Experimental investigation on high-temperature hydrothermal carbonization of olive pomace in batch reactor

Biomass hydrothermal carbonization (HTC) is the thermochemical conversion of cellulose, hemicellulose, lignin and lipids into organic, homogenized, carbon rich and energy dense solid fuel, called hydrochar. Process occurs under high – temperature and pressure conditions in the presence of subcritical water. The HTC process represents an effective alternative solution to the common treatments for wet biomass, i.e. composting and anaerobic digestion, over which HTC has several advantages. It is faster compared to the conventional treatments, and it is not affected by inhibiting or toxic substances. Moreover, the HTC process generates a solid product exploitable in different fields such as energy production, soil improvement or raw material for high added value applications. In this study, olive pomace is analyzed as raw material in HTC. A batch reactor (Vreactor: 5.0 L; Pdesign: 100 bar; Tdesign: 310 °C) was designed and constructed to investigate the HTC process in terms of mass yield, product composition and High Heating Value of the generated hydrochar. Experiments with temperature in the range of 260 °C to 305 °C (remaining in subcritical conditions) and reaction time from 60 to 180 minutes were carried out to investigate the mass yield of the produced hydrochar, the concentration of carbon in the produced solid and the High Heating Value (HHV). Depending on the test conditions, the obtained char showed a HHv up to 31.14 MJ/kg, with a definitive increase with respect to 22.4 MJ / kg of the initial olive pomace biomass. The effects of the HTC high temperature process on the olive mill waste water were also investigated in terms of residual Biological Oxygen Demand (BOD)5.