Energy Hybridization with Combined Heat and Power Technologies in Supercritical Water Gasification Processes

Citation: Benjumea, J.M.; Jiménez-Espadafor, F.J.; López-Guirao, F.; Gutiérrez Ortiz, F.J. Energy Hybridization with Combined Heat and Power Technologies in Supercritical Water Gasification Processes. Appl. Sci. 2022,12, 5497. https://doi.org/ 10.3390/app12115497 Academic Editor: Luca Fiori Received: 28 April 2022 Accepted: 26 May 2022 Published: 28 May 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Energy Hybridization with Combined Heat and Power Technologies in Supercritical Water Gasification Processes JoséManuel Benjumea 1, Francisco JoséJiménez-Espadafor 1, Francisco López-Guirao 2 and Francisco Javier Gutiérrez Ortiz 2,* 1 Department of Energy Engineering, Escuela Técnica Superior de Ingeniería, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain; [email protected] (J.M.B.); [email protected] (F.J.J.-E.) 2 Department of Chemical and Environmental Engineering, Escuela Técnica Superior de Ingeniería, University of Seville, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain; [email protected] *Correspondence: [email protected] Abstract: Supercritical water gasification (SCWG) has been shown to be an effective technology to valorize a wide range of organic waste by transforming them into gases with high energy potential, such as hydrogen and methane. However, the industrial implementation of these processes is rarely extended due to the huge energy requirements during plant start-up and operation. The purpose of this study is to explore feasible ways of energy integration by hybridizing SCWG processes with combined heat and power technologies, such as exhaust gases coming from (i) internal combustion engines or (ii) gas turbines. The analysis focuses on energy consumption with the aim of optimizing the operation and design of plants. System configurations are simulated with Aspen Plus considering data from the literature for the gasification of glycerol and using typical plant capacities on an industrial scale. Results show the thermal power required in heat exchangers and the electricity generation from residual energy in hot effluents as a needed step to optimize the plant configuration and boost energy synergies with other technologies. Keywords: supercritical water gasification; hydrogen production; syngas production; hybridization technologies; combined heat and power; process modeling and simulation 1. Introduction Supercritical water gasification bases were provided for the first time at the end of the 1970s when Michael Modell 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 ◦ 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 [4–8]. 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 ◦ 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 fluidized Appl. Sci. 2022,12, 5497. https://doi.org/10.3390/app12115497 https://www.mdpi.com/journal/applsci Appl. Sci. 2022,12, 5497 2 of 20 bed 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 – 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 ◦ 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 ◦ 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 ◦C. Table 1. SCWG facilities at pilot and semi-pilot level. Institution (Country) Maximum Treatment Capacity (kg/h) Limit of Operating Conditions Reference Verena. Forschungszentrum Karlsruhe (Germany) 100.0 kg/h 700 ◦C, 350 bar [9–11] Pacific Northwest National Laboratory 38.0 L/h 350 ◦C, 210 bar [12] State Key Laboratory of Multiphase Flow in Power Engineering (SKLMF) (China) 19.2 kg/h 650 ◦C, 300 bar [13–18] University of Twente (The Netherlands) 30.0 kg/h 650 ◦C, 300 bar [19] Hiroshima University and Chugoku Electric Power (Japan) 41.7 kg/h 590 ◦C, 235 bar [20,21] TU Delft/Gensos (The Netherlands) 50.0 kg/h 600 ◦C, 240 bar [22] Paul Scherrer Institute PSI (Switzerland) 100.0 kg/h 400 ◦C, 300 bar [23] In spite of the successful demonstration in continuous pilot scale plants, at the industrial level, there are no well-known cases with reported results. Casademont 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 ◦ C and 220 bar). Industrial processes, where flowrates are high, boost the demanded energy necessary to operate the plant. Plant start-up prior to reaching steady-state operation is the step where a peak of energy should be supplied. Thus, energy recovery and heat integration in plants are key concepts that must be considered in the design and configuration steps of these plants. In this way, SKLMF researchers have analyzed different thermodynamic modeling at the simulation level to improve heat integration and power generation [ 25 , 26 ]. Additionally, to improve the economic feasibility of SCWG technologies, different studies on energy supplies have been reported. One of the most important advances was developed by a team from SKLMF to produce hydrogen from biomass by using SCWG technology promoted by solar energy. First, different systems were constructed and tested at the laboratory scale for proof-of-concept tests, based on concentrated solar energy processes with dish and toroidal surface heliostats [ 27 , 28 ]. Then, the first SCWG pilot plant based on solar energy was built [ 29 – 31 ] to analyze the possibilities of its commercialization. The KTH Royal Institute in Stockholm [ 32 , 33 ] was working at the simulation level on Appl. Sci. 2022,12, 5497 3 of 20 integrating the gasification technology (under non-supercritical conditions, to ensure the procurement of high conversion efficiency) with the supercritical water technology (to produce a clean syngas with low content of contaminants, such as tars, sour gas, and alkali compounds, necessary to feed the fuel cell). The product gas is routed to subsequent processes, including SOFC. Rainey et al., from Queensland University, carried out a study considering different possibilities of energy integration and process-energy configuration to increase the economic viability of the process. To do that, several alternative technologies, such as solar thermal technology (solar thermal and parabolic trough collector), natural gas combustion, and electrical energy, were analyzed [ 34 ]. Pie et al., from the Australian National University, also integrated the concentrated solar thermal technology to drive SCWG plants and its combination with the Fischer–Tropsch process [35,36]. At the same time, alternative ways were followed to try to move forward in the feasibility of SCWG by integrating it with other technologies, such as supercritical water oxidation (SCWO), hydrothermal liquefaction (HTL), or anaerobic digestion (AD), to improve, especially, the energy term. Wang et al. [ 37 ] developed a combined SCWG-SCWO system based on a cool-wall reactor. From the treatment of the sewage sludge, syngas was obtained from SCWG, while SCWO operated with the residual organic liquid to produce a harmless product and release heat. De Gisi et al. [ 38 ] worked on agricultural waste treatment (composed of buffalo sewage and maize silicate) by examining the potential of coupling SCWG and AD in a dual system. A liquid digestate fraction of AD is fed to the SCWG reactor to obtain biogas and value-added products, such as formic acid, xylitol, etc. Xu et al. [ 39 ] combined the HTL and SCWG to improve the recovery energy from algal biomass. The aqueous phase generated from HTL is entered the SCWG autoclave reactor for 60 min at 600 ◦ C. This integration could improve energy recovery between 5.53% and 18.30%. Among all the work carried out and the advances achieved in operating parameters conditions, equipment material and design or feedstock-treated energy requirements and economic feasibility in SCWG plants are actually limiting the industrial implantation of the technology. In order to explore new possibilities and alternative ways to improve that, the present work tackles the limitation of plant scale-up by proposing new energy supplies and integrated systems based on hybridization with conventional and renewable technologies. To the best knowledge of the authors, not many studies have reported an energy analysis of SCWG hybridization with combined heat and power technologies as a way to increase the viability of these plants while its use is promoted. Hence, this paper deepens the analysis of feasible configurations with the proposed technologies with the aim of obtaining a general overview of the potential possibilities to increase the profitability of SCWG technology on a larger scale. 2. Simulation Methodology The analysis was carried out by simulation using Aspen Plus. Figure 1depicts the flowsheet of the simulated plant (base scenario), including a pumping section to obtain supercritical pressure, a preheating step to reach the supercritical temperatures, a further preheating to the reaction temperature, the gasification in a tubular reactor, as well as the cooling and depressurization to reduce the temperature and pressure, respectively. Finally, separation processes are required to obtain clean syngas. Appl. Sci. 2022,12, 5497 4 of 20 Figure 1. Energy integration with hot effluent from the SCWG reactor (base scenario). Flowsheet of the simulated process in Aspen Plus. In this plant, the hot effluent leaving the reactor (hot fluid) feeds a heat exchanger (preheater) to increase the temperature of the feed (cold fluid) after pressurizing from atmospheric to 250 bar. This configuration does not allow the energy self-sufficient process and external supply energy (heater) is required to achieve the inlet temperature to the reactor at the setting values in a range between 450 and 600 ◦ C (imposed by the ICE and GT used). The specifications of the process units used in the simulation are shown in Table 2. Table 2. Specifications of the process units used in the Aspen Plus simulation. Component Code Equipment Specifications PUMP Pump Discharge pressure: 250 bar PREHEAT 2 stream countecurrent heat exchanger Hot stream outlet temperature: Minimum temperature approach: 10 ◦C HEATER Heater Outlet temperature: 450–600 ◦C Pressure: 250 bar R-GIBBS Reactor Heat Duty: 0 kW Pressure: 250 bar COOLER Cooler Outlet temperature: 50 ◦C Pressure: 250 bar BPR Back-pressure regulator valve Outlet pressure: 2 bar GL-SEP Gas–liquid separator Temperature: 30 ◦C Pressure: 2 bar In order to study SCWG plants at the industrial level, and based on the current SCWG plants’ data reported and exposed in the previous section, this work will consider a model plant with a nominal treatment capacity of 1000 kg/h. Therefore, although the main interest of SCWG is the valorization of organic waste, the focus of this work is to analyze the feasibility of hybridizing an SCWG plant through combined power and heat technologies. In this sense, a well-known compound, such as glycerol, is used as a feedstock since numerous experimental tests have been carried out by different research groups and the published results are widely available in the literature [ 40 – 42 ]. To calculate the thermodynamic properties of compounds, the equation of state (EOS) used to obtain a better representation of changes in the supercritical region is the predictive Soave–Redlich– Kwong (PSRK) EOS, as reported in the literature [ 43 ]. Pump efficiencies are set at 80%. The preheater is designed as a countercurrent heat exchanger, having a minimum temperature difference between the hot and cold streams set in the range from 5 to 25 ◦ C, depending on each case study’s requirements. The gasification reactor is simulated considering chemical equilibrium based on the Gibbs free energy minimization model, with high isolation that guarantees no heat losses. This methodology is widely used as a powerful tool to solve chemical equilibrium when kinetic data are not available or reliable enough [ 44 ]. The final Appl. Sci. 2022,12, 5497 5 of 20 separation of the gas mixture is modeled by a flash separator. The pressure losses are low and have not been considered in the simulation. In order to make possible an objective comparison of the results obtained for the proposed case studies, the simulation of the plant configuration corresponding to each case study is carried out considering the common operating parameters shown in Table 3. Table 3. Common operating conditions for all case studies. Operating Parameters Range of Values Reactor inlet temperature (◦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/◦C) 2/30 Minimum pinch temperature in the heat exchanger ( ◦ C) 10–25 The base scenario is representative of many SCWG plants that take advantage of the hot effluent leaving the reactor to preheat the feed stream. 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 1by hybridizing this plant with different potential heat sources. For this purpose, Figure 2shows 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. Figure 2. Flow diagram of a conventional SCWG plant without energy use (base scenario) or hybridization modes (case studies). 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 Appl. Sci. 2022,12, 5497 6 of 20 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 refrigerate the ICE allow the fluid temperature to increase to between 450 and 520 ◦ 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 ◦ C [ 45 ], so there is enough temperature available to heat the feed. In addition, the cooling water used at ICE is limited to 90 ◦ 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 ◦ C and 530 ◦ 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 ◦ C in the feed stream with maximum cycle efficiency to optimize the plant performance. Plant schemes of case studies A and B are represented in Figure 3. In both configurations, the hot effluent leaving the reactor is fed to a recovery boiler of a Rankine Vapor Cycle or an Organic Rankine Cycle to take advantage of the potential thermal power of the stream. In addition, it is important to emphasize the integration of cogeneration processes (combined heat and power, CHP), clearly encouraged by the current legislation in most countries, as an effective way to increase the energy efficiency of the plant. Figure 3. Hybridization modes for the preheating section using the exhaust gases from an internal combustion engine (ICE) or a gas turbine (GT) fed with external fuel (case studies A and B). Plant configurations for case studies A and B are quite similar, but for case study B only a heat exchanger is used, which is fed by exhaust gases coming from GT to heat the feed stream, while for case study A the heating is performed in two steps (separated heat exchangers) since there are two available heating sources. First, small heating is reached by using the cooling water coming from ICE refrigeration, and then, energy exchanged with the exhaust gases from the ICE combustion allows increasing the temperature of the feed stream to the reactor inlet set point. 3. Results and Discussion Starting from the base scenario, specific results will be shown for each case study, analyzing the selection of ICE and GT parameters, available thermal energy exchanged in the preheater, electricity generation, and the temperature–thermal power (TQ) curve in the preheater. Mass and energy balances are provided in the Supplementary Material. Appl. Sci. 2022,12, 5497 7 of 20 3.1. Base Scenario This process configuration has been set to obtain the energy requirements of the plant in the base scenario. Demanded energy was parametrized as a function of the inlet temperature of the reactor and the glycerol mass fraction, which was the selected feedstock in this study. Then, the energy exchanged in the preheater, where the effluent from the reactor is the ‘hot fluid’ and the feed stream is the ‘cold fluid’, was analyzed. In the scope of this work, heat exchanger calculations have been performed using a reference value for the overall heat exchanger transfer coefficient (0.85 kW/m 2· K) [ 46 ] and determining the temperature of the two outlet streams based on the heat and material balances. Thus, by imposing a minimum pinch point temperature of 25 ◦ C, an average area is obtained for each glycerol mass fraction considered. This design area is calculated to ensure that there is no temperature cross between hot and cold streams throughout the heat exchanger. The values for the areas ranged from 8.3 m 2 (10 wt.% glycerol) to 8.9 m 2 (30 wt.% glycerol). When selecting these data as the mean value to operate under these conditions, the outlet temperature of the ‘hot fluid’ leaving the preheater ranged from 212 ◦ C to 223 ◦ C. Figure 4a represents the variation of the thermal power exchanged in the preheater, observing an increase in the thermal power exchanged at low glycerol concentrations. Likewise, Figure 4b shows the thermal power required in the heater to achieve the reactor inlet temperature set. In this case, the energy requirement decreases as the amount of gasified glycerol increases. The total energy required for heating the feed stream (from ambient conditions) is represented by the sum of the thermal power exchanged in the preheater and that supplied in the heater. Figure 4. ( a ) Thermal power exchanged in the preheater; ( b ) Additional thermal power required in the heater to achieve the reactor inlet temperature in a range from 450 to 600 ◦ C. Both figures are parametrized as functions of the glycerol mass fraction in the feed stream. Clearly, the reduction in glycerol concentration and the increase in reactor inlet temperature maximize the exchanged thermal power in the preheater. This result comes from the great influence of the heat capacity variation of the mixture of glycerol and water on the thermal energy exchange. Figure 5shows the mass heat capacity of the glycerol–water mixture with a concentration of 10 wt.% and 30 wt.%. There are differences between both concentrations, especially around the supercritical region, where the maximum heat capacity for 10 wt.% glycerol is 30.0 kJ/kg K at 390.1 ◦ C, while for 30 wt.% glycerol, the maximum value decreases to 15.8 kJ/kg K at 406.2 ◦C. Appl. Sci. 2022,12, 5497 8 of 20 Figure 5. Variation of the heat capacity of the glycerol–water mixture for two different concentrations of glycerol mass concentrations. It is also important to analyze the behavior of the outlet temperature of the reactor with respect to the inlet temperature, as shown in Figure 6. In all cases, there is an increase in the temperature of the reactor outlet stream with respect to the reactor inlet. When the glycerol mass concentration remains constant, this temperature difference decreases as the reactor inlet temperature increases. For example, operating at 20 wt.% glycerol, when the reactor inlet temperature is set at 450 ◦ C, the temperature of the effluent leaving the reactor is 494.1 ◦ C, thus, increasing the temperature in 44.1 ◦ C. However, when the reactor inlet temperature is 600 ◦ C, the temperature at the reactor outlet increases in 30.7 ◦ C. Likewise, when the glycerol concentration increases, the temperature change between the reactor inlet and the outlet increases substantially. For example, operating at the lowest glycerol concentration (10 wt.%), the temperature changes from 15.3 ◦ C to 0.2 ◦ C throughout the reactor, when the reactor inlet temperature varies from 450 to 600 ◦ C, respectively. However, operating at the highest glycerol concentration (30 wt.%), the increase in the reactor temperature is significantly higher, changing from 71.3 ◦ C for a reactor inlet temperature of 450 ◦ C to 61.7 ◦ C when the reactor inlet temperature is 600 ◦ C. Figure 6. Reactor outlet temperature of the effluent leaving the reactor versus the reactor inlet temperature for different glycerol mass concentrations. Another important factor to consider in SCWG plants is the production of fuel syngas (H 2, CH 4 , and CO) in the gas effluent. Figure 7a–c shows the hydrogen, methane, and carbon monoxide flowrates, respectively. These results are consistent with experimental results obtained using glycerol as feedstock without catalysts [ 41 , 42 ], or using alkali catalysts [ 40 ], or a Ni-based catalyst supported on Al 2 O 3 and SiO 2 [ 4 ], where glycerol conversion was Appl. Sci. 2022,12, 5497 9 of 20 almost complete and hydrogen yields were very close to those values predicted by equilibrium at a short residence time, such as achieved in this study. Moreover, the model reliability is in line with other works where Aspen Plus is used for SCWG processes [ 47 – 54 ]. As expected, hydrogen production increases when the reactor inlet temperature increases. Furthermore, the increase in hydrogen production for a higher glycerol concentration is more noticeable at higher reactor inlet temperatures. The trend of carbon monoxide is similar to that of hydrogen but with more dependence on the reactor inlet temperature. Likewise, methane production decreases monotonically with the increase in the reactor inlet temperature. Figure 7. ( a ) Hydrogen flowrate in the gas stream. ( b ) Methane flowrate in the gas stream. ( c ) Carbon monoxide flowrate in the gas stream. Figures are parametrized as functions of glycerol mass concentration in the feed stream and flowrates are represented versus the reactor inlet temperature in the range from 450 to 600 ◦C. 3.2. Case Study A: Hybridization with an ICE (Internal Combustion Engine) Fed with External Fuel In this case study (Figure 3), the feed heating to the reaction temperature is reached using exhaust gas and cooling water from an ICE fed with external fuel. Instead, the hot effluent leaving the reactor is used for energy recovery (Organic Rankine Cycle or Vapor Rankine Cycle), and a syngas fuel stream is available for other uses. Configuration parameters of ICE [ 55 ] were selected considering the thermal power required to heat the feed stream from ambient to the reactor inlet temperature. Exhaust gas has a temperature between 450 ◦ C and 550 ◦ C that will allow a maximum temperature of approximately 530 ◦ C at the reactor inlet (considering 20 ◦ C as the minimum pinch Appl. Sci. 2022,12, 5497 16 of 20 the exhaust gas flowrate from ICE/GT when compared with cases A and B, which also decreases the size of the engines and, therefore, the capital cost of the plant. A rough estimate gives a power reduction of approximately 70%. Another option is hybridization with solar thermal energy, thus, considering energy storage, by heating a heat transfer fluid (HTF) by means of cylindrical-parabolic solar concentrators. This HTF (for example, Therminol) can be heated to 400 ◦ C without degradation and would be suitable for the first preheating in the SCWG process. With this solution, there would be an important reduction in fuel consumption, thus, maximizing the overall efficiency of the entire plant. Additionally, the storage of thermal energy (STE) by molten salts would allow a continuous mode of operation in the plant 24 h per day. However, the maximum allowable temperature of Therminol can affect the conversion of organic waste and an additional heat source is necessary to achieve at least 500 ◦ C at the reactor inlet, thus, increasing conversion. For this purpose, three different methods can be conceived using: − the energy available in the exhaust gases coming from ICE and GT in a second preheater; −thermal energy from the output of the reactor in a second preheater, or; − an autothermal reforming (ATR) process, involving a partial oxidation along with the gasification reactor inlet (SCWG). In this way, enough oxygen/air should be entered the preheater outlet to achieve the set point temperature (~500 ◦ C). This solution would reduce the produced syngas as a fraction of the organic waste that is oxidized. One more feasible alternative is the hybridization with an SOFC-type fuel cell, in which electric power is generated by feeding with two streams, one is air and the other is H 2 (or light hydrocarbons), which could come from the SCWG process itself. H 2 is recirculated and the exhaust is a stream of steam and air (or gas, if hydrocarbons are used) between 900 and 1000 ◦ C, allowing the SOFC fuel cell to hybridize with the SCWG process. In addition, the temperature of the SOFC exhaust stream will be higher than 500 ◦ C at the exit of the exchange with the reactor, therefore, it can be used for the preheating section, thus, integrating both sections. Figure 14 shows a scheme of the plant with the fuel cell or the STE. Figure 14. Thermal power required by the feed stream to be heated to the reactor inlet temperature. The temperature range of the ICE exhaust gases is overlaid. The glycerol mass concentration is in the range from 10 to 30 wt.%. 4. Conclusions Supercritical water gasification is a promising technology for syngas generation from wet waste biomass. Several pilot scale plants have been built around the world to gain experience in the valorization of a wide range of feedstocks. However, commercialization and implementation at the industrial level are not fully developed because of the various drawbacks that result from investment and process operation, among other factors. To explore possible alternatives aimed at taking advantage of synergies with other processes; Appl. Sci. 2022,12, 5497 17 of 20 this work suggests the use of combined heat and power technologies as means to improve energy efficiency. The proposed analysis addresses the hybridization of SCWG plants with ICEs (internal combustion engines) and/or GTs (gas turbines). By integrating these technological solutions for the energy supply in an SCWG plant, the feed stream can be heated using exhaust gases, and electricity is generated at the same time. Additionally, a syngas stream is produced, thus, performing the valorization of a waste feedstock. To embrace the possible process configurations, two different case studies have been carried out from a base scenario, to hybridize the process with ICE and GT, respectively, but using external fuel for feeding in both cases. The electrical power generated with the ICE solution is in the range from 1611 to 2381 kW, allowing the injection of power into the electrical grid. Similar efficiency is achieved with the GT solution, although in this case with an important reduction in mechanical power with respect to the ICE solution and, thus, with a lower electrical power generation ranging from 482 to 684 kW. In both cases, there is a stream of syngas (mainly H 2 , CH 4 , and CO) that can be used in other applications. The results obtained in the case studies analyzed, both energy recovery from exhaust gases and electricity generation, might be promising to improve energy use and consumption, but this requires further research. In fact, the two configurations studied in their present level of development involve a high external fuel flowrate, which is even higher than the syngas flowrate generated in the SCWG reactor; therefore, they should be disregarded for direct installation as studied for a new fully SCWG plant. However, it might make sense to implant an SCWG process in an existing plant, such as a sewage station where the needed electrical power may be supplied by ICEs or GTs to which could be added an SCWG process, i.e., when there are ICEs and/or GTs installed in a specific plant, an SCWG process could be integrated into it. Thus, this application might have sense in any power plant or subsystem in an existing plant where electricity was obtained from ICEs or GTs. Naturally, further optimization and hybridization studies including other technologies may reverse this preliminary conclusion, making the use of ICE and/or GT techno-economically feasible. As alternatives or supplements, new process configurations (with a partial external fuel supply) and the use of complementary technologies, such as autothermal SCWG, solar thermal energy with storage, or an SOFC-type fuel cell, open new research directions for future work. Supplementary Materials: Mass and energy balances for case studies. This supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app12115497/s1. Author Contributions: J.M.B.: Conceptualization, methodology, software, validation, formal analysis, investigation, data curation, writing—original draft preparation, writing—review and editing, visualization. F.J.J.-E.: Conceptualization, methodology, validation, formal analysis, resources, data curation, writing—original draft preparation, writing—review and editing, visualization, supervision, project administration, funding acquisition. F.L.-G.: Resources, writing—review and editing. F.J.G.O.: Discussion, formal analysis, validation, resources, writing—review and editing, supervision, project administration, funding acquisition. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Consejería de Economía y Conocimiento (Junta de Andalucía) for the financial support of his PAIDI 2020 (Research Project coded as P18-RT-2521). Informed Consent Statement: Not applicable. Conflicts of Interest: The authors declare no conflict of interest. References 1. Modell, M.; Reid, R.C.; Amin, S.I. Gasification Process. U.S. Patent US4113446A, 22 July 1978. 2. Modell, M. 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