Integration of the Calcium Silicate Hydrate (CaSil) Technology Into Geothermal Power Plants – Focus on Process Engineering Aspects

Abstract

*This paper is part 3 of 5 papers from our group covering this technology: 1. Overview of the technology (Johnston et al.), 2. Precipitation of CaSil (Fraser et al.), 3. Integration of the Technology (this paper), 4. CaSil technology under pressure (Borrmann et al.), and 5. Application of CaSil (Cairns et al.). A major challenge in geothermal energy utilisation is the formation and precipitation of amorphous silica scale. Over time, pipes, valves and heat exchangers are blocked by the rock-like scale, decreasing the efficiency of process equipment, which eventually becomes unfit for operation. Extensive periodical cleaning efforts or replacements are the consequence, necessitating high servicing and operational costs. Current mitigation approaches aim to delay the precipitation by decreasing the silica concentration through higher exit pressures and temperatures from the flashing process or by dosing of acid and additives to retard the polymerisation process. However, none of the currently implemented technologies can solve the problem wholly while being financially attractive. In a disruptive approach to address this issue we developed a new technology whereby we transform the dissolved silica species into a nanostructured calcium silicate hydrate (CaSil) material. The reaction takes place very rapidly and can transform - depending on the dosing regimen - enough dissolved silica to prevent the precipitation of silica scale or remove essentially all to enable downstream membrane or adsorbent methodologies for extraction of dissolved minerals, like lithium or zinc. Due to the surface chemistry of the CaSil material no adherence of the particles to metal parts can occur. Consequential, maintenance efforts are reduced greatly. Because dissolved silica is transformed definitively, additional energy can be extracted from geothermal brines without risking or promoting scale formation. This paper will focus on the process engineering challenges of this new disruptive technology. Using the example of our pilot plant, which is currently deployed in the Taupo Volcanic Zone in New Zealand, the individual unit operations of the technology will be explained. Our continuous pilot scale process is able to demonstrate the effectiveness and simplicity of the technology in a real world operational environment. Silica scaling can be wholly prevented, while at the same time ensuring an enhanced energy recovery without oversaturating the geothermal brine in dissolved silica again. Operational challenges during the development process will be highlighted and experiences and solutions discussed.