Multi-objective optimization of cascade waste heat recovery in a grate-kiln-annular cooler pelletizing system integrating a waste-heat boiler and organic Rankine cycle

Efficient recovery of medium- and low-grade waste heat is critical for reducing energy consumption and carbon emissions in iron-ore pelletizing. However, its application in grate-kiln-annular cooler pelletizing systems remains constrained by limited heat-source temperature levels and the inherent internal gas-recirculation structure. This study develops a full-process heat-integration model for a 2 Mt/y pelletizing system and proposes two retrofit schemes for graded waste-heat recovery. Scheme I introduces a waste-heat boiler to recover exhaust gas from annular-cooler stages II and III for steam generation, while Scheme II further integrates an organic Rankine cycle to convert main-exhaust-fan tail gas into electricity. Sensitivity analyses are conducted for stage-II extraction ratio, stage-III extraction ratio, and boiler pinch temperature difference. The annual carbon reduction, net income, and system exergy efficiency are simultaneously optimized using the NSGA-II algorithm. Results indicate that stage-II extraction plays a dominant role in steam recovery due to its higher temperature grade, whereas excessive stage-III extraction compromises internal heat recovery and exergy utilization. The optimized coupled scheme achieves 14361.40 tCO 2 /y carbon reduction, 357,116.77$/y net income, and 32.69% exergy efficiency, demonstrating an effective pathway for enhancing waste-heat utilization through integrated steam recovery and low-grade tail-gas power generation.