## 4 Conclusions  
In this work, we demonstrated that the hydrothermal carbonization process can be successfully adopted for the complete upgrading of the orange peel waste into hydrochar and value-added chemicals. The main processing variables, reaction temperature, initial pH and residence time affect the mass yield (MY), while the solid to liquid ratio was found to be insignificant for all L/S investigated. Indeed, there was a strong correlation between temperature and residence time, suggesting that the role of these two variables cannot be analyzed independently. The highest yield of hydrochar was obtained at a 210 \({}^{\circ}\)C reaction temperature, 180 min residence time, 6/1 \(w\)/\(w\) orange peel waste to water ratio and a 3.6 initial pH. The results suggest that the conversion of the citrus waste occurs during the hydrothermal carbonization process due to a series of reactions such as decarboxylation and dehydration, leading to an improvement of the chemical, structural and morphological characteristics of the optimized hydrochar. This makes it a carbonaceous material suitable for a wide range of applications, both in the chemical and energy fields. \begin{table}
\begin{tabular}{c c c c c} \hline \multirow{2}{*}{**Product**} & **Solvent** & **(0.1 M H\({}_{2}\)SO\({}_{4}\))** & **(0.1 M H\({}_{2}\)SO\({}_{4}\))** & **(0.1 M H\({}_{2}\)SO\({}_{4}\))** \\ \cline{2-5}  & water: methanol & & & \\ \cline{2-5} Methyl-levulinate & (50:50) & 7.1 (\% yield) & 24.0 (\% yield) & 14.2 (\% yield) \\ \hline \multirow{2}{*}{Ethyl-levulinate} & water: ethanol & & & \\  & (50:50) & 4.9 (\% yield) & 16.7 (\% yield) & 7.5 (\% yield) \\ \hline \end{tabular}
\end{table} 
Table 5: Effect of acid conditions on the preparation of methyl-levulinate and ethyl-levulinate starting from OPW by using HTC process (Reaction conditions: 20 g of OPW; temperature: 180 \({}^{\circ}\)C; time: 60 min; autogenous pressure; stirring: 600 rpm). Figure 7: Co-solvent effect of HTC reaction temperature on the production of alkyl levulinates (Reaction conditions: 20 g of OPW; temperature: 180 \({}^{\circ}\)C; time: 60 min; autogenous pressure; stirring: 600 rpm). The bio-products distribution strongly depends on the applied reaction conditions. Overall, 180 \({}^{\circ}\)C was found to be the best reaction temperature that maximizes the production of furfural and 5-HMF in the presence of pure water as a reaction medium. On the other hand, by using a sulfuric acid solution as reaction media, levulinic acid can be easily obtained as a main product. Accordingly, a good production in methyl levulinate and ethyl levulinate can be achieved in the presence of methanol or ethanol, respectively, with the best yield obtained with methanol in the esterification reaction. Future research will be devoted to a prior recovery of more valuable compounds from OPW (e.g., limonene, pectin) before their hydrothermal conversion into hydrochar and bulk chemicals with a cascade high-to-low value approach. The following are available online at [[https://www.mdpi.com/article/10.3390/app112210983/s1](https://www.mdpi.com/article/10.3390/app112210983/s1)]([https://www.mdpi.com/article/10.3390/app112210983/s1](https://www.mdpi.com/article/10.3390/app112210983/s1)). Figure S1. TGA curve in air of the hydrochar samples. Figure S2. SEM-EDS analyses of (a) S-HC\({}_{210\text{-}60}\) and (b) S-HC\({}_{300\text{-}60}\). The inset shows the elemental analysis of these samples. Figure S3. X-ray diffarttograms of the hydrochar obtained at different temperature after (a) 180 min of reaction time and (b) 300 min of reaction time. Figure S4. FT-IR analysis of the hydrochar samples at (a) 60 min of reaction time; (b) 180 min of reaction time; (c) 300 min of reaction time. Table S1. Hydrochar yield as function of L/S ratio. Conceptualization: E.P. and C.E. ; methodology: E.P., P.S.C., D.I., F.M. ([PERSON]) and C.E. ; formal analysis: A.S., V.B. and F.M. ([PERSON]); writing--original draft preparation: all authors; writing--review and editing.