Influence of Process Parameters on the Hydrothermal Carbonization of Olive Tree Trimmings: A 13C Solid-State NMR Study

Chars obtained from the hydrothermal carbonization (HTC) of agricultural wastes are increasingly being employed as solid biofuels. Their properties are strongly dependent on HTC process parameters. In this study, 13C solid-state NMR spectroscopy was applied to semiquantitatively investigate carbon functionalities present in olive tree trimming feedstock and in the corresponding hydrochar samples. Hydrochars were obtained by HTC under different conditions, that is, at two different temperatures (180 and 250 °C), with two different biomass/water ratios (B/W of 7 and 25% w/w) and with reaction times at peak temperatures of 30, 60, and 180 min. The NMR analysis was complemented by infrared spectroscopy experiments. A detailed analysis of carbon functionalities and their evolution during HTC allowed the transformation of feedstock into hydrochar to be followed and the structure of hydrochars to be correlated to the different reactions occurring during HTC in dependence on reaction time, temperature, and B/W ratio, as well as to the hydrochar properties fundamental for their application as solid biofuel reported in previous studies. 13C solid-state NMR spectroscopy revealed a powerful tool for explaining hydrochar properties as a function of HTC parameters.