Hydrogen production via a three-reactor chemical-looping (TRCL) process using perovskite-type oxygen-carrier materials.

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This project will investigate the use of perovskite-type oxygen-carrier materials for hydrogen production via chemical looping from the water-gas shift reaction (CO + H2 <=> CO2 + H2O), specifically focusing on the Three-Reactor Chemical Looping (TRCL) process that involves the addition of an air oxidation step and can use CH4 or syngas as the reducing agent. This process eliminates the need to separate the H2 and CO2 product streams since the reactants do not come into contact with each other; the products and the oxygen carrier material intermediate are in different phases.Perovskite-type materials are able to demonstrate a wide range of different oxidising potentials due to their non-stoichiometry. In addition, their ability to incorporate a number of different metal cations of differing valance states within their cubic ABO3 structure allows for materials engineering of desired properties with point defect chemistry. In chemical looping from the water-gas shift reaction, these materials have typically been strontium doped lanthanum ferrite oxides of the form La1-xSrxFeO3-o.CO is an ideal reducing agent to use in laboratory-scale experiments involving chemical looping as it oxidises directly to CO2, thereby avoiding selectivity complications and simplifying the modelling involved. However it is not a realistic feed gas to use on an industrial scale since it does not naturally occur in large concentrations and its main production method is via the reverse Boudouard reaction. CH4 from natural gas is more abundant and has been identified as one of the more likely alternatives as a reducing agent. However the large endothermic heat of reaction when the oxygen-carrier material is reduced via oxidation of CH4 results in an energy deficit in two-step chemical looping systems with the water-gas shift reaction. The TRCL process has been proposed as a solution to this problem, with the addition of the air oxidation step allowing for the process to potentially be operated auto-thermally on an industrial scale, and maintaining full oxidation of the oxygen carrier material. Studies of the system efficiency using a steam compression and combustion turbine with metal-oxide oxygen carrier materials have predicted that significant amounts of heat could be recovered from the gas streams to make the TRCL process near self-sufficient in terms of electrical power consumption.The TRCL process conventionally involves a fluidised bed system with a separate fuel reactor, steam reactor and air reactor. For this study, an existing single packed bed reactor with counter-current gas flows will be used. The perovskite-type oxygen carrier material, which is fixed in place within the reactor, undergoes a series of cyclic reductions and oxidations as separate feeds of carbon-based fuel (such as CH4) and H2O in a balance of inert followed by air enter the reactor, allowing for the production of a H2 stream. Following each step in a cycle, the reactor is purged with a stream of inert, typically argon, in order to remove any trace amounts of the reactants or products. This is to prevent the uncontrolled oxidation of the reactants or products existing in concentrations above their lower explosive limits, which is an important safety concern.This work will provide a comparison of the benefits of the TRCL process with two-step chemical looping configurations using perovskite-type oxygen carrier materials by developing a mathematical model that shows how the principle of chemical looping might be applied to an industrial scale. An existing model for the two-step process is based on a defect chemistry model that relates the virtual oxygen partial pressure to the delta parameter and the amount of strontium doping. A laboratory-scale reactor rig will be used in order to verify the model and further explore the potential of the TRCL configuration.