Important: A note on input values
Greenhouse gas emissions in this category are calculated using a mass-balance approach in which the carbon contained within the calcining products is subtracted from the initial carbon within the coke feedstock. It is therefore possible - by specifying unrealistic input values in which the quantity of the products is greater than the original source material - to generate a negative emissions quantity.
The Aluminium Sector Greenhouse Gas Protocol provides default values for most of the components involved in this calculation. The use of default values places a greater emphasis on methodological assumptions and generalisations and reduces the accuracy of emissions calculations. The default values provided in the IAI source documentation, when used in combination, produce a negative greenhouse gas emission, suggesting that some of the assumptions are incompatible with one another. The GHG Protocol worksheet suggests an alternative value for one specific component (the mass-of-calcined-coke-produced-per-mass-of-green-coke-feedstock; 0.68 t per t rather than 0.8 t per t), and it is this value which is provided by default. It must be stressed, however, that the use of facility-specific data is vastly preferable for making accurate and reliable emissions calculations.
How to use this category
To use this category, simply create a profile item - there are no drill choices.
The minimum requirement in order that a calculation be made is the specification of the quantity of green coke feedstock used during calcination. This should be specified using the massGreenCoke profile item value. The returned value represents CO2 emissions associated with the calcination of that quantity.
Users can also set values for other process attributes, using the appropriate profile item values. If left unspecified, default values are used in accordance with The Aluminium Sector Greenhouse Gas Protocol and the GHG Protocol worksheet.
Establishing the initial quantity of carbon
A number of parameters contribute to establishing the initial quantity of carbon fed into the calcining process. The proportion by weight of water, volatiles and sulphur within the green coke feedstock can be set using the greenCokeMoistureContent, greenCokeVolatilesContent and greenCokeSulphurContent profile item values respectively. These should be expressed as whole percentages, i.e. between 0-100.
Establishing the quantity of carbon retained
Users can also set several parameters which describe the quantity of carbon which is retained in the calcination products.
To set the quantity of calcined coke produced, users have two options: (1) the total quantity (i.e. mass) of calcined coke can be set using the massCalcinedCokeProduced profile item value; (2) the quantity of calcined coke can be derived from the quantity of green coke feedstock consumed by specifying the coke production per unit of feedstock, using the massCalcinedCokeProducedPerMass profile item value. In addition, users can specify the proportion by weight of sulphur in the calcined coke, using the calcinedCokeSulphur profile item value. This latter value should be expressed as a whole percentage, i.e. between 0-100.
Users similarly have two options for defining the quantity of coke dust retained. To specify an absolute quantity (i.e. mass) of coke dust, set the massCokeDust profile item value. Alternatively, the quantity of coke dust produced per mass of feedstock can be specified using the massCokeDustPerMass profile item value.
Finally, users can also specify the mass of under-calcined coke collected by setting the underCalcinedCokeCollected profile item value.