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

Alberto de la Calle, Ivan Ermanoski, James E. Miller, Ellen B. Stechel

Research output: Contribution to journalArticlepeer-review

2 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)1159-1168
Number of pages10
JournalInternational Journal of Hydrogen Energy
Volume72
DOIs
StatePublished - Jun 27 2024

Keywords

  • Counter-current reactor
  • Hydrogen generation
  • Redox reaction
  • Solar thermal
  • Thermochemical water splitting

ASJC Scopus subject areas

  • Renewable Energy, Sustainability and the Environment
  • Fuel Technology
  • Condensed Matter Physics
  • Energy Engineering and Power Technology

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