Formation of PCDDF in municipal solid waste incinerators: laboratory and modeling studies

The formation of dioxins in combustion systems

“…After examining the literature, Gullett et al [72] concluded that there is sufficient time in the downstream section of a MSW incinerator for the formation de novo PCCD/F to occur; their entrainment experiments bear this out. The formation rate in their equipment under typical furnace conditions was 1200 ng PCDD/F g −1 s −1 , which is midrange in the values proposed by Altwicker [172] for operating incinerators. Values of this order have also been reported from waste in the pilot incinerator at Umeå [58].…”

“…Studies on the adsorption of 2346TeCP onto flyash have indicated a range activities for the sites. Thus, Altwicker [172] has postulated the existence of "superactive" sites to explain the high reactivities found in the short term. Hell et al [66] have measured the rate of formation of PCDD from PCP adsorbed onto flyashes in batch experiments; they found a first-order reaction, with rate constants at 300 • C of the order of 10 −4 s −1 .…”

“…Altwicker [172] has noted that the net formation rates on the ash passing through the gas ducts of incinerators are of the order of 150 to 15,000 ng PCCD/F g −1 s −1 . If the ash concentration is a typical 0.5 g N m −3 , this is equivalent to 75 to 7500 ng PCCD/F N m −3 s −1 .…”

“…Altwicker [172] proceeded to extend the model to include superactive sites, using a similar four-stage process. Although the fraction of active sites is considered constant for normal sites, the concentration of superactive sites diminished exponentially with time due to accelerated destruction as well as formation.…”

“…This approach assumed that a similar mechanism applied to both precursor and de novo routes, because of a limit to the availability of reactant when the reaction temperature rose. The rate of adsorption of gasphase species on a surface can be estimated from kinetic theory [171,172], and applied both to precursor transfer and to HCl in the de novo route. The formation rate is determined by the reaction kinetics at lower temperatures, but is controlled by the diffusion rate of precursor at higher temperatures, typically above 300 • C [91].…”