JP7150268B2 - Scale removal method and scale removal composition - Google Patents

Description

TECHNICAL FIELD The present invention relates to a scale removing method and a scale removing composition for removing scale adhering to an exhaust gas system in which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus flows to a chimney.

BACKGROUND ART Many flue gas desulfurization facilities are installed as facilities for removing sulfur oxides (SOx) in exhaust gases. The mainstream of the flue gas desulfurization process in recent years is the wet method, and although there are differences in the type of absorbent and the amount of by-products, basically, (1) cooling / absorbing device, (2) ventilation device, (4) raw material (absorbent) storage and supply system; and (5) by-product treatment system. Among the wet methods, the lime-gypsum method is widely used as a desulfurization device for industrial large-scale boiler exhaust gas.

In addition to the above configuration, a mist removing device (mist eliminator) is provided to remove mist generated during SOx absorption processing by wet flue gas desulfurization. In some cases, an SGH (steam-gas heater) is provided to remove the mist that cannot be removed by the mist removing device. Exhaust gas from which mist has been removed by such equipment is sent to a GGH (gas-gas heater) reheater, heated, and released into the atmosphere from a chimney in a state of improved diffusivity.

As one of such flue gas treatment apparatuses, one according to Patent Document 1 is provided. In Patent Document 1, scattered mist in the exhaust gas is removed by heating and collision with the SGH. Therefore, when the SGH is used for a long period of time, the generated scale deteriorates the heating/collision function, and if left unattended, the mist removing function deteriorates. In addition, the remaining mist reaches the GGH reheater, and due to this remaining mist, scale adheres to the GGH reheater, and the scale clogs the elements of the GGH reheater, leading to an increase in differential pressure. I was afraid.

As a method for preventing scale in SGH and GGH caused by scattering mist in exhaust gas, as described in Patent Document 2, exhaust gas is heated upstream of an exhaust gas reheater that reheats exhaust gas that has passed through a flue gas desulfurization device. A method of providing a washable exhaust gas heating unit for removing mist in the exhaust gas has been proposed.
However, as described above, this method is a method for preventing scale adhesion in SGH and GGH, and is not a method for removing scale after adhesion, nor is it possible to completely prevent scale adhesion. rice field.

Here, although the flue gas to be treated by the flue gas desulfurization device has been treated in advance by a denitration device or an electrostatic precipitator, the flue gas contains a large amount of sulfurous acid gas (SO ₂ ) and a trace amount of chloride. (Cl), fluoride (F) and fly ash. Since these components are also absorbed in the absorber tower in the flue gas desulfurization system, the surrounding area of the absorber tower is exposed to a very severe corrosive and abrasive environment. In particular, since the exhaust gas after the sulfur oxides have been desulfurized in the absorption tower also contains mist as described above, the dry and wet state of the exhaust gas exists in the part where the temperature of the exhaust gas after desulfurization is increased, and the adhesion of scale. is likely to occur.

Although a method for removing gypsum scale caused by waste water discharged from a flue gas desulfurization apparatus has been proposed as in Patent Document 3, scale caused by exhaust gas discharged from the flue gas desulfurization apparatus is No removal method has been proposed so far.

Japanese Patent No. 3852820 Japanese Patent No. 5848032 JP-A-2000-24452

The scale generated in the exhaust gas system through which the exhaust gas discharged from the absorption tower of the flue gas desulfurization apparatus flows is difficult to remove because it contains various components. In the case of deposits, the operation of the flue gas desulfurization equipment was appropriately stopped, and the equipment and pipes constituting the exhaust gas system were repaired or replaced.
However, since repair and replacement require time and money, they are not performed frequently, and the deterioration of performance due to the adhesion of scale to the exhaust gas system becomes a bottleneck, and the operation of the flue gas desulfurization device and eventually the boiler. There are situations where it is necessary to reduce the rate.
Therefore, in the conventional flue gas treatment apparatus, a method for removing the scale deposited on the flue gas system through which the flue gas discharged from the absorption tower flows has been desired.

In view of the above-mentioned current situation, the present invention provides a method for removing scale adhered to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for the lime-gypsum method flows, and a scale removing composition used in this method. intended to provide

The present invention is a scale removal method for removing scale adhered to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for the wet lime-gypsum method flows, wherein a scale removal composition is brought into contact with the scale. wherein the scale removing composition comprises at least one alkali metal chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA). It contains a salt, an alkali carbonate, water, and a benzotriazole-based anticorrosive agent, and has a pH of 7 to 10 when diluted with water to a concentration of 1.0% by weight . and the method for removing scale , wherein the metal material of the exhaust gas system is carbon steel, and the scale contains at least calcium sulfate, silicon and iron .
Further, the scale removing composition is preferably an aqueous solution containing 3 to 25% by weight of an alkali metal salt of a chelating agent .

The present invention also relates to a scale removing composition used for removing scale adhered to an exhaust gas system in which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for the wet lime-gypsum method flows, comprising ethylenediaminetetraacetic acid (EDTA ), an alkali metal salt of at least one chelating agent selected from the group consisting of diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA), an alkali carbonate, water, and a benzotriazole-based anticorrosive. The scale removal composition has a pH of 7 to 10 when diluted with water to 1.0% by weight , the exhaust gas system metal material is carbon steel, and the scale is at least A descaling composition comprising calcium sulfate, silicon and iron .
Further, the scale removing composition of the present invention is preferably an aqueous solution containing 3 to 25% by weight of the alkali metal salt of the chelating agent .
The present invention will be described in detail below.
In addition, the "exhaust gas system" described in the present invention means a system in which the exhaust gas discharged from the absorption tower flows, and piping and equipment (mist eliminator, washable exhaust gas unit, SGH, GGH, reheater, etc.) includes.

In this specification, "X to Y" means "X or more and Y or less".

Flue gas desulfurization equipment for the wet lime-gypsum method is equipment for removing sulfur oxides contained in flue gas discharged from combustion plants (for example, boilers of power plants, etc.). In an absorption tower in which sulfur oxides in flue gas are removed, the flue gas is brought into contact with an absorption liquid containing lime as an absorbent, and the sulfur oxides in the flue gas are recovered as gypsum. Therefore, the flue gas discharged from the absorption tower basically does not contain sulfur oxides because it has been desulfurized, but it contains mist caused by the absorbing liquid used in the absorption tower, or the mist eliminator Even if the mist is removed by , SGH, etc., the exhaust is not completely dry and contains moisture.
According to the study of the present inventor, in addition to mist and / or moisture in the exhaust gas discharged from the absorption tower, the gypsum component (calcium sulfate) produced in the absorption tower and the chloride contained in the combustion exhaust gas ( Cl), fluoride (F) and components such as fly ash.
In this way, it is thought that the flue gas after desulfurization also contains the above-mentioned various components, albeit in very small amounts. It has become.

In addition, in the flue gas desulfurization equipment using the wet lime-gypsum method, the contact ratio between the combustion exhaust gas and the absorbing liquid is improved compared to the dry method, so the reaction rate is high. There is a problem that the flue gas temperature decreases and the flue gas after desulfurization becomes poorly diffused. To solve this problem, a method of increasing the exhaust gas temperature by installing reheating equipment is adopted, and an SGH (steam-gas heater) reheater and/or GGH (gas-gas heater) A reheater is installed.
Adhesion of scale in the exhaust gas system through which the exhaust gas discharged from the absorption tower flows is particularly noticeable in these reheaters, not only reducing the thermal efficiency of the equipment, but also increasing the gas differential pressure before and after the equipment. It is a problem that periodic water washing is required to reduce the differential pressure. The water washing work differs depending on the scale of the flue gas desulfurization system. unit stop is required, which is a major hindrance to stable operation.

In addition, since the scale deposited on the GGH reheater and the SGH reheater cannot be removed by regular water washing, the differential pressure in the reheaters increases significantly as the period of use increases. There is a tendency. Ultimately, therefore, it is a device that requires periodic repair and replacement, such as replacement of the tube of the reheater.

Here, according to the investigation of the present inventor, "gypsum" has been widely used for a long time when "molds" are used for crafts, arts, and dentistry. A gypsum dissolving agent for removing gypsum is used (see Japanese Patent Application Laid-Open No. 2014-065663). No method or descaling composition has been proposed for removing . The reason is that, as described above, the exhaust gas system contains various components such as chlorides, fluorides and fly ash contained in the flue gas in addition to calcium sulfate generated in the absorption tower. , It is completely unknown whether the gypsum dissolving agent used for construction purposes is effective or not.In addition, the maintenance of equipment and piping in the field of combustion exhaust gas treatment is generally performed by washing with water and repairing / replacing. This is considered to be due to the fact that there was a technical common sense that

As a result of intensive studies without being bound by the above-mentioned common technical knowledge, the present inventors have found a scale removal method for removing scale adhering to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for the wet lime-gypsum method flows, and A composition for removing scale was discovered, and the present invention was completed.

The scale removal method of the present invention comprises a step of bringing a scale removal composition into contact with the scale adhered to the exhaust gas system, wherein the scale removal composition comprises an alkali metal salt of a selected specific chelating agent, It contains an alkali carbonate and water.
The contacting means in the above step is not particularly limited, but for example, spraying means, spraying means, (high pressure) spraying means, etc. can be used.
Further, the scale removing method of the present invention can be used, for example, when the operation of the flue gas desulfurization apparatus is stopped during periodic inspection of the flue gas desulfurization apparatus or the like, the scale removal composition is used as it is or diluted with water. It can be carried out by contacting (spraying, etc.) the scale adhering to the exhaust gas system for a certain period of time.
In addition, the said scale removal composition is also one of this invention.
The scale-removing composition of the present invention will be described in detail below, but the details regarding the scale-removing composition can also be employed in the above-described scale-removing method.

The alkali metal salt of the chelating agent is an alkali metal salt of at least one chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA). be.
The alkali metal salt constituting the alkali metal salt of the chelating agent is, for example, a lithium salt, a sodium salt, or a potassium salt, preferably a sodium salt, and more preferably a trisodium salt.
The alkali metal salt of the chelating agent consists of EDTA.disodium salt, EDTA.2.5sodium salt, EDTA.trisodium salt, EDTA.3potassium salt, DTPA.3sodium salt, and CyDTA.3sodium salt. It is preferably at least one selected from the group, more preferably EDTA.trisodium salt.

The content of the alkali metal salt of the chelating agent in the scale removing composition of the present invention is preferably 3 to 25% by weight based on the total weight of the scale removing composition. If it is less than 3% by weight, it may not be possible to effectively remove the scale, and if it is more than 25% by weight, it may be difficult to handle it as an aqueous solution. Further, the content of the alkali metal salt of the chelating agent is more preferably 15 to 25% by weight.

Moreover, sodium carbonate, sodium hydrogencarbonate, potassium carbonate, potassium hydrogencarbonate etc. can be mentioned as said carbonate alkali salt. Among them, sodium hydrogen carbonate is preferable. This is because it is inexpensive, has high water solubility, and is easy to adjust to an appropriate pH.

The amount of the alkali carbonate compounded in the scale-removing composition of the present invention is preferably 2 to 5% by weight based on the total weight of the scale-removing composition. If it is less than 2% by weight, it may not be possible to effectively remove the scale, and if it is more than 5% by weight, it may be difficult to make an aqueous solution. Further, it is more preferable that the content of the alkali carbonate is 4 to 5% by weight.

In some cases, the above-mentioned composition for removing scale is diluted with water by about 3 times and used, but in terms of the effect of removing the scale attached to the above-mentioned exhaust gas system, it is used as it is without being diluted with water. is preferred.

The scale removing composition of the present invention has a pH of 7 to 10 when diluted with water so that the scale removing composition becomes 1.0% by weight. Therefore, when the amount of the alkali metal salt of the chelating agent in the scale removing composition of the present invention is 3 to 25% by weight with respect to the total weight of the scale removing composition, the scale removing composition The pH is 7-10 when the concentration of the alkali metal salt of the chelating agent is 0.03-0.25% by weight when diluted with water so that the substance becomes 1.0% by weight. If the pH is higher than 10, the scale adhering to the exhaust gas system may not be sufficiently dissolved, and if the pH is lower than 7, the alkali carbonate may not be sufficiently dissolved, which is not preferred. In addition, the method of measuring pH can use the method generally used. Examples of specific measurement methods are shown in Examples.

The weight ratio of the alkali metal salt and the alkali carbonate of the chelating agent in the scale removing composition is preferably in the range of 10:1 to 1:1, more preferably in the range of 5:1 to 2:1. is more preferable. This is because an increase in the dissolution rate of scale is expected.

In addition, the scale removing composition of the present invention preferably further contains a benzotriazole-based anticorrosive agent. This is because the inclusion of the benzotriazole-based anticorrosive agent can prevent metal corrosion of the exhaust-gas-related material. Benzotriazole-based anticorrosives are known as anticorrosives for copper and copper alloys, but have not been used as anticorrosives for carbon steel. However, as a result of intensive studies by the present inventor, it was found that a benzotriazole-based anticorrosive agent exerts a significant anticorrosion effect against carbon steel corrosion caused by the descaling composition of the present invention.

As the benzotriazole-based anticorrosive agent, a commonly used benzotriazole-based anticorrosive agent can be used. Specifically, for example, benzotriazole, 6-methyl-1H-benzotriazole, 7-methyl-1H-benzotriazole, 1-hydroxybenzotriazole, 5-methylbenzotriazole, 1-aminobenzotriazole, 1-phenylbenzotriazole , 1-hydroxymethylbenzotriazole, methyl 1-benzotriazolecarboxylate, 5-benzotriazolecarboxylic acid, 1-methoxy-benzotriazole, 1-(2,2-dihydroxyethyl)-benzotriazole, 1-(2,3 -dihydroxypropyl)benzotriazole, or 2,2'-{[(4-methyl-1H-benzotriazol-1-yl)methyl]imino}, commercially available from Ciba Specialty Chemicals as the "IRGAMET" series. Bisethanol, 2,2′-{[(5-methyl-1H-benzotriazol-1-yl)methyl]imino}bisethanol, 2,2′-{[(4-methyl-1H-benzotriazole-1- yl)methyl]imino}bisethane, or 2,2′-{[(4-methyl-1H-benzotriazol-1-yl)methyl]imino}bispropane and the like.
Among these, benzotriazole, 6-methyl-1H-benzotriazole and 7-methyl-1H-benzotriazole are preferably used. Benzotriazole-based anticorrosives can be used alone or in combination of two or more.

When a benzotriazole-based anticorrosive agent is added to the scale-removing composition of the present invention, the amount thereof is 0.2 to 1.0% by weight with respect to the total weight of the scale-removing composition. is preferred. If the content is less than 0.2% by weight, the anticorrosion effect may not be sufficient, and if the content is more than 1.0% by weight, no further anticorrosion effect can be expected. Further, the amount of the benzotriazole-based anticorrosive agent added is more preferably 0.3 to 0.5% by weight.

The scale removing composition of the present invention preferably further contains an organic acid, and more specifically, preferably contains hexahydroxyphthalate (salts such as Na, Li, K, etc.). This is because the dissolution rate of the scale adhering to the exhaust gas system increases.

When hexahydroxyphthalate is added to the scale removing composition of the present invention, the amount thereof is 0.1 to 1.0% by weight based on the total weight of the scale removing composition. is preferred. If the amount is less than 0.1% by weight, the dissolution rate of the scale attached to the exhaust gas system may not be improved. Because you can't. Further, the content of the hexahydroxyphthalate is more preferably 0.3 to 0.5% by weight.

In the method for removing scale of the present invention, it is preferable that the temperature of the composition for removing scale is 10 to 40° C. before contacting the scale adhered to the exhaust gas system. This is because the dissolution rate of the scale increases when the temperature of the scale removing composition that contacts the scale adhering to the exhaust gas system is 10° C. or higher. If the temperature of the scale removing composition that contacts the scale adhered to the exhaust gas system is higher than 40°C, decarboxylation, which is a side reaction, may occur and the reaction efficiency may decrease. This is because the desired effect may not be obtained.
Therefore, the descaling method of the present invention preferably includes a step of adjusting the temperature of the descaling composition. The means used in the step of adjusting the temperature is not particularly limited, and generally used cooling means, heating means, or the like can be used.

Moreover, the scale removing method of the present invention preferably has a step of recovering the scale removing composition after contact with the scale adhering to the exhaust gas system and bringing it into contact with the scale again. This is because the scale can be efficiently removed from the viewpoint of cost by recycling the scale removing composition of the present invention. When the scale removing composition is brought into contact with the scale after adjusting its temperature to 10 to 40° C., the temperature of the scale removing composition after contact with the scale has already been adjusted. This is because the reuse can reduce heating and/or cooling energy.

Further, the scale removing method of the present invention comprises the step of collecting the scale removing composition after contact with the scale adhering to the exhaust gas system, and measuring the residual concentration of the alkali metal salt of the chelating agent in the composition. It is preferable to have This is because the scale removal efficiency can be confirmed by measuring the residual concentration.
Further, in the method for removing scale of the present invention, the amount of the composition for removing scale added is adjusted based on the measured value of the residual concentration, and the composition for removing scale with the adjusted amount added is used to remove the collected scale. may have a step of adding to the composition for use. By adjusting the addition amount of the scale removal composition added to the collected scale removal composition (scale removal composition to be recycled (re)used) based on the measured value of the residual concentration, This is because the descaling of the exhaust gas system can be efficiently performed.

The method for measuring the residual concentration of the alkali metal salt of the chelating agent in the scale removing composition after contact with the scale adhering to the exhaust gas system is a method based on the commonly used chelate titration method. EDTA.3Na (19.0% by weight) as the alkali metal salt of the chelating agent, sodium bicarbonate (5.0% by weight) as the alkali carbonate salt, and the balance water for descaling The residual concentration of EDTA.3Na in the composition can be measured as follows.

<Chelate titration method>
EDTA (Kishida Chemical Co., Ltd.: Reagent special grade ethylenediaminetetraacetic acid)
100-fold diluted powder of NANA indicator (Kishida Chemical Co., Ltd.: 3-hydroxy-4-(2-hydroxy-sulfo-1-naphthylazo)-2-naphthalenecarboxylic acid) 48% sodium hydroxide aqueous solution liquid)
Water, chelating agent, 48% sodium hydroxide aqueous solution, and sodium hydrogen carbonate are mixed so that EDTA 3Na (19.0% by weight) and sodium hydrogen carbonate (5.0% by weight) are included, and used for scale removal. A composition was prepared.
Next, 2 g of the scale removing composition, which is a sample for measuring the residual concentration of the alkali metal salt of the chelating agent, was precisely weighed into a 200 mL Erlenmeyer flask, and 50 mL of water and 48% KOH (Osaka Soda Co., Ltd.: caustic One drop of triethanolamine (about 40 μL) was added to a mixture of 3 g of potash), and then NANA indicator (about 0.1 g) was added and stirred.
The EDTA concentration can be measured by titrating this with a 0.05 mol/L calcium chloride aqueous solution.
Titration volume (mL) × molar concentration of Ca ion (mol/L) ÷ sample amount (g) = EDTA/3Na concentration (mol/Kg)

According to the present invention, it is possible to remove scale attached to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for the wet lime-gypsum method flows.

FIG. 1 is a flow diagram showing one form of a flue gas desulfurization system for wet lime-gypsum method. FIG. 2 is a schematic diagram showing an experimental apparatus used in Example 11. FIG.

EXAMPLES The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these.

Attached to the fin tube, which is the heat transfer surface of the gas-gas heater reheater installed downstream of the absorption tower in the flue gas desulfurization equipment using the wet lime-gypsum method installed at a certain coal-fired power plant A. A scale (gray) was sampled and designated as test scale 1.
In addition, it adheres to the fin tube which is the heat transfer surface of the gas-gas heater reheater installed in the downstream of the absorption tower in the flue gas desulfurization equipment of the wet lime-gypsum method installed in a certain coal-fired power plant B. A scale (gray) that had been exposed was sampled and designated as test scale 2.
Table 1 below shows the results of fluorescence X analysis and the results of measuring the pH value of the 1% suspension water, which indicate the properties of these test scales 1 and 2.

In Examples 1 to 10 and Comparative Examples 1 to 11, water, a chelating agent, and a 48% sodium hydroxide aqueous solution or 48% An aqueous solution of an alkali metal salt of a chelating agent was prepared from an aqueous solution of potassium hydroxide, and then an aqueous solution of an alkali carbonate salt or an aqueous solution of sodium acetate was added to obtain aqueous solutions for testing according to Examples and Comparative Examples. 250 g of these test aqueous solutions were placed in a 300 mL beaker and allowed to stand in a constant temperature bath at 30°C. Here, the scale 1 or the scale 2 cut to about 1 cm x 1 cm x 1 cm was weighed (approximately 3 g), placed in a beaker containing the previously obtained aqueous solution for test, and after 7 hours. The mass was taken out and air-dried at room temperature (15 to 25° C.) for 24 hours, and then weighed to calculate the mass residual rate.
Table 2 shows the measurement results.

(chelating agent)
EDTA (Kishida Chemical Co., Ltd.: Reagent special grade ethylenediaminetetraacetic acid)
DTPA (Tokyo Chemical Industry Co., Ltd.: special grade diethylenetriaminepentaacetic acid)
CyDTA (Tokyo Chemical Industry Co., Ltd.: special grade trans-1,2-cyclohexanediaminetetraacetic acid monohydrate)
NTA (Tokyo Chemical Industry Co., Ltd.: special grade nitrilotriacetic acid)
PDTA (Tokyo Chemical Industry Co., Ltd.: primary 1,3-propanediamine-N,N,N',N'-tetraacetic acid)
DPTA-OH (Tokyo Chemical Industry Co., Ltd.: special grade 1,3-diaminno-2-propanol-N,N,N',N'-tetraacetic acid)
TTHA (Tokyo Chemical Industry Co., Ltd.: special grade triethylenetetramine-N,N,N',N'',N''',N'''-hexaacetic acid)
EDDS trisodium (Sigma-Aldrich Japan (same): (S,S)-ethylenediamine-N,N'-disuccinic acid trisodium salt solution 35% in H ₂ O)
48% sodium hydroxide aqueous solution (Kaname Yakuhin Co., Ltd.: caustic soda liquid)
48% potassium hydroxide aqueous solution (Osaka Soda Co., Ltd.: caustic potash solution)

(alkali carbonate)
NaHCO ₃ (Wako Pure Chemical Industries, Ltd.: reagent grade sodium hydrogen carbonate)
Na ₂ CO ₃ (Kishida Chemical Co., Ltd.: reagent special grade sodium carbonate anhydrous)
KHCO ₃ (Wako Pure Chemical Industries, Ltd.: Reagent special grade potassium hydrogen carbonate)

(Method for calculating residual scale mass)
Lump weight after 7 hours (g) ÷ Lump weight before input (g) × 100 = Scale lump residual ratio (%)
(Evaluation criteria)
If the lump residual rate on scale 1 and scale 2 is 5% or less: ○
If the lump residual rate on scale 1 or scale 2 is more than 5% and 20% or less: △
If the lump residual rate on scale 1 or scale 2 exceeds 20%: ×

(Method for measuring pH when each test aqueous solution according to Examples and Comparative Examples is diluted with water to 1.0% by weight)
1.5 g of each test aqueous solution according to the examples and comparative examples prepared above was weighed into a 200 mL beaker, and 148.5 g of ion-exchanged water was further added and mixed with stirring. This solution was diluted with water so as to contain 1.0% by weight of the aqueous solution for test to obtain a 1% by weight aqueous solution. The 1% by weight aqueous solution was stored in a constant temperature layer at 25° C. for 60 minutes, and the pH of each aqueous solution adjusted to the constant temperature was measured using a pH meter (manufactured by Horiba Ltd.: D-51). Table 2 shows the pH measurement results of the 1% by weight aqueous solution, together with the concentration (% by weight) of the alkali metal salt of the chelating agent contained in the 1% by weight aqueous solution.

From the results in Table 2 above, it was found that an aqueous test solution containing an alkali metal salt of a specific chelating agent, a carbonate, and water was diluted with water so that the aqueous solution for test was 1.0% by weight. It was confirmed that all of Examples 1 to 10, in which the pH was 7 or more and 10 or less, showed the ability to remove scale. On the other hand, it does not contain an alkali metal salt of a specific chelating agent, does not contain a carbonate, or has a pH outside the above-mentioned specific range when diluted with water so that the test aqueous solution has a pH of 1.0% by weight. It was confirmed that Comparative Examples 1 to 11 were significantly inferior in scale removal ability to Examples 1 to 10, and did not have practical scale removal ability.

(Example 11)
An alkali metal salt aqueous solution of a chelating agent was prepared from water, EDTA and a 48% aqueous sodium hydroxide solution in the same manner as in Example 4 above, and then an aqueous sodium bicarbonate solution was added to prepare 1500 g of the test aqueous solution according to Example 11. got This was placed in a 2 L beaker and allowed to stand in a constant temperature bath at 30°C. A screw pump is connected to this beaker, the aqueous solution for test is extracted from the bottom of the beaker, and the aqueous solution for test is circulated and discharged at a flow rate of 1200 mL / min from the top of a sieve (diameter 8 cm, mesh opening 2 mm) installed at the top of the beaker. A device was fabricated (see FIG. 2). A mass of 18.0 g of scale 1 (approximately 1 cm x 2 cm x 3 cm) was placed on the sieve in a wet state, and the discharge liquid was applied from the top of the mass. Lump weight (wet) after hours was measured. The results are shown in Table 3.

From the results of Example 11 shown in Table 3, in the scale removal method of the present invention, in the step of contacting the scale adhered to the exhaust gas system with the scale removal composition, the scale was coated with the composition. It was confirmed that the scale removal effect can be reliably obtained even in the method of spraying.

In Examples 12 to 16 and Comparative Examples 12 to 16, EDTA was added from water, a chelating agent and a 48% aqueous sodium hydroxide solution so that the components shown in Table 4 were finally contained in the amounts shown in Table 4. - A 3Na aqueous solution was prepared, an aqueous sodium bicarbonate solution was added thereto, and an anticorrosive agent was further added to prepare an aqueous solution for testing according to the above Examples and Comparative Examples. Each test aqueous solution was added to an 80-mL mayonnaise bottle up to 90% capacity, and a test piece (SPCC, 3 cm×5 cm, #400 polishing) was completely immersed therein and covered with a lid. The mayonnaise bottle was allowed to stand in a constant temperature bath at 30° C. After 72 hours (3 days), the immersed test piece was taken out, and the corrosion rate (MDD) was calculated from the weight difference before and after the immersion. The results are shown in Table 4 below.
(Evaluation criteria)
When MDD is 30 mg / dm ² · 24 h or less: ○
If MDD is greater than 30 mg/dm ² · 24 h: ×

(anticorrosive agent)
BT (benzotriazole) (Kishida Chemical Co., Ltd.: Reagent special grade 1,2,3-benzotriazole)
MBT (Wako Pure Chemical Industries, Ltd.: Class 1 5-methyl-1H-benzotriazole)
Sodium molybdate (Kishida Chemical Co., Ltd.: special reagent grade sodium molybdate dihydrate)
Sodium nitrite (Kishida Chemical Co., Ltd.: Reagent special grade sodium nitrite)
HEDP (Kishida Chemical Co., Ltd.: 1-hydroxyethylidene-1,1-diphosphonic acid monohydrate for chemical use)

From the results in Table 4, Examples 12 to 16, which contain EDTA/3Na, sodium hydrogen carbonate, and a specific anticorrosive agent in the test aqueous solution, all have test piece corrosion rates corresponding to Comparative Examples 12 to 16. It was confirmed that the anti-corrosion effect was significantly smaller than the results obtained from the test aqueous solution. The scale removing effect of the test aqueous solutions of Examples 12 to 16 and Comparative Examples 13 to 16 was comparable to the scale removing effect of the test aqueous solution of Comparative Example 12. Therefore, it was confirmed that the descaling effect was not affected by the presence or absence of the anticorrosive agent in the aqueous solution.

1 boiler 2 electric dust collector 3 gas-gas heater heat recovery device 4 gas-gas heater reheater 5 absorption tower 6 mist eliminator 7 chimney 10 flue gas passage 11 limestone silo 12 raw material pit 13 filtrate pit 14 gypsum dehydrator 20 Example 11 Test apparatus 21 beaker 22 comb 23 circulation pump 24 water bath used in

Claims (4)

A scale removal method for removing scale adhering to an exhaust gas system in which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for a wet lime-gypsum method flows,
a step of contacting the scale with a scale removing composition;
The descaling composition comprises an alkali metal salt of at least one chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA), and alkali carbonate. containing salt, water, and a benzotriazole-based anticorrosive agent ;
The scale removing composition has a pH of 7 to 10 when diluted with water to 1.0% by weight ,
The exhaust gas-based metal material is carbon steel,
A method for removing scale, wherein the scale contains at least calcium sulfate, silicon and iron . 2. The method of removing scale according to claim 1, wherein the composition for removing scale is an aqueous solution containing 3 to 25% by weight of an alkali metal salt of a chelating agent. A scale removing composition used for removing scale adhered to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus for a wet lime-gypsum method,
An alkali metal salt of at least one chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA), an alkali carbonate, water, and benzotriazole and a system anticorrosive agent ,
The scale removing composition has a pH of 7 to 10 when diluted with water to 1.0% by weight ,
The exhaust gas-based metal material is carbon steel,
A composition for removing scale, wherein the scale contains at least calcium sulfate, silicon and iron . The scale removing composition according to claim 3 , which is an aqueous solution containing 3 to 25% by weight of the alkali metal salt of the chelating agent.

Images