Co-hydrothermal carbonization of sewage sludge and coffee grounds: combustion characteristics and hydrochar properties

To simultaneously address the pressing challenges of sewage sludge (SS) disposal and coffee grounds (CG) valorization, herein, co-hydrothermal carbonization (Co-HTC) of SS and CG was performed at a dry mass ratio of 1:2 over a temperature range of 180–260 °C to evaluate hydrochar properties and synergistic interactions. Results revealed that hydrochar produced from SS alone (SS260) exhibited poor fuel quality with an ash content of 64.04% and a higher heating value (HHV) of merely 8.56 MJ·kg − 1 . In contrast, co-hydrochar (SC260) demonstrated substantially enhanced fuel characteristics: ash content reduced to 38.53%, fixed carbon increased to 10.53%, and HHV elevated to 15.11 MJ·kg − 1 (a 76.5% increase relative to SS260), achieving an overall energy recovery efficiency of 64%. Combustion performance was also markedly improved, with SC180 exhibiting a combustibility index (C i ) of 1.06 and a comprehensive combustion index (S) of 6.37, values far exceeding those of raw SS (0.75 and 2.92, respectively). Notably, significant synergistic effects were observed during Co-HTC: SC260 achieved a carbon retention synergy coefficient of 7.92% and a yield synergy coefficient of 3.05%. Additionally, the resulting co-hydrochar possessed a higher specific surface area (31.54 m 2 ·g − 1 ) and more developed porous structure compared to single-feedstock hydrochar. These findings demonstrate that Co-HTC of SS and CG is an effective strategy to upgrade hydrochar fuel quality, offering a technically viable approach for the clean and sustainable co-valorization of these two problematic organic wastes.