Agro-waste to solid biofuel through HTC: the role of process variables on secondary char formation and hydrochar energy properties

Several research studies have appeared recently where HTC is used to upgrade fuels from agro-industrial wastes. Since the interest in HTC increased, scientists have focused on the influence of process parameters on mass yields and energy properties of hydrochars. In the present work, we evaluate the influence of HTC reaction temperature, reaction time, and solid load on hydrochars obtained from three agro-industrial wastes: olive tree trimmings, olive pulp, and Opuntia ficus-indica cladodes. The following ranges of the HTC process variables were tested: reaction temperature: 120-250 °C; reaction time: 0.5-3 h; dry biomass to water ratio (by weight) B/W: 0.07-0-30. Hydrochars were characterized in terms of mass yield, ultimate and proximate analyses, HHV, thermal stability, and morphological features (SEM analysis coupled with EDS microanalysis). In the reported set of experiments, solid load proved to be a crucial parameter in determining the energy properties of hydrochars. The lower B/W, the lower was the degree of carbonisation (in terms of total carbon and fixed carbon content), and hence the energy properties of hydrochars. Temperature and solid load proved to be crucial in promoting “secondary char” (or “coke”) formation, which was found in large amount only at high solid load and reaction temperature. Secondary char showed a sphere-like structure formed by overlapping layers. EDS microanalysis showed that secondary char is characterised by a significantly higher carbon content than parent “primary char”, thus confirming its contribution towards enhancing the energy properties of hydrochars.