JPH0671145A - Method for removing nitrogen oxide in exhaust gas - Google Patents

Abstract

PURPOSE:To provide a method for removing nitrogen oxide wherein NOx exhausted from a diesel engine is efficiently removed, and decrease in denitrification rate accompanied with deterioration or the catalyst does not occurs, and a measure to a noxious gas is unnecessary and which is favorable from the viewpoint of the cost. CONSTITUTION:NOx in an exhaust gas is removed by introducing the exhaust gas generated from a diesel engine and contg. excess oxygen into a denitrification apparatus main body 2 wherein a catalyst prepd. by ion- exchanging an Na-containing Y-type zeolite with cobalt is provided in the presence of a hydrocarbon of heavy oil A sprayed from a nozzle to bring it into contact with the catalyst. The content of Co ion in the Na-contg. Y-type zeolite catalyst is about 0.1% at its optimum.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an method of removing nitrogen oxides in the exhaust gas (hereinafter abbreviated as NO _x), NO particularly from the NO _x containing gas in the exhaust gas excess oxygen, such as diesel generators are present Regarding how to remove _x .

[0002]

2. Description of the Related Art Generally, NO _x contained in exhaust gas emitted from diesel generators is harmful to humans,
Since it easily causes acid rain, development of an effective removal method is desired. Conventionally, as a NO _x treatment technology, a method of reducing the NO _x by using a catalyst has been put into practical use. This is roughly divided into (1) a method of using a three-way catalyst attached to a gasoline-fueled automobile (2) a selective catalytic reduction method using ammonia as a reducing agent (3) a zeolite containing various metals as a hydrocarbon Contacting with a gas containing NO _{x in} the presence of

The method utilizing the three-way catalyst of the above (1) is
This is a means that has been widely used in the past, and the selective catalytic reduction method (2) uses ammonia as a reducing agent and reacts selectively with NO _x even when oxygen coexists.
It is used to treat exhaust gas from diesel engines. In this case, as a catalyst, a noble metal such as Pt or various metal oxides supported on Al ₂ O ₃ , TiO ₂ or the like is used. Therefore, it is possible to process NO _x with a simple system, and yet N
Since O _x can be decomposed into harmless N ₂ and H ₂ O, there is an advantage that waste liquid treatment is unnecessary.

Further, the above-mentioned (3) is a method called direct decomposition method, which has been attracting attention in recent years as the most ideal method for removing NO _x , and in particular catalysts such as Cu-ZSM-5 zeolite and perovskite type complex compounds. Have been found.

[0005]

However, the method of reducing NO _x using such a catalyst has a drawback that the NOx removal rate is lowered due to the deterioration of the catalyst, and a countermeasure against harmful gas is required. However, there was a problem that the cost required for equipment and the like would rise sharply.

For example, in the method using the three-way catalyst of the above (1), the deterioration rate of the catalyst becomes extremely high due to the excess oxygen contained in the exhaust gas, which causes a difficulty in practical use.

Since the selective catalytic reduction method (2) uses harmful and dangerous ammonia gas as a reducing agent, it requires careful handling, requires various measures for safety, and requires expensive equipment. . Further, since the reduction catalyst is deteriorated by components other than NO _x in the exhaust gas, it is necessary to replace the catalyst, which is economically disadvantageous when a particularly expensive noble metal catalyst is used. Particularly, at high temperatures, inconveniences such as sintering of the catalyst components occur, and at low temperatures, ammonia reacts with water or SO _x , so that salts such as ammonium sulfate are formed on the catalyst surface and the denitrification rate decreases. . Therefore, there is a problem that the operating temperature range is limited to 320 to 450 ° C.

Further, in the above method (3), various metals contained as a component of the catalyst are covered with the sulfur oxide (SO _x ) contained in the exhaust gas, the catalytic activity is lowered, and denitration is performed. However, there is a problem in that it is difficult to obtain a high denitration rate over a long period of time.

The present invention has been made under the above background, and it is advantageous in terms of cost that the denitrification rate does not decrease due to the deterioration of the catalyst, no countermeasures against harmful gases are required, and it is cost effective. It is an object of the present invention to provide a method capable of efficiently removing NO _X discharged from a fuel _cell .

[0010]

In order to solve the above-mentioned problems, the present invention exhausts to a catalyst in which cobalt is ion-exchanged with alkali metal type zeolite in the presence of hydrocarbons in exhaust gas containing excess oxygen. Provided is a method for removing nitrogen oxides in exhaust gas, which is adapted to remove NO _X by bringing the gas into contact with it. The alkali metal-type zeolite is a Y-type zeolite containing Na, and the hydrocarbon is A heavy oil which is a fuel of a diesel engine. Further, the Co ion content in the Na-containing Y-type zeolite catalyst is optimally 0.1% by weight.

[0011]

According to such a method for removing nitrogen oxides, the exhaust gas containing excess oxygen discharged from the diesel engine is catalytically reacted with the Na-containing Y-type zeolite catalyst together with the A heavy oil as a reducing agent to generate NO _x. Are removed. By adding A heavy oil to the exhaust gas, SO acts by the action of hydrocarbons.
_{Since the} surface coating of the zeolite with ₂ gases is prevented, the catalytic active sites of cobalt do not deteriorate.

The Na-containing Y-type zeolite catalyst is Co
The NO _x removal rate shows the highest value when the exchange amount of ions is defined from a 0.01 molar aqueous solution. This corresponds to a Co ion content of 0.1% by weight.

[0013]

EXAMPLES Specific examples of a method for removing nitrogen oxides in exhaust gas according to the present invention will be described below. The catalyst adopted in this example is an alkali metal type zeolite, specifically Na-containing Y-type zeolite. A characteristic is that heavy oil A, which is a fuel for a diesel engine, is used as a reducing agent.

Usually, no clear limitation was made on the cobalt content and the NO _x removal rate for the catalyst which exchanged cobalt, but in the present example, by using A heavy oil as the reducing agent, the cobalt content was reduced. There are certain restrictions imposed on. Further, by using the heavy oil A, it is possible to prevent the catalyst deterioration even with respect to the sulfur oxides contained in the exhaust gas.

The chlorides used for ion exchange are cobalt nitrate and cobalt sulfate, and the chloride aqueous solution concentration is 0 to
The range was 0.1 molar. Na containing Y in this aqueous solution
Ion exchange is performed by immersing type zeolite.

Specific examples will be described below.

(1) Honeycomb body forming method Na-containing Y-type zeolite is HSZ-640N manufactured by Tosoh Corporation.
AA powder was used. Clay and glass fiber were mixed with this powder in an amount of 30% with respect to the zeolite powder, and the mixture was kneaded and then molded into a honeycomb shape. After drying this honeycomb in the sun for 1 day, it was dried in a dryer at 50 ° C for 10 hours and at 100 ° C for 5 hours.
The honeycomb type zeolite was completed by firing at 750 ° C. for 2 hours.

(2) Ion exchange method The formed honeycomb-type zeolite was immersed in a CO (NO ₃ ) ₂ aqueous solution for 2 hours to prepare a catalyst. First of all, CO
Five kinds of (NO ₃ ) ₂ aqueous solution were prepared, one for each of 0.005 molar concentration, 0.01 molar concentration, 0.05 molar concentration, 0.1 molar concentration and 0.3 molar concentration. The honeycomb-type zeolite was washed with ion-exchanged water and then immersed in each aqueous solution for 2 hours. That is, the pH of the CO (NO ₃ ) ₂ aqueous solution changes from 3.0 to 5.5, and at least 2 hours are required to complete the progress of ion exchange.

In the present embodiment, the above immersion was carried out once for 2 hours, but it was confirmed that if the honeycomb type zeolite is immersed again in a fresh aqueous solution of CO (NO ₃ ) ₂ , ion exchange will proceed.

Next, the honeycomb type zeolite immersed for 2 hours each was red-colored (CO (NO
₃ ) It was washed thoroughly until the color ₂ ) disappeared and dried at 100 ° C for 2 hours. Then, the honeycomb type zeolite catalyst was completed by firing in the air at 500 ° C. for 2 hours.

The catalyst obtained as described above was used in the experimental apparatus shown in FIG. 1 to determine the NO _X removal rate. In FIG. 1, 1 is a diesel generator body, 2 is a denitration device body, and exhaust gas discharged from the diesel generator body 1 flows into a lower portion of the denitration device body 2 through a flexible ventilation pipe 3. The ventilation pipe 3 is provided with a flow rate adjusting valve 4 to adjust the flow rate of the exhaust gas to the denitration device body 2. Reference numeral 5 denotes a bypass pipe, and the exhaust gas, the flow of which is blocked by the flow rate adjusting valve 4, is discharged to the outside through the bypass pipe 5.

A catalyst holder 6 is provided inside the main body 2 of the denitration apparatus, and the catalyst holder 6 holds the honeycomb type zeolite catalyst. Further, the exhaust gas inflow portion of the denitration device body 2 is provided with a nozzle 7 for spraying A heavy oil as a reducing agent, and the outer wall portion of the denitration device body 2 is further provided with a constant reaction temperature. The heater 8 is provided.

Further, a NO _x analyzer 10 and a SO ₂ analyzer 11 are installed at the exhaust gas outlet 9 of the main body 2 of the denitration device, and the change amounts of NO _x and SO ₂ are determined by both analyzers 10 and 11. Is measured.

Using such a device, a constant flow rate of exhaust gas discharged from the diesel generator body 1 is introduced from the ventilation pipe 3 into the denitration device body 2, and at the same time 0.1 cc of A heavy oil as a reducing agent is supplied from the nozzle 7. / Sec to 1 cc / sec was sprayed, and the exhaust gas was caused to catalytically react with the honeycomb-type zeolite catalyst held by the catalyst holder 6. Along with this, the exhaust gas flows through the inside of the denitration apparatus main body 2 in which the cobalt-supporting Na-containing Y-type zeolite is accommodated, undergoes NO _x treatment, and then flows out through the outlet 9.

In this example, a honeycomb-type zeolite made of Na-containing Y-type zeolite is used as a cobalt-supporting base material, and the reaction temperature is in the range of 300 to 600 ° C. The reaction pressure is not particularly limited, and the reaction proceeds at a normal exhaust pressure. The space velocity (SV value = the velocity at which the exhaust gas is introduced into the catalyst layer) varies depending on the conditions such as the reaction temperature and the required NO _x removal rate, and there are no particular limiting conditions, but about 500 to 100000 Hr ⁻¹ , preferably Is about 10
It was set to about 00 to 10,000 hr- ¹ .

When the heavy fuel oil A is added to the exhaust gas, the hydrocarbon has the effect of preventing the SO ₂ gas from covering the zeolite surface and preventing the catalytic active site of cobalt from deteriorating. This hydrocarbon burns at the exhaust gas temperature (300 to 500 ° C.), but since the soot contained in the exhaust gas is also burned at this time, catalyst poisoning due to the soot is also avoided and the catalyst life is greatly extended. You can

The exhaust gas, which has undergone the catalytic reaction in the presence of hydrocarbons as described above, is measured by the NO _x analyzer 10, and the correlation between the molar concentration of the ion exchange aqueous solution and the NO _x removal rate (%) is obtained. The results are shown in Table 1 and FIG.

[0028]

[Table 1]

The NO _x removal rate was calculated by the following formula.

The removal rate = (NO _x concentration of the exhaust gas - NO _x concentration in the treated gas) / from the result NO of the exhaust gas _x concentration Table 1 and Figure 2, Na-containing Y-type zeolite catalyst, replacement of Co ions It was found that the NO _x removal rate shows the highest value when is defined from a 0.01 molar aqueous solution. Table 2 shows the actual Co ion content measured by the atomic absorption method.

[0031]

[Table 2]

From the results shown in Table 2, it was found that the optimum Co ion content in the Na-containing Y-type zeolite catalyst was 0.1% by weight.

Further, the exhaust gas after the catalytic reaction is treated with SO ₂
Table 3 shows the result of measurement with the analyzer 11 and the change amount of SO ₂ .

[0034]

[Table 3]

From the results shown in Table 3, it was found that the Na-containing Y-type zeolite catalyst did not cause a large change such as SO ₂ adsorption. This indicates that the catalyst is less poisoned.

[0036]

As described in detail above, according to the method for removing nitrogen oxides in exhaust gas according to the present invention, exhaust gas containing excess oxygen discharged from a diesel engine is used as a reducing agent. NO _x can be efficiently removed by catalytically reacting with the A-containing heavy oil and the Na-containing Y-type zeolite catalyst. Further, by supporting cobalt on the Na-containing Y-type zeolite, the denitration ability of the zeolite can be enhanced.

Further, by adding the heavy fuel oil A to the exhaust gas, it is possible to obtain the effect that the hydrocarbons prevent the SO ₂ gas from covering the zeolite surface and the catalytic active sites of cobalt do not deteriorate. Further, the soot in the exhaust gas is combusted by the hydrocarbon, and the clogging of the zeolite due to such soot is suppressed, so that the denitration rate can be maintained high.

Therefore, according to the present invention, the reduction of the denitration rate due to the deterioration of the catalyst does not occur, and it is advantageous in terms of cost because the countermeasure against the harmful gas unlike the case of using the ammonia gas is unnecessary. Provided is a method capable of efficiently removing NO _X mainly discharged from a diesel engine.

[Brief description of drawings]

FIG. 1 is a schematic view of an apparatus showing a specific example of a method for removing nitrogen oxides in exhaust gas according to the present invention.

FIG. 2 is a graph showing the correlation between the NO _x removal rate of exhaust gas that has undergone the catalytic reaction to which the present invention has been applied and the molar concentration of the ion exchange aqueous solution.

[Explanation of symbols]

1 ... diesel generator body 2 ... denitration apparatus main body 3 ... vent pipe 4 ... flow control valve 5 ... (the A heavy oil) bypass pipe 6 ... catalyst holder 7 ... nozzle 8 ... heater 9 ... outlet 10 ... NO _x analyzer 11 ... SO ₂ analyzer

Claims (4)

[Claims] 1. In an exhaust gas containing excess oxygen,
A method for removing nitrogen oxides in exhaust gas, which comprises removing NO _x by bringing the exhaust gas into contact with a catalyst in which cobalt is ion-exchanged with an alkali metal zeolite in the presence of hydrocarbons. 2. The alkali metal-type zeolite is Na
The method for removing nitrogen oxides in the exhaust gas according to claim 1, wherein the method is a Y-type zeolite containing carbon. 3. The method for removing nitrogen oxides in exhaust gas according to claim 1, wherein the hydrocarbon is heavy fuel oil A which is a fuel for a diesel engine. 4. The method for removing nitrogen oxides from exhaust gas according to claim 1, wherein the Co ion content in the Na-containing Y-type zeolite catalyst is approximately 0.1% by weight.