Theoretical and Experimental Investigation of Solar Methane Reforming through the Nonstoichiometric Ceria Redox Cycle

The dry reforming of methane through the nonstoichiometric ceria (CeO₂–CeO_2−δ) redox cycle was examined theoretically and experimentally for converting high-temperature, solar process heat to syngas. The aforementioned cycle is composed of: (1) endothermic reduction of ceria and simultaneous partial oxidation of methane and (2) exothermic oxidation of the reduced ceria and simultaneous reduction of CO₂. In both steps, chemical equilibrium calculations indicate that isothermal operation is thermodynamically favorable under a wide range of conditions. The influence of the total amount of reactive gas, the operating temperature, and the inclusion of gas- or solid-phase heat exchangers on the cycle performance was determined through a holistic process model. A theoretical solar-to-fuel conversion efficiency, defined as the ratio of the difference between the calorific value of syngas (H₂ and CO) produced and methane converted to the total solar radiative input energy, of more than 45 % was predicted with no heat recuperation. Experimental validation was subsequently demonstrated in a packed-bed-type solar reactor using the high-flux solar simulator at the University of Florida at three discrete isothermal temperatures, namely, 950, 1035, and 1120 °C. Upon the completion of the reduction at each temperature, the bed-averaged oxygen nonstoichiometry equaled 0.07, 0.21, and 0.24, which yielded methane conversions of 9, 41, and 51 %, respectively. At 1120 °C, the extrapolated solar-to-fuel conversion efficiency was 9.82 %.