JP6317687B2 - Scale remover and boiler cleaning method using the scale remover - Google Patents

Description

  The present invention relates to a scale remover for use in chemical cleaning work such as a power plant, and in particular, a scale remover for effectively removing powder scale inside an evaporation pipe in a boiler in which oxygen treatment is performed on feed water, and this This invention relates to a chemical cleaning method using

  In boilers and the like of power plants, metal ions that are dissolved in a very small amount in boiler water deposit around a particularly high heat load such as an evaporation tube and adhere as a hardly soluble scale. Usually, the main component of the scale is iron oxide, accounting for 90% or more. The thermal conductivity of the scale adhering to the inner wall of the evaporator tube is 1/10 or less of that of the base metal, and when the amount of scale adhering increases, heat transfer is hindered and bulging due to overheating may occur. In addition, a wavy scale is formed with the growth, and the pressure loss of the boiler rises, and because the coefficient of thermal expansion of the base metal and the scale is different, the scale that has grown thick due to the thermal change at the time of boiler stop and startup It may break off and peel off, causing clogging at the bent part of the pipe. Therefore, depending on the degree of scale adhesion in the evaporation pipe, it is necessary to stop the boiler and remove these scales by chemical cleaning work.

  Among them, once-through boilers are difficult to completely discharge chemicals due to their structure, and chemicals may remain during chemical cleaning. Conventionally, chemical cleaning is a mixture of organic acids that are not corrosive, such as glycolic acid and citric acid. I have used acid.

On the other hand, there is oxygen treatment as a method for managing the quality of boiler feed water. This is to inject hematite (Fe ₂ O ₃ ) with lower solubility by injecting a small amount of oxygen into high-purity water and adjusting the dissolved oxygen concentration. The characteristics of oxygen treatment are that hematite is different from magnetite (Fe ₃ O ₄ ) generated in the case of volatile substance treatment, in which the growth of the scale is slow, the particle size of the agglomerates is small, and wavy scale is generated. May produce a scale with a smooth surface, thus reducing pressure loss.

  However, even in a boiler that performs oxygen treatment, when the amount of iron flowing into the boiler increases, the hematite scale grows. Further, the hematite scale is called a powder scale because it has a porous structure with small agglomerates, and is poor in thermal conductivity, causing a rise in the temperature of a metal that is a constituent member such as piping. Therefore, even if oxygen treatment is being performed, chemical cleaning work for the boiler is required.

  Hematite has a lower solubility than magnetite, and in chemical cleaning work, the powder scale dissolves and peels from the inner wall of the pipe, resulting in a slightly soluble sludge made of hematite and having a small particle size. Sludge is deposited in the portion of the pipe where the circulation flow rate is small, and is also dispersed in the chemical solution. Since this sludge has a small particle size, it is difficult to collect and the sludge is brought into the boiler by circulation of the cleaning liquid. Therefore, sludge remains in the boiler after chemical cleaning work, which may cause expansion of the evaporation pipe and the like.

  In order to remove small sludge, it has been proposed to provide a filter or a centrifuge in the cleaning liquid circulation path (Patent Document 1). However, it is necessary to use a plurality of fine filters, and the cleaning circulation path is enlarged. There is a problem that the operating cost is increased due to a decrease in circulating water flow due to clogging of the filter and frequent filter replacement.

On the other hand, as a cleaning method for dissolving and removing the iron oxide scale, a chelating agent and an organic acid salt of 0.05 × 10 ^{−3 to} 1.0 × 10 ⁻³ mol / L of a reducing divalent metal are allowed to coexist, and the pH is adjusted to 4. A method of dissolving and removing magnetite while maintaining in the range of 0 to 5.0 (Patent Document 2), malonic acid, glycolic acid, and L-ascorbic acid (reducing agent) are malonic acid: glycolic acid: reducing agent = 1: There has been proposed a chemical detergent composition (Patent Document 3) capable of dissolving and removing hematite contained at 0.5 to 1.5: 0.5 to 2 (weight ratio).

Japanese Patent No. 5556217 Japanese Examined Patent Publication No. 7-65204 Japanese Patent No. 3640148

  An object of the present invention is to dissolve and remove the low-solubility hematite formed by supplying oxygen-treated water in chemical cleaning performed to prevent scale adhesion in piping such as an evaporation pipe of a once-through boiler. It is to provide a descaling agent that can be used.

  It is another object of the present invention to provide an efficient once-through boiler chemical cleaning method that generates a small amount of fine sludge and can reduce operating costs.

  In view of the above problems, the present inventors have studied the development of a drug that dissolves hematite in order to prevent fine sludge made of hematite from remaining in piping such as an evaporation pipe of a once-through boiler. As a result, the present inventors have adopted this problem by adopting a scale remover containing oxalic acid and / or an ammonium salt of oxalic acid, containing a chelating agent, an organic acid different from oxalic acid, and a reducing agent. The present invention was completed by finding out that the problem can be solved.

According to the present invention, a scale remover and a boiler chemical cleaning method having the following configurations are provided.
[1] A scale remover containing oxalic acid and / or an ammonium salt of oxalic acid, containing a chelating agent, an organic acid different from oxalic acid, and a reducing agent.
[2] The chelating agent is at least one selected from ethylenediaminetetraacetic acid tetraammonium, ethylenediaminetetraacetic acid triammonium, ethylenediaminetetraacetic acid diammonium, ethylenediaminetetraacetic acid tetrasodium, ethylenediaminetetraacetic acid trisodium, and ethylenediaminetetraacetic acid disodium. The scale removing agent according to [1], which is described above.
[3] The organic acid is at least one selected from citric acid, glycolic acid, malic acid, gluconic acid, acetic acid, formic acid, lactic acid, tartaric acid, malonic acid, pyruvic acid, maleic acid, glutaric acid, and succinic acid. [1] or [2] scale remover [4] The reducing agent is at least one selected from L-ascorbic acid, L-ascorbate, erythorbic acid, erythorbate, and hydrazine. A scale remover according to any one of [1] to [3] [5] 0.38 to 19 wt% of the oxalic acid and / or ammonium salt of oxalic acid, 3.8 to 38 wt% of the chelating agent, The organic acid is contained in a range of 3.8 to 26.3 wt%, the reducing agent is contained in a range of 3.8 to 38 wt%, and the pH is in the range of 3 to 8, in any one of [1] to [4] Scale removal .
[6] A method for chemically cleaning an evaporator pipe of a boiler using water treated with oxygen,
A cleaning circulation path having an inlet and an outlet is provided at the bottom and the top of the evaporation pipe, respectively, and the scale remover of claim 1 is injected into the circulation path and circulated to form a scale particle having an average particle size of less than 20 μm. A chemical cleaning method for a boiler, characterized in that it is dissolved and removed.
[7] The scale remover has a pH in the range of 4 to 8, the oxalic acid and / or ammonium salt of oxalic acid is 0.1 to 5 wt%, the chelating agent is 1 to 10 wt%, and the organic acid is 1 The chemical cleaning method according to [6], wherein ˜7 wt% and the reducing agent is in the range of 1 to 10 wt%.
[8] The cleaning method according to [6] or [7], further including installing a centrifuge in the circulation path and removing sludge having an average particle size larger than 50 μm with the centrifuge.

  Trivalent iron ions and oxalic acid salts are soluble, but divalent iron ions and oxalic acid salts are extremely difficult to dissolve and form a film on the scale surface to inhibit the progress of the reaction. When oxalic acid and EDTA are combined, iron ions eluted by oxalic acid are retained by EDTA, and a film of iron (II) oxalate is not formed, and the dissolution reaction proceeds. When the amount of the reducing agent added is further increased, the solubility of hematite is improved.

  The scale remover of the present invention can dissolve 90% or more of the hematite constituting the powder scale, and can suppress the generation of fine sludge made of hematite.

  According to the chemical cleaning method of the present invention, not only soluble magnetite but also low-solubility hematite can be dissolved and removed, so that the generation of hardly soluble sludge can be greatly reduced.

  By providing a centrifugal separator in the washing circuit, it is possible to collect relatively coarse sludge having an average particle diameter of 50 μm or more, and further enhance the scale removal effect.

  Moreover, the organic mixed acid with low corrosiveness used conventionally is acidic (pH 2-4), whereas the scale remover of the present invention is used near neutral (pH 4-8). Safety in cleaning work is improved.

  Further, since the dissolution rate of the scale is larger than that of the organic mixed acid, the cleaning time can be shortened, and the corrosion rate of the base material is almost the same, so that the amount of corrosion can be suppressed.

It is a schematic explanatory drawing which shows the chemical cleaning method of the boiler using the scale removal agent of this invention. It is the experimental apparatus figure used in Example 2 and Example 3. FIG. 10 is a graph showing experimental results in Example 2. 10 is a graph showing experimental results in Example 2. 10 is a graph showing experimental results in Example 3.

[Scale remover]
The scale remover of the present invention contains oxalic acid and / or ammonium oxalate, a chelating agent, an organic acid different from oxalic acid, and a reducing agent.

  Oxalic acid and ammonium oxalate are preferred because they do not remain dissolved in the boiler wash water as oxalate ions and ammonium ions. As oxalates, for example, sodium salts and potassium salts are not suitable because sodium ions and potassium ions remain in the wash water, and these alkali metal ions may be precipitated as salts by heating.

  The chelating agent is a compound capable of forming a coordination bond with a metal ion and is not particularly limited as long as it is water-soluble, but ethylenediaminetetraacetic acid tetraammonium, ethylenediaminetetraacetic acid triammonium, ethylenediaminetetraacetic acid diammonium , Ethylenediaminetetraacetic acid tetrasodium, ethylenediaminetetraacetic acid trisodium, ethylenediaminetetraacetic acid disodium, etc. (EDTA) can be preferably used. Among them, in order to suppress the pH change due to the iron ion concentration accompanying the dissolution of the scale, it is desirable that the ionic product of the chemical solution is large, and from the viewpoint of the buffer effect by the weak base, ethylenediaminetetraacetic acid tetraammonium is preferable.

  As the organic acid, citric acid, glycolic acid, malic acid, gluconic acid, acetic acid, formic acid, lactic acid, tartaric acid, malonic acid, pyruvic acid, maleic acid, glutaric acid and oxalic acid can be preferably used. Among these, citric acid is preferred from the knowledge of the solubility in iron oxide and the formation of a complex with iron.

  As the reducing agent, L-ascorbic acid, L-ascorbate, erythorbic acid, erythorbate, and hydrazine can be preferably used. Of these, L-ascorbic acid is preferred.

  The scale remover of the present invention may contain a corrosion inhibitor in addition to the above essential components.

  The scale remover of the present invention comprises 0.38 to 19 wt%, preferably 0.38 to 3.8 wt% of the oxalic acid and / or ammonium salt of oxalic acid, and 3.8 to 38 wt% of the chelating agent, Preferably it contains 21.6-36.1 wt%, the organic acid 3.8-26.3 wt%, preferably 11.4-22.8 wt%, and the reducing agent 3.8-38 wt%, preferably It is preferable that 3.8-19 wt% is included and the pH is in the range of 3-8.

[Boiler chemical cleaning method]
Next, a chemical cleaning method for a boiler using the scale remover of the present invention will be described with reference to FIG.

  A cleaning circulation path d having an inlet and an outlet is provided at the bottom b and top c of a boiler evaporation pipe a using oxygen-treated feed water, and the scale remover is injected into the circulation path d. Circulate to dissolve and remove scale particles having an average particle size of less than 20 μm.

  The scale remover of the present invention is used after being diluted by being injected into boiler water circulating in the cleaning circuit. By dilution, the concentration of oxalic acid is 0.1 to 5.0 wt%, preferably 0.1 to 1.0 wt%, more preferably 0.5 wt%, the concentration of chelating agent is 1.0 to 10.0 wt%, Preferably 5.7 to 9.5 wt%, organic acid concentration is 1.0 to 7.0 wt%, preferably 3.0 to 6.0 wt%, reducing agent concentration is 1.0 to 10.0 wt%, preferably It is 1.0 to 5.0 wt%, more preferably 1.0 to 3.0 wt%.

  However, it is possible to retain about 1,900 mg / l of eluted iron ions at 1.0 wt% of the chelating agent, and the concentration of the chelating agent is adjusted by the expected value of the eluted iron ion concentration. It is not limited. Moreover, since an organic acid is used as a pH adjuster, it is not particularly limited to the above range as long as the scale remover can be adjusted to a desired pH range.

  As the amount of the reducing agent is increased, the solubility of hematite is improved, but the color of the chemical solution after the reaction tends to be dark and the finish of the cleaning surface tends to be poor. Therefore, the reducing agent is preferably in the above range. When the organic acid concentration is increased, the solubility in iron oxide is improved, and thus the above range is preferable.

  Further, when the pH is 3 or less, the chelating agent EDTA in the scale remover becomes EDTA · 4H and is insolubilized. Therefore, the pH at the time of chemical cleaning use of the scale remover of the present invention by dilution is 4 to 7, preferably pH 4.5 to It is desirable to use 5.5.

The scale amount of the boiler to be chemically cleaned is not particularly limited, but can be effectively removed if the adhesion amount is about 30 to 60 mg / cm ² and the iron ion concentration is about 15,000 mg / L. Has been confirmed.

  Sludge having an average particle size of less than 50 μm can be dissolved and removed using the scale remover of the present invention. In order to remove large sludge having an average particle size of more than 50 μm, a centrifugal separator g may be attached to the washing circuit e.

  Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited thereto.

[Preparation of scale remover]
Drug A (scale remover of the present invention)
Oxalic acid 0.5 wt%, ethylenediaminetetraacetic acid tetraammonium (EDTA) 5.7 wt%, citric acid 4.6 wt%, L-ascorbic acid 2 wt%, EBARACS-50A (corrosion inhibitor) 0.3 wt%, remaining amount is A scale remover A was prepared by weighing to water and dissolving in water.
・ Drug B (existing scale remover)
Citric acid 3 wt%, glycolic acid 3 wt%, L-ascorbic acid 0.3 wt%, EBARACS-50A 0.3 wt%, the remaining amount was measured to be water, and dissolved in water to prepare scale remover B .

[Example 1]
First, the difference in solubility between hematite and magnetite when using drugs A and B was examined. As the hematite and magnetite, commercially available reagents (hematite: iron (III) oxide manufactured by Wako Pure Chemical Industries, magnetite: ferric oxide (III) iron (II) manufactured by Wako Pure Chemical Industries, Ltd.) were used. Table 1 shows the experimental conditions and results.

  In the drug A which is the scale remover of the present invention, 3.00 g of hematite becomes 0.02 g of residue, and a 99.3% dissolution removal rate is obtained, which indicates that it is dissolved as iron ions.

  The drug B exhibits a certain degree of solubility in magnetite, but has low solubility in hematite.

[Example 2]
The evaporation tube was cut out from the boiler that had been subjected to oxygen treatment, incorporated into the apparatus shown in FIG. For comparison, the drug B was added and the experiment was performed under the same conditions.

  In FIG. 2, reference numeral 2 denotes an evaporating tube, and 1 denotes a control tube from which the scale of the extracted evaporating tube has been completely removed. Evaporation tube 2 and control tube 1 are overlapped and fixed with upper and lower flanges and sealed. The scale remover is introduced from the scale remover storage tank 4 and a washing circuit is formed so as to pass through the evaporation pipe 2 and the control pipe 1 to circulate the scale remover. A heater wire 6 is wound around the cleaning circuit and heated using a temperature controller 5 and a temperature sensor 7.

  The time-dependent change of the eluted iron ion concentration at this time is shown in FIG. In the experiment using the drug A, the concentration of eluted iron ions became almost constant in the time of about half to 1/3 of the experiment using the current drug B.

  The particle size distribution of the sludge after each dissolution experiment at this time is shown in FIG. In the cleaning using the medicine B, a peak of sludge having a relatively small particle diameter was observed in the vicinity of 20 μm. However, in the cleaning using the medicine A, this peak was not observed, and it was effective in removing sludge having a small particle diameter. It was shown that there is. Table 2 shows the residual ratio by sludge particle size.

  Table 3 shows the amount of corrosion of the inner wall of the control tube in this experiment ((weight before washing of control tube−weight after washing of control tube) / wetted area) and corrosion rate. The corrosion rate is approximately the same for both drug A and drug B, but the cleaning time is about half that of drug B for drug A, so the amount of corrosion was lower when drug A was used.

Example 3
In the same manner as in Example 2, the evaporation tube was cut out from the oxygen-treated boiler and incorporated in the apparatus shown in FIG. 2, and a dissolution experiment was performed at a temperature of 85 ° C. while adding and circulating the drug A. For comparison, an experiment was performed under the same conditions by adding the drug B to an apparatus incorporating an evaporation tube. The point at which the concentration of eluted iron ions became almost constant was taken as the end point, and the remaining sludge was collected and measured with an X-ray diffractometer. The crystal phase weight ratio is shown in Table 4 and FIG.

  In the experiment using the drug B, the ratio of hematite in the sludge is 85% and the ratio of the hematite in the scale before dissolution is 78%, which is almost the same, but in the experiment using the drug A, in the sludge The proportion of hematite was reduced to 6%.

a evaporating tube b evaporating tube bottom c evaporating tube top d cleaning circuit e centrifuge 1 corrosion loss control tube 2 dissolution experiment evaporating tube 3 venting 4 liquid reservoir 5 temperature controller 6 heating heater 7 temperature sensor 8 Circulation pump 9 Drain

Claims (8)

A scale remover containing oxalic acid and / or ammonium oxalate, a chelating agent, an organic acid different from oxalic acid, and a reducing agent, and dissolving and removing hematite or magnetite . The chelating agent is at least one selected from ethylenediaminetetraacetic acid tetraammonium, ethylenediaminetetraacetic acid triammonium, ethylenediaminetetraacetic acid diammonium, ethylenediaminetetraacetic acid tetrasodium, ethylenediaminetetraacetic acid trisodium, and ethylenediaminetetraacetic acid disodium. The scale remover according to claim 1. The organic acid is at least one selected from citric acid, glycolic acid, malic acid, gluconic acid, acetic acid, formic acid, lactic acid, tartaric acid, malonic acid, pyruvic acid, maleic acid, glutaric acid, and succinic acid. The scale remover according to claim 1 or 2. The scale removing agent according to any one of claims 1 to 3, wherein the reducing agent is at least one selected from L-ascorbic acid, L-ascorbate, erythorbic acid, erythorbate, and hydrazine. 0.38 to 19 wt% of the oxalic acid and / or ammonium salt of oxalic acid, 3.8 to 38 wt% of the chelating agent, 3.8 to 26.3 wt% of the organic acid, and 3.8 of the reducing agent The scale remover according to any one of claims 1 to 4, which is contained in a range of -38 wt% and a pH is in a range of 3-8. A chemical cleaning method for an evaporator tube of a boiler using oxygen-treated feed water,
A cleaning circulation path having an inlet and an outlet is provided at the bottom and the top of the evaporation pipe, respectively, and the scale remover of claim 1 is injected into the circulation path and circulated to form a scale particle having an average particle size of less than 20 μm. A chemical cleaning method for a boiler, characterized in that it is dissolved and removed. The scale remover has a pH in the range of 4 to 8, the oxalic acid and / or ammonium oxalate is 0.1 to 5 wt%, the chelating agent is 1 to 10 wt%, the organic acid is 1 to 7 wt%, The chemical cleaning method according to claim 6, wherein the reducing agent is in the range of 1 to 10 wt%.   The cleaning method according to claim 6 or 7, further comprising installing a centrifuge in the circulation path and removing sludge having an average particle diameter of more than 50 µm with the centrifuge.