Waste Heat Recovery from the Aluminium Casthouse: Comparison of Thermochemical and Thermomechanical Routes

Aluminium casthouses discharge large fractions of their input energy as low– to high–grade waste heat from flue gases and direct–chill (DC) cooling water. Conventional lines rarely implement systematic recovery beyond basic air preheating, leaving megawatts of continuous heat unutilised and increasing fuel and emissions intensity. For this reason, the f irst aim of the thesis is to evaluate the possibility of recovering heat from the primary cooling stage in DC casting using methanol, reaching its boiling point. Methanol was chosen for its low boiling temperature a hydrogen’s storage and handling challenges motivate liquid e-fuels such as methanol to couple heat recovery with on-site fuel synthesis or use. This thesis studies thermochemical and mechanical routes to valorise casthouse waste heat and to co-generate useful energy, while enabling fuel switching and CO2 management in furnaces. A framework for heat recovery was developed in this study, consisting of thermochemical recovery via methanol steam reforming (MSR) using recovered heat at ∼ 200–300◦C, feeding the reformate to the melter, substituting natural gas. In particular, air-blown and oxy-fuel melting furnaces based processes were compared, showing the advanatges and disadvantages of the two different configurations. In order to recover heat more efficiently, a mechanical/thermal recovery has been proposed to exploit flue-gas high temperature after powering the steam reforming operations. The thermochemical process includes feed preheating, catalytic methanol steam reforming in a multitubular reactor and routing the H2-rich reformate to the melter, avoiding natural gas consumption; oxy-fuel variants use internal CO2 recycle for temperature control and capture readiness. For the final heat integration, a supercritical CO2 power cycle, and district-heating exchangers have been proposed. The studied case shows that high-temperature flue-gas recovery can fully supply the reformer duty at ∼275◦C while sustaining air preheat and a small sCO2 bottoming cycle. Across representative lines, the integrated thermochemical route reduces net fossil fuel use, enables partial CO2 capture/readiness, and increases energy self-sufficiency; the mechanical route recovers additional work and heat, enhancing the plant energy efficiency. In conclusion, recovering heat from the casting operation represents a first step to run a thermochemical heat recovery, while casthouse flue-gas heat can drive methanol steam reforming at practical severity while maintaining safe furnace operation and providing a modest power bottoming cycle. Moreover, oxy-fuel variants facilitate temperature control and CO2 capture readiness, improving decarbonisation leverage when paired with thermochemical recovery.