## 3 Results and Discussion  
### The Influence of Temperature on Mass Yields during PW Recirculation  
Table 1 shows mass yields obtained after HTC tests performed with deionized water or recirculated PW as a solvent (deviations of data measured through Er% are shown in Table S1 in Supplementary Materials) As documented in the literature [16; 17], at step R0 (HTC runs carried out with deionized water), as temperature rises, a solid yield decrease and a gas yield increase were observed. More specifically, at 220 \({}^{\circ}\)C, the gas yield increased at the expense of the liquid one. Indeed, as reported in literature, only a small amount of cellulose has been degraded at 220 \({}^{\circ}\)C [52], therefore the increase in gas yield was due to decarboxylation of organic acids derived from hemicellulose hydrolysis and dehydration. At 250 \({}^{\circ}\)C, the enhanced carbonization of biomass led to a noticeable decrease of hydrochar mass yield and gaseous phase increase. The liquid fraction resulting from HTC process contains water soluble substances like carboxylic acids, sugars, and reactive intermediates such as furfural and HMF [31]. During recirculation steps (R1 and R2), these organic compounds reacted and influenced properties of the HTC products in a different way according to the process temperature. Dissolved organic matter in recirculated PW was involved in multiphase reactions. At HTC temperatures of 180 and 220 \({}^{\circ}\)C, a fraction of organic acids like acetic, lactic, propionic, levulinic, and formic [17] was degraded to result in the formation of further gaseous compounds. At 250 \({}^{\circ}\)C, recirculation of process water did not lead to an increase of gas mass yield, probably due to the relatively low concentration of these acids in the aqueous solution. An increase of hydrochar mass yield was observed at all three temperatures investigated; this effect was more pronounced at 180 \({}^{\circ}\)C (solid yield increase of about 6% after step R1). During second recirculation step, at temperatures of 180 and 220 \({}^{\circ}\)C, the solid yield increase was less pronounced while a modestly reverse trend was found at 250 \({}^{\circ}\)C.  
Effects of PW recirculation on mass yields could be explained by an interaction of different chemical reaction pathways. During HTC process, water in subcritical conditions catalyzes hydrolysis of hemicellulose, cellulose, and lignin to produce small-chain polymers and monomers [20]. These compounds are subsequently dehydrated to form reactive intermediates (e.g., furans and furfurrals) and organic acids [18; 53]. Organic acids, accumulated in the recirculated PW, could promote further hydrolysis and decomposition of biomass polymers leading to a growing concentration of soluble intermediates. The intermediates could be involved in condensation and polymerization reactions to form solid phase aromatic clusters [44]. When their concentration reaches a certain value, the \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Sample** & \multicolumn{3}{c}{**Mass Yields wt\% d.b.**} & \multicolumn{3}{c}{**Process Water**} \\ \hline  & **Solid** & **Liquid 1** & **Gas** & **pH** & **TOC (g/L)** \\ \hline
180\_R0 & 50.1\% & 43.7\% & 6.2\% & 3.87 & 16.06 \\
220\_R0 & 49.2\% & 40.7\% & 10.1\% & 4.40 & 13.30 \\
250\_R0 & 40.9\% & 44.4\% & 14.7\% & 4.66 & 9.70 \\
180\_R1 & 55.9\% & 37.6\% & 6.5\% & 3.95 & 18.82 \\
220\_R1 & 51.2\% & 37.6\% & 11.2\% & 4.49 & 16.92 \\
250\_R1 & 41.9\% & 43.4\% & 14.8\% & 4.69 & 14.25 \\
180\_R2 & 55.0\% & 38.1\% & 6.9\% & 3.97 & 20.08 \\
220\_R2 & 49.9\% & 38.7\% & 11.3\% & 4.50 & 17.79 \\
250\_R2 & 42.5\% & 42.8\% & 14.7\% & 4.70 & 18.06 \\ \hline \hline \end{tabular} 1  
\end{table} 
Table 1: Mass yields (dry basis) and process water characterization. Solid and gas fractions were measured after each HTC test performed in triplicate; average values showed (Er% \(\leq\) 1.5% for solid yield and 2.3% for gas yield). Average values of three pH and TOC measurements showed (Er% \(\leq\) 0.4% for pH and 0.5% for TOC).