Optimization of System Operation and Heat Exchanger Sizing in Rankine Cycles - a case Study on Aluminium Smelter Heat-to-power Conversion

orc2019 Tracking Number 174

In industry processes worldwide, large amounts of heat in the temperature range 125-250 °C are rejected to the ambient. Direct re-use or upgrading is the most efficient and cost-effective utilization of this surplus heat. However, lack of sufficient local heat demand can make heat-to-power conversion an attractive option for significant surplus heat utilization. Cost-effective heat-to-power conversion is challenging in the temperature range considered and optimizing system design and operation is therefore important. This work describes optimization of heat exchanger sizing and system operation of an ORC applied to heat-to-power conversion from aluminium smelter off-gas. The scenario involves 850 kW heat duty available at a temperature of 150°C. The overall system impact of considering heat source pressure drop was investigated in detail, as well as optimal distribution of heat transfer surface area between individual heat exchangers (HXs) in the system. The ORC system model is relatively detailed, including geometrically described HXs discretized along the flow direction with local evaluations of fluid states, heat transfer coefficients and pressure gradients. The system was optimized with net power as objective function and by simultaneously optimizing process conditions and HX geometries. The total HX surface area was constrained to 750 m2 during optimization. The effect of heat source pressure drop was investigated by calculating the fan work corresponding to the heat source HX pressure drop. Optimizing the system without accounting for the heat source pressure drop resulted in a net power production of 118 kW. Re-optimizing the system with a fan work penalty on pressure drop reduced net power by 21 %. The reduction was mostly caused by a lower heat transfer coefficient and higher exergy destruction in the heat recovery heat exchanger (HRHE). Optimizing heat transfer area distribution showed that the HRHE should account for 68 % of total HX area to maximize net power in a scenario with no penalty on heat source pressure drop. The large HRHE area requirement was caused by a poor heat transfer performance on the gas side of the HX. The fan work penalty on heat source pressure drop further increased HRHE area requirement to 76 % of total HX area.