Upgrading Post-Combustion Flue Gas from Fischer-Tropsch Processes into Green Fuels Using Plasma-Catalysis

In this work, we use plasma-catalysis to convert leftover methane (CH₄) and carbon dioxide (CO₂) in industrial flue gases (e.g., Fischer-Tropsch) into valuable fuels like methanol (MeOH). This approach tackles greenhouse gas emissions from fossil fuel heating while utilizing waste streams. However, optimizing this requires understanding the complex interplay between plasma excitation, catalyst activation, and gas flow for efficient conversion. Plasma pre-excites reactants, lowering the activation barrier for MeOH formation, and allowing the conversion of CH₄ and CO₂ at mild operating conditions beyond the catalyst's intrinsic capability. Copper (Cu) catalysts exhibit a synergistic effect with plasma, leading to a 10-fold higher MeOH yield compared to Nickel (Ni) catalysts. These experiments suggest that more active catalysts lower the selectivity of towards intermediate products, like liquid oxygenates, in plasma chemical processes. These findings are confirmed at mild operating conditions using online gas chromatography and in-situ Fourier-Transform Infrared Spectroscopy (FTIR) in Diffuse Reflectance (DRIFTS) and transmission mode. DRIFTS measurements on Cu surface reveals a favorable reaction pathway for MeOH production. Dissociative adsorption of vibrationally excited CO₂ and CH₄ on polycrystalline Cu surface, form intermediates, CH₃* and O*, which recombine to form CH₃O*, a key precursor in MeOH formation. The increase in CH₃O* concentration on the Cu surface correlates with the observation of higher MeOH concentration in gas phase. However, the presence of trace water vapor changes the reaction pathway. The transient increase in CH₃O* concentration followed by its decline implies a limited role for CO₂ dissociation in MeOH formation with trace water present. Further, the Damköhler number (Da) quantifies the interplay between species excitation and transport. Decreasing Da, achieved by optimizing transport, resulted in a 13-fold improvement in MeOH selectivity. This indicates faster transport of MeOH molecules from the catalyst surface before further reactions, maximizing yield. Further research aims at co-designing plasma and catalysts to minimize energy input and match dry reforming efficiency, paving the way for a sustainable Fischer-Tropsch process from waste gases.