JPS6233530A - Method and apparatus for removing sulfur oxide in waste gas - Google Patents

Abstract

PURPOSE:To decrease COD in waste water and to obtain a device which reduces driving power consumption by providing an oxidation part into a liquid tank in the bottom of an absorption column and returning the required amt. of the absorbent liquid which thoroughly oxidizes sulfite ions to the liquid tank by utilizing the difference in the liquid level in the stage of using magnesium hydroxide, etc., as an absorbent. CONSTITUTION:The waste gas introduced through an inlet 14 contacts with the absorbent liquid which contains magnesium hydroxide, magnesium oxide or magnesium carbonate and is sprayed from a spray device 3. The clean waste gas is discharged from an outlet 15. On the other hand, the absorbent liquid falls down and flows into an absorbent liquid tank 6 and an oxidation tank 7 according to the sectional area ratio partitioned by a partition plate 5 in common use as a weir. Air, etc., is supplied to the waste gas in the tank 7 to thoroughly oxidize the magnesium sulfite to the magnesium sulfate. Such absorbent liquid is partly discharged therefrom and is partly circulated to the tank 6 while the flow rate thereof is controlled with a flow rate control valve 12 by utilizing the difference in the height of the liquid level. The absorbent liquid in the tank 6 is circulated through a circulation line to the spray device 3.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention is a wet method for efficiently and economically removing sulfur oxides from exhaust gas using magnesium hydroxide, magnesium oxide, or magnesium carbonate as an absorbent. This invention relates to a flue gas desulfurization method and device.

[Conventional technology]

In wet flue gas desulfurization equipment that uses magnesium hydroxide, magnesium oxide, or magnesium carbonate as an absorbent, oxidize low-solubility magnesium sulfite to highly soluble magnesium sulfate to prevent formation of magnesium sulfite crystals in the absorption liquid. The method is well known and is an excellent method that has advantages such as easy operation and maintenance, and no scaling problems.

As the above-mentioned prior art, for example, (1) Unexamined Japanese Patent Publication No. 52-8
As shown in Japanese Patent Publication No. 1068 and Japanese Patent Publication No. 60-18449, absorption of SC2 and oxidation of sulfite are carried out in one column; Publication No. 81068, JP-A-57-42
As shown in Publication No. 13 and Japanese Utility Model Publication No. 55-23622, a system is known in which SO2 absorption and sulfite oxidation are carried out by providing independent columns.

As shown in Figure 3, method (1) is based on the SC2 in the exhaust gas.
The absorption of sulfite and the oxidation of sulfite are carried out in the absorption tower 1, and there are two methods: one in which sulfite ions are completely oxidized and one in which a portion of sulfite ions remain.

2 is a circulation pump, and 3 is a spray device.

In the method (2), as shown in Fig. 5, an oxidation tower 4 is installed in addition to the absorption tower 1, and the absorption liquid can be circulated between the absorption tower 1 and the oxidation tower 4 through connecting pipes 9a and 9b. The sulfite ion concentration in the absorption liquid is as follows:
This system allows adjustment by adjusting the amount of circulating liquid between the absorption tower 1 and the oxidation tower 4.

[Problem that the invention seeks to solve]

Of the above methods (1), the method of completely oxidizing sulfite ions involves adding a catalyst to completely oxidize to lower the equilibrium partial pressure of SC2, which lowers the COD in the wastewater but does not remove SO2 from the exhaust gas. There is a drawback that the amount of washing liquid (L/G) in the nodame becomes large. In addition, in the method where some sulfite ions remain, the amount of washing liquid (L/G) for removing SO2 is small because sulfite ions are present, but the COD in the wastewater becomes approximately 5 ooppm or more, and wastewater regulations are in place. In severe cases, it may be necessary to install a separate oxidation device.

On the other hand, method (2) can demonstrate stable desulfurization performance against fluctuations in the amount of absorbed SO7, and also reduce COD in wastewater.
However, on the other hand, power is required to circulate the liquid between the absorption tower and the oxidation tower, which requires a high cost of $02.
There is a drawback that increases as absorption increases.

The present invention was made in view of the above points, and a partition plate is provided in a part of the liquid tank at the bottom of the absorption tower to form an oxidizing section, and the absorption liquid after completely oxidizing sulfite ions in this oxidizing section is By utilizing the difference in liquid level between the oxidizing section and the bottom liquid tank, the necessary amount of liquid is sent from the oxidizing section to the absorption tower bottom liquid tank. The object of the present invention is to provide a method and an apparatus for the same, which can reduce the amount of energy consumed, reduce power consumption, and reduce the amount of absorbent.

[Failure to solve the problem]

The method for removing sulfur oxides from exhaust gas according to the present invention will be described based on FIGS. 1 and 2. Magnesium hydroxide, magnesium oxide, and magnesium carbonate, singly or in combination, are supplied to the absorption tower 1 as an absorbent. Then, the magnesium sulfite produced by absorbing sulfur oxides in the exhaust gas introduced into the absorption tower 1 is oxidized to magnesium sulfate so that crystals of magnesium sulfite do not precipitate in the absorption liquid, and a part of it is discharged. In the method, the bottom of the absorption tower 1 is divided into a total absorption liquid tank 6 and an oxidation tank 7, and the absorption liquid in the absorption liquid tank 6 is circulated to the top of the absorption tower 1 and sprayed to contact with the exhaust gas,
A part of the sprayed absorption liquid flows into the oxidation tank 7, and the remaining part of the absorption liquid flows into the absorption liquid tank 6. On the other hand, an oxygen-containing gas such as air is supplied to the oxidation tank 7 to reduce the concentration of the absorption liquid in the absorption liquid. Magnesium sulfite is oxidized to magnesium sulfate, part of the liquid in oxidation tank 7 is discharged, and oxidation 41! ! i7
A part of the liquid in the tank is circulated to the absorption liquid tank 6 while adjusting the flow rate by utilizing the difference in liquid level between the absorption liquid tank 6 and the oxidation tank 7.

Further, the apparatus of the present invention will be described based on FIGS. 1 and 2. A vertical weir-cum-partition plate 5 is provided at the bottom of the absorption tower 1 to partition it into an absorption 'Wl tank 6 and an oxidation tank 7. The bottom of the absorption liquid tank 6 and the spray device 3 provided in the upper part of the absorption tower 1 are connected by an absorption liquid circulation line 8 having a circulation pump 2, while an oxygen-containing gas supply pipe 10 is connected to the bottom of the oxidation tank 7. , a discharge pipe 11 is connected to the oxidizing tank 7, and an oxidizing liquid circulation line 13 having a flow rate control valve 12 connects the oxidizing tank 7 and the absorption liquid tank 6, and this oxidizing liquid circulation line 13 has a circulation pump. First, the oxidizing liquid is circulated from the oxidizing tank 7 to the absorbing liquid tank 6 by utilizing the difference in liquid level between the absorbing liquid tank 6 and the oxidizing tank 7.

14 is an exhaust gas inlet, 15 is a clean exhaust gas outlet, 16 is an agitator, 17 is a flow rate regulator, and 18 is an absorbent slurry supply pipe.

The location of the weir-cum-partition plate 5 is selected so that the amount of oxidizing liquid that is greater than the amount of oxidizing liquid transferred from the oxidizing tank 7 to the absorption liquid tank 6 by the oxidizing liquid circulation line 13 can be collected in the oxidizing tank 7. The liquid level is always maintained at the upper end of the weir/partition plate 5.

[Effect]

An absorbent consisting of a slurry of magnesium hydroxide, magnesium oxide, or magnesium carbonate is supplied into the absorption liquid tank 6, and the absorption liquid is circulated and sprayed to the spray device 3 through the absorption liquid circulation line 8, and the introduced exhaust gas and air are Contact with liquid S
Absorb O2. This absorption liquid falls and flows into the absorption liquid tank 6 and the oxidation tank 7 according to the partitioned cross-sectional area ratio. An oxygen-containing gas such as air is supplied to the oxidation tank 7 to completely oxidize the magnesium sulfite in the absorption liquid to magnesium sulfate, a part of which is discharged to the outside of the yarn through the discharge pipe 11, and a part of which is transferred to the absorption liquid tank 6. absorbing liquid tank 6 by utilizing the difference in liquid level between the
It circulates while adjusting the flow rate with the flow rate control valve 12. A flow rate regulator 17 provided in the oxidizing liquid circulation line 13 is connected to this flow rate regulating valve 12, and load signals such as fuel consumption and S○2 concentration in exhaust gas are input to the flow rate regulator 17. It is configured such that the circulating amount of the oxidizing liquid can be adjusted by operating the flow control valve 12. The purpose of adjusting the flow rate of the oxidizing liquid circulation line is to ensure stable desulfurization performance against large load changes.If there is almost no load change, the flow rate regulator of the oxidizing liquid circulation line should be Needless to say, it does not need to be provided.

〔Example〕

Examples and comparative examples will be described below.

Example An apparatus having the structure shown in Fig. 1 and Fig. 2 was used, and the inner diameter of the column was 7.
A heavy oil-fired boiler exhaust gas (SO2: 850pp
m) was introduced and treated. Gas amount is 10,000~20
．． 000 N, the amount of circulating absorption liquid was 60 d/H1, the pH of the absorption liquid was 5.5 to 6.5, and the Mg concentration in the absorption liquid was 8% by weight. The results are shown in Tables 1 and 2. Table 1 shows the case where the gas amount was fixed at 20,000 Ni/H, and Table 2 shows the case where the pH was fixed at 6.0.

(Margin below) Table 1 Table 2 In all of the above examples, the COD of the absorption liquid was 1,000 to 2,000 ppm Ninal! Although the amount of in-ring oxide was adjusted, no crystals of magnesium sulfite were observed in the absorption liquid.

Comparative Example Using the conventional apparatus shown in FIG.
) was tested. As a result, as shown in FIG.
In order to obtain pm, the COD of the absorption liquid (same as the wastewater) was required to be 0001)];) m or more. This means that the COD of the wastewater is also 1,000 pIlrf1 or more.

As shown in the examples, according to the present invention, a small liquid
It has the advantage that high desulfurization performance can be obtained at the gas ratio (L/G), the COD in the waste water is low, and no power is required for the oxidizing liquid v6 ring. When absorption and oxidation in the prior art are carried out in an integrated column (the case shown in Figure 3), the C of the absorption liquid (drainage water) to obtain the same high desulfurization performance as in the present invention.
The OD becomes high at 500 ppm or more. The reason is that the general desulfurization method is based on the chemical reaction equation (1) below, in which SO3, which is a COD source, reacts with SO2 dissolved in the liquid to desulfurize, so the SO3 concentration increases. If it decreases, the desulfurization performance will decrease.

S○3+SO2+H20→2HS○s (
1) 'For this reason, in order to obtain high desulfurization performance with low SO3 concentration, L/G must be increased, and in the method of completely oxidizing SO3 using a catalyst, the chemical reaction of equation (1) It can be regarded as physical absorption rather than absorption by
Furthermore, L/G must be increased.

In other words, normal physical absorption is as shown in equation (2), S○
Due to the absorption of the two gases by dissolution in water, the pH is lowered by the d-type sulfite ions produced, and the pressure of the S2 normal fraction increases, resulting in extremely low desulfurization performance. Therefore, the catalyst f is added to oxidize the acidic sulfite ions shown in equation (3) to sulfate ions, lowering the S○2 equilibrium partial pressure and improving the desulfurization performance, but the oxidation reaction shown in equation (3) This is due to the fact that the rate of reaction is slower than that of the chemical reaction of formula (1) above.

So2+ E(20-) H2SO4:H+HSO3(
2) HS○3+ + 1/202→H+S○a-, (3
) Mg(OH)++2H++504-MgSO4+2H
zO(4) Equation (4) shows a neutralization reaction with magnesium hydroxide.

On the other hand, in the conventional technology shown in Fig. 5, in which absorption and oxidation are performed in separate towers, power is required to circulate the absorption tower and the oxidation tower, but in the present invention, the difference in liquid level is used. , no power is required.

〔Effect of the invention〕

Since the present invention is configured as described above, it has various effects as described below.

(1) Compared to the conventional integrated type case (Figure 3), sulfite ions are present in the absorption tower washing liquid, and the sulfite ions can be easily adjusted according to the absorbed SO2 level. . Therefore, stable desulfurization performance can be obtained against fluctuations in the amount of absorbed SO2 (load, sulfur content in fuel, etc.).

(2) Compared to the conventional integrated type case (Fig. 3), SO2 is chemically absorbed by sulfite ions, and the amount of washing liquid (L/G'') in the absorption tower can be small.

(3) Compared to the conventional integrated type case (Figure 3), there is almost no COD in the wastewater extracted from the oxidation tank, and there is no need to install a separate oxidation device to meet strict wastewater regulations, making it easy to comply with. I can do it.

(4) Compared to the conventional stand-alone type case (Figure 5), there is no need for liquid circulation between the absorption tower and the oxidation tower, and only liquid transfer using height differences is required, reducing power consumption. .

(5) Compared to the conventional independent type (Figure 5), the absorption liquid tank and oxidation tank are integrated in one tower, making it more compact and reducing the installation area. .

(6) Since the oxidation tank is completely separated from the absorption tank,
There is no short bath of the absorbent supplied to the absorption liquid tank into the waste water, and the absorption liquid after SO2 absorption is stored in the oxidation tank and oxidized, so there is no alkali loss and the amount of absorbent can be reduced. .

[Brief explanation of drawings]

Fig. 1 is an explanatory diagram showing an example of the device for removing sulfur oxides in exhaust gas according to the present invention, Fig. 2 is a cross-sectional view taken along line A-A in Fig. 1, and Figs. 3 and 5 are diagrams showing a conventional device. An explanatory diagram showing an example, FIG. 4 is a graph showing the relationship between the SC2 concentration in the exhaust gas coming out of the absorption tower and the COD of the absorption liquid when an experiment was conducted using the apparatus shown in FIG. 3 (comparative example). . l...Absorption tower, 2...Circulation pump, 3...Spray device, 4...Oxidation tower, 5...Weir and partition plate, 6...
- Absorption liquid tank, 7... Oxidation tank, 8... Absorption liquid circulation line, 9a, 9b... Connection piping, IO... Oxygen-containing gas supply pipe, 11... Discharge pipe, 12...・Flow control valve,
13... Oxidizing liquid circulation line, 14... Exhaust gas inlet,
15... Clean exhaust gas outlet, 16... Stirrer, 17.
...Flow rate regulator, 18...Absorbent slurry supply pipe

Claims (1)

[Claims] 1. Produced by supplying magnesium hydroxide, magnesium oxide, and magnesium carbonate alone or as a mixture to an absorption tower as an absorbent and absorbing sulfur oxides in the exhaust gas introduced into the absorption tower. In this method, the absorbed magnesium sulfite is oxidized to magnesium sulfate so as not to precipitate magnesium sulfite crystals in the absorption liquid, and a part of it is discharged. The absorption liquid is circulated and sprayed to the upper part of the absorption tower and brought into contact with the exhaust gas, and a part of the sprayed absorption liquid flows into the oxidation tank, and the remainder of the absorption liquid flows into the absorption liquid tank.
Oxygen-containing gas is supplied to the oxidation tank to oxidize the magnesium sulfite in the absorption liquid to magnesium sulfate, and part of the liquid in the oxidation tank is discharged, and a part of the liquid in the oxidation tank is exchanged with the absorption liquid tank for oxidation. A method for removing sulfur oxides from exhaust gas, which is characterized by circulating the liquid into an absorption liquid tank while adjusting the flow rate by utilizing the height difference between the liquid level and the tank. 2. A vertical weir/partition plate is provided at the bottom of the absorption tower to separate the absorption liquid tank and the oxidation tank, and an absorption liquid circulation line with a circulation pump is used to connect the bottom of the absorption liquid tank and the spray device installed at the top of the absorption tower. On the other hand, connect an oxygen-containing gas supply pipe to the bottom of the oxidation tank, connect a discharge pipe to the oxidation tank, and connect the oxidation tank and absorption liquid tank with an oxidation liquid circulation line having a flow rate control valve. This oxidizing liquid circulation line does not have a circulation pump, and the oxidizing liquid is circulated from the oxidizing tank to the absorbing liquid tank by utilizing the difference in liquid level between the absorbing liquid tank and the oxidizing tank. Equipment for removing sulfur oxides inside.