The world’s aluminium smelters consume about 3.5% of the total global electric power. Globally, the aluminium industry emits around 450 MT of CO2-equivalents annually (around 1% of world’s total emission). These numbers are growing because an increasing share of the aluminium production is derived from electricity from fossil fuel. With the global demand for energy increasing steadily, and also with rising energy cost and increasing greenhouse gas emissions, energy saving in all parts of the production process will continue to be an important task for aluminium smelters in the coming years. Power being the single most important differentiator of cost position, due to the volatile LME prices of aluminium, cost cutting in aluminium smelting has been a major area of research.
There are reasons to why Aluminium Smelting is not a very energy efficient process. The cell resistance is high due to ohmic electrolyte and gas bubble resistances, plus ohmic resistances in the anodes and cathodes. The Anode-Cathode-Distance (ACD) must be kept above a certain minimum distance to avoid the back reaction of aluminium with CO2. Heat losses are necessary to maintain a frozen side ledge to protect the side walls, so extra heat has to be wasted.
In spite of the technical challenges, the process can be fine-tuned to optimum levels by reducing the cell specific energy consumption. It is necessary to significantly lower the heat losses dissipated by the cell’s external surface, viz., anode cover, anode conductors, shell sides and cathode conductors. Apart from these, there are several ways to reduce the cell voltage by design changes:-
- Larger anodes and/or larger and modified anode stubs and yoke
- Slotted anodes for better gas bubble drainage, reducing the anode effect
- Better anode rodding procedures to minimize external voltage drops
- Changes in current collector bar design and larger dimension (use of copper in the bars)
- Casting of cathode bus bars instead of ramming to obtain better contact resistance
- Modification of side lining from carbon to SiC
- Better side lining and steel shell ventilation
- Improved magnetic field compensation
- Conductor redesign and making a trade-off between voltage reduction and heat dissipation decrease
Many of the world’s leading smelters have already anticipated and signed up for the same. China, being the leading producer of Aluminium, is playing a substantial role in Energy Reduction Technologies. China Aluminium International Engineering Corporation Limited, also referred to as CHALIECO, is making significant contributions in energy reductions in leading smelters worldwide.
The whole subject of energy reduction encompasses the culmination of many entities. Simulations of these entities vary with every pot, and they need to be customized accordingly. Work is being carried out on simulation of the pot operation to fine-tune it to optimum levels. The lining design of bottom and side of the cell is being revamped based on expected temperature distribution models to obtain ideal techno-economic index. Cathode assembly and collector bars are being redesigned to account for horizontal current reduction, hence establishing pot stability.
Addition to this, an Integrated and Intelligent Control System for promoting energy efficiency in aluminium smelters is also being incorporated. The new generation MPPIC System and its supporting energy saving technology by CHALIECO has allowed to obtain substantially energy saving benefits and saved the cost of production along with automation improvements. It establishes a dynamic equilibrium between Superheat, Bath Temperature, Liquidus Temperature, Alumina Concentration, Noise Level, tapping quantity and AlF3 measurements. The following relationship exists between the parameters as depicted below:-
Technologically, the present aluminium production process can be a close-to-zero greenhouse gas producer. The first step, which is actually ongoing, is to focus on lower specific energy consumption as already discussed, and also to eliminate the occurrence of anode effects. Furthermore, it is possible to reduce the inherent production of CO2 by reducing the net carbon anode consumption, although this reduction can only be perhaps 10% or even less with the existing carbon anode technology. Here, an inert anode, if such a material can be developed for use in industrial aluminium production, would represent a remarkable technological breakthrough, because then oxygen is formed at the anodes instead of CO2. On the contrary, another alternative process, carbothermic production of aluminium, would increase the CO2 emissions if the produced CO is not captured and stored.
A natural step to save energy in the present electrolysis process would be to recover energy from the main heat loss sources of the cells, the cathode sidewalls and the anode gas exhaust systems. A future step may be CO2 gas capture and sequestration related to the electric power generation. Finally, collection and cleaning of the CO2 from the electrolysis process itself may perhaps be a technical possible scenario in the future.