JP3083915B2 - Removal method of low concentration nitrogen oxides - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a gas containing nitrogen oxides. For more information,
The present invention relates to a method for efficiently removing low-concentration nitrogen oxides contained in the air and the like.

[0002]

2. Description of the Related Art In recent years, air pollution by nitrogen oxides has become a major social problem. In order to prevent air pollution, countermeasures against emission sources have been emphasized in the past, and large boilers and heating furnaces in thermal power plants and chemical factories have been required to install denitration equipment to remove nitrogen oxides. In addition, an efficient denitration apparatus that meets this demand has been put to practical use. Nevertheless, the pollution of atmospheric nitrogen oxides is nevertheless increasing year by year. One of the pollution sources is a mobile emission source such as a car, and the pollution is particularly remarkable in a traffic-intensive area such as around a highway or a large city. Moreover, it is difficult to install a sufficient denitration device with these mobile discharge sources. Therefore, it is planned to directly remove nitrogen oxides from the periphery of a main road having a relatively high concentration of nitrogen oxides and the atmosphere of the tunnel.

[0003] Conventional techniques for removing nitrogen oxides have mainly been developed for treating flue gas as a countermeasure to the generation source. In the case of flue gas, the nitrogen oxides contained are 100-1
In addition to the relatively high concentration of 2,000 ppm by volume, a high temperature of 300 ° C. or more can be obtained relatively easily at the boiler outlet. Things could be removed efficiently.

[0004]

However, the concentration of nitrogen oxides in the atmosphere is very low, less than 1 ppm by volume, usually about 0.06 ppm by volume, and the temperature is as low as around room temperature. Therefore, the removal of nitrogen oxides directly from the atmosphere containing nitrogen oxides is a condition compared to the conventional technology for removing nitrogen oxides for treating flue gas as a source control measure. Quite differently, the conventional technology for removing nitrogen oxides for high temperature and high concentration was extremely inefficient. In addition, since removal of nitrogen oxides contained in the atmosphere is a non-productive business, the cost is severely restricted, and a more efficient method for removing low-concentration nitrogen oxides has been demanded from this viewpoint.

[0005] For example, Japanese Patent Publication No. 56-18246 discloses that activated carbon has a high nitrogen oxide adsorption ability, and nitrogen oxides in a gas are adsorbed on activated carbon and removed.
Then, a method is disclosed in which activated carbon is heated in ammonia gas to decompose and remove adsorbed nitrogen oxides. However, in this method, in removing nitrogen oxides, expensive and toxic ammonia must be added in a large excess of 1 to 10 times the amount of adsorbed nitrogen oxides, which is inefficient.

Accordingly, the present invention is to efficiently adsorb and remove nitrogen oxides even in a gas containing a low concentration of nitrogen oxides in order to adsorb and remove nitrogen oxides contained in the atmosphere and the like. It is an object of the present invention to provide a method for removing nitrogen oxides.

[0007]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a method for manufacturing a semiconductor device having a nitrogen oxide concentration of 50 vol.
m, a step of contacting a gas containing a low concentration of nitrogen oxides with activated carbon fibers to adsorb the nitrogen oxides to the activated carbon fibers ;
Each step of heating to 500 ° C. to obtain a gas containing concentrated nitrogen oxides by desorbing nitrogen oxides, and a step of removing nitrogen oxides from the gas containing concentrated nitrogen oxides And a method for removing low-concentration nitrogen oxides.

[0008] The present invention has the above configuration,
Low concentrations of nitrogen contained in the atmosphere, etc.
Elemental oxides can be efficiently removed. Here
Elemental oxides (NOx) are NO, NO_Two, N_TwoO, N_Two
O_Three, N_TwoO _Four, N_TwoO_FiveOr any mixture of them
Objects (eg NO and NO_TwoAnd a mixture thereof).

The "gas containing a low concentration of nitrogen oxides" in the present invention includes air containing nitrogen oxides and the like, for example, air along roads and in tunnels. Here, "low-concentration nitrogen oxides" means nitrogen oxides having a concentration low enough that it is difficult to efficiently treat them in a conventional denitration apparatus, and does not necessarily mean a specific value, but is usually , The concentration of nitrogen oxides is 50
The case where the capacity is not more than ppm, particularly 1 ppm or less corresponds to this.

The present invention provides a method for removing nitrogen oxides at a low concentration using an adsorbent, wherein activated carbon fibers are used as the adsorbent. The reason why the activated carbon fiber is employed in the present invention is as follows. That is, as the nitrogen oxide adsorbent, metal salts, silica gel, zeolite, activated carbon and the like can be considered. However, although metal salts, silica gel, and zeolites are effective for nitrogen dioxide (NO ₂ ), they have almost no adsorption capacity for nitrogen monoxide (NO), which is a main component of nitrogen oxides, and only activated carbon has NO. It is effective for the adsorption of nitrogen oxides. This is because activated carbon not only functions as an adsorbent but also as an oxidation catalyst.
Large oxidation capacity after oxidation to ₂ . The reason why activated carbon fiber is selected from activated carbon is that its nitrogen oxide adsorption capacity is higher than that of activated carbon such as granular activated carbon. This is described, for example, in Japanese Patent Publication No. 56-37865. This high adsorption capacity is particularly suitable for the present invention. In addition, the activated carbon fiber has an absorption / desorption speed which is nearly 10 times faster than that of the granular activated carbon, and enables efficient operation of the apparatus.

The activated carbon fibers used as the activated carbon in the present invention are fibrous, so that the efficiency of contact with the gas is good, so that the size of the apparatus can be reduced. In general, when installing a denitration apparatus along an existing road or in a ventilation system of a tunnel, the size of the apparatus is an essential requirement because the installation volume is largely restricted. For that purpose, the diameter of the activated carbon fiber is preferably 3 to 15 μm. The reason is that if it is less than 3 μm, the bulkiness of the fiber cannot be obtained and the airflow resistance increases, and if it exceeds 15 μm, the efficiency decreases because the outer surface area decreases.

The activated carbon fiber used in the present invention is obtained by converting an organic fiber into a porous carbon fiber through each of the steps of flame resistance, carbonization, and activation. Depending on the type of raw material, a polyacrylonitrile (PAN) system is used. In addition, various types such as rayon type, phenol type and pitch type are used. Among these activated carbon fibers, the PAN-based activated carbon fibers contain a nitrogen compound as shown in, for example, Japanese Patent Publication No. 56-37865, and are therefore excellent in the ability to adsorb nitrogen oxides. It is particularly advantageous in the invention.

The specific surface area of the activated carbon fiber is preferably 2
00 to 1000 m ² / g, particularly preferably 300 to 70
When it is 0 m ² / g, it has many pores having a diameter close to the molecular diameter of the nitrogen oxide to be adsorbed, and thus is particularly suitable for nitrogen oxide adsorption. When the specific surface area of the activated carbon fiber is 200
If it is less than m ² / g or more than 1000 m ² / g, the activated carbon fibers have many pores having a diameter different from the molecular diameter of the nitrogen oxide, so that the affinity for the nitrogen oxide decreases.

The activated carbon fiber can be used in the form of felt, woven fabric, tow, chop, and the like. Further, in the present invention, activated carbon fibers having a metal compound impregnated thereon and having an improved ability to adsorb nitrogen oxides may be used. These activated carbon fibers are used, for example, layered and packed in a column or the like. Next, the process of the method for removing low-concentration nitrogen oxides of the present invention will be described in more detail.

A large amount of raw gas containing a low concentration of nitrogen oxides is introduced into the packed bed of activated carbon fibers and brought into contact with the activated carbon fibers. Only the nitrogen oxides of the raw gas contacted with the activated carbon fibers are selectively adsorbed on the activated carbon fibers. The adsorption temperature is preferably 100 ° C. or less. The reason is that if the temperature exceeds 100 ° C., the adsorption capacity decreases so as not to withstand actual use. Further, the water content of the gas is preferably 3% by volume or less because it inhibits adsorption of nitrogen oxides, and it is also preferable to dehumidify the gas before introducing the activated carbon fiber into the packed bed.

A breakthrough curve when nitrogen oxides are adsorbed on activated carbon fibers can be generally shown schematically in FIG. The adsorption of nitrogen oxides can be performed until the activated carbon fibers are saturated with nitrogen oxides (A in FIG. 1).
Point), until the nitrogen oxides contained in the gas are no longer adsorbed and pass through the activated carbon fiber (point B in FIG. 1), or the nitrogen oxides that have passed have a permissible specific concentration. The process is performed until it exceeds (point C in FIG. 1).

Next, the nitrogen oxides are desorbed from the activated carbon fibers by heating the activated carbon fibers on which the nitrogen oxides are adsorbed in an inert gas. Inert gas is a gas that does not directly react with activated carbon fibers, and includes rare gases such as helium and argon, as well as nitrogen, carbon dioxide, water vapor,
Alternatively, oxygen may be contained in a trace amount of 1% by volume or less, and these gases may be used alone or as a mixture. In this case, by adjusting the flow rate of the inert gas and the desorption time with respect to the flow rate of the raw gas and the adsorption time, it is possible to obtain a gas containing nitrogen oxides concentrated to a concentration suitable for the denitration processing performed following the desorption processing. Can be. Since the inert gas is performed at a small flow rate with respect to the large flow rate of the raw gas, the nitrogen oxide to be desorbed can be a concentrated gas. At this time, the average concentration C of the gas obtained by the desorption
₂ (ppm by volume) is given by the following equation.

[0018]

(Equation 1)

Here, C ₁ is the concentration (ppm by volume) of low-concentration nitrogen oxides contained in the raw gas used for adsorption, F ₁
Is the flow rate (m ³ / hr), and t ₁ is the adsorption time (hr). On the other hand, F ₂ is the flow rate (m ³ / hr) of the concentrated gas obtained by desorption, and t ₂ is the desorption time (hr), which is equal to the time during which the inert gas having the flow rate F ₂ used for desorption is flowed. .

The temperature for desorption is preferably 100 to 500 ° C. The reason is that if the temperature is lower than 100 ° C., all the nitrogen oxides cannot be recovered due to insufficient desorption, and if the temperature exceeds 500 ° C., the wear of the activated carbon fibers increases even in an inert gas. In addition, the activated carbon fiber after desorption can be repeatedly used as an adsorbent. Finally, the gas containing the concentrated nitrogen oxides resulting from the desorption is sent to a conventional denitration apparatus for processing. The gas containing the concentrated nitrogen oxides may be adjusted to a concentration or a composition suitable for removal of nitrogen oxides before being transferred to the denitration apparatus.For example, the gas may be further diluted with air or oxygen. New components such as and ammonia may be added.

The conventional denitration apparatus used in the present invention may be any gas processing apparatus to which a known method for removing nitrogen oxides is applied. For example, the ammonia selective catalytic reduction method or the electron beam method is suitable for the present invention because its denitration apparatus is compact. Further, other methods of removing nitrogen oxides, such as an adsorption method using a chemical solution and a method of removing carbon materials such as coke by reducing them under heating, are also effectively used depending on the application.

Hereinafter, embodiments of the present invention will be described. The nitric oxide NO concentration in the examples was obtained by a chemiluminescence densitometer. The denitration rate was determined by the following equation.

[0023]

(Equation 2)

[0024]

Example 1 A specific surface area of 500 m ² / g and a fiber diameter of 1
A felt of 2 μm PAN-based activated carbon fiber having a diameter of 2.4
The column was formed by punching out a circular shape of 2.5 cm and laminating it on a quartz tube having an inner diameter of 2.4 cm until the layer height became 2.5 cm, and fixing the upper and lower sides of the felt with a quartz mesh.

First, in the adsorption step, the column of activated carbon fiber is placed at room temperature (25.degree.
Air containing 0.15 ppm by volume of O was flowed at a flow rate of 200 liters / hour, and was adsorbed for 750 hours until nitrogen monoxide NO could not be completely adsorbed on the activated carbon fibers and began to pass. Next, as a regeneration step, the column was heated to 400 ° C. while flowing nitrogen at a flow rate of 10 liters / hour through the column of activated carbon fibers to which the nitric oxide was adsorbed. The average concentration of nitrogen oxides obtained after 1 hour of regeneration was 2150 volume pp
m, and a sufficient concentration effect was obtained.

Further, while adding air and adding ammonia so that the concentration in the gas becomes 2,000 ppm by volume, a commercially available denitration catalyst kept at 350 ° C. at a flow rate of 13 liter / hour (TiO ₂ as a carrier and vanadium as a main component) is used. When the mixture was passed through a honeycomb-shaped denitration catalyst to which an active ingredient as a component was attached, the denitration rate was 88%.

[0027]

Example 2 The procedure of Example 1 was repeated five times to examine the NO adsorption capacity, the concentration of NO concentrated by desorption, and the change in the denitration rate. The results are shown in Table 1 below.

[0028]

[Table 1]

According to Table 1, activated carbon fibers have a high concentration effect of nitrogen oxides, and not only can exhibit a stable adsorption removal effect even when used repeatedly, but also adsorb low concentration nitrogen oxides. It can be seen that even when the removal treatment is performed and then the nitrogen oxides are removed, the denitration rate is high and the nitrogen oxides can be removed efficiently.

[0030]

The present invention has the following effects by adopting the above configuration. (1) Nitrogen oxides can be efficiently adsorbed on activated carbon fibers to a large amount of gas containing nitrogen oxides at a low concentration, and further removed. (2) The nitrogen oxide adsorbed on the activated carbon fiber can be converted into a nitrogen oxide concentrated by regenerating the activated carbon fiber, and the concentrated nitrogen oxide is removed by a conventional method. It can be efficiently removed.

Therefore, the efficiency is remarkably improved as compared with the method in which a large amount of low-concentration nitrogen oxide gas is directly denitrated. (3) Even if activated carbon fibers are repeatedly used for the treatment of removing and absorbing nitrogen oxides, the effect can be stably exhibited. (4) Activated carbon fibers also have a dust-removing ability, and therefore have a high effect of protecting the subsequent denitration device.

(5) The method for removing low-concentration nitrogen oxides of the present invention is suitable for a ventilation device provided along a road or in a tunnel.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a breakthrough curve when nitrogen oxides are adsorbed on activated carbon fibers.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ identification code FI B01J 20/34 (58) Investigated field (Int.Cl. ⁷ , DB name) B01D 53/56 B01D 53/34 B01D 53/81 B01J 20/20 B01J 20/28 B01J 20/34

Claims (3)

(57) [Claims] (1) When the concentration of nitrogen oxide is 50 vol.
adsorbing nitrogen oxides to the active carbon fiber of the gas containing less low concentration of nitrogen oxides m is contacted with activated carbon fibers, the adsorbed activated carbon fibers (2) the nitrogen oxide 300
A step of obtaining a gas containing concentrated nitrogen oxides by heating to 500 ° C. to desorb nitrogen oxides; and (3) a step of removing nitrogen oxides from the gas containing concentrated nitrogen oxides. A method for removing low-concentration nitrogen oxides, comprising the steps (1) to (3). 2. The step of heating the activated carbon fiber having adsorbed nitrogen oxide to desorb the nitrogen oxide to obtain a gas containing concentrated nitrogen oxide is performed in an inert gas. The method for removing low-concentration nitrogen oxides according to claim 1, wherein: 3. The activated carbon fiber has a fiber diameter of 3
１５15 μm, having a specific surface area of 200-1000 m ² / g
3. The method for removing low-concentration nitrogen oxides according to claim 1 or 2.