Upgrading of olive tree trimmings residue as biofuel by hydrothermal carbonization and torrefaction: a comparative study

A comparative study of hydrothermal carbonization (HTC) in a 50 ml batch reactor and low temperature pyrolysis (LTP) in a fixed bed reactor was carried out on olive tree trimming residue with the goal of obtaining solid biofuels with higher energetic properties. Olive tree trimmings (OT) milled and sieved to 425-850 mm particle size range, was pyrolysed at peak temperatures between 200 and 325 °C and hydrothermally carbonizated at peak temperatures between 120 and 250 °C and biomass to water ratio 0.25. Residence time at peak temperature of 30 minutes and an inert atmosphere of N2 was used for both the thermal processes. The results of comparison, on the basis of fuel qualities of biochars obtained, showed that the hydrothermally prepared biochar had higher energy Densification while the torrefied biochar had higher energy yield due to higher mass yield. The hydrochar obtained at 200 °C showed similar Mass Yield (MY), Energy Densification Ratio (EDR) and High Heating Value (HHV) of the torrefied char at 250 °C; (HTC char 200 °C: MY = 73.0 % , EDR = 1.17, HHV = 23,180 kJ/kg; LTP char 250 °C: MY = 76.3 % , EDR = 1.18, HHV = 23,326 kJ/kg). Hydrocarbonization processes lead to an increase on the high heating value of the biochars, up to 27,257 kJ/kg, (250 °C peak temperature) with a corresponding Energy Densification Ratio (EDR) of 1.37 when compared with the starting material. More severe conditions (300 °C peak temperature) were needed for torrefaction to obtain a biochar with the same EDR and a high heating value of 27,206 kJ/kg.