JPS58885B2 - HIGH SHIYUTSUGASUTUYUNO CHITSUSOSUKABUTSUNO - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an economical and practical method for removing nitrogen oxides contained in exhaust gas. Air pollution caused by nitrogen oxides contained in carbon dioxide has become a social problem.

Various measures to prevent the emission of nitrogen oxides have been studied, and among them, a method of reducing and removing nitrogen oxides using a reducing gas such as ammonia is attracting attention as a promising method for practical application.

Many catalysts have been proposed in the past in methods for reducing and removing nitrogen oxides using these reducing gases.

However, these catalysts are composed of expensive raw materials, and the cost of the catalyst in the method for reducing and removing nitrogen oxides is extremely large, so that they are not necessarily economically satisfactory.

Furthermore, when these catalysts are used to remove nitrogen oxides from exhaust gas, the dust in the exhaust gas is extremely difficult to adhere to the catalyst surface, resulting in a significant decrease in catalyst activity and an increase in pressure loss. This may cause a decrease in gas flow rate, etc.
It cannot be said that the catalyst is necessarily durable for long-term use.

In other words, most of these catalysts are those commonly used as carriers, such as silica and alumina.
They have the disadvantage of high raw material costs, and they also increase the specific surface area of the catalyst, resulting in an increase in the micropores of the catalyst and an increase in the surface roughness of the catalyst from a microscopic perspective. Therefore, in denitrification of flue gas containing dust, dust is deposited in the micropores and accumulated accordingly, making it impossible for the catalyst to maintain its activity over a long period of time.

As a result of various studies aimed at developing an economical and practical catalyst that improves the shortcomings of conventional catalysts, the inventors of the present invention have found that the catalyst components are made of inexpensive raw materials, and that they are resistant to the adhesion of soot and dust in exhaust gas. We have discovered a useful catalyst with low denitrification activity and excellent sustainability of denitrification activity over long periods of use.

The catalyst of the present invention not only improves the drawbacks of conventional catalysts, but also has the same or higher denitrification activity with a smaller amount of reducing agent for nitrogen oxides, and has an extremely long catalyst life including physical strength. It is an excellent product, and its advantages are extremely great not only economically but also in terms of practicality.

Therefore, an object of the present invention is to provide an economical and practical method for removing nitrogen oxides using an extremely inexpensive catalyst that has high denitrification activity and long-term activity sustainability.

The gist of the present invention is that when nitrogen oxides contained in exhaust gas are removed using a reducing agent, calcium sulfate, which has the property of shrinking in volume when heated, is used as the main ingredient.
This is a method for removing nitrogen oxides from exhaust gas, which is characterized by using a catalyst comprising this and a metal oxide.

Catalyst used in the method of the present invention (hereinafter referred to as the catalyst)
Since it uses calcium sulfate as its main ingredient, it is inexpensive and optimal in terms of usefulness.

In other words, calcium sulfate has a property that the volume of a gypsum molded body shrinks when heated, but in the present invention, we have focused on this property of calcium sulfate, and in particular, it has a small specific surface area, in other words, it has a microscopic shape. The pores are reduced as much as possible and the roughness of the surface of the molded catalyst is made as small as possible, that is, the catalyst has a very small internal surface with active surface, and its apparent geometric surface Only the denitrification reaction is made to act effectively on the denitrification reaction.

The catalyst molded body in the present invention has a specific surface area of about 30 when molded.
m2/g, but under combustion or denitrification reaction conditions, due to volumetric contraction, its specific surface area becomes 10 m2/g or less, which is extremely small compared to the specific surface area of ordinary catalysts.

Therefore, in the denitrification of flue gas containing dust, the deposition of dust in micropores and the accompanying accumulation do not occur, and the catalyst maintains its activity for a long period of time.

The weight ratio of the calcium sulfate used in the catalyst to the total weight of the catalyst can vary over a wide range, but is usually 10 to 99%, with the total weight of the catalyst being 100%.
9% by weight, preferably 50-99.5% by weight.

The metal oxide constituting the catalyst (hereinafter referred to as the metal oxide of the catalyst) may be one commonly used in this type of catalyst, such as oxidation of copper, silver, and gold in Group 1b of the periodic table. oxides of vanadium, niobium and tantalum of group 5b, oxides of chromium, molybdenum and tungsten of group 6b, oxides of manganese of group 7b and iron of group 8,
One or more metal oxides selected from oxides of cobalt, nickel, ruthenium, rhodium, osmium, iridium, and platinum are used.

Among these metal oxides, copper oxide iron oxide, cobalt oxide, nickel oxide, etc., which have good denitrification activity, are more preferable.

The metal oxide of the catalyst is produced from a metal oxide or a metal compound that becomes an oxide by heating and oxidation (hereinafter referred to as a raw material compound of the metal oxide).

Examples of metal compounds that become oxides when heated include metals themselves and organic carboxylates such as their hydroxides, nitrates, carbonates, and oxalates.The metal oxides constituting the catalyst are made from these compounds as raw materials. It can be done.

The amount of metal oxide used in the catalyst varies depending on the type of metal, and its weight ratio to the total weight of the catalyst is not particularly limited, but is usually 0.1 to 90% by weight, preferably 0.5 to 90% by weight.
It is 50% by weight.

Calcium sulfate used in the catalyst is cheaper and more easily available than compounds commonly used as carriers such as silica, alumina, and zeolite, and is less expensive than sulfuric acid produced during flue gas desulfurization using calcium hydroxide. Calcium can be used and is inexpensive and easily available in large quantities.

The components of the catalyst thus consist of inexpensive raw materials, which underlines the economical advantages of the process of the invention.

The catalyst is prepared by a commonly used method.

For example, calcium sulfate and the metal oxide of the catalyst or its raw material compound are mixed in a solid state and then molded into a predetermined size, or calcium sulfate and the metal oxide of the catalyst or its raw material compound are mixed in the presence of moisture. There is a method of mixing or kneading to form a powder with an appropriate particle size and molding it, or a method of impregnating a molded product of calcium sulfate with an aqueous solution of the raw material compound of the metal oxide.

When using a metal oxide raw material compound, it is desirable to combust it before mixing with calcium sulfate, between mixing and shaping, or after shaping.

In addition, when molding, compounds that act as binders such as starch and poval, and inorganic fiber compounds such as glass fiber and asbestos can be added to increase the strength of the molded product.In some cases, organic acids and organic Compounds called lubricants such as metal salts and paraffin can be added.

In the method for removing nitrogen oxides using the catalyst of the present invention,
Exhaust gases containing nitrogen oxides come from both stationary sources, such as boilers and chemical plants, as well as mobile sources, such as automobiles.

There is no problem even if the exhaust gas contains sulfur oxides, that is, sulfur dioxide, sulfur trioxide, and water vapor in addition to nitrogen oxides.

When using ammonia as a reducing agent, the amount used increases in proportion to the concentration of nitrogen oxides contained in exhaust gas and the amount of exhaust gas, but it is usually 0 in terms of molar ratio to nitrogen oxides, especially nitrogen monoxide. .3~3, O preferably 0.65~
It is 1.5.

Compared to conventional catalysts using silica or alumina as carriers, when the reducing agent is ammonia, the amount of ammonia used is smaller due to calcium sulfate, resulting in better denitrification activity, and as a result, the catalyst remains unreacted. The advantage of the present invention is obvious from the viewpoint of economy and prevention of pollution since almost no amount of ammonia is discharged.

The space velocity of exhaust gas containing nitrogen oxides relative to the catalyst is:
The concentration varies depending on the nitrogen oxide concentration before the reaction and the concentration after the reaction, but it is usually carried out at 500 to 100.0 OQHr-1, preferably 2,000 to 20,000Hr-1.

In the method using the catalyst, the reaction temperature is usually 200°C.
-550°C, preferably 300-450°C.

In carrying out the method of the present invention, exhaust gas and reducing gas may be continuously passed through a fixed bed, moving bed or fluidized bed packed with the catalyst from the top to the bottom of the reaction column or from the bottom to the top. can.

Compared to conventional catalysts using silica or alumina as carriers, the catalyst contains less soot and dust contained in exhaust gas and has excellent sustainability of catalytic activity, which makes the present invention economical and practical. The features are even clearer.

Hereinafter, the method of the present invention will be explained using Examples, but the Examples are not intended to limit the method of the present invention.

Example Calcium sulfate dihydrate, iron nitrate hexahydrate, and copper nitrate trihydrate were thoroughly kneaded in an aqueous medium, then burned at 500°C by blowing air through them, and the resulting powder was crushed to a diameter of The thus prepared catalyst, molded into a cylindrical shape of 8 mm and an average height of 6 stones, contained 91.0% by weight of calcium sulfate as anhydride and Fe2O.
The composition was 4.7% by weight as CuO and 4.3% by weight as CuO.

This catalyst 201 was packed in a cylindrical reactor with a diameter of 30 cm and a height of 30 cm, and a long-term activity experiment for 200 hours was conducted by passing exhaust gas from a boiler burning heavy oil A through the bottom of the reactor.

Note that the exhaust gas was heated with an electric heater so that the reaction temperature was 350 to 370°C.

Ammonia is used as the reducing gas, and the space velocity is 8,000.
Exhaust gas was passed through at Hr-1.

Average gas composition of exhaust gas is NNO30pp, 80225
oppm, 803 approximately 10 ppm, water vapor 9.5%, carbon dioxide gas 10.2%, oxygen 6.6%, nitrogen 73.7%, and the amount of soot and dust in the exhaust gas is 70 mg/Nm3 on average.

Ammonia was added at a molar ratio of NH3/NO of approximately 1.0.

The nitrogen oxide concentration in the gas at the inlet and outlet of the reactor was measured using a chemiluminescent NOx analyzer manufactured by Beckman, and the denitrification rate was determined.

After the denitrification rate gradually increases for several hours, it remains at a constant value of 86~
It showed 89%, and no decrease in activity was observed until 200 hours after the reaction was completed.

Furthermore, the pressure difference between the inlet and outlet of the reaction tower was 81 m immediately after the start of the reaction.
83mmaq and only 2mm at the end from maq
Only an increase in aq was observed.

Reference Example A comparison experiment with the catalyst of the present invention was conducted using a copper oxide-alumina catalyst as a representative example of a conventional catalyst.

After sufficiently kneading γ-alumina and copper nitrate trihydrate in an aqueous medium, they were calcined at 500°C by passing air through them, and the resulting powder was formed into a cylindrical shape with a diameter of 8 mm and an average height of 6 mm. .

The catalyst thus prepared has a composition of 91.3% by weight of γ-alumina and 8.7% by weight of CuO.

This catalyst 201 was packed into a cylindrical reactor with a diameter of 30 cm and a height of 30 cm, and was heated under almost the same conditions as the example of the present invention.
The reaction was carried out for 00 hours.

The denitrification activity increases with time, but it reached an almost constant value of 81% in a few hours, but after about 40 hours it started to decrease slightly, and by the 100th hour when the reaction ended, it was 75%.
It declined to .

Furthermore, the pressure difference between the inlet and outlet of the reaction tower is 85 m immediately after the start of the reaction.
An increase of 90 maq and 5 mmaq was observed at the end of the reaction.

As described above, compared to conventional catalysts, the catalysts shown in the examples of the present invention have superior denitrification activity under almost the same conditions and cause less soot and dust to settle in exhaust gas, so no decrease in denitrification activity was observed. Furthermore, it is clear that the pressure difference between the inlet and the outlet of the reaction tower increases by only 1 mmaq for 100 hours, which is extremely small compared to the 5 mmaq of the reference example.

Next, in order to further clarify the practical usefulness of the method using the catalyst of the present invention, some catalyst compositions and reaction results are shown in the table below.

Each of the catalysts shown was prepared as follows.

That is, after calcining a hydrate of calcium sulfate to 180°C, the raw materials for each metal compound for calcium sulfate were nitrate for copper, vanadium, iron, cobalt, nickel, and manganese, and ammonium chromate for chromium. there was.

The raw material compounds of calcium sulfate and metal oxide were sufficiently mixed in an aqueous medium, dried and pulverized, and the resulting powder mixture was molded into a cylindrical shape with a diameter of 5 mm and an average height of 4 mm.

The molded product was fired at 550°C and used in the reaction.

Each of the catalysts prepared in this way was divided into 20 ml pieces with a diameter of 3
cm reactor, and while raising the reaction temperature of the reactor to 360°C, a mixed gas containing nitrogen monoxide is passed through the top of the reactor, and after reaching a predetermined temperature, ammonia is added in the form of an aqueous solution to carry out the reaction. started.

Each reaction took 30 hours, and the space velocity was approximately 5,000 Hr-1.
I did it.

The composition of the mixed gas before passing through the catalyst layer is NN020.
0pp, NH3220ppm, 5SO2200pp, H
20, approximately 10.6%, CO29.3%, oxygen 2.3%, and nitrogen 77.8%.

The denitrification rate was determined by measuring the nitrogen oxide concentration at the inlet and outlet of the reactor in the same manner as in the previous example.

The denitrification rate generally shows an increasing trend after the start of the reaction, then reaches a constant value after several hours, and no decrease in activity is observed until the end of the reaction after 30 hours.

As described above, all of the catalysts of the present invention have good denitrification activity, and the strength of the catalyst after the reaction is the same as that before the reaction, demonstrating usefulness.

Claims (1)

[Claims] 1. When removing nitrogen oxides contained in exhaust gas using ammonia as a reducing agent, the main ingredient is calcium sulfate, which has the property of shrinking in volume when heated, and a catalyst consisting of calcium sulfate and a metal oxide is used. A method for removing nitrogen oxides from exhaust gas.