Elements partitioning during thermal conversion of sewage sludge

Abstract

The concentration of a high number of elements in the solid residue after the thermal conversion of dried sewage sludge was measured, mainly targeting the change in concentration of heavy metals under different thermochemical conditions. Experiments were conducted in a laboratory fluidized bed and in a packed bed reactor under conditions relevant for pyrolysis, gasification and combustion. The concentration of twenty elements in the solid residue was measured as a function of temperature, reaction time, oxidizing atmosphere (O2 and H2O), presence of Cl in the gas and type of reactor gas-solid contact (fluidized vs packed bed). The temperature and the oxidizing conditions in the gas were shown to play a dominant role, whereas the effect of reaction time was negligible. The concentrations of most of main elements increased with temperature, whereas those of most of minor elements followed the opposite trend. The presence of chlorine in the gas resulted in higher concentration of Fe, Mg, Cu, Ni, Zn Co, Sb and V in the residue, while it promoted almost complete volatilization of Pb and Sn. The concentration of minor elements in the residue was lower in packed bed than in fluidized bed, as a result of the higher dilution caused by the lower degradation of the fuel sample in packed bed conditions. The enrichment of the elements in the solid residue decreased with the temperature, being more significant in pyrolysis than in combustion and gasification.