JPS6388023A - Removal of mercury in exhaust gas - Google Patents

Abstract

PURPOSE:To prevent the thermal decomposition of hypochlorite and to efficiently remove Hg, by cooling Hg-containing exhaust gas in a precooling part and subsequently bringing the cooled exhaust gas into contact with a washing solution having hypochlorite added thereto. CONSTITUTION:The Hg-containing exhaust gas issued from a garbage incinerator 1 at 800 deg.C is cooled to 300 deg.C in an exhaust gas cooler 2 and dust is removed by an electric precipitator 3. Subsequently, the exhaust gas is supplied to a precooling tower 4 to be cooled to 80 deg.C by the NaOH-containing washing solution from a nozzle 8 and subsequently introduced into a washing tower 5 where a NaClO washing solution is sprayed to the exhaust gas from a nozzle 9 to remove Hg. Next, the exhaust gas enters a dehumidifying cooling part 15 through a mist separator 13 and is cooled by cooling water sprayed from a nozzle to be discharged through a mist separator 18.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for removing mercury from exhaust gas exhausted from a garbage incinerator or the like.

[Conventional technology] The exhaust gas from garbage incinerators contains harmful gases such as hydrogen chloride (HCl) and sulfur dioxide (SO2), which are subject to legal regulations. or wet method.

However, the exhaust gas contains mercury and its compounds in addition to the above-mentioned harmful gases, and the content may exceed the working environment standard value C0.05Wg/rn3). Sufficient mercury removal cannot be expected with this removal method. This is presumed to be due to the following reasons. In other words, although most of the mercury in exhaust gas is water-soluble mercury, the reason why mercury cannot be removed by conventional wet cleaning processes is because the cleaning solution itself absorbs reducing substances that are not sufficiently oxidized. This is to reduce and revolatilize the mercury compounds absorbed in the cleaning solution. Therefore, in order to prevent this re-volatilization, it is necessary to use an oxidizing agent to counteract the reducing property of the cleaning liquid.

For this reason, a wet smoke cleaning method has been developed in which a cleaning solution containing hypochlorite (for example, sodium hypochlorite) is brought into contact with exhaust gas to remove mercury from the exhaust gas. This wet smoke cleaning method is based on the following principle. First, as shown in equations (1) to (4) below, sodium hypochlorite acts on mercury other than water-soluble mercury (mainly metallic mercury), resulting in Hg
O is generated.

NaCf0−NaCJ+O=11) Na C1! O+HCJ! -Na C1! +HCl0-
A2') HCNO→HCNO+0...(3) Hg+O→HgO...(4) The HgO generated by the above formula (4) is expressed as follows (5), (6
) is stabilized in smoke washing water by reaction.

Hg O+2HC1→HgCノ2 +H20...(5
) Hg CI2 +2Na C No Na 2 Hg C
f4...(6) By the way, in order to put the above-mentioned wet smoke cleaning method into practical use as a system, it is conceivable to create a system similar to that used for general removal of harmful gases such as hydrogen chloride and sulfur dioxide. That is, using a wet type smoke cleaning device consisting of a garbage incinerator, an exhaust gas cooler (heat exchanger), an electrostatic precipitator, a pre-cooling section, and a cleaning tower, the cleaning liquid stored in the bottom of the cleaning tower is transferred to the pre-cooling section. A system is conceivable in which hypochlorite, which is involved in the removal of mercury described in (1) to (4), is added to the cleaning liquid sprayed into each pre-cooling section and the cleaning tower.

[Problems to be Solved by the Invention] However, the system sprays a cleaning liquid to which hypochlorite has been added to a pre-cooling section that pre-cools exhaust gas.

In this system, the temperature of the exhaust gas introduced into the pre-cooling section is as high as around 300°C, so hypochlorite is decomposed and volatilized in a single hour without being involved in the removal of mercury from the exhaust gas. As a result, there was a problem in that it was necessary to add a large amount of hypochlorite to remove mercury from the exhaust gas.

The present invention was made to solve the above-mentioned conventional problems, and when removing mercury by wet smoke cleaning in which mercury-containing exhaust gas is brought into contact with a cleaning solution containing hypochlorite, the present invention has been made to solve the above-mentioned conventional problems. Chlorite.

The purpose of the present invention is to provide a method for removing mercury from exhaust gas that can efficiently remove mercury by adding .

[Means for Solving the Problems] The present invention provides a method for removing mercury by wet smoke washing in which mercury-containing exhaust gas is brought into contact with a cleaning liquid to which hypochlorite is added, in which the mercury-containing exhaust gas is cooled in a pre-cooling section. This is a method for removing mercury from exhaust gas, which comprises bringing the cooled exhaust gas into contact with a cleaning liquid to which hypochlorite has been added to remove mercury from the gas.

[Function] In the present invention, the cooled exhaust gas after cooling the mercury-containing exhaust gas in a pre-cooling section is brought into contact with a cleaning liquid to which hypochlorite is added, so that hypochlorite does not participate in the oxidation of mercury. Since the loss of hypochlorite can be reduced by suppressing thermal decomposition, mercury in exhaust gas can be efficiently removed while the total amount of hypochlorite supplied is small.

[Embodiments of the invention] Hereinafter, the present invention will be explained in detail with reference to FIG.

Embodiment FIG. 1 is a schematic diagram showing one form of a two-unit wet type smoke cleaning device equipped with a pre-cooling section and a cleaning tower, and numeral 1 in the figure is a waste incinerator. The exhaust gas from the incinerator 1 is supplied to a precooling section 4 through an exhaust gas cooler 2 and an electrostatic precipitator 3. This precooling section 4 is connected to an adjacent washing tower 5.

At the bottom of the cleaning tower 5, a cleaning liquid 6 mainly containing sodium hydroxide is stored. This cleaning liquid is supplied to the first nozzle 8 of the pre-cooling section 4 through the pipe 7a, and from the nozzle 8, the cleaning liquid is sprayed as a cooling liquid onto the exhaust gas supplied into the pre-cooling section 4. A second nozzle 9 is arranged near the middle of the cleaning tower 5, and a cleaning liquid is supplied to this nozzle 9 through a pipe 7b branched from the pipe 7a. In addition, hypochlorite (for example, Na
Cf0) is supplied. Note that the cleaning liquid 6 stored at the bottom of the cleaning tower 5 is replenished with sodium hydroxide (NaOH), and the amount of sodium hydroxide to be replenished is determined by the amount of sodium hydroxide measured from the pH measuring meter lO that measures the pH value of the piping 7a. It is controlled by a pump 11 to which an output signal is input. In addition, the cleaning liquid 6 contains NaCJ! When the concentration rises to a predetermined value or higher, it is extracted through the pipe 7c and supplied to the wastewater treatment device 12.

A first mist separator 13 is arranged in the cleaning tower 5 above the second nozzle 9. A dehumidifying cooling section 15 having a third nozzle 14 for injecting cooling water is arranged in the cleaning tower 5 above the mist separator 13. The treated cooling water from the dehumidifying cooling unit 15 is introduced into the cooling water storage tank IB, and further supplied to the cooling tower 17. The cooling water from the cooling tower (7) is supplied to the third nozzle 14. In the cleaning tower 5 above the dehumidification cooling section 15, a second
A mist separator 18 is arranged.

Next, the mercury removal method of the present invention will be explained using the above-mentioned wet smoke cleaning device.

First, the mercury-containing exhaust gas discharged from the garbage incinerator 1 at a temperature of 800'C was cooled to 300''C in the exhaust gas cooler 2-, and then introduced into the electrostatic precipitator 3 to remove dust, etc. from the exhaust gas. Next, the exhaust gas collected from the dust collector 3 was supplied to the precooling base 4, where it was rapidly cooled to about 80° C. by a cleaning liquid mainly containing sodium hydroxide injected from the first nozzle 8. The cooled exhaust gas from the precooling base 4 is introduced into the cleaning tower 5, where the cooled exhaust gas is passed through a second nozzle 9.
The mercury was removed by spraying a cleaning solution to which Na 2 CO was added. Next, the process gas in the cleaning tower 5 is
The mist separator 13 transferred to the dehumidifying cooling unit 15, where it was cooled by cooling water (tap water) sprayed from a third nozzle, and further passed through the second mist separator 18 and discharged into the atmosphere through a chimney. Note that the conditions in this exhaust gas treatment step are as follows.

■Exhaust gas 2; 4577Z 3/hr ■Mercury 2 in exhaust gas; o, 5η/N77L3 ■Liquid gas ratio in precooling tower; 3
No. 1 ■Liquid-gas ratio in the cleaning section: 3 NO/TIL3 ■Processing gas temperature after cleaning: 72°C ■Processing gas temperature after cooling: 60'C Comparative example The cleaning liquid sprayed from the nozzle of the pre-cooling unit also had the following Mercury was removed from the exhaust gas in the same manner as in the example except that sodium chlorite was added.

Therefore, in a comparative example, we investigated the mercury removal rate in the treated gas exhausted from the cleaning tower with respect to the added amount of sodium hypochlorite in the cleaning liquid sprayed in the precooling group and the cleaning tower, and the results are shown in Figure 2. A characteristic diagram was obtained. In addition, in this example, when we investigated the mercury removal rate in the treated gas exhausted from the cleaning tower with respect to the added amount of sodium hypochlorite in the cleaning liquid sprayed in the cleaning tower, the characteristic diagram shown in Figure 3 was obtained. Obtained. Furthermore,
From the results shown in FIGS. 2 and 3, the mercury removal rate in the treated gas with respect to the total amount of sodium hypochlorite (Na C1o) was investigated, and the characteristic diagram shown in FIG. 4 was obtained. In addition, A in FIG. V is the characteristic line of this example, and B is the characteristic line of the comparative example. As shown in FIGS. 2 to 4, it can be seen that in this example, mercury in the exhaust gas can be efficiently removed with a smaller amount of sodium hypochlorite than in the comparative example.

In the above embodiment, the cleaning liquid stored in the bottom of the cleaning tower was used as the cleaning liquid sprayed from the pre-cooling base and the nozzle of the cleaning tower, but the present invention is not limited thereto. For example, as in the wet type smoke cleaning device shown in FIG.
It is also possible to spray from the nozzle 8. in this case,
Pipes 7a and 7b for introducing cleaning liquid into the precooling base 4 and the nozzle 8.9 of the cleaning tower 5 are arranged separately. Furthermore, the cleaning liquids 6' and 6 of the pre-cooling base 4 and the cleaning tower 5 are replenished with hydraulic soda, respectively, and the amount of replenishment of the hydraulic soda is the same as that of the piping 7a,
The output signal from the pH measuring meter 10'', 1o, which measures the pH value of 7b, is controlled by a manually operated pump, 11.Furthermore, the cleaning liquid 6.6' has a predetermined concentration of NaC therein. When the wastewater is raised to the second level, it is pulled out through the pipes 7c and 7c- and supplied to the wastewater treatment device 12.

In the wet type smoke cleaning equipment shown in Figure 1 and the wet type smoke cleaning equipment shown in Figure 2 used in the two examples, a cleaning solution to which sodium hypochlorite was added was sprayed through a nozzle in the cleaning tower. For example, the nozzles are arranged in two stages, and after the cleaning liquid is sprayed from the upper nozzle to cool the exhaust gas, the cleaning liquid added with sodium hypochlorite is sprayed into the cooled exhaust gas from the lower nozzle. Mercury removal may also be performed.

Although the wet smoke scrubbers shown in FIGS. 1 and 2 have a two-unit structure, the present invention is not limited to the two-unit structure. For example, as shown in FIG. 6, the washing tower 5 may have a structure in which a precooling section 19 in which a first nozzle 8 is disposed is provided on the lower side wall of the washing tower 5. However, in Fig. 6, the incinerator, exhaust gas cooler, and electric precipitator are omitted.

[Effects of the Invention] As detailed above, according to the present invention, when removing mercury by wet smoke cleaning in which mercury-containing exhaust gas from a garbage incinerator or the like is brought into contact with a cleaning solution containing hypochlorite, It is possible to provide a method for removing mercury from exhaust gas, which can efficiently remove mercury with the addition of a small amount of hypochlorite and has remarkable effects such as reducing running costs.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram showing one form of wet smoke cleaning equipment used in the method for removing mercury from exhaust gas of the present invention, and Figure 2 is a treatment for the amount of sodium hypochlorite added in the method for removing mercury in a comparative example. Figure 3 is a characteristic diagram showing the relationship between the mercury removal rate in the gas, and Figure 4 is a characteristic diagram showing the relationship between the mercury removal rate in the treated gas and the amount of sodium hypochlorite added in the mercury removal method of the example. is total sodium hypochlorite (
A characteristic diagram showing the relationship between the mercury removal rate in the process gas and Na C, l'o)H, and FIGS. 5 and 6 respectively show other forms of the wet smoke scrubbing device used in the mercury removal method of the present invention. FIG. 1... Incinerator, 2... Exhaust gas cooler, 3... Electrostatic precipitator, 4... Pre-cooling section, 5... Washing tower, 6.6゛・
...Cleaning liquid, 8.9.14... Nozzle, 12... Waste water treatment device, 15... Dehumidification cooling section, 19... Pre-cooling section. Applicant's agent Patent attorney Takehiko Suzue No. 1vjA

Claims (2)

[Claims] (1) In a method of removing mercury by wet smoke cleaning in which mercury-containing exhaust gas is brought into contact with a cleaning solution to which hypochlorite has been added, hypochlorous acid is added to the cooled exhaust gas after cooling the mercury-containing exhaust gas in a pre-cooling section. A method for removing mercury from exhaust gas, which comprises removing mercury from the gas by contacting with a cleaning liquid to which salt has been added. (2) Removal of mercury from exhaust gas according to claim 1, characterized in that the cleaning liquid to which hypochlorite has been added is sprayed into the cooled exhaust gas of a cleaning tower placed downstream of the precooling tower. Method.