JPH0722682B2 - Method and apparatus for decomposing nitrogen oxides - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method and an apparatus for adsorbing nitrogen oxides with a relatively low sound and then desorbing and decomposing the adsorbed and concentrated nitrogen oxides with a high sound.

[Conventional technology]

Nitrogen oxides such as NO ₂ and NO (hereinafter abbreviated as NOx) present in various exhaust gases are harmful to health and cause acid rain and photochemical smog. Is desired.

By the way, some methods of reducing NOx in exhaust gas have already been put into practical use.

For example, i. Three-way catalyst method for gasoline vehicles, b.
One example is a selective reduction method of NOx with ammonia that targets exhaust gas from large-scale emission sources such as boilers.

C. A method of directly decomposing NOx into harmless nitrogen or nitrous oxide and oxygen by using a noble metal-supported catalyst such as platinum, some metal oxide catalysts, or some kind of perovskite-based catalyst, and d. NOx using metal-supported zeolite catalyst, especially copper-supported zeolite catalyst with high decomposition catalytic activity
A direct decomposition method of is also reported. (Masakazu Iwamoto, J. Chem. S
oc., Chem. Commun., 1272 (1986)].

[Problems to be Solved by the Invention]

Above a. In principle, this cannot be applied to a system in which oxygen coexists, such as a diesel engine exhaust gas. Since ammonia uses huge equipment, it is technically extremely difficult to apply it when the exhaust gas generation source moves.

On the other hand, c. Has not been put to practical use because it has a low catalyst activity for use in purification of exhaust gas.

Also d. However, when the oxygen concentration in the gas is high and the NOx concentration is low, such as exhaust gas from a diesel engine, high catalytic activity cannot be expected, and it is far from practical use.

In order to overcome the above drawbacks of the prior art, the present invention
A method and apparatus for removing NOx at a high rate by desorbing and decomposing NOx by heating this processing layer after adsorbing and concentrating NOx in a processing layer consisting of an NOx adsorbent and a NOx decomposition catalyst. It is intended to be provided.

[Means for Solving the Problems]

The method for decomposing and removing NOx of the present invention to achieve the above object is NOx.
The contained gas is brought into contact with a NOx adsorbent (hereinafter abbreviated as an adsorbent) and a NOx decomposition catalyst (hereinafter abbreviated as a catalyst) to adsorb the NOx, and then the adsorbent and the catalyst adsorbing the NOx are removed. It is characterized in that the NOx is desorbed and decomposed into nitrous oxide, nitrogen and oxygen by heating.

And the apparatus of the present invention used for decomposing and removing such NOx is formed by connecting a pair of processing tanks filled with an adsorbent and a catalyst via a flow path switching valve, and the pair of processing layers are alternately desorbed by the NOx. It is possible to maintain the decomposition temperature and the NOx adsorption temperature, the treatment tank of the NOx adsorption temperature is always located downstream of the treatment tank of the NOx desorption decomposition temperature by switching the flow passage of the flow passage switching valve, and the NOx A flow path is formed by connecting a treatment tank having an adsorption temperature and a treatment tank having the NOx desorption decomposition temperature in series via the flow passage switching valve.

First, the method of the present invention will be described.

The NOx-containing gas to be treated by the method of the present invention,
Examples thereof include diesel automobile exhaust gas, cogeneration diesel engine exhaust gas, and gas engine exhaust gas. The NOx-containing gas is brought into contact with an adsorbent and a catalyst to adsorb NOx.

The adsorbent may be any kind of adsorbent capable of adsorbing NOx at a low temperature and desorbing at a high temperature as described later, and examples thereof include various metal-supported zeolites, activated carbon, and alkali salts.

Further, as the catalyst, copper-supported ZSM-5 type zeolite, alumina-supported platinum, cobalt oxide, silver-cobalt oxide, etc. can be used. Among these, the copper-supported ZSM-5 type zeolite has a high NOx decomposing activity, It is preferable in that the temperature is low.

ZSM-5 type zeolite is known to cause a unique catalytic reaction because it is the only zeolite currently known to have a 10-membered oxygen ring at the entrance of its pores. The method is, for example, JP-A-46-10064.
It has been proposed in the publication.

The mixing ratio when the adsorbent and the catalyst are mixed and used is not particularly limited. Adsorption amount of adsorbent and NOx of catalyst
This is because the optimum value can be set appropriately depending on the decomposition rate.

In the present invention, the adsorbent and the catalyst may be the same substance, and examples thereof include copper-supported ZSM-5 type zeolite, cobalt oxide, and silver-cobalt oxide.
M-5 type zeolite is particularly effective.

The optimum temperature of NOx adsorption depends on the adsorbent used, the type of catalyst, or the mixture ratio of the two, but in order to reduce the energy used, it is preferable that the temperature is about 100 ° C lower than room temperature to NOx decomposition temperature. Specifically, it is in the range of room temperature to 300 ° C.

The adsorbent and catalyst thus adsorbing NOx are then heated in the present invention to remove NOx from nitrogen, nitrous oxide,
Desorb and decompose into oxygen.

In addition, since NOx desorption decomposition does not proceed 100% completely,
It is preferable to carry out in the presence of a carrier gas for discharging undecomposed NOx together with decomposition products.Air or a NOx-containing gas itself is used as the carrier gas, and the amount of the carrier gas passing through the adsorbent and the catalyst is NOx. N at adsorption
By limiting the amount of Ox-containing gas, the NOx decomposition rate can be significantly improved.

Further, if the NOx-containing gas itself is used as the carrier gas, there is an advantage that the NOx-containing gas can be repeatedly decomposed and removed.

The temperature at which NOx is desorbed and decomposed also varies depending on the adsorbent, the type of catalyst, and the mixing ratio of the two.
When using type 5 zeolite and silver-cobalt oxide
The range of 300 to 600 ° C is particularly effective, and 600 to 800 ° C is preferable in the case of cobalt oxide and platinum supported on alumina.

The contact time between the adsorbent and the catalyst and the NOx-containing gas in the NOx adsorption process is not particularly limited, and the contact time with the carrier gas in the desorption decomposition / desorption process is also not particularly limited.

This is because the contact time can be set according to the concentration of NOx contained in the NOx-containing gas so that the decomposition activity and decomposition performance of the catalyst can be maximized.

However, as compared with the contact time at the time of adsorption, the contact time at the time of desorption decomposition is 100 times or more longer, resulting in extremely high N
Ox decomposition rate can be obtained.

Next, the device of the present invention will be described with reference to the drawings.

1 and 2 show a first embodiment of the device 1 of the present invention, in which a pair of processing tanks 2 and 3 filled with an adsorbent and a catalyst are connected via a flow path switching valve 6. There is.

That is, the inlet sides 2A and 3A of the processing tanks 2 and 3 are connected to the switching valve 6 by the pipelines 7A and 8A, respectively, and the outlet sides 2B and 3B of the processing tanks 2 and 3 are coupled to the switching valve 6 by the pipelines 7B and 8B. The processing tanks 2 and 3 are connected to each other, and the processing tanks 2 and 3 are heating means and cooling means (neither shown).
It is possible to keep high temperature (NOx desorption decomposition temperature) and low temperature (NOx adsorption temperature) alternately.

The switching valve 6 has a NOx-containing gas introduction pipe 4 and an NOx decomposition gas discharge pipe 5, and for example, a six-port valve as shown in the figure is used.

In the device of the present invention, preferably the conduits 7A, 7B
Are connected by a bypass line 9A with a valve 10A, while
The pipelines 8A, 8B are connected by a bypass pipeline 9B having a valve 10B.

These bypass lines 9A and 9B are connected to valves 10A and 1B as described later.
It has the function of reducing the flow rate of the NOx-containing gas when desorbing and decomposing NOx by opening and closing 0B.

FIGS. 3 and 4 show a second embodiment of the device 1 of the present invention, and the same parts as those in FIGS. 1 and 2 have the same part numbers.

In the second embodiment, the bypass line 9 in the first embodiment is used.
It differs from the first embodiment in that, instead of A and 9B, the conduits 7A and 8A are connected and a flow path switching valve 12, for example, a bypass pipeline 11 having a four-port valve is provided. Is provided with a NOx-containing gas introduction pipe 13 and a carrier gas introduction pipe 14,
The decomposition gas discharge pipe 5 is provided in the flow path switching valve 6.

The NOx-containing gas, the adsorbent, the catalyst, the NOx adsorption temperature and the desorption decomposition temperature, the carrier gas, the NOx-containing gas and the contact time between the carrier gas and the adsorbent and the catalyst in the apparatus of the present invention are the same as those of the present invention. As described regarding the method.

Also, different adsorbents and catalysts may be filled in the pair of treatment tanks, respectively, and the temperatures suitable for the adsorbents and catalysts may be controlled during NOx adsorption and desorption decomposition.

Regarding the theoretical background of the device of the present invention, the NOx treatment amount in the steady state (this is the NOx adsorbed in the treatment tank).
Defined as (amount x decomposition rate) and NOx amount X in the NOx-containing gas is X = NOx amount adsorbed in the treatment tank x decomposition rate, and the amount of NOx adsorbed in the treatment tank is large. If the decomposition rate is low and the amount of undecomposed NOx is high, it means that the adsorption capacity of the treatment tank should simply be increased.

Further, in order for the apparatus of the present invention to operate normally, it is clear that the treatment tank should have an adsorption capacity obtained by multiplying X / decomposition rate by a safety factor.

[Action]

The operation of the device of the present invention will be described with reference to FIGS. 1 and 2.

First, in FIG. 1, the treatment tank 2 is referred to as a high temperature treatment tank, that is,
It functions as a NOx desorption decomposition tank, and one of the processing tanks 3 functions as a low temperature processing tank, that is, a NOx adsorption tank.

The NOx-containing gas is introduced from the NOx-containing gas introduction pipe 4, and the pipeline 7A
Through the treatment tank 2 maintained at the NOx desorption decomposition temperature.

In the treatment tank 2, for example, NOx is adsorbed by the filled adsorbent, and this NOx is desorbed and decomposed at a predetermined temperature.

The decomposed gas and undecomposed NOx are discharged from the treatment tank 2 together with the NOx-containing gas, and by closing the valve 10B of the bypass pipe 9B, the pipe 7B, the flow path switching valve 6, and the pipe 8A are connected in series with the treatment tank 2. And the whole amount is sent to the low temperature treatment tank 3 located downstream of the treatment tank 2.

It should be noted that a part of the NOx-containing gas flowing in the bypass pipe 9A is combined with the exhaust gas from the processing tank 2 in the pipe 7B.

The treatment tank 3 is maintained at a predetermined NOx adsorption temperature, and undecomposed NOx and NOx in the NOx-containing gas are filled, for example, adsorbed by an adsorbent, and only the decomposed gas formed in the treatment tank 2 is discharged. It is discharged from the pipe 5.

Next, after cooling the treatment tank 2 to a predetermined NOx adsorption temperature, the flow path switching valve 6 is switched as shown in FIG.
The bypass valve 10A is closed and 10B is opened to maintain the temperature of the treatment tank 3 at a predetermined NOx desorption decomposition temperature.

Then, the adsorbed NOx is desorbed and decomposed in the treatment tank 3, and is connected in series with the treatment tank 3 as in the case of FIG.
In the treatment tank 2 located downstream of the treatment tank 3, undecomposed NOx
And adsorption of NOx in the NOx-containing gas occurs.

Although a part of the NOx-containing gas flows through the bypass pipe 9B, it is combined with the gas from the processing tank 3 in the pipe 8B.

By repeating such operations of FIGS. 1 and 2 at appropriate intervals, NOx in the NOx-containing gas can be continuously removed.

Next, the function of the apparatus shown in FIGS. 3 and 4 will be described.

First, in the apparatus shown in FIG. 3, the NOx-containing gas is supplied from the introduction pipe 13, is sent to the treatment tank 2 (adsorption tank) via the flow path switching valve 12 and the bypass pipe line 11, and is adsorbed with NOx. The gas is discharged from the discharge pipe 5 via the pipe 7B and the flow path switching valve 6.

On the other hand, the carrier gas is introduced from the introduction pipe 14 to the flow path switching valve.
12, the carrier gas accompanied by NOx decomposed gas and undecomposed NOx is sent to the treatment tank 3 (desorption decomposition tank) via the bypass pipe 11, and the treatment tank 2 via the pipe 8B, the flow path switching valve 6 and the pipe 7A. The undecomposed NOx and NOx in the carrier gas are sent to the (adsorption tank) and adsorbed to the treatment tank 2 (adsorption tank).

Next, after cooling the treatment tank 3 (desorption decomposition tank) to a low temperature for NOx adsorption, the flow path switching valves 6 and 12 are turned off as shown in FIG. 4 to raise the treatment tank 2 to the NOx decomposition temperature. For example, the NOx decomposed gas and the undecomposed NOx in the treatment tank 2 can be sent together with the carrier gas to the treatment tank 3 to be adsorbed. By repeating the operation of FIG. 3 and FIG. NOx can be continuously removed in the same manner as the apparatus shown in FIGS.

Examples of the method of the present invention will be described below.

〔Example〕

Example 1 (Preparation of copper supported zeolite) _{beaker, Al (NO 3) 3 ·} 9H 2 Put water O3.13g and 100 ml, with stirring dissolved with a magnetic stirrer, tetrapropylammonium bromide to the solution 7.98 g silica sol aqueous solution 60 g (SiO ₂ 31 wt%, Na ₂ O0.4% by weight, an aqueous solution containing Al ₂ O ₃ 0.03 wt%) was added.

A solution of 3.12 g of sodium hydroxide dissolved in 40 ml of water was gradually added to the obtained solution while stirring. The mixture was charged into an autoclave and crystallized at 160 ° C. for 72 hours with stirring.

After solid-liquid separation of the product, it is washed with water and dried to form the base Si / Al ₂
A ZSM-5 zeolite with a molar ratio of = 70 was obtained.

Next, a commercially available solution of 0.05 mol of copper acetate was prepared, and the above zeolite was added thereto, and the mixture was stirred for 24 hours and then centrifuged.

Repeat this operation 3 times, and finally wash with pure water 5 times.
Copper-supported ZSM-5 zeolite was prepared by drying overnight at 0 ° C.

(NOx removal reaction) Take 1 g of the copper-supported zeolite prepared as described above,
The reaction mixture was filled in an atmospheric pressure reactor, and helium gas containing 1000 ppm of nitric oxide (hereinafter referred to as NO) and 10% oxygen was flowed at room temperature for 10 minutes at a flow rate of 60 ml to adsorb NO.

Then, the zeolite layer was heated to 600 ° C. in 30 minutes while maintaining the same flow rate, and the decomposition rate of total NO in the outflowing gas was examined by a NOx analyzer, and it was 30%.

When the flow rate at the time of decomposition was set to almost 0 when the temperature was raised, and the decomposition rate when flowing out at 3 ml thereafter was also examined, it was improved to 60%.

The gas analysis was performed using a NOx analyzer and a gas chromatograph.

Comparative Example 1 1 g of the copper-supported ZSM-5 zeolite prepared in Example 1 was placed in an atmospheric flow reactor and 1000 ppm NO and 10% was added.
The helium gas containing oxygen was flowed at 600 ° C. at a flow rate of 60 ml / min, and the decomposition rate of NO was examined. As a result, it was 6%.

As compared with Comparative Example 1, the superiority of the method of Example 1 in which adsorption concentration was performed at room temperature is clear.

Example 2 (Preparation of cobalt oxide) cobalt nitrate _{[Co (NO 3) 2 · 6H} 2 O ] and 500ml of 72.52g was dissolved in pure water, sodium carbonate while stirring with a magnetic stirrer (Na ₂ CO ₃₎ 1 solution of 250 ml of 31.69 g
Dropwise over time to form a precipitate.

After stirring for another 1 hour, the precipitate was separated by a centrifuge.
The solid obtained is washed with pure water 5 times and then dried in a drier 110
Cobalt oxide was prepared by drying overnight at ℃, pulverizing, and calcination in an air stream to 400 ℃.

The firing conditions are 100 ℃ to 300 ℃, 1 ℃ per minute, 300 ℃ to 400 ℃.
The temperature was raised to 4.3 ° C./min and kept at 400 ° C. for 4 hours.

(NOx removal reaction) 1 g of the cobalt oxide prepared in this way was charged into a normal pressure flow reactor, and helium gas containing 1000 ppm NO and 10% oxygen at 200 ° C was flown at a flow rate of 60 ml / min. Run for 40 minutes
NOx was adsorbed. Next, the flow rate was kept as it was, the temperature of the cobalt oxide layer was raised to 600 ° C., and the decomposition rate of NO in the outflowing gas was examined. As a result, it was 19%.

Comparative Example 2 1 g of the cobalt oxide prepared in Example 2 was filled in an atmospheric pressure flow reactor, and helium gas containing 1000 ppm NO and 10% oxygen was flowed at 600 ° C. at a flow rate of 60 ml / min to obtain NO. The decomposition rate was 4%. The superiority of the method of Example 2 over this Comparative Example 2 is clear.

Example 3 The copper-supported ZSM-5 zeolite prepared in Example 1 was
g of the helium gas containing 500 ppm NO and 15% oxygen was charged for 30 minutes at a flow rate of 60 ml / min for 30 minutes, and then each of the treatment tanks 2 and 3 of the apparatus shown in FIG. The flow path was switched as described above and the same operation as in Example 1 was repeated, but almost no NOx flowing out from the outlet 5 was observed.

Example 4 The copper-supported ZSM-5 zeolite prepared in Example 1 was
g of the helium gas containing 500 ppm NO and 15% oxygen was filled in the treatment tanks 2 and 3 of the apparatus shown in FIG. 3 for 1 hour at a flow rate of 60 ml / min. Adsorbed.

Then, after switching the six-port valve 6 and the four-port valve 12, the air carrier gas flowing through 14 was temporarily stopped, and the air flow rate through the processing tank 2 was made almost zero. Next, the temperature was raised to 600 ° C., an air carrier gas was caused to flow, and the decomposition rate of NO in the gas flowing out of the processing tank 2 was examined.

A NO decomposition rate of about 60% was confirmed.

On the other hand, when the concentration of NOx finally flowing out from the device was also examined, almost no NOx was observed.

Similar results were obtained by repeating the adsorption and desorption decomposition.

Comparative Example 3 The same reaction as in Examples 3 and 4 was carried out at a gas flow rate of 60 ml / min using a simple atmospheric flow type reaction apparatus different from the apparatus of the present invention.
When carried out at 0 ° C., about 6% decomposition of NO was observed.
This result shows that the device of the present invention is extremely effective as a NOx removal device as compared with the results of Examples 3 and 4.

〔The invention's effect〕

As described above, according to the method of the present invention, NOx adsorbed and concentrated at a low temperature can be decomposed at a high temperature with a high efficiency of 20 to 30%, and NOx in a NOx-containing gas can be effectively removed. it can.

In the past, such a study was not made because it was not expected that NOx decomposition would occur with high efficiency in the presence of oxygen. Therefore, it was totally unexpected at first that a high NOx removal rate could be achieved by a simple method such as the method of the present invention.

Further, when a carrier gas is present at the time of desorption decomposition and the flow rate thereof is extremely reduced, 60% or more of the adsorbed NOx can be decomposed.

In general, when NOx is decomposed, oxygen is generated together with nitrogen, so that it is easily assumed that oxygen will inhibit the adsorbent and the catalyst, and it is unlikely that the NOx exhibits high decomposition activity.

Therefore, the achievement of a high removal rate of 60% or more is a very surprising result.

Further, according to the device of the present invention, a high NOx removal rate can be achieved with a simple device that is composed of only a pair of treatment layers filled with an adsorbent or a catalyst, a flow path switching valve, and a conduit connecting these. can do.

Furthermore, according to the apparatus of the present invention, by providing a bypass line having a bypass valve to reduce the gas flow rate flowing through the treatment tank, it is possible to surprisingly decompose and remove more than 60% of the adsorbed NOx. That is, by controlling the gas flow rate at the time of decomposition to be low, the throughput of the NOx-containing gas can be increased.

Further, in the device of the present invention, a pair of treatment tanks are alternately used as a low temperature adsorption tank and a high temperature desorption decomposition tank, and by operating the flow path switching valve, NOx-containing gas while almost completely preventing the outflow of undecomposed NOx. Can continuously decompose and remove NOx.

[Brief description of drawings]

1 and 2 are schematic diagrams showing an embodiment of the device of the present invention, and FIGS. 3 and 4 are schematic diagrams showing another embodiment of the device of the present invention. 2, 3 ... Processing tank, 6, 12 ... Flow path switching valve.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical display location B01D 53/56 53/81 53/86 ZAB B01J 20/10 ZAB C 7202-4G 23/74 ZAB 8017 -4G 23/75 23/89 ZAB A 8017-4G 29/76 ZAB A 9343-4G B01D 53/36 ZAB 53/34 129 A ZAB 8017-4G B01J 23/74 311 A

Claims (3)

[Claims] 1. A nitrogen oxide-containing gas is brought into contact with a nitrogen oxide adsorbent and a nitrogen oxide decomposition catalyst to adsorb the nitrogen oxide, and then the nitrogen oxide adsorbent adsorbing the nitrogen oxide, and A method for decomposing and removing nitrogen oxide, which comprises desorbing and decomposing the nitrogen oxide by heating a nitrogen oxide decomposing catalyst. 2. The method for decomposing and removing nitrogen oxides according to claim 1, wherein copper-supported zeolite is used as the nitrogen oxide adsorbent and the nitrogen oxide decomposition catalyst. 3. A pair of treatment tanks filled with a nitrogen oxide adsorbent and a nitrogen oxide decomposition catalyst are connected via a flow path switching valve, and the pair of treatment tanks are alternately desorbed and decomposed by the nitrogen oxide. The temperature and the nitrogen oxide adsorption temperature can be maintained, and the treatment tank at the nitrogen oxide adsorption temperature is always located on the downstream side of the treatment tank at the nitrogen oxide desorption decomposition temperature by the passage switching of the passage switching valve. And a flow path is formed by connecting the treatment tank at the nitrogen oxide adsorption temperature and the treatment tank at the nitrogen oxide desorption decomposition temperature in series via the flow passage switching valve. Decomposition and removal device.