Experimental analysis of CO2 splitting by perovskites-based Chemical Looping: isothermal redox cycling of SFNM-04 in a microreactor setup

Global electricity demand is growing faster than renewables, driving strong increase in generation from fossil fuels. As a result, carbon emissions from the electricity sector, which fell in both 2019 and 2020, are projected to increase in 2021 and in 2022. In a path towards meeting the international goals on climate change, the energy sector is going through epochal changes that involve in a transversal way the methods of supplying, transporting, storing and using energy. In this context, mitigation actions against CO2 emissions are becoming increasingly fundamental. A recent analysis published by IEA highlighted that one of the key technology areas for putting energy systems on a sustainable trajectory is represented by carbon capture, utilisation and storage (CCUS). One possible solution in the direction of renewable fuel-based economy is represented by the chemical looping (CL) technology that is an innovative carbon capture process. Exploiting the capability of an oxygen carrier to assume different oxidation states, the chemical looping process is divided into two half cycles: firstly, the oxygen carrier is reduced by using high temperature heat as energy driver either in an inert ambient or with the contribution of a reducing agent to lower the reduction temperature, then is regenerated by an oxidizer (e.g CO2 if the cycle is applied for CCU). In this study, the chemical looping technology is used to produce solar fuels by means of a renewable high temperature heat input, supported by hydrogen as reducing agent. The process necessitates of a bi-functional looping material able to split CO2 that combines both catalytic function for high fuel production and oxygen storage function for redox cycling. The key issue for the system performance is therefore represented by the oxygen carrier, that must accomplish several requirements. Although at present metal oxides dominate the research spectrum, in literature it is possible to find different materials such as perovskites, which have recently emerged as promising materials for this application as they feature high non-stoichiometric oxygen exchange capacities while offering the benefit of significantly reduced operating temperatures. This thesis arises from an international project involving PoliTo, UNIUD and MIT. The aim of this work is to propose a new double-perovskite structured material with composition Sr2FeNi0.4Mo0.6O6−δ for the thermochemical CO2 splitting cycle to produce CO. The material has been tested in a microreactor setup to assess the influence of the sample mass, gas flow rates and reactor configuration on the fuel yields. Isothermal redox cycles at 850°C have been performed with a crucible and with a tube-in-tube configuration. The results obtained in both configurations show that, at constant sample mass, the CO yields decreases when increasing the flow rates. While a decrease in the sample mass corresponds to an increase of the fuel yield at constant flow rate. The testing conditions in which the measured yield is independent from the gas flow rate have been identified. Finally, results obtained with SFNM-04 have been compared to data available in literature for LSM perovskites.