The event described here can be limited to specific settings and might not be encountered in every geothermal projects. More details about this topic are presented in a report that can be found on GEOENVI website.
The phenomenon of release of gases in the atmosphere, called degassing, may occur during the geothermal development if the geothermal fluids produced at the surface have a gas content. Geothermal fluids have a variable composition and gas concentration, depending on the geological formation of the reservoir, fluid temperature and depth. Water, which is the main constituent of geothermal fluids, in high temperature geothermal systems is produced at surface at temperature well above 100°C, and, if released in the atmosphere, is in gaseous conditions (water vapour in superheated conditions, usually called steam). High temperature geothermal fluids may also contain non-condensable gases (NCG), i.e. gases that do not condense at the same pressure and temperature conditions as water vapour but remain in the gas phase. The clear majority of the NCGs (95-99%) is typically CO2; other common compounds present in the gases of geothermal fluids are usually H2S, H2, Ar, NH3, N2 and CH4.
In flash and dry steam plants, the NCGs are released with the water vapour at the downstream of the condenser and at the outlet of the cooling towers, which are usually wet (water) cooling systems. In such case, if mitigation measures are adopted during geothermal operation, part of the NCGs produced can be treated instead of released to the atmosphere.
During the life cycle of a geothermal power plant, temporary degassing may also occur during the production tests in the well drilling phase, and during the geothermal plant maintenance operation and plant shutdown due to extraordinary events. After well abandonment, degassing might also occur in case the well has not been correctly sealed.
The effect of degassing to the environment depends on the gas amount, the toxicity, and the environmental background conditions. Geothermal gases have a natural origin, different from those generated by industrial combustion or other anthropic processes. However, European and national regulations enforce rules to guarantee air quality. Mitigation of degassing requires specific treatments, which are often applied also in the case of emission factors below the reference thresholds defined for human health and environmental safety.
Some harmful elements and compounds sometimes present in geothermal gases – such as mercury (Hg) and ammonia (NH3), as well as traces of arsenic (As) or antimony (Sb) – may be stripped by gases emitted at plants, included in aerosol particles (drift) emitted from cooling towers in power production plants, and then be deposited on soil and washed out by rain. Geothermal plants do not directly emit NOx, SO2 and primary particulate matter (PM), but secondary PM may form from the oxidation of H2S and NH3.
Monitoring and prevention measures are used only in the cases involving geothermal gas emissions. The degassing phenomena is generally monitored at different levels, e.g. by measuring emissions at geothermal plant level, and by monitoring the air quality in the surrounding environment, at degassing tanks closely associated with production wells, as well as changes in gas/temperature flux from soil in the surrounding environment.
Prevention and mitigation activities, such as the installation of abatement systems like the “AMIS” technology used in Italy, and the development of new technologies aiming at the complete reinjection of the fluid have been proven to be effective in reducing the potential consequence (in terms of both gravity and probability of appearance) related to the use of geothermal energy.
Potential emission and dispersion of geothermal fluids are treated in Liquid and solid effusions on surface. Emission of gases from engines used in surface operation are treated in Energy and water consumption and emissions.
The table here below provides an overview of this topic in term of risk and impact assessment i.e. its causes, consequences, the phases concerned, the influencing contexts or the principals monitoring and mitigation measures that can be adopted.
