Chemical Treatment Efficiency on Scaling and Corrosion at Rittershoffen, France, Upper Rhine Graben

Abstract

The Rittershoffen geothermal plant is a deep EGS geothermal project initiated in 2011 and located in Northern Alsace, French Upper Rhine Graben. This plant, in operation since 2016, comprises the first full and direct use of an EGS thermal plant. It has a nominal capacity of 25 MWth, which represents around 25% of the industrial heat demand of “Roquette Frères” bio-refinery at Beinheim, 10 km east of Rittershoffen. The heat transfer is ensured by a tubular heat exchanger from geothermal loop to a transport loop, reaching the industrial facility. A geothermal fluid, Na-Ca- Cl type with salinity of about 100g/L, is produced at a temperature of 170°C and is fully reinjected into the fractured granitic basement reservoir at 80-85°C. The fluid is enriched in dissolved gases, mainly CO2. This geochemical context induces barium sulphate precipitation during the heat transfer process. These strong deposits decrease notably the heat transfer efficiency of the heat exchangers, increasing operational costs like power consumption, cleaning operation and wastes. Therefore, to ensure a sustainable plant operation, a chemical treatment is required to inhibit this kind of secondary deposit. Moreover, arsenic, lead and antimony, minor and traces elements from geothermal brine, are precipitating while in contact with steel through electrochemical processes. It results in precipitations associated with sulphur. At the beginning of 2017, plant operator team and water treatment company built a partnership to select appropriate products tackling these operational issues and identify methodology and indicators to assess the chemical treatment efficiency of the heat exchangers. The selected scale inhibitor contains a powerful polymeric dispersant based on patented terpolymer technology. This technology has proven its efficiency on controlling a variety of salts, particularly on barium sulphate compounds as typically encountered in the Rhine Graben. By the application of a filming agent acting as corrosion inhibitor, the electrochemical processes leading to lead and arsenic sulphide deposition are significantly reduced, as well as contributing to the tubular heat exchanger, pipes and reinjection well casing integrity. The quantity, the structure, and the chemistry of scaling before and after the chemical treatment are compared. From the operational point of view, the ease of cleaning and the heat transfer capacity are also monitored. After more than two years of chemical treatment at Rittershoffen geothermal plant, operational monitoring shows a good efficiency of the applied treatment. The monitoring within the following months would be focused on corrosion rate assessment. New sampling campaign of scaling will be performed to improve the understanding of chemical treatment action and to start the dosage optimization.