JPS5853566B2 - Wet flue gas treatment method - Google Patents

Description

[Detailed description of the invention] The present invention relates to a wet flue gas treatment method.

The present inventors focused on the fact that aqueous solutions containing iodides of alkali metals, alkaline earth metals, or ammonia absorb nitrogen oxides, particularly nitrogen dioxide, extremely well, and based on that knowledge, they absorbed sulfur oxides and nitrogen oxides. Exhaust smoke containing
We have previously proposed a desulfurization and denitrification method for flue gas, which, after optional oxidation, is washed with a lime slurry containing iodides of alkali metals, alkaline earth metals, or ammonia.

(Patent application No. 49-44156).

However, in this case, it is necessary to increase the iodide concentration in order to obtain a sufficient nitrogen oxide removal rate;
A drawback is that the cost of recovering iodide increases when draining it out of the system.

Therefore, the present inventors researched a flue gas treatment method that can obtain a high nitrogen oxide removal rate without the drawbacks mentioned above, and investigated various additives that would promote the absorption of nitrogen oxides by iodide. As a result, it was discovered that even a very small amount of copper ions can bring about a great effect, and the present invention has been achieved.

That is, the present invention converts the exhaust gas into a lime slurry containing an alkali metal iodide, an alkaline earth metal iodide, or an ammonia iodide, or an alkali metal iodide, an alkaline earth metal iodide, or an ammonia iodide. The present invention relates to a wet flue gas treatment method characterized in that a copper compound is allowed to coexist in the cleaning liquid in a method of simultaneously removing sulfur oxides and nitrogen oxides from exhaust gas by cleaning with a limestone slurry containing the same.

The number of copper ions in the copper compound used in the present invention is C
Either u (I) or Cu (II) may be used, and Cu I
5CuC, g2 y CuSO4, etc. can be freely selected.

Even if the copper ion of the added copper compound is CuGI),
It is thought that CuI reacts with the marginal ions (') in the solution and exists as CuI according to the following reaction formula.

Cu (m + 2 I + Cu I + '/2 I
There is no problem because the generated 2 is easily returned to 2- by S02 in the exhaust gas by circulating the absorption liquid.

Further, the amount of copper ions is extremely small, and <0.005 mol/or less is sufficient.

The present invention will be explained in more detail using FIG.

The gas to be treated is introduced into a cooling and dust removing device 2 through a line 1, where it is cooled and humidified to approximately the saturation temperature and most of the dust contained in the gas is removed.

Moisture that evaporates into the atmosphere due to humidification is replenished through line 7 as supply water.

If NOx in the gas to be treated is mainly No, NOx
In order to increase the removal efficiency, gas is led to the No oxidizing device 3 as necessary, and the No oxidizing agent introduced from the line 4 oxidizes No to NO2.

Ozone, nitric acid, chlorine dioxide, etc. are often used as the No oxidizing agent.

Next, the gas is led to the gas cleaning device 5, and the alkali metal iodide is supplied from the absorbent preparation tank 15 through the line 16.
A cleaning solution containing lime slurry containing alkaline earth metal iodide or ammonia iodide, or limestone slurry containing alkali metal iodide, alkaline earth metal iodide, or ammonia iodide with a trace amount of copper ions added. Clean and NOx.

Remove SOx at the same time.

Taking as an example the case where CaI2 is used as an alkali metal iodide or an alkaline earth metal iodide, the reaction is as follows.

That is, SO2 becomes sulfite lime and bisulfite lime by the action of lime or limestone, and a part of it becomes gypsum by the action of 02 in the exhaust gas.

On the other hand, NO2 becomes nitrite lime due to Ca1t, and free 1
■2 is generated.

In particular, the addition of copper ions significantly accelerates this reaction.

Free calcium is converted back to CaI2 by lime sulfite and lime.

Next, bisulfite lime and nitrite lime quickly react to form hydroxylamine disulfonate lime and furthermore imidodisulfonate lime.

At the same time, the bisulfate produced becomes gypsum with lime.

Most of the NOx taken into the liquid side becomes the above-mentioned hydroxylamine disulfonate lime and imide disulfonate lime and accumulates in the absorbent liquid, but some of it becomes nitrite lime. It reacts and decomposes into N2.

The gas discharged from the gas cleaning device 5 passes through a line 6 as clean gas and is discharged from the chimney.

- The absorbent extracted from the gas cleaning device 5 is line 8
It is then guided to the sulfite lime oxidation device 9.

Here, excess lime content is neutralized with sulfuric acid from line 10, the pH value of the liquid is maintained at 6 or below, preferably 4.5 or below 3, and air from line 11 is devised to form fine bubbles. The gypsum is oxidized into gypsum by supplying it through the air supply port.

The reaction formula is as follows. Next, the liquid is gypsum separator 12
The gypsum separated here is collected from line 13,
The separated liquid is circulated through line 14 to absorbent preparation tank 15 .

In addition to the separation liquid, lime or limestone is supplied to the absorbent preparation tank 15 from a line 17 to prepare a lime slurry.

The above-mentioned nitrogen compounds accumulated in the separated liquid are decomposed in the next step.

That is, line 1 branches part of the separated liquid from line 14.
8 to a hydrolysis tank 19, and is hydrolyzed with a small amount of sulfuric acid from line 20 to form hydroxylamine monosulfonate lime, amidosulfonate lime, and bisulfate lime, at which time the pH value decreases to 2 or less. .

The reaction is expressed by the following formula, and is relatively fast *
*Proceeds quickly.

Particularly when the pH value is twice or less, the reaction is rapid and completes in about 1 hour.

The liquid extracted from the hydrolysis tank 19 is led to the decomposition device 21, where the temperature of the liquid is raised to 70°C to promote decomposition.
Heat above ℃.

The decomposition reaction is as follows.

The liquid is then sent to the gypsum displacement reactor 22, which is simultaneously supplied with lime slurry from line 23.

Ammonium bisulfate or ammonium sulfate produced in the above hydrolysis reacts with lime to form gypsum and ammonia.

The reaction is as follows. The amount of lime to be supplied here is equal to or more than the equivalent amount for the above reaction to maintain the pH value of the liquid at alkaline, and at the same time, the temperature of the liquid is increased to 70°C or higher and 100°C.
If the temperature is maintained below, most of the ammonia in the liquid will be dissipated into the gas.

- 10,000, good quality gypsum with large particles can be obtained as produced gypsum.

The liquid containing generated gypsum and excess lime is sent from line 24 to absorbent adjustment tank 15 and used as an absorbent.

On the other hand, the ammonia diffused into the gas is transferred from line 25 to N
Send it to the H3 processing equipment 26 and wash it with acid to neutralize it.
Decomposes at high temperatures of ~300°C in the presence of a catalyst.

Alternatively, since it is a concentrated ammonia gas, it can be recovered and effectively used as liquid ammonia.

EXAMPLE Using Ca2 water-soluble Q2J- containing CuI 2.5 x 10" mol/l as the absorption acid, NO2, SO2 containing gas (NO2120, SO
Absorption of 21,400 ppm and the remaining N2) was carried out.

The pH was maintained at 5 with Ca(OH)2. Gas flow rate 9Nl
The test was carried out for 1 minute, at a liquid temperature of 55°C, and a circulating liquid volume of 60 m'' for 7 hours.

When the concentration of CaI was varied from 0.024 to 0.17 mol/l under the above conditions and the NO2 absorption rate was investigated, the results shown in a of FIG. 2 were obtained.

The horizontal axis represents the CaI2 concentration (mol/A), and the vertical axis represents the NO2 removal rate (4).

For comparison, absorption was carried out under the same conditions as in the example except that CuI was not included, and the absorption rate was b.

It is clear that the addition of CuI increases the absorption performance compared to the case of CaI2 alone, and the degree of this increase increases as the CaI2 concentration increases.

Moreover, the SO2 absorption rate was 90% or more in all cases.

Substantially similar results can be obtained by using alkali metal iodides or ammonia iodides in place of CaI2, and C
Similar results were obtained when other copper salts were added in place of uI.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing one embodiment of the present invention, and FIG. 2 is a graph showing the improvement in NO2 absorption rate of the present invention.

Claims (1)

[Claims] 1. Exhaust gas is treated with lime slurry containing alkali metal iodide, alkaline earth metal iodide or ammonia iodide, or limestone slurry containing alkali metal iodide, alkaline earth metal iodide or ammonia iodide. A wet flue gas treatment method for simultaneously removing sulfur oxides and nitrogen oxides from exhaust gas, the method comprising coexisting a copper compound in the cleaning solution.