Partitioning of Cr, Cu, Pb and Zn in sewage sludge incineration by rotary kiln and fluidized bed furnaces

The behaviour of four metals (Cr, Cu, Pb, Zn) during sewage sludge incineration was studied in eight pilot plant tests performed with a rotary kiln (RK) and a fluidised bed (FB) furnace. To simulate sludge co-incineration with hazardous wastes, in three FB tests feed sludge was mixed with chlorinated organic compounds. Chromium and copper showed similar concentrations both in bottom or cyclone ash and in fly ash, even at high chlorine input and high combustion temperature. In contrast, zinc and lead concentrations in fly ash produced in RK tests are one or two orders of magnitude higher than those in bottom ash. However, not even these two metals showed significant enrichment in FB tests carried out at high chlorine input. The predictive capability of a thermodynamic model was checked by comparing predicted metal volatilisation in the combustion chamber with experimental metal enrichment in the fly ash. Large discrepancies were observed in FB tests carried out at high chlorine content, where Pb, Zn, and Cu are predicted to volatilise in great extent, andin RK tests where zinc volatilisation is not predicted. Likely explanations of these discrepancies are the very short solid residence times in the FB furnace (non equilibrium conditions) and the incomplete mixing conditions in the RK furnace (pyrolysis pockets).

From the environmental impact point of view, the pilot tests suggest that sludge incineration with fluidised bed furnace is safer than the one using rotary kiln furnace.