JP3257697B2 - Nitrogen oxide removal method - 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 removing nitrogen oxides (hereinafter sometimes simply referred to as "NOx") contained in various kinds of exhaust gas such as exhaust gas in a car tunnel and exhaust gas in an indoor car parking lot. The present invention relates to a method for removing nitrogen oxide which can adjust the injection amount of ammonia used for a decomposition reaction and prevent ammonia and unreacted NOx from being released out of the system.

[0002]

2. Description of the Related Art As a method of removing NOx in exhaust gas, a method of reacting with ammonia in a reactor filled with a reduction catalyst and denitrifying is used. If the injection amount of this ammonia is too large, it will release ammonia to the atmosphere,
Conversely, if the amount is too small, unreacted NOx will be discharged and the NOx removal rate will be low. Further, when the NOx concentration in the exhaust gas fluctuates greatly, it is necessary to adjust the ammonia injection amount in accordance with the fluctuation of the concentration. If the adjustment is not performed properly, the above-described inconvenience occurs.

For this reason, Japanese Patent Application Laid-Open No. 60-257823 and Japanese Patent Publication No. 63-12650 have been proposed. The former is NOx
A control method in which excess ammonia is adsorbed on the catalyst in the reactor when the amount is small, and when a large amount of NOx is used, a large amount of ammonia is injected in a short time by using the above-mentioned adsorbed ammonia. This is a technology that can ensure the denitration follow-up without having to do this. However, the adsorption capacity of the catalyst is originally low, and there is a high possibility that the adsorbed ammonia leaks out of the system, which may cause secondary pollution.

In the latter technique, the control accuracy is improved by controlling the injection amount of ammonia by feedforward control based on the NOx concentration at the inlet of the reactor and by feedback controlling the injection amount by the NOx concentration at the outlet of the reactor. This is a method for measuring the amount of ammonia to be injected in a short time even when the NOx concentration on the inlet side of the reactor changes suddenly. This control method can improve the control accuracy, but if the NOx amount increases rapidly, a large amount of ammonia must be immediately injected in a short time, and the ammonia injection pipe and the ammonia tank are set large. And there is a problem that the equipment cost is high.

[0005]

SUMMARY OF THE INVENTION Accordingly, the present inventors
With the aim of providing a method for removing nitrogen oxides that does not require a large-capacity ammonia injection facility and that does not release ammonia or unreacted NOx to the outside of the system, studies have been repeated and the present invention has been carried out. completed.

[0006]

According to the present invention, which has achieved the above objects, there is provided a nitrogen gas which is decomposed in a reactor filled with a reduction catalyst by adding ammonia to a desorption gas desorbed from a nitrogen oxide adsorbent. In the method for removing oxides, the desorption gas is circulated in a circulation system including the adsorbent and the reactor during the desorption step, and the nitrogen oxide concentration that changes depending on the amount of desorption on the reactor inlet side is During the period from the start of desorption to the time when it becomes smaller, a predetermined amount of ammonia calculated in advance is supplied into the circulation path, and after the change in the nitrogen oxide concentration at the reactor inlet side becomes smaller, The gist of the invention is to adjust the amount of ammonia supplied in accordance with the change in the flow rate of nitrogen oxides on the inlet side of the reactor. Further, in the method for removing nitrogen oxide decomposed in a reactor filled with a reduction catalyst by adding ammonia to the desorbed gas desorbed from the nitrogen oxide adsorbent, the desorbed gas may be an adsorbent in the desorption step. When the change at the reactor inlet side of the nitrogen oxide concentration, which varies with the amount of desorption, is small from the start of desorption, the nitrogen oxide flow rate at the reactor inlet side The ammonia supply amount may be adjusted according to the change.

[0007]

FIG. 1 is an explanatory view showing an embodiment of a NOx removing apparatus used in the present invention. Treated gas supply pipe 1 is the adsorption tower 4a of two towers, 4b automatically opening and closing valve (hereinafter simply referred to as a valve) through a V _1, V ₂ is connected to, adsorption column 4a,
The downstream side of the 4b is adsorbed exhaust gas discharge pipe 5 is connected via a valve V _3, V _4. The adsorption tower 4a, desorption gas exhaust pipe 2 via a valve V _7, V ₈ is connected to the downstream side of the 4b,
Exhaust extraction pipe 2 is connected through a valve V ₁₁ to the circulation pipe 8,
Blower 12 to the circulation pipe 8, a heater 7 and the reactor 10 is provided, the adsorption tower 4a downstream through the valve V _15, V _5, V ₆ of the reactor 10, on the upstream side of 4b Connected. And the blower 12 upstream in the circulation path 8 with connecting atmospheric air introducing pipe 6 via a valve V _12, on the upstream side of the reactor 10 for connecting the ammonia supply device 9 via a valve V _16.

[0008] The operation method of the above-mentioned removal device is as follows. By the gas to be treated for opening and closing the valves _{V 1, V 2, V 3} , V 4 containing NOx is introduced from the processing gas supply pipe 1 is introduced into one of the adsorption columns 4a, 4b, the tower NOx is adsorbed to the adsorbent, and the adsorbed exhaust gas from which NOx has been removed is discharged from the discharge pipe 5 to the atmosphere. This adsorption step is continuously performed by alternately using the adsorption towers 4a and 4b.

On the other hand, any of the adsorption towers in which the adsorption step is stopped performs, for example, a heating step, an adsorption tower cooling step, and a reactor preheating step, which will be described later, to perform desorption and decomposition of NOx adsorbed on the adsorbent. Regenerate the adsorption tower. Table 1 shows each step of the adsorption towers 4a and 4b and the open / closed state of each valve, and the horizontal axis shows one cycle of the adsorption tower. The stop step in the table indicates a step of stopping the operation of the adsorption tower and preheating the reactor.

[0010]

[Table 1]

FIG. 2 is an explanatory view showing a heating step of the adsorption tower 4b.
Performs an adsorption step of adsorbing NOx in the gas to be treated. Adsorption tower 4b in circulation passage C ₁ contrast
More valve V ₈ - the valve V ₁₁ - blower 12 heater 7 reactor 10-
The valve V ₁₅ - by flowing the desorbed gas in the circulation pipe 8 provided with a valve V _6, NOx to the desorption gas contained therein is heated is reduced and decomposed by ammonia. That is, the desorbed gas gradually desorbed from the adsorption tower 4b is heated and heated in the heater 7, and ammonia is added to the upstream side of the reactor 10 by an injection control method described later and introduced into the reactor 10. , Most of the NOx is reduced. A small amount of NO that could not be reacted in this reactor 10
x and ammonia flow to the downstream side, but since the NOx concentration in this gas is extremely low, this gas is returned to the adsorption tower 4b and used as a purge gas for desorbing NOx. And again the circulation pipe 8
Circulate. By repeating this circulation, the adsorption tower 4
All the NOx adsorbed on the adsorbent in b is desorbed, and this NOx is completely reduced by ammonia.

Therefore, no unreacted trace NOx or ammonia is released to the atmosphere. Incidentally valve V ₅ is left open for the by pressure relief of the circulation path pipe but suction pathway Ca
The amount of NOx and ammonia leaking to the side is very small, and since they are adsorbed via the adsorption tower 4a and released to the atmosphere, they are hardly released to the atmosphere.

FIG. 3 shows a cooling step of the adsorption tower.
As shown by the broken line, the air introduction pipe 6-blower 12-heater 7
(Stopped) - valve V ₁₃ - valve V ₁₄ - valve V ₈ - adsorption tower 4
through the system path C ₂ undergo b- valve V ₂ is introduced into the adsorption tower 4b fresh air, the temperature increase has been adsorbent in the heating step is cooled, suitable for adsorption of NOx in the next adsorption step Temperature.

[0016] FIG. 4 shows a preheating step in the reactor 10, the bypass pipe 13 a valve V ₁₄ - a closed system path C ₃ containing blower 12 heater 7 reactor 10 the valve V ₁₅ - valve V ₁₁ The formed gas is circulated while heating the internal gas, and the inside of the reactor 10 is preheated to a temperature suitable for the decomposition reaction of NOx and ammonia in the next heating step so that the reaction can be performed immediately. At this time, the adsorption tower 4b
Together with a complete rest, opening valve V ₅ leading to the suction pathway Ca, used as a pressure relief teeth boosted by heating.

Ammonia supplier 9 in the heating step
The control of the injection of ammonia from is performed as follows. FIG. 5 shows the NOx flow rate (solid line) on the inlet side of the reactor 10 when the adsorption tower is desorbed. The NOx amount (that is, the desorption amount) forms a peak in the early stage of desorption, and thereafter the flow rate gradually decreases. Therefore, the change in NOx concentration in the initial stage of desorption is large (0 to t in the desorption step), and thereafter, the change in NOx concentration is small. Therefore, in the present invention and injecting a certain amount of ammonia desorbed initial as shown by the broken line from the reactor 10 inlet side of the circulation path C _1, circulation path also remain ammonia or NOx unreacted Circulating in C ₁ (see FIG. 2),
These are prevented from being released out of the system and the reduction reaction proceeds. Therefore, a large amount of ammonia is not supplied in a short time, and it is not necessary to provide a large-capacity ammonia supply equipment, and the equipment cost can be reduced.

The ammonia injection amount in the initial stage of the desorption is preferably calculated based on the NOx adsorption integrated value in the adsorption step before the desorption step, and the ammonia injection amount suitable for the NOx desorption flow rate during this period is determined. decide.

After the change in the NOx concentration becomes small, the NOx concentration and the gas flow rate on the inlet side of the reactor 10 are measured, and N
Calculating the Ox flow, to adjust the ammonia injection amount according to the NOx flow rate change, unreacted ammonia and NOx during the desorption step is completed does not remain in the circulation path C ₁ while so as not to inject the excess ammonia Like This prevents NOx and ammonia from being released out of the system when the next adsorption tower cooling step is performed.

(Embodiment) Using the apparatus shown in FIG.
When the exhaust gas having an average concentration of 3 ppm was treated at a flow rate of 5000 Nm ³ / h, the denitration rate and ammonia leakage concentration were measured. However, one cycle of the adsorption-regeneration step in one adsorption tower is 24 hours, and the ammonia injection rate at the beginning of the desorption step (0.5 to 1 hour from the start of the desorption step) by the method of the present invention is 30 Nl / h. Was adjusted and controlled to an ammonia injection amount of 0.95 Nl / h per 1 Nl / h of NOx flow rate. However, ammonia was not injected until 0.5 hour from the start of the desorption step because the NOx concentration was low.

As a result, the adsorption rate in the adsorption step was 85%.
As described above, the NOx removal rate in the desorption step was 99% or more. Ammonia leaking into the atmosphere is 0.1%
It became possible to be below ppm. The apparatus used in the method of the present invention is not limited to the one shown in FIG. 1, but may be one in which a two-column type adsorption tower is replaced with a rotary rotor type adsorbent.

[0020]

Since the present invention is configured as described above, it is possible to prevent unreacted ammonia and NOx from being released to the outside of the system, and to reduce the equipment cost without requiring a large-scale ammonia supply equipment. be able to.

[Brief description of the drawings]

FIG. 1 is an explanatory view showing an embodiment of a NOx removing device used in the present invention.

FIG. 2 is an explanatory diagram showing a heating step of an adsorption tower 4b in the apparatus shown in FIG.

FIG. 3 is an explanatory view showing a cooling step of the adsorption tower 4b in the apparatus shown in FIG.

FIG. 4 is an explanatory view showing a preheating step of a reactor 10 in the apparatus shown in FIG.

FIG. 5 is a graph showing an example of adjustment of an ammonia injection amount according to the present invention.

[Explanation of symbols]

 4a, 4b Adsorption tower 8 Circulation pipe 9 Ammonia feeder 10 Reactor

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroshi Sato 6-70-2-C-1011 Shin-Matsudo, Matsudo City, Chiba Prefecture (72) Inventor Kiyoshi Chiba 2-16-11-204, Kinuta, Setagaya-ku, Tokyo (72) Inventor Naoya Konno 4-21-50-311 Mukodaicho, Tanashi-shi, Tokyo (72) Inventor Fumihiko Kasuya 1-3-18 Wakihamacho, Chuo-ku, Kobe-shi, Hyogo Kobe Steel, Ltd.Kobe Head Office (72) Invention Kobe Steel Co., Ltd.Kobe Steel Co., Ltd.Kobe Head Office (72) Inventor Kunio Osaka Kunio Osaka 1-3-1 Wakihamacho, Chuo-ku, Kobe City, Hyogo Prefecture Kobe Steel, Ltd. Kobe Head Office (72) Inventor Ryoyoshi Sugioka 1-3-18, Wakihama-cho, Chuo-ku, Kobe City, Hyogo Prefecture Kobe Steel, Ltd.Kobe Head Office (72) Inventor Akishi Fujimoto 1 Wakihama-cho, Chuo-ku, Kobe City, Hyogo Prefecture Kobe Steel, Ltd.Kobe Head Office (72) Inventor Shigeki Yamanaka 1-3-18, Wakihama-cho, Chuo-ku, Kobe City, Hyogo Prefecture Kobe Steel Co., Ltd.Kobe Head Office (56) References JP-A-4-78421 (JP, A) JP-A-3-188820 (JP, A) JP-A-3-186318 (JP, A) JP-A-5-192535 (JP, A) JP-A-5-253444 (JP, A A) JP-A-4-250822 (JP, A) JP-A-5-253445 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) B01D 53/34-53/94 B01J 20 / 00-20/34

Claims (3)

(57) [Claims] 1. A method for removing nitrogen oxides which decomposes in a reactor filled with a reduction catalyst by adding ammonia to a desorption gas desorbed from a nitrogen oxide adsorbent, wherein the desorption gas is used during the desorption step. Circulates in the circulation system including the adsorbent and the reactor, and the nitrogen oxide concentration that varies with the
Change of, time to decrease from the start of the desorption, and to supply precalculated quantity of ammonia into the circulation path, the change in the reactor inlet side of the nitrogen oxide concentration is reduced After that, the ammonia supply amount is adjusted in accordance with the change in the nitrogen oxide flow rate on the reactor inlet side. 2. The method according to claim 1, wherein the amount of ammonia supplied during a period until the change in the concentration of nitrogen oxides becomes small is calculated based on the integrated value of nitrogen oxide adsorption in the adsorption step.
The method for removing nitrogen oxides according to the above. 3. A desorption gas desorbed from a nitrogen oxide adsorbent.
Reactor with ammonia added to it and filled with a reduction catalyst
In the method for removing nitrogen oxides decomposed in a gas , the desorption gas includes an adsorbent and a reactor in the desorption step.
Circulates in the circulation system, at the inlet of the reactor where the nitrogen oxide concentration varies depending on the amount of desorption
The method for removing nitrogen oxides , characterized in that, when the change in the nitrogen oxides is small from the start of desorption, the amount of ammonia supplied is adjusted according to the change in the nitrogen oxide flow rate at the reactor inlet.