JP5149488B2 - Pyrolysis gas treatment method - Google Patents

Description

  The present invention relates to a pyrolysis gas treatment method for treating exhaust gas obtained by pyrolyzing organic waste such as sewage sludge.

For example, exhaust gas obtained by pyrolyzing organic waste such as sewage sludge contains harmful substances such as sulfur oxides, sulfides, cyanide, and organic substances. These harmful substances need to be reduced below the standard value. . As a method for treating such pyrolysis gas, as shown in Patent Document 1, a wet cleaning method has been used for a long time.

However, the conventional method only absorbs and separates the reducing substances or harmful substances in the exhaust gas with water or an alkaline agent, so there is a problem in the disposal of cleaning wastewater containing reducing substances or harmful substances generated by this treatment. It was. In addition, as in sewage treatment plants, washing wastewater is returned to the landing well as return water, but there is a problem that harmful substances in the washing wastewater adversely affect the sewage treatment. Furthermore, since the treatment is performed using an oxidizing agent such as ozone or sodium hypochlorite, there is a problem that the chemical cost becomes enormous and the treatment cost becomes high.
JP 2001-45819 A

  The present invention solves the above-mentioned problems and removes harmful substances such as sulfur oxides, sulfides, and cyanide from the exhaust gas obtained by thermally decomposing organic waste at low cost and efficiently. There is no problem even if the treated water is discharged as discharged water, and there is no problem even if it is returned to the receiving well as the returned water in the sewage treatment, and there is no adverse effect on the activated sludge treatment of the sewage It was completed for the purpose of providing the processing method.

In order to solve the above problems, the pyrolysis gas treatment method of the present invention supplies a slurry of an alkaline agent to a pyrolysis gas discharged from a pyrolysis furnace of organic waste at a high temperature of 400 to 600 ° C. After removing the obtained product with a ceramic filter, the pyrolysis gas discharged from the ceramic filter is introduced into a washing tower and washed with an alkaline washing water containing a cobalt compound, and then this washing water is circulated. It is circulated to the washing tower while aerated in the tank, and sulfur oxides, sulfides, and cyan absorbed in the washing water are oxidized and removed in the form of solids, and then the discharged water discharged from the circulation tank is removed. It is characterized by being guided to an oxidation tower and subjected to catalytic ozone treatment.

  In the present invention, sulfur oxide, sulfide, cyan, etc. are supplied by supplying an inexpensive alkali agent slurry to the exhaust gas, cleaning with an alkaline cleaning water containing a cobalt compound, and aeration of the cleaning water in a circulation tank. It is possible to reliably remove harmful substances and to keep the chemical cost of the oxidizing agent low. In addition, by incorporating the catalytic ozone treatment into the system, cyan and COD (organic substances) can be reliably removed, and safe treated water can be obtained.

Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a flowchart showing an outline of a process for treating pyrolysis gas discharged from a pyrolysis furnace. In the figure, 1 is a ceramic filter , 2 is a washing tower, 3 is a circulation tank, 4 is an oxidation tower, Is a bubbling device, and 6 is a pump.

First, a slurry of a hardly soluble alkali agent such as calcium hydroxide or magnesium hydroxide is added to the exhaust gas discharged from the pyrolysis furnace at a high temperature to promote the reaction of [Chemical Formula 1] below. Next, the product thus obtained is recovered by the ceramic filter 1 and the water in the slurry is converted into water vapor.

In this way, the pyrolysis gas contains harmful substances such as sulfur oxides, sulfides, and cyan, but since it is converted into solids by the reaction of [Chemical Formula 1], the ceramic filter 1 is more reliable. It will be filtered.
In this case, the filtration rate is preferably in the range of 0.5 to 1 m / min. If it is less than 0.5 m / min, the filtration efficiency is inferior. On the other hand, if the filtration is performed at a rate exceeding 1 m / min, the filtration pressure rises rapidly and the power of the exhaust gas fan increases (see FIG. 2).
Moreover, the gas temperature in this stage is 400-600 degreeC normally, and a water | moisture content will transfer to the waste gas side as water vapor | steam.

Next, the solid matter filtered by the ceramic filter 1 is backwashed with dust and discharged out of the system. Back washing is usually performed by pulse washing, but in the present invention, it is preferably carried out at a washing rate of about 5 to 20 times the normal filtration rate for 1 to 5 seconds (see FIG. 3). Further, the backwash interval is 20 to 30 minutes, and when pressure control is performed, backwashing is preferably performed so that the pressure loss of the ceramic filter 1 is maintained within 10 mmH ₂ O.

Next, the exhaust gas discharged from the ceramic filter 1 is introduced into the cleaning tower 2. The washing tower 2 is washed with an alkaline washing water containing a cobalt compound (about 1 mg / L as cobalt) and absorbs a small amount of sulfur oxides, sulfides, cyanide, etc. that could not be removed by the ceramic filter 1. The water that has become water vapor is also removed as water by humidification cooling. The exhaust gas is released into the atmosphere as a processing gas.

  On the other hand, when the alkali agent of the cleaning agent is sodium hydroxide, the substance moves to the cleaning water by the following reaction [Chemical 2].

As washing water, it is preferable to control pH to about 8.5-9. If the pH is less than 8.5, the reaction becomes insufficient, and if the pH exceeds 9, the chemical agent is expensive because the alkaline agent is consumed by absorption of carbon dioxide (see FIG. 4). Moreover, as a washing | cleaning condition of the washing tower 2, it is preferable that liquid / gas ratio (L / G) shall be about 2-4L / m < ³ > (refer FIG. 5).

In the circulation tank 3, the harmful substances absorbed in the washing water are oxidized and removed in the form of solids. That is, the above-described acidic sodium sulfite (NaHSO ₃ ) and sodium sulfide (Na ₂ S) are oxidized by aeration using cobalt as a catalyst to become solid sodium sulfate (Na ₂ SO ₄ ), which is discharged from a discharge port (not shown) to the outside. Taken out. The reaction formula is shown in the following [Chemical Formula 3].

In the case of using an oxidizing agent such as sodium hypochlorite as in the conventional case, this oxidation treatment is enormous in chemical costs. However, in the present invention, since oxidation is performed by aeration, only the electricity cost of the blower is used. Therefore, processing costs can be reduced. Moreover, since the washing water of the washing tower 2 is circulated and used with the circulation tank 3, the amount of drainage can be suppressed as much as possible.
Here, the conditions for aeration of the circulation tank 3 are normally air permeability: 0.5 to ² m ³ / m ³ . About 30 minutes is sufficient for the min and the aeration time. Moreover, as water temperature conditions of circulating water, 40 degrees C or less, Preferably 30 degrees C or less is good. This is because if the temperature of the circulating water is high, the oxygen melting efficiency during aeration is reduced and the amount of air required for aeration is increased. In some cases, a cooling device may be provided. Furthermore, in order to improve the aeration efficiency of the circulation tank 3, it is effective to increase the water depth of the air diffuser as much as possible and arrange it on the entire surface of the tank bottom.
However, if the wash water consumes a large amount of oxygen such as a large amount of acidic sodium sulfite, install a DO meter in the circulation tank and perform DO control based on the ventilation rate to reduce the risk of being discharged untreated. Can be eliminated. Moreover, if circulating water is circulated to the washing tower 2 in a state where DO is detected by DO control, for example, in a state where DO of about 1-2 mg / L can be detected, acidic sodium sulfite (NaHSO ₃ ), sodium sulfide (Na ₂ S) Is secured, and the propulsive force for absorption is improved from the relationship with the saturated concentration of these components, so that the efficiency of gas absorption is increased, which is preferable.

The washing water discharged from the circulation tank 3 contains cyan and a small amount of organic matter (COD) that could not be treated by aeration in the circulation tank 3. Therefore, they are guided to the oxidation tower, removed by catalytic ozone treatment, and then discharged as treated water.
The oxidation tower 3 is filled with electrolytic manganese dioxide as a catalyst, and decomposes cyan and a trace amount of organic matter by the following reaction [Chemical Formula 4].

The treatment conditions in the oxidation tower 4 are preferably SV: 1 to 3 / Hr and LV: 2 to 10 m / Hr. This is because when the operation is performed beyond this range, the removal rate of cyan and organic substances decreases (see FIG. 7).
The ozone equivalent for cyanide decomposition is 240/26 = 9.23, but organic matter decomposition also occurs at the same time, so the ozone addition rate needs to be 1.2 to 1.5 times the ozone equivalent. (See FIG. 8).

Although the treatment of the pyrolysis gas is completed as described above, as is apparent from the above description, the present invention has the following advantages.
(1) By using an inexpensive alkaline agent (such as slaked lime), components that consume a large amount of oxidizing agent in chemical treatment are removed, and the chemical cost can be kept low.
(2) Similarly, the chemical cost can be kept low by removing a component that consumes a large amount of the oxidizing agent by aeration.
(3) By incorporating into the system a catalytic ozone treatment that can reliably remove cyanide and COD, safe treated water can be obtained, and even if it is returned to the landing well as return water in the sewage treatment, it will adversely affect the activated sludge treatment of the sewage. There is no effect.

  As a result of treating the exhaust gas discharged from the pyrolysis furnace under the conditions shown in Table 1, it was confirmed that the content of sulfur oxides, sulfides, cyanide and the like in the treated water was sufficiently reduced. Moreover, COD was also removed sufficiently, and it was confirmed that even if it was returned to the landing well, it did not adversely affect the activated sludge treatment of sewage. In addition, the case where it processed by combining alkali cleaning and ozone oxidation was made into the conventional method, and the obtained result is shown in Table 1.

It is a flowchart which shows embodiment of this invention. It is a graph which shows the relationship between a filtration rate and an initial pressure loss. It is a graph which shows the relationship between the backwashing speed and the recoverability of pressure loss. It is a graph which shows the relationship between washing water pH and SOx removal rate. It is a graph which shows the relationship between a liquid-gas ratio and a SOx removal rate. It is a graph which shows the change of acidic sodium sulfite by aeration rate. It is a graph which shows the relationship between SV, LV, and CN removal rate. It is a graph which shows the relationship between ozone addition magnification and CN removal rate.

Explanation of symbols

1 Ceramic filter 2 Washing tower 3 Circulating tank 4 Oxidation tower

Claims (2)

The slurry of the alkali agent is supplied to the pyrolysis gas discharged from the pyrolysis furnace of organic waste at a high temperature of 400 to 600 ° C. , and the product obtained is removed by the ceramic filter and then discharged from the ceramic filter. The introduced pyrolysis gas is introduced into a washing tower and washed with an alkaline washing water containing a cobalt compound. Then, the washing water is circulated to the washing tower while being aerated in a circulation tank, and the sulfur absorbed in the washing water. A method for treating pyrolysis gas, characterized in that oxides, sulfides and cyanide are oxidized and removed in the form of solids, and then the discharged water discharged from the circulation tank is led to an oxidation tower for catalytic ozone treatment. .   The method for treating a pyrolysis gas according to claim 1, wherein the alkali agent is calcium hydroxide or magnesium hydroxide.

Images