| Abstract |
The performance of geothermal power plants is often interfered by the deposition of scales. In principle, the formation of scales can be reduced by addition of a chemical inhibitor. However, the selection of an appropriate inhibitor depends on the composition of the scales. The (partial) elimination of scales formed in geothermal plants in the Upper Rhine Graben was the aim of a project funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). Within the framework of this project – called SUBITO - several inhibitors or composites of inhibitors for scale reduction were tested. According to the state-of-technology sulfate scales were eliminated in all investigated geothermal sites. However, there are still scales being formed. At the outset of the investigation it was known that these scales consist of the elements lead (Pb), antimony (Sb) and arsenic (As), but information on the exact element and mineralogical composition was scarce. Thus, the analytical part of the project was focused on the chemical, mineralogical and radiological characterisation of scales and – in order to understand the process of scale formation – also on the respective analysis of the soluble and particulate fractions in the fluids. Altogether, fluids and scales from three different geothermal plants were sampled and analysed. The investigation comprises a pure sulfate-inhibition period (“referenceâ€) and a period with application of various new inhibitors. As a result of the solid-phase analysis of the “reference†scales it turned out that they are dominated by Pb in the form of PbS and Pb(0). The dominance of Pb was confirmed by RFA-measurements and by chemical analyses. In addition, these scales contain Sb and As as solid metals (Sb(0), As(0)). The ratio of radioactive 210Pb to inactive Pb is constant in all scales of one plant, pointing to a consistent source of both components. Fluids are highly mineralised with around 100 g/L of total dissolved solids. Among the three scale-forming elements (Pb, Sb, As) the most prominent one was As in all investigated fluids. One order of magnitude lower in concentration was Pb followed by even lower Sb. When compared to the ratio of the elements Pb, Sb, As in scales it can be concluded that the deposition of Pb and Sb is favoured compared to the deposition of As. In the context of scale deposition, the particulate concentration and radionuclide content in fluids were monitored. It was found that the ratio of the dissolved to the particulate proportion of Pb and Sb in the fluid varied remarkably. The variation was discussed in context with the application of the various inhibitors. During the investigation several inhibitors were applied in the three different geothermal plants. As result of the inhibition the normalized mass of scales was considerably reduced. Inhibition has moreover an influence on the content and the relative fractions of Pb, As, Sb, and Ba within the scales. The effect was revealed by an enhanced variation of their concentrations. In detail, the effect of the individual inhibitors on the particular elements was evaluated by a comparative analysis of chemical, mineralogical and radiological data. |