METHOD OF SECONDARY COMBUSTION PROMOTION FOR A FLUIDIZED BED INCINERATOR

Abstract: A secondary combustion method for combustible gases gene­ rated by thermal decomposition is disclosed for burning refuse such as municipal wastes. The refuse is fluidized, together with such fluidizing medium like sand, by the primary air, burned and decomposed. The pyrolysis gas is completely burned by the secondary air in a grid shape in multiple stages, which is formed inside the combustion chamber of the incinerator. The noncombustible gas and smut densities within exhaust gas are also reduced. Besides, the combustion chamber temperature can be maintained at a high level by carrying out secondary combustion swiftly.

Description: This invention relates to a method of incinerating sub­ stances such as municipal wastes and industrial wastes (called "refuse" hereinafter) while fluidizing them in a fluidized bed. More particularly, it is concerned with a method of the secondary combustion promotion for a fluid­ ized bed incinerator for post-combusting the combustible gas produced after pyrolysis in the upper part of an in­ cinerator after burning and decomposing the refuse in the fluidized bed. The fluidized bed incinerator is known for incinerating and disposing of refuse such as municipal wastes. The in­ cineration/disposition method of the refuse in this fluid­ ized bed incinerator is to burn the refuse while fluidiz­ ing them with air inside. In order to improve the fluidiza­ tion and combustion of the refuse, a fluidizing medium, such as sand, is fed together with the refuse in the fluid­ ized bed. A general type of fluidized bed incinerator is equipped with a number of air diffuser tubes or air diffuser plates (called "air diffusers" hereinafter) for blowing the air down to the lower section of the incinerator body. Further 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 fluidizing medium, thrown onto the air diffusers inside the incinerator body, are fluidized by the primary air blown from the air diffusers. The refuse generally contains a variety of materials such as low calorie refuse like food discards, high calorie refuse like plastics and rubber, refuse like shredded or chipped furniture, or refuse like fragmented metallic or vitreous containers, bottles or cans. The combustibles of the refuse fed to the fluidized bed are burned, the plastics and similar substances of which are melted by heat to generate pyrolysis gases, and the in­ combustibles, like metal and glass, are left unburnt (called "combustion residue" hereinafter). As the fluidizing medium is gradually fed onto the fluidized bed, a moving bed of fluidizing medium descends. Therefore, while the combustibles are burned or decomposed within the fluidized bed, the combustion residue goes down and out of the incinerator together with the fluidizing medium through the gaps among the air diffuser tubes at the lower section of the fluidized bed. The fluidizing medium is separated from the combustion residue and again fed to the fluidized bed. The secondary air is supplied into the upper section of the fluidized bed, 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 fluidized bed is dispersed uniformly under the presence of fluidizing medium to be dried, ignited, decomposed and burned instant­ aneously. Further, the ash and the dust produced therein are brought with 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 of almost completely, leaving behind some metallic, vitreous, or ceramic residue. The ratio of these substances to the refuse is usually 2%, and therefore the fluidized bed incinerator can dispose of 98% of the refuse. An advantage of the fluidized bed is that it can reduce the volume of combustion residue to 1/3 compared with a conventional mechanical incinerator like a stoker-type in­ cinerator. 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 onto the fluidized bed. Suppose that a large amount of the refuse is thrown at once onto the fluidized bed, then a large quan­ tity of pyrolysis gases and smuts are also generated simul­ taneously even though the refuse is burned and decomposed instantaneously. In this instance, it is impossible not only to completely combust a large quantity of pyrolysis gases a second time with the secondary air inside the in­ cinerator but it is also difficult to entirely collect the large quantity of smuts contained in the exhaust gas by means of the electric precipitator. The principal object of this invention is to provide a method for burning and decomposing the refuse slowly inside the fluid­ ized bed incinerator for combusting the generated combustible gases a second time in the upper section of the incinerator, for improving the combustion of the mixture of combustible gas and secondary air and for maintaining the temperature of the combustible gas in the incinerator at a high level. An additional object is to slow down the combustion of refuse in the fluidized bed by injecting secondary air into the com­ bustion chamber in order to carry out secondary combustion. These objects are solved by the features of the main claim. Further developments are described in the subclaims. The invention comprises such functions as fluidizing refuse such as municipal wastes and the fluidizing medium,both of which are supplied into the fluidized bed incinerator, with primary air. In order to form the fluidized bed, the refuse and the fluidizing medium are supplied in that fluidized bed, not only being burned but also decomposed, and the secondary air blows into the combustion chamber at the upper section inside the incinerator for secondary combustion of the com­ bustible gases which are produced by the thermal decomposi­ tion of the refuse. The secondary air, being blown into the combustion chamber, is blown out from a group of nozzles which are installed vertically in multiple stages and parallel to each other in the horizontal direction at least to one side of the incinerator walls so that the secondary air from each nozzle can flow horizontally across the combustion chamber. It will be appreciated in the following disclosure that this invention will develop the following excellent effects. (1) By blowing the secondary air into the fluidized bed in­ cinerator, the pyrolysis gas generated by thermal decomposi­ tion of the refuse can be burned a second time in a favorable manner by providing several stages of nozzle groups horizont­ ally in the vertical direction of the free-board section and blowing the secondary air in a grid shape. (2) The fluidized bed temperature can be controlled (mainly by being heated) with the secondary combustion flame by in­ stalling the lowest stage nozzle group to supply the air close to the upper surface of the fluidized bed. (3) Since the burning of pyrolysis gas in the lower section of the combustion chamber inside the incinerator can be done quickly, the combustible gas temperature within the combustion chamber can be maintained at a high level. The invention will now be further described with reference to the attached drawings in which Fig. 1 is a schematic sectional view of a fluidized bed in­ cinerator, according to this invention; Fig. 2 is a sectional view on line II-II of Fig. 1; Fig. 3 is a graph showing the chronological change of CO an O₂ gas densities within the exhaust gas in the case of this invention; Fig. 4 is a graph indicating the chronological change in the smuts in this invention; Fig. 5 is a graph showing the chronological change in CO and O₂ gas densities in the exhaust gas in the case of a conventional combustion method; and Fig. 6 is a graph indicating the chronological change of the smuts in a conventional method. Hereinafter, a preferred embodiment of the secondary com­ bustion promotion method for the fluidized bed incinerator according to this invention will be described referring to the attached drawings. In Fig. 1, the reference numeral 10 denotes an incinerator body made up of refractory walls 12 which comprises a rectangular wall 14, side walls 16 and an inverted rectangu­ lar pyramid bottom wall 18 connected to the lower section of the said side walls 16. The side walls 16 comprise an upper wall 16a in which a combustion chamber 20 (free-­ board section), described later, is formed, a wall 16b which is inclined inwardly from the said upper wall 16a and a vertical wall 16c extending vertically from the lower section of the said inclined wall 16b and connected with the bottom wall 18. An exhaust port 19 is provided on the top wall 14 and a discharge port 22 is provided at the end of bottom wall 18. In the space enclosed by the vertical wall 16c, a number of air diffuser tubes 24 are provided parallel to each other to blow the primary air forming the fluidized bed described later. The air diffuser tubes 24 extend through the vertical wall 16c, outside the incinerator body 10, and are connect­ ed to a fluidizing air charging tube 26. On either side of each air diffuser tube 24 nozzle holes 25 are provided along the length direction at intervals. A duct 30, through which refuse 18 such as municipal wastes is thrown onto the air diffuser tubes 24, is connected to the upper section wall 16a of the incinerator body 10 and a precipitator 31 is connected to the said duct 30. The precipitator 31 comprises a casing 34 connected to the duct 30 and a screw 36. This casing 34 comprises a hopper section 38 for the refuse 28. The refuse 28, thrown into this hopper section 38, is transferred to the duct 30 by the rotation of thescrew 36 via the duct 30 and fed onto the air diffuser tubes 24. On the upper section wall 16a of the incinerator body 10, a charging port 37 is provided to feed a fluidizing medium 32 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. A fluidizing air charging tube 26, not shown in the figure, is connected to an air charging source for supplying air to diffuser tubes 24, where the air comes out, as shown by the arrows in Figure 1, from each nozzle 25 of the air diffuser tubes 24. The refuse 28, along with the fluidizing medium 32 which is fed onto the air diffuser tubes 24, is fluidized by the air to form a fluidized bed 40. A screw conveyor 46 is connected to the discharge port 22 of incinerator body 10 for transference of the fluidizing medium 32 and the combustion residue of the refuse 28 to a separator 44 after these substances come through the gaps among the air diffuser tubes 24. 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 through a circulation line 50 structured with the verti­ cal conveyor etc. which is connected to the separator 44. To the upper section wall 16a, making up the combustion chamber 20 of the incinerator body 10, a number of nozzles 52 are installed and arranged vertically at multiple stages and also in the horizontal direction. The nozzles 52 are vertically provided in several stages in the incinerator body 10, for instance, installed in four stages as shown in Figure 1, where the lowest stage of nozzle group 52a and the 3rd stage nozzle group 52c are installed to the same side face of incinerator body 1, while the 2nd stage nozzle group 52b and the 4th stage nozzle group 52d are provided on the wall face opposite the lowest stage nozzle group 52a and the 3rd stage nozzle group 52c. These opposed nozzle groups 52 through 52d are installed in a manner so as to form a secondary air flow as shown by arrows 52A, 52B, 52C and 52D, respectively, toward the center line 0 of the incinerator body 10 as shown in Figure 1. Each nozzle group 52 shall be, as shown in Figure 2, installed so that a number of nozzles may be mounted in parallel to a header 56 and each of these nozzles may pass through the upper wall 16b and face the interior of the combustion chamber 20. These nozzles 54 have an inside diameter of 40 to 80 mm or a cross section from 30 mm × 60 mm to 40 mm × 100 mm, and the horizontal interval "l" for nozzles is from 200 to 600 mm. As shown in Fig. 1, not only is a secondary air charging tube 58 connected to the header tube 56 in each stage, but a damper 60 is also connected respectively. The secondary air supplied to the header 56 from the secondary air charg­ ing tube 58 is maintained at a pressure higher than 250 mmAq by means of the damper 60, and the secondary air from each nozzle 54 is blown across the combustion chamber 20, like the two-dot chain lines shown in Figure 2. The lowest stage nozzle group 52a is mounted in a position where the height "h" from the upper face of the fluidized bed 40 to the air flow 52 from those nozzles is 0,1 to 1,5 m. The primary air, blown out of the air diffuser tubes 24,and the secondary air, blown out of the nozzle groups 52a, through 52d are adjusted with a ratio of 1:3 to 3:2, or preferably 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 combustion of the refuse. The method for burning the refuse in the aforementioned fluidized bed incinerator will now be described. The refuse 28 is fed onto the air diffuser tubes 24 inside the incinerator body 10 from the precipitator 32 via the duct 30 and the fluidizing medium 32 is fed from the circulation unit 50 via the charging port 37. The fluidizing air is fed to each air diffuser tube 24 from the fluidizing air charging tube 26 and the primary air is blown out of nozzles 25 of the said air diffuser tubes 24, as shown by the arrows in Figure 1. The refuse 28 and the fluidizing medium 32 fed onto the air diffuser tubes 24 are fluidized by the primary air blown out of the nozzles 25. A number of start-up burners are installed inside the in­ cinerator body 10 (not shown in the figure) and the refuse 28,inside the fluidized bed 40,is burned by flames from burners when the operation is started. After the refuse 28 inside the fluidized bed 40 is burned with the fluidizing air, the ignition by burners is ceased. The flame is emitted over the entire surface of the fluidized bed 40 by the air flow 52 blown out in a grid shape from the lowest stage nozzle group 52a, and the flame over the said fluidized bed 40 cannot only be controlled but the pyrolysis gas generated by the thermal decomposition can also be dispersed uniformly. The combustion heat of the refuse 28 in the fluidized bed 40 causes some of the refuse 28 to be decomposed into pyrolysis gas. This pyrolysis gas, containing such com­ bustible gases as H₂, CO and hydrocarbonaceous gases, is burned a second time by the secondary air blown in from the nozzles 54 in the combustion chamber 20 at the upper part inside the incinerator body 10. The combustible gas produced is completely burned,while ascending in the combustion chamber 20,by the secondary air 52B, 52C and 52D from nozzle groups 52b, 52c, 52d with a velocity higher than 50 m/s which is blown in and has a grid-shaped form. Since these secondary air streams 52B, 52C and 52D are formed into a grid shape in some stages vertically across the combustion chamber 20 as shown in Figure 2, and the interior of the combustion chamber 20 is covered by the secondary air in the upper and lower stages, the combustible gas rising from the fluidized bed 40 is prevented from blowing through, and thus the com­ bustible gas can be burned positively, swiftly and stably in the combustion chamber 20 entirely. Further, the total air ratio required for combusting the refuse 28 in the conventional method is 1,7 to 2,0 against the theoretical air volume, which can be lowered to 1,4 to 1,7 by this invention, and the temperature inside the free-­ board section (combustion chamber) can also be maintained at a high level. The exhaust gas, which is generated with the combustion of the refuse 28 and the secondary combustion of pyrolysis gas, is brought out of the incinerator through the exhaust port 19. Since it has a high calorific value, this exhaust gas is used as a heat source for heating the water for boilers, etc. No smut is contained in the exhaust gas because it is removed by the electric precipitator after it has been used as a heat source. 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. The fluidizing medium 32 promotes the agitation and dispers­ ion of thrown refuse 28 and also forms the moving bed which descends inside the fluidized bed 40. Thereafter, the fluidizing medium 32 flows downward with the combustion residue 42 among the refuse, through the gaps among the air diffuser tubes 24, remains on the bottom wall 18 and forms the filling bed below the air diffuser tubes 24 with the fluidizing medium 32 and the combustion residue 42 contained therein. This filling bed regulates the level of the fluidized bed 40 which is formed over the air diffuser tubes 24. The filling bed, increased by the increment of combustion residue, is discharged by a screw conveyor which is installed in a lower position. The screw conveyor 46 transfers the fluidizing medium 32 and the combustion residue 42 to a separator 44. In the separator 44, the combustion residue 42 is separated from the fluidizing medium 42 by a sieve 48, and the combustion residue 42 is discharged out of the discharge port 45 while the fluidizing medium 32 is fed again to the fluidized bed 40 by a circulation line 50. Fig. 3 and Fig. 5 show the examples of chronological change of CO gas density and O₂ gas density when the refuse is burned in the fluidized bed incinerators according to the present invention and the conventional way. Municipal wastes are used as refuse in both the present invention and the conventional case, and fed 2,5 tons/h, while the method of blowing secondary air differs. In the example in Fig. 3, in addition to the present invention, the temperature of the fluidized bed is controlled to be 600°C. In the conventional example, the CO gas among pyrolysis gas, whose density is represented by "a", is periodically pro­ duced at a density higher than 5000 ppm as shown in Fig. 5, and the oxygen density "b" is also lower than 5% at that time. This means that the refuse is not stably burned in­ side the fluidized bed and that a large amount of pyrolysis gas represented by CO gas is generated due to changes in quality or volume of the refuse or in temperature or of the fluidized bed. It is known that the supply of second­ ary air for combustion of these gases cannot follow the subsequent changes so that oxygen density decreases, re­ sulting in an oxygen shortage. In contrast, a favorable mixture of the pyrolysis gas rising out of the fluidized bed with the secondary air is achieved in this invention, and sufficient secondary com­ bustion is carried out in the free board section, so the combustion inside the incinerator can be completed and the CO gas density ao can be suppressed to 1000 ppm or below, shown in Fig. 3, and it is also known that the oxygen density bo can be reduced to around 10%, thus having the pyrolysis gas burned stably. Fig. 4 and Fig. 6 show the cases of combining the means of fluidized bed temperature control with this invention and the chronological change of smut generated in a conven­ tional example, respectively. Smut from smoke is measured by Lingelman smoke density indicator, in both this invention and the conventional example, after the exhaust gas coming out of the fluidized bed incinerator is cooled down in the gas cooling unit and dust is removed by an electric precipitator. As indicated in Fig. 6, smoke with an indicated value higher than the critical point for vision (0,5) is often exhausted in the conventional example. In the present in­ vention, the smoke with a value higher than the critical point for vision (0,5) is only rarely exhausted as shown in Fig. 4.

Claims: 1. A method of secondary combustion promotion for a fluid­ ized bed incinerator comprising: (a) forming a fluidized bed by fluidizing with primary air such refuse as municipal wastes and a fluidiz­ ing medium supplied into the fluidized bed incinera­ tor; (b) feeding the refuse and the fluidizing medium into said fluidized bed; (c) burning and decomposing the refuse inside the fluid­ ized bed; and (d) supplying secondary air into the combustion chamber inside the upper section of the incinerator for secondary combustion of the combustible gas which is generated by the thermal decomposition of refuse, where the secondary air blown into the combustion chamber comes from a number of nozzle groups which are provided in several stages vertically and also in several rows horizontally. 2. The method of claim 1, wherein the fluidizing medium with the combustion residue is taken out of the lower section of the fluidized bed and fed back to the fluid­ ized bed after being separated from the combustion residue. 3. The method of claim 1 or 2, wherein the fluidizing medium is sand. 4. The method of anyone of claims 1 to 3, wherein the fluid­ ized bed is formed by fluidizing the refuse and the fluid­ izing medium with the primary air blown from a number of nozzles provided along either side of air diffuser tubes arranged in a grid shape at the lower section of in­ cinerator body. 5. The method of anyone of claims 1 to 4, wherein a plural­ ity of nozzle groups are provided in multiple stages vertically and also parallel to each other in horizontal direction, respectively, on opposite incinerator walls inside the combustion chamber. 6. The method of claim 5, wherein the secondary air blown out of the nozzle group in the lowest stage is supplied toward the flames from the refuse within the fluidized bed, in order to disperse the flames uniformly. 7. The method of claim 6, wherein the nozzle group in the lowest stage mounted on the incinerator is disposed in such a way that the secondary air stream is formed at 0,1 to 1,5 m from the upper surface of the fluidized bed. 8. The method of anyone of claims 1 to 7, wherein the adjacent nozzle interval in horizontal direction is from 200 to 600 mm. 9. The method of anyone of claims 1 to 8, wherein each nozzle on every horizontal stage of a nozzle group is connected to a header to which the secondary air is supplied with a pressure higher than 250 mmAq to blow the secondary air out of each nozzle. 10. The method of anyone of claims 1 to 9, wherein the total air volume of the primary air and the secondary air is from 1,4 to 1,7 times that of the theoretical air volume for refuse. 11. The method of claim 10, wherein the primary and the secondary air are approximately in the ratio of 1:1.

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