Production of hydrochar fuel by microwave-hydrothermal carbonisation of olive pomace slurry from olive oil industry for combustion application

This work is the first investigation on microwave-assisted hydrothermal carbonisation (MHTC) of real olive pomace (OP) slurry from the olive oil industry to produce hydrochars with improved fuel properties for combustion applications (e.g., in boilers). Experiments were conducted based on the central composite design of response surface methodology with two main process variables: temperature (180–250 °C) and holding time (2–30 min). Severity factors (log R₀) were calculated from the above variables and used to explain the process effect in a simpler way. Increasing the MHTC severity resulted in significant changes in the structure of OP (studied by FTIR and NMR analyses) as well as reductions in yield, bulk density, volatile matter, and ash content in the hydrochars. The high-severity hydrochar was positioned in the lignite zone (van Krevelen diagram). It also exhibited substantial reductions in the alkali index (86.53 %), slagging index (76.89 %), and fouling index (96.07 %) compared to the raw material. Overall, the best conditions for hydrochar production with improved combustion characteristics were found to be 250 °C for 30 min (HHV = 28.45 MJ/kg, energy densification ratio = 1.25, equilibrium moisture content = 31.1 mg/g, comprehensive combustibility index = 2.94 × 10⁻⁷%² min⁻² ºC⁻³). These properties indicate that high-severity hydrochars could be utilised as biofuels for energy applications.