speed. 2. The method of claim 1,wherein the high speed fluidizing air is supplied from the air diffuser tubes at 1,0 to 2,5 times the fluidizing starting speed, while the low speed fluidizing air is supplied at 0,5 to 1,5 times the fluid­ izing starting speed. 3. The method of claim 1 or 2, wherein a strongly fluidized area for the refuse and the fluidizing medium is formed above the air diffuser tubes from which high speed air blows, and a weakly fluidized area is formed above the air diffuser tubes from which low speed air blows, so that strongly and weakly fluidized areas are formed alternately. 4. The method of claim 3,wherein the refuse is quickly burned in the more fluidized areas, while the refuse is slowly burned in the lesser fluidized areas. 5. The method of claim 3 or 4, wherein both the refuse and the fluidizing medium move upward in the strongly fluid­ ized areas and further flow toward the adjacent weakly fluidized areas, and descend into the weakly fluidized areas inside the fluidized bed. 6. The method of anyone of claims 1 to 5 ,wherein the pyrolysis gas generated by the thermal decomposition of the refuse inside the fluidized bed is burned by secondary air supplied to a combustion chamber at the upper section inside the incinerator. 7. The method of claim 6, wherein the total air volume of primary air supplied to the fluidized bed and of the secondary air supplied into the combustion chamber inside the incinerator is 1,4 to 1,7 times that of the theoretic­ al air volume for the refuse. 8. The method of claim 7, wherein the primary air and the secondary air are approximately at the ratio of 1:1 in volume. 9. The method of anyone of the foregoing claims, wherein the air diffuser tubes are connected to a first header and to a second header at every other tube, and the fluidizing air at high pressure and low pressure is supplied alternate­ ly to said first header and said second header. 10. The method of anyone of the foregoing claims, wherein a plurality of air diffuser tubes are equipped with nozzles provided at either side at intervals along the length direction of the said tubes.

Inventors: INOUE, SATOSHI, KIYOTOMO, MIKIO, NARISOKO, MINORU
Applicants: ISHIKAWAJIMA-HARIMA HEAVY INDUSTRIES CO., LTD.
Application year: 1987
Main classification: F23G5
IPC codes: F23G5, F23C10