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

[Display omitted] •The mass yield of TSS in GP torrefaction was lower than that in AP torrefaction.•GP-torrefied sludge had higher ash content and better hydrophobicity.•GP torrefaction involves secondary reactions and deeper oxidation reactions.•Comprehensive combustion characteristic index of TSS obtained at 200 °C was highest.•GP torrefaction can promote the fixation of Pb, Zn and Cd into the residual fraction. 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 °C were much higher than those obtained at torrefaction temperature of 300 °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.