JP2019155339A - Scale removal method and composition for scale removal - Google Patents

Abstract

To provide a removal method of scale attached to an exhaust gas system through which exhaust gas exhausted from an absorption tower of a flue gas desulfurization apparatus of a lime-gypsum method is caused to flow, and to provide a composition for scale removal used in this method.SOLUTION: A scale removal method for removing scale attached to an exhaust gas system through which exhaust gas exhausted from an absorption tower of a flue gas desulfurization apparatus of a lime-gypsum method is caused to flow includes a process of bringing a composition for scale removal into contact with scale. Therein, the composition for scale removal contains alkali-metal salt of at least one kind of chelate agent selected from a group which consists of ethylenediamine tetraacetic acid (EDTA), diethylenetriamine pentaacetic acid (DTPA), and cyclohexanediamine tetraacetic acid (CyDTA), an alkali-carbonate, and water, and a pH when being diluted with water such that the composition for scale removal becomes 1.0 wt.% is 7 to 10.SELECTED DRAWING: None

Description

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

  Many flue gas desulfurization facilities are installed as equipment for removing sulfur oxides (SOx) in exhaust gas. The mainstream of the flue gas desulfurization process in recent years is a wet method, and there are differences in the type of absorbent and the amount of by-products, but basically (1) Cooling / absorption device, (2) Ventilation device, (3 ) Gas reheating device, (4) raw material (absorbent) storage and supply device, and (5) by-product fraction processing device. Of the wet methods, the lime-gypsum method is often used as a desulfurization apparatus for commercial boiler exhaust gas.

  In addition to the above-described configuration, a mist removing device (mist eliminator) for removing mist is provided to treat mist generated during SOx absorption processing by wet flue gas desulfurization. And in order to remove the mist which cannot be removed with the said mist removal apparatus, SGH (steam-gas heater) may be provided. The exhaust gas from which the mist has been removed by such a device is sent to a GGH (gas-gas heater) reheater, heated, and released from the chimney to the atmosphere with improved diffusibility.

  As such a flue gas treatment apparatus, a device according to Patent Document 1 is provided. In Patent Document 1, scattered mist in exhaust gas is removed by heating and collision with SGH. Therefore, when SGH is used for a long period of time, the function of heating / collision is lowered due to the generated scale, and if this is left untreated, the function of removing mist is lowered. Further, the remaining mist reaches the GGH reheater, and due to the remaining mist, the scale is fixed to the GGH reheater, the scale blocks the element of the GGH reheater, and the differential pressure increases. There was a fear.

As a method of preventing scale in SGH and GGH generated by scattered 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, this method is a method for preventing the adhesion of scales in SGH and GGH as described above, and is not a method for removing scales after adhesion, and it has not completely prevented the adhesion of scales. It was.

Here, although the combustion exhaust gas to be treated by the flue gas desulfurization apparatus has been treated in advance by a denitration apparatus or an electrostatic precipitator, the combustion exhaust gas contains a large amount of sulfurous acid gas (SO ₂ ) and a small amount of chloride. Components (Cl), fluoride (F), and fly ash are included. Since these components are also absorbed by the absorption tower in the flue gas desulfurization apparatus, the periphery of the absorption tower is exposed to a very severe corrosion and wear environment. In particular, since the exhaust gas after the sulfur oxides are desulfurized in the absorption tower also contains mist as described above, there is a dry and wet state of the exhaust gas in the portion where the temperature of the exhaust gas after desulfurization is raised, and the scale adheres. The environment is likely to occur.

  In addition, although the method of removing the gypsum scale resulting from the waste_water | drain discharged | emitted from a flue gas desulfurization apparatus like patent document 3 is proposed, about the scale resulting from the waste gas discharged | emitted from a flue gas desulfurization apparatus, Until then, no removal method was proposed.

Japanese Patent No. 3852820 Japanese Patent No. 5848032 JP 2000-24442 A

The scale generated in the exhaust gas system that flows the exhaust gas exhausted from the absorption tower of the flue gas desulfurization equipment contains various components, so it is difficult to remove it. Conventionally, the scale adheres to the exhaust gas system. In the case of accumulation, the operation of the flue gas desulfurization apparatus was stopped as appropriate, and the equipment and piping constituting the exhaust gas system were repaired or replaced.
However, repairs and replacements are time consuming and expensive, so they are not performed frequently, and the deterioration in performance due to the scale adhering to the exhaust gas system becomes a bottleneck, and the operation of the flue gas desulfurization equipment and the boiler There are also scenes where the rate must be reduced.
Therefore, in the conventional flue gas treatment apparatus, there has been a demand for a method of removing the scale deposited and deposited on the exhaust gas system through which the exhaust gas discharged from the absorption tower flows.

  In view of the above situation, the present invention provides a method for removing scales attached to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus using the lime-gypsum method, and a composition for removing scales used in this method. The purpose is to provide.

The present invention relates to a scale removal method for removing scales attached to an exhaust gas system through which exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus using a wet lime gypsum method is removed, and the scale removing composition is brought into contact with the scale. The descaling composition is an alkali metal of at least one chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA) and cyclohexanediaminetetraacetic acid (CyDTA) A descaling method comprising: a salt, an alkali carbonate salt, and water, and having a pH of 7 to 10 when diluted with water so that the descaling composition is 1.0% by weight. It is.
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 scale removing composition preferably further contains a benzotriazole anticorrosive.

The present invention also relates to 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 of a wet lime gypsum method is used, and 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 salt, and water, and the composition for removing scale described above A pH-reducing composition having a pH of 7 to 10 when diluted with water so that the product becomes 1.0% by weight.
Moreover, 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.
Moreover, it is preferable that the composition for scale removal of this invention contains a benzotriazole type anticorrosive further.
The present invention is described in detail below.
The "exhaust gas system" described in the present invention means a system through which exhaust gas discharged from the absorption tower flows, and piping and equipment (mist eliminator, washable exhaust gas unit, SGH, GGH, reheater, etc.) Is included.

In the present specification, “X to Y” means “X or more and Y or less”.

A wet lime gypsum method flue gas desulfurization apparatus is a facility for removing sulfur oxides contained in combustion exhaust gas discharged from a combustion plant (for example, a boiler of a power plant). In an absorption tower in which sulfur oxides in combustion exhaust gas are removed, an absorption liquid containing lime as an absorbent is brought into contact with the combustion exhaust gas, and sulfur oxides in the combustion exhaust gas are recovered as gypsum. Therefore, the exhaust gas discharged from the absorption tower does not basically contain sulfur oxide because it is desulfurized, but it contains mist due to the absorption liquid used in the absorption tower, or a mist eliminator. Even if the mist is removed with SGH or the like, the exhaust gas is not completely dry and contains moisture.
According to the inventor's consideration, in the exhaust gas discharged from the absorption tower, in addition to mist and / or moisture, the gypsum component (calcium sulfate) generated in the absorption tower and the chloride ( Cl), fluoride (F), and components such as fly ash are considered to be contained.
As described above, the exhaust gas after desulfurization is considered to contain the above-mentioned various components even in a trace amount, and in reality, in the exhaust gas system in which the exhaust gas discharged from the absorption tower flows, there is a problem that scale adheres. It has become.

In addition, in the flue gas desulfurization apparatus using the wet lime gypsum method, the contact rate between the combustion exhaust gas and the absorbing liquid is higher than that using the dry method. There is a problem that the flue gas temperature decreases and the exhaust gas after desulfurization becomes poorly diffused. In order to solve this problem, a method of increasing the exhaust gas temperature by using a reheating facility is employed, and an SGH (steam-gas heater) reheater and / or a GGH (gas-gas) is provided downstream of the absorption tower. Heater) A reheater is installed.
The adhesion of scale in the exhaust gas system in which the exhaust gas discharged from the absorption tower described above flows is particularly noticeable in these reheaters, and not only the thermal efficiency of the equipment is reduced, but also the gas differential pressure before and after the equipment is increased. In order to reduce the differential pressure, it is a problem that regular water washing work is required. Although the water washing operation varies depending on the scale of the flue gas desulfurization device, for example, in the flue gas desulfurization device used for the treatment of the combustion exhaust gas discharged from the thermal power plant, about 14 days are required for carrying out the water washing operation. It is necessary to stop the unit, which is a big obstacle to stable operation.

  Moreover, since the scale deposited on the GGH reheater and the SGH reheater cannot be removed by regular water washing operations, the differential pressure in the reheater increases remarkably as the period of use increases. There is a tendency. Therefore, in the end, the equipment is required to be periodically repaired or replaced, such as replacing the tube of the reheater.

  Here, according to the inventor's investigation, when performing “molding” for work, art, and dental use, “gypsum” has been used for a long time, and adhered to containers and instruments used during processing. A gypsum dissolving agent for removing gypsum is used (see Japanese Patent Application Laid-Open No. 2014-066563), but the scale adhered to the exhaust gas system through which the exhaust gas discharged from the absorption tower in the flue gas desulfurization apparatus of the lime gypsum method flows. No method or descaling composition has been proposed for removing slag. The reason is that, as described above, the exhaust gas system includes various components such as chloride, fluoride and fly ash contained in the combustion exhaust gas in addition to the calcium sulfate produced in the absorption tower. It is unclear whether or not the gypsum solubilizer used in applications such as work has any effect, and the maintenance management of equipment and piping in the field of flue gas treatment is generally water washing and repair / replacement This is thought to be due to the existence of technical common sense that

  As a result of earnest study without being bound by the above-mentioned common general knowledge, the present inventor, as a result, a scale removal method for removing scale attached to an exhaust gas system for flowing exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus of a wet lime gypsum method, and A descaling composition was found and the present invention was completed.

The scale removal method of the present invention has a step of bringing the scale removal composition into contact with the scale attached to the exhaust gas system, and the scale removal composition comprises an alkali metal salt of a selected specific chelating agent, It contains an alkali carbonate and water.
The means for contacting in the above step is not particularly limited, and for example, a spraying means, a spraying means, a (high pressure) spraying means, or the like can be used.
In addition, the scale removal method of the present invention can be used, for example, when the operation of the flue gas desulfurization device is stopped during periodic inspection of the flue gas desulfurization device, or by diluting the scale removal composition as it is or with water. It can be carried out by contacting (e.g. spraying) the scale attached to the exhaust gas system for a certain time.
The scale removing composition is also one aspect of the present invention.
Hereinafter, although the composition for scale removal of this invention is explained in full detail, the content regarding the composition for scale removal is employable also in the said scale removal 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). is there.
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 comprises EDTA.2 sodium salt, EDTA.2.5 sodium salt, EDTA.3 sodium salt, EDTA.3 potassium salt, DTPA.3 sodium salt, and CyDTA.3 sodium salt. It is preferably at least one selected from the group, and more preferably EDTA.3 sodium salt.

  In addition, it is preferable that the compounding quantity of the said alkali metal salt of the said chelating agent in the composition for scale removal of this invention is 3-25 weight% with respect to the total weight of the said composition for scale removal. If the amount is less than 3% by weight, the scale may not be removed effectively. If the amount is more than 25% by weight, it may be difficult to handle as an aqueous solution. The blending amount of the alkali metal salt of the chelating agent is more preferably 15 to 25% by weight.

  Examples of the alkali carbonate include sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate. Of these, sodium bicarbonate is preferable. This is because it is inexpensive, highly water-soluble, and easy to adjust to an appropriate pH.

  In addition, it is preferable that the compounding quantity of the said alkali carbonate salt in the composition for scale removal of this invention is 2 to 5 weight% with respect to the total weight of the said composition for scale removal. If it is less than 2% by weight, it may not be possible to remove the scale effectively, and if it is more than 5% by weight, it may be difficult to obtain an aqueous solution. The blending amount of the alkali carbonate is more preferably 4 to 5% by weight.

  Moreover, although the said scale removal composition may be diluted about 3 times with water and used, in terms of the scale removal effect adhering to the said exhaust gas system, it is used as it is without diluting with water. It is preferable.

  The scale removal composition of the present invention has a pH of 7 to 10 when diluted with water so that the scale removal composition is 1.0% by weight. Therefore, when the blending amount of the alkali metal salt of the chelating agent in the composition for removing scale of the present invention is 3 to 25% by weight with respect to the total weight of the composition for removing scale, the composition for removing scale described above. It is diluted with water so that the product becomes 1.0% by weight, and the pH when the concentration of the alkali metal salt of the chelating agent becomes 0.03 to 0.25% by weight is 7 to 10. 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 less than 7, the alkali carbonate may not be sufficiently dissolved. In addition, the method of measuring pH can use the method generally used. Examples of specific measurement methods are shown in the examples.

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

  Moreover, it is preferable that the composition for scale removal of this invention contains a benzotriazole type anticorrosive further. This is because inclusion of a benzotriazole-based anticorrosive can prevent metal corrosion of the exhaust gas-based material. The benzotriazole-based anticorrosive is known as an anticorrosive for copper and copper alloys, but has not been used as an anticorrosive for carbon steel. However, as a result of intensive studies by the inventor, it has been found that a benzotriazole-based anticorrosive exhibits a significant anticorrosion effect against carbon steel corrosion caused by the composition for removing a scale of the present invention.

As the benzotriazole anticorrosive, a commonly used benzotriazole anticorrosive 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] i, commercially available from Ciba Specialty Chemicals as the “IRGAMET” series. Mino} bisethanol, 2,2 ′-{[(5-methyl-1H-benzotriazol-1-yl) methyl] imino} bisethanol, 2,2 ′-{[(4-methyl-1H-benzotriazole- 1-yl) methyl] imino} bisethane, 2,2 ′-{[(4-methyl-1H-benzotriazol-1-yl) methyl] imino} bispropane, and the like.
Among these, benzotriazole, 6-methyl-1H-benzotriazole, 7-methyl-1H-benzotriazole and the like are preferably used. 1 type (s) or 2 or more types can be used for a benzotriazole type anticorrosive.

  In addition, when mix | blending a benzotriazole type | system | group anticorrosive with the composition for scale removal of this invention, the compounding quantity is 0.2 to 1.0 weight% with respect to the total weight of the said composition for scale removal. It is preferable. This is because if the blending amount is less than 0.2% by weight, the anticorrosion effect may not be sufficient, and even if blending more than 1.0% by weight, no further anticorrosion effect can be expected. The blending amount of the benzotriazole anticorrosive is more preferably 0.3 to 0.5% by weight.

  Moreover, it is preferable that the composition for scale removal of this invention contains an organic acid further, Specifically, it is preferable that hexahydroxy phthalate (Na, Li, K, etc. are included as a salt). This is because the dissolution rate of the scale attached to the exhaust gas system is increased.

  In addition, when mix | blending hexahydroxy phthalate with the composition for scale removal of this invention, the compounding quantity is 0.1-1.0 weight% with respect to the total weight of the said composition for scale removal. It is preferable. If the blending amount is less than 0.1% by weight, the dissolution rate of the scale attached to the exhaust gas system may not be expected to improve. Even if the blending amount is more than 1.0% by weight, further increase in the dissolution rate is expected. This is because it cannot be done. Further, the blending amount of the hexahydroxyphthalate is more preferably 0.3 to 0.5% by weight.

In the scale removal method of the present invention, it is preferable that the temperature of the scale removal composition before contacting the scale attached to the exhaust gas system is 10 to 40 ° C. This is because when the temperature of the scale removing composition in contact with the scale attached to the exhaust gas system is 10 ° C. or higher, the dissolution rate of the scale is increased. Further, when the temperature of the scale removing composition that contacts the scale attached to the exhaust gas system is higher than 40 ° C., decarboxylation as a side reaction may occur, and the reaction efficiency may be lowered. This is because a desired effect may not be obtained.
Therefore, the scale removal method of the present invention preferably includes a step of adjusting the temperature of the scale removal composition. The means used in the step of adjusting the temperature is not particularly limited, and generally used cooling means, heating means, etc. can be used.

  Moreover, it is preferable that the scale removal method of this invention has the process of collect | recovering the compositions for scale removal after contacting the scale adhering to the said waste gas system, and making it contact with the said 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. In addition, when the temperature of the scale removing composition is adjusted to 10 to 40 ° C. and brought into contact with the scale, the temperature of the composition for removing the scale after contacting the scale is already adjusted. It is because heating and / or cooling energy can be reduced by reusing.

Further, the scale removal method of the present invention includes a step of collecting the scale removal composition after contacting the scale attached 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 removal efficiency of the scale can be confirmed by measuring the residual concentration.
Furthermore, the scale removal method of the present invention adjusts the addition amount of the scale removal composition based on the measurement value of the residual concentration, and the recovered scale removal is performed on the scale removal composition with the addition amount adjusted. You may have the process of adding to the composition for use. By adjusting the amount of scale removal composition added to the recovered scale removal composition (circulating (reused) scale removal composition) based on the measurement value of the residual concentration, This is because the scale of the exhaust gas system can be efficiently removed.

  The method for measuring the residual concentration of the alkali metal salt of the chelating agent in the composition for removing the scale after contacting the scale attached to the exhaust gas system is a method based on a chelating titration method that is generally used. For example, EDTA.3Na (19.0% by weight) as an alkali metal salt of a chelating agent, sodium hydrogen carbonate (5.0% by weight) as an alkali carbonate, and the balance water are removed. The residual concentration of EDTA · 3Na in the composition for use can be measured as follows.

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

  ADVANTAGE OF THE INVENTION According to this invention, the scale adhering to the waste gas system which flows the waste gas discharged | emitted from the absorption tower in the flue gas desulfurization apparatus of a wet lime gypsum method can be removed.

FIG. 1 is a flow diagram showing an embodiment of a flue gas desulfurization apparatus using a wet lime gypsum method. FIG. 2 is a schematic diagram showing an experimental apparatus used in Example 11.

  Examples of the present invention will be shown below and will be described more specifically, but the present invention is not limited to these.

付 着 It adheres to the fin tube, which is the heat transfer surface of the gas-gas heater reheater installed downstream of the absorption tower in the wet lime gypsum flue gas desulfurization unit installed in coal-fired power plant A The scale (gray) was collected and used as test scale 1.
Moreover, 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 wet lime gypsum flue gas desulfurization device installed in the coal-fired power plant B The scale (gray) that had been collected was collected and used as test scale 2.
Table 1 below shows the results of fluorescence X analysis representing the properties of these test scales 1 and 2 and the pH value measurement results of 1% suspension water.

In Examples 1 to 10 and Comparative Examples 1 to 11, water, a chelating agent and a 48% aqueous sodium hydroxide solution or 48% so that the components shown in Table 2 are finally included in the amounts shown in Table 2. An aqueous alkali metal salt solution of a chelating agent was prepared from an aqueous potassium hydroxide solution, and then an aqueous alkali carbonate solution or an aqueous sodium acetate solution was blended to obtain aqueous test solutions 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 × 1 cm × 1 cm is weighed (about 3 g), and then placed in a beaker containing each of the test aqueous solutions obtained before, and after 7 hours. The lump was taken out and air-dried at room temperature (15 to 25 ° C.) for 24 hours, and then weighed to calculate the lump residual rate.
The measurement results are shown in Table 2.

(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 .: 1st grade 1,3-propanediamine-N, N, N ′, N′-tetraacetic acid)
DPTA-OH (Tokyo Chemical Industry Co., Ltd .: special grade 1,3-diamineno-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.3 sodium (Sigma Aldrich Japan (same): (S, S) -ethylenediamine-N, N′-disuccinic acid trisodium salt solution 35% in H ₂ O)
48% aqueous sodium hydroxide solution (chemical required: caustic soda liquid)
48% potassium hydroxide aqueous solution (Osaka Soda Co., Ltd .: Kakei Potash Solution)

(Alkaline carbonate)
NaHCO ₃ (Wako Pure Chemical Industries, Ltd .: Special reagent grade sodium bicarbonate)
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)

(Calculation method of scale lump survival rate)
Lump weight after 7 hours (g) ÷ lump weight before charging (g) x 100 = scale lump residual rate (%)
(Evaluation criteria)
When the lump residual rate in scale 1 and scale 2 is 5% or less: ○
When the lump residual rate in Scale 1 or Scale 2 is more than 5% and 20% or less:
When the lump residual rate in scale 1 or scale 2 exceeds 20%: ×

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

  From the results of Table 2 above, a test aqueous solution containing an alkali metal salt of a specific chelating agent, a carbonate, and water was diluted with water so that the test aqueous solution was 1.0% by weight. It was confirmed that Examples 1-10 whose pH at that time is 7 or more and 10 or less show scale removal ability. On the other hand, it does not contain an alkali metal salt of a specific chelating agent, does not contain a carbonate, or the pH when diluted with water so that the aqueous test solution becomes 1.0% by weight is out of the above specific range. As for Comparative Examples 1-11, scale removal capability was remarkably inferior compared with Examples 1-10, and it was confirmed that it does not have practical scale removal capability.

(Example 11)
In the same manner as in Example 4 above, an alkali metal salt aqueous solution of a chelating agent is prepared from water, EDTA and a 48% aqueous sodium hydroxide solution, and then mixed with an aqueous sodium hydrogen carbonate solution to give 1500 g of an aqueous test solution according to Example 11. Got. This was placed in a 2 L beaker and left in a constant temperature bath at 30 ° C. A screw pump is connected to this beaker, the aqueous test solution is extracted from the bottom of the beaker, and the aqueous test solution is circulated and discharged from the top of the sieve (diameter 8 cm, opening 2 mm) at a flow rate of 1200 mL / min. A device was made (see FIG. 2). Place 18.0 g of scale 1 lump (about 1 cm x 2 cm x 3 cm) in a wet state on the sieve, apply the discharge liquid from the top of the lump, 15 minutes later, 1 hour later, 3 hours later, 6 The lump weight (wet state) after the time 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 bringing the composition for scale removal into contact with the scale adhered to the exhaust gas system, the composition is applied to the scale. Also, it was confirmed that the scale removal effect can be surely obtained even in the spraying method.

In Examples 12 to 16 and Comparative Examples 12 to 16, EDTA was used from water, a chelating agent, and a 48% sodium hydroxide aqueous solution so that the components shown in Table 4 were finally included in the amounts shown in Table 4. -3Na aqueous solution was prepared, sodium hydrogencarbonate aqueous solution was mix | blended here, and also the anticorrosive was mix | blended, and the aqueous solution for a test which concerns on the said Example and said comparative example was prepared. Each test aqueous solution was added to an 80 mL mayonnaise bottle up to 90%, and a test piece (SPCC, 3 cm × 5 cm, # 400 polished) was completely immersed therein and capped. The mayonnaise bottle was allowed to stand in a constant temperature layer at 30 ° C. After 72 hours (3 days), the test piece after immersion was taken out, and the corrosion rate (MDD) was calculated from the weight difference before and after immersion. The results are shown in Table 4 below.
(Evaluation criteria)
When MDD is 30 mg / dm ² · 24 h or less: ○
When MDD is greater than 30 mg / dm ² · 24 h: x

(Anticorrosive)
BT (benzotriazole) (Kishida Chemical Co., Ltd .: reagent grade 1,2,3-benzotriazole)
MBT (Wako Pure Chemical Industries, Ltd .: 1st grade 5-methyl-1H-benzotriazole)
Sodium molybdate (Kishida Chemical Co., Ltd .: reagent-grade sodium molybdate dihydrate)
Sodium nitrite (Kishida Chemical Co., Ltd .: reagent special grade sodium nitrite)
HEDP (Kishida Chemical Co., Ltd .: Chemical 1-hydroxyethylidene-1,1-diphosphonic acid monohydrate)

  From the results of Table 4, in Examples 12 to 16 in which the test aqueous solution contains EDTA · 3Na, sodium bicarbonate, and a specific anticorrosive, the corrosion rate of the test pieces is related to Comparative Examples 12 to 16. It was confirmed that it was significantly smaller than the results obtained from the aqueous test solution and exhibited an anticorrosive effect. In addition, the scale removal effect by the aqueous test solution of Examples 12 to 16 and Comparative Examples 13 to 16 was similar to the scale removal effect by the aqueous test solution of Comparative Example 12. Therefore, it was confirmed that the scale removal effect is not affected by the presence or absence of the anticorrosive agent in the aqueous solution.

DESCRIPTION OF SYMBOLS 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 Exhaust gas passage 11 Limestone silo 12 Raw material pit 13 Filtration pit 14 Gypsum dehydrator 20 Example 11 Test equipment used in 21 Beaker 22 Comb 23 Circulation pump 24 Water bath

Claims (6)

A scale removal method for removing scale adhered to an exhaust gas system for flowing exhaust gas discharged from an absorption tower in a flue gas desulfurization apparatus of a wet lime gypsum method,
Contacting the scale with a composition for scale removal,
The descaling composition includes 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 an alkali carbonate. Salt and water,
The scale removal method, wherein the pH is 7 to 10 when diluted with water so that the descaling composition is 1.0% by weight.   The scale removal method according to claim 1, wherein the scale removing composition is an aqueous solution containing 3 to 25% by weight of an alkali metal salt of a chelating agent.   The descaling method according to claim 1 or 2, wherein the descaling composition further contains a benzotriazole anticorrosive. 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 wet lime gypsum flue gas desulfurization apparatus flows.
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, and water,
A scale removing composition having a pH of 7 to 10 when diluted with water so that the scale removing composition is 1.0% by weight.   The scale removing composition according to claim 4, which is an aqueous solution containing 3 to 25% by weight of an alkali metal salt of a chelating agent. Furthermore, the composition for scale removal of Claim 4 containing a benzotriazole type | system | group anticorrosive.

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