Comparative Analysis of Energy Extraction Systems for High Temperature, High Pressure Geothermal Steam Considering Silica Precipitation

Abstract

This paper presents the results from an analytical comparison of energy extraction systems from high temperature, high-pressure geothermal steam when taking into consideration dissolved silica according to quartz equilibrium in the reservoir. The comparisons include backpressure turbines, steam reheat, binary and hybrid solutions, conventional steam turbines and combinations of these. Experimental and computational research from the last four decades are summarized to address where and how amorphous silica may become problematic in the systems considered. A combination of thermodynamic calculations of super-saturation index, models for kinetics of particle formation and -growth, and computational fluid dynamic modelling of suspended particle deposition in nanoscales is used for the evaluations. The fluid in consideration is supercritical steam from a 4-5 km deep well with a nearby magmatic heat source, giving wellhead temperatures ranging between 400 and 550 ℃ at 350 bar pressure. The reservoir fluid is supercritical, while the conditions at wellhead may be superheated steam depending on the pressure loss during production. The analysis show that the dissolved silica content is high enough to necessitate some form of purification before entering a conventional turbine. The equipment upstream the scrubber needs to be robust with regard to silica precipitation/scaling and the pressure reduction and downstream equipment optimized to ensure as little energy loss as possible with as little downstream silica contamination as possible. The alternatives presented are rated with regard to power output, energy utilization efficiency and their robustness against silica scaling.