Past research has suggested that the presence of sulfur (S) in municipal waste combustors (MWCs) can decrease the downstream formation of chlorinated organic compounds, particularly polychlorinated dibenzop-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Thus, co-firing a MWC with coal, because of the S species from coal, may reduce PCDD and PCDF emissions. Experiments were carried out to test this hypothesis and to determine the role of S. A fieldsampled MWC fly ash was injected into the EPA's pilot-scale reactor, doped with hydrogen chloride (HCl).The tests involved either natural gas or coal combustion. Besides the combustion environment, MWC fly ash injection temperature and sulfur-to-chlorine ratio (S/Cl) were varied. Flue gas was sampled and analyzed for PCDD and PCDF to determine in-flight formation. In the natural-gas-fired reactor, when S was added (as sulfur dioxide, SO 2 ), the PCDD and PCDF formation decreased dramatically at S/Cl ratios as low as 0.64, and with varying furnace conditions, the inhibitory effect was consistent for S/Cl ratios of about 1. In tests with the coal-fired furnace, the S inhibitory effect was again observed at S/Cl values of 0.8 and 1.2, respectively, for the two coals tested. S inhibition mechanisms were studied in a bench-scale reactor. Results show that the depletion of molecular chlorine (Cl 2 ), an active chlorinating agent, by SO 2 through a gas-phase reaction appears to be a significant inhibition mechanism in addition to previously reported SO 2 deactivation of copper catalysts.

The ability of sorbent injection technology to reduce the potential for trace metal emissions from coal combustion was researched. Pilot scale tests of high-temperature furnace sorbent injection were accompanied by stack sampling for coal-based, metallic air toxics. Tested sorbents included hydrated lime, limestone, kaolinite, and bauxite injected at 1000, 1150, and 1300 °C, and through the burner at several feed rates. Continuous gas monitoring and solids sampling by particle size determined changes in metal concentrations from baseline measurements. The impact of sorbent injection on trace metal emissions is a function of metal type, sorbent type, and injection mode. Reductions in submicrometer concentrations of antimony, arsenic, mercury, and selenium were observed when hydrated lime and limestone were injected. The total amount of captured arsenic, cadmium, and lead increased when using, variously, hydrated lime, limestone, and kaolinite.

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Local instantaneous changes in heat-

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.