METAL PARTITIONING IN CYCLONE AND FILTER ASH DERIVED FROM SLUDGE INCINERATION TESTS IN A PILOT CIRCULATING FLUIDIZED-BED COMBUSTOR

摘要

Cd, Cr, Cu, Mn, Ni, Pb, Sn, and Zn behaviour during sewage sludge incineration was investigated in pilot tests in a circulating fluidized-bed furnace. Dewatered sludge was fed either alone or spiked with chlorinated organic compounds. The behaviour was studied by comparing metal concentration in the cyclone ash and bag filter ash. Cyclone ash contained a large fraction of sand carried over from the furnace. The dilution effect of this sand on the metal concentration was accounted for by using titanium as a tracer. The correction of metal concentration by the Ti method was validated by X-ray diffraction measurements of the sand content. Metal enrichment was quantified by an enrichment factor defined as the ratio of corrected metal concentrations between filter and cyclone ash. For Cr, Cu, Mn, Ni, Sn, and Zn the enrichment factors never differed significantly from 1 (no enrichment). In contrast, Cd and Pb enrichment factors increased with increasing chlorine concentration in the feed and with decreasing particle size. A comparison of the experimental data with the percentage of metal volatilization predicted by a thermodynamic model showed satisfactory agreement for Cd, Pb, Cr, and Sn. On the other hand, discrepancies were noted between model predictions and experimental results in respect of Zn, Cu, Mn, and Ni behaviour at high chlorine dosage. The failure of the model was ascribed both to kinetic limitations of metal vaporisation and to oversimplification of the model chemical system.