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Energy & Fuels 2006, 20, 1382-1387
Chemical Looping Combustion Using NiO/NiAl2O4: Mechanisms and Kinetics of Reduction-Oxidation (Red-Ox) Reactions from In Situ Powder X-ray Diffraction and Thermogravimetry Experiments Jennifer E. Readman, Anja Olafsen, Jens B. Smith, and Richard Blom* SINTEF Materials and Chemistry, P.O. Box 124, Blindern, Oslo, N-0314, Norway ReceiVed December 23, 2005. ReVised Manuscript ReceiVed April 5, 2006
In situ powder X-ray diffraction has been used to study NiO supported on NiAl2O4 during several reductionoxidation cycles, mimicking chemical looping combustion. Hydrogen and methane were used as fuel (reducing agents). Direct reduction and reoxidation of NiO/Ni is observed, and NiAl2O4 remained inert during the reduction and reoxidation processes. Thermogravimetric analyses of the material under the same reducing conditions, using a 90-210 µm particle fraction suitable for fluidized-bed applications, showed that, first, a rapid reduction occurs where oxygen transport to the particle surface not is rate-limiting. The rapid reduction is followed by a much slower reduction, where oxygen transport through the particle is expected to be ratelimiting. The fast reduction reaction is determined to be first order, with respect to H2, whereas an order slightly smaller than unity is observed when using CH4 as a reducing agent. Reoxidation is observed to be first order, with respect to O2. At low reactive gas concentrations, the reaction rates decreases in the following order: CH4 > H2 > O2.
Introduction The increasing concern over the amount of carbon dioxide (CO2) that is released into the atmosphere during fossil fuel combustion, and the subsequent effects that this phenomenon has on the planet’s climate has led to research into ways of reducing these emissions. In power plants, one possible method for CO2 separation is chemical looping combustion (CLC), in which CO2 is removed internally during the combustion process.1,2 The method involves the use of a two-reactor system: an “oxidizer” and a “regenerator”. In the oxidizer, a reducible metal oxide, which acts as an oxygen carrier, oxidizes the fossil fuel. The CO2 and H2O that are produced in the oxidizer can be easily separated and the CO2 used in other processes, whereas, in the regenerator, the reduced metal oxide is reoxidized using air. The effluent gases from the two reactors can be used to drive turbines for power production. The oxygen carrier material is continuously circulated between the two reactors. In situ powder X-ray diffraction (XRD) methods have previously been used to study the reduction and detection of intermediate phases of metal oxides of iron, cobalt, and manganese using hydrogen gas3-11 and to study the catalyst * Author to whom correspondence should be addressed. Fax: +47 22067350. E-mail:
[email protected]. (1) Richter, H. J.; Knoche, K. ACS Symp. Ser. 1883, 235, 71. (2) Ishida, M.; Jin, H. Energy 1994, 19, 415. (3) Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Kim, J. Y.; Pe´rez, M. J. Am. Chem. Soc. 2002, 124, 346. (4) Kim, J. Y.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L.; Rodriguez, J. A. J. Phys.: Condens. Matter 2004, 16, 3479. (5) Richardson, J. T.; Scates, R. M.; Twigg, M. V. Appl. Catal., A 2004, 267, 35. (6) Rossignol, S.; Gerard, F.; Mesnard, D.; Kappenstein, C.; Duprez, D. J. Mater. Chem. 2003, 13, 3017. (7) Kim, J. Y.; Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L. J. Am. Chem. Soc. 2003, 125, 10684. (8) Richardson, J. T.; Scates, R.; Twigg, M. V. Appl. Catal., A 2003, 246, 137.
degradation/stability during the partial oxidation of methane by co-feeding the reactants.12-16 In the present work, we report the use of in situ powder XRD to study not only the reduction of a supported binary metal oxide (in this case, NiO supported on NiAl2O4) using hydrogen or methane as a reducing agent, but also the subsequent reoxidation. Methane was of particular interest, because the ultimate objective of the experiments was to mimic the conditions for CLC using natural gas as fuel.17-19 To our knowledge, this is the first example of the study of the cycling of redox reactions with in situ powder XRD. One critical property for an oxygen carrier material that is used in a CLC process is that it must withstand a high number of redox cycles, both chemically and physically. The oxygen capacity also must be significant, depending on the specific reactor setup used for the process. In previous investigations, NiO/NiAl2O4 has been proven to be a suitable oxygen carrier.19-21 (9) Pirovano, C.; Vannier, R. N.; Nowogrocki, G.; Boivin, J. C.; Mairesse, G. Solid State Ionics 2003, 159, 181. (10) Rodrigeuz, J. A.; Kim, J. Y.; Hanson, J. C.; Perez, M.; Frenkel, A. I. Catal. Lett. 2003, 85, 247. (11) Rodriguez, J. A.; Kim, H. Y.; Hanson, J. C.; Brito, J. L. Catal. Lett. 2002, 82, 103. (12) Pickering, I. J.; Maddox, P. J.; Thomas, J. M. Chem. Mater. 1992, 4, 994. (13) Jones, R. H.; Ashcroft, A. T.; Waller, D.; Cheetham, A. K.; Thomas, J. M. Catal. Lett. 1991, 8, 169. (14) Ashcroft, A. T.; Cheetham, A. K.; Jones, R. H.; Natarajan, S.; Thomas, J. M.; Waller, D.; Clark, S. M. J. Phys. Chem. 1993, 97, 3355. (15) Widjaja, H.; Sekizawa, K.; Eguchi, K.; Arai, H. Catal. Today 1999, 47, 95. (16) Katoh, M.; Orihara, M.; Moriga, T.; Nakabayashi, I.; Sugiyama, S.; Tanaka, S. J. Solid State Chem. 2001, 156, 225. (17) Mattisson, T.; Lyngfelt, A.; Cho, P. Fuel 2001, 80, 1953. (18) Villa, R.; Cristiani, C.; Groppi, G.; Leitti, L.; Forzatti, P.; Cornavo, U.; Rossini, S. J. Mol. Catal. A: Chem. 2003, 204-205, 637. (19) Ishida, M.; Yamamoto, M.; Ohba, T. Energy ConVers. Manage. 2002, 43, 1469. (20) Ryu, H.-J.; Lim, N.-Y.; Bae, D.-H.; Jin, G.-T. Korean J. Chem. Eng. 2003, 20, 157. (21) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83, 1215.
10.1021/ef0504319 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/20/2006
Chemical Looping Combustion Using NiO/NiAl2O4
The objective of the present study is to establish procedures to obtain information about any possible intermediate phases formed during the redox reactions, and to determine if there is any participation of the support. Because the process under investigation is cyclic, it is possible to investigate the reversibility of the process, with respect to both complete reduction/ oxidation and sample integrity. In addition, we have performed thermogravimetry (TG) measurements to explore the kinetics of the reduction reactions, using the two different reducing agents as well as the reoxidation in air.
Energy & Fuels, Vol. 20, No. 4, 2006 1383 Table 1. Reproducibility Test of Kinetic Parameters from Thermogravimetry (TG) Experiments Reduction Rate, ∂ω/∂t (min-1)
experiment
mass (mg)
15% H2
5% H2
I II IIIc IVc V
0.0748 0.1603 0.0732 0.0732 0.1195
-0.0426 -0.0410 -0.0426 -0.0391 -0.0420
-0.0148 -0.0132 -0.0150 -0.0151 -0.0161
15% CH4 5% CH4 -0.0403 NPb -0.0428 -0.0414 -0.0438
-0.0254 NPb -0.0191 -0.0234 -0.0247
oxidation rate, ∂ω/∂t, with 20% O2 (min-1) 0.0569 0.0268 0.0476 0.0445 0.0503
a The total flow in all experiments is 400 mL/min (reacting gas + argon). Not performed. c Experiments III and IV were conducted before and after 4 days of annealing at 800 °C.
b
Experimental Section NiO/NiAl2O4 that contained 60 wt % NiO was prepared in a similar manner as that described by Ishida et al.:19 101.75 g (0.271 mol) of Al(NO3)3‚9H2O (>98.5%, from Merck) and 179.03 g (0.616 mol) of Ni(NO3)2‚6H2O (>98%, from Fluka) were dissolved in 50/50 mixtures of 2-propanol and water, making two solutions that have ∼0.9 mL of solvent per gram of nitrate salt. The two solutions were mixed together and heated to ∼230 °C with continuous stirring. After the desired viscosity had been attained, the mixture was transferred to an oven at 150 °C for 2 days. The dry product was transferred to a mortar and crushed. The resulting powder was calcined in air, using the following program: 5 °C/min to 1200 °C, then kept at 1200 °C for 6 h, before cooling down to ambient temperature in the oven. The final powder was then crushed and sieved (70-170 mesh, 90-210 µm). The specific surface area was measured to 2.0(2) m2/g from three one-point BET measurements, using a Quantachrome Monosorb instrument. (The value given in parentheses represents the estimated error in the last digit of the measurement.) In situ powder XRD experiments, using finely ground powders, were performed on a Siemens model D5000 diffractometer, using Cu KR radiation (λ ) 1.5418 Å). The diffractometer was configured in θ-2θ reflection mode (Bragg-Brentano geometry), and a scintillation counter was used as the detector. The temperature control unit, sample holder (with platinum filament), gas feeding system, and the in situ cell (volume estimated to be ∼1 L) used has been described in detail elsewhere.22 For the experiments using hydrogen as the reducing agent, 10% H2 in argon (flow rate of 10 mL/min, equating to 1 mL/min H2), air (flow rate of 5 mL/min, equating to 1 mL/min O2), and helium (flow rate of 62.5 mL/min) were used. For the methane experiments, the flow rates for air and inert were kept at the same values. A flow rate of 2 mL/min of methane was used. This was diluted in helium, giving a total flow rate of 12 mL/min and an overall concentration of 16.7% CH4 in helium. For all experiments, a heating rate of 15 °C/min was applied from room temperature to 800 °C. The mass-change measurements were conducted at 800 °C in a TG balance that was built in-house.23 Separate experiments were performed to verify that the derived rates were independent of the mass of the sample and gas flow rates used in the experiments. Kinetically relevant results were obtained using small sample masses and wide platinum cups where the powder can be placed flat, to minimize diffusion distances. The change in the mass conversion with time (∂ω/∂t) is given in Table 1, using different amounts of sample. The mass conversion (ω) is defined as ω ) m/mox, where m is the instantaneous mass of the carrier and mox is the mass of the fully oxidized carrier. The experiments that were conducted with the highest sample mass gave underestimated kinetic parameters, whereas, for the two lower sample masses, the results were within the error of the experiment, which indicated negligible gas diffusion limitations in these experiments. The flow-rate dependency was also determined to be insignificant for total flows of 300-500 mL/min, which is the range chosen to secure a rapid change in atmosphere when going from one gas (22) Readman, J. E.; Olafsen, A.; Larring, Y.; Blom, R. J. Mater. Chem. 2005, 15, 1931. (23) Larring, Y.; Haugsrud, R.; Norby, T. J. Electrochem. Soc. 2003, 150, B374.
composition to another. Using the measurable difference in gas viscosities of different gas mixtures, we could measure that the time for complete transformation from one gas composition to the next is
