(185c) Integration of renewable energy and reversible solid oxide cells towards decarbonizing the secondary aluminium production and urban systems

Secondary aluminium is responsible for 2% of industrial emissions, as it depends on natural gas [1]. Efforts are sought to reduce fossil fuel dependency increasing renewable energy (RE) share, using advanced energy systems and untaping savings potentials, like waste heat from furnaces stacks or casting water [2]. A combination of technologies including optimal storage, oxycombustion, carbon capture, power-to-gas (PtG), biomass conversion and electrification are devised to improve process integration and resiliency [3, 4]. In this work, a scenario of waste heat valorization and RE integration is presented, in which waste heat recovery from aluminium plant is used to reduce energy input for scrap metal preheating [2, 5] and space heating purposes [6]. A high temperature reversible solid oxide cell (rSOC) produces power or renewable hydrogen, which can be combined with biogenic CO₂ to produce synthetic natural gas to replace fossil fuel in the aluminum plant [7]. Electricity generated is used in electrical furnaces, rolling processes, and heat pumping systems of district heating network (DNH). Tail gas from rSOC partly supplies heat to melter furnace. These approaches showed promising potential to attain negative emissions and add flexibility to energy and carbon management (Figure 1) [8].

In the aluminium plant, pure aluminium is preheated (250°C) and melted together with scrap aluminium (660°C). Direct chill casting (DCC) solidifies aluminium into ingots producing hot water (50 °C). In the advanced scenario, casting heat is integrated into a CO₂ DHN (~50 bar, 14°C) composed of a closed loop that gathers waste heat from the aluminium plant, biomass conversion and PtG systems and supply space heating or domestic hot water to a nearby urban system. In rolling plant, ingots are treated in pusher furnaces (550°C), hot and cold rolled, and annealed in a continuous line (ACL). In the gasifier (900°C), biomass is converted into syngas using steam [9], which is compressed and purified to produce hydrogen or synthetic natural gas. Apart from CHP production, waste heat is used to generate steam to feed a solid oxide electrolyzer (SOEC, 750°C, thermoneutral, 1.29 V) that produces hydrogen and oxygen [10]. PtG is expected to play a role in harvesting renewable electricity into storable chemicals and fuels [11]. rSOC also works as fuel cell (SOFC, 750°C) consuming hydrogen and oxygen (or air) to produce electricity [12], used to drive other heating furnaces and appliances in aluminium plant, as well as DHN heat pumps [13]. Thus, the number of decarbonization options increases, requiring a systemic analysis to evaluate the most suitable and carbon neutral technologies to supply the energy requirements. The systems are modeled via Aspen ® software or Coolprop, ensuring constraints defined by the minimum energy requirements. Table 1 compares energy consumption remarks of the conventional scenario (i.e. natural gas-fired furnaces and biomass boiler-based DHN) and the integrated solution proposed to decarbonize the aluminium plant and the city demands. A MILP problem is conducted using OSMOSE [14] to maximize waste heat recovery and define favorable configurations, considering the temporal variation of the prices and energy demands.

The integrated scenario (Figure 1) consumes 27% more energy than the conventional scenario, because biomass is converted internally. Yet, conventional scenario is dependent on fossil gas, subject to volatile market conditions and increasing surveillance, due to direct and indirect atmospheric emissions. It is worth noting an opportunity to increase the share of RE in aluminium industry and urban systems by importing nine times more electricity from the grid. In rSOC, the power consumption during the electrolysis mode averages 57 MW, whereas fuel cell power generation is 15 MW. rSOC together with storage systems help managing CO₂ and SNG flows. CO₂ produced and captured (see Table 2) can be later used to generate SNG that is stored in summer season and consumed later in winter season. By harnessing electricity in summer, total biomass energy consumption drops by 25% of the total amount consumed in inefficient biomass boilers. Table 2 summarizes the CO₂ emissions balance including direct and indirect emissions for both studied scenarios. Net CO₂ emissions of the conventional scenario can be reduced more than five times if an integrated carbon management and sequestration system is employed. Captured and injected biogenic CO₂ allows offsetting indirect emissions associated to the supply chains of electricity and biomass.

Figure 2 illustrates seasonal storage profiles for SNG and CO₂ gases. Loading of SNG tanks occurs during summer season when surplus electricity from RE resources is available. It renders more attractive to harvest electricity and store it in the form of fuel to be used later in the colder season. Biomass resource harvested during the summer season can be used to produce syngas or synthetic natural gas consumed in the SOFC. The CO₂ gas is used to refill the storage tanks that store CO₂ until is reacted in a methanation section to produce SNG in summer season. The benefit to store RE in the form of gas is twofold: biomass consumption is drastically reduced and biogenic CO₂ produced is used to offset the CO₂ losses. Annualized investment and operational costs calculated for the conventional and integrated systems, are summarized in Table 3. Total cost of advanced technologies is 28% higher than in the conventional route, due to twenty times higher annualized investment. Yet, the annualized operational expenditure of the conventional solution is higher (11%), due to cheaper biomass resources and use of electricity during the summer season to produce and store fuel using biogenic CO₂.

References

[1] IAI. Aluminium Sector GHG Pathways to 2050, At: https://international-aluminium.org/resource/aluminium-sector-greenhouse... (25 March 2023).

[2] Kumar RA, et al. A Method for Recovering the Waste Heat to Achieve Overall Energy Conservation in Aluminum Casting Industries. Recent Advances in Energy Technologies. Singapore: Springer, pp. 331–342.

[3] Yu M, et al. Waste Heat Recovery from Aluminum Production. In Energy Technology 2018. Springer International, pp. 165–178.

[4] Clos, D, et al. Enabling Efficient Heat Recovery from Aluminium Pot Gas. In Light Metals 2017. Springer International Publishing, pp. 783–791.

[5] Arink T. Metal Scrap Preheating using Flue Gas Waste Heat. Energy Procedia 2017; 105: 4788–4795.

[6] Stijepovic MZ, et al, Optimal waste heat recovery and reuse in industrial zones. Energy, 36, 7, pp 4019-4031.

[7] Borge-Diez, D, et al. Analysis of Power to Gas Technologies for Energy Intensive Industries in European Union. Energies 2023; 16: 538.

[8] Florez-Orrego D. et al, A systemic study for enhanced waste heat recovery and renewable energy integration towards decarbonizing the aluminium industry. 36th ECOS 2023, June 25-30, Canaria, Spain.

[9] Flórez-Orrego D, et al. Comparative exergy and economic assessment of fossil and biomass-based routes for ammonia production. EnConvMan 2019; 194: 22–36.

[10] Fan L et al. Advances on methane reforming in solid oxide fuel cells. Renewable and Sustainable Energy Reviews 2022; 166: 112646.

[11] Domingos M, et al. Sustainable Methanol Production Using a Multi-Time Energy Integration and CO2 Management Approach as an Energy Carrier for Synthetic Natural Gas Synthesis. AIChE Annual Meeting. Orlando USA: AIChE, 2023.

[12] Salomone, F, et al. Techno-economic modelling of a Power-to-Gas system based on SOEC and CO2 methanation in a RES-based electric grid. Chemical Engineering Journal, 377 , 2019, pp 120233.

[13] Jensen SH, et al. Large-scale electricity storage utilizing reversible solid oxide cells combined with underground storage of CO2 and CH 4. Energy&EnvironmentalSci, 2015; 8: 2471–2479.

[14] Flórez-Orrego D, Ribeiro Domingos M., Maréchal F. A systematic framework for the multi-time integration of industrial complexes and urban systems, 7th CPOTE 2022, Warsaw, Poland, 20-23 September, 2022.