As mentioned previously, the solar thermal cracking of natural gas can be achieved in two types of reactors. In a direct heating reactor (Kogan and Kogan 2003b; Kogan et al., 2005; Trommer et al., 2004; Hirsch and Steinfeld, 2004; Abanades and Flamant, 2008; Rodat et al., 2010b), the particles absorb heat from solar radiation and the reactor is seeded for the increase of the adsorption and nucleation sites. The disadvantage of direct heating is the potential deposition of particles on the window of the reactor. In the indirect heating reactor (Dahl et al., 2004; Wyss et al., 2007; Rodat et al., 2010b), the solar irradiation zone is separated from the reacting flow by an opaque wall that serves as a heat transfer medium that allows convection of heat from the solid wall to the gas flow. A weakness of the indirect configuration is that it demands higher temperatures due to the heat transfer wall (Abanades and Flamant, 2006a).
In the scope of the European project SOLHYCARB (http://www. promes. cnrs. fr/ACTIONS/Europeenes/solhycarb. htm), which aims at the production of hydrogen and carbon black nanoparticles from methane cracking, a 20 kW laboratory-scale reactor and a 50 KW pilot-scale reactor are being developed based on the indirect solar heating configuration (Fig. 20.8). The 20 kW solar reactor consists of a cubic blackbody-cavity receiver that absorbs the concentrated solar radiation through a hemispherical quartz window placed at the front. Inside the reactor’s cavity, four graphite tubular reaction zones are arranged vertically. Each of the four consists of two concentric graphite tubes. The reaction gas first enters the inner tube and flows out through the space in-between the two tubes. This mainly serves to increase the gas residence time and the better preheating of the reactants (Rodat et al., 2010b). The 50 kW pilot-scale reactor (Fig. 20.8(c)) was designed on the same principle. The reactor body is made of an aluminum shell (800 x 780 x 505 mm) and a water-cooled front face with a 13 cm diameter aperture for concentrated solar radiation entry. The radiation is absorbed by the graphite cavity (360 x 400 x 300 mm) that approaches black-body behavior. To avoid contact of graphite with the oxidizing atmosphere, the opening is protected by a domed quartz window (outer diameter of 360 mm) swept by a nitrogen flow to avoid overheating. The reaction occurs in seven horizontal graphite tubes (single tubes).
Typical results for the 20 kW and 50 kW solar reactors are illustrated in Fig. 20.9. Experimental data show clearly that complete conversion of methane is achievable in the solar reactors. However, the significant amount of C2H2 that is produced lowers the carbon yield (Fig. 20.9(b)). The production at pilot-scale is 200 g/h H2 (88% H2 yield), 330 g/h CB (49% C yield), and 340 g/h C2H2. The thermal and thermochemical performances of the pilot reactor (50 kW) are shown in Fig. 20.10.