Aluminium PFC Overvoltage

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. Some process control systems characterize anode effects by calculating the Anode Effect Overvoltage (AEO) - a parameter which represents the additional cell voltage above the target operating voltage (calculated as the mean overvoltage through time). This parameter has been shown to be a good predictor of PFC emissions.

This methodology enables the calculation of aluminium-associated CF₄ and C₂F₆ emissions on the basis of conversion factors (overvoltage coefficient; i.e. kg / t·mV) which describe the relationship between anode effect overvoltage (mV) the rates at which CF₄ is emitted in relation to the quantity of aluminium produced. By specifying the quantity of aluminium produced and the anode effect overvoltage, a calculation for the associated CF₄ emissions can be made. C₂F₆ emissions are calculated as a fixed proportion of the CF₄ emissions. This proportion can be specified on a facility specific basis, but is represented by a default value if this data is unavailable.

This methodology represents the IPCC Tiers 2/3 approaches, the distinction being the use of facility specific data (which corresponds to tier 3).

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 two scenarios, differentiated by technology type (e.g. Centre Work Prebake, Side Work Prebake). Each technology type is represented by default values for the overvoltage coefficient and the CF₄/C₂F₆ weight fraction, 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 three quantities, all of which must be provided in order to calculate: (1) the quantity of aluminium produced (i.e. mass); (2) the anode effect overvoltage; and (3) the aluminium production process current efficiency.

In addition, the methodology enables the specification of facility-specific values for the overvoltage coefficient and the CF₄/C₂F₆ weight fraction where these are available.

Calculation and results

CF₄ and C₂F₆ emissions are calculated according to the activity data specified, and 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

Other IPCC methodologies for aluminium-associated PFC emissions are available. These include an alternative tiers 2/3 approach (the slope method), and the simpler tier 1 methodology.

In addition, IPCC methodologies for aluminium-associated CO₂ emissions are available, including the generic (tier 1) approach and more detailed, technology-specific prebake and Søderberg methodologies.