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7/30/2019 Hydrogen Without CO2 Release
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Production of Hydrogen without CO2 Release by Application of Solar
Thermal Methane Splitting
Abraham Kogan*
Solar Research Facilities Unit, Weizmann Institute of Science,
P.O. Box 26, Rehovot 76100, Israel
Abstract
It is widely accepted that in order to prevent global warming by excessive release of CO2 to the
atmosphere, the energy carrier in the foreseeable future will have to be hydrogen produced by
water splitting by nuclear and/or solar energy. At present, the direct use of hydrogen as a fuel is
insignificant. Its world production is approximately 50106
ton/year, about 95% of the total
being consumed in synthesis of ammonia and of methanol and in petroleum refining. More than
90% of hydrogen is produced from fossil fuels by steam reforming and none by solar energy.
Ironically, the hydrogen production industry is a heavy pollutant, releasing more than 7 ton
CO2/ton H2 to the atmosphere. During the next 30 to 40 years, while natural gas will still be the
dominant primary energy source and the main source of CO2 emissions, tremendous efforts will
have to be made to alleviate the danger of severe ecological changes caused by extravagant
release of CO2. The ultimate solution to the CO2 pollution problem during the transition period,
*Corresponding author. Tel.: +972-8-934-3782; fax: +972-8-934-4117.
E-mail address: [email protected] (A. Kogan).
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until hydrogen produced by alternative energy sources will become the predominant energy
carrier, may be the one suggested by Marketti [1], to steam-reform natural gas using heat
supplied by high temperature nuclear reactors and to separate and dispose CO2 away from the
atmosphere by injecting it into the ground or into the ocean. Such an immense engineering
undertaking will require in itself more than a decade to mature. In this context, solar thermal
methane splitting (STMS) appears to offer a propitious method for hydrogen production in an
ecologically clean manner that could contribute significantly to alleviate the impending CO2
pollution problem in the near future. STMS projects take place currently in a number of
institutions [2, 3, 4, 5]. The main technical obstacle that appears to have blocked fast
implementation of STMS technology is connected with the difficulty to prevent contact of
incandescent powder particles with the reactor window, in order to obviate its destruction by
overheating. This problem has been solved in our laboratory by maintaining inside the solar
reactor cavity a certain flow pattern akin to the natural tornado phenomenon. This flow pattern
enables effective reactor window screening by an auxiliary gas flowrate less than 5% of the main
gas flowrate [5]. The present paper discusses the progress of our STMS project, which is now
entering its pilot plant stage.
Keywords: thermal methane splitting, hydrogen production.
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1. Introduction
Solar thermal methane splitting (STMS) appears to be a very attractive process for
production of hydrogen by the use of solar energy. The reaction
2)s(4 H2CCH +
can be accomplished at atmospheric pressure in the temperature range 1500-2200 K, the
products of reaction form a suspension of carbon particles in hydrogen gas, a mixture that can be
separated relatively easily and when the carbon co-product is not burnt, the process is free of
CO2 emission. The economic loss due to abstention from burning the generated carbon is
compensated by the price of granular carbon, a widely used raw material in the rubber industry
[6]. The ecological benefit resulting from the replacement of the currently used processes of
hydrogen and carbon black production by STMS has been estimated to consist of an overall
avoided fossil fuel consumption and CO2-equivalent emissions of -277 MJ and -13.9 Kg-
equivalent CO2/KgH2 produced [7].
Certain inherent problems are encountered in the development of STMS which must be
solved in order to make this process technically feasible and competitive. These will be
discussed in the following.
2. Protection of the reactor window from contact with solid particles
2.1. Two alternative approaches
The presence of solid carbon particles in suspension in the reaction chamber causes much
difficulty in the design of the STMS reactor. When concentrated solar radiation is admitted
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directly into the reaction chamber through a transparent window, the irradiated powder particles
become incandescent. If allowed to come into contact with the transparent window, they lead to
its instantaneous destruction by overheating. In the design of the solar reactor, we are faced with
the tough problem how to prevent the solid particles in suspension from contacting the window
surface.
Some researchers tried to bypass this difficulty by developing a solar reactor in which
methane is heated by concentrated sunlight indirectly. Dahl et al. [7], by way of example, use a
reactor which consists of an outer quartz protection tube and an inner graphite tube. Argon flows
in the annular region between the two tubes and provides an inert atmosphere that prevents
oxidation of the graphite tube while preventing any decomposition products from depositing on
the inner quartz wall. The reactor is placed at the focal point of the solar concentrator.
This approach has been applied successfully for the performance of small proof-of-concept
experiments of methane splitting, in which reactions were carried out in the temperature range of
1533 < T < 2144 K with residence times between 0.1 and 1.5 seconds. It is doubtful, however,
whether it will be possible to operate an industrial size STMS plant by the indirect heating
method in this temperature range with an economically meaningful efficiency.
In view of the potential technical and economic advantages of a volumetric solar reactor, in
which the concentrated radiation is admitted directly into the reaction chamber through a quartz
window, we decided to try and develop an effective method for screening a solar reactor
window.
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Fig. 1. Reactor model M1. Flow visualization test
2.2. Some exploratory work on window screening by a gas curtain
A preliminary reactor model M1 was tested at the Solar Research Facilities Unit of the
Weizmann Institute of Science. It consists of a cylindrical Pyrex vessel divided by an annular
partition into a reaction zone and a narrow buffer zone near the top of the vessel (Fig. 1). A main
stream of gas flows into the reaction zone, while an auxiliary gas stream is directed into the
buffer zone. The auxiliary stream discharges from the buffer zone through the annular partition
into the reaction zone, where it mixes with the main stream. The gas mixture leaves the reactor
through a port at the bottom of the reactor. The gas both in the main and auxiliary stream was
nitrogen.
A series of flow visualization tests was performed with reactor M1. One of the two gas
streams was made visible by charging it with a heavy smoke, while the other gas stream was left
in its natural transparent condition.
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In all the flow visualization tests performed with reactor M1, the main gas stream entered
into the cylindrical reactor zone tangentially, generating in it a whirling gas flow. In some of the
tests, the auxiliary stream was introduced into the buffer zone also tangentially, in order to
induce a whirling motion concurrent with the motion of the main stream and to facilitate their
smooth merging. In other tests, the auxiliary stream entered the buffer zone radially.
These tests demonstrated that it is possible to completely eliminate the main gas from the
buffer zone by using an auxiliary gas stream, flowing at a rate of about 30% of the main gas flow
rate.
min = 0.3
It was also found that by moving the annular partition towards the top of the pyrex vessel,
which simulates the reactor window, min decreased slightly.
Surprisingly, the introduction of the auxiliary stream radially into the buffer zone gave
somewhat better results than those obtained in tests with a whirling auxiliary flow. This
unexpected result became clear only during our more recent work, as it will be explained below.
2.3. Design of Reactor Model M2
Following the preliminary flow visualization tests performed with reactor model M1, we
proceeded with the design of reactor model M2, which was intended to address simultaneously a
number of separate technical issues and to solve them in a coherent way:
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The piping for admission of the main stream (methane) and of the auxiliary gas streamshould be designed in a compact way, in order to avoid obstacles in the path of
propagation of the concentrated radiation that fans out from the Compound Parabolic
Concentrator (CPC), through the quartz window, into the reactor chamber.
For the same reason, the quartz window must be installed in the immediate vicinity of theCPC. It should be clamped and sealed between the CPC structure and the reactor metal
casing.
The quartz window should be cooled in order to remove heat of absorption. The main gas stream should be introduced into the reaction chamber preferably in a
direction away from the window, in order to cooperate with the auxiliary stream in the
task of preventing direct contact between solid particles and the window.
The radiation emerging out of the CPC diverges through a very wide angle. This dictatesa thin, poor insulation ceiling to the reactor. Introducing the main (methane) flow
through the ceiling should help in recycling heat from the ceiling back to the reaction
chamber.
These goals were met by replacing the discrete entry tubes, which were used in reactor M1
for introduction of the main and auxiliary streams into the reactor, by two narrow annular
passages (Fig. 2).
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Fig. 2. Reactor model M2b-CPC assembly
The annular passage defined by spacers placed between the top flange and a second metal
flange enables the introduction of the auxiliary stream into the reactor cavity in the form of a
planar thin film of gas adjacent to the inner surface of the window. This stream flows radially
from the periphery to the axis of symmetry.
A second annular passage, defined by spacers between the second metal flange and the top-
end ceramic annular structure, serves for the delivery of the main gas stream into the reactor
cavity in the shape of a hollow conic film, which flows in directions away from the window.
With this design we achieve an unobstructed passage for concentrated solar radiation from
the CPC through the quartz window into the reactor cavity. The annular partition used in
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reactor M1 for the definition of a buffer zone does not appear in the reactor M2 design. The two
gas streams are delivered to the reactor cavity in the form of continuous thin films.
Moreover, in the reactor M2 design the quartz window and the metal flanges are cooled
effectively by the two high-velocity gas streams.
To enable generation of a whirling main gas stream inside the reactor cavity, an impeller-like
disk (Fig. 3) was inserted into a circular groove cut in the top-end ceramic structure. The main
gas stream flowing through the annular passage into the reactor cavity is deflected by the slant
channels of the impeller-like disk, thus acquiring an angular momentum.
Fig. 3. Impeller-like disk
2.4. Flow Visualization Test Series with Reactor Model M2: The Tornado Effect
Different versions of reactor M2 were built around a common cylindrical metal casing. The
only practical way to scrutinize the flow inside the reactor cavity appeared to be by charging one
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of the two gas streams with smoke, by illuminating the cavity and by observing it through the
quartz window.
Figure 4 is a cross section of reactor model M2a, with the CPC replaced by a flange for ease
of visual observation.
Fig. 4. Reactor model M2a
Flow patterns inside reactor M2a, obtained by introducing radially a smoke-charged auxiliary
gas stream into the reactor cavity at a rate of 2 L/M are illustrated in Fig. 5. No main gas stream
was admitted to the reactor in the test shown in Fig. 5a. The impression one gets from
observation of this picture is that the auxiliary flow progressed for some distance along the
window surface without separation, but it definitely detached itself from the solid surface of the
window before reaching the reactor axis of symmetry. During the test shown in Fig. 5b, a main
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gas stream of 10 L/M was also introduced radially into the reactor cavity. In this case the
annular area of the window, which appears to be swept by the auxiliary stream, is smaller than
the corresponding area in Fig. 5a, since the auxiliary stream is sucked away from the window by
the main gas stream.
Fig. 5. M2b flow pattern. Smoke-charged radial auxiliary flow 2 L/M
While it is possible to get from these pictures some general impression about the flow
characteristics, there is no way to determine from them what is the thickness distribution of the
auxiliary flow layer, nor can we ascertain from them whether the main gas stream achieved
direct contact with the window.
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The following optical arrangement was used in order to enable visual inspection of a cross
section of the flow inside the reactor (Fig. 6). A laser beam directed towards the reactor window
is diffracted by passage through a transverse cylindrical glass rod. The laser radiation emerges
from the glass cylinder as a planar sheet of light that illuminates a cross section of flow inside the
reactor cavity.
Fig. 6. Planar laser beam optical configuration
Figure 7 is a visualization of the flow patterns of Fig. 5, obtained by the method of laser
cross-section illumination. Many flow details that could not be discerned in Fig. 5 are visible in
Fig. 7. We found out soon that this simple illumination technique can be a powerful tool in the
investigation of complex flow configurations.
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Fig. 7. Flow pattern of Fig. 5 visualized by laser cross-section illumination. Smoke-
charged radial auxiliary flow 2 L/M
The flow configuration visualized in Fig. 8 was also generated by introducing a main stream
of 10 L/M and an auxiliary smoke-charged stream of 2 L/M into the reaction chamber. The
auxiliary stream entered the chamber radially, as in the case of Fig. 5. The main stream,
however, was introduced into the chamber tangentially. The gas exits the reaction chamber
through a port at the end of the chamber opposite the window, centered on the reactor axis of
symmetry, as in the case depicted in Fig. 5.
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Fig. 8. M2b flow pattern. Smoke-charged radial auxiliary flow 2 L/M. Tangential main
flow 10 L/M
By introducing the main gas stream into the reaction chamber tangentially, instead of
radially, a very spectacular change was obtained in the fluid flow pattern inside the chamber. In
the case of radial admission of the main flow into the chamber, the auxiliary flow separates from
the window surface immediately upon its entry into the chamber, as shown clearly in Fig. 7b.
On the other hand, when the main gas is flown into the chamber tangentially, the auxiliary flow
adheres to the window surface as a thin boundary layer all the way to the window center, where
it changes abruptly its flow direction by 90, coalescing with all the other radial streamlines, to
form a common axial exit jet that reminds one of a tornado funnel (Fig. 8).
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Fig. 9. Consecutive stages in the evolution of a tornado flow pattern in a reaction
chamber, as described in the present invention. Smoke-charged radial auxiliary flow 2
L/M
Fig. 9 illustrates the evolution of this Tornado flow pattern inside the reaction chamber, when
the auxiliary gas stream is introduced into the reaction chamber at a constant flowrate of 2 L/M,
while the main gas flowrate is increased progressively from zero to 15 L/M. In the absence of a
whirling main gas stream (Fig. 9a), 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 higher flowrates (Figs. 9 b-d), 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.
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It covered the entire window surface area and it left finally the reaction chamber through a
narrow axially oriented funnel.
Synergy between the whirling flow of the main gas and the boundary layer flow of the
auxiliary gas is here exploited in order to protect effectively the reactor window. The synergy is
expressed by the fact that the auxiliary flow, which is desired to form a stable, continuous and
non-separated protective layer on the window surface, is not disturbed by the whirling main
stream. It is rather stabilized by it. 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.
We shall refer to the flow configuration illustrated in Figs. 8 and 9 as the Tornado flow
configuration or the Tornado windscreen.
Many tests have been performed with five reactor models that encompass a variety of
boundary conditions [5]. The three conditions required for the generation of a Tornado
windscreen have been demonstrated by experiment:
The main gas stream must be introduced tangentially into the reaction chamber. The products of reaction should be extracted from the reaction chamber in the axial
direction, preferably through a narrow exit port along the axis of symmetry of the reactor.
The auxiliary gas stream should be introduced essentially radially into the reactionchamber and adjacent to the window.
A patent has been issued recently for a reaction chamber with a protected surface, which is
based on the application of the Tornado flow configuration described above [8].
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2.5. Conditions for prevention of boundary layer separation A review of some basic
concepts in Fluid Dynamics
The very different flow patterns observed in Fig. 7b and Fig. 8 can best be explained by
referring to some basic concepts of Fluid Dynamics. Around 1750 dAlembert calculated the
flow of a perfect fluid around a circular cylinder with the assumption that at the surface of the
cylinder the fluid flows tangentially to the surface (Fig. 10). The calculated pressure distribution
on the cylinder in this solution is symmetric with respect to the axial plane normal to the
direction of undisturbed flow. This leads to the conclusion that the flow does not exert any drag
on the cylinder, a conclusion which contradicts our common experience.
Fig. 10. Ideal flow of a perfect fluid around a circular cylinder
It took more than 150 years until Prandtl proposed his Boundary Layer theory [9], by which
the dAlembert Paradox can be explained. Prandtl postulates that a real fluid in contact with a
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stationary solid surface is not moving parallel to it, it is just at rest (the no-slip assumption). For
fluids of low viscosity, the velocity approaches its main stream value at a very small distance
from the solid-fluid interface. Within this thin boundary layer friction plays a prominent role.
Friction with the stationary solid boundary tends to retard the flow of fluid within the boundary
layer while friction with the fluid outside the boundary layer tends to accelerate it by pulling it in
the opposite direction.
Fig. 11. Flow of a real low-viscosity fluid past a circular cylinder
Figure 11 illustrates the flow of a real low-viscosity fluid past a circular cylinder [10]. The
streamlines on the upstream side of the cylinder resemble those calculated by dAlembert for the
perfect fluid case. Point A0 is the upstream stagnation point, where the pressure reaches its
maximum value. Along streamline segment A0C0 the pressure decreases and the fluid velocity
outside the boundary layer is accelerated. The outside flow entrains apparently the fluid inside
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the boundary layer by friction strongly enough to compensate for its loss of momentum by
friction with the solid body. At point C0, however, the outside flow starts to decelerate, due to
the switch of pressure gradient from negative to positive. The flow outside the boundary layer
loses at this point its ability to compensate for the loss of momentum of the fluid within the
boundary layer. The boundary layer thickens and the streamlines outside the boundary layer do
not follow the contour of the cylinder anymore. The pressure in the region of detached flow is
close to the low pressure at the point of detachment and a net drag force is exerted by the fluid on
the cylinder.
The flow around a circular cylinder represents a typical illustration of the important boundary
layer separation phenomenon. Boundary layer separation is not confined to flow around blunt
bodies. It occurs also in internal flows, such as in diffusers, in which the flow is decelerated in a
diverging tube. Separation of a boundary layer at a plane or curved solid boundary occurs
whenever the fluid velocity outside the boundary layer decreases in the flow direction to a
sufficient extent. Batchelor [11] summarizes that in practical terms, a steady boundary layer
usually separates after very little retardation of the external stream. We may add that an
effective way to prevent boundary layer separation from a solid wall is to ensure that the
pressure in the fluid along the wall decreases in the direction of flow outside the boundary layer.
2.6. Explanation of the physical background of the Tornado windscreenNotice that when the three requirements for the generation of a Tornado flow configuration
as enunciated in paragraph 2.4 are fulfilled, the flow in the reaction chamber outside the
boundary layer approximates a free vortex flow. Neglecting compressibility effects, the
tangential velocity u and the pressurep in a free vortex flow are given by
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u = c1/r (1)
and
p0 p = c2/r2
(2)
Thus the whirling motion of the main gas stream produces a radial pressure variation inside
the reaction chamber, the pressure being highest at the periphery of the reaction chamber and
lowest at the reactor axis of symmetry, and the pressure decreasing steeply towards the axis of
symmetry. This pressure distribution is ideal for prevention of flow separation from the reactor
window surface, as explained in paragraph 2.5.
2.7. Some preliminary results on the effectiveness of the Tornado windscreen for protection ofa reactor window exposed to concentrated solar radiation
2.7.1. Protection of the window of a Solar Particle Receiver [12]
R. Bartocchi, a PhD student at the Solar Research Facilities Unit, WIS, encountered
difficulties in the protection of the quartz window of a Solar Particle Receiver from destruction
caused by contact with dispersed carbon black particles. After unsuccessful attempts to protect
the window by a simple aerodynamic curtain [13], he succeeded to solve the problem by
application of the Tornado windscreen method described above. He summarized in his PhD
thesis [14]:
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The test setup did not enable photographing the resulting flow pattern. However, it was
found that the principle of Kogan flow was superior to a simple aerodynamic curtain
Although highly effective, the Kogan flow did not succeed in keeping the window
entirely particle free.
Bertocchi was able to run inert gas solar heating tests that lasted for up to 30 minutes,
reaching a maximum air temperature of 1987 K. At the end of test, the window of this Particle
Receiver was slightly stained by powder, yet structurally undamaged.
2.7.2. Protection of the window of an unseeded STMS reactor
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 [15] and Yuen et al. [16], 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 gas very 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.
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The primary aim of the experimental work described below was to determine under realistic
STMS conditions to what extent the tornado flow configuration offers a reliable method for
protection of the reactor window from contact with hot powder particles. Another aim was to
determine the extent of reaction that can be attained in an STMS reactor without recourse to
active powder seeding.
The results of this experimental program were presented by the description of four
representative tests [17].
One of the tests endured for 37 min. By the end of the test, the temperature of reaction
products was 1320 K and the extent of reaction reached 27.3%. The reactor window, as
observed on the TV screen, remained clear throughout the test duration. Such performance
repeated itself in additional tests, in which the auxiliary gas was helium, demonstrating the
potential of the tornado effect to protect the reactor window from contact with powder particles
generated by methane dissociation.
The test duration of 37 min was not surpassed by anyone test in the present series. In most
cases, the crisis that led to test termination was provoked by plugging of the reactor exit port by
carbon deposition. This behavior appears to be intrinsically connected with the mode of gas
heating in a surface receiver-type reactor. The endothermic reaction in a surface receiver is
initiated in a narrow thermal boundary layer along the irradiated walls of the reaction chamber.
Our tests demonstrated the tendency of the carbon particles generated within this thermal
boundary layer to cling to the adjacent irradiated solid surface, forming a very hard carbon
deposit, which interferes eventually with the outflow of reaction products from the reaction
chamber.
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The outcome of our tests also dispelled the assumption that carbon particles generated within
the thermal boundary layer near the irradiated wall might start a volumetric radiation absorption
process, which could spread throughout the reaction chamber. The maximum extent of reaction
achieved in the present test series was only 28.1%. Methane flowing through the reaction
chamber along streamlines remote from the chamber wall did not get obviously heated enough to
undergo dissociation.
The method of volumetric gas heating by seeding the reaction chamber with radiation
absorbing particles is expected to offer an effective solution to the problems discussed. It will
certainly make possible to attain a much higher extent of reaction than that obtained so far. It
will also alleviate the problem of formation of carbon deposits on the reaction chamber walls,
since in a volumetric receiver the generation of carbon particles by methane splitting will take
place mainly in the very hot central region of the chamber. The walls of the chamber will be
shaded by the cloud of particles from direct solar irradiation and their temperature will remain
moderate. Hard carbon deposits will not build up on the walls under such conditions. To
counteract any tendency of formation of a soft carbon deposit, it will be possible to flush the
walls by a flowing gas film. The next phase of our research will proceed along these lines.
3. Simulation tests of a seeded reactor operating at room temperature in the Tornado flow
configuration
In view of the above mentioned results of the STMS tests with an unseeded reactor [17], we
decided to study the problems inherent to a powder seeded volumetric solar reactor.
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3.1. A peculiar property of the tornado flow pattern
Some exploratory tornado flow configuration tests with powder seeding were performed at
room temperature in order to gain experimental information about the paths followed by solid
particles entrained by the gas flow. During these tests neither one of the two gas streams
entering the reaction chamber were stained by smoke, but the main gas stream was charged with
a small amount of carbon black (CB) powder before its entry into the reactor.
Fig. 12 is a picture of the reactor window taken at the end of one such test. The inner surface
of the window was wetted before the test by a thin layer of lubricating oil.
Fig. 12. Reactor window stained by powder deposition after a 10-min CB seeding Tornado
flow test. The window surface was wetted by lubricating oil before start of test
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The following mechanism is suggested as a plausible explanation of the observed powder
flow behavior. The ascending gas current along the reactor cavity wall is part of a weak flow
perturbation generated by friction between the energetic gas stream in the tornado funnel and the
gas surrounding it. The fast axial flow of gas in the tornado funnel entrains by friction the
relatively quiescent gas around it. Fig. 13 is a qualitative picture of the streamlines in the
perturbed quiescent region. These are helical streamlines wrapped around torroidal surfaces.
This flow perturbation is, of course, a very weak effect that could be neglected, were it not for
the presence of the very fine powder particles that may be entrained by the torroidal flow
towards the reactor window. At close proximity to the entraining jet, solid particles suspended in
the perturbed gas are expected to move downward, concurrent with the entraining funnel jet.
Upon approaching the reactor cavity bottom, they diverge towards the periphery, turning in
sequence upwards towards the reactor window, then inwards along the window.
3.2. A trial to suppress the torroidal perturbation flow by the braking action of a
countercurrent flow: Reactor D3
The negative influence of the torroidal flow perturbation on the performance of the tornado
flow configuration could be eliminated by the introduction of a weak flow of gas along the
reactor cavity wall in countercurrent direction to the perturbation flow.
Reactor D3 (Figs. 14 and 15) was designed to enable the introduction of such flows
tangentially to the wall of the reactor cavity, at appropriate locations.
Referring to Figs. 14 and 15, the main reactor body is composed of three segments, a, b and
c, made of Darlene. By assembling these segments, two annular plenum chambers, d and e, and
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two corresponding narrow annular passages, f and g, are defined. A tertiary gas stream, F3,1, can
be introduced into the reactor cavity through chamber d and gap f. Stream F3,1 is discharged
from gap f smoothly and tangentially to the cavity wall along a circle situated not far from
window w. Another tertiary gas stream, F3,2, may be discharged similarly into the reactor cavity
through chamber e and gap g, along a circle more distant from window w.
Fig. 14. Components of reactor D3
Flows F1 and F2 are introduced into the D3 reaction chamber in the way they were introduced
into reactors D1 and D2. The seeding gas stream Fs can be injected into the D3 reaction chamber
optionally through either one of the ducts d1, d2 or d3.
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Fig. 15. Axial cross section of reactor D3
The main geometric dimensions of reactor D3 are:
Overall cavity length 26.40 cm Maximum cavity diameter 13.30 cm Window aperture diameter 6.20 cm Exit section diameter 1.20 cm Total cross-section area of the 18 slanted grooves in the
impeller ring
0.54 cm2
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Thirteen room temperature seeding simulation tests were performed with reactor D3. The gas
in flows F1, Fs, F3,1, and F3,2 was nitrogen, while F2 was a flow of helium. The technical details
of these tests and the obtained results were reported in Reference [18].
It was determined by experiment that for the specific shape of reactor D3, operating under the
particular conditions of the described room temperature simulation tests, the tornado effect can
provide perfect screening of the reactor window, when the seeding stream Fs is injected into the
reaction chamber through a radial duct situated at a distance from the window not less than 22%
of the chamber length. The simultaneous injection of tertiary streams F3,1 and F3,2 at a flowrate
of 1 L/min is sufficient to guarantee perfect protection of the window from contact with
entrained powder particles.
The streamlined design of reactor D3 solved also the problem of powder deposition on the
walls of the reaction chamber. During the entire powder-seeding test program performed with
reactor D3, the reaction chamber walls remained totally free of powder deposition.
Conclusions
The whirling flow system described above appears to be an ideal solution for the window-
screening job. It is a very efficient method, requiring an auxiliary gas flow rate of less than 5%
of the main gas flow rate. Its effectiveness is not limited to small size windows. The auxiliary
gas sweeps the window surface as a thin and very fast film that covers the surface completely. It
also cools effectively the window material, due to the high heat transfer coefficient of the thin
boundary layer flow. On the other hand, since the auxiliary stream does not detach from the
window surface and does not mix appreciably with the main stream, it does not remove much
process heat from the reacting gas.
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A perturbation of the tornado flow configuration caused by friction between the swift gas jet
leaving the reaction chamber along the tornado funnel and the surrounding quiescent gas was
identified by observing the path lines of solid particles suspended in the gas inside a solar reactor
model. This torroidal flow perturbation has a detrimental influence on the capability of the
tornado flow pattern to protect the reactor window from contact with powder particles.
The main aim of the present study was to find a practical way to suppress the negative
influence of the torroidal flow perturbation by injection of weak countercurrent flows into a
specially designed reactor model. The design of reactor D3 yielded a satisfactory solution.
It was determined by experiment that for the specific shape of reactor D3, operating under the
particular conditions of the described room temperature simulation tests, the tornado effect can
provide perfect screening of the reactor window, when the seeding stream Fs is injected into the
reaction chamber through a radial duct situated at a distance from the window not less than 22%
of the chamber length. The simultaneous injection of tertiary streams F3,1 and F3,2 at a flowrate
of 1 L/min is sufficient to guarantee perfect protection of the window from contact with
entrained powder particles.
Much experimental and computational work remains still to be done in order to obtain an
envelope of parameters (reactor size and shape, flowrates, temperature, viscosity, etc.) within
which the Tornado flow configuration offers perfect window protection from contact with
entrained particles.
The streamlined design of reactor D3 solved also the problem of powder deposition on the
walls of the reaction chamber. During the entire powder-seeding test program performed with
reactor D3, the reaction chamber walls remained totally free of powder deposition.
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Acknowledgements
This study was supported by the Heineman Foundation for Research, Education, Charitable
and Scientific Purposes and the Rose Family Foundation, NY, USA. The authors gratefully
acknowledge the generous support of these Foundations.
Nomenclature
c1, c2 Constants
CB Carbon Black
CPC Compound Parabolic Concentrator
F1 Main gas flowrate (SLM)
F2 Secondary (or auxiliary) gas flowrate (SLM)
F3,1, F3,2 Tertiary gas flowrates (SLM)
Fs Seeding gas flowrate (SLM)
p Pressure (bar)
p0 Stagnation pressure (bar)
r radial distance from reactor axis of symmetry (m)
STMS Solar Thermal Methane Splitting
T temperature (K)
u velocity (m/sec)
F2/F1
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