Two-step thermochemical cycles for splitting CO 2 with Zn/ZnO and FeO/Fe 3 O 4 redox pairs using concentrated solar energy are considered. Thermogravimetric-based kinetic analyses were performed for the reduction of CO 2 to CO with Zn and FeO. Both reactions are characterized by an initial fast interface-controlled regime followed by a slow diffusion-controlled regime, which are described using a shell-core kinetic model. In the interface-controlled regime, a power rate law is applied with apparent activation energies 113.7 and 73.4 kJ mol -1 , and corresponding reaction orders 0.339 and 0.792, for the Zn/CO 2 and FeO/CO 2 systems, respectively. In the diffusion-controlled regime, limited by the ion mobility through the oxide shells, the apparent activation energies are 162.3 kJ mol -1 for Zn/CO 2 and 106.4 kJ mol -1 for FeO/CO 2 . Additional reaction mechanisms above the Zn melting point for Zn/CO 2 reactions are postulated.

Two-step thermochemical cycles for CO 2 reduction via Zn/ZnO and FeO/Fe 3 O 4 redox reactions are considered. The first, endothermic step is the thermal dissociation of the metal oxide into the metal or a reduced valence metal oxide and O 2 using concentrated solar energy as the source of high-temperature process heat. The second, nonsolar, exothermic step is the reaction of the reduced metal/metal oxide with CO 2 , yielding CO and/or C, together with the initial form of the metal oxide that is recycled to the first step. Chemical equilibrium compositions of the pertinent reactions are computed as a function of temperature and pressure. A second-law thermodynamic analysis for the net reaction of CO 2 ) CO + 0.5O 2 indicates a maximal solar-chemical energy conversion efficiency of 39 and 29% for the Zn/ZnO and FeO/Fe 3 O 4 cycle, respectively. Efficiencies are lower for both cycles yielding C. Major sources of irreversibility are associated with the re-radiation losses of the solar reactor operating at 2000 K and the quenching of its products to avoid recombination.

The rate of MgO carbothermal reduction was studied at temperatures 7 from 1350-1650°C and pressures from 0.1-100kPa based on product gas 8 analysis at near isothermal conditions. For all temperatures the initial 9 rate of carbothermal reduction increased inversely with pressure, and between conversions of 20-35% a transition occurred after which the reaction rate was maximum at 10kPa. Analysis of reacted pellets showed that the reaction stoichiometry, the ratio of C to MgO reacted, was less than unity and decreased with pressure indicating CO 2 generation was more prevalent at elevated pressures. SEM imaging revealed the dissolution of C and MgO contact with conversion, andisoconversional analysis points to a change inthe rate determining step between 1 and 10kPa. The given experimental observations argue the importance of mass transfer and gaseous intermediates. A kinetic model is formulated based on a macroscopic species balance with CO 2 as the reaction intermediate.

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The production of ammonia via a two-step cyclic process is considered, consisting of an endothermic carbothermic reduction of Al2O3 in a N2 atmosphere to form AlN, followed by an exothermic steam hydrolysis of AlN to produce NH3 and re-form Al2O3. Four carbon sources, namely, wood charcoal, petroleum coke, carbon black, and activated carbon, were examined as reducing agents for the Al2O3-reduction step in the range 1500−1700 °C by means of thermogravimetry and gas chromatography. Rate laws and Arrhenius kinetic parameters were determined by applying solid−solid and gas−solid kinetic models. The cyclability of the two-step process was studied by carrying out four subsequent cycles, yielding an increase in the reaction rate of the Al2O3-reduction step and in the ammonia yield of the AlN-hydrolysis step, attributed to the increasing specific surface area after each cycle.

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A high-temperature pressurized air-based receiver for power generation via solar-driven gas turbines is experimentally examined and numerically modeled. It consists of an annular reticulate porous ceramic (RPC) foam concentric with an inner cylindrical cavity-receiver exposed to concentrated solar radiation. Absorbed heat is transferred by combined conduction, radiation, and convection to the pressurized air flowing across the RPC. The governing steady-state mass, momentum, and energy conservation equations are formulated and solved numerically by coupled finite volume and Monte Carlo techniques. Validation is accomplished with experimental results using a 3 kW solar receiver prototype subjected to average solar radiative fluxes at the CPC outlet in the range 1870–4360 kW m−2. Experimentation was carried out with air and helium as working fluids, heated from ambient temperature up to 1335 K at an absolute operating pressure of 5 bars. The validated model is then applied to optimize the receiver design for maximum solar energy conversion efficiency and to analyze the thermal performance of 100 kW and 1 MW scaled-up versions of the solar receiver.