Synchronous pyrolytic oil regulation and char deashing for waste passenger car tires via molten salt thermal treatment: Products characterization and mechanism analysis

Pyrolytic resource recovery of waste tires has been widely implemented at engineering scale, with truck tires as the dominant feedstock, whereas passenger car tires (PCTs) remain difficult to valorize because they generally yield low-value pyrolysis products. In this study, a NaOH–Na 2 CO 3 eutectic molten-salt strategy was investigated for regulating tar composition and simultaneously producing low-ash, low-sulfur char. Passenger car tires (PCTs) and truck tires (TTs) were used as feedstocks to compare product distribution, tar composition, gas properties, and the migration behavior of ash and sulfur in chars under conventional pyrolysis and molten salt pyrolysis. The molten salt system markedly regulated waste-tire pyrolysis product distribution, although the governing pathways were strongly feedstock-dependent. For PCT, molten salt pyrolysis increased tar yield from 33.00% to 38.20%, raised aliphatic hydrocarbons from 34.85% to 56.56%, and limonene accounted for 64.02% of the aliphatic hydrocarbon fraction in PCT-MS. These changes suggest that the molten salt medium promoted the release of primary volatiles and suppressed the secondary aromatization of high-value aliphatic products. Meanwhile, at 425 °C, the ash removal efficiency of PCT char increased from only 1.42% under conventional pyrolysis to 89.08% under molten-salt pyrolysis, while its total sulfur content decreased from 2.59 wt% to 0.75 wt%, indicating effective ash and sulfur removal. By contrast, the molten salt mainly weakened the strong char-forming tendency of TT, reducing the char yield from 60.67% to 43.65% and increasing the lower heating value (LHV) of the pyrolysis gas from 10.23 to 15.49 MJ/Nm 3 . Overall, NaOH-Na 2 CO 3 molten salt can contribute to the simultaneous recovery of high-quality tar and low-ash, low-sulfur char from PCT by enhancing heat and mass transfer, regulating secondary reactions of volatiles, and promoting ash and sulfur migration. The approach offers a feasible route for the high-value pyrolytic utilization of underused passenger car tires.