Geochemical characterization of fouling on mine water driven plate heat exchangers in Saxon mining region, Germany

Abstract

Fouling strongly impedes the long-term operational efficiency of geothermal heat exchangers by affecting heat transfer resistance and the flow hydrodynamics. In this study, we have compared the geochemical properties of fouling from four different mine water-driven plate heat exchangers in the Saxon mining area, Germany. The elemental composition of fouling was analyzed by means of micro X-ray fluorescence and scanning electron microscopy with energy dispersive X-ray analysis. Inflow and outflow waters from the heat exchanges were collected and analyzed hydrochemically in repeated sampling campaigns. Two main types of fouling were observed in all heat exchanger plates; easily removable reddish deposits were identified on the top of black encrustation. Generally, both fouling types were mainly composed of amorphous, slightly crystalline, and organically bound iron hydroxides. Easily reducible manganese oxides were also observed in the black encrustation of the Freiberg area and Oberschule sites. Arsenic also plays a significant role in fouling mechanisms of Ehrenfriedersdorf sites. The composition, structure, and quantity of fouling vary considerably on the physicochemical condition of the mine waters, and the maturity of the plates. High element contents in the feed water are not necessarily found in the fouling deposits. On average, a significant proportion (over 90%) of the iron in the mine water belonged to the >1.2 μm fraction. Most of the iron and arsenic found on the heat exchanger plates could be sourced from colloidal deposition and biofouling. The manganese could be real precipitation on plates that could trigger the fouling mechanisms in most cases. PHREEQC modeling shows that inflow mine waters only required 1 × 10−5–3 × 10−2 atm of pO2 to become over-saturated with manganese oxides.