JPS61133125A - Denitration process using ultraviolet ray - Google Patents

Abstract

PURPOSE:To perform denitration at low cost by irradiating a gas contg. NOx with ultraviolet rays in the absence or presence of NH3 after passing the gas through a mixture of TiO2 or ZnO and/or compds. of alkali or alkaline earth metal compd. CONSTITUTION:An NOx-contg. gas is passed through TiO2 or ZnO or their mixture, or through a mixture consisting of the above-described compd. or a mixture thereof with an oxide or hydroxide of alkali metal or alkaline earth metal. During passing of the gas, the gas is irradiated with near ultra violet rays of 200-400nm wavelength or far ultra violet rays having <=200nm wavelength. In this case, NH3 may be present or absent. When NH3 is absent, NOx is transformed to HNO3 and fixed to the metal oxide. When NH3 is present, it is decomposed to N2 and H2O.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an ultraviolet ray denitration treatment method for oxygen-containing gas containing nitrogen oxides. More specifically, in the presence of titanium oxide or zinc oxide alone or a mixture thereof, or a mixed metal oxide obtained by adding an alkali metal or alkaline earth metal oxide or hydroxide to these, as necessary. The present invention relates to a method for effectively removing or decomposing nitrogen oxides by irradiating an oxygen-containing gas containing nitrogen oxides with ultraviolet rays in the presence of ammonia.

[Conventional technology] Nitrogen oxides (NOx), such as nitric oxide (NO) and nitrogen dioxide (Not), contained in automobile exhaust gas and factory exhaust gas, are attracting attention as environmental pollutants and are strictly regulated. ing. Conventionally, methods for purifying NOx contained in exhaust gas from fixed sources such as factories include:
Generally employed are a non-catalytic reductive cracking method in which the reaction is carried out with ammonia at a high temperature of 800° C. or higher, and a catalytic cracking method in which the reaction is carried out by heating to a temperature of 200° C. or higher in the presence of a catalyst. In addition, for automobile exhaust gas, 300℃
The method described above uses a catalyst to reduce NOx with carbon monoxide and hydrogen contained in exhaust gas.

However, all of these utilize high-temperature reactions, and it is necessary to heat a large amount of gas containing a trace amount of NOX to a high temperature.

Therefore, in order to apply this method for the purpose of treating NOx-containing gas at room temperature, it is necessary to heat it to a high temperature, denitrate it, and then cool it again, making it unsuitable as a room temperature denitrification method.

Recently, N has been oxidized by the action of ozone or by CI Ot.
The oxidation absorption method involves oxidizing all Ox to 802 and absorbing it in an alkaline solution, or the oxidation absorption method in which all Ox is oxidized to 802 and NO is absorbed in an aqueous solution of Fe (It-ethylenediaminetetraacetic acid) to form Fe(1) (EDTA
) (NO) complex and reacting it with sodium sulfite has been proposed as a room temperature treatment method, but it requires expensive ozone generator equipment and requires very complicated wastewater treatment equipment. The problem is that processing costs are high due to the necessity of

[Problems to be Solved by the Invention] An object of the present invention is to solve the problems of the prior art described above and to provide a method for effectively and economically treating 140x-containing gas without special heating. There is a particular thing.

[Means for Solving the Problem] The NOX-containing gas treatment method of the present invention includes adding titanium oxide or zinc oxide alone or in combination, or an alkali metal or alkaline earth metal oxide to an oxygen-containing gas containing NOx. or in the presence of mixed metal oxides consisting of hydroxides.

It is characterized by irradiating ultraviolet rays in the presence of ammonia if necessary.

[Operations and Examples] The present inventors have been conducting research on excitation chemistry using ultraviolet rays for some time, and as one of the studies, when a gas containing NOx and ammonia is irradiated with vacuum ultraviolet rays with a wavelength of 200 ni or less at air temperature. They discovered that the two can react and decompose into water and nitrogen, rendering them harmless. However, since the vacuum ultraviolet ray generation efficiency of artificial light sources is low, the economic efficiency of denitrification and detoxification methods that apply this reaction cannot necessarily be said to be excellent.

As a development of these basic studies, the present inventors have discovered that titanium oxide or zinc oxide alone or in combination, or a mixed metal oxide consisting of an oxide or hydroxide of an alkali metal or alkaline earth metal, coexists with titanium oxide or zinc oxide. In some cases, the present inventors have discovered that the decomposition reaction can occur efficiently even by irradiation with near ultraviolet rays having a wavelength of 200 nll1 or more, and have completed the present invention. According to the method of the present invention, in addition to being able to use inexpensive low-pressure mercury lamps that are normally used as germicidal lamps, near-ultraviolet rays contained in sunlight can also be used. Economic efficiency will improve dramatically.

NOx in the present invention refers to NO or NO□ alone or as a mixture, and examples of the oxygen-containing gas containing these NOx include combustion gas or air. Further, the present invention provides NOx with a NOx concentration in the range of 1 to 10,000 pp-.
Although it is suitably applied to the treatment of oxygen-containing gases, there is essentially no concentration limit.

As mentioned above, the metal oxide used in the present invention is composed of titanium oxide or zinc oxide alone or in a mixture, or an oxide or hydroxide of these and an alkali metal or alkaline earth metal. Although the titanium oxide or zinc oxide in the present invention may have any crystal structure, the anatase type of titanium oxide exhibits particularly high activity. In addition, examples of the alkali metal oxides or hydroxides in the present invention include oxides or hydroxides of alkali metals such as lithium, sodium, potassium, rubidium, and cesium. Potassium oxides or hydroxides are preferred.

Further, examples of the oxides or hydroxides of alkaline earth metals include oxides or hydroxides of barium, magnesium, calcium, barium, and the like. The content form of these alkali metal or alkaline earth metal oxides or hydroxides is not particularly limited. A composite oxide may also be used, in which titanium oxide or zinc oxide alone or in a mixture is impregnated with an alkali metal or alkaline earth metal hydroxide such as sodium hydroxide, potassium hydroxide or barium hydroxide, or an oxide such as barium oxide. A mixture may also be used.

When ammonia coexists, 1 for NOx
Use ~2 times the amount of ammonia.

In addition, the composition of titanium oxide or zinc oxide alone or as a mixture and an alkali metal or alkaline earth metal oxide or hydroxide may be 100% of titanium oxide or zinc oxide alone or as a mixture when ammonia is present.
0.1 to 30 parts by weight, or NO when fixing NOx without coexisting ammonia.
An amount of the alkali metal or alkaline earth metal oxide or hydroxide is used in an amount equal to or more than the amount of x removed.

In the present invention, the method of contacting these mixed metal oxides with the oxygen-containing gas containing NOx includes a method of scattering these mixed metal oxides in powder form in an oxygen-containing gas containing NOx, a method of scattering the mixed metal oxides as powder in an oxygen-containing gas containing NOx, and a method of scattering the mixed metal oxides in powder form in an oxygen-containing gas containing NOx. These metal oxides can be brought into direct contact with oxygen-containing gases including NOx, such as by leaving the powder still in the road, or by kneading it into a plaster-like shape and applying it to the inner wall of the reactor, allowing ultraviolet rays to efficiently oxidize the metal. Any method may be used as long as it is directly absorbed into objects. It is also possible to adopt a method of dispersing these metal oxides in a paint or pigment and painting the inner wall of the reaction device.

The ultraviolet rays in the present invention are near ultraviolet rays having a wavelength of 400 to 200 nl, but may also include vacuum ultraviolet rays having a wavelength of 200 rv or less. These ultraviolet rays are generated using ultra-high-pressure mercury lamps, high-pressure mercury lamps, xenon lamps, and low-pressure mercury lamps, either alone or in combination, but improvements have been made so that mercury and a third component coexist in the discharge tube to have wavelength distribution characteristics that match the purpose. A light source can also be used. It is also possible to use near ultraviolet rays in sunlight.

The principle of action of metal oxides in the present invention is currently under detailed research, but n-type semiconductors such as titanium oxide and zinc oxide absorb ultraviolet rays, and electrons in the conductive band of the n-type semiconductor are excited to the conductive band. , the holes generated in the conductive band oxidize the hydroxyl group present on the surface of the metal oxide with one electron to generate OH radicals, and these 0■ radicals oxidize NOx and change it to nitric acid, which becomes alkali. It is thought that it is captured by oxides or hydroxides of metals or alkaline earth metals. In addition, if ammonia coexists, N
The -electron oxidation reaction of H generates NH3 cation radical, which dissociates into NH2 radical and H9, and NH2
It is thought that the radicals react with NO and change into water and nitrogen. On the other hand, the electrons excited in the conductive band reduce 802 by one electron to produce nitrite ions, which react with water to produce nitrous acid, which is roughly oxidized to nitric acid, which becomes a metal intensifier. It is thought that it will be fixed. In general, it is thought that when ammonia coexists, the generated nitrous acid reacts with ammonia to produce ammonium nitrite, which decomposes into nitrogen and water on the metal oxide. In addition, oxides or hydroxides of alkali metals or alkaline earth metals decompose NOx by increasing the amount of NOx adsorbed on the surface of the metal oxide or by increasing the amount of hydroxyl groups on the surface of the metal oxide. It is presumed that the speed is increased.

Therefore, especially when ammonia is not allowed to coexist, NOX is fixed to the metal oxide as an alkali metal salt or an alkaline earth metal salt of nitric acid. Furthermore, when ammonia is present, NOX is not fixed on the metal oxide but is decomposed by the ammonia into nitrogen and water.

These reactions occur even in the coexistence of oxidizable organic or inorganic compounds such as hydrocarbons and hydrogen sulfide, and
This is a very advantageous denitrification method because it can occur at air temperature or lower temperatures without the need for heating, or even at high temperatures.

The present invention will be described below with reference to Examples, but the present invention is not limited to these Examples.

Examples 1 to 8 and Comparative Examples 1 to 5 Metal oxides 25011;l having the compositions shown in Table 1 were prepared by the crosstalk method and placed in a high-purity quartz cylindrical reactor (diameter 4.23, length 15α) so that the installation area is as large as possible (installation area 1
0a ri. 20o at 2.51 below the bottom of the metal oxide installation
A 4 watt sterilizing mercury lamp was installed, emitting near ultraviolet light of wavelengths above n-.

This reactor was filled with air and NO was introduced to a concentration of 1100 pp, a sterilizing mercury lamp was turned on, ultraviolet rays were irradiated from the outside, and the change in NOx concentration over time was measured to determine the half-life. The results are shown in Table 1.

[Left below] As can be seen from the results of Example 1 and Comparative Examples 1 to 3,
It was shown that when titanium oxide coexists with ultraviolet rays, the half-life of NOx is significantly shortened, and that irradiating titanium oxide with ultraviolet rays significantly increases the NOx reduction rate. Moreover, from the results of Examples 1.3.4.5, it can be seen that when titanium oxide is made to coexist with an alkali metal or alkaline earth metal hydroxide and irradiated with ultraviolet rays, the NO reduction rate further increases. Potassium hexatitanate, which is a composite oxide of potassium oxide and titanium oxide, also has great activity under ultraviolet irradiation.

On the other hand, the results of Comparative Example 4 and Examples 7 and 8 show that zinc oxide also exhibits activity under ultraviolet irradiation. It should be noted that when the nitrate ions produced on the gold silt were quantified after the reaction using these metal oxides, the amount decreased.

Nitrate ions were detected in an amount equivalent to NOx.

This fact indicates that NOX was oxidized to nitric acid and fixed to the metal oxide.

Examples 9 to 15 and Comparative Examples 6 to 8 The half-life of NOx was investigated by carrying out the same operation as in Example 1 except that all the metal oxides having the compositions shown in Table 2 were added and 20 Opp of ammonia was coexisting. The results are shown in Table 2.

[Left below] As can be seen from Comparative Example 6.7.8 and Example 9.12, when ultraviolet rays are irradiated in the presence of ammonia and titanium oxide and zinc oxide, the half-life time of NOx is significantly reduced. It can be seen that it is shortened. In addition, Example 1G,
11.13.14, it can be seen that the reaction rate increases roughly by adding an alkali or using a composite oxide with an alkali oxide.

Comparative Example 9 The same operation as in Example 10 was performed except that the reaction vessel was filled with nitrogen instead of air so that oxygen did not coexist, and when the half-life of NOx was investigated, the half-life was 120 minutes or more. However, NOx levels hardly decreased.

Claims (1)

[Claims] 1. Ultraviolet rays of an oxygen-containing gas containing nitrogen oxides, characterized in that the oxygen-containing gas containing nitrogen oxides is irradiated with ultraviolet rays in the presence of titanium oxide or zinc oxide alone or a mixture thereof. Denitration treatment method. 2. The treatment method according to claim 1, which is carried out in the presence of a mixed metal oxide consisting of titanium oxide or zinc oxide alone or a mixture thereof and an alkali metal or alkaline earth metal oxide or hydroxide. 3 Ultraviolet denitrification of an oxygen-containing gas containing nitrogen oxides, which is characterized by irradiating the oxygen-containing gas containing nitrogen oxides with ultraviolet rays in the presence of titanium oxide or zinc oxide alone or in a mixture thereof in the presence of ammonia. Processing method. 4. The treatment method according to claim 3, which is carried out in the presence of a mixed metal oxide consisting of titanium oxide or zinc oxide alone or a mixture thereof and an oxide or hydroxide of an alkali metal or alkaline earth metal.