Energy recovery from biomass via hydrothermal processes‎: the case of olive oil mill waste in Morocco

Résumé

Olive oil production is a significant and widely practiced agricultural activity in Morocco and Mediterranean countries, covering an area of 11 million hectares. However, this industry is associated with the generation of huge amount of solid, liquid and semi-solid waste. The lack of a suitable management technique of these waste effluents can result in serious environmental problems for the producing countries. Two-phase olive mill waste is generated from the olive oil production via the 2-phases extraction method and consists of a mixture of waste waters and olive cake. Given its characteristics (high organic load and moisture content, acidic properties, rich in polyphenols) this residue has a highly polluting effect and needs to be well managed to avoid its negative impacts. Indeed, hydrothermal processing is a promising conversion technique that enables to process wet biomass, without a drying step, into carbon-rich materials (e.g. hydrochars) for energy, environmental, or agricultural applications. The primary objective of this work is to investigate the hydrothermal conversion of two-phase olive mill waste produced locally in Morocco. Therefore, several experimental studies were conducted using a 300 mL autoclave batch reactor. The effect of aqueous phase recirculation on hydrothermal carbonization of Moroccan two-phase olive mill waste was examined. The aqueous phase recirculation exhibited positive effects on the hydrochars yields and properties which can significantly contribute to the improvement of the hydrothermal carbonization efficiency and reduce it environmental impact. Hydrothermal liquefaction of two-phase olive mill waste was also studied at different operating conditions of temperature (260-300 °C), reaction time (0-60 min) and dry-matter/water ratios (0.05-0.15). The two-phase olive mill waste appears to be suitable for hydrothermal liquefaction and results in bio-oils and hydrochar with high energy contents. However, the slow heating rate (5 °C/min) resulted in higher hydrochar yields and limited the effect of reaction time due to the extended heating periods. Consequently, hydrothermal liquefaction using fast heating rate (60 °C/min) was performed in order to compare the effect of heating rate, temperature (260-300 °C) and reaction time (0-30 min). As expected, the experiments with fast heating rate resulted in lower hydrochar yields compared to slow heating. Interestingly, the effect of temperature and reaction time was also influenced by the increase in heating rate. Based on the obtained results, hydrothermal conversion of two-phase olive mill waste can represent an interesting solution for the use of this olive oil industry by-product for energy production purposes.

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