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ORIGIN OF CARBON IN POLYCHLORINATED DIOXINS AND FURANS FORMED DURING SOOTING COMBUSTION
Citation:
Wikstrom**, E., S Ryan*, A. Touati, D. Tabor, AND B K. Gullett*. ORIGIN OF CARBON IN POLYCHLORINATED DIOXINS AND FURANS FORMED DURING SOOTING COMBUSTION. ENVIRONMENTAL SCIENCE & TECHNOLOGY. American Chemical Society, Washington, DC, 38(13):3778-3784, (2004).
Impact/Purpose:
Published Journal Article
Description:
The paper discusses results of an investigation on the importance of solid- and gas-phase carbon precursors for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) during sooting combustion. Experiments were performed in an entrained flow reactor (EFR) at various CH4 flame equivalence ratios and with or without gas-phase chlorine and fly ash added to provide a realistic environment for carbon reactions and PCDD/F formation. Selected experiments were conducted with labeled 13CH4 and 37Cl2 to investigate the relative roles of different carbon and chlorine species for the formation of PCDD/F. The presence of soot particles and ash were the two major factors controlling the PCDD/F yields. The sixteen PCDD/F homologues, as well as other analyzed chlorinated aromatics, were formed by different reaction pathways dependent on the degree of chlorination. The mono- and di-chlorinated homologues were formed by non-catalytic reactions between gas-phase flame products, either during soot formation or in the lower temperature (<650 C) post-combustion zone. Meanwhile, the higher (tri- to octa-) chlorinated homologues were formed mainly in the post-combustion zone by fly-ash-catalyzed de novo synthesis of the soot. Of these, the PCDD/Fs were formed by release of high number carbon (>C12) fragments from the solid soot structure whereas the PCDDS, at least in part, were formed by reaction of two C6 fragments. The tri- to hexa-chlorinated DD/F homologues were formed via a relatively fast de novo synthesis occurring during the first minutes of reactions on the continuously formed soot particles, whereas de novo synthesis on an aged soot matrix was the major pathway for the hepta- and octa-chlorinated congeners.
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ORIGIN OF CARBON IN POLYCHLORINATED DIOXINS AND FURANS FORMED DURING SOOTING COMBUSTION