JPH05137963A - Method for removing nitrogen oxide - Google Patents

Abstract

PURPOSE:To effectively remove NOx with hydrocarbon as a reducing agent while inhibiting the production of N2O as a by-product even in the presence of excess gaseous oxygen. CONSTITUTION:When NOx is removed from NOx-contg. exhaust gas in an oxidizing atmosphere in the presence of hydrocarbon, sulfur oxide is allowed to coexist and a platinum-contg. catalyst is used. The concn. of the sulfur oxide is especially set <=2 times as high as the concn. of the NOx.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for efficiently removing nitrogen oxides contained in exhaust gas discharged from a boiler, an internal combustion engine, a nitric acid manufacturing plant or the like.

[0002]

BACKGROUND OF THE INVENTION As environmental conservation problem is the global scale of the challenges, gathered a lot of interest in the problem removal of NO _X is said to be the cause of acid rain (nitrogen oxides) and SO _X (sulfur oxide) ing. The main cause of generation of this NO _X is exhaust gas from internal combustion engines such as boilers and automobiles, or nitric acid plants.

Conventionally, for example, in a vehicle using gasoline, a three-way catalyst containing platinum, rhodium and cesium has been used. In a gasoline engine, the fuel is burned under the conditions of the theoretical air-fuel ratio (A / F = 14.6), so the oxygen concentration in the exhaust gas is low, and therefore CO, NO and hydrocarbons in the exhaust gas are It can be removed simultaneously with a three-way catalyst. In this catalyst, platinum is mainly C
O and the role of oxidizing hydrocarbons, rhodium is N
A role to reduce O _X, and cesium plays mainly the role of or stored or release oxygen depending on the composition of the exhaust gas, respectively.

However, when the fuel becomes lean, that is, oxygen-rich, even if rhodium, which has a high denitrification activity, loses its activity, the three-way catalyst will no longer function. For this reason, such a three-way catalyst cannot be practically used for a diesel engine, a boiler, etc. in which a large excess of oxygen exists in the exhaust gas.

Therefore, as a method for removing nitrogen oxides under such an excessive oxygen condition, for example, V ₂ O _{5 is used.}
Using ammonia with using -TiO ₂ catalyst as a reducing agent, a method for selectively reducing nitrogen monoxide is employed. On the other hand, this reduction method has a problem that a large amount of N ₂ O is produced as a by-product, depending on the composition of the exhaust gas. Further, since ammonia, which is accompanied by many dangers, is used for its handling, a catalyst using a hydrocarbon as a reducing agent instead of such ammonia has been desired. So far, some reports have been made for such purpose, but for example, a catalyst obtained by exchanging copper ion with H ⁺ or Na ⁺ of zeolite has been proposed (Japanese Patent Laid-Open No. 2004-242242). Sho 63-10091
No. 9).

[0006]

However, in the above-mentioned conventional copper ion-zeolite catalyst, there is a problem that especially zeolite is expensive and, in addition, it takes time to exchange copper ions. Further, there is a problem that the copper catalyst is easily poisoned by SO _X. In view of such circumstances, the present invention uses a hydrocarbon as a reducing agent, and suppresses the production of N ₂ O as a by-product even in the presence of excess oxygen and effectively removes nitrogen oxides. An object is to provide a method for removing oxides.

[0007]

The method for removing nitrogen oxides according to the present invention is a method for removing nitrogen oxides from exhaust gas containing nitrogen oxides in the presence of hydrocarbons in an oxidizing atmosphere. It is carried out by coexisting and using a platinum-containing catalyst. That is, the exhaust gas containing nitrogen oxides is brought into contact with a catalyst containing platinum in the presence of hydrocarbons in an oxidizing atmosphere to efficiently remove the nitrogen oxides in the exhaust gas. The platinum-containing catalyst can be increased in activity by supporting it on a carrier and maintaining a large surface area, and as the carrier, for example,
Alumina, silica, titania, zirconia, aluminum phosphate, silica-alumina, boria-silica-alumina, etc. can be used, and they are used alone or in combination of any two or more. You can also

Further, in particular, in the method of the present invention, the concentration of the sulfur oxide is set to be not more than twice the concentration of the nitrogen oxide.

[0009]

According to the present invention, by using the platinum-containing catalyst, hydrocarbons react preferentially with nitrogen oxides over oxygen in an oxidizing atmosphere. As a result, nitrogen oxides can be efficiently removed from the exhaust gas. Here, the oxidizing atmosphere means a state in which oxygen is present in excess of the amount required to convert all of carbon monoxide, hydrogen and hydrocarbons in the exhaust gas into carbon dioxide gas and water.

The method for preparing the catalyst is not particularly limited as long as platinum can be dispersed on the carrier. As the hydrocarbon, any hydrocarbon capable of reducing nitrogen oxides may be used. For example, the hydrocarbon may be present in the exhaust gas, or if it is still insufficient, it may be introduced from the outside. Examples of the introduced hydrocarbons include paraffinic hydrocarbons such as propane, butane and cyclohexane, olefinic hydrocarbons such as ethylene and propylene, and aromatic hydrocarbons such as toluene and xylene. The amount of these types of hydrocarbons introduced is such that the existing hydrocarbons in the exhaust gas are combined and then stoichiometrically reacted with nitrogen oxides in the exhaust gas to produce N ₂ , H ₂ O or CO ₂ . It is set to an amount necessary and sufficient to generate. Further, SO _X may be existing in the exhaust gas or may be introduced from the outside. Since SO ₂ is the SO ₃ is oxidized by O ₂ on the platinum, it may be used SO _2.

In the method of the present invention, since SO ₂ has an affinity for platinum, the presence of SO ₂ causes the presence of SO ₂ rather than (NO) ₂ Pt (NO) (SO _x ).
Pt is easily generated on the platinum surface, and the mobility of nitric oxide on the platinum surface is also reduced. As a result, N ₂ O
Opportunities for the formation of by-products are reduced, and the production rate of by-products is reduced in the reaction process. In particular, when the concentration of sulfur oxide exceeds twice the concentration of nitrogen oxide, the effect of suppressing N ₂ O generation no longer increases, and when excessive addition of sulfur oxide occurs, deterioration of the catalyst carrier occurs. Or, further, environmental problems caused by SO _X will occur.

[0012]

EXAMPLE A first example of the nitrogen oxide removing method according to the present invention will be described below. First, in the first embodiment, the catalyst is made as follows. 75g of Al a (NO _{3) 3} 9H ₂ O and 23 g H ₃ PO ₄ of dissolved in 1 liter of water, the final pH 7-8 This aqueous ammonia was added dropwise at a rate of 1 ml / min. After stirring for about 2 hours, the mixture is left standing for 24 hours, then suction filtered, washed with pure water, and dried at a temperature of 110 ° C. for 24 hours. Further thereafter, the aluminum phosphate is obtained by baking in air at a temperature of 500 ° C. for 3 hours.

Further, to 15 g of the aluminum phosphate obtained as described above, 0.2 g, 0.8 g and 2 g of solutions containing chloroplatinic acid were added, respectively, and each was evaporated to dryness with stirring. To do. Then, after being dried at a temperature of 110 ° C. for 20 hours and then calcined at a temperature of 500 ° C. for 3 hours, three kinds of platinum-containing catalysts, namely 0.5 w
t%, 2 wt% and 5 wt% Pt / AlPO, respectively
You get ₄ .

Next, the activity test conducted using the above three types of catalysts will be described. In this activity test, reduction was carried out at a temperature of 500 ° C. for 2 hours in 20% hydrogen before using the catalyst, and the reaction temperature of the activity test was set to 200 ° C. First, 1 g of each catalyst was charged into a stainless steel reaction tube having an inner diameter of 10 mm, and the reaction gas was charged with 3 g of gas.
Circulate at a flow rate of 0 ml / min. The composition of this reaction gas as a model gas is NO: 1000 pp.
_{m, C 3 H 6: 1000ppm} , O 2: 5%, He: Use dilution ones.

By the way, the method of the present invention is carried out by adding 100 ppm of SO ₂ to such a reaction gas. The concentration of NO and NO ₂ of each of the above three types of catalysts at the outlet of each reaction tube is of the chemiluminescence type. It is analyzed by NO _X meter. Then, the NO conversion rate (%) represented by the following equation (1) is adopted as the performance evaluation standard of each catalyst. Further, N ₂ O, which is a by-product, is detected by a gas chromatograph, and the N ₂ O selectivity (%) represented by the following formula (2) is adopted as the catalyst performance evaluation standard for this N ₂ O. To do.

Here, a comparative example 1 with respect to the first embodiment
In the above, the test was conducted under exactly the same conditions as in the first example except that SO ₂ was not added to the reaction gas. Furthermore,
As a modification of the first embodiment, H ₂ is further added to the source gas.
This was done by adding only 5% O. Table 1 below shows the calculation results of the NO conversion rate and N ₂ O selectivity as test results in these cases.

Next, a second embodiment of the nitrogen oxide removing method according to the present invention will be described. In the second embodiment, the catalyst is made as follows. 200 g of 0.4 g of chloroplatinic acid
23 g of Al (OH) _{3 in} a solution of water
Powder (aluminum hydroxide manufactured by Nippon Ketchen; Grade G,
BET surface area 340 m ² / g) is added and evaporated to dryness with stirring. Further, after drying this at 110 ° C. for 20 hours, it is baked at 500 ° C. for 3 hours to obtain 1 wt% Pt / Al.
PO ₄ is obtained. The activity test is carried out using such a platinum-containing catalyst. In this activity test, the reaction temperature was set at 200 ° C and 250 ° C.

In particular, in the second embodiment, as in the first embodiment, the reaction gas is added by adding 100 ppm of SO ₂ , but in the comparative example 2 with respect to the second embodiment, the reaction gas is The experiment was carried out under exactly the same conditions as in the second example except that SO ₂ was not added. Table 2 below shows the test results in these cases calculated by the equations (1) and (2).

Further, a third embodiment of the nitrogen oxide removing method according to the present invention will be described. In the third embodiment, the catalyst is made as follows. Put 49.11 liters of water in a 100-liter stainless steel reaction tank equipped with a stirrer, add 9540 g of an aluminum sulfate aqueous solution containing 774 g of Al ₂ O ₃ , and heat and maintain the mixture at 70 ° C. and stir it. 1275g as Al ₂ O ₃ while
PH by adding an aqueous solution of aluminic acid containing
An alumina hydrate slurry of 9.0 is obtained. Next, 55 g of nitric acid with a concentration of 30% was added to this slurry to adjust the pH.
5.4, while stirring this to obtain SiO ₂ 510
Alumina-silica hydrate having a pH of 8.5 is obtained by dropping an aqueous sodium silicate solution containing g. Then, this alumina-silica hydrate is filtered and washed to obtain an alumina-silica hydrate cake. 6760 g of such alumina-silica hydrate cake (1 as alumina-silica)
(Including 014 g) 94.4 g of reagent grade orthoboric acid
The mixture to which (53.4 g as B ₂ O ₃ ) was added is heated and kneaded in a kneader, and is further molded by an extrusion molding machine having a die with a diameter of 5.0 mm. Then, this molded product was dried at 110 ° C. for 15 hours and then dried in an electric furnace for 6 hours.
It is calcined at 00 ° C. for 2 hours, and after this calcining, it is pulverized to obtain boria-silica-alumina (B ₂
O ₃ : 5 wt%, SiO ₂ : 20 wt%, Al ₂ O ₃ :
75 wt%) is obtained.

A solution of 0.4 g of chloroplatinic acid dissolved in 200 ml of water is added to 15 g of the above boria-silica-alumina, and the mixture is evaporated to dryness while stirring. And 11
After drying at a temperature of 0 ° C for 24 hours, it is dried at 500 ° C for 3 hours.
By firing in air for 1 hour, 1 wt% Pt / B
₂ O ₃ —SiO ₂ —Al ₂ O ₃ is obtained. The activity test is carried out with the catalyst thus prepared, which catalyst is reduced in 20% hydrogen for 2 hours at a temperature of 500 ° C. before use. In particular, in the third embodiment, SO ₂ to be added to the reaction gas is set to three kinds of 100 ppm, 500 ppm and 1000 ppm, and the reaction temperature is set to 2
The setting was performed at 00 ° C and 250 ° C.

Table 3 above shows the test results of the third embodiment. In the table, in Comparative Example 3, the SO ₂ concentration in the reaction gas was 0.
It was done as. The composition of the reaction gas is N
O: 500 ppm, propylene: 500 ppm, O ₂ :
5%.

[0022]

As described above, according to the present invention, by using a platinum-containing catalyst, nitrogen oxides contained in exhaust gas can be efficiently removed, and SO ₂ is also added to the reaction gas. By making them coexist, the production of N ₂ O can be suppressed and the activation of the catalyst can be promoted. Further, there is an advantage that the nitrogen oxide removing action can be effectively exhibited even in the presence of water vapor in the reaction gas.

Claims (2)

[Claims] 1. A method of removing nitrogen oxides from an exhaust gas containing nitrogen oxides in the presence of hydrocarbons in an oxidizing atmosphere, wherein sulfur oxides are allowed to coexist and a platinum-containing catalyst is used. And a method for removing nitrogen oxides. 2. The method for removing nitrogen oxides according to claim 1, wherein the concentration of the sulfur oxides is set to not more than twice the concentration of the nitrogen oxides.