Edward A. Fletcher

Hydrogen- and Oxygen from Water

The limitations of thermochemical energy storage devices are the limitations of Carnot devices. Entropy production entailed in product separation further limits the efficiency of thermochemical processes. Thus, high upper temperatures and few reaction steps are desirable. In this article, the one-step effusional separation of water into hydrogen and oxygen is considered. Membrane materials, design, and fabrication techniques are suggested. A parametric analysis of the process suggests that the idea is a tantalizing possibility.

Metals, nitrides, and carbides via solar carbothermal reduction of metal oxides

We have examined the thermodynamics of the carbothermic reduction of metal oxides in either nitrogen or argon atmospheres. The reaction, which is given generally by (where the M denotes metal) MxOy + C (+ N2;) → {Mx′Ny′, Mx″Cy″, M} + CO, is highly endothermic and proceeds at temperatures from 1300 to 2350 K for the systems considered. Exploratory experimental studies were conducted in a solar furnace using concentrated solar radiation. The nature of the solid-phase products was determined by X-ray powder diffraction. Nitrides (AlN, TiN, Si3N4, ZrN) were formed for systems run in N2; carbides (Al4C3, TiC, SiC, CaC2) were formed for runs in Ar; and metals (Mg, Zn) were obtained from their oxides in an Ar atmosphere. The use of solar energy as the source of high-temperature process heat offers the possibility of reducing emissions of greenhouse gases and other pollutants to the environment when it replaces the burning of a fuel for process heat. Furthermore, the availability of process heat at very high temperatures may make alternative processes economically feasible.

High Temperature Solar Electrothermal Processing-III. Zinc from Zinc Oxide at 1200-1675K Using a Non-Consumable Anode.

The electrolytic decomposition potential of ZnO has been studied in a solar furnace in the temperature range 1200–1675 K. The electrolyte consisted of various mixtures of CaF2 and Na3AlF6. The measured potentials were close to the thermodynamically predicted values for the reaction ZnO(s) → Zn(g) + 0.502(g). The zero current overvoltages, surprisingly, increased with increasing temperature and the concurrent change in composition. The specific conductances of the electrolytes were estimated in the temperature range 1200–1500 K. They increased with increasing temperature and the concurrent change in composition. Various materials were tested for use as electrodes and crucibles. Some of our experiences and our experimental techniques are described.

High temperature solar electrothermal processing—II. Zinc from zinc oxide

The electrolytic decomposition potential of ZnO was studied in a solar furnace in the temperature range 600–1400 K and in three electrolytes (NaOH, 0.33ZnF2 0.67NaF, and 0.13AlF3 0.87NaF). From 600 to 1200 K, the measured potentials were close to the thermodynamically predicted values for the reaction ZnO(s) → Zn(stable phase) + 0.5O2(g), if provision is made for reasonable overvoltages. The overvoltages decreased with increasing temperature. In the range 1200–1400 K, the direct chemical reaction of ZnO with the graphite cathode resulted in the formation of a gas film in a process analogous to a transition from nucleate to film boiling, which greatly decreased the current. Various materials were tested for use as electrodes and cell casings. Their behavior, experimental problems and methods for dealing with them, as well as the apparatus, are described and discussed.

Reaction of steam with cellulose in a fluidized bed using concentrated sunlight

Steam gasification of cellulose at high temperatures can yield synthesis gas in a process that stores and transports highly concentrated solar energy. We have characterized the composition of the product gas in the temperature range 1050–1600 K in opaque and transparent fluidized-bed reactors. Bed material was either a pure tabular alumina or a crushed platinum-on-alumina catalyst. In both cases, the carbon present in the cellulose feedstock was well converted to a gaseous product. However, in the tabular alumina bed, the product gas had the high hydrocarbon and low hydrogen content characteristic of a rapid or flash pyrolysis process.

Hydrogen sulfide as a source of hydrogen

Abstract We suggest that hydrogen sulfide (that which is removed from fossil fuels as an unwanted waste product, as well as that which might be sought as a mineral in its own right), should be considered as a source of hydrogen. We discuss several techniques by means of which hydrogen sulfide might be thus exploited. We address, very briefly, the apparent problem of finding materials of construction for use in such processes.

Theoretical and experimental investigation of the carbothermic reduction of Fe2O3 using solar energy

The amount of coke needed to reduce iron ores to iron could be substantially reduced if it were used exclusively as a reducing agent and process heat were supplied by an alternative, e.g., solar, energy source. It could be reduced still further if CO2 rather than CO were the only gaseous product of the reduction. We have examined thermodynamically how this might be achieved and describe an idealization of a blast furnace for achieving it. We conducted some exploratory experiments on the reduction of Fe2O3 with graphite in the nominal temperature range 1300–2390 K in a solar furnace to test and gain experience with a new kind of receiver-reactor. These experiments are described. In the course of the work, we obtained encouraging results when using a plastic (polycarbonate) window to isolate our system from the atmosphere and protect a graphite reactor up to 1380 K.

High temperature solar electrothermal processing—Zinc from zinc oxide

We examined the feasibility of using sunlight to reduce the amount of electric power and/or fossil fuel needed to win metals from their ores and other compounds. The uniqueness of highly concentrated sunlight as a source of high-temperature process heat makes it attractive. We also examined, in some detail, ZnO as a candidate substance for exploratory experimental studies. We have mapped out, in temperature-pressure coordinates, the properties of the working substance and the thermodynamic performance of an archetype device as a guide to the temperature-pressure regime in which experimental studies might most profitably be undertaken.

A study of the use of Y2O3 doped ZrO2 membranes for solar electrothermal and solarthermal separations

We measured ion and (implicitly) electron hole conductivities and O2 semipermeabilities and their activation energies of commercial slip-cast Y2O3 doped ZrO2 under conditions simulating their practical application. We believe that this material shows promise as an O2 semipermeable membrane for ROC and electro-ROC type water splitters, as well as components of devices for the gas-phase electrolytic separation of metals (Zn in this study) from their oxides. The higher activation energy for hole transport vis-a-vis ion transport and the associated transition from electrically-dominated to pressure-dominated transport of O2 across such membranes makes an almost continuous transition from electrolytic to thermolytic processes possible. This is especially important in solar processes, where, at high temperatures, the difference between electrochemical and thermochemical processes fades away. This transition greatly increases the latitude with which process variables can be adjusted for optimization.

Hydrogen and sulfur from hydrogen sulfide—IV. Quenching the effluent from a solar furnace

Solar thermochemical production of H2 and sulfur from H2S were studied, using a 4.2 m solar furnace as a source of process heat. We used two reactor configurations. The independent variables were temperature, feed rate and pressure. Hydrogen production rate, yield, and the quench fraction (fractional yield) were measured. High yields (of the order 0.5) and quench fractions (of the order 0.7) were obtained over a range of temperatures and feed rates. Yields were a monotonically increasing, almost linear, function of the temperature.