JPH07194934A - Exhaust gas treatment method - Google Patents

Abstract

PURPOSE:To utilize an absorbent liquid, to desulfurize exhaust gas at a high yield, and to obtain gypsum with stable quality by a method in which cooled exhaust gas is brought intocontact with absorbent liquid while oxygen being introduced to remove harmful substances in the gas, and the pH of the absorbent liquid is regulated at a given value or lower in the process. CONSTITUTION:Exhaust gas 11 containing harmful substances such as sulfur oxides and fluorine compounds is introduced into an absorption column 12, where the gas is sprayed with an absorbent liquid containing a calcium compound for gas-liquid contact, and the treated gas is cooled, dust-removed, and desulfurized. The gas, after the contained mist being removed by a mist eliminator 13, is discharged as treated gas 14. In this process, air for oxidation is introduced into the lower part of the absorption column 12, where oxygen is absorbed, dissolved sulfite is oxidized, the calcium compound is dissolved and neutralized, and gypsum is produced. A part of the absorbent slurry is sent to a solid-liquid separator 17 to separate solid containing gypsum, while the mother liquor is sent to an absorbent preparation tank 19, where the calcium compound is added. The pH of the absorbent liquid is made below 4.5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for removing harmful substances from exhaust gas containing at least sulfur oxides or fluorine compounds. In particular, it relates to a method of treating coal combustion exhaust gas with high efficiency while sufficiently maintaining the utilization efficiency of an absorbent such as a calcium compound.

[0002]

2. Description of the Related Art Recently, with the diversification of energy,
Coal is often used as a boiler fuel in power plants and factories. The combustion exhaust gas of this coal contains dust containing fluorine compounds and aluminum compounds, etc., at high concentrations as high as several tens of times higher than those of heavy oil combustion exhaust gas, and each of these components contributes to desulfurization performance. It is known to have various adverse effects. That is, when hydrogen fluoride and other fluorine compounds and aluminum compounds are mixed in the absorption liquid of the flue gas desulfurization device, CaCO
_A sparingly soluble compound (apatite) is generated on the surface of the absorbent such as ₃ and causes a phenomenon of inhibiting dissolution of the absorbent. It has been reported that once this phenomenon occurs, no matter how the operating conditions are changed, the dissolution of the absorbent itself becomes extremely difficult (hereinafter referred to as "decrease in the dissolution activity of the absorbent"). In particular, it is said that when the so-called dissolution activity is remarkably lowered, it hardly acts as an alkali. As a result, the utilization efficiency of the absorbent that originally contributes to desulfurization cannot be sufficiently maintained, and suddenly, there arises a problem that the desulfurization rate cannot be maintained at a predetermined value.

In order to solve such a problem, conventionally,
Investigations have been made from various angles, and various improved techniques have been proposed. For example, Japanese Patent Laid-Open No. 62-2
Japanese Patent No. 44426 discloses that a calcium-based absorbent and an alkaline agent are added to the slurry after absorption and oxidation in an absorption tower to adjust the pH to 4.5 or more to prevent a decrease in calcium carbonate dissolution activity. Proposals have been made. Further, in Japanese Patent Laid-Open No. 53-129167, an alkali additive is added to the absorbing solution before the absorption treatment to increase the pH to 5.0 or higher and reduce the concentrations of dissolved aluminum and dissolved fluorine, respectively. Has proposed to prevent the decrease of the dissolution activity of calcium carbonate. Further, in JP-A-59-228926, a manganese compound and sodium hydroxide are added to an absorbing solution, and especially by adding sodium hydroxide, the pH is raised and the aluminum compound in the fly ash is dissolved. It has been proposed to prevent deterioration of desulfurization performance by decreasing

Each of these conventional proposals has a pH of 4.
It is a treatment method of 5 or more in a relatively high pH range.

[0005]

However, the present inventors have found that the so-called dissolution activity of the absorbent and the operating conditions are as follows.
As a result of intensive studies, it has been found that the following problems occur under the conventionally proposed operating conditions in a relatively high pH range. That is, using a so-called soot mixing type processing apparatus in which cooling (dust removal) and absorption / removal are performed in one tower, the absorption liquid is kept in a relatively high pH region (for example, pH 4.5 or higher) and fluorinated. When an operating experiment was conducted in which fly ash containing hydrogen and other fluorinated compounds and aluminum compounds was mixed, it was found that a significant decrease in the dissolution rate of calcium carbonate (absorbent) could occur. Therefore, as shown in FIG. 7, when a facility for supplying a sodium compound (particularly, NaOH, Na ₂ SO ₄ ) is provided and operated, a problem that the operation cost increases due to a large amount of the sodium compound being charged occurs. Or, the problem of instability of gypsum quality as a by-product due to a temporary decrease in solubility occurs, or the installation of continuous measuring instruments and control devices for special absorption liquid compositions for the addition of sodium compounds, etc. There were various problems such as an increase in construction costs due to

By the way, a so-called soot separation type processing apparatus in which a cooling tower and an absorption tower are separated as shown in FIG. 8 is provided with a cooling tower of a different flow as a pre-treatment.
In this cooling tower, cooling and dust removal are performed in advance, and moreover,
The structure is such that entrained mist is removed by the mist eliminator before reaching the absorption tower in the next step. For such structural reasons, hydrogen fluoride, fly ash, etc. are removed in the cooling tower where the pretreatment is performed, and the solubility of calcium carbonate in the absorbing solution used in the next step is reduced. It was believed that the problem would not occur. However, by using this soot separation type processing device, a region where the pH of the absorbing liquid is relatively high (for example,
When a confirmation experiment was performed by operating at pH 4.5 or higher), even in the soot separation system, the entrainment (F and Al-containing compound) in the exhaust gas discharged from the cooling tower, which is the pretreatment, was extremely small. Although it is an amount, in some cases, it may be mixed in the absorbing solution in the removal step of the next step, and in this case, it was found that the decrease in the dissolution rate of the absorbent is particularly remarkable. This is because the pH in the cooling step, which is the first-stage treatment, is extremely low at about 0.5 to 1.5, and the fluorine and aluminum are easily dissolved or already dissolved (particularly in the form of AlF complex). ) And the pH of the absorption liquid is higher than that in the cooling step, it is considered that the influence on the decrease in the dissolution activity of the absorption agent will be extremely large even if the entrainment amount is small (reverse). In the soot mixing method, the proportion of Al ₂ O ₃ , CaF _{2 and the} like that are difficult to dissolve is relatively high). Furthermore, in the soot separation method,
It was found that the greater the pH difference between the cooling step and the removal step, the greater the influence on the decrease in dissolution activity.

As described above, regardless of the soot mixing method or the soot separation method, the operation of operating in a high pH region (for example, pH 4.5 or higher) may reduce the dissolution rate of the absorbent. It has been confirmed that the methods proposed so far are not sufficient, and there is a demand for improvement techniques to cope with this.

The present invention was devised under such circumstances, and its purpose is to maintain high utilization efficiency of the absorbent without lowering the dissolution activity of the absorbent, and to improve the utilization efficiency of the absorbent. An object of the present invention is to provide an exhaust gas treatment method capable of obtaining a desulfurization rate and obtaining gypsum of stable quality.

[0009]

[Means for Solving the Problems] In order to achieve such an object, the inventors of the present application have earnestly studied, and as a result, in an actual device, for example, apatite which inhibits the dissolution of the absorbent is used. A reaction in which a different substance is formed so as to cover the surface of the absorbent, and a reaction in which the absorbent (calcium carbonate, etc.) dissolves before being covered by apatite (reaction with SO ₂ absorption and hydrogen ions generated by oxidation) However, if the latter's dissolution reaction can always be prioritized (increased), it is possible to control the decrease in dissolution activity caused by the formation of substances such as apatite. The present invention has been accomplished by finding operating conditions that can realize it.

That is, according to the present invention, a cooling step of cooling an exhaust gas containing a harmful substance such as a sulfur oxide and a fluorine compound, and the exhaust gas that has undergone the cooling step are brought into gas-liquid contact with an absorption liquid containing a calcium compound. And a removal step of introducing an oxidizing gas containing oxygen into the absorption liquid to remove harmful substances in the exhaust gas, and a solid-liquid separation of solid content containing gypsum from the absorption liquid that has undergone the removal step. An aluminum compound including four steps of a separation step and a charging step for charging a calcium-based compound into the mother liquor after the solid-liquid separation step to replenish the calcium content removed by the solid-liquid separation step. Is a method for treating exhaust gas, wherein the pH of the absorption liquid in the removal step is
To less than 4.5.

Hereinafter, as an example of an apparatus for carrying out the method of the present invention, a simplified apparatus diagram of a soot mixing system is illustrated in FIG. 1, and the present invention will be described in accordance with it. First, the form of the absorption tower 12 will be described as a general multi-stage spray type.

Exhaust gas 11 containing harmful substances such as sulfur oxides and fluorine compounds is introduced into an absorption tower 12 having a liquid storage section having a retention time for absorbing liquid according to the properties of the exhaust gas. The absorption tower 12 has a cooling step and a removing step. That is, in the absorption tower 12, the exhaust gas is brought into gas-liquid contact with an absorption liquid containing a calcium-based compound sprayed from a multi-stage spray by, for example, a pump, and is cooled, dust-removed and desulfurized, and then the mist eliminator 13 is used. After the entrained mist is removed by, the process gas 14 is discharged from the absorption tower 12. Absorption tower 12
Air for oxidation is introduced into the liquid in the lower liquid reservoir, where oxygen is absorbed and dissolved sulfur oxide is oxidized, and the calcium compound is dissolved and neutralized, Gypsum production and crystal growth takes place. As the calcium compound, calcium carbonate, calcium hydroxide, calcium oxide or the like is used.

A part of the absorption liquid slurry which has been subjected to the removal step in the absorption tower 12 is sent to the solid-liquid separator 17.
Here, a solid-liquid separation step of solid-liquid separating the solid content containing gypsum is performed. The mother liquor after this separation step is further sent to the absorbent mixing tank 19, and here, in order to mix and supplement the calcium component removed by the solid-liquid separation step, a calcium compound is added to the mother liquor after the solid-liquid separation step. A charging step for carrying out is performed. The absorption liquid prepared after the charging step is the absorption tower 1
Returned to 2. In this figure, as an emergency measure, as shown by the dotted line, NaOH, Na ₂ SO ₄ , Na
₂ CO ₃ , Mg (OH) ₂ and other hydroxides, carbonates or sulphates, such as the absorption liquid reservoir below the cooling / absorption tower, the mother liquor after solid-liquid separation, the line to the solid-liquid separator, etc. Alternatively, it is preferable to be able to batch-charge at least one place. Even when operating below pH 4.5
As a measure against the occurrence of temporary decrease in dissolution activity due to troubles such as high pH operation due to accuracy error of meter reading value, drainage failure of absorbing solution due to drainage system trouble, etc. Input so that it does not fluctuate significantly. This pH is 6.5 or less, preferably (set pH) ± 1.0, practically (set pH) ±
It is 0.5. This is because the desulfurization rate does not change significantly. Therefore, among the above-mentioned compounds to be added in a batch, sulfate is particularly preferable. This can prevent a decrease in pH and a decrease in desulfurization rate due to a temporary decrease in dissolution activity.

Such an exhaust gas treatment method of the present invention is intended to include the above-mentioned four steps and an aluminum compound mixed in any of the above-mentioned four steps. The requirement that the aluminum compound is mixed in any of the four steps can be easily understood from the balance with the above problem to be solved by the present invention. That is, the aluminum component flowing into the exhaust gas treatment device is contained not only in the fly ash but also in the calcium compound absorbent (eg, calcium carbonate), and also flows from this absorbent. The aluminum component in fly ash is about 5 to 1 as aluminum.
About 5% is contained, and the fly ash concentration in the exhaust gas at the outlet of the electrostatic precipitator is 50 to 50% under normal operating conditions.
200 mg / m ³ N, and when the sum of the inflow amount of aluminum and the inflow amount of aluminum from the calcium-based compound is summed, an amount sufficient to cause a decrease in the dissolving activity of the absorbent is flowing into the absorbent solution. Because.

Further, in the exhaust gas treating method of the present invention, the pH of the absorbing solution in the removing step is less than 4.5, preferably 4.0 or less, more preferably 2.0 to 4.0.
It is said that If this value is 4.5 or more, it is not preferable because the dissolution rate of the absorbent decreases remarkably as will be seen from the examples described later. In addition, the amount of the absorbent far exceeding the theoretical equivalent of the absorbent has to be added, and the quality of the gypsum as a byproduct is deteriorated, and in some cases, pH control cannot be performed. On the other hand, if this value becomes too low, for example, less than 2.0, the pH is low,
As the neutralization curve shows, the responsiveness in relation to the amount of absorbent input and pH is blunt, pH control becomes difficult, and p
If the absorption liquid has a H value of less than 2.0, the desulfurization efficiency may decrease, and the power consumption may increase. Such control of the pH value is performed by adjusting the addition amount of the calcium-based compound in the absorbent preparation tank 19, adjusting the supply amount of the mother liquor containing the calcium-based compound after the separation step to the absorption tower (removal step), and the like. Done by In addition, the pH of the absorption liquid in the present invention means the pH of the absorption liquid existing at the bottom of the absorption tower that is used for gas-liquid contact in principle, and exceptionally in the middle of a so-called spray type absorption liquid circulation line. For those equipped with a mechanism for adding calcium compounds, it means the pH at the spray outlet.

By the way, particularly in the combustion gas of coal, about 90% of the fluorine compounds contained in the coal are generally contained.
Although it is separated and removed from the exhaust gas together with the ash in a boiler or an electric dust collector to some extent, the exhaust gas at the inlet of the exhaust gas treatment apparatus contains about 10% of the fluorine compound in the coal. HF gas, volatile fluorine compound in the fly ash in the exhaust gas (e.g., CaF _2, etc.) absorbents and calcium-based compounds (e.g., calcium carbonate) fluorine compound (e.g., CaF _2, etc.) the total amount of fluorine in However, it is 1.5 when converted to the gas concentration in the exhaust gas as fluorine.
Even in the case of ppm or more, since the pH of the absorbing solution is regulated in the predetermined range in the exhaust gas treating method of the present invention, it has been confirmed that the dissolution activity of the absorbent can be prevented from lowering. By the way, when the total amount of fluorine is 1.5 ppm or more, the conventional high pH treatment method remarkably decreases the dissolving activity of the absorbent. Converting the total amount of fluorine to the gas concentration in the exhaust gas means converting the total amount of fluorine into mol and
The volume content is calculated by multiplying 2.4 to obtain the hydrogen fluoride-equivalent gas amount in the standard state, and dividing it by the exhaust gas amount.

Further, as will be seen from the examples described later, the decrease in the dissolution activity becomes more remarkable in the coexistence of chlorine, but even under such conditions, the exhaust gas treatment method of the present invention absorbs It has been confirmed that a decrease in the dissolution activity of the agent can be prevented.

With respect to the above exhaust gas treatment apparatus, the form of the absorption tower 12 has been described by taking a general multi-stage spray type as an example. The form of the absorption tower 12 is a so-called jet as shown in FIG. It is preferable to replace it with a bubbling reactor 12 '. In this case,
In particular, since the gas-liquid contact method in the removing step is a gas bubbling method, a high desulfurization rate can be obtained with high efficiency even in a particularly low pH range. The basic technical idea regarding the jet bubbling reactor 12 'is, for example,
JP-B-55-37295, JP-B-60-47
No. 26 publication.

This jet bubbling reactor 12 '
2 will be briefly described with reference to FIG. 2. Exhaust gas 11 flows near the exhaust gas inlet duct of the jet bubbling reactor 12 ′ and / or the inlet plenum 22.
It is cooled in between, and jetted into the absorbing liquid 30 through a large number of super jar pipes 24 attached to the deck plate 23 to absorb and remove SO ₂ . The desulfurized processing gas 14a passes through the gas riser pipe 25 penetrating the inlet plenum 22 to the outlet plenum 26, and is discharged from the stack through the outlet duct. Here, the oxidizing air is
The calcium carbonate for the neutralization reaction is dispersedly supplied from the bottom of the reactor into the absorption liquid, and is supplied as slurry 33 into the reactor. The generated gypsum grows in the reactor and is extracted as the slurry 35.

Further, as described above, the problem to be solved by the present invention occurs in so-called soot mixing system and soot separation system, both types of devices, so the present invention can be applied to any type. Therefore, it is not limited to the soot mixing method given as an example for the sake of description.

The treatment method of the present invention basically includes four steps, but it goes without saying that various known steps may be added to these steps, if necessary.

[0022]

EXAMPLES The present invention will be described in more detail below by showing specific examples of the present invention. Example 1 The soot mixing method shown in FIG. 1 (however,
As the absorption tower, a jet bubbling reactor 12 'type shown in FIG.
An exhaust gas treatment experiment was conducted.

As processing conditions, the processing gas amount: 5 m ³ / h
r, inlet SO ₂ concentration: 620 ppm-dry, calcium-based compound: CaCO ₃ (particle size 200 mesh-90% pass, purity 98.5%), the total fluorine concentration as a fluorine compound in the absorption liquid, and the theory of calcium carbonate The relationship between the excess amount with respect to the equivalent amount and pH was investigated. The Cl concentration in the absorbing solution was 5000 mg / l, the Al concentration as an aluminum compound was 250 mg / l, and the Mg concentration was 500 mg / l. The results are shown in the graph of FIG. From the graph showing the results of this experiment, it was confirmed that when the pH of the absorbing solution was less than 4.5, the decrease in the dissolution rate due to the incorporation of the fluorine compound (the decrease in the so-called dissolution activity of calcium carbonate) was extremely effectively prevented. (Example 2) An exhaust gas treatment experiment was conducted using the same device as in Example 1 except that a soot separation type simulation device, that is, a cooling tower was newly provided as a pre-stage treatment. In this experiment, a part of the liquid in the cooling tower was introduced into the absorption tower (jet bubbling reactor 12 ′), and the amount of Al as an aluminum compound and the amount of fluorine as a fluorine compound flowing into the absorption tower were adjusted to those in Example 1. The amount was 1/20 of the case. As a result, as in the case of Example 1, when the pH of the absorbing solution was less than 4.5, the excess amount of calcium carbonate was rapidly reduced, and the decrease in the dissolution activity of calcium carbonate was extremely effectively prevented. It was confirmed. (Example 3) An exhaust gas treatment experiment was conducted using the same apparatus as in Example 1. Fly ash containing 60 mg / kg of a fluorine compound as fluorine was set to 100 mg / m ³ N,
Furthermore, calcium carbonate containing 100 mg / kg of a fluorine compound was used as fluorine, and the other conditions were the same as in Example 1 above. In this experiment, HF in exhaust gas
The concentration was changed and the relationship between the total amount of fluorine converted into the HF concentration in the exhaust gas, the excess amount with respect to the theoretical equivalent of calcium carbonate, and pH was examined. The results are shown in the graph of FIG. From the graph showing the results of this experiment, the concentration of the fluorine compound (HF) in the exhaust gas in which the decrease in the dissolution rate becomes remarkable is 1.
It was confirmed that even when the content is 5 ppm or more, when the content is less than H4.5, the dissolution rate decrease (so-called dissolution activity decrease of calcium carbonate) due to the incorporation of the fluorine compound is extremely effectively prevented. (Example 4) An exhaust gas treatment experiment was conducted using the same apparatus as in Example 1 to investigate the relationship between the concentration of chlorine added in the absorbing solution, the excess amount of calcium carbonate relative to the theoretical equivalent, and pH. The total F concentration as the fluorine compound in the absorption liquid was 1000 mg / l, the total Al concentration as the aluminum compound was 250 mg / l, and the Mg concentration was 500 mg / l. The results are shown in the graph of FIG. From the graph showing this experimental result,
It was confirmed that when the pH of the absorbing solution was less than 4.5, the decrease in the dissolution rate (the decrease in the calcium carbonate dissolution activity) due to chlorine contamination was extremely effectively prevented. (Example 5) The pH dependence of the dissolution rate of calcium carbonate in the presence of HF and Al (OH) ₃ was measured. The results are shown in the graph of FIG. 6 below. From the graph showing the results of this experiment, it was confirmed that when the pH of the absorbing solution was 4.5 or more, the dissolution rate rapidly decreased (decrease in calcium carbonate dissolution activity).

[0024]

ACTION AND EFFECT From the above results, the effect of the present invention is clear. That is, the present invention, a sulfur oxide, a cooling step of cooling the exhaust gas containing harmful substances such as fluorine compounds, the exhaust gas after the cooling step, in gas-liquid contact with an absorbing liquid containing a calcium-based compound, and A removal step of introducing an oxidizing gas containing oxygen into the absorption liquid to remove harmful substances in the exhaust gas; and a solid-liquid separation step of solid-liquid separation of a solid content containing gypsum from the absorption liquid that has passed through the removal step. And a charging step for charging a calcium-based compound into the mother liquor after the solid-liquid separation step in order to replenish the calcium content removed by the solid-liquid separation step, and the aluminum compound is added to the above-mentioned 4 steps. A method for treating exhaust gas which is mixed in any one of the steps, wherein the pH of the absorption liquid in the removal step is
Is configured to hold below 4.5,
A high desulfurization rate can be obtained and gypsum with stable quality can be obtained while sufficiently maintaining the utilization efficiency of the absorbent without reducing the dissolution rate of the absorbent.

[Brief description of drawings]

FIG. 1 is a simplified device diagram of a soot mixing system as an example of a device for carrying out the method of the present invention.

FIG. 2 is a schematic device diagram of a jet bubbling reactor.

FIG. 3 is a graph showing the relationship between the total fluorine concentration in the absorbent, the excess amount of calcium carbonate with respect to the theoretical equivalent, and pH.

FIG. 4 is a graph showing a relationship between a pH obtained by changing the HF concentration in exhaust gas and converting the total amount of fluorine into an HF concentration in exhaust gas, an excess amount with respect to a theoretical equivalent of calcium carbonate, and pH.

FIG. 5 is a graph showing the relationship between the concentration of chlorine added to the absorbent, the excess amount of calcium carbonate with respect to the theoretical equivalent, and pH.

FIG. 6 is a graph showing the pH dependence of the dissolution rate of calcium carbonate in the coexistence of HF and Al (OH) ₃ .

FIG. 7 is a schematic device diagram for explaining a so-called soot mixing type processing device.

FIG. 8 is a schematic device diagram for explaining a so-called soot separation type processing device.

[Explanation of symbols]

 11 ... Exhaust gas 12 ... Absorption tower 13 ... Mist eliminator 17 ... Solid-liquid separator 19 ... Absorbent mixing tank

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location B01D 53/34 ZAB 53/68 C01F 11/46 102 C 9040-4G B01D 53/34 134 C

Claims (6)

[Claims] 1. A cooling step of cooling an exhaust gas containing a harmful substance such as a sulfur oxide and a fluorine compound, and bringing the exhaust gas after the cooling step into gas-liquid contact with an absorption liquid containing a calcium compound, and A removal step of introducing an oxidizing gas containing oxygen into the absorption liquid to remove harmful substances in the exhaust gas, and a solid-liquid separation step of solid-liquid separation of solids containing gypsum from the absorption liquid that has undergone the removal step, In order to replenish the calcium component removed by the solid-liquid separation step, a charging step for charging a calcium-based compound into the mother liquor after the solid-liquid separation step is included, and the aluminum compound is added to the above-mentioned 4 steps. The exhaust gas treatment method of mixing into any one of the steps, wherein the pH of the absorption liquid in the removal step is maintained at less than 4.5. 2. The pH of the absorbing solution in the removing step is 2.0.
It is -4.0, The exhaust gas processing method of Claim 1 characterized by the above-mentioned. 3. The exhaust gas treating method according to claim 1, wherein the cooling step and the removing step are performed by one unit and one tower. 4. The exhaust gas treatment method according to claim 1, wherein the exhaust gas is generated by combustion of coal. 5. The total amount of fluorine in the exhaust gas introduced into the cooling step and the fluorine compound contained in the calcium-based compound is 1.5 ppm or more in terms of fluorine-containing gas concentration in the exhaust gas. Claim 1 characterized by
5. The exhaust gas treatment method according to claim 4. 6. The exhaust gas treatment method according to claim 1, wherein a gas-liquid contact method in the removing step is a gas bubbling method.