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When geothermal fluid are discharged, trace amounts
of gas are also released. The largest proportion
is CO2 with lesser amounts of hydrogen
sulphide and methane. These are discharged naturally
though surface features, but can also be carried
by fluids during energy extraction from geothermal
reservoirs.
CO2 emissions for geothermal power
plants are normally in the range of 10-400 g/kWh
compared to 900-1000 g/kWh for oil and coal-fired
plants or 400 g/kWh for gas-fired combined
cycle plant. Atmospheric emissions from geothermal
plants average only about 5% of the emissions
from equivalent sized fossil fuel power plants.
While details of carbon tax implementation in
New Zealand are yet to be finalised, geothermal
discharge of CO2 is likely to attract
a carbon charge.
Gas Contents And CO2
Emissions From New Zealand Geothermal Developments
| Field |
Gas
% |
CO2
(kt/year) |
CO2
(g/kWh) |
| Ohaaki |
2.86 |
86 |
249 |
| Wairakei |
0.59 |
44 |
32 |
| Poihipi Road |
0.43 |
7 |
35 |
| Rotokawa |
2.00 |
22 |
105 |
| Mokai |
1.30 |
28 |
66 |
| Kawerau |
2.82 |
59 |
226 |
| Ngawha |
1.32 |
46 |
597 |
|
| Average |
1.62 |
42 |
100 |
Geothermal plant will emit non-condensable gases
and some water vapour. Even these can be chemically
treated if required, though use of this technology
has not be considered necessary at any plant in
New Zealand. Hydrogen sulphide is usually the
main contributor to odour nuisance. Some geothermal
cooling systems also produce toxic sludge, but
the quantities are small and can be disposed of
in sealed landfills.
Most modern geothermal plants adopt total reinjection
of separated water and condensate, and so have
zero liquid emissions. Wairakei is an exception
in that it produces large liquid emissions both
of separated water and of condensate, as it uses
the Waikato River in a once-through direct contact
cooling system. It also adds a significant amount
of heat to the river. An application to continue
these discharges for the next 25 years has resulted
in more stringent conditions being attached to
the resource consent.
The Kawerau development is another exception
to the reinjection rule, with some separated water
flowing to the Tarawera River after passing through
a cooling pond. For both Kawerau and Wairakei,
reinjection trials have been implemented.
Links
http://www.ew.govt.nz/enviroinfo/geothermal/index.htm
http://www.boprc.govt.nz/Water/Geothermal/Geothermal-Resource.asp
http://www.doc.govt.nz/Explore/001~Other-Places/003~Bay-of-Plenty/Geothermal-Treasures.asp
Publications
A Guide to Geothermal Energy and the Environment
Alyssa Kagel, Diana Bates, & Karl Gawell
http://www.geo-energy.org/Facilities/Links/GeothermalGuide.pdf
Practical methods of minimizing or mitigating
environmental effects from integrated geothermal
developments; recent examples from New Zealand
Chris Bromley
http://www.jardhitafelag.is/papers/PDF_Session_12/S12Paper067.pdf
Houghton, B.F. 1989: Inventory of New Zealand
Geothermal Fields and Features. Geological Society
of NZ
B.F. Houghton 1982. Geyserland: A Guide to the
Volcanoes and Geothermal Areas of Rotorua. Geological
Society of New Zealand Guidebook N. 4.
B.F. Houghton, E.F. Llyod and R.F. Keam 1980:
The Preservation of Hydrothermal System Features
of Scientific and Other Interest - A Report to
the Geological Society of New Zealand.
Parliamentary Commissioner for the Environment
2003. Electricity, energy and the environment.
Part A making the connections.
See Also Other Environmental
Aspects
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