Comparison of atmospheric pressure and gas-pressurized torrefaction of municipal sewage sludge: Properties of solid products

Gas-pressurized (GP) and atmospheric (AP) torrefaction of municipal sewage sludge were carried out under pure nitrogen and 9%-oxygen nitrogen atmosphere, in order to investigate the effects of reactor and oxygen content on the physicochemical properties, combustion/pyrolysis characteristics and heavy metal availability of solid products. Compared with the solid products from AP torrefaction, the GP torrefied sludge had higher ash content, lower H/C ratio and better hydrophobicity. The oxidative torrefaction produced torrefied sludge of similar properties (except for higher heating value) as normally torrefied sludge in AP and GP, respectively. The activation energy (Ea) required for torrefied sewage sludge (TSS) pyrolysis was 23.56-37.07 kJ/mol by the Coats-Redfern method. Comprehensive combustion characteristics of GP torrefied sewage sludge obtained at torrefaction temperature of 200 degrees C were much higher than those obtained at torrefaction temperature of 300 degrees C. Interestingly, GP torrefaction can significantly promote the transformation of heavy metals (e.g. Pb, Zn and Cd) into the residual fraction. The secondary interaction between volatiles and solids in GP torrefaction via gas-pressure-induced deoxygenation behavior dominated the difference of derived-solid products from GP torrefaction and AP torrefaction.