Production of char from sewage sludge employing hydrothermal carbonization: Char properties, combustion behavior and thermal characteristics

A comprehensive study was conducted on the properties of chars which derived from hydrothermal carbonization (HTC) of sewage sludge (SS) while varying the operation parameters as reaction temperature 180–300 °C, reaction time 30–480 min. The properties of chars derived from municipal sewage sludge HTC were evaluated by Elemental analysis, FTIR spectra and thermogravimetric analysis. Elemental analysis showed that O/C and H/C atomic ratios reduce to 0.07 and 1.84, respectively at 300 °C, 30 min due to dehydration and decarboxylation reactions were significant during HTC process. Higher heating values of the chars have improved 1.02–1.10 times of SS (10.97 MJ/kg) when reaction temperature rose to 260 °C with holding time between 30 min and 90 min. The approximate equilibrium moisture content becomes stable at 48.5% after 60 min reaction. The FTIR spectra determined that the HTC reaction improved dehydration on the OH band in SS. Higher temperature improved carbon aromaticity during HTC of SS. The thermogravimetric analysis is an effective means to analyze combustion behavior and thermal characteristics. It was also used to determine combustion kinetics of SS and chars. The result shows that the ignition temperature of chars was higher and has better performance on safe handling, storing and transportation than SS.