Migration characteristics of heavy metals during sludge pyrolysis.

• The migration characteristics of heavy metals during sludge pyrolysis were studied. • The activation energy of sludge pyrolysis was determined by the Model-free method. • As the pyrolysis temperature increased, the concentration of CO decreased and then increased. • Cu, Cr, and Ni were seldom involved in migration during pyrolysis. • As, Cd, and Hg readily migrated even at low pyrolysis temperatures. A comprehensive study was conducted to investigate the pyrolysis characteristics of municipal sludge, and the activation energy of sludge pyrolysis was determined using the Model-free method. The detailed migration characteristics of heavy metals in the pyrolysis products were also investigated at different pyrolysis temperatures (250–850 °C). The results demonstrate that sludge pyrolysis is a multi-step process; the activation energy of pyrolysis increased with the pyrolysis conversion rate, and the average activation energy was calculated as 79.59 kJ mol−1. As the pyrolysis temperature increased, the char yield decreased, the tar yield increased then decreased, and the gas yield increased. At 850 °C, the thermal volatilities of heavy metals followed the sequence Cu < Cr < Ni < Mn < Pb < As < Zn < Cd = Hg. In addition, Cu, Cr, and Ni were seldom involved in migration during pyrolysis while As, Cd, and Hg readily migrated even at low pyrolysis temperatures. The results provide a theoretical basis for sludge pyrolysis technologies and heavy metals migration control. [ABSTRACT FROM AUTHOR]