From sewage sludge and lignocellulose to hydrochar by co-hydrothermal carbonization: Mechanism and combustion characteristics

This investigation focuses on fabrication of hydrothermal carbon (HTC) using the co-hydrothermal carbonization (Co-HTC) method from the raw materials of sewage sludge (SS) and lignocellulose. The as-prepared hydrochar (HC) was characterized and analyzed by using ultimate analysis, proximate analysis, scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, and Fourier transform infrared spectroscopy (FTIR) to investigate its fuel properties, surface morphology, and molecular structure. Particularly, the HC obtained from the Co-HTC process with pine sawdust as raw material exhibited several advantages in terms of carbon content (40.40 %), fixed carbon (FC) content (41.82 %), and high heating value (HHV, 14.87 MJ/kg). Synergistic effects were achieved from the components in the Co-HTC system, leading to increased carbon content, retention of organics, energy yield, HHV, and HC yield. Notably, the Co-HTC treatment of pine sawdust and SS demonstrated the most notable synergistic effect. Thermogravimetric analysis and combustion characteristic index (CCI) values showed that softwood lignin, represented by pine sawdust, exhibited better performance during combustion, with a CCI value of 21.78 (10−7 min−2 °C−3). In short, Co-HTC of hydrochar from lignocellulose and SS can represent a promising approach to resource utilization for obtaining solid fuel.