From olive waste to solid biofuel through hydrothermal carbonisation: The role of temperature and solid load on secondary char formation and hydrochar energy properties

tHydrothermal carbonisation was used to upgrade fuels from two types of agro-industrial wastes: olivetree trimmings and olive pulp. Hydrochar yield, elemental and proximate analyses, thermal stability,higher heating value (HHV), and energy yield at different reaction temperatures (120, 150, 180, 200, 220,235 and 250◦C) and solid load (biomass to water ratios − B/W − equal to 7, 10, 15 and 25%) were assessedfor a fixed reaction time of 30 min. HHV varied linearly with hydrochar mass yield and reaction temper-ature in the temperature range 180–250◦C. Solid load proved to be a crucial parameter in determiningthe energy properties of hydrochars. The higher B/W, the higher were the degree of carbonisation (interms of fixed and total carbon), the hydrochar HHV, and the hydrochar yield. Elemental analysis showedthat during HTC, olive pulp samples underwent a greater degree of carbonisation when compared to thecorresponding olive tree trimmings residues. High solid load and high reaction temperature promotedsecondary char formation. Secondary char showed a sphere-like structure formed by overlapping layers.EDS microanalysis showed that secondary char is characterised by a significantly higher carbon contentthan parent primary char, thus confirming its contribution towards enhancing the HHV of hydrochars.