| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 104 === Mineral scaling is a major problem for geothermal power plants. Serious scaling was the key factor for shutting down the Chingshui geothermal power plant. Carbonates and silicas are the most common precipitated minerals as scaling in a geothermal system. To avoid the pollution of chemical inhibitors, our studies focus on hydrothermal experiments using high pressure thermal vessel to simulate the mineral precipitation and test how to prevent the mineral scaling.
This study chose the Chingshui geothermal area as the target objects. Pure water and saturated CO2 with slates are the starting materials in our experiments. In reservoir simulation experiments (R), single autoclave was used to simulate the roles of pressure at 300℃. In scale simulation experiments (S), three autoclaves have been applied to simulate the different steps when hot water raise up from reservoir to surface. Finally, we analyze the solution by the IC and ICP-AES, and precipitated minerals by SEM-EDS.
Experiments R show that large amounts of secondary chlorite and pyrite appeared at 300℃. When the pressure was up to 277 bars, hexagonal pyrrhotite occurred due to the higher pressure dissolving with more Fe. Calcite, gypsum and illite are the major products at saturated water vapor with pressure at 200℃ in the experiment S. When we injected CO2 with pressure of 200 bars into the system, calcite and gypsum disappeared and other secondary precipitated minerals decreased obviously. Therefore, if the CO2 injected into the reservoir or production well, the CaCO3 and CaSO4 deposits will inhibit and scaling dissolve again in reservoir. However, the concentrations of HCO3-, Na+ and K+ of solutions in our experiments were lower than the ones of thermal water in the Chinshui geothermal area. In the future, the NaHCO3 solution should design for experiments to get more CaCO3 precipitations. Meanwhile, more experiments need to be design to understand the effects of CO2 on scaling prevention in the future. For the silica scale, the Si4+ concentration is more oversaturated than the solubility of amorphous silica, if we inject CO2 to the system. Accordingly, amorphous silica would deposit as scaling. Therefore, concentration of the Si4+ is an important parameter for designing a heat exchanger with different temperature.
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