(310h) Decarbonizing the Production of Primary Aluminium Using Renewable Resource

Aluminium is the second most widely used metal in the world, with a variety of applications ranging from transportation to food packaging and construction. While many aluminium producers target increased utilization of recycled aluminium to produce rolled aluminium sheets, the need to continuously supply a fraction of pure aluminium persists in order to maintain product quality. In fact, with the continuously expanding industrial activities in many sectors the demand for primary aluminium is expected to increase by 34% in the coming 20 years. Production of primary aluminium is a highly energy intensive process with its smelting contributing by 8% of all worldwide industrial electricity consumption [1].

Aluminium production starts with the mining step of bauxite ore (a mixture of Al₂O₃.3_H2O, Fe₂O₃, and SiO₂), followed by its refining through the Bayer process to alumina (Al₂O₃.3_H2O). Molten aluminium (Al_(l)) is then extracted from alumina via an electrolytic smelting technique called the Hall-Heroult process, the most carbon intensive step in aluminium production. This process requires a huge amount of electricity (~13 – 15 kWh/kg Al) to overcome the strong chemical bond between aluminium and oxygen and relies on the use of a carbon anode as the reducing agent [2].

The electricity source used in the smelting step significantly contributes to the indirect carbon emissions of the process due to the huge multiplication factor associated with the consumption rate of electricity. A shift to a renewable electricity source such as hydropower or solar electricity can offset that effect. On the other hand, direct emissions from the smelting process related to the oxidation of the carbon anode during the Hall-Heroult process are a major source of CO₂ impacting the entire aluminium value chain and considered unavoidable based on current commercial technologies. More than 400 kg of carbon anode is consumed per tonne of aluminium produced, resulting in the formation of more carbon dioxide than aluminium (approximately 1.5 tonne CO₂ / tonne of aluminium). The expected value from the balanced chemical reaction Eq. (1) is approximately 1.22 tonne CO₂ / tonne Al suggesting the remaining emissions are due to process inefficiencies. By simulating the reduction process using Aspen plus® modelling tool the exergy of the system is found to be approximately 5.0 kWh/ kg Al based on pure component availabilities (H-T₀S). This value is roughly half that reported in literature for actual electricity consumption of modern day smelters, suggesting a process efficiency of ~50%. [3]_.

In order to eliminate the direct emissions resulting from primary aluminium production and mitigate the aluminium industry’s effects on global warming potential novel methods in reducing alumina must be adopted. Current research efforts and testing investments have focused on the development of inert anodes to reduce alumina following the chemical reaction in Eq. (2). Materials such as ceramics, metal alloys, and cermats are all being considered as potential substitutes to the current carbon electrode [4].

A major challenge in selecting the suitable inert anode material lies in meeting the performance requirements under harsh working environment during electrolysis, while having satisfactory electrical conductivity, high resistance to thermal shock, and sufficient corrosion resistance for a reasonable lifetime. These material performance optimizations and potential high cost of supply represent persistent threats to the full-scale deployment of inert anodes in the aluminium smelting industry.

A more environmentally friendly solution evaluated in this work is the utilization of hydrogen or biogenic char as the reducing agents in the production of primary aluminium. Hydrogen is a non-polluting, renewably available resource that could be produced via water electrolysis using green electricity sources, or from biomass gasification. Both considered renewable resources of increasing relevance and importance in the global energy mix. If hydrogen replaced the carbon anode in alumina reduction, the chemical reaction would follow Eq. (3) producing only H₂O vapor as a reaction by-product and the net CO₂ emissions of the process would be drastically reduced.

Integrating the production of primary aluminium with a biomass gasification process for fuel production presents the opportunity of utilizing part of the char produced during biomass pyrolysis or the hydrogen produced during gasification. This carbon although behaving similarly from an aluminium industry perspective is carbon neutral in terms of CO₂ emissions due to is biogenic origin. Experimental studies have been reported for both hydrogen and biochar utilization in such applications for the aluminium and steel industries with sufficient data supporting the plausibility of these concepts [5, 6].

In this work a systemic process optimization approach through mass and heat integration is adopted to enable the use of hydrogen or biogenic char as potential replacements to fossil-based carbon anodes in the production of primary aluminium. Total site optimization and integration with a biomass gasification system is evaluated. These process alternatives would have disruptive technological impacts on reducing the overall carbon intensity of the aluminium industry. A sensitivity analysis on different electricity mix resources based on varying geographical locations is conducted to quantify the indirect CO₂ emissions of using renewable versus grey energy sources in the smelting process.

Figure 1 represents the Aspen flowsheet simulation for the use of hydrogen in the alumina reducing reaction and compares this value with reported literature references for experimental evaluations. The Gibbs free energy of the reaction Eq. (3) is estimated at approximately -250 kJ/ mol Al₂O₃ at 700°C using Aspen modelling software with its basic Rstoich model. This value is in very well agreement with experimental results in Figure 1 (b) and confirms that the reaction is spontaneous below 2000°C with negative Gibbs free energy.

Finally, Figure 2 shows the process flow diagram of the alternative routes considered in this study for the decarbonization of alumina smelting production. The primary aluminium produced is also integrated as a feed to an aluminium remelting foundry for which mixed integer linear programming algorithms are applied to solve the heat and mass integration problem. By reducing the carbon emissions and energy intensity of the alumina reduction step this enhances overall process efficiency and reduces scope 3 emissions for aluminium remelting facilities.

References

[1] Pedneault J, Majeau-Bettez G, Krey V, et al. What future for primary aluminium production in a decarbonizing economy? Glob Environ Change 2021; 69: 102316.

[2] Kvande H, Drabløs PA. The Aluminum Smelting Process and Innovative Alternative Technologies. J Occup Environ Med 2014; 56: S23–S32.

[3] Choate WT, Green JAS. U.S. Aluminum Production Energy Requirements: Historical Perspective, Theoretical Limits, and New Opportunities.

[4] He Y, Zhou K, Zhang Y, et al. Recent progress of inert anodes for carbon-free aluminium electrolysis: a review and outlook. J Mater Chem A 2021; 9: 25272–25285.

[5] Braaten O, Kjekshus A, Kvande H. The possible reduction of alumina to aluminum using hydrogen. JOM 2000; 52: 47–53.

[6] Huang X, Ng KW, Giroux L, et al. Interaction Behavior of Biogenic Material with Electric Arc Furnace Slag. Fuels 2021; 2: 420–436.