Reducing cogeneration and trigeneration emissions

Gas-fired cogeneration and trigeneration are greenhouse-friendly ways of generating electricity using fossil fuels because they waste little energy.  Even so, they can emit significant amounts of oxides of nitrogen (NOx). The EPA has published a policy describing best available techniques for minimising emissions and has set limits reflecting these practices.  

Cogeneration is the simultaneous generation of thermal energy and electrical or mechanical energy in one process. It is also referred to as combined heat and power (CHP) because it productively uses the heat that is normally rejected as waste in conventional power generators.

Trigeneration includes a third output, cooling, and is also known as combined heat, cooling, and power (CHCP). The cooling is another output from the heat from burning fuel, and some designs alternate between them.

The Interim Nitrogen Oxide Policy for Cogeneration in Sydney and the Illawarra (PDF 3.1MB) sets out EPA’s current framework for dealing with emissions from cogeneration and trigeneration proposals in NSW.

The framework includes a requirement for proposals to meet best available techniques (BAT) emission control. These techniques cover all aspects of a proposal including fuel source, technology selection, and controls.

  • achieve the highest general level of environment protection of the environment (best)
  • are economically and technically viable for the relevant industry sector (available)
  • include both the technology used and the way an installation is designed, built, maintained, operated, and decommissioned (techniques)

EPA’s policy sets a lower NOx emission limit for a particular class of engines used for co-generation and tri-generation.  The limit is 250 mg/Nm3 for natural-gas-fired reciprocating internal combustion engines (ICEs) with a capacity to burn less than 7 megajoules per second (MJ/s) of fuel.  This limit applies in the Sydney and Wollongong Metropolitan Area and in the Wollondilly local government area. Outside of these locations, the limit is 450 mg/m3 as set out in the Protection of the Environment Operations (Clean Air) Regulation 2010.  Controlling NOx emissions to 250 mg/Nm3 will have a minor impact on project financial performance and is unlikely to impact project viability.

The NOx emission standard for natural-gas-fired reciprocating ICEs with a capacity to burn 7 MJ/s of fuel or more has not been proposed and is to be determined on a case-by-case basis.  This should include a site specific assessment of the feasibility and financial viability of NOx post combustion controls.  The financial viability of post combustion controls depends on the proposed operation of the plant and improves as the capacity of the engine increases.  

The EPA has defined BAT emission standards for natural gas fired reciprocating ICEs due to the significant interest in cogeneration and trigeneration applications. BAT will be developed for other technologies and industries on a case-by-case basis.

Local air quality impacts still need to be considered even if NOx emissions performance is consistent with BAT.

Emission standards do not necessarily protect air quality in the surrounding area as they do not account for site-specific features such as

  • meteorology
  • background air quality
  • terrain
  • building downwash

The impact of emissions from activities and plant on the surrounding area is determined through an air quality impact assessment conducted in accordance with the Approved methods for the modelling and assessment of air pollutants in NSW.

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