Method for removing scale in geothermal power plants

Abstract

One embodiment of the present invention is a removal method for removing deposited scale precipitated from a geothermal fluid containing dissolved silica in a geothermal power plant comprising a water pump that pumps the geothermal fluid from a production well, a steam separator that separates the geothermal fluid into geothermal water and geothermal steam, and a turbine that rotates as a result of being supplied with the geothermal steam separated by the steam separator, wherein in a first area leading to the turbine from the production well, the surfaces of sites in the geothermal plant that are in contact with the geothermal fluid are formed from diamond-like carbon, in areas other than the first area, at least the surfaces of such sites in the geothermal power plant are formed from at least one substance selected from among diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, and a scale removal liquid is injected into a flow path that includes the surfaces of such sites.

Description

Technical Field

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[0001] The present invention relates to a method for removing scale in a geothermal power plant.

Background Art

[0002] Geothermal power generation involves collecting high-temperature geothermal fluid (geothermal water and geothermal steam) from production wells and generating electricity using the steam separated from the geothermal fluid. The geothermal fluid collected from production wells contains more dissolved silica than well water or river water.

[0003] The dissolved silica in the geothermal water collected from production wells is concentrated by being depressurized in a geothermal power plant, cooled while flowing through pipes, and its solubility decreases. When the silica contained in the geothermal water becomes supersaturated, it polymerizes into amorphous silica and precipitates as silica scale. Since silica scale may adhere to the inner walls of pipes and cause problems such as pipe blockage, the adhesion of silica scale is a problem in geothermal power plants.

[0004] For pipes and the like to which silica scale adheres, there is a known method of injecting oxidizing agents such as hydrochloric acid, sulfuric acid, nitric acid, and hydrofluoric acid, and alkaline agents such as sodium hydroxide, sodium carbonate, and sodium bicarbonate as chemicals for dissolving and cleaning the silica scale into the pipes and the like. For example, Patent Document 1 discloses a method of removing scale by contacting a scale removal solution containing tropones and acids such as hydrochloric acid, sulfuric acid, and nitric acid with the scale.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, with conventional scale removal methods, even when chemicals are brought into contact with the scale, especially in areas with thick scale, the chemicals may not easily reach the surface where the scale is attached, potentially leaving scale on the inner walls of pipes and resulting in insufficient cleaning.

[0007] One aspect of the present invention provides a method for removing scale that can suppress poor cleaning of scale. [Means for solving the problem]

[0008] One aspect of the present invention relates to a geothermal power plant comprising a feed pump for pumping up geothermal fluid from production wells, arranged in order from the upstream side of the geothermal fluid; a steam-water separator for separating the geothermal fluid into geothermal water and geothermal steam; and a turbine that rotates by supplying the geothermal steam separated by the steam-water separator, wherein a scale removal method is provided for removing scale deposited and adhering to the geothermal fluid containing dissolved silica, wherein in a first region from the production well to the turbine, the surface of the portion of the geothermal power plant that comes into contact with the geothermal fluid is formed of diamond-like carbon, and in regions other than the first region, at least the surface of the portion of the geothermal power plant is formed of at least one selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, and a scale removal liquid is injected into the flow path including the surface of the portion. [Effects of the Invention]

[0009] According to one aspect of the present invention, poor cleaning of scale can be suppressed. [Brief explanation of the drawing]

[0010] [Figure 1] This is a schematic diagram of a geothermal power plant according to one embodiment. [Figure 2] This graph shows the amount of silica scale attached to each material piece. [Figure 3] This is a SEM image showing the surface of a material piece made of PVC. [Figure 4]This is a SEM image showing the surface of a material piece made of 13% Cr steel. [Modes for carrying out the invention]

[0011] The embodiments for carrying out the present invention will be described below with reference to the drawings.

[0012] Figure 1 is a schematic diagram of a geothermal power plant according to one embodiment. As shown in Figure 1, the geothermal power plant 100 includes, arranged in order from the upstream side of the geothermal fluid, a feed pump 2 that pumps up geothermal fluid from a production well 1, a steam-water separator 3 that separates the geothermal fluid into geothermal water and geothermal steam, and a turbine 4 that rotates by being supplied with the geothermal steam separated by the steam-water separator 3. In the geothermal power plant 100, the region from the production well 1 to the turbine 4 is referred to as the first region 10. That is, the first region 10 includes the production well 1, the feed pump 2, the steam-water separator 3, and the turbine 4. The turbine 4 is connected to the generator 5. The first region is the region in the geothermal power plant 100 that is under a high temperature and high pressure environment.

[0013] The geothermal power plant 100 may include piping L1 for introducing geothermal fluid (geothermal water and geothermal steam) pumped from production well 1 to feedwater pump 2, piping L2 for introducing geothermal fluid discharged from feedwater pump 2 to steam-water separator 3, and piping L3 for introducing geothermal steam separated in steam-water separator 3 to turbine 4. That is, the first region 10 may include piping L1 to L3.

[0014] The geothermal power plant 100 may include a condenser 6 that condenses the geothermal steam discharged from the turbine 4, a cooling tower 7 that cools the condensed water in the condenser 6, and a circulation pump 8 that sends the cooled water from the cooling tower 7 back to the condenser 6. In the geothermal power plant 100, the region from the condenser 6 to the point where the condensed water is cooled and returns to the condenser 6 is defined as the second region 20. That is, the second region 20 may include the condenser 6, the cooling tower 7, and the circulation pump 8. The second region is the region in the geothermal power plant 100 that is under a low-temperature, low-pressure environment.

[0015] The geothermal power plant 100 may include piping L4 for introducing condensed water (hot water) condensed in the condenser 6 to the cooling tower 7, piping L5 for introducing the cooling water cooled in the cooling tower 7 to the circulation pump 8, and piping L6 for returning the cooling water discharged from the circulation pump 8 to the condenser 6. In other words, the second region 20 may include piping L4 to L6.

[0016] The geothermal power plant 100 may include a retention tank 9 located in the flow path through which geothermal water separated by the steam-water separator 3 flows, and a reduction pump 11 that returns the geothermal water discharged from the retention tank 9 to the reduction well 14. The retention tank 9 allows the polymerization reaction of silica in the geothermal water to proceed and retains the water until the silica-based insoluble components have sufficiently coagulated and settled. In the geothermal power plant 100, the region from the outlet where the geothermal water flows out of the steam-water separator 3 to the reduction well 14 is defined as the third region 30. That is, the third region 30 may include the retention tank 9, the reduction pump 11, and the reduction well 14. The third region is a region in the geothermal power plant 100 that is under a high temperature and medium pressure or medium temperature and high pressure environment.

[0017] The geothermal power plant 100 may be equipped with piping L7 for introducing geothermal water separated by the steam-water separator 3 into the retention tank 9. That is, the third region 30 may include piping L7.

[0018] Next, the flow of geothermal fluid in the geothermal power plant 100 will be explained. In Figure 1, the flow of geothermal fluid is shown by solid arrows in each pipe. Production well 1 is a well that brings geothermal water, geothermal steam, or a mixture thereof (geothermal fluid) from the underground geothermal reservoir to the surface. The geothermal fluid pumped up from production well 1 is introduced to the feedwater pump 2 through pipe L1 and sent to the gas-liquid separator 3 through pipe L2. In the gas-liquid separator 3, it is separated into geothermal steam, which is the gaseous component, and geothermal water, which is the liquid component. The separated geothermal steam is sent to the turbine 4 through pipe L3 and used to rotate the turbine 4, which generates electricity with the generator 5.

[0019] The geothermal steam that has passed through the turbine 4 is sent to the condenser 6 and condensed. The condensed condensate is further sent to the cooling tower 7 through the pipe L4 and cooled. The cooled cooling water is introduced into the circulation pump 8 through the pipe L5, returned to the condenser 6 through the pipe L6, and used as the cooling water for the geothermal steam that has passed through the turbine 4.

[0020] The geothermal fluid contains dissolved silica. The method for removing the scale (silica scale or amorphous silica) deposited and adhering from the geothermal fluid in the geothermal power plant 100 is that in the first region 10, the surface of the location where the geothermal fluid of the geothermal power plant 100 contacts is formed with diamond-like carbon, and in the regions other than the first region, at least the surface of the location where the geothermal fluid of the geothermal power plant 100 contacts is formed with at least one selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, and a scale removal liquid is injected into the flow path including the surface of the location where the geothermal fluid contacts.

[0021] Generally, the parts of a geothermal power plant where geothermal fluid contacts, such as pipes and turbines, are formed of steel. Since the hydroxyl groups on the surface of steel are likely to bond with silica, silica scale is likely to adhere to the parts where geothermal fluid contacts. Therefore, the inventor focused on the fact that on the surfaces of diamond-like carbon materials, polytetrafluoroethylene materials, and polyvinyl chloride materials, there are fewer hydroxyl groups that are likely to bond with silica compared to the surface of steel, and particularly, there are even fewer hydroxyl groups on the surface of diamond-like carbon materials. In this embodiment, in the first region of the geothermal power plant 100 where the dissolved silica concentration is high, by forming the surface of the part where geothermal fluid contacts with diamond-like carbon, the starting point of silica scale adhesion on the surface of the part where geothermal fluid contacts can be effectively reduced, and the bonding force between the surface and silica scale can be reduced. In regions other than the first region, by forming at least the surface of the part where geothermal fluid contacts with at least one selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, the bonding force between the surface of the part where geothermal fluid contacts and silica scale can be reduced. Therefore, by injecting a scale removal liquid into the flow path including the surface of the part where geothermal fluid contacts, silica scale can be easily peeled off, dissolved, and removed, and poor cleaning of the scale can be suppressed.

[0022] Also, in the first region of the geothermal power plant 100 that is in a high-temperature and high-pressure environment, by forming the surface of the part where geothermal fluid contacts with diamond-like carbon that has excellent heat resistance and pressure resistance, it is possible to suppress deterioration or change of the surface of the part where geothermal fluid contacts, and the bonding force between the surface and silica scale can be reduced. In regions other than the first region, by forming at least the surface of the part where geothermal fluid contacts with at least one selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, it is possible to suppress deterioration or change of the surface of the part where geothermal fluid contacts, and the bonding force between the surface and silica scale can be reduced.

[0023] Specifically, the surfaces of the geothermal fluids that come into contact with the geothermal power plant 100 include the inner surfaces of pipes L1 to L7 and the outer surface of the turbine 4. That is, a method for removing scale in the geothermal power plant 100 may be to form the inner surfaces of pipes L1 to L3 and the outer surface of the turbine 4 with diamond-like carbon in the first region 10, and at least the inner surfaces of pipes L4 to L7 in the second and third regions with at least one material selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, and inject a scale removal liquid into the flow path including the inner surfaces of pipes L1 to L7 and the outer surface of the turbine 4.

[0024] In the scale removal method for the geothermal power plant 100, if the material used for piping L4 to L7 in the second and third regions is polytetrafluoroethylene or polyvinyl chloride, it is preferable to form piping L4 to L7 with polytetrafluoroethylene or polyvinyl chloride. Generally, in piping with a coating layer made of resin material on the inner surface, the coating layer may peel off, exposing the base material such as steel, and the bonding force between the inner surface of the piping and the silica scale may become stronger. In the scale removal method of this embodiment, since the entire piping is made of polytetrafluoroethylene or polyvinyl chloride, even if the inner surface of the piping peels off or is damaged, the bonding force between the inner surface of the piping and the silica scale can be reduced. Therefore, by injecting a scale removal liquid into the flow path including the surface of the area in contact with the geothermal fluid, the silica scale can be more easily peeled off, dissolved and removed, and poor cleaning of the scale can be further suppressed.

[0025] The scale removal solution may be acidic or alkaline. Using an acidic or alkaline solution allows for easier peeling, dissolution, and removal of silica scale, thereby further suppressing poor cleaning performance against scale.

[0026] The acid is not particularly limited, but examples include organic acids such as formic acid, oxalic acid, and acetic acid, or inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid. The alkali is not particularly limited, but examples include hydroxides such as sodium hydroxide and potassium hydroxide, carbonate compounds such as sodium carbonate, bicarbonate compounds such as sodium bicarbonate and potassium bicarbonate, sodium acetate, potassium acetate, ammonia, and organic amines. Among these, the scale removal solution preferably contains at least one selected from hydrochloric acid, sulfuric acid, hydroxides, carbonate compounds, bicarbonate compounds, and salts thereof. This prevents the surface of areas in contact with the geothermal fluid, which are formed from diamond-like carbon, polytetrafluoroethylene, or polyvinyl chloride, from dissolving or changing due to the scale removal solution, thereby suppressing the strengthening of the bonding force between the surface and silica scale. Therefore, by injecting the scale removal solution into the flow path including the surface of areas in contact with the geothermal fluid, silica scale can be more easily peeled, dissolved, and removed, and poor cleaning of scale can be further suppressed.

[0027] In the scale removal method for the geothermal power plant 100, if the geothermal fluid is geothermal steam, the scale removal liquid can be sprayed into the geothermal steam and mixed at the points where it comes into contact with the geothermal steam. Specifically, for example, the scale removal liquid can be sprayed into the geothermal steam and mixed in the piping L3 and turbine 4 of the first region 10. This allows the scale removal liquid to diffuse with the flow of the geothermal steam and come into contact with the entire area where it comes into contact with the geothermal steam. Therefore, silica scale can be peeled off, dissolved and removed more easily, and cleaning defects related to scale can be further suppressed.

[0028] In the scale removal method for the geothermal power plant 100, if the geothermal fluid is geothermal water, the scale removal solution can be supplied to the geothermal water and mixed at the points where it comes into contact with the geothermal water. Specifically, for example, the scale removal solution can be supplied to the geothermal water and mixed in the piping L1 and L2 of the first region 10, the piping L4 to L6 of the second region, and the piping L7 of the third region. This allows the scale removal solution to diffuse with the flow of the geothermal water and come into contact with the entire area where it comes into contact with the geothermal water. Therefore, silica scale can be peeled off, dissolved and removed more easily, and cleaning defects related to scale can be further suppressed.

[0029] In the method for removing scale in the geothermal power plant 100, while the geothermal power plant 100 is in operation, a scale removal solution is supplied to the geothermal fluid flowing through the channel, and while the geothermal power plant 100 is stopped, the channel may be filled with the scale removal solution or geothermal water mixed with the scale removal solution and left to stand. This allows scale removal work to be carried out while the geothermal power plant 100 is in operation. At the same time, while the geothermal power plant 100 is stopped, even if there is scale that cannot be completely removed during operation, scale removal work can be carried out intensively. Therefore, scale can be peeled off, dissolved and removed more easily, and cleaning failures due to scale can be further suppressed.

[0030] In the method for removing scale in the geothermal power plant 100, it is preferable to inject a scale removal solution into at least one of the following channels during the operation of the geothermal power plant 100: the channel between the production well 1 and the feedwater pump 2, the channel between the steam-water separator 3 and the turbine 4, the channel between the condenser 6 and the cooling tower 7, and the channel between the steam-water separator 3 and the retention tank 9. This allows the scale removal solution to be injected into channels where temperature drops are likely to occur and silica scale is likely to form, thereby further suppressing inadequate cleaning against scale.

[0031] In the method for removing scale in the geothermal power plant 100, a scale removal solution may be injected into the upstream flow path in each of the first region 10, the second region 20, and the third region 30 during the operation of the geothermal power plant 100. This can further suppress inadequate cleaning of the scale.

[0032] In the method for removing scale in the geothermal power plant 100, it is more preferable to inject a scale removal solution into at least the flow path between the production well 1 and the feedwater pump 2 and the flow path between the steam separator 3 and the retention tank 9 during the operation of the geothermal power plant 100. This further suppresses inadequate cleaning of scale.

[0033] An example of a configuration for injecting scale removal liquid into each of the aforementioned flow paths will now be described. As shown in Figure 1, the geothermal power plant 100 may have scale removal liquid tanks 12 connected to each of the following: pipe L1 which constitutes the flow path between the production well 1 and the feedwater pump 2; pipe L3 which constitutes the flow path between the steam-water separator 3 and the turbine 4; pipe L4 which constitutes the flow path between the condenser 6 and the cooling tower 7; and pipe L7 which constitutes the flow path between the steam-water separator 3 and the retention tank 9. Each scale removal liquid tank 12 is connected to pipes L1, L3, L4, and L7 via a pump (not shown). It is preferable that the scale removal liquid tanks 12 are located further upstream in each of the pipes L1, L3, L4, and L7.

[0034] In the scale removal method for the geothermal power plant 100, the scale removed by the scale removal liquid may be collected downstream of the point where the scale removal liquid is supplied into the geothermal fluid. This allows for the capture and collection of scale fragments that detach from the surface of areas in contact with the geothermal fluid, such as the inner surface of piping, and float in the geothermal fluid, thereby further suppressing poor cleaning of scale.

[0035] In the scale removal method for the geothermal power plant 100, it is preferable to collect scale in the flow path between the steam-water separator 3 and the retention tank 9. This allows for effective scale collection in the flow path where temperature drops are likely to occur and silica scale is likely to form, thereby further suppressing poor cleaning due to scale. Furthermore, scale may also be collected in the flow path between the circulation pump 8 and the condenser 6.

[0036] An example of a configuration for collecting the scale described above will now be explained. As shown in Figure 1, the geothermal power plant 100 may have a collector 13 connected to piping L7 which constitutes the flow path between the steam-water separator 3 and the retention tank 9. The collector 13 may be, for example, a container having a wire mesh inside to capture scale fragments, or a container having a similar function to the retention tank 9. The geothermal power plant 100 may further have a collector 13 connected to piping L6 which constitutes the flow path between the circulation pump 8 and the condenser 6.

[0037] In the scale removal method for the geothermal power plant 100, the supply flow rate or concentration of the scale removal solution may be controlled according to the mass of the collected scale. This allows the cleaning power to be adjusted according to the amount of scale generated, and poor cleaning of the scale can be further suppressed. The geothermal power plant 100 may have a mass meter for measuring the mass of the collected scale, a flow meter for detecting the supply flow rate of the scale removal solution, and a control unit for controlling the supply flow rate or concentration of the scale removal solution. The control unit determines the supply flow rate or concentration of the scale removal solution based on the mass obtained from the mass meter. If the concentration of the scale removal solution is determined, the control unit calculates the supply flow rate of the scale removal solution based on the determined concentration and the flow rate of the geothermal fluid. The control unit then operates a pump (not shown) connected to the scale removal solution tank 12 to achieve the determined supply flow rate or concentration.

[0038] In the scale removal method for the geothermal power plant 100, a scale remover may be supplied to the geothermal fluid before supplying the scale removal liquid to the geothermal fluid flowing through the channel. The scale remover can reach the surface of the area in contact with the geothermal fluid in the geothermal power plant 100, further reducing the bonding force between the surface and the silica scale. Therefore, by injecting the scale removal liquid into the channel including the surface of the area after further reducing the bonding force between the surface and the silica scale, the silica scale can be more easily peeled, dissolved, and removed, and poor cleaning of the scale can be further suppressed.

[0039] The scale remover preferably has a high affinity for diamond-like carbon, polytetrafluoroethylene, or polyvinyl chloride, which constitute the surface of the area in contact with the geothermal fluid. Examples of scale removers include 2-(furan-2-ylmethyldisulfanylmethyl)furan, ascorbic acid, and nicotinic acid. Among these, the scale remover preferably contains 2-(furan-2-ylmethyldisulfanylmethyl)furan. This makes it possible to further reduce the bonding force between the surface of the area in contact with the geothermal fluid in the geothermal power plant 100 and the silica scale.

[0040] Next, an experimental example of a scale removal method in geothermal power plant 100 will be described. One material piece made of 13% chromium steel (13% Cr steel), three material pieces (DLC-1, DLC-2, DLC-3) with a 1 mm thick diamond-like carbon (DLC) layer formed on the surface of the 13% chromium steel, one material piece made of polyvinyl chloride (PVC), and one material piece made of polytetrafluoroethylene (PTFE) were prepared. Each material piece was the same size. Each material piece was placed in the retention tank of the geothermal power plant, removed after 107 days (2675 hours), ultrasonically cleaned, and the amount of silica scale attached to each material piece was evaluated.

[0041] The amount of silica scale deposition was evaluated by performing elemental analysis on five randomly selected points on the surface of each material sample using energy-dispersive X-ray spectroscopy (EDX) attached to a scanning electron microscope (SEM), and calculating the average value of the five points. The amount of silica scale deposition on the resulting 13% Cr steel material sample was normalized to 100%, and the amount of silica scale deposition on each material sample was compared.

[0042] Figure 2 is a graph showing the amount of silica scale deposited on each material piece. The amount of silica scale deposited on the vertical axis is displayed on a logarithmic scale. As shown in Figure 2, compared to 13% chromium steel, DLC-1, DLC-2, DLC-3, PVC, and PTFE had less silica scale deposited, confirming that the bonding strength between the surface of these material pieces and the silica scale was weaker. In particular, DLC and PVC had significantly less silica scale deposited compared to 13% chromium steel, confirming that the bonding strength between the surface of these material pieces and the silica scale was significantly weaker.

[0043] Furthermore, the surfaces of material pieces made of 13% Cr steel and PVC after ultrasonic cleaning were observed using a scanning electron microscope (SEM). Figure 3 shows an SEM image of the surface of the PVC material piece, and Figure 4 shows an SEM image of the surface of the 13% Cr steel material piece. Figure 3 was observed at a magnification of 3630x, and Figure 4 was observed at a magnification of 11150x. As shown in Figures 3 and 4, the SEM images also confirmed that, compared to 13% chromium steel, PVC had significantly less silica scale adhesion, and the bonding force between the surface of the PVC material piece and the silica scale was significantly weaker.

[0044] As described above, embodiments have been explained, but these embodiments are presented as examples only, and the present invention is not limited by these embodiments. The above embodiments can be implemented in various other forms, and various combinations, omissions, substitutions, and modifications are possible without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents.

[0045] This international application claims priority under Japanese Patent Application No. 2023-022943, filed on 16 February 2023, which is incorporated herein by reference in its entirety. [Explanation of symbols]

[0046] 1 Production well 2. Water supply pump 3 Steam water separator 4 Turbines 5 Generators 6. Condenser 7 cooling tower 8. Circulation pump 9 Retention tank 10 First area 11. Reduction pump 12 scale removal solution tanks 13 Collector 14 Reinforcement well 20 Second area 30 Third area 100 Geothermal power plants L1, L2, L3, L4, L5, L6, L7 piping

Claims

1. In a geothermal power plant comprising a feed pump arranged in order from the upstream side of the geothermal fluid to pump the geothermal fluid from a production well, a steam-water separator that separates the geothermal fluid into geothermal water and geothermal steam, and a turbine that rotates by supplying the geothermal steam separated by the steam-water separator, a removal method for removing scale deposited and adhering to the geothermal fluid containing dissolved silica, In the first region from the production well to the turbine, the surface of the portion of the geothermal power plant that comes into contact with the geothermal fluid is formed of diamond-like carbon. In areas other than the first region, at least the surface of the aforementioned part of the geothermal power plant is formed of at least one material selected from diamond-like carbon, polytetrafluoroethylene, and polyvinyl chloride, A method for removing scale in a geothermal power plant, characterized by injecting a scale removal liquid into a flow channel that includes the surface of the aforementioned location.

2. The method for removing scale in a geothermal power plant according to claim 1, wherein the scale removal liquid is an acid or an alkali.

3. The method for removing scale in a geothermal power plant according to claim 2, wherein the scale removal solution comprises at least one selected from hydrochloric acid, sulfuric acid, hydroxide, carbonate, bicarbonate, and salts thereof.

4. A method for removing scale in a geothermal power plant according to claim 1, wherein, when the geothermal fluid is geothermal steam, the scale removal liquid is sprayed into the geothermal steam at the location in contact with the geothermal steam and mixed.

5. The method for removing scale in a geothermal power plant according to claim 1, wherein, when the geothermal fluid is geothermal water, the scale removal liquid is supplied to the geothermal water at the location in contact with the geothermal water and mixed with it.

6. A method for removing scale in a geothermal power plant according to claim 5, wherein the scale removed by the scale removal liquid is collected downstream of the position where the scale removal liquid is supplied into the geothermal fluid.

7. A method for removing scale in a geothermal power plant according to claim 6, wherein the supply flow rate or concentration of the scale removal liquid is controlled according to the mass of the collected scale.

8. During operation of the geothermal power plant, the scale removal liquid is supplied to the geothermal fluid flowing through the channel. The method for removing scale in a geothermal power plant according to claim 1, wherein, while the geothermal power plant is shut down, the flow path is filled with the scale removal liquid or geothermal water mixed with the scale removal liquid and left to stand.

9. The geothermal power plant comprises a condenser for condensing the geothermal steam discharged from the turbine, a cooling tower for cooling the condensed water from the condenser, and a retention tank located in the flow path through which the geothermal water separated by the steam-water separator flows. A method for removing scale in a geothermal power plant according to any one of claims 4 to 8, wherein, during the operation of the geothermal power plant, the scale removal liquid is injected into at least one of the following passages: the passage between the production well and the feedwater pump, the passage between the steam-water separator and the turbine, the passage between the condenser and the cooling tower, and the passage between the steam-water separator and the retention tank.

10. A method for removing scale in a geothermal power plant according to claim 9, comprising collecting the scale in the flow path between the steam-water separator and the retention tank.

11. A method for removing scale in a geothermal power plant according to claim 10, wherein a scale remover is supplied to the geothermal fluid in advance before supplying the scale removal liquid to the geothermal fluid flowing through the channel.

12. The method for removing scale in a geothermal power plant according to claim 11, wherein the scale removal agent comprises 2-(furan-2-ylmethyldisulfanylmethyl)furan.

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