## 3 Results and Discussion  
### Characterization of the Hydrochars  
Table 1 shows the hydrochar yields and the proximate and ultimate analyses of the raw GSs and hydrochars. Hydrochar yields were almost constant (\(\approx\)60-65 wt.%) in the HTC runs performed witha GS relative amount higher than 20% in the GS + water mixture within the temperature range of 220-280 \({}^{\circ}\)C (approx.). At a lower GS percentage the HC yield decreased since a higher relative amount of water favors the extraction and transfer of solid components to the liquid phase [12; 50]. The effect of the temperature in the HTC process can be seen at fixed GS percentage in the starting mixture (40 wt.%) and temperatures below and above the aforementioned range. At temperatures \(\leq\)200 \({}^{\circ}\)C, high HC yields were obtained associated to lower carbonization as can be seen from the C content of the samples. At temperature above 280 \({}^{\circ}\)C gasification reactions gain significance, giving rise to lower yield but a more carbonized solid [51]. It is well known that dehydration and decarboxylation reactions are the main responsible of the mass loss upon HTC [52; 53].  
A van Krevelen diagram was plotted from the ultimate analyses (Figure 1) where the points corresponding to the HCs have been placed. As the HTC temperature increases, the resulting HCs evolve from a typical peat composition towards the lignite region, approaching even sub-bituminous coal at the highest HTC temperature tested (300 \({}^{\circ}\)C).  
Table 2 summarizes the energy properties of hydrocarbons, where, clearly, there was a significant increase of the higher heating value as the HTC temperature increased. The energy density of the most carbonized solid was improved by one-third with respect to the starting precursor. Taking into account the GS to HC yield, the highest energy recovery efficiency does not correspond to the highest HTC temperature tested but to a significantly lower value somewhere around 200 \({}^{\circ}\)C. However, since the energy density of the resulting fuel is also an important issue, the information provided by Table 2 points to an optimum HTC temperature in the range of 260-280 \({}^{\circ}\)C. Moreover, this determination, also known as a BUE\({}_{\text{E}}\), provides an extra information because it allows a comparison between different fuels and options for biomass utilization [43].  
Figure 1: Van Krevelen diagram for grape seeds and hydrochars obtained at different temperatures (180–300 \({}^{\circ}\)C) with an initial solid content of 40 wt.%.  
The 77 K N\({}_{2}\) adsorption isotherms of the hydrocarbons (supporting material) corresponded to Type II of the IUPAC classification, associated to non-porous solids. The highest BET surface area was only 8 m\({}^{2}\):g\({}^{-1}\), corresponding to the GS-220-40 sample. To learn more on the porous texture of these hydrocarbons the CO\({}_{2}\) adsorption isotherms at 273 K were obtained (Supplementary Material). The surface area values from those isotherms (S\({}_{\rm DA}\)) varied from 100 to 845 m\({}^{2}\):g\({}^{-1}\), the highest corresponding to the above mentioned sample, with a micropore volume of 0.513 cm\({}^{3}\):g\({}^{-1}\). These huge differences between the BET and S\({}_{\rm DA}\) values are indicative of microporous texture consisting essentially of very narrow micropores, the so-called ultramicropores (\(\leq\)0.5 nm width), corresponding most probably to slit-shaped pores. This type of microporosity has been also observed in chars from HTC of different sugars at 180 \({}^{\circ}\)C calcined at high temperatures in an inert atmosphere [23].