JPS592534B2 - Processing method for gas containing nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for efficiently decomposing nitrogen oxides in nitrogen oxide-containing exhaust gas.

NO and N02 (hereinafter abbreviated as NOx) are harmful to the human body, and if absorbed directly into the body, they can cause harm such as reducing internal functions, and recently they have also become a cause of photochemical smog called oxidants. ing.

Therefore, there is a strong demand for establishing a method for removing NOx from exhaust gas.

Many methods have been proposed in the past for making NOx in exhaust gas harmless, and a method using a solid catalyst is known as a typical method.

However, this method generally has problems in terms of industrial implementation, such as the expensive catalyst used, the need for efficient contact equipment, and the need for periodic catalyst regeneration and replacement. This is not necessarily an advantageous method.

As a means to avoid such problems, studies have been conducted on methods that do not use solid catalysts, and a method has been proposed in which NOx is selectively reduced in the coexistence of ammonia and oxygen (for example, Japanese Patent Application Laid-Open No. 7774/1983) (See published patent publication).

Broadly speaking, this publication discloses two methods.

The first is to combine a NOx-containing twisting gas stream with ammonia or an ammonia precursor in the presence of a sufficient amount of oxygen to
Contact with N at 2000'F (871-1093°C)
This is a method of selectively reducing Ox.

In the second method, in addition to the oxygen and ammonia used in the first method, a substance selected from the group consisting of H2 and various other reducing substances is present, and the temperature is 1300 to 2000°F (704°F).
This is a method for selectively reducing NOx by performing gas contact in a temperature range of 1093°C to 1093°C.

This method can industrially advantageously carry out denitrification of various exhaust gases by simply adding ammonia or ammonia and a reducing substance to high-temperature exhaust gas without using an expensive catalyst.

However, when ammonia or its precursor is used in this method, the denitrification rate rapidly decreases below 800°C, which has the disadvantage that the denitrification rate desired industrially cannot be achieved.

In industrial combustion equipment for the purpose of heat recovery, such as boilers, industrial heating furnaces, and incinerators, the temperature of the flue gas flowing through it is controlled by the presence of heat transfer tubes arranged in sequence in the flue gas flow path. It is unavoidable that the temperature range in the flue gas flow path will fluctuate due to a rapid drop in the temperature or fluctuations in the load on the equipment.

Therefore, when applying the above denitrification method to these industrial combustion facilities, consideration must be given to ensuring a sufficient denitrification reaction zone, but securing the required denitrification reaction zone is not necessarily easy in terms of design. That's not the point.

Reiko's existing boilers, etc., cannot be modified to ensure a temperature range suitable for the denitrification reaction, as there are restrictions due to legal and technical factors, and applying the denitration method described above is accompanied by various difficulties. .

As a result of studying various non-catalytic denitrification methods for combustion exhaust gas, the present inventors found that when chlorine is mixed in with NOx-containing exhaust gas in addition to ammonia or its precursor,
It was discovered that effective denitrification of NOx-containing twisting exhaust gas can be carried out in the high temperature range of 00 to 1100°C, and also in the relatively low temperature range of 400 to 800°C (7 Achieved an invention.

That is, the present invention is a method for easily removing NOx from exhaust gas in an extremely wide temperature range without using a catalyst. This is a method for treating exhaust gas that converts NOx in the gas into nitrogen and water by maintaining the temperature at 400 to 1100°C.

The present invention will be explained in detail below.

The method of the present invention reduces NOx in exhaust gas when burning fuel such as heavy oil or coal for boilers, or exhaust gas when burning various wastes discharged from chemical factories, to 10 ppm or more, usually 50 to 50 ppm or more. It is most commonly used for gases containing about 500 ppm.

These exhaust gases are passed through a heat exchanger for effective use of thermal energy, but the treatment of the present invention
Effective denitrification can be carried out anywhere in the temperature range up to 0°C, and especially in the temperature range of 400 to 800°C, where denitrification methods using only ammonia or its precursors have not been able to perform effective denitrification. can do.

The amount of ammonia used is 0.6 per NOx in exhaust gas.
The molar ratio is selected from a range of 1 to 10 molar ratio or more, especially in consideration of economical efficiency.

Ammonia precursors such as ammonium formate, ammonium oxalate, and ammonium carbonate can also be used in place of ammonia.

The amount of chlorine used is 0.1 to 5% relative to NOx in exhaust gas.
A molar ratio of about 0 molar ratio, especially about 1 to 10 molar ratio is practical,
If the molar ratio is less than 0.1, the desired denitrification will not be carried out, and if it exceeds 50 molar ratio, the equipment for removing excess chlorine after denitrification will have to be enlarged, which is industrially disadvantageous. It is.

Chlorine or ammonia may be introduced in diluted form with water or other active gas so that mixing into the gas is rapid.

Note that when ammonia is introduced into the high-temperature exhaust gas through a metal pipe or the like, it is preferable to provide a heat shielding means to prevent the ammonia itself from being decomposed within the metal pipe.

There are no particular restrictions on the timing of mixing ammonia or its precursor and chlorine into the exhaust gas;
They can be added simultaneously or separately into the exhaust gas maintained at ~1100°C.

The residence time at this temperature is 0.01 seconds or more, especially 0.
．． A range of 1 second to 4 seconds is practical.

Specifically, the high temperature exhaust gas flowing out from the combustion zone is 110
A method is adopted in which ammonia and chlorine are added simultaneously or separately in an arbitrary order at an appropriate stage when the temperature is lowered to 0° C. or lower.

In addition, a method of adding ammonia and chlorine to exhaust gas discharged at a temperature slightly lower than 400°C and then raising the temperature to 400°C or higher, or adding ammonia and chlorine after heating exhaust gas at a temperature lower than 400°C to 400°C or higher. A method to do so will be adopted.

When adding these ammonia and chlorine, since the exhaust gas flowing out from ordinary boilers, industrial heating furnaces, incinerators, etc. is usually at a temperature of 1000°C or higher, it is necessary to add them to the exhaust gas flow path in the above temperature range. It is preferable to use a plurality of nozzles to disperse and introduce the material so as to uniformly disperse it.

Combustion exhaust gas from factories producing vinyl chloride, chlorinated paraffin, and other chlorine derivatives contains a considerable amount of free chlorine or its precursors along with NOx, so chlorine should be added to such exhaust gases. It is only necessary to add ammonia or its precursor in the above-mentioned predetermined temperature range without having to add the ammonia or its precursor, which is extremely advantageous in terms of industrial implementation.

If chlorine and ammonia are mixed into the NOx-containing exhaust gas according to the method of the present invention, the temperature of the high-temperature exhaust gas of 1100°C will drop below 400°C during the process of flowing out from the combustion zone, no matter how rapidly the temperature drops. Since the denitrification reaction is effectively carried out until the denitrification reaction is completed, extremely high denitrification efficiency can be achieved even while the exhaust gas is flowing down the flue gas pipe or the like.

The reason for this is not clear, but it appears to be because the presence of chlorine promotes the reduction of NOx by ammonia.

Chlorine and chlorides contained in the exhaust gas after denitrification treatment can be removed by contacting the exhaust gas with an alkaline solution such as aqueous caustic soda solution or water, so they can be easily removed along with sulfur compounds using known wet desulfurization equipment. can do.

According to the present invention, the temperature range required for denitrification is 4
Since the temperature range is extremely wide from 00 to 1100°C, denitrification can be performed stably and effectively even when the temperature of the exhaust gas to be treated is unstable or rapidly drops.

Further, when treating exhaust gas that is lower than the above-mentioned required temperature, the temperature is required, but in that case, only a slight increase in temperature is required, so the cost is low.

Next, the present invention will be explained with reference to examples.

Example 1 Ammonia 400 ppm (400 ppm) was added to the makeup gas obtained by mixing NOx 1 air and nitrogen, with a NOx content of 200 ppm, oxygen concentration of 2%, and the remaining nitrogen.
NH3/NOx: 2 molar ratio) was mixed, and this mixed gas and 60 ppm capacity of chlorine gas (C12/NOx: 0
．． 3 molar ratio) was passed through one end of a quartz tube maintained at a predetermined temperature in an electric furnace for a residence time of 0.4 seconds, and NOx in the exhaust gas from the other end of the quartz tube was measured.

Note that for the measurement of NOx, a chemiluminescence measuring device ECL-77V manufactured by Yanagimoto Seisakusho was used.

The results are shown in the table below. Note that NOx removal (1) was calculated using the following formula.

Example 2 The amount of chlorine added was set at a 2 molar ratio ((J!2/N0
Denitration was carried out in the same manner as in Example 1, except for x). The results are shown in the following table.

Example 3 The amount of chlorine added was adjusted to 5 molar ratio (C12/N0x
) Denitrification was carried out in the same manner as in Example 1, except that the results were as shown in the following table.

Example 4 The amount of chlorine added was set at a molar ratio of 2.5 to NOx (C12/N
0x), and the amount of ammonia added is 4 molar ratio (NH
Denitration was carried out in the same manner as in Example 1 except that the temperature was set to s/N0x), and the results are shown in the following table.

Comparative Example Denitration was carried out under the same conditions as in Example 1 without adding any chlorine. The results are shown in the table below.

Comparing the results of Examples 1 to 4 and Comparative Examples above in terms of reaction temperature (horizontal axis) and NOx removal rate (vertical axis), as shown in Figure 1, when using the method of the present invention, only ammonia was added. Compared to the comparative example, the denitrification temperature range is clearly wider.
In particular, in the comparative example, no denitrification occurs at temperatures below 700°C, whereas in the method of the present invention, sufficient denitrification occurs even at 400°C.

[Brief explanation of drawings]

Figure 1 of the attached drawings is a diagram showing the relationship between the holding temperature of NOx-containing gas and the NOx removal rate in the gas, and the vertical axis is the NOx
The removal rate is shown, and the horizontal axis shows the gas retention temperature.

Claims (1)

[Claims] 1. A method for treating a nitrogen oxide-containing gas, which comprises maintaining a nitrogen oxide-containing gas in which chlorine and ammonia or an ammonia precursor are mixed at a temperature of 400 to 1100°C.