## 2 Methodology
### Process Arrangements from the Feed Stream to the Reactor  
To illustrate the methodology, orange peel was used. The industries related to orange juice, the jam industry, and fresh orange fruits produce millions of tons of citrus waste [22], and the number of papers on the valorization of orange peel is increasing [23]. As it has a large amount of water, a hydrothermal conversion process, such as supercritical water gasification (SCWG), could effectively convert this citrus waste into H2 or biofuels. Orange peel has been chemically characterized by a number of components [24; 25] (wt.%): soluble sugar, 16.90; starch, 3.75; cellulose, 9.21; hemicellulose, 10.50; lignin, 0.84; pectin, 42.50; ash, 3.50; fats, 1.95; protein, 6.50; others, 4.35. This composition is given with tolerance ranges and could be approached by chemical compounds representative of those that are more complex without available data. Thus, cellulose and hemicellulose, polymeric in nature, have been widely studied, and glucose and xylose can be accepted as  
Figure 4: Arrangements where the RYield block is placed (**a**) at the end of the heat exchangers, (**b**) at the beginning, for a wet solid biomass, and (**c**) at the beginning, for a dry biomass mixed with a liquid water stream. model compounds [25; 26] since they can be considered as repeated units. Similarly, natural lignin is a polymer mainly built from three monomers, and p-couamyl alcohol is one of them (H-lignin) [27; 28]. Under supercritical water conditions, at a temperature higher than 400 \({}^{\circ}\)C, the polymeric species can be completely converted into these representative model compounds and oligomers, as has already been reported regarding the conversion of cellulose to glucose [29]. The three most common soluble sugars are glucose, fructose, and sucrose [30]. Sucrose is composed of a glucose molecule and another fructose molecule, so the same amount is considered for each sugar (5.63 wt.%). Regarding proteins, proline is an amino acid present in orange peel [31]. Likewise, galacturonic acid is representative of pectin [30; 32]. Starch generally has 20-25 wt.% amylose and 75-80 wt.% amylopetin [33], so 25 and 75 wt.% are adopted. Triolein, oleic acid, and glycerol are model compounds of fats [34; 35], and the same proportion is used for each one. Finally, the remaining 4.35 wt.% can be modeled by ascorbic acid (3.5 wt.%) and other acids with a proportion of 0.25 wt.% (citric, malic, malonic, and oxalic acids) [32]. The elemental composition (wt.%) is C, 39.1; N, 0.8; H, 5.9; and O, 50.7 (dry basis but with 3.5 wt.% ash). Different arrangements/flowsheets are designed to be evaluated (Figure 5)  
1. ARR-1: The K?ield plus RGibbs blocks operate under reactor conditions. The RYield block decomposes the feedstock into C, H\({}_{2}\), and O\({}_{2}\) (Figure 5a). 2. ARR-2: The same as ARR-1, but RYield operates at the RGibbs inlet temperature using a preheater for the feed stream (Figure 5b). Figure 5: Flowsheets or arrangements: ARR-1 (**a**), ARR-2 (**b**), ARR-3 and -4 (**c**), and ARR-5 (without pump) and -6 (**d**). 3. ARR-3: The same as ARR-2, but RYield operates at ambient temperature, and the stream that leaves the block is heated (Figure 5c). 4. ARR-4: The same as ARR-3, but RYield decomposes solid biomass into its representative liquid model compounds (Figure 5c). 5. ARR-5: There is a unique feed stream for RYield since water is considered as moisture in the solid, thus simulating wet solid biomass. RYield operates at ambient temperature, and the stream leaving the block is heated. Solid biomass is decomposed into C, H2, and O2 (Figure 5d). A pump cannot be added because the stream is a mixture of solid and gas. 6. ARR-6: The same as ARR-5, but RYield decomposes the wet solid biomass into a series of representative liquid model compounds (Figure 5d). A pump is added to the system to increase the pressure to 240 bar, so the electrical input is calculated. To perform the simulation, the thermodynamic method used in the RYield and RGibbs blocks must be the same.