Sulfur Scale Abatement System Using a Surfactant in Geothermal Power Plant Circulating Water

Abstract

A lot of sulfur rich scale accumulates in the cooling water system of geothermal power plants, which markedly lowers cooling system efficiency. Numerous sulfur bacteria were detected in the sulfur scale. It is thought that sulfur mainly originates from sulfur bacteria activity when oxidizing H2S (gas) to S (solid) in the circulating water (geothermal steam condensed water). In this study, we examined the prevention effect of surfactant on sulfur scale formation. At the Sumikawa Geothermal Power Plant (50MW, Akita Prefecture, Japan), we injected surfactant into the cooling tower circulating water at 1ppm concentration, which reduced the sulfur scale to 40-50% of that before surfactant injection. This chemical injection was successful to some degree in prolonging the plant inspection interval but the plant inspection interval is 1 year and still too short. With 1ppm surfactant, the number of sulfur bacteria was less than 10 cells/l in the circulating water but 103-104 cells/g in the sulfur scale. This means that the bacterial activity was inhibited in the circulating water but not in the scale. This might be the reason why sulfur scale accumulates in the 1 ppm surfactant cooling water system. Our laboratory and miniplant experiments showed that the bacterial activities were inhibited at concentrations over 0.5 ppm in the circulating water but not at concentrations less than 160 ppm in the sulfur deposit. Once the sulfur scale contacts over 160 ppm high concentration surfactant for a short time, signs of sulfur scale growth are no longer found. Now we are preparing to apply this idea to the actual geothermal power plant.