JPH05245340A - Treatment of combustion exhaust gas - Google Patents

Abstract

PURPOSE:To unnecessitate a storage facility, etc., of liquid ammonia in a denitration process by recovering ammonia included in combustion exhaust gas for CO2 removing treatment which is brought into contact with water solution of alkanolamine to use it as a reducing agent in the denitration process. CONSTITUTION:NH3 in gas given by CO2 removing treatment is reacted with CO2 in CO2 contg. water to form ammonium carbonates which are absorbed by water. Since the ammonium carbonates are decomposed into NH3, water and CO2 at >=70 deg.C, NH3 is easily regenerated by a method, such as that of blowing in air of >=100 deg.C. The regenerated NH3 together with CO2 which is similarly regenerated in the gaseous state is separated from water in a NH3 separation drum to be conveyed to a denitration process B. Therefore, a storage facility, etc., of NH3 which is difficult to handle are not required for the denitration process. On the other hand, diffusion of NH3 from a CO2 removing treated combustion exhaust gas exhaust opening to the outside of the system is effectively prevented and NH3 recovered from the CO2 removing process is effectively used as a reducing agent for the denitration process.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for treating combustion exhaust gas. More specifically, NOx contained in combustion exhaust gas
In the processing step of the combustion exhaust gas having a leaving CO ₂ removing the CO ₂ (carbon dioxide) by denitration step and aqueous alkanolamine solution to remove (nitrogen oxides),
Ammonia contained in de CO ₂ processing combustion exhaust gas (NH
₃ ) is collected and used as a reducing agent in the denitration process, and relates to a method for treating combustion exhaust gas.

[0002]

2. Description of the Related Art In recent years, the greenhouse effect of CO ₂ has been pointed out as one of the causes of the global warming phenomenon, and there is an urgent need to take countermeasures internationally in order to protect the global environment. CO ₂
As a source of the generation of carbon dioxide, it extends to all human activity fields that burn fossil fuels, and there is a tendency that the demand for emission control thereof becomes stronger. Along with this, for power generation facilities such as thermal power plants that use large amounts of fossil fuels, a method and a method for recovering CO ₂ in a combustion exhaust gas by contacting the combustion exhaust gas from a boiler with an alkanolamine aqueous solution, etc. A method of storing the stored CO ₂ without being released to the atmosphere has been vigorously studied.

Further, although fossil fuels vary in degree depending on their types, NOx (nitrogen oxide) and SO are generated by combustion.
Generate pollutants such as x (sulfur oxide). These are the causes of air pollution and acid rain, and their emission standards tend to be strengthened. Along with this, measures have been taken to treat the combustion exhaust gas of the boiler in the denitration process and the desulfurization process. Among them, it is known that in the denitration step, NH ₃ is used as a reducing agent and NOx is reduced in the presence of a catalyst to decompose into nitrogen and water. NH ₃ is NOx in combustion exhaust gas
It is usually used so that the concentration becomes 50 to 150 ppm, though it depends on the amount.

[0004]

As described above, NH ₃ is used as a reducing agent for NOx in the denitration step of combustion exhaust gas. Therefore, it is normally necessary to use liquid N
A storage tank for H ₃ is provided. However, liquid NH ₃ must be kept at a low temperature and a high pressure for storage, and NH ₃ itself is toxic and is a flammable substance that becomes an explosive mixed gas when mixed with air. It is regulated by various regulations such as the High Pressure Gas Control Law and the Odor Control Law. As described above, the denitration process for removing NOx from the combustion gas requires an NH ₃ storage and supply facility that is cumbersome to handle.

On the other hand, in the CO ₂ removal step, CO removal
_{2 A} small amount of NH ₃ is detected in the exhaust gas from the treated flue gas. It is presumed that the origin of this NH ₃ is that part of the alkanolamine decomposes in the process system. Even if it is a very small amount, if NH ₃ is directly released to the atmosphere together with the CO ₂ removal combustion exhaust gas, it may cause a new environmental problem. Therefore, it is necessary to recover this and detoxify the recovered NH ₃ for disposal. There's a problem.

[0006]

The present inventors have been
The above-mentioned NH related to the denitration process of the combustion exhaust gas that is being performed.
₃Of CO and CO removal that has become important in recent years₂
The NH generated in the process ₃Of earnestly studying the issue of
De-CO₂NH contained in the treated combustion exhaust gas ₃Collect
And using it in the denitration process all at once.
To complete the present invention by finding that it can be solved
I was able to.

That is, the present invention uses NH as a reducing agent.₃To
Denitration process of flue gas used and flue gas and alkali
Included in combustion exhaust gas by contacting with an aqueous solution of anolamine
CO ₂To remove CO₂Combustion exhaust gas treatment process
In the physical process, contact with an alkanolamine aqueous solution
CO removal₂Ammonia contained in the treated combustion exhaust gas is recovered.
It is collected and used as a reducing agent in the denitration step.
It is a method of treating combustion exhaust gas. Hereinafter, the present invention will be described in detail.
Explained.

[0008]

FIG. 1 shows an example of a combustion exhaust gas treatment process to which the present invention is applied. In FIG. 1, A is a boiler that uses coal, naphtha, heavy oil, etc. as fuel, B is a denitration process, C is a dust collection process such as an electric dust collector, D is a denitration process, E is a CO ₂ removal process, F is a chimney, and G is a stack. Is a transfer line for the recovered NH ₃ .

The temperature of the flue gas introduced into the denitration process varies depending on the type of fuel and boiler, but usually 2
It is 00-400 degreeC. N contained in the combustion exhaust gas
Ox includes NO, NO ₂ and N ₂ O, but the main component is NO, and the concentration in the combustion exhaust gas differs depending on the fuel and combustion conditions. NO concentration is usually 50-in LNG-fired
It is 100 ppm, 100 to 150 ppm for heavy oil burning, and 200 to 500 ppm for coal burning. The ratio of NO removed in the denitration step is usually about 50 to 80%. Examples of the catalyst used in the denitration step include those using titanium oxide as a carrier and a transition metal oxide such as vanadium pentoxide as an activator. Although the supply of NH ₃ used as a reducing agent varies depending on the target denitrification rate, it is injected into the denitrification process so as to be about 50 to 80% of the NOx concentration. The optimum temperature for injecting NH ₃ varies depending on the type of fuel, but is 200 to 400 ° C. in the case of LNG burning. In the present invention, although it depends on the operating conditions of the CO ₂ removal process, NH ₃ normally required in the denitration process can be covered by NH ₃ recovered in the CO ₂ removal process.

In the flue gas treatment process to which the present invention is applied, the flue gas is usually desulfurized by a desulfurization process as shown in FIG. 1 and then contacted with an alkanolamine aqueous solution by a CO ₂ removal process to remove CO ₂ . Remove. As this desulfurization step, various dry methods and wet methods have been conventionally proposed, but a so-called wet lime-gypsum method in which water slurry of limestone (calcium carbonate) powder absorbs SOx and collects it in gypsum is representative. It can be listed as an example.

In the CO ₂ removal step, examples of the alkanolamine aqueous solution that absorbs CO ₂ include monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, and the like, or a mixed solution thereof. However, an aqueous solution of monoethanolamine (MEA) is usually preferred.

The process for removing CO ₂ contained in the combustion exhaust gas using an alkanolamine aqueous solution, especially an MEA aqueous solution is not particularly limited, but a preferable example thereof will be described with reference to FIG. In FIG. 2, only the main equipment is shown and the auxiliary equipment is omitted. In FIG. 2, 201 is de-C
O ₂ column, 202 is a lower filling section, 203 is an upper filling section or tray, 204 is a CO ₂ removal tower combustion exhaust gas supply port, 20
5 is a CO ₂ -treated flue gas exhaust port, 206 is an MEA aqueous solution supply port, 207 is a first nozzle, 208 is a flue gas cooler provided as necessary, 209 is a nozzle, 21
0 is a filling part, 211 is a humidification cooling water circulation pump, 212 is a makeup water supply line, 213 is a CO ₂ absorption MEA aqueous solution discharge pump, 214 is a heat exchanger, 215 is an MEA aqueous solution regeneration tower (hereinafter also referred to as “regeneration tower”). ) 216 is a first nozzle, 217 is a lower filling part, 218 is a regenerative heater (reboiler), 219 is an upper filling part, 220 is a reflux water pump, 2
21 is a CO ₂ separator, 222 is a recovered CO ₂ discharge line,
223 is a regeneration tower reflux condenser, 224 is a second nozzle, 22
5 is a regeneration tower reflux water supply line, 226 is a combustion exhaust gas supply blower, 227 is a cooler, 228 is a reclaimer (accumulated heat stable salt removing device).

Further, 229 is a second nozzle of the CO ₂ removal tower 201, 230 is an NH ₃ removal filling section, 231 is a tray,
238 is a third nozzle, 232 is an NH ₃ separation drum, 23
3 is a steam supply line, 234 is a cooling water supply line,
235 is a cooler, 236 is a CO ₂ supply line, 237 is a CO ₂ mixer, and G is an NH ₃ transfer line in FIG. 1.

In FIG. 2, the combustion exhaust gas that has undergone the desulfurization step is pushed into the combustion exhaust gas cooler 208 by the combustion exhaust gas supply blower 226, and is brought into contact with the humidified cooling water from the nozzle 209 at the filling section 210 to be humidified and cooled, and then desiccated. It is guided to the CO ₂ removal tower 201 through the CO ₂ tower combustion exhaust gas supply port 204. The humidified cooling water that has come into contact with the combustion exhaust gas collects in the lower portion of the combustion exhaust gas cooler 208, and is pumped by the pump 211.
Recycled to 9. Since the humidified cooling water is gradually lost by humidifying and cooling the combustion exhaust gas, it is replenished by the makeup water supply line 212.

CO removal₂Combustion exhaust pushed into tower 201
The gas is a constant concentration of M supplied from the first nozzle 207.
The EA aqueous solution is brought into countercurrent contact with the lower filling section 202 and burns.
CO in burning exhaust gas₂Is absorbed and removed by the MEA aqueous solution
CO removal₂The treated combustion exhaust gas is directed to the upper filling section 203.
U De-CO₂The MEA aqueous solution supplied to the tower 201 is CO
₂Is absorbed and the heat of reaction due to the absorption absorbs the supply port 20.
6 is higher than the temperature in₂Absorbed MEA water
Sent to the heat exchanger 214 by the solution discharge pump 213,
It is heated and guided from the first nozzle 216 to the regeneration tower 215.
It

In the regeneration tower 215, the lower filling section 217
The MEA aqueous solution is regenerated by being heated by the regenerative heater 218 in FIG. 1, the regenerated MEA aqueous solution is cooled by the heat exchanger 214, and if necessary, a cooler provided between the heat exchanger 214 and the MEA aqueous solution supply port 206. It is returned to the CO ₂ removal column 201 via 227.

In the upper part of the regeneration tower 215, the CO ₂ separated from the MEA aqueous solution comes into contact with the reflux water supplied from the second nozzle 224, is cooled by the regeneration tower reflux condenser 223, and is CO ₂ separator 221. The steam entrained in CO ₂ is separated from the condensed reflux water and is guided to the CO ₂ recovery process through the recovered CO ₂ discharge line 222. When the CO ₂ removal step is continued, a part of MEA forms a stable salt and is accumulated in the system. Therefore, the reclaimer 228 of FIG. 2 is periodically operated to process the bottom liquid of the regeneration tower 215, for example.

In the CO ₂ removal step shown in FIG. 2, NH ₃ is contained in the CO ₂ removal combustion exhaust gas which is contacted with the alkanolamine (MEA) aqueous solution as described above. N
The content of H ₃ depends on the operating conditions of the CO ₂ removal process and is not generally determined, but is usually 30 to 80 ppm.
Is. In the present invention, NH ₃ contained in the flue gas discharged from CO ₂ removal is recovered, and the NH ₃ transfer line G in FIG.
To transfer to the denitration process.

CO removal₂NH in treated combustion exhaust gas₃Recovery of
As a method, CO removal₂CO removal in tower 201₂processing
Later combustion exhaust gas and CO₂Contact with water containing
preferable. CO₂As water containing water, CO removal from FIG.₂Tower
NH as shown at the top of 201₃Absorb water for 3rd
Cheat 238, NH₃Filling unit for removal 230, tray 23
1, NH₃Between the separation drum 232 and the third nozzle 238
Circulate, with CO₂Supplied from supply line 236
CO₂CO₂I prefer the one mixed in the mixer 237.
It can be mentioned as something new. Other CO₂
As water containing water,₂Second nozzle 22 of tower 201
It is also possible to use the recirculation tower reflux water supplied to
And use the humidified cooling water of the combustion exhaust gas cooler 208
Is also possible. CO in these waters₂At that temperature
It can be included at or near saturation.
For example, if the temperature is around 40 ℃, about 400ppm before
Later CO ₂It is included.

CO removal₂CO removal in the lower filling section 2 of the tower 201
₂The treated flue gas is usually at 50-80 ° C,
Large amount of MEA vapor equivalent to the vapor partial pressure at the temperature of
And a small amount of NH₃And water vapor flow upward and
Provided from the regeneration tower 215 through the raw tower reflux water supply line 225.
Contact with the above-mentioned reflux water to be supplied. And MEA and NH ₃
Is a weak base, CO₂Is almost saturated
Recycle tower recirculation water and CO removal₂Top contact with treated combustion exhaust gas
MEA or NH₃Is
It is easily absorbed by the reflux liquid of the regeneration tower. However NH₃
Is considered to be a decomposition / degradation product of MEA and is accumulated in the system.
To the upper part of the upper filler 203 after the saturated state.
Emitted as steam. As a preferred embodiment of the present invention,
This NH ₃Degassing the steam as described above₂The smell at the top of the towers
CO₂Absorb with water containing water and take it out of the system
It is transferred to the denitration step B shown in 1 for use.

The NH ₃ de CO ₂ processing combustion exhaust gas by reacting with CO ₂ in water with the CO _2, occur either in the reaction shown by the following formula, is absorbed as ammonium carbonate salts in water. NH ₃ + CO ₂ + H ₂ O → NH ₄ HCO ₃・ ・ ・ Equation (1) 2NH ₃ + CO ₂ + H ₂ O → (NH ₄ ) ₂ CO ₃・ ・ ・ Equation (2)

Ammonium carbonate produced by the above reaction formula
All salts are NH at 70 ℃ or higher₃, Water and CO₂To
Since it decomposes, in the process illustrated in FIG.
CO ₂NH taken out of the tower 201₃Water containing
Steam above 100 ° C from team supply line 233
It is easy to use NH by blowing it in.₃To play
You can NH played₃Is also regenerated in gaseous form
CO₂NH with₃Separation from water with separation drum 232.
Done, NH₃Denitration process B shown in FIG. 1 by transfer line G
Be transferred to. On the other hand, NH₃The water that has been regenerated and removed is cold
After being cooled by the cooling unit 235, CO₂To the mixer 237
Therefore CO₂CO supplied from the supply line 236₂To
Dissolve and de-CO₂In the third nozzle 238 at the top of tower 201
Supplied.

According to the present invention, the denitration process does not require a cumbersome NH ₃ storage facility, while de-CO 2
_{2 The} emission of NH ₃ from the treated combustion exhaust gas outlet 205 to the outside of the system is effectively prevented, and the NH ₃ recovered from the CO ₂ removal process is
₃ will be effectively used as a reducing agent in the denitration process.

[0024]

EXAMPLES The present invention will be specifically described below with reference to examples. (Example) The combustion exhaust gas was treated using the process and apparatus shown in FIGS. 1 and 2. In the denitration step B, TiO ₂ was used as the denitration catalyst, and the denitration temperature was 300 ° C. At that time, NH ₃ used as a reducing agent is absorbed and recovered from the CO ₂ removal step by using the reflux water of the regeneration tower, and 1 kg / cm ²
The entire amount regenerated with G steam was used. Table 1
Shown in.

[Table 1]

[0025]

INDUSTRIAL APPLICABILITY As described in detail above, in the combustion exhaust gas treatment process including the denitration process and the CO ₂ removal process according to the present invention, NH contained in the CO ₂ -treated combustion exhaust gas is removed.
By recovering ₃ and using it as a reducing agent in the denitration process, a storage facility for liquid NH ₃ in the denitration process became unnecessary. Further, most of NH ₃ contained in the CO ₂ -exhausted combustion exhaust gas discharged from the CO ₂ removal step was recovered, and it was possible to solve the environmental problem of NH ₃ secondary generated although it was a trace amount.

[Brief description of drawings]

FIG. 1 is a view showing an example of a combustion exhaust gas treatment process to which the present invention is applied.

FIG. 2 is a diagram showing an example of a CO ₂ removal step to which the present invention is applied.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Zenji Hotta 3-3-22 Nakanoshima, Kita-ku, Osaka-shi, Osaka Kansai Electric Power Co., Inc. (72) Kenji Kobayashi 3-chome Nakanoshima, Kita-ku, Osaka-shi, Osaka No. 22 in Kansai Electric Power Co., Inc. (72) Inventor Kunihiko Yoshida 3-3 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture No. 22 Inside Kansai Electric Power Co., Inc. (72) Shigeru Shimojo 3-3 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture No.22 in Kansai Electric Power Co., Inc. (72) Inventor Koichi Kitamura 3-3 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture No. 22 Inside Kansai Electric Power Co., Inc. (72) Masami Kawasaki 3-chome, Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture No. 22 in Kansai Electric Power Co., Inc. (72) Inventor Mutsunori Karazaki 2-5-1, Marunouchi, Chiyoda-ku, Tokyo Sanryo Heavy Industries Co., Ltd. (72) Inventor Masaki Iijima 2-5-1, Marunouchi, Chiyoda-ku Sanryo Heavy Industries Co., Ltd. (72) Inventor Toru Seto 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute (72) Inventor Mitsuoka Kaoru 4-6-22 Kannon-shinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Prefecture Mitsubishi Heavy Industries Ltd. Hiroshima Research Institute

Claims (1)

[Claims] 1. A process for flue gas having a leaving CO ₂ removing the CO ₂ contained in the denitration step and the combustion exhaust gas and the combustion exhaust gas is brought into contact with an aqueous alkanolamine solution for combustion exhaust gas using the NH ₃ as a reducing agent ,
A method for treating combustion exhaust gas, which comprises recovering ammonia contained in the combustion exhaust gas treated for de-CO ₂ treatment brought into contact with an alkanolamine aqueous solution and using it as a reducing agent in the denitration step.