IPCC methodology for aluminium production PFC emissions - general approach
This methodology represents carbon tetrafluoride (CF4) and carbon hexafluoride (C2F6) emissions associated with the production of aluminium. The data and calculation methodology is sourced from the Greenhouse Gas Protocol (GHGP) worksheet tool GHG emissions from the production of aluminum, version 2.0, which is ultimately based on the methodologies described in Volume 3, Chapter 4 - Metal Industry Emissions of their 2006 IPCC Guidelines for National Greenhouse Gas Inventories.
Aluminium is extracted from its ore (aluminium oxide (Al2O3), or 'bauxite') using an electrolysis cell and carbon anodes. During electrolysis, aluminium oxide is dissolved in a fluoride melt comprising about 80% (by weight) cryolite (Na3AlF6). Perfluorocarbons (CF4 and C2F6 collectively referred to as PFCs) are formed from the reaction of the carbon anode with the cryolite melt in cases where the concentration of aluminium oxide in the electrolyte is too low to support the standard anode reaction (a condition known as an anode effect).
This methodology enables the calculation of aluminium-associated CF4 and C2F6 emissions on the basis of emissions factors which represent the rates at which CF4 and C2F6 are typically emitted in relation to the quantity of aluminium produced. Multiplying a quantity of aluminium produced by these emissions factors results in an estimate of the emissions associated with that quantity.
This methodology represents the IPCC Tier 1 approach, and is considered advisable in cases where there is insufficient activity data available on the process anode effects.
The rates at which CF4 and C2F6 are emitted depend on the specific type of technology employed. This methodology provides emissions factors representing the typical unit emissions associated with four scenarios, differentiated by technology type (e.g. Centre Work Prebake, Vertical Stud Soderberg). Each technology type is represented by emissions factors for CF4 and C2F6 (i.e. kg [emissions] per kg [aluminium]), although these values can be specified on facility-specific basis if data is available.
In addition, the global warming potentials of CF4 and C2F6 are also used in order to convert absolute emissions quantities into CO2e quantities - i.e. the quantity of CO2 which would exert the same atmospheric warming effect.
Activity data required
CF4 and C2F6 emissions are directly proportionate to the quantity of aluminium produced (i.e. mass), which therefore must be provided in order to calculate. In addition, the methodology enables the specification of facility-specific CF4 and C2F6 emissions factors where these are available.
Calculation and results
CF4 and C2F6 emissions are calculated by simply multiplying the specified quantity of aluminium produced by the respective emissions factors. These quantities are converted into a total CO2e quantity using the associated global warming potentials.
Three quantities are ultimately returned: CF4, C2F6, and total CO2e. These emissions represent those attributable to the specified quantity of aluminium produced.
In addition, IPCC methodologies for aluminium-associated CO2 emissions are available, including the a generic (tier 1) approach and more detailed, technology-specific prebake and Søderberg methodologies.