JPS5831968B2 - High End Datsushi Youhouhou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating industrial exhaust gas for removing nitrogen oxides from industrial exhaust gas containing nitrogen oxides.

Nitrogen oxides (NOx) are generated from thermal power generation boilers and the like, and together with sulfur oxides (SOx) are major substances that pollute the air, and many methods of removing them are being studied.

Of these, many methods for removing SOx have been put into practical use, but those for NOx are still at the development stage.

Dry and wet methods are considered as NOx removal methods.

The dry method is a method of catalytic reduction using a reducing agent such as ammonia, but it has the drawback that the catalyst is easily deteriorated by the influence of SOx and dust.

As wet denitration methods, two methods are considered: an oxidative absorption method in which NOx is oxidized and absorbed to form nitrates or nitrites, and a wet method in which NOx is reduced using a reducing agent such as sulfite.

However, the nitrates or nitrites produced by the oxidation absorption method have a high solubility in water, and it is very difficult to separate and recover them, and furthermore, they cannot be used in many ways.

There is also a method of converting these products into nitrogen by chemical methods or biological treatment, but these methods are not economically viable.

The wet reduction method has the advantage that NOx is directly reduced to nitrogen, but has the disadvantage that the amount of reducing agent used is enormous because the reducing agent used is consumed by oxygen in the exhaust gas.

As a result of extensive research into wet processing methods, the present inventors have overcome the above-mentioned drawbacks and have completed an excellent process.

That is, the present invention provides (1) a method for treating exhaust gas containing nitrogen oxides to remove nitrogen oxides, in which (a) part of the nitrogen monoxide in the exhaust gas is oxidized by a catalyst or by addition of an oxidizing agent; A first step that produces a mixed gas of -nitrogen oxide and nitrogen dioxide by oxidizing it to nitrogen dioxide or adding nitrogen dioxide to the exhaust gas.
Step (b) Monovalent aliphatic carboxylic acid, oxalic acid, aromatic carboxylic acid or UIJ, which is the exhaust gas treated in the first step
The second step (c) absorbs and removes nitrogen oxides from the exhaust gas by washing with an aqueous solution containing an ammonium salt of a weak acid selected from phosphoric acids. a third step of decomposing the oxide into nitrogen) adding ammonia or aqueous ammonia to the aqueous solution containing the weak acid produced in the third step to regenerate the ammonium salt of the weak acid, and circulating this to the second step; A flue gas denitrification method characterized by comprising four steps.

(First invention) (2) In the above method, when generating a mixed gas of nitrogen monoxide and nitrogen dioxide in the first step, an appropriate amount of the cleaning liquid taken out from the second step is decomposed with an acid. , a flue gas denitrification method characterized by adding generated nitrogen dioxide to flue gas.

(Second invention) The main points are as follows.

Hereinafter, the present invention will be explained step by step starting from the first invention.

The first step is an oxidation step in which a portion of nitrogen monoxide NO in the exhaust gas is oxidized to nitrogen dioxide NO2.

The oxidation method is catalytic oxidation using an oxidation catalyst or oxidation by adding an oxidizing agent such as ozone.

2 NO-1 → NO2 (1) (Catalyst) NO+03 → NO2+02 (2) This oxidation is performed by converting NO and NO to be absorbed in the next second step.
This is because it is necessary to coexist with NO2 in approximately equal amounts, and a method of simply adding NO2 to the exhaust gas may also be used.

The second step is an absorption step in which nitrogen oxides are absorbed into ammonium nitrite by washing the exhaust gas with an aqueous solution containing an ammonium salt of a weak acid.

NO+NO2+H20→2HNO2(3)HNO2+N
H4,・Particularly effective are acetate, monovalent aliphatic carboxylates with a larger number of carbon atoms, oxalates, benzoates, and other aromatic carboxylates and phosphates. .

The concentration of the salt is an effective concentration for absorption and will vary depending on the type of salt, but will be at least about 1.0% to minimize the need for processes such as salt regeneration.

The third step is a decomposition step in which the ammonium nitrite produced in the second step is heated to convert it into nitrogen and water.

NH, NO2 → N2 + H20 (5) The decomposition temperature should be 50°C or higher, but in terms of decomposition rate it is 7.
The temperature is preferably 0°C or higher.

The decomposition reaction rate is expressed by the following pseudo-equation regarding nitrite ion NO2, and the value of the reaction rate constant is experimentally determined = -: 290X10 'see'(70'
C) (7) was asked.

Therefore, this reaction is a fast reaction and can be easily carried out industrially.

Further, when the operating temperature in the second step is high, decomposition can be achieved to some extent in the second step.

The fourth step is a step of adding ammonia to the aqueous solution treated in the third step to regenerate ammonium salt.

H,X+NH3→NH4,X (8) The regenerated aqueous solution is recycled and used in the second step.

The above is a detailed explanation of the first aspect of the present invention, and next, the second aspect of the present invention will be explained.

The gist of the second invention is a unique method for producing NO2 in the case where NO2 is added to the exhaust gas in the first step of the first invention. Ammonium nitrate is decomposed with an acid such as sulfuric acid to generate a mixed gas of highly concentrated NO2NO2, and this is converted to oxygen or air to oxidize the NO in the mixed gas to NO2 to produce NO2. It is added to the exhaust gas of the process.

The mixed gas of N02NO2 generated by the decomposition has a much higher concentration than nitrogen oxides in the exhaust gas, so it is easily oxidized.

The reaction formula is as follows.

NO2 gas is sent to the first step, ammonium sulfate generated in the liquid is removed from the thread by solid-liquid separation, and the remaining liquid is recycled and used as a cleaning liquid.

As described above, the present invention overcomes the drawbacks of the conventional methods.

That is, conventional NaOH. Ca(OH)2tMg(OH
) Problems such as corrosion of equipment for absorption with strong alkaline aqueous solutions such as 2 can be solved by using LJ ammonium salt aqueous solutions that are not weakly alkaline.

Furthermore, the treatment of absorbed NOx is more economical and simpler than conventional methods in that it requires simple heating.

Furthermore, the method of the present invention does not cause accumulation of nitrates and the like and is not affected by coexisting gases in exhaust gas.

These advantages are thought to be based on the use of the properties of ammonia, and cannot be inferred from conventional methods, but are completely new discoveries made by the present inventors.

The present invention is applicable to boiler exhaust gas, sintering furnace exhaust gas, nitric acid plant exhaust gas, etc.

Hereinafter, the operation of the present invention will be specifically explained with reference to the accompanying drawings.

FIG. 1 is a flowchart showing an example of implementing the first invention of the present invention, and FIG. 2 is a flowchart showing an example of implementing the second invention of the present invention.

In FIG. 1, I is a nitrogen oxide oxidation process using a catalytic oxidation device or an ozone addition device, (i) is an absorption process to remove NOx, (i) is a decomposition process to decompose ammonium nitrite,
(2) is a regeneration process in which ammonium salt, which is an absorbent, is regenerated.

The exhaust gas enters the oxidation process (2) through line 1, then enters the absorption process (2) through line 2, where it is cleaned and discharged into the atmosphere through line 3.

A part of the sintering liquid from the absorption process (2) is extracted from line 4 and sent to the decomposition process (2).

The decomposed liquid is sent to the regeneration step (2) through line 5, regenerated by ammonia introduced through line 6, and then circulated through line 7 to the absorption step (2).

In Figure 2, ■ is the manufacturing process for nitrogen dioxide, in which a part of the cleaning solution discharged from the absorption process ■ in Figure 1 is decomposed with an acid, such as sulfuric acid, and then oxidized to produce nitrogen dioxide. This is the process of manufacturing.

To this, sulfuric acid is added through line 8, decomposed and oxidized nitrogen dioxide is sent to the first step I through line 9, ammonium sulfate as a by-product is taken out through line 10, and the remaining liquid is explained with reference to FIG. In the same way as above, the liquid taken out from the decomposition process ■ passes through line 5 to the regeneration process ■
sent to.

The other details are the same as described in connection with FIG.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these.

Example 1 The results of a test conducted according to the flow shown in FIG. 1 are shown below.

Example 2 Under the same conditions as in Example 1, but when the composition of the absorbent was changed, the following denitrification rates were obtained.

CH3COONH410 section Denitrification rate 92.1 yen (C
OONH4)2 4% Denitrification rate 86.5

[Brief explanation of the drawing]

FIGS. 1 and 2 are flowcharts showing the flow of implementing the present invention.

Claims (1)

[Scope of Claims] 1. A method for treating exhaust gas containing nitrogen oxides to remove nitrogen oxides, comprising: (a) oxidizing a portion of nitrogen monoxide in the exhaust gas by catalytic oxidation using a catalyst or by adding an oxidizing agent; A first step that generates a mixed gas of -nitrogen oxide and nitrogen dioxide by converting it into nitrogen dioxide or adding nitrogen dioxide to the exhaust gas.
Step (b) The exhaust gas treated in the first step is washed with an aqueous solution containing an ammonium salt of a weak acid selected from monovalent aliphatic carboxylic acid, oxalic acid, aromatic carboxylic acid, or phosphoric acid. The second part absorbs and removes nitrogen oxides in the
Process←→C The cleaning liquid taken out from the second process is heated and absorbed.
(a third step of decomposing the nitrogen oxides into nitrates) adding ammonia or aqueous ammonia to the aqueous solution containing the weak acid produced in the third step to regenerate the ammonium salt of the weak acid;
A flue gas denitrification method comprising a fourth step of circulating this to a second step. 2. In a method of treating exhaust gas containing nitrogen oxides to remove nitrogen oxides, (a) adding nitrogen dioxide generated by decomposing an appropriate amount of the cleaning liquid taken out from the second step with acid to the exhaust gas; - The first step of producing a mixed gas of nitrogen oxide and nitrogen dioxide (b) The exhaust gas treated in the first step is converted into monovalent aliphatic carboxylic acid, oxalic acid, aromatic carboxylic acid or U The second step (c) absorbs and removes nitrogen oxides from the exhaust gas by cleaning with an aqueous solution containing an ammonium salt of a weak acid selected from nitrogen acids. in the third step of decomposing the nitrogen oxides into nitrogen) Adding ammonia or aqueous ammonia to the aqueous solution containing the weak acid generated in the third step to regenerate the ammonium salt of the weak acid, and circulating this to the second step. A flue gas denitrification method characterized by comprising a fourth step of: