Thermodynamic Analysis of Composite Metal Oxygen Carriers for Biomass Chemical Looping Gasification Coupled with CO2 Splitting

Biomass chemical looping gasification coupled with CO2 splitting (BCLGCS) presents a promising carbon-negative route for simultaneous syngas production and CO2 utilization, where the selection of oxygen carriers (OCs) is critical. Compared to single-metal oxides, composite metal OCs offer thermodynamic advantages. This study aims to evaluate the thermodynamic performance of composite metal OCs (LaFeO3, BaFeO3, CaFe2O4, and Ca2Fe2O5) in BCLGCS to overcome the thermodynamic limitations of conventional biomass-CO2 gasification. Gibbs free energy minimization calculations were performed to predict gas compositions and oxygen carrier phase transformations under varying operating conditions. Results show that steam addition promotes gasification by increasing H2 content and lowering required temperatures, but substantially reduces CO2 conversion in the splitting reactor by consuming residual char. Ca2Fe2O5 demonstrates superior adaptability with tunable H2/CO ratios, while LaFeO3 requires high OC loading and BaFeO3 undergoes deactivation via BaCO3 formation. This work reveals inherent thermodynamic conflicts between gasification and CO2 splitting steps, indicating that the optima for syngas production and CO2 utilization are mutually exclusive, an insight not previously quantified in BCLGCS literature. The findings provide theoretical guidance for designing carbon-tolerant OCs and optimizing process parameters, advancing BCLGCS toward practical carbon-negative applications.