Geothermal Power Generation

Flashed steam/dry steam condensing system; resource temperature range from about 320°C to some 230°C.

Flashed steam back pressure system; resource temperature range from about 320°C to some 200°C.

Binary or twin-fluid system (based upon the Kalian or the Organic Rankin cycle); resource temperature range between 120°C to about 190°C.

· Power house equipment: Comprising of turbine/generator unit complete with condenser, gas exhaust system.

· Automatic control and communication system: Consisting of frequency control, servo valve control, computer system for data collection, resource and maintenance monitoring, internal and external communication etc.

· Cooling system: Cooling water pumps, condensate pumps, fresh water (seawater) cooling, or cooling towers.

· Particulate and/or droplet erosion: This is an erosion problem that is typically associated with the parts of the system where the fluid is accelerated (e.g. in control valves, turbine nozzles, etc.) and/or abruptly made change direction (e.g. via pipe bends, T-fittings or wanes).

· Heat exchangers: These are either of the plate or the tube and shell type. These are generally only used in binary and hybrid type conversion systems, and/or in integrated systems.

· Gas evacuation systems: High temperature geothermal fluid contains a significant quantity of noncondensable gases (C02, N2, H2S, and others). These have to be removed for instance from the condensing plant for reasons of conversion efficiency. Some countries require the gas to be cleaned of H2S or Hg to minimize atmospheric pollution.

· Re-injection system: Comprising liquid effluent collection pipelines, injection pumps, injection pipelines, injection wells and control system.

· Chemical injection system: In order to reduce scaling of calcite in production wells sometimes a scale inhibitor is injected through capillary tubing down hole. Similar injection is applied with caustic soda to neutralize acid wells to reduce the corrosively. Acid is used for pH modification in order to arrest the scaling of silica in waste water going to reinjection, for cases where the water is supersaturated.

Power House equipment

Turbine:

· The problems potentially associated with the turbine are scaling of the flow control valve and nozzles (primarily in the stator inlet stage); stress corrosion of rotor blades; erosion of turbine (rotor and stator) blades and turbine housing.

· The rate and seriousness of scaling in the turbine are directly related to the steam cleanliness, i.e. the quantity and characteristics of separator “carry-over“. Thus the operation and efficiency of the separator are of great importance to trouble free turbine operation. Prolonged operation of the power plant off-design point also plays a significant role.

Generator:

· It must be pointed out here that high-temperature steam contains a significant amount of carbon dioxide CO2 and some hydrogen sulphite H2S and the atmosphere in geothermal areas is thus permeated by these gases.

· All electrical equipment and apparatus contains a lot of cuprous or silver components, which are highly susceptible to sulphite corrosion and thus have to be kept in an H2S free environment. This is achieved by filtering the air entering the ventilation system and maintaining slight overpressure in the control room and electrical control centres.

· The power generator is either cooled by nitrogen gas or atmospheric air that has been cleaned of H2S by passage through special active carbon filter banks.

 Condenser:

· The steam-water mixture emitted from the turbine at outlet contains a significant amount of non-condensable gases comprising mainly CO2 (which is usually 95–98% of the total gas content), CH4 and H2S, and is thus highly acidic.

· Since most high-temperature geothermal resources are located in arid or semi-arid areas far removed from significant freshwater (rivers, lakes) sources, the condenser cooling choices are mostly limited to either atmospheric cooling towers or forced ventilation ones.

· The application of evaporative cooling of the condensate results in the condensate containing dissolved oxygen in addition to the non-condensable gases, which make the condenser fluid highly corrosive and require the condenser to be clad on the inside with stainless steel; condensate pumpsto be made of stainless steel, and all condensate pipelines either of stainless steel or glass reinforced plastic.

· Addition of caustic soda is required to adjust the pH in the cooling tower circuit. Make-up water and blow-down is also used to avoid accumulation of salts in the water caused by evaporation.

· A problem sometimes encountered within the condenser is the deposition of almost pure sulphur on walls and nozzles within the condenser.

· This scale deposition must be periodically cleaned by high pressure water spraying etc.