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Guidance for Voluntary, Corporate Greenhouse Gas Reporting: Data and methods for the 2008 calendar year

• 1 Guidance on reporting
• 2 Emission factors and methods – context
• 3 Emission factors and methods 2008
• 4 References
• Appendix: Derivation of fuel emission factors

Appendix: Derivation of fuel emission factors

A1.      Importance of calorific value

Because the energy content of fuels may vary within and between fuel types, emission factors are commonly expressed in terms of energy units (eg, tonnes CO₂/TJ). This generally provides more accurate emissions estimates than if emission factors are expressed in terms of mass or volume. Converting to emission factors expressed in terms of mass or volume (eg, kg CO₂-e/litre) requires an assumption around which default calorific value should be used.

It is therefore useful to show how the per activity unit (eg, kg CO₂-e/litre) emission factors have been derived, and which calorific values have been used. It is important to note that if you are able to obtain fuel use information in energy units, or know the specific calorific value of the fuel which you are using, then you can calculate your emissions more accurately. All calorific values are sourced from the New Zealand Energy Data File 2009.

Note that gross calorific values have been used.

A2.      CH₄ and N₂O emission factors used in this guide

As stated above, although CO₂ emissions remain constant regardless of the way in which a fuel is combusted, CH₄ and N₂O emissions depend on the precise nature of the activity in which the fuel is being combusted. The emission factors for CH₄ and N₂O therefore vary depending on the combustion process. Table 10 shows the default CH₄ and N₂O emission factors (expressed in terms of energy units) which have been used in this guide. The calculations below show how these have been converted to per activity unit (eg, kg CO₂-e/kg) emission factors. All emission factors contained in Table 10 are sourced from the Energy Greenhouse Gas Emissions (2009) publication. This publication contains further CH₄ and N₂O emission factors for a range of other users (eg, residential).

Note that gross emission factors have been used.

A3.      Oxidation factors used in this guide

All oxidation factors contained in Table 10 are sourced from New Zealand’s National Greenhouse Gas Inventory 1990–2007. Oxidation factors have been applied only to the CO₂emission factors (and therefore by default to the CO₂-e emission factors) and have not been applied to the CH₄ and N₂O emission factors. This approach is consistent to that adopted by the National Greenhouse Gas Inventory.

A4.      Reference data

Table 10:     Underlying data used to derive the per activity unit emission factors – 2008

Table 11 contains the GWPs for CO₂, CH₄ and N₂O that have been used in converting to
CO₂-equivalent emission factors.

Table 11:     Global Warming Potentials for CO₂, CH₄ and N₂O

A5.      Example derivation of emission factors

The sub-bituminous coal emission factors for commercial use are derived as follows:

Note that if you knew that the calorific value of your coal was different to the default calorific value used in the above calculation, you could substitute your specific calorific value and obtain a more accurate (specific) emission factor.

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