Combination flash-bottoming cycle geothermal power generation: a case history

High- and low-enthalpy, liquid-dominated geothermal resources are commonly exploited by steam flash and binary power cycles, respectively. Cooled brine from both flash and binary power plants is typically reinjected into the geothermal reservoir to replenish the aquifer, maintain reservoir pressure and to protect the environment. Silica tends to precipitate from geothermal brine at almost every stage of brine processing as the temperature is reduced, either as hydrous, amorphous opal or as metal silicate. In 1994, binary cycle units comprising 16 MWe were installed at the Bulalo, Philippines geothermal field utilizing flashed waste brine that was previously injected directly to the reservoir. The binary bottoming cycle consists of six energy converters each employing a preheater and a vaporizer. The brine temperature is reduced across the heat exchangers from 450° to 408°. During the heat recovery process, over-saturation of silica in the injection brine increases from <10 to >100%, and scaling rates are predicted to increase thirty-fold from 0.1 to 3 mm/yr. Siliceous scaling in heat exchangers, cooled injection brine piping, injection wells and near-wellbore formation is inhibited by acidification of brine sent to the binary plant. The brine pH is reduced from 6.5 to 5.5 by treatment with only 10 mg concentrated sulfuric acid per kg of brine. The heat recovery process, incorporating silica scale control by pH modification has successfully operated at the Bulalo field for two years. Silica scaling, as a result of flashing and cooling brine has been reduced to less than 1 mm/yr