JP2679598B2 - Method and equipment for removing dioxins from fly ash in a refuse incinerator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention relates to the removal of dioxins in fly ash recovered from refuse incinerators, especially fluidized bed refuse incinerators.

[0002]

2. Description of the Related Art Polychlorinated dioxins and polychlorinated dibenzofurans, which are generally called dioxin, have strong carcinogenicity and teratogenicity, and are extremely harmful substances when accumulated in the human body. Experimental report ("Dioxin generation and prevention technology in municipal waste incineration facility" Hanai-Safety Engineering Vol 27. NO.
6.1988), dioxins are mostly generated not in the incinerator but in the electrostatic precipitator. There is a process in which incineration of dust causes carbon-containing polymer solids to undergo thermal decomposition to lower molecular weight, vaporize and combine with oxygen in the air to change to carbon dioxide gas. Organochlorine compounds are formed when combined with chlorine supplied from plastics. However, since the inside of the incinerator is at a high temperature, the components vaporized by thermal decomposition are immediately oxidized, so there is almost no chlorination reaction, or it is further oxidized. Since the retention time of ash is long, it has been confirmed that the chlorination reaction proceeds, not the oxidation reaction.

The problem of removing harmful dioxins from fly ash collected by a dust collector of an incinerator is naturally an important and urgent subject among environmental problems, and therefore many conventional techniques have been disclosed. The main technology is the efficient thermal decomposition of the recovered fly ash. For example, an inert gas treatment is performed under heating and V / TiO ₂ is used as a catalyst.
-138150, JP-A-5-146772, which relates to a furnace structure for uniformly heating and cooling dioxin,
JP-A-5-154461 discloses a method in which high-temperature combustion gas is directly contacted with fly ash for thermal decomposition.

On the other hand, in Japanese Patent Laid-Open No. 5-33914, water or a suitable binder is added to the collected fly ash as necessary, and then the fly ash is introduced into a granulator and pressure-molded, and the granules are trash The organic chlorine compound is thermally decomposed by throwing it into the feeding hopper together with dust and burning it. It shows a method of disassembling fly ash after processing from the bottom of the incinerator and discarding it.

[0005]

Among the above-mentioned prior arts, the former three are all devised in setting the conditions for heating and cooling, and are effective in improving the thermal decomposition efficiency. However, heating the recovered material containing fly ash, which is fine powder, as is is likely to cause another problem. When fine powder and granules are mixed, the width of the residence time distribution in the kiln widens and the decomposition rate varies, which is inconvenient for thorough decomposition. In addition, the fly ash of fine powder is melted or softened by coming into contact with hot air of 300 to 500 ° C and adheres and grows on the inner wall of the kiln, and if the adhered layer accumulates, it will hinder continuous operation. You have to stop and remove this deposit. In addition, if a large amount of hot air is passed through while containing fine powder, the dust will be severely scattered, the amount of exhaust gas collected by the dust collector will increase, and it may be troubled by the vicious cycle of re-introducing into the kiln.

In this respect, in the fourth prior art, fly ash is granulated by a granulator to form fine powder into granules, so that the considerable problems in the above prior art can be solved. However, incinerators for municipal waste are roughly classified into a stoker (grate) type and a fluidized bed type. In the case of a stoker type, if fly ash collected by a dust collector is granulated and then thrown into the furnace together with the waste. However, the ash content that has been pyrolyzed and rendered harmless can be recovered from the bottom of the furnace, but in the case of the fluidized bed type, most of the ash content generated by combustion is recovered by an electric dust collector. Since dioxins are generated as described above under operating conditions of about ℃, even if the collected fly ash is granulated and put into the furnace again, the same dioxins generation reaction is repeated in the electrostatic precipitator again. It just continues a vicious circle.

Further, when granulated into granules, there is no risk of melting and softening due to heating like fly ash, but a lot of time is wasted until the inside and outside of the particles are heated to the same temperature.
A long residence time is required to completely pyrolyze the interior. According to a paper by Umemura (“Pyrolysis equipment for dioxins in fly ash in a refuse incinerator”, May 1992, Industrial Machinery), maintaining oxygen deficiency at a constant temperature (about 400 Maintaining the retention time (about 1 hour), further required depending on the dioxin concentration Emission at 140 ° C or less after retention is mentioned, but the above problems remain for pyrolysis in the form of fine powder and solidification The problem remains when granulating.

An object of the present invention is to solve all of the above-mentioned problems, and to provide a method and an apparatus for removing dioxins that efficiently treat fly ash and contribute to environmental improvement.

[0009]

A method for removing dioxin in fly ash of a refuse incinerator according to the present invention is a pair of moldings in which only fly ash powder collected from exhaust gas of a refuse incinerator is rotated in mutually opposite directions. Squeeze between rolls to form a thin plate,
The above-mentioned problems were solved by crushing the thin plate molded body to subdivide it into flat strips, and heating the flat strips from which fine powder was removed in a rotary kiln in a low oxygen atmosphere.

More specifically, the thickness of the thin plate molded body is 3 to.
It should be in the range of 5 mm, the size of the flat strips should be sieved in the range of 0.5 to 8 mm, and the inside of the rotary kiln during heating should be in an atmosphere with an oxygen concentration of 5 Vol% or less. This is a very preferred embodiment.

The apparatus used in the above method includes a pair of forming rolls 11 which are rotatably supported by mutually parallel rotating shafts and rotate in opposite directions with a desired opening, and a dust incinerator between the forming rolls. A thin plate of fly ash powder including a hopper 12 to which only the fly ash powder collected from the exhaust gas is supplied, a cylindrical barrel 13 connected to the lower bottom of the hopper, and a screw 14 rotating on the central axis of the barrel. A molding machine 1, a crusher 2 for subdividing the thin plate compact into flat strips, a vibrating sieve 5 for sieving fine powder contained in the subdivided flat strips, and the flat strips into a horizontally elongated tubular body. It is a requirement to have a configuration of a rotary kiln 3 in which the body 31 is charged and the body 31 is rotated in a low oxygen atmosphere while rotating while being kept substantially horizontal.

In the construction of this apparatus, the thin plate molding machine 1 starts deepest on the inner peripheral surface of the barrel 13 at the beginning of the barrel and gradually inclines gradually, and disappears at an intermediate point left at least one pitch of the screw from the end of the barrel. It is most effective to achieve the purpose that the rotary kiln 3 is provided with a plurality of gradient grooves 15 and the rotary kiln 3 is of an external heating type, and the body 31 has a substantially hexagonal cross section.

[0013]

ACTION One of the features of the action of the present invention is that only the dry powder is strongly pressed between the rolls to form the fly ash in the form of fine powder without adding any water or binder. Naturally, dry molding without added water or binder is extremely advantageous at the time of intended thermal decomposition, but in order to actually enable such molding, it is necessary to form thin plates between molding rolls. The method of squeezing is excellent. Next, it is also important to disintegrate the thin plate-shaped compact into fine flat pieces and remove fine powder by sieving. The flat shape and the small pieces, unlike the granules produced by the granulator, takes a short time until the heat is evenly distributed over the whole body, resulting in extremely high thermal efficiency.
Generally, the optimum thermal decomposition of dioxins is around 400 ° C, and the whole is heated up to this proper temperature quickly, and after decomposition, 150 ° C.
It is essential to quench below. In that respect, it is extremely advantageous that the flat strips are rapidly soaked to the proper temperature, and there is no mixing of fine powder, so there is no concern that they will melt and soften and adhere to the furnace wall, and that the pyrolysis reaction will be completed efficiently. It can be said that it is an action.

At the time of heating with an external heat type rotary kiln, as the body of the furnace rotates, the object to be heated is lifted along the wall of the furnace and falls at a certain point due to its own weight. Figure 6 shows the concept of the movement. That is, the cascade motion is A
-It can be considered to be divided into rotational ascending motion, B-direction change due to gradual collapse of layer, C-avalanche motion, and D-stop zone, but in the case of fly ash mainly composed of fine powder, it is lifted along the furnace wall. Slippage occurs at the part to be lifted and cannot be lifted up,
Since it is not possible to induce avalanche movement in the upper part, it is often the case that the objects to be heated are not exchanged with each other and the stirring phenomenon is weakened, resulting in a marked decrease in thermal efficiency. On the other hand, in the case of flat strips, the mutual friction coefficient increases and they are entangled with each other and restrained from each other. This is also an advantageous action, but if the device is limited to a specific one as in the embodiments described later, this action can be promoted more effectively.

[0015]

1 is a schematic front view showing a flow of an embodiment of the present invention. As a pre-process in the figure, a fluidized bed waste incinerator F, an exhaust gas cooling chamber C, an air preheater H, an electric dust collector E and an induced air blower B are provided, and fly ash is an ash bunker 4 and a constant quantity feeder 41. And is supplied to the hopper 12 of the thin plate forming machine 1. The thin plate molding machine 1 is a dry powder molding machine called a roller compactor as a model, and its details are shown in FIG.
As shown in (A), the screw 14 is rotated around the axis of the funnel-shaped hopper 12 to forcibly push fly ash into the barrel 13. Below the barrel, there is a pair of forming rolls 11 that rotate in mutually opposite directions, and the fly ash is pinched and squeezed into a thin plate.

This model is originally superior to other models in molding powder materials that have a small friction coefficient and are difficult to mold. However, in molding fly ash containing a large amount of fine powder, fly ash and the inner circumference of the barrel Since the friction with the surface of the blade of the screw is greater than the friction with the surface, there is a concern that slip may occur and fly ash may co-rotate with the blade and escape. For that purpose, the conventional technique of Japanese Utility Model Publication No. 3-12478 developed by the applicant is diverted, and as shown in FIG. (B), a middle point left at least one screw pitch from the start end of the inner surface of the barrel An embodiment in which a plurality of sloped grooves 15 that disappear in step S1 are engraved gives good results. That is, the fly ash filled in the gradient groove 15 is caught by the concave side surface thereof and is restrained and locked, and a shearing action is exerted between the fly ash and the periphery of the blade that tries to rotate together, so that the fly ash feeding action is enhanced. Of. In this case, it is a feature of the present invention that only pure fly ash is supplied during molding, and no addition of other water or binder is performed.
This is a factor that promises to improve the thermal efficiency in the subsequent thermal decomposition process. It is convenient that the thin plate at the time of molding has a thickness of 3 to 5 mm, especially about 4 mm.

It is the crusher 2 that subdivides the thin plate-shaped compact into flat strips, one example of which is the coarse crusher 21 (breaker) shown in FIGS. 3 (A) and 3 (B) and FIG. 4 (A). It is formed by connecting with a granulator 22 (granulator) shown in (B). The crusher crushes the thin plate molded body supplied from the charging port 26 between the fixed teeth 25 attached to the charging port and the rotary shaft teeth 24 planted on the rotary shaft 23 to form a flat strip having an appropriate size. It is sent to the granulator 22 below. The sieving machine is crushed by a crushing blade 28 into the corner of a hollow cylindrical body 27 with a hexagonal cross section that is horizontally laid horizontally in the center.
And a screen 29 such as a wire net or a perforated plate is provided around the outer periphery of the hollow cylinder, and the flat pieces of an appropriate size supplied to the hollow cylinder are crushed by a crushing blade 28. After sizing to a size equal to or smaller than the screen opening, the particles are discharged to the outside of the screen, and after collection, they are passed through a vibrating sieve 5 to reliably remove fine powder and supplied to the rotary kiln 3.

Rotary kiln 3 applied in this embodiment
Is based on the applicant's own invention, Shokai 62-145098.
The conventional technique disclosed in the publication is used. This prior art is shown in FIG.
As shown in (A) and (B), this is a type of rotary kiln equipped with an external heating furnace 32, but its body 31 has a cross section forming an approximately hexagonal shape, and a fly ash thermometer 3 is provided approximately in the center.
It has three. As described above, if the cascade motion of fly ash in the furnace shown in FIG. 6 remains as fine powder, a sliding phenomenon along the inner wall of the furnace occurs and the stirring action between the materials is significantly hindered. In the present invention, this problem is dealt with as a flat strip, but if the inner wall of the furnace wall is smooth, even if it is a flat strip that is much less slippery than in the fine powder state, sliding motion -G still occurs. Because of concern, the rotary kiln consisted of a fuselage with a rectangular cross-section that most effectively prevents slipping. As a result, the cascade motion in the furnace is almost completely prevented from slipping as shown in Fig. (B), the agitation between the pieces is sufficiently advanced, and the loaded fly ash pieces are simultaneously heated substantially evenly. , The thermal decomposition reaction proceeds rapidly.

As shown in FIG. 1, a nitrogen generator 32 is connected to one end of the rotary kiln 3 so that a nitrogen gas stream constantly flows into the furnace during heating to create a non-oxidizing atmosphere. The pyrolyzed ash discharged from the rotary kiln 3 is sent to a cooler 33, cooled in a nitrogen atmosphere, discharged, and finally stored in an ash bunker 34 and collected. A dust collector 35 and an induced air blower 36 are provided on the supply side of the rotary kiln to collect fine powder again and return it to the thin plate molding machine 1.

[0020]

INDUSTRIAL APPLICABILITY As described above, the method and apparatus for removing dioxins in fly ash of a refuse incinerator according to the present invention have a higher thermal decomposition efficiency for dioxins than any of the conventional techniques and are harmful. Nearly complete removal of components can be achieved. An example of the effect will be described in the following embodiment. The conditions for sample preparation in the examples are as follows: fly ash obtained by collecting the exhaust gas from the fluidized bed incinerator with an electric dust collector is used as a starting material. At this time, the fly ash had an average particle size of 27 μm and a bulk specific gravity of 0.66. This fly ash was applied to a roller compactor with a rotation speed of 8 rpm and a compression pressure of 2.5 t / cm ² to form a thin plate with a plate thickness of 4.3 mm and a plate width of 60 mm, which was further subjected to a crushing machine and a sizing machine, and then finely powdered with a vibrating sieve. 72.5% was recovered as a flat strip of 0.5 to 8 mm and fine powder of 0.5 mm or less was 27.5%, which was returned to the roller compactor again and supplied together with fly ash. The oxygen concentration inside the externally heated rotary kiln was 1%, the heating temperature was 410 ° C at the temperature of fly ash, and the residence time of the heated portion in the furnace was 60 minutes. The pyrolysis conditions in the rotary kiln were substantially the same as those in this example, and the process up to the flat strip, which was a pretreatment, was omitted, and a simple ash fly ash was used as a starting material for a comparative example. The thermal decomposition results of each dioxin are listed in Table 1.

[0021]

[Table 1]

As is clear from the table, when the examples and the comparative examples are seen, the examples show that the chlorinated dioxin (PCDD) and chlorinated dibenzofuran (PCDF) have excellent removal rates. In particular, the reduction rate of TEQ (toxic equivalent concentration) in the examples is excellent, and the contribution to the environmental problems when actually operating in the field is certainly expected. In addition to the removal rate, in the comparative example, fine powder was welded and adhered to the inner wall surface of the body of the rotary kiln, and stable operation could not be performed for a long time due to the growth of the adhered substance. Without this, stable operation was possible.

[Brief description of the drawings]

FIG. 1 is a flow chart showing steps of the present invention.

FIG. 2 is a vertical cross-sectional front view of the thin plate molding machine in the examples (A).
It is a transverse front view (B) of the barrel.

FIG. 3 is a vertical front view (A) and a side sectional view (B) of the crusher.
It is.

FIG. 4 is a vertical front view (A) and a side cross-sectional view (B) of the granulator.
It is.

FIG. 5 is a front view (A) of a rotary kiln and a sectional view (B) of a body.

FIG. 6 is a sectional view of a body of a rotary kiln showing a problem in the related art.

[Explanation of symbols]

 1 Thin Plate Forming Machine 2 Crushing Machine 3 Rotary Kiln 4 Ash Bunker 5 Vibrating Sieve 11 Forming Roll 12 Hopper 13 Barrel 14 Screw 15 Gradient Groove 21 Coarse Crusher 22 Granulator 23 Rotating Shaft 24 Rotating Teeth 25 Fixed Teeth 26 Input 27 Cylindrical 28 Crushing blade 29 Screen 31 Body 32 Nitrogen generator 33 Cooler 34 Ash bunker 35 Dust collector 36 Induction blower F Fluidized bed waste incinerator C Exhaust gas cooling room H Air preheater E Electric dust collector B Induction blower

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masami Kamagata 1-12-19 Kitahorie, Nishi-ku, Osaka City, Osaka Prefecture Kurimoto Iron Works Co., Ltd. (56) Reference JP-A-4-84977 (JP, A) Kaihei 5-146772 (JP, A) JP 5-15867 (JP, A) JP 5-104073 (JP, A) JP 5-228462 (JP, A)

Claims (5)

(57) [Claims] 1. A fly ash powder collected from the exhaust gas of a refuse incinerator is pressed and squeezed between a pair of forming rolls rotating in opposite directions to form a thin plate, and the thin plate formed body is crushed. A method for removing dioxins in fly ash of a refuse incinerator, characterized by heating the flat strips, which have been finely divided into flat strips, and fine powders have been removed, in a rotary kiln in a low oxygen atmosphere. 2. The thin plate molding according to claim 1, wherein the plate thickness is in the range of 3 to 5 mm, and the flat strip has a size of 0.5 to 8 m.
A method for removing dioxins from fly ash of a refuse incinerator, which is characterized by being screened in a range of m. 3. The method for removing dioxins in fly ash of a refuse incinerator according to claim 1 or 2, wherein the inside of the rotary kiln during heating is in an atmosphere having an oxygen concentration of 5 Vol% or less. . 4. A pair of forming rolls 11 supported by rotating shafts parallel to each other and rotating in opposite directions with a desired opening.
A hopper 12 to which only fly ash powder collected from the exhaust gas of the refuse incinerator is supplied between the forming rolls, a cylindrical barrel 13 connected to the lower bottom of the hopper, and rotating on the central axis of the barrel. A fly ash powder thin plate forming machine 1 equipped with a screw 14, a crusher 2 for dividing the thin plate formed body into flat strips, and a vibrating screen for sieving fine powders contained in the flat strips. 5 and a rotary kiln 3 for loading the flat strip into a horizontally long tubular body and heating the body 31 in a low oxygen atmosphere while rotating while keeping the body 31 substantially horizontal. Equipment for removing dioxins in fly ash. 5. The thin plate molding machine 1 according to claim 4, wherein the thin plate forming machine 1 starts deepest on the inner peripheral surface of the barrel 13 at the barrel start end, and gradually inclines shallowly at an intermediate point at least one pitch of the screw left from the end of the barrel. Multiple disappearing gradient grooves 1
5. The apparatus for removing dioxins in fly ash of a refuse incinerator, comprising a rotary kiln 3 according to claim 5, which is of an external heating type, and a body 31 having a substantially hexagonal cross section.