Title: 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 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 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 identified in the west direction 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 places are located outside of the predominant plume
pathway during the modelled period January-December 2012.
A comparison of the model predictions with averaged measured concentrations in the area shows 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 modelling, 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 analyse 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.