## 3 Results and Discussion
### Hydrochar Solid Fraction
#### 3.1.1 Hydrochar Yields and Chemical-Structural Characteristics
As evidenced in Figure S3, the reaction time is irrelevant on the crystalline structure of the samples obtained at different reaction temperatures. This phenomenon can be explained by considering that the crystalline structure of cellulose in the lignocellulosic biomass has well-packed long chains characterized by strong hydrogen bonding networks, which maintain the sugar ring assembly promoted by the hydrolysis reaction occurring during the hydrothermal carbonization process [35]. Figure 2: SEM analysis of (**A**) S-HC\({}_{210-60}\) and (**B**) S-HC\({}_{300-60}\). FTIR spectra at different reaction temperatures and reaction times are reported in Figure S4 in ESI. As can be seen from the FT-IR spectra, the reaction time does not affect the structural properties of the samples obtained at different reaction temperatures, therefore an example of a summary of the structural properties and N\({}_{2}\) adsorption-desorption isotherms obtained at various temperatures and at a fixed reaction time of 60 min is reported in Table 2. The adsorption bands at 1608 cm\({}^{-1}\) and 1701 cm\({}^{-1}\) correspond to the C=C vibrations and the C=O band, respectively, displaying the asymmetric stretch of aromatic rings, carbonyl, quinone, ester or carboxylic groups probably with a small quantity of amide, revealing the decarboxylation reaction and the aromatization of the orange peel waste during the hydrothermal process. The peaks at 1026 cm\({}^{-1}\) can be attributed to the C-O stretching vibrations of carboxylic acids and esters or carbohydrates, while the band at about 3300 cm\({}^{-1}\) suggests the O-H stretching of hydroxylic or acboxyl functionalities. Aliphatic C-H bands are found at 3000-2800 cm\({}^{-1}\). The spectrum of S-HC\({}_{180\text{--}60}\) reveals a peak at 1120-1050 cm\({}^{-1}\), possibly related to a C-O band of the lignocellulosic component of the orange peels still present. According to XRD data, this peak decreases in intensity when increasing the treatment temperature. The presence of a peak at 1527 cm\({}^{-1}\) strongly suggests the presence of a nitro-compound characterized by the N-O asymmetric stretching. The peak at 1431 cm\({}^{-1}\) may also correspond to the asymmetric and symmetric stretching of the carboxylate (COO-) group. The fragment values became stronger and broader when experiments are carried out at a higher temperature, in agreement with TGA and EDS analysis confirming that during the HTC process a series of dehydration, decarboxylation and aromatization reactions occur, with a consequent decrease in the hydrochar yield as well as an enrichment in C content. Moreover, the presence of an absorptions peak at 1026 cm\({}^{-1}\) in the S-HC\({}_{180\text{--}60}\) sample, related to the C-O stretching vibration, can be again related to the presence of small amount of crystalline cellulose. As expected, upon increasing the reaction temperature, the absence of such a peak is indicative of the full conversion of OPW into biochar and bio-oil. Values of BET surface area and pore volume, reported in Table 2, relative to S-HC samples, at various temperatures and at a fixed reaction time of 60 min in accordance with other hydrochar prepared in similar experimental conditions [37] show that both parameters increase about three-four times when increasing the reaction temperature going from 180 to 300 \({}^{\circ}\)C.  
Figure 3: X-ray diffractograms of the hydrochar obtained at different temperatures after 60 min of reaction time. (\(\bullet\) reflections of crystalline cellulose; \({}^{\star}\) reflections of turbostratic structure of disordered carbon).