Upgrading of green waste into carbon-rich solid biofuel by hydrothermal carbonization: The effect of process parameters on hydrochar derived from acacia

Abstract

This study investigated the influence of operating parameters on the physical and chemical properties of solid and liquid products after the hydrothermal carbonization (HTC) of green waste, e.g. - acacia: residence time (2 and 4 h), biomass to water ratio (1:10, 1:8, and 1:5) and temperature (180, 200 and 220 °C). The hydrochars had a higher carbon content (from c.a. 50–67%) which resulted in a two times higher fixed carbon and calorific value. Additionally, the structural features were investigated by a scanning electron microscope, van Soest fibre analysis (hemicellulose, cellulose and lignin), and Fourier Infrared Spectroscopy to confirm any changes in the chemical structure. Moreover, the thermal behaviour of the hydrochars under combustion conditions were studied. Collaterally, the liquid products were analysed by measuring pH and conductivity, which confirmed their acidic and polar character. Chemical Oxygen Demand with very high values was conducted indicating that the liquid phase contained a high concentration of organic matter and nutrients. The collected and analysed data revealed that residence time and reaction temperature were the most prominent factors which caused enhancement of the energetic properties of the hydrochars. Whereas, biomass to water ratio was found to be negligible.