JPS5845887B2 - Method for removing nitrogen oxides from exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently reducing and removing nitrogen oxides contained in exhaust gas into nitrogen at high temperatures.

To detoxify nitrogen oxides (hereinafter abbreviated as NOx) generated from various fixed sources such as nitric acid production plants and thermal power plant boilers, ammonia gas is added to the exhaust gas containing nitrogen oxides and brought into contact with the catalyst layer. There is a way to turn it into nitrogen gas.

The reaction temperature generally used is 250'C to 450C.

The catalyst used is a titanium oxide-based catalyst invented by the present inventors (Japanese Patent Application Laid-Open No. 50-128680,
Many catalysts are known, including Japanese Patent Publication No. 52-6954, Japanese Patent Publication No. 52-17830, Japanese Patent Publication No. 52-35342, and Japanese Patent Publication No. 54-2912).

However, these conventional catalysts mainly exhibit relatively high activity in the temperature range of 300 to 500°C.

According to a detailed study by the present inventors, among conventional catalysts, catalysts made of titanium oxide and vanadium oxide are particularly effective at 350°C.
It was found that the activity before and after was high.

By the way, it has been found that conventional catalysts such as those mentioned above have low high-temperature activity and are not practical in denitrification processes that treat high-temperature gases of 500° C. or higher, such as gas turbine exhaust gas.

The reason for this is thought to be that ammonia added as a reducing agent not only reacts with NOx but also reacts with oxygen, and some of the ammonia is oxidized to NOx, resulting in a decrease in the denitrification rate.

According to the studies conducted by the present inventors, it has been found that among conventional catalysts, the catalyst composed of titanium oxide and tungsten oxide invented by the present inventors (Japanese Patent Publication No. 52-35342) has relatively good activity at high temperatures. Ta.

However, even with this catalyst, the activity decreased at temperatures above 500°C, particularly around 600°C, and it was found that this catalyst was not sufficient for practical use.

Furthermore, as a method for treating high-temperature exhaust gas, a gas phase reduction method in which ammonia is reacted with NOx at a temperature of 800 to 1100° C. in the absence of a catalyst is also known.

However, this method requires a temperature of 800 DEG C. or higher, and a high denitrification rate cannot be obtained at 500 DEG to 700 DEG C., which is the objective of the present invention, and this reaction also has the drawback of being somewhat low in selectivity.

An object of the present invention is to provide a method for efficiently reducing and removing nitrogen oxides in exhaust gas to nitrogen at high temperatures, eliminating the drawbacks of the prior art described above.

The feature of the method of the present invention is that the catalyst used in the method includes titanium oxide as the first active component, tungsten oxide as the second active component, and tin oxide, nickel oxide, manganese oxide, and antimony oxide as the third active component. The atomic ratio of tungsten to titanium is 0.01 to 0.01 to titanium.
5, the atomic ratio of at least one component selected from tin, nickel, manganese, and antimony is in the range of 0.01 to 0.1 to 1 titanium, and 500 to 850 'C-fired material is used.

According to research conducted by the present inventors, a catalyst combining titanium oxide and tungsten oxide showed relatively good performance in the reaction between ammonia and NOx at high temperatures of 400°C to 500°C. It was found that the addition of this substance reduces the decrease in activity due to the oxidation reaction of ammonia in the high-temperature section above 500°C, resulting in a catalyst with even higher performance.

In this case, the catalyst composition should preferably be within the above-mentioned range; for example, if the atomic ratio of tungsten to titanium is less than 0.01 or more than 0.5, the activity will be low and this is not practical.

Further, if the atomic ratio of the third component to titanium is less than 0.01, the effect of the addition of the third component will not be apparent and the activity will be low, and if it is more than 0 or 1, the activity will decrease.

The second feature of the method of the present invention is that the catalyst is calcined at 500°C.
The above is to be carried out at a temperature of 850°C or lower.

It has been found that catalysts calcined at temperatures below 500° C. are unstable under the conditions in which they are used, and not only do they not exhibit stable high activity, but also have low activity overall.

In addition, if the catalyst is fired at a temperature of 850°C or higher, sintering of the catalyst progresses and the specific surface area rapidly decreases, resulting in low activity.

In the present invention, the reaction temperature is selected to be 500°C or higher and 700°C or lower.

In this catalyst, high activity can be obtained in this temperature range.

At temperatures below 500°C, conventional catalysts other than the present catalyst, such as titanium oxide-tungsten oxide, can also achieve a high denitrification rate, so there is no particular advantage to using the catalyst of the present invention, and at temperatures above 700°C, the catalyst added as a reducing agent is Since ammonia reacts with oxygen to generate NOx, the NOx removal rate decreases.

In the present invention, the amount of ammonia added as a reducing agent is preferably in the range of O15 to 2.0 in molar ratio to NOx.

The method for preparing the catalyst used in the method of the present invention is not particularly limited, and it can be easily produced by conventional methods such as precipitation, cross-contact method, and impregnation method that are commonly used in the production of catalysts.

Further, as the final method for molding the catalyst, any molding method can be employed depending on the purpose, such as the usual extrusion molding method, tablet molding method, rolling granulation method, etc.

Further, as a method for producing the catalyst, it is also possible to use a method in which a slurry of catalyst components is coated on a carrier, preferably a honeycomb-shaped carrier.

The shape of the catalyst may be pellets, spheres, plates, honeycombs, pipes, envelopes, or the like.

As raw materials for preparing the catalyst used in the method of the present invention, those that produce oxides upon calcination, such as oxides, hydroxides, nitrates, sulfates, and chlorides, can be used.

Another preferred method is to add a precipitant such as aqueous ammonia or sodium carbonate solution to a solution of various salts to precipitate it to produce hydroxides, carbonates, etc., and then thermally decompose it to produce oxides.

In addition to the above-mentioned components, the catalyst of the present invention may also contain 50 or less by weight of alumina, silica/alumina, magnesia, and calcium oxide.

Of course, when a carrier made of these components, particularly a honeycomb-shaped carrier, is coated with the components of the present invention, the total weight of the carrier may be 50% or more.

Hereinafter, the content of the present invention will be explained in more detail with reference to Examples.

Example 1 The catalyst used in the method of the invention was prepared as follows.

Example Catalyst 1 Take 189.7 g of titanium tetrachloride (T iC4),
Pour into ice water from step 11 and add 3N ammonia water to neutralize.

Separate the resulting precipitate and wash thoroughly with distilled water.

The cake thus obtained was 173.59 (T t 02
(converted to 32.0 g, equivalent to 0.4 mol),
Add 200 g of ammonium paratungstate to this.
Add 2.33 g of nickel nitrate to 1 m of distilled water at 200 mA.
Add the mochumoto dissolved in each of the distilled water and evaporate the water while thoroughly kneading.

After drying the obtained cake, distilled water is added and wet milling is performed for 30 minutes, followed by extrusion molding into a diameter of 6 mounds.

The obtained molded product is dried and fired in a Matsufuru furnace at 800° C. for 5 hours.

The catalyst thus obtained had a metal atomic ratio of Ti:W:N1=
It has a composition of 1:0.05:0.02.

This was cut into 6 lengths and used in the following reaction.

The reaction tube is a quartz reaction tube with an inner diameter of 40 mm and an outer diameter of 4 mm inside.
It has a quartz thermocouple insertion tube, and the outside is heated with an electric furnace.

The reaction gas has the following composition. NO 300 ppm NH 3 360 ppm 02 15% N20 8 tons CO2 8 rice cake N2 The remainder A gas having this composition is passed through at a space velocity (NTP equivalent space column standard, hereinafter referred to as SV) of 10,000 hr-1.

Table 1 shows the NOx removal rates obtained by sequentially changing the reaction temperature.

Note that the NOx analysis was performed using a chemiluminescence type NOx analyzer, and the NOx removal rate was determined using the following formula.

Comparative Example 1 Example Catalyst 1 except that the raw materials were a hydrous titanium oxide cake prepared from titanium tetrachloride and ammonium metavanadate in the same manner as Example Catalyst 1, and the calcination temperature was changed to 450°C.
Comparative Example Catalyst 1 was prepared in the same manner as above.

This catalyst has a composition of Ti:V=1:0.05 in atomic ratio.

The results of performance tests performed in the same manner as in Example 1 are also listed in Table 1.

Note that the activity was too low at temperatures above 600'C, so measurements were not carried out.

Comparative Example 2 Comparative Example Catalyst 2 was prepared in the same manner as Example Catalyst 1 except that nickel nitrate was not added to Example Catalyst 1 and the calcination temperature was 450°C.

This catalyst has a composition of Ti:W=1:0.05 in atomic ratio, and the performance was tested in the same manner as in Example 1.
Also listed in the table.

Examples 2 to 8 Example 1 was prepared using a catalyst prepared in the same manner as Example Catalyst 1, except that the ratio of catalyst raw materials was changed and the catalyst composition was changed.
The results of the reaction were also shown in Table 1.

Examples 9-16 Examples 1-8 except that the third component of the catalyst was changed to tin oxide
Table 2 shows the results of testing the performance of catalysts prepared in the same manner as above.

Examples 17-24 Example 1 except that the third component of the catalyst was changed to manganese oxide
Table 3 shows the results of testing the performance of catalysts prepared in the same manner as in 8.-8.

Examples 25 to 32 Table 4 shows the results of testing the performance of catalysts prepared in the same manner as in Examples 1 to 8, except that the third component of the catalyst was changed to antimony oxide.

Example 33 Table 5 shows the results of testing the performance of catalysts prepared in the same manner as Example Catalyst 1 except that the calcination temperature was changed.

Example 34 Table 6 shows the results of testing the performance of a catalyst prepared in the same manner as Example Catalyst 9 except that the calcination temperature was changed.

Example 35 Table 7 shows the results of testing the performance of a catalyst prepared in the same manner as Example Catalyst 17 except that the calcination temperature was changed.

Example 36 Table 8 shows the results of testing the performance of a catalyst prepared in the same manner as Example Catalyst 25 except that the calcination temperature was changed.

Claims (1)

[Claims] 1. A method in which exhaust gas containing nitrogen oxides is brought into contact with a catalyst together with ammonia, and nitrogen oxides in the gas are reduced and removed at 500 to 700°C, wherein the catalyst has a temperature of 500 to 8
It is obtained by firing at a temperature of 50°C, and contains titanium oxide as the first active component, tungsten oxide as the second active component, and among tin oxide, nickel oxide, manganese oxide, and antimony oxide as the third active component. The atomic ratio of tungsten to titanium is in the range of 0.01 to 0.5 to 1 titanium, and at least one selected from tin, nickel, manganese, and antimony. A method for removing nitrogen oxides from exhaust gas, characterized in that the atomic ratio of one or more components to titanium is in the range of 0.01 to 0.1.