JP2002126448A - Method and apparatus for treating combustion exhaust gas - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for treating a combustion exhaust gas, in which ammonia contained in the waste water obtained by desulfurization or the waste water obtained by washing the collected fly ash is recovered and the recovered ammonia is added to the gas after denitration. SOLUTION: The solution obtained by desulfurization is filtered by using calcium carbonate, the pH of the obtained filtrate is adjusted and carbon dioxide and/or carbonate aqueous solution are added to the pH-adjusted filtrate, to which a flocculant is then added so that solid component is settled and separated and ammonia is recovered from the supernatant liquid.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a method and an apparatus for treating flue gas.

[0002]

2. Description of the Related Art Generally, an exhaust gas treatment device for a boiler for thermal power generation includes a denitration device for removing nitrogen oxides (NOx), an electric dust collector for removing dust, and a sulfur oxide (mainly SO _2). ) Is provided. It is known that in a boiler for thermal power generation, when heavy oil and petroleum coke containing a high sulfur content are used as fuel, SO ₃ and / or sulfuric acid mist are contained in the exhaust gas together with SO ₂ . It is difficult to sufficiently remove SO ₃ and / or sulfuric acid mist contained in the exhaust gas using a desulfurizer using calcium carbonate. To remove this, ammonia is injected into the exhaust gas downstream of the denitrifier, and harmless ammonium sulfate is removed. (Hereinafter also referred to as “ammonium sulfate”) or ammonium hydrogen sulfate (hereinafter “acid ammonium sulfate”)
Also called. ), And separation and removal are performed by an electric dust collector and a desulfurization device.

[0003] Ammonia supplied to the downstream of the denitration apparatus needs to be supplied in excess of a theoretical amount in order to completely remove SO ₃ and / or sulfuric acid mist. In the desulfurization unit. In addition, ammonium sulfate or ammonium acid sulfate, which is a reaction product of SO ₃ and / or sulfuric acid mist with ammonia, is mostly removed by an electric precipitator at the subsequent stage, and the unremoved ammonium sulfate and ammonium acid sulfate are removed together with unreacted ammonia in a desulfurization unit. Is absorbed by. Therefore, the wastewater discharged from the desulfurization apparatus contains a large amount of nitrogen such as ammonia, ammonium sulfate, and acidic ammonium sulfate, and cannot be discharged without treatment under recently tightened wastewater regulations.

[0004] Dust ash separated and removed by an electric dust collector contains ammonium sulfate and ammonium acid sulfate. The wastewater from which the dust ash is washed contains a large amount of nitrogen such as ammonia, ammonium sulfate and ammonium acid sulfate. Therefore, wastewater cannot be discharged without treatment under the current nitrogen wastewater regulations. Therefore, in order to make the nitrogen content in the wastewater equal to or less than the regulation value, a method of removing nitrogen by denitrification by biological treatment using nitrifying bacteria in a wastewater treatment device has been adopted. However, the method of biologically treating nitrogen dissolved in wastewater using nitrifying bacteria has a problem that the processing equipment capacity is increased and equipment costs are increased, and the method is added to remove SO ₃ and / or sulfuric acid mist. It is necessary to reduce the amount of ammonia used.

[0005] In order to reduce the amount of ammonia used, it is desirable to recover and recycle ammonia from wastewater from a desulfurization unit containing ammonia and wastewater from which dust ash has been washed. However, the filtrate after desulfurization by the lime gypsum method using calcium carbonate and separation of the generated calcium sulfate contains a large amount of Ca, and this waste water is introduced into a heater or a recovery tower to remove ammonia. When the recovery operation is performed, Ca is precipitated and clogged in the heater or the recovery tower, causing a problem that long-term continuous operation cannot be performed.

[0006] Therefore, gypsum (Ca
SO ₄ ) was separated by a filter, and the pH was adjusted with an alkali metal hydroxide such as NaOH.
It is common practice to settle and separate the a component together with the flocculant. However, in the filtrate obtained by adding a flocculant after the pH adjustment and performing sedimentation separation, the Ca content, which is mostly in the form of Ca (OH) ₂ , usually contains at least 200 ppm
include. When this solution is supplied to a heater or a recovery tower and heated, a small amount of ammonium carbonate releases carbonic acid, and the released carbonic acid reacts with Ca (OH) ₂ to precipitate calcium carbonate, There is a problem that it hinders long-term continuous operation.

[0007]

SUMMARY OF THE INVENTION The present invention has been made under such a background. The present invention relates to a method for treating combustion exhaust gas, wherein the wastewater obtained after desulfurization or the wastewater obtained by washing dust ash is washed. Recover the ammonia contained in the
An object of the present invention is to provide a method and an apparatus for treating combustion exhaust gas in which recovered ammonia is added to a denitrated gas.

[0008]

Means for Solving the Problems As a result of intensive studies to solve the above-mentioned problems, the present inventors filtered a desulfurized solution using an aqueous solution containing calcium carbonate, and adjusted the pH of the filtrate to 9 to 12. At the same time as the adjustment, an aqueous solution of carbon dioxide and / or carbonic acid is added, or the pH is adjusted to 9 to 12 after the addition of an aqueous solution of carbon dioxide and / or carbonic acid, and a supernatant is obtained by adding a flocculant to precipitate and separate solids. The present inventors have found that the above-mentioned problems can be solved by recovering ammonia from water, and have completed the present invention. The present invention provides the following [1] to
A method and an apparatus for treating combustion exhaust gas shown in [6].

[1] In the method for treating flue gas, ammonia is added to the denitrified gas of the flue gas to add S
O ₃ and / or sulfuric acid mist are converted to ammonium sulfate and / or
Alternatively, the solution is converted to ammonium hydrogen sulfate, the formed ammonium sulfate and / or ammonium hydrogen sulfate is removed together with the dust, and then the desulfurized solution is filtered using an aqueous solution containing calcium carbonate to separate calcium sulfate and a filtrate. Is adjusted to 9 to 12 and simultaneously adding an aqueous solution of carbon dioxide and / or carbonic acid, or adding the aqueous solution of carbon dioxide and / or carbonic acid and then adjusting the pH to 9 to 12.
The supernatant liquid obtained by adding a flocculant to precipitate and separate the solid content is led to a separately provided ammonia recovery step, steam is introduced, ammonia is recovered by concentration by distillation, and the recovered ammonia is recovered. A method for treating combustion exhaust gas, wherein the method is added to the denitrated gas. [2] The method for treating flue gas according to [1], wherein the flue gas is discharged from a boiler. [3] The removed ammonium sulfate and / or ammonium hydrogen sulfate is mixed with water and heavy oil together with dust, and separated into a solid containing dust and heavy oil and an aqueous solution containing ammonium sulfate and / or ammonium hydrogen sulfate, and the solid is used as fuel. The method of treating a combustion exhaust gas according to the above [1] or [2], wherein the pH of the aqueous solution is adjusted to 9 to 12, a coagulant is added, and the supernatant obtained by sedimenting and separating the solid is introduced into the ammonia recovery step. .

[4] The method for treating combustion exhaust gas according to the above [1] or [2], wherein the concentration of Ca contained in the supernatant introduced into the ammonia recovery step is 5 ppm or less. [5] Guided to the ammonia recovery step, the concentration of ammonia contained in the wastewater after the recovery of ammonia is 40 ppm
The method for treating combustion exhaust gas according to any one of the following [1] to [4], which is: [6] an electric dust collector for removing at least dust;
A flue gas treatment device comprising: a desulfurization device using calcium carbonate; and a waste water treatment device for treating desulfurization waste water discharged from the desulfurization device, wherein the following (1) to (4) A device for treating combustion exhaust gas, characterized in that ammonia collected using a wastewater treatment device including the device described in (1) is returned to the upstream side of the electric dust collector. (1) Filtration device for filtering calcium sulfate (2) pH adjusting device having a function of supplying an aqueous solution of carbon dioxide and / or carbonic acid (3) Sedimentation separation device for sedimenting and separating solids after adding a flocculant (4) A concentrator that collects ammonia by introducing steam into the supernatant liquid from which solids have been separated by settling.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. The method for treating flue gas of the present invention can be used, for example, for treating flue gas discharged from a boiler. First, ammonia is added to a denitrified gas of flue gas to convert SO ₃ and / or sulfuric acid mist into ammonium sulfate and And / or convert to ammonium bisulfate, and remove the formed ammonium sulfate and / or ammonium bisulfate together with the dust. Next, the solution desulfurized using an aqueous solution containing calcium carbonate is filtered to separate the solution into calcium sulfate and a filtrate, and the pH of the filtrate is adjusted to 9 to 12 while simultaneously adding an aqueous solution of carbon dioxide and / or carbon dioxide. The pH is adjusted to 9 to 12 after the addition of an aqueous solution of carbonic acid, the coagulant is added, the solid content is settled and separated, and the supernatant is led to a separately provided ammonia recovery step, steam is introduced, and distillation and concentration are performed. By doing so, ammonia is recovered, and the recovered ammonia is added to the above-mentioned denitrated gas.

The method used for the denitration treatment of the combustion exhaust gas from the boiler is not particularly limited, and for example, a method of treating NOx using ammonia in the presence of a catalyst can be used. Dust After denitration, the addition of ammonia to convert the SO ₃ and / or sulfuric acid mist into ammonium sulfate and / or ammonium hydrogen sulfate, ammonia and SO ₃ and / or ammonium sulfate produced by the reaction of sulfuric acid mist and / or hydrogen sulfate ammonium Collect with. Next, the gas from which most of the solid content has been removed is desulfurized by treating it with an aqueous solution containing calcium carbonate.

After desulfurization, the formed gypsum (CaSO ₄ ) is filtered, and the pH of the filtrate is adjusted to 9 to 9 using an alkali metal hydroxide.
At the same time as adjusting the pH to 12, an aqueous solution of carbon dioxide and / or carbonic acid is added, or the pH is adjusted to 9 to 12 after adding the aqueous solution of carbon dioxide and / or carbonic acid, and a coagulant is added to precipitate Ca as calcium carbonate. To separate. As described above, in the conventional method of adding a flocculant after pH adjustment, the Ca content still remains at least about 200 ppm. However, the pH is adjusted to 9 to 12 and carbon dioxide and / or an aqueous solution of carbon dioxide is added at the same time. After adding the carbon and / or carbonic acid aqueous solution, the pH is adjusted to 9-1.
2, the Ca content can be reduced to 5 ppm or less by using the method of the present invention in which the Ca content is converted to calcium carbonate, and a coagulant is further added to treat the Ca content. Can be prevented.

Further, by adjusting the pH to 9 to 12, the Ca component in the wastewater is precipitated and separated as a carbonate, and the condition can be set such that the ammonia can be easily stripped in the recovery step. The recovery rate of ammonia in the recovery step can be adjusted by the supply amount of steam, and the concentration of ammonia remaining in the effluent from the recovery step can be reduced to 40 ppm or less.

Further, the method for treating combustion exhaust gas of the present invention comprises:
The dust collected together with the ammonium sulfate and / or ammonium hydrogen sulfate generated by adding ammonia to the denitrated gas is mixed with water and heavy oil, and the solid containing dust and heavy oil and ammonium sulfate and / or ammonium hydrogen sulfate are contained. The supernatant may be divided into aqueous solutions, and after adjusting the pH of the aqueous solution to 9 to 12, a coagulant may be added, and the supernatant liquid from which solids have been separated by settling may be introduced into the ammonia recovery step. When the treatment method of the present invention is used for treating combustion exhaust gas from a boiler, solids can be used as fuel.

The apparatus for treating combustion exhaust gas of the present invention comprises an electric dust collector for removing at least dust, a desulfurization apparatus using calcium carbonate, and a wastewater treatment apparatus for treating desulfurization wastewater discharged from the desulfurization apparatus. And can be used to treat SO ₃ and / or sulfuric acid mist contained in the combustion exhaust gas discharged from the boiler with ammonia. In the processing apparatus of the present invention,
(1) a filtration device for filtering calcium sulfate, (2) a pH adjusting device having a function of supplying an aqueous solution of carbon dioxide and / or carbonic acid, and (3) a solid after adding a flocculant from wastewater from the desulfurization device. Ammonia collected by using a wastewater treatment device including a sedimentation separation device for sedimentation and separation of sediment, and (4) a concentration device for introducing steam into the supernatant liquid from which sedimentation of solids is separated and recovering ammonia, is provided by an electric precipitator. It is returned to the upstream side. In the pH adjusting device of (2), the pH of the filtrate obtained after filtering calcium sulfate is adjusted to 9-1.
The pH is adjusted to 9-1.
Any of a method of adjusting the pH to 2 and adding an aqueous solution of carbon dioxide and / or carbonic acid and a method of adjusting the pH to 9 to 12 after adding the aqueous solution of carbon dioxide and / or carbonic acid can be used.

Next, a method and an apparatus for treating combustion exhaust gas according to the present invention will be described with reference to the drawings. 1 and 2
All use the method for treating combustion exhaust gas of the present invention,
FIG. 4 is a schematic view showing a method for treating combustion exhaust gas discharged from a boiler, but the present invention is not limited to these methods and apparatuses. In FIG. 1, first, after the combustion exhaust gas from the boiler 1 is denitrified by the denitration device 2, ammonia 34 and the recovered ammonia 31 are supplied to remove SO ₃ and / or sulfuric acid mist in the exhaust gas. S
O ₃ and / or sulfuric acid mist converted into ammonium sulfate and / or acidic ammonium sulfate. Here, ammonia is supplied in an amount of 2 to 2.5 times the amount of SO ₃ and / or sulfuric acid mist. The generated exhaust gas containing ammonium sulfate and / or acidic ammonium sulfate is collected as dust ash 21 together with the unburned ash in the combustion exhaust gas in the subsequent electric dust collector 3. The exhaust gas from which solids have been removed in the electric dust collector 3 contains unreacted ammonia supplied in excess of the theoretical amount, and the exhaust gas is supplied to the desulfurizer 4. In the desulfurization device 4, calcium carbonate 22 is circulated as an absorbent, and unreacted ammonia is also absorbed together with SO ₂ , and the inert component in the exhaust gas is converted into the atmospheric emission gas 23.
Is released as This absorption liquid is supplied to the filtration device 5, and the formed gypsum (calcium sulfate) 24 is separated and discharged out of the system, but the filtrate 25 is collected in the pH adjustment tank 6, the coagulation reaction tank 7, the sedimentation separation tank 8, and the recovery. It is sent to an ammonia recovery system consisting of a tower 9.

In the pH adjusting tank 6 of the ammonia recovery system, the Ca content in the filtrate 25 is adjusted to a pH of 9 to 12 by the aqueous sodium hydroxide solution 26 so that the Ca component in the sedimentation / separation tank 8 tends to settle. This pH is a condition under which Ca is likely to settle and separate, and also a condition under which ammonia is easily diffused in the recovery tower 9. Further, in the pH adjusting tank 6, a liquid containing a carbonic acid aqueous solution 27 is supplied in order to settle and separate the Ca component in the filtrate 25. The liquid that has left the pH adjusting tank 6 is supplied to the coagulation reaction tank 7 and mixed with the coagulant 28. The liquid that has exited the flocculation reaction tank 7 is supplied to a sedimentation tank 8 where salts and heavy metals including calcium carbonate are sedimented and separated, and are discharged out of the system as solids from the bottom of the sedimentation tank 8. Next, the supernatant liquid 29 is supplied to the recovery tower 9, where steam 32 is blown from the bottom of the recovery tower 9, ammonia 31 is recovered as an aqueous solution from the top of the recovery tower, and ammonia is removed from the bottom of the tower. Then, the nitrogen content is discharged as out-of-system waste water 33 whose water content is equal to or less than the waste water regulation value.

As described above, ammonia supplied to remove SO ₃ and / or sulfuric acid mist contained in the gas discharged from the denitration device 2 is supplied in excess of the theoretical amount, but ammonia is supplied. It is absorbed into the desulfurization circulation liquid without being released from the desulfurization device 4 to the atmosphere. The calcium sulfate generated in the desulfurization circulation liquid is filtered off, and thereafter, it is possible to continuously recover ammonia by adjusting the Ca concentration to 5 ppm or less using the treatment method of the present invention. If recycled for removing SO ₃ and / or sulfuric acid mist, newly supplied ammonia can be reduced and an economical process can be realized. At the same time, external wastewater discharged from the bottom of the recovery tower can sufficiently clear the wastewater regulations.

FIG. 2 shows that, similarly to the apparatus shown in FIG. 1, the flue gas from the boiler 1 is denitrified by the denitration apparatus 2, and then SO ₃ and / or sulfuric acid mist in the exhaust gas is recovered as ammonia 34. The ammonium sulfate and / or acid ammonium sulfate that has been removed using ammonia 31 is collected as dust ash 21 in the electric dust collector 3 together with unburned ash in the combustion exhaust gas. The dust ash 21 containing ammonium sulfate and / or acid ammonium sulfate is transported to the mixing tank 10 and mixed with the water 35.

The mixing tank 10 has a stirrer, and a mixture of dust ash 21 and water 35 is sent to an ash granulator 11 and mixed with a binder (heavy oil) 36, and the ash is granulated in a form that can be easily supplied to a boiler. Is done. The granulated ash 37 is sent to the moisture separator 12 to separate it from moisture, where the granulated ash 37 and the moisture are separated, and the granulated ash 37 is recycled and used as boiler fuel. Moisture includes ammonia and ammonium sulfate and / or acid ammonium sulfate, pH
Is adjusted to 9 to 12, a coagulant is added, and the supernatant liquid from which the solid content is separated by settling is sent to an ammonia recovery system.

[0022]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited to these examples. (Example 1) To a denitrated exhaust gas containing about 100 volppm (based on dry gas) of SO ₃ and / or sulfuric acid mist, ammonia equivalent to 2 to 2.5 times the molar amount of SO ₃ and / or sulfuric acid mist is supplied. did. Unreacted ammonia was about 50 volppm (based on dry gas).
Most of the SO ₃ and / or sulfuric acid mist reacted with ammonia became ammonium sulfate or acidic ammonium sulfate and was removed together with the dust. Exhaust gas containing unreacted ammonia was introduced into a slurry calcium carbonate aqueous solution to convert SO ₂ into gypsum and to absorb unreacted ammonia into the circulating absorbent. The amount of SO ₃ and / or sulfuric acid mist in the atmospheric emission gas discharged after desulfurization was about 2 volppm (on a dry gas basis).

Next, a part of the absorbent used for desulfurization was extracted, and the formed gypsum was filtered. At this time, the extracted liquid contained about 9000 ppm of ammonia, and after removing the gypsum, the pH of the filtrate was adjusted to 10 to 12 with an aqueous sodium hydroxide solution. The pH-adjusted solution contains about 300 ppm of Ca, and if the solution is supplied to the recovery tower without reduction, it may cause blockage. Therefore, an aqueous carbonate solution was supplied to convert the Ca to calcium carbonate. The aqueous carbonate solution was supplied in an amount containing 10 times the amount of Ca. When this solution was mixed with a flocculant to precipitate solids, the Ca content in the supernatant was reduced to 5 ppm or less. The supernatant was heated in a heat exchanger and then supplied to an ammonia recovery step.

The conditions for ammonia recovery are as follows:
An aqueous ammonia solution having a range of 0.1 kg / cmG and an ammonia concentration of 5 to 8% by mass was recovered. After the recovered ammonia was vaporized, it was supplied to the denitrated gas for removing SO ₃ and / or sulfuric acid mist. As a result,
It is possible to reduce the amount of ammonia supplied from outside the system, and the amount of ammonia in the discharged atmospheric emission gas is 1 vol.
lppm or less, ammonia nitrogen in the wastewater outside the system is 40p
pm or less.

[0025]

As described above, in the exhaust gas treatment method for supplying ammonia for removing SO ₃ and / or sulfuric acid mist contained in the combustion exhaust gas, the pH of the desulfurization waste water is adjusted to adjust the carbon dioxide and / or Alternatively, by supplying a carbonated aqueous solution, the dissolved Ca component is removed as calcium carbonate, and continuous operation of the recovery tower can be performed without causing clogging due to precipitation or the like in the recovery tower. Also, ammonia was recovered from the top of the tower and
By recycling for removing O ₃ and / or sulfuric acid mist, the amount of ammonia used can be reduced, and the treated wastewater from the bottom of the tower can meet the nitrogen regulations.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a method for treating combustion exhaust gas discharged from a boiler, showing an example of a method for treating combustion exhaust gas of the present invention.

FIG. 2 is a schematic diagram of a method for treating combustion exhaust gas discharged from a boiler, showing an example of the method for treating combustion exhaust gas of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Boiler 2 Denitration apparatus 3 Electric dust collector 4 Desulfurization apparatus 5 Filtration apparatus 6 pH adjustment tank 7 Aggregation reaction tank 8 Sedimentation separation tank 9 Ammonia recovery tower 10 Mixing tank 11 Ash granulator 12 Water separator 21 Dust ash 22 Calcium carbonate 23 Air release gas 24 Gypsum (calcium sulfate) 25 Filtrate 26 Sodium hydroxide 27 Carbonic acid aqueous solution 28 Coagulant 29 Supernatant 30 Solid content 31 Recovered ammonia 32 Steam 33 Outside drainage 34 Ammonia 35 Water 36 Binder (heavy oil) 37 Granulated ash 38 Drainage

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) B03C 3/013 B01D 53/34 125E C01C 1/10 B03C 3/01 B C02F 1/52 (72) Inventor Tatsuharu Arai 5-1 Ogimachi, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Showa Denko Co., Ltd. (72) Inventor Tsuneyoshi Kuriyama 5-1 Ogimachi, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Showa Denko K.K. (72) Inventor Toshitaka Hiro 5-1 5-1 Ogimachi, Kawasaki-ku, Kawasaki-shi, Kanagawa Prefecture Showa Denko Co., Ltd. Industry, Technology Div portion F-term (reference) 4D002 AA02 AC01 BA02 BA03 BA14 BA16 DA05 DA07 DA16 EA07 EA08 EA13 EA14 FA03 HA06 4D011 AA13 AA15 4D015 BA19 BA24 BB05 CA20 EA15 EA16 EA32 FA01 FA02 FA15 FA25 4D054 AA02 EA02 EA10

Claims (6)

[Claims] In a method for treating a combustion exhaust gas, ammonia is added to the denitrated gas of the combustion exhaust gas to convert SO ₃ and / or sulfate mist into ammonium sulfate and / or ammonium hydrogen sulfate. Ammonium hydrogen sulfate is removed together with the dust, and then the desulfurized solution is filtered using an aqueous solution containing calcium carbonate to separate calcium sulfate and the filtrate, and the pH of the filtrate is adjusted to 9 to 12 and at the same time carbon dioxide and / or carbon dioxide is removed. After adding an aqueous solution or adding an aqueous solution of carbon dioxide and / or carbon dioxide, the pH is adjusted to 9 to 12, and a supernatant obtained by adding a flocculant to precipitate and separate a solid content is subjected to an ammonia recovery step provided separately. Ammonia is recovered by introducing steam, introducing steam, and concentrating by distillation. Method of processing a combustion exhaust gas which comprises adding A to the gas after denitration of the. 2. The method for treating flue gas according to claim 1, wherein the flue gas is discharged from a boiler. 3. The removed ammonium sulfate and / or ammonium hydrogen sulfate is mixed with water and heavy oil together with dust, and separated into a solid containing dust and heavy oil and an aqueous solution containing ammonium sulfate and / or ammonium hydrogen sulfate. The method for treating combustion exhaust gas according to claim 1 or 2, wherein a coagulant is added after adjusting the pH of the aqueous solution to 9 to 12, and a supernatant liquid obtained by settling and separating solids is introduced into the ammonia recovery step. 4. The method for treating combustion exhaust gas according to claim 1, wherein the concentration of Ca contained in the supernatant liquid introduced into the ammonia recovery step is 5 ppm or less. 5. An ammonia recovery process, wherein the concentration of ammonia contained in the wastewater after recovering the ammonia is 4%.
The method for treating combustion exhaust gas according to claim 1, wherein the amount is 0 ppm or less. 6. A flue gas treatment apparatus comprising: an electrostatic precipitator for removing at least soot; a desulfurization apparatus using calcium carbonate; and a wastewater treatment apparatus for treating desulfurization wastewater discharged from the desulfurization apparatus. A combustion exhaust gas characterized in that ammonia recovered from desulfurization wastewater using a wastewater treatment device including the following devices (1) to (4) returns to the upstream side of the electric dust collector. Processing equipment. (1) Filtration device for filtering calcium sulfate (2) pH adjusting device having a function of supplying an aqueous solution of carbon dioxide and / or carbonic acid (3) Sedimentation separation device for sedimenting and separating solids after adding a flocculant (4) A concentrator that collects ammonia by introducing steam into the supernatant liquid from which solids have been separated by settling.