What is the effect of non-condensable gases on steam turbine performance in a geothermal plant?

Non-condensable gases (NCGs), such as carbon dioxide (CO2), hydrogen sulfide (H2S), and nitrogen (N2), are often present in geothermal steam and significantly affect steam turbine performance in a geothermal plant. Their presence reduces the partial pressure of the steam, leading to a lower overall pressure in the turbine and a reduced temperature difference across the turbine stages. This decreased temperature difference directly diminishes the amount of energy that can be extracted from the steam, lowering the turbine's power output. Furthermore, NCGs increase the specific volume of the steam-NCG mixture, which means the turbine must handle a larger volume of fluid for the same mass of steam. This can lead to increased frictional losses as the mixture flows through the turbine blades and passages, further reducing efficiency. NCGs also accumulate in the condenser, increasing the back pressure on the turbine. A higher back pressure reduces the pressure drop across the turbine, again lowering power output. To mitigate these effects, geothermal plants employ gas extraction systems, such as steam jet ejectors or vacuum pumps, to remove NCGs from the condenser. Inefficient removal of NCGs can lead to turbine derating, meaning the plant must operate at a lower capacity than its design specifications. Corrosion can also be accelerated by the presence of certain NCGs, like H2S, increasing maintenance costs and potentially shortening the lifespan of turbine components.