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Title: Overcoming water-gas-shift equilibrium via chemical looping
Author: Mak, Cheuk-Man
ISNI:       0000 0004 7971 2819
Awarding Body: Newcastle University
Current Institution: University of Newcastle upon Tyne
Date of Award: 2018
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This thesis is focused on overcoming the equilibrium limitation of the water-gas-shift reaction (WGS), a common way to produce hydrogen, via chemical looping using iron-containing perovskite materials. The WGS reaction is separated into reduction and oxidation half-cycle by using an oxygen carrier material (OCM) to act as an intermediate via chemical looping, therefore, water is the only impurity in the hydrogen product stream. WGS conversions are thermodynamically limited to a maximum of 50 % by equilibrium constant using a metal with two oxidation states, metal or metal oxide, during steady state operation at 817 ▫C (equal amount to oxygen is removed/replaced from the material during reduction/oxidation half-cycle). Here we show that this limitation can be overcome by using a material that is able to produce hydrogen without undergoing phase transitions and with high structural stability via chemical looping in a counter-current flow fixed bed reactor. Initial experiments were performed to select a perovskite material from the La-Sr-Fe series that remain single crystal structure and capable of achieving high redox reactivity at 820 ▫C where the WGS equilibrium constant is close to unity. La0.6Sr0.4Fe3-δ (LSF641) was selected for further investigation as it was able to overcome WGS equilibrium limitation and was showing 80 % conversions for both half-cycles during steady-state which was the highest among other materials in the series. As water and CO were fed separately in opposite directions, an oxidation state profile of the bed was established during steady state. This oxidation state profile was determined by combining theoretical thermodynamic data with lattice parameters obtained from synchrotron in-situ x-ray diffraction (XRD). WGS conversions were further improved by using shorter redox duration. The stability of LSF641 was investigated by performing longterm redox cycling and it was showing constant conversions over 270 redox cycles. In addition, composite materials consisting of perovskite material and iron oxide were investigated to improve the overall OCM oxygen capacity. La0.7Sr0.3Fe3-δ perovskite with 11 wt.% iron oxide was able to produce hydrogen 15 times higher than iron oxide alone at the 200th cycles. Different phases were found after the reduction in CO as shown in XRD experiments, in particular La2-ySryFeO4-δ was found with the highest intensity in in-situ XRD experiment during the reduction in hydrogen, suggests that this phase is partly responsible for the increase in hydrogen production. The increase in hydrogen production was also related to the increase of porosity which was determined by using micro computed-tomography imaging to inspect the change in morphology of the OCM. The OCM porosity increased from 0.8 % to 5.6 % and 13% at 70th and 140th cycle, respectively.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available