JPS588890B2 - Chitsusosan Kabutsurenzokushiyorihohou - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for continuously removing nitrogen oxides from waste gases such as flue gases.

Nitrogen oxides include nitric oxide (NO) and nitrogen dioxide (N
There are various compounds such as dinitrogen trioxide (N20a), dinitrogen monoxide (N20), and dinitrogen pentoxide (N203), but the most important one due to its effect on the human body is monoxide. Nitrogen and nitrogen dioxide; other substances are small in quantity and can be ignored in terms of air pollution.

Generally, the combination of nitrogen monoxide and nitrogen dioxide is called nitrogen oxide (NOx).

Nitrogen oxides are not only physiologically harmful to the human body even in minute amounts, but they also play the role of triggering photochemical smog that damages not only the human body but also agricultural crops, and even in extremely small amounts they are harmful. was recently revealed.

Nitric oxide easily converts into nitrogen dioxide in the atmosphere, but this nitrogen dioxide is decomposed into nitrogen monoxide and atomic oxygen by sunlight, and this atomic oxygen converts oxygen molecules in the atmosphere into ozone, an air pollutant. It is said that the first stage of photochemical smog is caused by converting hydrocarbons, which are one of the two, into aldehydes.

A known method is to remove nitrogen oxides by catalytically reducing the nitrogen oxides using a carbon catalyst at a high temperature of 150° C. to 400° C. while blowing ammonia gas into the waste gas containing nitrogen oxides.

In such a method, in a system where there are no sulfur oxides in the exhaust gas, the carbon catalyst can be used as a nitrogen oxide reduction catalyst substantially permanently.

However, in systems where sulfur oxides coexist in the exhaust gas, reactions between sulfur oxides and ammonia occur, especially at temperatures below 350°C, and the products adhere to the catalyst over time. The efficiency (hereinafter referred to as denitrification rate) decreases after a certain period of time, and along with this, the pressure loss of the catalyst layer increases, the gas flow rate decreases, and in severe cases, it becomes difficult to operate the device.

(The p time until the denitrification rate decreases varies depending on the concentration of sulfur oxides in the waste gas and the treatment temperature, but as an example, at 200 ppm of nitrogen oxides, 500 ppm of sulfur oxides, and a treatment temperature of 230°C, it takes about 80 It takes about an hour.

) To overcome these drawbacks, reactions are generally reduced to 350
It is carried out at a high temperature of about ℃ or higher, or sulfur dioxide (S
It has been practiced to previously oxidize O2) to sulfur trioxide (SO), react it with ammonia, collect it as ammonium sulfate, and then treat only the nitride oxide.

However, in the former case, energy for heating the waste gas is wasted, and in the latter case, the reaction becomes complicated.

Therefore, as an improvement method for the above, when these sulfur compounds adhere at low temperatures below 300°C, the carbon catalyst is separated from the nitrogen oxide treatment reaction because the catalyst is not permanently poisoned by sulfur oxides. Thereafter, methods have often been used to regenerate the material by washing it with water or contacting it with a high temperature (usually 300° C. or higher) inert gas that does not contain oxygen, and then subjecting it to the reaction again.

However, with this method, the nitrogen oxide reduction treatment may be temporarily interrupted, or in order to make it continuous, it is necessary to equip another similar treatment device or use a carbon catalyst with sulfur compounds attached. It becomes necessary to take them out continuously.

Furthermore, a device for regenerating the attached carbon catalyst is also required.

When a carbon catalyst with sulfur compounds attached is regenerated by washing with water, a large amount of water is required, and these compounds cause corrosion of the equipment, so corrosion-resistant equipment that can withstand temperatures of 200°C or higher is required. There is a drawback that a large amount of heat is required for drying in order to subject the material to a treatment reaction again after regeneration.

A method of regenerating the carbon catalyst with a high-temperature inert gas that does not contain oxygen has also been considered, but in this case as well, an inert gas generator must be attached.

In addition, considerable efforts must be made to eliminate oxygen in the inert gas.

The present inventors have developed a continuous treatment method for nitrogen oxides that can be carried out at relatively low temperatures and that can regenerate carbon catalysts without using such water or inert gas or attaching special regeneration equipment. As a result of this study, we have arrived at the present invention.

That is, the present invention uses a catalytic reduction catalyst, and the temperature is 350°C.
When treating nitrogen oxide-containing waste gas that coexists with sulfur oxides at temperatures below ℃, ammonia is introduced into the middle of the catalyst layer, and only the waste gas is allowed to pass through the first half of the catalyst layer, allowing the catalyst to be regenerated by the waste gas. Then, before the waste gas flows to the latter half, ammonia is mixed in, and the nitrogen oxides in the waste gas are subjected to catalytic reduction treatment in the latter half of the catalyst layer, and when the catalytic ability of the latter half of the catalyst layer decreases. This is a continuous nitrogen oxide treatment method characterized by reversing the flow of the waste gas.

In order to use one reactor tower and perform the catalyst regeneration and the nitrogen oxide reduction reaction simultaneously and continuously in the same reactor, the following procedure may be performed.

In other words, only the waste gas to be treated passes through the first half of the catalyst layer, and the waste gas heated to the reaction temperature removes substances adhering to the catalyst and regenerates the catalyst. Ammonia is mixed into the reactor, and nitrogen oxides are treated by catalytic reduction in the latter half of the catalyst layer, decomposing them into safe nitrogen and water.

After that, when the denitrification rate in the rear half of the column begins to decrease, the flow of waste gas is switched and reversed, and the latter half before switching is used as the first half to regenerate the catalyst, and the first half is used as the second half for denitration. All you have to do is make them consecutive.

In addition, when two reactors A and B are used and an ammonia inlet is provided between the two catalyst-filled towers (i.e. reaction towers), the reactors A and B may be arranged in series. , the reaction column A may be the first half of the catalyst layer, and the section B may be the second half of the catalyst layer, as described above.

In this case, if there is a large amount of waste gas, in addition to A and B2 towers, two towers, A/ and B/, should be provided separately, and A and B2 towers would be installed.
They may be arranged in parallel to the tower and the timing of the reversal of the waste gas flow may be staggered.

The waste gas treated by the method of the invention is generated by burning a sulfur-containing solid fuel such as coal or a sulfur-containing liquid fuel such as heavy oil with air in a burner, and the flue gas (waste gas) thus generated is gas) is oxygen by using excess air, 0.5-10% by volume,
It usually contains trace amounts of sulfur trioxide (SO3), up to about 0.3% by volume of sulfur dioxide (SO2), and 100-1500 ppm of nitrogen oxides (NOX).

In addition to flue gas, the present invention can also be applied to waste gas from glass melting furnaces, waste gas from sintering furnaces, and the like.

The catalyst that can be used in the present invention may be any catalyst based on carbonaceous material, alumina, silica, or diatomaceous earth as long as it can selectively catalytically reduce nitrogen oxides in the presence of ammonia, and there are no particular restrictions. do not have.

By applying the present invention, about 90% of nitrogen oxides (NOx) can be continuously removed.

Regenerating the catalyst used in the present invention using waste gas at a temperature that causes nitrogen oxides and ammonia to react is a completely new method, which has not been found in any conventionally known method.

EXAMPLES The present invention will be explained in detail below with reference to Examples, but the present invention is not limited to these Examples.

Example 1 Two catalytic reaction towers A and B each having a cross section of 500 mm square were each filled with 6M of carbonaceous catalyst.

The height of each solution layer was 250 mm.

A reaction was carried out by introducing boiler flue waste gas into this reaction column.

The waste gas contains 250 ppm of nitrogen oxides (NOX), 500 ppm of sulfur oxides (SOX), and oxygen (02).
4%, water vapor (H20) 12%, carbon dioxide gas (C02) 1
With the remaining composition being 0% nitrogen (N2), the temperature was 250°C and the gas amount was 43 Nm3/H.

The reaction was started by adding 500 ppm of ammonia to the B column from the ammonia inlet provided between the reaction columns A and B, and the reaction was carried out in the B column.

The space velocity (SV) of the reaction was 710 hS.

The nitrogen oxide (NOx) concentration at the B outlet after treatment is 2 2
ppm, and the denitrification rate was 91%.

90 hours after the start of the reaction, the pressure drop in the B catalyst layer was 28 mmAq.
Since the denitrification rate decreased from 87 mmAq to 87%,
The reaction column was newly used as the second half for the reaction, and the B column, which had been undergoing the reaction, was used as the first half, and the catalyst in the B column was regenerated using the waste gas.

After switching the reaction tower from B tower to A tower, the denitrification rate was 92%.
Met.

In addition, 72 hours after the changeover, the pressure drop in Tower B, which had increased to 87 mmAq, recovered to 347 l1mAq.

Then, the catalyst 10 is placed on the catalyst in the B chamber immediately before switching.
Sulfur content per 0mA is 6 in terms of sulfur dioxide (SO2)
The amount equivalent to 230 mg was adsorbed, but after regeneration (72 hours), the amount was 738 mg per 100 ml.

On the other hand, the pressure drop in tower A used for the reaction was 28 imAq to 60 imAq.
It had risen to mmAq.

Thereafter, the gas flow was switched every 72 hours to continuously treat nitrogen oxides, but the denitrification rate remained constant at 92%.

Example 2 Each of cylindrical catalytic reaction towers (A and B towers) with a diameter of 100 mm was filled with 1 liter of catalyst in which ferrous sulfur (FeSO4) was supported on an alumina carrier.

The same flue gas as in Example 1 was introduced, and the temperature was 270°C.
The reaction was carried out at Sv2,000hr'.

The reaction was started by adding 500 ppm of ammonia to the B column from an ammonia inlet provided between the reaction columns A and B, and nitrogen oxides were reduced in the B column.

After treatment, the concentration of nitrogen oxides (NOx) at the outlet B is 60
ppm, and the denitrification rate was 76%.

100 hours after the start of the reaction, the pressure drop in the B catalyst layer was 2 mmAq.
9 miAq, the gas flow was changed in the same manner as in Example 1, and the reaction tower was switched from B tower to A tower.

After switching, the catalyst in column B was almost regenerated in 60 hours, and the pressure drop recovered to 3 mmAq.

Thereafter, nitrogen oxides were treated continuously while changing the gas flow every 60 hours, but the operation was successful without any problems.

Claims (1)

[Claims] 1 When using a catalytic reduction catalyst to treat nitrogen oxide-containing waste gas that coexists with sulfur oxides at a temperature of 350°C or lower, ammonia is introduced into the middle of the catalyst layer, and the first half of the catalyst layer is used only for the waste gas. The catalyst is regenerated by the waste gas, and then ammonia is mixed in before the waste gas flows to the latter half of the catalyst layer, and the nitrogen oxides in the waste gas are subjected to catalytic reduction treatment in the latter half of the catalyst layer. A continuous nitrogen oxide treatment method characterized by reversing the flow of the waste gas when the catalytic ability of the latter half of the catalyst layer decreases.