Enhanced waste heat recovery from hot-rolled aluminium coils: numerical modelling, system design, and utilization potential

In secondary aluminium production, hot rolling of annealed ingots into coils generates waste heat that must be dissipated. Following hot rolling, the coils must be cooled from approximately 400 • C to around 80 • C before they can be cold rolled. This untapped waste heat recovery potential accounts for approximately 20% of the total energy consumption in the rolling plant. Therefore, enhanced waste heat recovery from cooling aluminium coils must be investigated, along with the opportunities to upgrade waste heat. Computational fluid dynamics simulations of the heat transfer between coils and cooling air under various operating conditions are conducted to determine relevant thermal parameters, such as metal and fluid temperatures. Although increasing the cooling speed of the coils enables an increased throughput of the product from the rolling plant, it also leads to a condition of low temperature (low grade) waste heat recovery. Accordingly, the proposed configuration and selected operating condition maximizes the waste heat recovery and the exergy efficiency of the cooling system. Thermal parameter values, together with the geometrical characteristics of the coil, have been used in a datadriven approach to correlate the heat transfer coefficient for different transient regimes, based on dimensionless numbers, to reduce computational time. Next, a tunnel cooling system is designed to handle 90 coils per day to assess the waste heat recovery, as well as the waste heat utilization opportunities based on energy, environmental, and economic indicators. As a result, the coil cooling time is reduced from two days in regular operation (under natural cooling) to 18 h using the proposed forced cooling tunnel. The associated cooling air stream recovers around 79 kWh of heat per tonne of aluminium processed, amounting to 84% of the initial energy content of the hot coils. Recovered heat can be used in a transcritical CO 2 cogeneration cycle integrated with a heat pump system and subsequently cascaded into the district heating network of an urban system. The low-grade waste heat recovered contributes 7.6% of the total city heating demands and avoids 43 kgCO 2 /MWh associated with the heating of the urban system, when compared to the use of natural gas.