Combined Ceria Reduction and Methane Reforming in a Solar-Driven Particle-Transport Reactor

Modeling Analysis of a Polygeneration Plant Using a CeO2/Ce2O3 Chemical Looping

In the current context of complexity between climate change, environmental sustainability, resource scarcity, and geopolitical aspects of energy resources, a polygenerative system with a circular approach is considered to generate energy (thermal, electrical, and fuel), contributing to the control of CO₂ emissions. A plant for the multiple productions of electrical energy, thermal heat, DME, syngas, and methanol is discussed and analyzed, integrating a chemical cycle for CO₂/H₂O splitting driven using concentrated solar energy and biomethane. Two-stage chemical looping is the central part of the plant, operating with the CeO₂/Ce₂O₃ redox couple and operating at 1.2 bar and 900 °C. The system is coupled to biomethane reforming. The chemical loop generates fuel for the plant's secondary units: a DME synthesis and distillation unit and a solid oxide fuel cell (SOFC). The DME synthesis and distillation unit are integrated with a biomethane reforming reactor powered by concentrated solar energy to produce syngas at 800 °C. The technical feasibility in terms of performance is presented in this paper, both with and without solar irradiation, with the following results, respectively: overall efficiencies of 62.56% and 59.08%, electricity production of 6.17 MWe and 28.96 MWe, and heat production of 111.97 MWt and 35.82 MWt. The fuel production, which occurs only at high irradiance, is 0.71 kg/s methanol, 6.18 kg/s DME, and 19.68 kg/s for the syngas. The increase in plant productivity is studied by decoupling the operation of the chemical looping with a biomethane reformer from intermittent solar energy using the heat from the SOFC unit.

Facile CO2 separation and subsequent H2 production via chemical-looping combustion over ceria-zirconia solid solutions

A novel chemical-looping combustion scheme is proposed, where facile gas separation via steam condensation enables the production of sequestrable CO2 from alkanes, such as CH4, and pure H2 from H2O. This cycle consists of two steps, namely, (1) the endothermic reduction of a ceria-based solid solution via the complete oxidation of CH4, followed by (2) the exothermic oxidation of the reduced metal oxide via H2O splitting. Relative to iron oxide-based materials and undoped ceria, ceria-zirconia solid solutions possess favorable partial molar enthalpic and entropic properties; this promotes selective production of complete combustion products, H2O and CO2, during the reforming reaction. Thermodynamic predictions suggest that the complete oxidation of CH4 is possible by increasing the Zr content to 20 mol%, operating below 600 °C, increasing total pressure, or reducing the amount of delivered reactant. Furthermore, any H2, CO, or unreacted CH4 that may persist is thermodynamically favored to oxidize if exposed to unreacted oxide downstream, as is typical for a packed-bed or downer reactor configuration. Experiments were performed to validate the thermodynamic trends using isothermal thermogravimetry coupled with residual gas analysis, which confirmed that high selectivity towards H2O and CO2 is attainable for methane-driven reduction of Ce0.9Zr0.1O2; selectivities greater than 0.70 were observed at initial reaction extents. Importantly, metal oxide oxidation via H2O splitting and selective production of H2 (or CO if CO2 is the delivered oxidant) is also thermodynamically favored at the operating conditions considered for the first step. This work ultimately presents a viable avenue for the carbon-neutral conversion of CH4 (or other alkanes) to H2 if a renewable energy resource, such as solar energy, is leveraged to supply process heat.