ht. J. Hydrogen Energy, Vol. 23, No. 2, pp. X9-98, 1998 CC 1997 International Association for Hydrogen Energy Elsevier Science Ltd Pergamon PII: SO360-3199(97)00038-4 All rights reserved. Printed in Great Britain 036&3199/98 $19.00+0.00 DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS-II. EXPERIMENTAL FEASIBILITY STUDY A. KOGAN Solar Research Facilities Unit, Weizmann Institute ol’science, Rehovot, Israel Abstract-The (development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane. The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussedin some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures. 0 1997 International Association for Hydrogen Energy INTRODUCTION Hydrogen is one of the most important base materials in the chemical industry. It is used mainly in the production of ammonia and of methanol and in processes of pet- roleum refining. During the last twenty years hydrogen has been considered increasingly as a possible alternative to fossil fuel. As a fuel hydrogen has some unique proper- ties. It is a clean fuel, the only product of its combustion being water. Its heating value is more than trite the heat- ing value of gasoline. Moreover, the thermodynamic availability of its energy content is very high. With pre- sent day fuel cell technology it can be used to produce electricity with an efficiency of 70%. Hydrogen could thus become an efficient, non-polluting energy carrier of the future if a process of hydrogen production could be developed that does not involve release of pol- lutants to the atmosphere. The present economical and most widely used method of hydrogen production is by steam reforming of hydro- carbons [l]. Light hydrocarbons (methane to naphtha) and steam are converted catalytically to hydrogen and carbon oxides. The reforming reaction is highly endo- thermic. The heat of reaction is supplied by combustion of fuel gas or oil. Much CO* is thus generated in this process. Other hydrogen production processes that utilize chea- per heavy fossil fuels as raw material, such as partial oxidation of heavy oil and coal gasification, are even more detrirnental to the environment since they involve higher specific generation of COZ. The possibility of using hydrogen as an alternative or simultaneous energy carrier with fossil fuels was con- sidered first in the sixties, when nuclear energy was believed to be the main energy source in the post-fossil fuel era. The ISPRA studies (IEA) of multi-step ther- mochemical processes, in which water is split by the use of heat supplied by high temperature gas-cooled nuclear reactors at ,about 9OO”C, were initiated at that time [2]. HYDROGEN BY SOLAR THERMAL WATER SPLITTING (HSTWS) The prospect of utilization of solar radiation, which is a “clean” renewable and holds abundant energy, for the production of hydrogen by direct thermal splitting of water generated a considerable amount of research dur- ing the period 1975-1985. Fletcher and his collaborators at the University of Minnesota stressed the thermodynamic advantages of a one-step process with heat input at as high a temperature as possible 13-61. The theoretical and practical aspects of the reaction were examined by Olalde [7], Lede [S-lo] and Ounalli [l l] in France, by Bilgen [12, 131in Canada and by Ihara [14, 151 in Japan. The main emphases in 89
Transcript
ht. J. Hydrogen Energy, Vol. 23, No. 2, pp. X9-98, 1998 CC 1997 International Association for Hydrogen Energy
Elsevier Science Ltd Pergamon
PII: SO360-3199(97)00038-4 All rights reserved. Printed in Great Britain 036&3199/98 $19.00+0.00
DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS-II.
EXPERIMENTAL FEASIBILITY STUDY
A. KOGAN
Solar Research Facilities Unit, Weizmann Institute ol’science, Rehovot, Israel
Abstract-The (development of a process of hydrogen production by solar thermal water splitting (HSTWS) presents a formidable technological task. The process has, however, great potential from the thermodynamic point of view and, when combined with fuel cell technology, it can lead to efficient conversion of solar energy to power. In the process under development at the Weizmann Institute of Science, water vapor is partially dissociated in a solar reactor at temperatures approaching 2500 K. Hydrogen is separated from the hot mixture of water splitting products by gas diffusion through a porous ceramic membrane.
The paper describes the problems encountered during the development of the HSTWS process. The following topics are discussed in some detail: (a) achievement of very high solar hydrogen reactor temperatures by secondary concentration of solar energy; (b) materials problems encountered in the manufacture of the solar reactor; (c) development of special porous ceramic membranes that resist clogging by sintering at very high temperatures. 0 1997 International Association for Hydrogen Energy
INTRODUCTION
Hydrogen is one of the most important base materials in the chemical industry. It is used mainly in the production of ammonia and of methanol and in processes of pet- roleum refining. During the last twenty years hydrogen has been considered increasingly as a possible alternative to fossil fuel. As a fuel hydrogen has some unique proper- ties. It is a clean fuel, the only product of its combustion being water. Its heating value is more than trite the heat- ing value of gasoline. Moreover, the thermodynamic availability of its energy content is very high. With pre- sent day fuel cell technology it can be used to produce electricity with an efficiency of 70%. Hydrogen could thus become an efficient, non-polluting energy carrier of the future if a process of hydrogen production could be developed that does not involve release of pol- lutants to the atmosphere.
The present economical and most widely used method of hydrogen production is by steam reforming of hydro- carbons [l]. Light hydrocarbons (methane to naphtha) and steam are converted catalytically to hydrogen and carbon oxides. The reforming reaction is highly endo- thermic. The heat of reaction is supplied by combustion of fuel gas or oil. Much CO* is thus generated in this process.
Other hydrogen production processes that utilize chea- per heavy fossil fuels as raw material, such as partial
oxidation of heavy oil and coal gasification, are even more detrirnental to the environment since they involve higher specific generation of COZ.
The possibility of using hydrogen as an alternative or simultaneous energy carrier with fossil fuels was con- sidered first in the sixties, when nuclear energy was believed to be the main energy source in the post-fossil fuel era. The ISPRA studies (IEA) of multi-step ther- mochemical processes, in which water is split by the use of heat supplied by high temperature gas-cooled nuclear reactors at ,about 9OO”C, were initiated at that time [2].
HYDROGEN BY SOLAR THERMAL WATER SPLITTING (HSTWS)
The prospect of utilization of solar radiation, which is a “clean” renewable and holds abundant energy, for the production of hydrogen by direct thermal splitting of water generated a considerable amount of research dur- ing the period 1975-1985.
Fletcher and his collaborators at the University of Minnesota stressed the thermodynamic advantages of a one-step process with heat input at as high a temperature as possible 13-61. The theoretical and practical aspects of the reaction were examined by Olalde [7], Lede [S-lo] and Ounalli [l l] in France, by Bilgen [12, 131 in Canada and by Ihara [14, 151 in Japan. The main emphases in
89
90 A. KOGAN
these investigations were the thermodynamics and the demonstration of feasibility of the process. However, no adequate solution to the crucial problem of separation of the products of water splitting has been worked out so far.
Much effort has been devoted to demonstrate the possi- bility of product separation at a low temperature, after quenching the hot gas mixture by heat exchange cooling [6], by immersion of the heated target in a reactor full of water [7], by rapid turbulent gas jets [S, 91 or by self- cooling [lo]. Based on a theoretical evaluation, Lapique [9] concluded that by quenching under optimal con- ditions it should be possible to recover up to 90% of the hydrogen formed by thermal water splitting.
The quenching technique is a highly irreversible process. It cannot be considered a practical solution to the hydrogen separation problem, notwithstanding the fact that it may be very effective in preventing H,-0, recombination. Consider a water splitting reactor opera- ting at 2500 K and 0.05 bar. Even under such extreme conditions only about 25% of the water is dissociated. By quenching the mixture of dissociation products most of the heat energy contained in the remaining 75% of undissociated water becomes irretrievable, rendering the process uneconomic.
Fletcher and Moen [3] suggested the use of porous or screen-like membranes for a partial separation of hydro- gen by effusion in the Knudsen flow regime. In a sub- sequent paper [4] Fletcher and Yu studied experimentally the separation effectiveness in the effusional separation of helium-argon mixtures at Knudsen numbers of 2.5, 10 and 5. A helium-argon mixture is a good model of a mixture of products of water splitting, the two mixtures having almost the same heavy-to-light component molec- ular weight ratio.
THE WEIZMANN INSTITUTE OF SCIENCE (WIS) PROGRAM
The WIS hydrogen production program by solar ther- mal water splitting (HSTWS) is based on the assumption that a HSTWS process of high thermal efficiency may eventually evolve only if the water splitting reaction is conducted at a very high solar reactor temperature, and if the separation of hydrogen from the mixture of water splitting products is effected while the mixture is still hot.
Consider the water mole fraction in a stoichiometric mixture of hydrogen and oxygen under thermodynamic equilibrium as a function of temperature and pressure (Fig. 1). At a pressure of 0.05 bar water dissociation is barely discernible at 2000K. By increasing the tem- perature to 2500K, 25% of water vapor dissociates at the same pressure. A further increase in temperature to 2800 K under constant pressure causes 55% of the vapor to dissociate. These basic facts indicate the difficulties that must be overcome in the development of a practical HSTWS process: (a) attainment ofvery high solar reactor temperatures, (b) solution of the materials problems con- nected with the construction of a reactor that can contain
0.01 0.1 I 10
P (bar) Fig. 1. Thermal water splitting.
the water splitting products at the reaction temperature and (3) development of an effective method of in situ separation of hydrogen from the mixture of water split- ting products.
SECONDARY CONCENTRATOR OPTICAL SYSTEM
In order to attain efficient collection of solar radiation in a solar reactor operating at about 2500 Kit is necessary to reach a radiation concentration of the order of 10 000. This is a rather stringent requirement. By way of example, consider the WIS 3 MW solar tower facility, which con- sists of a field of 64 slightly curved heliostats, each one capable of concentrating solar radiation approximately by a factor of 50. Even by directing all the heliostats to reflect sun rays towards a common target, a con- centration ratio of only about 3000 may be obtained.
It is possible, however, to enhance the concentration ratio of an individual heliostat by the use of a secondary concentration optical system. We adopted this method for our laboratory scale experiments (Fig. 2). Solar radi- ation was directed to our temporary work station on the roof of the solar tower (floor 14) by one of the heliostats of the WIS solar facility. The heliostat consists of 20 curved facets with a total area of 56m*. It has a focal distance of 63 m and forms a solar image at a distance of about 3m in front of the reactor location. A parabolic mirror of 0.63 m diameter and 0.174 m focal length is used as a secondary concentrator of the radiation coming from the heliostat. It forms a secondary image of the sun, 0.024m in diameter, in the vicinity of its focal plane.
A zirconia target (m.p. 2715°C) was exposed at the secondary focal plane for 10 s while the solar radiation intensity was 687 W/m2. The effect of the concentrated solar radiation upon the target is shown in Fig. 3.
DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS-11 91
A water-cooled calorimeter was used to determine the radiation flux at the secondary focal plane (Fig. 4). With a calorimeter aperture diameter of 3cm a maximum of 3.2 kW was absorbed by the calorimeter. The estimated overall concentraiion ratio obtained by the heliostat- secondary concentrator system for a calorimeter aperture diameter of 2.4cm is 10 600 suns.
In April 1993 the solar hydrogen test installation was moved from the roof of the solar tower to its new location on floor 7. Here the secondary concentrator-solar reactor unit is installed with its axis of symmetry in the vertical direction and a plane water-cooled mirror is used in order to deflect the radia tion from the heliostat to the secondary concentrator (Fig. 5).
Fig. 3. Zirconia disc exposed at the focal plane of the secondary concentrator.
Aperture diameter Aperture diameter oA=3cm oA=3cm l A=5cm l A=5cm
-5 -3 -I I 3 5
Distance from focus (cm) Distance from focus (cm) Fig. 4. Radiation flux to water-cooled calorimeter.
Fig. 5. Solar hydrogen test installation on floor 7
92 A. KOGAN
SOLAR REACTOR MATERIALS OF CONSTRUCTION
The correct design and fabrication of a high tem- perature solar reactor depends greatly on the availability and the right selection of construction materials. Work at the temperature range of about 2500 K requires use of special materials. Ordinary steels cannot resist tem- peratures above a few hundred degrees centigrade, while the various stainless steels, including the more exotic ones, fail around 1000°C. In the range lOOO-15OO”C, alumina, mullite or fused silica may be used. As the temperatures in this investigation were about 2500 K, the choice of candidate materials becomes restricted to a few natural or man-made materials. Among them we considered only oxide type ceramics. Carbide or nitride composites are likely to react with the water splitting products at the high temperatures needed for the reac- tion The list of candidate materials is restricted, there- fore, to high temperature oxide ceramics shown in Table 1, in order of increasing melting point from yttria to thoria.
The solar reactor components are currently made of zirconia and of magnesia. These materials have mech- anical and thermal stability at temperatures up to 2500 K and fair thermal shock resistance. In the future, when higher reactor working temperatures are attempted, we will consider switching to thoria.
HYDROGEN SEPARATION
The method chosen for separation of hydrogen from the mixture of water splitting products is by gas diffusion through a porous ceramic membrane. This method is very effective for our application in view of the high molecular weight ratio of oxygen to hydrogen.
At the present time we use commercially available zir- conia crucibles as gas separation elements. They have an open porosity of 2&30% and an average pore size of 5/*m. In order to achieve efficient separation by gas diffusion it is necessary to maintain a Knudsen flow
regime across the porous wall. The molecular mean free path d in the gas must be greater than the average pore diameter 4.
By Kinetic theory
i=j2 2nvd2
where d = molecular diameter (cm), v = molecular density = 273.15 p v,/T, p = pressure (bar), T = temperature (K), v, = 2.685 x 10i9. molecules/cmi.
At p = 1 bar, T= 2500 K this gives
i = 0.59/*m.
Thus it is evident that the pressure inside the solar reactor, in which commercially available crucibles are used as separation membranes, must be sub-atmospheric. Table 2 indicates reactor pressures suitable for main- tenance of Knudsen flow across membranes with average pore sizes in the range of 0.25540pm.
According to these figures the pressure inside the reac- tor, when fitted with the 5 pm pore size zirconia crucible as gas separating element, must not exceed 50 mbar.
SOLAR REACTOR DESIGN
Figure 6 is a schematic representation of the solar reactor. This reactor consists of a cylindrical zirconia housing of 1Ocm inside diameter and 20cm length and insulated by a 2 in thickness of Zircar felt and board. One end of the housing is closed by a circular disc with a central aperture 3cm in diameter. A zirconia crucible having a porous wall is installed at the opposite end of the housing. The insulated housing is enclosed in a cylindrical quartz bell jar, backed by a water-cooled metal flange. The unit is installed coaxially with the sec- ondary concentrator. The distance between the apex of the parabolic mirror and the center of cavity aperture is adjusted so as to allow penetration of the maximum amount of radiation into the reactor cavity.
Superheated steam at a constant preset flow-rate is introduced into the cavity. Steam flow regulation is achieved with the aid of a simple throttled nozzle arrange- ment by control of the upstream temperature and pres- sure.
Table 2. Average pore diameter and maximum reactor pressure for Knudsen flow
4~ (w9
40 5 1 0.25
P(W
0.0063 0.05 0.25 1 .oo
DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS--II 93
Water- metal
To Hydrogen pump
Fig. 6. Solar reactor cross-section.
The temperature: reached at the base of the ceramic crucible is measured by an optical pyrometer placed in front of the cavity aperture along the axis of the sec- ondary radiation concentrator.
When the crucible wall temperature exceeds 2000K steam adjacent to the crucible begins to dissociate. Two streams of gas are extracted in parallel from the reactor cavity. One stream, is extracted by diffusion across the crucible porous walll. Under suitable pressure and tem- perature conditions this stream will be enriched in hydro- gen. The second stream by-passes the crucible wall and is depleted of hydrogen.
Upon leaving the reactor both streams are passed through cold traps, where the unreacted steam is condensed. Samples of the hydrogen-enriched stream are analyzed by a calibrated gas chromatograph (GC) in order to determine the hydrogen yield.
SOLAR HYDROGEN PRODUCTION TESTS
The first HSTWS test was performed on March 23, 1993. The intensity of solar radiation during the test was
900-950 W/m’. Steam was admitted to the reactor cavity through a calibrated orifice at a rate of 5g/min. The shutter in front of the secondary concentrator was opened very slowly, in 20 consecutive steps, over a period of 94 minutes. The temperature of the zirconia membrane inside the Isolar reactor increased during the test to 2050 K (Fig. 7). The following decrease in reactor inner wall temperature corresponds to the afternoon drop in solar radiation intensity.
First traces of hydrogen were detected by the GC at a reactor wall temperature of 1920 K. The hydrogen flow- rate from the reactor increased with reactor wall tem- perature ta 30 ml/min.
During the period of steam flow to the reactor a differ- ence of pressure was recorded across the porous membrane. At the time of maximum hydrogen flow-rate the upstream pressure was 45mb and the downstream pressure --7mb. These values are close to the design pressures.
In a subsequent series of HSTWS tests reactor tem- peratures esf up to 2250 K were reached, but the hydrogen yield decreased in consecutive tests and ultimately fell below the maximum yield of the first hydrogen pro- duction test.
In order to validate the suspicion that the porous zir- conia mernbrane became clogged up by progressive sin- tering during the solar tests, the membrane was replaced by an identical new membrane. A HTSWS test was con- ducted on January 3, 1995, with the new membrane installed inside the reactor, in order to determine a reactor temperature beyond which the pores of the commercially available membrane close down by sintering.
The reactor temperature was controlled in this test by changing i he distance between the reactor aperture plane and the secondary concentrator focal plane. At the start of the test the reactor temperature went up to 2 130 K for a short period and then stabilized at about 2100 K (Fig. 8). The hydrogen flow-rate leaving the steam traps on its way to the pump was determined by GC. Within the first
1 First traces of Hydrogen
rate of 30mUmm was registered ! by the gas chromatograph i-r
Fig. 7. Evolution of reactor temperature during the first hydro- gen production test.
94 A. KOGAN
600
300
0
.
_-
_-
I
T
\-
14:30 14:50 Time of day (hour)
Fig. 8. Hydrogen production test of January 3, 1995
0.14
0.12
0.10
20 minutes the rate approached an asymptotic value of 0.104 SLM, then it dropped drastically. The decrease in hydrogen flow-rate was accompanied by a widening of the difference in pressure between the front of the mem- brane and behind it.
Thus, it became evident that rapid sintering of the zirconia membranes we were using limits their usefulness in our application to temperatures below 2100 K.
In order to overcome this limitation we solicited the collaboration of the Israel Ceramics and Silicates Insti- tute at the Technion to develop a zirconia-based material for preparation of porous membranes characterized by improved stability with respect to pore closure at high temperatures.
DEVELOPMENT OF ZIRCONIA-BASED CERAMIC MEMBRANES WITH THERMALLY
STABLE POROSITY
The clogging of a porous zirconia membrane by pro- gressive sintering at an elevated temperature is well illus- trated in Figs 9 and 10. These are scanning electron microscope pictures of the surface of a porous zirconia crucible after 2 h and after 22 h of exposure to 1750°C respectively.
Two different approaches were proposed by ICSI in order to overcome this difficulty. Following one approach, the zirconia powder is doped with other metal oxide powders to interfere with the sintering process. The other approach is to start with a zirconia powder
prepared by the SOL-GEL method. The powder then consists of spherical particles that are difficult to sinter. Figure 11 illustrates the micro-structure of a stabilized zirconia cermic made from a micro-spherical powder pre- pared by the SOL-GEL method. Figure 12 demonstrates the effect of micro-spherical powder particle size on stab- ility of a porous ceramic at high temperatures.
The trials at ICSI are still at an early stage. The indi- cations so far are that the SOL-GEL approach has better chances of leading to a satisfactory solution. There is also the possibility of combining the two approaches in order to hinder completion of sintering up to very high tem- peratures.
A small solar furnace was built at the Solar Research Facilities Unit, WIS in order to enable testing of the ceramic samples supplied by ICSI at temperatures that cannot be reached in their electric furnaces. The micro- structure and phase of the samples prepared by ICSI are studied by SEM and XRD at the Technion. The gas permeability of the samples is then determined at WIS before exposure to a predetermined elevated temperature in the solar furnace. The samples are then returned to ICSI to start a next cycle of tests before exposure in the solar furnace to a next higher temperature.
CONCLUDING REMARKS
The hydrogen yield of a HSTWS plant has been esti- mated by a simplified theoretical model, in which it was assumed that the steam entering the reactor is preheated
DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS~PII 95
Fig. 9. Crucible from (- lOO/+ 325) after 2 h at 1750°C.
Fig. 10. Crucible from (~ lOO/ + 325) after 22 h at 1750°C
96 A. KOGAN
Fig. 11. Porous ‘ :eran iics sin te :red at 1700°C from ZY-8 micro-spherical powders; the powder pa 55-65 pm in size.
.rticles v iere (a) 45-55 pm a nd ( 3)
to reactor wall temperature [16]. The hydrogen yield in the January 3, 1995 test exceeded the value predicted by the simplified theory by 20%. It is plausible to assume that this result is related to the fact that the gas in the solar reactor that by-passes the gas separating membrane leaves the reactor at a temperature much lower than the reactor wall temperature (857K and 2250K, respec- tively). The very steep thermal gradient in the vicinity of the hot ceramic membrane induces a thermal gas sep- aration effect that drives light particles (hydrogen) pref- erentially towards the hot membrane surface. This effect enhances the main separation process obtained by gas diffusion.
The fact that the gas stream by-passing the porous membrane does not approach by far the reactor wall temperature has an important consequence on the chances of our method becoming of practical value. The heat supplied to the reactor by concentrated solar radi- ation has a very high energy. If it were all used for water splitting in a reversible way, the process would be very
efficient indeed. Yet part of the heat supplied to the reac- tor is used to heat up the steam that does not undergo dissociation. The lower the temperature to which this steam is heated, the more efficient is the hydrogen pro- duction process.
In this connection it is also of great interest to develop a method of recycling part of the hot water splitting products back to the reaction zone, using the process make-up steam as a driving fluid in a steam injector [ 171. The design of an appropriate injector, in which the driven fluid is a reactive gas mixture, is being undertaken by computational fluid mechanics methods in a joint effort with the Department of Aerospace Engineering, Tech- nion-IIT.
Acknowledgements-This work was supported by the Heineman Foundation for Research, Education, Charitable and Scientific Purposes, Inc., Rochester, N.Y. We are grateful for the generous support of the Foundation.
DIRECT SOLAR THERMAL SPLITTING OF WATER AND ON-SITE SEPARATION OF THE PRODUCTS--II 97
Powder partic size <44pm; sintering at 1750”C/5h
:le
This sample heat-treated at 1900°C/l.5h+ 1980W1.5h
Powder particle size 275pm; sintered at 1750W5h and heat-treated at 1900°C/1.5h+ 1980°C/1.5h
Fig. heat
12. SEM photographs of ZY-8 porous ceramics. (a) Powder particle size <44pm, sintering at 1750 C for 5 h; (b) Same sample -treated at 1900-C for I .5 h and at 1980’ C for 1.5 h; (c) Powder particle size 2 751Lrn. sintered at 1750 C for 5 h and heat-
treated at 19OO’C for 1.5h and 19XO’C for 1.5h.
98 A. KOGAN
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