Solar-Thermal Processing of Methane to Produce Hydrogen and Syngas

Energy & Fuels 2001, 15, 1227-1232

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Solar-Thermal Processing of Methane to Produce Hydrogen and Syngas Jaimee K. Dahl, Joseph Tamburini, and Alan W. Weimer* University of Colorado, Department of Chemical Engineering, Boulder, Colorado 80309-0424

Allan Lewandowski, Roland Pitts, and Carl Bingham National Renewable Energy Laboratory, Golden, Colorado 80401-3393 Received March 15, 2001. Revised Manuscript Received June 12, 2001

A solar-thermal aerosol flow reactor has been constructed, installed, and tested with the HighFlux Solar Furnace (HFSF) at the National Renewable Energy Laboratory (NREL). Experiments were successfully carried out for the dissociation of methane to produce hydrogen and carbon black and for the dry reforming of methane with carbon dioxide to form syngas (hydrogen and carbon monoxide). Approximately 90% dissociation of methane was achieved in a 25-mm diameter quartz reaction tube illuminated with a solar flux of 2400 kW/m2 (or suns). The carbon black produced was amorphous and had a particle size of 20 to 40 nm. Approximately 70% conversion was achieved for dry reforming using a solar flux of 2000 kW/m2. The experimental results for both processes are very encouraging and support further work to address the technical issues and to develop the processes.

Introduction The primary driver for the development of renewable energy strategies is current concern over the potential, irreversible environmental damage that may occur with the continued or accelerated use of fossil fuels. Movement toward a hydrogen (H2) based economy is an essential component of an international program to address that concern and will, in addition, address concerns over pollution in cities and associated health costs. However, current methods for producing hydrogen incur a large environmental liability because fossil fuels are burned to supply the energy to reform methane (CH4). We propose an alternate strategy using highly concentrated sunlight as the energy source that does not result in an increase of environmental liability. Indeed, it represents a route for utilizing current natural gas reserves that fixes carbon as well as increases the energy content of the fuel. The research presented here is oriented at developing a cost-effective, solar-thermal method of deriving hydrogen from natural gas. This research is also directed toward developing a cost-effective, solar-thermal method of dry reforming methane with carbon dioxide (CO2) to form syngas. An advantage of dry reforming is that it is a method for producing hydrogen that consumes carbon dioxide. In addition, there are a vast number of natural gas reserves that are currently not being utilized because they contain a significant amount of carbon dioxide, and it is expensive to separate the carbon dioxide from the methane and sequester and store it properly. Therefore, * Author to whom all correspondence should be addressed. Tel: 303492-3759. Fax: 303-492-4341. E-mail: [email protected].

dry reforming is a highly desirable process because the two components can be reacted to form syngas, which can be used in the production of longer chained hydrocarbons. Background Steinberg1-6 and Steinberg et al.7 have been major proponents of the thermal decomposition of methane process for hydrogen production. Methane is dissociated to carbon (C) black and hydrogen according to

CH4 f C + 2H2 ∆H298K ) 74.9 kJ/mol

(1)

Methane is a preferred choice for the production of hydrogen from a hydrocarbon because of its high hydrogen-to-carbon ratio (H/C ) 4), availability, and relatively low cost. Furthermore, the carbon produced can be sold as a coproduct into the carbon black market (inks, paints, tires, batteries, etc.) or sequestered, stored, and used as a clean fuel for electrical power generation. The sequestering or storing of solid carbon requires much less development than sequestering and storing gaseous carbon dioxide. Gibbs free energy minimization calculations have been carried out (P ) 0.1 MPa; 600 K e T e 2273 K) for the methane dissociation system (i.e., CH4 + heat f equilibrium products) to determine equilibrium (1) Steinberg, (2) Steinberg, (3) Steinberg, (4) Steinberg, (5) Steinberg, (6) Steinberg, (7) Steinberg, 797-820.

M. Int. J. Hydrogen Energy 1986, 11, 715-720. M. Energy Sources 1987, 9, 161-171. M. Int. J. Hydrogen Energy 1994, 19, 659-665. M. Energy Convers. Manage. 1995, 36, 791-796. M. Int. J. Hydrogen Energy 1998, 23, 419-425. M. Int. J. Hydrogen Energy 1999, 24, 771-777. M.; Cheng, H. C. Int. J. Hydrogen Energy 1989, 14,

10.1021/ef0100606 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/27/2001

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Figure 1. Primary methane decomposition equilibrium products P ) 0.1 MPa. Figure 2. Effect of residence time and temperature.

products. Thermodynamically favored products (Figure 1) indicate that dissociation is occurring above 600 K and that temperatures greater than 1500 K are required to achieve nearly complete dissociation. Although not shown, trace products at 1500 K include acetylene (C2H2), ethylene (C2H4), butylene (C4H8), propylene (C3H6), ethane (C2H6), and other species at quantities less than 1 × 10-3 moles. Although thermodynamics can predict when a reaction will not occur, it cannot predict whether a reaction will indeed occur in practice. Activation energies, transport processes (e.g., heating rate), and other reaction kinetic considerations are needed in order to determine if a reaction can be completed for a given amount of time in a chemical reactor design. Such kinetic data have been reported for reaction 1 using an electrically heated pilot-scale aerosol flow reactor.8 Matovich8 showed that the decomposition of methane could be carried to completion in a short residence time aerosol reaction tube at temperatures greater than T ) 2088 K. The reactor consisted of a 0.0762-m diameter by 0.914-m long (3 in. ID × 3 ft long) graphite aerosol reaction tube heated indirectly by radiation from external electrodes heated directly by electrical resistance. Later studies included work carried out in 0.305-m ID by 3.66-m long (1 ft ID × 12 ft long) reaction tubes.9 A small amount of carbon black was introduced in the methane feed stream to serve as a radiationabsorbing target to initiate the pyrolytic reaction. Due to the high temperatures involved and the difficulty in heating a gas to those temperatures (by convection from the reactor walls), the carbon particles are the key to this process. Reactions were carried out in the temperature range of 1533 < T < 2144 K with residence times between approximately 0.1 and 1.5 s. The fraction of methane dissociated was determined by measuring the thermal conductivity of the effluent gas after filtering the carbon black particles from the sample. Hydrogen flowed radially through a porous reaction tube, providing a fluidwall to prevent carbon black from depositing on the wall. The residence time in the reactor was controlled by the inlet flow of methane, the radial flow of hydrogen, and the reactor temperature. Some reported results8 where data were available for both a minimum residence time (tr(min)) of 0.2 s and a maximum residence time (tr(max)) of 1 s are summarized in Figure 2. It is clear from these (8) Matovich, E. Thagard Technology Company, U.S. Patent 4,095,974, 1978. (9) Lee, K. W.; Schofield, W. R.; Lewis, D. S. Chem. Eng. 1984, 4647.

results that residence time has little effect on dissociation for temperatures greater than T ) 1900 K and that complete dissociation can be achieved in aerosol flow reactors for temperatures greater than approximately T ) 2100 K for reaction times of t ) 0.2 s. The second process being studied is the dry reforming of methane with carbon dioxide to form syngas:

CH4 + CO2 f 2H2 + 2CO ∆H298K ) 250 kJ/mol (2) This is also a highly endothermic reaction and high conversions are achieved at temperatures greater than about T ) 973 K.10 Typically, nickel-based catalysts are used for reaction 2.10-13 However, undesirable side reactions (2CO f C + CO2; CH4 f C + 2H2) take place that produce carbon, which poisons the catalysts.10,12 NiO/MgO solid-solution catalysts have been found to inhibit carbon deposition.14 Dry reforming experiments have been carried out using solar powered reactors. Levy et al.15 used a solar heated metal reactor packed with a rhodium catalyst, so they had to prevent carbonforming side reactions from taking place. Worner et al.16 conducted experiments using catalytically active absorber systems. Coke deposition took place during the experiments, which caused degradation of the catalyst. The solar-thermal process investigated here can obtain good conversions of methane and carbon dioxide to syngas without the use of a catalyst. Carbon formation is not detrimental to the process. In fact, carbon particles are preferably flowed with reactant gas through the reactor tube as radiation absorbers to facilitate heating and reaction. The processes investigated here are the high-temperature thermal dissociation of methane and dry reforming of methane with carbon dioxide using a solarthermal aerosol flow reactor. The energy required to drive reactions 1 and 2 is supplied by concentrated sunlight. An experimental reactor apparatus was constructed and interfaced to NREL’s HFSF.17-20 There is no need for auxiliary cooling at the optical source. The (10) Halmann, M. M.; Steinberg, M. Greenhouse Gas Carbon Dioxide Mitigation; Lewis Publishers: Boca Raton, FL, 1999. (11) Inui, T.; Hara, H.; Takeguchi, T.; Ichino, K.; Kim, J. B.; Iwamoto, S.; Pu, S. B. Energy Convers. Manage. 1997, 38, S385-S390. (12) Choudhary, V. R.; Uphade, B. S.; Mamman, A. S. Appl. Catal. A 1998, 168, 33-46. (13) Pen˜a, M. A.; Go´mez, J. P.; Fierro, J. L. G. Appl. Catal. A 1996, 144, 7-57. (14) Ruckenstein, E.; Hu, Y. H. Chem. Innovation 2000, 30, 39-43. (15) Levy, M.; Levitan, R.; Rosin, H.; Rubin, R. Sol. Energy 1993, 50, 179-189. (16) Worner, A.; Tamme, R. Catal. Today 1998, 46, 165-174.

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Figure 4. Schematic of solar-thermal aerosol flow reactor. Figure 3. Schematic of the HFSF.

reactor is operated as a cold wall process, because the beam is delivered directly on target. In addition, the control of solar radiation (on/off) is almost instantaneous. Absorbing surfaces exposed to concentrated solar radiation can reach temperatures of between 1000 and 3000 K in fractions of a second. The process produces hydrogen or hydrogen and carbon monoxide (CO) using high efficiency direct solar-thermal heating with no associated carbon dioxide generation. Experimental Work High-Flux Solar Furnace. The HFSF facility at NREL in Golden, CO was used for this research. A schematic diagram of the HFSF is shown in Figure 3. NREL is the primary national laboratory in the United States for renewable energy research. The HFSF uses a series of mirrors that concentrate sunlight to a focused beam at maximum power levels of 10 kW into an approximate diameter of 10 cm. The solar furnace’s long focal length and its off-axis design give researchers flexibility and control over the delivered flux and also allow the use of secondary concentrators to obtain higher temperatures. It operates with a heliostat that has an area of 31.8 m2 and a 92% solar-weighted reflectivity. The heliostat reflects sunlight to a primary concentrator consisting of 25 hexagonal facets that are spherical mirrors ground to a 14.6-m radius of curvature. The total surface area of the primary concentrator is 12.5 m2 and it reflects radiation from the entire solar spectrum (300 nm to 2500 nm). Under optimal conditions, the primary concentrator can achieve maximum flux intensities of 2500 suns. Secondary concentrators that achieve intensities of more than 20 000 suns and refractive designs approaching 50 000 suns can be installed at the primary concentrator’s focal point to increase the intensity further. The furnace is easily capable of delivering flux densities on the order of 100-1000 W/cm2. No secondary solar concentration was used in these studies. Reactor System. Experiments were carried out using a modified reactor system originally built for previous experiments in fullerene production.21 The reactor consists of a particle and gas feed mechanism, quartz reactor tube, an internal graphite “target” feed tube, and a filter housing. The (17) Jenkins, D.; Winston, R.; O’Gallagher, J.; Bingham, C.; Lewandowski, A.; Pitts, R.; Scholl, K. Sol. Eng. 1996, 29-33. (18) Lewandowski, A.; Bingham, C.; O’Gallagher, J.; Winston, R.; Sagie, D. Sol. Energy Mater. 1991, 24, 550-563. (19) Lewandowski, A. Mater. Technol. 1993, 8, 237-249. (20) Pitts, J. R.; Tracy, E.; Shinton, Y.; Fields, C. L. Crit. Rev. Surf. Chem. 1993, 2, 247-269. (21) Mischler, D.; Pitts, R.; Fields, C.; Bingham, C.; Heben, M.; Lewandowski, A. In International Symposium on Solar Chemistry; Villigen, Switzerland, 1997.

reactor operates at atmospheric pressure with gas flow driven and controlled through a series of mass flow controllers. An in-line Horiba model TCA-300 hydrogen detector was inserted downstream of the particle filter. This detector is based on thermal conductivity measurements and was calibrated for 5% hydrogen in argon (Ar). Gas samples were also taken and analyzed using an off-line gas chromatograph (GC). Methane and produced hydrogen were kept outside flammability limits by operating with a dilute 5% or 10% methane in argon feed gas mixture and a pure argon purge stream for the dissociation reactions. For the dry reforming experiments, a dilute gas mixture consisting of 10% methane in argon was fed as well as a gas stream containing pure carbon dioxide. For all of the experiments, the ratio of methane to carbon dioxide was CH4/ CO2 ) 2:1. A 2:1 ratio of CH4/CO2 was intended to carry out reactions 1 and 2 simultaneously. The temperature inside the quartz tube reactor is exceedingly difficult to determine. However, the temperature of the quartz tube is carefully monitored using an infrared camera positioned on the side of the reactor. The quartz temperature is monitored to avoid warping or even melting the reactor wall with concentrated sunlight. A schematic of the reactor system is shown in Figure 4. A key aspect of the reactor operation is the heating means for the feed gas. The reactor has been designed for three alternative heating methods: (1) heating a 9.5 mm outside diameter by 6 mm inside diameter “target” graphite tube with concentrated sunlight, the heated target then heating the methane-containing feed gas by conduction; (2) heating the “target” graphite tube, but with radiation absorbing fine carbon black particles suspended in the methane-containing feed gas stream so the inside wall of the “target” radiates to the flowing particles that subsequently heat the flowing feed gas by particle surface conduction in addition to the wall conduction; and (3) heating the suspended carbon black particles directly with concentrated sunlight, the sunlight directed above the top of the graphite tube. As will be shown later, the experimental results indicate that alternative (2) is the best method to use to heat the reactant gas stream. The graphite tube can be seen contained within the quartz tube reactor assembly as shown in Figure 5. The quartz tube extends beyond the limits of the concentrated solar flux provided by the HFSF. The quartz tube was positioned at the nominal focus of the normal to the optical axis of the HFSF with its axis “vertical”. Alternatives (2) and (3) involve volumetric absorption of light by a gas-solid suspension. The particle suspension is generated in a feed mechanism located below the quartz tube. Lightly compacted carbon black particles (“Shawinigan” acetylene carbon black; product of Chevron Chemical Co., Houston, TX) are fed against a rotating steel brush that conveys the particles to a space where they are mixed with the 5% or 10% methane/argon feed gas. The suspension then flows through a set of nozzles that destroy any particle agglomerates. The nozzle diameter varies between

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Figure 5. Photographs of reactor tube assembly: (a) before heating, (b) immediately after heating (shutter closed).

Figure 6. Signal response to solar flux (with particles fed) (0.22 mol % CH4 (0.45% after 18 min) in total argon feed). 0.33 mm and 0.64 mm. Before the particle cloud passes through the focal area, two streams of “sweep” argon gas are “wrapped” around the gas-solid suspension. This “fluid-wall” is designed to prevent particles from reaching the quartz glass in locations where heating by the highly concentrated sunlight might soften or melt the tube. The gas and particles are fed from bottom to top of the quartz reactor. With the ratio of “sweep” argon to feed 5% or 10% methane/argon on the order of 10:1, the overall percentage of methane or hydrogen was relatively low for these experiments. These low concentrations are used as a safety precaution for early studies.

Results and Discussion The signal response for a typical dissociation experiment is shown in Figure 6. This was a very clear day with direct normal irradiance at approximately 1000 W/m2. The estimated flux on the target (reactor) was about 2100 kW/m2 or 2100 suns. The gas temperature was monitored by a thermocouple downstream of the reactor and the quartz temperature was monitored with an infrared video camera (IR in Figure 6). First the flow of sweep argon gas was initiated at 2 L/min, and then

the particle feed and 5 mol % methane/argon (at a flow of 0.1 L/min) were started at about 4 min (0.22 mol % methane/argon total). The particles and methane/argon feed gas mixture entered the reaction tube through a 0.15-m long internal graphite tube as described earlier. A change in the hydrogen % can be seen since the thermal conductivity of methane is higher than argon (there is a time delay for the flow to reach the hydrogen detector of about 20 to 30 s). This signal was allowed to steady, and then the concentrated sunlight was introduced by opening a fast-acting shutter. A nearly immediate increase in the hydrogen signal can be seen in the dashed trace. The shutter was closed at about 11 min and a corresponding decrease in the hydrogen signal can be seen. At 16 min the flow of methane/argon feed gas was stopped, then restarted, stopped again and restarted at 0.2 L/min at 18 min (0.45 mol % methane/ argon total). The changes in the hydrogen signal clearly indicate that the hydrogen production is following these flow manipulations. A sample bag was filled for subsequent analysis from about 20 to 25 min. The shutter was closed at about 27 min. The subsequent off-line GC analysis (0.8 mol % hydrogen, 180 ppm methane, 180 ppm acetylene, 520 ppm ethylene, 0.06 mol % carbon monoxide, 0.05 mol % carbon dioxide, 1.5 mol % air, balance argon) of the collected gas sample indicated an 88% dissociation of methane. The air was introduced when the sample bag was detached from the collection valve and possibly also when attaching it to the GC system. The carbon monoxide and carbon dioxide indicate that there is some air in the system. The ethylene and acetylene are incomplete reaction products. At the flows of methane and argon in this experiment, complete dissociation of methane to hydrogen would have yielded

Solar-Thermal Processing of Methane

Figure 7. Signal response to solar flux (no particles fed) (1.7 mol % CH4 in total argon feed).

0.91 mol % hydrogen. These results indicate that, with constant solar flux, an increase in the methane feed rate results in an increase in the hydrogen synthesis rate. Successful dissociation experiments were also carried out (Figure 7) without carbon particle co-feed and with a higher concentration of methane in the gas stream (1.7 mol % methane/argon total). When the amount of solar flux striking the reactor increased, the signal from the hydrogen detector increased accordingly. In Figure 7, the initial solar flux was 1160 kW/m2, resulting in little dissociation of methane. When the flux was raised to 1760 kW/m2 the % hydrogen signal quickly jumped to over 1 mol % hydrogen (about 32% dissociation). As the solar flux continued to increase to 2060 kW/m2 and 2360 kW/m2, the hydrogen signal increased accordingly to values of 1.4 and 1.5 mol %, respectively. These hydrogen concentrations correspond to 42 and 45% conversion, respectively. These results indicate that, for constant methane feed rate, an increase in solar flux

Figure 8. Synthesized carbon black (no particles fed).

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results in an increase in the hydrogen synthesis rate. The test that was run with carbon particle co-feed yielded a conversion of 88% for a flux of 2100 suns whereas the test that was run without carbon particle co-feed resulted in a conversion of 42% for a flux of approximately 2100 suns. These results suggest that having carbon particles flowing in the reactant gas stream does increase heat transfer to the gas stream and therefore increases conversion of methane to carbon and hydrogen. The quality of carbon black being produced by the dissociation process was also studied. These experiments were run without co-feeding any carbon black, so the gas stream was heated using the graphite tube. The amorphous particles were collected on a Teflon filter located downstream of the reactor exit. The quality of the carbon black produced by solar-thermal dissociation of methane appears to be of the highest quality with an average particle size of between 20 and 40 nm (Figure 8). Several experiments were also conducted to determine the temperature inside the graphite tube at the focal point. A type C thermocouple was inserted into the top of the reactor so the tip of the thermocouple was located just inside of the hollow graphite tube. The results of these experiments are summarized in Figure 9. Typical studies were made with solar fluxes ranging from 1500 to 2000 suns. This range corresponds to an average reactor temperature between T ) 1650 and 1800 K. The experimental results reported here are consistent with the results of Matovich8 summarized in Figure 2.

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Figure 9. Temperatures inside the graphite tube as a function of solar flux.

flux (kW/m2)

% CO2

% CH4

% CO

% H2

none 1500 2000

0.45 0.23 0.11

0.84 0.20 0.07

0 0.35 0.62

0 1.55 1.60

sample revealed that there was no detectable carbon monoxide or hydrogen in the feed. Several tests were then run at fluxes of 1500 and 2000 suns, and the results were averaged to obtain the values reported in Table 1. During the dry reforming reaction, hydrogen was produced in excess of carbon monoxide. This occurred because an excess of methane was added to the reactor, which resulted in the methane dissociation reaction occurring simultaneously. Carbon particles were observed on the filter located at the exit of the reactor. The tests at 1500 and 2000 suns resulted in approximately 40% and 70% conversion of methane and carbon dioxide to syngas, respectively. These conversions were calculated using the amount of carbon monoxide formed as compared to the amount of carbon dioxide fed to the reactor. Conclusions

Figure 10. Signal response to solar flux for dry reforming experiment (no particles fed).

A typical signal response for a dry reforming experiment is shown in Figure 10. For this experiment, 0.01 L/min carbon dioxide and 0.02 L/min of methane were fed with an annular argon flow of 2 L/min. The experiment was run on a clear day with direct normal irradiance at approximately 900 W/m2. The flux on the reactor was 2000 suns. No carbon particles were fed. The gas flows were started at about 5 min and the reactor was placed on sun at about 7 min. While the reactor was on sun, the thermal conductivity detector reported approximately 1.8% hydrogen. The shutter was closed at about 12 min, and a corresponding decrease in the % hydrogen reading was observed. After reopening the shutter at about 17 min, the hydrogen signal settled out once again at about 1.8% hydrogen. Several dry reforming experiments were run where gas samples were collected and analyzed using an off-line GC. The results are reported in Table 1. Analysis of a feed gas

Experiments to produce hydrogen and syngas were successfully carried out. High conversions were achieved using a solar-thermal reactor not designed for the specific processes and with only primary solar concentration. Conversions of approximately 90% and 70% were attained during the dissociation reactions and dry reforming reactions, respectively. In addition, when dissociating methane, a carbon black product of good quality was produced. These results warrant further study of both processes using secondary concentration to achieve higher solar fluxes and reactor temperatures than can be obtained with the primary concentrator alone. Acknowledgment. The authors thank the DOE Hydrogen Program, the University of Colorado, and BP (Anchorage, AK) for financially supporting this work under Grants DE-FC36-99GO10454, DE-PS3699GO10383, and appropriate cost share. EF0100606