Effects of sulfur on PCDD/F formation under stable and transient combustion conditions during MSW incineration

SO2 levels in the flue gas from a laboratory-scale fluidized bed reactor combusting artificial municipal solid waste (MSW) were varied (resulting in four different SO2:HCl ratios 0, 0.2, 0.7 and 2.7 (by mass)) to study the effects of sulfur on the formation of polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs) and polychlorinated dibenzothiophenes (PCDTs). Sampling was performed simultaneously at three fixed points in the post-combustion zone with temperatures of 400, 300 and 200 °C, under normal combustion conditions and both during and after transient combustion conditions. The findings indicate that sulfur has a greater inhibitory effect on PCDF formation than on PCDD formation and that the PCDD/PCDF ratio in the flue gas depends on both the SO2:HCl ratio in the flue gas and memory effects arising from transient combustion conditions. The results also indicate that the relative importance of different pathways shifts in the post-combustion zone; condensation products increasing with reductions in temperature and increases in residence time. However, these changes appear to depend on the SO2:HCl ratio in the flue gas and combustion conditions. Sulfur seems to inhibit the chlorination of PCDFs. A tendency for increased SO2 levels in the flue gas to increase levels of PCDTs was also detected, but the increases were much less significant than the reductions in PCDF levels.