Gasification is an effective technology for the thermal
disposal
of municipal solid waste (MSW) with lower dioxin emission compared
to the prevailing incineration process. Nevertheless, the mechanism
of dioxin formation in the reducing atmosphere during the gasification
process was seldomly explored. Herein, the effects of the atmosphere,
temperature, and chlorine source were systematically investigated
in terms of dioxin distribution. With CO2 and H2O as gasification agents, a reducing reaction atmosphere was formed
with abundant H2 which effectively suppressed the generation
of C–Cl, contributing to a substantial decrease of dioxin concentration
by ∼80% compared to the incineration process. The formation
of dioxin was favored at temperatures below 700 °C with its peak
concentration achieved at 500 °C. It was unveiled that inorganic
chlorine played a dominant role in the reducing atmosphere, with a
lower proportion of C–O–C/O–CO on residual
slag compared to an oxidizing atmosphere. Additionally, the generated
H2 reduced the concentration of dioxins by attacking C–Cl
and inhibiting the crucial Deacon reaction for dioxin formation, validated
by density functional theory calculation. Eventually, the formation
route paradigm and the reaction mechanism of dioxin formation from
MSW gasification were revealed, facilitating and rationally guiding
the control of dioxin emission.