51316-7902-Arsenic Enrichment on Cast Iron and Mild Steel Surfaces: Trigger or Result of Electrochemical Corrosion

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Product Number: 51316-7902-SG

ISBN: 7902 2016 CP

Author: Julia Scheiber

Publication Date: 2016

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Economic feasibility of geothermal power plants relies on continuous and constant operation of the geothermal loop. Constraints like unscheduled shut-down periods intense maintenance operations and follow-up costs reduce significantly their reliability. The Soultz-sous-Forêts EGS site operates a binary power plant and its efficiency depends mainly on down-hole pump heat exchanger unit and productivity/injectivity of the wells. Insufficient performance or malfunctioning of one of these components affects consequently the complete system. An intense monitoring of power plant performance during operation allowed in most of the cases characterization and identification of scale and corrosion related issues. In some cases these issues resulted in unexpected long-term shut down times of the Soultz power plant and high follow-up cost.The high salinity with 100 g/L TDS of the Na-Cl-Ca brine and the production of cuttings causes not only electrochemical corrosion but also erosion issues at surface and subsurface installations of the geothermal loop. Down-hole pump performance decreased to a non-operable state and metallurgy had to be changed from cast iron to duplex. The pump has to stand high temperature and pressure at rotation rates of 1500 – 2000 rpm which creates high flow velocities and turbulences. Based on the specific geochemical conditions; the impact of oxygen is negligible during production. But it turned out that As and Sb need to be considered as redox species which are involved in electrochemical reactions with the pump material resulting in deterioration of the cast iron. Moreover erosion issues related to the production of suspended particles at certain flow rates lead to the complete failure of the cast iron pump.Additionally scale formation complicates proper material performance of the surface equipment for example at the heat exchangers. Intensive mechanical cleaning procedures were required in the past in order to remove inorganic deposits mainly made of strontium rich barite (Ba0.6Sr0.4SO4) galena (PbS) and minor fractions of mixed sulfides ((FeSbAs)Sx). Sulfide formation is very probably related to electrochemical reactions between less noble iron alloys and brine. Moreover a progressing precipitation front of barite towards the natural fractured granitic reservoir was observed.