JPH05317646A - Waste gas treating method - Google Patents

Abstract

PURPOSE:To provide a treating method in which a desulfurization method excellent in SOx absorbing ability even at low pH and a flue gas denitrification method having a few loss of carbonaceous material are combined. CONSTITUTION:Nitrogen oxide contg. waste gas after removing most of sulfur oxide by a wet limestone-gypsum flue gas desulfurization method is heated to 90-200 deg.C and after adding ammonia, the gas is introduced into reaction layers 17-19, 24 incorporating carbonaceous material, where nitrogen oxide is reduced to nitrogen to remove nitrogen oxide from the waste gas. Simultaneously, the carbonaceous material whose catalytic ability was lowered by ammonium sulfate formed on the carbonaceous material is washed with water to regenerate it and water solution of ammonium sulfate obtained by the water washing is introduced into a liquid absorbent of the wet limestone-gypsum flue gas desulfurization method. Alkali is added to waste water discharged by the wet limestone-gypsum flue gas desulfurization method to double-decompose ammonium sulfate in the waste water into alkali sulfate and ammonia and the generated ammonia-contg. gas is added to the waste gas before the latter is introduced into the reaction layers.

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 exhaust method using a carbonaceous material. The present invention relates to a method for treating exhaust gas organically combined with a smoke denitration method.

[0002]

[Prior Art] SOx such as various combustion exhaust gas and factory exhaust gas
SOx and N are usually used to treat 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 highly demanded gypsum, is the mainstream. However, in this wet limestone-gypsum method, the absorption liquid is operated at a low pH of 3.5 to 5.5 in order to smooth the oxidation of the absorbed sulfite and prevent the scaling due to the generated calcium sulfite. The SOx absorption capacity is low due to its low dissolution rate, a large liquid / gas ratio is required, and the required power is large. 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 the 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 the 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 and corrosion. It had the drawback of causing it.

In addition, when there is no place to install a denitration reactor on the upstream side of the air heater, the ammonia-based SCR may be installed with a heating means after the wet flue gas desulfurization. There has been a problem that the fuel cost for heating to a high temperature of 400 ° C increases. Against 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 an 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, ammonia is added to the exhaust gas and introduced into carbonaceous materials such as activated carbon.
X is removed as ammonium sulfate or acidic ammonium sulfate, and NOx is decomposed into nitrogen and water. The ammonium sulphate and ammonium acid sulphate produced here accumulate on the surface or in the pores of the carbonaceous material and reduce the denitration reaction activity.Therefore, the carbonaceous material is usually withdrawn as a moving layer from the reactor and the carbonaceous material is inactive. About 400 ℃ in gas
The carbonaceous material is regenerated by heating and desorbing ammonium sulfate or acidic ammonium sulfate to reduce and desorb it.

However, according to this method, carbonaceous material and ammonia are consumed for the reductive desorption of ammonium sulfate and acidic ammonium 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. Furthermore, the reductive desorption product of ammonium sulfate and acidic ammonium sulfate is sulfur dioxide (SO ₂ ), which is converted to sulfuric acid or elemental sulfur for recovery, but the equipment cost for conversion is high, and these are plaster. There was a problem that the demand was small compared to the above and there were restrictions on location conditions.

[0006]

The problem to be solved by the present invention is to eliminate the drawbacks of the above conventional desulfurization and denitration methods. That is, by organically combining the wet limestone-gypsum method, which is currently known as the best flue gas desulfurization method, and the flue gas denitrification method using a carbonaceous material that can be performed in the low temperature region downstream of the air heater. The purpose of the present invention is to provide a technology that combines the flue gas desulfurization method, which has excellent SO _X absorption capacity even at low pH, with the flue gas desulfurization method, which causes less wear of carbonaceous materials.

[0007]

A method for treating exhaust gas according to the present invention which solves the above-mentioned problems, is a method for treating exhaust gas containing nitrogen oxides after removing most of sulfur oxides by wet limestone-gypsum flue gas desulfurization method. ~ 200 ℃, and after adding ammonia to the heated exhaust gas, introduce the carbonaceous material into a reaction layer containing a carbonaceous material to reduce the nitrogen oxides to nitrogen and remove the nitrogen oxides from the exhaust gas. The carbonaceous material whose catalytic ability has been lowered by ammonium sulfate formed on the carbonaceous material is washed with water to regenerate the carbonaceous material, and the regenerated carbonaceous material is subjected to removal of the nitrogen oxides. The ammonium sulphate aqueous solution obtained by washing with water is introduced into the absorbent of the wet limestone-gypsum flue gas desulfurization method, and alkali is added to the wastewater discharged from the wet limestone-gypsum flue gas desulfurization method to remove sulfur in the wastewater. Ammonium was metathesis sulfate alkali and ammonia, characterized in that the addition of generated ammonia-containing gas to the exhaust gas prior to introduction into the reaction layer.

The present invention will be described below based on the steps shown in FIG. SOx emitted from combustion equipment such as boilers,
The exhaust gas containing NOx at 130 to 150 ° C. is heat-exchanged with the exhaust gas after flue gas desulfurization in the gas-gas heat exchanger 2 through the conduit 1, and then introduced into the exhaust gas cooling tower 4 through the conduit 3. Cooling tower 4
The exhaust gas cooled by the spray of the aqueous solution supplied from the conduit 5 is introduced into the lower part of the desulfurization tower 7 by the conduit 6. On the other hand, in the upper part of the desulfurization tower 7, the pH containing limestone
The absorption liquid of 5 to 5.5 is supplied by the conduit 8 and is sprayed by means such as a spray, and comes into countercurrent contact with the cooled exhaust gas rising in the desulfurization tower 7 to remove most of SOx in the exhaust gas. ..

NOx and a small amount of SO are discharged from the upper part of the desulfurization tower.
The exhaust gas containing x passes through the conduit 9 and the gas-gas heat exchanger 2
After being heated to 80-110 ° C. at 100 ° C., it enters the exhaust gas temperature controller 11 through the conduit 10, where the temperature of the exhaust gas is adjusted to NO if necessary by a well-known suitable method such as an afterburner.
The temperature at which x is easily reduced on the carbonaceous material is adjusted to 80 to 200 ° C.

The exhaust gas from the temperature controller 11 is mixed with the ammonia gas from the ammonia supply conduit 13 in the middle of the conduit 12 and introduced into a reaction bed containing a carbonaceous material, for example, a fixed bed packed tower. Here, as the packed tower, a plurality of fixed bed packed towers as shown in the figure, for example, four fixed bed packed towers of 17, 18, 19 and 24 as shown in the figure are used. NOx reduction is performed at 18 and 19,
On the other hand, in the packed tower 24, the carbonaceous material is regenerated as described later. That is, the exhaust gas mixed with the ammonia gas is branched from the conduit 12 into the conduits 14, 15, 16 and introduced into the fixed bed packed towers 17, 18, 19 respectively filled with the carbonaceous material.

Here, the supply amount of ammonia gas Q (kgmol / h
r) is determined by the amount of NOx and SOx in the exhaust gas, and is usually used in the range of the following formula. 0.7 × (a + 2b) <Q <1.2 × (a + 2b) a is the amount of NOx in the exhaust gas (kgmol / hr), b is the amount of SOx (kgmol / h)
r). The exhaust gas introduced into the packed towers 17, 18, 19 ... 24 comes into contact with the carbonaceous material packed in the towers, and NOx in the exhaust gas
Is reacted with ammonia by the catalytic action of carbonaceous material as shown in the following formula, and is converted into harmless nitrogen and water.

4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O 6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O SOx in the exhaust gas is adsorbed and oxidized by the carbonaceous material, and then reacts with ammonia to form ammonium sulfate. Generate,
Removed from exhaust gas. The exhaust gas thus cleaned passes through the conduits 20, 21, 22 and merges with the conduit 23 to be discharged to the outside of the system. 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,
Examples thereof include those carrying one or more selected from Cr, Mn, Fe, Co, Ni, Cu, V, Mo, W and the like.

Ammonium sulfate deposited and deposited on the carbonaceous material increases with time and gradually reduces NOx reduction reaction activity. Therefore, it is necessary to regenerate the carbonaceous material before the NOx reduction reaction activity falls below a predetermined value. The time until the NOx reduction reaction activity falls below a predetermined value is set in advance according to various conditions of the exhaust gas to be treated, properties of the carbonaceous material used, exhaust gas treatment conditions, and the like.

While the exhaust gas is being treated in the packed towers 17, 18, and 19, the carbonaceous material is regenerated in the packed tower 24 as described above. The regeneration operation is carried out with washing water, and the flue gas desulfurization absorbent and / or water is used as the washing water. The flue gas desulfurization absorption liquid in which gypsum is separated from the conduit 25 and / or water is supplied to the upper part of the packed tower 24 from the conduit 26 to wash the carbonaceous material in the tower. As a cleaning method, a method of spraying from the upper part of the packed tower, a method of closing the lower valve 31 and immersing, etc. are used. By this washing, ammonium sulfate attached to and deposited on the carbonaceous material is eluted into the washing water, and the washing water is introduced into the washing water tank 28 through the conduit 27 at the lower part of the packed tower. The carbonaceous material is regenerated by this washing operation.

Drying of the carbonaceous material in the packed tower 24 which has completed the washing operation can be carried out by opening the dampers 29 and 30 and closing the valves 31 and 32 to allow the carbonaceous material to pass through the exhaust gas tower. After the regeneration, the packed tower 24 returns to the exhaust gas treatment process, and at the same time, the dampers 33 and 34 are closed and the valve is closed.
The packed column 19 then enters the regeneration process by opening 35 and 36.

Thus, exhaust gas can be continuously treated by sequentially repeating the exhaust gas treatment step and the regeneration step while switching the damper and valve installed in each packed column. Further, as described above, in the fixed bed packed bed, the carbonaceous material does not move and the carbonaceous material is regenerated by washing with water, so that there is an advantage that the carbonaceous material is hardly worn.

On the other hand, the washing water containing ammonium sulfate stored in the tank 28 is mixed with the desulfurization absorption liquid in the lower part of the desulfurization tower 7 through the conduit 29. As a result, the absorption liquid supplied to the upper part of the desulfurization tower for absorbing SOx by the conduit 8 contains ammonium sulfate, and the absorption of SOx is promoted by the pH buffering function of ammonium sulfate. The concentration of ammonium sulfate accumulated in the absorbing solution according to the present invention varies depending on the amount of waste water from the flue gas desulfurization process, but is 0.1 to 1.0 mol / l.
Usually, it will be in the range of 0.2 to 0.7 mol / l. FIG. 2 shows the pH buffering effect of the ammonium sulfate aqueous solution, and it can be seen that it exhibits a greater buffering effect on SO ₂ absorption than the gypsum slurry solution containing no ammonium sulfate. Therefore, compared to the case where ammonium sulfate is not contained, the decrease in pH due to the absorption of SOx is less, and SOx can be removed efficiently even if an absorbing solution having a low pH is used. This makes it possible to reduce the liquid / gas ratio (L / G) in the desulfurization tower,
The pump power can be reduced.

The absorption liquid that has absorbed SOx is piped at the bottom of the desulfurization tower.
Upon contact with the air supplied from 37, the sulfurous acid in the absorbing solution is oxidized to sulfuric acid. Further, the produced sulfuric acid reacts with limestone which is an alkali source supplied from the conduit 38 to produce gypsum. Even in the lower part of the desulfurization tower, the pH buffering effect of ammonium sulfate causes only a small increase in pH in the vicinity of the surface of limestone, and a high dissolution rate of limestone can be achieved. As a result, unreacted limestone is reduced, and the utilization rate of limestone can be increased.

The desulfurization tower 7 of FIG. 1 shows a gas continuous, liquid dispersion type absorption operation using a spray or the like. A liquid continuous for absorbing SOx by dispersing exhaust gas by bubbling in the absorption liquid, Gas-dispersed type, that is, SOx in exhaust gas in a liquid-phase continuous acidic aqueous solution that suspends gypsum stored in one tank
In the case of the process of absorbing water, oxidizing the generated sulfurous acid to sulfuric acid, neutralizing the sulfuric acid with limestone, and crystallizing the generated gypsum, the pH of the absorbing solution is usually 3.5 to 4.5 and the exhaust gas is Contact with. On the other hand, in the gas continuous and liquid dispersion type described above,
The pH is usually 5.0 to 5.5, and at a low pH such as in a liquid continuous or gas dispersion type, the pH buffering effect of ammonium sulfate is more remarkable, as shown in FIG. 2, and SOx absorption is promoted. As a result, the pressure loss of the exhaust gas in the desulfurization tower can be reduced, and the exhaust gas fan power can be reduced. Furthermore, since the neutralization reaction of limestone proceeds smoothly due to the buffering action of ammonium sulfate, the utilization rate of limestone can be further increased.

The absorption liquid containing gypsum is sent by a conduit 39 to a gypsum separator 40, where gypsum is separated. The absorption liquid from which the gypsum has been separated is stored in the filtrate tank 42 by the conduit 41, and is further sent to the thickener 44 and the limestone slurry tank 45 by the conduit 43. In the thickener 44, the fine gypsum remaining in the absorbing liquid is removed, and the clear absorbing liquid is stored in the absorbing liquid tank 47 by the conduit 46.
It is returned to the desulfurization tower by a conduit 48 from the lower part of 44. The clarified absorption liquid stored in the absorption liquid tank is sent to the packed tower 24 as the cleaning water for regeneration by the conduit 49, and is used for cleaning the carbonaceous material in the tower.

On the other hand, a part of the clarified absorbing solution in the absorbing solution tank 47 is sent to the exhaust gas cooling tower 4 by the conduit 50 and used for cooling the exhaust gas.
Sent to 52. The wastewater from which harmful substances have been removed in the wastewater treatment step 52 is sent to the metathesis tank 54 by the conduit 53, and ammonium sulfate in the wastewater reacts with the alkali supplied from the conduit 55 to decompose into ammonia and alkali sulfate. Disassembly
It is desirable to use a pH of 9-12. The decomposed ammonia is dissipated from the wastewater by the air supplied from the lower part of the metathesis tank 54 from the conduit 56, is sent to the ammonia supply conduit 13 by the conduit 57, and is again used for the denitration and desulfurization reaction in the carbonaceous material packed tower. To be done.

As the alkali for metathesis, sodium hydroxide, potassium hydroxide, slaked lime and the like can be used. However, when slaked lime is used, gypsum is produced due to the metathesis.
The gypsum slurry is sent from the lower part of the metathesis tank 54 to the desulfurization tower 7 by a conduit 58. The wastewater that has been subjected to the double decomposition in the metathesis tank 54 is sent to the drainage thickener 60 from the conduit 59, after separating the fine gypsum etc., is sent to the pH adjusting tank 62 in the conduit 61, the pH by the acid added from the conduit 63. Is adjusted to 6 to 8 and then discharged.

Although FIG. 1 shows the case where four carbonaceous packed towers are used, the number of towers is not particularly limited, and it is selected in consideration of the exhaust gas treatment conditions and economic efficiency in the treatment. Is desirable. Further, FIG. 1 exemplifies a case where a fixed bed packed tower of carbonaceous material is used for the flue gas denitration part, but as shown in FIG. 3, a carbon material can be used for the moving bed. That is, the exhaust gas mixed with an appropriate amount of ammonia is introduced into the moving bed reaction column 90, comes into contact with the carbonaceous material moving from the top to the bottom of the reaction column, and is denitrated. The carbonaceous material extracted from the bottom of the tower is introduced into the lower part of the water washing regenerator 91 by the transfer device 92, and comes into contact with the flue gas desulfurization absorbent and / or water supplied by means such as a spray from the upper part of the water washing regenerator, Ammonium sulfate deposited and deposited on the carbonaceous material is removed.

The carbonaceous material from which ammonium sulfate has been removed is introduced from the top of the water washing / regenerating apparatus 91 to the top of the reaction tower 90 by the transfer device 92 and is again used for exhaust gas treatment. The wash water from which ammonium sulfate is eluted is introduced into the desulfurization tower through the wash water tank 93. Compared with the fixed bed, this moving bed causes some wear of the carbonaceous material due to the movement of the carbonaceous material, but has the advantage that the number of reaction towers is small and the installation area is small.

Further, it is also effective to use a fixed bed packed tower of carbonaceous material and a moving bed reaction tower in combination. FIG. 4 shows an example in which moving bed reaction towers are provided at the front and rear of a fixed bed packed bed of carbonaceous material. The exhaust gas mixed with an appropriate amount of ammonia is introduced into the moving bed reaction column 94, contacts the carbonaceous material moving from the tower top to the tower bottom, and
Most of the SOx is removed here in the form of ammonium sulfate. The exhaust gas from which SOx has been removed is introduced into the fixed bed packed tower 95 and denitrated. The exhaust gas denitrated in the fixed bed packed tower 95 is introduced into the moving bed reaction tower 96, where a trace amount of ammonia remaining in the exhaust gas is removed. On the other hand, moving bed reaction tower 94
The carbonaceous material extracted from the bottom of the column is introduced into the lower part of the water washing regenerator 98 by the transfer device 97, is washed and regenerated by the method described above, and is then moved from the upper part of the water washing regenerator 98 to the moving bed reaction tower by the transfer device 97. Introduced to the top of 96 towers. The carbonaceous material introduced into the reaction column 96 is dried while removing ammonia in the exhaust gas while moving from the top to the bottom of the column. The carbonaceous material extracted from the bottom of the reaction tower 96 is introduced to the top of the reaction tower 94, and is again subjected to exhaust gas treatment. Fixed bed packed tower 95
Although only a trace amount of SOx remains in the exhaust gas introduced into the reactor, the activity decreases due to the formation of ammonium sulfate over the long term.At that time, water is supplied from the top of the fixed bed packed tower by means such as spraying. , Wash and regenerate carbonaceous material. The wash water from which ammonium sulfate is eluted is introduced into the desulfurization tower through the wash water tank 99. In the case of this combination of the moving bed and the fixed bed, the fixed bed is not affected by SOx and high denitrification activity can be obtained.
Since SOx and a small amount of slip ammonia, which are easier to remove than x, are targeted, the volume of the reaction tower can be small, and there is an advantage of being compact as a whole.

In the present invention, since ammonium sulfate is introduced into the absorption liquid in the flue gas desulfurization process, the flue gas desulfurization performance is significantly improved, but the desulfurization rate is increased and the frequency of washing the carbonaceous material for denitration in the subsequent stage is reduced. Whether or not to reduce the absorption tower circulating liquid amount in the flue gas desulfurization process or the exhaust gas fan ΔP to maintain the desulfurization rate in the flue gas desulfurization process at the level before the introduction of ammonium sulfate, It may be selected in consideration of economical efficiency.

Further, in the present invention, a desulfurization reaction also occurs in the packed bed of the carbonaceous material for denitration, so that the desulfurization rate through all steps is 10
Although the value is close to 0%, it is possible to greatly simplify the flue gas desulfurization process if, for example, a desulfurization rate of 95% is sufficient, and this is also affected by the increase in the frequency of washing carbonaceous material for denitration in the subsequent stage. In consideration of the above, the optimum method may be selected. Examples of the present invention will be described below.

[0028]

EXAMPLE A simulated exhaust gas having the following composition was treated according to the process shown in FIG. Simulated exhaust gas at a temperature of 50 ° C. was introduced into the upper part of the desulfurization tower 70 having a tower diameter of 40 mmφ and a tower height of 2,000 mm. The desulfurization tower 70 is a liquid continuous, gas dispersion type absorption device, and introduces the introduced exhaust gas into a gas dispersion pipe 71.
More dispersed in the absorbent. The liquid depth to the gas dispersion port at this time was 200 mm as a static liquid depth, and the liquid depth from the gas dispersion port to the bottom of the desulfurization tower was 1,000 mm. Simultaneously oxidizing air 12Nl
/ hr was introduced from the lower part of the desulfurization tower, and further, a slurry solution containing calcium carbonate was introduced so that the pH immediately below the gas dispersion port was maintained at 4.0.

At this time, the desulfurization rate was 90%. The slurry solution containing the generated gypsum is sent from the lower part of the desulfurization tower to a gypsum separator 75 by a pump, and the gypsum is separated from the separated solution by a thickener 76, and then the absorption tank
Introduced into 77, part of which was supplied to a calcium carbonate slurry tank 78 to disperse calcium carbonate in a slurry state, and then introduced into a desulfurization tower.

Exhaust gas discharged from the upper part of the desulfurization tower is heated to 110 ° C. by an exhaust gas heater 85, and ammonia gas 5
After mixing with 0 Nl / hr, it was introduced into the denitration tower 72. Denitration tower 7
Nos. 2 and 73 have a tower diameter of 50 mmφ and a tower height of 1,000 mm, and are filled with 4 mmφ × 5 mm denitration activated carbon catalyst with a bed height of 700 mm. At this time, the denitrification rate was 88%, and the desulfurization rate was 99% including the desulfurization step of the previous stage. The exhaust gas treated in the denitration tower was discharged outside the system.

When this simulated exhaust gas was continuously treated, the NOx removal rate gradually decreased, and after 10 days it had decreased to 80%, so the operation was continued by switching the NOx removal tower from 72 to 73. After switching the denitration tower, the activated carbon catalyst of the denitration tower 72 is washed with 1,000 ml of the absorption liquid from the absorption liquid tank of the desulfurization process,
It was further washed with 1,000 ml of water. Exhaust gas was introduced in parallel with the denitration tower 73 in the denitration tower 72 after washing in order to dry the activated carbon catalyst. On the other hand, the cleaning liquid was introduced into the absorbing liquid in the desulfurization step. As a result, the concentration of ammonium sulfate in the absorption liquid was 0.2 m.
Since it became ol / l and the desulfurization rate in the desulfurization process increased to 97%,
The liquid depth to the gas dispersion port was reduced to 85 mm as the static liquid depth, and the desulfurization rate was maintained at 90%.

Then, when the denitrification rate reached 80%, the denitrification tower 73 was washed with water in the same manner as above, and the catalyst was further dried. In addition, a cleaning liquid was also introduced into the desulfurization process absorption liquid in the same manner as above, and the increased desulfurization rate was maintained at 90% by adjusting the liquid depth of the gas dispersion port. As a result, 7 ml / hr was discharged from the system through the conduit 82. The absorption liquid reduced by this drainage was replenished with water for cleaning gypsum, and a constant amount was maintained although there was a periodic fluctuation due to the introduction of the denitration tower cleaning liquid.

Exhaust gas was treated continuously for 97 days while switching the denitration towers and cleaning and regenerating by such a method. As a result, from the middle of continuous operation, the concentration of ammonium sulfate in the absorption liquid in the desulfurization process was 0.5 to 0.6 mol / l, the liquid depth to the gas dispersion port at the desulfurization rate of 90% was 60 mm as the stationary liquid depth, and the denitration The column switching interval was 9 to 10 days, and the total desulfurization rate was 99%, which was a substantially constant value.

Further, since the almost steady state has been reached, the drain valve 83 is closed, 84 is opened, and slaked lime is added to the drainage discharged to the outside of the system. While keeping the pH at 11, air is introduced from the lower part to remove ammonia. Was stripped. The air containing stripped ammonia was introduced into the exhaust gas at the denitration tower inlet. 1% with introduction of stripping ammonia
The introduction of ammonia gas was reduced to 30 Nl / hr. Exhaust gas treatment was continued for 10 days while recovering ammonia from the wastewater. As a result, the desulfurization and denitration performances before the ammonia recovery could be almost maintained.

[0035]

As described above, the method for treating exhaust gas according to the present invention is provided with an ammonia catalytic reduction flue gas denitration step by a reaction layer of a carbonaceous material after the wet limestone-gypsum flue gas desulfurization step, and ammonia catalytic reduction is performed. The ammonium sulfate generated on the carbonaceous material in the reaction layer of the method smoke denitration process is removed by washing with water to prevent consumption of the carbonaceous material, and the resulting ammonium sulfate solution is absorbed by the wet limestone-gypsum method flue gas desulfurization process. By introducing it into the liquid, the pH buffering effect of ammonium sulfate on SOx absorption is utilized, and even at low pH.
An excellent absorbent that can absorb SOx can be provided, and flue gas desulfurization performance can be further improved. Further, by adding alkali to the wastewater discharged from the flue gas desulfurization step, ammonia is recovered and supplied again to the reaction layer for flue gas desulfurization, which makes it possible to achieve an effective configuration of the ammonia circulation system. The method can be said to be a method for treating exhaust gas that solves the problems of the conventional techniques.

[Brief description of drawings]

FIG. 1 is a diagram showing a process of the present invention when a fixed layer of a carbonaceous material is used in a denitration process.

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

FIG. 3 is a partial process diagram of the present invention when a moving layer of carbonaceous material is used in the denitration process.

FIG. 4 is a partial process diagram of the present invention in which a fixed layer and a moving layer of a carbonaceous material are used in the denitration step.

FIG. 5 is a diagram showing a process of an example of the present invention.

[Explanation of symbols]

7 Desulfurization tower 17, 18, 19, 24, 95 Carbonaceous material fixed bed packing tower 28 Wash water tank 42 Filtrate tank 70 Desulfurization tower 72, 73 Denitration tower 90, 94, 96 Carbonaceous material moving bed packed tower

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

Claims (3)

[Claims] 1. A nitrogen oxide-containing exhaust gas after removing most of sulfur oxides by a wet limestone-gypsum flue gas desulfurization method
The mixture is heated to 80 to 200 ° C., ammonia is added to the heated exhaust gas, and then the carbon oxide is introduced into a reaction layer containing the carbonaceous material to reduce the nitrogen oxides to nitrogen to remove the nitrogen oxides from the exhaust gas. At the same time, the carbonaceous material whose catalytic ability is lowered by ammonium sulfate formed on the carbonaceous material is washed with water to regenerate the carbonaceous material, and the regenerated carbonaceous material is subjected to the removal of the nitrogen oxides. The ammonium sulphate aqueous solution obtained by the above water washing of
Introduced into the absorption liquid of gypsum flue gas desulfurization method, by adding alkali to the wastewater discharged from the wet limestone-gypsum flue gas desulfurization method to decompose ammonium sulfate in the wastewater into alkali sulfate and ammonia, and the generated ammonia-containing gas Is added to the exhaust gas before being introduced into the reaction layer. 2. The method according to claim 1, wherein the reaction layer comprises a fixed layer of carbonaceous material and a moving layer, and the moving layer is arranged before and after the fixed layer. 3. In the wet limestone-gypsum flue gas desulfurization method, absorption of sulfur oxides in exhaust gas and formation of sulfurous acid in a liquid-phase continuous acidic aqueous solution in which gypsum contained in a tank is suspended. The method according to claim 1 or 2, which is a method of performing oxidation to sulfuric acid, neutralization of sulfuric acid with limestone, and crystallization of the produced gypsum.