Simulation and optimization of waste heat recovery process in coiled ascension pipes of coke ovens

This study investigates a newly installed coiled-tube heat exchanger integrated into the ascending pipe of a coke oven at a steel plant. A novel two-phase numerical model was established on the Fluent platform to enhance the accuracy of numerical simulations, with simultaneous consideration of the transient thermophysical properties of raw coke oven gas (RCOG) and the phase change process of the working water. This model was used to systematically investigate the effects of three key parameters – RCOG inlet velocity (0.84–6.56 m/s), inlet temperature (650–800°C), and working fluid inlet velocity (0.01–0.20 m/s) – on the heat transfer efficiency of the exchanger and the tar deposition characteristics on the pipe wall. The results indicate that RCOG inlet temperature is the primary factor determining heat transfer efficiency, and a reasonable increase in RCOG inlet velocity and temperature can significantly reduce tar accumulation on the inner surface of the ascending pipe. The optimization process determined the following optimal operating conditions: RCOG inlet velocity of 2.99 m/s and inlet temperature of 750°C, and working water inlet velocity of 0.14 m/s. Under these conditions, the system’s waste heat recovery efficiency is enhanced by approximately 12% compared with conventional operating conditions, and tar deposition is effectively suppressed. Furthermore, a significant temperature gradient was observed in the high-heat-flux region within the first 0.9 m of the ascending pipe, which may lead to localized thermal stress concentration. To reduce the risk of thermal damage, it is recommended to incorporate localized enhanced cooling or thermal compensation structures in engineering applications. The conclusions of this study provide theoretical support and engineering guidance for the optimal design and safe, efficient operation of waste heat recovery systems in coke oven ascending pipes.