Custom emissions factors

Where data on CO₂ emission factors for specific fuels are not available, an emissions factor can be derived if the carbon content of the fuel is known. This is based on the known relationship between a mass of carbon and the mass of CO₂ which is produced on burning together with an assumption of 100% burning efficiency - i.e. 100% carbon converted to CO₂.

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C to CO₂ stoichiometry

During combustion of carbonaceous materials, 1 carbon atom (from the material) combines with 2 oxygen atoms (from the environment), producing CO₂. The atomic mass of carbon, C, and oxygen, O, are 12 and 14 respectively, and therefore the molecular mass of CO₂ is 44. It follows that the ratio of masses of atomic carbon to CO₂ is 44/12, or approximately 3.667. By multiplying a quantity (mass) of carbon by this factor, we can derived the equivalent quantity of CO₂ which is produced on combustion.

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Carbon content

If the concentration of carbon - or 'carbon content' - of a given material is known, then it is straightforward to calculate the absolute mass of carbon which is burned:

mass_carbon = mass_fuel x %_carbon ,

where %_carbon is expressed on a decimal basis (i.e. 0-1). For example, if 80% of a particular material comprises carbon, then the quantity of carbon burned is simply the mass of fuel burned multiplied by 0.80. Therefore, if one knows the carbon content of a given fuel and the amount burned, the corresponding CO₂ emissions can be calculated thus:

mass CO₂ = mass_fuel x %_carbon x 3.667,

where %_carbon * 3.667 represents the 'emissions factor' for the given fuel (or material more generally).

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Expressing in other units

In some cases, an emissions factor which is applicable to other units of quantity - e.g. energy or volume - are appropriate. These can be derived by using the correct conversion factors.

A 'heating' or 'calorific' value is a value representing the energy contained within a given material or fuel. These are usually expressed in units such as GJ per L or kWh per t. Energy-based emissions can be calculated using a mass-based heating value x

mass CO₂ = energy consumed / energyContent_fuel x %_carbon x 3.667 ,

where energyContent_fuel is the heating value for the particular fuel (i.e. energy per mass). Similarly, one can calculate on the basis of volumetric quantities by converting volume into an equivalent mass using a value for material density (i.e. mass per volume):

mass CO₂ = volume consumed x density_fuel x %_carbon x 3.667,

where density_fuel is the density (i.e. mass per volume) of the given material.

All of these derivations assume 100% conversion of carbon into CO₂.