## 1 Introduction  
Hydrothermal carbonization (HTC) is a thermochemical process that can be used to convert fresh biomass into valuable carbon-rich materials, generally referred to as hydrocarbons [1]. In recent years, with the increasing attention towards environmentally friendly processes and renewable energy sources, HTC has attracted much interest given the low energy consumption of the process and the possible application of hydrochars as solid fuels. In fact, HTC is carried out at relatively low temperatures (180-250 \({}^{\circ}\)C) and under water pressure; hence, it is possible to use wet biomass without a preliminary energy-consuming drying process [2]. Moreover, hydrochars have a much higher calorific value than the initial biomass [3; 4]. Relevant to the use of hydrochars as fuel is, among other parameters, the higher heating value (HHV), which is related to char composition, and, combined with the mass yield of the process, determines the energy yield. In general, higher fixed carbon content, HHV and degree of aromatization render carbon materials more apt to replace fossil carbon sources. The structure and properties of chars produced by HTC depend on process parameters, such as the highest temperature reached, the residence time at the peak temperature, and the solid load expressed as the biomass-to-water ratio (B/W), with an interplay between the different parameters [4; 5]. Temperature has been found to determine the degree of carbonization, the calorific value, and mass yield, the first two increasing and the latter decreasing with increasing temperature [6]. The residence time has been observed to have a similar effect, although less pronounced [4; 7]. In general, higher energy yields are obtainedin milder conditions (i.e., temperature \(<\) 230 \({}^{\circ}\)C and residence time \(\leq\) 6 h) since, under more severe conditions, the HHV does not significantly increase, whereas the mass yield decreases [8]. Contrasting results are reported in the literature regarding the effects of the B/W ratio. HHV and mass yield were found to decrease with decreasing B/W [6], but in other cases the opposite trend in mass yield was observed [9]. Also feedstock type affects the HTC process. In fact, although all biomass mainly consists of the same three components, i.e., lignin, cellulose, and hemicellulose, in variable proportions, the interactions between lignin (less prone to degradation) and the other two components may alter their degradation pathway and kinetics [4]. Less important is the inorganic content since, due to dissolution in water, ashes in hydrochars are typically much less than in the initial feedstock. The low ash content represents an added value since it reduces the possibility of fouling, slagging, and corrosion during the combustion process [3; 10]. Given the issues related to organic waste management, the application of HTC to biomass from municipal, industrial, or agricultural wastes has been extensively investigated [4; 11]. Among the agricultural wastes investigated, olive tree trimmings, widely available in Mediterranean countries, have been recently studied [6; 12; 13; 14; 15; 16]. Olive pruning contains about 31-35% of water and therefore it is suitable for HTC, whereas it would require a drying step if pyrolysis, gasification or torrefaction were used. All studies have demonstrated the good energy densification of olive tree pruning hydrochar. For a thorough physico-chemical characterization of hydrochars, several instrumental techniques and analytical methods have been used, and among them \({}^{13}\)C solid-state Nuclear Magnetic Resonance (SSNMR) spectroscopy has revealed to be of great relevance [5; 17; 18; 19; 20; 21]. Indeed, this technique allows carbon functionalities of complex organic matrices, such as feedstocks and chars, to be (semi)quantitatively determined on whole samples, irrespective of their crystalline or amorphous nature, and without requiring any sample pretreatment.