Process systems were investigated for syngas production from CO 2 and renewable energy (solar) by the reverse water-gas shift (RWGS) and the reverse water-gas shift chemical looping (RWGS-CL) process. Thermodynamic analysis and optimization was performed to maximize the solar-to-syngas (StS) efficiency η StS. Special emphasis was laid on product gas separation. For RWGS-CL, a maximum StS efficiencies of 14.2 and 14.4% were achieved without and with heat integration, respectively. The StS efficiency is dictated by the low overall efficiency of H 2 production. RWGS-CL is most beneficial for the production of pure CO, where the StS efficiency is one percent point higher compared to that of the RWGS process with heat integration. Heat integration leads to significant reductions in external heat demand since most of the gas phase process heat can be integrated. The StS efficiencies for RWGS and RWGS-CL achieve the same level as the reported values for solar thermochemical syngas production.

Carbon monoxide production from carbon dioxide via isothermal reverse water-gas shift chemical looping (RWGS-CL) is studied with a modified iron oxide oxygen carrier material (80 wt% Fe 2 O 3-Ce 0.5 Zr 0.5 O 2). The material is characterized by TEM, XRD and thermogravimetry at temperatures from 750 • C to 850 • C and gas mole fractions of H 2 and CO 2 from 0.05 to 0.75, respectively. High temperature and high reactant concentrations favor the oxidation and reduction of the material during repeated redox cycles. The reaction rate of reduction is always faster than that of oxidation applying the same gas concentration of H 2 and CO 2 , respectively. The long term stability of the material is investigated with 500 redox cycles in a plug flow reactor. The material shows gradual deactivation lowering the CO yield during the first 100 redox cycles. After that, a steady state CO yield is reached for the next 400 redox cycles. Deactivation is attributed to surface sintering which results in slower reaction kinetics. TG data was used for a kinetic analysis applying the master plot method. The experimental data for oxidation and reduction indicated reaction mechanisms, which are well described by a reaction order and a geometrical contraction model. After parameter estimation, a good agreement between the model and the TG data was achieved with the reaction order and geometrical contraction model for oxidation and reduction, respectively. The RWGS-CL process can be used for sustainable CO production from CO 2 if the energy for the process and for H 2 production is supplied by renewable sources.

A fixed bed and a fluidized bed reactor design for the reverse water-gas shift chemical looping (RWGS-CL) process with 80 wt% Fe 2 O 3 − Ce 0.5 Zr 0.5 O 2 as oxygen storage material (OSM) are analyzed and compared by process simulation. The influence of gas inlet flow velocity on the the reaction regime is investigated and feasible regions of operation for continuous CO production are identified. Two performance indicators are defined for the process: 1) the average CO concentration and 2) the OSM utilization. Optimization problems are solved to maximize the performance indicators and analyze their mutual dependency. The results are discussed in a Pareto plot. It is shown that a fixed bed reactor design is advantageous for the RWGS-CL process because of more degrees of freedom for operation compared to the fluidized bed reactor design. A quasi-continuous production of CO is demonstrated for both reactor designs. A steady CO production with an average CO mole fraction of x CO = 0.64 and an OSM utilization of 83.9% can be realized with two fixed bed reactors. The fluidized bed reactor configuration can potentially minimize problems associated with material sintering but the average CO concentration and OSM utilization are significantly lower compared to the fixed bed due to the limited operational freedom. The results indicate the importance of dynamic simulations for understanding and exploiting the kinetic and thermodynamic aspects of inherently dynamic processes like RGWS-CL to maximize process performance.

Hydrogen and carbon monoxide production from H2O and CO2 is experimentally investigated using a two-step chemical looping process based on redox cycles of iron-alumina mixed oxides. The reduction of Fe3O4 in first endothermic step is followed by splitting of CO2 or H2O in a second exothermic step. The iron-aluminum oxides are more reactive with H2O than with CO2 in the range 650-750 °C. In situ XRD shows that deactivation results from different processes: iron oxide sintering and formation of spinel (FeAl2O4) with lower oxygen storage capacity. However, FeAl2O4 also takes over the role of Al2O3 and mitigates the iron oxide sintering. Deactivation at 650 °C is predominantly governed by sintering, while further loss of activity is due to combined sintering and spinel formation. At 750 °C the spinel formation is more dominant. A mixed oxide of Fe2O3 and Al2O3 with mass ratio of 70:30 was found most active and stable for H2O and CO2 splitting in chemical looping.