Chemical characteristics of the Bandaiko hyper-acidic hot spring in Kusatsu Town, Gunma prefecture, and evaluation test of scale for binary power generation

Résumé fourni par la source

The Bandaiko hot spring, located in Kusatsu Town, Gunma Prefecture, is characterized by its hyper-acidic nature with a pH level of 1.7 and remarkable flow rate of approximately nine tons per minute. Gunma Prefecture is exploring the possibility of modifying its existing heat exchange system to establish a binary power generation facility utilizing this hyper-acidic hot spring, marking a potential first in the world. However, in the existing heat exchange facility, scale accumulation of the current facility necessitates water washing roughly once a month, but the type of scale and the rate of accumulation rate are still unknown. To address this, hot spring water was sampled and chemically analyzed, and field tests simulating scale adhesion were conducted using this water. For evaluation, quartz glass coupons and a batch-type scale sensor were utilized. The accumulated scale was identified as elemental sulfur. The use of quartz glass coupons did not enable an assessment of the scale deposition rate based on weight changes. By contrast, an 18-d immersion test with batch-type scale sensor indicated a gradual decrease in transmittance over time. The decrease in this sensor's transmittance serves as a proxy indicator for material accumulation under static immersion conditions. As it is not possible to analyze the scale adhering to the titanium heat exchanger during operation, it does not directly indicate the rate of scale deposition. Analysis of deposits on the sensor surface and scale recovered during the water washing of the heat exchanger revealed that the scale was elemental sulfur. Further analysis revealed that the elemental sulfur, derived from the chemical composition of the hot spring water, was undersaturated within the tested temperature range. The hot spring water is transported via pipeline to the heat exchange facility after gas-liquid separation at the source—located 2 km away—yet it is determined that atmospheric air enters the water at the source. This suggests that the precipitation of sulfur is caused either by the oxidation of trace amounts of H 2 S in the hot spring water by atmospheric oxygen or, alternatively, by bacterial activity. A subsequent bacterial analysis detected sulfur-oxidizing bacteria. These findings suggest that the formation of elemental sulfur scale likely results from the precipitation of sulfur caused by H 2 S oxidation—driven either by microbial activity or atmospheric oxygen—during the transport of the hot spring water. While installing sterilization equipment is an option for inhibiting bacterial activity, it entails significant costs. Given that the periodic water cleaning currently employed effectively restores heat exchange efficiency, this method remains the most appropriate approach for the time being. However, if the concentration of H 2 S in the hot spring water increases, reconsideration of a sterilization approach will be necessary. The novelty of this study is to connect observed sulfur scale, water chemistry, field sensor testing, equilibrium modelling, and microbial evidence in an actual utilization setting.

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