Incineration of doped sludges in fluidized bed. Fate and partitioning of six targeted heavy metals. I. Pilot plant used and results

Abstract

Incineration of sewage sludge doped with several heavy metals was studied at small pilot plant scale in a bubbling fluidized bed of 15 cm i.d. and 5.2 m height. Some ceramic and metallic filters were tested at a relatively high temperature (600–700°C) to check their usefulness for partitioning of heavy metals in the flue gas. The work was focused on the fate of six selected heavy metals (Cr, Cd, Ni, Zn, Cu, Pb). In this process, there were four exit flows or discharges for these metals: bottom ash, coarse fly ash, cake filter or fine fly ash and flue exit gas. The distribution or partitioning of each heavy metal (HM) among these four exit flows was studied. Only cadmium and sometimes lead showed any difference between the different HMs considered. All other HMs seems to have the same fate, distribution or partitioning. Such distribution is governed or ruled by the fluid dynamics in the incinerator, cyclone and ceramic filter. Most of the HMs do not have enough residence time in this incinerator type to diffuse out of the ash particle and so remain in the particle. The amount of each HM in each exit flow in this process is governed by fluid dynamics and kinetics and not at all by thermodynamics.