Corrosion and Scaling in Geothermal Heat Exchangers
Round 1
Reviewer 1 Report
This review covers two critical issues in geothermal heat exchanges: corrosion and scaling, since both co-occur. Throughout the manuscript, the authors combine some fundaments and literature findings about the corrosion and scaling problems, emphasizing the materials (metal alloys and coatings) and the use of chemicals to minimize such problems.
Some physical parameters (in addition to the chemical water composition, pCO2, and pH2S) are important for corrosion and scaling, namely pressure and temperature. The authors mainly emphasized the temperature effect (justifiable regarding the heat exchanger systems) on both phenomena (corrosion and scaling). Still, they could also discuss the pressure effects on the scaling one unless pressure drops never occur in such heat exchanger systems. Also, for the CaCO3 scale, this is an important issue as a pressure decay may favor this scale formation. Similarly, in a CO2-containing environment, higher pressures (not considering the effect of temperature) may increase the corrosion rate and avoid the CaCO3 scaling due to a more acidic environment.
Finally, I recommend that the authors search and describe if geochemical modeling (e.g., Phreeqc) may be or is used in studies dealing with the corrosion and scaling in geothermal heat exchangers to support similar studies or discussions.
Author Response
Query 1: Some physical parameters (in addition to the chemical water composition, pCO2, and pH2S) are important for corrosion and scaling, namely pressure and temperature. The authors mainly emphasized the temperature effect (justifiable regarding the heat exchanger systems) on both phenomena (corrosion and scaling). Still, they could also discuss the pressure effects on the scaling one unless pressure drops never occur in such heat exchanger systems. Also, for the CaCO3 scale, this is an important issue as a pressure decay may favor this scale formation. Similarly, in a CO2-containing environment, higher pressures (not considering the effect of temperature) may increase the corrosion rate and avoid the CaCO3 scaling due to a more acidic environment.
Response 1:
We appreciate the reviewer's insightful comments on our paper. The effect of pressure on the solubility of minerals is negligeable compared to that of temperature at conditions relevant to heat exchangers (<25 bar). Therefore, the effect of pressure is rarely discussed for scaling in heat exchangers. Pressure mainly influences mechanical loading and gas solubility. However, influence on mechanical loading is out of scope. As pointed out by the Reviewer, CO2 degassing can have an influence on scaling. Therefore, the following text was added to Section 5 of the manuscript. The references are not formatted below, but have been formatted in the revised manuscript.
“In geothermal power plants, pressure primarily diminishes during the vertical trans-fer of fluids from the reservoir to the surface. Within tube and shell heat exchangers, the pressure is typically maintained between 18 to 25 to avoid detrimental degassing [59]. The pressure drop across the heat exchanger is influenced by the presence of rough scale deposits, leading to drag effects, yet it remains constrained within the range of approximately 3.5-5 bar [55]. Under these conditions, temperature pre-dominantly governs mineral solubility, with pressure exerting a secondary influence. Nevertheless, pressure significantly impacts gas solubility, and any alterations in pressure conditions can result in CO2 degassing. CO2 degassing is highly undesirable since it raises acidity levels by forming carbonic acid and promotes precipitation of calcite [54]. To mitigate CO2, it is advisable to operate geothermal systems at higher pressures (18-25 bar). However, turbulence-induced localized pressure fluctuations can still cause degassing and exacerbate calcite precipitation, as reported by Bosh et al. [54].”
Query 2: Finally, I recommend that the authors search and describe if geochemical modeling (e.g., Phreeqc) may be or is used in studies dealing with the corrosion and scaling in geothermal heat exchangers to support similar studies or discussions.
Response 2:
Thank you for the excellent suggestion. Indeed, open-source software, particularly PHREEQC, has been effectively utilized for modelling scale formation mechanisms in geothermal heat exchanger systems.
To address this valuable input, we have incorporated the following passage into Section 5 of the manuscript:
“Information on scale precipitation mechanisms may be obtained using geochemical modeling software such as PHREEQC. PHREEQC is an open-source geochemical modeling software developed by the United States Geological Survey (USGS), designed for the scientific analysis of complex chemical reactions in aqueous systems [60, 61]. Its accessibility has imposed it as a widely used tool in the field of geochemistry to calculate hydrochemical parameters such as mineral saturation states and dissolved gas partial pressures to better understand scale precipitation [32, 53, 54, 62-64]. However, this software does not yet model corrosion processes.”
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Manuscript ID: applsci-2662077
Title: Corrosion and Scaling in Geothermal Heat Exchangers
This review examines the challenges posed by corrosion and scaling in geothermal heat exchangers and discusses mitigation strategies such as corrosion-resistant alloys, protective coatings, and anti-scaling agents. The paper highlights recent advancements and aims to provide valuable insights for material selection and heat exchanger design in geothermal energy production. Some comments to improve the review:
1. What are the implications of these challenges related to scaling and corrosion on the efficiency of geothermal power plants?
2. How do CRAs (Corrosion-Resistant Alloys) address corrosion problems in geothermal heat exchangers? Can you provide more information on the considerations related to environmentally assisted cracking (EAC) and residual stress relief during the design and manufacturing stages?
3. What are the limitations of CRAs in terms of preventing scaling and their thermal conductivity compared to carbon steel? How do these factors impact the cost-effectiveness of CRAs?
4. Can you discuss the different types of coating systems that can be used to mitigate both scaling and corrosion in geothermal heat exchangers? What are the advantages and disadvantages of using coated carbon steel in this context?
5. Have the existing coating options undergone rigorous testing in service conditions that involve large and intricate surfaces, dynamic fluid behavior, and temperature cycling? What are the key criteria for evaluating the effectiveness of a coating system?
6. What are the desired characteristics of an ideal coating for geothermal heat exchangers in terms of thermal conductivity and corrosion reduction? How can a balance be achieved between maintenance and production costs?
7. How can heat exchangers be designed to adapt to unforeseen issues in geothermal systems? Can you provide some general recommendations for maximizing efficiency in heat exchanger design?
8. How do anti-scaling agents, inhibitors, and coating systems work together to maximize efficiency in geothermal heat exchangers? Can you discuss the synergies and potential challenges of employing a combination of these mitigation strategies?
9. Ultrasonic vibration has been used as a cleaning technology so far. Recently, the use of ultrasonic vibrations has been introduced as another way to improve the efficiency of thermal systems. It is suggested to mention this in the introduction for readers’ information. Referring to the following articles will be helpful: 10.1016/j.icheatmasstransfer.2022.106098; Doi: 10.1007/s10973-023-12102-7.
10. Can you explain the complexities and potential limitations of accurately replicating in-service conditions in laboratory testing? How can the accuracy and representativeness of laboratory results be ensured?
Minor editing of the English language is required.
Author Response
We are grateful to the reviewers for their constructive comments which have helped us improve the revised script. Revisions that have been added to the revised manuscript and the rebuttal document is attached.
Author Response File: Author Response.pdf
Reviewer 3 Report
I have no comments, in my opinion it is fine.
Author Response
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Query 1: I have no comments, in my opinion it is fine. |
Response: Thank you very much for your review. Your positive feedback is greatly appreciated. |
