Co-Incineration of Real Municipal Sewage Sludge in a Fluidized Bed: Effects of the Coexistence of P, Fe, and Na on Agglomeration and Heavy Metal Emissions

Abstract

This study investigated the effects of the coexistence of P, Fe, and Na during the fluidized bed incineration of municipal sewage sludge on agglomeration, defluidization, and heavy metal emissions. The results indicate that increasing the temperature promotes the melting of eutectic compounds, promoting agglomeration and accelerating defluidization. Conversely, using smaller bed material particle sizes or increasing the operating gas velocity enhances particle mobility and mixing, delaying agglomeration. A comparison of the roles of P and Fe revealed that an increase in their content generally extended the defluidization time. However, high P concentrations weakened the inhibitory effect of Fe, leading to a shorter defluidization time for the P/Fe/Na ratio of 0.6/5/1 than for 0.4/5/1. Differences in heavy metal emission concentrations were negligible before defluidization; however, emissions gradually increased once agglomeration began, with Cu and Pb concentrations increasing significantly after defluidization. The Cu emission concentration was lower than that of Pb because of its higher melting and boiling points. Experimental results show that Fe reacts with P to form high-melting-point FePO4, extending fluidized bed stability. Low Fe content allows P to form low-melting-point compounds like NaPO3, increasing agglomeration risks. Therefore, to ensure long-term stability during cocombustion, operators must maintain an excess Fe/P ratio (Fe > P).