Reykjanes Geothermal Field: Control of Silica Scaling in Separator by Flashing of Brine in Critical Nozzles

Abstract

In the Reykjanes geothermal power plant, the steam is separated from brine at high pressure and the hot brine disposed off. Bottoming unit is now being developed in which the hot brine will be flashed in two stages and the medium- and low-pressure steam used in a dual stage turbine to generate 30 MW of electricity. Because of high silica concentration and high salinity severe scaling will occur when the brine is flashed to lower temperature in a separator. The use of acid for scale mitigation, although effective, is considered unattractive and therefore a new separator technology was developed to control the silica scaling without use of chemicals. Experience shows that silica scaling rate in two-phase steam and brine is considerably less than in single phase brine and the idea was that by operating the separator with minimum brine level the scaling problem would be minimized. Also, by controlling the flow with fixed diameter nozzles with critical flow, the separator would be valve-less and cleaning of the scale would be possible by reaming the nozzles without stopping the flow. A double flash experimental critical nozzle separator was designed for 25 kg/s of 207 °C brine, which was flashed in two stages at 155 °C and 108 °C. The pressure in the low-pressure separator was controlled by barometric pipe in an atmospheric flashing tank designed to allow easy access for cleaning. The tests were successful and showed that the separators could be operated with minimal maintenance. The design is now being scaled up for full size separation of 410 kg/s of brine.