## 4 Conclusions  
Hydrothermal carbonization of agro-waste has been shown to be a viable technology to convert lignocellulosic waste biomass into energy dense solid biofuels. To mitigate the environmental effect and increase the economic appeal of HTC, the produced liquid fraction, which is highly concentrated in organic by-products such as carboxylic acids, sugars, and other water-soluble substances, must be properly valorized. This work described the effects of process water recirculation on solid and liquid products obtained after HTC tests carried out at temperatures of 180, 220, and 250 \({}^{\circ}\)C using dried lemon peel waste as starting material. The aim was to detect temperature influence, during the use of recirculated PW as a solvent, on hydrochar yield and properties and to suggest chemical pathways that could be involved during recirculation. Results have shown that PW recirculation can influence hydrocarbons properties positively. Hydrochar yield increased slightly with PW recycling and this effect was more pronounced at lower temperatures. Moreover, PW reuse has resulted in an increase of the energy yield. Hydrolysis of biomass macro-components could be promoted by organic acids dissolved in the recirculated PW leading to an increase of such compounds in the liquid phase promoting their reactivity. These compounds could undergo condensation reactions to yield more complex solid aromatic structures causing a solid mass yield increase. In the first recirculation step, as HTC temperature increased, the hydrochar yield increase was less pronounced. Indeed, an increase of about 6% was observed at 180 \({}^{\circ}\)C while solid yield increase decreased to 2% and 1% at 220 and 250 \({}^{\circ}\)C, respectively. In the second recycle step, at temperatures of 180 and 220 \({}^{\circ}\)C, a lower solid yield increase, when compared to the first recycle step (solid yield increase of 5% and 1% respectively), while at 250 \({}^{\circ}\)C, the delayed accumulation of reactive organic matter probably led to a greater increase in hydrochar production (hydrochar yield increase of 2%). Volatile matter, fixed carbon, and HHV of hydrocarbons were only slightly influenced. Further experiments are required to assess how HTC operating conditions and number of recirculation steps could improve hydrochars physicochemical properties and optimize the process for different biomasses as well as supporting the suggested reaction pathways. Moreover, an energy balance is required in view of an actual use of this strategy for large-scale applications. In conclusion, PW recirculation appears to be a suitable method to utilize the liquid by-product of HTC process, recovering energy and mitigating the possible environmental impact of the process itself. This strategy is aimed at both further carbon and energy recovery and reduction of water consumption for a cleaner production system of renewable solid biofuels. The following are available online at [[https://www.mdpi.com/2571-577/4/1/19/s1](https://www.mdpi.com/2571-577/4/1/19/s1)]([https://www.mdpi.com/2571-577/4/1/19/s1](https://www.mdpi.com/2571-577/4/1/19/s1)), Figure S1: Piping and Instrumentation diagram (P&Id) of the HTC experimental system (TC = thermocouple; PI = pressure gauge; PID = temperature controller), Figure S2: HTC apparatus: (a) HTC system during heating phase; (b) HTC reactor cooled down through a massive stainless steel disc at \(-10\)\({}^{\circ}\)C; (c) HTC reactor cooled down through a stainless steel radiator with flowing tap water, Table S1: Mass yields (dry basis) and process water characterization (Er% with three replications), Table S2. Proximate analysis and energy properties of raw LP and hydrocarbons (Er% with two replications). Conceptualization, A.P., M.V., and A.M.; methodology, A.P., M.V., and A.M.; validation, A.P. and M.V. ; investigation, A.P. ; resources, A.M., G.D.B., and M.G.G. ; data curation, A.P., M.V., and A.M.; writing, original draft preparation, A.P. ; writing--review and editing, M.V. and A.M.; visualization, A.M. and G.D.B.