Heat transfer characteristics of direct solid-solid heat exchange for waste heat recovery from high-temperature slags

The moving bed with direct solid-solid heat exchange has become one of the most promising methods for waste heat recovery from high-temperature slags produced in industry production processes. However, the lack of deep understanding of heat transfer control mechanisms, reliable experimental data and accurate heat transfer prediction models limits its technological development and engineering application. In this study, the heat transfer characteristics of steel slags in a moving bed heat exchanger were experimentally investigated under conditions representative of industrial practice, including a wide particle size distribution of 0–30 mm, solid temperatures of 300–700 °C, and particle descending velocities of 0.15–0.9 mm/s. Key thermophysical properties of the slags were characterized prior to the experiments. The results show that the bed-to-wall overall heat transfer coefficient ( h overall ) increases with rising solid temperature and particle descent velocity, while decreasing with increasing particle diameter. The h overall ranges from approximately 20 to 60 W/(m 2 ·K), depending on operating conditions. Existing empirical correlations were found to predict the experimental data with large relative errors often exceeding 20%. Thus, a mechanism-based heat transfer prediction model was developed. The proposed model shows good agreement with experimental data, with an average prediction error of 11%. Moreover, the intrinsic dominant heat transfer mechanisms and the corresponding shift in governing mechanisms under varying conditions were systematically revealed. This work not only provides a validated prediction model for engineering applications, but also offers theoretical insights into the governing heat transfer mechanism underlying the key parameter variations, thereby supporting both academic advancement and practical engineering implementation.