Modeling and Analysis of Power-to-hydrogen based Alumina Hydrate Calcination for Low-carbon Aluminum Making

Abstract

The aluminum industry is a highly energy-intensive sector with high-temperature processes currently mainly driven by fossil fuels. The demand, production, and use of aluminum are increasing and so are the emissions. It is estimated that the aluminum industry consumes 95 MJ of energy and emits 11.5 tons of carbon dioxide (CO2) per ton of aluminum produced, resulting in 0.45-0.5 Gt of global CO2 equivalent emissions annually. Alumina smelting and refining aluminum are responsible for over 90% of aluminum production’s direct CO2 emissions, the rest being from recycled production, anode production, and casting. Although aluminum smelting is the most energy-intensive process step, the energy input is essentially electricity and can therefore be decarbonized. However, the Bayer process where alumina is produced from bauxite is thermal energy intensive due to the calcination step carried at up to 1100 °C, which is dependent on the use of fossil fuels. Climate change targets call for the development of smart strategies and technologies for not only decarbonizing electricity generation, but also heat supply. One of the most promising technologies to switch from fossil fuels in high-temperature industrial processes is power-to-gas (PtG). This process also provides the possibility to store surplus energy from intermittent energy sources in the form of hydrogen or synthetic natural gas. Since hydrogen combustion produces no carbon emissions, power-to-hydrogen (PtH₂) is a potential solution to significantly reduce alumina hydroxide calcination emissions depending on technical, cost, and retrofit considerations. The objective of this Thesis is to reduce greenhouse gas (GHG) emissions in the calcination process through the use of PtH₂ in place of conventional fuels. This is achieved using process modeling to evaluate the energy consumption and emissions of PtH₂-based calcination in comparison with natural gas-based calcination. Steady-state mass and energy conservation are applied to each system component. The thermodynamic model is validated by comparison of stream properties and overall energy consumption and emissions with reference data for a conventional natural gas-based calcination process. The energy consumption and emissions of the low-carbon hydrogen-based process are compared with that of the conventional natural gas-based calcination process. The energy consumption of the low-carbon hydrogen-based calcination process, 2.8068 GJ per ton of alumina (equivalent to 5.3462 GJ per ton of aluminum), excluding water electrolysis for hydrogen production, is similar to that of the conventional process under the modeling assumptions made. In parallel, at an electricity emission factor of for example 10 gCO2 per kWhe, calcination emissions are almost eliminated (i.e., reduced by 0.1554 tCO2 per ton of alumina, equivalent to 0.296 tCO2 per ton of aluminum (13.8% of total conventional system emissions). Sensitivity analysis reveals that alumina calcination energy consumption for both the conventional and hydrogen-based process is influenced by the efficiency of the alumina direct cooling stages. The total emissions from the low-carbon hydrogen system are primarily sensitive to the electricity emission factor. At an electricity emission factor of 40 gCO2 per kWh or lower, the H2-based calciner and aluminum smelting systems have lower emissions than the conventional system.

Date of Award	20 Dec 2023
Original language	American English
Supervisor	VALERIE Eveloy (Supervisor)