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 sulfide emissions (H2S), 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 emissions and occupational safety. Solution-mineral equilibria considerations and assumed steam consumption by the 72 MW geothermal power plant in San Jacinto-Tizate were used to constrain the annual H2S emission at 1436.2 tons/year. Dispersion modeling 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 identified in the W direction of the power plant, spreading towards WNW and WSW. The modeling results show that the operation of the San Jacinto-Tizate geothermal power plant does not have significant impact on air quality of the neighboring communities, since populated places are located outside of the predominant plume pathway during the modeled period January-December 2012.

A comparison of the model predictions with averaged measured concentrations in the area showed an underestimation of the measured values in most of the monitoring points, suggesting that the model predictions should be considered indicative rather than accurate. Different sources of discrepancy were identified, namely the source input data used for modeling, the characteristics of the location of the control points, the operation conditions of the power plant when the measurements are done as well as the influence of natural sources of H2S in the reported measured concentrations The results, however, provide useful information to analyze the spatial distribution and the extents of the plume during a given period, and can assist policy makers and project developer to review and improve the air quality monitoring plans.