The Challenge of Water Chemistry

The characteristic chemical composition of the mine water depends on the mine itself, the surrounding rock, the residence time, and the physical conditions of the surrounding environment. Even within a mine, significant differences may arise. In combination with the mode of operation, dissolved and undissolved components can lead to a decrease in the system operability. This is especially true in the case of built-in heat exchangers, where fouling can result in lower heat transfer coefficients and increased pressure drop, either reducing the water flow rate or requiring more energy to pump the same amount of water through the system. Thus, the question arises: How suitable is the existing water for energy utilization and what measures must be taken to ensure reliable operation?

The temperature change that takes place during energy recovery is a determining factor in the extent of fouling.  As solubility limits change with changing temperature, some minerals could precipitate out of solution if their solubility limits are crossed, leading to deposit formation on heat exchanger surfaces [1].

Grubenwasserzusammensetzung in einem erzgebirgischen Zinnbergwerk (Grafik: TU Bergakademie Freiberg)

Over the course of operation, the combination of changes in physical parameters such as pressure and temperature with dissolved and entrained material present in the water, can cause precipitation and, as a result of chemical-physical interactions with the solid surface, adherence to the heat exchanger surfaces. The effects of fouling on total heat transfer capacity are two-fold. On one hand, a reduction in the heat transfer coefficient occurs as a result of the reduced thermal conductivity and relatively thick mineral layers; on the other hand, either the volumetric flow rate is reduced due to increased pressure drop or the energy needed to pump the same amount of water through the system increases. Thus, the efficiency and reliability of the system are significantly compromised [1].

 

Ausbildung von Foulingschichten im Betrieb befindlicher Grubenwassergeothermieanlagen (Grafik: TU Bergakademie Freiberg)

Therefore, project Vodamin II seeks to investigate the fouling behavior of different mining water systems. In addition to analyzing the composition of the fouling deposits resulting from different water sources and modes of operation, different heat exchanger coatings and materials of construction will be tested. To this effect, both laboratory experiments and in-situ testing on actual operational systems will be conducted.

Literatur

[1] Wenzke B., Grab T., Ussath M., Fieback, T. (2018): Energetische Nutzung von Grubenwässern und der Einfluss der Wasserbeschaffenheit auf den Anlagenbetrieb, Deutscher Geothermiekongress 2018