| Abstract |
Binary cycle units comprising I6 MWe were installed at the Bulalo, Philippines geothermal field utilizing flashed waste brine that was previously injected directly to the reservoir, In this bottoming cycle, the brine temperature is reduced from 177oC to 135 oC across heat exchangers. Over-saturation of silica during conductive cooling of the brine increases to >100%. As a result, silica scaling rates in heat exchanger tube sand brine disposal systems are predicted to increase thirty fold from 0. I to >3 mm/yr. Silica scaling in heat exchanger tubes and downstream injection piping must be controlled to maintain optimum heat transfer and to prevent injection piping and well scaling. Siliceous scaling in heat exchangers, cooled injection brine piping, injection wells and near-wellbore formation is inhibited by UNOCAL's patented pH modification process. This process consists of acidification of brine entering the binary plant heat exchangers. Brine pH is reduced from about 6.5 to 5.5 by treatment with only 10 mg concentrated sulfuric acid per kg of brine. The pH modification process has successfully controlled silica scaling in heat exchangers and injection brine piping for over two years. Silica scaling, as a result of flashing and cooling brine, has been reduced to less than 1 mm/yr at a relatively low operating cost. |