## 3 Results and Discussion
### Experimental Data Validation  
It has already been mentioned that, despite the enormous potential of the SCWG processes, its industrial implementation presents some limitations that are still to be overcome. For that reason, the analysis of its kinetic parameters and the subsequent validation with experimental results are essential if successful new reactors are to be designed. Moreover, as already mentioned in the Introduction, different reactions may take place in SCWG processes, which would make any rigorous study on its kinetics that intends to cover all the possible pathways a task of enormous difficulty. The present study has, therefore, focused on a really simplified kinetic model where the results obtained from our experiments are compared. This should hopefully cast some light on the discussion that has been presented in previous sections with regards to the results obtained in this study. [PERSON] et al. [32] demonstrated that cellulose is rapidly and completely converted into glucose at temperatures around 400 \({}^{\circ}\)C. Hence, in order to simulate the gasification efficiency of the different experiments in this paper, kinetics based on glucose can be used. [PERSON] et al. [58] found that, although different reaction pathways of glucose decomposition have been proposed, glucose hydrolysis along with water-gas shift are the principal reactions to be taken into account with regards to H2 yields. [PERSON] et al. [32] proposed a simplified model based on a pseudo-first-order kinetics for the production of hydrogen from glucose (Equation (11)). \[\mathrm{C_{6}H_{12}O_{6}+6H_{2}O\to 6 CO_{2}+12H_{2}} \tag{11}\]  
The different kinetic parameters proposed in this model were determined and validated with experiments carried out at temperatures ranging from 480 to 750 \({}^{\circ}\)C, at 280 bar and with 0.6 M of glucose, no oxidant and a residence time of 10-50 s. Therefore, the reaction rate for the simulation of glucose conversion would be calculated as follows:  
\[-r=10^{3.09\pm 0.26}\exp{\left(\frac{-67.6\ \pm 3.9}{RT}\right)}[glucose] \tag{12}\]  
where, r (mol/L-s), E\({}_{a}\) (kJ/mol), Temperature (K) and [glucose] is (mol/L). Figure 7: Effects of an alkali catalyst, specifically KOH at 0.1 wt %, on the SCWG of cellulose at 250 bar, 535 \({}^{\circ}\)C, with an initial concentration of cellulose = 0.3 g/L, and n = 0.4. For the validation of the experimental data, the reactor was considered to be a plug flow reactor, since this reactor type has a L/D ratio \(>>\) 100 and the flow is turbulent in most of the experiments (Reynolds number is greater than 2200) [59]. In fact, the Reynolds number reached just 1708 in the last of the experiments, where a catalyst was used, and the residence time was 10 s. Nevertheless, the plug flow reactor assumption was held even for the lowest Reynolds number (1708) that was obtained at 535 \({}^{\circ}\)C and with a 1.3 mL/min flow, since the surface reactions that took place were negligible [60]. [PERSON] et al. [61] reported that a number of effects should also be taken into account for the idealization of a tubular flow reactor as plug flow. This approach requires that seven time constants associated to the reactor and its operating conditions are previously determined. Such time constants are then used to verify a number of criteria. In their experimental reactor, [PERSON] et al. did not manage to meet the criterion related to radial diffusion nor the one associated with surface reactions and, according to their report, such criteria are not relevant when surface reactions are not significant. It could, therefore, assume that our reactor can be idealized as a plug flow reactor under the given test conditions. Equation (12) was validated according to the data from the experiments that had been carried out at residence times of 10-50 s, and it is a simplified expression of the overall gasification reaction (Equation (11)).