The olive oil industry is an important source of agricultural residues throughout its value chain, ranging from intermediate process slurries to relatively dry content pruning residues. Among them, crude olive pomace (COP) is of particular interest since it is abundant, low cost and can be a promising source for bioenergy. Nevertheless, because COP is phytotoxic and has a high moisture content and low energy density, it represents a challenge to conventional processes that usually require a dry and homogenous material. The main novelty of this study is the use of a transition metal catalyst and a central composite design (CCD) approach to optimize the conversion of COP through hydrothermal liquefaction (HTL) into valuable products. Results show that catalytic HTL is capable of converting up to half of the COP into bio-oil. Higher process temperatures resulted in lower bio-oil yields but larger higher heating value (HHV) and lower N content. The bio-oils produced at higher temperatures also show lower concentration of phenols and regarding biochar, a low inorganic content. Without any further upgrading, COP bio-oils produced by HTL are rich in valuable compounds such as oleic acid, phenolic compounds and ketones that can be used in the polymer industry or as chemical intermediates. The highest bio-oil yield was 51.96 wt% at 330 ºC for 30 min and 7.5 wt% catalyst with a HHV of 22.0 MJ/kg. At those operational conditions, the biochar yield was 16.49 wt% with a HHV of 8.9 MJ/kg. The major minerals found in the biochars (CaO, SiO₂ and P₂O₅) suggests that biochar could be well-suited for use in soil applications or as materials for adsorption, especially the non-catalytic ones. Furthermore, the experimental results acquired from HTL of COP were used to develop a global kinetic model. Using an explicit Runge-Kutta method, the kinetic parameters were calculated. After comparing the global kinetic model with a linear system of ordinary differential equations (ODEs) based on the CCD models, results indicate that this approach is more effective in predicting the yields of HTL products.

An urgent ecological issue is the threat posed by invasive species, which are becoming more widespread especially in Africa. These encroachments damage ecosystems, pose a threat to biodiversity, and outcompete local plants and animals. This article focuses on converting Acacia Mellifera from Namibia, commonly known as encroacher bush (EB) into high-quality drop-in intermediates for the chemical and transport industry via hydrothermal liquefaction (HTL). HTL tackles the growing need for sustainable energy carriers while simultaneously halting the spread of the invasive species. A surface response methodology was used to optimize the HTL process for the following operational conditions: temperature (250–340 °C), residence time (5–60 min) and catalyst loading (0–10 wt%). The catalyst of choice was determined after evaluating the energy recovery (ER) of four different catalysts (Zeolite, La₂O₃, Hydrotalcite, Ni/SiO₂–Al₂O₃) under the same HTL operational conditions. The results indicate that the addition of hydrotalcite results in high yields of bio-crude oil (13–28 wt%), without compromising the high heating value (HHV, 26–31 MJ/kg), water content (0.47 wt%) or increasing the content of oxygenated compounds compared to the non-catalytic experiment. For the experimental conditions tested, we observed a global maximum in conversion in the 330 °C and 30 min range. Our findings indicate that the most significant factor on the conversion of EB into bio-crude oil was temperature, followed by the catalyst loading. Furthermore, biochars produced at 330 °C and 30 min show potential as solid biofuels with HHVs up to 28.30 MJ/kg.

The oil palm industry has been under public scrutiny during the last decades due to environmental and social issues related to its practices. Oil palm (Elaeis guineensis Jacq.) trunks (OPTs) are of special interest as they are left idle in the field after the replanting process which is performed every 25 years. This common practice results in harvesting challenges, phytosanitary risks, and a loss of bioenergy potential. Due to their high moisture content and fibrous nature, OPTs present a problem for traditional conversion processes that require a dry and homogeneous material. This study evaluates the feasibility of converting OPTs into a bio-crude oil and biochar to increase the sustainability of the oil palm sector. To date, research efforts have primarily focused on hydrothermal liquefaction (HTL) of OPT without catalysts, resulting in a limited understanding of the potential of OPTs. Thus, the main novelty of this work is the evaluation of the effects of catalyst dosage (0–5 wt%) on the bio-oil yield, reaction temperature (260–300^∘C), and residence time (15–60 min) using a half-fraction experimental design methodology. For this, OPTs extracted from two plantations in Guatemala were used. The maximum bio-oil yield (26.77 ± 3.60 wt%) was found at 260^∘C for 15 min and 5 wt% catalyst with a high heating value (HHV) of 19.29 ± 1.33 MJ kg⁻¹. Nonetheless, the bio-oils produced without a catalyst at 300^∘C and 15 min have higher HHV (27.63 ± 1.35 MJ kg⁻¹) and are similar to Diesel fuel based on their H/C and O/C ratio. These results indicate that there is a potential trade-off between the bio-crude oil mass yield and HHV when using the catalyst.