Ind. Eng. Chem. Res. 2002, 41, 1935-1939
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Direct Solar Reduction of CO2 to Fuel: First Prototype Results Ann J. Traynor and Reed J. Jensen* Renewable Energy Corporation, 112 C Longview Drive, Los Alamos, New Mexico 87544
Direct solar reduction of CO2 to CO and oxygen has been demonstrated using only solar energy. Known thermochemical, kinetic, and spectral properties of the CO2/CO/O2 system enable the process. In this first prototype system, a solar focusing mirror and secondary concentrator were used to provide high solar intensity around a ceramic rod. This high-temperature, high solar irradiance environment provided strong heating of CO2 with the resultant dissociation to CO and oxygen. Quenching of the back reaction was provided by the geometry and gas dynamics of the system and by cool gas quencher jets just downstream. The best measured net conversion of CO2 to CO was near 6%, which is compared to a plant design target of 12%. The peak observed conversion of solar energy to chemical energy was 5%. Calculations indicate that a mature system will yield 20% solar-to-chemical energy conversion with an additional 25% electrical energy. Introduction We present a process for the thermochemical and photochemical reduction of CO2 to fuel. It addresses directly the accumulation of CO2 in the atmosphere by either drawing CO2 from the atmosphere or, more likely, intercepting an effluent stream from a coal-fired power plant or a cement plant and converting it to CO by a high-temperature solar process. The CO is subsequently converted to useful fuels by a range of catalytic processes that have already been demonstrated or are in use. Alternatively, the process can be used to produce hydrogen from sunlight and water only, using the CO2/ CO system as continuously recycled process gases. After an introduction to the process, we describe the design of the prototype, present first results, and assess their significance for the full process implementation. Process The dissociation of CO2 to CO at high temperature is well-known from thermodynamics. It can also be dissociated by direct photolysis, as discussed here. The results of the described work will show that CO2 can be dissociated and stabilized as CO with relative ease. The process is represented by the equation
CO2 + hν or heat f CO +1/2O2
(1)
Direct solar absorption in CO2 is enabled by intense preheating, which results in a radical spreading of the absorption spectrum of CO2 to longer wavelengths, allowing it to absorb solar light rather than vacuum ultraviolet only.1 The heating imparts vibrational bending in the CO2 molecule to enable strong optical connections by symmetry and Frank-Condon factors to a set of bent intermediate states.2-5 These states then serve as stepping stones to rapid bulk heating and photolysis. The dramatic increase in the negative Gibbs free energy of formation of CO with increasing temperature pulls the reaction in the direction of the CO product as the feed stream is heated both directly and through recombination. The chemistry can then be * Corresponding author. Email:
[email protected]. Phone: 505-672-2000. Fax: 505-672-0209.
quenched and frozen against back reaction by rapid cooling, both by contact with cooler reactor walls and by injection of an excess of unheated CO2. In the process presented here, CO2 is preheated to near 1900 °C, at which temperature direct photon absorption and further heating can occur. The calculated absorption cross sections for hot CO2 and the implied heating in an irradiance of 5000 suns are given in ref 6. This induced absorption results in further heating, photolysis, and pyrolysis of CO2 to CO by concentrated solar light. Key developments that stimulated this work are the elucidation of the nature of the intermediate states of CO2,2-5 followed in 1990 by a dramatic demonstration by Koshi et al.7 of the increase in cross section on the red side of the CO2 ultraviolet absorption for shock heated CO2. A systematic investigation of the ultraviolet absorption of CO2 by Jensen et al.1 conclusively demonstrated optical absorption in the solar region of the spectrum. Without this information, it would have seemed that the molecule would absorb significantly only in the deep UV, in broad bands centered at 147, 133, and 112 nm.8 Toward implementation, the proposed process would be deployed in modular units. The modules include a mirror that is actively pointed at the sun during operation and that has an attached converter and processor system that is always maintained at the focal point of the mirror. The conversion to CO, the quenching, and the primary power extraction would be performed at the converter. The separations, shift reaction, and synthesis would be done collectively at a ground station. Prototype Design The apparatus consists of three primary parts: the solar concentrator dish, the converter, and the instrumentation. The solar concentrator comprises square mirror segments of 0.327 m2 each, fabricated from thin flat silvered glass mirror, aluminum honeycomb, sheet steel, and appropriate adhesives. The mirror segment components were vacuum assembled against a spherical mandrel to impart a 12.8 m radius of curvature to the glass mirror segments and the over all mirror system. After assembly, the mirror segments were removed from the mandrel and mounted on a steel superstructure
10.1021/ie010871x CCC: $22.00 © 2002 American Chemical Society Published on Web 03/08/2002
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Figure 1. Converter assembly for direct solar reduction of CO2. The overall length of the apparatus represented is about 20 cm and the overall diameter is about 16 cm.
that is approximately parabolic itself. The converter was mounted at the end of a 6.4 m boom that is integral with the superstructure. In this work, 20-28 mirror segments were employed at a time, providing a reflecting area of 6.5-9.2 m2 at an overall mirror system f number of about 1.5, depending on the mirror segment configuration on the superstructure. The system typically delivered 4-5 kW of solar power into the converter window (a circle that is 8.1 cm in diameter). Because the reflecting medium itself was 1-mm thick rearsurface silvered glass, there was little solar ultraviolet light available for the process. The converter design is shown schematically in Figure 1. Its function is to convert CO2 to CO and O2. It does this through three subfunctions: preheating the CO2 to about 1900 °C, dissociating the CO2 through further heating or direct photon absorption, and limiting the back reaction of CO + O to CO2. At the heart of the converter is the metal reflector cone, which measures 15 cm along the axis. For most of our runs, this was made of 99.9% silver machined to configuration and crudely hand polished inside to a reflectivity of about 75-85%, depending on the run and the most recent polish. The piece did tarnish somewhat during its run cycles. Feed CO2 gas for the converter was preheated by routing the feed gas through an annular path that allowed for contact with the outside of the reflector and finally injecting the gas into the converter funnel through a set of 16 1.6-mm diameter holes in the hot aluminum injector ring. These holes are part of a flow path designed to equilibrate the feed gas temperature with that of the injector ring. A critical part of the apparatus is the 0.63 cm diameter ZrO2 rod cantilevered in the exhaust channel so that it extends into the funnel a distance of 2-4 cm upstream of the throat, which is the hottest part of the system. Fine jets are built into
the exit channel of the converter to introduce cool CO2 into the flow to quench the back reaction. They are situated just 1 cm downstream from the throat. They comprise a set of four 1-mm diameter holes at a 30° angle to the flow, placed evenly around the circumference of the channel and connected to a relatively highpressure feed plenum. They can provide a gas flow of 0-75 L/min. The instrumentation provides several channels of temperature histories as well as the histories of CO and O2 concentration in the exhaust gas. Other process parameters such as flow rates and pressures were noted manually as appropriate. The heart of the data gathering was a Fluke Hydra Series II data logger connected to a laptop PC. The CO was monitored with a Bacharach Monoxor II H electronic gas analyzer. The O2 was monitored with a Bacharach Oxor II electronic gas analyzer. The sampling hoses for these instruments were shortened to improve the response time, and the electronic output was routed directly to the data logger. We typically recorded three or four channels of temperature histories, one for the metal reflector, one for the aluminum injector ring, and one or two at various locations in the exhaust channel to monitor the exhaust gas temperature. Results We have performed over 50 experimental runs where appreciable CO was produced using only solar energy. Temperature and product gas composition histories from one such test run are shown in Figure 2. In these run histories, data are gathered and recorded from each channel every 2 s. Mirror segments were deployed (uncovered) a few at a time in this and most runs to avoid thermal shock to the ZrO2 rod. The CO production
Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1937 Table 1. Test Runs with Jet Flow conversion peak highest CO of solar into feed flow jet flow exhaust percentage chemical energy run (L/min) (L/min) temp (°C) (%) (%) 30 32 58 62
Figure 2. Temperature and gas concentration histories for a test run, along with an in-process CO meter calibration.
rises in steps in response to the increased power. For example, in the test run shown in Figure 2, one mirror was deployed from the start. Then, at 9:47, six mirrors were deployed. Fourteen were deployed by 9:48, and finally, all 22 were deployed by 9:49. Significant CO is first observed when just six segments were deployed, and the temperature of the exhaust gases measured by the thermocouple 6 cm downstream of the throat read about 300 °C. At this time, the temperatures in the injector ring and the body of the silver reflector had barely begun to rise above ambient. By about 9:50, a quasi-steady radiation intensity and temperature in the hot zone were obtained. The product gas reached a CO concentration near 4%, and the O2 level reading, while troubled by a higher noise level ((0.8%), indicated near 1.5%. In other runs with tighter seals, the oxygen shows a more faithful response of one-half of the CO reading. As is visible in this run, the O2 sampling and monitoring system has a faster response time than the CO sampling and monitoring system. In the test run shown in Figure 2, the CO2 feed rate was 10 L/min, and the jet flow rate was zero. Immediately after the run, at 9:55:30, the converter was taken off of the sunlight focus, and a calibration sample was injected through the jet gas line into the system while the primary feed was unchanged. The temperatures were still rather high, as can be read from the figure. The point at which the converter was taken offsun is obvious from the temperature slope reversal seen in the figure. The injected calibration sample was 405 cm3 of Matheson CP-grade CO at ambient temperature and pressure. Analysis of the area under the calibration curve at its known injection flow rate provides a calibration factor for this run and other runs with identical monitor and sampling configuration. In this configuration, the resultant calibration factor was 1.05, a very modest adjustment to the direct reading. Other calibration checks indicated that the meter was operating within specifications. We completed many test runs to establish the consistency of the prototype and to characterize its behavior. Most of the runs were performed with a CO2 feed flow rate of 10 L/min STP, all at ambient pressure. In the many test runs performed without jet flow (such as the example in Figure 2), we recorded a range of relatively high CO percentages, with the highest being 6.0% and with many over 4%. The efficiency of conversion of solar energy into chemical energy was generally between 2% and 3%. This efficiency is the enthalpy change between the products and the reactants at room temperature divided by the energy delivered to the converter. There were fluctuations in the exhaust gas
12 10 10 10
76 63 60 18
220 183 332 433
1.2 1.0 0.9 2.1
5.14 3.78 3.12 2.97
temperature because of imperfect solar tracking, but the exhaust temperature 6 cm downstream of the throat was between 500 and 600 °C during most of the fullpower running time. Temperatures were measured at other positions in the exhaust channel in various runs in the set, as discussed in the following section. In some test runs, jets were turned on at various flow rates, as shown in Table 1. The immediate effect is a drop in the product gas temperature and the CO and O2 concentrations. These decreases are primarily the result of dilution. It will be noted, however, that the CO concentration is generally a little higher than would be expected from simple dilution. This is probably because the cool gas injection by the jets quenches the back reaction more rapidly than when no jet gas is flowing, leaving a larger net production of CO. We see a higher conversion of solar to chemical energy for these runs than we do for those without jet flow. The best run in this regard is run 30, which yielded 5% solar-to-chemical energy conversion despite many compromises in optics and flow in this first set of experiments. In certain test runs, steam was inserted into the feed stream to determine whether catastrophic poisoning of the CO production reaction would occur. We found a modest quenching effect when 2.9 L/min ambient pressure flow of steam was added to the 10 L/min flow of CO2 feed. The steam injection brought the CO production percentage from 1.6% down to 0.9%, an amount that is not much greater than the feed gas dilution alone would cause. We did other experiments with nitrogen as a feed gas, and they showed that the CO production went to zero, as expected. In Table 2 we show the measured and estimated temperatures for test runs without jet flow at various points in the process gas channel and the temperatures of the reflector cone and the feed ring. We refer to these latter two structures, which typically stay within a few degrees of each other, as the metal core. There are no choke points in the flow. All cooling was by convection with some radiation at high temperatures. All of the temperatures below 1400 °C were measured directly in multiple runs. They represent an estimate of the weighted average (over several runs) early in the period of high CO production. The temperature values differ a little from run to run and vary within a run but by not more than about 100 °C. The reaction zone temperature is estimated by the behavior of the ZrO2 rods. They are advertised as operable up to 2400 °C. After many of the runs, we found the rod partially melted or largely glazed. With a parabolic reflector, which gives higher optical intensity at its focal point than a straight-wall reflector, the ZrO2 rods melt completely. We use the observation of a partial melt as an indicator of 2400 °C. We know that the CO2 gas becomes absorbing at these temperatures1,6 and tends to aid in the heating of the bulk gas as it moves along in this nominally laminar flow. To get a sense of how the process may perform, it is worthwhile to
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Table 2. Converter and Exhaust Channel Operating Temperatures without Jet Flow reaction zone position downstream from the throat (cm) temp (°C)
throat
jet position
exhaust funnel
tail pipe
metal core
-4 to 0
0
1.0
2.2
6
n/a
2350
2300
1350
850
550
250
estimate how much energy is getting into the gas and how it is distributed between sensible heat and dissociation of the CO2 to CO for temperatures of 2350 and 2500 °C. In these calculations, we chose the thermodynamic path of heating the gas to a temperature and then adding the enthalpy of dissociation to CO + 1/2O2 at that temperature. We simply use an average constant value of 11. 9 cal/(°C mol) for the heat capacity of CO2 over this range and 65.7 kcal/mol for the enthalpy of dissociation to CO + 1/2O2 at this temperature.10 This simple method neglects the fact that some of the O2 product is also dissociated and, therefore, somewhat underestimates the energy in dissociation. At 2350 °C, at the measured flow rates during operation, about 1251 W of energy (30% of the total from the mirror) are deposited initially in the gas: 388 W as dissociation energy and 862 W as thermal energy. This assumes that dissociation is in thermal equilibrium (at 2350 °C CO2 is 19% dissociated). At the higher temperature of 2500 °C, we would have 1490 W of energy (35% of the total from the mirror) deposited in the gas: 572 W as dissociation energy and 918 W as thermal energy. At 2500 °C, the CO2 is 28% dissociated.
Figure 3. Calculated concentrations of CO, O, and O2 in the CO2 process gas, initialized at the throat of the reflector cone and flowing down the exit channel at the typical flow rate of the actual tests.
Discussion The back reactions, as discussed here, contribute several hundred watts of heat load to the exhaust channel. This heat source is indicated by the fact that the melting of the ZrO2 rod extends several millimeters downstream from the throat of the reflector cone and at least 1 cm downstream from the zone of highest irradiation. The rapid cooling of the exhaust channel indicates a strong participation of radiant heat transfer in this system, which is also consistent with the optical participation of CO2 in the heat balance of the system. Some cooling also derives from a very slight leakage of air between the flanges at the rear of the apparatus. This is supported by the observation that the oxygen monitor never indicates zero. Its base reading in the experiments seems to be more like 0.5-1%, which when added to the 0.8% noise, explains the observed oxygen monitor baseline traces. We performed a simple, one-dimensional calculation of the concentrations of the species CO, O, O2, and CO2 using the known rates9 for the four salient chemical reactions
CO + O + M ) CO2 + M
(2)
CO2 + O ) CO + O2
(3)
CO + O2 ) CO2 + O
(4)
O + O + M ) O2 + M
(5)
The measured flow rates and temperatures (linearly interpolated between the four measured or estimated points in Table 2) along the converter exit channel were used as model inputs. The calculation was based on the simple assumption that all of the CO and O were
Figure 4. Monte Carlo ray tracing calculation of optical absorption in, and optical intensification by, the straight-walled conical reflector in the converter.
instantly formed in a single step at the throat of the exit channel. The results of the calculation are shown in Figure 3. The model shows a significant amount of back reaction, to the extent of approximately 50-70% of the initially formed CO. The model also provides a basis for understanding our observed CO percentage as it relates to temperatures in the reaction zone and along the exit channel. Observing about 5% CO in the final exhaust gas implies an initial concentration of about 12% CO at the throat. The model shows that, by the position of the jets (1 cm down the channel), only 20% of the original O atom concentration remains. This is in agreement with our observation that operation of the jets in their current position can increase the net CO product by only about 20-25%. We have performed Monte Carlo-type ray tracing calculations on the performance of the mirror and concentrator system as a function of the reflectivity of the concentrator. In this simple model, we neglected the energy absorbed by the gas itself. The results are summarized in Figure 4. It shows that to absorb 70% of the energy into the reflector, as we are doing, implies a primary reflectivity of only 80%. This corresponds to
Ind. Eng. Chem. Res., Vol. 41, No. 8, 2002 1939
Figure 5. Schematic showing energy conversion goals for a direct solar reduction system based on 100 kW initial solar input.
a maximum radiation density of only 4.5 times the incident irradiance, which is 44% lower than the possible 8-fold concentration in the case of a reflectivity of 98%. If the reflectivity could be increased to 95%, only 26% of the energy would be absorbed in the reflector cone, and the peak radiation density would increase to 7 times its initial irradiance. The salient result of this initial study is that with common materials and modest solar concentration, net CO2 to CO conversion approaching the useful range can be achieved. Due diligence was used to investigate whether the CO could have originated from some reaction other than eq 1. The hot metals (200-350 °C) exposed to the process gas were the aluminum feed ring and the silver reflector. They showed no signs of attrition or oxidation through the dozens of runs. The steel outside envelope of the apparatus achieved temperatures no greater than 120 °C and showed no noticeable oxidation. The temperature of the steel that comprised the jet ring remained ambient when the jets were running and increased only somewhat during the test runs without jets. It received no direct exposure from the sun. This steel area showed minimal oxidation or even tarnish after 70 runs. Furthermore, the operation of the overall system showed a rapid and faithful CO production response to the brightness of irradiation that was directed to the rear of the reflector funnel. All evidence supports the model that the concentrated light on the small ZrO2 rod initiated strong heating in the CO2 gas, which then dissociated to CO and O. The back reaction was partially quenched by the rather sudden cooling at the relatively cool reflector walls so that substantial amounts of CO and O2 were detected downstream. The small dimensions of the annular exit channel hasten the cooling. The fact that the oxygen monitor showed a concentration of oxygen approximately one-half that of the CO is strongly confirmatory of the model, as is the observation that, in many of the runs, up to 1/4 mol of CO was produced. If the CO came from metal oxidation by the CO2, over several runs, this would amount to a substantial consumption of the critical metal parts in the apparatus.
The overall process energy balance goals are shown in Figure 5. We believe that all of these values are achievable simultaneously with modest improvements in the mirror system and design changes in the exhaust channel/quenching system. The values in the figure are based on a hypothetical design with an optical power input of 100 kW. Of the 92 kW reaching the converter, 22 kW remain captured as chemical energy in the CO. The rate of energy capture of the final fuel product will depend somewhat upon the synthesis process chosen, but will be in the range of 18-20 kW. Most of the 70 kW of waste heat will pass at very high temperatures, enabling the harnessing of 25 kW as electrical power using rather standard steam turbine technology. The design of the converter system can easily recycle some of the waste heat to preheat the CO2 feed. At these efficiencies, we believe that the renewable energy costs from this system will be competitive at an early date. Acknowledgment We acknowledge the contribution of Dr. Melvin Prueitt for the Monte Carlo ray tracing calculations and those of Mr. Ara Stevens for expert assembly and performance of the many experimental runs required for this study. Literature Cited (1) Jensen, R.; Guettler, R.; Lyman, J. The Ultraviolet Absorption Spectrum of Hot Carbon Dioxide. Chem. Phys. Lett. 1997, 277, 356. (2) Cossart-Magos, C.; Launay, F.; Parkin, J. E. High-resolution absorption spectrum of CO2 between 1750 and 2000 Å. 1. Rotational analysis of nine perpendicular-type bands assigned to a new bent-linear electronic transition. Mol. Phys. 1992, 75, 835. (3) Spielfiedel, A.; Feautrier, N.; Cossart-Magos, C.; Chaumbaud, G.; Rosmus, P.; Werner, H. J.; Botschwina, P. Bent valence excited states of CO2. J. Chem. Phys. 1992, 97, 8382. (4) Knowles, P. J.; Rosmus, P.; Werner, H. J. On the Assignment of the Electronically Excited Singlet States in Linear CO2. Chem. Phys. Lett. 1988, 146, 230. (5) Dixon, R. The carbon monoxide flame bands. Proc. R. Soc. London 1963, A275, 431. (6) Jensen, R.; Lyman, J. Solar Conversion of CO2 to Fuel. Proceedings of the 4th International Conference on Greenhouse Gas Control Technology, Aug 30-Sept 2, 1998, Interlaken, Switzerland. (7) Koshi, M.; Yoshimura, M.; Matsui, H. Photodissiciation of O2 and CO2 from vibrationally excited states at high temperatures. Chem. Phys. Lett. 1991, 176, 519. (8) Rablais, J. W.; McDonald, J. M.; Scherr, V.; McGlynn, S. P. Electronic Spectroscopy of Isoelectronic Molecules. II. Linear Triatomic Groupings Containing Sixteen Valence Electrons. Chem. Rev. 1971, 71, 73 and references therein. (9) Tsang, W.; Hampson, R. F. Chemical Kinetic Data Base for Combustion Chemistry, Part 1, Methane and Related Compounds. J. Phys. Chem. Ref. Data 1986, 15, 1087. (10) JANAF Thermochemical Tables, Dow Chemical Company: Midland, MI, 1965.
Received for review October 24, 2001 Revised manuscript received January 23, 2002 Accepted January 30, 2002 IE010871X
