## 3 Results
### \({}^{13}\)C SSNMR Spectroscopy
A more detailed and quantitative description of the transformation of organic matter during the different HTC processes could be obtained by performing a deconvolution of the  
Figure 2: \({}^{13}\)C CP-MAS NMR spectra of FS (**a**) and hydrochar samples HTC_25_180_30 (**b**), HTC_7_250_30 (**c**), HTC_25_250_30 (**d**), HTC_7_250_60 (**e**), HTC_25_250_60 (**f**), HTC_7_250_180 (**g**), and HTC_25_250_180 (**h**). \({}^{13}\)C CP-MAS NMR spectra. The procedure was first applied to the FS spectrum (Figure 2a) to analyze signals from the different feedstock components in detail. The deconvolution of the hydrodynamic spectra (Figure 2b-h) was performed, keeping the chemical shift values of the feedstock signals fixed and using the minimum number of additional signals required to achieve a good reproduction of the experimental spectra. In the aliphatic region of the hydrochar spectra, the main contributions were found at 41, 36, 30, 26, and 17 ppm, some of them already present in the FS spectrum, increasing in intensity with treatment time and temperature. In the region between 110 and 165 ppm, signals at 110, 120, 125, 129, 132, 139, and 144 ppm were found, in addition to those of lignin at -147 and 153 ppm and signals with lower intensity at 149 and 158 ppm. The additional signals can be ascribed to aromatic and furanic structures formed during HTC by the transformation of carbohydrates, while the signal at 110 ppm can be ascribed to furan units. Those at 125 and 132 ppm are assigned to arena units and those at 147-152 ppm to both arene and furan units [51; 52]. Based on the relative intensities of these signals, the aromatic structures were found to prevail over the furanic ones for all hydrochar samples. The integral intensities of the signals obtained from the spectral deconvolutions were therefore determined over the seven spectral regions indicated in the Materials and Methods section. The relative intensities are reported in Figure 3.  
As can be observed from Figure 3, carbohydrates, mainly responsible for the intensity in the 60-110 ppm region, are the main component of olive tree trimmings, and they are only partially decomposed during HTC, the decomposition being more severe when the process is conducted at higher temperature, higher solid load, and for more extended periods. In the HTC process alkyl and aromatic structures are formed, which add to those already present in the feedstock due to recalcitrant waves, cutin, suberin, and lignin. As a result, aromatic components progressively increase by increasing HTC temperature, time, and B/W ratio, whereas allyl components increase by increasing HTC temperature and B/W ratio but do not show a clear trend with reaction time. The lignin O-CH\({}_{3}\) signal between 45 and 60 ppm is present in all spectra, confirming lignin recalcitrance to hydrothermal carbonization. The contribution of carboxyl structures decreases by prolonging the HTC  
Figure 3: Relative integral intensities of signals in the indicated spectral regions of \({}^{13}\)C CP-MAS NMR spectra of FS and hydrochar samples. process, whereas an opposite trend is observed for carbonyl groups, the formation of which is observed only after treatment at 250 \({}^{\circ}\)C and is favoured by the higher solid load.