The effects of combustion and feeding conditions on Polycyclic Aromatic Hydrocarbons (PAH) and PCDD/F formation and appearance in the emissions at the stack during sludge incineration are discussed in this paper. Partitioning in the solid streams of Cd, Cr, Cu, Mn, Ni, Pb and Zn is also analyzed. Tests were performed on a demonstrative plant equipped with a fluidized bed furnace (FBF) using sewage sludge either as is or spiked with chlorinated organic compounds (tetrachloroethylene or a mixture of tetrachloroethylene, chlorobenzene and toluene) to study the chlorine effect on the presence of micropollutants in the different streams. Exhaust gases were sampled both before and after the treatment system (bag house and wet scrubber). In the untreated flue gas the highest values of PCDD/F and PAH were detected when the afterburning chamber was not in use or operating at low temperatures. Operation of the afterburning chamber at temperature higher than 850–900 °C was sufficient to keep organic micropollutants concentrations in the untreated flue gas at reasonably low levels. No significant correlation of the operating conditions with emissions at the stack was found. High copper concentration in the feed enhanced PCDD/F formation, with exception of tests carried out with high afterburning temperature. The homologue profile of PCDD/F and PAH depended on test conditions. Preferential accumulation of heavy metals in the filter ash with respect to cyclone ash was quantified in terms of an enrichment factor. Out of the seven metals considered, only Cd and Pb undergo significant enrichment in the filter ash. The enrichment increased with increasing chlorine content of the feed. In contrast, Cu, Cr, Mn, Ni, and Zn behaved as refractory (non-volatile) elements even at high chlorine dosage. In accordance with the widely accepted hypothesis that metal enrichment is due to metal vaporization in the combustion chamber and subsequent condensation onto the filter ash particles, a thermodynamic model of the combustion process was able to satisfactorily predict the different metal behavior and the effect of chlorine dosage on metal enrichment.
Study on the Flow Characteristics of Desulfurization Ash Fine Particles in a Circulating Fluidized Bed