There are three Tiers presented in the 2006 IPCC Guidelines for estimating emissions from fossil fuel combustion. In addition a Reference Approach is presented. It can be used as an independent check of the sectoral approach and to produce a first-order estimate of national greenhouse gas emissions if only very limited resources and data structures are available to the inventory compiler.
The 2006 IPCC Guidelines estimate carbon emissions in terms of the species which are emitted. During the combustion process, most carbon is immediately emitted as CO2. However, some carbon is released as carbon monoxide (CO), methane (CH4) or non-methane volatile organic compounds (NMVOCs). Most of the carbon emitted as these non-CO2 species eventually oxidises to CO2 in the atmosphere. This amount can be estimated from the emissions estimates of the non-CO2 gases (See Volume 1, Chapter 7).
In the case of fuel combustion, the emissions of these non-CO2 gases contain very small amounts of carbon compared to the CO2 estimate and, at Tier 1, it is more accurate to base the CO2 estimate on the total carbon in the fuel. This is because the total carbon in the fuel depends on the fuel alone, while the emissions of the non-CO2 gases depend on many factors such as technologies, maintenance etc which, in general, are not well known. At higher tiers, the amount of carbon in these non-CO2 gases can be accounted for.
Since CO2 emissions are independent of combustion technology whilst CH4 and N2O emissions are strongly dependent on the technology, this chapter only provides default emission factors for CO2 that are applicable to all combustion processes, both stationary and mobile. Default emission factors for the other gases are provided in subsequent chapters of this volume, since combustion technologies differ widely between source categories within the source sector "Combustion" and hence will vary between these subsectors.
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