JPH0691132A - Exhaust gas treatment - Google Patents

Abstract

PURPOSE:To effectively desulfurize and denitrify exhaust gas by combining a flue gas desulfurizing method by the limestone-gypsum method and a flue gas denitrifying method not causing the wear of carbonaceous material. CONSTITUTION:Limestone-gypsum slurry solution contg. ammonium sulfate is fed to the upper part of a 1st desulfurizer 7 from the lower part of a 1st desulfurizer 7 and dispersed and is brought into countercurrent contact with rising cooled exhaust gas to remove SOx in the exhaust gas by 90-95%. Next, the exhaust gas is introduced into the lower part of a 2nd desulfurizer 9 and is brought into countercurrent contact with water solution of ammonium sulfite dispersed by a means, such as spray to remove residual SOx in the exhaust gas to several ppm or less. The exhaust gas contg. a small quantity of stripping ammonia is heated by a heat exchanger 2 through a mist eliminator 12 and temperature is adjusted by an exhaust gas temperature controller 15. The exhaust gas is mixed with ammonia gas fed from a duct 18 to remove NOx and to purify the exhaust gas by a reactor 19 incorporating carbonaceous material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating exhaust gas containing sulfur oxides (SOx) and nitrogen oxides (NOx). More specifically, it relates to a wet flue gas desulfurization method and a dry exhaust method using a carbonaceous material. The present invention relates to an exhaust gas treatment method in which a smoke denitration method is efficiently combined.

[0002]

[Prior Art] SOx such as various combustion exhaust gas and factory exhaust gas
SOx, N is usually used as a method for treating exhaust gas containing NOx and NOx.
Ox is often treated separately, and for SOx, the wet limestone-gypsum method, which uses limestone as a neutralizing absorbent and recovers it as useful and high-demand gypsum, is the mainstream. On the other hand, the mainstream of NOx is the selective catalytic reduction method (SCR) with dry ammonia using metal oxide as a catalyst at a high temperature of 250 to 400 ° C.

However, since SCR with this metal oxide catalyst requires a high temperature, a denitration reactor must be installed upstream of the air heater of the boiler to denitrate, and sulfur trioxide (SO 3) contained in exhaust gas (SO ₃ ) and SO ₃ produced on the denitration reaction catalyst react with ammonia blown for denitration, and the produced ammonium sulfate or ammonium acid sulfate deposits and deposits on the downstream air heater or gas gas heat exchanger to prevent clogging or corrosion. It had the drawback of causing it.

In addition, when there is no place to install the denitration reactor on the upstream side of the air heater, the SCR by this ammonia may be installed with a heating means after the wet flue gas desulfurization. There is a problem that the fuel cost for heating to a high temperature of 400 ° C increases. For these,
In recent years, a simultaneous desulfurization denitration method using a carbonaceous material such as activated carbon has been proposed in a low temperature range of 130 to 150 ° C downstream of the air heater.
(For example, JP-A-50-104774, JP-A-50-104775, JP-A-55-81728 and JP-A-1-274826) have been put to practical use.

In this method, SOx is removed as ammonium sulfate or acidic ammonium sulfate by adding ammonia to exhaust gas and introducing it into a carbonaceous material such as activated carbon,
NOx decomposes into nitrogen and water. The ammonium sulphate or ammonium acid sulphate produced here accumulates on the surface or pores of the carbonaceous material and reduces the denitration reaction activity.Therefore, the carbonaceous material is usually withdrawn as a moving layer from the reactor and the carbonaceous material is inactive. The carbonaceous material is regenerated by heating to about 400 ° C in gas to reduce and desorb ammonium sulfate and acidic ammonium sulfate.

In this method, however, the carbonaceous material and ammonia are consumed for the desorption and desorption of ammonium sulfate and ammonium acid sulfate, and the strength of the carbonaceous material is reduced by this operation, so that the carbonaceous material is often worn away. Therefore, there is a problem that the operating cost increases. In addition, the large amount of wear of the carbonaceous material makes it difficult to use a highly active catalyst supporting an active metal, resulting in an increase in size of the reactor. Furthermore, the reductive desorption product of ammonium sulfate and acidic ammonium sulfate is sulfur dioxide (SO ₂ ),
This is converted into sulfuric acid or elemental sulfur and recovered, but there are problems that the equipment cost for conversion is high, and that demand for these is smaller than that of gypsum, and there are restrictions on site conditions.

[0007]

The problem to be solved by the present invention is to eliminate the drawbacks of the conventional smoke exhaust and denitration methods. That is, the present invention is to effectively combine a flue gas desulfurization method that does not cause wear of a carbonaceous material even if it is carried out in a low temperature region downstream of an air heater with a flue gas desulfurization method by a wet limestone-gypsum method.

[0008]

The exhaust gas treating method of the present invention for solving the above-mentioned problems is characterized in that it comprises the following steps a, b and c. B. Exhaust gas is brought into contact with ammonium sulfate-containing limestone-gypsum slurry solution having a pH of 3.5 to 5.5 to absorb and remove most of the sulfur oxides in the exhaust gas, and to remove sulfurous acid formed by the absorbed sulfur oxides. A step of oxidizing with an oxygen-containing gas to produce gypsum, b. Contacting the exhaust gas discharged from the above step a with an ammonium sulfite solution having a pH of 5.5 to 7 to substantially remove residual sulfur oxides, and A step of introducing a part of the ammonium sulfite solution into the limestone-gypsum slurry solution in the above step a. C. A part of the solution after removing the gypsum from the above step a. Is extracted, and slaked lime is added to the solution. Ammonium sulphate therein is metathesized into ammonia and gypsum, and air or nitrogen is blown into this metathesis reaction solution to diffuse ammonia, and the diffused ammonia-containing air or nitrogen is subjected to the above-mentioned step. .
In addition to supplying ammonia to the ammonium sulfite solution, the gypsum slurry solution after ammonia emission is added to the limestone-gypsum slurry solution in the above step b.
Alternatively, after the gypsum is separated, the gypsum is added to the limestone-gypsum slurry solution in the above step a., And the separated liquid is discharged out of the system. D. The residual sulfur oxide discharged from the above step b. Is substantially removed. A step of adding ammonia to the generated exhaust gas and then introducing it into a reaction layer containing a carbonaceous material to reduce and remove nitrogen oxides in the exhaust gas.

The present invention will be described below based on the steps shown in FIG. SOx emitted from combustion equipment such as boilers,
Exhaust gas containing NOx is supplied to the gas-gas heat exchanger 2 via the conduit 1, desulfurized as described later, and after heat exchange with the exhaust gas after flue gas desulfurization supplied in the conduit 13, the exhaust gas is cooled by the conduit 3. Introduce to tower 4.

The exhaust gas cooled in the cooling tower 4 by spraying the aqueous solution supplied from the conduit 5 is introduced into the lower part of the first desulfurization tower 7 by the conduit 6. On the other hand, in the upper part of the first desulfurization tower 7, a limestone-gypsum slurry solution containing ammonium sulfate and having a pH of 4.5 to 5.5 (hereinafter, abbreviated as absorption liquid)
Is supplied from the lower part of the first desulfurization tower 7 through a conduit 8 and dispersed by means such as a spray, and comes into countercurrent contact with the cooled exhaust gas rising in the first desulfurization tower 7 so that SOx in the exhaust gas is 90%.
~ 95% (or most) removed.

The exhaust gas from which most of SOx has been removed in the first desulfurization tower 7 is then introduced into the lower part of the second desulfurization tower 9 to
During the ascending of the desulfurization tower, a pH of 5.5, which is supplied to the upper part of the second desulfurization tower 9 by a conduit 10 and dispersed by means such as spraying
By countercurrent contact with an ammonium sulfite aqueous solution of ~ 7, residual SOx in exhaust gas is thoroughly removed to a few ppm or less.

In the second desulfurization tower 9, residual SOx is removed and NOx
Exhaust gas containing a small amount of stripping ammonia generated from the ammonium sulfite aqueous solution is sent to the mist eliminator 12 via the conduit 11, and after the mist is removed, is supplied via the conduit 13 to the gas-gas heat exchanger 2 by the conduit 1. It is reheated to 80 to 110 ° C. by heat exchange with the exhaust gas and is sent to the exhaust gas temperature controller 15 through the conduit 14. In the exhaust gas temperature controller 15, the temperature of the exhaust gas is adjusted to 80 to 200 ° C., which is a temperature at which NOx is easily reduced on the carbonaceous material, by a well-known appropriate method such as an afterburner. The exhaust gas leaving the temperature controller 15 is mixed with the ammonia gas supplied by the conduit 18 after the concentration of the stripping ammonia in the exhaust gas is analyzed by the ammonia analyzer 17 in the middle of the conduit 16, and the carbonaceous material is contained therein. Is introduced into the reaction layer 19. The supply amount Q (kgmol / hr) of ammonia gas from the conduit 18 is determined by the amount of NOx and stripping ammonia in the exhaust gas, and is usually in the range shown by the following equation.

0.7a-b <Q <1.2a-b where a is the amount of NOx in the exhaust gas (kgmol / hr), and b is the amount of stripping ammonia (kgmol / hr). Reaction layer 19
The exhaust gas introduced into the exhaust gas comes into contact with the carbonaceous material filled in the bed, and NOx in the exhaust gas reacts with ammonia by the catalytic action of the carbonaceous material as shown in the following equation, and is converted into harmless nitrogen and water. It

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O The purified exhaust gas is discharged to the outside of the system through a conduit 20. As the carbonaceous material used in the present invention, for example, a specific surface area of 50 to 1500 m ² / g is usually a carbonaceous material called activated carbon or activated coke, or Ti, C
Examples thereof include those carrying one or more selected from r, Mn, Fe, Co, Ni, Cu, V, Mo, W and the like.

In the present invention, SOx in the exhaust gas is thoroughly removed to a level of several ppm or less before entering the denitration reaction layer, so there is almost no ammonium sulfate formed in the carbonaceous material, and the reduction reaction of NOx over a long period of time. The activity is maintained. Therefore, the operation of regenerating the carbonaceous material may be carried out, for example, about once every six months to one year, and can also be carried out at the time of regular maintenance of combustion equipment such as a boiler. The carbonaceous material can be regenerated by, for example, washing with water, heat treatment, or the like, and it is also possible to take out only the catalyst from the denitration reaction layer.

As described above, the present invention has the advantage that the carbonaceous material used as a catalyst does not move and the carbonaceous material does not need to be regenerated for a long period of time, so that the carbonaceous material is hardly worn. Therefore, it can be said that it is particularly effective to use the carbonaceous material supporting the above-mentioned active metal having higher activity as the catalyst. On the other hand, the first
The absorption liquid that has absorbed most of SOx in the desulfurization tower 7 comes into contact with an oxygen-containing gas, for example, air supplied by the conduit 21 in the lower part of the first desulfurization tower 7, and the sulfurous acid in the absorption liquid is oxidized to form gypsum. To generate.

In the present invention, since the absorbing solution contains ammonium sulfate, the pH buffer action of ammonium sulfate causes
SOx absorption is promoted. The concentration of ammonium sulfate in the absorbing solution in the present invention is usually 0.01 to 1.0 mol / l, preferably 0.1 to 0.7 mol / l. FIG. 2 shows the pH buffering action of the aqueous ammonium sulfate solution at 50 ° C., which shows a greater buffering action for SO ₂ absorption than the gypsum slurry solution containing no ammonium sulfate. Therefore, compared with the case where ammonium sulfate is not included, the absorption of SOx
Since the decrease in pH is small, a high desulfurization rate can be achieved.

Also in the lower part of the first desulfurization tower 7, the vicinity of the surface of the limestone is
A low pH rise is required and a high limestone dissolution rate can be achieved. As a result, unreacted limestone is reduced and the utilization rate of limestone can be increased. The absorption liquid containing gypsum produced in the first desulfurization tower 7 is sent to the gypsum separator 23 via the conduit 22, the absorption liquid from which the gypsum has been separated is stored in the filtrate tank 25 by the conduit 24, and further by the conduit 26. The limestone sent to the limestone slurry tank 27 and slurried from the limestone supplied through the conduit 28 is supplied to the first desulfurization tower 7 through the conduit 29.

On the other hand, a part of the absorption liquid is taken out from the filtrate tank 25 by the conduit 30, is sent to the exhaust gas cooling tower 4, is used for cooling the exhaust gas supplied by the conduit 3, and then is treated by the conduit 31 in the waste water treatment process 32. Sent to. The absorption liquid from which the harmful substances have been removed in the wastewater treatment process 32 is transferred to the metathesis tank 34 through the conduit 33.
Ammonium sulphate in the effluent, which is sent to, reacts with the slaked lime slurry supplied by conduit 35 to metathesis into ammonia and gypsum.

The metathesis is preferably carried out at pH 9-12.
The produced ammonia is diffused from the wastewater by the air or nitrogen 36 supplied from the lower part of the metathesis tank 34, and is blown into the ammonium sulfite aqueous solution of the second desulfurization tower absorption liquid tank 38 by the conduit 37 to be absorbed. The removed air or nitrogen is introduced into the second desulfurization tower 9 via the conduit 39.

The gypsum slurry produced in the metathesis tank 34 is taken out by the conduit 40 and sent to the first desulfurization tower 7.
Further, the absorption liquid which has been subjected to the double decomposition in the double decomposition tank 34 is sent as drainage to the drainage thickener 42 by the conduit 41, and after separating the fine gypsum etc. here, it is sent to the pH adjusting tank 44 by the conduit 43,
It is discharged after adjusting the pH to 6 to 8 with an acid added from 45.

On the other hand, in the second desulfurization tower 9, the absorption liquid which has absorbed the residual SOx in the exhaust gas is sent to the absorption liquid tank 38 by the conduit 46, and the ammonia-containing air blown into the absorption liquid tank 38 is absorbed. Alternatively, by contacting with nitrogen to absorb ammonia and absorbing residual SOx, the acidic ammonium sulfite produced is activated and regenerated to ammonium sulfite.

The ammonium sulfite concentration in the absorbent used in the second absorption tower of the present invention is usually 0.001 to 0.
It is 1 mol / l, preferably 0.005 to 0.05 mol / l. Ammonium sulfite has a large pH in the pH range of 5.5 to 7 used.
Since it has a buffering effect, it can be efficiently absorbed even if the SOx concentration in the exhaust gas is low. Further, in the present invention, since ammonium sulfite is used as an absorbing liquid, it is usually several ppm.
~ A few tens of ppm of ammonia is released into the exhaust gas, but the amount of ammonia supplied for the denitration reaction in the latter stage can be adjusted according to the amount of ammonia released in the exhaust gas, and the overall ammonia balance can be maintained. .

Although the concentration of ammonium sulfite in the absorption liquid increases with time due to absorption of residual SOx in the second desulfurization tower, a part of the absorption liquid should be introduced into the absorption liquid in the lower part of the first desulfurization tower by a conduit 47. Can maintain a constant concentration. Ammonium sulfite introduced into the absorption liquid in the lower part of the first desulfurization tower is oxidized by the air supplied from the lower part of the first desulfurization tower to become ammonium sulfate as described above,
Eventually, slaked lime will be double decomposed and recovered as gypsum.

The desulfurization tower 7 in FIG. 1 shows a gas continuous, liquid dispersion type absorption operation using a spray or the like. The liquid continuous, gas is used for absorbing SOx by dispersing exhaust gas by bubbling in the absorption liquid. Dispersion type, that is, pH 3.5 stored in one tank
Absorption of SOx in exhaust gas in a liquid phase continuous solution of a limestone-gypsum slurry containing ammonium sulfate of about 4.5,
It is also possible to use a process of oxidizing the sulfurous acid produced and crystallizing the produced gypsum. In this case, the absorption liquid has a pH of 3.5.
Although it is lower than that of the gas continuous type and the liquid dispersion type, the pH buffering effect of ammonium sulfate is more remarkable as shown in FIG. 2, SOx absorption is promoted, and a high desulfurization rate can be achieved. Furthermore, the neutralization reaction of limestone proceeds smoothly due to the buffering action of ammonium sulfate, so that the utilization rate of limestone can be further increased.

In order to carry out this liquid continuous, gas dispersion type absorption operation, a jet bubbling reactor (JB) is generally used.
Preference is given to using the device referred to as R). Here, the SOx removal in JBR will be outlined with reference to the drawings. FIG. 3 is a schematic diagram of a typical JBR. The exhaust gas is blown through the inlet plenum 50 and a large number of sparger pipes 51 extending below the liquid level to 100 to 400 mm below the liquid level to form a jet bubbling layer 52. SOx is absorbed by highly efficient gas-liquid contact in this layer. The desulfurized gas is discharged to the outside through the gas riser 53.
Under the jet bubbling layer 52, a continuous oxygen dissolution region 54 is formed by the supply of oxidizing air, and SOx absorbed in the jet bubbling layer is immediately oxidized in situ to sulfate ions (SO ₄ ²⁻ ). To be done. The absorption liquid moves to the lower part of the device after the exhaust gas bubbles are separated, and the limestone slurry is injected into the absorption liquid for neutralization and supply of calcium ions. The absorption liquid then moves to the oxygen dissolution region. In this region, the oxygen-dissolved absorbing solution moves to the jet bubbling layer and acts again as a medium for SOx absorption and oxidation. The generated gypsum crystals exist in a state of being suspended in the absorption liquid, but as part of the absorption liquid is pulled out from the lower part of the tank, it is discharged out of the tank and subjected to solid-liquid separation.
As described above, since JBR is subjected to operations of absorption, oxidation, neutralization and crystallization in one tank, the apparatus is extremely compact and efficient desulfurization can be performed.

Furthermore, since the JBR has the upper space 55 at the outlet of the gas riser 53, it is possible to remove the residual SOx with the ammonium sulfite absorption liquid by utilizing this portion. In this case, it can be carried out by a method of spraying the absorbing liquid into the upper space 55, and the absorbing device is extremely compact as a whole. Examples of the present invention will be shown below.

[0028]

【Example】

Example 1: Desulfurization Using a device having the configuration shown in FIG. 4, a simulated exhaust gas having the following composition was treated. The simulated exhaust gas at a temperature of 50 ° C. was introduced into the upper part of the first desulfurization tower 60 having a tower diameter of 45 mmφ and a tower height of 2,000 mm. The desulfurization tower 60 is a liquid continuous, gas dispersion type absorption device, and the absorption liquid is a limestone-gypsum slurry solution containing 0.3 mol / l ammonium sulfate.
The introduced exhaust gas was dispersed in the absorbing liquid through the gas dispersion pipe 61. The liquid depth to the gas dispersion port at this time is the static liquid depth.
150 mm, liquid depth from gas dispersion port to bottom of desulfurization tower is 1,000 m
It was m. Simultaneously, 12 Nl / hr of oxidizing air was introduced from the lower part of the desulfurization tower, and a slurry solution containing calcium carbonate was introduced as follows so that the pH immediately below the gas dispersion port was maintained at 4.0. The desulfurization rate at this time was 95%. The slurry solution containing the generated gypsum is sent to the gypsum separator 62 by a pump from the lower part of the desulfurization tower, the solution in which the gypsum has been separated is introduced into the absorption liquid tank 63, and a part is supplied to the calcium carbonate slurry tank 64. The calcium carbonate was dispersed in a slurry state and then introduced into the desulfurization tower 60.

The exhaust gas discharged from the upper part of the desulfurization tower 60 was introduced into the lower part of the second desulfurization tower 65. The second desulfurization tower 65 has a tower diameter of 45 mmφ,
With a tower height of 800 mm, a magnetic Raschig ring of 5 mmφ x 7 mm has a layer height of 40
It is filled with 0 mm. The exhaust gas introduced into the lower part of the desulfurization tower 65 was contacted with an absorption liquid of ammonium sulfite 0.02 mol / l of pH 6.5 supplied at a flow rate of 5 l / hr from the upper part of the desulfurization tower 65, and then discharged from the upper part of the desulfurization tower 65.

The absorption liquid that has absorbed SO ₂ in the exhaust gas is desulfurized
It is extracted from the lower part of 65 and introduced into the absorption tank 66. Since the absorption liquid pH decreases as SO ₂ is absorbed, the pH is adjusted to 6.5 by the ammonia water supplied from the tank 67, and then the desulfurization tower 65 again. Feed to the top. Furthermore, since the amount of ammonium sulfite in the absorbent increased with the absorption of SO ₂ , a part of the absorbent was introduced into the absorbent of the first desulfurization tower 60 and the desulfurization test was continued. At this time, the SO ₂ concentration in the second desulfurization tower outlet gas was 0.6 p
The pm and stripping ammonia concentrations were 15 ppm.
The desulfurization rate in the second desulfurization tower was 98.8%, and the total desulfurization rate including the first desulfurization tower was 99.94%. Example 2: Denitration Based on the results of the desulfurization test, a simulated exhaust gas having the following composition was prepared and subjected to denitration treatment.

[0031] 30Nl ammonia gas with a concentration of 1% in simulated exhaust gas at a temperature of 50 ℃
After mixing / hr, it was introduced into a denitration reactor in a tubular electric furnace. The catalyst layer in the denitration reactor is 40 mmφ × 320 mm, 4 mmφ
× 5 mm of vanadium-supported activated carbon catalyst was loaded. The temperature of the catalyst layer was set to 100 ° C., and the exhaust gas was treated under the condition of SV about 2,500 hr ⁻¹ . The denitrification rate was 88% at the beginning, but gradually increased and stabilized at 91 to 93%, and the denitrification rate did not decrease after 4,000 hours.

[0032]

As described above, the exhaust gas treatment method of the present invention is intended to provide optimum conditions for an ammonia catalytic reduction flue gas denitration process using a carbonaceous material as a catalyst, which is capable of denitration in a low temperature region downstream of an air heater. The exhaust gas desulfurization method based on the wet limestone-gypsum method in the previous stage is intended to thoroughly remove SOx to several ppm or less, which makes it necessary to regenerate the carbonaceous material that is the denitration catalyst for a long time. Since there is no loss of carbonaceous material, the volume of the denitration reaction layer can be reduced by using a highly active catalyst supporting an active metal, and the by-product is only gypsum. It can be said that this is an exhaust gas treatment method that solves the above problems.

[Brief description of drawings]

FIG. 1 is a diagram showing a process of the present invention.

FIG. 2 is a diagram showing a buffering effect of an aqueous ammonium sulfate solution on SO ₂ absorption.

FIG. 3 is a schematic view of a liquid continuous jet gas bubbling reactor of limestone-gypsum slurry for absorbing most of SOx.

4 shows a simulated exhaust gas desulfurization apparatus used in Example 1. FIG.

[Explanation of symbols] 7 First desulfurization tower 9 Second desulfurization tower 18 Ammonia gas conduit 19 Carbonaceous material reaction layer 21 Oxygen-containing gas conduit 38 Second desulfurization tower absorption tank 34 Metathesis tank 35 Slaked lime supply conduit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoichi Umehara 2-12-1, Tsurumi Chuo, Tsurumi-ku, Yokohama-shi, Kanagawa Chiyoda Kako Construction Co., Ltd. (72) Inori Naoki Sonehara 2 Tsurumi-chu, Tsurumi-ku, Yokohama-shi Chome 12-1 Chiyoda Kako Construction Co., Ltd.

Claims (1)

[Claims] 1. A method for treating exhaust gas, which comprises the following steps a, b, c and d: B. Exhaust gas is brought into contact with ammonium sulfate-containing limestone-gypsum slurry solution having a pH of 3.5 to 5.5 to absorb and remove most of the sulfur oxides in the exhaust gas, and to remove sulfurous acid formed by the absorbed sulfur oxides. A step of oxidizing with an oxygen-containing gas to produce gypsum, b. Contacting the exhaust gas discharged from the above step a with an ammonium sulfite solution having a pH of 5.5 to 7 to substantially remove residual sulfur oxides, and A step of introducing a part of the ammonium sulfite solution into the limestone-gypsum slurry solution in the above step a. C. A part of the solution after removing the gypsum from the above step a. Is extracted, and slaked lime is added to the solution. Ammonium sulphate therein is metathesized into ammonia and gypsum, and air or nitrogen is blown into this metathesis reaction solution to diffuse ammonia, and the diffused ammonia-containing air or nitrogen is subjected to the above-mentioned step. .
In addition to supplying ammonia to the ammonium sulfite solution, the gypsum slurry solution after ammonia emission is added to the limestone-gypsum slurry solution in the above step b.
Alternatively, after the gypsum is separated, the gypsum is added to the limestone-gypsum slurry solution in the above step a., And the separated liquid is discharged out of the system. D. The residual sulfur oxide discharged from the above step b. Is substantially removed. A step of adding ammonia to the generated exhaust gas and then introducing it into a reaction layer containing a carbonaceous material to reduce and remove nitrogen oxides in the exhaust gas.