JPH06954B2 - A chemical cleaning method for scales in geothermal power plants, etc. - Google Patents

Description

TECHNICAL FIELD The present invention relates to a chemical cleaning method for dissolving and removing carbonate scale adhering to steam separators and pipes in a steam production well of a geothermal power plant or above-ground equipment. .

[Conventional technology]

In geothermal power plants in geothermal fields where the geology is mainly composed of limestone layers, the scale mainly composed of calcium carbonate precipitates near the flashing point of the hot water in the steam producing well, causing blockage of the well. It is known that this causes a decrease in steam production and a reduction in turbine output. Moreover, even after the steam production well, scale adheres to the above-ground equipment such as steam separators and piping, which hinders continuous operation for power generation.

By the way, generally, the scale produced and attached in the steam production well of a geothermal power plant is a high temperature stable type calcite (calcite) hard crystal, but the scale produced and attached in ground equipment and pipes is a low temperature stable type. It is a soft crystal of Aragonite. An example of the chemical composition of the scale sampled from the well is CaCO ₃ 78.2%, SrCO ₃
19.5%, BaCO ₃ 0.19%, MgCO ₃ 0.53%, SiO ₂ 0.19%
There is also an example in which CaCO ₃ is 99%, and thus the scale is composed of a carbonate containing calcium as a main component.

As a measure for removing the scale containing calcium carbonate as a main component, conventionally, a boring method and an acid cleaning method, in which steam production is temporarily stopped, have been adopted. The boring method is a method of making a hole in a well with a cutting machine, and the acid cleaning method is a method of dissolving and removing the scale by using a hydrochloric acid solution.

[Problems to be solved by the invention]

The boring method, which has been conventionally used as a measure for removing scales containing calcium carbonate as a main component, can remove the blockage scale in a few days, but it takes more than one month to prepare and remove the rig. Because it also requires
It has the drawback that the cost and the economic loss due to the stoppage of steam production during that period are very large. In addition, the acid cleaning method requires a working period of about 10 days, which is economically superior to the boring method, but a corrosion inhibitor for hydrochloric acid having a heat resistance of about 200 ° C has not yet been developed. Therefore, there is a problem of steel pipe material corrosion in steam production wells and danger of hydrogen gas generation due to the steel material corrosion, and odor pollution due to organic corrosion inhibition.

〔Purpose〕

Therefore, the calcium carbonate generated in the steam production well,
Since it is necessary to provide a cleaning agent which has excellent ability to dissolve and remove scales of magnesium carbonate, barium carbonate, strontium carbonate, etc., and which has no problems such as corrosion of steel pipe materials and odor pollution in a steam production well, The present inventors have completed the present invention by discovering an excellent cleaning agent as a result of conducting research. That is, the present invention aims to provide a chemical cleaning method of scale in a geothermal power plant or the like, which can easily remove the scale composed of the above-mentioned carbonate, and which does not cause steel corrosion and odor pollution at all. And

[Means for solving problems]

And, as a means for achieving the above object, the present invention is to use a mixed solution (pH 5 to 10) obtained by adding an alkaline agent to an aminopolycarboxylic acid aqueous solution as a cleaning agent.
That is, according to the present invention, a mixed solution prepared by adding at least one alkaline agent to at least one aqueous solution of aminopolycarboxylic acid and adjusting the pH to 5 to 10 is injected into a steam production well of a geothermal power plant. However, it is a chemical cleaning method for a scale in a geothermal power plant or the like, which is characterized by dissolving and removing the scale generated and adhered in the steam production well.

Examples of the aminopolycarboxylic acid used in the present invention include ethylenediaminetetraacetic acid, cyclohexanediaminetetraacetic acid,
One or a mixture of two or more of diethylenetriaminepentaacetic acid, ethyletherdiaminetetraacetic acid, glycoletherdiaminetetraacetic acid and the like can be mentioned. As the alkaline agent, ammonia, sodium hydroxide, potassium hydroxide,
One or a mixture of two or more of hydrazine, monoethanolamine, diethanolamine, triethanolamine and the like is used.

The scale to be removed according to the present invention is calcium carbonate, magnesium carbonate, barium carbonate, strontium carbonate, or the like.

A cleaning agent comprising a mixed solution of aminopolycarboxylic acid and an alkaline agent in the present invention (hereinafter referred to as a mixed cleaning agent).
Has a very good dissolving power for scales composed of carbonates such as calcium, magnesium, barium and strontium, when used under a temperature condition of preferably around 200 ° C (temperature inside steam production well). In addition, the corrosion and corrosion of the steel material can be neglected without adding a corrosion inhibitor.

The cleaning method according to the present invention will be specifically described. In the present invention, after the steam production of the geothermal power plant is temporarily stopped, the mixed cleaning agent is injected into the steam production well in the vicinity of the flushing point. For this reason, the mixed cleaning agent injection pipe is inserted up to near the flushing point of the steam production well.

The dissolving power of the carbonate scale by the mixed detergent of the present invention is
It depends on the concentration of the aminopolycarboxylic acid. Therefore, the concentration of the aminopolycarboxylic acid in the mixed detergent is determined by the scale amount of the carbonate formed and attached to the cleaning object. However, in order to achieve effective cleaning, a concentration of 5-10% by weight is preferred. The alkaline agent itself in the mixed detergent does not have any dissolving effect on the carbonate scale, but the alkaline agent is used in combination with the aminopolycarboxylic acid to adjust the pH of the aminopolycarboxylic acid aqueous solution. By adjusting this pH, the solubility of aminopolycarboxylic acid in water can be increased. Therefore, the combined addition of the alkaline agent has an auxiliary effect on the dissolution of the carbonate scale, and plays an important role. For this reason, the pH of the mixed detergent of the present invention is 5
10 to 10, preferably 6 to 10. That is, if the pH is less than 5, for example, even if an alkaline agent is added and the pH of the mixed detergent is 4, the solubility of the aminopolycarboxylic acid becomes very small, and the amino in the mixed detergent is The polycarboxylic acid concentration does not reach the desired value, and as a result, the dissolving action on the carbonate scale is reduced.
Furthermore, it is not preferable because it will corrode the steel pipe material in the steam production well. On the other hand, when the pH is higher than 10, H in the COOH group of aminopolycarboxylic acid is almost consumed by the alkaline agent, and as a result, the dissolving action on the carbonate scale is reduced. For the above reasons, in the present invention, the pH of the mixed detergent is 5 to 10, preferably 6 to 10.
It is preferably adjusted to 10.

In the cleaning method according to the present invention, the effective cleaning temperature for dissolving and removing carbonate scale is such that the higher the temperature is, the higher the dissolving power is, and if possible, the temperature slightly lower than the decomposition temperature of aminopolycarboxylic acid is the most preferable. preferable. Fortunately, the temperature in the steam production well of a geothermal power plant where carbonate scale is generated and adhered is usually 200 to 250 ° C,
No heat is required for cleaning, and it is sufficient to inject the mixed cleaning agent into the scale-attached portion.

Examples of the dissolution action of a carbonate scale by a mixed detergent comprising an aminopolycarboxylic acid and an alkaline agent for pH adjustment used in the present invention, ethylenediaminetetraacetic acid as an aminopolycarboxylic acid, and sodium hydroxide as an alkaline agent As shown in the following formulas (1) to (4), this is because the chelating action of ethylenediaminetetraacetic acid with respect to calcium, magnesium, barium and strontium strongly works at high temperature, and the carbonate scale is reduced. Presumed to be dissolved.

CaCO ₃ + Na ₃ HY → Na ₂ ⁺ + CaY ² + + NaHCO ₃・ ・ ・ ・ (1) Formula MgCO ₃ + Na ₃ HY → Na ₂ ⁺ MgY ² + + NaHCO ₃・ ・ ・ ・ (2) Formula BaCO ₃ + Na ₃ HY → Na ₂ ⁺ BgY ^{2 −} + NaHCO ₃ ··· (3) Formula SrCO ₃ + Na ₃ HY → Na ₂ ⁺ SrY ² + + NaHCO ₃ ··· (4) Formula (However, Na ₃ HY is ethylenediaminetetraacetic acid Represents a sodium salt, and Y ⁻ represents an anion of ethylenediaminetetraacetic acid.) [Examples] Table 1 shows examples of the present invention. 100 ml of the mixed detergent having the composition shown in this table was placed in a Teflon-lined iron container, and a strip-shaped test piece (surface area 32 cm ² ) of STPT material (JIS standard steel material) was immersed in the container and collected from the steam production well of the actual machine. Carbonate scale (CaCO ₃ 64%, M
A test was conducted in which gCO ₃ 4.0%, SrCO ₃ 25.2%, BaCO ₃ 6.4%) and high-purity natural calcite (99.0% CaCO ₃ ) were crushed into about 5 mm square pieces. Then, as this test method, the test piece is immersed in the mixed detergent in the Teflon-lined iron container, and the carbonate scale and the crushed product of natural calcite are mixed into the mixed detergent separately, and After sealing the container, keep it in the air dryer at a predetermined temperature and heat it for 6 hours, (take out the container once every 30 minutes and move it left and right to stir the mixed detergent in the container. After cooling with water, open the lid of the closed container, and in the case of the above test piece, take it out and reduce its corrosion weight (m
g / cm ² · hr), and in the case of carbonate scale and natural calcite, dissolution rate of carbonate scale and calcite
(%) (Quantitative analysis by atomic absorption spectrophotometer) was measured. For comparison, an acid corrosion inhibitor was added to a hydrochloric acid solution and a similar test was conducted. However, as for the cleaning temperature, there is a risk of hydrogen gas generation due to corrosion of the test piece (steel material). It was carried out at 100 ° C, which is 100 ° C lower. The results of these tests are shown in Table 1.

As is clear from Table 1, test Nos. 1 to 12, which are examples of the present invention, add a pH adjusting agent (alkali agent) to aminopolycarboxylic acid alone to adjust the pH to a range of 5 to 11. The prepared mixed cleaning agent is used. (In addition, pH4
In the following, the solubility of aminopolycarboxylic acid was so small that it was not used. In any of the examples, in the range of pH 5 to 10 excluding pH 11, the dissolution rate of the carbonate scale is 92 to 100% on the actual scale and 80 to 95% for the high-purity natural calcite. When the pH becomes 5, the corrosion loss of the test piece (STPT material) becomes large and the aminopolycarboxylic acid is consumed by the steel material, so the dissolution rate of the carbonate scale is slightly lower than at pH 6. Also, the pH is 11
However, the dissolution rate of the carbonate scale suddenly decreased, but when the pH reached 11, the CO of aminopolycarboxylic acid
It is considered that since the H of the OH group is completely consumed by the pH adjusting agent (alkali agent), the chelating effect on the carbonate scale is weakened.

Test Nos. 13 to 20 of the examples are mixed detergents in which the pH is adjusted within the range of 5 to 11 by adding two types of pH adjusting agents (alkali agents) to two types of aminopolycarboxylic acids in combination. There is no particular difference from the results of Example Test Nos. 1 to 12.

On the other hand, test No. 21 of the comparative example shows the result of hydrochloric acid cleaning by the conventional method, but the corrosion rate of the test piece (STPT material) is about 1/2 of that of the actual machine while the dissolution rate in the carbonate scale is high. It is very large even at the cleaning temperature, and if it is tested at 200 ° C, which is the same level as the actual equipment, the amount of corrosion further increases, and there is a risk of penetrating the steel pipe material in the steam production well and a risk associated with hydrogen gas generation. This is what causes sex.

[Effects of the Invention] As described in detail above, the present invention uses a mixed detergent (pH 5 to 10) composed of an aminopolycarboxylic acid and an alkaline agent. It is not necessary to add a corrosion inhibitor, and the corrosion of the steel pipe material in the steam producing well can be neglected, and the dissolution in the carbonate scale acts preferentially to produce the effect of efficient cleaning. Further, in the present invention, as described above, since the corrosiveness to the steel pipe material in the steam producing well can be neglected, there is no risk associated with the generation of hydrogen gas, and moreover, an organic type corrosion inhibitor is used. Since there is no need to use additives and the like, there is no problem of odor pollution. Moreover, in the present invention, as compared with the conventional boring method or hydrochloric acid washing method, almost no cleaning is required after the carbonate scale is dissolved and removed, so that the working period can be greatly shortened, and the economic merit due to it can be reduced. This is a very large effect.

Claims (1)

[Claims] 1. A mixed solution prepared by adding at least one alkaline agent to at least one aqueous solution of aminopolycarboxylic acid and adjusting the pH to 5 to 10 is injected into a steam production well of a geothermal power plant. A method for chemically cleaning scales in a geothermal power plant, characterized by dissolving and removing scales adhering to the inside of the steam production well.