Design Optimization of Waste Heat Recovery System around Cement Rotary Kiln

Portland cement is produced by one of the highest energy-consumptive industrial processes. Within the process, the rotary kiln represents one of the major sources of thermal energy loss. Based on the lengthwise temperature profile of the kiln, an optimal placement for heat recovery is identified based on the highest surface temperatures. This study aims to optimize the design of an arc-shaped thermal absorber parallel to the rotary cement kiln for heat recovery by thermoelectric generators (TEGs). A comprehensive numerical study is carried out by considering the combined effects of convective heat transfer, thermal radiation, and rotation of the kiln to find the optimum properties of the thermal absorber. Thus, a two-dimensional (2D) incompressible and unsteady turbulent flow horizontally perpendicular to the kiln is investigated. Different parameters such as curvature radius, arc angle, and angular position of the absorber, temperature of the kiln surface, wind velocity, and kiln rotational speed are studied for optimal design of the absorber. Multiplication of the length and average temperature of the absorber is a conceptual definition applied to explore the optimum design. On the other hand, power generation by using several commercial TEGs (size of 56×56  mm2) is evaluated for all studied absorbers. Given the kiln surface temperature of 500°C at an optimal position along the kiln, the results show that the case with the absorber radius of 2.5 m has the best performance of studied cases and can generate a total power between 26.449 and 48.889 kW, corresponding to the kind of studied commercial thermoelectric modules.