(186f) Thermodynamic and Computational Modeling of Thermochemical Syngas Production Via the CeO2-CH4-CO2 Redox Reforming Cycle in a Tubular Reactor

Solar thermochemical processing offers an efficient pathway for producing sustainable fuels using concentrated solar energy as the source of high-temperature process heat. The targeted precursor fuel is syngas − a specific mixture of H₂ and CO that can be further processed to drop-in transportation liquid fuels. Within this context, a 2-step dry redox reforming process is considered, using CeO₂ as the redox material and CH₄ and CO₂ as a reducing and oxidizing agent, respectively. The thermodynamics of this process have been extensively studied [1][2] and its on-sun feasibility was demonstrated on a reasonable scale [3]. The dry redox reforming process can operate isothermally in the range 800-1200°C and achieve high non-stoichiometries (δ>0.05), indicating a high proportion of O₂ exchange and therefore a high specific fuel output per reactive mass. The dry redox reforming process is represented by two steps [2]:

Endothermic reduction:

CeO₂ + δCH₄ ↔ CeO_2-δ + δCO + 2δH₂ (eq. 1)

Exothermic oxidation:

CeO_2-δ + δCO₂ ↔ CeO₂ + δCO (eq. 2)

We report on the thermodynamic investigation, and its implementation into a computational fluid dynamic model using a customized solver in OpenFOAM. The solver considers all conservation equations in gas and solid phases (species mass, continuity, energy, and momentum) and allows one to model reversible heterogenous reactions of a non-stoichiometric solid whose inherent properties are dependent on δ. Validation of the model was carried out via experiments in a tubular reactor (19 mm-diameter, 300 mm length). The tubular reactor was filled with a packed bed of ceria pellets, and was heated by an electrical furnace. The main operating conditions for purposes of validation were: 1000°C, ambient pressure, mass flowrates of 1 L/min (L: normal liters), and up to 5% reactive gas concentrations diluted in Ar. The outlet product composition was used as the primary means of model validation. Subsequently, the model is used in parametric studies for the purposes of scale-up.

References

[1] B. Bulfin et al., "Statistical thermodynamics of non-stoichiometric ceria and ceria zirconia solid solutions," Phys Chem Chem Phys, vol. 18, no. 33, pp. 23147-54, Aug 17 2016, doi: 10.1039/c6cp03158g.

[2] Bulfin, B., et al., Thermodynamic comparison of solar methane reforming via catalytic and redox cycle routes. Solar Energy, 2021. 215: p. 169-178.

[3] M. Zuber et al., "Methane dry reforming via a ceria-based redox cycle in a concentrating solar tower," Sustainable Energy & Fuels, 2023, doi: 10.1039/d2se01726a.