Corrosion And Scaling Processes At A Shallow Aquifer Thermal Energy Storage Site And Monitoring Using Redox Potential Measurements

摘要

Corrosion and scaling phenomena were investigated in a shallow aquifer thermal storage site in Berlin, Germany, for heat (depth ~ 300 m, NaCI 30 g L~(-1)) and cold (depth ~ 80 m, CaS04 500 mg L~(-1), NaCI 100 mg L~(-1), IC 5,5 mM) during 10 years of geochemical monitoring and successful operation. Ferric hydroxide precipitation and microbiologically induced clogging as well as corrosion processes were observed and studied. Ferrous iron oxidation kinetics, both biotic and abiotic, and ferrous iron speciation were studied using oxidation experiments and redox potential measurements as well as the waterchemical analysis of the thermal fluids. It was shown, that speciation and oxidation kinetics of ferrous iron in the investigated thermal fluids depends on ligand competition between solution species involving ferrous iron. Main Fe~(2+) complexing anions were CO_3~(2-) and OH for the heat store and CI~-, SO_4~(2-) and organic compounds for both heat and cold storage aquifers. In the heat store, a Fe(OH)CO_3~- - complex was found to partly determine the ferrous iron activity. In the cold store, ligand competition between CI~- and SO_4~(2-) takes place. With respect to corrosion and scaling processes, a detailed yet generalizable full picture, involving biotic and abiotic iron and sulfur cycling due to different microenvironments and oxygen contents of 20 - 200 μg L~(-1) in the pumped fluids, has been recognized. It was found that redox potential measurements can quantitatively be interpreted to calculate ferrous iron activities in the investigated fluids, effectively serving as a ferrous iron sensor that can be used to predict ferric hydroxide precipitation rates. It can be further interpreted with respect to corrosion processes.The method given is proposed to be used for monitoring corrosion and scaling processes in water bearing systems.