Thermoeconomic evaluation of waste heat recovery system in aluminium smelters using a parallel two-stage organic Rankine cycle

Abstract

The primary aluminium industry stands as one of the most energy-consuming and, at times, the most inefficient, with approximately 50 % of energy being lost in the form of waste heat. The multiplicity of wasted heat sources in aluminium smelters presents a challenge in how to recover and integrate them, given their variations in both quantity and temperature levels. In this context, the study adopts the Parallel Two-stage Organic Rankine Cycle (PTORC) to separately integrate the wasted heat from the cathode sidewalls and the exhaust gases within a unified recovery system. The influence of primary and secondary evaporation temperatures, their pinch points, the number of integrated aluminium pots, and the working fluid on the thermodynamic performance and economic feasibility of PTORC are examined. At a given design condition, the findings indicate that decreasing the primary evaporation temperature while increasing the secondary evaporation temperature achieves the optimal operating condition of the system, resulting in a significant improvement in both output power and economic performance, while also reducing exergy destruction. At the primary evaporation temperature of 111.5 °C and the secondary evaporation temperature of 78.5 °C, the net output power reaches the optimal value of 3,840 kW. Furthermore, maintaining a lower pinch temperature difference in both evaporators proves advantageous for enhancing PTORC performance. Pentane, R236ea, and isopentane demonstrate outstanding maximum net power output at a constant secondary evaporation temperature, respectively. Meanwhile, R236ea and isobutane emerge as the most suitable working fluids for PTORC from an economic standpoint.