## 1 Introduction  
Supercritical water gasification (SCWG) is a promising technology through which renewable hydrogen can be obtained from biomass. This process presents some advantages in comparison with other conventional processes, since electricity can be obtained when the gaseous product is directly burnt in engines or turbines. Moreover, traditional gasification technologies generally produce a low-quality gas that requires costly purification procedures in order to meet essential quality standards [1]. Furthermore, biomass needs to be previously dried and this procedure represents additional running costs [2]. This is why we consider that SCWG appears to be a feasible solution to these issues. In SCWG processes, supercritical water (i.e., temperature and pressure higher than 374 \({}^{\circ}\)C and 221 bar, respectively), acts as both a solvent for organic materials and a reactant, which means that a previous biomass drying procedure is not required. Furthermore, compared to conventional gasification processes, SCWG requires lower temperature levels to produce hydrogen. In fact, SCWGprocesses need a temperature over 600 \({}^{\circ}\)C and pressure level above water critical point, plus the addition of a small amount of oxygen to mainly produce hydrogen. Moreover, supercritical water has the capacity to dissolve the organic components that are found in biomass from plants, such as cellulose or lignin. This favors the hydrolysis reactions that degrade the organic polymeric structures in contrast with the pyrolysis reactions that are typical in conventional gasification techniques. In the hydrolysis reactions that take place in SCWG processes, the products are solved in hot compressed water. Thus, the formation of char and tar that takes place in pyrolysis reactions is prevented. Nevertheless, SCWG of biomass is a complex process where a number of competitive reactions (hydrolysis, water-gas shift, oxidation, methanation, etc.) take place to some extent, depending on the reaction specific conditions [3]. The complete procedure of a SCWG process comprises a complex pattern of pathways, but the reactions involved could be explained in a simplified manner as follows:  
Oxidation reactions [4; 5]:  
\[\mathrm{H_{2}+1/2\;O_{2}\to H_{2}O;\;\;\;\Delta H^{o}\;298=-242\ kJ/mol} \tag{1}\]  
\[\mathrm{CO+1/2\;O_{2}\to CO_{2};\;\;\;\Delta H^{o}\;298=-283\ kJ/mol} \tag{2}\]  
\[\mathrm{CH_{4}+2O_{2}\to CO_{2}+2H_{2}O\;\;\;\Delta H^{o}\;298=-802\ kJ/mol} \tag{3}\]  
Methanation reactions [6]:  
\[\mathrm{CO+3H_{2}\leftrightarrow CH_{4}+H_{2}O\;\;\;\;\Delta H^{o}\;298=-206.17 \ kJ/mol} \tag{4}\]  
\[\mathrm{CO_{2}+4H_{2}\leftrightarrow CH_{4}+2H_{2}O\;\;\;\;\Delta H^{o}\;298=-16 5.01\ kJ/mol} \tag{5}\]  
Hydrolysis reaction:  
\[\mathrm{C_{n}H_{m}O_{y}+(n-y)\;H_{2}O\to nCO+(n-y+m/2)\;H_{2}\;\Delta H= endothermic, hydrocarbon dependent} \tag{6}\]  
Water-gas shift reactions [6]:  
\[\mathrm{CO+H_{2}O\leftrightarrow CO_{2}+H_{2}\;\;\;\;\Delta H^{o}\;298=-41.15 \ kJ/mol} \tag{7}\]  
The high water content triggers a water-gas-shift reaction (CO + H_{2}O\leftrightarrow CO_{2}+H_{2}\)) when gas products are formed from the intermediates. Hence, a H_{2}-rich gas is obtained with a low CO yield. Hydrogen is a clean fuel with a higher energy content (120 MJ/kg) than conventional fuels [7]. Furthermore, the gas obtained from SCWG processes comes out at high pressure levels, which often represents an additional benefit towards its subsequent use [8]. The above-mentioned benefits of SCWG have made it an attractive and frequent object of study over the last two decades. Some of those research works have focused either on cellulose, hemicellulose, lignin, glucose and fructose as model compounds [9; 10; 11; 12; 13] or on real biomass such as straw, sawdust, cornstarch, starch and potato waste [14], viruses [15], sewage sludge [16] or oil mill wastewater as feedstock [17]. SCWG is carried out with no addition of oxidant or with a small amount of dissolved oxygen.