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SOLAR ENERGY RESEARCH FACILITY

A Kogan, M Kogan, S BarakProf. Emeritus, Dept. of Aerospace Eng., Technion I.I.T.

Visiting Scientist, the Weizmann Institute of Science

The process of co-production of H2 and CB by Solar Thermal Methane Splitting (STMS) is a

profitable alternative to the classical method of H 2 production by Methane steam reforming. When

the CB by-product is not burnt but used as a valuable raw material in the rubber industry, STMS

becomes a non-polluting endothermal process that can be achieved by the use of concentrated

solar energy. The two products of reaction can be easily separated by filtration. The estimatedpotential impact of the introduction of STMS on reduction of CO2 emission and on energy saving

are 13.9 Kg-equivalent CO2 and 277 MJ fossil fuel/Kg H2 produced, as compared to the separate

production of H2 and CB by the prevailing conventional processes [1] .

An intensive STMS program is underway at t he Solar Research Facilities Unit at the WeizmannInstitute of Science since 1999. Three intrinsic problems have been identified in the early stages

of this work.

(a) An effective method had to be found to protect the reactor window from contact with theincandescent CB particles generated in t he reaction chamber.

(b) An efficient way had to be worked out to enable absorption of the concentrated solar energyby Methane, which is a transparent gas.

(c) A way had to be found to prevent deposition of incandescent CB particles on the reactorwall and the formation of Pyrocarbon deposit.

SCREENING THE SOLAR REACTOR WINDOW BY THETORNADO EFFECT [2]

The quartz window through which concentrated solar radiation enters into the reaction chamber

must be protected from contact with solid carbon particles generated by the STMS reaction. These

irradiated particles are heated to incandescence. If allowed to come in touch with the window

surface, they might stick to it leading to window destruction by overheating. The usual method of

screening the window by flooding its surface with a curtain of an auxiliary gas stream requires

very substantial auxiliary gas flowrates and the heat absorbed by the gas represents a major lossof energy. In an effort to reduce the auxiliary gas flowrate to a minimum, a certain flow pattern akin

to the natural tornado phenomenon has recently been developed in our laboratory which enables

effective reactor window screening by an auxiliary gas flowrate less than 5% of the main gas

flowrate. Details of the tornado effect are discussed in [3].

Following is a brief exposition of the physical background and illustrations of this phenomenon

(Figs. 1 and 2).

Fig. 1: Axial cross section of an early reactor

configuration

Fig. 2: Consecutive stages in evolution of tornado flow

pattern in reaction chamber

The axisymmetric chamber of the STMS reactor is provided with a transparent window located at

one end of the chamber, transversally to the longitudinal axis (Fig. 1). A flow of methane is introduced

nto the chamber in a manner whirling around the axis, while the reaction products are withdrawn at

the opposite end of the chamber through a narrow central tube oriented along the longitudinal axis.

The gas flow inside the chamber approximates then a free vortex flow, characterized by a drop of

pressure from the periphery of the chamber to its axis.

An auxiliary flow of protecting gas introduced at the periphery of the window is directed towards

the window central area. It is accelerated by the negative pressure gradient generated by the free

vortex flow. The auxiliary boundary layer flow at the window surface is thereby stabilized and it

remains attached to the surface all the way to the center of the window. There the radially converging

streamlines turn abruptly by 900

in the axial direction, forming a typical tornado-like funnel along the

reactor axis.

Synergy between the free vortex flow of the main gas and the boundary layerflow of the auxiliary gas

s here exploited in order to effectively protect the reactor window. The synergy is expressed by thefact that the auxiliary flow which is desired to form a stable, continuous and non-separated protective

ayer on the window surface is not disturbed by the whirling main stream. Rather it is stabilized by

t. Consequently, the auxiliary flow does not need to be injected with high velocity or with a great

flowrate in order to adhere to the surface to be protected, because it uses the energy of the whirling

main stream against which protection is sought.

The tornado effect has been demonstrated in a series of simulation tests at room temperature with

the reactor model shown in Fig. 1. The main gas stream was flown from an annular plenum chamber

through a narrow annular gap towards the upper part of the reaction chamber. An impeller-like ring

was implanted in the annular gap. The main gas stream acquired an angular momentum during its

passage through slanted grooves in the impeller ring and it entered the reactor cavity in a whirlingmotion. The auxiliary gas stream was flown radially from a second annular plenum chamber through

a second narrow annular gap towards the periphery of the inner surface of t he window. B oth streamsconsisted of nitrogen gas.

The auxiliary stream was made visible by charging it with smoke, while the gas in the main stream

was left in its natural transparent condition. In order to enable visual inspection of a cross section of

the flow inside the reaction chamber, a laser beam directed towards the reactor window was diffracted

by passage through a t ransverse cylindrical glass rod. The monochromatic laser beam emerged from

the glass rod as a planar sheet of light that illuminated a cross section of flow inside the reaction

chamber.

The four tornado configuration tests illustrated in Fig. 2 were performed with an auxiliary smoke-charged

gas maintained at a constant flowrate of 2 L/min. In the absence of a whirling main gas stream (Fig. 2a),

the auxiliary flow separated from the window surface immediately upon its entry into the reaction chamber.

When the whirling main stream was introduced into the reactor cavity at successively higherflowrates

(Fig. 2bd), the auxiliary stream became progressively stabilized as a thin boundary layer. For a main gas

flowrate of 15 L/min, the auxiliary gas moved at high speed in the thin boundary layer near the window

surface. It covered the entire window surface area and it left finally the reaction chamber through a narrowaxially oriented funnel

By further increasing the rate of swirling flow up to 20 L/M a transition point was reached. The flow became

unstable, flipping alternatively into a diffuse flow pattern, devoid of the characteristic axial funnel of the

tornado flow, and back to the regular tornado flow (Fig. 3)Then by raising the flowrate beyond the transition

value the flow became stable, assuming continuously the diffuse flow pattern (Fig. 4)

The reactor model shown in Fig.1 had a maximum diameter of 12.9 cm and it was equipped with a grooved

impeller ring with 18 slanty grooves that guide the Methane stream entering the reaction chamber througha total normal cross section of 1.08 cm

2to swirl around the reactor axis of symmetry. The Ekman number

at the flow transition point was

Methane is a transparent gas. Radiation propagating into the solar reactor is not absorbed directly by

Methane. It heats the reactor wall and part of the heat is transferred to the gas by conduction and convection

(surface heating)

Following a method proposed by Hunt [4], a gas may be heated by concentrated radiation throughout

the volume of the reaction chamber by dispersing small particles in the gas, to form an opaque cloud.

Radiation is absorbed by the particles in suspension, which in turn exchange heat with the surrounding gasvery effectively, in view of the very large surface area per unit mass of particles (volumetric gas heating).

It should be noticed that even in the absence of active seeding, solid carbon particles are generated

near the hot surface of the reaction chamber by the methane splitting reaction. These particles start a

volumetric absorption process that may spread in a chain reaction into the bulk of the reaction chamber.

It was not clear a priori whether this effect is strong enough to render active seeding superfluous.

The results of our early STMS tests (1999/2000) with an unseeded solar reactor proved that this is not

the case. The maximum extent of reaction achieved in that test series was only 28.1%. Methane flowing

through the reaction chamber along streamlines remote from the chamber wall obviously was not heated

enough to undergo dissociation.

In recent STMS tests the reactor was seeded with CB particles. The extent of reaction jumped up f rom

28% to 80%.

The early STMS tests with the unseeded reactors were performed at temperatures of up to 1320K.Most of the carbon generated in the process clung to the irradiated reactor wall and it formed a very hard

deposit. In most cases, the tests were terminated when the reactor exit port became choked by the accrued

Pyrocarbon deposit.

At this early s tage we understood that formation and depositi on of Pyrocarbon on the reactor wall is a basi c

problem that must be treated and solved.

The conditions that promote carbon deposition on the reactor walls were studied during extensive

STMS tests at the WIS Solar tower.

We were guided by this information in the evolution of the shape of the axial cross section of the

reaction chamber. Its profile is smooth and slender and means are provided for cooling certain

critical locations along the inside wall of the reaction chamber. The possibility is also provided for

introduction of auxiliary streams of gas into the reaction chamber tangentially to the chamber wall,so as to promote the flow of the Carbon Black (CB) suspension towards the chamber exit port.

Fig. 5 is an axial cross section

of the reactor protected from

Pyrocarbon deposition, following

the method developed at WIS.

In this design part of the zirconia

insulation underneath the reactor

window is replaced by a hollow

stainless steel water-cooledflange (1); part of the zirconia

structure at the exit end of the

reactor is replaced by a water-

cooled shaped cylinder made of

Copper(2).

A thin metal sheet covers the

wall of the reaction chamber

in the region (3). It is fastened

to the upper end of the copper

piece (2). The temperatures of

the external surfaces of parts (1)

and (2) are kept down by out ofcontact water cooling. Part (3) is

partly cooled by a tertiary stream

of blowing gas and partly by

heat conduction to the Copper

piece.

SEQUENCE OF OPERATIONS WHEN STARTING ASTMS TEST (Fig 5).

1. The two cooling waterflows F(CW), the external cooling air flow F (CA) and the two Nitrogen flows

F(N2) (for boundary layer blowing and for quenching of products) are started.

2. The secondary flow F2(He) is started.

3. The confined tornado flow configuration is established in the reaction chamber by starting the

whirling flow F1(N2).

4. Concentrated solar radiation is admitted to the reactor window.

5. When the reactor wall reaches a local predetermined temperature the whirling flow of MethaneF1(CH4) is started. It enters on the periphery of the reaction chamber at four points disposed

symmetrically around the reactor below the reactor aperture plane, pointing to the hottest region

in the reaction chamber. The directions of these four streams deviate from the radial direction,

so as to generate a whirling Methane stream compatible with the whirling Nitrogen stream

F1(N2).

6. The CB seeding stream F(N2-CB) is started.

7. When a steady state and steady flow is reached, the whirling N2flow F1(N2) may be reduced

appreciably.

e process of co-production of H2 and CB by Sola

INTRODUCTION

TRANSFER OF RADIATION ENERGY TO THE REACTANT GAS

DEVELOPMENT OF MEANS TO COUNTERACT PYROCARBONFORMATION AND DEPOSITION.

Effective solutions were developed to solve the three intrinsic problems encountered

in the STMS system. The project is now mature for up-scaling to an industrial

size module of a demonstration plant. Besides its potential impact on reduction of

CO2 emission and on energy saving, our non-polluting method of co-production

of Hydrogen and Carbon Black by STMS is also expected to be economically

competitive with the conventional method of Natural gas steam reforming for

H2 production and CB production by standard methods. The expected cost of

Hydrogen for large scale solar plants depends on the price of CB: 14 /GJ forlowest CB grade sold at 0.66/kg, and 10/GJ for CB sold at 0.8/kg

Effectvesolut onswered velo edtosolv

CONCLUSION

T(K) (CH4)(m2/sec) /293K F(CH4) max(L/min)

293 0.143 x 10-4 1.0 20

1000 1.369 x 10-4 8.4 168

1500 2.636 x 10-4 16.2 324

1900 3.834 x 10-4 23.5 470

2000 4.159 x 10-4 25.5 510

Fig.4 Smoke flow visualization of a degenerated

tornado flow configuration

5

max

10413.3

==

sw

tr

VDE

Fig. 3 Smoke flow visualization of an unstable tornado

flow

REFERENCES1. Dahl, JK et al (2004), Rapid solar thermal dissociation of natural gas in an aerosol flow reactor, Energy, 29, 715-725.

2. Kogan, A and Kogan, M, US Pat. No. 6,827,082 B1, Dec. 7, (2004).

3. Kogan, A and Kogan M (2002), The tornado flow configuration an effective method for screening of a solar reactor

window, J. Solar Energy Engineering, 124, 206-214.

4. Hunt, AJ, (1979), A new solar thermal receiver utilizing a small particle heat exchanger, Proc. 14th Intersociety EnergyConversion Engineering Conference, Boston, MA, USA.

ACKNOWLEDGEMENTSThe STMS R&D program at WIS was supported steadfastly by the Heineman Foundation for Research, Education, Charitable

and Scientific Purposes and the Rose Family Foundation, Rochester, NY during 1997-2006. In March 2006 WIS became a

participant of the SOLHYCARB Consortium, a specific targeted research project financed by the 6th Framework P rogram

of the European Commission. The author wishes to express his deep gratitude to the Heineman and Rose Foundations and

the European Commission for their generous support of our project.

Fig. 5 Axial cross section of the WIS

10 Kw prototype reactorENVELOPE OF PERFORMANCE OF THE CONFINEDTORNADO FLOW

A NON-POLLUTING SOLAR CHEMICAL PROCESS FOR

CO-PRODUCTION OF H2 AND CARBON BLACK (CB)

BY SOLAR THERMAL METHANE SPLITTING

where is the kinematic viscosity of Methane at 25C and Vsw is the swirling velocity of gas at its entry

into the reaction chamber.

We notice that the kinematic viscosity of gases increases considerably with temperature. By changingfrom room temperature to 2000 K the kinematic viscosity of Methane goes up by a factor of 25 (Table 1)!

This is a fortunate natural circumstance. It enables practical scale up of a Solar Thermal Methane Splitting

(STMS) pilot plant to an industrial plant size.

(b) Tangential main flow - 5 L/M (d) Tangential main flow - 15 L/M Table 1. Temperature dependence of maximum Methane flowrate through Reactor A for which the

tornado effect provides perfect window screening.

(c) Tangential main flow - 10 L/M(a) Tangential main flow - 0 L/M