ROLE OF SULFUR IN REDUCING PCDD AND PCDF FORMATION

Past research has suggested that the presence of sulfur (S) in municip al waste combustors (MWCs) can decrease the downstream formation of ch lorinated organic compounds, particularly polychlorinated dibenzo-p-di oxins(PCDDs) and polychlorinated dibenzofurans (PCDFs). Thus, co-firin g a MWC with coal, because of the S species from coal, may reduce PCDD and PCDF emissions. Experiments were carried out to test this hypothe sis and to determine the role of S. A field-sampled MWC fly ash was in jected into the EPA's pilot-scale reactor, doped with hydrogen chlorid e (HCl). The tests involved either natural gas or coal combustion. Bes ides 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 n atural-gas-fired reactor, when S was added (as sulfur dioxide, SO2), t he PCDD and PCDF formation decreased dramatically at S/Cl ratios as lo w 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-fire d furnace, the S inhibitory effect was again observed at S/Cl values o f 0.8 and 1.2, respectively, for the two coals tested. S inhibition me chanisms were studied in a bench-scale reactor. Results show that the depletion of molecular chlorine (Cl-2), an active chlorinating agent, by SO2 through a gas-phase reaction appears to be a significant inhibi tion mechanism in addition to previously reported SO2 deactivation of copper catalysts.