JPS589692B2 - Method for reducing effluent ammonia in ammonia catalytic reduction denitrification - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for catalytic reduction and denitrification of nitrogen oxides in combustion exhaust gas from a boiler or the like using ammonia, and to a method for reducing ammonia flowing out as an unreacted component during the denitrification process.

In recent years, nitrogen oxides in exhaust gas generated from combustion devices such as boilers have become an environmental and social problem, and measures to eliminate them have been called for.

Various methods have been developed to remove it, but the current method is to add ammonia as a reducing agent and bring it into contact with nitrogen oxides in the exhaust gas in the presence of a catalyst, decomposing it into harmless nitrogen and water vapor. is generally widespread.

Various catalysts have been developed, and the denitrification reaction temperature is in the range of the exhaust gas temperature at the outlet of the boiler economizer of 270 to 420°C, usually around 350°C.

More than 95% of the nitrogen oxides (NOx) in the combustion exhaust gas are occupied by nitrogen monoxide NO, and the ammonia catalytic reduction reaction is expressed by the following equation.

NO+NH3+1/4O2→3/2H20...
・・・・・・(1) In this way, NO and NH3 are 1:1
However, in order to improve the denitrification reaction, NH3
is usually added in excess of No, usually in an amount of 1.0 to 1.5 times the amount of NO.

In addition to reacting with NO shown in equation (1), NH3 also reacts with 02 in the exhaust gas and undergoes oxidative decomposition, but the amount is small, and normally when the equivalent amount of NH3 to the reacted NO is consumed. It may be considered.

Therefore, in the ammonia catalytic reduction denitrification method, the lower the denitrification rate and the greater the excess rate of added ammonia, the more NH3 will flow out from the outlet of the catalytic reaction tower, and since ammonia is a harmful component, its outflow becomes a problem. .

Conventionally, measures have been taken to deal with this effluent ammonia by reducing the excess rate of added ammonia as much as possible and lowering the space velocity to increase the denitrification rate, but the disadvantage is that the smaller the concentration of effluent ammonia, the larger the catalyst volume becomes. It had

An object of the present invention is to provide a method for reducing harmful effluent ammonia in the ammonia catalytic reduction denitrification method described above.

The inventors of the present invention have intensively investigated methods for reducing effluent ammonia in ammonia catalytic reduction denitrification methods, and have discovered that the coexistence of hydrogen peroxide can reduce effluent ammonia without reducing denitrification efficiency, resulting in the present invention. Ta.

That is, after ammonia and hydrogen peroxide are added to the exhaust gas, it is passed through a catalyst layer, and nitrogen oxides are reduced and decomposed into harmless nitrogen and water vapor. At the same time, excess ammonia is oxidized and decomposed in the catalyst layer to become harmless nitrogen. This is a method of converting it into water vapor and reducing effluent ammonia.

The present inventors found that at temperatures above 450°C, hydrogen peroxide and ammonia react directly in the gas phase and ammonia is decomposed, but the reduction in ammonia in this case is due to the denitrification reaction in the subsequent denitrification reaction tower. It has been found that hydrogen peroxide must be added at a temperature of 450° C. or lower, which is not preferable because it lowers the temperature.

The reaction between hydrogen peroxide and ammonia in the gas phase takes place at 450°C.
Substantially no reaction occurs below, and a catalyst is required for the reaction.

Most metal oxides such as iron, cobalt, nickel, titanium, vanadium, and tungsten oxides were effective as catalysts.

Therefore, most of the current practical denitrification catalysts based on ammonia catalytic reduction are based on transition metal oxides, and it is convenient to use these catalysts because denitrification and ammonia decomposition can be carried out at the same time.

The amount of hydrogen peroxide added is preferably about 0.5 times the mole or more of the ammonia flowing out when hydrogen peroxide is not added.
The larger the amount added, the less ammonia flows out.

Even if hydrogen peroxide is added in excess, it is harmless because it decomposes on the catalyst.

Hereinafter, the effects of the present invention will be specifically illustrated by examples.

NOx200ppm (including NO196ppm), SO2
500 ppm, CO28%, H208%, 0
25% heavy oil combustion exhaust gas 50Nm'/hr and NH322
Add NH3 to 0 ppm, and add this gas to Ti
．． Pass through a catalyst bed filled with 02 series catalyst at a temperature of 340°C.
When denitration was carried out at SV 10,000 hr-1, the denitrification rate was 95% and the amount of NH3 flowing out was about 23 ppm.

Under the same conditions as above, 30% by weight of H202 water was further sprayed and a predetermined amount was added from the top of the catalyst layer, and the denitrification rate and effluent ammonia at the outlet of the catalyst layer were measured, and the results shown in the table below were obtained. .

As described above, by adding H202, the denitrification rate did not change significantly, but the effect of reducing the outflow NH3 was obtained.

Claims (1)

[Claims] 1 In a denitrification method in which ammonia is added to combustion exhaust gas and nitrogen oxides in the exhaust gas are catalytically reduced and removed at a temperature of 450°C or lower using a metal oxide catalyst, the temperature at 450°C upstream of the catalyst is
In the catalytic reduction denitrification method, hydrogen peroxide is added simultaneously with ammonia or downstream of ammonia injection at a gas temperature of Decrease method.