JPH0866612A - Removing method of nitrogen dioxide - Google Patents

Abstract

PURPOSE: To remove NO2 conveniently and at low cost by passing a gas to be treated containing relatively low concn. NO and NO2 through a layer filled with a nitrogen dioxide adsorbent and/or a reducing agent. CONSTITUTION: The gas to be treated such as ventilated gas from an automobile tunnel or closed automobile stall containing both nitrogen monoxide and nitrogen dioxide and having <=6ppm total concn. of these is treated. This gas is passed through the layer filled with the nitrogen dioxide adsorbent such as carbonaceous adsorbent and/or reducing agent. The nitrogen dioxide in the gas is removed by adsorption and/or reduction. In this way, the NO2 harmful essentially is removed and its concn. is lowered to less than an environmental reference value of NO2 concn. by a convenient device and at low cost.

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 highly efficiently removing NO ₂ in a relatively low concentration of NO _X contained in the air of a living space or a medical facility in a region where the

[0002]

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

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]

In [0008] the conventional of ozone addition method, a problem that for the concentration of NO _X fluctuation large target (gas to be treated) it is difficult to accurately control the addition rate of the ozone There is a point. That is, if ozone is added excessively for safety in order to prevent insufficient ozone addition, NO
The total amount of NO in _X is oxidized to NO ₂ , but if the NO ₂ removal rate is not kept high, the regulated NO ₂ concentration will increase. In addition, ozone itself is a harmful oxidant, and at the same time, components that are easily oxidized in the gas to be treated are oxidized, and oxidants having high activity may be by-produced.
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 concentration of NO _{X in the} gas to be treated is 3 ppm (NO ₂ is about 0.3 ppm in automobile tunnel ventilation gas, and the rest is NO).
), And oxidizing all of the NO in the NO _x to NO ₂ ,
In order to control the NO ₂ concentration at the treatment equipment outlet to 0.06 ppm or less, which is similar to 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, adsorption Even if the removal rate by chemicals etc. is 100%, the above NO ₂ concentration at the outlet:
There is a problem that 0.06ppm or less cannot be achieved and oxidants are also leaked at that ratio. If the addition of ozone is suppressed to avoid the above problems and dangers, unoxidized
There is a problem that the NO concentration increases and the overall NO _X removal rate decreases.

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 characterized in that it does not form an oxidizing atmosphere in the entire gas phase, so it does not generate oxidants, and the adsorbent has a very high NO ₂ adsorption and removal performance, so NO ₂ is almost completely removed. Will be 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, in common with each of the above methods, NO is replaced with NO ₂
Along with this, there is an economic problem in targeting removal.
In other words, the total amount of NO _X emitted from fixed sources is NO, but even if it is released or diluted in the atmosphere, it will be oxidized to NO ₂ over a long period of time, so the removal of this NO is restricted by the total amount regulation of the entire region. It is effective as a countermeasure. In contrast, the subject of the gas to be treated of the present invention (such as ventilation gases from automobiles tunnel or closed car parking) Although not help total reduction for even small amount low NO _X concentration, NO ₂ is total NO 10 to 50% of _X is included, and because of this, some measurement points that exceed environmental standards have appeared, so local environmental measures are desired. As a countermeasure, the above-mentioned methods that target the entire NO _X have been proposed.However, in each of these methods, the adsorption and reactivity are low, and NO that accounts for the majority of the concentration is also targeted for removal. However, there is a problem in that it is inevitable to increase the size and increase the cost.

Therefore, it can be considered to remove only NO ₂ more easily. At that time, there is a problem that unremoved NO is oxidized to NO ₂ in the atmosphere, but 0.1ppm
If the oxidation rate of NO at a concentration of (contaminated atmospheric NO concentration level) to several ppm (NO concentration level in ventilation gas from automobile tunnels or closed-type automobile parking lots) is extremely slow, Considering diffusion, NO is a simple method.
It is also effective to set ₂ as the main target of removal.

The present invention has been made by paying attention to such a situation, and an object thereof is to increase the size and cost of an apparatus which is common to the above-mentioned conventional methods in which the entire NO _X is removed. It is possible to eliminate the economical problem of aging, remove NO ₂ which is essentially harmful with a simpler device and at lower cost, and to meet the environmental standard value of essentially NO ₂ concentration of 0.06 ppm or less. The present invention intends to provide a NO ₂ removal method capable of achieving the above.

[0012]

In order to achieve the above object, the NO ₂ removal method according to the present invention has the following constitution. That is, in the method for removing NO _{2 according to} claim 1, the nitrogen dioxide adsorbent and / or the reducing agent is filled with the gas to be treated which contains both nitric oxide and nitrogen dioxide and has a total concentration of 6 ppm or less. It is a method of removing nitrogen dioxide, which is characterized in that it is connected to a layer.

A method for removing NO _{2 according to} a second aspect of the present invention is to fill a nitrogen gas adsorbent and / or a reducing agent with a gas to be treated containing both nitric oxide and nitrogen dioxide and having a total concentration of 6 ppm or less. The nitrogen dioxide in the gas is removed by adsorbing and / or reducing the nitrogen dioxide in the gas.

The method for removing NO _{2 according to} claim 3 is the method for removing nitrogen dioxide according to claim 2, wherein the gas after removing the nitrogen dioxide is released into the atmosphere.

The method for removing NO _{2 according to} claim 4 is the method for removing nitrogen dioxide according to claim 1, 2 or 3, wherein the nitrogen dioxide adsorbent and / or the reducing agent is a carbonaceous adsorbent. The method for removing NO _{2 according to} claim 5, wherein after the adsorption and reduction with the carbonaceous adsorbent is completed, the carbonaceous adsorbent is treated with 10
The method for removing nitrogen dioxide according to claim 4, wherein the adsorption gas component is desorbed by heating to a temperature of 0 to 200 ° C. to recover the adsorption and reduction activities of the carbonaceous adsorbent. The method for removing NO _{2 according to} claim 6, wherein after cooling the desorbed gas component, it is passed through a packed bed containing a carbonaceous adsorbent to adsorb and / or reduce nitrogen dioxide in the gas component. The method for removing nitrogen dioxide according to claim 5, which prevents leakage of nitrogen dioxide into the atmosphere.

[0016]

[Action] NO for NO _X concentration is lower the gas to be treated as such ventilation gas from automobiles tunnel or closed car parking
We have studied various _X removal methods, but in the case of such low-concentration NO _X gas, if the NO oxidation rate is extremely slow as described above, NO ₂ is removed as a simple method considering the gas diffusion during that period. Based on the idea that it is also effective to make it the main target of NOx, we examined to confirm the correctness, and in the case of such low-concentration NO _X gas, even if the entire amount of NO _X was not targeted for removal. , If you remove the NO ₂ contained in it,
It has been clarified that it is effective for locally reducing NO ₂ concentration and that the object of the present invention can be achieved, 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. NO _X contained in the gas (the average concentration of usually 1-3 ppm, of which about 10% NO _2, remaining NO) instead of removing, after removal by reducing the adsorption or NO to NO _2,
When released to the atmosphere, the time required to reach the surface (ie 3
The concentration of NO ₂ generated by air oxidation within about 5 minutes)
It is an environmental standard value, including a part of NO _{2 in} the gas that reaches the surface of the earth that cannot be removed by the above adsorption or reduction.
If it is 0.06 ppm or less, it is possible to essentially meet the environmental standard value of NO ₂ concentration: 0.06 ppm or less. Therefore, the initial conditions for the NO ₂ concentration to become the 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)) are clarified. In order to obtain
Adjusting various gases (air) with NO concentration) of several ppm,
Fill it in a translucent container, leave it under various conditions,
By measuring the change with time of the concentration of NO ₂ which is an oxidation product of NO,
As a result of examining the relationship between the change and the initial NO concentration of gas, 3
When the NO ₂ concentration reaches 0.06 ppm in minutes, the initial NO concentration is 8.
It was evaluated that the initial NO concentration was about 6.4 ppm when the NO ₂ concentration reached 0.06 ppm in about 5 ppm in about ₂ ppm. The results are based on literature (Toshikazu Okita, Atmospheric Conservation Science, Industrial Books (198
2), Table 4.7) shows almost the same as the result calculated based on the NO oxidation rate constant. From these results, the initial release NO concentration for the above-mentioned NO ₂ concentration to become the environmental standard value: 0.06 ppm or less is about 6.4 ppm when the longest time to reach the surface of the earth is 5 minutes. ,
If the initial release NO concentration (that is, the NO concentration in the gas after removal of NO ₂ ) is kept at 6 ppm or less, when it is released into the atmosphere, it will reach the surface of the earth within about 3 to 5 minutes. The NO ₂ concentration produced by air oxidation is 0.06, which is an environmental standard value.
ppm or less, essentially NO ₂ concentration environmental standard value: 0.06
Therefore, it will be possible to cope with ppm or less. Here, in order to make the initial release NO concentration: 6 ppm or less, the NO _X concentration in the original gas before removal of NO ₂ (that is, the total concentration of NO and NO ₂ )
Should be 6ppm or less. Note that the allowable concentration of NO ₂ that is released without being removed by the adsorption or reduction (NO ₂
The allowable concentration) is NO _X (= NO + in the gas to be treated, as described later).
NO ₂ ) concentration, more strictly (or directly), depends on the initial release NO concentration, and even if such allowable release NO ₂ concentration is included, NO ₂
The NO _x concentration in the original gas before removal 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 contains both NO (nitric oxide) and NO ₂ (nitrogen dioxide), and their total concentration is 6 ppm. The following gases to be treated are communicated with the layer filled with the NO ₂ (nitrogen dioxide) adsorbent and / or the reducing agent. Therefore, in this layer (packed bed), NO ₂ in the gas to be treated is adsorbed or reduced to NO,
Alternatively, the gas at the outlet of the packed bed is adsorbed and reduced to NO and removed, so that the NO concentration of NO ₂ removed is:
Even if it becomes a gas of 6 ppm or less (that is, a gas having an initial release NO concentration of 6 ppm or less) that is released into the atmosphere, as can be seen from the above findings, air is not reached within the time to reach the ground surface (about 3 to 5 minutes). The NO ₂ concentration produced by oxidation is 0.06 ppm or less, and it is possible to essentially meet the environmental standard value of the NO ₂ concentration: 0.06 ppm or less. In addition, NO concentration with NO ₂ removed: 6 ppm
The following gases (that is, gases with an initial release NO concentration of 6 ppm or less) do not mean that NO ₂ should not be contained at all, and the NO _X (NO + NO ₂ ) concentration in the gas to be treated, more directly, Corresponding to the initial release NO concentration, it is acceptable to some extent (the release NO ₂ allowable concentration may be included) as described above (the details will be described later).

At this time, if the gas targeted for NO ₂ removal (purification treatment) is a low-concentration NO _X gas having a NO _X concentration (that is, the total concentration of NO and NO ₂ ) of 6 ppm or less, it is directly (pre-purification treatment). However, the gas to be treated in the present invention is a low-concentration NO _X gas, especially ventilation gas from an automobile tunnel or a closed type automobile parking lot, or the atmosphere. Since it is air such as living space or medical facilities (NO _X concentration of about 1 to 3 ppm) in a heavily polluted area, it can be directly passed through the packed bed, and there is no need for prior purification treatment. Further, the present invention method, as described above, NO _X
It is intended to remove NO _{2 in} NO _X , not the whole to be removed, and the proportion of NO _{2 in} such NO _X is low, such as ventilation from an automobile tunnel or a closed car parking lot. In the case of gas, it is as low as about 10% as described above, and therefore the amount of removal by adsorption and / or reduction can be small. The problem described above can be solved, and the operation can be performed with a simpler device at low cost.

Therefore, according to the method for removing NO ₂ according to the present invention, as compared with the above-mentioned conventional methods, for low-concentration NO _X gas such as ventilation gas from an automobile tunnel or a closed type automobile parking lot, With a simpler device and at a lower cost, it is possible to remove essentially harmful NO _2, and it is possible to meet the environmental standard value of the NO ₂ concentration of 0.06 ppm or less.

In the packed bed of the NO ₂ adsorbent and / or the reducing agent, the NO ₂ in the gas to be treated is adsorbed as described above, reduced to NO, or adsorbed at the same time.
It is reduced to NO and removed (NO ₂ removal method according to claim 2). At the outlet of this packed bed, the gas to be treated becomes a gas having NO concentration removed from NO _{2 as} described above: 6 ppm or less, which may be directly discharged into the atmosphere without post-treatment (claim 3). NO ₂ removal method).

The NO ₂ adsorbent and / or the reducing agent is preferably an adsorbent which does not oxidize NO in low concentration NO _X to NO ₂ as much as possible and at the same time can adsorb and remove NO ₂ with high efficiency. As a result of repeated research on a large number of adsorbents in order to search for such an adsorbent, it was concluded that the carbonaceous adsorbent is optimal. Therefore, from this point, it is preferable to use the carbonaceous adsorbent as the NO ₂ adsorbent and / or the reducing agent, that is, to use the carbonaceous adsorbent packed bed as the packed bed (NO in claim 4). ₂ removal method). here,
The carbonaceous adsorbent generally refers to activated carbon, but it is one that does not activate only by carbonization (activated coke, etc.), a composite material of another material and activated carbon, or one in which activated carbon is supported on another material. May be.

When a polar adsorbent such as zeolite or activated alumina was used in place of the carbonaceous adsorbent, the adsorbent adsorbed a considerable amount of NO ₂ under dry conditions, but had low performance under high humidity. In contrast, if the hydrophobic carbonaceous adsorbent performance degradation even at high humidity range is small, while others are rather improved performance, also difficult to very releasing NO _2, NO
Even through NO ₂ which did not contain NO, the total amount of NO 2 which was broken through without being adsorbed was NO. This is because NO ₂ is NO due to surface carbon.
This is due to the breakthrough after being reduced to, and it is a feature not found in other adsorbents that NO ₂ can be almost completely removed even under high space velocity conditions.

The carbonaceous adsorbent which adsorbs a small amount of NO ₂ and NO becomes reusable by heating and regeneration when its performance deteriorates. That is, after the adsorption and reduction with the carbonaceous adsorbent is completed, the carbonaceous adsorbent is treated with 100 to 20
The adsorbed gas component can be desorbed by heating to a temperature of 0 ° C. to recover the adsorption and reduction activities of the carbonaceous adsorbent (the method for removing NO _{2 according to} claim 5). At this time, it is preferable that the heating temperature (regeneration temperature) is 150 to 200 ° C. because desorption occurs more completely. NO by heating above
Most of ₂ (or nitric acid formed by reacting with coexisting water) reacts with surface carbon and is desorbed by being reduced to NO. NO ₂ desorbed as compared to zeolite and activated alumina which desorb NO _{2 as it is}
It is advantageous in that it is not necessary to consider the removal of.

[0025] During the desorption, the NO ₂ concentration and the like which has been adsorbed, after slight but it is also possible to desorb remain NO _2, but which has cooled the gas components desorbed in which case, the carbon Through a packed bed containing a porous adsorbent,
It is advisable to adsorb and / or reduce NO ₂ , which can prevent leakage of nitrogen dioxide into the atmosphere (the method for removing NO _{2 according to} claim 6). When a plurality of packed beds are provided and the gas to be treated is continuously purified, the desorbed gas may be passed through the packed bed during the adsorption operation in the packed bed.

When the carbonaceous adsorbent packed bed is used as described above (the method for removing NO _{2 according to} claim 4), NO ₂ such as zeolite or activated alumina is provided upstream of the carbonaceous adsorbent packed bed.
When the adsorbent packed bed is provided, NO _{2 in the} carbonaceous adsorbent packed bed is provided.
The load can be reduced to extend the life of the carbonaceous adsorbent. However, according to this case, when the thermal regeneration of the adsorbent, because the zeolite or activated alumina desorbs remains NO ₂ as described above, the desorbed NO ₂ is detoxification (the aforementioned selective catalytic reduction method or the like It must be released after treatment). On the other hand, when a large amount of carbonaceous adsorbent is required in a large air volume device or the like, there is an advantage that the filling amount can be reduced and the replacement frequency can be reduced.

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. Here, c ₀ : NO concentration before the start of reaction (concentration when released from ventilation equipment or the like in the present invention), c ₁ : NO ₂ concentration before start of reaction (when released from ventilation equipment or the like in the present invention) Concentration, i.e. could not be removed
NO ₂ concentration), k: reaction rate constant, t: reaction time (in the present invention, release-time to reach the ground surface), c: time t
NO ₂ concentration. Here, in the NO-containing air as described above
As a result of the same experiment as the measurement of the change with time of the concentration of NO ₂ which is an oxidation product of NO, the concentration was determined to be a molar fraction ([Example] 3 pp
In the case of m → 3 × 10 ⁻⁶ ), it was found that when the time is expressed in minutes, k = 300 [min ⁻¹ ] at room temperature.

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

[0030]

As mentioned above, the time to reach the ground surface (3 to 5 minutes)
It suffices if the total NO ₂ concentration on the subsequent surface is 0.06 ppm or less.
Therefore, if the allowable value of the total NO ₂ concentration after 5 minutes is 0.06 ppm or less, c ₁ or the allowable NO ₂ concentration at the time of release (at the time of release)
The NO ₂ permissible concentration) is obtained, and the concentration shown in the last column of Table A above (NO ₂ permissible concentration upon release) is obtained. Therefore, for general targets whose NO concentration at the time of release (initial NO concentration of release) is about 1 to 3 ppm, the removal performance may be set so that the allowable NO ₂ concentration at release is about 0.05 ppm. Conversely, 6 ppm
When it reaches a certain level, it is necessary to remove it to less than 0.01ppm. Thus, the allowable concentration of NO ₂ at release depends on the initial NO concentration, and the smaller the initial NO concentration is, the larger it is. When the initial NO concentration of 6 ppm is 0.006 ppm or less, when the initial NO concentration of 1 ppm is 0.058 ppm or less. is there. From these facts, the concentration of NO _X in the original gas before NO ₂ removal it can be seen that it is sufficient if not exceed 6 ppm.

Generalizing the above, total NO after 5 minutes
₂ NO ₂ permissible concentration at release (molar fraction) required to reduce the concentration to 0.006 ppm or less (to be less than or equal to the permissible value of total NO ₂ concentration on the ground surface) is the initial release NO concentration (molar fraction) the when C ₀ is represented by the following formula, it is represented by the following formula to represent the concentration at this time in ppm. (0.06 × 10 ^-6 )-(1500C ₀ ² ) / (1500C ₀ +1) -------- Formula 0.06− (1500 × 10 ^-6 C ₀ ² ) / (1500 × 10 ^-6 C ₀ +1) ) ---- expression

From the above equation, NO _X (NO + NO ₂ ) concentration: 6
Gas to be treated below ppm is passed through a packed bed of NO ₂ adsorbent and / or reducing agent, and when removing NO ₂ by adsorbing and / or reducing NO ₂ in the gas, the NO concentration at the outlet of the packed bed is adjusted. X
Then, the NO ₂ concentration at the packed bed outlet is [0.06− (1500
It can be said that the control should be performed so that it becomes less than or equal to × 10 ^-6 X ² ) / (1500 × 10 ^-6 X + 1)] ppm.

[0034]

[Example] As a carbonaceous adsorbent, commercially available granular palm shell activated carbon GX (hereinafter, adsorbent A) manufactured by Takeda Pharmaceutical Co., Ltd. and NO ₂ of NO
Phenol resin activated granular charcoal (Kobe Steel Co., Ltd.) which was cooled in a nitrogen atmosphere after activation to develop a pore diameter of 4 to 10Å to enhance the oxidation catalytic activity to Hereinafter, an adsorbent B) was prepared.
In addition, Sumitomo Chemical Co., Ltd. granular activated alumina KHD (hereinafter,
Adsorbent C) was also prepared. Then, these adsorbents are separately filled into an adsorption tube with an internal size of 3.2 cm to form an adsorbent packed bed with a packing height of 150 mm, and these packed beds have a temperature of 35 ° C, a relative humidity of 60% and NO. _X concentration: 5.0ppm (NO: 4.5ppm, NO 2: 0.5
ppm) air was passed as the gas to be treated. At this time, the adsorption temperature was 35 ° C., the space velocity was 12000 / h, and the treatment time was continuous 60 hours.

In the above treatment, the NO concentration and NO _X concentration at the adsorbent packed bed outlet were measured by a chemiluminescence type NO _X analyzer (minimum measurement range: 0-2 ppm), and NO ₂ was calculated from the difference between the two concentrations. The concentration was determined. Table 1 shows the NO and NO ₂ concentration changes thus obtained. As can be seen from Table 1, the NO concentration at the outlet of the adsorbent packed bed is 1.1 to 4.4 ppm, which is 6 ppm or less in any case. On the other hand, the NO ₂ concentration at the packed bed outlet is at the level of ND (: not detected [detection lower limit 0.02 ppm]) during the treatment time of 60 hours in the case of the packed bed of adsorbent A, and in the case of the packed bed of adsorbent B, Since the treatment time is up to 24 hours and the level of ND, these correspond to a gas having an initial release NO concentration of 6 ppm or less (that is, a NO ₂ removed NO concentration: gas having a concentration of 6 ppm or less), and therefore, it is directly exposed to the atmosphere. Even if released to, the NO ₂ concentration generated by air oxidation within the time to reach the surface (about 3 to 5 minutes) is
0.06ppm becomes less (NO ₂ concentration at the surface even assuming that NO ₂ of about 0.02ppm had been released at all times becomes less 0.06ppm), essentially NO ₂ concentration of environmental standards: 0.06ppm or less Can be dealt with. In the case of the adsorbent B packed bed, the NO ₂ concentration at the packed bed outlet during the treatment time of 36 to 60 h is
0.03 to 0.10 ppm, of which, after the treatment time of 48 hours, if released directly into the atmosphere, the environmental standard value of NO ₂ concentration is: 0.06 pp
It is difficult to cope with m or less, but at least processing time 36h
Even if it is directly released into the atmosphere, the environmental standard value of NO ₂ concentration is: 0.
It is possible to achieve a level below 06ppm. Further, in the case of the adsorbent C packed bed, the NO ₂ concentration at the packed bed outlet at the treatment time of 12 to 60 hours is 0.03.
It is ~ 0.37ppm, of which, if it is directly discharged into the atmosphere after the treatment time of 24h, it is difficult to meet the environmental standard value of NO ₂ concentration: 0.06ppm or less, but it is directly released into the atmosphere until the treatment time of 12h. Even if the environmental standard value of NO ₂ concentration: 0.06pp
It is possible to deal with m or less.

Comparing the performances of the three adsorbents A, B, and C, adsorbent C showed almost no NO adsorbing capacity, and from the beginning, NO
A breakthrough of ₂ was observed, with the worst performance. On the other hand, the adsorbents A and B were excellent in performance, and comparing them, the adsorbent B was superior in the removal performance of the entire NO _X including NO, but when only focusing on NO ₂ , A was superior, and no breakthrough of NO ₂ was observed during the experimental period (treatment time 60 h). Here, the adsorption of NO on the activated carbon is due to the fact that NO is oxidized to NO ₂ by the oxidation catalytic ability of the activated carbon. Therefore, the adsorbent B with enhanced oxidation catalytic ability has the same operation (treatment) time. In comparison with the above, a larger amount of NO ₂ was treated (a part was adsorbed, and a part was re-reduced and re-released to NO by the reaction with the surface carbon). Therefore, for removal of only NO ₂ as in the case of the method of the present invention, the NO adsorption activity is enhanced as in the adsorbent A, as compared with the expensive activated carbon having the enhanced NO adsorption activity as in the adsorbent B. Inexpensive activated carbon, which is not cheaper, has higher performance than conventional NO _X.
It is more economical than the whole method.

[0037]

[Table 1]

As can be seen from the above embodiments, according to the present invention, NO _X removal target for purifying low concentration NO _X gas such as ventilation gas from an automobile tunnel or a closed type automobile parking lot is NO. _By limiting the number to ₂ , it becomes possible to perform the purification treatment at a high space velocity using a general inexpensive adsorbent. In this case, in particular, when a carbonaceous adsorbent such as the adsorbent A is used, almost all of NO ₂ is desorbed in the form of NO as a breakthrough gas during the adsorption operation or as a desorption gas during the regeneration operation. Therefore, there is no problem of treatment of NO ₂ generated by the adsorbent regeneration operation, which is economical.

[0039]

Method of removing NO ₂ according to the present invention, upon purification of such low concentration NO _X gas such as ventilation gases from automobiles tunnel or closed car parking, NO ₂ is essentially harmful
It eliminates the economic problem of increasing the size and cost of the equipment that is common to each conventional method that removes the entire NO _X and eliminates the economic problems. However, it is possible to remove essentially harmful NO ₂ at a lower cost with a simpler device, and the environmental standard value of the essentially NO ₂ concentration is:
This has the effect of making it possible to meet the requirement of 0.06 ppm or less. Further, since the method of the present invention does not add ozone and does not require dehumidification, it has an effect that it can be solved without problems such as the conventional ozone addition method and zeolite filling method.

Claims (6)

[Claims] 1. A carbon dioxide containing both nitrogen monoxide and nitrogen dioxide, the total concentration of which is 6 ppm or less, is passed through a layer filled with a nitrogen dioxide adsorbent and / or a reducing agent. How to remove nitrogen. 2. A treated gas containing both nitric oxide and nitrogen dioxide, the total concentration of which is 6 ppm or less, is passed through a layer filled with a nitrogen dioxide adsorbent and / or a reducing agent, and the dioxide in the gas is discharged. A method for removing nitrogen dioxide, which comprises removing nitrogen by adsorbing and / or reducing it. 3. The method for removing nitrogen dioxide according to claim 2, wherein the gas after removing the nitrogen dioxide is released into the atmosphere. 4. The method for removing nitrogen dioxide according to claim 1, 2 or 3, wherein the nitrogen dioxide adsorbent and / or the reducing agent is a carbonaceous adsorbent. 5. After completion of adsorption and reduction with the carbonaceous adsorbent, the carbonaceous adsorbent is heated to a temperature of 100 to 200 ° C. to desorb the adsorbed gas component, and the carbonaceous adsorbent is adsorbed. And the method for removing nitrogen dioxide according to claim 4, wherein the reducing activity is restored. 6. After cooling the desorbed gas component,
6. The method for removing nitrogen dioxide according to claim 5, wherein nitrogen dioxide in the gas component is adsorbed and / or reduced by passing through a packed bed containing a carbonaceous adsorbent to prevent leakage of nitrogen dioxide into the atmosphere.