JP7201491B2 - Dehydrated sludge incineration method - Google Patents

Description

The present invention relates to a method for incinerating dewatered sludge obtained by dewatering sewage sludge generated from a sewage treatment plant or the like.

Waste incinerators can be broadly classified into fixed bed incinerators and fluidized bed incinerators. As shown in Patent Document 1, the fixed bed incinerator is a furnace having a structure in which the incinerated material is placed on a fire grate and incinerated, and the fluidized bed incinerator is shown in Patent Document 2. It is a furnace with a structure that incinerates while flowing together.

Fixed-bed incinerators are suitable for incinerating waste that has a large amount of non-combustible materials and low adhesiveness, such as municipal waste. However, dehydrated sewage sludge is highly adhesive and has a high ventilation pressure loss, making it difficult for combustion air to enter inside the dewatered sludge. This tendency is particularly noticeable in dehydrated sludge flocculated with a polymer flocculant. For this reason, even if the dewatered sludge is put into a fixed-bed incinerator, it cannot be uniformly combusted, and incomplete combustion and local combustion tend to occur. And it is not easy to incinerate in a fixed bed incinerator because clinker may be generated in the local high temperature field.

Fluidized bed furnaces are widely used to incinerate dewatered sludge, but fluidized bed furnaces require a large amount of air to form a fluidized bed inside the furnace, requiring a large amount of power costs. As a result, there is a problem that the running cost increases. In addition, in the fluidized bed furnace, incineration ash is transferred to the exhaust gas treatment system together with the exhaust gas, so there is a problem that a large-scale exhaust gas treatment facility using a cyclone or bag filter is required.

Japanese Patent Application Laid-Open No. 2002-181311 JP-A-8-261427

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to solve the above-mentioned conventional problems and to provide a novel dewatered sludge incineration method capable of uniformly burning highly sticky dehydrated sludge without causing it to flow.

The method of incinerating dewatered sludge of the present invention, which has been made to solve the above problems, is to add granular aggregate to dewatered sludge , which has a particle size of 1 to 20 mm and a melting point higher than the sludge incineration temperature . The mixed sludge is mixed to improve dispersibility, and the mixed sludge is put into a vertical furnace for layered combustion.

As the granular aggregate, it is preferable to use a mixture of granulated dewatered sludge and granular aggregate. In addition, (weight of granular aggregate)/(dry solid weight of dehydrated sludge) in the mixed sludge is preferably 1-4.

In a preferred embodiment, air is supplied from the lower part of the furnace body of the vertical furnace, a cooling section is formed below the stack combustion section, and combustion air enters the stack combustion section from below. In addition, the incineration ash and granular aggregate falling from the lower surface of the laminated combustion section are cooled in the cooling section, taken out of the furnace, classified, and the granular aggregate is reused. As the granular aggregate, granules of sand or incinerated ash can be used.

In the dewatered sludge incineration method of the present invention, the dewatered sludge is mixed with granular aggregates having a particle size of 1 to 20 mm and having a melting point higher than the sludge incineration temperature, thereby improving dispersibility. Since the sludge is put into the furnace and burned in layers, the combustion air can easily enter the layered combustion layer formed inside the furnace body. Therefore, even highly sticky dehydrated sludge can be uniformly incinerated while being stacked without being fluidized, and the generation of clinker can be prevented. In addition, the running cost can be reduced because power for fluidity is no longer required.

Further, in the dewatered sludge incineration method of the present invention, the incineration ash falls below the laminated combustion layer and is taken out from the lower part of the furnace body. Therefore, incineration ash does not move to the exhaust gas treatment system, and large-scale exhaust gas treatment equipment becomes unnecessary. Other configurations and effects will be described together with the embodiments.

1 is a schematic cross-sectional view of a vertical furnace used in an embodiment; FIG. It is a graph which shows the result of having measured the ventilation differential pressure of the mixed sludge of a dewatered sludge and a granular aggregate. It is a graph which shows a temperature change when lamination|stacking combustion of only dehydrated sludge is carried out with an experimental furnace. It is a graph which shows a temperature change when layered combustion of mixed sludge is carried out by an experimental furnace.

Preferred embodiments of the present invention will be described below. First, the structure of a vertical furnace used in the embodiments will be described.

(Furnace structure)
FIG. 1 is a schematic cross-sectional view thereof, and 10 is a furnace body of a vertical furnace. The dehydrated sludge is mixed with granular aggregates and fed by a feeding device 11 such as a spreader at the upper part of the furnace body 10 , and is dropped and accumulated in the lower part to form a layered combustion section 12 . Air is supplied from an air supply pipe 13 at the bottom of the furnace body 10 , and a cooling section 14 is formed below the stack combustion section 12 . The air supplied to the cooling section 14 enters the layered combustion section 12 and becomes combustion air to burn the mixed sludge. A gas combustion section 15 is formed above the stack combustion section 12 . An appropriate number of air supply pipes 16 can also be provided in this gas combustion section 15 . A transfer means 17 such as a screw feeder is provided below the cooling section 14, and a classifying device 18 such as a vibrating sieve or a wind classifier based on the difference in specific gravity is provided at the outlet side. The classifier 18 is a device for classifying the incineration ash and granular aggregate that have fallen below the stacked combustion section 12 .

(Mixed sludge)
As described above, since the dehydrated sludge of sewage is sticky, it was not easy to burn it uniformly while it is stacked as shown in FIG. Therefore, in the present invention, dehydrated sludge is mixed with granular aggregates to obtain mixed sludge with improved dispersibility, and the mixed sludge is charged into a vertical furnace for layered combustion.

Granular aggregates play a role in improving the dispersibility of dewatered sludge by entering between dewatered sludge. For this reason, granular materials having a melting point higher than the sludge incineration temperature are used. The sludge incineration temperature is about 750 to 950°C, the same as in conventional sludge incinerators. If the melting point is lower than the sludge incineration temperature, it melts and cannot function as a granular aggregate. As the granular aggregate, sand or granules of incineration ash can be used. Granules of incineration ash have a porous surface and can absorb water in dewatered sludge, so they are excellent in the effect of improving the dispersibility of dewatered sludge.

In order to improve the dispersibility of the dewatered sludge, it is desirable to form a large number of fine voids between the granular aggregate and the dewatered sludge when they are mixed. For this purpose, it is most preferable to granulate the dehydrated sludge as well and mix the granules with each other.

It is preferable that the particle size of the granular aggregate is 1 to 20 mm. If the particle size is finer than this range, the gap formed becomes smaller, making it difficult for the combustion air to enter the layered combustion section 12 . Conversely, granules with a diameter larger than 20 mm are not preferable because they are difficult to granulate, and the strength is lowered, and breakage and chipping are likely to occur. When fracture or the like occurs, the granular aggregate becomes fine particles, resulting in filling the gaps.

The particle size of the sludge particles is also preferably 1 to 20 mm for the same reason. The value of (particle size of granular aggregate/particle size of dewatered sludge) should be about 0.5 to 2, and extreme differences are not preferred. This is because if the particle size of the granular aggregate and the particle size of the dewatered sludge are significantly different, the small-diameter particles will fill the gaps formed by the large-diameter particles, reducing the effect of improving the dispersibility.

The ratio (weight of granular aggregate)/(dry solid weight of dewatered sludge) in the mixed sludge is preferably in the range of 1-4. The dry solid weight of dewatered sludge is a value called DS (Dry Solid). When this value is less than 1, that is, when the weight of the granular aggregate is less than the dry solid weight of the dewatered sludge, the effect of improving dispersibility becomes insufficient. Conversely, if the weight of the granular aggregate exceeds four times the weight of the dry solids of the dewatered sludge, the dewatered sludge is relatively reduced, resulting in a reduction in treatment capacity. As a means for mixing the dewatered sludge and the granular particles, a well-known agitating mixing device such as a paddle mixer can be used.

(Incineration method)
The mixed sludge described above is charged from the charging device 11 into the furnace body 10 of the vertical furnace shown in FIG. The amount of air supplied from the air supply pipe 13 in the lower part of the furnace body 10 is preferably 0.2 to 1.0 in terms of the ratio of air to mixed sludge charged.

As described above, the mixed sludge is in a state of improved dispersibility, so the air supplied to the cooling section 14 dispersively enters the layered combustion section 12 and becomes combustion air to burn the mixed sludge. Combustion therefore takes place mainly in the lower surface portion of the layered combustion section 12, which is in direct contact with the air. In addition, fine voids are maintained inside the mixed sludge, which increases the specific surface area of the dehydrated sludge. This homogenizes the gas flow. The gas includes air remaining after combustion, pyrolysis gas of sludge, and flue gas generated by combustion. This combustion proceeds evenly over the entire lower surface of the stacked combustion section 12, suppressing the formation of a local high-temperature field and suppressing the generation of clinker. The incineration ash generated by the combustion of the mixed sludge and the granular aggregate contained in the combustion part fall from the bottom surface of the layered combustion part 12, but the mixed sludge of the same amount as the burned part is charged from the charging device 11. Thus, the stack combustion section 12 is always maintained at a fixed position. The sludge incineration temperature is 750-950°C.

The incineration ash and granular aggregate that have fallen from the lower surface of the stacked combustion section 12 are cooled by the air in the cooling section 14 and sent to the classification device 18 by a conveying means 17 such as a screw feeder. As the classifier 18, for example, a known vibrating sieve or an air classifier using a difference in specific gravity can be used. The classified incineration ash is discharged out of the system. On the other hand, the granular aggregate is returned to the mixing device 19 and reused. The air supplied to the cooling section 14 is heated by heat exchange with the incineration ash and enters the stack combustion section 12, so that the combustion efficiency can be improved.

The combustion gas generated by the combustion and the unburned gas generated inside the heated stacked combustion section 12 are transferred to the gas combustion section 15 above the stacked combustion section 12, and the unburned gas is supplied from the air supply pipe 16. It is completely combusted by the air. When the ratio of air supplied from the lower air supply pipe 13 is 0.5, if the total ratio of air supplied from the upper air supply pipes 16 is set to 0.5 or more, complete combustion can be achieved in the gas combustion section 15. can be made Combustion gas is discharged from the upper part of the furnace body 10 and treated by an exhaust gas treatment system. In the present invention, since layered combustion is performed and incinerated ash is taken out from below the furnace body 10, almost no incinerated ash is contained in the combustion gas. As a result, the load on the exhaust gas treatment system is reduced compared to the conventional system, and the need for a cooling tower, bag filter, etc. is eliminated, and the equipment can be simplified.

As described above, the granular aggregate is cyclically used, but as it is repeatedly used, it gradually becomes finer, so it is necessary to add new granular aggregate. Then, a part of the classified incineration ash can be granulated and sintered to make a new granular aggregate. As a result, the incineration ash can be effectively used, and the running cost can be reduced.

(Measurement of breathability)
Using dehydrated sludge granulated to have a particle size distribution of 1 to 20 mm, the air permeability of mixed sludge obtained by mixing with granular aggregate was measured. The dehydrated sludge used was sewage dewatered sludge flocculated using a polymer flocculant at a sewage treatment plant in Aichi Prefecture. As granular aggregates, three types of silica sand No. 1, silica sand No. 2 and silica sand No. 5 were used. Each particle size is shown in Table 1.

Using the granular aggregate described above, the following four types of mixed sludge samples were prepared.
(Sample 1) Dehydrated sludge only. Its moisture content is 75 wt%.
(Sample 2) A mixture of No. 5 silica sand equal to the dry solid weight of dehydrated sludge.
(Sample 3) A mixture of No. 2 silica sand twice the dry solid weight of the dehydrated sludge.
(Sample 4) A mixture of No. 1 silica sand twice the dry solid weight of the dewatered sludge.

Each sample was packed in a cylinder with a diameter of 10 cm and a height of 1 m, and compressed air was supplied from the lower end to measure the ventilation differential pressure. The results are shown in FIG. The horizontal axis is the weight of the filled sample and corresponds to the fill height.

As shown in the graph of FIG. 2, in sample 2, the particle size of No. 5 silica sand is too small compared to the particle size of the dewatered sludge, so even if it is added, there is not much difference from sample 1 with only dewatered sludge, and the air permeability improvement effect was weak. Sample 3 was mixed with No. 2 silica sand having a particle size slightly larger than that of the dewatered sludge, and the air permeability was greatly improved. Sample 4 is a mixture of No. 1 silica sand having a particle size approximately twice as large as that of the dehydrated sludge, but its air permeability is slightly worse than that of sample 3. The reason for this is presumed to be that the voids formed by the No. 1 silica sand were filled with particles of the dehydrated sludge.

(Combustion experiment using an experimental furnace)
Next, using a small cylindrical experimental furnace, an experiment in which the dewatered sludge alone was subjected to layered combustion and an experiment in which the mixed sludge of Sample 3 was subjected to layered combustion were conducted. The experimental furnace has the structure shown in FIG. 1, but does not have the transfer means 17, and has a structure in which incinerated ash and granular aggregates are taken out batchwise from the lower part of the furnace every 10 hours. The combustion temperature was set at 850°C.

Two temperature sensors were placed in the lower part of the laminated combustion layer and two in the middle part to measure temperature changes during combustion. The results are shown in FIGS. 3 and 4. FIG. The temperature sensor is stationary and does not move with the stacked combustion layers.

In the experiment in which the dehydrated sludge shown in FIG. 3 was stacked and burned alone, the lowest part described as the lower part 1 was first burned at 850 ° C. After 10 hours, the incinerated ash and granular aggregate were batch-type from the lower part of the furnace body. Upon removal, the section labeled Lower 2 subsequently became the lowest stage, and combustion of that section began. Although the same applies to the following, it can be seen that the dewatered sludge alone has poor dispersibility and combustion of the dewatered sludge is not uniformly performed, so that a local high temperature field is formed and the combustion temperature reaches 1000° C. for some time. In addition, it can be seen that the combustion temperature in the lower part of the laminated combustion layer is less likely to be transmitted to the upper layer due to the poor air permeability of the dewatered sludge.

On the other hand, in the experiment of layered combustion of the mixed sludge of sample 3, the temperature of the upper layer gradually increased when the combustion of the lower part 1 burned, and the combustion temperature was 850 ° C as set without the formation of a local high temperature field. It can be seen that the This experiment confirmed the superiority of the dehydrated sludge incineration method of the present invention.

As described above, according to the present invention, dehydrated sewage sludge with high adhesiveness can be uniformly combusted without fluidization, the power cost of fluidizing air can be eliminated, and the exhaust gas treatment system can be simplified. Therefore, the cost of sludge incineration can be greatly reduced.

10 Furnace body 11 Feeding device 12 Stacked combustion unit 13 Air supply pipe 14 Cooling unit 15 Gas combustion unit 16 Air supply pipe 17 Transfer means 18 Classifying device 19 Mixing device

Claims (6)

Dewatered sludge is mixed with granular aggregate consisting of granular substances with a particle size of 1 to 20 mm and a melting point higher than the sludge incineration temperature to obtain mixed sludge with improved dispersibility, and then put into a vertical furnace. A method of incinerating dewatered sludge, characterized by burning the dehydrated sludge in layers. 2. The method of incinerating dewatered sludge according to claim 1 , wherein the dewatered sludge is granulated and mixed with granular aggregate. The dewatered sludge incineration method according to any one of claims 1 and 2 , wherein (weight of granular aggregate)/(dry solid weight of dewatered sludge) in the mixed sludge is 1-4. Air is supplied from the lower part of the furnace body of the vertical furnace, a cooling section is formed below the stacked combustion section , and combustion air is introduced into the stacked combustion section from below. A method for incinerating dewatered sludge according to any one of the above. The incineration ash and granular aggregate falling from the lower surface of the stacked combustion section are cooled in the cooling section, taken out of the furnace, classified, and the granular aggregate is reused. The method for incinerating dehydrated sludge according to 1. The method of incinerating dehydrated sludge according to any one of claims 1 to 5 , wherein the granular aggregate is granulated sand or incinerated ash.

Images