Modelling H2S Dispersion from San Jacinto-Tizate Geothermal Power Plant, Nicaragua

Abstract

One of the main environmental concerns during the operation of geothermal power plants is related to hydrogen sulphide (H2S) emissions, which negatively affect the air quality in its environs. National ambient air guidelines for H2S have not been developed in Nicaragua; hence, project developers voluntarily follow international guidelines for atmospheric concentration and occupational safety. Solution-mineral equilibria considerations and assumed steam consumption of the 72 MW power plant in San Jacinto-Tizate were used to constrain the annual H2S emission at 1400 tons/year. Dispersion modelling of the H2S emission from the power plant was carried out using AERMOD model to predict H2S ground level concentrations in the vicinity of the project. The prevailing spatial distribution of the plume was to the west of the power plant, spreading towards west-northwest and west-southwest. The modelling results show that the operation of the San Jacinto-Tizate geothermal power plant does not have significant impact on air quality of the neighbouring communities, since populated areas are located outside of the predominant plume pathway during the modelled period January-December 2012. A comparison of the model predictions with measured concentrations averaged over 24 hours within 1.5 km distance from the power plant shows an underestimation of the measured values at most of the monitoring points, suggesting that the model predictions should be considered indicative rather than accurate. Several sources of uncertainty that may contribute to this differences were identified, namely the source input data used for modelling, the characteristics of the location of the control points, the operation conditions of the power plant when the measurements were made as well as the influence of natural sources of H2S in the measured concentrations The results, however, provide useful information to analyse the spatial distribution and the extent of the plume during a given period, and can assist policy makers and project developer to review and improve the air quality monitoring plans.