JPH07114980B2 - Waste incinerator exhaust gas treatment method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for treating exhaust gas from a refuse incinerator, and more particularly to a refuse incinerator with reduced emission of harmful organic chlorine compounds, particularly aromatic chlorine compounds. The present invention relates to a method for treating exhaust gas.

(Prior Art) Organic chlorine compounds harmful to the human body, especially aromatic chlorine compounds, for example, highly toxic aromatic chlorine compounds such as PCDD (polychlorinated dibenzodioxin) and PCDF (polychlorinated dibenzofuran) , A pesticide by-product or as a secondary product when incinerating waste, is known to pollute the environment (K.Olie et al, Chemosphere,
6 , 455 (1977), and T. Wakimoto et al, Environme.
ntal Health Perspectives, 59 , 159 (1985)). Among them, as a method of preventing the organic chlorine compounds generated during refuse incineration, conventionally, the structure of the incinerator and the incineration conditions have been studied, and a scrubber removal device has been attached. However, the causes of these harmful substances are not always clear, and effective prevention, etc. could not be taken.

(Problems to be Solved by the Invention) An object of the present invention is to treat waste incinerator exhaust gas capable of significantly reducing the amount of organic chlorine compounds, particularly aromatic organic chlorine compounds, contained in the waste incinerator exhaust gas. To provide a method.

(Means for Solving Problems) The first aspect of the present invention is to provide a method for treating municipal waste incinerator exhaust gas, in which waste gas incinerator exhaust gas is cooled and then dust-removed by an electric dust collector, in an electric dust collector outlet exhaust gas. It is characterized in that the amount of the organic chlorine compound is detected and the inlet gas temperature of the electrostatic precipitator is controlled so that the detected value becomes a predetermined value or less.

A second aspect of the present invention is a method for treating an exhaust gas of an incinerator that removes dust from the electric dust collector after cooling the exhaust gas of the incinerator of the municipal waste, detecting the amount of an organic chlorine compound in the exhaust gas of the outlet of the electrostatic dust collector, The temperature of the inlet gas of the electrostatic precipitator and the current density of the dust collecting electrode are controlled so that the detected value becomes equal to or less than a predetermined value.

A third aspect of the present invention is a method for treating a waste incinerator exhaust gas, which cools the waste incinerator exhaust gas and then removes dust with an electric dust collector,
It is characterized by detecting the amount of an organic chlorine compound in the exhaust gas from the outlet of the electrostatic precipitator and controlling the inlet gas temperature of the electrostatic precipitator and the residence time of the ash so that the detected value becomes a predetermined value or less. .

In the present invention, the “ash residence time” refers to the residence time in the electric dust collector of the ash collected from the waste incinerator exhaust gas in the electric dust collector.

Next, the background of the present invention and the experimental results will be described.

FIG. 4 is a flow sheet of a typical conventional incineration facility. The present inventors collected ash and exhaust gas samples from before and after the electrostatic precipitator of such an incineration facility, and quantitatively analyzed the organic chlorine compounds contained therein. The results are shown in Table 1. Further, Table 2 shows the results of obtaining the amount of the generated organic chlorine compound from the product of the value indicating the concentration of each organic chlorine compound in Table 1 and the emission amount (average value). In Table 2, the values of the cooling tower ash, the electrostatic precipitator (EP) ash, and the EP outlet gas correspond to the discharge amount (mg / H) per unit time from the waste incineration facility.

From the results shown in Table 1, it is generally recognized that the chlorine compounds in the gas are higher in the electric precipitator from the inlet side to the outlet side, and the organic chlorine compounds in the ash are higher in the electric precipitator ash than in the cooling tower ash. For example, 6 to 8 CDD (numbers are chlorine numbers), 4 to 8 CDF, 3 to
It can be seen that the chlorinated benzene of 6 and the lower chlorine compound increased remarkably in the electrostatic precipitator. For example, 8CDD is below the detection limit (1 ng / g) in cooling tower ash, but is below 150 ng / g in EP ash, and 7CDF is below the detection limit (2 ng / g) in cooling tower ash.
g) or less, EP ash is 430 ng / g, pentachlorobenzene is 5 μg / m ³ at the EP inlet side, but 15 at the EP outlet side.
It has increased to 0 μg / m ³ . However, the polycyclic aromatic hydrocarbons show a similar or decreasing tendency.

Next, from the results in Table 2, comparing the ratio of ash from the electrostatic precipitator and chlorine compounds contained in the exhaust gas, 6CDD is
87%, 8CDD is 72%, 4CDF is 95%, and 8CDF is 76% in the exhaust gas, and most of the produced chlorine compounds are released from the stack to the atmosphere. Moreover, in all cases, the higher the chlorine number, that is, the lower the vapor pressure, the higher the ratio of adsorption to ash was recognized. From the above results, it was found that when the gas temperature passing through the electrostatic precipitator is low, the chlorine compound mainly exists in ash, and when the gas temperature is high, the chlorine compound has a high abundance ratio in the exhaust gas.

Next, the present inventors added 1N hydrochloric acid to the cooling tower ash and EP ash, and examined the amount of H ₂ generated when left standing at room temperature (20 ° C.). The results are shown in FIG. H ₂ was rapidly generated for 30 min in both EP ash and cooling tower ash, and gradually increased thereafter. When the chlorine absorption amount is calculated from the H ₂ generation amount after 150 minutes, the cooling tower ash is 16 mg / g and the EP ash is 43 mg / g. It is considered that chloride corresponding to this amount was produced.

Further, the present inventors conducted a heating test by changing the heating temperature and time in vacuum and in the presence of oxygen, for a total of four types of samples of the cooling tower ash and the EP ash that were treated with hydrochloric acid and not treated with hydrochloric acid. The results are shown in Table 3.

As is clear from Table 3, both the cooling tower ash and the EP ash treated with hydrochloric acid were heated to 300 ° C. in an oxygen atmosphere (Experiment No. 17,
18, 35 and 36), an extremely large amount of chlorine compounds was produced. For example, a sample obtained by heating cooling tower ash treated with hydrochloric acid for 60 minutes in an oxygen atmosphere (experiment
From No.18), chloroform 4800ng / g, carbon tetrachloride 11000
ng / g, trichloroethylene 7700 ng / g, tetrachlorobenzene 7000 ng / g, pentachlorobenzene 19000 ng / g, and hexachlorobenzene 23000 ng / g were produced. The amounts produced were extremely low in vacuum and at 100 ° C. It was also found that chlorine compounds are produced by heating both the cooling tower ash and EP ash, although not as significantly as hydrochloric acid-treated ash.
Especially, chloroform and trichlorophenol produced a large amount even at 200 ℃. The chlorobenzenes did not form unless heated to 300 ° C.

Next, the discharge experiment of the cooling tower ash was conducted by the discharge experiment device shown in FIG. 3 simulating the electrostatic precipitator. The DC power supply (high-voltage power supply) consists of a transformer and a rectifier, and the output voltage can be changed by adjusting the voltage on the input side of the high-voltage transformer with a slider (maximum voltage −6kv, maximum current 100m.
A). One vacuum bottle was used as the reaction container, and the discharge negative electrode was inserted in the center thereof. Cooling tower ash 20 in the reaction vessel
After adding 0 mg and evacuating the interior, introducing 100 ml of oxygen and changing the discharge current, the current density of the dust collector, the temperature, the time (ash retention time), and the HCl concentration in the gas phase, Was measured. The results are shown in Table 4.
As a representative example of chlorine compounds, the relationship between the amount of pentachlorobenzene produced, discharge current, temperature, reaction time, and HCl concentration is shown in FIGS. 5 (a), (b), (c), and (d).

From these results, regarding the discharge current characteristics, carbon tetrachloride, trichloroethylene, tetrachloroethylene, tetrachlorobenzene, and pentachlorobenzene significantly increase with the increase of the discharge current value, and the chlorination reaction is promoted by the discharge. I understood. In Table 4, Experiment No. 37 at a current value of 0 shows the amount produced by the thermal reaction at 300 ° C. by the reaction system of cooling tower ash, oxygen, hydrogen chloride and steam.

Regarding the temperature characteristics, it was found that the production amount of lower chlorine compounds other than chloroform drastically increased from 200 to 400 ° C, while the production amount of chlorobenzene with chlorine number 3 to 6 drastically increased from 300 ° C to 400 ° C. .

Regarding time characteristics (ash retention time), a tendency was observed to increase over 0 to 40 minutes, but aromatic chlorine compounds (chlorinated benzene, etc. increased significantly over 40 to 60 minutes). Trichlorethylene decreased from 40 to 60 minutes.

Regarding hydrogen chloride concentration characteristics, hydrogen chloride concentration 0 to 3.65 mg
The production amount increased toward /, but no clear relation was found above that.

From the above experimental results of cooling tower ash and electrostatic precipitating ash from the refuse incineration facility, most of the organic chlorine compounds that were thought to be generated in the incinerator, that is, PCDD, PCDF, chlorinated benzene, and lower chlorine compounds Was found to form during passage through the electrostatic precipitator. According to the consideration of the present inventors, the combustion of dust is a process in which a polymer containing carbon and a solid are reduced to low molecular weight by thermal decomposition, vaporized and combined with oxygen in the air to be converted into carbon dioxide gas. . Here, when the low-molecular organic compound is not oxygen but is combined with chlorine supplied from chlorine-containing plastic or the like, an organic chlorine compound is produced. The temperature of the incinerator is about 750 to 950 ° C, and the gas of organic compounds generated by thermal decomposition is instantaneously oxidized, so there is almost no chlorination reaction, or even if chlorine compounds are generated, it is further oxidized. It On the other hand, the temperature of the electrostatic precipitator is as low as 300 ℃, and the ash retention time is long, so
It is considered that the chlorination reaction proceeds without receiving the oxidation reaction.

The problem here is what is the origin of the reaction and whether discharge is involved in the reaction. Unburned exhaust gas components of the furnace such as acetylene and benzene can be considered as the substances to be chlorinated, but these unburned components increase temporarily when incompletely burned, but at normal concentration levels around 1 ppm. It is low and it is unlikely that the unburned components in the gas phase will be the origin. As the origin substance other than the gas phase, a high molecular organic component which is not vaporized from the ash and remains may be considered. As described above, the present inventors conducted a series of experiments on the production process of organic chlorine compounds using this ash as a sample, and as shown in FIG. 2, a large amount of hydrogen was generated by the reaction between the ash and hydrochloric acid. Indicated. Many kinds of metals are contained in ash, and among them, Fe and Zn are large in amount, and these metals react with hydrochloric acid to generate H ₂ and chloride. In the case of Fe, FeCl ₂ is produced, and Fe ₃ O ₄ produces FeCl ₂ and FeCl ₃ . Further, FeCl ₂ reacts with steam to produce FeCl ₃ . As is well known, FeCl ₃ catalyzes the chlorination reaction of hydrocarbons. It is also known that chlorine is generated when heated in oxygen. In addition, a large amount of chlorine compounds were generated when the cooling tower ash treated with hydrochloric acid and EP ash were heated in an oxygen atmosphere according to the experiments conducted by the present inventors, which means that Fe contained in the ash was FeCl _{3 in the} above-described process. Changes to
It is presumed that it contributed to the chlorination of organic substances. The large amount of organic chlorine compounds produced indicates that the ash contains a large amount of these substances. It has been recognized that when the vapor pressure of the generated chlorine compound is high, most of it migrates into the gas phase and the amount adsorbed to ash decreases. On the other hand, cooling tower ash and EP ash that were not treated with hydrochloric acid
Chlorine compounds are formed when heated above 200 ℃. It is thought that this is because part of Fe in the ash had already been converted to chloride by the hydrogen chloride produced by combustion.

In addition, in the ash heating experiment, trichlorophenol was formed at 200 ° C and showed a decreasing tendency at 300 ° C, probably because the reactivity of trichlorophenol was high and it changed to other components. 4CDD if trichlorophenol is condensed
It is an interesting component because it changes to.

In the discharge experiment of cooling tower ash, it is considered that the amount of chlorine compounds produced increases with the discharge current because HCl is dissociated by the discharge and highly reactive Cl and Cl ₂ are generated to promote the chlorination reaction.

The present inventors have arrived at the present invention based on the above findings. That is, the present invention detects the concentration of an organic chlorine compound (for example, carbon tetrachloride, tetrachloroethylene, trichlorophenol, chlorobenzenes, etc.) in the exhaust gas at the outlet of the electrostatic precipitator, and the detected value is a predetermined value. By controlling at least the gas concentration at the entrance of the electrostatic precipitator so as to be below the value, the generation of harmful organic chlorine compounds is suppressed below a predetermined value. If the gas concentration at the entrance of the electrostatic precipitator is low, the dust collection efficiency tends to decrease.
From the viewpoint of suppressing organic chlorine compounds, the temperature is preferably 250 ° C or lower, more preferably 100 to 199 ° C. If the temperature does not reach 100 ° C, the dust collection efficiency decreases and it becomes impractical. Further, the temperature range of around 250 ° C. which has been conventionally performed is also applicable, but it may be difficult to reduce the organic chlorine compound in the exhaust gas to a predetermined value or less only by controlling the temperature. It is necessary to reduce the current and the ash residence time. Hereinafter, the present invention will be described in more detail with reference to the drawings.

FIG. 1 is a diagram showing a flow of a method for treating exhaust gas from an municipal waste incinerator showing an embodiment of the present invention. The exhaust gas line of the electrostatic precipitator is provided with an organic chlorine compound detection device 1 and a control device 7. The control device 7 receives the signal from the detection device 1 via the control line 4, compares the organochlorine compound concentration with a set value, and when this exceeds the set value, the control device 7 sends the signal to the gas cooling tower via the control line 3. Send a signal to increase the cooling capacity of the gas cooling tower. The control device 7 is also connected to the electrostatic precipitator voltage adjusting device via the line 5 so that the discharge power supply can be adjusted. The control device 7 is also connected to a hammering cycle adjusting device of the electrostatic precipitator via a line 9 so that the ash retention time can be adjusted by changing the cycle.

In the above configuration, first, the concentration of the organic chlorine compound in the exhaust gas from the outlet of the electrostatic precipitator is detected, and, for example, the cooling capacity (cooling water amount) of the gas cooling tower is increased so that the detected value becomes a predetermined value or less. However, if this is insufficient or because of economical reasons, the discharge current of the electrostatic precipitator or / and the residence time of ash are further changed so that the concentration of the organochlorine compound in the exhaust gas falls below a predetermined value. To control.

In the present invention, the temperature of the exhaust gas introduced into the electrostatic precipitator (gas temperature at the gas cooling tower outlet) is 25
The temperature is preferably 0 ° C or lower, more preferably 100 to 199 ° C. 250 ° C
If it exceeds, the temperature may not be enough to control the organic chlorine compound in the exhaust gas from the electrostatic precipitator outlet to a predetermined value or less. Further, when the gas temperature is less than 100 ° C, water vapor is condensed, and the ash collected in the electrostatic precipitator is less likely to fall off, and the dust collection efficiency is reduced.

Gas chromatography is suitable as the organic chlorine compound detection device 1 used in the present invention.

As means for adjusting the outlet gas temperature of the gas cooling tower from the control signal 3 from the control device 7, for example, the amount of cooling water in the cooling tower may be increased or decreased by a valve or the like, whereby the exhaust gas introduced into the electrostatic precipitator. The temperature can be maintained at a predetermined temperature. A boiler or the like may be used instead of the cooling tower. Further, in order to suppress the generation of the organochlorine compound by the discharge current (or the current density of the dust collector) of the electrostatic precipitator, the voltage (current) of the electrostatic precipitator is similarly supplied from the control device 7 via the control line 5. It suffices to send a signal to the adjusting device and adjust the current (or current density) so as to be below a predetermined value. The control of the reaction time (ash retention time) in the electrostatic precipitator is performed by the control device 7 via the control line 9 such as the reaction time control element of the electrostatic precipitator, for example, the number of hammers (cycles) of the electrostatic precipitator. ) Should be increased or decreased.

In the present invention, the predetermined value of the organic chlorine compound in the exhaust gas, which serves as a reference for the set value of the control device 1, can be appropriately determined according to the allowable amount of the organic chlorine compound in the exhaust gas. Organochlorine compounds to be detected include highly toxic PCDD or
PCDF etc. may be measured directly, but sampling of these,
Since analysis may be difficult, therefore, substances that are relatively easy to analyze, such as carbon tetrachloride, tetrachloroethylene, trichlorophenol, chlorobenzenes, etc., corresponding to the generated amounts thereof may be used as an index for measurement.

In December 1983, the Ministry of Health and Welfare established the “Dioxin Expert Meeting on Waste Management” and in May 1984, the expert meeting report was compiled. Based on the survey results, the maximum concentration of dibenzodioxins (TCDDs) in exhaust gas was set to 1250 ng / Nm ³ and the maximum daily intake of humans through the atmosphere was 1.58 × 10 ^-3 ng considering the diffusion in the atmosphere. Calculated as / kg / day and calculated this as TCDD
In comparison with the 0.1 ng / kg / day that was set for 2,3,7,8-TCDD, which is said to be highly toxic among the , 2,3,7,8-dibenzodioxin tetrachloride conversion value may be 1250 ng / Nm ³ or less, and the emission standard of other indicator substances may be less than or equal to the corresponding value.

As the control elements in the present invention, the inlet gas temperature of the electrostatic precipitator, the discharge current (or current density), and the ash residence time were taken up. As shown in FIG. Since the HCl concentration in the dust inlet gas is also related to the formation of organic chlorine compounds, it can be used as an index.

Although FIG. 1 shows a method of continuously monitoring and continuously controlling the amount (concentration) of the organic chlorine compound in the exhaust gas, the present invention is not limited to continuous monitoring, When the concentration of the organic chlorine compound is relatively stable, the concentration of the organic chlorine compound may be measured intermittently or periodically after the first condition is set and controlled accordingly.

(Examples) Hereinafter, the present invention will be specifically described with reference to Examples.

Example 1 Using the refuse incinerator exhaust gas treatment equipment shown in FIG. 1, the gas temperature at the entrance of the electrostatic precipitator was changed to 150 to 300 ° C., and the concentration of the organochlorine compound in the exhaust gas at the entrance and exit of the electrostatic precipitator, The concentrations of organic chlorine compounds in the cooling tower ash and EP ash were measured. The results are shown in Table 5 (Part 1) (Part 2) and Table 6 (Part 1) (Part 2).

Further, the relationship between the temperature of the inlet of the electrostatic precipitator and the concentration of the organic chlorine compound in the exhaust gas of the outlet of the electrostatic precipitator shown in Table 5 is shown in Fig. 6 (a), (b), (c), (d), (e). , (F),
It shows in (g).

From the results shown in Table 5, taking pentachlorobenzene as an organic chlorine compound that serves as an index, and determining the gas temperature at the inlet of the electrostatic precipitator such that this organic chlorine compound is, for example, 100 μg / m ³ or less, is 250 ° C or less. It can be seen that it is. The concentration of this indicator organochlorine compound was regularly detected at the outlet of the electrostatic precipitator, while operating at an inlet gas temperature of the electrostatic precipitator of 250 ° C or lower. It was confirmed that the concentration of the indicator organochlorine compound was not more than the above-mentioned predetermined value. Further, when the concentration of the index organochlorine compound exceeds the set value, the concentration of the organochlorine compound could be adjusted to a predetermined value or less by increasing the cooling water amount of the gas cooling tower and lowering the temperature. . From the analysis results of EP ash, it was confirmed that the concentration of pentachlorobenzene was almost constant, and that the concentration in EP ash did not rise even if the concentration of organic chlorine compounds in the gas was reduced. Furthermore, such a phenomenon was also confirmed with other organic chlorine compounds. It should be noted that, as the temperature of the gas at the entrance of the electrostatic precipitator decreases, the dust collection efficiency decreases. Therefore, it has been found that it is preferable to keep the temperature as high as possible as long as the organic chlorine compound in the exhaust gas is below a predetermined value.

Therefore, the gas temperature at the entrance of the electrostatic precipitator is set to 20 compared to the above case.
℃ higher, whereas, as a result of reducing the current density of the dust collecting electrode by 0.2 mA / m ^2, to control the index organochlorine compound concentration of the as well as in electrostatic precipitator outlet gas below a predetermined value did it. It was also confirmed that the concentration of the index organochlorine compound can be controlled to be equal to or lower than a predetermined value also by changing the residence time by increasing the number of hammers of the dust collecting electrode instead of changing the discharge current.

(Effects of the Invention) According to the present invention, by controlling the inlet gas temperature of the electrostatic precipitator, the discharge current (current density of the dust collecting electrode), the residence time of the ash, etc., the harmful gas discharged from the refuse incinerator is harmful. It is possible to reduce the concentration of the organochlorine compound to the allowable value or less. In addition, by controlling the gas temperature at the entrance of the electrostatic precipitator and the discharge current (electrode density) or (and) gas residence time in combination, the organic chlorine compounds in the exhaust gas are allowed to reach the allowable value without significantly reducing the dust collection efficiency. It can be reduced to the following.

[Brief description of drawings]

FIG. 1 is a diagram showing a schematic flow of a method for treating exhaust gas from an municipal waste incinerator according to the present invention, and FIG. 2 is a diagram showing reaction time and H ₂ production amount when cooling tower ash and EP ash are treated with hydrochloric acid, respectively. FIG. 3 is a diagram showing a relationship, FIG. 3 is an explanatory diagram showing an experimental apparatus simulating an electric precipitator according to the present invention, and FIG. 4 is a diagram showing a schematic flow of a conventional method for treating exhaust gas from an MSW incinerator, FIG. 5 (a), (b), (c), (d) and FIG. 6 (a),
(B), (c), (d), (e), (f), (g)
It is a figure which shows the experimental result of this invention, respectively. 1 ... Chlorine compound detection device 3, 4, 5, 9 ... Control line, 7 ... Control device.

Front page continued (72) Inventor Sosaku Yamada 5-417-249 Makuhari-cho, Chiba-shi, Chiba Sharman 1-602 Makuhari (72) Inventor Hiroaki Harada 318-151 Minami-Nakone, Kasukabe, Saitama 151 (72) Inventor Gentaro Takasuka 4-19 Hamamatsucho, Nishi-ku, Yokohama-shi, Kanagawa

Claims (6)

[Claims] 1. A method for treating municipal waste incinerator exhaust gas, which comprises removing dust with an electric dust collector after cooling the waste incinerator exhaust gas,
The amount of organic chlorine compounds in the exhaust gas of the electrostatic precipitator outlet is detected, and the inlet gas temperature of the electrostatic precipitator is controlled so that the detected value becomes a predetermined value or less. Processing method. 2. A method for treating waste gas from a refuse incinerator according to claim 1, wherein an inlet gas temperature of the electrostatic precipitator is controlled to be 250 ° C. or lower. 3. A method for treating municipal waste incinerator exhaust gas, which comprises removing dust with an electric dust collector after cooling the waste incinerator exhaust gas,
Detecting the amount of organic chlorine compounds in the exhaust gas from the outlet of the electrostatic precipitator, and controlling the inlet gas temperature of the electrostatic precipitator and the current density of the precipitator so that the detected value is below a predetermined value. Waste incinerator exhaust gas treatment method. 4. The method of treating exhaust gas from a refuse incinerator according to claim 3, wherein the inlet gas temperature of the electrostatic precipitator is controlled to 250 ° C. or less and the current density of the dust collector is controlled to 0.8 mA / m ² or less. . 5. A method for treating municipal waste incinerator exhaust gas, which comprises removing dust with an electric dust collector after cooling the waste incinerator exhaust gas,
It is characterized by detecting the amount of an organic chlorine compound in the exhaust gas from the outlet of the electrostatic precipitator and controlling the inlet gas temperature of the electrostatic precipitator and the residence time of the ash so that the detected value becomes a predetermined value or less. Waste incinerator exhaust gas treatment method. 6. A method of treating exhaust gas from a refuse incinerator according to claim 5, wherein an inlet gas temperature of the electrostatic precipitator is 250 ° C. or lower and a ash residence time is 40 minutes or shorter.