Slow-pyrolysis and hydrothermal carbonization of exhausted olive pomace: A comparative approach focused on the agro-chemical use of the solid char

Exhausted olive pomace (EOP) represents the main waste stream recovered from olive oil processing. This hardly exploitable waste biomass shows a prevailing contribution of non-structural water-soluble extractives (mainly phenolic compounds), proteins and inorganics. In this work, the comparison between slow-pyrolysis and sub-critical hydrothermal carbonization (HTC) of EOP has been investigated, in both cases aimed at the selective production of the char as the primary target product. In addition to the use of such carbonaceous materials as green adsorbents, also the development of agronomic applications, such as soil amendment, is attractive for improving environmental sustainability. For the exploitation of the “agro-char”, the optimization of the carbonization extent is fundamental, at the same time preserving the inorganics, which are indispensable components for the plant-soil system. After each thermochemical treatment, an exhaustive characterization of all the involved streams, with particular attention to the agrochemical characteristics of the solid char, was carried out, identifying the most appropriate reaction conditions for the successful fractionation of the biomass components. Regarding the HTC thermal treatment, the prevailing aliphatic behaviour of the corresponding hydrochars was demonstrated, highlighting only a limited carbonization progress, if compared with that of the pyrolysis-derived biochars. A hydrothermal processing temperature of 240 °C was appropriate to achieve the maximum EOP carbonization degree, compatibly with such sub-critical technology. The largest contribution of the labile carbon of hydrochars makes them suitable for short-term agronomic uses, such as the improvement of the physical properties of degraded soils and for bio-remediation purposes, exploiting their rich surface reactivity. On the other hand, developing the slow-pyrolysis approach, it was found that 600 °C resulted the most appropriate operating temperature, allowing the synthesis of a bio-char in good yield and with satisfactory agronomic properties, in terms of good carbon stability, alkalinity, soil exchange capacity and presence of valuable plant nutrients. This temperature was also effective for maximizing the recovery of a bio-oil with satisfactory energetic properties, at the same time keeping under control the formation of non-condensable gases. Bio-oils obtained from both thermal treatments were characterized for their physicochemical properties, identifying the presence of many oxygenated compounds, such as carboxylic acids, ketones, esters, and aromatics. Bio-oil could be directly used as a bio-fuel and/or for the production of added-value bio-products, after further appropriate upgrading, for example by hydrodeoxygenation path. Lastly, the analysis of the corresponding gaseous fraction has been performed, thus valorizing all the involved streams, in the perspective of the full valorization of the waste biomass, in agreement with the principles of the circular bio-economy.