Within the calculator the general approach to calculating combustion emissions is by multiplying quantities of fuels used by a relevant emissions factor. A simplification of this approach is enabled by the fact that,on an energy basis, fuels of the same physical state have the same emissions factor (or similar enough to ignore the difference).
This fact may seem somewhat unintuitive, as correctly one would expect that burning a tonne of coal would produce more emissions that burning a tonne of wood. Burning a tonne of coal would, however, generate far more energy as the calorific content per gram is far higher. By considering this on an energy basis, effectively normalises by the energy output, such that burning 1kWh of coal produces the same emissions as burning 1kWh of wood. (The mass of the kWh of wood would be greater than the 1kWh of coal.)
The same logic applies across all solid fuels, and the same principal applies to liquid and gaseous fuels. This allows for a simplified set of emissions factors to be used within the model, with just one for each GHG (CO2, CH4 and N2O) for each physical state of fuels. The emissions factors are stated on the ‘constants’worksheet and referenced where required from this single location. This approach allows these factors to be edited easily, as they only need to be changed once (rather than multiple times as would have been the case had the factors been stated separately within each relevant sector worksheet).
NOTE FROM ED: Need something here about how this is different from the UNFCCC reporting basis. Why we are confident this is a valid methodology and some of the questions we often get asked.
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