**3. Results and Discussion**

*3.1. Derivative Thermogravimetric Curves*

Figure 2 shows the derivative thermogravimetric curves (DTGA) of grape seeds and hydrochars under pyrolytic and oxidative conditions, at different heating rates. Table 3 shows a sum of the position of local peaks, with their temperatures and intensities.

**Figure 2.** *Cont*.

**Figure 2.** Derivative thermogravimetric (DTGA) curves: (**a**) during pyrolysis at 10 ◦C/min; (**b**) during oxidation at 10 ◦C/min; (**c**) effect of the heating rate during pyrolysis of hydrochar 180 ◦C; (**d**) effect of the heating rate during pyrolysis of hydrochar 220 ◦C; (**e**) effect of the heating rate during pyrolysis of hydrochar 250 ◦C. HC: hydrochars, dα/dt: derivative conversion curve of the extent of conversion α with respect to time t.

**Table 3.** DTGA peak temperature and decomposition rate of pyrolysis and oxidation (10 ◦C/min).


Overall, both grape seeds and hydrochars mostly decompose between 200 and 500 ◦C, indicating that most of the volatile matter is removed in this range. Grape seeds present three main decomposition peaks, occurring at 280, 343, and 421 ◦C under a pyrolytic atmosphere. These peaks can be attributed to the overlapped decomposition of hemicellulose, lignin plus cellulose, and oil, which are the main constituents of grape seeds (around 7% cellulose, 31% hemicellulose, 44% lignin, and 10–15% oil [41,42]). Indeed, it is well known that under pyrolytic conditions, due to its amorphous and little polymerized structure, hemicellulose is very reactive at low temperatures (in the range of 250–300 ◦C [21,43]). In the DTGA curves, this reactivity translates into the first decomposition peak. Meanwhile, the second peak can be attributed to the overlapped decomposition of lignin and cellulose, which both generally decompose in the 300–350 ◦C range [43]. Considering the low content of cellulose (around 7% [41,42]), lignin clearly dominates at this temperature. Beyond this decomposition, lignin also contributes by forming a decomposition "baseline" to the profile, overlapping the other components. Indeed, due to its high polymerized structure and higher heterogeneity, it decomposes also (less intensively than at 300–350 ◦C) in a broad range of temperatures, from around 230◦ up to 600 ◦C [43]. Therefore, during the pyrolysis of grape seeds, its decomposition could overlap to that of the other constituents, forming a "baseline" to the overall profile. Meanwhile, comparing the measured curve with that of

grape seed oil from previous work by our group [28], it is clear that the peak occurring at 423 ◦C can be attributed to the decomposition of the oil. Regarding the hydrochars, through HTC, grape seeds undergo carbonization, decreasing their atomic O/C and H/C ratios with a natural reduction of the volatile matter as the harshness of the process increases [32]. In DTGA curves, this translates in flattening the first decomposition peak at around 280 ◦C, which is indeed absent for all the hydrochars. Therefore, the authors can affirm that hemicellulosic compounds are mostly degraded during HTC. Interestingly, the 250 ◦C hydrochar exhibits a broad and not intense decomposition between 200 and 300 ◦C. Since hemicellulose degrades during HTC, this reactivity can be explained by the presence of re-polymerized compounds from the aqueous phase. Indeed, during HTC, sugar-derived compounds dissolved in the aqueous phase (like 5-HMF) can undergo condensation and re-polymerization, forming a solid phase called "secondary char" [44]. Beyond this, fatty acids formed during the hydrolysis of the oil could also be embedded in the solid phase, conferring to the sample an extra-reactivity at low temperatures. Both grape seeds and 180 and 220 ◦C hydrochars show their highest reactivity at around 350 ◦C, attributable to the degradation of lignin and cellulosic derived compounds. Differences can be due to both the heterogeneity of the feedstock and the synergistic behavior among the constituents, which are present in different ratios in the various samples. Conversely, the 250 ◦C hydrochar, after the initial slow decomposition at 200–300 ◦C, shows only one main decomposition peak at 400 ◦C. Therefore, while the 180 and 220 ◦C hydrochars tend to follow the feedstock profile (2nd and 3rd peaks), the 250 ◦C hydrochar highly deviates from that, highlighting the impact of the HTC operating temperature on the devolatilization profile. Overall, it is important to highlight to positive effect of the HTC treatment: after HTC, curves tend to shift towards the right region of the graph, demonstrating a higher thermal stability during the degradation process.

Regarding oxidative conditions (Figure 2b), as expected by typical combustion profiles of biomass fuels [20], all the profiles can be divided into two regions: a first phase during which the feedstock devolatilizes to char, and a second phase where char oxidation occurs. The first phase, between 250 and 450 ◦C, resembles the pyrolytic behavior: hemicellulose, lignin plus cellulose, and oil volatilize in the same temperature ranges as during pyrolysis. Then, the second phase, 480–520 ◦C, corresponds to the oxidation of the char formed during the first phase. This phase is predominant with respect to the previous phase. As for pyrolysis, the 180 and 220 ◦C hydrochars resemble the behavior of grape seeds, exhibiting three main DTGA peaks. Meanwhile, the 250 ◦C hydrochar presents only two DTGA peaks, demonstrating the harshness of severe HTC conditions on the feedstock structure and composition. Oxidation and pyrolysis TGA curves where (1-α) is plotted vs. T are reported in Supplementary Materials Figure S1.

Figure 2c–e show the effect of the heating rate on DTGA curves. The heating rate does not affect the shape of the decomposition profile. Meanwhile, the DTGA peak moves towards higher temperatures at increasing heating rates, which can be due to heat and mass transfer phenomena and intrinsic devolatilization kinetics. Indeed, at higher rates, the mismatch between the temperature in the furnace and the particle is higher, and therefore there is a delay between the temperature measured and the decomposition stage [45]. In addition, the dα/dt is higher at higher rates because, at a fixed α, dt is smaller. In addition, higher rates cause a bigger gradient between the surface and core temperature of the sample [45].

#### *3.2. DSC Curves*

Figure 3 shows DSC curves under nitrogen and oxidative atmosphere. Under an inert atmosphere, both grape seeds and hydrochars show an initial endothermic phase followed by an exothermic one at higher temperatures. The transition from an endoto an exothermic behavior occurs in a temperature range between 200 and 290 ◦C. The heat request comprises both the heat necessary to evaporate water and that to trigger endothermic reactions. Water derives from both some residual moisture and some produced

during condensation reactions occurring at the beginning of pyrolysis. Meanwhile, after the transition temperature, heat is released during pyrolysis. This behavior was already observed in literature with hydrochars derived from other feedstock [40,46] and can be attributed to the presence of non-oxidized oxygen on the hydrochar, like in the form of carboxylic groups. Indeed, this oxygen is enough to oxidize the elemental carbon, leading to a heat release. As shown in Table 4, the net energy balance along the temperature spectrum 40–600 ◦C is positive for all the samples, highlighting that overall pyrolysis releases thermal energy. Not surprisingly and consistently with elemental analysis (Table 1), a higher heat release is associated with a higher volatile matter loss (Table 4). Details on the progressive integration of DSC data are given later in the paper (namely, in Section 3.5).

**Figure 3.** Differential scanning calorimetry (DSC) curves of grape seeds and hydrochars (HC) at 10 ◦C/min: (**a**) under inert and (**b**) oxidative atmosphere (> 0 exothermic, < 0 endothermic); per grams of dry feedstock.

**Table 4.** Details of DSC curves: integrals, mass loss, and transition point from endothermic to exothermic behavior; per kilograms of dry feedstock. The positive values of "Net energy" correspond to exothermal behavior during thermal treatments.


<sup>1</sup> Integral and mass losses computed/measured in the range 40–600 ◦C.

As for DTGA curves, samples show heat profiles with local peaks. Indeed, grape seeds show three main peaks, at 280, 352, and 410–425 ◦C, in correspondence with the degradation of hemicellulose, lignin plus cellulose, and oil. The 180 and 220 ◦C hydrochars show two peaks, while the 250 ◦C shows only one at 420 ◦C. A comparison between DTGA and DSC curves under pyrolytic atmosphere is reported in Supplementary Materials Figure S2.

As expected from the nature of the process, under oxidative conditions (Figure 3b), the amount of heat released increases as the carbonization degree of the sample increases. All the samples show a slight endothermic behavior at low temperature, justified by the absence or low presence of oxidative reactions at this condition. Overall, the heat release profile is broad and rises to the final observed temperature. Indeed, at 600 ◦C, a certain quantity of matter that undergoes oxidation is still present. Even if less sharp than under an inert atmosphere, all the samples show a peak at 350–450 ◦C that can be associated with

the oxidation of lignin-like compounds. Grape seeds also show a small inflection at 280 ◦C, probably due to the oxidation of hemicellulosic compounds.
