A rigorous simulation model of geothermal power plants for emission control

Geothermal power plant (GTPP) operating conditions and associated emissions mainly depend on the endogenic fluid used to generate power, and the case of GTPPs located in Tuscany (Italy) is considered in this study. Since measuring on-line the quantity of emitted pollutants is a difficult task, a process simulation model featuring all unit operations of the GTTP is developed using UniSim Design® to forecast and control pollutant emissions. An accurate identification of the thermodynamic correlation parameters for the solubility of the considered pollutants (mercury and hydrogen sulfide) in water has been performed to match literature values. A data reconciliation procedure is used to match the simulation model outcome with real measurements of two (20 MWe and 40 MWe) GTTPs. Results are fully satisfactory as the mercury emission forecasted is always slightly above the measured data, evidencing the model is conservative, and hence reliable in ensuring satisfaction of emission limits established by law. For the 20 MWe plant, the simulated total mercury emissions are 3.31 g/h exceeding the measured ones by 27%, while for the 40 MWe plant, they are 1.38 g/h above the measured ones by 4%. In addition, for the 20MWe plant, pollutants emission and net power production are both considered in a performance analysis. The worst case scenario for power generation (18.7 MW) is in summer conditions, while for pollutant emission is in winter conditions with 5.22 g/h of total mercury and 20.46 kg/h of hydrogen sulfide. Finally, energetic performances result to be independent from the environmental measures adopted.