JPH0775717A - Method for reducing nox in waste combustion gas - Google Patents

Abstract

PURPOSE:To efficiently denitrate waste combustion gas immediately after the start of a combustor through the normal running by forming a bypass having an ozonizer and an NOx adsorber to a passage for treated gas and bypassing the gas when the low-temp. waste combustion gas is treated immediately after the start. CONSTITUTION:A bypass 9 with an ozonizer 6 and an NOx adsorber 5 is formed to a passage 7 for the treated gas leaving an ammonia- -reducing denitrator 3 (denitrator). The treated gas contg. unreacted NOX leaving the denitrator 3 is passed through the bypass 9 when the denitrator 3 does not function sufficiently immediately after the start to oxidize the NO in the treated gas to the NO2 and N2O5 which are absorbed, and the unreacted NOx are removed. The bypass 9 is closed when the denitrator 3 functions sufficiently. Besides, a high-temp. desorbing gas is passed through the adsorber 5 while the bypass 9 is closed to regenerate the adsorbent.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing nitrogen oxides in flue gas discharged from a combustion device such as a gas turbine or a boiler.

[0002]

2. Description of the Related Art Conventionally, as a method for reducing nitrogen oxides in flue gas discharged from a combustion apparatus such as a gas turbine or a boiler, ammonia is used as a reducing agent and a catalyst such as vanadium is used to remove the nitrogen oxides from nitrogen and water. A general method is to use an ammonia reduction denitration device that decomposes into water and becomes pollution-free. This method has a high catalytic activity in a temperature range of about 250 to 400 ° C., has been practically used in a large-scale apparatus by an efficient denitration method, and has a lot of achievements. However, in the case of this method, the activity of the catalyst is extremely small in a low temperature range of 200 ° C. or lower, and a practical denitration rate cannot be obtained. On the other hand, immediately after starting a combustion device such as a gas turbine or a boiler, combustion exhaust gas containing a low temperature and a considerably high concentration of nitrogen oxides is generated, and the ammonia reduction denitration device functions sufficiently.
It takes about 30 minutes to about 1 hour to reach the temperature of 0 degree or higher.

Further, as a denitration method capable of obtaining a high denitration rate in a low temperature range, there is a method of adsorbing and removing nitrogen oxides with an adsorbent such as alumina or activated carbon. However, in the case of this method, since NO, which is the main component of nitrogen oxides contained in the combustion exhaust gas, is hardly adsorbed,
It is necessary to oxidize the adsorbent into a form of NO ₂ or N ₂ O ₅ which is easily adsorbed, and moreover, it is necessary to frequently regenerate the adsorbent because the adsorption capacity of the adsorbent is lowered by the adsorption of nitrogen oxides.

[0004]

DISCLOSURE OF THE INVENTION The present invention solves the above-mentioned problems of the prior art, and with a simple device configuration, it is possible to perform efficient denitration immediately after starting the combustion device and during steady operation.
An object is to provide a method for reducing nitrogen oxides in combustion exhaust gas.

[0005]

Means for Solving the Problems The present invention is as follows: (1) In a method of removing nitrogen oxides by passing combustion exhaust gas from a combustion device through an ammonia reduction denitration device, a process gas flow path from the ammonia reduction denitration device is provided. By providing a bypass equipped with an ozone oxidation device and a nitrogen oxide adsorption treatment device, and when the ammonia reduction denitration device is not functioning sufficiently immediately after starting the combustion device, etc., unreacted nitrogen oxides discharged from the denitration device are removed. A processing gas containing the gas is passed through the bypass, and NO in the processing gas is replaced with NO ₂ and / or N ₂ O ₅
Nitrogen oxidation in flue gas, which is characterized by removing unreacted nitrogen oxides by performing adsorption treatment after oxidization into nitrogen, and closing the bypass when the ammonia reduction denitration equipment is fully functional. How to reduce things. (2) In the above-mentioned method, when the ammonia reduction denitration apparatus is fully functioning and the bypass is closed, high temperature desorption gas is passed through the adsorption treatment apparatus to regenerate the adsorbent,
In the method for reducing nitrogen oxides in combustion exhaust gas, a desorption gas containing desorbed nitrogen oxides is introduced into the combustion exhaust gas before entering the ammonia reduction denitration apparatus.

FIG. 1 is a schematic flow chart showing one embodiment of the present invention. In FIG. 1, exhaust gas from a combustion device such as a gas turbine or a boiler is supplied from a combustion exhaust gas passage 1, is added with NH ₃ as a reducing agent from an NH ₃ addition passage 2, and is filled with a denitration catalyst such as vanadium. It is also introduced into the ammonia reduction denitration device 3. Since the combustion exhaust gas has a temperature of 250 to 400 ° C. during steady operation, it is sufficiently denitrated by the ammonia reduction denitration device 3, and the treatment gas is heat-recovered by the heat exchanger 4 and is then treated at a temperature of about 100 ° C. It is sent to the chimney 8 via the flow path 7 and discharged into the atmosphere.

A bypass 9 in which an ozone generator 6 and a nitrogen oxide adsorption treatment device 5 are installed is provided in the treatment gas passage 7 through a switching valve (not shown). When the exhaust gas temperature is lower than about 250 ° C. immediately after starting the combustion device, the denitration efficiency in the ammonia reduction denitration device 3 is poor, and the treated gas contains a large amount of unreacted nitrogen oxides. It cannot be released into the atmosphere as it is. Therefore, the process gas is guided to the bypass 9 by operating the switching valve. The processing gas that has entered the bypass 9 oxidizes NO in the gas by the ozone supplied from the ozone generator 6 according to the following equation to form NO ₂ or N ₂ O ₅ that can be adsorbed, and then the adsorption processing device 5 Led to NO
Denitration treatment is performed by adsorbing and removing ₂ and N ₂ O ₅ ,
It is emitted from the chimney 8 into the atmosphere. NO + O ₃ → NO ₂ + O ₂ (1) 2NO ₂ + O ₃ → N ₂ O ₅ + O ₂ (2)

Examples of the adsorbent to be filled in the adsorption treatment device 5 include an alumina-based adsorbent, a carbon-based adsorbent such as activated carbon, and a zeolite-based adsorbent. Γ-as an adsorbent
When an adsorbent having a large adsorption capacity such as alumina is used, denitration of 80 to 90% is possible only by the oxidation up to the above formula (1), but when an adsorbent having a low adsorption capacity is used or even higher. When the denitration rate is desired, it is necessary to increase the rate of oxidation up to (2).

The amount of ozone added is the same as in the above (1) and (2).
As can be seen from the formula, 1 to 1.
It is about 5 mol. Further, ozone may be supplied by installing an ozone generator 6 such as a commonly used ozonizer, but a plasma generating portion is provided in the combustion exhaust gas flow path or the like, and ozone generated by discharging there is generated ozone. It can also be used.

When the temperature of the combustion exhaust gas rises to about 250 ° C. with the lapse of time from the start of operation of the combustion device, the activity of the denitration catalyst in the ammonia reduction denitration device 3 becomes sufficiently high,
Since the concentration of nitrogen oxides in the processing gas discharged from the ammonia reduction denitration apparatus 3 decreases to such an extent that it can be released into the atmosphere as it is, the bypass 9 is closed at that point, and the processed gas is processed gas flow path 7 To be sent to the chimney 8. Normally, the exhaust gas temperature at the time of starting a combustion device such as a boiler starts to rise near room temperature and rises to around 250 ° C in about 30 minutes to 1.5 hours depending on the type and scale of the combustion device.
The switching from the bypass 9 to the processing gas flow path 7 is performed so that the temperature of the combustion exhaust gas is sufficient for the ammonia reduction denitration device 3 to function.
The time when the temperature reaches around 0 ° C is used as a guide, but it is not absolute. Depending on the conditions of each device, the concentration of nitrogen oxides in the combustion exhaust gas, the type of denitration catalyst, the capacity of the denitration device, the amount of nitrogen oxides, etc. The switching timing may be set appropriately in consideration of the release allowable concentration and the like.

The adsorbent that has adsorbed the nitrogen oxides in the adsorption treatment device 5 can be easily regenerated during the normal operation period when the bypass 9 is closed. That is, the adsorbed nitrogen oxides can be desorbed by passing a high-temperature desorption gas of about 300 to 400 ° C. through the adsorption treatment device 5 or by reducing the pressure inside the adsorption treatment device 5. The gas containing nitrogen oxides after desorption is extremely small compared to the mainstream combustion exhaust gas, and it may be treated by separately providing a treatment device such as a small incinerator with a denitration device. A method of introducing the flue gas into combustion exhaust gas is preferable. Also, FIG.
As indicated by a broken line in FIG. 3, a part of the high-temperature processing gas discharged from the ammonia reduction denitration apparatus as the high-temperature desorption gas is supplied to the adsorption processing apparatus 5 through the high-temperature desorption gas flow path 10 to desorb the adsorbed nitrogen oxides. If the desorbed gas containing the desorbed nitrogen oxides is introduced into the combustion exhaust gas before the denitration process through the desorption gas flow channel 11, all of them can be processed in the same facility, which is the simplest method. Is practical.

According to the method of the present invention, nitrogen oxides in low temperature combustion exhaust gas generated at the time of starting a combustion apparatus such as a gas turbine or a boiler, for which there is no effective means at present and which countermeasures are urgently needed. Can be easily reduced.

[0013]

EXAMPLES The method of the present invention will be described in more detail with reference to the following examples.
To explain. (Example) According to the schematic flow chart shown in FIG.
The effectiveness of the adsorption treatment equipment used was tested. Figure 2 (a)
Shows the process of adsorption test, and FIG. 2 (b) shows the process of desorption test. First
According to the flow of FIG. 2A, the adsorbent 14 (diameter
Made of quartz filled with about 3 mm granular activated alumina)
The temperature of the adsorbent in the reaction tube 13 (inner diameter 24 mm) is set in the electric furnace 22.
Is heated so that the temperature becomes 100 ° C, and the test gas introduction pipe 1
8 to NO ₂: 20 ppm, CO₂: 5%, O₂: 18
%, Remaining N₂2 N-liter / min of test gas prepared in
The adsorption treatment was performed by supplying at a flow rate of. Filling the adsorbent 14
The amount is SV = 5000h^-1So that Exhaust gas
A chemiluminescence type NOx analyzer 16 is used in
NOx (NO₂+ NO) content is measured
I kept 2.8ppm for 30 minutes.
It was

2 (b), the introduction of the sample gas is stopped, the temperature of the adsorbent is raised to 350 ° C., and the amount of SV = 50 h ⁻¹ from the desorption gas supply line 17 is reached. Was introduced for 60 minutes to perform desorption treatment. Such adsorption and desorption operations were repeated 73 times, but the NO ₂ content in the exhaust gas during adsorption was about 2.8 p until the final test.
It maintained pm and showed a good denitration effect.

Next, when a similar adsorption test was conducted by adding NO: 20 ppm instead of NO ₂ : 20 ppm in the test gas, almost no adsorption was observed, and NOx in the exhaust gas was hardly adsorbed.
(NO ₂ + NO) remained at 20 ppm. From these test results, it is not possible to adsorb and remove NO in the reaction tube 13 using an adsorbent, but it is possible to adsorb and remove efficiently by changing the form of NO ₂ , and the adsorbent is also regenerated and repeatedly You can see that it can be used.

[0016]

According to the method of the present invention, there is no effective means at present, and countermeasures against it are urgently needed. Oxides can be easily reduced. The method of the present invention can be carried out simply by adding simple equipment to an exhaust gas treatment system using a conventional ammonia reduction denitration apparatus, and its industrial utility value is extremely great.

[Brief description of drawings]

FIG. 1 is a schematic flowchart showing one embodiment of the present invention.

FIG. 2 is a schematic flow chart showing the steps of the effectiveness test of the adsorption treatment apparatus performed in the examples.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Masayoshi Murata 5-717-1, Fukahori-cho, Nagasaki-shi, Nagasaki Sanryo Heavy Industries Ltd. Nagasaki Research Institute (72) Inventor Akira Serizawa 1-1, Akinoura-cho, Nagasaki-shi, Nagasaki No. Mitsubishi Heavy Industries, Ltd. Nagasaki Shipyard (72) Inventor Atsushi Morii 1-1, Atsunoura-machi, Nagasaki-shi, Nagasaki Mitsubishi Heavy Industries Ltd. Nagasaki Shipyard (72) Satoshi Uchida 2-5 Marunouchi, Chiyoda-ku, Tokyo No. 1 Sanryo Heavy Industries Co., Ltd.

Claims (2)

[Claims] 1. A method for removing nitrogen oxides by passing flue gas discharged from a combustion device through an ammonia reduction denitration device, wherein an ozone oxidation device and a nitrogen oxide adsorption treatment are performed in a flow path of a processing gas emitted from the ammonia reduction denitration device. A bypass provided with an apparatus is provided, and when the ammonia reduction denitration apparatus is not functioning sufficiently immediately after the start of the combustion apparatus or the like, a processing gas containing unreacted nitrogen oxides, which is discharged from the denitration apparatus, is passed through the bypass, The unreacted nitrogen oxides are removed by oxidizing NO in the process gas to NO ₂ and / or N ₂ O ₅ and then adsorbing it, and bypassing is performed at the time when the ammonia reduction denitration device becomes fully functional. A method for reducing nitrogen oxides in combustion exhaust gas, which comprises closing the exhaust gas. 2. The method according to claim 1, wherein the adsorbent is regenerated by passing a high-temperature desorption gas through the adsorption treatment device during operation when the ammonia reduction denitration device is fully functioning and the bypass is closed. The method for reducing nitrogen oxides in combustion exhaust gas, comprising introducing the desorbed gas containing nitrogen oxides into the combustion exhaust gas before entering the ammonia reduction denitration apparatus.