The olive industries, especially in the Mediterranean countries are known to produce a significant amount of residues such as crude olive pomace (COP) and olive tree pruning waste (OTPW) which can lead to various environmental and societal issues if not treated or further utilized (Garcìa Martìn et al., 2020). Considering the fact that not all olive oil industries make use of these olive residues (COP and OTPW) for further processing, they seem to be a promising candidate to produce renewable energy due to their bioenergy potential and physicochemical properties. These residues could be valorized to produce liquid biofuels which would help creating a circular economy. This work aims to expand the current state-of-art of hydrothermal liquefaction (HTL) of olive residues such as COP and OTPW, which will contribute towards the profitability and sustainability of olive oil industries. This work will also serve as a basis to future studies such as development of a continuous process to support the high commercial demand of the liquid biofuel and carrying out a techno-economic assessment of the HTL process of COP and OTPW at this scale.

Due to the characteristics of the olive residues, especially their high moisture content and organic compounds (Ribeiro et al., 2020), the use of HTL has been proposed for the development of the thermochemical conversion process for bio-oil production. So far, only non-catalytic HTL processes have been investigated in batch scale laboratory setups. Thus, the main novelty of this work is that the effects of catalyst dosage (0 - 10 wt.%) along with reaction temperature (250 - 340 °C) and residence time (5 - 60 min) using a central composite design of experiments has been evaluated. The bio-oil and biochar fractions resulting from HTL were characterized by elemental analysis, bomb calorimetry, X-ray fluorescence (XRF), X-ray Diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy coupled with Energy dispersive spectroscopy (SEM-EDS), and Fourier-transform infrared spectroscopy (FTIR). From the experiments, it was proven that the production of liquid biofuels from olive residue biomass from Spain is possible under both, catalytic and non-catalytic routes. By implementing the central composite design experimental campaign, a set of mathematical models were developed for predicting the mass yields of the bio-oil, biochar, aqueous, and gas phase produced under combinations of the evaluated process variables.

The main finding of this work is that by using a catalyst, significantly higher mass yields of bio-oil were achieved with lower operational conditions when compared with the existing literature. The maximum bio-oil yield was found out to be 56.0% wt. on treating COP at 330°C for 60 minutes and 10 wt. % catalyst whereas for OTPW, the maximum bio-oil yield was found out to be 56.3% wt. at 330 °C for 30 minutes and 7.5 wt. % catalyst. The bio-oil produced under these conditions showed an average energy content of 23.21 MJ/kg. The highest high heating value (HHV) of 32.09 MJ/kg was obtained under non-catalytic HTL route at 340 °C and 15 minutes. The determination of the energy content (HHV) of the bio-oil fractions showed that there is a potential trade-off between the respective mass yield and HHV when using the catalyst.