Numerical investigation and optimisation of microwave-assisted continuous-flow preheating of waste cooking oil for pyrolysis applications

Microwave heating offers rapid, volumetric, and selective energy transfer, making it a promising alternative to conventional thermal methods across various industries, including food processing and energy recovery. Despite its potential, most studies have focused on batch systems or continuous-flow reactors with simplified geometries, limiting their applicability to real-world industrial processes. This study presents a novel microwave-assisted continuous-flow preheating system featuring inclined baffles within a tube placed inside a commercially available microwave cavity. Waste cooking oil (WCO) was used as a model feedstock to investigate the thermal and hydrodynamic characteristics of the preheating stage prior to pyrolysis. The analysis focused on stabilisation time, maximum achievable temperature, energy efficiency, and fluid residence time under different design configurations. A sensitivity analysis was conducted to assess the effects of baffle inclination angle and baffle spacing on system response. The results showed that increasing baffle spacing enhanced both energy efficiency and fluid residence time. Additionally, when the baffle spacing was 10 mm, stabilisation required more time at a 20° inclination angle (21.78 s) compared to 0° (11.05 s). Coupling computational fluid dynamics (CFD) simulations with polynomial regression techniques demonstrated strong agreement and effectively identified optimal design parameters. It should be noted that the present study is limited to non-reactive heating and does not consider pyrolysis kinetics, phase change, or product evolution. Nevertheless, the findings provide valuable guidance for the design and optimisation of microwave-assisted continuous-flow preheating systems and establish a foundation for future investigations under reactive pyrolysis conditions.