JPS6099B2 - Method of oxidizing nitrogen monoxide in combustion exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for oxidizing nitrogen monoxide in combustion exhaust gas, and more specifically, by introducing a mixed gas obtained by mixing combustion exhaust gas with a specific oxidation promoter into a corona discharge zone. , relates to a method for oxidizing nitrogen monoxide in combustion exhaust gas to nitrogen dioxide.

Conventionally proposed methods for removing nitrogen oxides from combustion exhaust gas are broadly divided into wet methods and dry methods. In general, most of the nitrogen oxides in combustion exhaust gas are nitrogen monoxide, which is difficult to absorb into water. Therefore, in the linear method, some or all of the nitrogen monoxide is oxidized to nitrogen dioxide or dinitrogen pentoxide, which is more easily absorbed, and then absorbed. Here, as a method of oxidizing nitrogen monoxide, a method using ozone is known, but since the cost of ozone is relatively high and the cost of oxidation increases, there is a desire to reduce the cost. In addition, in recent years, a method has been developed in which combustion exhaust gas containing nitrogen monoxide is passed through the corona discharge zone to oxidize it to nitrogen oxide (Japanese Unexamined Patent Publication No. 114370/1983), but this method is several to several tens of times faster than the ozone oxidation method described above. It is expensive and impractical as it is. Therefore, the present inventors have conducted extensive research in order to overcome the drawbacks of the above-mentioned techniques and develop an economically advantageous and effective method for oxidizing nitrogen monoxide.

As a result, before introducing the flue gas into the corona discharge zone,
The present inventors have discovered that nitrogen monoxide can be efficiently oxidized by mixing it with a specific oxidation promoter, and have completed the present invention. That is, the present invention provides a method for treating combustion exhaust gas containing nitrogen monoxide in the presence of oxygen, water vapor, and carbon dioxide gas with an olefinic hydrocarbon, a paraffinic hydrocarbon, and an oxygen-containing hydrocarbon. One or more compounds are added to nitrogen monoxide in the combustion exhaust gas.
The mixed gas is mixed at a ratio of 0.1 to 3.0 moles per mole, and then the resulting mixed gas is mixed.

The present invention provides a method for oxidizing nitrogen monoxide in combustion exhaust gas, which is characterized in that nitrogen monoxide in combustion exhaust gas is introduced into a corona discharge zone formed by applying a high voltage between a corona discharge electrode and a corona discharge electrode. The method of the present invention consists of a first step of mixing combustion exhaust gas with a specific hydrocarbon as an oxidation promoter, and a second step of introducing the resulting mixed gas into a corona discharge zone to oxidize nitrogen monoxide to nitrogen dioxide. The process can be divided into two steps.

In the first step of the present invention, it is necessary to mix the combustion exhaust gas and the oxidation promoter in the presence of oxygen, water vapor, and carbon dioxide. In other words, it is important that the mixed gas to be introduced into the corona discharge zone contains oxygen, water vapor, and carbon dioxide. Therefore, if the combustion exhaust gas already contains desired amounts of oxygen, water vapor, and carbon dioxide, there is no need to further supply these. The reason why oxygen, water vapor and carbon dioxide gas are contained in the mixed gas is as follows.

That is, first, regarding oxygen, oxygen in the mixed gas receives discharge energy in the corona discharge zone and becomes atomic oxygen or ozone, which acts on nitrogen monoxide to produce nitrogen dioxide.

This atomic oxygen or ozone also acts on hydrocarbons and the like in the mixed gas to form oxide radicals that promote the oxidation of nitrogen monoxide. As described above, oxygen is essential for the progress of the oxidation reaction of the present invention, and is directly or indirectly involved in the oxidation of nitrogen monoxide. In the method of the present invention, the amount of oxygen required for oxidizing nitrogen monoxide is usually 1% by volume or more, preferably 4 to 2% by volume, based on the total amount of combustion exhaust gas.
It is. Note that since the amount of oxygen in the combustion exhaust gas is generally 4 to 5% by volume of the total amount of the exhaust gas, there is no need to separately supply oxygen in the method of the present invention. Next, water vapor also plays an important role in the oxidation of nitrogen monoxide, and in experiments conducted by the present inventors, a mixed gas containing no water vapor and a mixed gas containing water vapor at 9.2% by volume of the combustion exhaust gas were compared. Even if the same discharge power was used, the oxidation rate of nitric oxide in the latter was superior to the former by about one needle. The amount of water vapor required in the present invention varies depending on the concentration of nitrogen monoxide, oxygen, carbon dioxide, etc. in the exhaust gas and cannot be determined unambiguously, but it is usually 5 to 13% by volume based on the total amount of combustion exhaust gas. preferable. Note that since the combustion exhaust gas generally contains about 1% by volume, there is often no need to further supply water vapor. On the other hand, carbon dioxide gas also plays an important role in the oxidation of nitrogen monoxide, and according to experiments by the present inventors, a mixed gas containing no carbon dioxide gas and a mixed gas containing 12% by volume of carbon dioxide gas of the combustion exhaust gas. When compared, the oxidation rate of nitric oxide in the latter was about four times better than the former even when the same discharge power was used.

The amount of carbon dioxide required in the present invention varies depending on the concentration of nitrogen monoxide, oxygen, water vapor, etc. in the exhaust gas, but it is usually preferably 3 to 1 percent by volume of the combustion exhaust gas. Note that the amount of carbon dioxide contained in normal combustion exhaust gas is sufficient. The oxidation promoter to be mixed with the combustion exhaust gas in the first step of the method of the present invention is one or more compounds selected from olefinic hydrocarbons, paraffinic hydrocarbons, and oxygenated hydrocarbons, as described above. be.

This compound becomes an oxide radical in the corona discharge zone described below, which plays a role in promoting the oxidation of nitrogen monoxide. Therefore, the mixing amount of this oxidation promoter should be determined according to the content of nitrogen monoxide in the combustion exhaust gas, and is generally 0.1 per mole of nitrogen monoxide contained in the exhaust gas.
The ratio is determined to be 3.0 mol. If it is less than 0.1 mol, it has almost no effect, and if it exceeds 3.0 mol, the oxidation rate of nitrogen monoxide does not increase, and the amount of unreacted hydrocarbons, which are oxidation promoters, increases and becomes secondary. This is not desirable as it may cause pollution.

Compounds used as oxidation promoters include olefinic hydrocarbons, paraffinic hydrocarbons, and oxygen-containing hydrocarbons as described above, but more specifically, they are as follows.

First, preferred examples of olefinic hydrocarbons include ethylene, propylene, isobutene, n-butene, etc., and preferred compounds of paraffinic hydrocarbons include methane, ethane, propane, butane, and bentane. On the other hand, suitable oxygen-containing hydrocarbons include alcohols such as methanol, ethanol, propanol, and butanol, and aldehydes such as formaldehyde and acetaldehyde, and ethers, ketones, and the like can also be used. In the method of the present invention, the above-mentioned compounds are suitable oxidation promoters because they have a strong oxidation promoting effect and there is no risk of secondary pollution. There is no problem even if it is hydrogen or the like. If the oxidation promoter is liquid at room temperature, it is made into a gas in a vaporizer before being mixed with the combustion exhaust gas, and then mixed with the exhaust gas to form a mixed gas.
It is preferable to lead it to a corona discharge zone. Furthermore, there is no particular restriction on the temperature of the mixed gas that should be adjusted when introducing the mixed gas generated in the first step of the present invention into the corona discharge zone, but as described above, water vapor should be present in the mixed gas. It is preferable to set the temperature to 100° C. or higher because of necessity.

On the other hand, heating to 40,000 or higher does not significantly increase the oxidation rate of nitrogen monoxide, which is economically disadvantageous as it becomes necessary to heat the exhaust gas or use heat-resistant materials. Therefore, in the present invention, it is advantageous to adjust the temperature of the mixed gas within the range of 100 to 400 qo. Next, the second step of the present invention,
That is, the process of introducing the above-mentioned mixed gas into the corona discharge zone to oxidize nitrogen monoxide to nitrogen dioxide will be described.

The corona discharge zone used in the method of the present invention may be of any shape as long as it generates corona discharge in the flow path of the mixed gas, such as a flat electrode type structure,
They can have any structure, such as those with the same cylindrical electrode structure.

In addition, since the wire electrode that generates corona has a highly corrosive atmosphere in the reaction system, it is necessary to use SUS to obtain a stable oxidation rate.
It is desirable to choose a corrosion-resistant material such as stainless steel such as 304 or SUS31. On the other hand, the counter electrode is placed at an appropriate distance from the line electrode. The polarity and waveform of the voltage are not particularly limited and may be appropriately selected depending on the reaction conditions and the like. In determining the discharge power used for the corona discharge of the present invention, the present inventors conducted the following experiment.

That is, a mixed gas in which an oxidation promoter was mixed with exhaust gas was subjected to corona discharge at various electric powers to oxidize nitrogen monoxide, and the relationship between the discharge power and the oxidation rate was investigated. The relationship when propylene is used as the oxidation promoter is shown in FIG. 1, and the relationship when isobutene is used is shown in FIG. 2 in comparison with the case where no oxidation promoter is mixed. The amount of the oxidation promoter added was adjusted to be equimolar to the nitrogen monoxide in the exhaust gas in both cases of propylene and isobutene. As can be seen from Figures 1 and 2, in general, the higher the discharge power, the better the oxidation rate, but in practice, the applied voltage and the It is preferable to select a current value. However, if the discharge power is large enough to reach the spark discharge power, spark discharge will occur and the oxidation rate of nitrogen monoxide will be significantly lowered, which is not preferable. Therefore, it is preferable that the discharge power used for the corona discharge of the present invention is greater than /hr in terms of IW/N and less than the spark discharge power. Since the value of this spark discharge power is determined by the electrode structure, electrode arrangement, etc., it is important to select a device with an electrode structure and electrode arrangement that can generate stable corona discharge without causing spark discharge. The oxidation of nitric oxide in the method of the present invention is carried out in a corona discharge zone, and is thought to proceed mainly through a radical reaction between oxide radicals such as hydrocarbons, which are oxidation promoters, and nitric oxide. The details are not clear. However, according to the experiments of the present inventors,
When propylene is used as an oxidation promoter, as shown in FIG. 1, the following equation '1} holds true as the oxidation rate equation for nitrogen monoxide.

d[N.

]:K.・５・・・・・・・・・'・〕dt〔NO〕：
Nitric oxide concentration P: Discharge power V: Cylindrical electrode tube volume Ko: Rate constant, that is, the oxidation rate of nitric oxide is proportional to the discharge power per unit volume of the cylindrical electrode tube.

When a compound other than propylene, such as isobutene, is added as an oxidation promoter, the behavior is as shown in FIG. 2, and the following equation (2) is established as the oxidation rate equation of nitrogen monoxide.

d [za? ]=K. - [N.

]-Rise-...--{2' [NO], P and V are as above. K, indicates the rate constant. That is, in this case, the oxidation rate of nitric oxide is proportional to the product of the nitric oxide concentration and the discharge power per unit volume of the cylindrical electrode tube. An example of an apparatus for carrying out the method of the invention is shown in FIG.

In FIG. 3, the cylindrical electrode 1 is used as a ground electrode and the bran electrode 2 is used as a discharge electrode, and a voltage of several KV obtained from a high voltage generator 3 is applied between the two electrodes to generate a corona discharge. Combustion exhaust gas is introduced from the combustion exhaust gas inlet 4, and if the oxidation promoter is gaseous at room temperature, it is introduced from the position 5, mixed with the combustion exhaust gas, and guided to the corona discharge zone. When the oxidation promoter is liquid at room temperature, it is introduced from position 6, converted into gas by vaporizer 7, and then mixed with combustion exhaust gas. The nitrogen monoxide in the combustion exhaust gas mixed with the oxidation promoter in this manner is oxidized to nitrogen dioxide in the corona discharge zone, and is discharged from the exhaust gas outlet 8. The following examples were carried out using the apparatus shown in Fig. 3, and the conditions were as follows: discharge type: direct current (
10) Corona discharge, cylindrical electrode 3 diameters 44 ribs, length 800 bars, wire electrode: diameter 0.2 ribs or 0.4 ribs, processing gas amount:
271 (N so/hr), and the composition of the mixed gas to be further treated is 20 hours of nitrogen monoxide, 1 hour of sulfur dioxide.
50 pm, carbon dioxide gas 12% by volume, water vapor 9.2% by volume
, a predetermined concentration of oxygen, a predetermined concentration of oxidation promoter, and the remainder was nitrogen gas.

Example 1 The mixed gas temperature was 150 pm, the oxygen concentration was 2% by volume, the wire electrode diameter was 0.2 cm, and the oxidation promoter was F.

. A predetermined amount of pyrene was used, and the other conditions were as described above. Measuring the device inlet and outlet concentrations of nitric oxide and the device inlet and outlet concentrations of nitrogen oxides (N○x),
It was organized using the above equation {1). The results are shown in FIG. 4 as the relationship between the rate constant and the amount of propylene added. Example
2 The same operation as in Example 1 was carried out except that the diameter of the wire electrode was 0.4 mm and the oxidation promoter was a predetermined compound other than propylene, and the results were organized using the above formula (2).

The results are shown in Figure 5. As can be seen from Figures 4 and 5, the concentration of the oxidant is 0.1 in molar ratio to nitrogen monoxide.
If it is less than 3.0, it is not very effective, and on the other hand, even if it is made more than 3.0, the rate constant will not become so large. Example 3 The same machine as in Example 1 was operated except that in Example 1, the propylene concentration was set to 1.0 (molar ratio) to nitrogen monoxide in the exhaust gas, and the oxygen concentration was varied variously.

The results are shown in Figure 6. As is clear from FIG. 6, in order for the oxidation reaction to proceed smoothly, the oxygen concentration in the combustion exhaust gas must be at least 1% by volume, preferably at least 4% by volume.

Example 4 In Example 1, the oxygen concentration was 4% by volume, and the wire electrode diameter was 0.4.
side, the propylene concentration is 1 relative to nitrogen monoxide in the exhaust gas.
．． The same operation as in Example 1 was performed except that the mixed gas temperature was set at 0 (molar ratio) and the mixed gas temperature was variously changed.

The results are shown in FIG. Example 5 The same operation as in Example 4 was performed except that the oxygen concentration was 2% by volume and the oxidation promoter was propane or formaldehyde.

The results are shown in FIG. Figures 7 and 8 show that when propylene and formaldehyde are used as oxidation promoters, the rate constant is constant regardless of temperature, but when propane is used, the higher the temperature, the larger the rate constant. I understand.

As described above, when carrying out the method of the present invention, there is a temperature range that is effective for the oxidation reaction of nitrogen monoxide depending on the type of oxidation promoter used, so a suitable temperature range should be selected.

Example 6 Example 1 except that the propylene concentration was set to 1.0 (molar ratio) to nitrogen monoxide in the exhaust gas in Example 1.
The operation was carried out under the same conditions as above, and the power consumption corresponding to the oxidation rate of nitric oxide was measured.

Compare this power consumption with the power consumption of the conventional ozone oxidation method (the power efficiency of the ozonator when oxidizing nitrogen monoxide at a concentration of 20 pm to nitrogen dioxide is 2 W・hr/k9-03). and shown in Table 1. Table 1 As is clear from Table 1 above, according to the method of the present invention,
It can be seen that the power consumption is less than 50% compared to the conventional ozone oxidation method.

Furthermore, since increasing the inner diameter of a cylindrical electrode tends to increase the rate constant, it is possible to further reduce the discharge power on a practical scale. Therefore, the method of the present invention is fully effective industrially.

The method of the present invention is preferably carried out in an existing electrostatic precipitator; in this case, it is possible to simultaneously remove fine dust coexisting in the combustion exhaust gas and oxidize nitrogen monoxide, and it is also more economical. It is. ,

[Brief explanation of drawings]

FIGS. 1 and 2 are graphs showing the relationship between the discharge power per unit gas flow rate and the oxidation rate of nitrogen monoxide. FIG. 3 shows an example of an apparatus for carrying out the method of the invention. FIG. 4 is a graph showing the results of Example 1;
The figure is a graph showing the results of Example 2. 6 is a graph showing the results of Example 3, FIG. 7 is a graph showing the results of Example 4, and FIG. 8 is a graph showing the results of Example 5. In Fig. 3, 1 is a cylindrical electrode, 2 is a wire electrode, 3 is a high voltage generator, 4 is a combustion exhaust gas inlet, 5 is a gaseous oxidation promoter inlet, 6 is a liquid oxidation promoter inlet,
7 indicates a carburetor, and 8 indicates an oxidized exhaust gas outlet. Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8

Claims (1)

[Scope of Claims] 1. In the presence of oxygen, water vapor and carbon dioxide gas, a compound selected from the group consisting of olefinic hydrocarbons, paraffinic hydrocarbons and oxygen-containing hydrocarbons is applied to combustion exhaust gas containing nitrogen monoxide. 0.1 to 3.0 of one or more compounds per mole of nitrogen monoxide in the combustion exhaust gas.
A type of combustion exhaust gas characterized in that the mixed gas is mixed in a molar ratio and then the resulting mixed gas is introduced into a corona discharge zone formed by applying a high voltage between a corona discharge electrode and a ground electrode. How to oxidize nitric oxide. 2 The discharge power in the corona discharge zone is 1W/Nm^3/
2. The method according to claim 1, wherein the spark discharge power is greater than or equal to hr and less than or equal to the spark discharge power. 3. The method according to claim 1, wherein the olefinic hydrocarbon is one or more compounds selected from the group consisting of ethylene, propylene, isobutene, and n-butene. 4. The method according to claim 1, wherein the paraffin hydrocarbon is one or more compounds selected from the group consisting of methane, ethane, propane, butane, and pentane. 5. The method according to claim 1, wherein the oxygen-containing hydrocarbon is an alcohol and/or an aldehyde. 6. The method according to claim 5, wherein the alcohol is one or more compounds selected from the group consisting of methanol, ethanol, propanol, and butanol. 7. The method according to claim 5, wherein the aldehyde is formaldehyde and/or acetaldehyde. 8. The method according to claim 1, wherein the temperature of the mixed gas is 100°C to 400°C. 9. The method according to claim 1, wherein the amount of oxygen present is 1 to 20% by volume of the total amount of combustion exhaust gas. 10. The method according to claim 1, wherein the amount of water vapor present is 5 to 15% by volume of the total amount of combustion exhaust gas. 11. The method according to claim 1, wherein the amount of carbon dioxide present is 3 to 15% by volume of the total amount of combustion exhaust gas.