Suppression of dioxins after the post-combustion zone of MSWIs

Thiourea was selected as representative of combined S- and N-inhibitors and injected after the post-combustion zone of two full-scale municipal solid waste incinerators (MSWIs) using a dedicated feeder. Firstly, the operating conditions were scrutinised by monitoring the concentrations of SO2, NH3 and HCl in the clean flue gas. The suppression experiment showed that in MSWI A thiourea could reduce the total I-TEQ value in flue gas by 73.4% from 1.41 ng I-TEQ/Nm3 to 0.37 I-TEQ/Nm3, those in fly ash by 87.1% from 14.3 ng I-TEQ/g to 1.84 I-TEQ/g and the total dioxins emission factor by 87.0 wt.%, with a (S + N)/Cl molar ratio of 9.4. The suppression efficiencies of PCDD/Fs in flue gas and fly ash in MSWI B could be up to 69.2% and 83.0% when the (S + N)/Cl molar ratio attained 7.51. Furthermore, the congener distributions of dioxins were also analysed in the flue gas and fly ash, before and after addition of thiourea, to find cues to some suppression mechanism. In addition, the filtered fly ash was explored by the Scanning Electron Microscope (SEM) and Energy Dispersive Spectrometer (EDS) analysis of fly ash. These results suggest that poisoning the metal catalyst and blocking the chlorination are most probably responsible for suppression.