METHOD OF COMBUSTION FOR FLUIDIZED BED INCINERATORS

Abstract: A method of controlling the combustion in the fluidized bed for fluid­ ized bed incinerators for disposing of refuse such as municipal wastes is dis­ closed. According to this invention method, a fluidized bed in which to burn the refuse is formed out of the refuse such as municipal wastes and a fluidizing medium by the primary air ; the pyrolysis gas generated in said combustion is burnt with the secondary air in the upper, or the freeboard, space of the fur­ nace body ; the gas generated in said secondary combustion is discharged out of the furnace body as an exhaust ; and a part of said discharged exhaust gas is returned to the furnace body, thus achieving effective control over drying, combustion, and pyrolysis of the refuse.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method of combustion for fluidized bed in­ cinerators for disposing of refuse such as municipal wastes. More particularly, it is concerned with a method of stabilizing combustion by recirculating the ex­ haust gas. 2. Description of the Prior Art Fluidized bed incinerators are constructed in such a way that the sub­ stances such as municipal wastes that are to be incinerated (called "refuse" hereinafter) are incinerated away in a fluidized bed, formed together with a fluidizing medium such as sand (called "fluidizing medium" hereinafter) over the air diffuser which is provided within the fluidized bed incinerator body (called "furnace body" hereinafter), as they are thrown thereonto, both by means of the primary air that is injected from said air diffuser. The free­ board part of the furnace body above and over said fluidized bed constitutes a combustion chamber in which to burn the pyrolysis, or the thermal decomposition, gas generated in the fluidized bed in afterburning (called "afterburning cham­ ber" hereinafter) with the secondary air introduced thereinto. The exhaust gas that has been generated in the afterburning chamber and in the fluidized bed is discharged out of the furnace body from the furnace top, cleaned up, then re­ leased to the atmosphere. The conventional method of combustion for such fluidized bed incinerator is simply to provide the primary and the secondary airs to burn the refuse. However, due to the fact that such properties of refuse as calorific value, water content, and size of lumps are changing from moment to moment, maintenance of stable combustion is difficult. Though this difficulty can theoretically be overcome by controlling the feeding rates of the primary and the secondary airs, but, because of the instantaneous changes in the combustion state, such controlling, hence mainte­ nance of stable combustion, has been difficult. Especially as the temperature of the fluidized bed becomes higher, the drying, pyrolysis, and combustion of refuse tend to be more vigorous, and as these reactions become more vigorous, the amount of the unburnt gas becomes greater, giving rise to the problem of in­ creasing nitrogen oxides (called "NOx" hereinafter). SUMMARY OF THE INVENTION This invention has been done to solve these problems mentioned above. Thus, it is the object of the present invention to provide a method of combus­ tion for fluidized bed incinerators that is capable of controlling the rise of the temperature of the fluidized bed, thereby making the drying, pyrolysis, and combustion of refuse sluggish and stabilized. It is another object of the present invention to prevent generation of unburnt gas and to decrease NOx. According to this invention method, a fluidized bed in which to burn the refuse is formed out of the refuse such as municipal wastes and a fluidizing medium by the primary air ; the pyrolysis gas generated in said combustion is burnt with the secondary air in the upper, or the freeboard, space of the fur­ nace body ; the gas generated in said secondary combustion is discharged out of the furnace body as an exhaust ; and a part of said discharged exhaust gas is returned to the furnace body, thus achieving effective control over drying, combustion, and pyrolysis of the refuse. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing an example of a fluidized bed incin­ erator piping system to practice the method of this invention ; and Fig. 2 is a schematic diagram showing another example of a fluidized bed incinerator piping system to practice the method of this invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now the preferred embodiments of this invention method are described in detail with reference to the attached drawings. As shown in Fig. 1, the fluidized bed incinerator 1 is equipped with an air diffuser comprising a plurality of air diffuser tubes 2, whose duty it is both to form a fluidized bed 3 out of the substances to be incinerated a such as municipal wastes (the refuse) and a fluidizing medium b such as sand (the fluid­ izing medium) and to burn the refuse therein with the primary air sand air dif­ fuser tubes 2 inject thereinto. The air diffuser tubes 2 are positioned in the lower part of the incinerator body (the furnace body) in a plural number and in parallel to each other, each tube having a large number of nozzles from which to blow the primary air in. A primary air charging tube 6 is connected to the air diffuser tubes 2 through a primary air blower 5. The freeboard space of the furnace body above and over the fluidized bed 3 serves as the secondary combus­ tion chamber (afterburning chamber) 7 in which to burn the pyrolysis gas gener­ ated in the fluidized bed 3. In the top of the furnace body, there is provided an uptake 9 to lead the exhaust gas discharged out of the afterburning chamber 7 to a stack 8 through a gas cooler 10, a gas clean-up unit 11 such as an electrostatic precip­ itator, and an induced draft fan 12 in said order. To the afterburning chamber 7, there is connected a secondary air charg­ ing tube 14 through a secondary air blower 13 to supply the secondary air there­ into. Also provided to the afterburning chamber 7 are a refuse charging port 15 and a fluidizing medium charging port 20, respectively through which the refuse a and the fluidizing medium b are thrown thereinto. Furthermore, for the purpose of recirculating a part of the exhaust gas once discharged out of the furnace body, an exhaust returning tube 16 is pro­ vided to the uptake 9 at a place downstream to the induced draft fan 12 to be connected to the afterburning chamber 7 through an exhaust circulator fan 17, said exhaust returning tube 16 being equipped with a valve 21 at a place down­ stream of the exhaust circulator fan 17. A branch pipe 18 that branches off the exhaust returning tube 16 at a place between the valve 21 and the exhaust circulator fan 17 is connected to the primary air charging tube 6 at a place upstream of the primary air blower 5 so as to supply the exhaust gas to the air diffuser tubes 2. To the branch pipe 18, there is provided a valve 19. Beneath the fluidized bed 3, there is formed a packed bed 4 made up of the combustion residue c of refuse a and the fluidizing medium b . At the bottom of the furnace body, there is provided a screw conveyor 22 to take out the combustion residue c and the fluidizing medium b ; the combustion residue c and the fluidizing medium b taken out of the furnace body thereby are separated from each other by the separator 23, the fluidizing medium b returning into the furnace body through a returning line, then through the charging port 20. In the arrangement described above, a part of the exhaust gas that has been discharged out of the afterburn­ ing chamber 7, cooled down to generally below 300°C in the gas cooler 10, and treated for dust and the like in the gas clean-up unit 10 is fed either to the after­ burning chamber 7 or to the fluidized bed 3 or to the both, the proportion of said returning gas being 10 to 30% of the discharged exhaust gas. This is because a deficiency in the amount of the fed back exhaust will result in insufficient controlling of combustion, whereas an excess therein will result in an overcontrolling. That is to say, the exhaust gas supplied to the air diffuser tubes 2 together with the primary air decreases the oxygen content of the primary air and increases the gas volume in the fluidized bed 3, thus lowering the temperature of the fluidized bed 3, hence making the drying, pyrolysis, and combustion of the refuse more sluggish, and consequently making the combustion stabilized. In this case, since the exhaust has been cooled in the gas cooler 10, the temperature of the fluidized bed 3 is decreased, and the combustion is stabilized more easi­ ly. Also, since the flame temperature of the afterburning chamber 7 is controlled by the fed back exhaust, the temperature of the fluidized bed 3 is indirectly controll­ ed, aiding stabilization of combustion in the fluidized bed 3. Furthermore, owing to the supply of the exhaust gas, NOx is decreased. As for another method of returning a part of the ex­ haust to the fluidized bed 3, the branch pipe 18 of the exhaust returning tube 16 may be led directly into the fluidized bed 3 as shown in Fig. 2. As for yet another method of returning a part of the exhaust to the afterburning chamber 7, the exhaust return­ ing tube 16 may be connected to the secondary air charging tube 14 at a place upstream of the secondary air blower 13 as shown in Fig. 2. Furthermore, the amount of the exhaust gas to be fed back may be automatically controlled through operating the valve 19 and the valve 21 by measuring the temperature of the fluidized bed 3. Thus it will be appreciated in what has been disclosed heretofore that the present invention will develop excel­ lent effects as follows. (1) Owing to the part of the exhaust gas returned to the furnace body, the combustion in the fluidized bed is made more sluggish, the rise of the temperature of the fluidized bed controlled, and the drying, pyrolysis, and combustion of refuse in the fluidized bed are made more sluggish and stabilized; and (2) Owing to the drying, pyrolysis, and combustion in the fluidized bed being made more sluggish, the generation of unburnt gas is prevented and NOx decreased.

Claims: 1 A method of combustion for fluidized bed incinerators in the method of combustion for fluidized bed incinerators that comprises: (a) burning the substances to be incinerated such as municipal wastes (refuse) while forming a fluidized bed out of the refuse and a fluidizing medium such as sand (fluidizing medium) as they are thrown into the incinera­ tor body (furnace body) both by means of the primary air; (b) burning the pyrolysis gas generated in the combustion of (a) with the secondary air within the upper space (freeboard) of the furnace body; and (c) discharging the exhaust gas generated in the combustion of (b) out of the furnace body, a method of controlling the combustion by returning a part of the exhaust gas discharged out of the furnace body in (c) to the furnace body. 2 The method of claim 1 wherein the primary air is injected into the fur­ nace body by means of the air diffuser provided within the furnace body. 3 The method of claim 2 wherein the part of the discharged exhaust of claim 1 is returned to the furnace body by means of the air diffuser of claim 2. 4 The method of claim 1 wherein the part of the discharged exhaust is returned to the fluidized bed separately from the primary air. 5 The method of claim 1 wherein the part of the discharged exhaust is returned to the freeboard of the furnace body together with the secondary air. 6 The method of claim 1 wherein the part of the discharged exhaust is returned to the freeboard of the furnace body separately from the secondary air.

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