Aluminium prebake electrolysis

Summary

This methodology represents carbon dioxide (CO₂) emissions associated with the production of aluminium using prebake technologies. 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, the aluminium (Al) is separated according to the following equation:

1. 2Al₂O₃ + 3C => 4Al + 3CO₂

This methodology enables the calculation of aluminium-associated CO₂ emissions based on a mass balance approach that assumes that the carbon (C) contained within the prebaked anodes is ultimately emitted as CO₂. By determining the quantity of C which enters the process, the quantity which is emitted as CO₂ can be calculated. This balance may be complicated by impurities present within the anodes (sulphur and ash), and the respective quantities of these impurities can be additionally considered in the mass balance equation. The methodology does, however, provide industry-typical values for these which can be used in those cases where facility specific data is not available.

Once the consumed quantity of carbon has been calculated, this is converted into the corresponding quantity of CO₂ on the basis of their relative molecular/atomic masses.

This methodology represents both Tier 2 and Tier 3 approaches under the IPCC guideliines, the Tier 3 approach corresponding to the use of facility-specific data.

Model data

This methodology provides default values for several process components:

• Typical sulphur content in baked anodes

• Typical ash content in baked anodes

Activity data required

CO₂ emissions are directly proportionate to the quantity of aluminium produced (i.e. mass) and the quantity of baked anodes consumed per unit of aluminium produced (i.e. mass [anodes] per mass [aluminium]). Both of these values must therefore be provided in order to calculate.

In addition, the methodology enables the specification of facility-specific data for baked anode sulphur and ash contents where this is available.

Calculation and results

CO₂ emissions are calculated by considering all of the carbon inputs to the Prebake process and multiplying by the CO₂/C mass ratio. These emissions represent those attributable to the specified quantity of aluminium produced.

Related methodologies

Emissions from the combustion of fossil fuels used in the production of baked anodes can be calculated using the IPCC methodology for stationary fuel combustion. Two other sources of related CO2 emissions are represented by the IPCC methodology for anode baking furnaces. This methodology covers (1) the combustion of volatile matter released during the baking operation; and (2) the combustion of baking furnace packing material (coke), but excludes the emissions associated with fossil fuel used to produce heat/energy.

Alternative IPCC methodologies are available for aluminium-associated CO₂ emissions including the generic (tier 1) approach and an alternative tiers 2/3 methodology (representing the Søderberg process).

IPCC methodologies for aluminium-associated PFC emissions (CF₄ and C₂F₆) are also available, including a simple tier 1 approach and two alternative approaches for tiers 2/3: the slope and overvoltage methods.