Effect of Cofiring Coal on Formation of Polychlorinated Dibenzo-p-Dioxins and Dibenzofurans during Waste Combustion

The effect of cofiring coal with municipal waste on formation of polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) was examined by cofiring minor amounts (<7% by wt) of high (3.4% by wt) or low (0.7% by wt) sulfur (S) coal in a municipal waste combustor. PCDD/F concentrations were reduced up to 80% during the 13-run, 6-day effort. Both current and past operating conditions (fuel type and rate, gas concentrations) had an effect on current levels of PCDD/F formation. The influence of past conditions is consistent with a PCDD/F formation mechanism involving combustor wall deposits as sites for formation and indicates that PCDD/F measurements on field units may be affected in part by past operating conditions, at least up to 6 h. These wall deposits act as sources and receptors of PCDD/F precursors, reactants, and/or catalysts. It is proposed that an effect of higher sulfur dioxide concentration from cofiring coal is to displace the sulfate/chloride equilibrium in the deposits, thereby decreasing chlorine contact with active sites and/or reducing catalytic activity through formation of metal sulfates rather than metal chlorides. PCDD versus PCDF levels were affected by mostly different operating parameters, suggesting that their mechanisms of formation are somewhat distinct.