Abstract
The thermochemical conversion of methane (CH 4) and water (H 2 O) 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 CH 4 and H 2 O splitting over non-stoichiometric ceria (CeO 2 /CeO 2−δ) redox cycle were experimentally investigated in a volumetric solar reactor prototype. The cycle consists of (i) the endothermic partial oxidation of CH 4 and the simultaneous reduction of ceria and (ii) the subsequent exothermic splitting of H 2 O and the simultaneous oxidation of the reduced ceria under isothermal operation at ∼1,000 • C, enabling the elimination of sensible heat losses as compared to non-isothermal thermochemical cycles. Ceria-based reticulated porous ceramics with different sintering temperatures (1,000 and 1,400 • C) were employed as oxygen carriers and tested with different methane flow rates (0.1-0.4 NL/min) and methane concentrations (50 and 100%). The impacts of operating conditions on the foam-averaged oxygen non-stoichiometry (reduction extent, δ), syngas yield, methane conversion, solar-to-fuel energy conversion efficiency as well as the effects of transient solar conditions were demonstrated and emphasized. As a result, clean syngas was successfully produced with H 2 /CO ratios approaching 2 during the first reduction step, while high-purity H 2 was subsequently generated during the oxidation step. Increasing methane flow rate and CH 4 concentration promoted syngas yields up to 8.51 mmol/g CeO 2 and δ up to 0.38, at the expense of enhanced methane cracking reaction and reduced CH 4 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 CH 4 conversion were achieved in the range of 2.9-5.6% and 40.1-68.5%, respectively.
Highlights
Most hydrogen production is currently achieved via conventional steam reforming of natural gas (Equation 1) (Zheng Q. et al, 2014)
Chemical looping reforming of methane and H2O splitting from isothermal ceria redox cycle for efficient syngas and hydrogen production have been assessed thermodynamically and experimentally
A parametric study considering the influence of inlet CH4 concentration, CH4 flow rate, and annealing temperature on syngas production rate, yield, foam-averaged oxygen non-stoichiometry (δ), CH4 conversion, and reactor performance was performed
Summary
Most hydrogen production is currently achieved via conventional steam reforming of natural gas (Equation 1) (Zheng Q. et al, 2014). The heat source is provided by combustion of up to 41% of the methane feedstock, causing 24% reduction in product energy content compared to the feedstock (Simakov et al, 2015; Krenzke et al, 2017), and costly catalysts are necessary to conduct such reactions (Dincer and Rosen, 2013). This unavoidably results in a significant portion of methane feedstock consumption, as well as greenhouse gas emissions (especially CO2), which contribute to climate change and global warming (Nejat et al, 2015).
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