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 degrees 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 degrees C (HCB: 4 x 10(4) ng/g of C2H2), while Cl(x)Ph production (2.5 x 10(4) ng/g of C2H2) was maximized at 500 degrees C (ca. 10(4)-10(5) ng/g of C2H2). PCDF displayed a dramatic peak at 500 degrees C (TCDF: 2 x 10(6) pg/g of C2H2), in contrast to the generally accepted de novo and aromatic precursor formation temperature of 300 degrees C. Comparison of gasphase 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 degrees C de novo synthesized Cl(x)Bz demonstrated the relevance of mid-temperature (600 degrees C) heterogeneous combustion reactions of C-2 aliphatics in real combustion systems.