JP3248956B2 - Exhaust gas treatment method - Google Patents

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), and more particularly to a wet flue gas desulfurization method and a dry flue gas using 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 of various combustion exhaust gas and factory exhaust gas
As a method of treating exhaust gas containing NOx and NOx, SOx, N
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 demanding gypsum, is dominant. On the other hand, as for NOx, dry catalytic selective catalytic reduction (SCR) using ammonia at a high temperature of 250 to 400 ° C is dominant.

However, since the SCR using this metal oxide catalyst requires a high temperature, a denitration reactor must be installed upstream of the air heater of the boiler to perform denitration, and the sulfur trioxide (SO ₃ ) and SO ₃ generated on the denitration reaction catalyst reacts with ammonia blown for denitration, and the generated ammonium sulfate or ammonium acid sulfate precipitates and deposits on a downstream air heater or a gas-gas heat exchanger to cause clogging and corrosion. There was a drawback to wake up.

[0004] In addition, when there is no place to install a denitration reactor upstream of the air heater, the SCR by ammonia may be installed by providing heating means after wet flue gas desulfurization. There is a problem that fuel cost for heating to a high temperature of 400 ° C. increases. In contrast,
In recent years, a simultaneous desulfurization and denitration method using a carbonaceous material such as activated carbon has been proposed in the 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, JP-A-1-74826, etc.) have been put to practical use.

In this method, ammonia is added to exhaust gas and introduced into a carbonaceous material such as activated carbon to remove SOx as ammonium sulfate or acidic ammonium sulfate.
NOx decomposes into nitrogen and water. The ammonium sulfate or ammonium acid sulfate produced here accumulates on the surface or pores of the carbonaceous material and reduces the denitration reaction activity.Therefore, the carbonaceous material is usually extracted from the reactor as a moving bed, and the carbonaceous material is inerted. Heating to about 400 ° C in a gas reduces and desorbs ammonium sulfate and ammonium ammonium sulfate to regenerate carbonaceous materials.

However, in this method, a carbonaceous material or ammonia is consumed for reducing and desorbing ammonium sulfate or ammonium ammonium sulfate, and furthermore, the strength of the carbonaceous material is reduced by this operation. Therefore, there is a problem that the operating cost increases. Further, the large amount of wear of the carbonaceous material makes it difficult to use a highly active catalyst carrying an active metal, and causes an increase in the size of the reactor. Further, the product of reductive desorption of ammonium sulfate and ammonium ammonium sulfate is sulfur dioxide (SO ₂ ),
This is converted into sulfuric acid or elemental sulfur and recovered. However, equipment costs for the conversion are increased, and there is a problem that these are less demanded than gypsum, and there are restrictions on location conditions.

[0007]

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks of the conventional smoke exhaust and denitration methods. That is, the present invention is to effectively combine a flue gas denitrification method that does not cause the loss of carbonaceous material even when it is performed in a low temperature region downstream of an air heater with a flue gas desulfurization method using a wet limestone-gypsum method.

[0008]

The method for treating exhaust gas according to the present invention, which solves the above-mentioned problems, is characterized by comprising the following steps (a), (b) and (d). A. The exhaust gas is brought into contact with a limestone-gypsum slurry solution containing ammonium sulfate 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 the sulfurous acid formed by the absorbed sulfur oxides. A step of oxidizing with an oxygen-containing gas to form 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; A step of introducing a part of the ammonium sulfite solution into the limestone-gypsum slurry solution of the above step a. C. Extracting a part of the solution after removing the gypsum from the above step a., Adding slaked lime to the solution, Ammonium sulfate contained therein is metathesized into ammonia and gypsum, and air or nitrogen is blown into this metathesis reaction solution to diffuse ammonia, and the ammonia-containing air or nitrogen released in the above step .
While supplying ammonia to the ammonium sulfite solution to absorb ammonia, and adding the gypsum slurry solution after ammonia release to the limestone-gypsum slurry solution of the above step a.
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 oxides discharged from the above step b. Are substantially removed. A step of adding ammonia to the exhaust gas and introducing it into a reaction layer containing a carbonaceous material to reduce and remove nitrogen oxides in the exhaust gas.

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

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

The exhaust gas from which most of the SOx has been removed in the first desulfurization tower 7 is then introduced into the lower part of the second desulfurization tower 9 and
During the ascent of the desulfurization tower, the pH 5.5 supplied by a conduit 10 to the upper part of the second desulfurization tower 9 and dispersed by means such as spraying.
(7), the remaining SOx in the exhaust gas is thoroughly removed until the residual SOx in the exhaust gas becomes several ppm or less.

The remaining SOx is removed in the second desulfurization tower 9 and NOx is removed.
Exhaust gas containing a small amount of stripping ammonia generated from an aqueous solution of ammonium and sulfite is sent to a mist eliminator 12 via a conduit 11 and supplied by a conduit 1 in a gas gas heat exchanger 2 via a conduit 13 after mist is removed. It is reheated to 80 to 110 ° C. by heat exchange with the exhaust gas to be sent, and sent to an exhaust gas temperature controller 15 through a 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 that has exited 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 contains the carbonaceous material. Into the reaction layer 19. The supply amount Q (kgmol / hr) of the ammonia gas from the conduit 18 is determined by the amounts of NOx and stripping ammonia in the exhaust gas, and is usually in a range represented 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 bed 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 formula, and is converted into harmless nitrogen and water. You.

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 out of the system through the conduit 20. As the carbonaceous material used in the present invention, for example, a carbonaceous material having a specific surface area of 50 to 1500 m ² / g, usually called activated carbon or activated coke, or Ti, C
One supported by at least one selected from the group consisting of r, Mn, Fe, Co, Ni, Cu, V, Mo, W and the like can be mentioned.

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

As described above, in the present invention, there is no movement of the carbonaceous material used as a catalyst, and since there is no need to regenerate the carbonaceous material for a long period of time, there is an advantage that the carbonaceous material is hardly worn. Therefore, it can be said that it is particularly effective to use a carbonaceous material supporting the above-mentioned active metal having higher activity as a catalyst. Meanwhile, 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 a conduit 21 at the lower part of the first desulfurization tower 7, and the sulfurous acid in the absorption liquid is oxidized to form gypsum. Generate.

In the present invention, since the absorption solution contains ammonium sulfate, the pH buffering action of ammonium sulfate causes
SOx absorption is promoted. In the present invention, the concentration of ammonium sulfate in the absorbing solution 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 ammonium sulfate aqueous solution at 50 ° C., and shows that it has a larger buffering action on SO ₂ absorption than the gypsum slurry solution containing no ammonium sulfate. Therefore, due to SOx absorption compared to the case without ammonium sulfate
Since the decrease in pH is small, a high desulfurization rate can be achieved.

In the lower part of the first desulfurization tower 7, the pH near the surface of the limestone is reduced due to the pH buffering action of ammonium sulfate.
The pH rise is small and a high limestone dissolution rate can be achieved. As a result, the amount of unreacted limestone is reduced, and the utilization rate of limestone can be increased. The gypsum-containing absorbent produced in the first desulfurization tower 7 is sent to a gypsum separator 23 via a conduit 22, and the absorbent from which the gypsum has been separated is stored in a filtrate tank 25 by a conduit 24, and further by a conduit 26. The limestone supplied to the limestone slurry tank 27 is slurried by the conduit 28 and then supplied to the first desulfurization tower 7 by the conduit 29.

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

The metathesis is preferably carried out at pH 9-12.
The produced ammonia is released from the wastewater by air or nitrogen 36 supplied from the lower part of the double decomposition tank 34, and is blown into the aqueous solution of ammonium sulfite in 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 a conduit 39.

The gypsum slurry produced in the double decomposition tank 34 is taken out by the conduit 40 and sent to the first desulfurization tower 7.
Absorbent solution decomposed in the metathesis tank 34 is sent as wastewater to a drainage thickener 42 by a conduit 41, where fine gypsum and the like are separated, and then sent to a pH adjustment tank 44 by a conduit 43.
After adjusting the pH to 6 to 8 with an acid added from 45, the mixture is discharged.

On the other hand, in the second desulfurization tower 9, the absorbent containing the residual SOx in the exhaust gas is sent to an absorbent tank 38 by a conduit 46 and blown into the absorbent in the absorbent tank 38. Alternatively, it absorbs ammonia by contacting with nitrogen, and the ammonium acid sulfite generated by absorption of residual SOx is activated and regenerated by ammonium sulfite.

The concentration of ammonium sulfite in the absorbing solution used in the second absorption tower in the present invention is usually 0.001 to 0.1.
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
Since it exhibits a buffering action, it can be efficiently absorbed even when the SOx concentration in the exhaust gas is low. In addition, in the present invention, since ammonium sulfite is used as an absorbing solution, usually several ppm
Up to several tens of ppm of ammonia are diffused into the exhaust gas, but the amount of ammonia supplied for the subsequent denitration reaction can be adjusted according to the amount of diffused ammonia in the exhaust gas, and the overall ammonia balance can be maintained .

Although the concentration of ammonium sulfite in the absorbing solution increases with time due to the absorption of residual SOx in the second desulfurization tower, a part of the absorbing solution must be introduced into the absorbing solution at the lower part of the first desulfurization tower by the conduit 47. Can maintain a constant concentration. Ammonium sulfite introduced into the absorption liquid at the lower part of the first desulfurization tower is oxidized by the air supplied from the lower part of the first desulfurization tower to form ammonium sulfate, as described above.
Finally, it is double-decomposed by slaked lime and collected as gypsum.

In the desulfurization tower 7 of FIG. 1, a continuous gas and liquid dispersion type absorption operation using a spray or the like is shown. Dispersion type, that is, pH 3.5 contained in one tank
Absorption of SOx in flue gas in a liquid phase continuous solution of limestone-gypsum slurry containing ~ 4.5 ammonium sulphate;
Processes that oxidize the resulting sulfurous acid and crystallize the formed gypsum can also be used. In this case, the pH of the absorbing solution is 3.5
The pH buffer action of ammonium sulfate is even more remarkable, as shown in FIG. 2, but the absorption of SOx is promoted, and a high desulfurization rate can be achieved. Further, the neutralization reaction of limestone also 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 the liquid continuous and gas dispersion type absorption operation, a jet bubbling reactor (JB
It is preferable to use a device called R). Here, the removal of SOx in the JBR will be outlined with reference to the drawings. FIG. 3 is a schematic diagram of a typical JBR. The exhaust gas is blown 100-400 mm below the liquid level through an inlet plenum 50 and a number of sparger pipes 51 extending below the liquid level to form a jet bubbling layer 52. In this layer, highly efficient gas-liquid contact is performed and SOx is absorbed. The desulfurized gas is discharged to the outside through the gas riser 53.
A continuous oxygen dissolution region 54 is formed under the jet bubbling layer 52 by the supply of oxidizing air, and the SOx absorbed by the jet bubbling layer is immediately oxidized to sulfate ions (SO ₄ ^2- ) on the spot. Is done. The absorbing liquid moves to the lower part of the apparatus after the exhaust gas bubbles are separated, and limestone slurry is injected into the lower part for neutralization and supply of calcium ions. The absorbent then moves to the oxygen dissolution zone. The absorbing solution in which oxygen is dissolved in this region moves to the jet bubbling layer and works again as a medium for absorbing and oxidizing SOx. The generated gypsum crystals exist in a suspended state in the absorbing liquid, but are discharged out of the tank as part of the absorbing liquid is withdrawn from the lower part of the tank, and subjected to solid-liquid separation.
As described above, since the operations of absorption, oxidation, neutralization, and crystallization are performed in a single tank, the apparatus is extremely compact, and efficient desulfurization can be performed.

Furthermore, since the JBR has an upper space 55 at the outlet of the gas riser 53, the residual SOx can be removed by using this portion with an ammonium sulfite absorbing solution. In this case, it can be carried out by a method of spraying the absorbing liquid into the upper space 55 or the like, and an extremely compact absorbing device as a whole is obtained. Hereinafter, examples of the present invention will be described.

[0028]

【Example】

Example 1 Desulfurization A simulated exhaust gas having the following composition was treated using the apparatus having the configuration shown in FIG. 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 solution is a limestone-gypsum slurry solution containing 0.3 mol / l ammonium sulfate.
The introduced exhaust gas was dispersed in the absorption liquid through the gas dispersion pipe 61. At this time, the liquid depth up to the gas dispersion port is the stationary liquid depth.
150mm, liquid depth from gas dispersion port to bottom of desulfurization tower is 1,000m
m. At the same time, 12Nl / hr of oxidizing air was introduced from the lower part of the desulfurization tower, and a slurry solution containing calcium carbonate was introduced as described below so that the pH immediately below the gas dispersion port was maintained at 4.0. At this time, the desulfurization rate was 95%. The slurry solution containing the formed gypsum is sent from the lower part of the desulfurization tower to a gypsum separator 62 by a pump, and the gypsum separated solution is introduced into an absorption liquid tank 63, and a part is supplied to a calcium carbonate slurry tank 64. After the calcium carbonate was dispersed in a slurry state, it was 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,
A magnetic Raschig ring with a tower height of 800 mm and a diameter of 5 mmφ x 7 mm has a layer height of 40
0mm is filled. The exhaust gas introduced into the lower part of the desulfurization tower 65 was discharged from the upper part of the desulfurization tower 65 after coming into contact with 0.02 mol / l of ammonium sulfite having a pH of 6.5 supplied from the upper part of the desulfurization tower at a flow rate of 5 l / hr.

The absorbing solution having absorbed SO ₂ in the exhaust gas is supplied to a desulfurization tower.
It is extracted from the lower part of 65 and introduced into the absorption tank 66. Since the pH of the absorbing solution decreases with the absorption of SO ₂ , the pH is adjusted to 6.5 with ammonia water supplied from the tank 67, and then the desulfurization tower 65 is returned. Feed to the top. Further, since the amount of ammonium sulfite in the absorbing solution increases with the absorption of SO ₂ , a part of the absorbing solution was introduced into the first desulfurizing tower 60 absorbing solution to continue the desulfurization test. At this time, the SO ₂ concentration in the outlet gas of the second desulfurization tower was 0.6 p
pm, stripping ammonia concentration was 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 a denitration treatment.

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

[0032]

As described above, the method for treating exhaust gas of the present invention is intended to provide optimum conditions for the ammonia catalytic reduction flue gas denitration process using a carbonaceous material capable of being denitrated in a low temperature region downstream of an air heater. It is intended to thoroughly remove SOx to several ppm or less by flue gas desulfurization based on the wet limestone-gypsum method in the previous stage, and it is necessary to regenerate carbonaceous materials as a denitration catalyst for a long period of 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 a method of treating exhaust gas that solves the problem.

[Brief description of the drawings]

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

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

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

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

[Explanation of symbols]

7 First desulfurization tower 9 Second desulfurization tower 18 Ammonia gas pipe 19 Carbonaceous material reaction layer 21 Oxygen-containing gas pipe 38 Second desulfurization tower absorption tank 34 Double decomposition tank 35 Slaked lime supply pipe

Continuation of the front page (72) Inventor Naoki Sonehara 2-1-1, Tsurumichuo, Tsurumi-ku, Yokohama-shi, Kanagawa Prefecture Inside Chiyoda Chemical Construction Co., Ltd. (56) References JP-A-53-22870 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) B01D 53/50 B01D 53/77

Claims (1)

(57) [Claims] 1. A method for treating exhaust gas, comprising the following steps (a), (b), (c) and (d). A. The exhaust gas is brought into contact with a limestone-gypsum slurry solution containing ammonium sulfate 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 the sulfurous acid formed by the absorbed sulfur oxides. A step of oxidizing with an oxygen-containing gas to form 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; A step of introducing a part of the ammonium sulfite solution into the limestone-gypsum slurry solution of the above step a. C. Extracting a part of the solution after removing the gypsum from the above step a., Adding slaked lime to the solution, Ammonium sulfate contained therein is metathesized into ammonia and gypsum, and air or nitrogen is blown into this metathesis reaction solution to diffuse ammonia, and the ammonia-containing air or nitrogen released in the above step .
While supplying ammonia to the ammonium sulfite solution to absorb ammonia, and adding the gypsum slurry solution after ammonia release to the limestone-gypsum slurry solution of the above step a.
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 oxides discharged from the above step b. Are substantially removed. A step of adding ammonia to the exhaust gas and introducing it into a reaction layer containing a carbonaceous material to reduce and remove nitrogen oxides in the exhaust gas.