Insights into the mechanisms of volatile organic compounds formation during the hydrothermal carbonization of olive tree sawdust, rice husk and wine pomace

Hydrothermal carbonization is increasingly used to convert wet biomass residues into carbon-rich hydrochar, yet fresh hydrochar can exhibit strong odours that indicate retained volatile organic compounds. In this study, hydrochars were produced from wine pomace, olive tree sawdust, and rice husk at two hydrothermal carbonization temperatures (180 and 220 °C) and two residence times (2 and 6 h). The volatile organic compounds emissions were quantified in a microchamber at 23, 40 and 80 °C, collected on Tenax under a controlled nitrogen flow, and analysed by thermal desorption gas chromatography and mass spectrometry. Results are reported as mass-specific emission factors (micrograms per gram per hour) using toluene-equivalent semi-quantification and class-resolved interpretation.Across all conditions, olive tree sawdust hydrochars emitted the highest concentrations of volatile organic compounds, followed by rice husk and wine pomace hydrochars, with grand mean values of 86.1, 33.5, and 9.2 µg⋅g−1⋅h−1, respectively. Peak emissions were observed for olive tree sawdust hydrochar produced at 220 °C for 6 h and extracted at 80 °C (152.7 µg⋅g−1⋅h−1); corresponding maxima for rice husk and wine pomace were substantially lower. Increasing microchamber temperature consistently mobilized semi-volatile compounds and shifted profiles towards less volatile fractions, supporting temperature stepping as a rapid diagnostic for batch comparison.Oxygenated compounds dominated overall. Rice husk hydrochars showed a stronger carbohydrate-derived furanic signature, whereas olive tree sawdust hydrochars additionally released abundant lipid- and phenolic-related compounds. Feedstock selection emerged as the primary lever to reduce emissions, while process severity and handling temperature further shaped both magnitude and composition.