JP3502966B2 - Nitrogen dioxide removal method - Google Patents

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to nitrogen dioxide (NO).
₂ ) Regarding the removal method, more specifically, it relates to a method for removing NO ₂ in exhaust gas generated during regeneration of an adsorbent that adsorbs and removes NOx in ventilation gas in a road tunnel or during production of nitric acid by selective catalytic reduction.

[0002]

2. Description of the Related Art A conventional method for removing nitrogen dioxide is to remove NO ₂ in a gas to be treated by reducing and decomposing NO ₂ with a reducing agent such as ammonia or urea using a vanadium-supported titania catalyst.

[0003]

However, in the above-described denitration method, as will be described later, when the NOx component molar ratio in the gas to be treated is NO ₂ > NO, the catalytic activity is higher than when NO ₂ ≦ NO. There was a problem of low.

FIG. 1 shows NO / at a reaction temperature of 250 ° C.
The relationship between the NOx ratio and the denitration rate is shown. The denitration conditions in this case are as follows.

Area velocity (AV): 35 Nm ³ / m ² ·
h, the gas to be treated is air + H ₂ O (about 3%), NOx:
90 ppm, NH ₃ : 90 ppm.

Here, the NO / NOx ratio is 0.5 (NO:
When NO ₂ = 1: 1), the denitration performance of the catalyst becomes maximum,
NO / NOx ratio becomes lower than 0.5 (NO ₂ rich)
The denitration performance decreases as it goes. It is considered that one of the causes of the decrease in the catalytic activity is a decrease in NH ₃ adsorption points due to an excess of oxygen on the catalytic active points. That is, 1) In the case of NO removal, NO + NH ₃ + 1 / 4O ₂ → N ₂ + 3 / 2H ₂ O catalyst active sites are reduced and oxygen becomes insufficient, but oxygen is supplemented by the active site adsorption of O _{2 in} the gas to be treated. Be seen. When the reaction temperature is a low temperature of 200 ° C. or lower, re-oxidation of the catalyst due to the gas phase oxygen hardly occurs, and the denitration performance sharply decreases.

2) In the case of removing NO ₂ When NO ₂ + NH ₃ → N ₂ + 3 / 2HO + 1 / 4O ₂ high concentration oxygen exists in the gas to be treated, the oxygen generated on the catalytic active point is easily vaporized. It cannot be detached into a phase. Therefore, ammonia adsorption is hindered by excess oxygen on the catalyst active site, and as a result, the denitration performance of the catalyst is reduced.

3) NO + NO ₂ (molar ratio 1: 1) NOx NO + NO ₂ + 2NH ₃ → 2N ₂ + 3H ₂ O There is no excess or deficiency of oxygen and the maximum NOx removal performance of the catalyst can be obtained.

An object of the present invention is to adsorb and remove NOx.
An object of the present invention is to provide a nitrogen dioxide removing method which does not deteriorate the denitration performance even when the NOx component molar ratio in the exhaust gas generated during regeneration of the binder or during production of nitric acid is NO ₂ > NO.

[0010]

The method for removing nitrogen dioxide according to the present invention is used for regenerating an adsorbent from which NOx has been adsorbed and removed, and nitric acid.
NO ₂ in the exhaust gas produced during the production of NH
In nitrogen dioxide removal method decomposed by selective catalytic reduction using a _3, denitration reaction temperature at a low temperature (300 ° C. or less)
When the NOx component molar ratio in the exhaust gas is NO ₂ > NO
In case of reacting with excess oxygen on the catalytic active site and at 300 ° C
It is characterized in that a substance that is oxidized below, preferably an organic compound, is used as a reduction aid.

When ammonia water or urea water is used as the NH ₃ supply source, a substance that reacts with excess oxygen on the catalytic active site and is oxidized at 300 ° C. or lower (this is called the reduction auxiliary agent) is used. ) Is added to the reducing aid and N
It is preferable to use H ₃ together.

The reducing aid is required to react with excess oxygen on the catalytic active site and be oxidized at a low temperature (300 ° C. or lower) regardless of the gas phase oxygen.

The amount of the reducing aid injected is preferably as small as possible in consideration of generation of unreacted substances and by-products.

[0014] The reducing Motosuke agent, alcohol such as charcoal <br/> hydrogen or 2-propanol and propylene is exemplified.

The denitration catalyst is not particularly limited as long as it has an increased oxidizing power so that the reduction aid can be easily oxidized. Specific examples thereof include a vanadium-supported titania catalyst and the like.

NO ₂ in the gas to be treated is replaced with NH 3 which is a reducing agent.
_{In the} NO ₂ removal that decomposes by selective catalytic reduction using ₃ , when the denitration reaction temperature is high (more than 300 ° C.),

[Chemical 1] NO is generated due to the equilibrium relationship of. Further, by burning ammonia, NH ₃ + 5 / 4O ₂ → NO + 3 / 2H ₂
The reaction of O produces NO. These generated N
O deprives the catalyst surface (on the active site) of excess oxygen, so that the denitration performance of the catalyst does not deteriorate so much.

On the other hand, when the denitration reaction temperature is low (300 ° C. or lower), NO generation can hardly be expected, and therefore excess oxygen that inhibits ammonia adsorption exists on the catalyst surface (on the active site). According to the method, by using a substance that reacts with excess oxygen on the catalytic active site and is oxidized at 300 ° C. or lower as a reduction aid, the excess oxygen is consumed in the oxidation of the reduction aid, resulting in a catalyst. Ammonia adsorption on the active site is not hindered by the above-mentioned excess oxygen, so that NO ₂ is reliably removed.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Next, examples of the present invention will be described.

1. Preparation of Catalyst Ceramic paper consisting of ceramic fibers (thickness 0.
25 mm) was impregnated and supported with a titania colloidal solution (solid content: 32 wt%) obtained by the nitrate hydrolysis method, dried at 110 ° C. for 1 hour, and then calcined at 400 ° C. for 3 hours to give anatase-type titania. 90 g / m
^Two retained plate-like carriers were obtained.

This plate-shaped carrier was immersed in a saturated aqueous solution of ammonium metavanadate (normal temperature) and then dried at 200 ° C. for 30 minutes. Then, after repeating this operation once more, the obtained dried product was calcined at 400 ° C. for 1 hour to obtain a vanadium-supported titania plate-shaped catalyst.

2. Example 1 Removal of Nitrogen Dioxide Using Hydrocarbon as the Reduction Aid Example 1 Using the above vanadium-supported titania plate catalyst as a denitration catalyst, ammonia as a reducing agent, and propylene as the reduction auxiliary, under the following conditions: To remove nitrogen dioxide.

Comparative Examples 1 and 2 Nitrogen dioxide was removed by using only ammonia (Comparative Example 1) and propylene (Comparative Example 2) as reducing agents without using the reducing aid.

[0023] In the above Example 1 and Comparative Examples 1 and 2, the reducing agent and the reducing Motosuke agent was used in the gas to be treated with an equal volume.

Denitration conditions: Area velocity (AV): 35 Nm ³ / m ² · h, gas to be treated is air + H ₂ O (about 3%), NO ₂ : 50 ppm, N
H _3: 50ppm, propylene: As seen in 50ppm Figure 2, when the propylene co-Example 1 was used as the reducing Motosuke agent with ammonia as a reducing agent, compared with the case of Comparative Example 1 using only ammonia 300 ° C
It is recognized that the nitrogen dioxide removal performance is high at the following low temperatures. Further, in the case of Comparative Example 2 using only propylene, since nitrogen dioxide was scarcely removed, propylene functions only as the reducing aid that deprives the catalyst of excess oxygen, and the reducing agent is ammonia. Was confirmed to work.

3. Nitrogen dioxide removal using alcohol as the reduction aid Examples 2 to 5 Using the above vanadium-supported titania plate-like catalyst as a denitration catalyst, ammonia was used as a reducing agent in the same amount as the gas to be treated, and at the same time as the reduction aid. Nitrogen dioxide was removed under the following conditions by using 2-propanol sequentially reduced from the same amount as the gas to be treated.

Denitration conditions: Area velocity (AV): 35 Nm ³ / m ² · h, gas to be treated is air + H ₂ O (about 3%), NO ₂ : 50 ppm, N
H _3: 50 ppm, 2-propanol: 5 to 50 ppm 2-propanol, 5 ppm (Example 2), 12.5
ppm ( Example 3), 25 ppm ( Example 4), 50 p
Each aqueous solution of pm ( Example 5) was heated and vaporized, and then used together with ammonia.

As shown in FIG. 3, in the case of each example in which 2-propanol was used as the reducing aid together with ammonia as the reducing agent, it was higher than in the case of the comparative example 1 using only ammonia. Nitrogen dioxide removal performance is recognized.

Further, in the case of each of the examples using 2-propanol as the reducing aid, higher nitrogen dioxide removal was performed in the low temperature range of 200 to 250 ° C. than in the case of the above-mentioned Example 1 using propylene as the reducing aid. Performance is recognized.

Further, it can be seen that the amount of 2-propanol used as the reduction aid is higher than the amount of the gas to be treated to a certain extent, and the nitrogen dioxide removing performance is higher.

2-Propanol is more easily oxidized than propylene in a low temperature range, and the catalyst is excessively reduced by excessive injection of 2-propanol, or unreacted 2-propanol is used.
It is considered that propanol tends to inhibit the denitration performance by adsorbing to the catalyst.

Here, the oxidation reaction of 2-propanol is C ₃ H ₇ OH + 9 / 2O ₂ → 3CO ₂ + 4H ₂ O, and the injection amount of 2-propanol is 1 mol of NO.
It can be said that 1/9 mol or less for ₂ is sufficient.

[0032]

According to the method for removing nitrogen dioxide according to the present invention, N
When the adsorbent from which Ox has been adsorbed and removed is regenerated or nitric acid is produced,
In the nitrogen dioxide removal method that decomposes NO ₂ in flue gas by selective catalytic reduction using NH ₃ as a reducing agent, the NOx removal reaction temperature is low (300 ° C. or less)
When the NOx component molar ratio in the NOx is NO ₂ > NO,
Reacts with excess oxygen on the active site and oxidizes below 300 ° C
Since a substance composed of hydrocarbons or alcohols used is used as a reduction aid, NOx in exhaust gas at low temperature
When the component molar ratio of NO ₂ > NO, the excess oxygen generated on the catalyst surface (on the active site) is consumed for the oxidation of the reduction aid, and therefore the excess oxygen is on the catalyst surface (on the active site).
It does not interfere with ammonia adsorption in. Therefore, there is an effect that the adsorption of ammonia on the catalytically active point is surely performed .

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a NO / NOx ratio and a denitration rate by a conventional denitration method.

2 is a graph showing the NOx removal efficiency according to a comparative example of examples and with which the said reducing Motosuke agent present invention using propylene as.

3 is a graph showing the NOx removal efficiency according to a comparative example of examples and with which the said reducing Motosuke present invention using 2-propanol as agent.

─────────────────────────────────────────────────── ─── Continuation of the front page (56) Reference JP-A 5-220348 (JP, A) JP-A 7-227523 (JP, A) JP-A 53-26262 (JP, A) JP-A 62- 163731 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B01D 53/00-53/96

Claims (4)

(57) [Claims] 1. When regenerating an adsorbent from which NOx is adsorbed and removed,
In nitrogen dioxide removal method decomposed by selective catalytic reduction using NH ₃ as a reducing agent NO ₂ in exhaust gas produced during the manufacture of nitric acid, the denitration reaction temperature low (300 ° C. or less
(Below), the NOx component molar ratio in the exhaust gas is NO ₂ >
In the case of NO, reacts with excess oxygen on the catalytic active site and
A method for removing nitrogen dioxide, which comprises using a substance that is oxidized at 00 ° C. or lower as a reduction aid. As a source of wherein NH _3, claim 1, together with the use of aqueous ammonia or urea water, which comprises using the said reducing Motosuke agent and NH ₃ by dissolving the reducing Motosuke agent thereto an aqueous solution at the same time Nitrogen dioxide removal method described. 3. The method for removing nitrogen dioxide according to claim 1, wherein the reducing aid is a hydrocarbon. 4. The reduction aid is alcohol.
Or the method for removing nitrogen dioxide according to 2.