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PRINCIPAL PUBLICATION AND AUTHORS
Production of high purity H2 through chemical-looping WGS at reforming temperatures the importance of non-stoichiometric oxygen carriers, C. de Leeuwe (a,b), W. Hu (a), J. Evans (c), M. von Stosch (a), I.S. Metcalfe (a), Chem. Eng. J. 423, 130174 (2021); https:/doi.org/10.1016/j.cej.2021.130174. Material from this article has been reproduced and modified under the CC BY license (http:/creativecommons.org/licenses/by/4.0/). (a) School of Engineering, Newcastle University, Newcastle upon Tyne (UK) (b) Department of Chemical Engineering and Analytical Science, University of Manchester, Manchester (UK) (c) Department of Chemistry, Durham University, Durham (UK)
REFERENCES
[1] I.S. Metcalfe et al., Nat. Chem. 11, 638-643 (2019). [2] C. de Leeuwe et al., J. Solid State Chem. 293, 121838 (2020).
eq. 1 eq. 2 eq. 3
Fig. 134: δ of a 2.2 g of La0.6Sr0.4FeO3-δ bed at 20 s increments simulated using a thermodynamically limited model for a 60 s feed of 5% H2O compared to values measured using operando SXRD. An uncertainty of one standard deviation is plotted on the experimental data. Figure modified from the principal publication.
mixed with the H2O and/or H2 and the need for product gas separation is eliminated:
CO +H2O D CO2 +H2 m CO + La0.6Sr0.4FeO3-δ → m CO2 + La0.6Sr0.4FeO3-δ-m m H2O + La0.6Sr0.4FeO3-δ-m → m H2 + La0.6Sr0.4FeO3-δ
The sub-reactions can be performed using a system of packed beds that are alternately fed with CO and H2O. Previous work has shown that chemical-looping H2 production in such systems can produce high-purity H2 and CO2 (H2:H2O and CO:CO2 > 19:1 at the reactor outlets, on a molar basis) as separate streams at temperatures around 1073 K [1]. Under these conditions, there are no kinetic limitations.
Here we work with one single packed bed reactor with alternate feeds on 5% CO in argon and 5% water in argon. The non-stoichiometric OC, La0.6Sr0.4FeO3-δ, was chosen for its stability over a wide range of partial pressures of oxygen, Po2, and the fact that the dependence of the oxygen non-stoichiometry, δ , on Po2 and the relationship between unit cell parameter and oxygen content have been studied in detail [2]. This means that when operando synchrotron powder X-ray diffraction (SXRD) is performed on a working reactor, the oxygen content of the carrier as a function of time can be inferred from unit cell parameters and this can be compared to thermodynamic simulations of the reactor.
A thermodynamic model with no free parameters has been developed that predicts the spatiotemporal variation of δ in the La0.6Sr0.4FeO3-δ bed under steady cyclic operation. Snapshots from the model of the expected δ during the oxidation half cycle of the chemical-looping WGS can be seen in Figure 134. These are compared to the measured variations in δ along the reactor (obtained from operando SXRD at beamline ID22 using the technique detailed by de Leeuwe et al [2]) at the beginning and end of each half cycle.
The simulated results reproduce the experimental oxygen content at the oxidised end of the bed. In fact, the change in oxygen content of the bed measured by operando SXRD (9.0 ± 0.5 x 10 5 mol) and by measuring the outlet gas composition (9.0 ± 0.3 x 10 5 mol) both agreed with the value predicted by the simulation (8.9 x 10 5 mol) to within 3%.
Having a numerical model that accurately predicts the performance of the reactor is immensely useful. This is especially so when the model only requires thermodynamic properties available from the literature. Armed with the model, it is possible to predict how different materials will behave in the reactor and even optimise the composition of these materials. Furthermore, the model can be employed to help design larger-scale reactor systems for real-world applications.
