In situ desulfurization mechanism of molten salt thermal treatment for waste tires

Abstract

Pyrolysis is regarded as a practical way for waste tires to achieve waste-to-resource objectives. However, the environmental risk of pollution emissions in product utilization caused by the migration of sulfur is an important issue in the waste tire disposal industry. This study introduced a solution to using the binary molten salt system (NaOH-Na2CO3) as a heat transfer and desulfurization medium for the thermal utilization of waste tires. The results demonstrated that molten salts had in-situ desulfurization capability at 425 °C −575 °C and inhibited the migration of sulfur to gas–liquid-solid products with the total sulfur ratio reduced by 50 % − 67 %. The sulfur-containing radical intermediates could be extensively consumed by molten salt, leading to a low concentration level of H2S and COS in gaseous products. Simultaneously, molten salt blocked the traditional pathway of the sulfhydryl radical with inorganic additives, weakening the sulfur enrichment to char. Additionally, molten salt enhanced heat transfer during pyrolysis, facilitating the decomposition of thiazole and the polymerization of sulfur-containing benzene ring (2,3-dimethyl-thiophene to benzothiophene). The online monitoring further corroborated that COS release was preceded by H2S at high temperatures (>500 °C) along with the full degradation of the waste tires. Higher desulfurization efficiency of molten salt was achieved with feedstocks of small particle size, as it allows for adequate exposure of the reaction contact sites. This approach provides a new opportunity for in-situ desulfurization of waste tire pyrolysis via molten salt thermal treatment.