JP2000254619A - Treatment of solid and waste gas, containing dioxins - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a treatment process for simply and conveniently treating solid waste containing dioxins at a low cost and also to provide a treatment process for removing dioxins from a combustion waste gas by utilizing a wet type gas absorption column used for acidic gas removal. SOLUTION: This treatment process for solid waste containing dioxins comprises: dispersing a solid containing dioxins into a hydrochloric acid-acidic aqueous solution containing a reaction catalyst in a dissolved state, at <100 deg.C; and exposing the resulting aqueous dispersion to an oxygen-containing gas so that the redox potential of the aqueous solution (liquid phase of the dispersion) is >=500 mV with the standard hydrogen electrode as the reference electrode, to detoxify the dioxins. This treatment process for a waste gas containing dioxins comprises: bringing e.g. a combustion waste gas containing at least gaseous sulfur dioxide and dioxins into contact with an absorbing liquid which is maintained at <100 deg.C and at a pH value of 2.0-6.0, to remove gaseous sulfur dioxide and dioxins from the combustion waste gas; exposing the absorbing liquid containing sulfur dioxide and dioxins, each removed from the waste gas, to an oxygen-containing gas so that the redox potential of the resulting absorbing liquid is >=500 mV with the standard hydrogen electrode as the reference electrode, to oxidize the sulfur dioxide and to detoxify the dioxins.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating solids and exhaust gas containing harmful dioxins. Examples of such solids include fly ash generated by incineration of waste, used adsorbent used for removing dioxins in flue gas, and the like. Further, the exhaust gas to be treated according to the present invention contains at least a sulfurous acid gas and dioxins. Examples of such a combustion exhaust gas include an exhaust gas generated when incinerating municipal waste and industrial waste. is there.

[0002]

2. Description of the Related Art The emission of organic chlorine compounds, particularly dioxins, into the environment accompanying the combustion of municipal waste and industrial waste has been regarded as a problem. In the past, Japan did not regulate the emission of dioxins from incineration facilities for municipal solid waste or industrial waste, but the danger of dioxin emission was pointed out and this became a major social problem. Published “Guidelines on Prevention of Dioxins Generation in Waste Disposal” in January 2010, and set the concentration of dioxins in exhaust gas discharged from the newly installed continuous furnace at 0.1 ng-
They are instructed to make TEQ / Nm ³ or less. Also,
With the revision of the Air Pollution Control Law in December 1997, the Environment Agency has designated dioxins as designated hazardous substances, and has set limits for dioxins in exhaust gas generated not only from general waste but also from incineration of industrial waste. It was decided to provide.

[0003] In general, organic chlorine compounds include DDT and PCB.
There are many things that have a harmful effect on the human body and environmental ecosystems when released into the environment like dioxins, but dioxins, which have recently become a problem from waste treatment facilities, The most toxic 2,3
7,8-tetrachlorodibenzo-p-dioxin (2,
3,7,8-TCDD) is said to be a very toxic substance far exceeding the above-mentioned DDT and PCB. Generally, dioxins refer to the above 2,3,7,8-TCDD and its analogous compounds, and 1 to 8 are added to the dibenzo-p-dioxin nucleus.
Polychlorinated dibenzo-p-dioxins (PCDDs) substituted with two chlorine atoms, and 1 to the dibenzofuran nucleus
It is a general term for polychlorodibenzofurans (PCDFs) in which eight chlorine atoms are substituted. Note that coplanar PCBs, which are a part of PCB homologs, are also PCDDs and PCDFs.
Since it exhibits toxicity very similar to s, it may be referred to as dioxins including this, but since regulation of dioxins currently assumed does not include this, when it is referred to as `` dioxins '' in this specification Coplanar PC
B is not included. The present invention aims at detoxification of dioxins, but provides an effective means for detoxification of harmful organic chlorine compounds in general, including not only PCDDs and PCDFs but also related compounds and coplanar PCBs.

[0004] Usually, dioxins are rarely taken up by taking only specific ones out of them, and various dioxins are often taken up as a problem. However, since the degree of harmfulness of various dioxins differs from one another, a scale for distinguishing and evaluating the harmfulness of different dioxins is required to evaluate the harmfulness of the mixture of various dioxins as a whole. For this reason, based on the short-term toxicity evaluation results of various dioxins, a coefficient (toxic equivalent coefficient) for converting the amount of each dioxin into the amount of 2,3,7,8-TCDD having the same degree of toxicity. The value obtained by adding the value obtained by multiplying the actual amount of each dioxin by this toxic equivalent coefficient is called a toxic equivalent conversion value (TEQ), and represents the emission amount and concentration of dioxins. It is used for

[0005] Unlike DDT and PCB, dioxins are not chemical substances artificially synthesized for a certain purpose of use, but are formed as by-products in the synthesis of useful organic chlorine compounds and the like, and those organic chlorine compounds are not used. It is generated when the compound burns. And it is said that the largest cause of the emission of dioxins into the environment is incineration of municipal solid waste and industrial waste. For this reason, while measures have been sought to control the emission of dioxins during waste incineration, various technologies described below have been developed and the results are being demonstrated.

[0006] First, dioxins are often formed by incomplete combustion of organochlorine compounds.
There is no doubt that thorough combustion or stable combustion is effective in suppressing the production of dioxins. However, this has led to the centralization or enlargement of waste treatment facilities on the one hand, and it cannot be denied that there are factors that may cause new problems in the location of the facilities.

Second, dioxins are generated not only by incomplete combustion but also by the contact of fly ash with precursors such as chlorobenzene and chlorophenol in the exhaust gas during the process of cooling the exhaust gas. This is a process in which dioxins are produced by using metal components in fly ash as a catalyst, and is called de novo synthesis.
It occurs at about 600 ° C., and synthesis at about 300 ° C. is said to be the most active. For this reason, the boiler, where the exhaust gas is cooled, has a structure that prevents fly ash from accumulating, thereby avoiding contact between the exhaust gas and the fly ash in the cooling process, and spraying water directly at the boiler outlet to reduce the temperature of the exhaust gas. Is rapidly cooled to 200 ° C. or less to minimize the chance of de novo synthesis.

Third, as a method of treating exhaust gas containing dioxins, for example, use of a low-temperature bag filter has been proposed (Japanese Patent Laid-Open No. 6-47224). This is 20
It uses a bag filter at an operating temperature of 0 ° C or less,
According to this, the particulate dioxins are captured by the filter, and the dioxins are adsorbed and removed from the exhaust gas by the attached fly ash layer. It is said that a bag filter operated at 150 ° C. can obtain a removal rate of 90 to 95%. It has also been proposed to blow powdered activated carbon into exhaust gas at the inlet duct of a bag filter and collect the powdered activated carbon adsorbing dioxins with the bag filter (for example, Japanese Patent Application Laid-Open No. Hei 5-2031).
No. 27). According to this, a removal rate of 95% or more is obtained. Another method for removing dioxins in exhaust gas is to oxidize and decompose dioxins in exhaust gas in a temperature range of 200 ° C. or higher using an oxidation catalyst based on a denitration catalyst (Japanese Patent Application Laid-Open No. 4-503772). etc)
Also, there has been proposed a method of performing adsorption removal using a fixed bed or a moving bed filled with activated carbon pellets.

Fourth, dioxins are contained in furnace ash and fly ash more quantitatively than exhaust gas.
It has been said that the effects on the environment are not as great as those contained in exhaust gas if the contents contained in furnace ash and fly ash are properly buried. However, in reality, not all of the furnace ash and fly ash are buried under proper management; they are washed away by rainwater and flow into rivers, and penetrate underground and contaminate groundwater. The situation seems endless. For this reason, it has been attempted to bury the furnace ash or fly ash after performing some processing without burying it as it is. As one example, there is a technique in which furnace ash and fly ash are melted and solidified at a temperature of 1250 to 1450 ° C. or more to form a glassy slag. According to this, the volume can be reduced, the heavy metal can be fixed in the network of glass, and the dioxins are decomposed by the high temperature during melting. As another example for fly ash, fly ash was collected under a low oxygen atmosphere.
A method of heating at 0 to 550 ° C. and detoxifying dioxins by a dechlorination reaction (Japanese Patent Publication No. 6-38863).
No.) or a method of introducing fly ash into supercritical water (374 ° C. or higher and 218 atm or higher) to oxidatively decompose dioxins (Takeshi Sako, Kagaku, Vol. 52, No. 10, 31-34 ( 1997)).

[0010]

Among the various countermeasures described above, the first and second ones attempt to suppress the production of dioxins themselves, while the third and fourth ones attempt to suppress the production of dioxins themselves. It can be said that those which try to remove the generated dioxins. By the way, the methods for treating solid waste such as furnace ash and fly ash containing dioxins mentioned above as the fourth various technologies are all for treating solids containing dioxins at high temperatures. It requires relatively expensive equipment and sophisticated operation management, and may not be adopted in all incineration facilities. Also,
As described above, the solid waste containing dioxins may contain activated carbon and the like generated when dioxins in exhaust gas are adsorbed and removed, in addition to furnace ash and fly ash. Therefore, if a method for easily treating such solid waste in a lump can be developed, it is apparent that the method can be advantageously used in a waste incineration facility or the like. That is,
A first problem to be solved by the present invention is to provide a method for collectively and inexpensively and conveniently treating solid waste containing dioxins.

On the other hand, the conventional methods for removing dioxins in exhaust gas, which have been mentioned as the above-mentioned third various techniques, are used to remove dioxins by adsorption or oxidative decomposition and to remove dioxins. Requires some new equipment. In particular, those that are adsorbed and removed generate solid waste such as fly ash, powdered activated carbon, and granular activated carbon that have adsorbed dioxins. Therefore, it is also necessary to treat these to make the dioxins harmless. In view of these points, it is clear that it would be cost-effective to develop a method that does not require the construction of a new device in removing and detoxifying dioxins in exhaust gas. That is, a second problem to be solved by the present invention is that dioxins are added to a wet gas absorption tower conventionally required for removing acidic gases (hydrogen chloride, sulfurous acid gas, etc.) in combustion exhaust gas. It is intended to provide a function of removing all dioxins such as solid and gaseous dioxins from exhaust gas to make them harmless.

[0012]

According to the present invention, in order to solve the first problem, a solid containing dioxins is dissolved in an aqueous hydrochloric acid solution containing a reaction catalyst in a dissolved state at a temperature lower than 100 ° C. A method for detoxifying the dioxins by dispersing and aerating an oxygen-containing gas so that the oxidation-reduction potential of the aqueous solution is 500 mV or more based on a standard hydrogen electrode.

The solid containing dioxins is typically a fly ash produced by incineration of waste or a used adsorbent used for removing dioxins in combustion exhaust gas.

The reaction catalyst is generally a metal ion which can have a low valence and a high valence. Such metal ions include ions of iron, manganese, copper, nickel, molybdenum, chromium, vanadium, tungsten, silver, tin, and the like. These metal ions may be used as a mixture of two or more kinds. The metal ions may form complex ions. Particularly preferred metal ions are copper ions or iron ions. The amount of the reaction catalyst contained in the aqueous solution is
Although not particularly limited, in the case of copper ions, as Cu, 2
0-10000 mg / liter, preferably 100-5
It may be about 000 mg / liter. Abundance is 10
Even if it exceeds 000 mg / liter, an increase in the effect as a catalyst cannot be expected. Also, when other metal ions are used, their abundances may be similar to those of copper ions.

[0015] These catalytic metal ions can be generally supplied in the form of metal oxides or metal salts such as oxides, chlorides, carbonates and sulfates. The aqueous hydrochloric acid solution as a reaction treating agent for detoxifying dioxins used in the present invention contains these metal oxides and metal salts in a dissolved state. The reaction catalyst may partially contain undissolved components, but if such undissolved components are in the process of shifting to the dissolved state, they will also effectively act as reaction catalysts.

A metal ion capable of taking a low valence and a high valence contributes to detoxification of dioxins while changing its valence, and therefore, in an aqueous solution, both valences are generally used. Things are mixed. Therefore, when the catalyst metal ions are supplied from the outside, it is not always necessary to add those having the same valence, and a mixture of low valence metal ions and higher valence metal ions may be added. For example,
The addition of cuprous chloride and cupric chloride is also a preferred embodiment of the present invention.

The pH of the aqueous solution is generally 0.5-7.
0, particularly preferably in the range of 2.0 to 6.0. Alternatively, a multi-stage treatment may be performed, and a different pH may be adopted in each stage. The concentration of chloride ions in the aqueous solution is preferably from 10 to 3000 mmol / l. The oxygen-containing gas is typically air, and such gas is typically aerated for 0.1-100 hours in an aqueous solution. In order to promote the detoxification reaction of dioxins, a persulfate can be added to the aqueous solution.

Further, the present invention provides a method for treating an exhaust gas containing at least sulfur dioxide gas and dioxins, wherein the exhaust gas has a pH of less than 100 ° C. and a pH of 2.0. Contact with an absorbent maintained in the range of ~ 6.0 to remove sulfurous acid gas and dioxins from the exhaust gas, and contain oxygen so that the oxidation-reduction potential of the absorbent is 500 mV or more based on a standard hydrogen electrode. Provided is a method for oxidizing sulfur dioxide gas removed from exhaust gas to detoxify dioxins by aerating the gas.

The rate of the wet decomposition reaction of dioxins can be increased by setting the oxidation-reduction potential of the absorbing solution to 500 mV or more based on the standard hydrogen electrode. However, in a reaction system controlled to the oxidation-reduction potential, the reaction rate is reduced. It is considered that the sulfurous acid gas removed from the exhaust gas or the sulfite ion supplied from the outside is oxidized to generate persulfuric acid having high oxidizing ability, which further promotes the detoxification of dioxins.

The absorbing solution generally contains a metal ion which can have a low valence and a high valence, and this functions as a reaction catalyst when detoxifying dioxins. It is particularly preferable that such metal ions include copper ions or iron ions. The absorption solution preferably has a chloride ion concentration higher than a sulfate ion concentration. In addition, a persulfate can be added to the absorbing solution to promote the detoxification reaction of dioxins. The method of the present invention is particularly preferably performed using a jet bubbling reactor. The dioxins in the exhaust gas usually exist mainly in a form adsorbed by fly ash, but the jet bubbling reactor has a high trapping efficiency of such fly ash. If the efficiency of capturing fly ash in the absorbing solution is not sufficient, the exhaust gas separated from the absorbing solution may be further passed through a dust collector to collect the remaining fly ash. In this case, it is preferable to reduce the relative humidity of the separated exhaust gas to such a level that water does not condense in a downstream dust collector. Further, by spraying activated carbon circulated and supplied into the exhaust gas on the upstream side of the dust collector and collecting it by the dust collector, the residual concentration of dioxins in the exhaust gas can be further reduced. Fly ash and activated carbon collected by the dust collector may be returned to the absorbent to detoxify the adsorbed dioxins.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows one preferred embodiment of the present invention for solving the first problem. In FIG. 1, in a reaction tank 1, fly ash or activated carbon adsorbing dioxins is dispersed in an aqueous solution containing a reaction catalyst in a dissolved state. An oxygen-containing gas such as air is blown from a gas blowing pipe 2 provided near the bottom of the tank to maintain the dispersion state and supply oxygen into the aqueous solution.

As described above, the reaction catalyst is generally a metal ion which can have a low valence and a high valence, and is particularly preferably a copper ion or an iron ion. These metal ions are usually contained in the fly ash and are dissolved in the aqueous solution. Therefore, a sufficient amount (generally, 1 mg) of these metal ions is required to act as a catalyst for the oxidative decomposition reaction of the dioxins.
/ L or more) is present in the liquid without any additional addition, but if necessary, a salt or the like containing these metal elements may be added to the liquid.

The pH of the solution is generally maintained at 0.5 to 7.0, preferably 2.0 to 6.0, in order to efficiently perform the detoxification reaction of dioxins. For adjusting the pH, an acid such as sulfuric acid or hydrochloric acid, or an alkali such as caustic soda, soda ash, slaked lime, limestone, or magnesium hydroxide can be used. The solution is hydrochloric acid, that is, an acidic aqueous solution containing chlorine ions (Cl ⁻ ). The chloride ion concentration of the liquid is preferably from 10 to 3000 mmol / L, more preferably 100 mmol / L or more. Thereby, dioxins can be detoxified efficiently. The temperature of the solution may be 100 ° C. or less, and if it is 10 ° C. or more, there is no particular hindrance to the progress of the reaction, but it is more preferably 30 ° C. or more. Thus, in the present invention, since the reaction is generally carried out under normal pressure and at 100 ° C. or lower, the treatment is extremely inexpensive and simple compared to the conventional method requiring high temperature or high pressure conditions for the decomposition of dioxins. be able to.

The oxygen-containing gas used for aeration is usually air, but pure oxygen or air to which pure oxygen is added can also be used. When using air, it is preferable to perform aeration for 0.1 to 100 hours. If the time is less than 0.1 hour, the detoxification of dioxins is insufficient.
Aeration for more than 00 hours does not improve the effect, but causes a problem that the volume of the reaction tank becomes too large to secure a certain processing capacity. When aeration is performed with an oxygen-containing gas, the concentration of dissolved oxygen in the liquid increases, and the oxidation-reduction potential of the liquid increases. In the present invention, aeration is performed so that the oxidation-reduction potential of the liquid becomes 500 mV or more based on the standard hydrogen electrode, thereby promoting the detoxification reaction of dioxins. As will be described later, under such a strong oxidizing atmosphere, if sulfite ions are present in the solution, they are oxidized to generate persulfate ions having high oxidizing ability, which further promotes the detoxification reaction of dioxins. It is likely. Therefore, in the case where sulfite ions are hardly contained in the liquid, it is effective to add a persulfate.

The liquid preferably contains a substance that promotes contact between the liquid and the solid (a contact promoter). Such contact promoters include surfactants and alcohols.
The type of surfactant is not particularly limited, and various surfactants such as anionic, cationic, nonionic and amphoteric surfactants can be used. The surfactant is preferably contained in the liquid in an amount of 0.005 to 1% by weight, more preferably 0.01 to 0.5% by weight. As the alcohols, lower alcohols are preferably used, and specific examples thereof include methanol, ethanol, and propanol. Alcohols are preferably contained in the liquid in an amount of 0.5 to 10% by weight, more preferably 1 to 10% by weight. In order to promote the contact between the solid and the liquid,
Ultrasonic waves may be applied to the processing area.

FIG. 2 shows one preferred embodiment of the present invention for solving the second problem. Exhaust gas containing at least sulfur dioxide and dioxins is blown into the absorbent in the liquid-phase continuous-type wet desulfurizer 11. At this time, the sulfurous acid gas is absorbed by the absorbing solution and becomes sulfite ions. In the exhaust gas, dioxins are mainly present in a form adsorbed by fly ash, and this fly ash is also trapped in the absorbent.
Since an oxygen-containing gas such as air is separately blown into the absorbing solution, the supplied dissolved oxygen oxidizes the sulfite ions in the solution to sulfate ions. Since the exhaust gas often contains an acid gas such as a hydrogen chloride gas in addition to the sulfurous acid gas, the pH of the absorbing solution gradually decreases due to the generated sulfate ions and chloride ions, so that an alkaline component for neutralizing the pH is used. Is supplied to the liquid, and the pH of the liquid is maintained in the range of 2.0 to 6.0. For the supply of the alkali component, calcium compounds such as limestone (calcium carbonate) and slaked lime (calcium hydroxide), sodium compounds such as sodium hydroxide and sodium carbonate, and magnesium compounds such as magnesium hydroxide can be used. When an alkali agent for neutralization is added, sulfate ions in the solution react with calcium ions and the like in the added alkali agent to form a precipitate such as gypsum (calcium sulfate). These precipitates are separated from the absorbing liquid by the solid-liquid separator 15 together with the fly ash. Part of the absorbent from which the precipitate and fly ash have been removed is circulated and used as the absorbent, and part of the absorbent is discharged after being subjected to wastewater treatment 16. When the sulfate is gypsum, it may be considered to separate and collect gypsum precipitate and fly ash by appropriate means. Solid ash separated fly ash does not contain harmful organic chlorine compounds such as dioxins, and heavy metals are extracted and removed by the acidic absorbing solution. May be performed. In the case where a waste direct melting furnace is used as the incineration equipment, it is also possible to return the solid-liquid separated fly ash to the direct melting furnace to form slag.

The alkali used for neutralizing fly ash and acid gas collected in the absorbing solution usually contains metal ions acting as a reaction catalyst. The dioxins contained in the fly ash are decomposed and made harmless by the action of the dissolved oxygen. In this case, the oxygen-containing gas is excessively aerated to oxidize the sulfite ions so that the oxidation-reduction potential (ORP) of the absorbing solution is sufficiently increased to reduce dioxins contained in the exhaust gas in the absorbing solution. It is very effective for oxidative decomposition at the same time.
More specifically, oxygen-containing gas such as air is aerated,
The oxidation-reduction potential of the absorbing solution is 500 mV based on the standard hydrogen electrode.
As described above, increasing the pressure to preferably 650 mV or more is effective for decomposing dioxins. Thereby, the oxidizing ability of the reaction catalyst can be increased. Furthermore, if sulfite ions are present in the solution while maintaining the absorbing solution in a strong oxidizing atmosphere, it will be oxidized to form persulfate ions with high oxidizing ability in the solution, which will be contained in the exhaust gas in the form of gas or fly ash. It is thought that it contributes to the oxidative decomposition of the dioxins contained in the form adsorbed on, and promotes the detoxification of the dioxins. In fact, the present inventors have found that the concentration of persulfate ions reaches about 100 mg / L when the oxygen-containing gas is aerated so that the oxidation-reduction potential of the absorbing solution is about 800 mV by treating the exhaust gas containing the sulfur dioxide gas. Make sure that. At this time, it is presumed that the detoxification of dioxins is also promoted at the same time.

In order to simultaneously and efficiently absorb and oxidize sulfur dioxide gas and decompose and decompose dioxins, the pH of the absorbing solution must be maintained at 2.0 to 6.0. pH
A value of 6.0 or more is not preferred because metals that act as catalysts for the production of persulfuric acid or the decomposition of dioxins are difficult to dissolve. If the temperature of the solution is 10 ° C. or higher, there is no particular hindrance to the progress of the reaction, but 30 ° C. or higher is more preferable. In the present invention, in general, the reaction is carried out at normal pressure and at 100 ° C. or lower, so that dioxins can be removed at extremely low cost and easily compared to the conventional high temperature or high pressure conditions required for the decomposition of dioxins. it can.

The concentration of various ions in the absorbing solution is not particularly limited. However, since chlorine ions have a function of keeping the concentration of catalytic metal ions effective for decomposing dioxins high, they are preferably present at a relatively high concentration. In contrast, sulfate ions react with the catalyst metal in the liquid to precipitate it,
It is preferable that the concentration is not too high, since it may have a negative effect of forming a complex to increase steric hindrance and reduce catalytic activity. Specifically, the chloride ion concentration is preferably higher than the sulfate ion concentration, and particularly preferably 2000 mg / L or higher than the sulfate ion concentration. On the other hand, the sulfate ion concentration is preferably 8000 mg / L or less. Since chloride ions are unlikely to precipitate or form a complex, it is considered that even if they are present at a high concentration, the catalytic activity of metal ions is hardly impaired. When treating incineration exhaust gas from general waste such as municipal solid waste, since the exhaust gas often contains a large amount of hydrogen chloride gas and chlorine-containing fly ash, the chloride ion concentration in the absorbing solution is inevitable. And its concentration is usually more than 5000 mg / L.

The exhaust gas treated by the wet desulfurization unit 11 contains almost no acidic gas and a small amount of residual fly ash. However, dioxins which were not adsorbed to the fly ash and existed in a gaseous state were one of them. Since the exhaust gas after the wet desulfurization treatment may be heated by the heat exchange heater 12, powdered activated carbon may be blown into the exhaust gas to adsorb and remove the remaining dioxins. The blown powdered activated carbon is collected by the bag filter 13 at the subsequent stage together with the fly ash remaining in a small amount without being collected by the absorbing liquid, and can be circulated and supplied again to the exhaust gas. By returning the used powdered activated carbon to the wet desulfurization unit 11, the adsorbed dioxins can be decomposed. The exhaust gas treated by the bag filter is further heated by the heat exchange heater 14 in order to prevent white smoke, and then discharged from the chimney.

FIG. 3 schematically shows the structure and operation of a jet bubbling reactor 21 which can be suitably used as the above-mentioned wet desulfurization apparatus. Combustion exhaust gas containing at least sulfur dioxide and dioxins (often adsorbed on fly ash) passes through an inlet plenum 24 partitioned by a lower deck 22 and an upper deck 23, and a reaction chamber below the lower deck. It is blown into the absorbing liquid from a sparger pipe 26 protruding into the absorbing liquid in 25. Since a large number of holes are provided on the side surface of the sparger pipe, the exhaust gas spouts out of those holes into the absorbing liquid to form fine bubbles. As a result, the exhaust gas and the absorbent have a large gas-liquid contact area, so that the sulfurous acid gas (and other acidic gases) in the exhaust gas is rapidly dissolved and absorbed in the liquid, and fly ash in the exhaust gas is also absorbed. Is trapped on the gas-liquid contact surface. The dioxins adsorbed on the fly ash are decomposed by dissolved oxygen in the absorbing solution by the catalytic action of metal ions contained in the fly ash. If there are few such metal ions in the fly ash, a solution such as an iron salt can be added to the absorbing solution through the catalyst metal supply pipe 27. The exhaust gas, which has become fine bubbles, slowly rises in the absorbent, is separated from the absorbent, passes through the gas riser 28, and is discharged through the outlet plenum 29 formed on the upper deck. Air is blown into the absorbing solution in the reaction chamber from a gas distributor 30 provided near the bottom, and oxidation of sulfite ions generated by dissolving sulfurous acid gas in the absorbing solution and oxidation of dioxins adsorbed on fly ash. Contributes to decomposition. The absorbing liquid is loosely stirred by the stirring blade 31, and the lower liquid and the upper liquid slowly circulate. The sulfate ions generated by the oxidation of the sulfite ions react with calcium supplied into the liquid from the limestone slurry supply pipe 32 for neutralization to form gypsum. The gypsum and fly ash settled at the lower part are continuously extracted as slurry from the slurry drawing pipe 33 and processed. Most of the absorbent separated from the slurry is returned to the reaction chamber.

Sulfurous acid gas is generated when a substance containing sulfur is burned, and is well known to be contained in a large amount in the combustion exhaust gas of a heavy oil combustion boiler. For this reason, flue gas desulfurization equipment is an indispensable exhaust gas treatment facility at thermal power plants with large heavy oil combustion boilers, etc.
In other waste incineration facilities, etc., since sulfur is contained in the waste and heavy oil may be used as an auxiliary fuel, the generation of sulfurous acid gas cannot be avoided. It is expected that the installation of a smoke desulfurization unit will be required. Usually, wet flue gas desulfurization equipment (lime gypsum method) is operated under neutral to weakly acidic conditions, and calcium ions are added as an alkaline component. Therefore, the method of the present invention is already installed in many incineration facilities. It can be carried out using a wet flue gas desulfurization device. In this case, in order to prevent fly ash contained in the flue gas from hindering the operation of the desulfurization unit, a bubbling system that uses a continuous liquid phase device and blows the flue gas into the absorbent in the form of a large number of fine bubbles It is very advantageous to use a type of jet bubbling reactor, such as the one shown above. The jet bubbling reactor absorbs sulfurous acid gas and the like in the absorption solution adjusted to a predetermined pH, and oxidizes generated sulfite ions to sulfate ions,
Since a series of processes of adding an alkaline agent such as a calcium compound to precipitate and recover sulfate precipitates is performed by one apparatus, it is also preferable in that the entire apparatus becomes compact.

In a flue gas desulfurization apparatus for treating an exhaust gas containing 500 ppm or more of sulfurous acid gas, such as in a thermal power plant having a heavy oil combustion boiler, air is generally introduced into the absorbing solution to oxidize sulfurous acid to sulfuric acid. ing. However, in a general waste incineration facility, etc., the sulfur dioxide gas concentration in the exhaust gas is 500 ppm or less. In such a case, the oxygen in the exhaust gas can oxidize the entire amount of sulfurous acid without introducing air. At present, air is not introduced into the absorbent even if a flue gas desulfurization unit is provided. The method of the present invention is an incineration plant for general waste containing dioxins, by introducing a gas containing oxygen into the absorbing solution of a wet flue gas desulfurization unit to increase the oxidizing ability of the absorbing solution, so that only sulfurous acid gas is used. However, dioxins are also oxidatively decomposed at the same time.

In the present invention, the acidic exhaust gas containing at least sulfur dioxide and dioxins is treated by a wet exhaust gas treatment device to remove those harmful substances at the same time, but other forms are also applicable. It is possible. For example,
Using an existing desulfurization wet exhaust gas treatment device, dioxin-containing solid waste generated separately from the exhaust gas can be treated by being introduced into the absorption liquid of the device, and sulfur dioxide gas can be contained. Exhaust gas containing no dioxins and exhaust gas containing dioxins but containing little sulfur dioxide can also be treated using the same exhaust gas treatment device. That is, in the present invention, it is not always necessary that the sulfurous acid gas and the dioxins are simultaneously introduced into the absorption solution, and the oxidation of the sulfite ions generated in the solution by the absorption of the sulfurous acid gas and the dioxins taken in the solution are not necessarily required. All the cases including the process in which the decomposition proceeds simultaneously are included.

[0035]

EXAMPLE 1 Using the experimental apparatus shown in FIG. 4, a decomposition experiment of dioxins contained in fly ash was performed according to the following operation and conditions. (Processing operation and conditions) Cu, Mn, Fe, Mn, and C were added to the flask shown in FIG.
2.0 L of pure water containing 100 mg / L each of r and Ni (dissolved as chloride) was added, a small amount of hydrochloric acid was added thereto, and the mixture was heated and stirred, and the conditions of pH 3.5 and temperature of 65 ° C. were maintained. 2. Subsequently, 400 g of fly ash was added thereto, and stirring was continued at 65 ° C. for 48 hours under a flow of air at 50 NL / hour while maintaining pH 3.5 by adding hydrochloric acid. During this time, the oxidation-reduction potential of the liquid was 600 mV. After stirring for 3.48 hours, the slurry was subjected to suction filtration to obtain a treated liquid and treated fly ash. 4. Dioxins in raw ash, treatment liquid, treatment fly ash and exhaust gas were analyzed.

(Treatment results) 1) The change in weight of the fly ash before and after the wet treatment is as shown in Table 1. 400 g before the treatment was reduced to 150 g (37.5% of that before the treatment) after the treatment. This is probably because soluble salts such as NaCl were dissolved.

[Table 1]

2) The concentration of dioxins in the fly ash before and after the wet treatment is as shown in Table 2, and the concentration in the fly ash after the treatment is generally lower than the concentration in the fly ash before the treatment. It has become quite low. The abbreviations in Tables 2 and 3 have the following meanings. T4CDDs Tetrachlorodibenzoparadioxin P5CDDs Pentachlorodibenzoparadioxin H6CDDs Hexachlorodibenzoparadioxin H7CDDs Heptachlorodibenzoparadioxin O8CDD Octachlorodibenzoparadioxin Total PCDDs All polychlorodibenzoparadioxin T4CDFs Tetrachlorodibenzofuran P5CDFs Hexachlorodibenzofuran HDF Chlorodibenzofuran O8CDFs Octachlorodibenzofuran Total PCDFs Total polychlorodibenzofuran Total Total dioxins

[Table 2]

3) Table 3 shows the dioxin removal rates. The removal rate is that the fly ash weight after treatment is 37.5% of the original weight.
In consideration of the above, it was calculated by the following equation 1.
Since dioxins in the exhaust gas and the treatment liquid were negligible in terms of the material balance, the removal rate was calculated only from the analysis value of the dioxin concentration in fly ash.

(Equation 1)

[Table 3]

Example 2 Except that the type and / or flow rate of the oxygen-containing gas was changed, the same operation as in Example 1 was carried out to treat dioxins in fly ash. The dioxin concentration in the ash before and after the treatment was analyzed, and the dioxin removal rate was determined. Table 4 shows the conditions and results at that time.

Table 4 Test 1 Test 2 Treatment liquid pH 3.5 3.5 Redox potential 750 mV 850 mV Treatment temperature 65 ° C. 65 ° C. Treatment time 48 hours 48 hours Oxygen-containing gas Air Pure oxygen Gas flow rate 100 NL / h 50 NL / H Dioxin removal rate 94% 98.5%

Comparative Example 1 Dioxins in fly ash were treated in the same manner as in Example 1 except that the oxygen concentration of the oxygen-containing gas was changed. The dioxin concentration in the ash before and after the treatment was analyzed, and the dioxin removal rate was determined. Table 5 shows the conditions and results at that time.
Shown in

[Table 5] Treatment liquid pH 3.5 Oxidation-reduction potential 400 mV Treatment temperature 65 ° C Treatment time 48 hours Working gas oxygen concentration 3% by volume Gas flow rate 50 NL / h Dioxin removal rate 35%

Example 3 The same treatment as in Example 1 was carried out except that the concentration of sulfate ions in the treatment solution was changed, and the removal rate of dioxins was determined. The results are shown in Table 6 in comparison with Example 1.

Table 6 Example 1 Example 3 Sulfate ion concentration (mg / L) 1500 6500 Dioxin removal rate (%) 85 44

Example 4 The apparatus shown in FIG. 4 was modified so that a gas containing 500 ppm of sulfurous acid gas could be blown into the liquid, and was operated under the conditions shown in Table 7 (the conditions not shown are the same as in Example 1). The same treatment as in Example 1 was carried out, and the dioxin removal rate was determined.

Table 7 Oxygen-containing gas Pure oxygen Gas flow rate 50 NL / h Treatment liquid pH 4.5 Redox potential 750 mV Dioxin removal rate 96%

[0043]

According to the present invention, a solid containing dioxins, such as furnace ash or fly ash or activated carbon used for removing dioxins from exhaust gas, can be processed at low cost and simply. . Further, desulfurization treatment and decomposition treatment of dioxins can be simultaneously performed on combustion exhaust gas containing sulfurous acid gas and dioxins using an existing desulfurization apparatus, which is extremely advantageous.

[Brief description of the drawings]

FIG. 1 is a schematic view illustrating one embodiment of the present invention.

FIG. 2 is a schematic view showing another embodiment of the present invention.

FIG. 3 is a schematic view of a jet bubbling reactor that can be suitably used as a wet desulfurization apparatus in the embodiment of FIG.

FIG. 4 shows an apparatus for performing an experiment for decomposing dioxins.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Reaction tank 2 Gas injection pipe 11 Wet desulfurization apparatus 12 Heat exchange heater 13 Bag filter 14 Heat exchange heater 15 Solid-liquid separation 16 Drainage treatment 17 Insolubilization treatment 21 Jet bubbling reactor 22 Lower deck 23 Upper deck 24 Inlet plenum 25 Reaction chamber 26 Spar Jar pipe 27 Catalyst metal supply pipe 28 Gas riser 29 Outlet plenum 30 Gas distributor 31 Stirrer blade 32 Neutralization limestone slurry supply pipe 33 Slurry extraction pipe

 ────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Kazushige Kawamura 2-12-1, Tsurumichuo, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture F-term (reference) 2E191 BA12 BC01 BD13 4D002 AA02 AA17 AB01 AC04 BA02 BA05 BA14 DA01 DA04 DA14 DA21 DA26 DA35 EA02 GA01 GB03 GB08 GB09 GB20 4D004 AA37 AA50 CC01 CC02 CC03 CC09 CC11 CC12 DA03 DA06 DA10 DA12 DA20

Claims (20)

[Claims] 1. A solid containing dioxins is treated with 10
The dioxin is dispersed at a temperature lower than 0 ° C. in a hydrochloric acid aqueous solution containing the reaction catalyst in a dissolved state, and the oxygen-containing gas is aerated so that the oxidation-reduction potential of the aqueous solution is 500 mV or more based on a standard hydrogen electrode. How to make harmless. 2. The method according to claim 1, wherein the solid containing dioxins is fly ash generated by incineration of waste or used adsorbent used for removing dioxins in combustion exhaust gas. Method. 3. The method according to claim 1, wherein the reaction catalyst comprises a metal ion capable of having a low valence and a high valence. 4. The method according to claim 1, wherein the reaction catalyst comprises copper ions or iron ions. 5. The method according to claim 1, wherein the reaction catalyst contains undissolved components, and the undissolved components are in a process of transitioning to a dissolved state. 6. The method according to claim 1, wherein the pH of the aqueous solution is 0.5 to 7.0. 7. The method according to claim 1, wherein the pH of the aqueous solution is 2.0 to 6.0. 8. The concentration of chlorine ions in the aqueous solution is 10 to 10.
The method according to any one of claims 1 to 7, wherein the amount is 3000 mmol / liter. 9. The method according to claim 1, wherein the oxygen-containing gas is aerated for 0.1 to 100 hours. 10. The method according to claim 1, wherein a persulfate is added to said aqueous solution. 11. An exhaust gas containing at least sulfur dioxide and dioxins having a pH of less than 100.degree.
The solution is brought into contact with an absorbent maintained in the range of 0 to 6.0 to remove sulfur dioxide and dioxins from the exhaust gas, and the oxygen is adjusted so that the oxidation-reduction potential of the absorbent is 500 mV or more based on a standard hydrogen electrode. By aerating the contained gas,
A method of oxidizing sulfur dioxide removed from the exhaust gas to detoxify dioxins. 12. The method according to claim 11, wherein the absorbing solution contains a metal ion capable of taking a low valence and a high valence. 13. The method according to claim 11, wherein said absorbing solution contains copper ions or iron ions. 14. The method according to claim 11, wherein the absorption solution has a chloride ion concentration higher than a sulfate ion concentration. 15. The method according to claim 11, wherein a persulfate is added to the absorbing solution. 16. The method according to claim 11, wherein the method is performed using a jet bubbling reactor. 17. The method according to claim 11, wherein the dioxins in the exhaust gas are present mainly in a form adsorbed on fly ash. 18. The method according to claim 17, wherein the exhaust gas separated from the absorbent is passed through a dust collector after the relative humidity of the exhaust gas has been reduced to a level that does not condense water in the subsequent stage, thereby collecting residual fly ash. . 19. The method according to claim 18, wherein the activated carbon circulated and supplied into the exhaust gas on the upstream side of the dust collector is sprayed and collected by the dust collector. 20. The method according to claim 18 or 19, wherein fly ash and / or activated carbon collected by said dust collector are returned to said absorbing solution.