| Abstract |
Hatchobaru power station started to operate with the first unit of 55MWe in 1977, and was expanded with another unit of 55MWe in 1990. Although high efficiency in power generation has been attained by adoption of a double-flash system, the low temperature (90oC) of separated brine yielded high silica-scaling potential. The retaining tank system was originally developed in the Hatchobaru-Otake power station as a countermeasure against scaling problems. This system aimed to reduce the silica-scaling rate in and around reinjection well by polymerizing silica in brine after retaining for about one hour, and had succeeded for long years. However, during the long period of operation, the installation of a number of make-up production wells that produces fluids exhibiting a variety of chemistry resulted in the change of chemical conditions of reinjection brine. This prevented silica polymerization of brine in the retaining tank, and raised the silica-scaling rate. Consequently, pH modification system has been adopted as an alternative scale prevention method from 2003. The purpose of this method is to reduce the silica-scaling rate by adjusting brine pH with sulfuric acid. Although diluted sulfuric acid is generally utilized for pH adjustment in Japan, concentrated sulfuric acid is directly injected into brine in the Hatchobaru because of restriction of available clear water for dilution and limitation of space for water tanks in the field. From the results of silica polymerization experiments, it was confirmed that adjustment of pH at 5-6 brings sufficient effect for reducing silica-scaling rate. To date, the pH modification system has been successfully operated, and reduction of scaling rate and decrease of decline rate of reinjection capacity have been observed. This paper introduces scale prevention effects under various brine chemical conditions based on the several kinds of monitoring data taken during the operation of Hatchobaru power station. |