Hydrogen sulphide abatement during discharge of geothermal steam from well pads : a case study of well pad TR-18, El Salvador

Abstract

The emission of hydrogen sulphide (H2S) during geothermal development is a significant environmental aspect that must be considered in an environmental management plan. Wells TR-18 and TR-18A are part of the expansion of the Berlín geothermal power plant in El Salvador. H2S emissions were estimated for these two wells during well discharge, and calculated to be 7.1 and 42.8 g/s, respectively. Maximum H2S concentrations in the air were also determined in the nearby Bob Graham community using a dispersion model (SCREEN3). Four cases were evaluated: nominal flow, 35% of the nominal flow, nominal flow with 90% removal efficiency, and nominal flow with double stack height. Estimated maximum concentrations in the air in the community were 54.9 and 174.2 (μg/m3 24-hour averaged) during TR-18 and TR-18A well discharges, respectively. These estimated concentrations exceeded USEPA-California (43 μg/m3 1-hour averaged) and Icelandic (50 μg/m3 24-hour averaged) regulations.

The aim of this project was to suggest mitigating actions and to fulfil recommended environmental regulations. Three methods for H2S removal during well discharge were evaluated: caustic scrubbing, caustic scrubbing with peroxide and scrubbing with iron sulphate. The first method was found to be the most appropriate based on its 90% efficiency and its low chemical costs when compared to the other two methods. Another measure recently introduced by LaGeo is the implementation of the chemical tracer method, which uses isopropanol for steam phase and sodium benzoate for liquid phase. This method is most suitable for flow measurement because it avoids H2S emissions in flow testing.