Solar reactor technology for splitting CO2via a 2-step thermochemical redox cycle using concentrated solar radiation.

Efficient heat transfer of concentrated solar energy and rapid chemical kinetics are desired characteristics of solar thermochemical redox cycles for splitting CO2. We have fabricated reticulated porous ceramic (foam-type) structures made of ceria with dual-scale porosity in the millimeter and micrometer ranges. The larger void size range, with dmean = 2.5 mm and porosity = 0.76-0.82, enables volumetric absorption of concentrated solar radiation for efficient heat transfer to the reaction site during endothermic reduction, while the smaller void size range within the struts, with dmean = 10 μm and strut porosity = 0-0.44, increases the specific surface area for enhanced reaction kinetics during exothermic oxidation with CO2. Characterization is performed via mercury intrusion porosimetry, scanning electron microscopy, and thermogravimetric analysis (TGA). Samples are thermally reduced at 1773 K and subsequently oxidized with CO2 at temperatures in the range 873-1273 K. On average, CO production rates are ten times higher for samples with 0.44 strut porosity than for samples with non-porous struts. The oxidation rate scales with specific surface area and the apparent activation energy ranges from 90 to 135.7 kJ mol(-1). Twenty consecutive redox cycles exhibited stable CO production yield per cycle. Testing of the dual-scale RPC in a solar cavity-receiver exposed to high-flux thermal radiation (3.8 kW radiative power at 3015 suns) corroborated the superior performance observed in the TGA, yielding a shorter cycle time and a mean solar-to-fuel energy conversion efficiency of 1.72%.

The European consortium SOLARJET has experimentally demonstrated the first ever production of jet fuel via a thermochemical H 2 O/CO 2 -splitting cycle using simulated concentrated solar radiation. The key component of the production process of sustainable "solar kerosene" is a high-temperature solar reactor containing a reticulated porous ceramic (RPC) foam structure made of pure CeO 2 undergoing a 2-step redox cyclic process. During the first endothermic reduction step at 1450− 1600 °C, the RPC was directly exposed to concentrated thermal radiation with power inputs ranging from 2.8 to 3.8 kW and mean solar flux concentration ratios of up to 3000 suns. In the subsequent exothermic oxidation step at 700−1200 °C, the reduced ceria was stoichiometrically reoxidized with CO 2 and/or H 2 O to generate CO and/or H 2 . The RPC featured dual-scale porosity: millimeter-size pores for volumetric radiation absorption during reduction and micrometer-size pores within its struts for enhanced oxidation rates. For a cycle duration of 25 min, mean reduction rates were 0.17 mL O2 min −1 g −1 CeO2 and mean oxidation rates were 0.60 mL CO min −1 g −1CeO2 . The solar-to-fuel energy conversion efficiency was 1.72%, without sensible heat recovery. A total of 291 stable redox cycles were performed, yielding 700 standard liters of syngas of composition 33.7% H 2 , 19.2% CO, 30.5% CO 2 , 0.06% O 2 , 0.09% CH 4 , and 16.5% Ar, which was compressed to 150 bar and further processed via Fischer−Tropsch synthesis to a mixture of naphtha, gasoil, and kerosene.

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