Redox degrees of iron-based oxygen carriers in cyclic chemical looping combustion using thermodynamic analysis

Abstract

Chemical looping combustion (CLC) provides a sustainable production of energy while inherently capturing carbon dioxide using oxygen carriers (OCs). To efficiently operate the reactor of the CLC, a thermodynamic analysis of OC is essential. This study focuses on the complete thermodynamic analysis of CLC employing iron-based OCs. In the analysis, methane as a reduction gas and air as an oxidizer along with the fuel and air reactors operated at 900 °C. Six different methane-to-hematite molar ratios (M/H) and O2-to-hematite molar ratio (O/H) on iron oxide redox reactions are evaluated. The redox degree, including reduction and oxidation degrees of OCs in the fuel and air reactors, is introduced to figure out their performance. The results reveal that M/H = 1/15 with O/H = 1 yields the highest CO2 and H2O yields. Under six O/H ratios with M/H = 1/12, the highest CO2 and H2O yields are achieved at O/H ≥ 0.18. These operation conditions are conducive to carbon capture. In most cases of reduction and oxidation reactions, Fe3O4 and Fe2O3 account for the largest shares of iron in the OCs, respectively. The results show that a decrease in oxygen input to the air reactor leads to more carbon formed in the system. An enthalpy balance indicates an overall exothermic reaction behavior. Introducing the redox degree suggests that the optimal operation conditions are at M/H = 1/12 and O/H = 0.18. This study has provided crucial information about the operation and reactivity of iron oxides with methane in the chemical looping combustion process.