A numerical model for determining the disperson of hydrogen sulphide plume from Olkaria geothermal power station

Abstract

The concentration variation of hydrogen sulphide arising from its dispersion around Olkaria geothermal power station has been determined using an Eulerian model that was developed. The model quantitatively simulates the atmospheric interactions and relates the emission of hydrogen sulphide to the air quality. The fundamental principle of this thesis is that the dispersion of hydrogen sulphide in the atmosphere can be determined by solving a set of governing equations. This is in accordance with the existing mass conservation law. The governing equations are responsible for advection and diffusion (transport), depletion (sink) and accumulation (source) of hydrogen sulphide in the atmosphere. The equations were solved explicitly using the finite difference method. A computer program was developed using C++ programming language. The program divides the Olkaria geothermal field and its environs into blocks and vertical layers. Then it distributes the hydrogen sulphide emitted from a source within the field to all the blocks and layers depending on the prevailing meteorological conditions. The results presented for the hypothetical situation show that the model responded well to wind speed, direction as well as gravitational settling as some of the factors that influence the advection and diffusion of hydrogen sulphide. Model validation indicates that hydrogen sulphide concentrations around the power station vary with wind speed among other meteorological conditions. Generally, higher concentrations were recorded near the source_ However the model underestimated all the concentrations compared to the measured concentrations and these was attributed to the models inability to accommodate the terrain and change in wind direction. Low levels of concentration were also attributed to high background concentrations as a result of emissions from local sources such as evaporation at cooling towers, silencers, drainage and traps in the vapor ducts among others.