Evaluation of the clean characteristics and combustion behavior of hydrochar derived from food waste towards solid biofuel production

•Hydrothermal carbonization of food waste for clean solid biofuel was developed.•Aromatic-S and sulfate-S increased in contrast to sulfoxide-S.•The combustion behavior and gas emissions pattern were estimated.•Temperature above 220 °C increased NO release for the nitrogen forms and ratio. This study investigated the clean properties and combustion behavior of hydrochar from food waste (FW). The hydrochar was characterized by proximate analysis and HHVs, and the chemical forms were investigated by FTIR, XPS (Sulfur), and XANES (Chlorine). TG-FTIR was used to assess the combustion behaviors, and XRF was used to assess the fouling and slagging inclinations. Results showed that increasing temperature from 180 °C to 260 °C enhanced the removal of N, S, and Cl for hydrochar. Especially, aromatic-S and sulfate-S increased in contrast to sulfoxide-S, and more inorganic-S was produced in hydrochar; all samples had mainly inorganic-chlorine. Additionally, the release of HCl, SO2, and NO of hydrochar combustion were significantly reduced and the SI and FI index decreased to ranges of 0.18–0.16 and 0.32–0.17, respectively. However, hydrochar produced at temperature above 220 °C led increased NO emission possibly due to formation of more pyridine-N and quaternary-N.