Co-pyrolysis of waste tires and used oil: RSM optimization and prediction of fuel properties

• Co-pyrolysis of waste tires and used oil optimized using RSM and CCD. • Predictive models for fuel properties achieved R 2 > 0.90 for most responses. • Optimal conditions yielded 63.4% oil with diesel-like cetane index and viscosity. • Calcined cockle shells reduced sulfur and chlorine as a green catalyst alternative. • Predictive equations integrated into a web-based tool for fuel property estimation. End-of-life tires (ELTs) and waste lubricant oil are abundant hydrocarbon-rich wastes that can be upgraded into liquid fuels instead of being landfilled or openly burned. This study investigates the co-pyrolysis of waste tires and used lubricant oil in the presence of CaO-based catalysts and applies response surface methodology (RSM) with a central composite design to optimize the process. Three coded factors, final temperature (386.36 to 453.63 °C), used oil ratio (12.96 to 97.04 wt%), and CaO loading (6.59 to 23.41 wt%), were varied in 19 experiments. Quadratic models were developed for seven responses, namely oil yield, cetane index, specific gravity, density, viscosity, and sulfur and chlorine contents, with R 2 mostly above 0.90, indicating good predictive capability. The optimum conditions (437 °C, 79.29 wt% used oil, and 12.05 wt% CaO) produced 63.4 ± 0.28 wt% oil with a cetane index of 51.5 ± 0.71 and a kinematic viscosity of 3.97 ± 0.07 cSt, which are close to diesel fuel specifications. Replacing commercial CaO with calcined cockle shell CaO under the same conditions slightly improved viscosity (3.90 ± 0.11 cSt) and reduced sulfur and chlorine levels to 0.242 ± 0.06 wt% and 18.55 ± 1.03 ppm, respectively, demonstrating the feasibility of using waste-derived CaO as a catalyst. The final set of RSM models was also implemented in a web-based calculation tool to enable rapid estimation of fuel properties from operating conditions.