JPH0899016A - Method for denitrification of exhaust gas - Google Patents

Abstract

PURPOSE: To decrease NOx in an exhaust gas and to solve thereby a problem of unreacted ammonia by mixing NH3 in the exhaust gas contg. NOx and irradiating this mixed gas with an ultraviolet ray with a specified wavelength. CONSTITUTION: Test gases such as NO, N2 , CO2 , O2 and NH3 controlled into an arbitrary flow rate by means of a gas flow rate controller 4 are mixed uniformly by means of a gas mixer 5 and are introduced into a reactor 2 with irradiation windows 3 made of a synthetic quartz on its both ends. H2 O in the test gases is fed by means of a humidifier 6. The test gases coming out of the reactor 2 pass through a filter for collecting reaction products and are guided into an NH3 analyzer 9 equipped with a recorder 10 and are then exhausted into the atmosphere. The gas mixer 5 and the reactor 2 are stored in a thermostat bath which kept at 350 deg.C by imaging the outlet of a denitrification apparatus. As the light source of ultraviolet rays, an excimer laser 1 is used and the ultraviolet rays with wavelength of 170-230nm are used for irradiation. As the result, NH3 is decomposed into (NH2 +H), which is reacted with NOx to decrease NOx.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for denitration of exhaust gas using ammonia as a reducing agent.

[0002]

2. Description of the Related Art Recently, a dry flue gas denitration device that catalytically reduces nitrogen oxides in exhaust gas discharged from various combustion devices in the presence of ammonia in view of environmental regulations has many advantages over a wet denitration device. Since it has, it has come to be used frequently in recent years.

Above all, a denitration apparatus using a catalyst for removing nitrogen oxides having a honeycomb-shaped, lattice-shaped or plate-shaped structure has a simple structure and a small pressure loss, and the catalyst is clogged by dust contained in exhaust gas. It is most practically used because it has many excellent advantages such as low number.

However, in the above method, it is very difficult for NOx and ammonia to react in the presence of a catalyst, and for NOx to be completely reacted with ammonia and NOx. Moreover, the performance of the catalyst used for denitration generally deteriorates during use, but if the performance deteriorates, the unreacted ammonia concentration at the outlet of the denitration device increases. Also, like exhaust gas from heavy oil burning and coal burning, SOx, especially SO ₃
When present, the unreacted ammonia concentration reacts with unreacted ammonia to form ammonium ammonium sulfate, which causes problems such as clogging of a heat exchanger such as an air preheater.
It is considered to be the life of the catalyst when it reaches (ppm).

[0005]

As described above, in order to reduce unreacted ammonia, it is necessary to increase the required amount of catalyst, or to add or replace it, and the filling amount of the NOx removal catalyst increases. However, there is a problem in that the addition or replacement of the catalyst is frequently required and the work becomes complicated.

The present invention is an exhaust gas denitration method in which NOx in exhaust gas is reduced by decomposing ammonia into (NH ₂ + H) and reacting (NH ₂ + H) with NOx, thereby eliminating the problem of unreacted ammonia. Is provided.

[0007]

According to the present invention, an exhaust gas containing NOx is mixed with NH _3, and then the mixed gas is irradiated with a wavelength 1
It is characterized by irradiating UV light of 70 nm to 230 nm, decomposing NH ₃ into (NH ₂ + H) by the UV light, and reacting (NH ₂ + H) with NOx.

That is, in the present invention, the amount of ammonia to be injected (generally NH
It is possible to easily keep the denitration efficiency constant by increasing the ₃ / NOx molar ratio).

However, since the molar ratio is increased as shown in FIG. 1, unreacted ammonia is increased. The unreacted ammonia is limited by the problem of clogging of heat exchangers such as air preheaters installed downstream of the denitration equipment due to ammonium ammonium sulfate and the emission standard, which determines the life of the catalyst.
In addition, the performance of the denitration catalyst generally deteriorates due to poisoning components contained in the exhaust gas. In this case as well, the amount of unreacted ammonia increases, and as a countermeasure against this, addition and replacement of the catalyst are required.

From the above, by decomposing ammonia and reacting with NOx, an economical exhaust gas denitration device can be installed.

As a method of decomposing ammonia and reacting it with NOx, a photochemical reaction is used.
It is possible to decompose NH ₃ efficiently with UV light of nm.

Ammonia is 170 nm as shown in FIG.
It absorbs ultraviolet light of ˜230 nm and decomposes by the reaction shown by the following formula. NH ₃ + Hν → NH ₂ + H

Ammonia decomposed by the above formula reacts with NOx in the exhaust gas and is finally decomposed into nitrogen and water. The reaction formula is as follows. NO + NH ₂ → N ₂ + H ₂ O

As described above, denitration can be performed without producing unreacted ammonia.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 3 shows the overall structure of an experimental apparatus used for carrying out the method of the present invention, in which NO, N ₂ and CO adjusted to arbitrary flow rates by a gas flow rate controller 4. ₂ ,
Test gases such as O ₂ and NH ₃ are uniformly mixed by a gas mixer 5, and enter a reactor 2 having irradiation windows 3 made of synthetic quartz material at both ends. H ₂ O in the test gas is supplied by the humidifier 6. The test gas discharged from the reactor 2 passes through a filter 7 for the purpose of collecting reaction products, is guided to an NH ₃ analyzer 9 equipped with a recorder 10, and is discharged to the atmosphere. The gas mixer 5, the reactor 2 and the like are housed in a constant temperature bath, and the temperature is set to 350 ° C assuming the outlet of the denitration device. A low-pressure mercury lamp or an excimer laser can be used as a light source of ultraviolet rays, but in this device, an excimer laser 1 is used.
Irradiation with UV light of 3 nm. The feed gas composition used in the experiment is as follows. _{NO: 50ppm NH 3: 6.5ppm O} 2: 5%
CO ₂ : 10% H ₂ O 10% N ₂ : remaining

In order to confirm the analysis and removal of NH ₃ by ultraviolet light, the feed gas was fed into the reactor 2 at a space velocity of 3,000 h ^-1.
And was irradiated with 193 nm excimer laser light. The results are shown in Fig. 4, which shows that NH ₃ was completely decomposed by irradiating the laser, and an equivalent amount of NO ₃ was generated.
It can be seen that x has been removed.

[0017]

As described above in detail, according to the present invention, the exhaust gas containing NOx is mixed with NH _3, and then the mixed gas is irradiated with ultraviolet light having a wavelength of 170 nm to 230 nm, thereby eliminating the problem of unreacted ammonia. A denitration method that has been resolved can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a denitration rate, an unreacted ammonia concentration, and an ammonia injection amount.

FIG. 2 is a diagram showing an ultraviolet absorption spectrum of NH ₃ .

FIG. 3 is a cross-sectional explanatory view showing an experimental device of the present invention.

FIG. 4 is a diagram showing the result of decomposition of ammonia in ultraviolet light of 193 nm.

[Explanation of symbols]

1 Excimer Laser Transmitter 2 Reactor 3 Quartz Irradiation Window 4 Gas Flow Controller 5 Gas Mixer 6 Humidifier 7 Filter 8 Constant Temperature Bath 9 NH ₃ Analyzer 10 Recorder

Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location B01D 53/34 129 B

Claims (1)

[Claims] 1. A method for denitrifying exhaust gas, which comprises mixing NH ₃ with exhaust gas containing NOx and irradiating the mixed gas with ultraviolet light having a wavelength of 170 to 230 nm.