Coupling waste heat extraction based on phase change material with steam generation: evidence from steel industry

Waste heat recovery in steel industry represents one of the greatest opportunity to reduce the consumption of primary energy while increasing the sustainability of the steelmaking processes. One of the most important and challenging source of waste heat in steelmaking process is the off gas emitted by the Electric Arc Furnace (EAF), which represents about 30% of the total energy provided to the process. Due to the dynamic of the EAF process, the off gas parameters (temperature and mass flow rate) fluctuate intensively causing large variations in thermal power. Currently, the most modern heat recovery systems face these thermal power fluctuations generating saturated steam at around 220 °C and storing it in Ruth’s steam accumulator to feed an Organic Rankine Cycle (ORC). Since the average temperatures of the off gas are usually around 600 °C, in order to reduce exergy losses a better solution would be to generate superheated steam at higher temperature to directly feed a steam turbine. For this aim, high temperature phase change materials (PCMs) can be employed to reduce the fluctuating thermal power entering the steam generator. Previous findings have shown technical and economic performances of a PCM-based device able to smooth off gas temperature profiles. The introduction of a heat transfer fluid (HTF) to overcome overheating issues in PCM containers has also been investigated. In this paper, the opportunity of actively managing the HTF flowing through the PCM device to enhance the steam parameters level is investigated. To this end, the PCM-based device is coupled with a steam generator: the effects of both off gas temperature smoothing and extraction of stored heat in the PCM by HTF on steam parameters and performance of the related energy conversion system are analysed. An analytical model is implemented in Fortran90 to predict the thermal behaviour of the PCM device. The Fortran code is interfaced with the TRNSYS software to analyse the performance of the whole heat recovery system (PCM device, steam generator and steam turbine) and to develop a proper control strategy. All thermo-physical properties are estimated using the REFPROP 8.0 package. Calculations are carried out in transient regime, adopting off-gas flow rates and temperature profiles representative of real EAF processes. Finally, an economic performance assessment of the whole system is carried out.