Experimental Study on the Effect of SO₂ on PCDD/F Emissions:  Determination of the Importance of Gas-Phase versus Solid-Phase Reactions in PCDD/F Formation

Cofiring coal in municipal solid waste incinerators (MSWIs) has previously been reported to reduce polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) emissions due to increasing the flue gas SO₂ concentration. The present study was focused on understanding the primary mechanism responsible for the suppressant effect of SO₂ on total PCDD/F formation and toxic equivalent (TEQ) emissions. The addition of SO₂, simulating the effect of coal addition on the flue gas composition, resulted in significant reductions in the TEQ emissions due to reactions involving SO₂ in the postcombustion zone. However, emissions of total PCDDs/Fs, unlike the TEQ value, were dependent upon the Cl₂ and SO₂ injection temperatures due to increases in non-TEQ correlated isomers. The conversion of metal chlorides in the fly ash to sulfates, thus reducing the sites responsible for chlorination/oxidation reactions, was concluded to be the main suppressant mechanism; proposed reactions for copper and iron are presented. This mechanism was found to be independent of combustion conditions and could have prolonged effects on PCDD/F emissions from deposits formed with high flue gas S/Cl ratios.