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

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


Keller, M. - Presenter, The University of Tokyo
Otomo, J., The University of Tokyo
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 BaZr0.9Y0.1O3-δ and SrFe0.5Ti0.5O3-δ as support material. BaZr0.9Y0.1O3-δ support also exhibited favorable properties for methane cracking, while SrFe0.5Ti0.5O3-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 BaZr0.9Y0.1O3-δ, exhibited highly promising properties for this application.