Globally, and despite declining ore quality, the alumina refining industry reduced the energy intensity of its operations by 9% between 2006 and 2010, while increasing production by 15%.
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Aluminium is an “energy bank”; most of the original input can be recovered every time the product is recycled.
Bauxite mining requires relatively low energy inputs, compared to other steps in the aluminium production process – with less than 1.5 kilograms of fuel oil (mainly in the form of diesel for haul trucks) and less than 5 kWh of electricity consumed per tonne of bauxite extracted.
The bauxite refining process requires significantly higher 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 by the Bayer Process is very much dependent on the quality of the raw material, with böhemitic or diasporic bauxites requiring higher temperature digestion, often associated with a higher fuel input. Investments in cost effective technology upgrades at existing facilities can improve the energy efficiency with no change in input material, as can “sweetening” of the feedstock with small quantities of higher quality bauxite. Such improvements, along with the addition of new, best available technology, refining capacity has driven an almost 10% improvement in global refining energy efficiency in just 5 years. Today, the average specific energy consumption is around 14.5 GJ per tonne of alumina, including electrical energy of around 150 kWh/t Al2O3.
Cogeneration or combined heat and power (CHP), wherein fuel is combusted to generate both electricity and useful heat simultaneously, is increasingly being employed in refineries. While a significant capital investment is required to build a CHP plant, there can be significant benefits, both in terms of energy efficiency and as a valuable resource for local communities. In an alumina refinery, a cogeneration facility provides all the electricity needed to power the refining process and supporting systems (such as lighting, offices etc). 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. In some instances, excess or lower quality steam can also be exported.
The greenhouse gas emissions from alumina production are predominantly related to fuel combustion; therefore improved energy efficiency along with fuel switching, where viable and appropriate, is the primary means of reducing the greenhouse gas intensity of refining processes, which currently stands at around 1 tonne of CO2e per tonne of alumina produced.