JPS6044009B2 - Method for removing nitrogen oxides from exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for removing nitrogen oxides from exhaust gas by catalytic reduction using a catalyst bed by adding ammonia.

BACKGROUND OF THE INVENTION Combustion exhaust gas emitted from thermal power plants and various factories contains a large amount of nitrogen oxides (NOx), which has become a social problem as a major cause of air pollution.
There are two main methods for removing nitrogen oxides contained in this combustion exhaust gas: wet methods and dry methods.

However, the wet method requires large and complicated equipment, is very expensive, and the wastewater treatment technology has not been fully established, so there are major problems in treating the denitrification wastewater. In particular, the dry method has attracted attention. In the dry method for removing nitrogen oxides from exhaust gas, ammonia gas is blown into the combustion exhaust gas, and then Cr, Cu, and TiO are removed using a fire-resistant catalyst carrier.

This is a method of catalytic reduction removal using a catalyst bed supporting a catalytic material such as a catalytic metal oxide such as Fe. As a conventional method for removing nitrogen oxides in exhaust gas by catalytic reduction using a catalyst bed by adding ammonia, for example, as shown in FIG. A catalyst bed 4 made of granular catalyst 3 is installed, and an ammonia gas inlet 5 is arranged in the front flow path on the exhaust gas introduction side of the catalyst bed 4, and ammonia gas is introduced into the combustion exhaust gas 6 flowing through the pipe 1. A widely known removal method involves adding and mixing ammonia through the port 5, catalytically reducing nitrogen oxides in the exhaust gas in the catalyst bed 4, and releasing the nitrogen oxides into the atmosphere as harmless nitrogen gas.

However, such conventional removal methods add the entire amount of ammonia to the forward flow path of the first catalyst bed 4.
NH, HSO due to fluctuations in exhaust gas flow rate and temperature.

, (NH.). Ammonium sulfate compounds such as SO, (NH3), (SO. This has the fatal disadvantage of causing a poisonous phenomenon and significantly reducing removal efficiency. The method for removing nitrogen oxides from exhaust gas of the present invention is to remove nitrogen oxides from exhaust gas by catalytic reduction using a catalyst bed by adding ammonia. A plurality of catalyst beds carrying a catalyst material on the surface of the honeycomb structure are installed at predetermined intervals in the gas outflow direction, and an ammonia gas inlet is arranged on the gas inflow side of each catalyst bed, so that the inflow exhaust gas is This is a method for removing nitrogen oxides from exhaust gas that selectively utilizes the catalyst bed by adjusting and controlling the ammonia gas addition point so that ammonia gas is selectively supplied to the catalyst bed on the gas outlet side when the exhaust gas is reduced. , solving the above-mentioned conventional drawbacks,
There is a method for removing nitrogen oxides that reduces the phenomenon of catalyst poisoning caused by (NH4)x(SO4), etc., extends the life of the catalyst, and improves the removal efficiency.

The structure of the present invention will be explained based on FIG. 2 showing a specific example. The combustion exhaust gas pipe 7 is made of ceramic material such as cordierite, mullite, alumina, etc., and the cross-sectional shape of the through hole is circular. Alternatively, a plurality of catalyst beds 8a, 8b, 8c in which a catalyst substance made of a catalyst metal oxide is supported on the thin wall surface of a ceramic honeycomb structure made of polygons such as triangles, squares, pentagons, hexagons, and octagons. are installed at predetermined intervals in the exhaust gas outflow direction. Ammonia inlet ports 9a, 9b, 9c are provided on the exhaust gas inflow side of each of the catalyst beds 8a, 8b, 8c, so that ammonia can be injected according to fluctuations in the exhaust gas flow rate, temperature, nitrogen oxide content, etc. Addition of The locations are ammonia gas inlet ports 9a, 9b,
This is a nitrogen oxide removal method in which catalyst beds 8a, 8b, and 8c are selectively used as nitrogen oxide purification catalyst beds by adjusting and controlling the catalyst beds 9c. In other words, for example, the operating rate decreases and the amount of exhaust gas decreases. When the amount is reduced to about 1B, it is not necessary to use all the catalyst beds, so at that time, the addition of ammonia from the ammonia gas inlets 9a and 9b is stopped and purification is performed using only a part of the catalyst bed 8c.

That is, if the entire amount of ammonia gas is added from the front side of all the catalyst beds, in other words, from the introduction port 9a in front of the first catalyst bed 8a on the gas introduction side, all the catalyst beds 8a, 8b and 8C1
In particular, since 8a is poisoned by ammonium sulfate compounds, if the gas amount is small, the inlet 9 in front of the final catalyst bed 8c
Ammonia gas is introduced only through the catalyst bed 8c, and only the catalyst bed 8c on the gas outlet side is selectively utilized. In this case, since no ammonia gas is added to the catalyst beds 8a and 8b, no poisoning by ammonium sulfate occurs. When the amount of gas is large and a large amount of ammonium sulfate is generated, the addition of ammonia is divided into three parts, and 113 ammonia is added each through the inlet ports 9a, 9b, and 9c, and the purification effect is performed by dividing the ammonia into multiple catalyst beds. etc., the catalyst bed is selectively utilized by adjusting and controlling the noammonia addition port. In addition, since multiple catalyst beds are installed, if the fluctuating amount of exhaust gas is particularly large and the temperature of the exhaust gas in the pipes decreases significantly, use the same catalyst material for the entire catalyst bed. It is also possible to use a high-temperature, high-activity catalyst in the catalyst bed near the temperature, and then use a catalyst material with optimal purification properties as the temperature decreases.

Note that it is difficult to form one catalyst bed with one honeycomb structure, so it is of course possible to form a catalyst bed by combining a plurality of single honeycomb structures two-dimensionally or three-dimensionally. There are some good ones.

As described above, the method for removing nitrogen oxides from exhaust gas according to the present invention is to install a plurality of catalyst beds at predetermined intervals in the gas outflow direction in the exhaust gas pipe, and to install each catalyst bed on the gas inflow side of each catalyst bed. An ammonia gas inlet is provided to selectively utilize multiple catalyst beds by changing, adjusting or controlling the ammonia addition point according to fluctuations in exhaust gas temperature, flow rate, nitrogen oxide content, and other factors. Therefore, the poisoning of the catalyst due to the formation of ammonium sulfate, etc. is less likely to be localized to a specific catalyst bed, which greatly extends the life of the catalyst and improves the removal efficiency. It has been used as a method for removing oxides, and is an extremely useful method from the viewpoint of pollution prevention.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram schematically showing an example of a conventional method for removing nitrogen oxides from exhaust gas by adding ammonia gas.
FIG. 2 is an explanatory diagram schematically showing one specific example of the method for removing nitrogen oxides from exhaust gas according to the present invention. 1,7...Pipeline, 2...Frame, 3...
... Granular catalyst, 4, 8a, 8b, 8c... Catalyst bed, 5, 9a, 9b, 9c... Ammonia gas inlet, 6... Combustion exhaust gas.

Claims (1)

[Claims] 1. In a method for removing nitrogen oxides from exhaust gas by catalytic reduction using a catalyst bed by adding ammonia, a plurality of catalysts having catalyst substances supported on the surface of a ceramic honeycomb structure are used in the exhaust gas pipe. The beds are installed at predetermined intervals in the gas outflow direction, and an ammonia gas inlet is placed on the gas inflow side of each catalyst bed, so that when the inflow exhaust gas decreases, ammonia gas is given priority to the catalyst bed on the gas outflow side. A method for removing nitrogen oxides from exhaust gas, which comprises adjusting and controlling the point at which ammonia gas is added to selectively supply ammonia gas, thereby selectively utilizing a catalyst bed.