Formation and destruction of PCDD/PCDF during heat treatment of fly ash samples from fluidized bed incinerators

Abstract

PCDD/PCDF isomer distribution patterns of fly ash samples from eight Japanese fluidized bed incinerators (FBIs) for municipal waste differed greatly from those reported for grate fired municipal waste incinerators (MWIs), indicating a different de novo synthesis pathway. During thermal treatment of FBI fly ash samples in the presence of oxygen little or no de novo synthesis was observed, while during the same treatment with two fly ash samples from Japanese grate fired incinerators the PCDD/PCDF concentrations increased by a factor of about ten and therefore showed a comparable PCDD/PCDF formation behavior as previously reported for MWI ashes from Europe. The low de novo synthesis potential for the FBI fly ash samples can be explained by a comparatively high calcium content resulting in a rather high pH of the fly ash of 11.5-12.7. Inhibition of the de novo synthesis of PCDD/PCDF with model fly ash (MFA) after the addition of CaO confirms this hypothesis. The inability of CaCO3 to inhibit the de novo synthesis when added to MFA suggests that the inhibition mainly is the result of the high pH rather than directly from the calcium-ions. The relatively low amounts of PCDD/PCDF formed during the heat treatment of the fluidized bed ash samples suggest that the high PCDD/PCDF concentrations often found in old FBIs are primarily the result of the condensation of precursors rather than of direct PCDD/PCDF formation from carbon particulates in the fly ash.