KR800000279B1 - Exhaust gas scrubbing process - Google Patents

Abstract

An exhaust gas contg. NOx and/or SOx was simultaneously scrubbed by contact with a soln. which was PH>4.5 contg. thiourea, EDTA-Cu salt, and sulfite, hydroxide, carbonate of alk. earth, then solids obtained(sulfate and sulfite) were sepd., and the filtered soln. was reused., The treated gas contained 180 ppm NOx. CaSO4.2H2O was recovered. The method is useful for treating combustion gas or chemical plant waste gas.

Description

Exhaust Gas Cleaning Method

The present invention relates to exhaust gas containing nitrogen oxides (NOx), such as combustion exhaust gas, exhaust gas from a metal melting washing plant, exhaust gas from various chemical processes, and exhaust gas from other industrial fields, and the like. A wet exhaust gas cleaning method for removing nitrogen oxide (NOx) including nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ).

In more detail, the present invention oxidizes nitrogen monoxide (NO) to first convert it to nitrogen dioxide (NO ₂ ) and / or dinitrogen trioxide (N ₂ O ₃ ), and then in a scrubber, washing with sulfite of alkaline earth metals. The present invention relates to a wet method of removing nitrogen oxides (NOx) from such an exhaust gas by washing the exhaust gas with a deposit liquid.

In addition, the present invention is the combustion including the exhaust gas from the boiler, heating furnace, sintering furnace, roasting furnace, converter, melting furnace, incinerator, etc. The present invention also relates to an exhaust gas cleaning method for simultaneously removing nitrogen oxides (NOx) and sulfur oxides from exhaust gases.

There are mainly three methods for removing nitrogen oxides (NOx) from the exhaust gas. That is, (a) a reduction method of reducing nitrogen oxides (NOx) to nitrogen (N ₂ ) in a gas as a catalyst and a reducing agent, and (b) nitrogen oxides (NOx) from the gas as an adsorbent such as activated carbon. Adsorption method for adsorbing and (c) Adsorption method for cleaning nitrogen oxide from gas with a suitable liquid absorbent. Among these methods, water, aqueous ammonium solution, aqueous sodium hydroxide (NaOH) solution, aqueous solution of NaClOx (x = 1, 2 or 3), sulfuric acid in a spray tower, a sealed tower, or other various scrubbers Absorption methods for cleaning exhaust gases that have to be treated as liquid absorbents such as iron solutions and the like have been widely used.

However, it is not known how to remove nitrogen oxides (NOx) from exhaust gases containing such substances at high denitrification rates as sulfites of alkaline earth metals, such as calcium sulfite.

Various methods of removing NOx and / or SOx from exhaust gases are known in the art. The present inventors also simultaneously remove NOx and SOx from the exhaust gas using a cleaning solution of carbonate (or hydroxide) of alkali metal or ammonium and sulfite, as shown in Japanese Patent Application Publication No. 50-27763 (1975). The method was also developed.

However, there is also no known method of simultaneously removing SOx and NOx from an exhaust gas at high denitrification and desulfurization rates as carbonates (or hydroxides) of alkaline earth metals. It is therefore an object of the present invention to provide a method for removing NOx from exhaust gases at high denitrification rates using sulfite of alkaline earth metals, in particular calcium sulfite, which is available at a low price on the market.

It is still another object of the present invention to provide a method for simultaneously removing SOx and NOx from a combustion exhaust gas at high denitrification and desulfurization rates in a single scrubber using alkaline earth metal carbonates or hydroxides.

Other objects and advantages of the present invention will become apparent from the following description.

According to the present invention, a method for removing nitrogen oxides including nitrogen monoxide and nitrogen dioxide from an exhaust gas containing nitrogen oxides should be prepared by the following steps.

(1) Oxidation step of nitrogen monoxide converting nitrogen monoxide into nitrogen dioxide, dinitrogen trioxide, or mixtures thereof by adding an oxidizing agent to the exhaust gas.

(2) containing (i) sulfites of at least one alkaline earth metal, ii) thiosulfates of thiourea or alkali metals, alkaline earth metals and ammonium or mixtures thereof, and iii) copper ions or iron ions or mixtures thereof; Contacting the gas in the scrubber with a scrubbing liquid having a pH of no greater than 4.5.

According to an embodiment of the present invention for removing NOx from the exhaust gas, at least a part of the cleaning solution is a solid-liquid separation process for separating an alkaline earth metal sulfate produced by reacting nitrogen oxide with sulfite of alkaline earth metal in the cleaning solution. The resulting solution is circulated in the scrubber as a scrubbing liquid.

According to the present invention, there is also provided a method comprising the following steps to simultaneously remove sulfur oxides and nitrogen oxides from an exhaust gas containing sulfur oxides and nitrogen oxides.

(1) an oxidation step of converting nitrogen monoxide contained in the nitrogen oxide into nitrogen dioxide, dinitrogen trioxide, or mixtures thereof by adding an oxidant to the exhaust gas, and then

(2) in a scrubbing tower (i) at least one hydroxide or carbonate of an alkaline earth metal or mixture thereof and at least one sulfite of an alkaline earth metal, (ii) thiourea or thiosulfate of an alkali metal and ammonium, or mixtures thereof, And (iii) contacting the gas with a wash liquor containing copper ions or iron ions or mixtures thereof and having a pH of less than 4.5.

According to this embodiment of the method for simultaneously removing SOx and NOx from the combustion exhaust gas, at least a part of the washing solution separates sulfates of alkaline earth metals formed by reacting nitrogen oxides with sulfite salts of alkaline earth metals in the washing solution. It is used in solid-liquid separation process and the resulting solution is circulated in the washing tower as a washing liquid.

Conventionally, when the sulfite of alkaline earth metal was used as a gas detergent, the denitrification rate was extremely low, for example, about 20% in the case of calcium sulfite. Therefore, in the case of using alkaline earth metal and sulfite as a cleaner, a method of removing NOx from exhaust gas by using sulfite of alkaline earth metal in order to increase the denitrification rate has been studied and developed by the inventors in earnest and overall. .

As a result, in addition to the sulfite salts of alkaline earth metals, the use of a cleaning solution containing thiourea and / or liosulfate, and copper ions and / or iron ions, results in 85% or more carbonaceous NOx from the exhaust gas It can be removed as

Although the denitrification rate is increased when the exhaust gas containing nitric oxide is washed with a washing solution containing thiourea or thiosulfate added to the sulfite of alkaline earth metal, it is not yet suitable for practical use. For example, when a washing solution containing thiourea or thiosulfate and calcium sulfite is used, the denitrification rate is usually about 50%.

In addition, when copper ions or iron ions are added to the cleaning solution of sulfite salts of alkaline earth metals, the denitrification rate is considerably increased compared to the cleaning solution containing no copper ions or iron ions, but still remains at a favorable level. As an example, when a cleaning solution containing calcium sulfite, copper ions or iron ions is used as the cleaning agent, the denitrification rate is usually 40%.

Contrary to the above, when a washing solution containing thiourea or thiosulfate and copper or iron ions added to the sulfite of an alkaline earth metal is used to remove nitrogen oxides from the exhaust gas, a remarkably high denitrification rate can be obtained.

For example, when the exhaust gas containing nitrogen oxides is washed with a washing solution containing calcium sulfite, thiourea and copper ions, NOx can be removed from the exhaust gas at a denitrification rate of 90% or more. That is, it was found that the thiourea moth and the combination of thiosulfate and copper or iron ions were incorporated into a washing solution of sulfite salts of alkaline earth metals, and these additives showed a catalytic effect against denitrification.

The main reaction formula for removing nitrogen oxide in the present invention is represented by the following reaction formulas (1) and (2).

N₂(1)NO ₂ + ₂ MSO ₃ → ₂ MSO ₄ +

N ₂ (1)

N ₂ O ₃ +3 MSO ₃ → 3MSO ₄ + N ₂ (2)

[Where M represents an alkaline earth metal such as calcium, magnesium, etc.]

And, primarily, an oxidant, such as chlorine dioxide (ClO ₂ ) or ozone (O ₃ ), to oxidize nitrogen monoxide (NO) to nitrogen dioxide (NO ₂ ) and / or dinitrogen trioxide (N ₂ O ₃ ), An exhaust gas containing NO is added to the supply pipe or pipe through which the exhaust gas supplied to the scrubber passes. This oxidation reaction is represented as follows.

2NO + ClO ₂ + H ₂ O → NO ₂ + HCl + HNO ₃ (3)

NO + O ₃ → NO ₂ + O ₂ (4)

2NO + O ₃ → H ₂ O ₃ + O ₂ (5)

According to an embodiment of the method, the molar ratio of ClO ₂ to NO is adjusted to be 0.5, and the molar ratio of O ₃ to NO is adjusted to be 0.5 and 1.0.

This is then oxidized and the NO ₂ and / or N ₂ O ₃ originally contained in the exhaust gas contain, in a scrubber, sulfite, thiourea and / or thiosulfate, and copper and / or iron ions of alkaline earth metals. It wash | cleans secondly with the washing | cleaning trust liquid to make. The scrubber employed in the present manner may be any scrubber or absorber that has conventionally been used to clean or absorb the cutlet, but is in a wavy area Moredana, described in US Pat. No. 3,892,837 issued July 1, 1975. It is better to use a plate column (perforated plate or lattice tube tower without jaws and downcomers and having a free space ratio such as 0.300.60).

The denitrification process of the present application can be applied not only to the exhaust gas containing only NOx but also to the exhaust gas containing both SOx and NOx, such as combustion exhaust gas.

The reaction for removing sulfur oxides from the exhaust gas in the simultaneous denitrification and desulfurization process of the present application is represented by the following equations (6) and (7).

M (OH) ₂ + SO ₂ → MSO ₃ + H ₂ O (6)

MCO ₃ + SO ₂ → MSO ₃ + CO ₂ (7)

[Where M represents the same as defined above]

Thus, the sulfur dioxide contained in the exhaust gas is absorbed into the washing liquid as the sulfite of the alkaline earth metal by washing the gas with the washing liquid containing the hydroxide or carbonate of the alkaline earth metal together with the denitrifying agent in the washing machine.

In practice, for example, a large amount of carbon dioxide (CO ₂ ) contained in the combustion exhaust gas is dissolved in the washing solution, and the hydroxide or carbonate is reacted with dissolved CO ₂ to convert it into bicarbonate of an alkaline earth metal. For this reason, SO ₂ contained in the exhaust gas is mainly washed with bicarbonates of chopped earth metals as shown in the reaction (8) below.

M (HCO ₃ ) ₂ + SO ₂ → MSO ₃ + 2CO ₂ + H ₂ O (8)

Furthermore, the desulfurization reactions (6), (7) and (8) of the resulting sulfite salts of alkaline earth metals, as found in equations (1) and (2), replace NO ₂ and / or N ₂ O ₃ with N _2. Can be used directly as a reducing agent. When the exhaust gas containing a relatively large amount of SOx as compared to NOx is used simultaneously in the desulfurization and denitrification method of the present invention, there is an advantage that no additional sulfite is required.

Thiosulfate to be employed in the present method is, for example, sodium thiosulfate (Na ₂ S ₂ O ₃ ), potassium thiosulfate (K ₂ S ₂ O ₃ ), calcium thiosulfate (CaS ₂ O ₃ ), magnesium thiosulfate (MgS ₂ O ₃ ), ammonium thiosulfate ((NH ₄ ) ₂ S ₂ O ₃ ), and the like.

Copper ions and / or iron ions employed in the present method are, for example, cuprous sulfate, cupric sulfate, cuprous chloride, cupric chloride, ferrous sulfate, ferric sulfate, cuprous chloride. Iron, ferric chloride, or the like, or a chelate compound or complex compound of copper or iron, may be mixed into the wash solution. Chelating agents employed in the present methods are, for example, ethylene diaminetetraacetic acid (EDTA) ((CH ₂ ) ₂ N ₂ (CH ₂ COOH) ₄ ), citric acid (C ₆ H ₈ O ₇ ), tartaric acid Conventional chelating reagents such as (C ₄ H ₆ O ₆ ), gluconic acid (C ₆ H ₁₂ O ₇ ), and the like.

Copper and iron complex compounds include a copper ammonia complex [(Cu (NH ₃ ) ₄ ) ²⁺ ] iron cyanide complex [(Fe (CN) ₄ ) ^4- ] and the like.

In the present method for removing NOx or NOx and SOx from the exhaust gas, the pH of the washing solution is not less than 4.5, and 5 to 9 is suitable. When the pH of the washing solution is less than 4.5, the denitrification rate and the desulfurization rate are undesirably reduced because hydrogen sulfite ions are remarkably formed in the equilibrium relationship between sulfite ions and hydrogen sulfite ions. When the pH of the washing liquid is more than 9, a large amount of the alkali compound in the washing liquid is wasted to neutralize the CO ₂ contained in the exhaust gas.

In the present method of removing NOx or NOx and SOx from the exhaust gas, the content of the sulfite contained in the washing liquid is not less than 0.005 mol / l based on the total volume of the liquid, and 0.01 mol / l or more is suitable. When the content of sulfite is less than 0.005 mol / l, the denitrification rate is reduced to the point where it is not practical.

In the present method for removing MOx or NOx and SOx from the exhaust gas, the content of thiourea or thiosulfate contained in the wash solution is not less than 0.03 mol / l based on the total volume of the solution, and is 0.05 to 0.4 mol / L range is suitable. When the content of thiourea or thiosulfate is less than 0.03 mol / l, the denitrification rate is undesirably reduced. Although the content of thiourea or thiosulfate is greater than 0.4 mol /, it actually satisfies the denitrification effect but it is uneconomical.

The content of copper ions or iron ions contained in the washing liquid used in the present method is not less than 0.003 mol / L based on the total volume of the liquid, and the range of 0.01 to 0.04 mol / L is suitable.

When the content of copper ions or iron ions is less than 0.003 mol / l, the denitrification rate is reduced to the point where it cannot be used.

In contrast, when the content of copper ions or iron ions is 0.04 mol / l or more, the denitrification effect does not actually increase, which is uneconomical in practical use.

In an embodiment of the method of the present method of removing NOx or NOx and SOx from an exhaust gas, the cleaning solution is circulated in the scrubber. For this reason, at least a part of the circulating washing liquid should be taken out of the cleaning system and used for the solid-liquid separation treatment.

Thus, the solid mainly containing the alkali earth metal sulfate (MSO ₄ ) formed in the formulas (1) and (2) is separated from the entrainment and the resulting solution is returned to the cleaning system. Solid-liquid separation of the wash liquor can be readily accomplished with conventional techniques for separating solids from the wash liquor, such as filters, centrifuges, precipitation separators and the like.

The invention is described in detail in accordance with the following examples, but is not limited thereto.

Example 1

The gas absorption experiment was carried out using an absorbent liquid containing 0.03 mol / l calcium sulfite containing air containing 200 ppm by volume of NO ₂ gas, and thiourea or EDTA-Cu ²⁺ compound or thiourea and EDTA-Cu ²⁺ compound. It was carried out by blowing the foam into various absorbent liquid containing more. In the experiment, air containing 200 ppm of NO ₂ gas was blown into the absorbent bottle containing 500 ml of the absorbent liquid at 10 l / min.

The denitrification rate was obtained by measuring the concentration of NO ₂ gas in the intake and discharge of absorbent bottles using a chemiluminescent nitrogen oxide measuring device.

As a result, the intrinsic effect at the time of adding a thiourea and an EDTA-Cu ²⁺ compound is shown in Table 1.

TABLE 1

Example 2

In the gas absorption experiment, the absorption liquid containing 0.05 mol / L calcium sulfite, air containing approximately 200 ppm of NO ₂ gas by volume, and various absorptions containing thiourea or sodium thiosulfite and copper or iron ions It was carried out by blowing the foam into the solution.

In the experiment, air containing NO ₂ gas was bubbled into an absorbent bottle containing 500 ml of the absorbent liquid at 10 L / min.

The denitrification rate was determined by measuring the NO ₂ gas concentration at the suction and discharge of the absorption bottle by a chemiluminescent nitrogen oxide measuring apparatus.

The results are shown in Table 2, from which the inherent effects of the addition of thiourea or sodium thiosulfate and copper or iron ions are clearly understood.

TABLE 2

Example 3

A gas absorption experiment was carried out using an absorption liquid containing 0.18 mol / L calcium sulfite, air containing 200 ppm of NO ₂ gas by volume, thiourea or EDTA-Cu ²⁺ compound, or thiourea and EDTA-Cu ²⁺ This was carried out by blowing bubbles into various absorbent liquids containing the compound and generating bubbles.

In the experiment, air containing NO ₂ gas was bubbled into an absorbent bottle containing 500 ppm of the absorbent liquid at 10 L / min.

The denitrification rate was obtained by measuring the NO ₂ gas concentration at the suction and discharge of the absorption bottle by a chemiluminescent nitrogen oxide measuring device. The results are shown in Table 3, from which the intrinsic effects when adding thiourea and EDTA-Cu ²⁺ are clearly understood.

TABLE 3

Example 4

A gas absorption experiment was carried out using an absorbent liquid containing 0.37 mol / l calcium sulfite containing air containing approximately 200 ppm of NO ₂ gas by volume, and an EDTA chelate compound of thiourea or sodium thiosulfate and copper or iron. It was carried out by blowing the foam into various absorbent liquid containing more. In the experiment, the air containing NO ₂ gas was bubbled into the absorption bottle, even at 500 ppm of the absorbent liquid at 10 L / min.

The denitrification rate was determined by measuring the NO ₂ gas concentration at the suction and discharge of the absorbing bottle by a chemiluminescent nitrogen oxide measuring apparatus. The results are shown in Table 4, from which the inherent effects of addition of thiourea or sodium thiosulfate and EDTA chelate compounds of copper or iron are clearly understood.

TABLE 4

Example 5

Exhaust gases at 10,000 Nm ³ / hr from the metal washing plant, containing 200 ppm NO by volume and 250 ppm NO ₂ by volume, were denitrified according to the present method. 130 ppm ClO ₂ gas by volume was continuously added to the exhaust gas in the feed pipe into which the gas was fed into the scrubber to convert NO contained in the gas to NO ₂ . The exhaust gas then enters the bottom of the scrubber tower consisting of three Moredana stages (perforated plates without jaws and downcomers) with a free space ratio of 0.38 (opening area of the plate / cross section of the tower). The gas was continuously supplied, where the exhaust gas was brought into countercurrent contact with the wash solution circulated with the given composition under the following conditions.

Surface gas velocity of tower 5m / sec

Tower Liquid / Gas Ratio (L / G) Ratio 4

PH 6.2 of the cleaning solution

Composition of circulating washing liquid (mol / liter)

CaSO ₃ 0.05

CaSO ₄ 0.5

EDTA-Fe ²⁺ 0.03

Na ₂ S ₂ O ₃ 0.15

The NOx content at the exit of the Moredana washing tower was 20 ppm or less.

In contrast, the gas cleaning experiment of the exhaust gas was repeated using the same procedure as described above, except that EDTA-Fe ²⁺ compound and Na ₂ S ₂ O ₃ were not incorporated into the wash solution. At the same time, the NOx content in the scrubber discharge gas was 180 ppm.

In the gas absorption test of the present invention described above, the additional dose of calcium sulfite (CaSO ₃ ) was 30 kg / hr.

The circulating fresh water was continuously taken out of the system at a rate of 85 liters / hr and filtered using a pressure filter. Solid calcium sulfite was separated and the resulting filtrate was recycled to the circulation tank of the washing liquid.

Calcium sulfite containing the separated solid bundle was recovered by oxidation with calcium sulfate. The amount of CaSO ₄ H ₂ O thus obtained was 43 kg / hr.

Example 6

Exhaust gases of 10,000 Nm ³ / hr from industrial plants containing 250 ppm NO by volume and 100 ppm NO ₂ by volume were denitrified according to the present method.

260 ppm O ₃ gas by volume was continuously added to the exhaust gas in the feed pipe to which gas was supplied to the scrubber, thereby converting NO contained in the gas to NO ₂ . Then, the exhaust gas is continuously supplied to the bottom of the washing tower, which is composed of mordan plates (perforated tubes having no jaws and down pipes) having a free space ratio of 0.38, where the exhaust gas is placed under the following conditions. And a countercurrent contact with the circulating washing liquid having a predetermined composition.

Surface gas velocity of tower 5m / sec

Tower Liquid / Gas Ratio (L / G) Ratio 4

PH 6.2 of the cleaning solution

Composition of circulating washing solution (mol / liter)

CaSO ₃ 0.05

CaSO ₄ 0.5

EDTA-Cu ²⁺ 0.03

Thiourea 0.15

The NOx content was 16 ppm or less for the scrubber exhaust gas.

The additional dose of calcium sulfite (CaSO ₃ ) was 36 kg / hr. The circulating wash liquor was continuously taken out of the system at a rate of 100 liters / hr and filtered using a pressure filter. The solid calcium sulfite was thus separated and the resulting filtrate was recycled back to the circulation tank of the wash liquor.

Calcium sulfite contained in the separated solid was recovered by oxidation with calcium sulfate (CaSO ₄ .2H ₂ O). The CaSO ₄ .2H ₂ O content thus obtained was 52 kg / hr.

Example 7

Exhaust gases from the metal washing plant at 10,000 Nm ³ / hr, containing 250 ppm NO by volume and 100 ppm NO ₂ by volume, were denitrified according to the present method. The NO contained in the gas was converted into NO ₂ by continuously adding 130 ppm of ClO ₂ gas by volume to the exhaust gas in a feed pipe through which the gas was supplied into the scrubber. Then, the exhaust gas is continuously supplied to the bottom of the washing tower, which is composed of a mordena plate (perforated plate having no jaw and downcomer) having a free space ratio of 0.38, where the exhaust gas is placed under the following conditions. And a countercurrent contact with the circulating washing liquid having a predetermined composition.

Surface gas velocity of tower 5m / sec

Tower Liquid / Gas Ratio (L / G) Ratio 4

PH 6.2 of the cleaning solution

Composition of circulating washing solution (mol / liter)

CaSO ₃ 0.05

EDTA-Cu ²⁺ 0.03

Thiourea 0.15

NOx content was 15 ppm or less (denitrification rate 95.7%) in the mortar scrubber exhaust gas.

In contrast, the gas cleaning experiment of the exhaust gas was carried out using the same procedure as described above except that the EDTA-Cu ²⁺ compound and thiourea were not incorporated into the cleaning solution. The NOx content in the exhaust gas was 180 ppm (48.6% denitrification rate).

As is apparent from the above results, the synergistic effect of thiourea and copper from the viewpoint of denitrification is remarkable when calcium sulfite is used as the cleaning agent.

Example 8

The gas absorption experiment was conducted by absorbing an air containing 0.18 mol / liter of calcium sulfite and 0.2 mol / liter of calcium carbonate in air containing 500 ppm of NO ₂ gas and 1,000 ppm of SO ₂ gas, and thiourea Or it was carried out by blowing in a variety of absorption liquid containing more than the EDTA chelate (copper) compound of copper or a mixture thereof. In the experiment, air containing NO ₂ gas was bubbled into an absorbent bottle containing 500 ml of the absorbent liquid at 10 liters / min. NO ₂ and SO ₂ concentrations in the intake and discharge gas of absorbent bottles were measured by chemiluminescent nitrogen oxide measuring device and solution conductivity type sulfur oxide measuring device, respectively.

The results are shown in Table 5, where it can be seen that the intrinsic effects when adding the EDTA chelate compounds of thiourea and copper become suitable.

TABLE 5

Example 9

The gas absorption experiment was carried out by absorbing an air containing 0.37 mol / liter of calcium sulfite and 0.20 mol / liter of calcium carbonate containing air containing 500 ppm of NO ₂ gas and 100 ppm of SO ₂ gas, and thiourea or It was carried out by bubbling in a variety of consignments further containing sodium thiosulfate and copper or iron ions.

In the experiment, air containing NO ₂ gas was bubbled in an absorbent bottle containing 500 ml of absorbent liquid at 10 liters / min. The concentrations of NO ₂ and SO ₂ in the intake and discharge gas of absorbent bottles were measured by chemiluminescent nitrogen oxide measuring device and solution conductivity type sulfur oxide measuring device, respectively. The result is shown in Table 6, and it turns out that the intrinsic effect at the time of addition of thiourea or sodium thiosulfate, and copper or iron ion becomes suitable.

TABLE 6

Example 10

Volume of the exhaust gases out to a (燒結爐) 25,000Nm ³ / hr from the sintering furnace containing the SO ₂ to 400ppm of NO ₂ and the volume of NO to the volume of 10ppm to 190ppm was treated in accordance with the present method to a. A 100 ppm ClO ₂ gas by volume was continuously added to the exhaust gas in the feed pipe through which the gas was supplied into the scrubber, thereby converting NO contained in the gas into NO ₂ . Then, the exhaust gas was continuously supplied to the bottom of the washing tower composed of four mordena plates having a free space ratio of 0.33, where the exhaust gas was brought into countercurrent contact with the washing liquid circulated with the composition under the following conditions.

Surface gas velocity of tower 5m / sec

Tower liquid and gas (L / G) ratio 4.5

PH 6.1 of the cleaning solution

Composition of circulating washing solution (mol / liter)

CaSO ₃ 0.05

CaSO ₄ 0.5

CaCO ₃ 0.2

SC (NH ₂ ) ₂ 0.1

EDTA-Cu ²⁺ 0.04

The content of SO ₂ and NO x in the scrubber discharge gas was 3 ppm or less and 15 ppm or less, respectively.

The additional dose of calcium carbonate (CaCO ₃ ) was 50 kg / hr, and no calcium sulfite was additionally supplied.

The circulating wash liquid was continuously taken out of the system at a rate of 200 l / hr and filtered using a pressure filter. The solid calcium sulfite was separated and the resulting filtrate was recycled to the circulating tank of the wash solution.

Calcium sulfite contained in the separated solid was recovered by oxidation with calcium sulfate (CaSO ₄ .2H ₂ O). The CaSO ₄ .2H ₂ O content thus obtained was 61 kg / hr.

Example 11

Exhaust gases at 10,000 Nm ³ / hr from an industrial plant containing 250 ppm NO by volume, 100 ppm NO ₂ by volume and 450 ppm SO ₂ by volume were treated according to the present method. 130 ppm ClO ₂ gas by volume was continuously added to the exhaust gas in the feed pipe through which the gas was supplied into the scrubber, thereby converting NO contained in the gas into NO ₂ . And the exhaust gas was continuously supplied to the bottom of the scrubber tower consisting of three mordan plates (perforated plates without jaws and downcomers) with a free space ratio of 0.38, where the exhaust gas underneath In countercurrent contact with a circulating wash solution having a constituent composition.

Surface gas velocity of tower 5m / sec

Tower liquid and gas (L / G) ratio 4

PH 6.2 of the cleaning solution

Composition of circulating washing solution (mol / liter)

CaCO ₃ 0.5

CaSO ₃ 0.05

CaSO ₄ 0.5

EDTA-Cu ²⁺ 0.03

Na ₂ S ₂ O ₃ 0.12

The content of SO ₂ and NO x in the scrubber discharge gas was, however, 10 ppm or less and 18 ppm or less, respectively.

On the contrary, when the gas cleaning experiment of the exhaust gas was repeated using the same procedure as described above, except that EDTA-Cu ²⁺ compound and Na ₂ S ₂ O ₃ were not mixed in the wash solution. The concentrations of SO ₂ and NO x in the scrubber discharge gas were 10 ppm or less and 180 ppm, respectively.

In the gas cleaning experiment of the present invention, the additional dose of calcium carbonate (CaCO ₃ ) was 22 kg / kr.

The circulating wash liquid was continuously taken from the system at a rate of 75 l / hr and filtered using a pressure filter. The solid calcium sulfite was separated and the resulting filtrate was recycled to the circulating tank of the wash solution.

Calcium sulfite contained in the separated solid was recovered by oxidation with calcium sulfate. The CaSO ₄ .2H ₂ O content thus obtained was 38 kg / hr.

Example 12

Exhaust gases at 10,000 Nm ³ / hr from an industrial plant containing 250 ppm NO by volume, 100 ppm NO ₂ by volume, and 450 ppm SO ₂ by volume were purged according to the present method.

130 ppm ClO ₂ gas by volume was continuously added to the exhaust gas in the feed pipe through which the gas was supplied into the scrubber to convert NO contained in the gas into NO ₂ . And the exhaust gas was continuously supplied to the bottom of the scrubber tower consisting of three mordan plates (perforated plates without jaws and downcomers) having a free space ratio of 0.38, where Backflow contact with the circulating washing liquid having the composition was carried out.

Surface gas velocity of tower 5m / sec

Tower liquid and gas (L / G) ratio 4

Composition of Washing Liquid (mol / liter)

CaSO ₃ 0.5

CaCO ₃ 0.5

EDTA-Cu ²⁺ 0.03

Thiourea 0.15

The contents of SO ₂ and NO x in the gas from the mortar or washing tower were 10 ppm or less and 15 ppm or less, respectively.

In contrast, the gas cleaning experiment of the exhaust gas is not repeated when the same test as described above is repeated, except that the EDTA-Cu ²⁺ compound and thiourea are not mixed in the wash solution. The contents of SO ₂ and NO x in the gas from the washing tower were 10 ppm or less and 180 ppm, respectively.

Claims (1)

Oxidizing the nitrogen monoxide contained in the nitrogen oxide into nitrogen dioxide, dinitrogen trioxide, or mixtures thereof by adding an oxidant to the exhaust gas, and then (I) at least one hydroxide or carbonate or mixture of alkali earth metals and sulfites of at least one alkaline earth metal, (ii) thiourea or alkali metals or alkaline earth metals of at least one or more alkaline earth metals And (iii) a sulfuric acid is converted to sulfite by contacting a gas washing solution containing thiosulfate of ammonium or mixtures thereof and (iii) copper ions or iron ions or mixtures of 0.003 mol / l or more in a washing tower and nitrogen oxides. And removing the sulfur oxides and the nitrogen oxides simultaneously from the exhaust gas containing the sulfur oxides and the nitrogen oxides.