## 3 Results and Discussion
### Hydrochar Solid Fraction
#### 3.1.1 Hydrochar Yields and Chemical-Structural Characteristics
Termogravimetric (TGA) and energy-dispersive spectroscopy (EDS) analysis (shown in Figures S1 and S2 in ESI) of the different hydrochar samples, obtained under different hydrothermal temperatures at a fixed residence time of 180 min, confirm the noticeable role of reaction temperature in the yield and elemental composition of hydrochar. Apart from the S-HC\({}_{180-180}\) sample, the breakdown of all samples in the air's atmosphere takes place in three stages. Until 200 \({}^{\circ}\)C the weight loss (stage I) is very low (less than 2%), probably due to the hydrophobic nature of the hydrocarbons, which hinders the absorption of a large water quantity inside their structures. When increasing the temperature a large weight loss (stage II) starts, centered at around 300 \({}^{\circ}\)C and associated with the release \begin{table}
\begin{tabular}{c c c} \hline \hline
**Sample** & **LogR\({}_{0}\)** & **HC Yields (wt\%)** \\ \hline S-HC\({}_{150-60}\) & 3.25 & 13.27 \\ S-HC\({}_{180-60}\) & 4.13 & 18.74 \\ S-HC\({}_{180-180}\) & 4.61 & 35.37 \\ S-HC\({}_{180-300}\) & 4.83 & 36.40 \\ S-HC\({}_{210-60}\) & 5.01 & 49.10 \\ S-HC\({}_{210-180}\) & 5.49 & 54.05 \\ S-HC\({}_{210-300}\) & 5.71 & 36.13 \\ S-HC\({}_{240-60}\) & 5.90 & 29.45 \\ S-HC\({}_{240-180}\) & 6.37 & 28.66 \\ S-HC\({}_{240-300}\) & 6.59 & 24.48 \\ S-HC\({}_{270-60}\) & 6.78 & 24.53 \\ S-HC\({}_{270-180}\) & 7.26 & 22.34 \\ S-HC\({}_{270-300}\) & 7.48 & 20.39 \\ S-HC\({}_{300-60}\) & 7.66 & 20.40 \\ S-HC\({}_{300-180}\) & 8.14 & 20.87 \\ S-HC\({}_{300-300}\) & 8.36 & 19.69 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Hydrochar yields as a function of logR\({}_{0}\). of organic volatile matters due to the decomposition of hemicellulose and cellulose. The successive stage (III) is related to surface and bulk hydrochar oxidation and suggests that the progressive loosing of the organic constituents, originally present in the orange peels, and the surface groups of hydrochar are formed at different hydrothermal temperatures, leading to samples production characterized by a different chemical composition. Furthermore, EDS spectra of S-HC\({}_{210-60}\) reveals that at a lower temperature of reaction, many residual oxygenated groups still remain on the surface, evidenced by a high content of oxygen (about 20 wt%) and a lower content of carbon (ca. 78 wt%). On the contrary, an increasing of the reaction temperature up to 300 \({}^{\circ}\)C causes a decrease of the oxygen content (12.1%), ascribed to dehydration processes from cellulose to hydrochar [35] and a consequent enhancement of the carbon content (ca. 88 wt%). As above mentioned, the reaction temperature influences not only the yields of the obtained hydrochar samples but also its morphology and composition, as revealed by SEM experiments. The morphology of samples prepared at lower hydrothermal temperature (Figure 2A) is not very dissimilar to that of the pectin and lignocellulosic (the main components of the orange peels raw material) [36]. On the other hand, at 300 \({}^{\circ}\)C the material produced shows the presence of the characteristic hydrochar microspheres. XRD analysis was used for the determination of crystalline structures in the hydrochar samples (Figure 3). The reflections of crystalline cellulose at 16 and 22.6 theta are noticeably evidenced in hydrochars obtained at the lowest temperature. As the temperature of the hydrochar preparation increases, a gradual shift from orderly crystallites to transition crystallites can be noticed in XRD spectra. Upon further increases of the hydrothermal temperature, these peaks disappear, whereas new broad peaks, at around 25 and 40 2-theta (*), related to turbostratic structure of disordered carbon coming from the (100) plane of graphite appear and gradually grow in intensity. Further increasing of the temperature to 300 \({}^{\circ}\)C leads to a complete conversion of the \"crystalline stage\" to the \"amorphous stage\", as diffraction peaks at 300 \({}^{\circ}\)C suffer from broad shape, low intensities and low signal to noise ratio.