Experimental and numerical analysis of thermochemical cycles using cerium oxides and iron oxides

The present work deals with both experimental and numerical analyses of thermochemical cycles. This technological pathway typically aims at producing hydrogen or synthesis gas starting from water or water/carbon dioxide mixture, respectively. In case both of these latter feeds are supplied to the process, the resulting synthesis gas mixture, made up of hydrogen and carbon monoxide, can be further treated downstream to obtain higher complexity chemicals, e.g. through Fischer-Tropsch processing. Nowadays, the importance of developing processes capable to treat and valorise carbon dioxide emissions lies in the possibility to face as effectively as possible climate changes and global warming, in the framework of a decarbonization global policy. It is noteworthy that such thermochemical path should be powered by renewable energy sources, typically concentrating solar power (CSP) facilities, for assuring ideally a zero-impact as regards carbon dioxide net emissions. Under the experimental point of view, thermogravimetric analysis (TGA) was conducted on two different well-known materials, namely cerium oxides and iron oxides, to experimentally characterize their behaviour and performances in the given working conditions. Both these metal oxides were already extensively studied so far for thermochemical cycles applications, and several previous works from the literature review will be reported herein for sake of comparison and completeness. The TGA procedures allowed to conclude that using iron oxides leads to higher productivity in terms of oxygen released during the reduction phase, that corresponds - in principle - to higher fuel productivity during the following oxidation phase. Thus, iron oxides were selected for performing a numerical simulation in COMSOL Multiphysics environment as a further step of the workflow. The numerical model simulates the cylindrical solar receiver-reactor of the CSP system installed at the Energy Center research pole, Turin (Italy), and in particular the iron oxide transient reduction reaction that takes place with the given boundary conditions imposed. In the real system, the receiver is placed in the focus of the paraboloidal concentrator to absorb as much solar radiation as possible. The concentrator was not considered inside the numerical simulation, apart from a thermal flux boundary condition imposed on the external walls of the cylinder. Starting from the TGA experimental outcomes, all the kinetic parameters of interest were estimated through an optimization procedure and were used in the model as a thermochemical boundary condition. The functional form for the reaction rate, in which the parameters were implemented, was retrieved from literature, where the same mathematical expression was used in similar experimental conditions and with similar materials. The model allowed to obtain as a main output the evolution in time of the oxygen concentration at the outflow section. A possible future work will focus on the numerical model validation on field, through direct experimental runs on the real CSP system just described.