JPS5817363B2 - Nitrogen oxide reduction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention uses gas containing nitrogen oxides, such as combustion exhaust gas, to
This is a method of reducing nitrogen oxides in the gas by mixing it with a low calorific value reducing gas containing hydrocarbons and burning it at a low temperature.

Nitrogen oxides in the atmosphere are the main cause of photochemical smog generation, and it is extremely important to remove them from the gas in order not to pollute the atmosphere, and various methods have been developed.

Representative methods include solution absorption methods, solid adsorption methods, and catalytic methods.

These methods each have some drawbacks that hinder their practical application, such as requiring various post-treatments in addition to the gas removal step, or the problem of catalyst poisoning in the catalytic method. ing.

Furthermore, as a method for burning nitrogen oxide-containing gas together with combustible gas, Japanese Patent Application Laid-Open Nos. 50-121159 and 1972-
There is a method disclosed in No. 104470.

All of these methods have drawbacks such as complicated steps and high costs in practice.

The process of the present invention is simpler than these conventional methods, and Patent I
This invention relates to a method for treating nitrogen oxide-containing gas that does not require post-treatment and is easy to put into practical use.

That is, the present invention is a mixture of a gas containing nitrogen oxides, a hydrocarbon, and a gas with low activity or inertness, and which does not self-combust even if the theoretical amount of air necessary for combustion is introduced in an adiabatic system. A large amount of reducing gas with a low calorific value having the properties is introduced into a reduction combustion furnace and mixed, and the stoichiometric oxygen ratio of the total gas at the mixing pressure is maintained at 1 or less and 0.5 or more, and the temperature is 700℃ to 110℃.
A method for treating nitrogen oxide-containing gas, which comprises reducing nitrogen oxides to nitrogen by combustion while maintaining a temperature of 0°C, and then adding air to burn the combustible components. It is.

The present invention will be explained based on process diagrams shown in the drawings.

When fuel and air are introduced into a combustion furnace and burned, exhaust gas containing nitrogen oxides is produced.

This gas contains a low concentration of oxygen.

The combustion furnace shown in the drawings is not specified and may be any combustion furnace such as a waste combustion furnace or a boiler, and the gas containing nitrogen oxides is not specified.

Next, this nitrogen oxide-containing gas and a large amount of reducing gas with a low calorific value, which is a mixture of hydrocarbons and a low-activity or inert gas, are introduced into a reduction combustion furnace and mixed. Reduction combustion is carried out in .

Inside this reduction combustion furnace, the nitrogen oxide-containing gas and the introduced reducing gas mix and react.

In the process of this reaction, the reducing gas is not self-combustible, so reducing agents such as hydrocarbons can be uniformly dispersed without burning, so local combustion does not occur, and thermal nitrogen oxides are generated due to local heating. occurrence does not occur.

Furthermore, even after the reaction, the temperature does not rise because it is a reducing gas with a low calorific value, and no thermal nitrogen oxides are generated, and the nitrogen oxides are completely reduced inside the reduction combustion furnace.

Nitrogen oxides are reduced and burned by hydrocarbons, but in this case, the volume ratio of the nitrogen oxide-containing gas to the reducing hydrocarbon is small (meaning the volume ratio is close to 1).

), there is no need to provide a special mixing device to bring both substances into a diffusion-mixed state, or to increase the volume of the reducing combustion chamber in order to lengthen the residence time in the mixed state of both substances. .

In other words, a reducing gas (reducing gas with a low calorific value) whose volume is reduced by mixing a low-activity or inert gas in a hydrocarbon chamber is used, which allows the gas containing nitrogen oxides to The volume ratio to the reducing gas can be made small.

A reducing gas must meet the following conditions.

(a) In an adiabatic system, self-combustion does not occur even when the theoretical amount of air required for combustion is introduced.

(b) The smaller the amount of oxygen contained, the better; at most, it should not exceed the theoretical amount of oxygen required for combustion of the hydrocarbons, etc. contained.

In addition, the hydrocarbons used in the present invention are methane, ethane, propylene, butane, ethylene, acetylene, and propylene, which are gaseous at room temperature, and liquid at room temperature, and from 100°C to 3.
Means hexane, kerosene, light oil, gasoline, and alcohols that are gaseous at 00°C.

These can be used alone or in combination.

Further, as less active or inert gases, nitrogen, argon, carbon dioxide, waste gas with a low oxygen concentration, water vapor, etc. are used.

It is sufficient that these contain 95% by volume or more of components that do not participate in the combustion reaction.

The gas containing nitrogen oxides and the reducing gas having a low calorific value are mixed so as to satisfy the following conditions.

(1) The amount of hydrocarbons contained in the reducing gas is small, and the oxygen contained in the gas containing nitrogen oxides and the oxygen in the reducing gas are reduced, and the nitrogen oxides are reduced. exists in the theoretical amount necessary for

(2) The mixing ratio of the nitrogen oxide-containing gas to the reducing gas is such that both gases can reach a mixed diffusion state simply by mixing, that is, the volume ratio is small, and the reducing combustion temperature of the mixing pressure is low. be within a range that can be controlled within the temperature range described below.

(3) Furthermore, the reaction temperature of both gases at the mixed pressure is 700℃~11
It is necessary to control the temperature so that the temperature reaches 00°C.

Temperature control can be easily achieved by various known means, such as providing a heat recovery device such as a water pipe on the inner wall of the furnace in which reductive combustion is carried out.

(4) Also, the theoretical oxygen ratio of the gas at the mixed pressure, that is, the value obtained by dividing the total oxygen amount by the theoretical oxygen amount required to completely burn the combustible components in the mixed gas, is preferably 1 or less and 0.5 or more. is 0
．． It is preferably in the range of 6 to 0.99.

(5) The temperature of the reducing gas when mixing it with nitrogen oxides should be 400°C or less, but the temperature inside the reduction combustion furnace, the volume ratio of both gases, the hydrocarbons in the reducing gas, etc. Determined by type, operating conditions, and energy efficiency.

In this way, both gases are mixed and heated to 700°C to 1100°C.
By carrying out reduction combustion while maintaining the temperature at 5, most of the nitrogen oxides are reduced, and most of the hydrocarbons are also converted to carbon monoxide and hydrogen.

Furthermore, since the maximum temperature in reductive combustion is 1100° C., there is no high-temperature part due to combustion of hydrocarbons, and no thermal nitrogen oxides are generated.

Note that if the combustion temperature at the source does not reach 700°C, the reduction reaction will not proceed sufficiently.

Next, gas containing oxygen is added to the reduced nitrogen oxide gas after reduction combustion to cause oxidation combustion.

In the drawing, air is introduced into the oxidation combustion chamber as the gas to contain oxygen.

Further oxidation thus occurs, the remaining hydrocarbons are oxidized and burned, and the carbon and hydrogen oxides become carbon dioxide and water vapor.

Oxygen in this oxidative combustion can be carried out by using a slight excess of N compared to the theoretical amount of oxygen, but the reaction temperature, gas mixture state, average residence time, and furnace for causing this oxidation can be controlled. The excess oxygen concentration is freely determined by the structure of .

The oxidized combustion exhaust gas that is finally discharged is still at a high temperature, so if necessary, heat recovery is performed using a heat recovery device to reduce the temperature to 100°.
Cool to about 300°C and discharge.

Since this gas has an inert component as its main component, it can also be used as an inert gas to be mixed with hydrocarbons and the like to generate a reducing gas with a low calorific value.

In this case, it is possible to reduce the amount of hydrocarbons consumed by the oxygen in the reducing gas, which is advantageous because there is less heat generated by combustion of the reducing gas and the temperature rise during reductive combustion can be suppressed. .

As described above, the method of the present invention makes it possible to treat gas containing nitrogen oxides more advantageously than conventional methods.

Although the method of the present invention has the above-mentioned characteristics, it is further superior in the following points.

During reductive combustion containing nitrogen oxides, a large amount of inert or less active gas is mixed with the reducing hydrocarbon, so almost no waste is generated.

Therefore, there is no need to use a large excess of oxygen (usually air) compared to the theoretical amount in the latter stage of oxidation combustion.
Hydrocarbons, etc. remaining after reduction combustion can be combusted with an amount slightly exceeding the required theoretical amount.

Thus, it is easy to reduce the oxygen content of the oxidized combustion exhaust gas to 1% or less, and it is not impossible to reduce it to 0.

In other words, since no extra oxygen is required, there is no heat loss due to it, and therefore the heat recovery rate is high.

In addition, since there is a wide range of choices regarding the amount of oxygen in oxidative combustion, operation is easy.

In reductive combustion, the amount of reducing gas input is small, and even if the temperature of the nitrogen oxide-containing gas fluctuates, it is easy to absorb the fluctuation, and during combustion, the amount of heat generated by combustion per unit ( Since the heat of combustion per unit volume is not large,
Temperature control becomes extremely easy.

Although the process diagram shown in the drawing shows a typical example of implementing the present invention, the combustion furnace, reduction combustion furnace, and oxidation combustion furnace may be integrated, and nitrogen oxide-containing The gas is not limited to the system shown in the drawings, and the heat recovery device and circulation of the oxidized combustion exhaust gas are also provided as necessary.

Example 1 A combustion furnace with an inner diameter of 350 nmφ and a length of 1,350 nm and having a fireproof and insulated structure was charged with liquefied natural gas at 0.8 normal cubic meters per hour and air containing 200 ppIII of NO at a volume of 34 mm per hour.
The concentration of nitrogen oxides in the flue gas discharged from the combustion of 4 normal cubic meters was determined by the chemiluminescence method, and the concentration of nitrogen oxides was found to be 560 tons by volume.

The composition of the combustion exhaust gas at this time was as shown in Table 1.

Next, 36 cubic meters per hour of this combustion exhaust gas containing nitrogen oxides was fed into a reduction combustion furnace with an inner diameter of 350mm and a length of 1500mm, and at the same time nitrogen containing 1.8% nitrogen by volume 50 normal cubic meters per hour, the average temperature is about 920℃ (890℃~940℃), and the theoretical oxygen ratio is 0.7.
1 was used for reduction combustion.

The gas discharged from the reduction combustion furnace was 88.2 normal cubic meters per hour, and the nitrogen oxides were so small that the volume could not be measured, and the other compositions were as shown in Table 2.

This product was introduced into an oxidation combustion furnace having an inner diameter of 350 mmφ and a length of 1360 mm, and air was introduced into the furnace at a rate of 15 normal cubic meters per hour.

The oxygen concentration of the exhaust gas at the outlet is 1.9% by volume in terms of dry gas.
Met.

Furthermore, the nitrogen oxide concentration is 54 volume pprn in terms of dry gas.
The denitrification rate was approximately 72%.

The amounts of hydrocarbons and carbon monoxide were measured using a gas chromatograph, but they were less than 1 ppm by volume, making quantitative analysis impossible.

The carbon dioxide concentration was 5.4% by volume in terms of dry gas, with the remainder being nitrogen.

Comparative Example 1 36 cubic meters per hour of combustion exhaust gas having a nitrogen oxide concentration of 560 ppffl by volume, obtained in exactly the same manner as in Example 1, was transferred to the reduction combustion furnace of Example 1 containing 1.8% by volume of propane. 50 normal cubic meters of nitrogen per hour and 15 normal cubic meters of air per hour
The composition of the exhaust gas at the outlet is as follows:
1.9% by volume of oxygen, 5.4% by volume of carbon dioxide, 92% by volume of nitrogen.
At 7% by volume, the nitrogen oxides were 260 ppm by volume.

Example 2 A combustion furnace, a reduction combustion furnace, and an oxidation combustion furnace exactly the same as in Example 1 were used.

Combustion in the combustion furnace was performed in exactly the same manner and produced a flue gas containing exactly the same nitrogen oxides.

The composition of this exhaust gas was the same as in Example 1.

This combustion exhaust gas is introduced into a reduction combustion furnace at a rate of 36.0 cubic meters per hour, and a low calorific value reducing gas containing 1.2% by volume of butane (the composition is shown in Table 3) is introduced at a rate of 50 normal cubic meters per hour. The average temperature of the mixture is approximately 960°C (from 940°C to 970°C).
), and reductive combustion was carried out at a stoichiometric oxygen ratio of 0.85.

The gas discharged from the reduction combustion furnace was 87.5 normal cubic meters per hour, nitrogen oxides and oxides were 0.0065% by volume, and the other compositions were as shown in Table 4.

This material was introduced into an oxidation combustion furnace, and air was supplied at a rate of 3.3 per hour.
Normal cubic meter gas was introduced into the same reactor, and the oxygen concentration of the exhaust gas at the outlet was 0.2% by volume.

Furthermore, the concentration of nitrogen oxides in the exhaust gas released from this oxidation combustion furnace was 65 volumes in terms of dry gas, the denitrification rate was approximately 85%, and no hydrocarbons or carbon monoxide were detected at all. 11) I)m or less.

The composition of the released exhaust gas was as shown in Table 5.

In this example, a portion of the exhaust gas from the oxidation combustion furnace was circulated and mixed with butane to produce the above-mentioned low calorific value reducing gas.

The amount of gas released outside the subtraction system was 46.8 normal cubic meters/hour.

The effects of the present invention are clear from the above Examples and Comparative Examples.

[Brief explanation of drawings]

The drawing is an example of a process diagram for an embodiment of the present invention.

Claims (1)

[Claims] 1. A reducing gas that is a mixture of a hydrocarbon and a low-activity or inert gas and that does not self-combust even if the theoretical amount of air necessary for combustion is introduced in an adiabatic system. The resulting reducing gas and the exhaust gas containing nitrogen oxides are led to a reduction combustion furnace and burned at a temperature of 700°C to 1100°C while maintaining a stoichiometric oxygen ratio of 1 or less and 0.5 or more, and then reduced combustion The combustion gas burned in the furnace is guided to the oxidation combustion furnace,
A method for reducing nitrogen oxides characterized by burning and discharging residual hydrocarbons together with air. 2. The method for reducing nitrogen oxides according to claim 1, wherein the stoichiometric oxygen ratio of the reducing gas led to the reduction combustion furnace and the exhaust gas containing nitrogen oxides is maintained at 0.99 to 0.6. 3 Hydrocarbons include those that are gaseous at room temperature such as methane, ethane, propane, butane, ethylene, acetylene, and propylene, hexane, kerosene, diesel oil, gasoline, and alcohols.
The reduction of nitrogen oxides according to claim 1, which is a gas at 00 to 300°C, is also