Investigation of aqueous phase recirculation on co-hydrothermal carbonization of sewage sludge and lignite: Hydrochar properties and heavy metal chemical speciation

Abstract

Excessive process water remained an important challenge in hydrothermal carbonization (HTC). In this work, co-HTC of sewage sludge (SS) and lignite (LIG) along with the aqueous phase recirculation (APC) was employed to explore the feasibility of producing clean solid fuels at 230 °C. Chemical speciation and concentration of heavy metals (HMs) in hydrochars were evaluated, as well as fuel properties. The results showed that the co-HTC with LIG was beneficial for the decline of total concentrations of Cu, Cr, Ni, Mn, and Zn. Furthermore, APC further decreased the risk of Ni without expanding the pollution of HMs. Compared with the hydrochar produced from SS, the coalification degree, higher heating values (5.61─6.19 MJ/kg), and combustion behaviour of the hydrochar derived from co-HTC were upgraded and improved. The yield of hydrochar was enhanced via APC but several fuel properties changed slightly. These findings offered a viable technology that could lower the output of wastewater and realize the energy utilization of SS.