# Hydrogen Production in Methane Decomposition Reactor Using Solar Thermal Energy
## 1 Introduction
developed a methane decomposition reactor consisting of a graphite tube encased by stainless steel. The reactor was directly heated by solar heat concentrated through quartz, and at 0.7 L/min, a conversion rate of 53.3-54.9% was achieved. In addition, a performance prediction model was developed and verified by comparing it with experimental results [14; 15; 16]. Magg et al. developed a 5 kW direct heating solar methane decomposition reactor of a cylindrical cavity shape, with quartz mounted in a circular opening. In the experiment, a combination of argon, methane, and carbon particles was injected to improve the conversion rate. A methane conversion rate of 98.8% was achieved when methane was 0.8 L/min in 10.4 L/min of the gas mixture at 1043 \({}^{\circ}\)C [17]. In the same reactor, Hirsch et al. added a spiral flow path to the inner wall of the reactor to rotate methane to secure residence time and develop a reactor with improved heat transfer from the high temperature wall [18]. Ozalp et al. also performed a simulation on the decomposition of methane, according to the presence or absence of a helical cavity inside the reactor, to compare its performance [19]. Yeheskel et al. developed a direct-heated volumetric reactor optimized through CFD and conducted an experiment to convert methane into hydrogen and carbon particles using a reactor characterized by high reaction temperature, quartz protection, no boundary layer separation, and directional flow [20]. Most indirect reactors are tubular. Dahl et al. developed an indirect heated reactor consisting of three concentric vertical tubes with a porous graphite tube at the innermost wall, a solid graphite tube in the middle, and a quartz fluid wall at the outer side. A secondary concentrator was installed on the front of the reactor to reflect light from a 10 kW solar furnace. In their experiment, a methane conversion rate of 90% was achieved at 1860 \({}^{\circ}\)C, with a methane flow rate of 1 L/min [21]. Rodat et al. also developed a 10 kW reactor. A concentrated solar heat was passed through the opening, the graphite tube reactor located in the graphite cavity was heated,and the methane was passed through the tube and separated in a high temperature environment. In addition, heat transfer and reaction were calculated through heat flow analysis, and compared with the experimental results [22; 23]. Subsequently, the experiment was performed in a 1 MW solar furnace by expanding it to a 50 kW class methane decomposition reactor. A conversion rate of 98% was achieved when the methane flow rate was 21 L/min at 1525 \({}^{\circ}\)C [24]. Valdes-Parada et al. developed a reactor in which a graphite tube was vertically mounted inside a hexahedral cavity with quartz that absorbs concentrated solar heat. The residence time was secured by improving the flow path in the reaction tube, which was annular. The reactor temperature distribution and the methane-hydrogen conversion rate were numerically analyzed at the time of supplying the argon-methane mixture, and the relationship between the temperature and conversion rate was explained by comparing the results with the experimental results [25]. The direct heating type reactor is preferred for heating the reactants during heat transfer because solar heat is directly transferred to the inside, as shown in Figure 1. However, due to the short transmission distance between the reactor and the solar heat irradiated, there is a substantial decrease in distance within the reactor. Furthermore, carbon particles generated are deposited on the quartz, which reduces its ability to effectively transmit light. This increases the quartz temperature, resulting in damages. In addition, it is difficult to expand a reactor for treating methane with a high flow rate due to the limited size of the quartz-manufacturing process. Prior to this study, the cavity shape was modified, and the insulation performance was improved by optic and heat transfer calculation of CFD [26].