JP2786562B2 - Treatment method of combustion exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for treating flue gas. More specifically, NOx contained in flue 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 ) A method for treating combustion exhaust gas, wherein the method comprises recovering and using it as a reducing agent in a denitration step.

[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 countermeasures have been urgently required internationally to protect the global environment. CO ₂
As a source of the occurrence, all human activity fields burning fossil fuels tend to be increasingly demanded for emission control. Along with this, a method and method for contacting boiler flue gas with an alkanolamine aqueous solution to remove and recover CO ₂ in flue gas for power generation facilities such as thermal power plants that use large amounts of fossil fuels Methods for storing the released CO ₂ without releasing it to the atmosphere are being vigorously studied.

[0003] Although fossil fuels vary in degree depending on the type, fossil fuels emit NOx (nitrogen oxides) and SO by combustion.
Generates pollutants such as x (sulfur oxide). These are considered to cause air pollution and acid rain, and their emission standards tend to be strengthened. Accordingly, measures have been taken to treat boiler combustion exhaust gas in a denitration process or a desulfurization process. Among them, a denitration step using NH ₃ as a reducing agent and reducing NOx in the presence of a catalyst to decompose it into nitrogen and water is known. NH ₃ is NOx in flue gas
Although it depends on the amount, it is usually used so that the concentration may be 50 to 150 ppm.

[0004]

As described above, NH ₃ is used as a NOx reducing agent in the denitration step of the combustion exhaust gas. Therefore, the denitration device for the flue gas usually contains the liquid N
An H ₃ storage tank is provided. However, to store liquid NH ₃ , low temperature and high pressure must be maintained, and NH ₃ itself is toxic, and is a flammable substance that becomes an explosive mixed gas when mixed with air. First, it is regulated by various regulations such as the High Pressure Gas Control Law and the Odor Control Law. As described above, the denitration step for removing NOx from the combustion gas requires a cumbersome NH ₃ storage / supply facility.

On the other hand, in the CO ₂ removal step, CO ₂ removal
₍₂₎ NH ₃ is detected in the treated flue gas, albeit in a trace amount. It is presumed that the origin of this NH ₃ is that a part of the alkanolamine is decomposed in the process system. Despite the small amount, if NH ₃ is directly released into the atmosphere together with the CO ₂ -treated flue gas, it can pose a new environmental problem, so it must be collected, and the recovered NH ₃ must be rendered harmless and disposed of. There's a problem.

[0006]

Means for Solving the Problems The present inventors have conventionally proposed
The NH related to the flue gas denitration step being performed
_ThreeProblems and CO 2 removal that has recently been regarded as important_Two
The NH generated in the process _ThreeOf the issue
Fruit, de-CO_TwoNH contained in treated combustion exhaust gas _ThreeCollect
These problems at a glance
Complete the present invention with the knowledge that it can be solved
Was completed.

That is, the present invention relates to a method for reducing NH_ThreeTo
Denitration process of flue gas used and flue gas and alkali
It is contained in combustion exhaust gas by contacting aqueous solution of nolamine
CO _TwoTo remove CO_TwoCombustion exhaust gas treatment
Contact with an aqueous alkanolamine solution
CO removal_TwoRecovers ammonia contained in treated flue gas
And used as a reducing agent in the denitration step.
This is a method for treating combustion exhaust gas. Hereinafter, the present invention is described in detail.
Will be described.

[0008]

FIG. 1 shows an example of a process for treating combustion exhaust gas to which the present invention is applied. In Figure 1, A coal, naphtha, boiler for heavy oil and the like as fuel, B is denitrification process, C is dust collection process such as an electrostatic precipitator, D is the denitration process, E is de-CO ₂ process, F is the chimney, G Is a transfer line for recovered NH ₃ .

The temperature of the combustion exhaust gas guided to the denitration process varies depending on the type of fuel and the type of boiler.
00 to 400 ° C. In addition, 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 varies depending on the fuel and combustion conditions. In LNG firing, NO concentration is usually 50 ~
100 ppm, 100-150 ppm for heavy oil burning, and 200-500 ppm for coal burning. The ratio of NO removed in the denitration step is usually operated at about 50 to 80%. Examples of the catalyst used in the denitration step include those using titanium oxide as a carrier and using a transition metal oxide such as vanadium pentoxide as an activator. The supply of NH ₃ used as a reducing agent varies depending on the target denitration rate, but is injected into the denitration step so as to be about 50 to 80% of the NOx concentration. The optimal temperature for starting the injection of NH ₃ varies depending on the type of fuel. In the present invention, although it depends on the operating conditions of the CO ₂ removal step, the NH ₃ usually required in the CO ₂ removal step can be covered by the NH ₃ recovered in the CO ₂ removal step.

In the process of treating flue gas to which the present invention is applied, usually, as shown in FIG. 1, after the flue gas is desulfurized in a desulfurization step, the flue gas is brought into contact with an aqueous alkanolamine solution in a deCO ₂ step to remove CO ₂ . Remove. As the desulfurization step, various dry methods and wet methods have been conventionally proposed. A typical example is a so-called wet lime-gypsum method in which SOx is absorbed into a water slurry of limestone (calcium carbonate) powder and recovered as gypsum. It can be mentioned as a typical thing.

In the step of removing CO ₂ , the aqueous solution of an alkanolamine that absorbs CO ₂ may be an aqueous solution of monoethanolamine, diethanolamine, triethanolamine, methyldiethanolamine, diisopropanolamine, diglycolamine, or a mixed aqueous solution thereof. In general, an aqueous monoethanolamine (MEA) solution is preferably used.

The process for removing CO ₂ contained in the combustion exhaust gas using an alkanolamine aqueous solution, particularly an MEA aqueous solution, is not particularly limited, but a preferred 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.
O ₂ tower, 202 is a lower filling section, 203 is an upper filling section or tray, 204 is a CO ₂ tower flue 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 needed, 209 is a nozzle, 21
0 is a filling unit, 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 abbreviated as “regeneration tower”). 216, a first nozzle, 217 a lower filling section, 218 a regenerator (reboiler), 219 an upper filling section, 220 a reflux water pump,
21 is a CO ₂ separator, 222 is a recovered CO ₂ discharge line,
223 is a regenerator cooling condenser, 224 is a second nozzle, 22
Reference numeral 5 denotes a regeneration tower reflux water supply line, reference numeral 226 denotes a combustion exhaust gas supply blower, reference numeral 227 denotes a cooler, and reference numeral 228 denotes a reclaimer (storage heat stable salt removing device).

Further, 229 is the second nozzle of the CO ₂ removal tower 201, 230 is a filling section for removing NH ₃ , 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.

In FIG. 2, the flue gas that has undergone the desulfurization step is pushed into a flue gas cooler 208 by a flue gas supply blower 226 and is humidified and cooled by contact with humidified cooling water from a nozzle 209 at a filling section 210. It is led 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 flue gas accumulates in the lower portion of the flue gas cooler 208 and is pumped by the pump 211 into the nozzle 20.
It is recycled to 9. Since the humidified cooling water is gradually lost by humidifying and cooling the combustion exhaust gas, it is replenished through the makeup water supply line 212.

Removal of CO_TwoCombustion exhaust trapped in tower 201
The gas is supplied from the first nozzle 207 at a constant concentration of M
The EA aqueous solution is brought into countercurrent contact with the EA aqueous solution at the lower filling portion 202,
CO in flue gas_TwoIs absorbed and removed by the MEA aqueous solution.
And remove CO_TwoThe treated flue gas flows to the upper filling section 203
U. CO removal_TwoThe aqueous MEA solution supplied to the column 201 is CO
_TwoAnd the heat of reaction due to the absorption causes the supply port 20
6 is higher than the temperature in_TwoAbsorption MEA water
It is sent to the heat exchanger 214 by the solution discharge pump 213,
It is heated and guided to the regeneration tower 215 from the first nozzle 216.
You.

In the regeneration tower 215, the lower filling section 217
The MEA aqueous solution is regenerated by heating by the regenerative heater 218, and 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 tower 201 via 227.

At the upper part of the regeneration tower 215, 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 cooler 223, and is cooled by the CO ₂ separator 221. The water vapor accompanying the CO ₂ is separated from the condensed reflux water, and guided to the CO ₂ recovery step through the recovered CO ₂ discharge line 222. When the operation of the CO ₂ removal step is continued, a part of MEA forms a stable salt and is accumulated in the system. For this purpose, the reclaimer 228 shown in FIG. 2 is operated periodically to process, for example, the bottom liquid of the regeneration tower 215.

In the CO ₂ removal step shown in FIG. 2, NH ₃ is contained in the CO ₂ -treated combustion exhaust gas brought into contact 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.
It is. In the present invention, the NH ₃ contained in the CO ₂ -treated flue gas is recovered and the NH ₃ transfer line G shown in FIG.
To the denitration step.

DeCO2_TwoNH in treated combustion exhaust gas_ThreeCollection
The method is to remove CO_TwoDe-CO2 in tower 201_Twoprocessing
After combustion exhaust gas and CO_TwoCan be brought into contact with water containing
preferable. CO_TwoThe water containing_TwoTower
NH at the top of 201_ThreeNo. 3 water for absorption
Zulu 238, NH_ThreeRemoval filling section 230, tray 23
1, NH_ThreeBetween the separation drum 232 and the third nozzle 238
Circulated, in which case CO_TwoSupplied from the supply line 236
CO_TwoTo CO_TwoMixing with the mixer 237 is preferred.
It can be mentioned as a new thing. Other CO_Two
The water containing_TwoSecond nozzle 22 of tower 201
It is also possible to use regeneration tower reflux water supplied to 9
And using the humidified cooling water of the combustion exhaust gas cooler 208
Is also possible. These waters contain CO_TwoAt that temperature
At or near its saturation level
For example, when the temperature is around 40 ° C, about 400 ppm before
Later CO _TwoIt is included.

Removal of CO_TwoDe-CO2 in lower filling section 2 of tower 201
_TwoThe treated flue gas is usually at 50-80 ° C,
Relatively large amount of MEA vapor corresponding to the partial pressure of the vapor at the same temperature
And a small amount of NH_ThreeContaining water and water vapor,
The raw tower reflux water supply line 225 supplies the water from the regeneration tower 215.
It comes into contact with the reflux water supplied. And MEA and NH _Three
Is a weak base, so weakly acidic CO_TwoIn almost saturation
Regeneration tower reflux water and CO2 removal_TwoUpper contact with treated combustion exhaust gas
MEA or NH_ThreeIs
It is easily absorbed by the reflux liquid in the regeneration tower. However, NH_Three
Is considered to be a decomposition / degradation product of MEA, and accumulated in the system.
From the saturated state to the top of the upper filler 203
Emitted as steam. As a preferred embodiment of the present invention,
This NH _ThreeDegas the steam as described above_TwoAt the top of the towers
CO_TwoAbsorb with water containing
This is transferred to the denitration step B shown in FIG.

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 ... Formula (1) 2NH ₃ + CO ₂ + H ₂ O → (NH ₄ ) ₂ CO ₃ ... Formula (2)

Ammonium carbonate formed by the above reaction formula
All salts are NH4 above 70 ° C_Three, Water and CO_TwoTo
In the process illustrated in FIG.
CO _TwoNH taken out of the tower 201_ThreeContaining water
Steam over 100 ° C from the team supply line 233
Easily NH3 by blowing_ThreeTo play
Can be. Regenerated NH_ThreeIs also regenerated in gaseous form
CO_TwoNH with_ThreeSeparated from water by separation drum 232
And NH_ThreeDenitration process B shown in FIG.
Is transferred to On the other hand, NH_ThreeThe recycled / removed water is cold
After being cooled by the recirculator 235, CO 2_TwoTo the mixer 237
Therefore CO_TwoCO supplied from the supply line 236_TwoTo
Dissolve and remove CO_TwoIn the third nozzle 238 at the top of the tower 201
Supplied.

According to the present invention, the denitration step does not require a cumbersome NH ₃ storage facility, while the CO 2 removal
_{2 The} emission of NH ₃ from the treated flue gas outlet 205 to the outside of the system is effectively prevented, and the NH recovered from the CO ₂ removal step
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. (Embodiment) Combustion exhaust gas was treated using the process and apparatus shown in FIGS. In the denitration step B, TiO ₂ was used as a denitration catalyst, and the denitration temperature was 300 ° C. At that time, all the NH ₃ used as a reducing agent was absorbed and recovered from the CO ₂ removal step using the reflux water in the regeneration tower, and 1 kg / cm ²
The whole regenerated with G steam was used. Table 1 shows the conditions
Shown in

[Table 1]

[0025]

As described above in detail, in the processing of the flue gas including the denitration step and the CO ₂ removal step according to the present invention, the NH contained in the CO ₂ -treated combustion flue gas is reduced.
By recovering ₃ and using it as a reducing agent in the denitration step, a storage facility for liquid NH ₃ in the denitration step becomes unnecessary. The NH ₃ contained in the de-CO ₂ process the combustion exhaust gas discharged from the de-CO ₂ process is almost recovered, it was possible to solve the environmental problems of the NH ₃ which secondarily occurs although the trace.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a process for treating combustion exhaust gas 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.

──────────────────────────────────────────────────の Continued on the front page (72) Inventor Zenji Hotta 3-3-22 Nakanoshima, Kita-ku, Osaka-shi, Osaka Inside Kansai Electric Power Company (72) Inventor Kenji Kobayashi 3-chome, Nakanoshima, Kita-ku, Osaka, Osaka No. 22 Kansai Electric Power Co., Inc. (72) Kunihiko Yoshida 3-3-2 Nakanoshima, Kita-ku, Osaka-shi, Osaka No. 22 Kansai Electric Power Co., Inc. (72) Koichi Kitamura, Inventor 3-3-1 Nakanoshima, Kita-ku, Osaka City, Osaka Prefecture (72) Inventor Masami Kawasaki, 3-chome Nakanoshima, Kita-ku, Osaka City, Osaka No. 22 Kansai Electric Power Co., Inc. (72) Inventor Mutsunori Karasaki 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Heavy Industries, Ltd. (72) Inventor Masaki Iijima Tokyo 2-5-1 Marunouchi, Chiyoda-ku, Tokyo Mitsubishi Heavy Industries, Ltd. Headquarters (72) Inventor Toru Seto 4--22 Kannonshinmachi, Nishi-ku, Hiroshima-shi, Hiroshima Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory (72) Inventor Kaoru Mitsuoka Akira 4-6-22 Kanon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima, Japan Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory (56) References JP-A-5-184866 (JP, A) JP-A-50-143778 (JP, A) 56-56529 (JP, A) (58) Field surveyed (Int. Cl. ⁶ , DB name) B01D 53/62 B01D 53/56

Claims (1)

(57) [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, comprising recovering ammonia contained in a CO ₂ -treated combustion exhaust gas brought into contact with an alkanolamine aqueous solution and using the ammonia as a reducing agent in the denitration step.