JP2001304537A - Exhaust gas treatment method - Google Patents

Abstract

(57) [Abstract] [Problem] To stably remove dioxins in exhaust gas generated from a melting furnace or preheating furnace for ferrous scrap, a melting furnace for non-ferrous metals or ferroalloys, a melting furnace for dust and incinerated ash, and the like. I do. SOLUTION: After exhaust gas from a melting furnace 1 and a preheating furnace 2 is completely burned in a combustion tower 3 and maintained at a high temperature, the temperature of the exhaust gas is lowered in a cooling tower 4 and the atmosphere is dehumidified by a dehumidifier 5, and then the exhaust gas is discharged. The dioxin is removed at the same time as dust removal. In addition to the atmosphere, various dry gases and gas sources such as dry ice and liquid nitrogen can be used. Further, the exhaust gas that has exited the cooling tower 4 can be supplied to a building-based suction gas, mixed and diluted, and then introduced into a building-based dust collector to remove dioxins simultaneously with dust removal.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a melting furnace and a preheating furnace for ferrous scrap, a melting furnace for non-ferrous metals and alloyed iron,
The present invention relates to an exhaust gas treatment method for removing dioxins in exhaust gas generated from a furnace for melting incinerated ash and the like.

[0002]

2. Description of the Related Art In recent years, environmental regulations have become increasingly strict. Scrap melting furnaces and preheating furnaces, melting furnaces for melting and refining non-ferrous metals and alloyed irons, melting furnaces for melting dust and incinerated ash, etc. Alternatively, regarding exhaust gas generated from an incinerator or the like that incinerates general garbage, it is necessary to reduce not only floating dust but also various harmful substances to below a regulation value. In particular, dioxins are the focus of public attention, and the regulatory values themselves are becoming increasingly strict.

Conventionally, as shown in FIG. 4, the reduction of dioxins in the exhaust gas of the melting furnace 1 and the preheating furnace 2 is performed by reducing the combustion tower 3
It is important to keep the temperature at a high temperature for a certain period of time while completely burning the mixture, and then quickly cool it in the cooling tower 4. In this regard, the Ministry of Health and Welfare guidelines for municipal waste incinerators have already been presented. The exhaust gas after cooling is removed by a dust collector 7 using a bag filter, and dioxins adhere to the dust and are removed.

[0004]

[Problem to be solved by the invention]
The temperature of the exhaust gas in the dust collector 7 is controlled at about 170 ° C., and at the time of stable steady operation at this temperature, dioxins in the exhaust gas are sufficiently removed and reduced without resynthesis. However, dioxins may not be sufficiently removed or reduced at the start-up / end of operation, during unsteady times when combustion is unstable, or at small incinerators or old incinerators. There is still room for improvement.

On the other hand, in exhaust gas treatment of a melting furnace such as an electric furnace which uses a large amount of scrap, a cooling tower 4 is used.
When the subsequent temperature was around 170 ° C. as in the case of incineration of municipal garbage, it was found that the resynthesis of dioxins proceeded. As a result of detailed studies by the present inventors, depending on the type of scrap used, if not cooled down to about 80 ° C. or less, dioxins are decomposed and removed in the combustion tower 3 once. It was found that resynthesis occurred between the cooling tower 4 and the dust collector 7 and could not be removed together with the dust. The cause is said to be largely due to the oil content attached to the scrap, but the clear mechanism is expected to be clarified by detailed research in the future.

[0006] In the cupola type melting furnace, the concentration of dioxins is lower than that of an electric furnace equipped with a preheating furnace because of the reducing exhaust gas containing CO. Not. Therefore, similarly to other melting furnaces, it is necessary to rapidly cool the exhaust gas after maintaining the exhaust gas temperature at a high temperature in the combustion tower. Regulation values cannot be satisfied.

By the way, the exhaust gas heated to a high temperature in the combustion tower 3 is usually directly spray-cooled with water to be rapidly cooled in the cooling tower 4. For this reason, the steam pressure in the exhaust gas sharply increases, and the dew point temperature increases. 1 in urban garbage incineration
There is no industrial problem up to about 70 ° C, but if it is lower than this, the temperature will be lower than the acid dew point, and there will be a problem of corrosion of ducts and dust collectors. And the problems such as clogging of the filter of the dust collector occur.

[0008] Instead of extremely lowering the temperature after the cooling tower 4, if a catalyst tower 19 is installed as shown in FIG. Can be processed. However, due to the performance of the conventional catalyst, the exhaust gas
It is necessary to raise the temperature by reheating. It is not preferable to cool the exhaust gas once and then reheat it from the viewpoint of energy saving, and the equipment scale becomes large and the investment amount expands. Therefore, the installation of the catalyst tower 19 cannot be said to be the best measure.

Accordingly, the present invention is applicable to a melting furnace and a preheating furnace for ferrous scrap, a melting furnace for non-ferrous metals and ferroalloys, a melting furnace for dust and incinerated ash, and the like without causing problems such as corrosion of equipment and clogging of filters. An object of the present invention is to reduce dioxins in exhaust gas to a level that satisfies regulation values while removing dioxins together with dust by a filtration type dust collector.

[0010]

The present invention provides the following (1):
It is as (2).

(1) After exhaust gas is completely burned in a combustion tower and kept at a high temperature, the temperature of the exhaust gas is lowered by direct spray cooling or a combination of indirect cooling and direct spray cooling, and a dry gas or a gas source is mixed and added. An exhaust gas treatment method characterized in that dioxins are removed at the same time as dust removal.

(2) After the exhaust gas is completely burned in a combustion tower and kept at a high temperature, the temperature of the exhaust gas is lowered by direct spray cooling or a combination of indirect cooling and direct spray cooling, and the mixture is supplied to a building system suction gas to be mixed with a building system. An exhaust gas treatment method comprising introducing dioxins into a dust collector and removing dust at the same time.

[0013]

FIG. 1 shows an example of an apparatus for carrying out the exhaust gas treatment method of the present invention.

The exhaust gas generated in the melting furnace 1 passes through a scrap preheating furnace 2 and is completely burned and maintained at a high temperature in a combustion tower 3.
It is cooled directly by the spray cooling tower 4, dust is removed by the dust collector 7, and is discharged from the stack 9 to the atmosphere while being sucked by the suction fan 8. Here, when the exhaust gas heated at a high temperature in the combustion tower 3 is directly cooled in the spray cooling tower 4, the dew point temperature increases,
If the exhaust gas temperature drops to about 80 ° C. in the dust collector 7 in this process, problems such as corrosion of the duct / device and clogging of the filter of the dust collector will occur. Then, the air introduction valve 14 is opened to suck air into the exhaust gas duct through the dehumidifier 5, and the sucked air and the exhaust gas are sufficiently uniformly mixed in the mixing tower 6 and sent to the dust collector 7. As a result, the exhaust gas is diluted by the dried atmosphere, and the dew point temperature is greatly reduced, so that problems such as corrosion of the duct / device and clogging of the filter of the dust collector are greatly reduced. In addition, the temperature of the exhaust gas itself can be lowered, so that dioxins can be prevented from being re-synthesized by the dust collector 7 in the exhaust gas of the melting furnace, and the amount of spray cooling in the cooling tower 4 can be reduced to reduce the dew point. It is also possible to suppress a rise in temperature.

By installing a dew point meter or a hygrometer after the cooling tower 4 and measuring the moisture content in the exhaust gas, the flow rate of the introduced atmosphere can be controlled by the flow control valve 15. This makes it possible to optimize the introduction air flow rate, reduce the dew point temperature, and suppress the equipment scale of the duct / device after the introduction into the atmosphere.

The introduced atmospheric moisture varies greatly with the seasons in Japan. In general, the dehumidifying device 5 may not be necessary because the humidity is low in a dry season in winter. However, when the humidity and the temperature are high as in summer, the dehumidifying device is indispensable. The mixing tower 6 of the exhaust gas and the introduced atmosphere has a sufficient distance from the confluence of the two and the dust collector 7, and there is no problem if it is omitted if there is some means for mixing efficiently.

FIG. 1 shows an example in which the atmosphere is introduced, but if a large amount of dry gas is easily available, it may be used. In addition to gas, gas sources such as solids such as dry ice and liquids such as liquid nitrogen can be expected to cool the exhaust gas with a large heat absorbing effect during vaporization, and after vaporization, as a dry gas to reduce the dew point. It can be used because it can contribute. It can be selected from a dry gas or a gas source in consideration of location conditions and economy.

FIG. 2 shows an example in which an indirect cooling machine 10 is installed between the combustion tower 3 and the direct spray cooling tower 4. When all the high-temperature exhaust gas discharged from the combustion tower 3 is cooled to a predetermined temperature by direct spray cooling, the dew point temperature becomes extremely high. Therefore, after cooling with the indirect cooling machine 10 as much as possible, direct spray cooling is performed to lower the temperature to a predetermined temperature. As a result, an increase in the dew point temperature is suppressed, the flow rate of the introduced air can be reduced, and the scale of the device after merging can be reduced. In the cooling by the indirect cooler 10, as the temperature becomes lower, the efficiency of heat exchange becomes worse and the equipment scale becomes larger. When the cooling rate becomes too slow, dioxins are resynthesized. Therefore, the cooling temperature in the indirect cooling machine 10 is determined while sufficiently grasping the decomposition and resynthesis behavior of dioxins in the exhaust gas in consideration of the scrap properties and the like used in the melting furnace 1 and the preheating furnace 2. .

In FIG. 2, the indirect cooler 10 is installed between the combustion tower 3 and the direct spray cooling tower 4. In this case, heat is exchanged from a high temperature, so that by-products such as steam are obtained. On the other hand, when it is installed directly downstream of the spray cooling tower 4, as described above, the cooling efficiency is significantly deteriorated with a normal cooling medium such as water. However, liquid nitrogen, liquid argon,
If an evaporator such as liquid carbonic acid is used, high-temperature exhaust gas can be used by connecting it to this exhaust gas duct as an alternative to steam that is generally used as a heating medium. In factories that usually have a scrap melting furnace,
Use gases such as argon, carbon dioxide and oxygen. These are generally purchased as liquids, except when they are made by themselves, and vaporized near the factory. Therefore, it is sufficiently possible to utilize the indirect cooling machine 10 as these vaporizing media as long as the conditions are met.

As an indirect cooling method, besides providing a cooling tower, there is a method of cooling exhaust gas by cooling a duct with water. Generally, the cooling efficiency is lower than that of a dedicated cooling tower, but it is effective in reducing capital investment and suppressing the rise in dew point temperature. However, as described above, it is necessary to pay sufficient attention to the resynthesis of dioxins due to a decrease in the cooling rate of the temperature.

FIG. 3 shows a method of mixing the exhaust gas from the melting furnace 1 with the building suction gas and treating it with the building dust collector 12.
In general, a factory having a melting furnace 1 has a direct dust collection system for sucking exhaust gas generated from the melting furnace 1 and an indirect system for collecting exhaust gas leaking from the direct dust collection system and other dust generating points of the factory into a building. There is a dust collection system. Since the indirect dust collection system sucks a considerable amount of air, the suction air volume is much larger than that of the direct dust collection system, and the exhaust gas temperature near the dust collector is close to normal temperature. Therefore, as shown in FIG. 3, when the exhaust gas discharged from the cooling tower 4 is supplied to and mixed with the suction gas of the building system by the booster fan 17, the exhaust gas of the direct dust collection system is diluted by the suction gas of the building system and the dew point is reduced. The temperature drops significantly and the temperature drops. Therefore, resynthesis of dioxins can be prevented, and by collecting dust in the building-based dust collector 12, it becomes possible to reduce the emission control value of dioxins to a level that satisfies the requirements.

However, since the indirect dust collection system has a large air volume and a large duct diameter, it is necessary to take measures to uniformly mix the two. It is sufficient if the dust collector can be connected to the duct sufficiently upstream of the building-based dust collector 12. However, due to the limitation of the installation space, the dust collector is actually connected to a duct near the dust collector 12. In this case, the mixing tower 11 is effective for mixing the two gases in a short time. Mixing tower 1
As an alternative to 1, it is also possible to install a baffle plate or the like in the duct of the indirect dust collection system after merging to achieve sufficient mixing. It is sufficient that the exhaust gas introduced into the dust collector 12 is sufficiently mixed so that both the dew point temperature and the gas temperature are at levels at which corrosion of the apparatus and clogging of the filter can be prevented.

FIG. 3 shows only the direct spray cooling tower 4, but when the dew point temperature and the gas temperature after mixing reach predetermined values,
A combination with the indirect cooling shown in FIG. 2 is also possible.

[0024]

According to the present invention, the dew point temperature increased by direct spray cooling after the combustion tower can be efficiently reduced. This avoids equipment troubles such as corrosion of ducts and equipment and clogging of dust collector filters.
Dioxins in the exhaust gas can be reduced to a level at which the regulation target value can be sufficiently cleared. Furthermore, the equipment investment required for this can be made extremely small because the equipment specifications can be minimized.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of an apparatus for performing an exhaust gas treatment method of the present invention.

FIG. 2 is a diagram showing another example of an apparatus for performing the exhaust gas treatment method of the present invention.

FIG. 3 is a diagram showing another example of an apparatus for performing the exhaust gas treatment method of the present invention.

FIG. 4 is a diagram showing an exhaust gas treatment apparatus provided with a conventional catalyst tower.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Melting furnace 2 Preheating furnace 3 Combustion tower 4 Direct spray cooling tower 5 Dehumidifier 6 Mixing tower 7 Dust collector 8 Suction fan 9 Stack 10 Indirect cooler 11 Mixing tower 12 Building dust collector 13 Building suction fan 14 Atmospheric introduction valve 15 Flow control Valve 16 Building system suction duct 17 Booster fan 18 Heating device 17 Catalyst tower

Claims (2)

[Claims] 1. After exhaust gas is completely burned in a combustion tower and kept at a high temperature, the temperature of the exhaust gas is lowered by direct spray cooling or a combination of indirect cooling and direct spray cooling, and a dry gas or a gas source is mixed and added to the dust collector. An exhaust gas treatment method characterized in that dioxins are removed simultaneously with introduction of dust. 2. After exhaust gas is completely burned in a combustion tower and kept at a high temperature, the temperature of the exhaust gas is lowered by direct spray cooling or a combination of indirect cooling and direct spray cooling, and supplied to a building system suction gas to be mixed with the building system dust collector. An exhaust gas treatment method characterized in that dioxins are removed simultaneously with dust removal.