# Hydrogen Production in Methane Decomposition Reactor Using Solar Thermal Energy
## 4 Development of Methane Decomposition Reactor
### Indirect Heating Reactor 1  
To improve the quartz breakage and gas leakage in the reactor, a rotary cylindrical reactor was developed as shown in Figure 8, and its dimensions are shown in Table 1. The total length of the reactor was 400 mm, and a 315 mm catalyst was placed inside the reactor above the bottom cone. The upper end of the reactor was fastened with a flange to facilitate catalyst replacement, and a graphite seal was used between the flanges and the reactor to prevent gas leakage. When the concentrating position on one side of the reactor is fixed, rapid thermal damage may occur owing to continuous exposure. As a result, a motor and reducer were installed to supply a uniform heat flux in the circumferential direction, so that the reactor could be rotated. In addition, rotary joints serving as inlet and outlet were connected to the upper and lower ends of the reactor. Inside the reactor, a catalyst coated with carbon black (CB I) on NiCrAl metal foam was installed to reduce the reaction temperature and activation energy required for methane decomposition [29]. The NiCrAl metal foam was dipped into the binder solution followed being mixed with powder. The powder-dipped metal foam was dried at room temperature overnight, and heat treated at 160 \({}^{\circ}\)C and 300 \({}^{\circ}\)C for 4 and 2 h, respectively, under static air. It is a circular catalyst with a diameter of 200 mm and thickness of 9.5 mm. Because the structure is metal, it has excellent conductivity, and can quickly transfer the amount of condensed heat supplied through the outer wall of the reactor to the inside of the reactor and the catalyst itself. Prior to the metal foam catalyst, a pellet catalyst was used, but heat transfer from the outer wall of the reactor to the center of the reactor was slow; hence, it took approximately 2 h for the catalyst temperature to reach 1000 \({}^{\circ}\)C. This could be shortened to approximately 1 h using a metal foam catalyst. Figure 7: Quartz window of reactor (after experiment). CB I contains oxygen functional groups on the surface, such as carbonyl, carboxyl, quinone, and epoxy. The improved coke resistance of carbon catalysts has been reported with the aid of oxygen functional groups [30]. Methane conversion using CB I-coated NiCrAl foam at different temperatures and gas hourly space velocity (GHV) is shown in Figure 9. On the one hand, the methane conversion under 1000 \({}^{\circ}\)C rapidly decreased with the increase in GHV. On the other hand, above 1100 \({}^{\circ}\)C, the methane conversion in various GHSVs was stable at 80%. These results show the characteristics of the catalyst used in this study, and indicate that a sufficient methane decomposition reaction takes place above 1000 \({}^{\circ}\)C, especially above 1100 \({}^{\circ}\)C.  
In the experiment, a multiple temperature sensor was mounted along the rotation axis through the upper rotary joint, as shown in Figure 8, to measure the temperature of the catalyst center according to the height inside the reactor. To reduce the heat loss of the reactor, a cavity filled with insulation material with a thickness of 50-100 mm was manufactured, and the reactor was positioned. The front of the cavity had an opening at the focal position to allow the concentrated light to pass through, and the diameter was initially 200 mm; however, to reduce the heat loss, the size of the concentrating area at the focal point was measured and reduced to be optimized at a diameter of 150 mm. In a simple analysis, 40 L/min of methane is supplied to the reactor, and 1.7 kW is required to heat it to 1000 \({}^{\circ}\)C, and 2.75 kW is required to convert 80% of the methane to hydrogen during an endothermic reaction. In addition, the heat loss of 5.83 kW occurs. At the same time the cavity also heated up, and its radiation and convection heat were transferred to the environment in spite of the high insulation of the cavity.