(322h) CO2 Activation By Methane in a Dual-Bed Configuration Via Methane Cracking and Iron Oxide Lattice Oxygen Transport – Concept and Materials Development

In this work, we explore a novel process configuration for CO₂ activation into CO by methane in a dual-bed reactor configuration by circulating supported iron oxide/iron particles with deposited carbon between two reactors. Iron was chosen as the active phase due to its ability to effectively crack methane at relevant temperatures, its favorable properties for lattice oxygen transport between the two reactors, and its low cost. In reactor 1 Fe₃O₄ is reduced to metallic iron by H₂ that is produced by catalytic cracking of methane fed to the bottom of the reactor. The reduced iron particles and the carbon deposited on them from methane cracking are then introduced to reactor 2, to which CO₂ is fed. There, the carbon is gasified by the reverse Boudouard reaction and the iron is reoxidized back to Fe₃O₄, thereby activating the CO₂ into CO. The merit of cracking the methane in reactor 1 instead of fully combusting it lies in the concentration of the entire carbon fed to the system in the product gas stream from reactor 2. This further allows oxidation of some hydrogen in reactor 1 with air to provide heat for this otherwise highly endothermic process without generating any CO₂ that is diluted with nitrogen from air.

To demonstrate the feasibility of this concept, we pursued a two-fold approach. First, the theoretical potential of the process was evaluated by thermodynamic equilibrium calculations. For both reactors, moving bed configurations with counter-current gas-solid flow and well-mixed fluidized bed configurations were investigated. Moving beds were approximated with a multi-stage equilibrium model, whereas fluidized beds were modeled as CSTRs, with two separate stages for reactor 1 (a cracking and an iron oxide reducing stage, respectively). Results indicated that moving beds offer significant advantages over well-mixed fluidized beds for both reactors. With both reactors realized as counter-current moving bed reactors, about 2.7 mol CO₂ per mol of CH₄ could be potentially activated while providing a product stream of about 82 mol-% CO, with the remainder being CO₂, at a moderate reactor temperature of 850°C.

Second, suitable materials for this process were developed. To prevent sintering of the iron/iron oxide and to facilitate fast methane cracking, reduction and oxidation kinetics, it needs to be combined with a suitable support material. Several potential support materials were investigated based on their electronic, ionic and protonic conduction properties, and cermet materials with 30 wt-% Fe₂O₃ and 70 wt-% support were produced. The reduction kinetics with H₂, the kinetics for methane cracking and the reoxidation kinetics with CO₂ were then evaluated in a TGA in a cyclic manner. Materials were further characterized by X-ray diffraction and SEM. Iron oxide reduction kinetics were fastest with BaZr_0.9Y_0.1O_3-δ and SrFe_0.5Ti_0.5O_3-δ as support material. BaZr_0.9Y_0.1O_3-δ support also exhibited favorable properties for methane cracking, while SrFe_0.5Ti_0.5O_3-δ proved to be an ineffective support for methane cracking.

In conclusion, our results indicate the high potential of this process for CO₂ activation as evidenced by the high achievable ratio of CO₂ activated per mol of CH₄ in a dual moving-bed configuration. Iron/Iron oxide supported on Zr-based ceramics, in particular BaZr_0.9Y_0.1O_3-δ, exhibited highly promising properties for this application.