Methane dry reforming via a ceria-based redox cycle in a concentrating solar tower

Abstract

Drop-in fuels produced using solar energy can provide a viable pathway towards sustainable transportation, especially for the long-haul aviation sector which is strongly dependent on jet fuel. This study reports on the experimental testing of a solar reactor using concentrated solar energy for the production of syngas, a mixture of mainly H₂ and CO, which serves as the precursor for the synthesis of kerosene and other liquid hydrocarbon fuels. The thermochemical conversion route is based on the dry reforming of CH₄via a 2-step redox cyclic process utilizing the intermediation of non-sacrificial ceria (CeO₂), comprising: (1) the endothermal reduction of CeO_2−δox with CH₄ to form CeO_2−δred and syngas (δ denoting the non-stoichiometry); and (2) the exothermal oxidation of CeO_2−δred with CO₂ to form CO and the oxidized state of CeO_2−δox. The solar reactor consists of a cavity-receiver lined with a reticulated porous ceramic (RPC) structure and an axial tubular section at the cavity's rear filled with a packed-bed of agglomerates, both RPC and agglomerates made of pure ceria. Testing is performed at a high-flux solar tower at conditions and scale relevant to industrial implementation. For a solar radiative power input of 10 kW (corresponding to a mean solar flux of 560 suns) at temperatures in the range 800–1000 °C, with reacting gas flow rates of 105 normal L min⁻¹ and concentrations of CH₄ (reduction step) and CO₂ (oxidation step) of up to 20% in Ar, the solar-driven redox reforming process yields a peak CH₄ molar conversion of 70% and a peak H₂ selectivity of 68%. Co-feeding of CH₄ and CO₂ during the reduction step resulted in the highest solar-to-fuel energy efficiency of 27%, defined as the ratio of the higher heating value of the syngas produced over the sum of the solar radiative power input through the solar reactor's aperture and the higher heating value of CH₄ fed to the solar reactor. Regardless of the operational mode, the syngas product composition was similar at equal δ attained during the reduction. The addition of the tubular packed bed increased the syngas yield by 32%.