Origin of Carbon in Polychlorinated Dioxins and Furans Formed during Sooting Combustion

摘要

The importance of solid- and gas-phase carbon precursors for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDDs/Fs) during sooting combustion was investigated in an entrained flow reactor (EFR).Experiments were performed at various methane (CH_4) flame equivalence ratios with or without gas-phase chlorine (CI_2) and fly ash,to provide a realistic environment for carbon reactions and PCDD/DF formation.Selected experiments were conducted with labeled ~(13)CH_4 and ~(37)Cl_2 to investigate the relative roles of different carbon and chlorine species for the formation of PCDD/DF.The presence of soot and ash were the two major factors controlling the PCDD/DFyields.The 16 PCDD/DF homologues as well as other analyzed chlorinated aromatics were formed by reaction pathways that varied with degree of chlorination.The mono- and dichlorinated homologues were formed by gas-phase,catalytic,or noncatalytic flame product reactions,occurring during soot formation in the near flame zone and/or at lower reaction temperatures (C_(12)) fragments in the solid soot structure,while the PCDDs,at least in part,were also formed by reaction of two C_6 fragments.The tri- to hexachlorinated DD/DF 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 octachlorinated congeners.