## 3 Results  
### FTIR Spectroscopy  
The FTIR spectra of the feedstock (FS) and char samples after HTC in different conditions are reported in Figure 1. The FS spectrum (Figure 1a) showed features typical of lignocellulosic materials, with peaks mainly arising from functional groups in carbohydrates, i.e., cellulose and hemicellulose, and lignin [18; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34]. The bands in the 3000-2800 cm\({}^{-1}\) region and the broad band between 3700 and 3100 cm\({}^{-1}\) were ascribable to asymmetric and symmetric C-H stretching vibrations in methyl and methylene groups and to the O-H stretching vibrations in hydroxyl and carboxyl groups, respectively. These functional groups are present in both carbohydrates and lignin, while cutin, suberin, and extractives containing methylene and methyl groups may contribute to the bands between 3000 and 2800 cm\({}^{-1}\)[24]. Bands from carbohydrates were observed at 1732, 1375, 1210, 1165, 1060, and 1033 cm\({}^{-1}\), ascribable to C=O, C-H, C-O-C, and C-O deformation and stretching vibrations of different groups. In particular, the band at 1732 cm\({}^{-1}\) was assigned to C=O stretching vibrations of the carboxyl and acetyl groups in hemicellulose. Moreover, the bands at 1318 and 895 cm\({}^{-1}\) arise from the CH\({}_{2}\) rocking and the C-H deformation in cellulose, while the band at 1375 cm\({}^{-1}\) is assigned to C-H deformation in cellulose and hemicellulose. Bands ascribable to aromatic C=C and C-O stretching and bending vibrations of different lignin groups were detected at 1610, 1512, and 1265 cm\({}^{-1}\), while signals arising from aromatic C-H deformations were found between 900 and 700 cm\({}^{-1}\). Bands from C-H and C-O deformation, bending, and stretching vibrations of groups in carbohydrates and lignin were observed at 1457, 1430, and 1110 cm\({}^{-1}\). Upon HTC treatment, several changes occurred in the FTIR spectra (Figure 1b-h), associated with the transformation of olive trimming components during the carbonization process [5; 14; 18; 24; 26; 27; 28; 30; 31; 32; 33; 35; 36; 37; 38]. The broad O-H stretching, observable in the spectra of all hydrocarbons, showed a decrease in intensity with increasing treatment time for samples with B/W = 25% \(w/w\) and a reaction temperature of 250 \({}^{\circ}\)C; at the same time, the band maximum shifted to higher wavenumbers. On the other hand, this band was not affected by HTC at 180 \({}^{\circ}\)C, while the treatment at 250 \({}^{\circ}\)C for samples with B/W = 7% \(w/w\) only caused the band shift. A band at 3065 cm\({}^{-1}\) due to aromatic C-H stretching vibration was visible in the hydrocarbon spectra, although with very low intensity. The alkyl C-H stretching signals in the 3000-2800 cm\({}^{-1}\) region slightly increased in intensity with increasing treatment time and temperature. The band of hemicellulose at 1732 cm\({}^{-1}\), clearly distinguishable in the  
Figure 1: FTIR spectra of FS (**a**) and hydrocarbon samples HTC_25_180_30 (**b**), HTC_25_250_30 (**c**), HTC_25_250_60 (**d**), HTC_25_250_180 (**e**), HTC_7_250_30 (**f**), HTC_7_250_60 (**g**), and HTC_7_250_180 (**h**). feedback, remained visible after HTC at 180 \({}^{\circ}\)C, whereas it appeared as a shoulder of the adjacent C=O stretching bands at 1700 and 1652 cm\({}^{-1}\) upon treatment at 250 \({}^{\circ}\)C. The latter bands showed a significant increase in intensity with increasing treatment temperature, while they initially increased and then decreased again with increasing treatment time. The aromatic or furanic C=C and C=O stretching bands at 1610, 1512 and 1427 cm\({}^{-1}\), hardly visible in the feedstock spectrum, increased upon treatment at 180 \({}^{\circ}\)C and 250 \({}^{\circ}\)C for 30 min, remaining almost constant for longer treatments.