## 3 Results and Discussion
### Experimental Data Validation
In a first step, only the results from the tests that correspond to gasification with a maximum n value of 0.4 and for 10 s residence time have been represented in Figure 8. It can be seen from this figure that the experiments that present a satisfactory agreement between the model and the actual experimental data are those with optimal oxygen coefficient values (around n = 0.4). Thus, the gasification process can be validated according to this simplified reaction rate expression (Equation (12)). However, there are two different groups of experiments that present some disagreement, as can be seen in Figure 8. There are two pathways in partial oxidation SCWG processes: gasification and oxidation, two competing reactions [15]. In experiments where oxygen levels are low (around n < 0.4), the steam reforming reaction is predominant over the oxidation one, so that hydrogen yields are enhanced when oxygen coefficients are higher. Therefore, when the oxygen coefficient is roughly lower than 0.4, the hydrogen yield predicted by the equation is generally higher than the actual yield obtained from the experiment. This is probably due to a shortage in oxygen supply, and in turn, of energy to adequately support hydrogen production. On the contrary, higher experimental gasification efficiency than the predicted ones are obtained when the lowest initial concentration rates are fed (0.06 and 0.1 g cellulose/L), as can be seen in Figure 8. These disagreements could be explained by the enhancement of hydrolysis and water-gas shift reactions that takes place when an initial biomass with a low concentration is fed, which, as mentioned above, results in greater hydrogen yields. The predicted gasification efficiency compare with the experimental data from all experiments are shown in Figure 9. As can be seen, the closest agreement between both sets of data is observed when the experiments are carried out with an oxygen coefficient n = 0.4. However, some of the data presented considerable discrepancies. The two groups of experiments that presented noticeable disagreements are the same ones that are present in Figure 8. A higher predicted gasification efficiency than the experimental was obtained from the experiments where the oxygen coefficient was different from n = 0.4. In those experiments where the oxygen coefficient was close to or greater than 0.5, the oxidation reactions became predominant and, as a consequence of this, hydrogen yields were smaller. To sum up, when the oxygen coefficient has its optimal value (around n = 0.4) the gasification process can be validated according to this simplified reaction rate expression. Probably, although the authors in this work [32] do not supply any particular amounts of oxygen into the system, there may be some oxygen in the system that has not been quantified and it can produce an effect similar to that obtained in this work. On the other hand, as can be seen in Figure 9, all the experiments where a low initial concentration of biomass and a catalyst were used reached actual experimental gasification efficiency levels above the predicted ones. These disagreements could be attributed to the positive effect of a low initial biomass concentration, which, combined with the positive effect of an alkali catalysts, particularly especially KOH and K\({}_{2}\)CO\({}_{3}\), enhances water-gas shift reactions [48] and results in larger hydrogen yields. Therefore, in this way, Equation (12) is a simple and valid representation of the experimental data registered when cellulose the SCWG process is carried out on an initial organic concentration in the range 0.3-0.6 g cellulose/L and for n = 0.4 oxygen coefficient. However, any variation from these ideal conditions has a remarkable effect on other chemical reactions, such as oxidation, which takes a more predominant role over the whole process.