## 3 Results and Discussion
### Effects of the Operating Conditions on the SCWG of Cellulose
#### 3.1.1 Effects of Temperature on the SCWG of Cellulose  
Figure 2 describes the influence of temperature on the SCWG process when cellulose is used as feed. The experiments were carried out at 250 bar, 0.4 as oxygen coefficient, and a cellulose aqueous solution feed at 0.3 g/L concentration was employed. The effect of specific reaction temperatures was studied within the range 450-560 \({}^{\circ}\)C and over three different residence times (6, 8 and 10 s). The results in terms of H\({}_{2}\) yield (%) are represented in Figure 2. It is well known and, naturally expected, that higher temperatures result in considerably greater hydrogen yields [48]. Nevertheless, it is important to point out that at 560 \({}^{\circ}\)C and with a very short residence time (6 s), efficiency levels are really close to 100%. The hydrogen obtained through SCWG may come either from the cellulose or from the water. Moreover, although at low temperatures the effect of the residence time is almost negligible, when the temperature goes up it has a much more significant effect on the outcome. Therefore, longer residence times and higher temperatures usually produce greater gas yields since the thermal cracking reactions of the organic compounds are enhanced [48]. Furthermore, the number of the different reactions between the intermediates resulting from such thermal cracking reactions is also increased. Therefore, in general, higher residence times favor the hydrogen generation, and cause increased participation in other reactions when the concentration is high. Therefore, hydrogen gasification efficiency of the process may be reduced from a longer residence time, being more pronounced at higher temperatures. As can be seen from the experiment results included in Table 2, CH\({}_{4}\) yields appeared at 560 \({}^{\circ}\)C, while no CO is registered at this temperature. This enhanced hydrogen yield with low carbon monoxide formation as temperature is increased has been previously obtained by other authors [15; 17; 49; 50]. However, the equilibrium rate of the water-gas shift direct reaction is slow and its equilibrium constant decreases as temperature increases in the reaction medium. The above-mentioned authors observed a fast-type water-gas shift reaction in their studies on glucose gasification by means of supercritical water. Moreover, in the present work, the LHV of gases increased from 1189.4 kJ/Nm\({}^{3}\) at 450 \({}^{\circ}\)C to 8371.8 kJ/Nm\({}^{3}\) at 560 \({}^{\circ}\)C with a residence time of only 6 s. This increment in LHV is due not only to the greater amount of hydrogen in the gas phase obtained at 560 \({}^{\circ}\)C, but also to the presence of methane in small quantities. In fact, this LHV is higher than the maximum LHV obtained in a previous study where the SCWG of lactose as model compound was studied [51]. The authors obtained an LHV of 3513 kJ/Nm\({}^{3}\) at 700 \({}^{\circ}\)C and 60 s.  
Figure 2: Effect of the reaction temperature on the SCWG of cellulose at 250 bar with an initial concentration of cellulose = 0.3 g cellulose/L and n = 0.4. Figure 3 shows a chromatogram obtained by GC-MS, where the peaks corresponding to the volatile compounds obtained in the liquid effluent from the SCWG of cellulose at 560 \({}^{\circ}\)C can be seen. According to Figure 4, most of the compounds detected in the liquid effluent obtained at 560 \({}^{\circ}\)C, are alcohols (39.9 %) and benzenes (42.8%). Therefore, under these conditions (560 \({}^{\circ}\)C and a low residence time) most of the initial organic matter is probably removed from the liquid phase and the production of gases is promoted. This is in agreement with the gas yields obtained at 560 \({}^{\circ}\)C (Table 2), where efficiency was close to 100% at 560 \({}^{\circ}\)C and with 6 s residence time. To conclude, and according to previous studies with similar results [51; 52], gas formation is probably enhanced by some sort of free-radical reaction that takes place under these conditions.