Co-hydrothermal carbonization of coal waste and food waste: fuel characteristics

The goal of this study was to upgrade fuel properties of two different bituminous coal wastes (CW) by reducing their ash, sulfur, and chloride content through co-hydrothermal carbonization (co-HTC) with food waste (FW). CW and FW were mixed in 1:1 (dry wt. basis), ratio and co-HTC experiments were performed at three different temperatures of 180, 230, and 280 °C for 30 min. Mass yield, proximate and ultimate analyses, energy content, and combustion indices of hydrochars were evaluated for the fuel properties. Results showed that co-HTC at 230 °C produced hydrochar with maximum fixed carbon, elemental carbon, and higher heating value (HHV), as well as minimum sulfur and ash for both CW samples. At 230 °C, the elemental carbon of co-HTC hydrochar was increased to a maximum of 46.7 and 49.5%, and the energy content increased to 22.4 MJ/kg and 23.0 MJ/kg for CW1FW (co-HTC hydrochar of CW1 and FW) and CW2FW (co-HTC hydrochar of CW2 and FW), respectively. Additionally, the presence of FW in the treatment process reduced the sulfur content to 2.1 and 1.4% compared with the raw CW values of 9.6 and 8.5% for CW1 and CW2, respectively. Combustion indices also indicated that CW2FW-H230 hydrochar had combustion indices similar to bituminous coal.