JPH0679136A - Method for treating air containing small amount of nitrogen oxide - Google Patents

Abstract

PURPOSE:To provide a method for purifying a large volume of air containing a small amount of NOx efficiently using an NOx decomposition catalyst. CONSTITUTION:Air containing a small amount of NOx is guided to an NOx adsorption tower 17, so that NOx is removed by adsorption and purified gas 6 is discharged to an atmosphere. During this process, the previously adsorbed NOx is desorbed in an NOx adsorption tower 18. The desorbed NOx gas is supplied to NOx concentration towers 19, 20 where NOx is adsorbed and desorbed again for concentration. The NOx gas thus concentrated is heated by a heat exchanger 13 into a gas 12 and further, it is heated to a temperature suited for NOx decomposition reaction by a heater 14. After that, this heated NOx gas is conducted to an NOx decomposition tower 21 filled with an NOx decomposition catalyst, where NOx is decomposed into nitrogen and oxygen. An outlet gas 15 coming out of the NOx decomposition tower 21 is used as a desorption purge gas in adsorption towers 19, 20 for NOx concentration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for purifying air containing nitrogen oxides (hereinafter referred to as NOx), for example, purification of polluted air for efficiently purifying air polluted with nitrogen oxides generated from exhaust gas of automobiles. Regarding the method.

[0002]

2. Description of the Related Art NOx of about 5 ppm is recognized in tunnel roads, underpasses, roads with noise prevention covers, underground parking lots, etc. in metropolitan areas such as Tokyo, and also in underground tunnel motorways, which are expected to increase in the future. Although it is considered that similar pollution will progress, conventionally, a method for purifying large-capacity air having such a low pollution level has not been put into practical use.

The method for purifying polluted air is roughly divided into a dry method and a wet method, but the wet method requires a wastewater treatment device and is difficult to put into practical use, and the development of the dry method is the mainstream.

As a dry method, polluted air is treated with activated carbon or moisture.
Cura sieve, silica gel, alumina, metal oxide, etc.
Adsorption method of adsorbing NOx through the adsorbent of air, contaminated air
To NH ₃Is injected and heated, and NOx is converted to N using a catalyst.₂Return to
Selective catalytic reduction method or NH in polluted air₃With
Then, it is irradiated with electron beam to remove NOx and SOx from nitric acid or sulfuric acid mist.
And NH₃In reaction with ammonium nitrate, ammonium sulfate
Electric dust collectors that generate um and double salts of both
There is an electron beam irradiation method for recovering this.

Or, ideally, NO which decomposes NOx into harmless N ₂ and O ₂ in the presence of a catalyst without the addition of a reducing agent.
x Catalytic decomposition method, etc. [Industrial Pollution Control Association, "Survey research on automobile exhaust gas technology" (consigned by Japan Highway Public Corporation), March 1983, pp. 37-38] However, first, for the adsorption method, general silica gel, alumina, and molecular sieve were used. NO what you had
It is not economical because it requires a large amount of adsorbent because its adsorption performance for is extremely small.

Next, in the NH ₃ injecting NOx selective catalytic reduction method or electron beam irradiation method, an equivalent amount of NOx contained in a small amount of about 5 ppm in large volume polluted air, which is the object of the present invention, is required. It is difficult to uniformly mix a small amount of NH _3, and there is a risk of secondary pollution such as outflow of unreacted NH ₃ . [Industrial Pollution Control Association, "Survey research on automobile exhaust gas treatment technology" (consigned by Japan Highway Public Corporation) March 1984
Mon, p. 49] Further, in the NOx catalytic decomposition method without addition of a reducing agent, the reaction temperature is high and the temperature is 400 to 600 ° C, and the NOx concentration becomes low.
NOx decomposition rate decreases rapidly, [(decomposition by catalytic reaction of nitrogen oxides) Masakazu Iwamoto, PETROTECH, No. 12
Vol. 11, No. 11 (1989), p. 34] It was not practically applicable to low NOx concentration and large volume contaminated air at room temperature.

In order to solve the above problems, the present inventors have proposed an adsorption method using an adsorbent having a large NOx adsorption characteristic.
A method for purifying large-capacity NOx-contaminated air was proposed by combining the NOx catalytic cracking method (JP-A-3-186318).

The above proposed method is generated from automobile exhaust gas
From air polluted with NOx, etc., NO
x is adsorbed and removed, and discharged as clean air out of the system, while
The NOx adsorption / desorption step of desorbing adsorbed NOx from the adsorbent to obtain a concentrated desorption gas, and the desorption gas is catalytically decomposed at high temperature by a NOx decomposition catalyst to render it harmless, and the decomposition gas heats the desorption gas mutually. After exchange and heat recovery of the decomposition gas, NO
x A method for purifying contaminated air, which comprises a NOx decomposition step that is returned to the adsorption / desorption step.

In the above-mentioned proposed method, as the NOx adsorbent, a copper ion-exchanged zeolite obtained by subjecting a Y-type or X-type zeolite to a copper ion exchange treatment is used, and as the NOx decomposition catalyst, a mol of an oxide in a dehydrated form is used. Expressed as a ratio, (1.0 ± 0.4) R ₂ O ・ [aM ₂ O ₃・ bAI ₂ O ₃ ] ・ ySi
O ₂ (in the formula, R: alkali metal ion and / or hydrogen ion, M: one or more element ions of the group consisting of Group VIII elements, rare earth elements, titanium, vanadium, chromium, niobium, and antimony, a + b = 1 , A ≧ 0, b ≧ 0, y> 1
A catalyst obtained by performing copper ion exchange using a crystalline silicate having the chemical composition of 2) as a carrier is used.

According to the research conducted by the present inventors, it has been found that the NOx adsorbent used in the above-mentioned proposed method is also effective when ZSM-5 type zeolite is subjected to a copper ion exchange treatment. It has been confirmed that the above-mentioned crystalline silicate, which is a precursor of the decomposition catalyst, preferably has the X-ray diffraction pattern shown in Table 1. Furthermore, it has been confirmed that this NOx decomposition catalyst efficiently promotes NOx decomposition by containing or adding an organic substance such as propane or propylene as a reducing agent.

[0011]

[Table 1]

[0012]

[SUMMARY OF THE INVENTION However, the copper with the crystalline silicate catalyst ion-exchanged, when decomposing the concentrated NOx, the performance of the catalyst is greatly influenced by the gas composition to coexist, especially O ₂ It was found that there is a problem that a large amount of catalyst or a large amount of reducing agent is required because the NOx decomposition rate decreases when is present in the coexisting gas.

As described above, the present invention concentrates the dilute NOx in the atmosphere previously proposed by the present inventors by an adsorption / desorption process using an adsorbent, and desorbs the desorbed gas containing the enriched NOx by a NOx decomposition catalyst. lean NOx which is adapted to denitration without addition of NH ₃
An object of the present invention is to provide a method for purifying lean NOx-containing air that can eliminate this problem due to the coexistence of O _{2 in the} method for purifying air containing oxygen.

[0014]

According to the present invention, the lean NOx in the atmosphere is adsorbed and removed by an adsorbent at room temperature to release clean air into the atmosphere, while the adsorbed NOx is desorbed from the adsorbent. Eliminates the problems caused by coexisting O ₂ in the purification method of NOx-containing air that has a NOx adsorption / desorption process to obtain a desorption gas containing concentrated NOx and a NOx decomposition process to denitrate the desorption gas without adding NH ₃ by a NOx decomposition catalyst. For the adsorption
As a purge gas for desorbing NOx from the adsorbent,
The gas from the NOx decomposition process is used.

Examples of the NOx adsorbent used in the present invention include copper ion exchanged zeolite obtained by subjecting ZSM type, Y type or X type zeolite to copper ion exchange treatment. The NOx decomposing catalyst is a dehydrated oxide. Expressed as a molar ratio of (1.0 ± 0.4) R ₂ O ・ [aM ₂ O ₃・ bAI ₂ O ₃ ] ・
ySiO ₂ (wherein R is an alkali metal ion and / or hydrogen ion, M is a group VIII element, a rare earth element, one or more element ions of the group consisting of titanium, vanadium, chromium, niobium, and antimony, a + b = 1 , A ≧ 0, b ≧ 0, y>
A catalyst obtained by carrying out copper ion exchange using a crystalline silicate having a chemical composition of 12) and having an X-ray diffraction pattern shown in Table 1 as a carrier is disclosed (see Japanese Patent Laid-Open No. 3-143547).

[0016]

In the method of the present invention, the polluted air containing lean NOx is introduced into the packed tower filled with the adsorbent, NOx is adsorbed by the adsorbent, and the purified air is discharged from the packed tower. It The adsorbent has a large NOx adsorption capacity, and is selected and used. However, as NOx adsorption progresses, it becomes more difficult to adsorb, and thus adsorbed NOx is desorbed to regenerate the adsorbent.

Further, when NOx adsorption and desorption performances deteriorate as the adsorption and desorption are repeated for a long period of time, the desorption temperature is temporarily raised and accumulated in the adsorbent.
It is also possible to completely desorb other adsorbing components such as SOx compounds to regenerate the adsorbent. The purge gas used for desorption preferably contains O ₂ as little as possible and can be supplied by an inexpensive method. As an example, the gas after the NOx decomposing column can be supplied as the purge gas.

The desorbed NOx-containing gas is then introduced into a NOx decomposition column packed with a NOx decomposition catalyst. Where NOx is nitrogen N ₂
And catalytically decomposed to oxygen O ₂ .

NO → N ₂ + O ₂ …………………… (1) NO when adding a reducing agent such as propane or propylene
The decomposition reaction formula of x is expressed as follows.

CH ₂ +1/2 O _2- > CH ₂ O (2) -i) CH ₂ O + O _2- → CO ₂ + H ₂ O (2) -ii) CH ₂ O + 2NO- → N ₂ + H ₂ O + CO ₂ (2) -iii) In these formulas, CH ₂ represents one example of an organic substance, and CH ₂
O represents an activated oxygen-containing organic compound.

At this time, the NOx-containing gas introduced into the NOx decomposing tower is heated to a temperature suitable for the NOx decomposing reaction, and the desorption gas at room temperature and the NOx decomposing tower outlet gas at a high temperature are alternately heat-exchanged. It is preferable to reduce the heating load.

The concentrated NOx gas after being desorbed from the adsorbent by using the gas having a very low O ₂ concentration that has left the NOx decomposition step as the purge gas for desorbing the adsorbed NOx from the adsorbent according to the present invention is a conventional air purge method. Since the coexisting O ₂ concentration is extremely low compared to, the above catalytic reaction (1),
(2) can be easily progressed and the amount of catalyst used is small (in the case of formula (1)) or the amount of reducing agent used is also small (in the case of formula (2)), so the catalyst load is reduced. It becomes possible.

[0023]

Embodiments of the present invention will be described in detail with reference to FIG.

In FIG. 1, a blower 2 for supplying air 1
After removing the soot and dust in the air by means of the dust collector 3, the exhaust gas 5 from the NOx concentrating tower 19, which will be described later, is added to form the NOx adsorbing tower inlet gas 4 and supplied to the NOx adsorbing tower 17. Most of the NOx is adsorbed and removed here, and is discharged as the clean gas 6.

Since most of the exhaust gas 5 from the adsorption tower 19 which functions as a NOx concentrating tower, which will be described later, is composed of N ₂ gas, it may be released into the atmosphere as it is. Assuming that NOx gas leaks to the atmosphere, the recycled gas 5 is recycled to the inlet of the NOx adsorption tower 17.

In the adsorption tower 18 functioning as a NOx desorption tower, the NOx adsorbed earlier is desorbed to the gas side. For this reason, the inside of the NOx desorption column 18 is degassed by the vacuum pump 9 and placed under a reduced pressure condition. In the NOx gas 7 degassed from the adsorbent to the gas side under reduced pressure, the NOx concentration is the NO of the inlet gas 1.
Although it is concentrated about 100 times as much as the x concentration and the gas flow rate is also reduced by about 1/100, in order to further reduce the amount of gas supplied to the NOx decomposition process described later and increase the NOx concentration, It is also possible to provide the NOx concentration step again (the adsorption tower 19 serving as the NOx enrichment tower in FIG. 1). The embodiment of FIG. 1 shows such an example.

The operation will be described below. Desorbed NOx
The gas 7 is supplied to the NOx concentration tower 19. As a result, NOx
Most of the NOx is adsorbed and removed in the concentrating tower 19, and most of the other nitrogen and oxygen gases pass therethrough.
It is returned to the NOx adsorption tower 17 as the x-concentration process recycled gas 5.

Adsorption tower 2 acting as a concentrated NOx desorption tower
At 0, as in the case of the NOx desorption column 18 described above, the vacuum pump 38 degasses and puts it under a reduced pressure condition.

NOx desorbed from the adsorbent to the gas side under reduced pressure
The gas 10 becomes the gas 12 heated by the heat exchanger 13 from the outlet gas 11 of the NOx concentrating vacuum pump 38, further heated by the heater 14 to a temperature suitable for the NOx decomposition reaction, and then filled with the NOx decomposition catalyst. The NOx decomposing tower 21 is introduced.

The NOx concentration of the gas introduced into the NOx decomposing tower 21 is further concentrated and the amount of gas is greatly reduced, and the outlet gas 10 purged by the outlet gas 15 after the NOx decomposing tower 21 is O ₂ Since it does not contain much NO, it is in a state suitable for directly decomposing NOx by the catalyst packed in the NOx decomposing tower 21, and most of NOx is decomposed into nitrogen and oxygen according to the above formula (1) with a small amount of catalyst. It

On the other hand, when propane gas 39 is added as a NOx reducing agent to the rear side of the heater, the NOx decomposing reaction formula (2) is used to denitrate the NOx decomposing tower 21 with the catalyst. In this case, since the O ₂ concentration is not so high, the side reaction of the reaction formula (2) -ii) is suppressed and the amount of propane as a reducing agent to be added is selectively advanced in the reaction of (2) -iii) to be reduced. Is possible.

The adsorbent used in the present invention also has a property of adsorbing sulfur oxides SOx, carbon monoxide CO, hydrocarbons and aldehydes in polluted gas, and most of these adsorbed gases decompose NOx after desorption. In the tower 21, it is detoxified according to the following formula.

CO ＋1/2 O ₂ ─ → CO ₂ ………… (3) CH ₄ + 2O ₂ ─ → CO ₂ + 2H ₂ O ………… (4) HCHO + O ₂ ─ → H ₂ O + CO ₂ … (5) In addition, in FIG. 1, 22-29, 30-37 show the switching valve of each tower.

Experimental examples for demonstrating the effect of the method according to the present invention will be shown below. (Experimental Example 1) Table 2 below shows the sequence method of the NO adsorption towers 19 and 20 in FIG.

[0035]

[Table 2]

Table 3 shows the gas composition and gas flow rate in line 8 using Y-type zeolite with copper ion exchanged as the adsorbent. In the purging process of the above sequence, the switching valve 36 is opened as the purge gas to use the NOx enrichment column 19 passing gas (almost air is equivalent to the conventional method), or the switching valve 37 is opened to the outlet gas after the NOx decomposition column 21. Table 3 shows the gas composition and the gas flow rate in the line 10 in the case of using.

From Table 3, it can be seen that the O ₂ concentration greatly differs depending on the type of purge gas. A direct decomposition test of NO was performed with this gas composition.

[0038]

[Table 3]

The catalytic crystalline silicate 1.2CuO-copper was ion exchanged _{[0.2Fe 2 O 3 · 0.8Al 2 O} 3 ] · 25
A NOx decomposition test of the above gas compositions A and B was conducted using SiO ₂ (JP-A-3-143547). Table 4 shows the GHSV values necessary to obtain a NOx decomposition rate of 80% under the reaction conditions of a temperature of 450 ° C. Experimental Example 2 Using the gas composition obtained in Experimental Example 1 (Table 3), NO was decomposed using propane as a reducing agent. A catalyst similar to that described above was used as a catalyst, and a NOx decomposition test of gas compositions A and B was conducted. The reaction temperature is 350
Table 5 shows the amount of added C ₃ H ₈ to obtain 80% NOx decomposition rate at ℃
Shown in.

[0040]

[Table 4]

[0041]

[Table 5]

[0042]

As shown in the examples, the NOx concentrated in the adsorbent by the method of the present invention is purged and desorbed by using the gas discharged from the NOx decomposition step to remove O _{2 in} the gas.
Since the ratio of NOx is small, it is a preferable gas composition when directly decomposing NOx or using a catalyst for the denitration reaction using organic substances such as propane, and NOx is decomposed without adding NH ₃ with a small amount of catalyst. It became possible to do.

[Brief description of drawings]

FIG. 1 is a process chart showing an embodiment of a method for treating lean NOx-containing air according to the present invention.

[Explanation of symbols]

 17-20 NOx adsorption tower 21 NOx decomposition tower 22-37 Switching valve

Claims (1)

[Claims] 1. Dilute nitrogen oxide in the atmosphere is adsorbed and removed by an adsorbent at room temperature to release clean air into the atmosphere, while nitrogen adsorbed and desorbed from the adsorbent is concentrated nitrogen. In a method for purifying lean nitrogen oxide-containing air, which comprises a nitrogen oxide adsorption / desorption step for obtaining a desorption gas containing oxides, and a nitrogen oxide decomposition step for denitrifying the desorption gas with a nitrogen oxide decomposition catalyst without adding ammonia. A method for treating diluted nitrogen oxide-containing air, characterized in that the gas discharged from the decomposition step is used as a purge gas for desorbing the adsorbed nitrogen oxides from the adsorbent.