JP5396955B2 - Exhaust gas treatment apparatus and exhaust gas treatment method - Google Patents

Description

  The present invention relates to an exhaust gas treatment device and an exhaust gas treatment method for removing dioxins in exhaust gas discharged from a heat treatment device. In particular, the present invention relates to an exhaust gas treatment apparatus and an exhaust gas treatment method for removing dioxins in exhaust gas generated from an electric furnace for steelmaking.

Exhaust gas discharged from heat treatment equipment that requires treatment includes combustion and melting in steelmaking electric furnaces, metal smelting furnaces, municipal waste incineration facilities, industrial waste incineration facilities, and other combustion devices, melting devices, heating devices, etc. Exhaust gas discharged along with the heating step can be mentioned. Here, taking the exhaust gas from the steelmaking electric furnace as an example, since dioxins contained in the exhaust gas from the steelmaking electric furnace are discharged together with the exhaust gas, it is regulated to perform a process for preventing this. The emission standards of dioxin concentration of the exhaust gas of the steel-making electric furnace, the enforcement of the "Law Concerning Special Measures against Dioxins", existing for the facility 5ngTEQ / ^{m 3,} new reference value of 0.5ngTEQ / ^{m 3} for the facility The following are regulated.

The following methods are used as a method for removing dioxins contained in the exhaust gas of an electric furnace for steel making. The exhaust gas discharged from the steelmaking electric furnace is heat-treated in the secondary combustion furnace at a sufficient temperature and residence time (850 ° C or higher, 2 seconds or longer) to decompose dioxins, and then the exhaust gas is re-synthesized with dioxins. In order to suppress and to liquefy gaseous dioxins, it is rapidly cooled to 100 ° C. or lower. Dust contained in the exhaust gas from steelmaking electric furnaces contains a lot of metal components, and these metal components have a high catalytic activity for dioxin resynthesis. Therefore, secondary combustion furnaces are used to suppress dioxin resynthesis. It is necessary to rapidly cool the exhaust gas heated at 100 ° C. or lower. When cooled below 100 ° C, gaseous dioxins in the exhaust gas liquefy and become adsorbed or adhered to the dust. By removing the dust by introducing the cooled exhaust gas to a filtration dust collector (bag filter) Dioxins can be removed. By such a method, most of the dioxins in the exhaust gas can be removed by collecting the dust adsorbed or adhered thereto, and the concentration of dioxins in the exhaust gas is about 0.1 to 0.5 ng- up to about TEQ / Nm ³ can be reduced.

  On the other hand, in the exhaust gas treatment system of an electric furnace for steel making, as described in Patent Document 1, a system that directly sucks exhaust gas generated from an electric furnace installed in a building from the electric furnace, and an electric furnace There is an indirect suction system (the sucked gas is called environmental dust collection exhaust gas) that sucks the exhaust gas leaked outside from the upper part of the building together with the surrounding air for ventilation.

  In order to reduce the concentration of dioxins in the exhaust gas to an extremely low concentration, the exhaust gas is obtained by joining the exhaust gas with high temperature directly sucked from the steel furnace for electric furnace and the environmental dust collection exhaust gas (nearly air) near the normal temperature sucked indirectly. By diluting the concentration, the exhaust gas temperature is rapidly cooled to about 60 to 80 ° C., suppressing the resynthesis of dioxins, increasing the ratio of dioxins existing in the state of being adsorbed or adhering to dust, and further increasing the concentration Dilution is done. The concentration of dioxins in the exhaust gas is reduced to an extremely low concentration by subjecting the cooled and diluted exhaust gas to dust removal using a filtration dust collector (bug filter).

  On the other hand, in a waste incinerator, the exhaust gas discharged from the incinerator is heat-treated at a sufficient temperature and residence time (850 ° C. or more, 2 seconds or more) in the secondary combustion furnace to decompose dioxins, and then the exhaust gas is removed. Rapid cooling to suppress resynthesis of dioxins and to liquefy gaseous dioxins. When cooled, gaseous dioxins in the exhaust gas liquefy and become adsorbed or adhered to dust, and dioxins can be removed by introducing the cooled exhaust gas to a filtration dust collector to remove the dust. Has been done. In addition, when reducing to extremely low concentrations, spray activated carbon powder between the rapid cooling tower and the filtration dust collector, and adsorb the dioxins on the powder activated carbon to improve the removal efficiency of the dioxins in the filtration dust collector. Has been done.

JP-A-5-10687

In removal processing of dioxins contained in exhaust gas discharged from an electric furnace for steelmaking, Patent Document 1 tries to reduce dioxins to a very low concentration such as a dioxin concentration of 0.1 ng-TEQ / Nm ³ or less. Then, as described above, the environmental dust collection exhaust gas is merged with the exhaust gas discharged from the steelmaking electric furnace, cooled, diluted, and dust-removed with a filtration dust collector (bug filter). I have the following problems.
(1) The amount of exhaust gas to be treated is increased by merging environmental dust collection exhaust gas with electric furnace exhaust gas. For this reason, it is necessary to install a large filtration dust collector, resulting in an increase in equipment costs and operating costs. . Incidentally, the amount of environmental dust collection exhaust gas is about 4 to 6 times the amount of electric furnace exhaust gas.
(2) After the environmental dust collection exhaust gas is combined with the electric furnace exhaust gas, the exhaust gas amount is increased and diluted, and after dioxins are collected by the filtration dust collector, the dioxin concentration in the exhaust gas discharged from the chimney is Although it is reduced, the total amount of dioxins discharged into the environment is not necessarily reduced when considered together with the collected dioxins.
(3) Dust is removed by a filtration dust collector to remove dioxins from the exhaust gas. However, there is a limit to further increasing the capture rate of dioxins to be captured by the filtration dust collector.

  Also, in the removal process of dioxins contained in the waste incinerator exhaust gas, when trying to reduce dioxins to a very low concentration, as described above, powdered activated carbon is sprayed upstream of the filtration dust collector and dioxins are collected. Is adsorbed on powdered activated carbon to improve the removal efficiency of dioxins in a filtration dust collector. However, in order to maintain high removal efficiency, it is necessary to spray a large amount of powdered activated carbon. However, since powdered activated carbon is collected together with exhaust gas dust by a filtration dust collector, the amount of discharged dust increases significantly, and the processing cost increases. The problem of increasing arises.

  The present invention has been made in view of such circumstances, and increases the amount of exhaust gas to be treated in removing dioxins in exhaust gas discharged from a heat treatment apparatus such as an electric furnace for steelmaking or a waste incinerator. It is another object of the present invention to provide an exhaust gas treatment apparatus and an exhaust gas treatment method capable of efficiently reducing the concentration of dioxins in exhaust gas to an extremely low concentration without significantly increasing the amount of dust collected. .

  Prior to the description of the present invention, as a result of detailed analysis of the form of dioxins and the particle size distribution of dust in the electric furnace exhaust gas for steelmaking, the following was found, and the analysis results will be described.

<Existence form of dioxins in electric furnace exhaust gas for steelmaking>
In the temperature range where the exhaust gas temperature is 100 ° C. or less, most of the dioxins contained in the exhaust gas become liquid and exist in a form adsorbed or adhered to dust.

<Particle size distribution of dust in electric furnace exhaust gas for steelmaking>
The particle size distribution of dust collected from the exhaust gas discharged from the steelmaking electric furnace was measured, and the results are shown in FIG. In FIG. 2, the horizontal axis indicates the particle diameter and the vertical axis indicates the frequency. In FIG. 2, the particle size distribution when the operating conditions are different at the start of operation of the electric furnace for steelmaking, during normal operation, at the time of steel production, etc. is shown in curves 1 to 3, and the particle distribution was obtained as follows.

Curve 1: Median diameter = 1.02 μm, particle ratio of 1 μm or less is 25%
Curve 2: Median diameter = 1.95 μm, particle ratio of 1 μm or less is 16%
Curve 3: Median diameter = 2.45 μm, particle ratio of 1 μm or less is 5%
From the above results, the dust in the electric furnace exhaust gas for steelmaking has a median diameter of 1 to 2.5 μm, a ratio of the particle diameter of 1 μm or less is 5 to 25%, and dust with a very fine particle diameter of 1 μm or less is found. Many were recognized. Such very fine dust having a particle diameter of 1 μm or less cannot be sufficiently captured by a normal filter type dust collector such as a bag filter, and often passes through.

  By grasping the above situation, the inventor has invented an exhaust gas treatment apparatus and an exhaust gas treatment method for effectively removing dioxins in the electric furnace exhaust gas for steelmaking.

The present invention relates to an exhaust gas treatment device, in an exhaust gas treatment device that removes dioxins in exhaust gas discharged from a heat treatment device, a cooling tower that cools the exhaust gas to 100 ° C. or less, and an exhaust gas cooled by the cooling tower. Receiving a dust filter having a particle diameter of 1 μm or more and removing dust having a particle diameter of less than 1 μm at the outlet to a concentration of 0.5 mg / m ³ N or less, and the exhaust gas removed by the bag filter. And a HEPA filter that removes the concentration of dust having a particle diameter of less than 1 μm at the outlet to 0.03 mg / m ³ N or less.

As for the exhaust gas treatment method, in the exhaust gas treatment method for removing dioxins in the exhaust gas discharged from the heat treatment apparatus, a cooling step for cooling the exhaust gas to 100 ° C. or less by a cooling tower, and an exhaust gas cooled by the cooling tower First dust removing step of removing dust having a particle diameter of 1 μm or more with a bag filter, and removing dust having a particle diameter of less than 1 μm at the outlet of the bag filter to 0.5 mg / m ³ N or less. And a second dust removing step of removing dust having a particle diameter of less than 1 μm at the outlet of the HEPA filter to 0.03 mg / m ³ N or less with respect to the exhaust gas removed by the bag filter. It is characterized by comprising.

  The operation of the exhaust gas treatment apparatus and method for removing dioxins in the electric furnace exhaust gas for steelmaking of the present invention is as follows.

  (1) By cooling the electric furnace exhaust gas for steel making to 100 ° C. or less with a cooling tower, resynthesis of dioxins is suppressed, and most of the dioxins contained in the exhaust gas change from a gaseous state to a liquid state, It becomes a form adsorbed or adhered to dust.

(2) Dust in the exhaust gas having a particle diameter of 1 μm or more is removed by a bag filter, and the dust concentration in the outlet having a particle diameter of less than 1 μm is reduced to 0.5 mg / m ³ N or less, thereby removing dust in the exhaust gas. Most of the dust is collected. Since most of the dioxins are attached to or adsorbed to the dust, most of the dioxins are removed from the exhaust gas by this dust collection treatment.

(3) the discharged flue gas from the bag filter was introduced into HEPA filter, by filtration through HEPA filter off fine dust such be captured by the bag filter, dioxins are removed adhering or adsorbed to the fine dust The As a result, the concentration of dioxins in the exhaust gas at the outlet of the HEPA filter is reduced to an extremely low concentration of 0.1 ng-TEQ / Nm ³ or less. In this way, dust with a particle size of 1 μm or more is removed by the bag filter, and the dust concentration at the outlet of the bag filter with a particle size of less than 1 μm is reduced to 0.5 mg / m ³ N or less. Since the amount of dust in the introduced exhaust gas becomes extremely small, the load on the HEPA filter does not become excessive , clogging is reduced, and the lifetime is maintained for a long time .

  The HEPA filter (High Efficiency Particulate Air Filter) is an ultra-high performance air filter. According to JIS Z8122, the collection efficiency of DOP particles (dioctyl phthalate particles) of 0.3 μm at the rated airflow is 99.97% or more. And an air filter having an initial pressure loss of 245 Pa or less ”. It is used as a final filter in a clean room.

  Combining the collection of most of the dust with a bag filter and the collection of fine dust that could not be sufficiently collected with the bag filter with a HEPA filter under a low load, the dust is highly efficient. Can collect dust almost completely, so that dioxins adhering to or adsorbing to dust can be removed almost completely efficiently.

  By combining the two types of dust collectors under optimum conditions, each feature can be utilized as follows.

  The bag filter can remove dust adhering to the filter for a long time by removing the dust adhering to the surface opposite to the surface of the filter to which dust has adhered. However, there is a disadvantage that the collection rate of fine dust having a particle diameter of less than 1 μm is not so high.

  On the other hand, the HEPA filter has an advantage that the collection performance of fine dust having a particle diameter of less than 1 μm is very high, but since dust cannot be removed, it reaches the stage where a certain amount of dust has adhered. There is a drawback that the filtration performance cannot be maintained and the filter itself needs to be replaced.

  It is necessary to combine the advantages of these two types of dust removers under optimum conditions according to the state of the exhaust gas to be treated. In the present invention, the optimum conditions have been found.

That is, in the present invention, in order to reduce the dust concentration in the exhaust gas introduced into the HEPA filter as much as possible, the dust concentration at the outlet can be reduced to 0.5 mg / Nm ³ or less upstream of the HEPA filter. A bag filter is provided, and most of the dust in the exhaust gas is removed by this bag filter, and the exhaust gas from the outlet of the bag filter is introduced into the HEPA filter. By doing this, the life of the HEPA filter is maintained for a long time, fine dust that could not be collected by the bag filter is removed under a low load, and dioxins in the exhaust gas are stably removed for a long time. It can be removed efficiently.

As described above, the present invention removes dust having a particle diameter of 1 μm or more with a bag filter, and removes the dust having a particle diameter of less than 1 μm at the outlet of the bag filter to 0.5 mg / m ³ N or less. Therefore, the amount of dust in the exhaust gas introduced into the HEPA filter is extremely small, so the load on the HEPA filter is not excessive , clogging is reduced, and the life can be maintained for a long period of time. The effect that the dioxins in it can be removed stably for a long time with high removal efficiency is obtained.

It is a schematic block diagram of the apparatus of one Embodiment of this invention. It is a figure which shows the particle size distribution of the dust obtained corresponding to the different operating conditions of the electric furnace for steelmaking. It is a figure which shows the relationship between the exhaust gas temperature and the dioxin removal effect obtained by the apparatus of FIG.

  Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.

  FIG. 1 is a schematic configuration diagram of the present embodiment. In the figure, reference numeral 1 denotes an electric furnace for steel making as a heat treatment apparatus. As described above, the electric furnace 1 discharges exhaust gas containing dust having a particle size distribution as shown in FIG. 2 under different operating conditions such as the start of operation, the normal operation, and the time of steel output.

  A secondary combustion furnace 2 is connected to the electric furnace 1. In the secondary combustion furnace 2, the combustible component in the exhaust gas discharged from the electric furnace 1 is combusted, and the exhaust gas is heated to a sufficient temperature and residence time (850 ° C. or higher) to thermally decompose dioxins in the exhaust gas. 2 seconds or more).

  A cooling tower 3 is connected to the exhaust part of the secondary combustion furnace 2. The cooling tower 3 is not limited in its form, and may be one that sprays water on exhaust gas, for example. In the cooling tower 3, the exhaust gas is cooled to 100 ° C. or lower.

  A bag filter 4 and a HEPA filter 5, which will be described later, are sequentially connected to the cooling tower 3, and exhaust gas discharged from the HEPA filter 5 is discharged into the atmosphere from the chimney 6.

  In the present embodiment, an experiment was conducted on the bag filter and the HEPA filter, and the optimum filter characteristics for the apparatus of the present embodiment were selected.

  First, an experiment was conducted to examine the relationship between the exhaust gas temperature and the dioxin removal effect.

  The electric furnace exhaust gas for steel making is cooled to a temperature of 100 ° C. or less, and gaseous dioxins in the exhaust gas are liquefied and adsorbed or adhered to the dust. The dust is removed with a bag filter to reduce the concentration of dioxins in the exhaust gas. An experiment was conducted to confirm that it could be reduced.

  In the vapor pressure data of dioxins (Berchtold F Rordorf, Chemosphere, Vol. 18, Nos. 1-6, pp783-788, 1989), dioxins are liquid or solid at temperatures below 100 ° C.

  The temperature of the exhaust gas discharged from the steelmaking electric furnace and combusted in the secondary combustion furnace was lowered by a rapid cooling device and introduced into a bag filter. The exhaust gas temperature at the bag filter inlet was changed in the range of 70 ° C. to 130 ° C., and the concentration of dioxins in the exhaust gas at the bag filter outlet was measured. Also, the concentration of dioxins in the exhaust gas introduced at the bag filter inlet is measured, and the removal rate of dioxins by the bag filter is obtained from the measured value of the dioxins concentration in the exhaust gas at the bag filter outlet, as shown in FIG. Results were obtained. FIG. 3 shows the exhaust gas temperature and the dioxin removal effect.

  In FIG. 3, the horizontal axis represents the exhaust gas temperature at the bag filter inlet, the vertical axis represents the dioxin concentration (indicated by ■) and the dioxin removal rate (indicated by ◇) in the exhaust gas at the bag filter outlet. As shown, the lower the exhaust gas temperature at the bag filter inlet, the lower the dioxin concentration at the bag filter outlet and the higher the dioxin removal rate.

  Dioxins adhering to or adsorbing to dust can be removed by cooling the electric furnace exhaust gas for steelmaking, making dioxins liquid, and adhering or adsorbing to dust, and removing the dust with a bag filter. It was confirmed that the concentration of dioxins in the exhaust gas can be reduced.

In addition, by cooling the electric furnace exhaust gas for steel making to 100 ° C. or less with a cooling tower, the concentration of dioxins in the exhaust gas at the bag filter outlet can be remarkably reduced to about 10 ng-TEQ / Nm ³ or less, thereby removing dioxins. It was recognized that the rate could be increased. That is, it has been found that it is important to set the bag filter inlet temperature to 100 ° C. or less of the exhaust gas.

  Next, the HEPA filter was examined. Since the HEPA filter collects dust and then cannot be removed like a bag filter and dust can be removed by gas injection, the pressure loss of the filter gradually increases as the amount of dust attached to the filter increases. When the dust adhesion amount reaches a certain amount (the upper limit value of the dust adhesion amount), the filter pressure loss increases to the extent that the exhaust gas cannot be fed, and the HEPA filter cannot be used. For this reason, the lifetime of the HEPA filter is determined by the dust concentration of the exhaust gas introduced into the HEPA filter. The lower the dust concentration, the longer the lifetime, and the higher the dust concentration, the shorter the lifetime.

Upper limit of the dust adhesion amount HEPA filter is about 300 g / ^{m 2,} Table 1, respectively approximate life expectancy when the dust concentration of the exhaust gas to be introduced into the HEPA filter is 0.1mg ^/ Nm ³ ~3mg ^/ Nm 3 Shown in From this lifetime estimation result, when the minimum lifetime that can be industrially realized when applying the HEPA filter is more than half a year, at least the dust concentration of the exhaust gas introduced into the HEPA filter is suppressed to 0.5 mg / Nm ³ or less. There is a need.

  In addition, we examined the bag filter to select the type of filter cloth.

Since it has already been found that the exhaust gas temperature at the bag filter inlet should be 100 ° C. or lower, what kind of filter cloth is used for the exhaust gas at 100 ° C. so that the estimated life of the HEPA filter is more than half a year, An experiment was conducted to determine whether the dust concentration can be suppressed to 0.5 mg / Nm ³ or less.

Various types of filter cloth were installed on the bag filter, dust was removed by introducing the electric furnace exhaust gas for steelmaking, and the dust concentration at the bag filter outlet was measured. The steel furnace electric furnace exhaust gas contains a dust concentration of about 10 g / Nm ³ , the filtration wind speed is set to 1.5 m / sec, the exhaust gas temperature at the bag filter inlet is cooled to 100 ° C., and is introduced into the bag filter. . The results are shown in Table 2.

As shown in Table 2, when a filter cloth made of (i) an aramid resin fiber surface-coated with a fluororesin and (ii) a polyphenylene sulfide resin fiber surface-coated with a fluororesin is used, dust at the bag filter outlet The concentration could be reduced to 0.5 mg / Nm ³ or less. On the other hand, with other filter cloths, the dust concentration at the bag filter outlet was 0.5 mg / Nm ³ or more, and the dust could not be sufficiently collected. Therefore, it was found that (i) and (ii) are preferable as the filter cloth of the bag filter.

<Example>
The electric furnace exhaust gas for steel making is cooled to 100 ° C, introduced into a bag filter using a filter cloth of aramid resin fiber that has been surface-treated with a fluororesin, and dust is removed, and the exhaust gas after dust removal is introduced into a HEPA filter for further dust removal. The dust concentration and dioxin concentration in the exhaust gas at the bag filter inlet, the HEPA filter inlet, and the HEPA filter outlet were measured.

  Table 3 shows the setting conditions. Table 4 shows the measurement results of dioxins and dust concentration.

As is clear from Table 4, by combining the HEPA filter with the bag filter, the concentration of dioxins in the exhaust gas could be reduced to 0.0041 ng-TEQ / Nm ³ . Dioxin concentration in the exhaust gas to be introduced into the dust HEPA filter bag filter is 1.0ng-TEQ / Nm ^3, since it is reduced to 0.0041ng-TEQ / Nm ^3, by HEPA filter It was confirmed that the collection of fine dust was very effective in reducing dioxins.

Moreover, it can be confirmed that the dust concentration in the exhaust gas introduced into the HEPA filter can be reduced to 0.2 mg / Nm ³ by using an aramid resin fiber whose surface is coated with a fluorine-based resin for the filter cloth of the bag filter. It was also confirmed that the lifetime of the HEPA filter can be maintained for a long time by applying the bug filter using

  By installing an activated carbon adsorber or catalytic cracker on the downstream side of the HEPA filter, gaseous harmful substances (nitrogen oxides, acid gases, volatile organic compounds, etc.) and odor components (methane, ammonia, sulfur compounds, etc.) ) Can also be removed by adsorption or decomposition. According to the present invention, even fine dust is removed, so that it is expected that the removal efficiency by activated carbon, a catalyst, etc. can be increased and the life can be extended.

  It can be used in the industrial field of exhaust gas treatment of steelmaking electric furnaces.

1 Heat treatment equipment (electric furnace for steelmaking)
3 Cooling tower 4 Bag filter 5 HEPA filter

Claims (2)

In the exhaust gas treatment device that removes dioxins in the exhaust gas discharged from the steelmaking electric furnace ,
A secondary combustion furnace in which the exhaust gas is heated at a temperature of 850 ° C. or more for a residence time of 2 seconds or more to decompose dioxins;
A cooling tower for cooling the exhaust gas heated in the secondary combustion furnace to 100 ° C. or lower to liquefy gaseous dioxins and adsorb or adhere to the dust ;
Upon receiving the exhaust gas cooled by the cooling tower, the dust concentration is reduced to 0.5 mg / m ³ N or less, and most of the dust in the exhaust gas is collected, and most of the dioxins in the exhaust gas are removed to discharge the exhaust gas. A bug filter that drains from
A HEPA filter that receives the exhaust gas removed by the bag filter, collects fine dust that could not be captured by the bag filter, and removes dioxins adsorbed or adhered to the fine dust ;
Bei to give a,
The bag filter removes most of the dust in the exhaust gas, reduces the amount of dust in the exhaust gas introduced into the HEPA filter, and removes fine dust under a low load in the HEPA filter to reduce clogging. An exhaust gas treatment apparatus characterized by maintaining a long life . In an exhaust gas treatment method for removing dioxins in exhaust gas discharged from an electric furnace for steel making ,
A heating step in which the exhaust gas is heat-treated at a temperature of 850 ° C. or more for a residence time of 2 seconds or more in a secondary combustion furnace to decompose dioxins;
A cooling step in which the exhaust gas heated in the secondary combustion furnace is cooled to 100 ° C. or less by a cooling tower to liquefy gaseous dioxins and adsorb or adhere to dust ,
Dioxins in the exhaust gas are collected by removing the dust concentration at the outlet of the bag filter to 0.5 mg / m ³ N or less with the bag filter from the exhaust gas cooled in the cooling tower to collect most of the dust in the exhaust gas. A first dust removal process that removes most of the
A second dust removing step for collecting fine dust that could not be captured by the bag filter with respect to the exhaust gas removed by the bag filter , and removing dioxins adsorbed or adhered to the fine dust by the HEPA filter; ,
In the first dust removal process, the bag filter removes most of the dust in the exhaust gas, reducing the amount of dust in the exhaust gas introduced into the HEPA filter, and removing fine dust under a low load in the HEPA filter. An exhaust gas treatment method characterized by removing dust and reducing clogging and maintaining a long life .

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