Energy Fuels 2009, 23, 6207–6212 Published on Web 10/05/2009
: DOI:10.1021/ef9007246
Coproduction of Syngas and Lime by Combined CaCO3-Calcination and CH4Reforming Using a Particle-Flow Reactor Driven by Concentrated Solar Radiation V. Nikulshina,† M. Halmann,‡ and A. Steinfeld*,†,§ †
ETH Zurich, Department of Mechanical and Process Engineering, 8092 Zurich, Switzerland, ‡Weizmann Institute of Science, Department of Environmental Sciences and Energy Research, Rehovot 76100, Israel, and §Solar Technology Laboratory, Paul Scherrer Institute, CH-5232 Villigen, Switzerland Received July 13, 2009. Revised Manuscript Received September 18, 2009
The combined CaCO3-calcination and CH4-reforming process is investigated using concentrated solar energy as the source of high-temperature process heat. The thermodynamic equilibrium composition indicates the coproduction of lime and syngas (H2/CO molar ratio = 1) at above 1200 K and 1 bar. Exploratory experimental runs were carried out with a 5 kW solar chemical reactor comprised of a windowed cavity-receiver containing a flow of CH4 with suspended CaCO3 particles and directly exposed to high-flux solar radiation. At an optimal reactor configuration, CaCO3 and CH4 conversions of 83% and 38%, respectively, were achieved during 11 min of irradiation at a solar concentration ratio of 1884 suns and a nominal temperature of 1223 K. Effecting simultaneously both reactions in a single solar-driven reactor eliminates CO2 emissions derived from the separate fossil-fuel-based production of these two energy-intensive material commodities.
converting CO2 into valuable chemicals.5-15 Effecting both processes simultaneously in a single reactor further improves energy efficiencies through concurrent high-temperature reactions. Supplying concentrated solar energy as the source of high-temperature process heat further avoids greenhouse gases and other pollutants derived from their conventional fossil-fuel-combustion-based production. The objective of the present study is to demonstrate the technical feasibility of the solar-driven combined calcination-reforming process.
1. Introduction In previous studies,1-4 a novel solar thermochemical process was proposed that combines the CaCO3-calcination with the CH4-reforming, in which the CO2 stream released by the calcination reaction (eq 1) is consumed by the dry-reforming reaction (eq 2), CaCO3 ¼ CaOþCO2
ΔH298K ° ¼ 164:9 kJ mol -1
CH4 þCO2 ¼ 2COþ2H2
ð1Þ
ΔH298K ° ¼ 247 kJ mol -1 ð2Þ
Thus, combining eqs 1 and 2 yields CaCO3 þCH4 ¼ CaOþ2COþ2H2 ¼ 425:2 kJ mol -1
2. Thermodynamic Analysis Equation 3 summarizes the overall process, but significant intermediate reactions of the syngas chemistry need to be considered, viz., water-gas shift : H2 þCO2 f H2 OþCO ΔH298K °
ΔH298K ° ð3Þ
Equation 3 proceeds endothermically at above 1000 K (ΔG1000K° = 0) to coproduce lime and syngas. The main benefit of such a combination is the considerable decrease of a-vis the separate production of these two CO2 emissions vis- energy-intensive material commodities2 and the possibility of
¼ -39:5 kJ mol -1 Boudouard : 2CO f CO2 þC
ΔH298K °
¼ -171 kJ mol -1
ð5Þ
CH4 -decomposition : CH4 f 2H2 þC
*To whom correspondence should be addressed. Telephone: þ41-446327929. Fax: þ41-44-6321065. E-mail:
[email protected]. (1) Halmann, M.; Steinfeld, A. Energy Fuels 2003, 17, 774–778. (2) Halmann, M.; Steinfeld, A. Stud. Surf. Sci. Catal. 2004, 153, 481– 486. (3) Nikulshina, V.; Hirsch, D.; Mazzotti, M.; Steinfeld, A. Energy 2006, 31, 1379–1389. (4) Halmann, M.; Steinfeld, A. Energy 2006, 31, 1533–1541. (5) Gadalla, A. M.; Bower, B. Chem. Eng. Sci. 1988, 43, 3049–3062. (6) Slade, D. A.; Duncan, A. M.; Nordheden, K. J.; Stagg-Williams, S. M. Green Chem. 2007, 9, 577–581. (7) Rostrup-Nielsen, J. R.; Hansen, J.-H. B. J. Catal. 1993, 144, 38– 49. (8) Laosiripojana, N.; Assabumrungrat, S. Appl. Catal., B 2005, 60, 107–116. (9) Bradford, M.; Vannice, M. A. Catal. Rev. Sci. Eng. 1999, 41, 1–42. (10) Hu, Y. H.; Ruckenstein, E. Adv. Catal. 2004, 48, 297–345. (11) Choudhary, T. V.; Goodman, D. W. J. Mol. Catal. A 2000, 163, 9–18. r 2009 American Chemical Society
ð4Þ
ΔH298K °
¼ 75 kJ mol -1
ð6Þ
C-gasification : CþH2 O ¼ COþH2
ΔH298K °
¼ 175 kJ mol -1
ð7Þ
all of which depend strongly on the temperature and pressure and determine the relative amounts of H2, H2O, CO, CO2, and (12) Wei, J. M.; Iglesia, E. J. Catal. 2004, 224, 370–383. (13) Sun, H.; Wang, H.; Zhang, J. Appl. Catal., B 2007, 73, 158–165. (14) Delmon, B. Appl. Catal., B 1992, 1, 139–147. (15) Pompeo, F.; Nichio, N. N.; Gonzalez, M. G.; Montes, M. Catal. Today 2005, 107-108, 856–862.
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: DOI:10.1021/ef9007246
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CH4 in the gas phase and CaCO3, CaO, and C in the solid phase. The equilibrium composition of the system constituted of 1 mol of CaCO3 and 1 mol of CH4 was computed16 at 1 bar total pressure and over a wide temperature range of interest. It is shown in Figure 1. Species whose mole fraction was less than 10-5 have been omitted. At ambient temperature, CaCO3 and CH4 are the thermodynamically stable components. Over a small temperature range above 700 K, CaCO3 decomposes into CaO and CO2 (eq 1) while CH4 decomposes into C and H2 (eq 6). Since catalysts are not employed in the present study, controlled carbon formation can be tolerated, which eventually is gasified by CO2 and H2O to form CO and H2 (eqs 5 and 7, respectively). At lower temperatures, RWGS consumes H2 and CO2 to produce additional CO, resulting in a higher conversion of CO2 than CH4. Equation 5 is exothermic while eq 6 is endothermic and favored at high temperatures. For obtaining high conversion, the dry-reforming reaction must be carried out at above 1100 K.17,18 The reaction goes to completion at above 1200 K to form CaO(s) and a gas phase consisting of an equimolar mixture of H2 and CO. At higher pressures, as preferred in industrial applications, the thermodynamic equilibrium of eq 3 is shifted to the left in such a way so as to relieve the pressure in accordance with the Le Chatelier’s principle.
Figure 1. Variation of the thermodynamic equilibrium composition with temperature for the system 1 mol of CaCO3 þ 1 mol of CH4 at 1 bar. Species whose mole fraction is less than 10-5 are omitted.
3. Solar Reactor Technology and Experimental Setup The solar reactor configuration is schematically shown in Figure 2. It consists of a cylindrical cavity-receiver containing a circular aperture, which is closed by a clear fused quartz window for the access of concentrated solar radiation. The aperture is equipped with a diverging water-cooled frustum for mounting the window at a plane where the radiative intensities are about 10 times smaller than those at the aperture and for providing a buffer zone where dust deposition is unlikely to occur. The window is actively cooled and kept clear from particles and/or condensable gases by means of an aerodynamic protection curtain created by combined tangential and radial flow nozzles. CH4 diluted with Ar and laden with CaCO3 particles is injected into the front part of the cavity through four inlet nozzles, located 30 mm behind the aperture, creating a gas-particle flow that progresses toward the rear part of the cavity as the reaction occurs. With this arrangement, this gas-particle flow is directly exposed to high-flux solar irradiation, providing efficient radiative heat transfer directly to the reaction site for driving the hightemperature highly endothermic process. Radiation entering the cavity undergoes multiple scattering among particles and multiple reflections within the cavity walls, until it is absorbed either by the particles or by the cavity walls, or eventually escapes through the aperture. Other mechanisms of heat transfer to the reacting flow include infrared radiation by hot particles and cavity walls, forced convection between the gas stream and the cavity walls and between the gas stream and particles, and conduction to particles that are swept across the hot reactor walls. This reactor concept has been successfully applied previously for the solar combined ZnO-reduction and CH4-reforming process,19 solar thermal cracking of CH4,17 and solar gasification of petroleum coke.20 A 5 kW prototype was fabricated with three geometrical configurations, as listed in Table 1. These different configurations evolved during the experimental campaign, as the reactor
Figure 2. Cross section of the solar reactor, comprised of a windowed cavity-receiver containing a flow of CH4 laden with CaCO3 particles and directly exposed to high-flux solar irradiation.
underwent a number of modifications aimed at improving the fluid dynamics and heat/mass transfer, reducing heat losses, avoiding solids sedimentation, and increasing the chemical yield and energy conversion efficiency of the process. The reactor wall temperatures were measured with type-K thermocouples (TC) inserted in the inner walls and not exposed to direct irradiation. Off-gas temperature was measured by type-S TC at the outlet tube. The highest measured wall temperature is taken as the reference temperature. A more indicative temperature of the reaction is the particle temperature, which can be calculated by radiative exchange in participating media21 and is estimated to be about 200 K higher than the wall temperature, as the directly irradiated particles serve as a radiation shield to the reactor walls. The experimental setup is depicted in Figure 3. Experimentation was carried out at the ETH High-Flux Solar Simulator (HFSS):22 a high-pressure Ar arc close-coupled to an elliptical reflector that delivers an external source of intense thermal radiation and simulates the heat transfer characteristics of highly concentrating solar systems, such as solar towers or dishes. The
(16) Roine A. Outokumpu HSC Chemistry for Windows; Outokumpu Research: Pori, Finland, 1997. (17) Hirsch, D.; Steinfeld, A. Int. J. Hydrogen Energy 2004, 29, 47–55. (18) Bharadwaj, S. S.; Schmidt, L. D. J. Catal. 1994, 146, 11–21. (19) Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, A.; Wuillemin, D. Energy 1998, 23, 803–814. (20) Z’Graggen, A.; Haueter, P.; Trommer, D.; Romero, M.; de Jesus, J. C.; Steinfeld, A. Int. J. Hydrogen Energy 2006, 31, 797–811.
(21) Maag, G.; Lipi nski, W.; Steinfeld, A. Int. J. Heat Mass Transfer, 2009, 52, 4997–5004. (22) Hirsch, D.; v. Zedtwitz, P.; Osinga, T.; Kinamore, J.; Steinfeld, A. ASME J. Solar Energy Eng. 2003, 125, 117–120.
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Table 1. Configurations of the 5 kW Solar Reactor Prototype cavity configuration I II III
outlet port
length/diameter [mm]
material
aperture’s diameter [mm]
200/100 200/100 210/120
steel alloy (10% Ni) steel alloy (10% Ni) Inconel 601 (60% Ni)
60 60 50
diameter [mm] 22 56/22 24
type tangential conical cylindrical
flow type vortex axial vortex
by thermogravimetry (TG, Netzsch STA 409 CD) and X-ray powder diffraction (XRD, Philipps XPert-MPD powder difractometer FeKR, λ = 1.937 40 A˚, 2θ = 20-80°, step with 0.05°) for the purpose of determining their degree of calcination. For selected samples, the particle size distribution was measured by laser scattering (HORIBA LA-950), and the particle morphology was examined by scanning electron microscopy (Toshiba TN-1000). The molar flow rate of gas species i (with i = H2, CO, CO2, CH4, Ar) is defined as n_ i ¼ n_ tot yi ð8Þ where yi is the molar fraction of species i measured by IR and gas chromatography (GC), and n_ tot is the total molar flow rate calculated from the molar flow rate and molar fraction of Ar. The methane conversion is defined as XCH4 ¼ 1 -n_ CH4, out ð9Þ n_ CH4, in The relative error of XCH4 was (12%, mainly due to fluctuations of the inlet gas flows caused by the flow controllers. The number of moles of CaCO3 and CaO in the product sample was determined from the weight loss obtained during the TG analysis, which enabled the calculation of the CaCO3 conversion (or degree of calcination), defined as nCaO XCaCO3 ¼ ð10Þ nCaCO3 þnCaO
Figure 3. Experimental setup at the ETH High-Flux Solar Simulator.
incoming radiative flux was measured optically by a calibrated CCD camera focused on a water-cooled Al2O3-plasma-coated Lambertian (diffusely reflecting) target and numerically integrated over the aperture area to yield the solar power input Qsolar with an accuracy of (913 %.20 The reactor was positioned with its aperture at the HFSS focal plane. The particle feeder consisted of a rotating brush connected to a piston of adjustable speed, calibrated prior to the runs. Inlet gas flows were controlled by electronic flow meters (Bronkhorst HI-TEC). The reactor was first heated under Ar atmosphere to the desired temperature that was limited by material constraints, e.g., melting point (mp) of the steel alloy 1350 °C. Once steady state was achieved, CH4 diluted in Ar and laden with CaCO3 particles was continuously fed at ambient temperature into the reactor. On the basis of previous studies with a similar reactor concept, the gas-particle flow underwent heating at rates exceeding 1000 K/s along the first centimeters after the inlet plane.23 The products exiting the reactor were quenched in the condenser and particles were filtered. The composition of the gaseous products was analyzed online by infrared (IR) gas analyzers for CO, O2, CH4, and CO2 (Siemens-23, 0.2% detection limit, 1 Hz sampling frequency) and H2 (Siemens Calomat-6, 50 ppm detection limit, 1 Hz sampling frequency). In addition, the gas composition was verified by gas chromatography (two-channel Varian Micro gas chromatograph, equipped with a Molsieve-5A and a Poraplot-U columns; 1/90 Hz sampling frequency). The reactor pressure was monitored with a pressure sensor (Keller PR/PAA-23 S Ei, 0-5 bar detection range) while a pressure safety valve was employed to prevent overpressure inside the cavity due to the thermal expansion and possible obstruction of the gas lines by solids accumulation that could lead to window destruction. Representative samples of solid products collected at the filter were analyzed
The hydrogen yield is calculated as nH2, out þnH2 Oout YH2 ¼ nCH4, in
ð12Þ
where nH2Oout is determined from the first mass drop in the TG analysis (to be shown in the next section), assuming that all released water vapor during the RWGS reaction is consumed by CaO according to CaO þ H2O = Ca(OH)2.
4. Results and Discussion Table 2 lists the reactor temperature, solar power input, mean solar concentration ratio (The mean concentration ratio C~ is defined as C~ = Qsolar/(IA), where Qsolar is the solar power intercepted by the aperture of area A. C~ is often expressed in units of “suns” when normalized to I = 1 kW/m2.), inlet mass/ molar flow rates of CaCO3, CH4, and Ar, chemical conversions of CH4 and CaCO3, H2 yield, and duration for nine solar experimental runs performed under approximate steady-state conditions. The gas composition and reactor temperature as a function of time for run no. 9 with the highest chemical conversion (XCH4 = 0.38, XCaCO3 = 0.83, YH2 = 0.39) are shown in Figure 4. The particle size distributions for the solid reactants CaCO3 and the solid product samples collected in the filter downstream are shown in Figure 5. The mean particle sizes were 6 μm for fed CaCO3 (all runs), and 35, 32, and 11 μm for agglomerated particles collected after experimental run nos. 7, 8, and 9. Figure 6 shows SEMs of the unreacted CaCO3 (Figure 6a) and of solid products of run no. 7 (Figure 6b) and no. 8 (Figure 6c). The reactants featured bundles of needles with diameters 1 in the gas products observed in all runs pointed out toward the occurrence of the RWGS reaction (eq 4) downstream, thermodynamically favored by lower temperatures. The second mass drop corresponds to the calcination of the remaining CaCO3. Configuration I with a tangential exit exhibited poor fluid dynamics resulting in undesired particle deposition and low CH4 conversions. Configuration II with an axial conical exit diminished particle deposition, reached faster steady-state conditions, and resulted in higher CH4 conversions. However both configurations I and II suffered from relatively high conduction and reradiation heat losses.24 Configuration III with an improved Al2O3/ZrO2 insulation and cavity geometry for efficient radiative absorption gave the highest XCH4, XCaCO3, and XH2. Generally, a higher operational temperature, an increase of CH4 concentration in the reacting gas, and a reduction of the total gas flow rate favored higher conversions as a result of faster kinetics and longer residence times. For all runs, XCH4 < XCaCO3, as the reaction rate for dryreforming of CH4 proceeded at a slower rate than that for the thermal decomposition of CaCO3. The kinetics of dry-reforming can be enhanced by employing catalysts,10-15,25 but their recovery from the solid product mixture would introduce a factor of complexity in a commercial large-scale operation. Inconel reactor walls (with 60% Ni) presumable catalyzed the reforming reaction; however, the catalytic effect is restricted by the limited inner cavity wall surface in contact with the flow. Particularly, it should be possible to considerably increase the CH4-reforming conversion, either by a moderate increase in the reaction temperature or by passing the CH4-CO2 product gas mixture over a dry-reforming catalyst to consume excess CO2. Both CH4 and CO2 are strongly absorbing in the infrared region and are, therefore, potent greenhouse gases. Under concentrated solar radiation, which has a large IR component, these gases and CaCO3 will undergo vibrational excitation, resulting in chemical reactions. At 2000 K, high CH4 and CO2 conversions to syngas were obtained without catalysts in a fluid-wall aerosol flow reactor subjected to concentrated solar irradiation.27 Downstream of the solar reactor, a condenser cooled the gas-particle flow, partially affecting the reverse water-gas shift reaction. The H2O produced reacted with CaO, forming Ca(OH)2, which decomposed during the first mass drop in the TG analysis.
Figure 4. Product gas composition and reactor temperature for solar run no. 9 with the highest chemical conversion.
Figure 5. Particle size distributions of solid reactants (CaCO3) and of solid products of experimental runs 7-9.
featured agglomerations of rhombohedral particles of sizes that were in agreement with the measured particle size distributions. Figure 7 shows the mass change during a dynamic TG analysis in Ar (180 mL/min) and in the 25-1000 °C range with the solid products collected from the solar experimental run nos. 7, 8, and 9. Two mass drops are observed. The first one in the temperature range 250-400 °C is attributed to the dehydration of Ca(OH)2, as verified by XRD. The presence of
(24) Schaller, J.; Vielle, P. Master Thesis, ETH Zurich, Zurich, Switzerland, 2006. (25) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117–127. (26) Welford, W. T.; Winston, R. High Collection Nonimaging Optics; Academic Press: San Diego, CA, 1989. (27) Dahl, J. K.; Weimer, A. W.; Lewandowski, A.; Bingham, C.; Bruetsch, F.; Steinfeld, A. Ind. Eng. Chem. Res. 2004, 43, 5489–5495.
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: DOI:10.1021/ef9007246
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Figure 6. SEMs of (a) solid reactants CaCO3, (b) solid products of experimental run no. 7, and (c) solid products of experimental run no. 8.
volume/surface ratio of the cavity, further reducing the portion of thermal energy lost by conduction heat transfer. Obviously, higher temperatures and longer residence times would result in higher chemical conversions and, consequently, higher ηsolar-to-chemical. 5. Conclusions We have demonstrated in exploratory experimental runs the technical feasibility of performing the combined CaCO3calcination and CH4-reforming process using simulated concentrated solar energy. The solar chemical reactor consisted of a gas-particle flow confined to a cavity-receiver and directly irradiated, providing efficient heat transfer to the reaction site. Tests were carried out in the temperature range 1123-1418 K and with solar concentration ratios up to 1884 suns. Chemical conversions without added catalysts were moderate, reaching 83% for CaCO3 and 38% for CH4, as residence times were less than 5 s and maximum temperatures were limited by the reactor materials. The solar-to-chemical energy conversion efficiency ηsolar-to-chemical varied in the range 7-10%. There is room for optimization toward maximizing ηsolar-to-chemical by reducing reradiation through the reactor aperture (e.g., incorporating a CPC) and conduction heat losses through the reactor insulation.
Figure 7. Relative weight loss and temperature as a function of time during a dynamic TG run in Ar with the solid products of the solar experimental runs 7-9.
The solar-to-chemical energy conversion efficiency ηsolar-to-chemical, defined as the ratio of the net absorbed energy by the chemical reaction to the solar power input, varied in the range 7-10%. No attempts were undertaken to optimize the reactor technology for maximum ηsolar-to-chemical. Major sources of irreversibility were reradiation losses through the reactor aperture (∼10% of Qsolar), conduction/transient losses through the reactor insulation (∼73% of Qsolar), and absorption/reflection at the window (∼7% of Qsolar). To some extent, reradiation losses can be minimized by incorporating a secondary concentrator (e.g., CPC26) at the reactor aperture to reduce its size while boosting C~ and keeping Qsolar constant. Scalingup to higher solar power levels would result in a larger
Acknowledgment. We thank J. Schaller and P. Vielle for the data collected during the experimental campaign with Configuration I.
Nomenclature C~ = mean solar concentration ratio d = particle diameter [m] I = direct normal solar irradiation m = mass [mg] 6211
Energy Fuels 2009, 23, 6207–6212
: DOI:10.1021/ef9007246
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m_ = mass flow rate [g/min] n_ = molar flow rate [mol/min] n = number of moles [-] q = frequency distribution fraction [%] RWGS = reverse water-gas shift T = reactor temperature [°C] t = time [min] Qsolar = solar power input [kW]
XCH4 = CH4 conversion [-] XCaCO3 = CaCO3 conversion [-] YH2 = hydrogen yield [-] y = molar fraction [-] ΔG = Gibbs free energy change ΔH = enthalpy change ηsolar-to-chemical = solar-to-chemical energy conversion efficiency
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