JPH11147024A - Flue-gas treatment method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the concentration of sulfurous acid gas contained in flue gas after treatment and also the concentration of ammonia contained in the flue gas after treatment at a low value by providing an absorbing solution atomization step to atomized the absorbing solution with an appropriate pH value for absorbing ammonia which is present in the flue gas, into the flue gas after a desulfurization process and before a mist removal process, and thereby absorb the ammonia. SOLUTION: An absorbing solution-atomization process is executed by using a spray nozzle 42. That is, an absorbing solution N with an appropriate pH value for absorbing ammonia by the spray nozzle 42 is atomized into a flue gas A3, in a process in which the flue gas A3 after desulfurization following contact with an absorbent slurry in an absorption tower 8, passes through the upstream of a flue 41. The pH value of the absorbing solution N is preferably set at a level of about 4.0 or below by, for example, adjusting the pH value by adding sulfuric acid or the like. In addition, in a mist removal process, a mist is eliminated by a mist eliminator 43 in the downstream side of the flue 41. That is, the mist entrained in the flue gas A3 after desulfurization after passing the desulfurization process and the absorbing solution atomization process, is collected and removed by the mist eliminator 43.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flue gas treatment technology for purifying flue gas, such as desulfurization treatment, and more particularly to the removal of sulfur dioxide in flue gas and the reduction of ammonia concentration in flue gas after treatment. The present invention relates to a flue gas treatment technology that can be easily realized.

[0002]

2. Description of the Related Art Conventionally, a flue gas treatment method for removing harmful substances such as sulfur oxides (mainly sulfur dioxide) from flue gas generated from a boiler or the like of a thermal power plant is shown in FIG. Are known. Hereinafter, the smoke exhaust treatment technology will be described.

[0003] As shown in FIG. 8, untreated flue gas A discharged from a boiler main body 1 is first introduced into a denitration device 2 disposed in a boiler house, and nitrogen oxides in the flue gas are decomposed. Is done. The denitration device 2 decomposes nitrogen oxides by an ammonia catalytic reduction method using a catalyst.
A denitration equivalent of ammonia B is injected into the flue gas.

Next, the smoke exhaust gas is introduced into an air heater 3 (heat exchanger) also installed in the boiler house, where the heat is recovered, and the air C supplied to the boiler main body 1 by this heat is recovered.
Is heated. The flue gas A1 leaving the air heater 3 is then led out of the boiler house by a flue 4 and led to a dry electric precipitator 5 installed outside the boiler house.
Here, the dust D in the flue gas is collected and removed.

In the case of an oil-fired boiler or the like, the flue gas is collected by an electric dust collector 5 to collect sulfur trioxide (SO ₃ ) in flue gas as ammonium sulfate ((NH ₄ ) ₂ SO ₄ ). In some cases, ammonia may be injected into the flue gas. On the other hand, in the case of a coal-fired boiler, since a large amount of dust such as fly ash is present in the flue gas, SO ₃ in the flue gas does not become a harmful mist (sub-micron particles) but is generated by the dust particles. The condensed state is collected by an electric dust collector 5 and an absorption tower 8 described later.
For this reason, in the case of a coal-fired boiler, the injection of ammonia into the flue 4 is generally omitted.

Next, the smoke exhausted from the electric precipitator 5 is discharged to a flue 6
Is led to the heat recovery section 7 (heat exchanger) of the gas gas heater, where the heat is recovered, and then introduced into the absorption tower 8 of the desulfurization apparatus as the pre-desulfurization flue gas A2. In the absorption tower 8, a calcium-containing slurry in which limestone is suspended as an absorbent (hereinafter, referred to as an absorbent slurry) comes into gas-liquid contact with the flue gas, so that mainly SO ₂ in the flue gas is contained in the absorbent slurry. As it is absorbed, the remaining dust is also trapped in the absorbent slurry.

In this case, as shown in FIG. 8, the absorption tower 8 is provided with a tank 21 to which the absorbent slurry E is supplied at the bottom, and a gas-liquid contact portion extending above the tank 21. , An absorption tower (liquid column tower) for bringing the flue gas and the slurry in the tank 21 into gas-liquid contact. In addition, the desulfurization performance (desulfurization rate) of this type of desulfurization apparatus has recently been as high as over 90%, and in some cases, sulfurous acid gas is hardly contained in the desulfurized flue gas A3 that has left the absorption tower 8 after desulfurization. It is desulfurized to the extent that it does not exist.

Next, the desulfurized flue gas A3 from which SO _{2 and the} like have been removed in the absorption tower 8 of the desulfurization apparatus is subjected to removal of dust remaining in a wet electric precipitator 9 if necessary, and then to a gas gas heater. In the reheating unit 10, after being heated by the heat recovered by the heat recovery unit 7 to a temperature preferable for atmospheric release,
Through a fan or chimney (not shown), the exhaust smoke after treatment A4
Released into the atmosphere as.

The wet-type electrostatic precipitator 9 is provided when it is necessary to particularly reduce the dust concentration in the treated exhaust gas A4 discharged to the atmosphere. The drainage liquid F containing the collected dust is discharged. Further, the heat recovery unit 7 of the gas gas heater may be arranged upstream of the electric dust collector 5 in order to improve the dust removing performance of the electric dust collector 5 and the like. By the way, all the heat recovery of the smoke exhaust is performed in front of the electric precipitator 5 and the temperature of the exhaust gas introduced into the electric precipitator 5 is further reduced. In particular, in the case of the exhaust gas of the coal-fired boiler, Due to the specific resistance, the removal rate of dust such as fly ash in the electric precipitator 5 is significantly increased, which is advantageous.

In addition, in the absorption tower 8, the slurry that has absorbed SO ₂ is oxidized, and gypsum is produced as a by-product by a reaction described below including a neutralization reaction. Here, the absorption tower 8 is provided with a flue gas introduction section 22 for introducing the flue gas A2 before desulfurization, and a flue gas lead-out section 23 for leading the flue gas A3 after desulfurization is formed at an upper end thereof. This is a so-called counter-current absorption tower in which smoke is introduced from below and flows upward.

A mist eliminator 24a is provided in the flue gas discharge section 23, and the accompanying mist in the flue gas generated by the gas-liquid contact is collected here. It is configured such that a large amount of mist containing ammonia or the like is not discharged. In this case, the mist (recovered liquid) collected by the mist eliminator 24a flows down from the lower end of the mist eliminator 24a and returns directly into the tank 21.

A plurality of spray pipes 24 are provided in the absorption tower 8 in parallel.
A plurality of nozzles (not shown) for spraying the slurry in the tank 21 upward in a liquid column shape are formed. Further, a circulation pump 25 for sucking up the absorbent slurry in the tank 21 is provided outside the tank 21, and the slurry is sent to each spray pipe 24 via a circulation line 26.

In the case of FIG. 8, as a means for blowing the oxidizing air G as fine bubbles while stirring the slurry in the tank 21, the stirrer 27 and the air G near the stirring blades of the stirrer 27 are used. And a blowing pipe 28 for blowing air into the tank 21. The slurry that has absorbed the sulfurous acid gas and the air in the tank 21 are efficiently brought into contact with each other to oxidize the entire amount to obtain gypsum.

That is, the spray pipe 24 is
The absorbent slurry E, which is ejected from the tank and is in gas-liquid contact with the flue gas A2 and flows down while absorbing sulfurous acid gas and dust (including ammonium salts such as ammonium sulfate) and ammonia gas, is discharged from the tank 21.
In the inside, while being stirred by the stirrer 27 and the blowing pipe 28, it is oxidized by contacting with a large number of air bubbles blown therein, and further causes a neutralization reaction to become a slurry containing a high concentration of gypsum.
The main reactions occurring during these treatments are represented by the following reaction formulas (1) to (3).

[0015]

## STR1 ## (absorption _{tower) SO 2 + H 2 O →} H + + HSO 3 - (1) ( ^{_{Tank) H + + HSO 3 - +}} 1 / 2O 2 → 2H + + SO 4 2- (2) 2H + + SO 4 2- + CaCO ₃ + H ₂ O → CaSO ₄ .2H ₂ O + CO ₂ (3)

In this way, a large amount of gypsum, a small amount of limestone as an absorbent, and a small amount of dust and ammonia collected from flue gas are usually suspended or dissolved in the tank 21. In this case, the slurry in the tank 21 is supplied to the solid-liquid separator 30 by a piping line 29 branched from the circulation line 26 in this case, and is filtered to reduce the amount of gypsum H having a small amount of water.
It is taken as. On the other hand, a part of the filtrate from the solid-liquid separator 30 is sent to the slurry adjusting tank 31 in this case as water constituting the absorbent slurry E, and the remaining part is desulfurized to prevent accumulation of impurities. It is discharged as wastewater K. Since ammonium salts such as ammonia and ammonium sulfate absorbed from the flue gas have high solubility, most of them are contained in the slurry E as ammonium ions, and are finally contained in the desulfurization wastewater K and discharged.

In this case, limestone as an absorbent is supplied to the operating tank 21 from the slurry adjusting tank 31 as an absorbent slurry E. The slurry adjusting tank 31 includes a stirrer 32
The powdery limestone L charged from a silo (not shown) and the supplied filtrate J are stirred and mixed to produce an absorbent slurry E. The internal slurry E is a slurry pump 33. , So that it is supplied to the tank 21 as appropriate.

Further, for example, make-up water M (industrial water or the like) is supplied to the tank 21 as appropriate, and the water that gradually decreases due to evaporation or the like in the absorption tower 8 is supplemented. In addition, the filtrate J is directly returned to the tank 21, and only the industrial water or the like supplied from outside the system may be used as the water constituting the absorbent slurry E (water supplied to the slurry adjusting tank 31). is there.

During operation, the tank 21 contains a predetermined concentration of gypsum or absorbent by adjusting the flow rate of makeup water to the tank 21 and the flow rate of slurry withdrawal from the piping line 29. Slurries are always kept within a certain range of levels.

During operation, in order to maintain a high desulfurization rate and gypsum purity, the boiler load (flow rate of flue gas A2), the concentration of sulfurous acid gas in the flue gas A2 introduced into the absorption tower 8, the tank The pH and the limestone concentration of the slurry in the tank 21 are detected by a sensor, and the supply amount of the limestone to the tank 21 and the like are appropriately adjusted by a controller (not shown) based on the detection results. Here, the pH in the tank 21
In order to promote the above-mentioned oxidation reaction and produce high-purity gypsum while maintaining the sulfur dioxide gas absorption performance at a high level, conventionally, it is usually adjusted to about 6.0.

[0021]

By the way, in the conventional flue gas treatment technology (especially the flue gas treatment using the above-mentioned counter-current type absorption tower), if the flue gas A2 before desulfurization contains ammonia. On the exit side of the absorption tower 8 of the desulfurization apparatus, there is a problem to be solved in that this ammonia is diffused as a gas and contained in a large amount in the desulfurized flue gas A3 and the treated flue gas A4 to be discharged.

That is, for example, in a flue gas treatment facility of an oil-fired boiler of a thermal power generation facility, as described above, sulfur trioxide (SO ₃ ) contained in the flue gas is collected as ammonium sulfate ((NH ₄ ) ₂ SO ₄ ). For this purpose, a process of injecting ammonia into flue gas in a stream upstream of a desulfurization apparatus is usually performed, and for this purpose, flue gas introduced into an absorption tower for desulfurization (in FIG.
2) contains a maximum of about 100 ppm of ammonia.

In the conventional countercurrent type absorption tower 8 as illustrated in FIG. 8, much of this ammonia is dissolved and absorbed in the slurry on the lower side in the absorption tower 8, and the ammonia in the exhaust gas is discharged. The concentration once drops. However, since the pH of the slurry which is injected into the upper part of the absorption tower 8 and comes into contact with the exhaust gas is as high as about 6, according to the characteristic of the equilibrium partial pressure of ammonia shown in FIG. As the partial pressure increases, much of the ammonia in the slurry is again radiated to the gas side here, and eventually, the ammonia concentration in the desulfurized flue gas A3 (and the treated flue gas A4) becomes about 50 ppm at the maximum.

Ammonia is also one of the odor-causing substances. From the viewpoint of preventing air pollution, it is desirable to minimize the ammonia concentration in the treated flue gas A4 that is released into the atmosphere, and to achieve high desulfurization performance and There has been a demand for a flue gas treatment method that realizes cost reduction and the like, and that emits a small amount of ammonia.

Therefore, the first object of the present invention is to provide a flue gas treatment method capable of keeping the concentration of ammonia in the flue gas after treatment low. Second, it is another object of the present invention to provide a flue gas treatment method capable of suppressing the concentration of ammonia in the flue gas after treatment with a simpler and less expensive structure.

[0026]

According to a first aspect of the present invention, there is provided a method for treating flue gas, comprising: introducing flue gas containing at least ammonia and sulfur dioxide gas into an absorption tower; A desulfurization step of absorbing and removing at least sulfur dioxide from flue gas by bringing the slurry with the calcium compound-containing slurry into a gas-liquid contact, and a mist removing step of removing mist from the flue gas after the desulfurization step and collecting the mist as a liquid. In the smoke treatment method, an absorbent having a pH suitable for absorbing ammonia present in the exhaust gas after the desulfurization step is sprayed into the exhaust gas after the desulfurization step and before the mist removal step. An absorption liquid spraying step for absorbing the ammonia is provided therein.

According to a second aspect of the present invention, in the method for treating flue gas, the recovered liquid recovered in the mist removing step is introduced into the absorption tower,
The calcium compound-containing slurry is used as a liquid.

[0028] The method for treating flue gas according to claim 3 is provided with a wet dust removal step of introducing the flue gas to a wet-type electrostatic precipitator downstream of the absorption tower to remove dust remaining in the flue gas. In the wet dust removing step, the waste liquid discharged from the wet electrostatic precipitator is used as at least a part of the absorbing liquid.

According to a fourth aspect of the present invention, in the desulfurization step, the slurry which is in gas-liquid contact with the flue gas and oxidizes and dehydrates the slurry that has absorbed the sulfurous acid gas to produce gypsum solids as a by-product. Part of the filtrate generated by dewatering the slurry,
It is characterized in that it is configured to be used as at least a part of the absorbing liquid.

According to a fifth aspect of the present invention, in the flue gas treatment method, the sulfur dioxide concentration in the flue gas after the desulfurization step is set higher than a target value of the sulfur dioxide gas concentration in the flue gas after the treatment, and the sulfur dioxide to be removed is removed. By allowing a part of the gas to remain in the flue gas after the desulfurization step, and setting the pH of the absorbing solution from a weakly acidic region to near neutral, the flue gas after the desulfurizing step is contained in the absorbing solution. It is characterized by absorbing sulfurous acid gas remaining in the flue gas after the desulfurization step together with ammonia present therein.

[0031]

An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing a flue gas treatment facility that implements the flue gas treatment method of the present embodiment. FIG.
The same components as in the prior art shown in FIG. 1 are denoted by the same reference numerals, and redundant description will be omitted, and some of them will be omitted in FIG. As shown in FIG. 1, the flue gas treatment facility of the present embodiment is provided with an upstream side of a horizontal flue 41 for guiding desulfurized flue gas A3 coming out of the absorption tower 8 to a wet type electric dust collector 9 for removing ammonia. A feature is that a horizontal type spray nozzle 42 for spraying the absorbing liquid N is provided, and a horizontal type mist eliminator 43 is also provided downstream of the flue 41.

Here, the mist eliminator 43 is capable of removing the mist originally entrained in the desulfurized flue gas A3 and the mist of the absorbing liquid N sprayed from the spray nozzle 42 in the upstream stream. May form a liquid film that makes gas-liquid contact with gas components in the flue gas. For example, a folded plate type mist eliminator, which has been generally used in this type of equipment, can be used.
Then, the recovered liquid P collected from the flue gas A3 by the mist eliminator 43 is discharged via the lower hopper 44, and is introduced into the tank 21 of the absorption tower 8 in this case.

Next, an example of the smoke exhaust treatment method of the present invention, which is performed by the above-described equipment configuration, will be described. In this case, the desulfurization step of the present invention is performed in the gas-liquid contact section in the absorption tower 8. That is, the flue gas A 2 before desulfurization that has exited the heat recovery unit 7 shown in FIG. 8 is guided to the absorption tower 8 and is suitable for absorbing sulfurous acid gas injected from the spray pipe 24.
By gas-liquid contact with the slurry containing calcium compound of H (usually 6.0), at least sulfur dioxide is absorbed and removed from flue gas A2. In this gas-liquid contact,
In addition to the sulfurous acid gas, the ammonia gas and dust in the flue gas as described above are also removed from the absorbent slurry.

Next, the absorbing liquid spraying step of the present invention is performed by the spray nozzle 42. That is, the absorption tower 8
During the process in which the desulfurized flue gas A3, which comes into gas-liquid contact with the absorbent slurry in the above, passes upstream of the flue 41, the spray nozzle 42 produces an absorbent N having a pH suitable for absorbing ammonia by the flue gas A.
Sprayed into 3.

Further, the mist removing step of the present invention comprises
1 is executed by the mist eliminator 43 on the downstream side. That is, the mist entrained in the post-desulfurization flue gas A3 that has passed through the desulfurization step and the absorbent spraying step is collected and removed by the mist eliminator 43. In this case, the collected liquid P composed of the collected mist is introduced into the tank 21 of the absorption tower 8 via the hopper 44, and becomes a part of the liquid constituting the slurry in the tank 21.

In the absorbing liquid spraying step and the mist removing step, the sprayed absorbing liquid N comes into gas-liquid contact with the desulfurized flue gas A3 to absorb and remove at least ammonia gas remaining in the flue gas A3. Along with the sulfur dioxide gas concentration in the post-treatment flue gas A4, the ammonia concentration in the post-treatment flue gas A4 can be significantly reduced.

The sprayed absorbent N and the exhaust gas A after desulfurization were used.
The gas-liquid contact 3 also occurs in the space in the flue 41, but also occurs in the mist eliminator 43, thereby realizing effective absorption of ammonia gas and the like. This is because, in the mist eliminator 43, a large amount of the liquid film surface composed of the collected mist is formed, and the gas component of the smoke exhaust A3 comes into contact therewith. This is because the eliminator 43 functions.

The absorbing liquid N sprayed in the absorbing liquid spraying step
The pH may be adjusted by adding sulfuric acid or the like to industrial water or the like supplied from outside the system, if necessary, but the liquid discharged from the flue gas treatment facility may be used. If this is the case, the amount of water used can be saved and the operating costs can be reduced. That is, for example, a part of the above desulfurization wastewater K (usually about pH 6) or the drainage F of the wet type electrostatic precipitator 9 (usually about pH 8)
If pH is adjusted and used as needed, there is no need to supply industrial water or the like in particular for removing ammonia.

The spraying amount of the absorbing liquid N may be basically set in accordance with the required ammonia removal rate (hereinafter, sometimes referred to as deaeration rate). As shown in FIG. 5, the ratio of the liquid amount to the gas amount (so-called liquid / gas ratio G / L) is 0.04 to 0.5.
If a small amount is sprayed so as to be about 10 [liter / m ³ N], a practical deaeration rate can be sufficiently achieved.

The pH of the absorbing solution N may be basically set according to the required ammonia removal rate.
Since ammonia is an alkali, it is natural that the pH should be as low as possible. That is, in order to absolutely absorb and remove ammonia, for example, sulfuric acid or the like is added to adjust the pH to adjust the pH to 4.0 or less (preferably 2.
0 or less).

However, as shown in the verification data (FIG. 3) described below, according to the study by the inventors, when a considerable concentration of sulfurous acid gas remains in the exhaust gas A3 after desulfurization, the following formula ( 4) Because of the reaction shown in (5), most of the absorbed ammonia is neutralized by the absorbed sulfurous acid.
It has been found that ammonia is absorbed and removed at a considerable removal rate even when the pH of the absorbing solution N is 4.0 or more.

[0042]

## STR2 ## _{NH 3 + H 2 O → NH} 4 + + OH - (4) 2NH 4 + + 2OH - + SO 2 + H 2 O → (NH 4) 2 SO 3 + 2H 2 O (5)

For this reason, the sulfurous acid gas concentration in the flue gas A3 after desulfurization (ie, the performance of the absorption tower 8 and the properties of the flue gas A2 before desulfurization)
In some cases, a weakly acidic to near neutral solution (about pH 4.0 to about 6.0) can be used as the absorbing liquid N. In this case, for example, the above-mentioned desulfurization effluent K or the like can be used as it is, or can be used after slightly adjusting the pH, and a chemical such as sulfuric acid for adjusting the pH becomes completely unnecessary, or at least the amount thereof is reduced. Can be significantly reduced.

Therefore, as a more detailed aspect of the present exhaust gas treatment method, it is preferable to set the following operating conditions or perform the following operation control. That is, after desulfurization flue gas A
3 is set higher than the target value of the concentration of sulfur dioxide in the flue gas A4 after the treatment, and a part of the sulfur dioxide to be removed is left in the flue gas A3 after desulfurization. By setting the pH of the absorbing solution N from a weakly acidic region to near neutrality, sulfuric acid gas remaining in the absorbing solution N together with ammonia is absorbed.

For example, when the sulfur dioxide gas concentration in the exhaust gas A3 after desulfurization is 50 ppm, the following verification data (FIG. 3)
As shown in Fig. 5, even when the pH of the absorbing solution N is about 6.0,
A deaeration rate of about 0% can be achieved. For this reason, if there is no problem with the deaeration rate of about 50%, the pH of the absorbing solution N may be about 6.0. Then, the pH of the absorbing solution N is 6.0.
In the case of the degree, as shown in the verification data (FIG. 4) described later, a desulfurization rate of about 50% is achieved by the absorbent spraying step and the mist removing step. For this reason, even if the sulfur dioxide gas concentration in the desulfurized flue gas A3 is 50 ppm, the sulfur dioxide gas concentration in the treated flue gas A4 is about 25 ppm.

In other words, even when the sulfur dioxide concentration in the treated flue gas A4 needs to be about 25 ppm, the sulfur dioxide gas concentration in the desulfurized flue gas A3 is 50 ppm, and the pH of the absorbing solution N is about 6.0. Then, the remaining sulfurous acid gas in the desulfurized flue gas A3 is absorbed and removed together with the ammonia in the desulfurized flue gas A3, so that a sulfur dioxide gas concentration of 25 ppm can be finally achieved. In addition, at the same time, since the deaeration rate of 50% can be achieved, the ammonia concentration in the exhaust gas A3 after desulfurization becomes, for example, 50%.
ppm, the ammonia concentration in the treated flue gas A4 is 25
It can be suppressed as low as about ppm.

The concentration of the sulfur dioxide in the flue gas A3 after desulfurization is set according to the parameters (slurry pH, slurry pH, etc.) that affect the performance (desulfurization rate) of the absorption tower 8 with respect to the sulfur dioxide gas concentration in the flue gas A2 before desulfurization. The setting and control of the circulation flow rate) may be performed indirectly. However, flue gas before desulfurization A2
Of flue gas after desulfurization A
In order to maintain the sulfur dioxide concentration in the smoke 3 at a predetermined value, for example, a sensor for detecting the sulfur dioxide concentration in the post-treatment smoke A3 in real time is provided, and the measured value of the sulfur dioxide concentration is maintained at a set value. As described above, the supply amount of the absorbent and the circulation flow rate of the slurry in the absorption tower 8 may be directly controlled.

In any case, as described above, the amount of sulfurous acid gas absorbed by the absorption tower 8 is positively reduced, and the absorption liquid spraying step and the mist removal step at the outlet side (flue 41) of the absorption tower 8 are performed as follows. In a mode in which the remaining sulfurous acid gas is absorbed and a predetermined amount of ammonia is absorbed and removed, in addition to the effect that the amount of ammonia released to the atmosphere is suppressed, the following practically excellent properties are obtained. The effect is obtained.

That is, first, since a relatively high pH solution can be used as the absorbing solution N, chemicals such as sulfuric acid for pH adjustment are not required at all, or the amount thereof can be reduced, and the operating cost and the like can be reduced accordingly. Can be reduced. Further, since the desulfurization rate in the absorption tower 8 itself may be lower than the conventional one, the circulation flow rate of the slurry (the amount of the slurry injected from the spray pipe 24) is reduced or the set value of the pH of the slurry is lowered. Thus, the amount of the raw material for the absorbent (in this case, limestone) can be reduced.
Therefore, from this point as well, a further reduction in operating cost and the like can be realized.

The fact that the pH of the slurry in the absorption tower 8 is lowered means that the amount of the raw material for the absorbent (in this case, limestone) can be reduced and the concentration of the unreacted absorbent in the slurry can be reduced. Therefore, there is also an advantage that the purity of the gypsum H produced as a by-product increases. Also,
When the pH of the slurry in the absorption tower 8 decreases, the pH of the desulfurization wastewater K decreases.
Similarly, the desulfurization wastewater K can be easily used as the absorbent N sprayed in the absorbent spraying step.

Further, when the pH of the slurry in the absorption tower 8 decreases, the concentration of ammonia released into the exhaust gas A3 after desulfurization tends to decrease due to the equilibrium partial pressure of ammonia shown in FIG. If the ammonium ion concentration in the slurry of the tower 8 is constant, the ammonia concentration in the post-treatment flue gas A4 tends to decrease. Conversely, conversely, it is possible to further increase the ammonium ion concentration in the slurry of the absorption tower 8 while maintaining the concentration of ammonia released into the exhaust gas A3 after desulfurization at a constant level.

According to the study by the inventors, when the concentration of ammonium ions in the slurry of the absorption tower increases, as shown in the data of FIG. 7, for example, as shown in the data of FIG. It is known that the desulfurization rate of the slurry is improved, and if the same desulfurization rate is achieved, the circulation flow rate of the slurry and the usage of the absorbent can be reduced accordingly.

In the case of this example, the ammonia present in the flue gas A2 before desulfurization is absorbed in the slurry of the absorption tower 8, so that ammonium ions are supplied to the slurry and the ammonia is absorbed. The recovered liquid P collected by the mist eliminator 44 is stored in the tank 21 of the absorption tower 8.
The ammonium ions are also supplied into the slurry in the absorption tower 8, thereby contributing to an improvement in desulfurization performance (or a reduction in the amount of absorbent used).

In other words, according to this example, a large amount of the ammonia which was conventionally contained in the treated flue gas A4 and could be released to the atmosphere in a large amount was recovered in a considerable amount and effectively used for improving the desulfurization performance and the like. An excellent effect is obtained.

The ammonium ion concentration in the slurry of the absorption tower accumulates and increases excessively if left unattended.
In this case, the wastewater is discharged out of the system as desulfurized wastewater K together with other harmful substances (for example, chlorine and heavy metals). The method of treating the desulfurized wastewater K is to recover the ammonia contained in the desulfurized wastewater K. However, it is preferable to adopt a method that can be reused as ammonia to be injected into flue gas before the above-mentioned denitration apparatus or absorption tower. For example, the existing non-drainage technology (so-called A) that mixes the dust D of the dry electric precipitator 5 with the desulfurization wastewater K and collects and reuses ammonia.
Adoption of (WMT) is advantageous in that ammonia can be circulated and used, and the required amount of ammonia separately supplied from outside the system can be significantly reduced.

In this embodiment, the wet dust removing step of the present invention is performed by the wet electrostatic precipitator 9. That is, the flue gas A3 is introduced into the wet electric precipitator 9 on the downstream side of the absorption tower 8 (in this case, on the downstream side of the mist eliminator 43), and the remaining dust is removed. Thereby, the dust concentration in the post-treatment smoke exhaust A4 becomes extremely low.

(Demonstration Data) Next, the results of experiments conducted by the inventors to verify the effects of the present invention (demonstration data)
Will be described. The experiment uses equipment having an absorbent spraying unit and a mist removing unit having the same configuration as the above-described flue 41, spray nozzle 42, and mist eliminator 43,
The gas corresponding to the exhaust gas A3 after desulfurization was supplied to the absorption liquid spraying section of this facility. And the gas flow rate is 836m ³ N
/ H, the gas flow rate is 5 m / s, and the gas temperature is 50
° C, and the ammonia concentration in the gas was 25 ppm. The temperature of the absorbing solution to be sprayed was adjusted to 30 ° C., and the concentration of ammonium sulfate in the absorbing solution was set to 200 mmol / liter.

Note that the spray nozzle is 4
They were used side by side. In addition, as the mist eliminator, a folded plate type mist eliminator, a Euroform type mist eliminator and a chevron type mist eliminator, were used, but there was no difference in performance between any of the mist eliminators. An experimental result when one (30 mm pitch) element is used will be described.

First, FIG. 3 shows the measured data of the removal rate (deaeration rate) of ammonia removed in the absorption liquid spray section and the mist removal section when the pH of the absorption liquid to be sprayed is changed as a parameter. Things. In this measurement, the sulfurous acid gas concentration in the gas supplied to the absorbent spraying unit was set to 0 p.
pm, 15 ppm and 50 ppm. In this case, the liquid-gas ratio (L / G) of the absorbing liquid to be sprayed was adjusted to a constant value of 0.04 [liter / m ³ N].

As a result of the experiment, when no sulfur dioxide gas was present in the flue gas, almost no ammonia was absorbed when the pH of the absorbing solution was 4.0 or more. Found that it was necessary to lower the pH of the absorbing solution to about 2.3. However, when about 15 ppm of sulfurous acid gas is present in the flue gas, the pH of the absorbing solution becomes 6.
A deaeration rate of 30% or more can be realized even at 0, and if the sulfur dioxide gas is present in the flue gas at about 50 ppm, the pH of the absorbing solution is reduced.
It has been proved that a deaeration rate of about 50% can be realized even with a pH of 6.0.

Next, FIG. 4 shows measured data of the removal rate (desulfurization rate) of sulfurous acid gas removed in the absorption liquid spray section and the mist removal section when the pH of the absorption liquid to be sprayed is changed as a parameter. It was done. In this measurement, the sulfurous acid gas concentration in the gas supplied to the absorbent spraying section was set to 50 p.
pm (constant). In this case, the liquid-gas ratio (L / G) of the absorbing liquid to be sprayed was adjusted to a constant value of 0.04 [liter / m ³ N].

As a result of the experiment, when the pH of the absorbing solution was 2.0, almost no sulfurous acid gas was absorbed. However, when the pH of the absorbing solution was, for example, 6.0, the absorption solution spraying step and the mist removing step of the present invention resulted in the following. It has been demonstrated that a desulfurization rate of about 50% can be realized.

Next, FIG. 5 shows the removal rate of ammonia (deaeration) in the absorbing liquid spraying section and the mist removing section when the liquid-gas ratio (L / G) of the absorbing liquid to be sprayed is changed as a parameter. 2 shows the measured data of (ratio). Note that this measurement was performed with the sulfur dioxide gas concentration in the gas supplied to the absorbent spraying unit being 0 ppm (constant). In this case, the pH of the absorbing solution to be sprayed was adjusted to 2.0 (constant). As a result of the experiment, it was found that the deaeration rate increased according to the spray amount of the absorbing solution.

Next, FIG. 6 shows the pressure loss in the absorbent spraying section and the mist removing section (flue 41 in FIG. 1) when the liquid-gas ratio (L / G) of the absorbing liquid to be sprayed is changed as a parameter. Or the pressure loss of the mist eliminator 43)
2 shows the measurement data of FIG. As a result of the experiment, it was found that even if the spray amount of the absorbing solution was changed, the pressure loss hardly changed, and it was always very small, about 3.0 mmH ₂ O.

It should be noted that the present invention is not limited to the above-described embodiment, and may have various aspects. First, as shown in FIG. 2, for example, a vertically installed spray nozzle 51 is used as a spray nozzle for performing the absorbent spraying step of the present invention, and this spray nozzle 51 is connected to the mist eliminator at the outlet side of the absorption tower 8. 24a, and is arranged on the upstream side (lower side) of the absorption tower 8a.
The embodiment in which the absorbing liquid spraying step and the mist removing step of the present invention are performed on the outlet side of the present invention.

The absorbing liquid spraying step and the mist removing step of the present invention may be performed in multiple stages by disposing a plurality of spray nozzles and mist eliminators for performing these steps in the flow direction of the flue gas. By doing so, the ammonia concentration can be further reduced. For example, in the case of providing two stages, even if the individual deaeration rate of each stage is 50%,
The overall deaeration rate is 75%. In this case as well, if the sulfurous acid gas is absorbed together with the ammonia in each stage, and the sulfurous acid gas concentration and the ammonia concentration in the exhaust gas after the final treatment are reduced to the target values or less, the smoke exhaust gas can be further cleaned. Can be realized, and the pH of the absorbing liquid sprayed at each stage can be made relatively high, so that chemicals such as sulfuric acid become unnecessary or the amount of use thereof can be significantly reduced.

The absorption tower for performing the desulfurization step of the present invention is not limited to the countercurrent type liquid column tower exemplified in the above embodiment, but may be a packed tower, a spray tower, or the like. A co-current absorption tower may be used. However, in the case of a countercurrent type absorption tower, the present invention is applied because the pH of the liquid at the outlet side of the absorption tower is relatively high, so that ammonia is easily diffused and the ammonia concentration of the exhaust gas after desulfurization is easily increased. The effect is particularly remarkable.

The spray amount of the absorbing solution and the pH of the absorbing solution in the absorbing solution spraying step of the present invention are set as constant values according to the average value (or the worst value) of operating conditions such as smoke emission properties. Alternatively, the following control may be performed so that the control of the desulfurization performance of the absorption tower can be performed so as to be able to determine and cope with fluctuations in smoke emission properties and the like in detail. For example, a sensor is provided for online detection of the ammonia concentration and the sulfur dioxide concentration of the desulfurized flue gas A3 and the treated flue gas A4 in FIG. 1, and based on the detection results of these sensors, FIG.
5 may be appropriately calculated based on the relationship between FIG. 5 and FIG. 5 to control the spray amount and pH to achieve the target performance, and control means for real-time controlling the spray amount and pH to achieve the optimum value may be provided.

[0069]

According to the flue gas treatment method of the present invention, an absorbent having a pH suitable for absorbing ammonia present in the flue gas after the desulfurization step during the flue gas before the mist removal step and before the mist removal step. Was sprayed to absorb the ammonia in the absorbing solution. For this reason, the sulfurous acid gas in the flue gas can be removed, and the ammonia concentration in the flue gas after the treatment can be much lower than before. For this reason, prevention of air pollution due to harmful substances in the smoke can be more fully achieved.

Further, in the flue gas treatment method according to the second aspect, the recovered liquid recovered in the mist removal step is introduced into an absorption tower for performing a desulfurization step, and a calcium compound-containing slurry for absorbing sulfurous acid gas is constituted. Use as a liquid to remove. For this reason, the amount of water used can be saved, and the ammonia that has conventionally been released to the atmosphere can be effectively used. That is, in this case, at least a part of the ammonia remaining in the flue gas after the desulfurization step is introduced into the slurry of the absorption tower in a form dissolved in the recovered liquid, and the desulfurization performance of the absorption tower Contribute to the improvement of

Further, in the smoke exhaust treatment method according to the third aspect,
Wet dust is introduced into the wet-type electrostatic precipitator on the downstream side of the absorption tower to remove dust remaining in the smoke, so that the dust concentration in the smoke after treatment can be particularly reduced. It can contribute to cleaner smoke emission. In this case, the drainage discharged from the wet electrostatic precipitator is used as at least a part of the absorbing liquid in the absorbing liquid spraying step, so that the amount of water used can be reduced.

Further, in the smoke exhaust treatment method according to the fourth aspect,
In the desulfurization process, the slurry that has been in gas-liquid contact with the flue gas and absorbed the sulfur dioxide gas is oxidized and dehydrated to produce gypsum solids as a by-product, and a part of the filtrate (desulfurization effluent) generated by the dehydration of this slurry is removed. Also, since the structure is used as at least a part of the absorbing liquid in the absorbing liquid spraying step, the amount of water used can be saved.

Further, in the smoke exhaust treatment method according to the fifth aspect,
The concentration of sulfur dioxide in the flue gas after the desulfurization process is set higher than the target value of the concentration of the sulfur dioxide gas in the flue gas after the treatment, and a part of the sulfur dioxide gas to be removed remains in the flue gas after the desulfurization process. At the same time, by setting the pH of the absorbing solution in the absorbing solution spraying step from a weakly acidic region to near neutrality (for example, 4.0 to 6.0), the absorbing solution contains in the exhaust gas after the desulfurization step. Together with ammonia present in the flue gas after the desulfurization step.

Therefore, taking into account the case where almost no sulfurous acid gas is present in the flue gas after the outflow step, p
H is set to, for example, about 2.0 to 4.0, and the pH of the absorbing solution becomes lower in each stage as compared with the case where ammonia in the flue gas must be absolutely removed. For this reason,
There is no need to add sulfuric acid or the like to adjust the pH of the absorbing solution, or at least the amount of addition can be significantly reduced. Further, it is possible to use the above-mentioned filtrate (desulfurization waste water) without adjusting the pH as it is, which is extremely advantageous in practical use.

In this case, on the other hand, the burden (desulfurization rate) of the desulfurization step itself in the absorption tower can be reduced as compared with the conventional case, so that the slurry circulation flow rate of the absorption tower and the supply amount of the absorbent can be reduced. An excellent effect of (reducing the pH of the slurry) is also obtained. When the supply amount of the absorbent in the absorption tower can be reduced (the pH of the slurry is reduced), the amount of the absorbent used can be reduced, the purity of gypsum by-produced, and the ammonia by the decrease in the slurry pH can be reduced. Various advantages are obtained, such as a reduction in the amount of radiation.

[Brief description of the drawings]

FIG. 1 is a diagram showing equipment for implementing a smoke exhaust treatment method which is an example of the present invention.

FIG. 2 is a diagram showing equipment for performing a smoke exhaust treatment method according to another example of the present invention.

FIG. 3 is data showing the relationship between the ammonia removing action and the pH of the absorbent by the absorbent spraying step and the mist removing step of the present invention.

FIG. 4 is data showing the action of removing sulfurous acid gas by the same step.

FIG. 5 is data showing the relationship between the ammonia removing action and the amount of absorbing liquid in the same step.

FIG. 6 is data of pressure loss in the same step.

FIG. 7 is data showing the relationship between the ammonium ion concentration in the slurry of the absorption tower and the desulfurization rate.

FIG. 8 is a view showing a conventional flue gas treatment facility.

FIG. 9 is a diagram showing the relationship between pH and the equilibrium partial pressure of NH ₃ .

[Explanation of symbols]

 8 Absorption tower 9 Wet electric dust collector 41 Flue 42 Spray nozzle 43 Mist eliminator A Untreated flue gas A2 Desulfurized flue gas A3 Desulfurized flue gas (smoke flue after desulfurization process) A4 Treated flue gas B Ammonia F Waste liquid H Gypsum solids K Desulfurization wastewater (part of filtrate) N Absorbent P Recovered liquid

Claims (5)

[Claims] 1. An exhaust gas containing at least ammonia and sulfurous acid gas is led to an absorption tower, and p-gas suitable for absorbing sulfurous acid gas is introduced.
A desulfurization step of absorbing and removing at least sulfur dioxide from flue gas by bringing the slurry containing the calcium compound containing H into gas-liquid contact, and a mist removing step of removing mist from the flue gas after the desulfurization step and collecting the mist as a liquid. In the flue gas treatment method, an absorbent having a pH suitable for absorbing ammonia present in the flue gas after the desulfurization step is sprayed into the flue gas after the desulfurization step and before the mist removal step, and this absorption is performed. A method for treating flue gas, comprising a step of spraying an absorbing liquid for absorbing the ammonia in the liquid. 2. The method according to claim 1, wherein the liquid collected in the mist removal step is introduced into the absorption tower and used as a liquid constituting the calcium compound-containing slurry. 3. A wet dust removing step for introducing flue gas to a wet-type electrostatic precipitator on the downstream side of the absorption tower to remove dust remaining in the flue gas, and in the wet dust removing step, 3. The method according to claim 1, wherein the waste liquid discharged from the dust collector is used as at least a part of the absorbing liquid. 4. A slurry containing gypsum by-produced by oxidizing and dewatering the slurry that has absorbed the sulfurous acid gas in gas-liquid contact with the flue gas in the desulfurization step. The method according to claim 1, wherein a part is used as at least a part of the absorbing liquid. 5. The method according to claim 1, wherein a concentration of the sulfur dioxide in the flue gas after the desulfurization step is set higher than a target value of the concentration of the sulfur dioxide gas in the flue gas after the treatment. By setting the pH of the absorbing solution from a weakly acidic region to near neutrality, the desulfurization is performed together with the ammonia present in the smoke after the desulfurization step. The method according to any one of claims 1 to 4, wherein sulfur dioxide gas remaining in the smoke after the process is absorbed.