Inorganic scaling and its mitigation in geothermal systems

Abstract

Geothermal energy systems provide an attractive, sustainable and renewable energy source. The majority of geothermal reservoir waters face metal sulfide and metal silicate scaling and deposition that raise serious problems to the proper operation of the geothermal installations. The formation, crystallization and deposition of such inorganic salts occur mainly due to their extremely low solubilities, as exemplified by their low thermodynamic solubility constants and, for some, their inverse solubility properties. In this work we focus on some of the most common geothermal scale types, such as sulfides (zinc and antimony), and silicates (aluminum and magnesium). From a geology point of view, both magnesium and aluminum silicates constitute a family of widely studied and well characterized geological formations. However, although these terms are commonly used by water treatment professionals to indicate any salt or deposit that contains both Si and Mg or Al, the true identity of such deposits remains elusive and bears little resemblance to their geological counterparts. The aim of our approach in tackling such scaling issues is the systematic study of the influence of chemical additives in the presence of Mg2+ and Al3+ ions and the correlation of chemical structure with inhibition chemistry. Zinc and antimony (III) sulfide are among the metal sulfides that pose problematic issues. Their extremely low Ksp values allow immediate precipitation at very low metal and sulfide concentrations. Classic threshold inhibition techniques, as well as dispersion approaches, are useful strategies to control their formation and/or deposition. Based on a plethora of experimental data, a number of useful functional insights have been generated, which add to building a more complete and comprehensive picture of the mechanism of magnesium and aluminum silicates and zinc and antimony sulfide formation and control. Finally, we attempt to focus on the “worst-case” in a scaling scenario, which is the formation of the so-called “composite fouling”. It is composed of more than one scaling salts and its composition becomes more complex in more challenging water systems, such as geothermal waters. There have been numerous studies on the inhibition of a single scaling salt and a few on the control of two-component scales. To our knowledge, there are no reports on tackling composite fouling as a whole.

Η ερευνητική εργασία έχει στόχο την εκτενή μελέτη συγκεκριμένων δυσδιάλυτων επικαθίσεων και την αποτροπή του σχηματισμού τους σε γεωθερμικές εγκαταστάσεις. Οι υπό μελέτη ενώσεις αφορούν δύο μεγάλες κατηγορίες, metal silicates και metal sulfides και ο κύριος στόχος της εργασίας είναι οι εξής τρεις: magnesium silicate, aluminium silicate, zinc sulfide. Επίσης, μελετήθηκαν οι ικανότητας παρεμπόδισης και διασποράς μιας οικογένειας πολυμερικών χημικών προσθέτων ενάντια σε πολύπλοκες επικαθίσεις μικτής φύσης (composite fouling).