## 4 Discussion
Moreover, some stable lignin crystalline structures may undergo demethylation and pyrolytic reactions without fragmentation [1]. As a result, minor chemical and physical modifications occur to lignin during the HTC process, especially under mild conditions [32; 59]. The interconnected network formed by non-dissolved pyrolyzed cellulose and lignin is usually referred to as primary char. The aliphatic structures increase upon HTC treatment. This trend indicates, on one side, the recalcitance to hydrolysis of aliphatic structures present in waves, cutin, and suberin and, on the other, the formation of new alkyl groups from the dehydration of the alkoxy ones [49; 60]. On the whole, the HTC treatment in the mild conditions reported here gives rise only to a partial carbonization of the feedstock, as previously observed for different biomasses [5; 18; 21; 35; 61; 62]. The transformation from a lignocellulosic composition to a mainly aromatic network, typically observed for biomasses upon HTC at much higher temperatures [31; 32], only partially occurred during the HTC processes carried out here. The biomass load (B/W = 25% \(w/w\)_vs._ B/W = 7% \(w/w\)) mainly affects the decomposition and carbonization rates which are higher for the higher B/W ratio (Figure 3). Our spectroscopic data (Figures 2 and 3) indicate that hydrochars are constituted of hydrophobic aromatic/aliphatic structures, but are also rich in reactive oxygen functional groups (i.e., hydroxyl/phenol, carboxyl, or carbonyl). Moreover, aromatic structures prevail over furanic ones, and residual feedstock components (mainly lignin) are conserved in the hydrochar samples. The obtained results can help to explain some properties of hydrochars from lignocellulosic biowaste at the molecular level and the HTC mechanisms reported in previous studies [6; 12; 13]. The increase in carbon content and decrease in oxygen content observed by increasing the HTC temperature (from 180 to 250 \({}^{\circ}\)C) and reaction time (from 30 to 180 min) is strongly correlated to the progressive carbonization of carbohydrates by dehydration and decarboxylation reactions, as well as to the preservation of lignin and alkyl components. The prevalence of aromatic over furanic structures suggests that primary char is mainly formed under the explored HTC conditions, in agreement with ref. [13; 21]. The structures detected by the NMR experiments are also compatible with the higher HHV, energy densification ratio (EDR), and fixed carbon found for hydrochar samples obtained at higher temperatures after longer treatments [6; 12; 13]. As far as the effect of solid load on hydrochar properties is concerned, the lower carbonization degree of carbohydrates observed at the lower B/W ratio agrees with the lower carbon increase and oxygen decrease found by ultimate analysis and with the lower HHV, EDR, and fixed carbon values reported in ref. [6]. Indeed, at lower biomass load, the greater capacity of the liquid phase to dissolve molecular fragments arising from biomass hydrolysis, as observed by the higher liquid mass yield [6], hinders the polymerization and condensation reactions leading to the formation of secondary char. Conceptualization, L.C. and C.F. ; methodology, L.C. and C.F. ; software, L.C. and C.F. ; validation, L.C. and C.F. ; formal analysis, L.C. ; investigation, L.C. and C.F. ; data curation, L.C. and C.F. ; writing--original draft preparation, L.C. and C.F. ; writing--review and editing, L.C. and C.F. ; supervision, L.C. and C.F. All authors have read and agreed to the published version of the manuscript. This research received no external funding. The data presented in this study are available on request from the corresponding author. [PERSON] and [PERSON] are kindly acknowledged for providing the feedstock and hydrochar samples. The authors declare no conflict of interest.