JPS5934413B2 - Treatment method for exhaust gas containing NOx - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides a method for reducing NOx (
This invention relates to a denitrification method in which imidodisulfonate produced in a wet reduction absorption method for nitrogen oxides is crystallized and separated as dipotassium imidodisulfonate, and then thermally decomposed to render it harmless as nitrogen.

Here, the NOx wet reduction and absorption method refers to a method in which NOx is collected as imidodisulfonate in an aqueous solution containing a small amount of iron salt and sulfite.

When NOx comes into contact with an aqueous solution containing an iron salt and a sulfite, it reacts as shown in the following formula to become an imidodisulfonate, which accumulates in the aqueous solution.

2 NO+ 6 MH8Os→2 NH(SOs M
) 2+M2 S 20a + 2 N20 Methods for rendering this imidodisulfonate N2 harmless include (1) hydrolyzing it under oxidizing conditions to form sulfamino acid, and then converting it to nitrogen with nitrite; or (2)
There is a known method in which hydrolysis is carried out under more severe conditions, and after hydrolysis to acidic ammonium sulfate, ammonia gas is generated as alkalinity, and then this ammonia is air oxidized to nitrogen using a catalyst.

However, method (1) requires an equimolar amount of nitrite to the amount of absorbed NOx, and method (2) requires a long and complicated process, so these methods are not preferred. do not have.

On the other hand, it is known that solid imidodisulfonate undergoes the following thermal decomposition reaction in an inert gas.

NH(SO3K)2 → 2SO4+SO2+1/3N2+1/3NH8NH
(SO3NH4)2 →SOs + SO2+H20+ 1/'3N2 +
1/3 N'HsNaN(S03Na)2 →￥2Na2SO4+1/2S +1/2N2 This reaction is an intramolecular redox reaction in which monotrivalent nitrogen is oxidized by hexavalent sulfur, and this reaction cannot be carried out industrially. When the process is carried out in a thermal thermal decomposition furnace, the sulfur produced as a by-product due to the oxygen present in the atmosphere becomes sulfur dioxide gas, and the ammonia produced is prevented from producing NOx by setting the ambient temperature to 900 to 1300°C. Since the absorbed NOx can be converted into nitrogen without any carbon dioxide, the absorbed NOx will be converted into nitrogen.

As a result of research into a method for easily and efficiently converting NOx contained in combustion exhaust gas into nitrogen, the inventors found that
It has been found that it is effective to collect Ox as an imidodisulfonate through wet reduction absorption, and then separate the solid as a solid and thermally decompose it.

Hereinafter, the configuration of the present invention will be explained in detail.

The absorption of NOx in the present invention is achieved by using an aqueous solution containing iron salts and sulfites as the absorption liquid, but a chelating agent such as ethylenediamine m-acid or nitrilotriacetic acid is added to the absorption liquid together with the iron salt to absorb iron. Preferably it is present as chelated iron.

Furthermore, by making an organic acid alkali having S02 absorbing ability, such as sodium acetate, exist in the absorbing liquid, S02 is simultaneously absorbed and sulfite is generated in the absorbing liquid, which acts as a NOx reducing agent.

When brought into contact with an absorption liquid containing NOx-containing exhaust gases, iron salts and sulfites, the NOx becomes imidodisulfonate and is collected in the absorption liquid.

Due to the presence of potassium ions in the absorption liquid, this imidodisulfonate can be easily separated from the absorption liquid by concentration and cooling. It is preferable because it has a lower solubility than natural salts.

However, imidodisulfonate can be separated as a basic salt by adding an alkali such as Ca(OH)2-NaOH to the absorption liquid, but there are problems such as decomposition of iron chelate, so it is not possible to separate it as a neutral salt. It is preferable to do so.

The separated dipotassium imidodisulfonate crystals are placed in an internal heating furnace to undergo a thermal decomposition reaction.

The thermal decomposition reaction temperature is 430 to 1300°C, preferably 500 to 700°C, and the reaction time is sufficient to be several seconds to several minutes, which is heat supply rate-limiting.

When the thermal decomposition reaction is carried out at 90 to 1300°C, almost all the nitrogen content of dipotassium imidodisulfonate is converted to nitrogen, but when the thermal decomposition reaction is carried out at 430 to 700°C, dipotassium imidodisulfonate is converted to nitrogen. 173 mol of ammonia is generated per 1 mol.

The generated pyrolysis gas containing ammonia is further heated to 900 to 1300° C. in an oxygen-containing atmosphere, whereby it is oxidized by oxygen in the combustion gas and becomes nitrogen.

Therefore, almost all of the nitrogen content of dipotassium imidodisulfonate was converted to N2 gas.

Note that a large amount of nitionate is present in the absorption liquid containing imidodisulfonate, and this nitionate can also separate the imidodisulfonate from the absorption liquid.

In the thermal decomposition of imidosulfonate, potassium nitionate is decomposed at a lower temperature of 200 to 250°C as shown in the following formula.

K2S20. →2SO4+SO2 Therefore, potassium nitionate accumulates in the absorption liquid,
There is no problem in being separated at the same time as dipotassium imidodisulfonate.

As a result of thermal decomposition, the remaining potassium sulfate is dissolved in the absorption liquid again, and a part of the potassium sulfite is oxidized by oxygen in the exhaust gas, and the potassium sulfate produced is reacted with calcium hydroxide or calcium carbonate to form sulfate radicals. Separates as calcium sulfate.

Since the gas after thermal decomposition contains a high concentration of S02 generated by the decomposition of dipotassium imidodisulfonate and potassium nitionate, this gas is brought into contact with the absorption liquid together with the exhaust gas containing NOx.

When absorbing 1 mole of NO, 3 moles of sulfite are consumed.For example, when treating exhaust gas pm containing N0200 and when there is no oxygen in the exhaust gas, 600 mol of sulfite is consumed.
If more than ppm of SO2 is contained, there is no need to supplement sulfite.

However, the exhaust gas usually contains a significant amount of oxygen, which causes a significant amount of sulfite to be oxidized and S02
However, by circulating the gas containing S02 after thermal decomposition, there is no need to replenish sulfite, especially when the oxygen content in the exhaust gas is high (and the amount of sulfite oxidized). It is effective when there are many

Hereinafter, an example of an embodiment of the present invention will be explained with reference to the accompanying drawings.

In the figure, 1 is an absorption tower. Exhaust gas 2 containing NOx and SO2 and pyrolysis gas 3 containing SO2 generated in a subsequent step are introduced into an absorption tower 1 and brought into contact with an absorption liquid 4 containing iron ethylenediaminetetraacetic acid chelate and sulfite.

Calcium hydroxide is added to the absorption tower effluent 5 in a pH adjustment tank 6 to adjust the pH to 5.5 to 6.0, and most of it is circulated to the absorption tower 1 as absorption liquid 4, while a part is branched. and used for separation of imidodisulfonate.

The branched liquid 7 (ζ contains calcium sulfate and dipotassium imidodisulfonate as solids, and is divided into a slurry 9 concentrated by a sedimentation separation tank or a liquid cyclone 8 and a supernatant liquid 10, and the concentrated slurry 9 Add hot water to dissolve dipotassium imidodisulfonate and separate calcium sulfate 11.

This filtrate 12 and the above supernatant liquid 10 are combined and cooled and crystallized in a cooling tank 13 to separate dipotassium imidodisulfonate 14, and the filtrate is sent to a pH adjustment tank 6.

During the cooling crystallization of dipotassium imidodisulfonate, potassium nitionate is precipitated at the same time (but it is not separated). Disassemble.

The pyrolysis gas 3 is sent to the absorption tower 1, and the pyrolysis residue, potassium sulfate, is sent to the cooling tank 13 to facilitate crystallization of dipotassium imidodisulfonate.

By doing as described above, the imidodisulfonate produced by absorbing NOx can be successively separated from the absorption liquid, and the nitrogen content of the imidodisulfonate can be converted to nitrogen and rendered harmless.

In addition, it is possible to simultaneously decompose the nitionate salts that inevitably occur in the absorption liquid, and it is also possible to recycle the S02 necessary for NOx absorption, eliminating the need for sulfite replenishment. (Become.

Example 1 Exhaust gas and pyrolysis gas having the following compositions were charged at 5ONm'/h and 36Nm'/h, respectively, into a packed absorption tower with a height of 8m and a height of 15mm.
NO was absorbed by supplying at a rate of 7Nl/h.

The composition of the absorption liquid supplied from the upper part of the absorption tower was adjusted as follows.

CH3CO0K 5.0% by weight K2S20. 6.
2S0. to 1% by weight. 1.0% by weight NH(SO, Ugu 1
．． 8% by weight Fe edtaK 5.0% by weight Ca5q
- 205.0% by weight of 2H20 and 4% by weight of 2SO4 PH = 5.5 Ca(OH)2 was added to the absorption tower effluent to adjust the pH to 5.5 to complete the absorption tower.

A portion of the liquid after pH adjustment (7.0 kg/h) was divided, a portion of the supernatant (2.6 kg/h) was returned to the absorption tank, and the remaining slurry was 1.41 t/h; hot water 1.4 t/h. After adding kq/h to dissolve NH(SO2K)2, it was filtered to separate the gypsum.

This plaster contained 2S04, so it weighed 11kg/
Wash with H water to dissolve K2SO4, filter again and remove C
9/h was obtained for a5O, -2H200,35.

The washing solution containing K2SO4 was concentrated and returned to the absorption solution.

This filtrate and supernatant liquid (3 kq/h) were cooled together to 35°C to obtain 0.182/cq/h (quantity) of precipitated crystals.

This crystal is NH(SO,K)256 weight east 2S20
It included 644 weight clerks.

This crystal mixture was put into a pyrolysis furnace at 0.182/h and pyrolyzed at 1050°C.

The generated gas of 367 N/h contained SO24.5 volume fraction.

The only solid in the thermal decomposition residue was K2SO4.

280 for this. When the was returned to the cooling tank and operated continuously,
The NO absorption rate was 82%.

[Brief explanation of the drawing]

The accompanying drawings are process diagrams showing an example of an embodiment of the present invention. In the figure, 1...absorption tower, 2...exhaust gas,
3...Pyrolysis gas, 4...Absorption liquid, 5
...Absorption tower effluent, 6...pH adjustment tank,
7...Branch liquid, 8...Liquid cyclone, 9...Slurry, 10...Supernatant liquid,
11... Calcium sulfate, 12... Filtrate, 13... Cooling tank, 14... Dipotassium imidodisulfonate, 15... Heat Decomposition furnace.

Claims (1)

[Claims] I: Exhaust gas containing NOx and S02 is brought into contact with an absorption liquid containing at least iron salts and potassium sulfite with a pH of 5.5 to 6.0, and the resulting imidodisulfonate is extracted from the absorption liquid as imidodisulfonic acid. After separating it as dipotassium, it is thermally decomposed, and the cracked gas containing nitrogen and S02 is brought into contact with the absorption liquid again, and potassium sulfate produced by the thermal decomposition is dissolved in the absorption liquid. A method for treating exhaust gas containing NOx, which comprises adding calcium hydroxide and separating sulfate radicals as calcium sulfate. 2. The method according to claim 1, wherein the thermal decomposition temperature is between 430 and 1300C, preferably between 500 and 700C. 3 The above imidodisulfonic acid dicalylami is 430 to 70
The method according to claim 1, wherein the pyrolysis gas generated by thermal decomposition at 0°C is further heated to 900 to 1300°C in an oxygen-containing atmosphere.