Experimental evaluation of the reactivity of iron oxides in different reaction conditions for the splitting of carbon dioxide in a Chemical Looping process

In this work, it is investigated one of the methods for reducing CO2 emissions into the atmosphere precisely because the problem of climate change is becoming of great interest worldwide, since it affects both the health of the planet and human health. There are various processes for the capture of carbon dioxide such as the Carbon Capture Storage which involves the extraction of CO2 from the flue gas of fossil fuels and then the storage of it, or the Carbon Capture Utilization which considers CO2 as a raw material that can be recycled and used as a product or converted into a new product. It can be used, in fact, in the food and beverage industry, as a packaging gas or for the synthesis of pharmaceutical products, but above all it can be used to obtain CO which in turn leads to the production of a clean fuel, i.e. syngas. The carbon dioxide molecule is highly stable; therefore, a high energy vector is required to break one of the C=O bonds. It is of great interest when this energy vector is produced from renewable resources, an example can be CSP (Concentrated Solar Power). Different processes have been proposed for the dissociation of CO2 but the one that is of greatest interest for research studies, especially for its large-scale application, is Chemical Looping. Today the Chemical Looping process is carried out with non-volatile cycles that use redox pairs of metal oxides, which generally are iron oxides, ceria, or perovskites. In this work, the experimental investigation of the behaviour of iron oxides during a Chemical Looping process is carried out with a focus on the oxidation phase with CO2 to obtain CO. The production of CO is analysed for a CO2 concentration range from 20-40 vol% for a temperature range from 700°C to 1000°C. During the experimental investigation it was noted that the material shows a rapid deactivation with the formation of Fe3C, namely carbon deposition, in the case in which only thermal reduction is performed, while a gradual deactivation occurs when the reduction is performed with hydrogen. This phenomenon is attributed to a sintering process on the surface of the material. In fact, during the reduction there is a loss of oxygen which leads to the creation of vacancies, while due to the sintering process there is less formation of vacancies which affect the yield of CO and the variation of iron oxide mass. Greater mass variation is a synonymous of greater release of oxygen during the reduction phase. Therefore, to obtain a good yield of CO, the temperature of the oxygen carrier must be between 800-1000 °C with a CO2 concentration of 20-40 vol%, but first a reduction process with 5 vol% of hydrogen is needed, since this gas tends to reduce the formation of carbon deposition in the material. As a final experimental investigation, iron oxides reaction during the reduction phase with methane is investigated, with oxidation phase performed by carbon dioxide. This cycle shows H2 formation by methane cracking in the reduction phase and CO2 splitting with CO formation in the oxidation respectively, thus having the production of useful products in both the steps of the cycle.