JPH0866619A - Treatment of gas containing low concentration nitrogen oxide - Google Patents

Abstract

PURPOSE: To efficiently remove NOx of low concentration without secondary pollution by keeping the concentration of desorbed NOx in a regenerating gas circulating route low, making a part of the circulating gas branch off to denitrify it at that time, and freely selecting the temperature. CONSTITUTION: After gas contg. dilute NOx 1 gets rid of dust, it is introduced into adsorption towers 6a, 6b to adsorb and remove NOx. Next, the absorption tower 6b which has finished adsorption enters a desorption process, and a denitrifying reactor 14 is heated, and a gas circulating route is formed of a denitrifying gas blower 11, a denitrifying gas heater 12 and a denitrifying reactor 14. A regenerating gas circulating route is separately formed of the adsorption tower 6b, a desorbed gas circulating blower 8 and a desorbed gas heater 9. Since when the adsorption tower 6b is regulated to the suitable temperature for desorption, the concentration of the desorbed NOx is increased, the gas contg. NOx of high concentration in the desorption tower heating and circulating route is introduced to the denitrified gas circulating route, and a reducing agent is fed to reduce and decompose the gas by the denitrifying reactor 14. When the concentration of NOx in the regenerating gas circulating route is lowered to a prescribed value, the adsorption tower 6b is cooled by a cooler 10 preparing for switching to adsorption.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently removing NOx from a gas containing low-concentration nitrogen oxides (hereinafter abbreviated as NOx), and in particular, ventilation in various tunnels such as underground tunnels, mountain tunnels and roads with shelters. Low concentrations of NOx contained in gas and exhaust gas from indoor car parking lots
The present invention relates to a method for treating low-concentration nitrogen oxides that efficiently removes nitrogen.

[0002]

2. Description of the Related Art Recently, various tunnel roads such as underground tunnel roads, roads with shelters, etc. and indoor car parking lots have rapidly increased along with the development of transportation networks and the increase of physical distribution mainly in large cities. The construction of a mountain road is remarkable. Such ventilation equipment for automobile tunnels is provided mainly for the purpose of protecting the health of pedestrians and improving the visual distance.
The ventilation system generally used at present is to supply fresh outside air into the tunnel and to ventilate contaminated air outside the tunnel. However, these ventilation gases contain about 5 ppm of NOx, and if they are released to the surroundings, the surrounding atmospheric environment becomes a problem.
For this reason, various methods have been proposed for removing low-concentration nitrogen oxides contained in ventilation gases for these automobile tunnels and the like.

[0003] These are a few p like exhaust from a car tunnel.
As a technique for treating NOx of about pm, there is an adsorption removal method using an adsorbent.
No. 016886 (Japanese Utility Model Laid-Open No. 2-108719), NOx in polluted air is previously oxidized to NO ₂ and then adsorbed and removed by an adsorbent (for example, activated carbon type), and high temperature air is adsorbed on the adsorbent. An apparatus is disclosed in which high-concentration NOx that is desorbed by being circulated is fractionated, a reducing agent is added, and nitrogen gas is decomposed through a denitration catalyst reactor and exhausted to the outside of the system.

On the other hand, Japanese Patent Application No. 4-110761 (Japanese Unexamined Patent Publication No.
5-277339), an adsorber during the desorption process,
A circulation system path including a heater, a denitration reactor, and a cooler after the denitration reaction is formed, and the desorbed gas of the adsorber is heated to a predetermined temperature suitable for denitration, and a reducing agent is added to the desorbed gas in the desorbed gas. A method has been proposed in which NOx is reduced, and then the heating gas on the outlet side of the denitration reactor is adjusted to an appropriate temperature for desorption of NOx by a cooler and then refluxed to the desorber.

[0005]

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention
In implementing the device shown in Japanese Patent No. 719,
By circulating hot air through the adsorbent in the desorption process, the NOx concentration in the circulating gas is gradually increased by the accumulation of desorbed NOx, and when it reaches a certain concentration, a part of the circulating gas is branched and introduced into the denitration reactor. As a result, NOx is reduced and decomposed to be harmless and discharged to the outside of the system. Therefore, air comparable to this branched gas is sucked into the system, so that the NOx concentration in the circulation system passage gradually decreases, and when the NOx concentration falls to a predetermined value, the introduction of the branched gas into the denitration reactor is stopped and reduced. Is to end. In this way NOx in desorption gas
Since the concentration decreases as the desorption time elapses, it is necessary to control the injection of the reducing agent with high accuracy according to this change in concentration, and it often fails to leak the reducing agent and NOx out of the system. There was a defect such as.

In the method disclosed in Japanese Unexamined Patent Publication No. 5-277339, the high temperature air is circulated through the adsorbent during the desorption process, which is the same as that disclosed in Japanese Utility Model Laid-Open No. 2-108719. However, a denitration reactor is installed in this circulation system, and a reducing agent is added to reduce and decompose NOx, and when the NOx concentration in the circulation system decreases to a predetermined concentration, denitration is terminated. Unreacted N
Ox and the reducing agent are never discharged out of the system.
However, since it is necessary to denitrate the whole amount of the high temperature circulating air, the denitration reactor becomes large as compared with the case where a portion of the circulating air is denitrated as shown in Japanese Utility Model Laid-Open No. 2-108719. There are drawbacks. Further, if the high temperature air used for regeneration is too high, it is disadvantageous in terms of energy economy and it is necessary to improve the heat resistance of the device material and strengthen the heat retention, which leads to cost increase. However, the temperature required for denitration is preferably about 250 to 300 ° C. In this method, for example, when desorption is performed at about 150 ° C., the circulating gas temperature is heated to the denitration temperature at the denitration reactor inlet to remove the denitration. It is necessary to cool down to the desorption temperature at the adsorbent inlet during the subsequent desorption, and there was an economical problem such as energy loss for heating and cooling and an increase in equipment cost.

In view of the above-mentioned state of the art, the present invention aims to provide a denitration method for a low-concentration NOx-containing gas that does not have the problems of the above-mentioned prior art.

[0008]

The present invention is a method of dry-treating a gas containing low-concentration nitrogen oxides in an adsorption tower to mainly adsorb and remove nitrogen oxides, and desorbing the adsorption tower with high temperature air to regenerate it. In, a desorption gas circulating blower for desorbing gas from the adsorption tower being regenerated, a regeneration gas circulation system passage consisting of desorption gas to be circulated to the adsorption tower being regenerated again via a desorption gas heater is formed, and the regeneration gas is A denitration gas blower that sucks and branches a regeneration gas consisting of a part of desorption gas from a circulation system, a denitration gas heater that raises the temperature of the regeneration gas to a temperature suitable for the denitration reaction, and a reducing agent is injected into the heated regeneration gas. A reducing agent supply pipe, a denitration reactor, and a denitration gas circulation system that joins the processing gas from the denitration reactor to the regeneration gas circulation system, and the regeneration gas circulation is performed as the temperature of the adsorption tower increases during regeneration. Inside the road When the concentration of nitrogen oxides in the desorption gas begins to rise, regeneration of a predetermined amount of the regenerated gas circulation system by the denitration gas blower into the denitration gas circulation system that has been heated to a temperature suitable for the temperature of the denitration reaction in advance. After introducing the circulating gas and injecting the reducing agent through the reducing agent supply pipe, the nitrogen oxide is reduced and decomposed by passing through the denitration reactor, and the denitration processing gas is merged with the regenerated gas circulation system passage again to produce the nitrogen oxide. When the concentration of the combined gas is recirculated and supplied to the adsorbing tower during regeneration, regeneration of the adsorbing tower proceeds, and when the concentration of nitrogen oxides in the regenerated gas circulation system is reduced to a predetermined value, the denitration reaction The method for treating a low-concentration nitrogen oxide-containing gas is characterized in that the operation of the vessel is stopped, the temperature of the adsorption tower after regeneration is cooled to room temperature, and the regeneration of the adsorption tower is terminated.

[0009]

In the present invention, when desorbing NOx adsorbed in the adsorbing tower during regeneration, the desorbed gas in the adsorbing tower during regeneration is a regenerated gas circulation system including a desorbed gas circulation blower and a desorbed gas heater. While heating to a predetermined temperature for desorption in the road,
It is circulated to the adsorption tower during regeneration, but a part of the circulating gas in the regeneration gas circulation system is branched and heated to a predetermined temperature for denitration, and a reducing agent is added to remove NOx in the denitration reactor. Regeneration is promoted by joining the reductively decomposed clean gas again with the circulation gas in the regeneration gas circulation path. In the regeneration operation described above, N contained in the gas introduced into the denitration reactor
The denitration reaction of Ox is performed very quickly, and NOx supplied to the denitration reactor is almost decomposed and removed by selecting an appropriate denitration temperature and appropriately supplying a reducing agent.

On the other hand, the NOx desorption rate from the adsorption tower is relatively slow and is gradually carried out over several tens of minutes. Therefore, what is introduced into the denitration reactor is a part of the circulating gas in the regeneration gas circulation system, but the NOx concentration in the circulation system is suppressed to a considerably low level, and NOx desorption proceeds rapidly. For this reason,
Even if the desorption of the adsorbed NOx is kept low, the desorption is sufficiently performed.

Next, even if the amount of reducing agent to be added becomes excessive or insufficient, the denitration apparatus outlet gas returns to the regeneration gas circulation system passage, so that NOx and reduction occur while circulating through the regeneration gas circulation passage. The concentration of the agent is gradually decreased, and unreacted NOx and reducing agent are not discharged outside the system. In particular, if the amount of reducing agent added is insufficient, the reducing agent will not flow into the circulating gas.

Further, NO desorbed from the adsorption tower during regeneration.
When x is reduced and desorption is completed, the NOx concentration in the regeneration gas circulation system path is rapidly decreased. Therefore, at this point, the denitration reaction in the denitration device is terminated, and the adsorption column after desorption is adsorbed. Cool to a suitable temperature and complete regeneration. This makes it possible to prepare for the next suction operation.

As the NOx adsorbent, one capable of efficiently removing NOx in the air is preferable, and activated carbon or a copper ion-exchanged zeolite proposed by the applicant of the present invention (Japanese Patent Application No. 1-325).
834) and the like. When NO ₂ is mainly used, activated alumina, activated silica and the like can be used. In addition, when heating the regeneration gas circulation system path, it is necessary to measure the homogeneity with the atmosphere due to the expansion of the gas in the system, and the escape gas for this must be released through the adsorption process inlet or a separate adsorbent. Is desirable.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIG. After introducing the lean NOx-containing gas 1 to the dust remover 3 by the suction fan 2 to remove the medium dust in the processing gas, the dust removing gas 4 is filled with the adsorbent through the automatic opening / closing valve V ₁ or V ₅ . It is introduced into either the NOx adsorption tower 6a or 6b. Here, NO is adsorbed and removed, and becomes the clean gas 5 or 7 through the automatic opening / closing valve V ₃ or V ₇ and is released to the atmosphere. In order to continuously purify NOx, two or more NOx adsorption towers should be installed, and one NOx adsorption tower should have no outlet NOx.
The adsorption tower is switched to another adsorption tower before the x concentration reaches a predetermined value.

Next, the following desorption process is carried out in the adsorption tower (hereinafter referred to as 6b) which has completed the adsorption process. First, just before the NOx adsorption tower 6b is heated, the denitration reactor 14 is heated. For this purpose, the denitration gas blower 11
→ Denitration gas heater 12 → Denitration gas circulation system connecting the denitration reactor 14 is formed. As a result, the denitration reactor 14
Is heated to a temperature above 200 ° C. suitable for the reaction. The pressure increase due to the heating of the system at this time is eliminated by the atmospheric pressure equalizing pipe 15.

Next, the automatic on-off valves V ₆ , V ₈ , V ₁₁ , V ₁₂
Is opened and the NOx adsorption tower 6b is being desorbed by NOx while the automatic opening / closing valve V ₆ → NOx adsorption tower 6b → automatic opening / closing valve V ₈ → desorption gas circulation blower 8 → moving opening / closing valve V ₁₁ → desorption gas heater 9 → moving A regeneration gas circulation system passage of the desorption tower having the opening / closing valve V ₁₂ is formed, and the NOx desorption tower 6b is heated and adjusted to an appropriate desorption temperature of about 100 to 200 ° C. When the desorbed NOx concentration starts to increase with the rise in temperature of the NOx desorption tower 6b, the high-concentration NOx-containing gas in the desorption tower heating circulation path is continuously guided to the denitration blower 11 to the denitration gas circulation path during operation. A reducing agent such as ammonia is supplied from the reducing agent supply pipe 13, and high-concentration NOx in the desorption gas is reduced and decomposed in the denitration reactor 14. When the NOx desorption of the NOx adsorption tower 6b approaches the end, and the NOx concentration in the regeneration gas circulation system path decreases to a predetermined value, the reducing agent supply from the reducing agent supply pipe 13 is stopped and the denitration gas heater 12 and the desorption gas are removed. Turn off the heater 9,
The automatic opening / closing valves V ₁₁ and V ₁₂ are closed, and the automatic opening / closing valves V ₉ and V ₁₀ are closed.
And the cooler 10 is operated to cool the NOx adsorption tower 6b to near room temperature to prepare for the next NOx adsorption step switching. The pressure increase during heating of the regeneration gas circulation system passage is prevented through the atmosphere equalizing pipe 15 of the passage.
The gas discharged from this pipe is introduced into the inlet of the NOx adsorption tower 6a to adsorb and remove a small amount of NOx and unreacted reducing agent contained in the exhaust gas.

[Experimental Example 1] The NOx removal performance was examined using the NOx removal device shown in FIG. 1 kg of granular activated carbon (3GS manufactured by Kuraray Chemical Co., Ltd.) was filled as an NOx adsorbent. NOx adsorption tower is used, and NO is 10 as adsorption condition.
Air added with ppm (temperature: 30 ° C., humidity: 2.6 Vo
Adsorption was carried out for 10 hours at a gas superficial linear velocity of 10 cm / sec using a gas to be treated of 1%) 200 liter / min. The NOx concentration at the outlet of the NOx adsorption tower at the beginning of the adsorption was 1.4 ppm, and was about 3.8 ppm after 10 hours.

After completion of the adsorption, in the denitration gas circulation system passage (denitration gas blower 11 → denitration gas heater 12 → denitration reactor 14) shown in FIG. 1, the denitration reactor 14 (honeycomb-like TiO 2).
₂ carriers V ₂ O ₅ supported catalyst 0.1 liter) 25
While maintaining at 0 ° C, the regeneration gas circulation system (automatic open / close valve V ₆ → NOx adsorption tower 6b → automatic open / close valve V ₈ → desorption gas circulation blower 8 → automatic open / close valve V ₁₁ → desorption gas heater 9 → automatic opening / closing The valve V ₁₂ ) is started to be heated, and the NOx adsorption part is at 100 ° C.
When the concentration of adsorbed NOx suddenly rises over the range, ammonia gas is supplied from the reducing agent supply pipe 13 to remove the denitration reactor 14
Then, denitration was started. Next, the NOx adsorption section of the NOx adsorption tower 6b is heated to a temperature of about 180 ° C. to continue NOx adsorption and denitration, and the regeneration is terminated when the NOx concentration in the regeneration gas circulation system passage becomes 10 ppm or less. .
Here, the gas flow rate is 5 liters / m in the denitration gas circulation path.
In, the flow rate was 40 liters / min in the regeneration gas circulation system, and the ammonia supply amount during denitration was about 95% (molar ratio) with respect to the NOx amount at the inlet of the denitration reactor 14.
During this period, the gas released from the atmospheric pressure equalizing pipe 15 in the denitration gas circulation system was supplied to the inlet of the NOx adsorption tower 6a in the adsorption process. Ammonia at the outlet of the NOx adsorption tower 6a was measured but was not detected.

[0019]

INDUSTRIAL APPLICABILITY The present invention is a method for removing low-concentration NOx, and since the desorbed NOx concentration in the regeneration gas circulation system can be kept as low as possible, NOx can be desorbed efficiently at a lower temperature. At this time, instead of denitrifying the entire circulating gas in the regenerated gas circulation system, a part of the circulating gas is branched and denitrated, so that the optimum desorption temperature and the optimum denitration temperature can be freely selected separately. It is possible to perform the regeneration process efficiently in terms of performance and energy. Also,
This is a practically effective method that does not release a reducing agent such as ammonia used for regeneration to the outside of the system and has no problem of secondary pollution.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of an embodiment of the present invention.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yora Mitsuhashi 2-1-1 Niihama, Arai-cho, Takasago-shi, Hyogo Mitsubishi Heavy Industries Ltd. Takasago Plant

Claims (1)

[Claims] 1. A method of dry-treating a gas containing low-concentration nitrogen oxides in an adsorption tower to adsorb and remove mainly nitrogen oxides, and desorbing the adsorption tower with high-temperature air to regenerate the adsorbing tower during regeneration. The desorption gas from the desorption gas circulation blower, a regeneration gas circulation system passage consisting of the desorption gas to be circulated again through the desorption gas heater to the adsorption tower during regeneration, and a part of the regeneration gas circulation system passage. A denitration gas blower that sucks and branches the regenerated gas consisting of a desorption gas, a denitration gas heater that raises the temperature of the regeneration gas to a temperature suitable for the denitration reaction, a reducing agent supply pipe that injects a reducing agent into the heated regeneration gas, and denitration A denitration gas circulation system passage is formed to join the processing gas from the reactor and the denitration reactor to the regeneration gas circulation system passage, and the desorption gas in the regeneration gas circulation system passage is formed as the temperature of the adsorption tower during regeneration increases. Of nitrogen oxides When the concentration starts to increase, a predetermined amount of regenerated circulation gas in the regenerated gas circulation system is introduced by the denitration gas blower into the denitration gas circulation system that has been heated to a temperature suitable for the temperature of the denitration reaction in advance, and After injecting a reducing agent from a reducing agent supply pipe, the nitrogen oxide is reduced and decomposed by passing through a denitration reactor, and the denitration treatment gas is re-merged into the regeneration gas circulation system passage to reduce the nitrogen oxide concentration. By promoting the regeneration of the adsorption tower by circulating and supplying gas to the adsorption tower during regeneration, when the concentration of nitrogen oxides in the regeneration gas circulation system is reduced to a predetermined value, the operation of the denitration reactor is stopped,
A method for treating a gas containing low-concentration nitrogen oxides, which comprises terminating the regeneration of the adsorption tower by cooling the temperature of the adsorption tower after the regeneration to room temperature.