Polychlorinated Benzene, Phenol, Dibenzo-p-dioxin, and Dibenzofuran in Heterogeneous Combustion Reactions of Acetylene

Acetylene was reacted with HCl/air under heterogeneous combustion conditions between 300 and 600 °C. Model catalyst mixtures of SiO2/metal oxides (Al2O3, Fe2O3, TiO2, and CuO) were compared with municipal waste incinerator (MWI) fly ash. Chlorinated benzenes (Cl x Bz), chlorinated phenols (Cl x Ph), and PCDD/F were detected in both gas-phase products as well as catalyst-adsorbed products. Cl x Bz production increased exponentially to 600 °C (HCB: 4 × 104 ng/g of C2H2), while Cl x Ph production (2.5 × 104 ng/g of C2H2) was maximized at 500 °C (ca. 104−105 ng/g of C2H2). PCDF displayed a dramatic peak at 500 °C (TCDF: 2 × 106 pg/g of C2H2), in contrast to the generally accepted de novo and aromatic precursor formation temperature of 300 °C. Comparison of gas-phase Cl x Bz congener distribution patterns revealed that the CuO-catalyzed reaction closely matched the Cl x Bz pattern produced in the reactions with MWI fly ash, providing further evidence of the importance of Cu in the fly ash matrix. Comparison of fly ash-catalyzed gas-phase Cl x Bz with MWI flue gas and 300 °C de novo synthesized Cl x Bz demonstrated the relevance of mid-temperature (600 °C) heterogeneous combustion reactions of C2 aliphatics in real combustion systems.