## 3 Results and Discussions  
The assessment of new porous carbon material should be based on its adsorption properties, evaluated through the well-established procedures. Table 2 lists the results of the BET analysis. The surface area and the average diameter of the pore \(D_{av,BJH}\) (obtained as 4\(\cdot\)V\({}_{\text{BJHa}}\)/A\({}_{\text{BET}}\), where V\({}_{\text{BJHa}}\) is the BJH desorption pore volume and A\({}_{\text{BET}}\) is the BET surface area) are critical parameters for physical adsorption. In general, the higher the superficial area, the better the sorbent capacity. The increase of porosity is also highlighted by SEM-EDS images (Figure 2), where, in the left-hand side (Figure 2a,c,e), the original wood structure is always present after hydrothermal treatment with several typical pits [46] and, at higher magnification (3000 and 10,000\(\times\)), 10 \(\upmu\)m macropores are also visible. On the right-hand side (Figure 2b,d,f), after the activation procedure, as expected, a well-developed sponge structure is evident at increasing magnification (400, 2000, and 10,000\(\times\)). \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample Parameters**} & **Surface Area** & **BJH Desorption** & **Average Diameter of** \\  & **(A\({}_{\text{BET}}\))** & **Pore Volume (V\({}_{\text{BJH}}\))** & **the Pores (D\({}_{av,BJH}\))** \\  & **[m\({}^{2}\)/g]** & **[cm\({}^{3}\)/g]** & **[Å]** \\ \hline HC\_200\_0 & 1.13 & 0.004 & 142 \\ HCA\_200\_0 & 881 & 0.241 & 11 \\ HC\_200\_120 & 1.33 & 0.008 & 241 \\ HCA\_200\_120 & 284 & 0.109 & 15 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: BET and BJH results of all materials tested. Figure 2: Scanning electron microscopy (SEM) images of HC\_200\_0 (left-hand side: (**a**,**c**,**d**) and HCA\_200\_0 (right-hand side: (**b**), (**d**,**f**) at different magnification: (**a**,**b**) 400\(\times\); (**c**) 3000\(\times\); (**d**) 2000\(\times\); (**e**,**f**) 10,000\(\times\). The EDS analysis, not reported here, shows the carbon as the main element with a distributed trace of chlorine, owing to the HCl washing step of the activation method. As far as the time course of the outflowing CO\({}_{2}\) and CH\({}_{4}\) concentration is concerned, a sigmoidal behavior was observed for all tests and at all values of the operating pressure, as shown in Figure 3. The recorded signals are reported as a normalized value, that is, divided by the inlet concentration. By inspection of Figure 3, the time of first detection (arrows) depends evidently on the operating pressure. The translation of blank curves is inherent in the operating modality of the apparatus and depends linearly on the pressure (data not reported for the sake of brevity). The translation of the response curves suffers from the superimposition owing to the presence of the adsorbent bed. The difference between the two delays increases steadily. Table 3 is a synopsis of all the results. The sorbent capacity of the activated hydrochar obtained at 200 \({}^{\circ}\)C and 0 min is higher than that of the corresponding material recovered after 120 min of retention in the HTC reactor. The results of the BET analysis confirm this finding, where the 0 minute sample shows a specific area as high as 210% (3.1 time bigger) and an average pore diameter decreased by 27% in comparison with sample HCA_200_120. These results warn that the HTC reaction time is a crucial parameter and that the existence of a possible optimal retention time will be worth study, and in the further developments, it will be coupled with a cost optimization aimed at industrial exploitation of the results. On the other hand, the performances of non-activated hydrochar denounce of an adsorbent capacity of an order of magnitude lower, and are thus not acceptable for industrial applications. Figure 3: CO\({}_{2}\)/N\({}_{2}\) adsorption curves at (a) 2 bar; (b) 3 bar; (c) 4 bar; and (d) 5 bar. Figure 4 shows typical adsorption curves obtained with the activated hydrochar at 2 and 5 bar (a and b, respectively) and the mixture CH4/CO2.