## 1 Introduction  
Supercritical water gasification bases were provided for the first time at the end of the 1970s when [PERSON] studied the production of a syngas with a high concentration of hydrogen that could potentially be used as a synthetic fuel [1]. Working beyond the critical temperature and pressure (374 \({}^{\circ}\)C and 220 bar), the concentration of gaseous products depends on the operating conditions employed, such as temperature, pressure, residence time, feedstock concentration, etc. [2, 3]. Many types of feedstock, such as biomass, organic waste, plastic, coal, sludge and slurry, etc., have been studied in previous decades achieving different results and optimizing the condition to maximize the production of syngas or fuel gases [4, 5, 6, 7, 8]. Despite the effort made by the scientific community up to nowadays, the scale up from laboratory to industrial level is still not fully established. Initially, several works at pilot scale were reported (Table 1), highlighting the VERENA pilot plant constructed by Forschungszentrum Karlsruhe [9, 10]. The usual operating flowrate was 50 kg/h with a temperature of approximately 660 \({}^{\circ}\)C and a pressure of 280 bar. Since its construction in the 2000s, the pilot plant has been working to provide a better understanding of SCWG; however, several challenges are still to be solved [11]. The Pacific Northwest National Laboratory developed a process known as the Thermochemical Environmental Energy System (TEES) [12] and demonstrated it in a continuous pilot scale operation with a nominal flowrate of 38 L/h. In China, the State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF) developed several tubular reactor systems and the first fluidizedbed reactor in supercritical water with the aim of solving the plugging problems due to the high concentration of organic compounds in the used feedstock, such as wet biomass, sewage sludge, or thermoset plastics [13; 14; 15; 16; 17; 18]. The University of Twente in collaboration with BTG B.V. designed and built a pilot plant in a concrete high pressure safety box. It was designed for temperatures up to 650 \({}^{\circ}\)C and pressures of 300 bar with a treatment capacity of 30 kg/h [19]. In Japan, Chugoku Electric Power in collaboration with Hiroshima University developed a pilot plant to generate flammable gas from chicken manure [20; 21], and the results showed an almost complete gasification of the feedstock, achieving a process efficiency of 70% or higher. A few years ago, the University of Delft in collaboration with Gensos constructed a pilot scale plant of biomass in a fluidized bed. The plant has a capacity of 50 kg/h and reaches temperatures of 600 \({}^{\circ}\)C at 240 bar [22]. At present, the HydroPilot project has started to run a pilot plant demonstration promoted by the Paul Scherrer Institute and the Fachhochschule Nordwestschweiz [23]. The capacity of this pilot system is 100 kg/h of wet biomass, working under a pressure from 280 to 300 bar and temperatures of approximately 400 \({}^{\circ}\)C.  
In spite of the successful demonstration in continuous pilot scale plants, at the industrial level, there are no well-known cases with reported results. [PERSON] et al. [24] conducted a patent review to analyze the most significant invention since 1995. This review shows the technical limitations and the large investment needed for industrial plant operation that currently limits the feasibility and subsequent commercial implementation of SCWG technology. To increase economic feasibility, energy recovery, and integration of power generation devices are proposed as the key to optimizing operation and reducing costs. From the energy viewpoint, one of the most important obstacles in supercritical processes is the energy required to achieve supercritical conditions (more than 374 \({}^{\circ}\)C and 220 bar). Industrial processes, where flowrates are high, boost the demanded energy necessary to operate the plant.