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.