Scale Analysis in the Framework of Optimization of Energy Production at the Soultz-sous- Forêts EGS Site

In order to increase the production of energy at the Soultz-sous-Forêts power plant (Rhine Graben, France), a prototype heat exchanger in which 10% of the whole water flow was diverted has been tested for decreasing the temperature from 70°C to 40°C. After 3 months of operation, it was opened for sampling of scales (deposited in the tubes and water boxes) and study of corrosion. The prototype heat exchanger has been designed with 6 different metallurgies (1.4539, 1.4547, 1.4462, 1.4410, 2.4858, TiGr.2) as described by Ravier et al. (2019). Due to high service conditions (150°C and 25 bar, high salinity brine) and the risk of scale formation, all the geothermal
plants in operation in the Rhine Graben are designed with shell and tube heat exchangers. The prototype heat exchanger was designed according to this technology. The alloy 1.4410 is the steel alloy currently used in the heat exchangers of the Soultz industrial power plant. The other metallurgies were already tested in extreme conditions in Iceland (Karlsdottir et al., 2014; 2015) and gave good results. The aim of this work is to present chemistry data about scales related to the different metallurgies, acquired with Inductively Coupled Plasma-Mass Spectrometry (ICP-MS; reference sample), X-ray Fluorescence (XRF; 6 metallurgies tested during 3 months) and Scanning Electron Microscopy coupled with an Energy Dispersive Sprectrometer (SEM-EDS; all samples). The reference sample
(obtained in normal industrial conditions in contact with 1.4410 alloy) contains mainly Pb (70%), Sb, As, S, Na, Fe and Ca. In Ti metallurgy, small spheres bearing Ti (probably TiO 2 ) are found in the scales at both the contact with tube and with the fluid whatever the temperature tested; when the temperature is lowered, the amount of Ti in the scales, obtained by XRF, decreases. As and Sb tend to increase in scales when the temperature decreases, whatever the metallurgy, while Pb and Cu tend to decrease, or decrease and then increase, depending on the metallurgy. According to XRF data, the chemistry of scales reflects only partly that of the metallurgy with which they are in contact. Constant features are encountered by SEM-EDS in all the scale samples at 60°C whatever the metallurgy: 1) granular matrix in contact with the fluid, containing Pb, S, As, Sb, 2) NaCl cubes either as big square patches embedded in the granular matrix or as small cubes protruding out of the surface (either in contact with the tube or with the brine). The scales for which a section could be obtained show a thickness of approximately 30-40µm over the 3-month test, that is to say about 10-13µm per month.