JPH0679135A - Method for treating air containing small amount of nitrogen oxide - Google Patents

Abstract

PURPOSE:To make it possible to purify a large amount of air containing a small amount of NOx efficiently by burning combustible gas contained in a gas which contains desorbed nitrogen oxide of high concentration using an oxidation catalyst prior to NOx decomposition, and using a combustion heat as a heat source for NOx decomposition catalyst. CONSTITUTION:Air 1 containing a small amount of NOx is guided to an adsorption tower 17 where the most of NOx is removed by adsorption and a purified gas 6 is discharged to an atmosphere. During this process, an adsorption tower 18 which has adsorbed NOx previously acts as an NOx desorption tower to allow the adsorbed NOx to be desorbed to the gas side. The desorbed NOx gas 7 is again adsorbed and desorbed using adsorption towers 19, 20 for concentration. The desorbed NOx gas 11 is heated by a heat exchanger 13, and further, is supplied to a heater 14 using an oxidation catalyst so that a contained combustible gas is burned. In addition, the NOx gas is heated to a temperature suited for NOx decomposition reaction, and is guided to an NOx decomposition tower 21.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a dilute nitrogen oxide (hereinafter
The present invention relates to a method for purifying contained air (referred to as NO _X ), for example, a method for purifying polluted air that efficiently purifies air polluted with nitrogen oxides generated from exhaust gas of automobiles.

[0002]

BACKGROUND OF THE INVENTION such as Tokyo metropolitan of the tunnel road, underpass, road with a cover for anti-noise, in the underground parking lot observed NO _X of about 5ppm, also in expected underground tunnel automobile road of increase in the future It is considered that similar pollution will proceed, but conventionally, a method for purifying a large volume of air having such a low pollution level has not been put into practical use. The method for purifying contaminated air is roughly classified into a dry method and a wet method. However, the wet method requires a wastewater treatment device and is difficult to put into practical use, and the development of the dry method has become the mainstream.

As a dry method, polluted air is treated with activated carbon or moisture.
Cura sieve, silica gel, alumina, metal oxide, etc.
NO through the adsorbent_XAdsorption method to adsorb, polluted air
To NH ₃Is injected and heated, and NO is_XTo N₂Return to
Selective catalytic reduction method or NH in polluted air₃Add
Then, irradiate the electron beam and NO_XAnd S0_XWith nitric acid or sulfuric acid mist
And NH₃Ammonium nitrate, ammonium sulfate in reaction with
Electrolyte particles as fine particles and double salts of both
There is an electron beam irradiation method in which this is collected using a container.

Or, ideally, NO which decomposes NO _X 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 treatment technology" (consigned by Japan Highway Public Corporation), March 1983, pp. 37-38]. However, first of all, regarding the adsorption method, when using general silica gel, alumina, and molecular sieve, NO
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 injection NO _X selective catalytic reduction method or electron beam irradiation method, it is necessary for the reaction with NO _X contained in a small amount of about 5 ppm in a large volume of contaminated air, which is the object of the present invention. It is difficult to mix an equivalent amount of NH ₃ evenly, and there is a risk of secondary pollution such as outflow of unreacted NH ₃ . [Industrial Pollution Control Association, "Survey and research on automobile exhaust gas treatment technology" (consigned by Japan Highway Public Corporation) 1984 3
Mon, p. 49].

Further, the NO _X catalytic decomposition method without addition of a reducing agent has a high reaction temperature of 400 to 600 ° C., and when the NO _X concentration becomes low, the NO _X decomposition rate sharply decreases.
[(Decomposition by catalytic reaction of nitrogen oxides) Masakazu Iwamoto, PETR
OTECH. Vol. 12, No. 11 (1989), p. 34] Low NO _X
It was not practically applicable to the concentration and large volume of 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 NO _X adsorption characteristic.
A method for purifying large-capacity NO _X contaminated air was proposed by combining the NO _X catalytic cracking method (JP-A-3-186318).

The above proposed method is generated from automobile exhaust gas
NO_XNO from the air polluted by the adsorbent at room temperature
_XIs removed by adsorption and discharged as clean air out of the system.
Wear NO _XNO to obtain concentrated desorption gas by desorbing NO_XAdsorption and desorption
NO in process and desorption gas_XDecomposition catalyst makes contact at high temperature
It decomposes to make it harmless, and the decomposition gas heats up with the desorption gas.
After exchange and heat recovery of the decomposition gas, NO_XAdsorption / desorption process
NO to return to_XContaminated sky characterized by consisting of a decomposition process
It is a method of purifying Qi.

In the above-mentioned proposed method, as the NO _X 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 NO _X decomposition catalyst, a dehydrated oxide is used. Expressed as a molar ratio of (1.0 ± 0.4) R ₂ O ・ [aM ₂ O ₃・ bAL ₂ O ₃ ] ・ ySi
O ₂ {wherein R is an alkali metal ion and / or a hydrogen ion, M is an element ion of at least one member selected from 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 a chemical composition of 2} as a carrier is used.

According to the research conducted by the present inventors, it has been found that the NO _X adsorbent used in the above-mentioned method is also effective if ZSM-5 type zeolite is subjected to copper ion exchange treatment. It has been confirmed that the above-mentioned crystalline silicate, which is a precursor of the NO _X decomposition catalyst, preferably has an X-ray diffraction pattern shown in Table 1 below. Further, it has been confirmed that the present NO _X decomposition catalyst efficiently promotes NO _X decomposition by containing or adding an organic substance such as propane or propylene as a reducing agent.

[0011]

[Table 1]

[0012]

However, when copper is added to the
With the crystalline silicate catalyst exchanged_XMinutes
In order to solve it, it is necessary to maintain a temperature of 300 ° C or higher.
NO separated and concentrated from the atmosphere at room temperature_XHeating gas
It is necessary to increase the energy required for heating.
There are dots. The present invention is a lean NO_XAtmosphere containing
NO concentrated from the adsorption and desorption process_XGas containing
Get this gas NO_XNH by decomposition catalyst₃Without adding
Energy consumption in the above-mentioned proposed method
Diluted NO that solved the problem of high cost _XPurification of contained air
The challenge is to provide a method.

[0013]

Means for Solving the Problems The present invention adsorbs and removes lean NO _X emitted from automobile exhaust gas, etc. in the atmosphere at room temperature using an adsorbent to release clean air into the atmosphere, while In order to solve the above-mentioned problems in the method for purifying dilute nitrogen oxide-containing air in which concentrated NO _X is desorbed from the adsorbent and the desorbed high-concentration NO _X- containing gas is decomposed into a harmless gas by a NO _X decomposition catalyst, a NO _X decomposition step is performed. In this method, a combustible gas contained in the desorbed high-concentration nitrogen oxide-containing gas is burned by an oxidation catalyst, and the combustion heat is used as a heat source for the NO _X decomposition catalyst.

Examples of the NO _X 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, and the NO _X decomposition catalyst is in a dehydrated form. Expressed as a molar ratio of oxides, (1.0 ± 0.4) R ₂ O ・ [aM ₂ O ₃・ bAl ₂ O ₃ ] ・
ySiO ₂ {in the formula, R: alkali metal ion and / or hydrogen ion, M: group VIII element, rare earth element, titanium, vanadium, chromium, niobium, one or more element ions in a group consisting of 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 (see JP-A-3-143547).

[0015]

In the present invention, the polluted air containing lean NO _X is introduced into the packed tower filled with the adsorbent, the NO _X is adsorbed by the adsorbent, and the purified air is discharged from the packed tower. To be done. The adsorbent has a large NO _X adsorption capacity, and is selected and used. However, as NO _X adsorption progresses, it becomes more difficult to adsorb, so the adsorbed NO _X is desorbed to regenerate the adsorbent.

[0016] Also, according to repeated desorption and long-term adsorption, when the performance of the NO _X adsorption and desorption as come reduced is accumulated in the adsorbent temporarily raising the desorption temperature
The adsorbent can also be regenerated by completely desorbing other adsorbing components such as S0 _X compounds. 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 NO _X decomposition column can be supplied as the purge gas.

The desorbed NO _X containing gas is then led to the NO _X decomposition catalyst NO _X decomposing column packed with. Where NO _X is nitrogen N ₂
And catalytically decomposed to oxygen O ₂ .

NO → N ₂ + O ₂ …………………………………………………… (1) When adding a reducing agent such as propane or propylene
The NO _X decomposition reaction formula 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) -ii) where CH ₂ Shows an example of an organic substance, and CH ₂ O means an activated oxygen-containing organic compound.

At this time, the NO _X containing gas to be introduced into the NO _X decomposition tower is heated to a temperature suitable for the NO _X decomposition reaction, but the desorption gas at normal temperature and the NO _X decomposition tower outlet gas at high temperature are alternated. Further, heat is exchanged with the fuel cell, and an oxidizing catalyst is disposed to burn the combustible gas, and the heat of combustion is used to reduce the heating load. The concentrated NO _X gas obtained by using the gas after the NO _X decomposition tower as the concentrated NO _X desorption gas has a very low O ₂ concentration that coexists as compared with the conventional air purging method, so that the above catalytic reaction (1), (2) Can be easily promoted 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 can be reduced. It has become possible.

In the polluted air, CO, formaldehyde, ethylene and the like are contained in addition to NO _X , and particularly CO is contained in tunnel exhaust gas in an amount more than twice that of NO _X. When the contaminated air is introduced to the NO _X separation concentration column, combustible gases such as CO as well as enriched gas and NO _X flows out to the downstream of the NO _X decomposing column.

As the oxidation catalyst to be installed in front of the NO _X decomposition catalyst device, it is preferable to support a noble metal such as Pt, Pd, Ru and Rh on an Al ₂ O ₃ carrier. In particular, recently on Au ₂ O ₃ support
The catalyst supporting C can burn CO even at a low temperature near room temperature, and is preferable as an oxidation catalyst used in this process.

[0023]

Embodiments of the present invention will be described in detail with reference to FIG. In FIG. 1, air 1 is guided to a dust remover 3 by a blower 2 to remove soot particles in the air, and then a NO _x enrichment tower 1 to be described later.
The exhaust gas 5 from 9 is added to form the NO _x adsorption tower inlet gas 4 and supplied to the NO _x adsorption tower 17. Most no here
_X is adsorbed and removed, and is discharged as a clean gas 6.

Incidentally, NO described later_XWorking as a concentration tower
Most of the exhaust gas 5 from the adsorption tower 19 is N₂From gas
Therefore, it may be released into the atmosphere as it is.
But, by any chance, rich NO_XAssuming that gas leaks to the atmosphere
NO as recycled gas 5 _XLisa at the entrance of adsorption tower 17
I try to cycle. NO_XAs a desorption tower
In the working adsorption tower 18, the previously adsorbed NO_XThe gas side
Attach and detach. NO for this_XVacuum inside the desorption tower 18
It is degassed by the pump 9 and placed under a reduced pressure condition.

NO _X desorbed from the adsorbent to the gas side under reduced pressure
In the gas 7, although the NO _X concentration is reduced to the gas flow rate is also about one hundredth are concentrated about one hundred times as compared to the NO _X concentration inlet gas 1, to be described later of the NO _X decomposition step In order to further reduce the supply gas amount and increase the NO _X concentration,
It is also possible to provide the NO _X concentration step (the adsorption tower 19 serving as the NO _X concentration tower in FIG. 1) again. The embodiment of FIG. 1 shows such an example.

The operation will be described below. Desorbed NO _X
The gas 7 is supplied to the NO _X concentration tower 19.

[0027] As a result, most of the NO in the NO _X concentration column 19
_X is removed adsorbed, other nitrogen or oxygen gas almost pass, NO _X concentration step the recycle gas 5 as described above
Is returned to the NO _X adsorption tower 17.

Adsorption tower 2 acting as a concentrated NO _x desorption tower
At 0, as in the case of the NO _X desorption tower 18, the vacuum pump 38 degassed and placed under reduced pressure conditions. The NO _X gas 10 desorbed from the adsorbent to the gas side under reduced pressure becomes the gas 12 heated by the heat exchanger 13 from the outlet gas 11 of the NO _X concentrating vacuum pump 38, and further the heater 14 filled with the oxidation catalyst. After heating to a temperature suitable for NO _X decomposition reaction at
It is introduced into the NO _X decomposition tower 21 filled with the _X decomposition catalyst.

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

On the other hand, the case of adding propane gas 39 as a reducing agent of the NO _X on the rear side of the heater 14 using an oxidation catalyst, which is packed in the NO _X decomposing column 21 catalyst, NO _X decomposition reaction formula ( Denitrate in 2). In this case, since the O ₂ concentration is not so high, the side reaction of the reaction formula (2) -ii) is suppressed (2) -iii).
It is possible to reduce the amount of propane which is a reducing agent to be added, by selectively advancing the reaction. In addition, in FIG.
Reference numerals 29 and 30 to 37 denote switching valves of each tower. Hereinafter, experimental examples for demonstrating the effect of the method according to the present invention will be shown. (Experimental Example 1) Table 2 below shows the sequence method of the NO adsorption towers 19 and 20 in FIG.

[0031]

[Table 2]

Table 3 shows the gas composition, gas flow rate, and gas temperature in the lines 1, 8, 10, and 16 using Y-type zeolite in which copper is ion-exchanged as the adsorbent. In the purging process of the above sequence, the line 1 when the switching valve 37 is opened and the outlet gas after the NO _X decomposition column 21 is used.
Table 3 shows the gas composition and the gas flow rate at 0. A direct decomposition test of NO was performed with this gas composition.

[0033]

[Table 3]

The catalyst is a crystalline silicate in which copper is ion-exchanged, 1.2Cuo. [0.2Fe ₂ O ₃ .0.8Al ₂ O ₃ ] .25.
A decomposition test of NO _X was carried out using SiO ₂ (JP-A-3-13547). Line 16 at GHSV 50 h ^-1 , 450 ° C
As a result of introducing this gas into the NO _X decomposition tower 21, a NO _X decomposition rate of 80% was obtained. Was carried out decomposition reaction of the NO _X using as a reducing agent of propane is used (Experiment Example 2) the gas composition obtained in Experimental Example 1 (Table 3). The same catalyst as in Experimental Example 1 was used, and the reaction temperature was 3
70 ℃, GHSV 5000 h ^-1 , propane 39 2000 pp
As a result of adding m 2 and introducing the gas into the NO _X decomposition tower, the NO _X decomposition rate was 81%.

When propane is added, the heat of the exit gas 15 rises due to the combustion heat of propane. Therefore, the heat exchange rate 13 in the case of Experimental Example 1 changes the heat exchange efficiency,
The temperature of each gas line was set as follows. Line 10
(130 ℃), Line 12 (240 ℃), Line 16
(370 ° C), line 15 (510 ° C), line 38
(380 ° C.).

[0036]

As shown in the examples, according to the method of the present invention, a heater using an oxidation catalyst is arranged before the NO _X decomposition step to burn combustible gas, and the combustion heat is converted into NO _X decomposition catalyst. It is possible to reduce the energy required for heating by using it as a heat source, and it has become possible to put into practice a preferable method for purifying air containing dilute nitrogen oxides.

[Brief description of drawings]

FIG. 1 is a process diagram showing an embodiment of a method for treating lean NO _X- containing air according to the present invention.

[Explanation of symbols]

17-20 NO _X adsorption tower 21 NO _X 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 oxide desorbed from the adsorbent is concentrated in nitrogen oxide. Nitrogen oxide adsorption / desorption step for obtaining desorption gas containing substances,
And a method for purifying dilute nitrogen oxide-containing air, which comprises denitrifying the desorbed gas with a nitrogen oxide decomposing catalyst without adding ammonia, in the desorbed gas before entering the nitrogen oxide decomposing step. Is burned, and the heat of combustion is used as a heat source in the nitrogen oxide decomposition step.