Experimental selection of optimal chemical looping conditions using iron oxides for applications in CSP systems

The persistent rise of the average temperature of the Earth shows that global warming is an obvious fact. This is connected to increasing greenhouse gas (GHG) emissions, pollution and the resulting climate changes, causing alterations to the environment. All this can be traced back to the use of fossil fuels as the world’s main energy source. One alternative to counteract global warming is to investigate technologies for capturing CO2 already generated by current fossil fuel energy systems and develop methods to convert CO2 into useful chemicals or combustible gases. Among the possible methods, one of the most promising is the chemical looping process composed of two-step thermochemical cycles for CO2 conversion to syngas. The objective of this dissertation is to investigate the feasibility to perform chemical looping processes for syngas production in the CSP system located on the rooftop of the Energy Center building. In this way, it would be possible to store intermittent solar energy in the form of chemical energy through the conversion of CO2 molecules. For this purpose, at first, a geometrical optics model and an interconnected heat transfer model have been developed, using COMSOL Multiphysics, to simulate and predict the temperature evolution of the receiver for the four seasons. Then, to find the best set-up of the system, different chemical looping processes at different temperatures, CO2 concentrations and reducing conditions were investigated at the CO2 Circle Lab (CCL) in Environment Park. An electrically heated tubular furnace is utilized to simulate a real solar thermochemical reactor and iron oxides powder is used as the oxygen carrier. The isothermal chemical looping cycle at 1000 °C with 5% H2 in reduction and 40% CO2 in oxidation with N2 performs best, among all those analyzed, in terms of process yield. The total CO production during a single cycle is 0.00422 mol/g, while the obtained CO peak production rate is 27.63 μmol/g/s. In a second moment, the CO produced can be mixed with H2 with a controlled H2/CO ratio. Based on the temperature evolution prediction of the Energy Center reactor obtained from the COMSOL model, is possible to consider the opportunity to translate this ideal test in real-world condition. A different minimum number of cycles per day can be performed depending on the weather season, except for winter, for which the above process is not applicable due to the low temperatures. In the last case, it can be considered to replicate the process but at a lower temperature and consequently with a lower process yield expected.