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This study aims at valorizing the residual aqueous phase from hydrothermal carbonization (HTC) of Sicilian agrowastes in order to enhance the hydrochar recovery, positively affecting the process energy balance. Process waters (PW) obtained from HTC and co-HTC using orange peel waste and fennel plant residues were used as recycled solvent in experiments carried out at the temperatures of 180 and 230 ◦C. The results showed that an additional hydrochar formation was promoted during recirculation of solvent, leading to average increments of solid mass yield of 10.5 wt% for tests conducted at 180 ◦C and 3.9 wt% for 230 ◦C. After five consecutive recirculation phases in co-HTC runs, the hydrochar yield increased up to 18.2 wt%. The low H/C and O/C atomic ratios values, found after recirculation, indicate that organic acids, accumulated in the PW, may catalyze the process and promote the biomass deoxygenation by boosting dehydration and decarboxylation. The recovered PWs from conversion steps with deionized water were also carbonized in absence of the solid feedstock in order to quantify their contribution in hydrochar formation during recirculation and thus the synergistic interactions. After recirculation, energy recovery averagely augmented by more than threefold, showing that the proposed strategy could significantly improve the sustainability of HTC.

Recirculation of hydrothermal carbonization (HTC) process water (PW) has been revealed as an effective strategy to promote the recovery of lost dissolved organics in the spent solvent. However, the role of operating parameters such as temperature, time and solid load during recirculation needs to be clarified and better investigated in order to achieve a successful PW valorization. In this paper, the effects of different reaction conditions during HTC with PW recirculation were studied by using orange peel waste as feedstock. The results showed that strong synergistic interactions occurred between the biomass and the recycled solvent, leading to a hydrochar mass yield increase between 0.5 and 11 wt% on a dry basis (d.b.), according to the reaction conditions. PW recirculation, especially at low conversion severity and solid load, increased hydrochar carbon content while drastically reduced the oxygen fraction. The reason was probably related to promoted dehydration and decarboxylation. As result, carbon and energy yields were respectively increased by 26.1 % and 27.0 %, on average. The energy balance outcomes showed that the energy recovery rose up to 8 times by recycling PW compared to phases with deionized water as HTC solvent.

This work explores the valorization of residual olive mill wastewater as a process aqueous medium for cohydrothermal carbonization (co-HTC) of typical Sicilian agro-wastes (tangerine and orange peel wastes). Co-HTC experiments were carried out at 180 and 220 ◦C to assess the interaction between the feedstocks. Synergistic effects increased the yield of hydrochar and gas phases while antagonism altered the formation of aqueous products. Compared to the expected value, hydrochar yield was increased by 37 wt%, on average, while the interaction effect on gas phase was weaker (+23 wt%, on average) and increased with temperature. Both the retention of unhydrolyzed primary char and the recapture of secondary char phases from process water enhanced the hydrochar recovery in different ways according to feedstock nature and co-HTC conditions. On the basis of hydrochars characterization through elemental analysis and surface functionality, the degree of carbonization was significantly improved after co-HTC due to promoted dehydration and decarboxylation reactions. As result, co-hydrochars exhibited a moderately higher energy content and a greater thermal stability compared to samples obtained from HTC of individual substrates. Co-hydrochars also retained relatively high amounts of nutrients such as phosphorus, potassium and calcium, which could enable their use as soil improvers.