Formation of PCDD/Fs from the Copper Oxide-Mediated Pyrolysis and Oxidation of 1,2-Dichlorobenzene

Abstract

Formation of polychorinated dibenzo-p-dioxins (PCDDs) has been demonstrated to occur via surface-mediated reactions of chlorinated phenols. However, polychlorinated dibenzofurans (PCDFs) are observed in much lower yields in laboratory studies than in full-scale combustors where PCDFs are in higher concentrations than PCDDs. This has led to the suggestion that at least PCDFs are formed from elemental carbon in the de novo process. However, the potential for PCDF formation from reactions of chlorinated benzenes has been largely overlooked. In this study, we investigated the potential contribution of chlorinated benzenes to formation of PCDD/Fs using 1,2dichlorobenzene as a surrogate for reactions of other chlorinated benzenes and CuO/silica (3 wt % Cu) as a surrogate for fly ash. Results were similar for oxidative and pyrolytic conditions with a slight increase in more chlorinated products under oxidative conditions. Reaction products included chlorobenzene, polychlorinated benzenes, phenol, 2-monochlorophenol (2-MCP), dichlorophenols, and trichlorophenols with yields ranging from 0.01 to 2% for the phenols and from 0.01 to 10% for chlorinated benzenes. 4,6-Dichlorodibenzo furan (4,6-DCDF) and dibenzofuran (DF) were observed in maximum yields of 0.2% and 0.5%, respectively, under pyrolytic conditions and 0.1% and 0.3%, respectively, under oxidative conditions. In previous studies of the pyrolysis of 2-MCP under identical conditions, 4,6-DCDF and dibenzo-p-dioxin (DD) were observed with maximum yields of ∼0.2% and ∼0.1%, respectively, along with trace quantities of 1-monochlorodibenzo-p-dioxin (1-MCDD). Under oxidative conditions, 1-MCDD, DD, and 4,6-DCDF were observed with maximum yields of 0.3%, 0.07% and 0.1%, respectively. When combined with the fact that measured concentrations of chlorinated benzenes are 10-100× that of chlorinated phenols in full-scale combustion systems, the data suggest surfacemediated reactions of chlorinated benzenes can be a significant source of PCDD/F emissions.

Polychlorinated Dibenzo-p-dioxin and Dibenzofuran (PCDD/F) Emission Behavior during Incineration of Laboratory Wastes. Part 2: PCDD/F Profiles and Characteristics of Output Materials

“…According to our previous study, metals which vaporized as the particulate phase reacted with Cl to form chloride in flue gas (Chang et al, 2012). During this stage, metal chloride plays an important role in catalyzing the formation of PCDD/Fs (Lu et al, 2007;Nganai et al, 2011). Therefore, the PCDD/F content of ash increased with increasing Cl level in the input materials, which was also reported in a previous study (Hatanaka et al, 2005).…”

Contribution of aluminas and aluminosilicates to the formation of PCDD/Fs on fly ashes

“…The concentration of other transition metals falls within bounds typically found in fly ashes. From the perspective of PCDD/F formation, the concentration of copper in fly ashes is of importance, as copper is considered one of the most potent PCDD/F formation catalysts and has been studied extensively in the past 9, 19, 20, 40, 41 . The fly ash studied in here contains only 0.02 % Cu, which is within the range observed in fly ashes from waste incineration processes 35–39 .…”

“…Reactions following the ‘precursor’ pathway involve a chlorinated aromatic molecule acting as a precursor to dioxin formation 10–17 . Previous research on the mechanism for the surface-mediated formation of PCDD/Fs has confirmed the involvement of surface radicals bound to metal centers on the fly ash surface 8, 9, 18, 19 . Though the original mechanistic studies were performed over copper oxide clusters on silica, the mechanism applies to any surface center that can form surface radicals.…”

“…Using a bottom up approach, systems containing either iron or copper oxides on a silica matrix were studied extensively and these metals were found to contribute to PCDD/F formation 8, 18, 19, 23–25 . The initial focus on iron and copper oxide was due to their relative high concentrations in fly ashes and their known redox properties 25–28 .…”

Chlorobenzene Formation from Fly Ash: Effect of Moisture, Chlorine Gas, Cupric Chloride, Urea, Ammonia, and Ammonium Sulfate

“…The concentration of CBz in flue gas of waste incinerator is much higher than dioxins, as well as 10-100 · that of CPh. The surfacemediated reactions of CBz is possibly a significant source of PCDD/F emissions (Nganai, et al, 2011), although the ratio of PCDFs to PCDDs is mostly more than 1.0. Considering the great correlation between CBz and PCDD/Fs, Ö berg and Bergstrom (1985) proposed CBz as a surrogate of PCDD/Fs.…”