JPH10211427A - Nitrogen dioxide absorber, and method for removing nitrogen dioxide using this absorber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a means to reduce NOx contained in a gas ventilated from a road tunnel and an underground closed space to the environmental standard level of 0.06ppm or less at a low cost and using a simple installation. SOLUTION: This nitrogen dioxide adsorber is an absorber which bears an alkaline metal on an Mn-Ti surface-moldified titania carrier. This absorber can adsorb only NO2 without being adversely affected by coexistent NO. The preferable adsorber is of a honeycomb shape. The method for removing nitrogen dioxide comprises treating an exhaust gas containing a low concentration nitrogen oxide with the nitrogen dioxide adsorber and remove the nitrogen dioxide contained in the exhaust gas by adsorption.

Description

[Detailed description of the invention]

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hazardous waste gas, which contains low concentrations of nitrogen oxides (NOx), i.e., nitrogen monoxide (NO) and nitrogen dioxide (NO ₂ ). It relates to a method for removing the substance NO ₂ and a nitrogen dioxide absorbent for use in this method. Specifically, the present invention is suitably applied to efficiently remove only NO ₂ in low-concentration NOx contained in ventilation gas from road tunnels, underground closed spaces, and the like.

[0002]

2. Description of the Related Art As a method for removing NOx from a stationary source such as a boiler, a selective catalytic reduction method (SCR method) has been established in which NOx is decomposed into nitrogen and water using ammonia as a reducing agent.

[0003] However, even if this method is applied to the removal of NOx in ventilation gas from road tunnels, closed underground spaces, etc., the amount of gas is enormous at room temperature and the concentration of NOx is low. In addition to being unable to obtain a removal rate, it requires a great deal of energy and is not a practical method.

Conventionally, the following methods have been proposed as methods for removing low-concentration nitrogen oxides, especially NO ₂ .

[0005] A method of injecting an oxidizing agent such as ozone (O ₃ ) into the exhaust gas to be treated to oxidize NO to NO ₂ and then adsorbing and removing NO ₂ with a specific composite oxide adsorbent (Japanese Unexamined Patent Application Publication No. 3-13). 275126).

[0006] A method of adsorbing and removing NO ₂ in exhaust gas using a special porous carbonaceous adsorbent
No. 76753), or by using a manganese oxide having NO oxidation activity at room temperature and a carbonaceous adsorbent together to remove NO in the exhaust gas.
₂ (JP-A-6-99028).

[0007]

The O ₃ injection method described above has the problem that it is difficult to control the amount of ozone injected accurately in response to fluctuations in the NOx concentration in the gas to be treated.

[0008] Since it is difficult to always inject the same amount of O ₃ as the concentration of NO in the gas to be treated fluctuates, an excess amount of O ₃ is usually injected. When O ₃ is injected in excess, all of the NO is converted to NO ₂ , so NO ₂ , which is a regulated substance for air pollution, falls below the environmental standard value (0.06).
ppm) or less, a high NO ₂ removal rate must be maintained.

[0009] In addition, since the unreacted portion of the excessively injected O ₃ is discharged as leak O ₃ , the unreacted leak O
It is also necessary to take measures to prevent secondary pollution of exhaust gas treated by ₃ .

[0010] On the other hand, the method using the porous carbonaceous adsorbent, that is, the activated carbon adsorbent has the following problems.

[0011] When a large amount of adsorbent is used to process a large amount of gas, air (oxygen) is supplied to the adsorbent, and the heat generated by oxidation is not released sufficiently, the adsorbent spontaneously ignites. There is a risk of causing

In the step of regenerating the activated carbon adsorbent that has adsorbed NO ₂ , activated carbon (C) is produced by heating and regenerating this in air.
reacts with oxygen (O ₂ ) in the air to become CO ₂ and the adsorbent is reduced in weight. In addition, if the adsorbent is heated in air at a temperature of 200° C. or higher, the activated carbon may be burned and the adsorbent may burn out. Therefore, regeneration becomes insufficient at temperatures below 200°C.

[0013] In order to completely regenerate the activated carbon adsorbent,
Since this must be regenerated by heating in an inert gas or in a reducing atmosphere, activated carbon-based adsorbents are usually discarded without being regenerated.

[0014] Therefore, the use of activated carbon-based adsorbents is impractical, especially in processes that process large volumes of gas.

NO in the atmosphere is oxidized by O ₂ in the air to become NO ₂ as shown in the following formula (1).

[0016]

[Formula 1] 2NO+O ₂ →2NO ₂ (1)

The reaction rate of this reaction is expressed by the following equation (2).

[0018]

[Formula 2] ([NO] is NO concentration, [O ₂ ] is O ₂ concentration, [N
O ₂ ] is the NO ₂ concentration, k is the reaction rate constant, and t is the time)

Since the oxygen concentration in air is constant, the equation
From (2), the production rate of NO ₂ in air is 2 of the NO concentration.
The higher the NO concentration, the higher the NO
₂ is easier to generate.

NOx emitted from stationary sources such as flue gas is mostly NO, and has a high concentration (several hundred pp
m), even if it is released into the atmosphere and diluted, the NO concentration is higher than that of ventilation gas from road tunnels, underground closed spaces, and the like. Therefore, as shown in the above reaction formula (1), NO
Due to the high rate of conversion to ₂ , NO must be removed. In other words, removal of NO ₂ alone does not solve the problem.

On the other hand, the NOx contained in the ventilation gas from road tunnels, underground closed spaces, etc., has a low concentration (1 to 5
ppm), which is contained about 10% in NOx, and NO ₂ subject to regulation is 0.1 to 0.5 ppm. now,
Assuming that only NO ₂ is removed and NO is not removed, the several ppm of NO released without being removed is diluted with the atmosphere, so the NO concentration is about 1/100 compared to the stationary source. It is considered that the amount of oxidation conversion from NO to NO ₂ is very small.

[0022] Therefore, with respect to the removal of NOx contained in ventilation gas from road tunnels, closed underground spaces, etc., it is possible to remove _only the regulated NO2, reduce equipment costs and operating costs, and install NOx removal equipment. This is one of the important options for reducing space.

From the above point of view, the present invention treats NOx contained in ventilation gas from road tunnels, underground closed spaces, etc., with simpler equipment and at a lower cost, and achieves a NO ₂ environmental standard of 0.06 ppm. The aim is to provide a means to reduce to:

[0024]

A nitrogen dioxide adsorbent according to the present invention is an adsorbent comprising an Mn--Ti surface-modified titania carrier carrying an alkali metal. This adsorbent can only adsorb NO ₂ without being affected by coexisting NO. A preferred form of the adsorbent is a honeycomb.

The method for removing nitrogen dioxide according to the present invention is a method of treating exhaust gas containing low-concentration nitrogen oxides with the above nitrogen dioxide adsorbent to adsorb and remove nitrogen dioxide in the exhaust gas.

[0026]

[Embodiment of the invention]

[Example 1] Anatase titania sol (solid content: 3
0% by weight) was impregnated into a 0.5 mm thick ceramic paper (manufactured by Nippon Muki Co., Ltd., silica:alumina=50:50). Next, the same paper was corrugated and dried to form a corrugated sheet holding 252 g/m ² of amorphous titania. A honeycomb structure was obtained by laminating a plurality of the titania-impregnated ceramic paper corrugated plates.

This honeycomb structure was immersed in a 2 mol/l manganese nitrate aqueous solution for 30 minutes, dried, and fired at 450° C. for 3 hours in an air stream to obtain a surface-modified honeycomb titania carrier.

This honeycomb carrier was immersed in a 3 ^mol / ^l potassium hydroxide aqueous solution for 30 minutes, dried, and dried at 120° C. for 3 hours in an air stream.
A prismatic potassium-supporting surface-modified titania adsorbent (A) was obtained.

This adsorbent (A) contains 3 each of titanium (Ti), manganese (Mn) and potassium (K).
It contained 8.6%, 8.6%, and 10.2%.

Adsorption time Adsorbent (A) was packed in a stainless steel reaction tube and
ppm of NOx (27ppm for NO, _3ppm for NO2
m) at 160 NL/h (space velocity: SV =
equivalent to 8,000 h ⁻¹ ), and the adsorbent
The NO ₂ and NO adsorption removal performance of (A) was measured. The measurement results are shown in FIG.

As can be seen in FIG. 1, the NO removal rate drops below 30% in about 20 hours after the start of adsorption.
It can be seen that the NO ₂ removal rate of 0% or more can be maintained for about 180 hours or longer.

[0032] From this, the adsorbent (A) scarcely adsorbs NO, and is primarily a NO ₂ adsorbent that selectively adsorbs and removes NO ₂ .
It turns out that it functions as an adsorbent.

In the adsorbent (A), NO ₂ in the gas reacts with potassium (K), which is an alkali metal, and is considered to be fixed in the adsorbent (A) as potassium nitrite or potassium nitrate. .

Influence of SV (amount of treated gas) One adsorbent (A) was filled in a stainless steel reaction tube, and 94
ppm of NOx (80ppm for NO, _14ppm for NO2
The relationship between the NO ₂ removal rate and the amount of treated gas was examined by circulating air containing m) through the reaction tube and varying the amount of gas. The results are shown in FIG.

As seen in FIG. 2, space velocity (SV),
That is, the amount of treated gas per unit volume of adsorbent and NO ₂
A linear relationship is recognized between the removal rate and the adsorption reaction, and it is found that the adsorption reaction can be arranged as a first-order reaction.

Effect of NO ₂ Concentration Adsorbent ( _A ) was filled in a stainless steel reaction tube, and the gas flow rate was maintained at a constant value (100 NL/h).
The relationship between the NO ₂ removal rate and the NO ₂ concentration was investigated by changing the concentration. The results are shown in FIG. The gas to be passed through the adsorbent (A) contains not only NO ₂ but also NO, and the NO ₂ /NO ratio was adjusted to about 0.15. Also, the space velocity (SV) was set to 5,000 h ^-1 .

As can be seen in FIG. 3, the NO ₂ removal rate
₂ It can be seen that it is almost constant regardless of the concentration.

Predicted Performance in Full-Scale Apparatus The performance in a full-scale apparatus using adsorbent (A) was predicted based on the following assumptions.

It is assumed that the NOx concentration in the exhaust gas to be treated in a full-scale system is about 1-3 ppm, and that NO ₂ is 0.1-0.3 ppm. again,
As for the amount of gas to be treated per unit volume of adsorbent, the space velocity (SV) was set to 6.0, focusing on reducing the facility capacity.
It was assumed to be 00h ^-1 .

[0040] Based on the above conditions, the NO ₂ concentration was set to 0.3
The NO ₂ adsorption removal performance expected as ppm is shown in FIG.

As seen in FIG. 4, it can be seen that the NO ₂ removal rate can be maintained above 80% for approximately 2,500 hours. One year (365
day), the annual operating time is 5.1
10 hours. The adsorption time of _2,500 hours in FIG.
In the case of pm, the adsorption operation can be continued for about half a year without exchanging the adsorbent (A). Naturally, NO ₂
When the concentration is 0.3 ppm or less, the service life of the adsorbent increases as the concentration decreases, and the concentration decreases to 0.15 ppm.
In the case of pm, the operation can be continued for about one year without replacing the adsorbent.

[0042]

Effect of the Invention The NO ₂ removal method according to the present invention adsorbs and removes only NO ₂ in the low-concentration NOx contained in the ventilation gas from road tunnels, underground closed spaces, etc., resulting in a long-lasting effect of about half a year or more. More than 80% of NO ₂ can be removed without changing the adsorbent over time.

FIG. 5 shows a schematic flow of the NO ₂ adsorption removal process according to the present invention for removing only NO ₂ and FIG.
1 shows a schematic flow of a conventional NOx adsorption removal process for removing Ox.

In the conventional process shown in FIG. 6, an electrostatic precipitator
(1) a plurality of NOx adsorbers filled with NOx adsorbent (1
2) and a regeneration system device (3) for regenerating the adsorbent is provided, and the regeneration system device (3) is a circulation blower (4), a heater
(5), SCR denitrification reactor (6), etc. A plurality of NOx adsorbers (12) and regeneration system equipment (3) are arranged in parallel.

[0045] In the conventional process having such a structure, after the ventilating gas containing low concentrations of NOx is dedusted through the electrostatic precipitator (1), it is passed through two of the multiple NOx adsorbers to adsorb NOx. Remove. One of the multiple NOx adsorbers is always in regeneration mode and the rest are in adsorption mode. One of the two NOx adsorbers in the adsorption mode is switched to the regeneration mode sequentially, and the adsorbed NOx
Hot air is passed through the adsorber to desorb NOx from the adsorbent to regenerate the adsorbent. At the same time, the desorbed high-concentration NOx reacts with NH ₃ in the SCR denitrification reactor (3) in the regeneration system and is decomposed into nitrogen and water to be rendered harmless. In addition, in the conventional process of adsorbing and removing NOx, the adsorption time to maintain a NOx removal rate of 80% or more is several days, so the NOx removal device must regenerate the adsorbent within the same device. , it cannot be a practical device, and regenerative system equipment is essential.

On the other hand, in the process according to the present invention for adsorbing and removing only NO ₂ , the adsorption and removal operation can be continued for half a year or more without regenerating the adsorbent. In this way, operation can be continued for more than half a year without replacing the adsorbent,
It is not necessary to regenerate the adsorbent within the NOx remover. _In the NO2 adsorption removal process according to the present invention, the used adsorbent can be sent to a regeneration plant for regeneration and the regenerated adsorbent can be reused.

N according to the present invention that adsorbs and removes only NO ₂
The O ₂ adsorption removal process requires only an electrostatic precipitator (1) and one NO ₂ adsorber (2), as shown in FIG.
Since no regenerative equipment is required, the process is extremely simple compared to the conventional process shown in FIG.

[0048] Therefore, in the NO ₂ removal process according to the present invention, compared with the conventional NOx removal process, the facility capacity can be significantly reduced, and the facility construction cost (initial cost) can be greatly reduced. Since regeneration of the adsorbent is not required, no energy is required for regeneration, and a significant reduction in operating costs (running costs) is possible.

[Brief description of the drawing]

FIG. 1 is a graph showing the relationship between adsorption time and removal rate of NO ₂ and NO.

FIG. 2 is a graph showing the relationship between space velocity and NO ₂ removal rate;

FIG. 3 is a graph showing the relationship between NO ₂ concentration and NO ₂ removal rate;

FIG. 4 is a graph showing the relationship between adsorption time and NO ₂ removal rate;

FIG. 5 is a flow sheet outlining the NO ₂ adsorption removal process according to the present invention;

FIG. 6 is a flow sheet showing an outline of a conventional NOx adsorption removal process;

[Description of symbols]

1: Electric dust collector 2: NO ₂ adsorber

──────────────────────────────────────────────────── ──── continuation of the front page (72) Inventor Seietsu Kikuchi Hitachi, 5-3-28, Nishikujo, Konohana-ku, Osaka Shipbuilding Co., Ltd. (72) Inventor Kaizo Kamitoshi Hitachi, 5-3-28, Nishikujo, Konohana-ku, Osaka Shipbuilding Co., Ltd. (72) Inventor Atsushi Fukuju Hitachi, 5-3-28, Nishikujo, Konohana-ku, Osaka Shipbuilding Co., Ltd. (72) Inventor Akihiro Usutani Hitachi, 5-3-28, Nishikujo, Konohana-ku, Osaka Shipbuilding Co., Ltd. (72) Inventor Teruo Iwamoto Hitachi, 5-3-28, Nishikujo, Konohana-ku, Osaka Shipbuilding Co., Ltd.

Claims (2)

[Claims] 1. A nitrogen dioxide adsorbent comprising an alkali metal supported on a Mn-Ti surface-modified titania support. 2. A method for removing nitrogen dioxide, comprising treating exhaust gas containing low-concentration nitrogen oxides with the nitrogen dioxide adsorbent according to claim 1 to adsorb and remove nitrogen dioxide in the exhaust gas.