## 1 Introduction
Oxidants may be added for two main purposes, namely, to favor the initial breakdown of refractory molecules and to increase the temperature thanks to the exothermic oxidation reactions that take place. Biomass supercritical water gasification is an endothermic reaction, and for this reason, some studies add a small amount of oxidant to favor the energy balance of the process. In partial oxidative gasification processes, heat is generated initially from the oxidation reaction, that heats up the reaction medium. This kind of internal heating method improves gasification efficiency by eluding the heat resistance that is present in external heating procedures [18; 19; 20]. Besides, the presence of a small amount of oxidant may contribute to a further gasification of some of the reactants, or some intermediate products may be obtained during the process (for instance, complex-structure compounds such as phenols). Such intermediate products might not be fully gasified in the absence of an oxidant that starts off the breaking of some of the molecule bonds [21]. For this reason, hydrogen peroxide is often used as a source of free radicals that help to decompose the phenolic compounds [22]. All in all, gasification efficiency is improved thanks to a reduction in the activation energy required to start the reaction process [23; 24]. For example, General Atomic is a company that has set up a demonstrative partial oxidative gasification plant for the production of hydrogen and intends to build an industrial-scale waste treatment plant based on this technology [25; 26]. Nevertheless, the number of running plants that produce hydrogen from biomass by means of a continuous flow supercritical reactor operating on water partial oxidative gasification principles is still rather short. Lignocellulosic biomass typical content includes 30-60% cellulose, 20-40% hemicellulose and 15-25% lignin [25]. Since diverse groups of compounds are produced from that cellulose, hemicellulose or lignin, it is necessary to study the behavior of their intermediates and their degradation or reformation routes into gases. Hence, in order to understand the behavior, mechanisms and the breakdown rates of complex lignocellulosic biomass under supercritical conditions, it is necessary to deepen into the development of SCWG processes when applied to their model compounds, such as cellulose, glucose, glycerol, lignin or phenolic compounds [27]. Many researchers have studied the supercritical water gasification of particular biomass types [28; 29; 30] and their model compounds [31; 32; 33] to explain such behavior and mechanisms in order to enhance the process efficiency. The production or large amounts of H\({}_{2}\) through the SCWG of biomass requires high temperature and pressure levels. For this reason, the effect of different catalysts such as alkali, transitional metals and activated carbon have been studied to reduce the operating conditions and improve hydrogen selectivity [34]. Moreover, the presence of a catalyst may reduce tar and char formation. Many authors have achieved highly efficient gasification levels using alkali metal catalysts such as Na\({}_{2}\)CO\({}_{3}\), KHCO\({}_{3}\), K\({}_{2}\)CO\({}_{3}\), KOH and NaOH. Alkali metal catalysts promote water-gas shift reaction and enhance H\({}_{2}\) yields [35]. [PERSON] et al. [15] showed that the addition of KOH increases the production of H\({}_{2}\) through the supercritical gasification of industrial wastewaters such as viruses or cutting oils. [PERSON] et al. [36] studied the effect of KOH, NaOH and Na\({}_{2}\)CO\({}_{3}\) used as catalyst on the SCWG of lactose and whey. The production of H\({}_{2}\) went up from 0.41 mol H\({}_{2}\)/mol lactose without a catalyst to 4.01 mol H\({}_{2}\)/mol lactose in presence of 0.1 wt % of Na\({}_{2}\)CO\({}_{3}\) at 225 bar, 550 \({}^{\circ}\)C, feed of 2.5 wt % in a continuous system at a feed flowrate of 5 g/min.