JPS581966B2 - High Gas No Shiyori Hohou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention simultaneously economically and efficiently removes harmful sulfur oxides SO2 and nitrogen oxides (NOx) from a large volume of waste gas such as boiler waste gas of a power plant. The present invention relates to a method for treating waste gas, in which the waste gas is released into the atmosphere as a harmless gas.

SO2 in waste gas is easily absorbed and removed by alkaline substances such as alkali metal hydroxides, alkaline earth metal hydroxides, and aqueous ammonia, but most of the NOx contained in waste gas from boilers, etc. This is nitric oxide (NO), and its concentration is about several hundred to 1000 ppm, so it is extremely difficult to absorb and remove NOx only by cleaning with the above-mentioned alkaline substance.

For this reason, conventional methods for removing NOx from waste gas include adding NO2 to the waste gas in an amount almost equal to NO and allowing the above alkaline substances to absorb the NOx (equimolar method); A method (ozone method) has been used in which NOx is oxidized and absorbed by adding NOx to the exhaust gas.

However, with the equimolar method, the NOx concentration may be diluted,
If NO,2/0 in the waste gas after NO2 is added is considerably smaller than 1, it becomes extremely difficult to absorb NOx, so the absorption equipment needs to be large, and NO can be absorbed to low concentrations.
It is economically disadvantageous to absorb and remove x.

Further, the ozone method has the advantage that the reaction between NOx and ozone proceeds very rapidly and quantitatively, so that absorption occurs easily, but it also has the disadvantage that it is not economical due to the high price of ozone.

The present invention aims to eliminate the above-mentioned drawbacks and provide a waste gas treatment method that simultaneously removes SO2 and NOx economically and efficiently. N02 is added cyclically and an aqueous alkaline solution (the aqueous alkaline solution in the present invention is an aqueous alkali metal hydroxide solution, an aqueous alkaline earth metal hydroxide solution, or an aqueous solution of ammonia).

) in the first absorption tower containing SO2 in the exhaust gas.
and some NOx are removed to generate sulfites and nitrites, and then the absorption liquid of the first absorption tower is supplied to the second absorption tower, and the ozone-containing gas is added to the gas that has passed through the first absorption tower. The solution of sulfite and nitrite is washed in a second absorption tower containing the sulfite and nitrite solution, and the remaining NOx is absorbed and removed to produce nitrite. This method is characterized by generating NO by acid decomposing the nitrite produced in the absorption tower, which is then oxidized to NO2, and is circulated and added to the waste gas to remove NOx and SO2 from the waste gas. .

SO2 in the waste gas is easily absorbed by the alkaline aqueous solution as shown in equation (1) and equation (2).

Further, in the equimolar method of NOx, it is absorbed as shown in equation (2).

M here: alkali metal, alkaline earth metal or NH4
etc.

n is 1 or 2. The same applies below. In this case, in NOx absorption according to equation (2), when the NO concentration is low or NO2/No<1, NOx
The rate of absorption is significantly lower.

Further, the oxidation of NO by ozone and the absorption of NO2 by the sulfite bath liquid proceed as shown in equations (3) and (4) below.

The reactions in equations (3) and (4) proceed very quickly and quantitatively even when the NO concentration is low. NO that remains unabsorbed is completely oxidized, and N02 is completely captured as nitrite according to equation (4).

In addition, the regeneration of N02 for circulation addition is performed by, for example, generating NO by decomposing nitrite with a mineral acid, such as sulfuric acid, as shown in equation (5), and oxidizing NO to NO2 as shown in equation (6). .

The present invention absorbs NOx to generate nitrite in the presence of sulfite, an alkaline aqueous solution and ozone as shown in formulas (3) and (4), simultaneously converts sulfite to sulfate, and further (5) , (6) to generate NO gas and further oxidize it to NO2.
) to (6), it secures the necessary amount of NO2 to be circulated and added to the NO-containing waste gas in the equimolar absorption method, thereby improving the absorption efficiency and reducing the amount of ozone. Ozone in law
By using only the low-temperature portion of Ox, the amount of expensive ozone used can be reduced, making it possible to treat waste gas containing NOx and SO2 extremely economically.

Furthermore, as shown in reaction formulas (1) to (6) above, alkali metal hydroxides,
Even when an alkaline earth metal hydroxide or ammonia water bath solution is used, there is no need to separate sulfite and nitrite, and the NO2 generated by the ozone oxidation of No according to equation (3) above can be converted into sulfite, In the operation of absorbing with a mixed solution of nitrite, sulfite is oxidized to sulfate as shown in equation (4), and further, nitrite is changed to nitrate as shown in equation (5).

Therefore, an operation for separating sulfite and nitrite is not necessarily necessary, and operations such as liquid extraction or crystallization depending on the properties such as solubility of the salt may be performed if necessary.

For example, in the case of a calcium-based absorption liquid, since the solubility of calcium sulfite is low, it can be separated into a nitrite solution and a sulfite-containing slurry by a simple solid-liquid separation operation. and operate a second absorption tower with an absorption liquid with a high concentration of sulfites,
This is intended to increase the absorption rate and reduce the amount of liquid held in the second absorption tower, and in principle this separation operation is not necessary.

Examples of the alkaline aqueous solution used in the present invention include alkali metal hydroxides such as sodium and potassium, calcium, alkaline earth metal oxides such as magnesium, and aqueous ammonia.

However, for alkaline earth metal hydroxides, such as calcium or magnesium hydroxides, which have extremely low bath solubility in water, their aqueous solutions are used as slurries containing solids.

Further, as the absorption tower, any gas-liquid contact device such as a packed tower or a spray tower can be effectively used.

Next, we will show experimental examples measured by the present inventor.
2 is added, and this waste gas is sent to a packed tower containing Mg(OH)2 slurry, and the Mg(OH)2 slurry is brought into countercurrent contact under the following conditions, and the total unit movement height (based on the gas phase) is H
OG) was determined as a function of NO2/NO and NO concentration.

The results are shown in Figure 1.

Packed tower operating conditions Absorption liquid temperature 55°C Absorption liquid PH 7-8 Liquid-gas ratio 5 In Fig. 1, as the NO concentration becomes lower than the results in Fig. 1, NO2
It can be seen that as /NO becomes smaller, HOG becomes larger and becomes more difficult to absorb.

Next, the present invention will be explained in detail by way of examples.

EXAMPLE 1 FIG. 2 shows a flow sheet of an embodiment of the apparatus used in the practice of the present invention.

The exhaust gas containing SO2 and NOx is cooled in a cooler 1, and the NO2 generated as described later is mixed with N in the exhaust gas in a mixer 2.
In the first absorption tower 3, the S
Most of the O2 and some of the NOx are absorbed and CaS03
and C a (NO2)2 are generated.

A portion of the slurry effluent from the first absorption tower 3 is subjected to solid-liquid separation in a solid-liquid separator 10.

The solid part CaSO3 and slurry separated in the separator 10 are led to the second absorption tower 5.

On the other hand, Ca(No2)2 in the solution part separated in the separator 10
is sent to the nitrite decomposer 7.

The waste gas exiting the first absorption tower 3 is mixed with O3 in an ozone oxidizer 4, and then transferred to a second absorption tower 5 containing Ca SO 3 and Ca (OH) 2 led from the first absorption tower. guided by.

The gas discharged from the second absorption tower 5 contains almost no SO2 and NOx and is discharged into the atmosphere.

In the second absorption tower 5, the remainder of NOx is absorbed, and C a (
NO2)2 and CaSO4 are produced.

Ca (NO2)2 and Ca generated in the second absorption tower 5
SO4 is partially extracted and CaS is extracted in the first gypsum separator 6.
The O4 crystals are separated and the C a (N
o2) 2 is decomposed in the nitrite decomposer 7 with H2SO4 added from 11, and as shown in the following equation (7), No.
, Ca S04 , Ca (NO3)2.

The generated NO is oxidized in the oxidizer 8 and sent to the mixer 2 as NO2.
lead to.

The generated CaSO4 is separated into gypsum crystals in the second gypsum separator 9, and the solution portion ca(NO3)2 is separated and collected as a by-product.

As shown in Example 1, the present invention deals with SO2 and N in waste gas.
Ox can be removed economically and efficiently at the same time, and its characteristics can be summarized as follows.

(b) When the concentration of NOx contained in the waste gas is high, NOx is absorbed by the equimolar method, which has good absorption efficiency, and when the concentration of NOx decreases, the ozone method is used to absorb NOx and dispose of the waste gas. NOx in gas can be removed economically and efficiently down to a low concentration.

(b) Since nitrite produced by the equimolar method and the ozone method can be effectively used as a NO2 raw material for circulating addition, it is always possible to set No2/NO>1 in the equimolar method, and It is possible to increase the denitrification rate and reduce the amount of ozone used at the same time.

(c) Since sulfite can be oxidized in the ozone absorption process, there is no need to specifically oxidize sulfite.

[Brief explanation of the drawing]

Figure 1 shows the NO concentration, NO2/
Relationship between NO and HOG. FIG. 2 shows a flow sheet of an embodiment of the apparatus used in the present invention. 1...Cooler, 2...Mixer, 3...
...First absorption tower, 4...Ozone oxidizer, 5
...Second absorption tower, 6th gypsum separator, 7...
...Nitrite decomposer, 8...Oxidizer, 9...
...Second gypsum separator, 10...Solid-liquid separator.

Claims (1)

[Claims] 1. NO2 generated as described below is added to the waste gas containing nitrogen oxides (NOx) and sulfur oxides SO2 by circulation, and the waste gas is mixed with an aqueous alkali solution (aqueous alkali metal hydroxide solution, alkaline earth metal The first absorption tower containing an aqueous hydroxide solution or aqueous ammonia solution removes SO2 and some NOx in the waste gas, and the first absorption tower generates sulfites and nitrites. The contained gas is added and mixed into the gas that has passed through the first absorption tower, and the remaining NOX is absorbed by washing in the second absorption tower, which contains the solution containing sulfite and nitrite from the first absorption tower. removing the nitrite to produce nitrite, acid decomposing the nitrite produced in the first absorption tower and the second absorption tower to produce NO, then oxidizing NO to NO2 and circulating it into the waste gas, A method for treating waste gas, characterized in that NOx and SO2 in the waste gas are removed by removing sulfites as sulfates.