To elucidate the effect of polyacrylic acid (PAA) on the formation of silica scales from geothermal water at geothermal power plants, two different experiments were carried out using geothermal water supplied from a production well, TP-2, at the Takigami geothermal power plant in Oita Prefecture, Japan. Polymerization of silicic acid in geothermal water and adsorption of silicic acid and aluminum on the surface of silica gel powder as a model compound of pre-formed silica scale were investigated in the presence and absence of PAA around 90℃. The polymerization of silicic acid which causes changes in monosilicic acid concentration with time and in hydrodynamic size of polysilicic acid particles was not affected by PAA. On the other hand, the adsorption behavior of aluminum was considerably different in cases between the absence and presence of PAA. Only a small amount of aluminum was adsorbed in the absence of PAA, but most of aluminum was adsorbed in the presence of PAA. The amount of silicic acid adsorbed in the presence of PAA was significantly larger than that in the absence of PAA. Consequently, the Al-PAA complex may accelerate the formation of silica scales from geothermal water.
The efficiency and maintenance of small geothermal power plants that use low-temperature hot spring water are affected by the adhesion scale. The precipitated scale samples from the binary power plant at Obama Hot Spring (Kyusyu, Japan) was analyzed and found to contain aragonite, quartz, and amorphous silica-based phases. Thermodynamic analysis revealed that aragonite, calcite, talc, sepiolite, and amorphous magnesium silicate were in supersaturated state in the geothermal water. Meanwhile, magnesium carbonate and amorphous silica were unsaturated. Mg2+ does not affect the precipitation of carbonate scale, and that of amorphous silicabased scale could be predicted thermodynamically after considering the influence of Mg2+. Based on the chemical composition of the geothermal water supersaturated with amorphous magnesium silicate, we extended a simple method to predict the precipitation of amorphous magnesium silicate. The saturation indexes of both components are decreased by decreasing the pH and the temperature; hence, this could be used to reduce the adhesion scale at Obama Hot Spring. At the pH is ≥ 9, the precipitation of amorphous magnesium silicate should be considered, even if amorphous silica is unsaturated in the hot spring water (assuming ionic strength I = 0.1–0.2). With regard to the temperature, the precipitation risk of amorphous magnesium silicate is high at around 100 ˚C, but greatly reduced by operating the geothermal water at around 75 ˚C.
A criterion to evaluate possible influence of geothermal development onto nearby hot spring aquifer, based on geochemical characteristic of the aquifer, was developed in a form of decision tree. Hydraulic and thermal connection of a geothermal reservoir to nearby hot spring aquifer may be expressed by five types of simplified structure models, which are, identical thermal water-, seepage thermal water-, steam heated-, heat conduction- and independent- systems. Since interference of geothermal fluid production differs depend on such types, required monitoring items for protection of hot spring aquifers also differ for each type. The type of relationship between a specific geothermal reservoir and a nearby hot spring aquifer can be roughly estimated by temperature and anion characteristics of the hot spring waters for volcanic geothermal systems. In a former study, a criterion was developed based on five types of geochemical characteristics of hot spring waters. However, such a categorization was not quite successful when volcanic gas or CO2 gas was supplied to the aquifer or geometrical setting was not appropriate. Although hydraulic relationship is well described by five types, geochemical characteristics of hot springs are not well represented by five types only. Therefore in this study, a new criterion was developed by adding two new types, CO2 gas supply and volcanic gas supply types, resulting in seven types in total. Spatial arrangement of the target aquifer and geothermal reservoir is also taken into consideration. The threshold values in this criterion are based on data sets obtained from hot spring wells and natural geothermal manifestations all over Japan. Then the new criterion was applied to hot spring waters in Kuju region, Oita, Japan where three geothermal power plants are in operation. As a result, all seven types were identified for this region. It was found that hot springs of seepage thermal water type and steam heated type densely exist near geothermal power plants, suggesting that such hot spring type identification may be an effective tool for geothermal exploration. The new criterion, which helps to estimate effects of geothermal development onto each nearby hot spring aquifer and enables to protect hot springs against excess geothermal development, may be applied for other volcanic geothermal regions.