Evaluation of biochar and hydrocar energy potential derived from olive mills waste: The case of Montenegro

Three different waste streams from olive oil production from Montenegro are utilized energetically by hydrothermal carbonization (HTC – hydrochar) and pyrolysis (biochar). Pyrolysis experiments of three-phase olive mill solid waste (3POMSW) were conducted for different pyrolysis temperatures of 450, 550, and 700 °C and a residence of 60 min for each experiment. HTC experiments for two-phase olive mill waste (2POMW) and a mixture of olive mill wastewater (3POMWW) and 3POMSW were carried out at 180, 200, and 220 °C, autogenous pressure of (1, 1.6, and 2.3 MPa) for 60 min. Increasing reaction temperature led throughout all experiments to a lower carbon recovery. Furthermore, all hydrochar and biochar samples had energy densification of more than one, indicating a higher mass-energy density than the initial feedstock. The highest energy yields were achieved for the 3POMW, with 46.48 % in hydrochar treated at 180 °C and 44.09 % in biochar treated at 450 °C. Concerning the hydrochar from 2POMW, the highest energy and mass yields of 62.53 % and 49.65 %, respectively obtained at 180 °C. Summarizing, the energy potential of Montenegro's olive mill waste (OMW) was estimated at 43.69 PJ, which roughly could cover the whole energy demand of the olive oil industry in Montenegro.