OCR Text |
Show The following conclusions and inferences can be drawn from the foregoing limited data: . There is positive evidence that PCDDs are present in MSW, and stack emissions of PCDDs are equal to or less than the quantities present in the raw refuse . . PCDFs are emitted from resource recovery facilities in significantly greater amounts than the quantities present in the raw refuse. The second theory, which involves thermal transformation of PCOD and PCDF precursors, could account for formation of these compounds during incineration. Polychlorophenols and polychlorobenzenes are considered potential precursors for PCDD emissions since PCDOs have been formed from these compounds in laboratory studies. PCBs have been found to convert to PCDF compounds during transformer fires. However, available data are insufficient to confirm or refute hypotheses concerning precursors of PCDDs and PCDFs forming PCDDsjPCDFs during the incineration process. The third theory for the occurrence of PCDFs and PCDDs in municipal incinerator emissions suggests that these materials are formed from unrelated materials under specific conditions (de novo synthesis). Pyrolysis of uncontaminated pure vegetable extracts in a laboratory combustor has yielded phenolic compounds which, when exposed to chlorine, resulted in parts ~n the per million-range of PCDD and PCDF emissions. These and other laboratory combustion experiments led researchers to suggest that PCDDs and PCDFs are synthesized in the incinerator systems from unchlorinated aromatic precursors and chlorine from various sources. Major source of unchlorinated precursors in MSW is ~ignin which is the predominant constituent of ~ood and paper. Pyrolysis of lignin releases various phenols and due to their structures they can dimerize to form the basic dioxin and furan structures. Based on the third hypothesis described above commoner6 suggested that PCDDs and PCDFs are formed from lignin phenolic derivatives as follows: Chlorination of phenolic compounds derived from lignin in the presence of HCI to form chlorophenols, followed by dimerization to form PCDDs. In the last step, lower-chlorinated PCDDs (or unchlorinated dibenzo-p-dioxin) adsorbed on fly ash are chlorinated by HCl to form higher chlorinated PCDDs. Considering that HCl is formed from chlorinated plastics (PVC) and possibly NaCl, he proposed a reduction of PCDDjPCDF emissions can be based on an overall strategy of lowering the chlorine content of the MSW by separating chlorinated plgstics and, if necessary, sodium chloride, from the refuse. Howe ver, the data available do not s~pport the Commoner strategy. Experiments conducted in Japan have shown that when plastic-free MSW was |