###### Abstract  
Hydrothermal carbonization (HTC) is a promising thermochemical pre-treatment to convert waste biomass into solid biofuels. However, the process yields large amounts of organic process water (PW), which must be properly disposed of or reused. In this study, the PW produced from the hydrothermal carbonization of lemon peel waste (LP) was recycled into HTC process of LP with the aim of maximize energy recovery from the aqueous phase while saving water resources and mitigating the overall environmental impact of the process. The effects of HTC temperature on the properties of solid and liquid products were investigated during PW recirculation. Experiments were carried out at three different operating temperatures (180, 220, 250 degC), fixed residence times of 60 min, and solid to liquid load of 20 wt%, on a dry basis. Hydrochars were characterized in terms of proximate analysis and higher heating values while liquid phases were analyzed in terms of pH and total organic carbon content (TOC). PW recirculation led to a solid mass yield increase and the effect was more pronounced at lower HTC temperature. The increase of solid mass yield, after recirculation steps (maximum increase of about 6% at 180 degC), also led to a significant energy yield enhancement. Results showed that PW recirculation is a viable strategy for a reduction of water consumption and further carbon recovery; moreover preliminary results encourage for an in-depth analysis of the effects of the PW recirculation for different biomasses and at various operating conditions. lemon peel waste; hydrothermal carbonization; hydrochar; process water recirculation; biofuel +
Footnote †: journal:  
_Applied system_  
_innovation_  
_Article_  
**Problemization of Lemon Peel Waste: Preliminary Results on the Effects of Temperature during Process Water Recirculation**  
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of the sector due to the possibility of direct treatment of wet biomass without the need of a drying pre-treatment [11]. HTC also known as wet torrefraction is a process where wet biomass is processed in a batch mode in presence of water at subcritical condition, temperature range typically between 180-250 \({}^{\circ}\)C and corresponding autogenous water pressure of 10-40 bar, for a residence time varying between few minutes and several hours (0.5-12 h) [12; 13; 14]. When compared to dry torrefraction, HTC revealed enhanced biomass degradation at same reaction temperature and residence time due to the catalytic role of water on biomass constituent degradation [5; 15]. Heterogeneous lignocellulosic structure of biomass is degraded by reaction routes mechanism such as hydrolysis, dehydration, decarboxylation, polymerization, poly-condensation, and aromatization [16; 17; 18; 19]. Some reactions are known to be exothermic, while others are endothermic. It has been recently reported that HTC process overall shows an exothermic behavior also depending on the nature of the starting material, reaction temperature, and solid load [13]. The exothermal character of HTC and of all hydrothermal processes of biomass is of high technological importance for the correct energy analysis and the design of the reaction system. The main advantage of HTC over other thermochemical conversion technologies (such as pyrolysis, gasification, and incineration) is its ability to convert the wet feedstock to a solid carbonaceous product (hydrochar) at relatively high yields without prior dewatering and drying [20]. Moreover HTC process has been efficiently applied to very low-value residues as the organic fraction of municipal waste [21; 22], sewage, and agro-industrial sludge to enhance sludge dewaterability and/or for the production of soil amendments [23; 24; 25; 26]. HTC has been also successfully used as a pre-treatment for the production activated carbons to be used as water remediation agents [27; 28]. Together with HTC, two other wet thermochemical technologies are nowadays being investigated to waste biomass upgrading.