Inhibition Effect and Mechanism of Thiourea on Electrophilic Chlorination of Aromatics in Combustion Flue Gas

Abstract

Thiourea can effectively inhibit the formation of highly toxic chlorinated aromatics in postcombustion zone. However, the inhibition mechanism was still not adequately understood. In this study, naphthalene was adopted as a model aromatic compound to investigate the inhibition effect and mechanism of thiourea on the formation of chlorinated aromatics via electrophilic chlorination over Cu and Fe chlorides. Thiourea addition resulted in the reductions of 77.6–99.8% and 36.4–98.1% in the yield of polychlorinated naphthalenes from naphthalene chlorination mediated by CuCl2 and FeCl3 at 150–300 °C, respectively. The inhibition efficiency of chlorination presented a decreasing tendency with increasing reaction temperature and O2 content in flue gas. X-ray diffraction analysis revealed that the thiourea-induced reduction of highly active Cu (II) and Fe (III) chlorides to less active Cu (I) and Fe (II) chlorides was a primary mechanism for inhibiting aromatic chlorination. [thiourea–Cu]+ complex generated during the reduction process could be transformed into CuS and Cu2S, and isomeric reaction product NH4SCN could react with Cu2+ to produce Cu­(SCN)2 and then also form Cu sulfides, suggesting sulfidization of Cu chloride was another important inhibition mechanism. Chlorination inhibition induced by the volatile decomposition products of thiourea might only play a minor role.

Exploring N/S-Based Polymers for Synergistic Inhibition of Multiple Unintentional Persistent Organic Pollutants during Iron Ore Sintering

“…The order of inhibitory effects for CBzs, PCBs, and PCDD/Fs was UFR < TSCFR < TUFR. N and S play major roles in the inhibition of UP-POPs during thermal processing, ,,, and the inhibitory efficiencies of the macromolecular inhibitors were directly correlated with their N and S contents in this study. The elemental compositions of the three inhibitors were analyzed, and the results are shown in Table .…”

“…13−15 of thiourea in combustion flue gas was also clarified. 16,17 Lonepair electrons of N and S in N/S-containing inhibitors are able to form stable complexes with catalytic metals to retard the activity of the catalyst. 18,19 Our group previously utilized thiourea in the iron sintering process and achieved total CBz and PCB emission concentration reductions of 78.2 and 83.0%, respectively.…”

“…Chemical inhibition is a low-cost and highly efficient technique to reduce UP-POP formation and emissions at the source. , Many previous studies used N/S-based inhibitors, such as urea, NH 3 , amidosulfonic acid, and other S-containing substances. − Inhibitory effects were observed both in the laboratory and in full-scale municipal solid waste incinerators, and the mechanism of action underlying the inhibitory effects of thiourea in combustion flue gas was also clarified. , Lone-pair electrons of N and S in N/S-containing inhibitors are able to form stable complexes with catalytic metals to retard the activity of the catalyst. , Our group previously utilized thiourea in the iron sintering process and achieved total CBz and PCB emission concentration reductions of 78.2 and 83.0%, respectively . The thermal properties and chemical structures of N/S-containing compounds may influence the decomposition of N/S-containing products .…”

“…Due to their excellent adsorption of metal ions, they are commonly used in sewage treatment . Compared to traditional inhibitors, , macromolecular inhibitors are more stable, have higher decomposition temperatures, and can be stored more easily, which may facilitate the development of effective inhibitors for use in real engineering applications. These compounds are inexpensive and nontoxic and work at temperatures within the range required for PCDD/F formation, making them applicable for use in real-world iron ore sintering processes.…”