PCDD/Fs’ suppression by sulfur–amine/ammonium compounds

• Thiourea could inhibit 97.3% of PCDD/Fs with a (S + N)/Cl molar ratio of 0.47.
• The amine functional group –NH2 is more efficient than ammonium NH4+.
• The inhibition effect of PCDD/Fs by thiourea is better at 350 °C than that at 650 °C.

Three distinct –S and –NH2 or NH4+ containing compounds, including ammonium thiosulfate, aminosulfonic acid and thiourea, were studied as polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) inhibitors. All these three –S and –N containing compounds tested show strong suppression of PCDD/Fs formation, especially for thiourea which has not been studied before. With a (S + N)/Cl molar ratio of only 0.47, thiourea could inhibit 97.3% of PCDD/Fs and even 99.8% of I-TEQ. At an unusually high de novo test temperature (650 °C), the PCDD/Fs’ formation was still very low but also the inhibition capacity of thiourea was weak, with an efficiency of 59% for PCDD/Fs when with a (S + N)/Cl molar ratio of 1.40. The results also revealed that the inhibition capability of the combined –S/–NH2 or –S/NH4+ suppressant was strongly influenced by both the nature of the functional group of nitrogen and the value of the molar ratio (S + N)/Cl. The amine functional group –NH2 tends to be more efficient than ammonium NH4+ and within a certain range a higher (S + N)/Cl value leads to a higher inhibition efficiency. Moreover, the emission of gases was continuously monitored: the Gasmet results revealed that SO2, HCN and NH3 were the most important decomposition products of thiourea. Thiourea is non-toxic, environment-friendly and can be sprayed into the post-combustion zone in form of powder or aqueous solution. The cost of thiourea at least can be partially compensated by its high inhibition efficiency. Therefore, the application of thiourea in a full-scale incinerator system is promising and encouraging.