P. Kuhn

Solar-Processed Metals as Clean Energy Carriers and Water-Splitters

Abstract Two-step solar thermochemical cycles and processes for the production of hydrogen, hydrocarbons, and synthesis gas are considered. The first step is based on the thermal, electrothermal, or carbothermal reduction of metal oxides, producing metals, metal nitrides, metal carbides, or lower-valence metal oxides. These are hightemperature highly endothermic reactions that can be driven by concentrated solar energy, reducing the consumption of fossil fuels and their concomitant emissions. The second step involves hydrolysis reactions. The thermodynamics of both reaction steps are examined and relevant experimental studies conducted using solar energy are reviewed.

Production of solar hydrogen by a novel, 2-step, water-splitting thermochemical cycle

A novel, two-step, water-splitting cycle is presented which, in contrast to previously proposed cycles that require upper operating temperatures above 2300 K, can be conducted at a moderate temperature. In the first endothermic step, Ni0.5Mn0.5Fe2O4 is thermally activated above 1073 K to form an oxygen-deficient ferrite. In the second step, activated ferrite is reacted with water below 1073 K to form hydrogen; the ferrite product is recycled to the first step. H2 and O2 are derived in different steps, eliminating the need for high-temperature gas separation. Both reactions have been demonstrated experimentally using concentrated solar radiation as energy source for the activation.

Comparative experimental investigations of the water-splitting reaction with iron oxide Fe1−yO and iron manganese oxides (Fe1−xMnx)1−yO

Abstract The oxidation of the non-stoichiometric iron oxide Fe1−yO and mixed iron manganese oxides (Fe1−xMnx)1−yO (x ≤ 0.3) by water forms molecular hydrogen. The course of this reaction was studied in a tubular fixed bed reactor at various temperatures by means of on-line mass spectrometry of the gas phase and by structural investigations of the solid products with powder X-ray diffraction and scanning electron microscopy.

Solar thermal production of zinc and syngas via combined ZnO-reduction and CH4-reforming processes

Abstract The combined thermal reduction of ZnO and reforming of CH4 has been thermodynamically and kinetically examined. The chemical equilibrium composition of the system ZnO + CH4 at 1200 K and 1 atm consists of a single gas phase containing Zn (vapor) and a 2:1 mixture of H2 and CO. The overall reaction can be represented as: ZnO(s) + CH4 = Zn(g) + 2H2 + CO. Thermogravimetric measurements on ZnO powder were conducted at various temperatures and CH4 concentrations of the reducing gas. The apparent activation energy obtained was 146 kJ mol−1. By aplying a shrinking-particle model, the reaction mechanism was found to be controlled by gas film diffusion in the Stokes regime. The reaction was also studied in a solar furnace using concentrated radiation as the energy source of high-temperature process heat (ΔH∘1200K = 440 kJ mol−1). Its technical feasibility was demonstrated. The solar receiver consisted of a fluidized-bed tubular quartz reactor coupled to a compound parabolic concentrator. Directly irradiated ZnO particles, fluidized in CH4, acted as heat absorbers and chemical reactants, while the Zn vapor produced was trapped in a cold-finger condenser. The proposed solar combined thermochemical process offers the possibility of simultaneous production of zinc and synthesis gas from zinc oxide and natural gas, without discharging greenhouse gases and other pollutants to the atmosphere. Furthermore, it provides an environmentally clean path for either recycling Zn-air batteries or producing H2 in a water-splitting scheme.

High-temperature solar thermochemistry: Production of iron and synthesis gas by Fe3O4-reduction with methane

Criteria for selecting thermochemical processes that use concentrated solar radiation as the energy source of high-temperature process heat are reviewed. We have thermodynamically examined the system Fe3O4 + 4CH4. At 1 atm and temperatures above 1300 K, the chemical equilibrium components consist of metallic iron in the solid phase and a mixture of 66.7% H2 and 33.3% CO in the gaseous phase. The total energy required to effect this highly endothermic transformation is about 1000 kJ/per mole of Fe3O4 reduced. We conducted exploratory experimental studies in a solar furnace using a solar receiver (with internal infrared mirrors) containing a fluidized bed reactor. Directly irradiated iron oxide particles, fluidized in methane, acted simultaneously as radiant absorbers and chemical reactants, while freshly produced iron particles acted as reaction catalysts. The proposed process offers simultaneous production of iron from its ores and of syngas from natural gas, without discharging CO2 and other pollutants to the environment.

Temporary phase segregation processes during the oxidation of (Fe0.7Mn0.3)0.99O in N2H2O atmosphere

A two-step thermochemical cycle with the ternary metal oxide system (Fe1 − xMnx)3O4(Fe1 − xMnx)1 − yO is applied to convert solar energy to chemical energy. Experimental investigations on the water splitting reaction of (Fe1 − xMnx)1 − yO revealed temporary formation of a manganese rich rock salt phase and an iron rich spinel phase due to phase segregation processes.

Thermoanalysis of the combined Fe3O4-reduction and CH4-reforming processes

The chemical equilibrium composition of the system Fe3O4 + 4CH4, at 1300 K and 1 atm consists of solid Fe and a 2:1 gas mixture of H2 and CO. Thermogravimetric (TG) analysis combined with gas Chromatographic measurements was conducted on the reduction of Fe3O4 (powder, 2-µm mean particle size) with 2.3, 5, 10, and 20 pct CH4 in Ar, at 1273, 1373, 1473, and 1573 K. The reduction proceeded in two stages, from Fe3O4 to FeO, and finally to Fe. CR, conversion and H2 yield increased with temperature, while the overall reaction rate increased with temperature and CH4 concentration. C (gr) deposition, due to the cracking of CR,, was observed. By applying a topochemical model for spherical particles of unchanging size, the reaction mechanism was found to be mostly controlled by gas boundary layer diffusion. The apparent activation energy reached a maximum at 30 pct reduction extent and decreased monotonically until completion. When compared with the results using instead either H2 or CO as reducing gas, the reduction achieved completion faster using CH4, at temperatures above 1373 K.

Coal gasification by the coal/CH4ZnOZnH2O solar energy conversion system

The coal/CH 4 -ZnO-Zn-H 2 O system has been proposed for the 2-step efficient chemical conversion system of the fossil fuels using a solar energy as the process heat and fossil fuel/ZnO redox system. The overall reaction is represented by CH x (coal) + CH 4 + 2H 2 O(g) = 2CO + (4 + x/2)H 2 The complete coal gasification to H 2 /CO gas mixture with the coal/ZnO redox system could be demonstrated with zero CO2 emissions at 1000-1100°C, eliminating the need to separate, while the steam gasification efficiency of coal remained 47% on a carbon basis. Zn-H 2 O reaction was conducted to generate H2 gas successfully. In the overall reaction, the product gas of the H2/CO mole ratio 2 could be obtained and can be directly used for synthesis of methanol and other chemicals.

CH4-utilization and CO2-mitigation in the metallurgical industry via solar thermochemistry

The industrial production of metals by carbothermic reduction of their oxides are high-temperature energy-intensive processes that release vast amounts of greenhouse gases and other pollutants to the atmosphere. A thermodynamic analysis and related experimental studies indicate the technical feasibility of reducing these emissions by combining the reduction of metal oxides with the reforming of natural gas for the co-production of metals and synthesis gas (as feed stock for methanol processing). Replacing fossil fuels with solar energy as the source of process heat further reduces CO 2 emissions to zero, and upgrades the calorific value of the products.