JPH08141362A - Nitrogen oxide removing method - Google Patents

Abstract

PURPOSE: To adsorb and remove NOX in a gas containing NOX in a low concentration by bringing the nitrogen oxides into contact with a specified crystalline silicate-based adsorbent carrying calcium or magnesium. CONSTITUTION: A crystalline silicate is used as a carrier and the silicate has a chemical formula, (1±0.8)R2 O.[aM2 O3 .bAl2 O3 ].cMeO.ySiO2 in a dehydrated state (wherein, R stands for an alkali metal ion and/or hydrogen ion; M is one or more elements selected from VIII-group elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium; Me is alkaline earth elements, a a>=0, b>=0, c>=0, a+b=1, y/c>12, y>12) and has an x-ray diffraction pattern shown in the table I. A gas is brought into contact with an adsorbent produced by carrying calcium or magnesium on the carrier to adsorb and remove nitrogen oxides at a normal temperature and the resulting adsorbent is heated in air to desorb the nitrogen oxides and to be reproduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention is mainly applied to road tunnels,
The present invention relates to a method for adsorbing and removing low-concentration nitrogen oxides (NOx) such as nitric oxide (NO) and nitrogen dioxide (NO ₂ ) contained in ventilation gas from a closed parking lot.

[0002]

2. Description of the Related Art Ventilation equipment in a road tunnel is mainly used for securing a clear visual distance due to dust and carbon monoxide (CO),
It is installed for the purpose of reducing harmful substances such as nitrogen oxides (NOx) to below the allowable value. The current ventilation system takes in fresh air from the outside of the tunnel, removes dust if necessary, and
The method of forced ventilation outside the tunnel is generally used. However, this method only diffuses ventilation gas containing harmful substances into the atmosphere, and is not a fundamental environmental improvement. Particularly in urban areas where air pollution due to automobile exhaust gas is becoming serious, a highly polluted area is expanded, which may hinder tunneling and shelter installation in road planning. Therefore, it is desired to develop a new ventilation method that saves energy and does not affect the surrounding environment.

However, since the tunnel ventilation gas has a peculiarity that the NOx concentration is 10 ppm or less at room temperature and a large capacity and the NOx concentration fluctuates drastically depending on the traffic volume, the NH which has already been put to practical use in boiler combustion exhaust gas. It is impossible to directly apply the NOx purification method using ₃ as a reducing agent.

Therefore, various dry methods and wet methods have been proposed as low-concentration NOx purification methods, but the wet method is difficult to put into practical use because it requires a wastewater treatment apparatus. The following dry methods are available.

NOx of tunnel ventilation gas is adsorbed on an adsorbent, and after separating and concentrating from the adsorbent by changing temperature and pressure, catalytic decomposition is performed in the presence of a catalyst without adding a reducing agent to decompose NOx into harmless nitrogen. Law.

An adsorption method in which NOx of tunnel ventilation gas is adsorbed by an adsorbent, separated and concentrated by changing temperature and pressure, and then NH ₃ is used as a reducing agent to make harmless nitrogen with a catalyst and removed.
Depending on the adsorbent, NO ₂ will give high performance, so
When oxidizing with ozone beforehand, propane may be used as a reducing agent instead of NH ₃ .

An electron beam irradiation method in which NH ₃ is added to a tunnel ventilation gas, and at the same time, an electron beam is irradiated to replace NOx with ammonium nitrate, and is collected and removed by an electric dust collector.

In consideration of the fact that such NOx purification equipment is installed in the city, the inventor of the present invention has a strong image of harmful substances and uses NH ₃ , O ₃ , which may generate secondary pollutants, and ammonium nitrate. I examined the purification method without the generation of solid by-products like the above and proposed the first method. In other words, it is a method of separating and concentrating NOx by PSA (pressure swing adsorption) method using a zeolite-based adsorbent and then decomposing it into nitrogen and oxygen with a NOx decomposition catalyst, but concentrating the NOx concentration several thousand times (about 1%). Otherwise, the NOx decomposing catalyst does not exhibit a predetermined decomposing performance, so that it is inferior in practicality in terms of the occupied volume of the NOx separating / concentrating section, equipment cost, required power, and the like.
Therefore, using the same adsorbent as above, a method of converting to nitrogen by a catalytic action by adding a reducing gas was proposed. Here, as the reducing gas, propane, which is a household fuel, was selected in place of NH _3, which has a strong image as a harmful substance in the event of a leak. However, this method was affected by saturated water content (about 2.6%) contained in the treated gas, and even if the hydrophobic zeolite adsorbent was used, the NOx adsorption capacity was reduced by repeated adsorption and desorption. . Therefore,
It is necessary to say that a means for reducing the water content of the treated gas to zero or increasing the adsorbent capacity is required, and it cannot be said that the effect of improving the NOx separation / concentration section is small.

[0009] Next, as a result of selecting an adsorbent having a small decrease in NOx adsorption capacity even after repeated adsorption / desorption in the presence of water and examining adsorption / desorption conditions, activated carbon was used as the adsorbent.
Adsorption and desorption by TSA (temperature swing adsorption) method, NH ₃
We have also obtained the prospect that a process of using nitrogen as a reducing agent to produce harmless nitrogen with a catalyst is also promising. However, ordinary activated carbon has a slow adsorption rate and is affected by the NO partial pressure. To achieve a predetermined adsorption performance, the treatment gas flow rate must be 0.1 m / sec or less, and the activated carbon must be regenerated by heating. If it is carried out in the air, there is a risk of ignition, so temperature control must be carried out sufficiently, so it cannot be said to be perfect for practical use.

[0010]

As described above, it can be said that a method for efficiently and economically removing the lean NOx in the tunnel ventilation gas has not been found yet. On the other hand, the tunnel ventilation gas contains soot and dust, and soot and dust removing equipment such as an electric dust collector has already been installed. In the electrostatic precipitator, when the applied voltage is increased, NO is converted to NO _2, and the conversion rate increases as the applied voltage increases. Therefore, by utilizing this phenomenon, or by adding ozone from the outside of the system to convert NO to NO ₂ and applying an adsorbent that selectively adsorbs NO ₂ , lean gas in tunnel ventilation gas is diluted. It is known that NOx can be removed efficiently. In view of such a state of the art, the present invention provides a low concentration of NO.
An object of the present invention is to provide a method capable of adsorbing and removing NOx in an x-containing gas for the purpose.

[0011]

According to the present invention, a dilute exhaust gas containing nitrogen oxide, particularly nitrogen dioxide as a main component, is brought into contact with a specific crystalline silicate-based adsorbent carrying calcium or magnesium to effect nitrogen oxidation. It is a method for removing nitrogen oxides, which comprises adsorbing and removing a substance at room temperature, and desorbing an adsorbent having adsorbed nitrogen oxides with heated air to regenerate it.

[0012] The crystalline silicate is dehydrated (1 ± 0.8) R ₂ O. [aM ₂ O ₃ .bAl
₂ O ₃ ] .cMeO.ySiO ₂ (wherein R is an alkali metal ion and / or a hydrogen ion, M is selected from the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, antimony, and gallium. One or more elements, Me is an alkaline earth element, a ≧ 0, b ≧ 0, c ≧
A crystalline silicate having a chemical composition of 0, a + b = 1, y / c> 12, y> 12) and an X-ray diffraction pattern shown in Table A below is used.

The amount of calcium or magnesium supported on the crystalline silicate is 0.3 to 40 wt% as an oxide, preferably 0.5 to 30 wt%. Examples of the method for supporting calcium or magnesium on the crystalline silicate include an ion exchange method and an impregnation method.

[0014]

[Table 1] VS: Very strong M: Intermediate S: Strong W: Weak (X-ray source Cu)

[0015]

The adsorbent used in the present invention is a diluted NO.
_It has an effect of efficiently adsorbing NO ₂ from the air containing ₂ at room temperature, and the adsorbed NO ₂ is desorbed by heating to about 400 ° C. Therefore, by repeating this adsorption / desorption reaction, the air containing the lean NO ₂ can be efficiently purified. Hereinafter, the present invention will be described in more detail with reference to examples.

[0016]

【Example】

(Prototype of adsorbent)

Preparation of powder adsorbent 1 Water glass No. 1 (SiO ₂ : 30%): 5616 g of water:
It was dissolved in 5429 g to prepare a solution A. On the other hand, water: 417
Aluminum sulfate: 718.9 g, ferric chloride:
110 g, calcium acetate: 47.2 g, sodium chloride: 262 g and concentrated hydrochloric acid: 2020 g were mixed and dissolved to prepare a solution B. Solution A and solution B were supplied at a constant ratio to generate a precipitate, which was sufficiently stirred to obtain a slurry having a pH of 8.0. Then, this slurry was charged into a 20 liter autoclave, tetrapropylammonium bromide: 500 g was further added, and hydrothermal synthesis was carried out at 160 ° C for 72 hours, followed by washing with water and drying, and further at 500 ° C for 3 hours. The crystalline silicate 1 is obtained by firing. This crystalline silicate 1 had the following composition formula expressed by the molar ratio of the oxide (dehydrated form), and the crystal structure was as shown in Table A above by X-ray diffraction.

[0019]

[Chemical formula 1] 0.5Na ₂ O ・ 0.5H ₂ O ・ [0.8Al
₂ O ₃ · 0.2Fe ₂ O ₃ · 0.25CaO] · 25S
iO ₂

The above crystalline silicate 1 was treated with 4N NH ₄ C.
and stirred 3 hours l aqueous solution (40 ° C.), was carried out NH ₄ ion exchange. After ion exchange, wash and wash at 100 ° C for 24 hours
After drying for an hour, it was baked at 400 ° C. for 3 hours to obtain H-type crystalline silicate 1. This H type crystalline silicate 1
(100 g) of calcium chloride solution (CaCl ₂ · 2
H ₂ O: 18.3 g / 100 cc of water) was impregnated, thoroughly kneaded, and then evaporated to dryness at 200 ° C. Then, by firing at 500 ° C. in an air atmosphere for 12 hours, 7 w of CaO is added.
A powder adsorbent 1 supporting t% was obtained.

Preparation of powder adsorbents 2 to 30 In the method for synthesizing crystalline silicate 1 in the preparation of powder adsorbent 1 above, instead of ferric chloride, cobalt chloride, ruthenium chloride, rhodium chloride, lanthanum chloride, chloride Cerium, titanium chloride, vanadium chloride, chromium chloride, antimony chloride, gallium chloride and niobium chloride,
Crystalline silicates 2 to 12 were prepared by repeating the same operation as in crystalline silicate 1 except that the same molar number as Fe ₂ O ₃ was added in terms of oxide. The crystal structure of these crystalline silicates is shown in Table A above by X-ray diffraction, and its composition is expressed by the molar ratio of oxides (dehydrated form) of 0.5Na ₂ O · 0.5H. ₂ O ・ (0.
2M ₂ O ₃ · 0.8Al ₂ O ₃ · 0.25CaO) · 2
5SiO ₂ . Where M is Co, Ru, Rh, L
a, Ce, Ti, V, Cr, Sb, Ga, Nb.

Furthermore, in the method of synthesizing the crystalline silicate 1,
Instead of calcium acetate, magnesium acetate,
Strontium acetate and barium acetate converted to oxides
Except that the same molar number as CaO was added.
Crystalline silicate 1
3-15 were prepared. Crystals of these crystalline silicates
The structure is that shown in Table A above by X-ray diffraction.
The composition of is expressed by the molar ratio of oxide (dehydrated form),
0.5Na₂O ・ 0.5H₂O ・ (0.2Fe ₂O₃・
0.8 Al₂O₃・ 0.25MeO) ・ 25SiO₂so
is there. Here, Me is Mg, Sr, or Ba.

Using the above crystalline silicates 2 to 15,
H-type crystalline silicate 2 in the same manner as the powder adsorbent 1
~ 15 were obtained, and this silicate had an aqueous calcium chloride solution (CaCl ₂ · 2H ₂ O: 18.3 g / 100 cc of water).
Were impregnated to obtain powder adsorbents 2 to 15 in the same manner as powder adsorbent 1.

An aqueous solution of magnesium chloride was used in place of the aqueous solution of calcium chloride, and an aqueous solution of magnesium chloride (MgCl ₂ .6H ₂ O: 35.3 g / 100 c of water) was added to the above H-type crystalline silicates 1 to 15 (100 g).
c) is impregnated, thoroughly kneaded, and then evaporated to dryness at 200 ° C.,
By firing in air at 500 ° C for 12 hours, MgO
Powder adsorbents 16 to 30 carrying 7 wt.

〇 60 parts of silica sol (Nissan Chemical's “Snowtex 0”, silica content: 20%) and alumina sol (Nissan Chemical, alumina content: 10%) were used as a slurry preparation binder for 100 parts of powder: 100 parts, respectively. After adding 10 parts and further 300 parts of water and mixing, the pH was adjusted to 4 by adding nitric acid to prepare a slurry for washcoat.

Coating and firing A cordierite monolithic honeycomb substrate (400 cells / inch ² ) was dipped in the above slurry, taken out, and the excess slurry was blown off with compressed air, and the temperature was increased to 30 ° C. at 30 ° C.
Dry for minutes. After drying, the honeycomb substrate is further dipped in a slurry, and these operations are repeated to coat a predetermined amount of adsorbent powder (about 60 g / m ³ substrate surface) on the honeycomb substrate, and at 600 ° C. The honeycomb adsorbents 1 to 30 were obtained by firing in an electric furnace for 3 hours.

Calcium and magnesium are added to the H-type crystalline silicate 1 carrier 0.5 times with CaO and MgO, respectively.
%, 2.0%, 10.0%, and 30.0 wt% of the powder adsorbents 31 to 38 were experimentally prepared.

Further, as a comparative example, powders (100 mesh or less) of activated carbon powder (Shirasagi PHC-5 manufactured by Takeda Pharmaceutical Co., Ltd.) and activated alumina (Sumitomo Chemical Co., Ltd., JGC Chemical Co., Ltd.) were prepared, and powder adsorbents 39 to 41 were used. Obtained. Then, these powder adsorbents 31 to 41 were also formed into a honeycomb by the same method as described above to obtain honeycomb adsorbents 31 to 41. A list of these honeycomb adsorbents is shown in Table B.

[0029]

[Table 2]

[0030]

[Table 3]

[0031]

[Table 4]

(Evaluation of Performance of Prototype Adsorbent) The adsorption and desorption performance of the prototype adsorbent was evaluated by setting adsorption and desorption test conditions as shown in Table C below.

[0033]

[Table 5]

Using the results of the second adsorption test after the first adsorption / desorption test as data, the breakthrough time (time when C / C ₀ = 0.2, C ₀ : inlet NOx concentration, C: outlet NOx concentration) )
And the NO ₂ adsorption capacity were determined. The test results are shown in Table D, and the adsorbents that were prototyped (honeycomb adsorbents 1, 16, 3,
Fig. 1 shows the breakthrough curve (a) and desorption curve (b) of Fig.
As shown in FIG.

Table D shows C / C _{0 at} an adsorption time of 60 minutes.
= 0.2, the breakthrough time was described as 60 minutes or more, and the NO ₂ adsorption capacity was the adsorption amount up to the adsorption time of 60 minutes.

[0036]

[Table 6]

From Table D, the crystalline silicate adsorbent supporting calcium or magnesium (Honeycomb Nos. 1 to 3)
0) has a long breakthrough time and a large NO ₂ adsorption capacity. On the other hand, although activated carbon (Honeycomb No. 39) has a large NO ₂ adsorption capacity, it has a very short breakthrough time and is not practical. On the other hand, activated alumina (Honeycomb Nos. 40 and 41) has a slight difference in performance due to the difference in physical properties, but the NO ₂ adsorption capacity and breakthrough time are inferior to those of the crystalline silicate adsorbent supporting calcium or magnesium. I understand that.

Further, the honeycomb adsorbent 3 in which the loading amount of calcium or magnesium on the crystalline silicate is changed
From 1 to 38, the following things became clear. The smaller the amount of calcium and magnesium carried, the shorter the breakthrough start time, and in conjunction with this, the breakthrough time (C / C ₀ = 0.
2) was also shortened, but the total breakthrough time under these conditions was 60 minutes or longer. On the other hand, the higher the loading amount of calcium or magnesium, the better the adsorption performance, but it is almost the same at 2 wt% or more as an oxide.

[0039]

INDUSTRIAL APPLICABILITY The NOx adsorbent obtained by the method of the present invention selectively adsorbs and removes lean NOx in the air at room temperature, and in particular, NO ₂ , and is not affected by moisture in the air. NOx can be efficiently adsorbed and removed.

[Brief description of drawings]

FIG. 1 is a chart showing a NOx adsorption breakthrough curve (a) and a NOx desorption curve (b) of a honeycomb adsorbent 1 used in the present invention.

FIG. 2 NOx of the honeycomb adsorbent 16 used in the present invention
The chart which shows an adsorption breakthrough curve (a) and a NOx desorption curve (b).

FIG. 3 is a chart showing a NOx adsorption breakthrough curve (a) and a NOx desorption curve (b) of a honeycomb adsorbent 39 shown as a comparative adsorbent.

FIG. 4 is a chart showing a NOx adsorption breakthrough curve (a) and a NOx desorption curve (b) of the honeycomb adsorbent 40 shown as a comparative adsorbent.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01J 20/04 B 20/34 F

Claims (3)

[Claims] 1. A gas containing dilute nitrogen oxides in a dehydrated state (1 ± 0.8) R ₂ O. [aM ₂ O ₃ ·.
bAl ₂ O ₃ ] .cMeO.ySiO ₂ (wherein R is an alkali metal ion and / or a hydrogen ion, M is a group VIII element, a rare earth element, titanium, vanadium, chromium, niobium, antimony, or gallium. At least one element, Me is an alkaline earth element, a ≧ 0, b ≧
0, c ≧ 0, a + b = 1, y / c> 12, y> 12), and a crystal having the X-ray diffraction pattern shown in Table A in the Detailed Description of the Invention section. A carrier made of water-soluble silicate is brought into contact with an adsorbent supporting calcium or magnesium to adsorb and remove nitrogen oxides at room temperature, and the adsorbent adsorbing nitrogen oxides is desorbed with heated air to be regenerated. And a method for removing nitrogen oxides. 2. Removal of nitrogen oxides according to claim 1, wherein an adsorbent having an amount of calcium or magnesium supported on the crystalline silicate carrier in the range of 0.3 to 30% by weight is used. Method. 3. The method for removing nitrogen oxides according to claim 1, wherein the dilute nitrogen oxide contains nitrogen dioxide as a main component.