Methodology

Underlying the curves is a detailed reconciliation between energy inputs and carbon emissions, normalised to consistent units of production. For each commodity, appropriate system boundaries - i.e. clearly defined cut-off points within the supply chain - have been applied. GHG data is presented from a smelters viewpoint - where mine, refinery and freight emissions are considered as upstream to the smelter and are generally included in Scope 3 reporting. Therefore E0 is the total GHG emissions produced at the smelter, while E1 includes emissions generated along the supply chain from mining, refining, freight and at the smelter to produce Primary Aluminium.

Alumina refining

The GHG emissions from alumina refining using the Bayer process are predominantly related to fuel combustion. The process requires energy primarily in the form of heat and steam. Natural gas, coal and oil are the main fuel sources and are combusted on site. The energy required is largely dependent on the quality of the raw material, with boehmitic or diasporic bauxites requiring higher temperature digestion, often associated with a higher fuel input, while gibbsite generally requires a lower temperature digestion and usually a lower fuel input. In an alumina refinery, a cogeneration facility, or combined heat and power (CHP) wherein fuel is combusted to generate both electricity and useful heat simultaneously provides all the electricity needed to power the refining process and supporting systems, such as lighting and offices. The waste heat from the generator is captured and used to produce steam for the refining process. The CHP plant is sometimes designed to produce surplus electricity for export to local communities, a local customer or to the grid.

Energy is also used in the drying process. Here calciners are used, most refineries now use energy efficient stationary calciners (also known as bed or fluid-flash), although some less efficient rotary calciners are still used at older operations.

Scope 1 (Direct) includes all energy used in the digestion and calcining operations, plus on-site power generation.

Scope 2 (Indirect) GHG emissions are generated at sources owned or controlled by another organisation. This is generally electricity purchased by the refinery from the grid. However, most refineries generate electricity onsite which is therefore incorporated in Scope 1 emissions.

Primary smelting

The GHG emissions from smelting using the electrolytic reduction process stem from the manufacture of the carbon anode, electrolysis and casting. For many smelters, the major source of emissions are those produced through the generation of electricity utilised by the smelter. This may be self-generated or purchased from the gird. Broadly speaking, hydroelectric power dominates in Canada, Brazil, Norway and Russia, gas in the Middle East and coal in Australia, India and China.

Scope 1 (Direct) includes all energy used in on-site anode manufacturing, where CO2 emissions result from fuel used for the baking process and also from process emissions due to the combustion of pitch volatiles from the anode and packing coke. During the electrolysis process CO2 emissions occur due to the carbon anode consumption and further direct PFC emissions resulting from anode effects. Energy used in the casting stage is also included. Emissions from company owned power plants are also included.

Scope 2 (Indirect) GHG emissions are from electricity purchased by the smelter from the grid or from a dedicated power plant. Upstream Distribution emissions for aluminium smelting are generally reported as Scope 3 (Other Indirect) Greenhouse Gas Emissions. These typically include freighting of bauxite from mine to refinery and refinery to smelter via rail, trucking, barging, pipeline or ocean freight. Estimates for these emissions where not owned by the aluminium company have been estimated and included in the accompanying analysis. Estimates are based on tonnes shipped per kilometre - appropriate CO2e factors have been applied depending upon the transport method, size of vessel and regional variation. Many producers are making significant efforts to use renewable energy, including electrification of mining fleet and rolling stock and introduction of LNG fueled ships.