JPH08252428A - Removal of nitrogen dioxide - Google Patents

Abstract

PURPOSE: To remove substantially noxious NO2 in a simple way and at a low cost, and thereby meet environmental standards by heating a nitrogen dioxide adsorbent-packed layer and introducing a gas containing the desorbed nitrogen dioxide into a nitrogen dioxide reducing agent-packed layer and then releasing it into the atmosphere. CONSTITUTION: An untreated gas 1 which contains nitrogen monoxide and nitrogen dioxide, the total concentration of which is equal to 6ppm or less, is absorbed by a blower 2, then is guided to a first NO2 adsorbent-packed layer 4a through a switching valve 3, and is released into the atmosphere from an exhaust port 6 through the switching valve 5 after the adsorption and removal of NO2 . The gas flow path is switched to a second NO2 adsorbent-packed layer 4b side prior to the arrival of the NO2 concentration to a set value, and the first NO2 adsorbent-packed layer is set to a regeneration stage. The regeneration gas 8 is introduced into a regeneration blower 9, flowing from a heater 10, a switching valve 11, the first NO2 adsorbent-packed layer 4a, switching valve 12 to an NO2 reducing agent-packed layer 13 in that order. In this case, the desorbed NO2 is reduced to NO in the NO2 reducing agent-filled layer 13.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing nitrogen dioxide.
NO _X ) That is, nitric oxide (hereinafter NO) and nitrogen dioxide (hereinafter
Relates to a method of removing NO ₂ is essentially harmful from the process gas containing NO _2), in particular, automobile tunnels and closed car parking (in the ventilation gas from the underground parking lot, etc.) or the like, or air pollution The present invention relates to a method for removing NO _{2 in} NO _X having a relatively low concentration contained in the air of a living space, a medical facility, etc., in a region with remarkable efficiency with high efficiency and at low cost.

[0002]

2. Description of the Related Art A selective catalytic reduction method in which NO _x is converted to nitrogen (N ₂ ) by using ammonia as a reducing agent has become popular as a technique for reducing NO _x from fixed sources, and high temperature (several 100 ° C.) and high concentration It is effective for targets (several 100 ppm).

However, as such ventilation gas from automobiles tunnel or closed car parking, NO _X concentration is relatively low (typically about 1-3 ppm), relative to the processed gas is cold (room temperature) is However, the selective catalytic reduction method cannot be directly applied (that is, it is not effective due to poor reaction efficiency and cannot be suitably applied). Therefore, the following methods have been proposed for such gas.

Ozone is added to the gas to be treated to change NO to NO.
Method for adsorbing and removing the NO ₂ with ₂ total amount of oxidizing NO _X to after the NO ₂ composite metal oxide sorbents have been proposed (JP-A-3-275126, JP-A 4-176335 Issue).
At that time, a method has also been proposed in which the decomposition of excess ozone is performed by a NO ₂ adsorbent (JP-A-6-275126). In order to adsorb NO as it is, a method using zeolite (Japanese Patent Laid-Open No. 1-155934) and a method using an adsorbent in which ruthenium is supported on titanium oxide (Japanese Patent Laid-Open No. 5-123568)
Issue). The present inventors have found that a carbonaceous adsorbent that suppresses surface oxidation, and in particular, a carbonaceous adsorbent with well-developed pores having a specific diameter has a high NO adsorbing power (Japanese Patent Laid-Open No. H11 (1999) -135242). 5-76753, Japanese Patent Application No. 4-329652). Further, the present inventors have found that a specific crystal form of manganese oxide has room temperature oxidation activity of NO (Japanese Patent Application Laid-Open No. 5-253474), and NO _x can be used in combination with a carbonaceous adsorbent. A removal method has been proposed (Japanese Patent Application No. 4-252872).

[0005]

[SUMMARY OF THE INVENTION However, in the ozone addition method of the prior art, it is difficult to accurately control the addition rate of the ozone with respect to concentration of NO _X fluctuation large target (gas to be treated) There is a problem. That is, if excess ozone is added with a margin to completely carry out the oxidation reaction of NO to NO ₂ , unless the NO ₂ removal rate is kept high, the concentration of NO ₂ that is the target of regulation will be higher than before treatment. On the contrary, it increases. Further, ozone itself is harmful, and at the same time, it may react with hydrocarbons in the gas to be treated to form oxidants having high activity as a by-product. At this time, even if these oxidants can be removed by an appropriate adsorbent or decomposing agent, in an actual device, some leakage from a damper or the like on the way from the ozone injecting section to the adsorbent (or decomposing agent) filling section. (Leakage) is unavoidable on a steady basis, and at that time, NO ₂ also leaks at the same time. For example, the NO _X concentration in the gas to be treated is 3 ppm (NO ₂ is about 0.3 ppm in the automobile tunnel ventilation gas, and the rest is NO).
_When oxidizing all NO in _X to NO ₂ ,
In order to reduce the NO ₂ concentration to 0.06ppm or less, which is on par with the environmental standard value, it is necessary to maintain a removal rate of 98% or more, including leakage, and if the leakage rate is 2% or more, remove it with an adsorbent. Even if the rate is 100%, the above-mentioned outlet NO ₂ concentration: 0.06 ppm or less cannot be achieved, and oxidants also leak at that rate. Conversely, when suppressing the addition of ozone to avoid the above problem and risk increases the unoxidized NO concentration, there is a problem that NO _X removal rate as a whole is lowered.

In the method of using zeolite among the methods of 1, the high NO adsorption performance cannot be obtained unless the gas to be treated is dehumidified in advance, which is economically disadvantageous. Incidentally, in the method described in JP-A-5-123568, NO ₂ removal performance is not mentioned,
Unknown.

In the system using the carbonaceous adsorbent according to the above, the adsorbent enhances the oxidation activity of NO to increase the NO ₂ having high adsorbability.
Is produced, oxidants are not produced, and the adsorbent has an extremely high NO ₂ adsorption / removal performance, so that NO ₂ is almost completely removed. However, since the cost of the adsorbent is higher than that of ordinary activated carbon, it is desired to reduce the filling amount or extend the life without impairing the removal performance.

[0008] In scheme, since the manganese oxide in a particular crystalline form having an oxidizing power of the suction force and NO ₂ NO, the by combining a high carbonaceous adsorbent of NO ₂ adsorption activity, NO _X Although it is possible to improve the adsorption performance, it was found that this oxide has low heat resistance, so that it is difficult to produce a high-strength granular product, and it is difficult to use it for a long time in an actual device.

Further, the following problems are common to each of the above methods. That is, in the gas to be treated targeted by the present invention (such as ventilation gas from an automobile tunnel or a closed type automobile parking lot), NO ₂ is contained in about 10% of all NO _X , which causes environmental standards. More measurement points have also appeared, and as a countermeasure, the above-mentioned methods that target the entire NO _X have been proposed.However, although these methods are effective in reducing the total NO _X amount, the adsorption Also, since NO, which has low reactivity and occupies the majority in terms of concentration, is also included in the removal target, the device becomes large and complicated (hereinafter referred to as "device upsizing"), and
Increasing costs is inevitable, and improvements are desired.

Therefore, depending on the conditions of the place where the device is to be installed, it is possible to more easily remove only NO ₂ . At that time, the concentration not removed: several ppm
The problem is that NO is oxidized to NO ₂ in the atmosphere, but the rate of oxidation of NO to NO ₂ is sufficiently slow between the time it is released into the atmosphere and the time it reaches the residential area (surface). If so, considering the gas diffusion during that time, it is also effective to make NO ₂ the main target of removal as a simple method.

The present invention has been made in view of such a situation, and an object thereof is to increase the size and cost of the apparatus which are common to the above-mentioned conventional methods in which the entire NO _X is removed. to solve the problem, more easily and remove the NO ₂ which is essentially harmful in low cost, environmental standards (NO ₂ concentration at the surface: 0.06 ppm or less) removal of obtaining work to correspond NO ₂ to It is intended to provide a method.

[0012]

In order to achieve the above object, the NO ₂ removal method according to the present invention has the following constitution. That is, the method for removing NO _{2 according to} claim 1 is a method in which a gas to be treated containing nitric oxide and nitrogen dioxide, the total concentration of which is 6 ppm or less, is introduced into the atmosphere after passing through the nitrogen dioxide adsorbent packed bed. Then, while supplying the gas to be treated or a gas other than the gas to the packed bed, the packed bed is heated to desorb nitrogen dioxide, and the gas containing the desorbed nitrogen dioxide is released as a nitrogen dioxide reducing agent. It is a method for removing nitrogen dioxide, which is characterized in that the nitrogen dioxide is discharged into the atmosphere after passing through a packed bed.

The method for removing NO _{2 according to} claim 2 comprises nitric oxide and nitrogen dioxide, the total concentration of which is 6 pp.
The gas to be treated having a volume of m or less is passed through the nitrogen dioxide adsorbent-packed layer, then through the nitrogen dioxide reducing agent-packed layer, and then released into the atmosphere. The packed bed is heated while supplying a gas or a gas other than the gas to desorb nitrogen dioxide, and the gas containing the desorbed nitrogen dioxide is passed into the nitrogen dioxide reducing agent packed layer and then released into the atmosphere. This is a method for removing nitrogen dioxide.

According to a third aspect of the present invention, in the method of removing NO ₂ , after the gas containing the desorbed nitrogen dioxide is passed through the nitrogen dioxide reducing agent-packed layer and before being released into the atmosphere, the nitrogen dioxide adsorbent-packed layer is filled. The method for removing nitrogen dioxide according to claim 1 or 2, wherein the method is connected to a second nitrogen dioxide adsorbent-packed bed different from the bed. The method for removing NO _{2 according to} claim 4, wherein the desorbed nitrogen dioxide-containing gas or the gas obtained after passing the gas through the nitrogen dioxide reducing agent-filled layer contains the desorbed nitrogen dioxide. 4. The method for removing nitrogen dioxide according to claim 1, 2 or 3, wherein a gas having a lower nitric oxide concentration is mixed. Furthermore, the method for removing NO _{2 according to} claim 5 is the method for removing nitrogen dioxide according to claim 1, 2, 3 or 4, wherein the nitrogen dioxide reducing agent-filled layer contains a carbonaceous material as a nitrogen dioxide reducing agent. Is.

[0015]

The present inventor has studied various methods for removing low-concentration NO _X , but if the NO oxidation rate is sufficiently slow as described above, only NO ₂ is to be removed as a simple method. Based on the idea that this is also effective, we examined the correctness and found that in the case of low-concentration NO _X gas, removal of NO ₂ contained in it reduces local NO ₂ concentration. Is effective and can achieve the object of the present invention, and the present invention has been completed based on such findings.

That is, it takes about 3 to 5 minutes for the ventilation gas from the automobile tunnel or the closed type automobile parking lot to reach the ground surface. Instead of removing NO _X (typical average concentration is 1 to 3 ppm, about 10% of which is NO ₂ and the rest is NO) contained in this gas, NO ₂ (which is unavoidably slightly co-adsorbed with NO ₂ ) is removed. )
If only adsorbed, or after reducing NO ₂ to NO and removing it, and then releasing it into the atmosphere, NO that has not been removed is oxidized and produced within the time to reach the surface of the earth, and NO ₂ , and The concentration that combines NO ₂ that could not be removed by adsorption or reduction (hereafter referred to as the surface NO ₂ concentration) is the environmental standard value.
If it is 0.06 ppm or less, it is possible to try to cope with the NO ₂ concentration below the environmental standard value. Therefore, the initial conditions for the surface NO ₂ concentration to become an environmental standard value: 0.06 ppm or less (that is, the NO concentration of the gas released into the atmosphere after the NO _{2 is} removed [hereinafter, the initial release NO concentration]) NO to clarify
Is diluted with air and NO concentration (hereinafter, initial NO concentration) is several ppm
In adjusting the different gas, filling it to the transparent envelope, and left under various conditions, NO ₂ the oxidation product of NO
As a result of measuring the change with time in the concentration of NO and examining the relationship between the change and the initial NO concentration of gas, when the NO ₂ concentration reaches 0.06 ppm in 3 minutes, the initial NO concentration is approximately 8.2 ppm in 5 minutes. The initial NO concentration was estimated to be about 6.4 ppm when the ₂ concentration reached 0.06 ppm. This value was almost unchanged whether the above NO was derived from standard gas or was collected from diesel vehicle exhaust gas or actual vehicle tunnel ventilation. This result almost agrees with the result calculated based on the oxidation rate constant of NO described in the literature (Toshikazu Okita, Atmospheric Conservation Science, Industrial Book (1982), Table 4.7). From these results, the above-mentioned surface NO ₂ concentration is the environmental standard value: 0.06 pp
The initial release NO concentration for reaching m or less is about 6.4 ppm when the longest 5 minutes to reach the surface is adopted, and therefore the initial release NO concentration (that is, in the gas after removal of NO ₂₎ Once you have a NO concentration) below 6 ppm, if it was released into the atmosphere, time to reach the surface (NO ₂ concentration is oxidized to generate 3-5 minutes or so) in the (ground NO ₂ concentration)
Is below the environmental standard value of 0.06ppm, and is essentially NO _2.
It is possible to meet the environmental standard value of concentration: 0.06ppm or less. Wherein such release initial NO concentration: 6 ppm in the following, NO ₂ removal before the original of the NO _X in the gas (= NO + NO ₂₎
The concentration should be 6ppm or less. Note that the allowable concentration of NO ₂ that is released without being removed by the above adsorption or reduction (at the time of release)
The allowable concentration of NO ₂ ) is the NO _X (=
NO + NO ₂ ) concentration, more strictly depending on the initial NO concentration released,
Even if the NO ₂ allowable concentration at the time of release is included, the NO _X concentration in the original gas before removal of NO ₂ should be 6 ppm or less.

The method for removing NO _{2 according to} the present invention has been completed based on the above findings, and the method for removing NO ₂
O) and nitrogen dioxide (NO ₂ ), the total concentration of which is 6ppm or less, the gas to be treated is passed into the nitrogen dioxide adsorbent-packed layer (layer filled with NO ₂ adsorbent) and then released into the atmosphere. I am trying to do it. Therefore, since NO ₂ in the gas to be treated is adsorbed in this packed bed, the gas at the outlet of the packed bed becomes a gas with NO ₂ removed and the initial release NO concentration: 6 ppm or less, which is released into the atmosphere. However, as can be seen from the above findings, the NO ₂ concentration generated by oxidation within the time to reach the surface (about 3 to 5 minutes) is 0.06 ppm or less, which is essentially the environmental standard value of NO ₂ concentration: Measures can be taken for 0.06ppm or less. In addition, gas with initial release NO concentration: 6 ppm or less means
It does not mean that NO ₂ should not be contained at all, and it is acceptable to some extent in accordance with the NO _X (= NO + NO ₂ ) concentration in the gas to be treated, more directly corresponding to the initial release NO concentration (the above-mentioned NO ₂ during release). It may be included up to the permissible concentration) as described above (the details will be described later).

At this time, the gas to be removed of NO ₂ is NO _X (= NO
+ NO ₂ ) concentration: If it is 6 ppm or less, it may be directly passed through the packed bed without preliminary purification treatment, and the gas to be treated in the present invention has a NO _X concentration of about 1 to 3 ppm and a NO _X concentration:
Since it is 6 ppm or less, it corresponds to it, and it may be directly passed through the packed bed. Further, the method of the present invention is not intended to remove the entire NO _X as described above, but is intended to remove only NO _{2 in} NO _X , and its content ratio is low (in the case of an automobile tunnel, 10%), because the amount of removal is small, NO
Eliminates the problems of device size increase and cost increase that are common to each of the above conventional methods in which the entire _X is removed.
Eliminates NO ₂ which is essentially harmful at a simpler and lower cost, and meets environmental standards (NO ₂ concentration on the ground surface: 0.06 ppm or less)
Can be dealt with.

As the NO ₂ adsorbent in the packed bed, for example, zeolite, aluminum oxide, titanium oxide, etc. are used, and NO _{2 is} adsorbed by the NO ₂ adsorbent, but NO _{2 is} gradually increased as the adsorption progresses. Will start to break through, so the adsorption operation will be stopped periodically to desorb the adsorbed NO ₂ .
NO ₂ adsorbent needs to be regenerated. Further, making the desorbed gas to lower the NO ₂ concentration before released into the atmosphere, i.e., it is necessary to release the NO ₂ concentration in the desorption gas from the below 0.06ppm to the atmosphere. Therefore, the method for removing NO _{2 according} to the present invention uses the above-mentioned adsorption operation (gas to be treated is NO ₂
(Discharge after passing through the adsorbent packed bed), and then supplying the gas to be treated or a gas other than the gas to the packed bed to heat the packed bed to desorb NO ₂ and desorb the NO _2. A gas containing NO ₂ (desorption gas) is passed through a nitrogen dioxide reducing agent-filled layer (layer filled with NO ₂ reducing agent) and then released into the atmosphere. With this NO ₂ reducing agent, NO ₂ in the desorbed gas is reduced to NO and becomes a gas with a NO ₂ concentration of 0.06 ppm or less, so that it can be released into the atmosphere without any trouble and the environmental standards can be satisfied. Even if the gas to be treated is used as the gas supplied to the packed bed during the desorption, it is included in the gas.
The NO ₂ concentration is much lower than the desorbed NO ₂ concentration, so other gases with lower NO ₂ concentrations (eg ambient air)
There is no difference in the effect from the case of using, so that it can be desorbed without any trouble.

Therefore, according to the NO ₂ removal method of the present invention, the entire NO _X is to be removed with respect to low concentration NO _X gas such as ventilation gas from an automobile tunnel or a closed type automobile parking lot. It eliminates the problems of increasing the size and cost of the equipment that each conventional method has in common. Compared to these methods, it is simpler, lower cost, and inherently harmful NO.
₂ can be removed, and environmental standards (NO ₂ concentration on the ground surface: 0.06 ppm
The following can be addressed.

It is desirable that the NO ₂ reducing agent contains a carbonaceous material (the method for removing NO _{2 according to} claim 5). That is, such reducing agents include activated carbon, activated coke, and those in which these carbonaceous materials are supported on other materials. On these carbonaceous materials, C (carbon) extracts O atoms from NO ₂ and NO. A reaction to reduce to occurs. on the other hand,
A method of using ammonia or hydrocarbons as a gaseous reducing agent to reduce it to nitrogen over a catalyst such as titanium oxide or zeolite is also conceivable, but even if the reduction to NO is stopped, the substantial effect does not change. Handling is also complicated. A material containing the carbonaceous material can be preferably used without the complexity of handling.

The reduction reaction to NO on the above carbonaceous material is 1
It progresses sufficiently at 00-200 ℃, but it also occurs at room temperature. Such reduction reaction can be suitably used in addition to the reduction of NO _{2 in} the desorbed gas, and also after the gas to be treated is passed through the NO ₂ adsorbent packed bed and before being released into the atmosphere. That is, the gas to be processed is NO ₂
If the NO ₂ adsorbent packed bed is connected to the NO ₂ reducing agent packed bed and then released to the atmosphere, some of the NO ₂ cannot be adsorbed and removed by the former NO ₂ adsorbent packed bed. also, since the NO ₂ is the latter NO ₂ reducing agent is reduced to NO in the filling layer is removed (NO ₂ concentration decreases), thereby improving the NO ₂ removal performance as a whole (removal of claim 2, wherein NO ₂ Method). However, when an NO ₂ reducing agent containing a carbonaceous material is used, part of NO ₂ is adsorbed on the carbonaceous material (NO ₂ is produced from NO and O ₂ in the gas to be treated and is adsorbed). However, it further reacts with C and is re-reduced to NO to be desorbed), which increases the consumption of the carbonaceous material. Therefore, from the required NO ₂ removal rate, etc.,
It suffices to decide whether or not to apply the above method (method for removing NO _{2 according to} claim 2).

After passing the desorbed gas through the NO ₂ reducing agent-packed layer and before releasing it into the atmosphere, a _second layer different from the NO ₂ adsorbent-packed layer for adsorbing and removing NO _{2 in} the gas to be treated is provided. When the NO ₂ to as to communicate the adsorbent filling layer, NO ₂ by the NO ₂ absorbent
Since NO is adsorbed, it is not necessary to set the NO ₂ reduction rate in the NO ₂ reducing agent-packed layer to be excessively high, and it can be set to be low, which has the advantage of reducing the burden (removal of NO _{2 according to} claim 3). Method).

The desorption gas is a high concentration of NO _x (mainly NO ₂ ).
Therefore, the gas after passing through the NO ₂ reducing agent packed bed contains a high concentration of NO. The rate of oxidation of NO to NO ₂ by O ₂ is proportional to NO concentration. Therefore, it is desirable to dilute the NO concentration before it is released into the atmosphere to reduce the NO concentration. For that purpose, a gas having a NO concentration lower than that of the desorbed gas may be mixed with the desorbed gas or with the gas after passing the desorbed gas through the NO ₂ reducing agent-packed layer (NO in claim 4). ₂ removal method). At this time, when the second NO ₂ adsorbent-packed layer is provided, it is preferable to mix it before that (to the gas after passing the desorbed gas to the NO ₂ reducing agent-packed layer).

The release initial NO concentration as described above: 6 ppm or less of gas may comprise a release time of NO ₂ OEL to _{NO 2, NO X (= NO} + NO 2) in the release time of NO ₂ OEL to be treated in the gas
The concentration depends more directly on the initial NO concentration released. The details will be described below.

The oxidation reaction of NO to NO _{2 in} air is 2NO +
It is expressed as O ₂ → 2NO ₂ , and its reaction rate is −d [NO] / dt ＝ d
Follow the formula [NO ₂ ] / dt = k [NO] ² (where [NO], [NO ₂ ]: concentration, k: reaction rate constant). If this is integrated, c = c ₁ + (k
The _equation tc ₀ ² ) / (c ₀ kt +1) is obtained. However, c ₀ : NO concentration before the start of the reaction (in the present invention, the concentration when released from the ventilation equipment, etc.), c ₁ : NO ₂ concentration before the start of the reaction (concentration when released from the ventilation equipment, etc., That is, the concentration of NO ₂ that could not be removed), k: reaction rate constant, t: reaction time (time to reach the ground surface), c: NO ₂ concentration at time t ₂ . here,
As a result of performing the same experiment as the measurement of the change with time of NO ₂ concentration in the NO-containing air as described above, when the concentration was expressed in mole fraction and the time was expressed in minutes, k = 300 [min ^-1 ]
It turns out that is represented by.

Using the above equation, for various c ₀ , t =
Calculating c for 3 and 5 [min] is as follows (Table 1 below). The NO ₂ concentration in the calculation result plus c ₁ is the actual expected NO ₂ concentration.

[0028]

As described above, upon reaching the surface of the earth (3-5 after release)
Since the total NO ₂ concentration of (after minutes) is 0.06 ppm or less,
Calculating the allowable value of total NO ₂ concentration after 5 minutes as 0.06ppm or less, c ₁ (allowable concentration of NO ₂ at the time of release) was calculated.
The concentration shown in the last column of is obtained. As can be seen, the permissible NO ₂ concentration during release depends on the initial release NO concentration, and the smaller the initial release NO concentration is, the larger the initial release NO concentration is 6 pp.
0.006ppm or less at m, 0 at initial NO concentration of 1ppm.
It is 058 ppm or less. Therefore, the removal performance may be set according to the initial NO concentration released, and if the NO ₂ removal performance is set sufficiently high, it is possible to cope with the NO _X concentration in the gas to be treated being about 6 ppm or less. Target NO _X concentration 1-3 ppm
It can be said that the NO ₂ removal performance may be lowered in the case of the gas to be treated.

Generalizing the above, total NO after 5 minutes
₂ NO ₂ allowable concentration at release (molar fraction) required to keep the concentration of 0.006 ppm or less (below the allowable value of total NO ₂ concentration on the ground surface)
Is expressed by the following equation, where the initial release NO concentration (ppm) is C ₀ . 0.06− (1500 × 10 ^-6 C ₀ ² ) / (1500 × 10 ^-6 C ₀ +1) ---- formula

From the above equation, NO _X (= NO + NO ₂ ) concentration: 6
When a target gas of ppm or less is passed through a NO ₂ adsorbent and / or a reducing agent packed bed and NO ₂ in the gas is adsorbed and / or reduced and removed, the NO concentration at the packed bed outlet is X. ,
The NO ₂ concentration at the packed bed outlet is [0.06− (1500 × 10 ⁻⁶ X ² ).
It can be said that control should be performed so that it is less than / (1500 × 10 ⁻⁶ X + 1)] ppm.

[0032]

EXAMPLE An NO ₂ removal treatment system 7 according to an example of the present invention is shown. An outline of the NO ₂ removal processing by this system 7 will be described below, particularly focusing on the gas flow. The gas to be processed 1 is sucked by the blower 2, passes through the switching valve 3 and
After being guided to the NO ₂ adsorbent-filled layer 4a and adsorbing and removing NO ₂ , the NO ₂ is admitted to the atmosphere through the switching valve 5 and the exhaust port 6. Before the NO ₂ concentration at the exhaust port 6 reaches the set value, the gas flow path is switched to the side of the second NO ₂ adsorbent packed bed 4b, and the first N 2
The O ₂ adsorbent-filled layer 4a moves to regeneration. Regeneration gas (generally ambient air) 8 is taken in by a regeneration blower 9 and heater 10
→ Switching valve 11 → 1st NO ₂ adsorbent packed bed 4a → Switching valve 12 →
The NO ₂ reducing agent filled layer 13 flows in this order. At this time, the first NO ₂
The NO ₂ desorbed from the adsorbent packed bed 4a is the NO ₂ reducing agent packed bed 13
Is reduced to NO. This reduced gas may be released into the atmosphere as it is, but it is sucked into the blower 2 so as to re-adsorb unreduced NO ₂ and not re-oxidize high concentration NO to generate NO _2. It is preferable that the gas to be treated is diluted with the gas to be treated 1 and then the gas is passed through the second NO ₂ adsorbent-filled layer 4 b and discharged from the exhaust port 6.

The NO ₂ removal treatment system 7 was used to carry out an NO ₂ removal treatment experiment under the following conditions. That is, aluminum oxide for catalyst carrier as NO ₂ adsorbent (cylindrical pellet, diameter 1.6m
m) was used to fill an adsorption tube having an inner size of 1.84 cm with NO ₂ adsorbent packed layers 4 a and 4 b having a packing height of 18 cm. On the other hand, as a NO ₂ reducing agent, coconut shell activated carbon (cylindrical pellets,
Using 7 to 12 mesh and a specific surface area of 1000 m ² / g, this was filled in an adsorption tube having an inner size of 1.84 cm to form a NO ₂ reducing agent packed layer 13 having a packed height of 6 cm. Then, in the filling layer 4a,
Temperature 25 ℃, relative humidity 60%, NO _X concentration: 2ppm (NO: 1.8pp
m, NO ₂ : 0.2 ppm) was passed as the gas to be treated 1.
At this time, the space velocity was 10000 / h and the processing time was continuous 40 hours.

Next, the flow path is switched to the side of the second NO ₂ adsorbent-filled layer 4b, and the gas to be treated 1 similar to the above is passed under the same conditions while the first NO ₂ adsorbent-filled layer 4b is filled. Air heated by the heater 10 as regeneration gas 8 is passed through the layer 4a, and NO
₂ is desorbed to regenerate the packed bed 4a and the packed bed 4a
Through desorption gas after passing the NO ₂ reducing agent packed layer 13, it was reduced to NO in the gas NO _2. At this time, the first NO ₂
The adsorbent-packed layer 4a and the NO ₂ reducing agent-packed layer 13 reached 180 ° C. in 3 hours, were held at 180 ° C. for 2 hours, and then cooled to room temperature in 3 hours. The space velocity was 2000 / h in the adsorbent-filled layer 4a and 6000 / h in the reducing-agent-filled layer 13.

[0035] adsorbing and removing the above-mentioned NO _2, NO ₂ desorption (regeneration) and repeating the reduction of the NO desorption gas NO _2, NO ₂
The NO ₂ concentration at the outlet of the adsorbent packed bed and the outlet of the NO ₂ reducing agent packed bed were measured by a chemiluminescence NO _x analyzer. The result is shown in FIG. As can be seen from Fig. 2, NO ₂ adsorbent packed bed
₂ was adsorbed and removed with high efficiency. A small amount of NO ₂ was observed during regeneration. Therefore, from the 4th playback, NO
_{2 The} gas discharged from the reducing agent packed bed was flowed through the NO ₂ adsorbent packed bed similar to the above (however, packing height 3 cm, space velocity: 1200
0 / h). As a result, NO ₂ was no longer recognized.

[0036]

INDUSTRIAL APPLICABILITY The method for removing NO _{2 according to} the present invention is intended to remove NO ₂ which is essentially harmful to low-concentration NO _X gas such as ventilation gas from an automobile tunnel or a closed type automobile parking lot. Therefore, it eliminates the economic problem of increasing the size and cost of the equipment that the conventional methods that remove all NO _{X in} common have in common, compared to the conventional methods,
Eliminates NO ₂ which is essentially harmful at a simpler and lower cost, and meets environmental standards (NO ₂ concentration on the ground surface: 0.06 ppm or less)
With this, it is possible to deal with the above.

[Brief description of drawings]

FIG. 1 is a diagram for explaining a NO ₂ removal treatment system according to an embodiment of the present invention, particularly focusing on a gas flow.

It is a diagram showing a relationship between NO ₂ concentration in the processing time and the filling layer outlet during NO ₂ removal process according to an embodiment of the present invention; FIG.

[Explanation of symbols]

1--gas to be treated, 2--blower, 3--switching valve, 4a--first
NO ₂ adsorbent packed bed, 4b--second NO ₂ adsorbent packed bed, 5--switching valve, 6--exhaust port, 7--NO ₂ removal treatment system, 8--
Regeneration gas, 9--regeneration blower, 10--heater, 11--switching valve, 12--switching valve, 13--NO ₂ reducing agent packed bed.

Claims (5)

[Claims] 1. A gas to be treated containing nitric oxide and nitrogen dioxide, the total concentration of which is 6 ppm or less, is passed into a nitrogen dioxide adsorbent packed bed and then released into the atmosphere, and then the packed bed is discharged. While supplying the gas to be treated or a gas other than the gas, the packed bed is heated to desorb nitrogen dioxide,
A method for removing nitrogen dioxide, characterized in that the desorbed gas containing nitrogen dioxide is passed through a nitrogen dioxide reducing agent-packed layer and then released into the atmosphere. 2. A gas containing nitrogen monoxide and nitrogen dioxide, the total concentration of which is 6 ppm or less, is passed through the nitrogen dioxide adsorbent-packed layer, and then through the nitrogen dioxide reducing agent-packed layer, and then the atmosphere. And then desorbing nitrogen dioxide by supplying the gas to be treated or a gas other than the gas to the nitrogen dioxide adsorbent packed bed to desorb the nitrogen dioxide, and to remove the desorbed nitrogen dioxide. A method for removing nitrogen dioxide, comprising passing a gas containing the nitrogen dioxide reducing agent-filled layer and then releasing the gas into the atmosphere. 3. A second nitrogen dioxide separate from the nitrogen dioxide adsorbent-packed layer after passing the desorbed nitrogen dioxide-containing gas through the nitrogen dioxide reducing-agent packed bed and before being released into the atmosphere. The method for removing nitrogen dioxide according to claim 1 or 2, which leads to the adsorbent packed bed. 4. The gas containing the desorbed nitrogen dioxide or the gas obtained by passing the gas through the nitrogen dioxide reducing agent-filled layer, the gas having a lower nitric oxide concentration than the gas containing the desorbed nitrogen dioxide. The method for removing nitrogen dioxide according to claim 1, 2 or 3, wherein: 5. The nitrogen dioxide reducing agent-filled layer contains a carbonaceous material as a nitrogen dioxide reducing agent.
3. The method for removing nitrogen dioxide according to 3 or 4.