The carbon anode, used in aluminum electrolysis (Hall–Heroult process), is over-consumed by air oxidation and carboxy-reaction (with CO2). Several anode features may affect this over-consumption, such as impurity content, graphitization level and anode porosity features (e.g., porosity volume fraction or pore size distribution). The two first parameters are basically related to the quality of raw materials and coke calcination conditions. Anode porosity is, however, greatly affected by anode manufacturing conditions, and is possible to be modified, to some extent, by adjusting the anode recipe and the processing parameters. This work aims to investigate the effect of anode porosity on its air reactivity. Baked anode samples were prepared in laboratory scale and then crushed into powder form (−4760 + 4000 µm). The recipe for anode preparation was similar to a typical industrial recipe, except that in the lab scale no butt particles were used in the recipe. Anode particles were then gasified at six different conversion levels (0, 5, 15, 25, 35 and 50 wt %) under air at 525 °C. The porosity was characterized in several pore size ranges, measured by nitrogen adsorption and mercury intrusion (0.0014–0.020, 0.002–0.025, 0.025–0.100, 0.1–40.0 and superior at 40 µm). The volume variation of each pore range, as a function of carbon conversion, was assessed and used to determine the size of the most active pores for air oxidation. The most active pore size was found to be the pores inferior at 40 µm before 15 wt % of gasification and pores superior at 40 µm between 15 and 50 wt % of carbon conversion. Limitation of pore size range could be used as an additional guideline, along with other targets such as high homogeneity and density, to set the optimum anode manufacturing parameters.

Greenhouse gas (GHG) accounting in industrial plants usually has multiple purposes, including mandatory reporting, shareholder and stakeholder communication, developing key performance indicators (KPIs), or informing cost-effective mitigation options. Current carbon accounting systems, such as the one required by the European Union Emission Trading Scheme (EU ETS), ignore the system context in which emissions occur. This hampers the identification and evaluation of comprehensive mitigation strategies considering linkages between materials, energy, and emissions. Here, we propose a carbon accounting method based on multilevel material flow analysis (MFA), which aims at addressing this gap. Using a Norwegian primary aluminum production plant as an example, we analyzed the material stocks and flows within this plant for total mass flows of goods as well as substances such as aluminum and carbon. The results show that the MFA-based accounting (i) is more robust than conventional tools due to mass balance consistency and higher granularity, (ii) allows monitoring the performance of the company and defines meaningful KPIs, (iii) can be used as a basis for the EU ETS reporting and linked to internal reporting, (iv) enables the identification and evaluation of systemic solutions and resource efficiency strategies for reducing emissions, and (v) has the potential to save costs.

Comparison of the Life Cycle Analysis of inert electrodes and the Hall-Heroult process in aluminum production Bethany Bronkema bethanyb@ru.is Guðrún Arnbjörg Sævarsdóttir gudrunsa@ru.isDavid C. Finger davidf@ru.is To be presented orally at EGU2023 – April 23rd-28th.Reykjavik University, School of Science and Engineering, Department of Engineering, Reykjavik, Iceland  The global production of pure aluminum consumes substantial amounts of energy and alone produces around 1.1 billion metric tonnes of carbon dioxide emissions (CO2, eq) each year, or around two percent of global emissions. The Hall-Heroult process is currently the only industrial process for primary aluminum production, producing up to two tonnes of CO2 per tonne of pure aluminum by electrolysis in a molten salt electrolyte using carbon anodes. However, the use of inert electrodes represents a low-carbon alternative to the Hall-Heroult process as direct emissions can be significantly reduced, lowering the CO2, eq footprint and the ecotoxicity of aluminum production. However, a transition to inert anodes implies a redesign of current electrolysis cells to optimize the energy requirement of the new process. In this study, we performed a life cycle analysis to compare the ecological footprint of the aluminum production process with inert electrodes and the Hall-Heroult process. The life cycle assessment was conducted using GaBi software linked to the ecoinvent database and complemented with primary data. We calculated the ecological footprint for five scenarios: i) using inert electrodes with a 13.5 kWh per kilogram of aluminum energy requirement, ii) using a 17 kWh per kilogram of aluminum energy requirement, iii) using Icelandic grid electricity (primarily renewable hydropower), iv) using a global energy mix (primarily based on fossil energy), and v) and “best case scenario” in which a renewable source of energy is assumed for the refinement stage preceding the smelting stage. Each of these scenarios were then compared with the ecological footprint for the Hall-Heroult process using carbon anodes. The preliminary results reveal that the energy mix always has the highest impact on the ecological footprint in the earlier refinement and electrolysis stages. However, using inert electrodes in smelters powered with renewable electricity can significantly lower the carbon footprint and ecotoxicity of aluminum production.