The transformation and fate of sulphur during CO₂ gasification of a spent tyre pyrolysis char

The transformation and fate of sulphur (S) in a spent tyre pyrolysis char during CO₂ gasification were studied by following the S species and contents using X-ray photoelectron spectroscopy (XPS). The spent tyre pyrolysis char (particle size fraction ≤150 μm), without and with 1 M HCl acid washing to remove inorganic S, were gasified in a fixed bed reactor. The effect of temperature (850, 950, 1050 °C), reaction time (1, 2, 3, 6 h) and CO₂ concentration (33.3, 50.0, 66.7 vol% in N₂ ) on the S species in the char samples were investigated. The main S species in the spent tyre pyrolysis char were ZnS and aliphatic sulphide. After CO ₂ gasification, aliphatic sulphide, thiophene, sulphoxide and sulphone became the dominant organic S while ZnS and CaSO₄ were the main inorganic S. The percentage of total S increased with increasing gasification temperature, time and CO ₂ concentration. The content of organic S increased with increasing gasification temperature and time, while, the content of inorganic S decreased. Increasing CO ₂ concentration had negligible effect on the content of organic S but led to significant reduction in the content of inorganic S since ZnS reacted with CO₂ to produce ZnO and SO ₂. Aliphatic sulphide, sulphoxide and sulphone were shown to have transformed to more stable thiophene. ZnS decomposed to release S X at > 900 °C while CaSO 4 reacted with CO and carbon to produce COS. Both S X and COS reacted with the organic matrix in the char to form sulphoxide and sulphone.