In-situ upgrading of waste passenger car tires via molten salt thermal treatment: Effects of particle size and temperature

Adaptable molten salt treatment yields similar products from car and truck tires. • Molten NaOH-Na 2 CO 3 enables in-situ upgrading of car tire pyrolysis products. • The absolute content of BTX in tire pyrolysis oil increased by 1.5-2.8 times. • Maximum ash and sulfur removal in tire pyrolysis char reached 71% and 65%. • The H 2 content in the tire pyrolysis gas increased by a maximum of 19 times. Waste passenger car tires present significant recycling challenges due to their low thermal conductivity and high ash and sulfur contents, which frequently result in suboptimal product quality during conventional pyrolysis. To overcome the limitations of complex post-treatment processes, a molten salt thermal treatment method utilizing NaOH-Na 2 CO 3 medium was proposed to achieve the in-situ upgrading of pyrolysis products. In this study, the effects of operating temperature (425-575 °C) and feedstock particle size (6-10 mm) on products were systematically evaluated. Experimental results indicate that the molten salt medium significantly enhanced heat transfer and catalytic cracking, which effectively fixed CO 2 and led to a 19-fold surge in H 2 production, peaking at 5.57 mmol/g. For the liquid-phase products, the introduction of molten salt significantly promoted the dealkylation of substituted aromatics, increasing the benzene concentration by 5.43 to 8.61 times. Furthermore, the molten salt effectively overcame the mass transfer resistance of larger particles, facilitating the extensive formation of monocyclic aromatic hydrocarbons. Notably, at the solid-phase interface, the molten salt acted as an efficient in-situ purifying agent, reacting directly with inorganic impurities (SiO 2 , CaCO 3 ) and sulfides to form highly water-soluble salts. This mechanism achieved a maximum deashing ratio of 71% and a desulfurization ratio of 36.74%, yielding a high-value pyrolytic char with a carbon content exceeding 90%. Overall, this approach demonstrates that the thermal conversion products of low-quality passenger car tires can be significantly upgraded, providing a highly efficient pathway for comprehensive resource recovery. Download: Download high-res image (371KB) Download: Download full-size image