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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.

Note that gross emission factors have been used.

A2 CH 4 and N2O emission factors used in this guidance

As stated above, although CO2 emissions remain constant regardless of the way in which a fuel is combusted, CH 4 and N2O emissions depend on the precise nature of the activity in which the fuel is being combusted. The emission factors for CH 4 and N2O therefore vary depending on the combustion process. Table 10 shows the default CH 4 and N2O emission factors (expressed in terms of energy units) which have been used in this publication. The below calculations show how these have been converted to per activity unit (eg, kg CO2-e/litre) emission factors. All emission factors contained in Table 10 are sourced from the Energy Greenhouse Gas Emissions publication. This publication contains further CH 4 and N2O emission factors for a range of other users (eg, residential).

Note that gross calorific values have been used.

A3 Oxidation factors used in this publication

All oxidation factors contained in Table 10 are sourced from New Zealand’s National Greenhouse Gas Inventory 1990–2005. Oxidation factors have been applied only to the CO2 emission factors (and therefore by default to the CO2-e emission factors) and have not been applied to the CH 4 and N2O emission factors.

A4 Reference data

Table 10: Underlying data used to derive the per activity unit emission factors – 2006

View underlying data used to derive the per activity unit emission factors – 2006 (large table).

Table 11 contains the GWPs for CO2, CH 4 and N2O that have been used in converting to CO2-equivalent emission factors.

Table 11: Global warming potentials for CO2, CH 4 and N2O

 

CO2

CH 4

N2O

Global warming potential

1

21

310

A5 Example derivation of emission factors

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

CO2 emission factor (kg CO2/litre) = [(Calorific value*tCO2 per TJ emission factor)*Oxidation factor]/1000

= [(22.40*91.2)*0.98]/1000

= 2.00 kg CO2/litre

CH 4 emission factor (kg CO2-e/litre) = [(Calorific value*tCH 4 per TJ emission factor)*GWP of CH 4]/1000

= [(22.40*0.0095)*21]/1000

= 0.00447 kg CO2-e/litre

N2O emission factor (kg CO2-e/litre) = [(Calorific value*tN2O per TJ emission factor)*GWP of N2O]/1000

= [(22.40*0.00133)*310]/1000

= 0.00924 kg CO2-e/litre

Total CO2-e emission factor (kg CO2-e/litre) = Sum of CO2, CH 4 and N2O emission factors

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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