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Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor
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International Journal of Hydrogen Energy 28 (2003) 1187 – 1198 www.elsevier.com/locate/ijhydene
Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor Meir Kogan, Abraham Kogan∗ Solar Research Facilities Unit, Weizmann Institute of Science, Room 208 240 Herzl St., PO Box 26, Rehovot 76100, Israel Accepted 28 October 2002
Abstract Solar thermal methane splitting was performed in a series of tests with an unseeded low capacity reactor. E/ective screening of the reactor window from contact with carbon particles was achieved by application of the tornado 0ow con1guration (J Solar Energy Eng 124 (2002) 206). The tests were performed at atmospheric pressure and at temperatures up to 1320 K. An 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 accrued carbon deposit. The results of the tests are discussed and ways to correct the problems encountered with the unseeded reactor are proposed. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal methane splitting; Hydrogen production
1. Introduction Hydrogen is a clean fuel in the sense that no CO2 is generated when it is burned. At present, the amount of hydrogen that serves as a fuel is negligible. It is used almost exclusively as a raw material in the manufacture of ammonia, methanol and petrochemicals. The predominant method of hydrogen production is by methane steam reforming, which involves massive coproduction of CO2 , about 7 t CO2 =t H2 . At the present level of world hydrogen consumption, which approaches 108 t=yr, the production of hydrogen is an important contributor to the anthropogenic release of CO2 to the atmosphere. Therefore, there is a strong incentive to develop methods of hydrogen production in which the emission of CO2 is reduced or, preferably, completely eliminated. The future role of hydrogen as an alternative to fossil fuels depends, inter alia, on the successful development of clean hydrogen production methods. ∗ Corresponding author. Tel.: +972-8-934-3782; fax: +972-8934-4117. E-mail address: [email protected] (A. Kogan).
Steinberg and Cheng [1] made an extensive study of modern and prospective technologies for hydrogen production from fossil fuels. A comparison of the speci1c cost of hydrogen obtained by the analyzed technologies shows that thermal methane decomposition has a signi1cant economic potential. The possibilities of decomposing hydrocarbons directly into carbon and hydrogen were also discussed by Sandstede [2] and Fulcheri and Schwob [3]. The development by a Norwegian company of a plasma arc process for splitting of natural gas was described by Lynum [4]. The economic aspects of this process were discussed by Gaudernack and Lynum [5]. They concluded that the cost of hydrogen would depend on the prices of the input commodities, natural gas and electricity, and on the revenues from the coproduct, carbon black. With conservatively estimated carbon black prices and with the very low price of electricity in Norway, the calculated cost of hydrogen came out competitive with hydrogen produced by hydrocarbon steam reforming. In many parts of the world, electricity is much more expensive than in Norway, but abundant solar energy, which can be utilized for hydrocarbon splitting, is available. There
0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(02)00282-3
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is therefore an incentive to develop a process of solar thermal 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 relatively moderate temperature. Fig. 1 illustrates some results of thermochemical equilibrium calculations of the (C, 2H2 ) system, obtained by the application of the NASA CET-85 computer program. At ambient pressure, the mol fraction of unreacted methane is less than 0.001 at 1800 K. Kinetic data on thermal splitting of methane were obtained experimentally by Matovich [6] and Lee et al. [7]. The reaction residence times reported in [6,7] fall in the time interval 0:2 s 6 tr 6 1 s. Above 1900 K, the extent of reaction was not much in0uenced by the residence time. Complete dissociation was achieved above 2100 K, a temperature that can be e/ectively attained in a solar reactor. Solar thermal methane splitting proof-of-concept experiments were conducted recently at NREL, in collaboration with the University of Colorado Department of Chemical Engineering [8]. A stream of dilute 5% CH4 in argon feed gas surrounded by a pure argon purge mixture was 0own through a reactor consisting of a 25 mm diameter quartz reactor tube. The reactor was illuminated with a solar 0ux of 2400 kW=m2 . The very low concentration of CH4 in argon was maintained in order to keep the concentration of CH4 and of H2 outside 0ammability limits. Approximately 90% dissociation of methane was reported. 2.2. Separation of reaction products Separation of the hydrogen and carbon product mixture does not present a diKcult technical problem, since the mixture components appear in di/erent phases. Technical details are reported in [9]. 2.3. Screening of a solar reactor window A more complicated technical problem is connected with the need to protect the reactor window 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 0ooding its surface with a “curtain” of an auxiliary gas stream requires very substantial auxiliary gas 0owrates and the heat absorbed by the gas represents a major loss of energy [10,11]. In an e/ort to reduce the auxiliary gas 0owrate to a minimum, a certain 0ow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory,
Fig. 1. Thermal splitting of methane—XCH4 vs. p, T .
which enables e/ective reactor window screening by an auxiliary gas 0owrate less than 5% of the main gas 0owrate. Details of the tornado e/ect are discussed elsewhere [12]. Here, we shall limit ourselves to a brief exposition of the physical background and to an illustration of this phenomenon. When a 0uid 0ows along a solid stationary boundary, its motion is retarded in a thin boundary layer by friction. The retarded 0uid boundary layer may thicken progressively in the direction of 0ow and ultimately it may detach from the solid boundary and mix with the main 0ow. Boundary layer detachment can be averted if care is taken to maintain a uniformly decreasing pressure in the direction of 0ow. The accelerated main 0ow entrains then the 0uid in the boundary layer strongly enough to counteract the 0ow retardation caused by friction with the stationary boundary. Turning to our particular application, 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. A 0ow of methane is introduced into 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 0ow inside the chamber approximates then a free vortex 0ow, characterized by a drop of pressure from the periphery of the chamber to its axis. An auxiliary 0ow 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 0ow. The auxiliary boundary
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Fig. 2. Cross section of reactor M2a.
layer 0ow at the window surface is stabilized thereby and it remains attached to the surface all the way to the center of the window. There, the radially converging streamlines turn abruptly by 90◦ into the axial direction, forming a typical tornado-like funnel along the reactor axis. Synergy between the free vortex 0ow of the main gas and the boundary layer 0ow of the auxiliary gas is here exploited in order to protect e/ectively the reactor window. The synergy is expressed by the fact that the auxiliary 0ow, 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 0ow does not need to be injected with high velocity or with a great 0owrate 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 e/ect has been demonstrated in a series of simulation tests at room temperature with the reactor model shown in Fig. 2 [12]. The main gas stream was 0own 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 whirling motion. The auxiliary gas stream was 0own radially from a second annular plenum chamber through a second narrow annular gap towards the periphery of the inner surface of the window. Both streams consisted 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 0ow inside the reaction chamber, a laser beam directed towards the reactor window was di/racted by passage through a transverse cylindrical glass rod. The monochromatic laser beam emerged from the glass rod as a planar sheet of light that illuminated a cross section of 0ow inside the reaction chamber. The four tornado con1guration tests illustrated in Fig. 3 were performed with an auxiliary smoke-charged gas maintained at a constant 0owrate of 2 l=min. In the absence of a whirling main gas stream (Fig. 3a), the auxiliary 0ow 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 0owrates (Fig. 3a–c), the auxiliary stream became progressively stabilized as a thin boundary layer. For a main gas 0owrate 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 1nally the reaction chamber through a narrow axially oriented funnel. 2.4. Transfer of radiation energy to reactant gas 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).
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Fig. 3. Four tornado 0ow con1guration tests with successively increasing main gas 0owrates. Smoke-charged secondary gas 0owrate was 2 l=min.
Following a method proposed by Hunt [13] and Yuen et al. [14], 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 e/ectively, 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 e/ect is strong enough to render active seeding super0uous.
3. Experimental The primary aim of the experimental work described below was to determine under realistic STMS conditions to what extent the tornado 0ow con1guration o/ers 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. 3.1. Experimental setup A diagram of the experimental setup designed for a preliminary study of STMS is shown in Fig. 4. It consists of a solar reactor, an optical concentration system, the piping requisite for the establishment and maintenance of a tornado 0ow con1guration inside the reactor chamber, instrumentation and auxiliary equipment. Two reactor con1gurations, designated M3-a and M4-f, were used during the tests described below. Their geometries are illustrated in Figs. 2 and 11, respectively. The optical concentration system consists of a heliostat followed by a secondary paraboloid concentrator and a tertiary compound paraboloid concentrator (CPC). The performance of the optical system was determined during preliminary calibration tests. The 0ux of radiation leaving the CPC exit section was measured by replacing the reactor
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Fig. 4. Diagram of STMS test setup.
by a water-cooled calorimeter. The radiation 0ux reaching the outer surface of the reactor window was 2064 W, when normalized for a solar irradiation of 850 W=m2 . This corresponds to a concentration factor of 781. The value of solar irradiation was supplied by the control room of the Solar Research Facilities Unit at the Weizmann Institute of Science.
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The reactor pressure in the tests described below was planned to be slightly above atmospheric pressure (1:010 atm 6 pr 6 1:105 atm), in order to evade the need for pumping. This pressure was monitored by a colored water U-tube manometer. An STMS test is started by 0owing methane and auxiliary gas through 0owmeters to the reaction chamber at predetermined rates F1 and F2 , respectively. When the predetermined gas 0owrates are reached, a shutter plate is raised and concentrated radiation is admitted to the reactor window. Hot products of reaction leaving the reaction zone are quenched by water sprays. The mixture of reaction products and quenching water enters a gravity separator. The quenching water 0owrate and temperature increase are determined by a rotameter and a T-type thermocouple, respectively. The carbon black slurry is discharged from the separator to the atmosphere through a reinforced plastic U-tube, while the gas is 0own through an upper exit pipe to the roof of the tower. The gas composition is determined by a mass chromatograph. A B-type thermocouple was installed to measure the temperature of reaction products at the exit port of the reaction chamber. Unfortunately, the temperatures registered by this thermocouple did not represent correctly the sought gas temperature, for reasons explained below. The temperature approached by the reaction products at the end of each test was therefore evaluated indirectly from an energy balance over the quenching process of the product gas mixture, as follows: f(T ) = {(1 − )[hS0CH4 (T ) − hS0CH4 (Tf )] + 2 [hS0H2 (T ) − hS0H2 (Tf )]}F1 + {hS0Aux (T ) − hS0Aux (Tf )}F2 − mqw cqw (Tf − Ti ) = 0:
Fig. 5. Quartz window and two chunks of carbon found near reactor exit port after test no. 1.
(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 were determined experimentally during the test. Thus, it was possible to evaluate f(T ) and to plot it as a curve in the f–T plane. The intersection of the curve with the T -axis yielded the gas temperature T before quenching. An alarm device was installed in the test rig. It is actuated when either one of the following events occurs: (1) The temperature of a certain thermocouple exceeds a predetermined value. (2) The reactor pressure reaches 1:10 atm. (3) A 0ammable gas sensor detects traces of H2 or CH4 above a preset threshold.
In case of alarm, the test is immediately terminated by discontinuing methane 0ow to the reactor and by releasing the shutter plate, in order to block radiation from reaching the reactor window. A close loop TV was installed in order to enable indirect observation of the reactor window during STMS tests and to detect promptly any incipient 0aw in the quartz window. 3.2. Experimental results The results of our experimental program will be presented by 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 0ow from the thermocouple duct.
3.2.1. Test no. 1 Test no. 1 was performed with reactor M3-a (Fig. 2). The reaction chamber of M3-a has the shape of a frustum of small dimensions (D1 = 8:0 cm; D2 = 1:0 cm; L = 10:0 cm). The aperture of the quartz window installed normal to the reactor axis of symmetry has a 5:8 cm diameter. An impeller ring with 18 slant grooves of 0:15 cm × 0:6 cm cross sections was installed in the reactor during test no. 1. The test parameters during test no. 1 were: I = 740 ± 6 W=m2 ; F1 = 20 SLM CH4 ; F2 = 2 SLM Ar. The stabilizing in0uence of the tornado e/ect was tried during this test under extremely adverse gasdynamic conditions. The temperature of reaction products increased steadily for 15 min from start of irradiation, leveling o/ at 885 K. At that time, the density of the hot reaction products
was more than six times smaller than that of the argon layer 0owing above it at near room temperature (a Taylor instability condition). Pulsed formation of black powder was observed at the central region of the window 5 min after start. At t = 10 min, the pulsations subsided. At t = 15 min, a milky opaque spot was seen in the central region of the window. It grew gradually in size, reaching a 3 cm diameter at t = 30 min, when the test was terminated. The extent of reaction was 6.1% at t = 5 min, then it dropped to 3% and 1nally, towards the end of the test, it rose again to 6%. The quartz window was then removed from the reactor for inspection. The window was found undamaged over most of its area, except for a central region, approximately 3 cm in diameter, which exhibited signs of molten quartz and of carbon deposition. Two chunks of soot, which were detached
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Fig. 8. Sketch of gas 0ow bypassing the tornado 0ow pattern during test no. 2.
from the window surface, lay superimposed at the bottom of the reaction chamber (Fig. 5). The lower chunk had a layered conical shape. Its base facing the window had a very shiny metallic hue, while its opposite side was black. The second chunk was a thin disc, shiny on both sides. The two chunks did not block completely the chamber exit port. The above observations support the following test interpretation: During an initial time period (5 min 6 t 6 15 min), when the window was still completely undamaged and unobstructed, the methane splitting reaction occurred at a low extent along the irradiated surface of ceramic insulation. However, the tornado 0ow con1guration did succeed only partially to overcome the e/ect of Taylor instability on the argon boundary layer 0ow along the window surface. The heavy boundary layer remained attached to the solid surface only about half way from periphery to center of window. The central region beyond the 0ow detachment line was not protected from contact with methane, nor was it effectively cooled by the detached gas 0ow. Quartz became eventually hot enough in this region to support methane splitting to some extent. The generated hot carbon particles stuck to the quartz surface, rendering it opaque. At this stage, radiation was trapped in a very thin layer of quartz at the quartz–carbon interface, due to the opacity of carbon and the very low thermal conductivity of quartz. A very thin layer of quartz melted. It acquired a white-milky appearance and it became re0ective. The net radiation 0ux entering the reaction chamber became thereby reduced, causing a temporary decrease in extent of the methane splitting reaction.
When the carbon deposit on the central area of the window became heavy enough, it detached from the window surface, which regained temporarily much of its transparency, prompting an increase in extent of reaction. 3.2.2. Test no. 2 Reactor M3-a was used also during test no. 2, but the impeller ring was replaced by a ring with 18 slant grooves with 0:07 cm × 0:2 cm cross sections. The test parameters during test no. 2 were: I = 834 ± 22 W=m2 ; F1 = 20 SLM CH4 ; F2 = 4 SLM He. Helium was used as an auxiliary gas instead of argon in test no. 2, in order to alleviate the adverse e/ect of Taylor instability (Notice that in an industrial STMS plant, hydrogen will serve as an auxiliary gas.) The normal cross sections of the slanted grooves in the reactor impeller ring were reduced by a factor of 6.43 and the angular momentum of the whirling methane stream at a given 0owrate increased accordingly. Twenty minutes after start of irradiation, the temperature of reaction products reached 1150 K. The extent of reaction reached at this time a maximum value, =8:7%. The pressure in the reaction chamber started to climb. Two more minutes later, the alarm was activated by the pressure rise. In an e/ort to release the pressure increase in the reaction chamber, both gas 0owrates to the reactor were reduced to 10 SLM CH4 and 2 SLM He, respectively. Despite this step, window damage was detected on the TV screen 3 min thereafter. Reactor pressure continued to climb and reactor temperature dropped drastically. The test was terminated.
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Fig. 10. Quartz window after test no. 3.
Fig. 9. Quartz window after test no. 2: (a) surface facing reactor cavity; (b) exterior surface.
The reactor insulation consisted of an inner Cotronics Rescor 760 zirconia casting surrounded by three 1tted consecutive cylinders of type ZYC Zircar. A 6 mm diameter duct drilled radially through the ceramic insulation served for installation of the B-type thermocouple. Initial thermal cracks were detected in the inner zirconia casting already in earlier tests. These cracks widened signi1cantly during test no. 2 (Fig. 6). Traces of carbon black imprints on the disassembled reactor components after test no. 2 revealed the following facts (Figs. 6 and 7): (1) Some gas was 0owing out of the reaction chamber radially along lines corresponding to the 1ssures in the ceramic insulation. (2) This gas returned to the lower side of the chamber, where the pressure is reduced, through the radial duct that houses the B-type thermocouple.
(3) The reactor port was completely plugged by powder and protruding veins of carbon were deposited on the wall 1ssures. The following sequence of events is suggested to explain the evolution of temperature, reactor pressure and hydrogen production rate during test no. 2: (1) Practically all the carbon black powder generated in the reactor cavity clung to the ceramic insulation wall. Ten minutes after start of irradiation, the tornado exit port was blocked to a considerable extent. As a result, the pressure inside the reactor went up. (2) Part of the primary (CH4 ) gas introduced into the reactor cavity bypassed the tornado 0ow pattern (Fig. 8), 0owing outwards from the high pressure region in the periphery of the reactor cavity through the cracks in the insulating wall, taking then a reverse shortcut route through the radial duct of thermocouple T , to discharge 1nally at the low pressure location of the exit port. Thus, “the wind was taken out of the sails” of the tornado. (3) Towards the end of the test, the obstruction of the exit port by powder agglomeration caused the reactor pressure to surpass the alarm limit.
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Fig. 11. Cross section of reactor M4f.
(4) In order to bring down the reactor pressure again below the preset limit, the primary and secondary gas 0owrates were reduced to half their original values. However, by this step the tornado e/ect was weakened even more. Within a few minutes, the window surface was covered up by carbon black and the window was disintegrated (Fig. 9). 3.2.3. Test no. 3 During test no. 3, the reactor and impeller ring were the same as those used during test no. 2. In preparation for test no. 3, the B-type thermocouple junction was covered by a titanium cap that 1tted tightly into the thermocouple duct. The 0ow of gas through this duct was thus blocked. A thin layer of zirconia felt was placed between the reactor ceiling 0ange and the zirconia insulation, in order to block passage of gas through any in-between gap. The cracks in the zirconia insulation of the reaction chamber were 1lled with a zirconia cement. The test parameters during test no. 3 were: I = 878 ± 10 W=m2 ; F1 = 10 SLM CH4 ; F2 = 4 SLM He. Test no. 3 lasted for 37 min. The extent of reaction reached 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. The test was terminated when some smoke appeared near the CPC and naphthalene odor was perceived at the exit of the quenching water stream. The quartz window was removed from the reactor. It was found slightly stained by carbon powder, but there was no indication of initial melting or any disintegration of the quartz
disc (Fig. 10). Chunks of brittle carbon were found near the chamber exit port, which was only partially blocked by them. 3.2.4. Test no. 4 Test no. 4 was performed with reactor M4-f (Fig. 11), equipped with the impeller ring used in tests no. 2 and 3. The reaction chamber was formed by casting zirconia into a stainless-steel mold. The ceramic casting was not removed from the mold. It was braced by it during solar tests. The formation of severe cracks in the casting, due to thermal stress, was thus prevented. Additional measures were taken, such as the use of a zirconia-felt gasket, in order to prevent degeneration of the tornado 0ow pattern by escape of gas from the reaction chamber. The dimensions of the chamber were D1 = 7:3 cm, D2 = 6:5 cm, L = 6:7 cm. The chamber exit port had a 1:0 cm diameter. An annular disc of titanium was installed transversally in the chamber, at a distance of 2:3 cm from the quartz window. The central hole of the annulus had a 2:3 cm diameter. The test parameters during test no. 4 were: I = 883 ± 5 W=m2 ; F1 = 10 SLM CH4 ; F2 = 4 SLM He. The amount of hydrogen formation during test no. 4 went up drastically, reminding the results obtained in test no. 3. Five minutes after start of irradiation, the methane splitting reaction attained an extent of 19%. Ten minutes after start, a maximum gas temperature of 1250 K was attained with = 26:2%. Despite this fact, the pressure rise inside the reactor was insigni1cant. Eighteen minutes 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 signi1cant reactor pressure increase appeared only 23 min after start and the test was then terminated. During the course of test no. 4, as well as during the previous three tests, the amount of carbon in the stream of quenching water leaving the reactor was negligible. Almost all the carbon generated by methane splitting was deposited on the hot reactor walls. The following facts were observed after reactor disassembly (Figs. 12–14): (1) Carbon deposition on the annular ring was very asymmetric. A sector of about one-third of the central hole was still unobstructed by carbon deposit. (2) The carbon deposited on the side of the disc facing the window (the irradiated side) consisted of stony, hard layers of material with a metallic tint. (3) Carbon was deposited also on the opposite side of the disc. This deposit had a 0u/y and 1brous appearance. (4) Much carbon black settled at the bottom of the reactor cavity. It had the appearance of a black soft or brittle material. It seemed to obstruct completely the reactor exit port.
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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 of tests to illustrate the performance of the tornado e/ect under realistic STMS conditions. Test no. 3 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 e/ect 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
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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 0ush the walls by a 0owing gas 1lm. The next phase of our research will proceed along these lines. Acknowledgements This study was supported by the Heineman Foundation for Research, Education, Charitable and Scienti1c Purposes, Inc., Rochester, NY, USA. The authors gratefully acknowledge the generous support of the Heineman Foundation. References
Fig. 14. View of reactor cavity exit port after test no. 4.
the adjacent irradiated solid surface, forming a very hard carbon deposit, which interferes eventually with the out0ow of reaction products from the reaction chamber. 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 0owing 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 o/er an e/ective 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
[1] Steinberg M, Cheng MG. Modern and prospective technologies for hydrogen production from fossil fuels. Int J Hydrogen Energy 1989;14(11):797–820. [2] Sandstede G. Decomposition of hydrocarbons into hydrogen and carbon for the CO2 -free production of hydrogen. Proceedings of the Ninth World Hydrogen Energy Conference, vol. 3. Paris, France, 1992. p. 1745 –52. [3] Fulcheri L, Schwob Y. From methane to hydrogen, carbon black and water. Int J Hydrogen Energy 1995;20(3):197–202. [4] Lynum S. CO2 -free hydrogen from hydrocarbons—the KvVmer CB & H process. Proceedings of the Fifth Annual US Hydrogen Meeting, Washington, DC, 1994, p. 6-45–6-56. [5] Gaudernack B, Lynum S. Hydrogen from natural gas without release of CO2 to the atmosphere. Proceedings of the 11th World Hydrogen Energy Conference, vol. 1. Stuttgart, Germany, 1996. p. 511–23. [6] Matovich E. Thagard Technology Company. US Patent No. 4095974, 1978. [7] Lee KW, Scho1eld WR, Scott Lewis D. Mobile reactor destroys toxic wastes in space. Chem Eng 1984;2:46–7. [8] Dahl JK, Tamburini J, Weimer AW, Lewondowski A, Pitts R, Bingham C, Glatzmaier GC. Thermal dissociation of methane using a solar coupled aerosol 0ow reactor. Proceedings of the AIChE Annual Meeting, Los Angeles, CA, USA, 2000. [9] Donnet JB, Bansal RC, Wang HJ. Carbon black—science and technology, 2nd ed. New York: Marcel Dekker, Inc., 1993. [10] Litterst T. Investigation of window damage by hot particles in solar heated circulating 0uidized beds. Proceedings of the Sixth International Symposium on Solar Thermal Concentrating Technologies, vol. 1. MojXacar, Spain, 1992. p. 359 – 69. [11] Steinfeld A, Brock M, Meier A, Weidenka/ A, Wuillemin D. A solar chemical reactor for co-production of zinc and synthesis gas. Energy—Int J 1998;23:803–14. [12] Kogan A, Kogan M. The Tornado 0ow con1guration—an e/ective method for screening of a solar reactor window. J Solar Energy Eng 2002;124:206–14. [13] Hunt AJ. A new solar thermal receiver utilizing a small particle heat exchanger. Proceedings of the 14th Intersociety Energy Conversion Engineering Conference, Boston, MA, USA, 1979. [14] Yuen WW, Miller FJ, Hunt AJ. Heat transfer characteristics of a gas–particle mixture under direct radiant heating. Lawrence-Berkeley Laboratory, 1985 (LBL-20289 preprint).
International Journal of Hydrogen Energy 28 (2003) 1187 – 1198 www.elsevier.com/locate/ijhydene
Production of hydrogen and carbon by solar thermal methane splitting. I. The unseeded reactor Meir Kogan, Abraham Kogan∗ Solar Research Facilities Unit, Weizmann Institute of Science, Room 208 240 Herzl St., PO Box 26, Rehovot 76100, Israel Accepted 28 October 2002
Abstract Solar thermal methane splitting was performed in a series of tests with an unseeded low capacity reactor. E/ective screening of the reactor window from contact with carbon particles was achieved by application of the tornado 0ow con1guration (J Solar Energy Eng 124 (2002) 206). The tests were performed at atmospheric pressure and at temperatures up to 1320 K. An 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 accrued carbon deposit. The results of the tests are discussed and ways to correct the problems encountered with the unseeded reactor are proposed. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Thermal methane splitting; Hydrogen production
1. Introduction Hydrogen is a clean fuel in the sense that no CO2 is generated when it is burned. At present, the amount of hydrogen that serves as a fuel is negligible. It is used almost exclusively as a raw material in the manufacture of ammonia, methanol and petrochemicals. The predominant method of hydrogen production is by methane steam reforming, which involves massive coproduction of CO2 , about 7 t CO2 =t H2 . At the present level of world hydrogen consumption, which approaches 108 t=yr, the production of hydrogen is an important contributor to the anthropogenic release of CO2 to the atmosphere. Therefore, there is a strong incentive to develop methods of hydrogen production in which the emission of CO2 is reduced or, preferably, completely eliminated. The future role of hydrogen as an alternative to fossil fuels depends, inter alia, on the successful development of clean hydrogen production methods. ∗ Corresponding author. Tel.: +972-8-934-3782; fax: +972-8934-4117. E-mail address: [email protected] (A. Kogan).
Steinberg and Cheng [1] made an extensive study of modern and prospective technologies for hydrogen production from fossil fuels. A comparison of the speci1c cost of hydrogen obtained by the analyzed technologies shows that thermal methane decomposition has a signi1cant economic potential. The possibilities of decomposing hydrocarbons directly into carbon and hydrogen were also discussed by Sandstede [2] and Fulcheri and Schwob [3]. The development by a Norwegian company of a plasma arc process for splitting of natural gas was described by Lynum [4]. The economic aspects of this process were discussed by Gaudernack and Lynum [5]. They concluded that the cost of hydrogen would depend on the prices of the input commodities, natural gas and electricity, and on the revenues from the coproduct, carbon black. With conservatively estimated carbon black prices and with the very low price of electricity in Norway, the calculated cost of hydrogen came out competitive with hydrogen produced by hydrocarbon steam reforming. In many parts of the world, electricity is much more expensive than in Norway, but abundant solar energy, which can be utilized for hydrocarbon splitting, is available. There
0360-3199/03/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/S0360-3199(02)00282-3
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is therefore an incentive to develop a process of solar thermal 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 relatively moderate temperature. Fig. 1 illustrates some results of thermochemical equilibrium calculations of the (C, 2H2 ) system, obtained by the application of the NASA CET-85 computer program. At ambient pressure, the mol fraction of unreacted methane is less than 0.001 at 1800 K. Kinetic data on thermal splitting of methane were obtained experimentally by Matovich [6] and Lee et al. [7]. The reaction residence times reported in [6,7] fall in the time interval 0:2 s 6 tr 6 1 s. Above 1900 K, the extent of reaction was not much in0uenced by the residence time. Complete dissociation was achieved above 2100 K, a temperature that can be e/ectively attained in a solar reactor. Solar thermal methane splitting proof-of-concept experiments were conducted recently at NREL, in collaboration with the University of Colorado Department of Chemical Engineering [8]. A stream of dilute 5% CH4 in argon feed gas surrounded by a pure argon purge mixture was 0own through a reactor consisting of a 25 mm diameter quartz reactor tube. The reactor was illuminated with a solar 0ux of 2400 kW=m2 . The very low concentration of CH4 in argon was maintained in order to keep the concentration of CH4 and of H2 outside 0ammability limits. Approximately 90% dissociation of methane was reported. 2.2. Separation of reaction products Separation of the hydrogen and carbon product mixture does not present a diKcult technical problem, since the mixture components appear in di/erent phases. Technical details are reported in [9]. 2.3. Screening of a solar reactor window A more complicated technical problem is connected with the need to protect the reactor window 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 0ooding its surface with a “curtain” of an auxiliary gas stream requires very substantial auxiliary gas 0owrates and the heat absorbed by the gas represents a major loss of energy [10,11]. In an e/ort to reduce the auxiliary gas 0owrate to a minimum, a certain 0ow pattern akin to the natural tornado phenomenon has recently been developed in our laboratory,
Fig. 1. Thermal splitting of methane—XCH4 vs. p, T .
which enables e/ective reactor window screening by an auxiliary gas 0owrate less than 5% of the main gas 0owrate. Details of the tornado e/ect are discussed elsewhere [12]. Here, we shall limit ourselves to a brief exposition of the physical background and to an illustration of this phenomenon. When a 0uid 0ows along a solid stationary boundary, its motion is retarded in a thin boundary layer by friction. The retarded 0uid boundary layer may thicken progressively in the direction of 0ow and ultimately it may detach from the solid boundary and mix with the main 0ow. Boundary layer detachment can be averted if care is taken to maintain a uniformly decreasing pressure in the direction of 0ow. The accelerated main 0ow entrains then the 0uid in the boundary layer strongly enough to counteract the 0ow retardation caused by friction with the stationary boundary. Turning to our particular application, 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. A 0ow of methane is introduced into 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 0ow inside the chamber approximates then a free vortex 0ow, characterized by a drop of pressure from the periphery of the chamber to its axis. An auxiliary 0ow 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 0ow. The auxiliary boundary
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Fig. 2. Cross section of reactor M2a.
layer 0ow at the window surface is stabilized thereby and it remains attached to the surface all the way to the center of the window. There, the radially converging streamlines turn abruptly by 90◦ into the axial direction, forming a typical tornado-like funnel along the reactor axis. Synergy between the free vortex 0ow of the main gas and the boundary layer 0ow of the auxiliary gas is here exploited in order to protect e/ectively the reactor window. The synergy is expressed by the fact that the auxiliary 0ow, 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 0ow does not need to be injected with high velocity or with a great 0owrate 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 e/ect has been demonstrated in a series of simulation tests at room temperature with the reactor model shown in Fig. 2 [12]. The main gas stream was 0own 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 whirling motion. The auxiliary gas stream was 0own radially from a second annular plenum chamber through a second narrow annular gap towards the periphery of the inner surface of the window. Both streams consisted 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 0ow inside the reaction chamber, a laser beam directed towards the reactor window was di/racted by passage through a transverse cylindrical glass rod. The monochromatic laser beam emerged from the glass rod as a planar sheet of light that illuminated a cross section of 0ow inside the reaction chamber. The four tornado con1guration tests illustrated in Fig. 3 were performed with an auxiliary smoke-charged gas maintained at a constant 0owrate of 2 l=min. In the absence of a whirling main gas stream (Fig. 3a), the auxiliary 0ow 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 0owrates (Fig. 3a–c), the auxiliary stream became progressively stabilized as a thin boundary layer. For a main gas 0owrate 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 1nally the reaction chamber through a narrow axially oriented funnel. 2.4. Transfer of radiation energy to reactant gas 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).
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Fig. 3. Four tornado 0ow con1guration tests with successively increasing main gas 0owrates. Smoke-charged secondary gas 0owrate was 2 l=min.
Following a method proposed by Hunt [13] and Yuen et al. [14], 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 e/ectively, 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 e/ect is strong enough to render active seeding super0uous.
3. Experimental The primary aim of the experimental work described below was to determine under realistic STMS conditions to what extent the tornado 0ow con1guration o/ers 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. 3.1. Experimental setup A diagram of the experimental setup designed for a preliminary study of STMS is shown in Fig. 4. It consists of a solar reactor, an optical concentration system, the piping requisite for the establishment and maintenance of a tornado 0ow con1guration inside the reactor chamber, instrumentation and auxiliary equipment. Two reactor con1gurations, designated M3-a and M4-f, were used during the tests described below. Their geometries are illustrated in Figs. 2 and 11, respectively. The optical concentration system consists of a heliostat followed by a secondary paraboloid concentrator and a tertiary compound paraboloid concentrator (CPC). The performance of the optical system was determined during preliminary calibration tests. The 0ux of radiation leaving the CPC exit section was measured by replacing the reactor
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Fig. 4. Diagram of STMS test setup.
by a water-cooled calorimeter. The radiation 0ux reaching the outer surface of the reactor window was 2064 W, when normalized for a solar irradiation of 850 W=m2 . This corresponds to a concentration factor of 781. The value of solar irradiation was supplied by the control room of the Solar Research Facilities Unit at the Weizmann Institute of Science.
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The reactor pressure in the tests described below was planned to be slightly above atmospheric pressure (1:010 atm 6 pr 6 1:105 atm), in order to evade the need for pumping. This pressure was monitored by a colored water U-tube manometer. An STMS test is started by 0owing methane and auxiliary gas through 0owmeters to the reaction chamber at predetermined rates F1 and F2 , respectively. When the predetermined gas 0owrates are reached, a shutter plate is raised and concentrated radiation is admitted to the reactor window. Hot products of reaction leaving the reaction zone are quenched by water sprays. The mixture of reaction products and quenching water enters a gravity separator. The quenching water 0owrate and temperature increase are determined by a rotameter and a T-type thermocouple, respectively. The carbon black slurry is discharged from the separator to the atmosphere through a reinforced plastic U-tube, while the gas is 0own through an upper exit pipe to the roof of the tower. The gas composition is determined by a mass chromatograph. A B-type thermocouple was installed to measure the temperature of reaction products at the exit port of the reaction chamber. Unfortunately, the temperatures registered by this thermocouple did not represent correctly the sought gas temperature, for reasons explained below. The temperature approached by the reaction products at the end of each test was therefore evaluated indirectly from an energy balance over the quenching process of the product gas mixture, as follows: f(T ) = {(1 − )[hS0CH4 (T ) − hS0CH4 (Tf )] + 2 [hS0H2 (T ) − hS0H2 (Tf )]}F1 + {hS0Aux (T ) − hS0Aux (Tf )}F2 − mqw cqw (Tf − Ti ) = 0:
Fig. 5. Quartz window and two chunks of carbon found near reactor exit port after test no. 1.
(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 were determined experimentally during the test. Thus, it was possible to evaluate f(T ) and to plot it as a curve in the f–T plane. The intersection of the curve with the T -axis yielded the gas temperature T before quenching. An alarm device was installed in the test rig. It is actuated when either one of the following events occurs: (1) The temperature of a certain thermocouple exceeds a predetermined value. (2) The reactor pressure reaches 1:10 atm. (3) A 0ammable gas sensor detects traces of H2 or CH4 above a preset threshold.
In case of alarm, the test is immediately terminated by discontinuing methane 0ow to the reactor and by releasing the shutter plate, in order to block radiation from reaching the reactor window. A close loop TV was installed in order to enable indirect observation of the reactor window during STMS tests and to detect promptly any incipient 0aw in the quartz window. 3.2. Experimental results The results of our experimental program will be presented by 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 0ow from the thermocouple duct.
3.2.1. Test no. 1 Test no. 1 was performed with reactor M3-a (Fig. 2). The reaction chamber of M3-a has the shape of a frustum of small dimensions (D1 = 8:0 cm; D2 = 1:0 cm; L = 10:0 cm). The aperture of the quartz window installed normal to the reactor axis of symmetry has a 5:8 cm diameter. An impeller ring with 18 slant grooves of 0:15 cm × 0:6 cm cross sections was installed in the reactor during test no. 1. The test parameters during test no. 1 were: I = 740 ± 6 W=m2 ; F1 = 20 SLM CH4 ; F2 = 2 SLM Ar. The stabilizing in0uence of the tornado e/ect was tried during this test under extremely adverse gasdynamic conditions. The temperature of reaction products increased steadily for 15 min from start of irradiation, leveling o/ at 885 K. At that time, the density of the hot reaction products
was more than six times smaller than that of the argon layer 0owing above it at near room temperature (a Taylor instability condition). Pulsed formation of black powder was observed at the central region of the window 5 min after start. At t = 10 min, the pulsations subsided. At t = 15 min, a milky opaque spot was seen in the central region of the window. It grew gradually in size, reaching a 3 cm diameter at t = 30 min, when the test was terminated. The extent of reaction was 6.1% at t = 5 min, then it dropped to 3% and 1nally, towards the end of the test, it rose again to 6%. The quartz window was then removed from the reactor for inspection. The window was found undamaged over most of its area, except for a central region, approximately 3 cm in diameter, which exhibited signs of molten quartz and of carbon deposition. Two chunks of soot, which were detached
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Fig. 8. Sketch of gas 0ow bypassing the tornado 0ow pattern during test no. 2.
from the window surface, lay superimposed at the bottom of the reaction chamber (Fig. 5). The lower chunk had a layered conical shape. Its base facing the window had a very shiny metallic hue, while its opposite side was black. The second chunk was a thin disc, shiny on both sides. The two chunks did not block completely the chamber exit port. The above observations support the following test interpretation: During an initial time period (5 min 6 t 6 15 min), when the window was still completely undamaged and unobstructed, the methane splitting reaction occurred at a low extent along the irradiated surface of ceramic insulation. However, the tornado 0ow con1guration did succeed only partially to overcome the e/ect of Taylor instability on the argon boundary layer 0ow along the window surface. The heavy boundary layer remained attached to the solid surface only about half way from periphery to center of window. The central region beyond the 0ow detachment line was not protected from contact with methane, nor was it effectively cooled by the detached gas 0ow. Quartz became eventually hot enough in this region to support methane splitting to some extent. The generated hot carbon particles stuck to the quartz surface, rendering it opaque. At this stage, radiation was trapped in a very thin layer of quartz at the quartz–carbon interface, due to the opacity of carbon and the very low thermal conductivity of quartz. A very thin layer of quartz melted. It acquired a white-milky appearance and it became re0ective. The net radiation 0ux entering the reaction chamber became thereby reduced, causing a temporary decrease in extent of the methane splitting reaction.
When the carbon deposit on the central area of the window became heavy enough, it detached from the window surface, which regained temporarily much of its transparency, prompting an increase in extent of reaction. 3.2.2. Test no. 2 Reactor M3-a was used also during test no. 2, but the impeller ring was replaced by a ring with 18 slant grooves with 0:07 cm × 0:2 cm cross sections. The test parameters during test no. 2 were: I = 834 ± 22 W=m2 ; F1 = 20 SLM CH4 ; F2 = 4 SLM He. Helium was used as an auxiliary gas instead of argon in test no. 2, in order to alleviate the adverse e/ect of Taylor instability (Notice that in an industrial STMS plant, hydrogen will serve as an auxiliary gas.) The normal cross sections of the slanted grooves in the reactor impeller ring were reduced by a factor of 6.43 and the angular momentum of the whirling methane stream at a given 0owrate increased accordingly. Twenty minutes after start of irradiation, the temperature of reaction products reached 1150 K. The extent of reaction reached at this time a maximum value, =8:7%. The pressure in the reaction chamber started to climb. Two more minutes later, the alarm was activated by the pressure rise. In an e/ort to release the pressure increase in the reaction chamber, both gas 0owrates to the reactor were reduced to 10 SLM CH4 and 2 SLM He, respectively. Despite this step, window damage was detected on the TV screen 3 min thereafter. Reactor pressure continued to climb and reactor temperature dropped drastically. The test was terminated.
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Fig. 10. Quartz window after test no. 3.
Fig. 9. Quartz window after test no. 2: (a) surface facing reactor cavity; (b) exterior surface.
The reactor insulation consisted of an inner Cotronics Rescor 760 zirconia casting surrounded by three 1tted consecutive cylinders of type ZYC Zircar. A 6 mm diameter duct drilled radially through the ceramic insulation served for installation of the B-type thermocouple. Initial thermal cracks were detected in the inner zirconia casting already in earlier tests. These cracks widened signi1cantly during test no. 2 (Fig. 6). Traces of carbon black imprints on the disassembled reactor components after test no. 2 revealed the following facts (Figs. 6 and 7): (1) Some gas was 0owing out of the reaction chamber radially along lines corresponding to the 1ssures in the ceramic insulation. (2) This gas returned to the lower side of the chamber, where the pressure is reduced, through the radial duct that houses the B-type thermocouple.
(3) The reactor port was completely plugged by powder and protruding veins of carbon were deposited on the wall 1ssures. The following sequence of events is suggested to explain the evolution of temperature, reactor pressure and hydrogen production rate during test no. 2: (1) Practically all the carbon black powder generated in the reactor cavity clung to the ceramic insulation wall. Ten minutes after start of irradiation, the tornado exit port was blocked to a considerable extent. As a result, the pressure inside the reactor went up. (2) Part of the primary (CH4 ) gas introduced into the reactor cavity bypassed the tornado 0ow pattern (Fig. 8), 0owing outwards from the high pressure region in the periphery of the reactor cavity through the cracks in the insulating wall, taking then a reverse shortcut route through the radial duct of thermocouple T , to discharge 1nally at the low pressure location of the exit port. Thus, “the wind was taken out of the sails” of the tornado. (3) Towards the end of the test, the obstruction of the exit port by powder agglomeration caused the reactor pressure to surpass the alarm limit.
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Fig. 11. Cross section of reactor M4f.
(4) In order to bring down the reactor pressure again below the preset limit, the primary and secondary gas 0owrates were reduced to half their original values. However, by this step the tornado e/ect was weakened even more. Within a few minutes, the window surface was covered up by carbon black and the window was disintegrated (Fig. 9). 3.2.3. Test no. 3 During test no. 3, the reactor and impeller ring were the same as those used during test no. 2. In preparation for test no. 3, the B-type thermocouple junction was covered by a titanium cap that 1tted tightly into the thermocouple duct. The 0ow of gas through this duct was thus blocked. A thin layer of zirconia felt was placed between the reactor ceiling 0ange and the zirconia insulation, in order to block passage of gas through any in-between gap. The cracks in the zirconia insulation of the reaction chamber were 1lled with a zirconia cement. The test parameters during test no. 3 were: I = 878 ± 10 W=m2 ; F1 = 10 SLM CH4 ; F2 = 4 SLM He. Test no. 3 lasted for 37 min. The extent of reaction reached 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. The test was terminated when some smoke appeared near the CPC and naphthalene odor was perceived at the exit of the quenching water stream. The quartz window was removed from the reactor. It was found slightly stained by carbon powder, but there was no indication of initial melting or any disintegration of the quartz
disc (Fig. 10). Chunks of brittle carbon were found near the chamber exit port, which was only partially blocked by them. 3.2.4. Test no. 4 Test no. 4 was performed with reactor M4-f (Fig. 11), equipped with the impeller ring used in tests no. 2 and 3. The reaction chamber was formed by casting zirconia into a stainless-steel mold. The ceramic casting was not removed from the mold. It was braced by it during solar tests. The formation of severe cracks in the casting, due to thermal stress, was thus prevented. Additional measures were taken, such as the use of a zirconia-felt gasket, in order to prevent degeneration of the tornado 0ow pattern by escape of gas from the reaction chamber. The dimensions of the chamber were D1 = 7:3 cm, D2 = 6:5 cm, L = 6:7 cm. The chamber exit port had a 1:0 cm diameter. An annular disc of titanium was installed transversally in the chamber, at a distance of 2:3 cm from the quartz window. The central hole of the annulus had a 2:3 cm diameter. The test parameters during test no. 4 were: I = 883 ± 5 W=m2 ; F1 = 10 SLM CH4 ; F2 = 4 SLM He. The amount of hydrogen formation during test no. 4 went up drastically, reminding the results obtained in test no. 3. Five minutes after start of irradiation, the methane splitting reaction attained an extent of 19%. Ten minutes after start, a maximum gas temperature of 1250 K was attained with = 26:2%. Despite this fact, the pressure rise inside the reactor was insigni1cant. Eighteen minutes 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 signi1cant reactor pressure increase appeared only 23 min after start and the test was then terminated. During the course of test no. 4, as well as during the previous three tests, the amount of carbon in the stream of quenching water leaving the reactor was negligible. Almost all the carbon generated by methane splitting was deposited on the hot reactor walls. The following facts were observed after reactor disassembly (Figs. 12–14): (1) Carbon deposition on the annular ring was very asymmetric. A sector of about one-third of the central hole was still unobstructed by carbon deposit. (2) The carbon deposited on the side of the disc facing the window (the irradiated side) consisted of stony, hard layers of material with a metallic tint. (3) Carbon was deposited also on the opposite side of the disc. This deposit had a 0u/y and 1brous appearance. (4) Much carbon black settled at the bottom of the reactor cavity. It had the appearance of a black soft or brittle material. It seemed to obstruct completely the reactor exit port.
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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 of tests to illustrate the performance of the tornado e/ect under realistic STMS conditions. Test no. 3 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 e/ect 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
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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 0ush the walls by a 0owing gas 1lm. The next phase of our research will proceed along these lines. Acknowledgements This study was supported by the Heineman Foundation for Research, Education, Charitable and Scienti1c Purposes, Inc., Rochester, NY, USA. The authors gratefully acknowledge the generous support of the Heineman Foundation. References
Fig. 14. View of reactor cavity exit port after test no. 4.
the adjacent irradiated solid surface, forming a very hard carbon deposit, which interferes eventually with the out0ow of reaction products from the reaction chamber. 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 0owing 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 o/er an e/ective 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
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