DIRECT SOLAR-THERMAL HYDROGEN PRODUCTION
USING NOZZLE/SKIMMER AND GLOW DISCHARGE
W.R. Pyle, M.H. Hayes, A.L. Spivak
H-Ion Solar Company
6095 Monterey Avenue
Richmond, California, 94805
Tel: (510) 237-7877 FAX: (510) 232-5251
An investigation of direct solar-thermal hydrogen and oxygen production from water is described. Nozzle jets and skimmers have been used for separation of the products and suppression of recombination. The dissociation of water vapor and the separation of its products was conducted in plasma-enhanced, non-equilibrium glow discharges.
In this report we describe the status of our work with a solar water dissociation gas phase reactor:
Solar radiant energy was concentrated by a parabolic mirror to produce high temperatures on a nozzle inside a solar reactor fed with water vapor (steam) at low pressure, to produce hydrogen and oxygen.
A sustainable, non-polluting energy currency is required to provide for human needs such as cooking, heating, transportation, electricity production, and refrigeration. An economically viable method for producing large quantities of hydrogen from water using sun-light would satisfy this need, in a "Hydrogen Economy".
Pioneering studies of solar hydrogen and oxygen production from water occurred throughout the world during the 1970's and early 1980's as a response to the oil shocks of 1973 and 1978/1979. After a lull in activities during the mid and late 1980's, interest in direct water dissociation and other hydrogen production technologies has again increased during the early 1990's due to increasing environmental problems.
WATER DISSOCIATION TO PRODUCE HYDROGEN AND OXYGEN
Our research objective is to develop a process that can attain a high level of water vapor dissociation and efficiently separate the hydrogen from the oxygen and un-converted water vapor.
At high temperatures, above about 1800 K, water vapor (steam) begins to dissociate into a mixture of H2, O2, H2O, O, H and OH. The extent of dissociation increases with increasing temperature and decreasing pressure. The water and the diatomic hydrogen and oxygen species completely dissociate into H(atomic hydrogen) and O (atomic oxygen) above about 3500 K under equilibrium conditions at 1 mm Hg absolute pressure.
ENDOTHERMIC WATER VAPOR DISSOCIATION REACTIONS
Endothermic reactions can be carried out with the aid of electricity, as in electrolysis. However, thermal power from concentrated solar radiation can be used without any mechanical or electrical input and without the aid of any catalyst to achieve water dissociation. The direct thermal method has two advantages: it is very simple in its principle and it is versatile and adaptable to many endothermic gas-phase reactions. This method does not suffer from the corrosive reagent problems of various multi-step lower temperature thermo-chemical cycles. New problems arise, however, in finding reactor materials capable of withstanding the higher temperatures required with one-step concentrated radiation water-thermolysis. The materials used in the reactor must be capable of withstanding the thermal cycling and shock brought about by the intermittent nature of solar diurnal and weather cycles.
The chemical reactions associated with high temperature dissociation of water vapor are shown below, along with their pressure equilibrium constants:
Where P is the total pressure and X is the mole fraction.
Equation (1) above is an over-simplification of the primary reaction, because surface reactions in this reactor are significant. The surface reactions have been well characterized by Lede et al 1982).
Water vapor dissociation has an activation energy threshold of about 135 kcal/mole (5.9 electron volts). Atomic hydrogen and oxygen are formed and then recombined to diatomic hydrogen and oxygen, losing about 78 kcal/mole in the recombination process.
In this investigation we are studying the non-isothermal low-pressure glow-discharge region shown in Figure 1 (Bockris et al, 1985). It can be seen that the enthalpy change may be greater than the water dissociation activation threshold of 135 kCal/mole(5.9ev).
Glow Discharge Region Between Feed Ring and Nozzle
NON-EQUILIBRIUM PLASMA-CHEMICAL REACTIONS ASSOCIATED WITH WATER VAPOR GLOW DISCHARGES
The process of decomposition of water in the plasma state can be accomplished at highest efficiency where the electron temperature is not sufficient for intense excitation of the electron states, and the larger portion of the electron's energy contributed to the discharge is expended for excitation of vibration modes and for dissociative attachment (Melik-Asloanova et al, 1978).
Dissociation of water vapor molecule is accomplished by successive stages of water oscillative excitation, population of highly excited states and, finally by reactions with participation of H2O* (where the * denotes the vibrationally excited state of water). The reaction is initialized in the bi-molecular act:
H2O* + H2O* => H + OH + H2O
The radicals H and OH initiate a reaction with participation of the oscillatively excited molecules:
H + H2O* => H2 + OH
OH + H2O* => H + H2O2
Water decomposition by dissociative attachment requires that in each reaction an electron disappears and a negative ion is produced. Plasmochemical dissociative attachment reactions become energetically effective when each electron produced in the plasma can repeatedly participate in the dissociative attachment process. Multiple use of the electron becomes possible due to a high rate of negative ion collapse by electron impact. The chain process initializes:
e + H2O => H- + OH
H- + e => H + e + e
Termination of the chain results from recombination with water and OH in a three-body collision:
H + OH + H2O => H2O + H2O
The parallel channel of the chain extension has poorer kinetics because of a higher activation barrier of the limiting process:
OH + H2O => H2 + HO2
HO2 + H2O => H2O2 + OH
The total efficiency of the process depends on the energy losses in the discharge, the efficiency associated with the negative contribution of the chain-breaking reaction, the heat losses of the excited-state reactions, and finally on the efficiency of the reaction relative to the oscillative relaxation. The energy transfer efficiency of vibrational excitation by electron impact is a function of the energy contribution to water: oscillations, translational motion, rotation, and dissociative attachment. The energy transfer efficiency of the chain process depends on the chain length.
DISSOCIATOR-NOZZLE APPLICATION TO PRODUCTION OF HYDROGEN AND OXYGEN FROM WATER VAPOR
A block diagram of the Photo-separatory Nozzle reactor (Pyle 1983) and the process under investigation is shown in Figure 2:
Block Diagram of Solar Reactor
CONCENTRATION AND ABSORPTION OF SUNLIGHT IN SOLAR REACTOR
Sun-light was concentrated using a parabolic mirror and directed through the entrance window of a high-temperature gas-phase reactor. Inside the solar reactor the concentrated radiant energy was focused onto a trumpet-shaped ceramic dissociator-nozzle. An optical pyrometer with a video camera looks through a prism and a hole in the concentrator mirror, then through the quartz reactor window to view the dissociator-nozzle surface and obtain nozzle surface temperatures.
The solar spectrum,at sea level on Earth with air mass of 1, loses some energy bands due to the water vapor absorption which occurs as the light is passing through the atmosphere, especially in the near UV. This absorption limits the energy spectrum available for water dissociation at the earth’s surface. We used re-radiation from a hot surface (the nozzle) to enhance gas heat transfer and create a surface reaction site.
Solar heat source temperature can be calculated as a function of the concentrator optical concentration ratio, reflection coefficient, and arriving solar power density.
By raising the temperature of the nozzle surface to a sufficiently high value (using the nozzle as a target for concentrated sunlight) and dropping the water vapor pressure to a sufficiently low value, we are attempting to produce significant water vapor dissociation.
GLOW-DISCHARGE ENHANCEMENT OF RADIATION AND CONVECTION HEAT TRANSFER AT NOZZLE ENTRANCE
A high voltage DC electric power supply (PS-1) was used in some of the experiments to create a water-vapor glow-discharge between the water vapor feed-ring and the entrance to the ceramic dissociator-nozzle to enhance dissociation (Pyle et al, 1994). See Figure 3.
Photo-separatory Nozzle Reactor
The applied voltage was measured with volt-meter V and the current with ammeter A, shown in the upper right of Figure 3. The circle shown in Figure 3, between the feed ring and the dissociator-nozzle, is where the solar image "fireball" and the water vapor glow-discharge region are located.
The partially dissociated water vapor was allowed to pass through the dissociator-nozzle throat and expand into a lower pressure region. The pressure inside the dissociator-nozzle tube (downstream) must be lower than at the entrance (dissociation region) to create the driving force for sonic flow through the nozzle throat (pressure ratio > 0.55) , and produce a jet and shock-wave down stream.
In our application a sonic dissociator-nozzle geometry was used because this was the easiest to fabricate using quartz tubing in the laboratory.
All gases at room temperature are excellent electrical insulators. In order to make them electrically conducting, a sufficient number of charge carriers have to be generated. If a sufficiently high electrical field is applied to a pair of electrodes separated by a volume of the gas, an electrical breakdown of the originally non-conducting gas establishes a conducting path through the electrode gap, producing an array of phenomena known as gaseous discharges.
We desire a steady-state dc glow discharge for our solar plasma reactor, to obtain a higher electron population density with relatively low electrical power consumption.
After breakdown, the current A rises and voltage V drops to steady-state values. The discharge voltage depends on the current and certain properties of the discharge tube, such as gas type, gas pressure, electrode material, and electrode temperature. A current suppression (or ballast) resistor is needed to limit the current through the gas-plasma after breakdown occurs. We used 10,000 to 18,000 ohm suppression resistors in the Photo-separatory Nozzle reactor.
Heat is supplied to the dissociator-nozzle (which is also the cathode electrode for the glow discharge) by the concentrated solar beam, and it operates in glow discharge with low cathode fall. Cooling of the nozzle surface by electron boil-off occurs under direct thermionic emission process conditions at very high temperatures. Indirect emission processes (secondary emission processes) are also important to consider: electrons are ejected from the surface of the nozzle by energetic particles such as ions and photons in the glow discharge.
GLOW DISCHARGE APPLIED BETWEEN DISSOCIATOR-NOZZLE AND SKIMMER SEPARATOR
A supersonic beam skimmer was installed behind the shock wave to separate hydrogen and oxygen from the partially dissociated water vapor (Becker et al 1955). See Figure 3.
The supersonic beam skimmer used was a knife-edged cone. The skimmer was positioned with micrometers so that its entrance was at the down-stream edge of the shock-wave envelope produced inside the dissociator -nozzle. An appropriate pressure ratio was applied between the low pressure dissociator-nozzle jet region and the beam skimmer interior to improve the separation factor for hydrogen and oxygen production. This jet separator functions by transverse gas diffusion. The hydrogen and oxygen (plus unconverted water vapor) are separated because the hydrogen gas diffuses transversely (relative to the jet flow axis, more rapidly than oxygen or water), due to its lower mass.
A second high voltage DC electric power supply (PS-2) was used to create a glow discharge between the sonic dissociator-nozzle and the conical skimmer nozzle to study separation enhancement by cataphoresis. Cataphoresis is the spatial gas density gradient which forms near a cathode within a glow discharge. Cataphoretic efficiency has been observed to decrease with increasing temperature, however.
The glow-discharge between the dissociator-nozzle and the skimmer-nozzle also allows shock-wave visualization by a video camera which is aimed through a viewing slit in the reactor wall. The video camera is attached to a coherent optical fiber bundle and illuminated with a He-Ne laser through another incoherent optical fiber bundle to allow observation of the dissociator-nozzle and skimmer separation distance, as the skimmer is moved in or out of the jet.
Enrichment of heavier-mass species occurs in the core of the gas jet. Enrichment of the lighter-mass species occurs in the periphery of the gas jet.
The free molecular diffusion separation arises from the disparity in the thermal velocity of light and heavy gas molecules, for gases with the same mass flow velocity and local temperature. The heavier species have a lower thermal velocity and expand less (laterally) after the transition plane, remaining more concentrated at the beam center.
In moving the skimmer transversely a qualitative measurement of the degree of beam spreading is obtained. A much broader spreading is observed for light species (hydrogen) than for heavy species.
In the case of a fixed skimmer-orifice dimension, an axially movable skimmer is necessary to achieve comparable beam intensities for different gases. The mass separation factor is approximately proportional to the mass ratio when the skimmer-interference and background-penetration effects are minimized.
Separation Factor =
[(Heavy Core Gas, M%) (Light Peripheral Gas, M%)] / [(Heavy Peripheral Gas, M%) (Light Core Gas, M%)]
GLOW DISCHARGE POWER SUPPLY REQUIREMENTS
In the dark (no concentrated sunlight), with water vapor pressures in the Photo-separatory Nozzle reactor's normal pressure regime, the power supply voltage required for sustaining normal glow may be about 700, whereas under concentrated sun-beam illumination the normal glow voltage is only about 200. The current requirement is on the order of 1-3 ma under either dark or illuminated conditions. The use of higher pressure requires the use of higher current. The glow-discharge power is only a small fraction of the power arriving via the concentrated sun-beam, about 1 to 2 watts typically. This electrical power requirement can easily be met by a small photovoltaic cell array.
The glow-discharge power for the lower pressure gas region between the dissociator-nozzle and the skimmer is of the same order as for the feed-ring to dissociator-nozzle power, 1 to 2 watts.
DESCRIPTION OF EXPERIMENTAL APPARATUS
The complete Photo-separatory Nozzle reactor assembly is shown in Figure 4.