Sensitivity analysis and dynamic modelling of the reinjection process in a binary cycle geothermal power plant of Larderello area

Highlights: • For a 5 MW power plant, the CO₂ compressor train consumes up to 176 kW. • The friction loss for the downward flow in the reinjection well is about 7.5 mbar/m. • The start-up of the reinjection takes about 20 min achieving the steady-state. • The CO₂-dependent saturation pressure of the drain is the reinjection process limit. • Previously dissolved CO₂/H₂S in water reduce the well capacity of receiving extra NCG. The surface equipment design for a binary cycle geothermal power plant including facilities required for the power cycle and the complete reinjection process of two-phase geothermal fluid (H₂O + non-condensable gases (NCGs)) in the Castelnuovo area of Larderello are modelled. The proposed scheme includes the configuration of closed-loop power plant, NCGs compression train and reinjection process at the wellhead. Steady-state and dynamic simulations are performed. The steady-state model is developed to find the correct operating solutions and conditions, while the dynamic model investigates the unsteady behaviour of the system. The study includes a sensitivity analysis of surface equipment design, demonstrating the effect of internal and external variables on the system performance, including power cycle, compression train and reinjection process. The maximum compressor power consumption (at 60 bar reinjection pressure) is 176 kW at 8% CO₂ mass fraction into the geothermal fluid. The unsteady system behaviour at startup and its response to step-changes of flow stream condition is assessed: it is found that the average response-time is about 20 min. Also, the system requires a drain pressure higher than the saturation level for effective reinjection. A new Antoine – based correlation for water-CO₂ mixture is proposed for geothermal applications.