Isothermal torrefaction kinetics for sewage sludge pretreatment

Abstract

Torrefaction of sewage sludge, a by-product from waste water treatment plants, has been receiving increased attentions in terms of sustainable sludge treatment, energy recovery and possibility to mitigate the environmental impacts. Torrefied sewage sludge is recognized as a better solid fuel than the dry sewage sludge, and it is also beneficial for subsequent thermochemical processes. In this study, sewage sludge was subjected to different torrefaction conditions in a thermogravimetric analyzer. The thermogravimetric data were then simulated using a two-consecutive reaction model to reproduce the mass losses during torrefaction. The obtained results from the torrefaction of the non-lignocellulosic biomass are also compared with other lignocellulosic biomass including a softwood and a hardwood. The findings from this study reveal that the non-lignocellulosic biomass is less thermally resistant and degrades much faster than the lignocellulosic biomass during the first stage of torrefaction. Moreover, the solid yield from the sewage sludge torrefaction at temperatures less than 280 °C is lower than that from the woody biomass torrefaction. However, this trend is inverted when the torrefaction temperature attains 280 °C and higher. The results obtained are useful for new design or retrofit of the sewage sludge torrefaction processes.