Conversion of sewage sludge and corncob into clean solid fuel: Hydrochar properties and heavy metal chemical speciation

Mitigating heavy metals (HMs) risk while improving fuel properties remains an important challenge in solid fuel production. Hence, ferric citrate (FC) was selected as an additive to analyse its effect on co-hydrothermal carbonization (HTC) of sewage sludge (SS) and corncob (CC). SS and CC were converted to hydrochar at 230–260 °C and its chemical forms of HMs as well as fuel properties were evaluated. Results showed that temperature profoundly affects the influence of FC. At 260 °C and with the help of FC, a low ecological risk index was obtained. The fuel ratio, higher calorific value and combustion performance of hydrochar from co-HTC were enhanced and improved compared to hydrochar from SS. Additionally, FC further promoted dehydration and decarboxylation reactions. This study demonstrates that co-HTC with FC assisted is a viable strategy for producing high-quality biofuel with mitigated HM risks.