AUTHOR=Chuayboon Srirat , Abanades Stéphane , Rodat Sylvain 

TITLE=High-Purity and Clean Syngas and Hydrogen Production From Two-Step CH4 Reforming and H2O Splitting Through Isothermal Ceria Redox Cycle Using Concentrated Sunlight

JOURNAL=Frontiers in Energy Research

VOLUME=Volume 8 - 2020

YEAR=2020

URL=https://www.frontiersin.org/journals/energy-research/articles/10.3389/fenrg.2020.00128

DOI=10.3389/fenrg.2020.00128

ISSN=2296-598X

ABSTRACT=The thermochemical conversion of methane (CH4) and water (H2O) to syngas and hydrogen via chemical looping using concentrated sunlight as a sustainable source of process heat attracts considerable attention. It is likewise a means of storing intermittent solar energy into chemical fuels. In this study, solar chemical looping reforming of CH4 and H2O splitting over nonstoichiometric ceria (CeO2/CeO2-δ) redox cycle was experimentally investigated on-sun in a volumetric solar reactor prototype. The cycle consists of (i) the endothermic partial oxidation of CH4 and simultaneous reduction of ceria and (ii) subsequent exothermic splitting of H2O and simultaneous oxidation of the reduced ceria under isothermal operation at ~1000 ºC, enabling the elimination of sensible heat losses as compared to non-isothermal thermochemical cycles. Ceria-based reticulated porous ceramics with different sintering temperatures (1000 and 1400 °C) were employed as oxygen carrier and tested with different methane flow-rates (0.1-0.4 NL/min) and methane concentrations (50 and 100%). The impacts on the foam-averaged oxygen non-stoichiometry (reduction extent, δ), syngas yield, methane conversion, solar-to-fuel energy conversion efficiency as well as transient solar conditions were demonstrated and emphasized. As a result, clean syngas was successfully produced with H2/CO ratios approaching 2 during first reduction step while high-purity H2 was subsequently generated during oxidation step. Increasing methane flow-rate and CH4 concentration promoted syngas yields up to 8.51 mmol/gCeO2 and δ up to 0.38, at the expense of enhanced methane cracking reaction and reduced CH4 conversion. Solar-to-fuel energy conversion efficiency, namely the ratio of the calorific value of produced syngas to the total energy input (solar power and calorific value of converted methane), and CH4 conversion were achieved in the range of 2.9-5.6% and 40.1-68.5%, respectively.