Most high-temperature geothermal resources are located in arid or semi-arid areas far removed from significant freshwater (rivers, lakes) sources. This mostly limits condenser cooling choices to either atmospheric cooling towers or forced ventilation ones. Freshwater cooling from a river is, however, used for instance in New Zealand and seawater cooling from wells on Reykjanes, Iceland.
In older power plants the atmospheric versions and/or barometric ones, the large parabolic ones of concrete, were most often chosen. Most frequently chosen for modern power plants is the forced ventilation type because of environmental issues and local proneness to earth quakes.
The modern forced ventilation cooling towers are typically of wooden/plastic construction comprising several parallel cooling cells erected on top of a lined concrete condensate pond. The ventilation fans are normally vertical, reversible flow type and the cooling water pumped onto a platform at the top of the tower fitted with a large number of nozzles, through which the hot condensate drips in counterflow to the airflow onto and through the filling material in the tower and thence into the condensate pond, whence the cooled condensate is sucked by the condenser vacuum back into the condenser.
To minimise scaling and corrosion effects the condensate is neutralised through pH control, principally via addition of sodium carbonate. Three types of problems are found to be associated with the cooling towers, i.e.
· Icing problems in cold areas.
· Sand blown onto the tower in sandy and arid areas.
· Clogging up by sulphitephylic bacteria.
1. The first mentioned is countered by reversing the airflow cell by cell in rotation whilst operating thus melting off any icing and snow collecting on the tower.
2. The second problem requires frequent cleaning of nozzles and condensate pond. The last mentioned is quite bothersome.
It is most commonly alleviated by periodic application of bacteria killing chemicals, and cleaning of cooling tower nozzles by water jetting. The sludge accumulation in the condensate pond, however, is removed during scheduled maintenance stops. A secondary problem is the deposition of almost pure sulphur on walls and other surfaces within the condenser. It must be periodically cleaned by high pressure water spraying etc., which must be carried out during scheduled turbine stops.
Condenser pumping system
The condensate pumps must, as recounted previously, be made of highly corrosion resistant materials,and have high suction head capabilities. They are mostly trouble free in operation.
The condensate pipes must also be made of highly corrosion resistant materials and all joints efficiently sealed to keep atmospheric air ingress to a minimum, bearing in mind that such pipes are all in a vacuum environment. Any air leakage increases the load on the gas evacuation system and thus the ancillary power consumption of the power plant.
Heat exchangers
· In high-temperature power generation applications heat exchangers are generally not used on the well fluid. Their use is generally confined to ancillary uses such as heating, etc. using the dry steam. In cogeneration plants such as the simultaneous production of hot water and electricity, their use is universal.
· The exhaust from a back pressure turbine or tap-off steam from a process turbine is passed as primary fluid through either a plate or a tube and shell type heat exchanger
· The plate type heat exchanger was much in favour in cogeneration plants in the seventies to nineties because of their compactness and high efficiency. They were, however, found to be rather heavy in maintenance.
· The second drawback was that the high corrosion resistance plate materials required were only able to withstand a relatively moderate pressure difference between primary and secondary heat exchanger media.
· Thirdly the plate seals tended to degenerate fairly fast and stick tenaciously to the plates making removal difficult without damaging the seals.
· The seals that were needed to withstand the required temperature and pressure were also pricy and not always in stock with the suppliers.
· This has led most plant operators to change over to and new plant designers to select the shell and tube configurations, which demand less maintenance and are easily cleaned than the plate type though requiring more room.
· In low-temperature binary power plants shell and tube heat exchangers are used to transfer the heat from the geothermal primary fluid to the secondary (binary) fluid. They are also used as condensers/and or regenerators in the secondary system.
· In supercritical geothermal power generation situation it is foreseen that shell and tube heat exchangers will be used to transfer the thermal energy of the supercritical fluid to the production of clean steam to power the envisaged power conversion system.
Gas evacuation system:
As previously stated the geothermal steam contains a significant quantity of non-condensable gas (NCG) or some 0.5% to 10% by weight of steam in the very worst case. To provide and maintain sufficient vacuum in the condenser, the NCG plus any atmospheric air leakage into the condenser must be forcibly exhausted. The following methods are typically adopted, viz.:
· The use of a single or two stage steam ejectors, economical for NCG content less than 1.5% by weight of steam.
· The use of mechanical gas pumps, such as liquid ring vacuum pumps, which are economical for high concentration of NCG.
· The use of hybrid systems incorporating methods 1 and 2 in series.
· The advantages of the ejector systems are the low maintenance, and high operational security of such systems.
· The disadvantage is the significant pressure steam consumption, which otherwise would be available for power production.
· The advantages of the vacuum pumps are the high degree of evacuation possible. The disadvantage isthe electric ancillary power consumption, sensitivity to particulate debris in the condenser, and high maintenance requirements.
· To reduce the ambient level of H2S in the proximity of the power plant, the exhausted NCG is currently in most countries discharged below the cooling tower ventilators to ensure a thorough mixing with the air as it is being blown high into the air and away from the power plant and its environs. In the USA and Italy H2S abatement is mandatory by law, and in Italy also mercury (Hg) and thus require chemical type abatement measures.
· In some of the older Geysers field power plants the H2S rich condenser exhaust was passed through abed of iron and zinc oxide to remove the H2S. These proved a very messy way of getting rid of the H2S and were mostly abandoned after a few years. In a few instances the Stretford process and other equivalent ones have been used upstream of the power plant to convert H2S gas into sulphur for industrial use. This has proved expensive and complex and is not in use in other geothermal fields than the Geysers field in California.
· The main H2S abatement methods currently in use worldwide are (only some are currently used for geothermal NCG):
1. Claus (Selectox).
2. Haldor Topsöe – WSA process.
3. Shell-Paques Biological H2S removal process/THIOPAC.
4. LO-CAT (wet scrubbing liquid redox system).
5. Fe-Cl hybrid process.
6. Aqueous NaOH absorbent process.
7. Polar organic absorbent process.
8. Photo catalytic generation process.
9. Plasma chemical generation process.
10. Thermal decomposition process.
11. Membrane technology.
A study into feasible H2S abatement methods for the Nesjavellir Geothermal Project was carried outby Matthíasdóttir (2006). Matthíasdóttir and Gunnarsson, from the Iceland Technology Institute,came to the conclusion that of the above listed methods the following four merited further study for Nesjavellir, i.e the Haldor Topsöe-WSA, THIOPAQ (with bacteria), LO-CAT and the Fe-Cl hybrid process.
