articleSTMS1

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Available online at www.sciencedirect.com

International Journal of Hydrogen Energy ( )

www.elsevier.com/locate/ijhydene

1

Production of hydrogen and carbon by solar thermal methanesplitting. I. The unseeded reactor3

Meir Kogan, Abraham Kogan

Solar Research Facilities Unit, Weizmann Institute of Science, Room 208 240 Herzl St., PO Box 26, Rehovot 76100, Israel5

Abstract

Solar thermal methane splitting was performed in a series of tests with an unseeded low capacity reactor. Eective screening7

of the reactor window from contact with carbon particles was achieved by application of the tornado ow conguration (J

Solar Energy Eng 124 (2002) 206). The tests were performed at atmospheric pressure and at temperatures up to 1320 K. An9

extent of reaction of 28% was attained. Most of the carbon generated in the process clang to the irradiated reactor wall and it

formed a very hard deposit. In most cases, the tests were terminates when the reactor exit port became choked by the accrued11

carbon deposit. The results of the tests are discussed and ways to correct the problems encountered with the unseeded reactor

are proposed.13

? 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.

Keywords: Thermal methane splitting; Hydrogen production15

1. Introduction

Hydrogen is a clean fuel in the sense that no CO 2 is gen-17

erated when it is burned. At present, the amount of hydro-

gen that serves as a fuel is negligible. It is used almost ex-19

clusively as a raw material in the manufacture of ammonia,

methanol and petrochemicals. The predominant method of21

hydrogen production is by methane steam reforming, which

involves massive coproduction of CO2, about 7 t CO2=t H2.23

At the present level of world hydrogen consumption, which

approaches 108 t=yr, the production of hydrogen is an im-25

portant contributor to the anthropogenic release of CO2 to

the atmosphere. Therefore, there is a strong incentive to de-27

velop methods of hydrogen production in which the emis-

sion of CO2 is reduced or, preferably, completely eliminated.29

The future role of hydrogen as an alternative to fossil fuels

depends, inter alia, on the successful development of clean31

hydrogen production methods.

Corresponding author. Tel.: +972-8-934-3782; fax: +972-8-934-4117.

E-mail address: [email protected] (A. Kogan).

Steinberg and Cheng [1] made an extensive study of mod- 33

ern and prospective technologies for hydrogen production

from fossil fuels. A comparison of the specic cost of hy- 35

drogen obtained by the analyzed technologies shows that

thermal methane decomposition has a signicant economic 37

potential. The possibilities of decomposing hydrocarbons

directly into carbon and hydrogen were also discussed by 39

Sandstede [2] and Fulcheri and Schwob [3].The development by a Norwegian company of a plasma 41

arc process for splitting of natural gas was described by

Lynum [4]. The economic aspects of this process were dis- 43

cussed by Gaudernack and Lynum [5]. They concluded that

the cost of hydrogen would depend on the prices of the 45

input commodities, natural gas and electricity, and on the

revenues from the coproduct, carbon black. With conserva- 47

tively estimated carbon black prices and with the very low

price of electricity in Norway, the calculated cost of hydro- 49

gen came out competitive with hydrogen produced by hy-

drocarbon steam reforming. 51

In many parts of the world, electricity is much more ex-

pensive than in Norway, but abundant solar energy, which 53

can be utilized for hydrocarbon splitting, is available. There

0360-3199/03/$ 22.00 ? 2002 Published by Elsevier Science Ltd on behalf of the International Association for Hydrogen Energy.

doi:10.1016/S0360-3199(02)00282-3
mailto:[email protected]:[email protected]


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is therefore an incentive to develop a process of solar ther-1

mal methane splitting (STMS).

2. Some technical aspects in the development of STMS

2.1. Temperature and residence time requirements

The thermal methane splitting reaction takes place at a rel-5

atively moderate temperature. Fig. 1 illustrates some results

of thermochemical equilibrium calculations of the (C, 2H2)7

system, obtained by the application of the NASA CET-85

computer program. At ambient pressure, the mol fraction of9

unreacted methane is less than 0.001 at 1800 K.

Kinetic data on thermal splitting of methane were obtained11

experimentally by Matovich [6] and Lee et al. [7]. The reac-

tion residence times reported in [6,7] fall in the time interval13

0:2 s6 tr6 1 s. Above 1900 K, the extent of reaction was

not much inuenced by the residence time. Complete disso-15

ciation was achieved above 2100 K, a temperature that can

be eectively attained in a solar reactor.17

Solar thermal methane splitting proof-of-concept exper-

iments were conducted recently at NREL, in collaboration19

with the University of Colorado Department of Chemical

Engineering [8].21

A stream of dilute 5% CH4 in argon feed gas surrounded

by a pure argon purge mixture was own through a reac-23

tor consisting of a 25 mm diameter quartz reactor tube. The

reactor was illuminated with a solar ux of 2400 kW=m2.25

The very low concentration of CH4 in argon was maintainedin order to keep the concentration of CH 4 and of H2 out-27

side ammability limits. Approximately 90% dissociation

of methane was reported.29

2.2. Separation of reaction products

Separation of the hydrogen and carbon product mixture31

does not present a dicult technical problem, since the mix-

ture components appear in dierent phases. Technical de-33

tails are reported in [9].

2.3. Screening of a solar reactor window35

A more complicated technical problem is connected with

the need to protect the reactor window from contact with37

solid carbon particles generated by the STMS reaction.

These irradiated particles are heated to incandescence. If39

allowed to come in touch with the window surface, they

might stick to it, leading to window destruction by overheat-41

ing. The usual method of screening the window by ooding

its surface with a curtain of an auxiliary gas stream43

requires very substantial auxiliary gas owrates and the

heat absorbed by the gas represents a major loss of energy45

[10,11]. In an eort to reduce the auxiliary gas owrate to a

minimum, a certain ow pattern akin to the natural tornado47

phenomenon has recently been developed in our laboratory,

Fig. 1. Thermal splitting of methaneXCH4 vs. p, T.

which enables eective reactor window screening by an 49

auxiliary gas owrate less than 5% of the main gas owrate.

Details of the tornado eect are discussed elsewhere [12]. 51

Here, we shall limit ourselves to a brief exposition ofthe physical background and to an illustration of this phe- 53

nomenon.

When a uid ows along a solid stationary boundary, its 55

motion is retarded in a thin boundary layer by friction. The

retarded uid boundary layer may thicken progressively in 57

the direction of ow and ultimately it may detach from the

solid boundary and mix with the main ow. Boundary layer 59

detachment can be averted if care is taken to maintain a

uniformly decreasing pressure in the direction of ow. The 61

accelerated main ow entrains then the uid in the bound-

ary layer strongly enough to counteract the ow retardation 63

caused by friction with the stationary boundary.Turning to our particular application, the axisymmetric 65

chamber of the STMS reactor is provided with a transparent

window located at one end of the chamber, transversally to 67

the longitudinal axis. A ow of methane is introduced into

the chamber in a manner whirling around the axis, while the 69

reaction products are withdrawn at the opposite end of the

chamber through a narrow central tube oriented along the 71

longitudinal axis. The gas ow inside the chamber approx-

imates then a free vortex ow, characterized by a drop of 73

pressure from the periphery of the chamber to its axis.

An auxiliary ow of protecting gas introduced at the pe- 75

riphery of the window is directed towards the window cen-

tral area. It is accelerated by the negative pressure gradient 77

generated by the free vortex ow. The auxiliary boundary


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M. Kogan, A. Kogan / International Journal of Hydrogen Energy ( ) 3

Fig. 2. Cross section of reactor M2a.

layer ow at the window surface is stabilized thereby and it1

remains attached to the surface all the way to the center of

the window. There, the radially converging streamlines turn3abruptly by 90 into the axial direction, forming a typical

tornado-like funnel along the reactor axis.5

Synergy between the free vortex ow of the main gas and

the boundary layer ow of the auxiliary gas is here exploited7

in order to protect eectively the reactor window. The syn-

ergy is expressed by the fact that the auxiliary ow, which9

is desired to form a stable, continuous and non-separated

protective layer on the window surface is not disturbed by11

the whirling main stream. It is rather stabilized by it. Conse-

quently, the auxiliary ow does not need to be injected with13

high velocity or with a great owrate in order to adhere to

the surface to be protected, because it uses the energy of the15whirling main stream against which protection is sought.

The tornado eect has been demonstrated in a series17

of simulation tests at room temperature with the reactor

model shown in Fig. 2 [12]. The main gas stream was19

own from an annular plenum chamber through a narrow

annular gap towards the upper part of the reaction cham-21

ber. An impeller-like ring was implanted in the annular

gap. The main gas stream acquired an angular momentum23

during its passage through slanted grooves in the impeller

ring and it entered the reactor cavity in a whirling motion.25

The auxiliary gas stream was own radially from a second

annular plenum chamber through a second narrow annular27

gap towards the periphery of the inner surface of the win-

dow. Both streams consisted of nitrogen gas. The auxiliary29

stream was made visible by charging it with smoke, while

the gas in the main stream was left in its natural transparent 31

condition. In order to enable visual inspection of a crosssection of the ow inside the reaction chamber, a laser 33

beam directed towards the reactor window was diracted

by passage through a transverse cylindrical glass rod. The 35

monochromatic laser beam emerged from the glass rod as a

planar sheet of light that illuminated a cross section of ow 37

inside the reaction chamber.

The four tornado conguration tests illustrated in Fig. 3 39

were performed with an auxiliary smoke-charged gas main-

tained at a constant owrate of 2 l=min. In the absence of a 41

whirling main gas stream (Fig. 3a), the auxiliary ow sep-

arated from the window surface immediately upon its entry 43

into the reaction chamber. When the whirling main streamwas introduced into the reactor cavity at successively higher 45

owrates (Fig. 3ac), the auxiliary stream became progres-

sively stabilized as a thin boundary layer. For a main gas 47

owrate of 15 l=min, the auxiliary gas moved at high speed

in the thin boundary layer near the window surface. It cov- 49

ered the entire window surface area and it left nally the

reaction chamber through a narrow axially oriented funnel. 51

2.4. Transfer of radiation energy to reactant gas

Methane is a transparent gas. Radiation propagating into 53

the solar reactor is not absorbed directly by methane. It heats

the reactor wall and part of the heat is transferred to the gas 55

by conduction and convection (surface heating).


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Fig. 3. Four tornado ow conguration tests with successively increasing main gas owrates. Smoke-charged secondary gas owrate was

2 l=min.

Following a method proposed by Hunt [13] and Yuen1

et al. [14], a gas may be heated by concentrated radiation

throughout the volume of the reaction chamber by dispersing3

small particles in the gas, to form an opaque cloud. Radiation

is absorbed by the particles in suspension, which in turn5

exchange heat with the surrounding gas very eectively, in

view of the very large surface area per unit mass of particles7

(volumetric gas heating).

It should be noticed that even in the absence of active9seeding, solid carbon particles are generated near the hot

surface of the reaction chamber by the methane splitting re-11

action. These particles start a volumetric absorption process

that may spread in a chain reaction into the bulk of the re-13

action chamber. It was not clear a priori whether this eect

is strong enough to render active seeding superuous.15

3. Experimental

The primary aim of the experimental work described be-17

low was to determine under realistic STMS conditions to

what extent the tornado ow conguration oers a reliable19

method for protection of the reactor window from contact

with hot powder particles. Another aim was to determine the 21

extent of reaction that can be attained in an STMS reactor

without recourse to active powder seeding. 23

3.1. Experimental setup

A diagram of the experimental setup designed for a pre- 25liminary study of STMS is shown in Fig. 4. It consists of

a solar reactor, an optical concentration system, the piping 27

requisite for the establishment and maintenance of a tornado

ow conguration inside the reactor chamber, instrumenta- 29

tion and auxiliary equipment.

Two reactor congurations, designated M3-a and M4-f, 31

were used during the tests described below. Their geometries

are illustrated in Figs. 2 and 11, respectively. 33

The optical concentration system consists of a heliostat

followed by a secondary paraboloid concentrator and a ter- 35

tiary compound paraboloid concentrator (CPC). The per-

formance of the optical system was determined during pre- 37

liminary calibration tests. The ux of radiation leaving the

CPC exit section was measured by replacing the reactor 39


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Fig. 4. Diagram of STMS test setup.

by a water-cooled calorimeter. The radiation ux reaching1the outer surface of the reactor window was 2064 W, when

normalized for a solar irradiation of 850 W=m2. This corre-3

sponds to a concentration factor of 781. The value of solar

irradiation was supplied by the control room of the Solar Re-5

search Facilities Unit at the Weizmann Institute of Science.

Fig. 5. Quartz window and two chunks of carbon found near reactor exit port after test no. 1.

The reactor pressure in the tests described below 7

was planned to be slightly above atmospheric pressure

(1:010 atm6pr6 1:105 atm), in order to evade the need 9

for pumping. This pressure was monitored by a colored

water U-tube manometer. 11An STMS test is started by owing methane and auxil-

iary gas through owmeters to the reaction chamber at pre- 13

determined rates F1 and F2, respectively. When the prede-

termined gas owrates are reached, a shutter plate is raised 15

and concentrated radiation is admitted to the reactor win-

dow. Hot products of reaction leaving the reaction zone are 17

quenched by water sprays. The mixture of reaction products

and quenching water enters a gravity separator. 19

The quenching water owrate and temperature increase

are determined by a rotameter and a T-type thermocouple, 21

respectively. The carbon black slurry is discharged from

the separator to the atmosphere through a reinforced plastic 23U-tube, while the gas is own through an upper exit pipe to

the roof of the tower. The gas composition is determined by 25

a mass chromatograph.

A B-type thermocouple was installed to measure the tem- 27

perature of reaction products at the exit port of the reaction

chamber. Unfortunately, the temperatures registered by this 29

thermocouple did not represent correctly the sought gas tem-

perature, for reasons explained below. The temperature ap- 31

proached by the reaction products at the end of each test was

therefore evaluated indirectly from an energy balance over 33

the quenching process of the product gas mixture, as follows:

f(T) = {(1 )[ h0

CH4 (T) h0

CH4 (Tf)]

+ 2[ h0H2

(T) h0H2 (Tf)]}F1

+ { h0Aux(T) h0Aux(Tf)}F2

mqwcqw(Tf Ti) = 0: (1)


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Fig. 6. Pictures of (a) reactor ceramic insulation with thermal cracks and (b) opposite surface of copper disc marked with traces of carbon

black imprints.

The temperatures Ti and Tf and the extent of reaction 1

were determined experimentally during the test. Thus, it was

possible to evaluate f(T) and to plot it as a curve in the3

fT plane. The intersection of the curve with the T-axis

yielded the gas temperature T before quenching.5

An alarm device was installed in the test rig. It

is actuated when either one of the following events7

occurs:

(1) The temperature of a certain thermocouple exceeds a9

predetermined value.

(2) The reactor pressure reaches 1:10 atm.11

(3) A ammable gas sensor detects traces of H2 or CH4above a preset threshold.13

In case of alarm, the test is immediately terminated by

discontinuing methane ow to the reactor and by releasing 15

the shutter plate, in order to block radiation from reaching

the reactor window. 17

A close loop TV was installed in order to enable indi-

rect observation of the reactor window during STMS tests 19

and to detect promptly any incipient aw in the quartz

window. 21

3.2. Experimental results

The results of our experimental program will be presented 23by the description of the following four tests.


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Fig. 7. Outer layers of ceramic insulation. Carbon black imprints indicate the divergence of ow from the thermocouple duct.

3.2.1. Test no. 11Test no. 1 was performed with reactor M3-a (Fig. 2). The

reaction chamber of M3-a has the shape of a frustum of small3

dimensions (D1 = 8:0 cm; D2 = 1:0 cm; L = 10:0 cm). The

aperture of the quartz window installed normal to the reactor5

axis of symmetry has a 5:8 cm diameter. An impeller ring

with 18 slant grooves of 0:15 cm 0:6 cm cross sections7was installed in the reactor during test no. 1.

The test parameters during test no. 1 were:9

I= 740 6 W=m2; F1 = 20 SLM CH4; F2 = 2 SLM Ar.The stabilizing inuence of the tornado eect was tried11

during this test under extremely adverse gasdynamic con-

ditions. The temperature of reaction products increased13

steadily for 15 min from start of irradiation, leveling o at

885 K. At that time, the density of the hot reaction products15

was more than six times smaller than that of the argon layerowing above it at near room temperature (a Taylor insta- 17

bility condition). Pulsed formation of black powder was

observed at the central region of the window 5 min after 19

start. At t= 10 min, the pulsations subsided. At t=15 min,

a milky opaque spot was seen in the central region of the 21

window. It grew gradually in size, reaching a 3 cm diame-

ter at t= 30 min, when the test was terminated. The extent 23

of reaction was 6.1% at t= 5 min, then it dropped to 3%

and nally, towards the end of the test, it rose again to 6%. 25

The quartz window was then removed from the reactor for

inspection. The window was found undamaged over most 27

of its area, except for a central region, approximately 3 cm

in diameter, which exhibited signs of molten quartz and of 29

carbon deposition. Two chunks of soot, which were detached


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Fig. 8. Sketch of gas ow bypassing the tornado ow pattern during test no. 2.

from the window surface, lay superimposed at the bottom1

of the reaction chamber (Fig. 5). The lower chunk had a

layered conical shape. Its base facing the window had a very3

shiny metallic hue, while its opposite side was black. Thesecond chunk was a thin disc, shiny on both sides. The two5

chunks did not block completely the chamber exit port.

The above observations support the following test in-7

terpretation: During an initial time period (5 min6 t6

15 min), when the window was still completely undamaged9

and unobstructed, the methane splitting reaction occurred at

a low extent along the irradiated surface of ceramic insula-11

tion. However, the tornado ow conguration did succeed

only partially to overcome the eect of Taylor instability on13

the argon boundary layer ow along the window surface.

The heavy boundary layer remained attached to the solid15

surface only about half way from periphery to center of win-dow. The central region beyond the ow detachment line17

was not protected from contact with methane, nor was it ef-

fectively cooled by the detached gas ow. Quartz became19

eventually hot enough in this region to support methane

splitting to some extent. The generated hot carbon parti-21

cles stuck to the quartz surface, rendering it opaque. At this

stage, radiation was trapped in a very thin layer of quartz23

at the quartzcarbon interface, due to the opacity of carbon

and the very low thermal conductivity of quartz. A very25

thin layer of quartz melted. It acquired a white-milky ap-

pearance and it became reective. The net radiation ux en-27

tering the reaction chamber became thereby reduced, caus-

ing a temporary decrease in extent of the methane splitting29reaction.

When the carbon deposit on the central area of the window 31

became heavy enough, it detached from the window sur-

face, which regained temporarily much of its transparency, 33

prompting an increase in extent of reaction.

3.2.2. Test no. 2 35

Reactor M3-a was used also during test no. 2, but the

impeller ring was replaced by a ring with 18 slant grooves 37

with 0:07 cm 0:2 cm cross sections.The test parameters during test no. 2 were: 39

I=83422 W=m2; F1 =20 SLM CH4; F2 = 4 SLM He.Helium was used as an auxiliary gas instead of argon in 41

test no. 2, in order to alleviate the adverse eect of Taylor

instability (Notice that in an industrial STMS plant, hydro- 43

gen will serve as an auxiliary gas.) The normal cross sec-

tions of the slanted grooves in the reactor impeller ring were 45reduced by a factor of 6.43 and the angular momentum of

the whirling methane stream at a given owrate increased 47

accordingly.

Twenty minutes after start of irradiation, the temperature 49

of reaction products reached 1150 K. The extent of reaction

reached at this time a maximum value, =8:7%. Thepressure 51

in the reaction chamber started to climb. Two more minutes

later, the alarm was activated by the pressure rise. In an eort 53

to release the pressure increase in the reaction chamber,

both gas owrates to the reactor were reduced to 10 SLM 55

CH4 and 2 SLM He, respectively. Despite this step, window

damage was detected on the TV screen 3 min thereafter. 57

Reactor pressure continued to climb and reactor temperature

dropped drastically. The test was terminated. 59


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Fig. 9. Quartz window after test no. 2: (a) surface facing reactor

cavity; (b) exterior surface.

The reactor insulation consisted of an inner Cotron-1

ics Rescor 760 zirconia casting surrounded by three t-

ted consecutive cylinders of type ZYC Zircar. A 6 mm3

diameter duct drilled radially through the ceramic in-

sulation served for installation of the B-type thermo-5couple.

Initial thermal cracks were detected in the inner zirconia7

casting already in earlier tests. These cracks widened sig-

nicantly during test no. 2 (Fig. 6).9

Traces of carbon black imprints on the disassembled reac-

tor components after test no. 2 revealed the following facts11

(Figs. 6 and 7):

(1) Some gas was owing out of the reaction chamber13

radially along lines corresponding to the ssures in the

ceramic insulation.15

(2) This gas returned to the lower side of the chamber,

where the pressure is reduced, through the radial duct17that houses the B-type thermocouple.

Fig. 10. Quartz window after test no. 3.

(3) The reactor port was completely plugged by powder 19

and protruding veins of carbon were deposited on the

wall ssures. 21

The following sequence of events is suggested to explain

the evolution of temperature, reactor pressure and hydrogen 23

production rate during test no. 2:

(1) Practically all the carbon black powder generated in 25

the reactor cavity clung to the ceramic insulation wall.

Ten minutes after start of irradiation, the tornado exit 27port was blocked to a considerable extent. As a result,

the pressure inside the reactor went up. 29

(2) Part of the primary (CH4) gas introduced into the

reactor cavity bypassed the tornado ow pattern 31

(Fig. 8), owing outwards from the high pressure

region in the periphery of the reactor cavity through 33

the cracks in the insulating wall, taking then a reverse

shortcut route through the radial duct of thermocouple 35

T, to discharge nally at the low pressure location of

the exit port. Thus, the wind was taken out of the 37

sails of the tornado.

(3) Towards the end of the test, the obstruction of the exit 39

port by powder agglomeration caused the reactor pres-sure to surpass the alarm limit. 41


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Fig. 11. Cross section of reactor M4f.

(4) In order to bring down the reactor pressure again be-1

low the preset limit, the primary and secondary gas

owrates were reduced to half their original values.3

However, by this step the tornado eect was weakenedeven more. Within a few minutes, the window surface5

was covered up by carbon black and the window was

disintegrated (Fig. 9).7

3.2.3. Test no. 3

During test no. 3, the reactor and impeller ring were the9

same as those used during test no. 2. In preparation for

test no. 3, the B-type thermocouple junction was covered11

by a titanium cap that tted tightly into the thermocouple

duct. The ow of gas through this duct was thus blocked.13

A thin layer of zirconia felt was placed between the reactor

ceiling ange and the zirconia insulation, in order to block15passage of gas through any in-between gap. The cracks in

the zirconia insulation of the reaction chamber were lled17

with a zirconia cement.

The test parameters during test no. 3 were:19

I=87810 W=m2; F1 =10 SLM CH4; F2 =4 SLM He.Test no. 3 lasted for 37 min. The extent of reaction reached21

at the end of test was 27.3%. At this time, the temperature

of gas at the reactor exit port was leveled at 1320 K. The23

test was terminated when some smoke appeared near the

CPC and naphthalene odor was perceived at the exit of the25

quenching water stream.

The quartz window was removed from the reactor. It was27

found slightly stained by carbon powder, but there was no in-

dication of initial melting or any disintegration of the quartz29

disc (Fig. 10). Chunks of brittle carbon were found near

the chamber exit port, which was only partially blocked by 31

them.

3.2.4. Test no. 4 33

Test no. 4 was performed with reactor M4-f (Fig. 11),

equipped with the impeller ring used in tests no. 2 and 3. 35

The reaction chamber was formed by casting zirconia into a

stainless-steel mold. The ceramic casting was not removed 37

from the mold. It was braced by it during solar tests. The

formation of severe cracks in the casting, due to thermal 39

stress, was thus prevented. Additional measures were taken,

such as the use of a zirconia-felt gasket, in order to prevent 41

degeneration of the tornado ow pattern by escape of gas

from the reaction chamber. The dimensions of the chamber 43

were D1 = 7:3 cm, D2 = 6:5 cm, L = 6:7 cm. The chamberexit port had a 1:0 cm diameter. An annular disc of titanium 45

was installed transversally in the chamber, at a distance of

2:3 cm from the quartz window. The central hole of the 47

annulus had a 2:3 cm diameter.

The test parameters during test no. 4 were: 49

I= 883 5 W=m2; F1 = 10 SLM CH4; F2 = 4 SLM He.The amount of hydrogen formation during test no. 4 51

went up drastically, reminding the results obtained in test

no. 3. Five minutes after start of irradiation, the methane 53

splitting reaction attained an extent of 19%. Ten minutes

after start, a maximum gas temperature of 1250 K was 55

attained with = 26:2%. Despite this fact, the pressure

rise inside the reactor was insignicant. Eighteen min- 57

utes after start, the quartz window became blackened. The


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Fig. 12. View of irradiated side of titanium annular disc after test

no. 4.

pressure still remained almost unchanged. A signicant1

reactor pressure increase appeared only 23 min after start

and the test was then terminated. During the course of3

test no. 4, as well as during the previous three tests, the

amount of carbon in the stream of quenching water leaving5

the reactor was negligible. Almost all the carbon gener-

ated by methane splitting was deposited on the hot reactor7

walls.

The following facts were observed after reactor disassem-9

bly (Figs. 1214):

(1) Carbon deposition on the annular ring was very asym-11

metric. A sector of about one-third of the central hole

was still unobstructed by carbon deposit.13

(2) The carbon deposited on the side of the disc facing the

window (the irradiated side) consisted of stony, hard15

layers of material with a metallic tint.

(3) Carbon was deposited also on the opposite side17

of the disc. This deposit had a uy and brous

appearance.19

(4) Much carbon black settled at the bottom of the reactor

cavity. It had the appearance of a black soft or brittle21

material. It seemed to obstruct completely the reactor

exit port.23

Fig. 13. View of backside of titanium annular disc after test no. 4.

4. Discussion and conclusions

The four tests described above were selected from a series 25

of tests to illustrate the performance of the tornado eect

under realistic STMS conditions. 27

Test no. 3 endured for 37 min. By the end of the test, the

temperature of reaction products was 1320 K and the extent 29

of reaction reached 27.3%. The reactor window, as observed

on the TV screen, remained clear throughout the test dura- 31

tion. Such performance repeated itself in additional tests, inwhich the auxiliary gas was helium, demonstrating the po- 33

tential of the tornado eect to protect the reactor window

from contact with powder particles generated by methane 35

dissociation.

The test duration of 37 min was not surpassed by anyone 37

test in the present series. In most cases, the crisis that led

to test termination was provoked by plugging of the reactor 39

exit port by carbon deposition. This behavior appears to be

intrinsically connected with the mode of gas heating in a 41

surface receiver-type reactor. The endothermic reaction in

a surface receiver is initiated in a narrow thermal boundary 43

layer along the irradiated walls of the reaction chamber.

Our tests demonstrated the tendency of the carbon particles 45

generated within this thermal boundary layer to cling to


12/12

UNC

ORRECT

EDPRO

OF


HE 1575

ARTICLE IN PRESS

Fig. 14. View of reactor cavity exit port after test no. 4.

the adjacent irradiated solid surface, forming a very hard1

carbon deposit, which interferes eventually with the outow

of reaction products from the reaction chamber.3

The outcome of our tests also dispelled the assumption

that carbon particles generated within the thermal bound-5

ary layer near the irradiated wall might start a volumetric

radiation absorption process, which could spread through-7

out the reaction chamber. The maximum extent of reaction

achieved in the present test series was only 28.1%. Methane9owing through the reaction chamber along streamlines re-

mote from the chamber wall did not get obviously heated11

enough to undergo dissociation.

The method of volumetric gas heating by seeding the reac-13

tion chamber with radiation absorbing particles is expected

to oer an eective solution to the problems discussed. It15

will certainly make possible to attain a much higher extent

of reaction than that obtained so far. It will also alleviate17

the problem of formation of carbon deposits on the reaction

chamber walls, since in a volumetric receiver the genera-19

tion of carbon particles by methane splitting will take place

mainly in the very hot central region of the chamber. The21

walls of the chamber will be shaded by the cloud of parti-

cles from direct solar irradiation and their temperature will23

remain moderate. Hard carbon deposits will not build up on

the walls under such conditions. To counteract any tendency 25

of formation of a soft carbon deposit, it will be possible to

ush the walls by a owing gas lm. The next phase of our 27

research will proceed along these lines.

Acknowledgements 29

This study was supported by the Heineman Foundation

for Research, Education, Charitable and Scientic Purposes,

Inc., Rochester, NY, USA. The authors gratefully acknowl-

edge the generous support of the Heineman Foundation.

References

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Hydrogen Energy 1989;14(11):797820. 33[2] Sandstede G. Decomposition of hydrocarbons into hydrogen

and carbon for the CO2-free production of hydrogen. 35Proceedings of the Ninth World Hydrogen Energy Conference,

vol. 3. Paris, France, 1992. p. 174552. 37[3] Fulcheri L, Schwob Y. From methane to hydrogen, carbon

black and water. Int J Hydrogen Energy 1995;20(3):197202. 39[4] Lynum S. CO2-free hydrogen from hydrocarbonsthe

Kvmer CB & H process. Proceedings of the Fifth Annual 41US Hydrogen Meeting, 1994.

[5] Gaudernack B, Lynum S. Hydrogen from natural gas without 43release of CO2 to the atmosphere. Proceedings of the

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[6] Matovich E. Thagard Technology Company. US Patent No. 474095974, 1978.

[7] Lee KW, Schoeld WR, Scott Lewis D. Mobile reactor 49destroys toxic wastes in space. Chem Eng 1984;2:467.

[8] Dahl JK, Tamburini J, Weimer AW, Lewondowski A, Pitts R, 51Bingham C, Glatzmaier GC. Thermal dissociation of methane

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[9] Donnet JB, Bansal RC, Wang HJ. Carbon blackscience and 55technology, 2nd ed. New York: Marcel Dekker, Inc., 1993.

[10] Litterst T. Investigation of window damage by hot particles 57in solar heated circulating uidized beds. Proceedings

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35969. 61[11] Steinfeld A, Brock M, Meier A, Weidenka A, Wuillemin

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[12] Kogan A, Kogan M. The Tornado ow congurationan 65eective method for screening of a solar reactor window. J

Solar Energy Eng 2002;124:20614. 67[13] Hunt AJ. A new solar thermal receiver utilizing a small

particle heat exchanger. Proceedings of the 14th Intersociety 69Energy Conversion Engineering Conference, Boston, MA,

USA, 1979. 71[14] Yuen WW, Miller FJ, Hunt AJ. Heat transfer characteristics

of a gasparticle mixture under direct radiant heating. 73Lawrence-Berkeley Laboratory, 1985 (LBL-20289 preprint).

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