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[Display omitted] • Chemical looping allows for H 2 production with inherent carbon separation. • It can achieve 95% conversion of H 2 O at 1073 K outperforming a WGS reactor. • A numerical model with no fitted parameters can simulate a working reactor. • This was confirmed using operando gas and solid oxygen capacity measurements. • This system is thermodynamically limited. H 2 is an important feedstock for many industrial processes and could be used as an energy carrier in a low carbon economy. This means that carbon neutral methods for H 2 production are of vital importance. Chemical looping allows for H 2 production with inherent carbon separation, making it an ideal system to produce low carbon H 2. This work generates insights into the production of high purity H 2 using a chemical looping packed bed reactor system containing an oxygen carrier of variable oxygen non-stoichiometry. Such a system has been shown to achieve 95% conversion of H 2 O to H 2 at 1073 K outperforming the maximum theoretical conversions of 50% achieved by a conventional water gas shift reactor at that temperature. A numerical model was developed from theoretical consideration, with no fitted parameters and used to simulate the working reactor. Operando measurement of gas conversions and changes in solid oxygen capacity, through synchrotron X-ray diffraction, were used to validate the numerical model and confirmed that the reaction was thermodynamically limited. The model the model was shown to reproduce the conversion of the oxygen carrier, the reactant conversion and the product evolution. Sensitivity analysis showed that the relationship between the oxygen carrier material oxygen content and the chemical potential of oxygen in the carrier was the key consideration for the design and operation of a packed bed chemical looping reactor using an oxygen carrier of variable non-stoichiometry. [ABSTRACT FROM AUTHOR]