Emission Characteristics of Heavy Metals and Organic Pollutants from the Combustion of Sewage Sludge in a Fluidized Bed Combustor

In this study, a bench-scale fluidized bed combustor (FBC) was used to investigate the partitioning of nine heavy metals (Cd, Co, Cr, Cu, Mn, Ni, Pb, V, and Zn) and organic hazardous air pollutants, including aliphatics, monoaromatic compounds, cyclic hydrocarbons, and polycyclic aromatic hydrocarbons (PAHs), in the gas and solid phases of emissions from combustion of seven sewage sludge samples. The concentrations of heavy metals (HMs) in two main ash streams (bottom ash and fly ash) were detected by inductively coupled plasma−optical emission spectroscopy (ICP−OES). It showed that the concentration of zinc was the highest in bottom and fly ashes, ranging from 1600−1900 and 2200−2900 mg/kg, respectively. The average concentrations of HMs analyzed in both bottom ash and fly ash followed the order of Zn > Cu > Mn > Cr > Ni > Pb > V > Cd > Co. Co and Cu were mostly found in bottom ashes, while Pb and Zn were mostly found in fly ashes; other HMs were almost equally distributed between bottom and fly ashes. Organic pollutants [particularly hazardous air pollutants (HAPs)] in flue gas trapped onto the XAD resin and in fly ash were extracted with dichloromethane and analyzed using a gas chromatography−mass spectrometry (GC−MS) analyzer. The results showed that PAHs were more likely emitted in flue gas than in fly ash and the concentrations of PAHs in both flue gas and fly ash of sludge combustion were in a reasonably comparable range, with a maximum concentration reaching 0.03 mg/L. Aliphatics were mainly recovered from fly ashes (>70%), while monoaromatic compounds were found in both flue gases and fly ashes and cyclic hydrocarbons were undetectable for most of the sludge samples. The total amounts of HAPs in the flue gases increased most likely with the increasing H/C ratios of the sludge samples.