High-reactivity hydrated lime prevents de novo dioxin formation in the 200-400 °C flue-gas window via chlorine scavenging and Damköhler-based design

Abstract

Formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) in the de novo window (∼200-400 °C) remains a barrier to combining strict emission control with efficient waste-to-energy operation. We evaluated high-reactivity hydrated lime (HR-Ca(OH)₂) as a dual-purpose reagent for mechanistic suppression of de novo PCDD/F formation under simulated municipal solid waste incineration flue-gas conditions. A bench-scale laminar-flow reactor was operated with phenol/p-chlorophenol precursors and Cu-bearing fly ash under compositions representative of full-scale stacks. Across five configurations spanning lab- and stack-relevant geometries, positioning HR-Ca(OH)₂ upstream of Cu-active fly-ash phases ("lime-first") reproducibly reduced TeCDD formation and total PCDD/F TEQ by up to one order of magnitude. Kinetic behavior was consistent with full-scale observations: conversions of phenol to chlorophenols (∼1.4 %) and p-chlorophenol to TeCDDs (∼5.2 %) matched reported ranges, supporting external validity. A chlorine mass balance showed that when the Ca:Cu mass ratio was maintained at ≥10, gas-phase HCl was effectively scavenged and chlorine availability for de novo chemistry was strongly depleted. A mechanistic design framework based on an effective Damköhler number, Da = keff·τ, was developed; achieving Da ≥3 yielded ≥95 % suppression under all tested conditions. Sensitivity analyses for SO₂, NOx, H₂O and other stack-relevant interferents indicated that these species modify keff and accessibility but do not shift the Ca:Cu or Da thresholds within typical operating ranges. These results provide quantitative criteria (Ca:Cu ≥ 10; Da ≥3) for low-dioxin, high-efficiency operation and support HR-Ca(OH)₂ as a practical route to in-window suppression in waste-to-energy facilities.