ANALYSIS OF TIME SCALES PERTINENT TO DIOXIN FURAN FORMATION ON FLY-ASH SURFACES IN MUNICIPAL SOLID-WASTE INCINERATORS

Formation of polychlorinated dioxins and furans (PCDD/F) in municipal solid waste incinerators has been reported extensively in the literature. Of specific interest are potential heterogeneous pathways: low temperature catalytic formation over fly ash in air pollution control devices (APCDs'). An intuitive analysis of APCDs identifies at least two different particle residence times: short and long time scales. Therefore, the PCDD/F measured in the stack gases may include contributions from formation on both time scales. During the residence of particles in the system, a sequence of mass transfer, precursor adsorption and surface reaction should take place to form PCDD/F. An analysis suggests that the mass transfer of precursors (chlorobenzenes/chlorophenols) may not be limiting for high reaction selectivities ( > 10%) while chemisorption is the more probable adsorption mechanism. Time scale estimates for surface reaction mechanisms indicate that both Eley-Rideal and Langmuir-Hinshelwood mechanisms are possible on the fly ash surface. In determining the relevance of each mechanism to incinerator conditions, several parameters, such as the degree of desorption of PCDD/F, extent of reentrainment of collected particles, particle collection efficiency, etc., play a key role; an approach to quantitative treatment of such parameters is the content of this communication.