Thermal Reduction of Iron Oxide under Reduced Pressure and Implications on Thermal Conversion Efficiency for Solar Thermochemical Fuel Production

Successful implementation of solar thermochemical metal oxide water splitting cycles is dependent upon the ability to reach low partial pressures of oxygen during the thermal reduction step. Low partial pressures of oxygen are required to provide a thermodynamic driving potential for the thermal reduction reaction and avoidance of recombination. Achieving low partial pressures of oxygen (e.g., <10(-2) may require a nontrivial energy input to the solar-to-fuel conversion process, negatively impacting the solar to fuel energy conversion efficiency. Three different strategies to reduce the partial pressure associated with oxygen generated during an iron oxide thermal reduction process were investigated using an open system thermodynamic analysis. These strategies include vacuum pumping, purging with an inert gas, and purging with steam. If the difficult to achieve solid-phase heat recuperation is neglected, open-system thermodynamic simulations show that vacuum pumping will have over twice the overall cycle energetic and exergetic efficiencies than those of inert purging; assuming oxygen separation is required every cycle in the case of inert or steam purging. To demonstrate the concept of vacuum pumping, thermal reduction of an iron zirconia bed in a tubular reactor was performed at low pressures of approximately 10(-4) bar at a temperature of 1450 degrees C. The maximum extent of reduction (14.2 +/- 1.7 mol %) was reached after approximately 1 h of reduction at 1450 degrees C, while the predicted theoretical extent of reduction ranges from 16.5 mol % at 10(-2) bar to 76.9 mol % at 10(-4) bar. In the present analysis, reaction kinetics are not considered, and its application is limited to the thermodynamically driven processes.