Production of hydrogen via an Iron/Iron oxide looping cycle: Thermodynamic modeling and experimental validation

An incremental thermodynamic equilibrium model has been developed for the chemical reactions driving a clean, hydrogen producing iron/iron oxide looping cycle. The model approximates a well-mixed reactor with continuous reactant gas flow through a stationary solid matrix, where the gas residence time is long compared to time constants associated with chemical kinetics and species transport. The model, which computes the theoretical limit for steam-to-hydrogen conversion, has been experimentally validated for the oxidation reaction using an externally heated, 21 mm inner diameter, tubular fluidized bed reactor. Experiments were carried out at 660 and 960 °C with steam flow rates ranging from 0.9 to 3.5 g/min. For small flow rates, i.e., for long residence times, the experimentally observed cumulative steam-to-hydrogen conversion approaches the theoretically predicted conversion. At a 960 °C operating temperature, the measured hydrogen yield approaches the theoretical limit (experimental yields are always within 50% of the theoretical limit), and the yield is insensitive to variations in the steam flow rate. In contrast, the measured hydrogen yield deviates significantly from the theoretical limit at a 660 °C operating temperature, and strong variations in hydrogen yield are observed with variations in steam flow rate. This observation suggests that the reaction kinetics are significantly slower at lower temperature, and the model assumption is not satisfied.