Hydrothermal carbonization of olive pomace and determining the environmental impacts of post-process products

This study mainly aims to understand the potential of producing fuels with high carbon content from waste biomass, which is difficult to treat and dispose, through a hydrothermal carbonization method and to determine the environmental impacts of its post-process products. In this study, olive waste from oil mills and decentralized olive oil industries was used because this waste is difficult to dispose owing to the contained pollutants. During the hydrothermal carbonization process, the effects of the process parameters such as varying temperatures (220, 240, 260, 280, and 300 °C), residence times (1, 2, 4, 8, 12, 16, and 24 h), and waste biomass-to-water ratios (25–50%) on the quality of hydrochar produced were examined. The results indicate that the energy values increased significantly with the increase in temperature, duration of testing, and biomass/water ratio, while the hydrochar yield decreased with increasing temperature and residence time. The hydrochar energy values varied between 25.51 ± 0.01 MJ/kg and 32.67 ± 0.20 MJ/kg, whereas the hydrochar yields varied between 43% and 70%. Furthermore, acidic wastewater containing a high load of organic and inorganic compounds and gas containing a high percentage of carbon dioxide (70–90%) were determined as the post-process products generated from hydrothermal carbonization of the olive pomace. Further, wastewater from the post-process products was observed as a potential substrate for anaerobic digestion in the production of biogas. A life cycle assessment was also performed by emphasizing the need to review the environmental impacts of hydrothermal carbonization. LCA results demonstrate that the combination of hydrothermal carbonization and anaerobic digestion is more feasible than incineration for all environmental impact categories due to the substitution of electricity by biogas and hydrochar. The reliability of the results was supported by sensitivity and uncertainty analyses. Furthermore, the sensitivity analysis indicated a directly proportional relationship between the increase in the energy recovery rate in HTC system and environmental benefits.