Towards chemical equilibrium in thermochemical water splitting. Part 2: Re-oxidation

• Published in: International journal of hydrogen energy Vol. 72; pp. 1159 - 1168
• Main Authors: de la Calle, Alberto, Ermanoski, Ivan, Miller, James E., Stechel, Ellen B.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 27-06-2024
• Subjects: Counter-current reactor; Hydrogen generation; Redox reaction; Solar thermal; Thermochemical water splitting; Thermochemical water splitting; Hydrogen generation; Counter-current reactor; Solar thermal; Redox reaction
• ISSN: 0360-3199; 1879-3487
• DOI: 10.1016/j.ijhydene.2024.05.298

Chemical equilibrium represents the highest efficiency achievable by a thermochemical cycle under specific operational conditions. This study delves into two-step thermochemical water splitting cycles, which dissociate water into hydrogen (H2) and oxygen (O2) via sequential thermal reduction and re-oxidation processes. Building on the thermal reduction analysis presented in Part 1, this paper zeroes in on the re-oxidation reaction within the optimal reactor framework – the counter-current reactor. It elucidates the equilibrium pathway, delineating the progression of chemical equilibrium at each incremental stage of re-oxidation. Using ceria (CeO2) as a model redox-active metal oxide, the investigation analyzes the influence of operational parameters on the re-oxidation reaction. Findings reveal the theoretical feasibility of maintaining near constant temperature (±2.5 °C) during re-oxidation, achieving a total extent of reduction of 0.038 and a hydrogen conversion yield of 32%. While near adiabatic operation is also achievable, practicality is constrained by a maximum total extent of reduction of 6.5·10−3. The study also explores the economic implications of thermochemical water splitting, particularly focusing on the steam-to-hydrogen output ratio. It highlights the severe economic hurdles associated with hydrogen yields below approximately 1%. Furthermore, the investigation reveals that the addition of inert gas to the re-oxidation step offers no significant advantages. Concluding the analysis, a comparative assessment exposes negligible differences in outcomes when substituting carbon dioxide (CO2) for water (H2O) as the oxidant in low-temperature scenarios (800 °C). This comprehensive study not only advances the understanding of re-oxidation dynamics in thermochemical water splitting but also informs practical and economic considerations crucial for the advancement of this technology.
•Reversible/equilibrium re-oxidation pathway defined for water splitting cycles.•Reversible quasi-constant temperature and quasi-adiabatic re-oxidation is feasible.•Limited impact of gas purity on re-oxidation at fixed flow/minimum temperature.•Inert gas addition offers no thermodynamic advantage in re-oxidation.•Low re-oxidation temperatures expand the solution space for high cycle efficiency.