| Abstract |
At an early stage of the development of the Ngatamariki field, Mighty River Power designed and constructed a test rig utilising geothermal fluids from NM7 to simulate and test a variety of plant designs and conditions. Both a flash test plant and a binary test plant were constructed and tested, to help design the optimal plant for Ngatamariki. Scaling rates and elemental analysis of the deposited scale were determined for test sections located throughout the process for each trial. Additional measurements included silica polymerisation times and acid titration curves. Three flash plant trials were conducted using a dual-flash design, all utilising acid dosing to a pH25 of 5.0. The low pressure (LP) separation pressure was varied between the trials, ranging from 1.7 bar(g) to 0.44 bar(g), corresponding to a temperature range between 130 degC and 110 degC. Results indicated that the scaling rate increased with a lower flash pressure and temperature, with the scaling rate being acceptable at a LP separation pressure of 1.7 barg, correlating to a silica saturation index (SSI) of 1.8.1.9. For all flash plant trials algae contamination of test pieces was observed, which is thought to be due to operational processes around the flushing and drying of pipes prior to analysis. Three binary trials were conducted with exit temperatures tested being 95 degC, 90 degC and 85 degC. For all trials, condensate was added to the brine prior to the preheater section to reduce the pH prior to increasing the SSI to >1.0 within the preheater. In all cases both antimony sulphide and arsenic sulphide deposited within the heat exchanger, but for an exit temperature of 85 degC, significant arsenic sulphide also deposited after the preheater, limiting the design exit temperature to 90 degC. Silica deposition was insignificant in all binary trials, indicating that acid dosing is not required to control silica scaling. |