Co-hydrothermal carbonization of sewage sludge and lignocellulosic biomass: Fuel properties and heavy metal transformation behaviour of hydrochars

In this study, co-hydrothermal carbonization (co-HTC) of sewage sludge (SS) and lignocellulosic biomass was conducted at 220 °C for 1 h, and transformation behaviour of heavy metals during co-HTC, along with fuel properties, were investigated. The results showed that the majority of Cr, Ni, Cu and Zn was still accumulated in hydrochar during individual HTC of SS. The addition of lignocellulosic biomass could effectively reduce (F1+F2) fractions of Ni and Cr. Owing to the positive effect on the reduction of (F1+F2) fractions of Cu, the addition of lignin exhibited the lowest potential ecological risk index. Furthermore, compared with the hydrochar derived from SS, it was found that fuel ratio of hydrochar derived from co-HTC increased to 0.08─0.39 and high HHV (6.86─12.90 MJ/kg) was also achieved. The TGA results displayed that the combustion behaviors of hydrochars derived from co-HTC were expected to be safer and stable than that of hydrochar derived from SS. These findings offered an effective approach to convert SS into clean solid fuel with the targeted regulation of heavy metals.