Sampling and analysis of H2S and Hg from evaporative geothermal cooling towers: The Italian experience

The Italian regulatory framework on geothermal power plants requires measurement of the emission of H₂S and Hg from cooling towers. Sampling of emitted air in geothermal plants is a quite complex task requiring specific non-standard methods in order to collect stable and representative samples. A direct application of standard methods for H₂S and Hg emissions to cooling tower is often not suitable or difficult to apply. The standard methods indeed were developed for sampling in the stacks of combustion plants such as power generation or cement production plants. The drastic difference in temperature, humidity, air flow composition and velocity and moreover geometrical difference between stacks with diameters of few meters compared to cells of cooling towers, very often over 5–9 m, required a special setup of methods for sampling and analysis. Enel Green Power and Consiglio Nazionale delle Ricerche (Italy) thus have carried out a screening of the existing methods and some of them were customized and extended for their application on cooling towers of geothermal power plants. The main results obtained provided suitable sampling and analysis methods for H₂S in the range 0.2–25 mg/Nm³ and for mercury between 20–500 µg/Nm³. The aforementioned sampling and analysis methods are currently adopted by EGP and by the regional agency for environmental protection (ARPAT) for the measurement of H₂S and Hg emission from cooling towers of geothermal power plants in Italy.