Application of low PO2 iron-based oxides for decarbonising steel industry via chemical looping

Abstract

This dissertation exploits the use of low oxygen partial pressure (PO2 ) of dicalcium ferrite (Ca2Fe2O5, C2F) in multiple applications for decarbonising the hard-to-decarbonise steel industry. Here, C2F) is proven to be a versatile metal oxide for (i) integrated calcium and chemical looping combustion (CaL-CLC), and (ii) hydrogen and syngas production using methane. C2F) comprises of CaO and Fe2O3 which are already utilised in the steel industry. This is important consideration for scalability. The first part of this thesis demonstrates the applicability of C2F) to simultaneously combust CO and capture CO2 in a fluidised bed and a thermogravimetric analyser (TGA). It was found that C2F) is able to react with hypothetical BFG with PCO⁄PCO2 ratio of ~1, despite of its reduction to CaO and metallic iron having the ratio of ~3, via a reductive-carbonation reaction (stage 1, in a carbonator at a lower temperature). This is only plausible under conditions when carbonate can form. The reduced-carbonated C2F) is regenerated (stage 2, at a higher temperature in a calciner) at much lower calcination temperature than CaCO3, i.e., ~750 vs 900 °C at PCO2 ) of 1 bar, driven from the reincorporation into a mixed phase. The proposed CaL-CLC was examined from a thermodynamic standpoint using Aspen Plus V10 and MTDATA. Different available databases were assessed since the choice of database was found to impact the system performance. The low PO2 property allows C2F) to perform endothermic combustion of CO/H2 in the carbonator. The process models reveal the reduced-carbonated C2F) chemically stores the heat of combustion effectively. Therefore, it can be utilised for sorbent regeneration in the calciner, essentially pumping the heat from the carbonator to the calciner. This system minimises the energy use in the calciner compared to the CaL-CLC using CaO/CuO. Although pure oxygen is still needed for sorbent regeneration, this eliminates the needs of oxygen in the carbonator where CaL alone is implemented. The level of carbon removal (CO+CO2) is greatly influenced by the PCO⁄PCO2 ratio at equilibrium, therefore lower temperatures cause an improvement. Assuming the kinetics of C2F) in combusting CO controlled by surface kinetics, led to the hypothesis that dopants of CeO2, CuO, and NiO impregnated onto the C2F) structure would improve rates of reaction. However, redox cyclic in the fluidised bed for over 40 cycles and isothermal reduction in the TGA indicate that the kinetics might be controlled by oxygen diffusion through the C2F) structure. It was found that impregnating CuO onto the C2F) improves the kinetics in combusting CO, as well as changing its energy activation. This indicates it might have different rate controlling step than the other studied materials. The applicability of C2F) to producing syngas and hydrogen using methane was demonstrated in the fluidised bed for ~40 cycles. Instead of experiencing deteriorating performance as in the reaction with CO, the C2F) performance improved as the cycles increased. The work here demonstrates the C2F) is able to be used for various applications, including (i) partial oxidation, (ii) steam or dry methane reforming, and (iii) autothermal reforming. The C2F) reduced form is an active pyrolysis catalyst, offering a new possibility to employ the carbon as looping material, being transported across stages, and adding a degree of freedom to the process configuration.