JPH03267116A - Method for decomposing nitrogen oxide in waste gas - Google Patents

Abstract

PURPOSE:To initiate the reduction reaction of NOx in waste gas at a considerably reduced temp. by adding gaseous hydrocarbon as well as ammonia to the waste gas and decomposing the NOx. CONSTITUTION:Gaseous hydrocarbon such as methane or hydrocarbon-contg. gas such as coke oven gas as well as ammonia is added to waste combustion gas. The reduction reaction of NOx takes place and the NOx is decomposed. By this method, a high rate of denitration is attained even at about 650 deg.C which is a very low temp. in the case of noncatalytic denitration. It is not necessary to heat even low-temp. waste gas such as waste gas from a heat treating furnace or an annealing furnace in an iron mill and the consumption of heat energy is very low.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for reducing and decomposing nitrogen oxides in combustion exhaust gas.

[Prior art] As a method of exhaust gas denitration to remove nitrogen oxides from combustion exhaust gas, there is a method that reduces and decomposes nitrogen oxides by simply adding ammonia to exhaust gas without using a catalyst. There is a so-called non-catalytic denitrification method.

The reduction reaction of nitrogen oxides is a chain reaction resulting from a combination of many elementary reactions, and can be expressed as a whole as follows.

NO+NH3+1/402 =N2 +3/2 H20
The non-catalytic denitrification method is a denitrification method that is very easy to operate because it does not use expensive catalysts and therefore does not suffer from the various serious problems associated with the use of catalysts. However, this method has a problem in that the above-mentioned reduction reaction does not start unless the exhaust gas temperature is extremely high (higher than about 900° C.).

A method to deal with these problems and lower the reaction initiation temperature has been announced (Journal of Japan Fuel Association, Vol. 56, No. 605).
In this improved method, as shown in FIG. 5, the reaction is started at a low temperature by adding hydrogen together with ammonia to the exhaust gas. FIG. 5 shows that the reduction reaction begins to proceed rapidly from around 700°C.

[Problems to be Solved by the Invention] However, when the above-mentioned improved method is carried out under practical conditions, the effect of hydrogenation is significantly reduced6. According to 700℃
If the reaction temperature is lowered to a certain degree, the degree of reduction of nitrogen oxides becomes very low. Therefore, even with this improved method, the reaction temperature cannot be lowered significantly;
When processing low-temperature exhaust gas, it is necessary to heat the exhaust gas, consuming a large amount of thermal energy.

An object of the present invention is to provide a method for decomposing nitrogen oxides in exhaust gas that further improves the above-mentioned improved method and can further significantly lower the nitrogen oxide reduction reaction initiation temperature.

"Means and effects for solving the problems" In order to achieve the above object, in the present invention, a hydrocarbon gas or a hydrocarbon-containing gas is added to the exhaust gas together with ammonia to decompose nitrogen oxides.

Further, even if coke oven gas is added to the exhaust gas together with ammonia, nitrogen oxides can be efficiently decomposed.

The present inventors conducted various tests with the aim of searching for an additive gas that has the effect of further lowering the reduction reaction initiation temperature of nitrogen oxides than reducing gases such as hydrogen, and found that carbonization of methane, etc. It has been found that the effect is remarkable when hydrogen gas is added.

In addition, when coke oven gas is added as an additive gas, coke oven gas contains methane, so this gas has the same effect, and also contains hydrogen and carbon monoxide, so these gases The effects of reducing gas can also be obtained.

Note that ammonia may be added in aqueous ammonia.

[Example] Examples of the present invention will be described below.

FIG. 6 is an explanatory diagram of an apparatus implementing the method of the present invention.

1 is a combustion furnace, 2 is a burner, 3 is an object to be heated charged in the combustion furnace, 4 is a denitrification reaction chamber, and 5 is a heat exchanger. The combustion gas generated from the burner 2 is transferred from the combustion furnace 1. Enter the denitrification reaction chamber 4. Ammonia is introduced into the denitrification reaction chamber 4,
Furthermore, additive gas such as hydrocarbon gas or coke oven gas is added from another pipe. By introducing these gases, a reduction reaction of nitrogen oxides occurs, and the nitrogen oxides are decomposed. Then, the treated exhaust gas is discharged after a portion of its sensible heat is recovered in the heat exchanger 5.

(Example 1) Using the apparatus shown in FIG. 6, methane was added together with ammonia to the combustion exhaust gas to perform a reductive decomposition reaction of nitrogen oxides.

Ammonia was added in the form of aqueous ammonia. The reaction temperature at this time was varied from around 650°C to around 900°C, and the fixed reaction conditions were: No. at the entrance of the denitrification reaction chamber, = 18 Qppm, (NH
3)/(NOx) molar ratio = 10, the denitrification reaction chamber population was set to 02-1%, and the results are shown in Figure 1. As shown in this figure, when methane was added at 1000 PPffi7, the temperature at 650°C
A good denitrification rate was obtained over the entire range from around 900°C to around 900°C.

Note that the reason why the denitrification rate of this example is lower than that of the conventional technology shown in FIG. NH3)/(NO!) molar ratio of 10, whereas in the conventional technology, the molar ratio (NH3)/(NO!) was much larger than this.
)/(NOx) molar ratio.

(Comparative Example) Under the same conditions as in Example 1, hydrogen gas was added to perform a reductive decomposition reaction of nitrogen oxides. As shown in FIG. 4, the results showed that the denitrification rate was low on the low temperature side, and there was a significant difference from the denitrification rate of Example 1. In this result, the reaction temperature at which a denitrification rate comparable to that of Example 1 can be obtained is at least 850°C.

(Example 2) Under the same conditions as in Example 1, a coke oven gas having the composition shown in Table 1 was added to carry out a decomposition reaction of nitrogen oxides. The denitrification rate is shown in Figure 2.

In this example, coke oven gas 3600pp■
When added, the temperature from around 650°C to 90°C is the same as in Example 1.
A good denitrification rate was obtained over the entire temperature range up to around 0°C.

FIG. 3 is a diagram showing the ammonia concentration in the treated exhaust gas when the ammonia is 150 PP11. The concentration of unreacted NH was very low, and it was confirmed that there was no concern about environmental problems caused by ammonia gas emissions.

Table 1 (vo1%) [Effects of the Invention] The present invention is a method of adding to exhaust gas a hydrocarbon gas or a hydrocarbon-containing gas that can significantly lower the nitrogen oxide reduction reaction initiation temperature. A good denitrification rate can be obtained even at around 650° C., which is a very low temperature range for catalytic denitrification.

Therefore, for example, even when denitrating low-temperature exhaust gas such as exhaust gas from a heat treatment furnace or annealing furnace in a steel mill, it can be carried out without heating the exhaust gas, and the thermal energy consumption is extremely low.

Further, when coke oven gas is added as the hydrocarbon-containing gas, since the coke oven gas also contains hydrogen and carbon monoxide in addition to methane, the effects of these reducing gases can also be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between reaction temperature and NOx removal rate in one embodiment of the present invention, FIG. 2 is a diagram showing the relationship between reaction temperature and NOx removal rate in another embodiment of the present invention, and FIG. 3 is a diagram showing the relationship between reaction temperature and NOx removal rate in another embodiment of the present invention. Figure 2 is a diagram showing the ammonia concentration in the treated exhaust gas in the test, Figure 4 is a diagram showing the relationship between reaction temperature and denitrification rate when the inventors carried out the test according to the conventional technique, and Figure 5 is a diagram according to the conventional technique. A diagram showing the relationship between reaction temperature and denitrification rate, and FIG. 6 is an explanatory diagram of a device W in which the method of the present invention was implemented. DESCRIPTION OF SYMBOLS 1... Combustion furnace, 2... Burner, 3... Material to be heated charged into the combustion furnace, 4... Denitrification reaction chamber, 5... Heat exchanger. I people〒Wenhai Figure 1 defeat M ('C) Figure 2

Claims (2)

[Claims] (1) In a method of decomposing nitrogen oxides by adding ammonia to exhaust gas containing nitrogen oxides, the nitrogen oxides are decomposed by adding a hydrocarbon gas or a hydrocarbon-containing gas to the exhaust gas together with the ammonia. A method for decomposing nitrogen oxides in exhaust gas, characterized by: (2) A method for decomposing nitrogen oxides in exhaust gas according to claim 1, wherein the hydrocarbon-containing gas is coke oven gas.