Aluminium PFC Defaults

Summary

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.

The methodology

Emissions model

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.

Model data

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.

Related methodologies

In cases where more detailed activity data are available, the IPCC slope, and overvoltage methodologies may provide more accurate emissions calculations.

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.