## 2 Simulation Methodology
Next, alternative case studies are analyzed by hybridizing this initial scenario with combined power and heat technologies as a means of increasing the energy efficiency of these plants. Thus, this work is focused on alternative heating solutions to the current heater in Figure 1 by hybridizing this plant with different potential heat sources. For this purpose, Figure 2 shows a modification of the base scenario, where the preheater is not fed with the reactor effluent. Except for the preheater, both plants have the same components and should be designed to convert as much organic waste as possible. With this new design, there are two main advantages:  
1. Heat from external sources can be used as heat input to the preheater; in this regard, different alternatives can be analyzed. 2. Full thermal energy at the reactor output is available in the cooler and, therefore, can be used for power generation by organic or steam Rankine cycles. From this configuration, two different case studies are considered to analyze the hybridization with conventional and renewable technologies. These case studies take advantage of the exhaust gas from an internal combustion engine (ICE) and a gas turbine (GT), respectively. In both cases, external fuel is supplied to feed ICE and GT, and the energy of the effluent leaving the reactor is recovered. In this study, natural gas was used as an external fuel to obtain a more homogeneous benchmark, although natural gas should be replaced by a similar but more sustainable fuel, such as biogas. The composition of this fuel depends on a number of factors that would add uncertainty to the analysis performed and, hence, the more relatively homogeneous natural gas was selected. In a techno-economic assessment, specific biogas should be selected. Finally, a general description of other case studies is also shown as a benchmark for future work, that is, the hybridization with solar  
Figure 2: Flow diagram of a conventional SCWG plant without energy use (base scenario) or hybridization modes (case studies). \begin{table}
\begin{tabular}{c c} \hline \hline
**Operating Parameters** & **Range of Values** \\ \hline Reactor inlet temperature (\({}^{\circ}\)C) & 450–600 \\ Operating pressure (bar) & 250 \\ Glycerol mass concentration (\%) & 10, 15, 20, 25, 30 \\ Total flowrate (kg/h) & 1000 \\ Pressure and temperature in the separator (bar/\({}^{\circ}\)C) & 2/30 \\ Minimum pinch temperature in the heat exchanger (\({}^{\circ}\)C) & 10-25 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Common operating conditions for all case studies. energy (parabolic trough concentrators) and solid oxide fuel cells (SOFC). Next, each case study is described in detail. Case study A: Hybridization with an internal combustion engine (ICE). The exhaust gases and cooling water used to refrigate the ICE allow the fluid temperature to increase to between 450 and 520 \({}^{\circ}\)C. The temperature in ICE exhaust gases, fed with natural gas (NG), biogas, H\({}_{2}\), or mixtures of NG and H\({}_{2}\), can reach between 450 and 550 \({}^{\circ}\)C [45], so there is enough temperature available to heat the feed. In addition, the cooling water used at ICE is limited to 90 \({}^{\circ}\)C and can also be used as a previous preheating medium. However, these temperature ranges depend on the operating parameters of the engine, such as the air-to-fuel ratio, the engine timing diagram, or the ICE load, among others. This requires determining an optimum motor configuration to achieve the required fluid temperature. Case study B: Gas turbine (GT) hybridization, using the exhaust gas stream of the GT, whose temperature can be between 450 \({}^{\circ}\)C and 530 \({}^{\circ}\)C for the power range achieved, depends on the design parameters of the GT, such as the pressure ratio and the temperature at the combustion chamber outlet. The GT cycle design should guarantee a temperature between 450 and 520 \({}^{\circ}\)C in the feed stream with maximum cycle efficiency to optimize the plant performance.