IPCC methodology for aluminium production CO2 emissions - Soderberg processes

Contents

Summary

This methodology represents carbon dioxide (CO2) emissions associated with the production of aluminium using Søderberg 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

Carbon (C) at the anode is therefore consumed during electrolysis and emitted as CO2. This net reaction accounts for the majority of process-related greenhouse gas emissions in aluminium production. The Søderberg process is one of two primary technologies employed in this process and involves the use of a coke/pitch electrode paste which is baked during electrolysis rather than prior to, as in the case of the prebake process.

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 Søderberg paste is ultimately emitted as CO2. By tracking the C which enters the process, and accounting for C which leaves the process via the aluminium product (and other waste-/by-products), the residual C which is emitted as CO2 can be calculated. This balance may be complicated by impurities present within the coke and pitch feedstocks (such as ash, sulphur and hydrogen), as well as the C which remains in the dust collected from the Søderberg cells. The respective quantities of these impurities/by-products can be additionally considered in the mass balance equation, and the methodology provides industry-typical values for each for 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

The rate at which CO2 is emitted depends on the specific type of technology employed. This methodology provides data representing four scenarios, differentiated by technology type (Vertical Stud Søderberg, Horizontal Stud Søderberg), and paste type (wet, dry). Each scenario is represented by default values for the paste binder content (%) and cyclohexane soluble matter emissions (per quantity of aluminium produced). Both of these values can be specified on facility-specific basis if data is available.

In addition, this provides default values for several process components:

  • Typical binder content in paste
  • Typical sulphur, ash and hydrogen contents in pitch
  • Typical sulphur and ash contents in coke
  • Typical quantity of dust carbon per unit aluminium production

The methodology also provides the mass ratio of CO2/C (44/12 = 3.66667) which is used to convert quantities of C into their corresponding quantity of CO2 (assuming all residual C is emitted as CO2).

Activity data required

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

In addition, the methodology enables the specification of facility-specific data for a number of components (e.g. paste binder content; pitch sulphur, ash and hydrogen content; coke sulphur and ash content) where this is available.

Calculation and results

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


Related methodologies

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


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