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 10% of the emissions from equivalent sized fossil fuel power plants.
Gas Contents And CO2 Emissions From New Zealand Geothermal Developments
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.
Geothermal Fluid Use and Emissions Trading Requirements
- Application – Electricity Generation and Industrial Heat
The New Zealand Emissions Trading Scheme applies to using geothermal fluid for generating electricity or industrial heat, where the emissions of carbon dioxide-equivalent (CO2-e) exceed 4,000 tonnes from a given installation per annum.
- Legislative Requirements
The Climate Change Response Act 2002 (update as at 8 Dec 2009) requires industries to register, to set up holding accounts, to gather data, to monitor emissions, to provide regular data returns for prescribed periods at specified times. Payment is according to default emissions factors for a given facility as specified in the Climate Change (Stationary Energy and Industrial Processes) Regulations 2009 unless an application for a unique emissions factor is made and approved under the Climate Change (Unique Emissions Factors) Regulations 2009.
Geothermal facilities supplying geothermal fluid for generating electricity or industrial heat are subject to the Climate Change (Stationary Energy and Industrial Processes) Regulations 2009. These regulations consider fluid supply as either geothermal steam (Schedule 2, Table 6, Part A) or geothermal fluid (Schedule 2, Table 6, Part B). Prescribed or default emission factors are defined in Schedule 2, Table 6 of the regulations for these two fluid types. The measured annual fluid production is multiplied by the prescribed emissions factor to derive the reportable annual emissions from a given facility.
There is an option for the prescribed emissions factor to be substituted with a unique emissions factor. The methodology to develop a unique emissions factor for a geothermal facility is covered in the Climate Change (Unique Emissions Factors) Regulations 2009, clauses 14 to 17. Aspects of determination of unique emissions factors covered are in a letter on the Climate Change Act – Geothermal Sampling Procedures dated 23 September 2010 from GNS Science to the New Zealand Geothermal Association. This letter identifies appropriate sampling methods that comply with the legislative requirements.
- Other Commentary and Information
A number of companies have analysed their processes, determined that it is cost effective to make an application for a unique emissions factor and have subsequently applied for and been granted a unique geothermal emissions factor.
The carbon emissions scheme effectively taxes industries for their emissions. For the geothermal industry, which has comparatively low carbon emissions, this increases their economic performance with respect to other higher emitting generators.
For more on the emissions trading scheme visit the NZ Government climate change web site
- A Guide to Geothermal Energy and the Environment
Alyssa Kagel, Diana Bates, & Karl Gawell
- Practical methods of minimizing or mitigating environmental effects from integrated geothermal developments; recent examples from New Zealand
- 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.