Effect of cofiring goal on formation of polychlorinated dibenzo-p-dioxins and dibenzofurans during waste combustion

The effect of cofiring coal with municipal waste on formation of polychlori nated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) w as examined by cofiring minor amounts (<7% by wt) of high (3.4% by wt) or l ow (0.7% by wt) sulfur (S) coal in a municipal waste combustor. PCDD/F conc entrations were reduced up to 80% during the 13-run, 6-day effort. Both cur rent 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 combu stor wall deposits as sites for formation and indicates that PCDD/F measure ments 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 PCD D/F precursors, reactants, and/ or catalysts. it is proposed that an effect of higher sulfur dioxide concentration from cofiring coal is to displace t he sulfate/chforide equilibrium in the deposits, thereby decreasing chlorin e contact with active sites and/or reducing catalytic activity through form ation of metal sulfates rather than metal chlorides. PCDD versus PCDF level s were affected by mostly different operating parameters, suggesting that t heir mechanisms of formation are somewhat distinct.