Aluminium prebake pitchcooking

Summary

This methodology represents carbon dioxide (CO₂) emissions associated with the production of baked anodes for aluminium production. 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. The Prebake process is one of two primary technologies employed in this process and involves the use of a coke/pitch electrode paste which is baked prior to electrolysis. In addition to emissions related to the combustion of fossil fuels, the anode baking process represents two other sources of CO2 emissions: (1) the combustion of pitch volatile matter released during the baking operation; and (2) the combustion of baking furnace packing material (coke).

This methodology enables the calculation of CO₂ emissions associated with pitch volatiles combustion and packing material in the anode baking furnace. Both types of emissions are calcualted on the basis of a mass balance approach that tracks the quantities of carbon (C) entering and leaving the baking process, and assuming that the discrepancy represents the C emitted as CO₂.

Pitch volatile matter combustion: The emissions associated with the combustion of pitch volatiles are calculated by considering the quantity of green anodes which is used, from which the appropriate quantities for the hydrogen content of the green anodes, baked anode production and waste tar are subtracted. The remaining quantity represents the carbon lost through pitch volatile combustion. This quantity is converted into the corresponding quantity of CO₂ on the basis of the relative molecular/atomic masses of CO₂ and C.

Furnace packing material: The emissions associated with the combustion of furnace packing material are calculated by considering the quantity of packing coke which is consumed, from which the appropriate quantities of impurities (ash, sulphur; specified as weight percentages, wt. %) are subtracted. The total quantity of coke consumed is converted into the corresponding quantity of CO₂ on the basis of the relative molecular/atomic masses of CO₂ and C.

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

Model data

CO₂ emissions associated with these processes depend partly on the type of furnace employed. This methodology represents two scenarios: Rieghammer furnaces and other furnaces. Both of these scenarios are represented by specific default data values for the following process components:

• Typical quantity of tar collected from furnace (as a percentage of green anode quantity)

• Typical ratio of green anode weight to baked anode weight

• Typical hydrogen content of green anode (wt. %)

• Typical packing coke consumption relative to baked anode production

• Typical sulphur content of packing coke (wt. %)

• Typical ash content of packing coke (wt. %)

Activity data required

Both calculations are dependent on the quantity of baked anodes produced and therefore this must be specified in order to calculate.

The pitch volatile calculation also has specific activity data requirement, although this is somewhat flexible. The quantity of green anodes used can be calculated using the quantity of baked anodes produced (required) together with the typical weight ratio of green anodes consumed and baked anodes produced (provided as a default value). Alternatively, users can specify the corresponding green/baked anode weights for the process if facility-specific data are available. Should the process quantity of green anodes be known, then the quantity of waste tar collected can be derived from the default factor provided (i.e. mass [waste tar] per mass [green anode]). Otherwise, the quantity of waste tar must be explicitly provided.

Otherwise, the methodology enables the optional specification of some quantities where facility-specific data is available. These include: the green anode hydrogen content, the quantity of packing coke used (per unit of baked anodes produced), and the packing coke sulphur and ash contents. Each of these is represented by default values if facility specific data is not available.

Calculation and results

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

Related methodologies

Emissions from the related electrolysis process (which occurs subsequent to the baking process and consumes the baked anodes) can be calculated using the IPCC methodology for prebake electrolysis.

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.