Insight into nitrogen transformation during the binary NaOH-Na2CO3 molten salt thermal treatment of waste tires

Abstract

Molten salt thermal treatment of solid waste is a promising way for energy recovery and pollutant removal. However, the migration of nitrogen during pyrolysis of waste tires poses a challenge for cleaner production. This study investigated nitrogen conversion pathways during waste tires pyrolysis using a binary NaOH-Na₂CO₃ salt at 425, 500, and 575 °C. The results demonstrated that amine-N and pyrrole-N were the main N-containing species in waste tires. Compared to conventional pyrolysis, the molten salt enhanced the deamination of the amine-N while increasing levels of NH₃ and aniline. Owing to the catalytic benefits of molten salt, the C-N bond cleavage of amine-N was initiated by electron transfer involving the combination of NH₂-Phenyl structures with Na⁺. The hydrocracking of Phenyl-NH-Phenyl in amine-N was facilitated by the available H radicals (or H₂). Furthermore, molten salt obstructed the contact of C(N) sites on the char surface, causing the suppression of HCN production. The SH radicals might be consumed by OH^– ions, which disabled the formation path of benzothiazole from aniline and SH radicals. Comparatively, pyrrole-N showed limited reactivity and remained in char. This work contributes to nitrogen regulation during the molten salt treatment of solid waste.