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Chemical Looping Combustion in a Rotating Bed Reactor Finding Optimal Process Conditions for Prototype Reactor † Silje Fosse Hakonsen and Richard Blom*,† †
SINTEF Materials & Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway
bS Supporting Information ABSTRACT: A lab-scale rotating bed reactor for chemical looping combustion has been designed, constructed, and tested using a CuO/Al2O3 oxygen carrier and methane as fuel. Process parameters such as bed rotating frequency, gas flows, and reactor temperature have been varied to find optimal performance of the prototype reactor. Around 90% CH4 conversion and >90% CO2 capture efficiency based on converted methane have been obtained. Stable operation has been accomplished over several hours, and also stable operation can be regained after intentionally running into unstable conditions. Relatively high gas velocities are used to avoid fully reduced oxygen carrier in part of the bed. Potential CO2 purity obtained is in the range 30 to 65% mostly due to air slippage from the air sector which seems to be the major drawback of the prototype reactor design. Considering the prototype nature of the first version of the rotating reactor setup, it is believed that significant improvements can be made to further avoid gas mixing in future modified and up-scaled reactor versions.
’ INTRODUCTION A chemical looping combustion (CLC) process was already suggested in the 1950s as a way to produce pure carbon dioxide.1 It was further developed as a combustion technique in the 1980s2 and later in the 1990s presented as a possible way to separate CO2 during fossil fuel combustion.3 The interest in CLC has boosted during the past decade due to its relatively high net energy efficiency4,5 and potential low cost of CO2 capture.6 CLC is a cyclic process where a metal oxide first is used to combust a fuel, and then the reduced metal oxide is reoxidized in air before a new cycle can be carried out. Such a red-ox cycle can in principle be carried out in two ways; either i) by moving the metal oxide between static gas streams or ii) by keeping the metal oxide static while switching the gas streams. Option i) is in most cases implemented by a circulating fluidized bed (CFB) reactor setup where the metal oxide powder circulates between a fuel reactor in which the combustion takes place and an air reactor where reoxidation takes place.7,8 CFB reactors have recently gained far the most attention within the CLC community since this reactor type already is commercial for combustion processes (boilers) and within refinery processes such as fluidized catalytic cracking (FCC). Option ii) most often involves one or more fixed bed reactors where complex valving sequences ensure cyclic gas feeding to the reactors and optimal gas separation. Early CLC experiments were carried out in single fixed bed reactors.3,9 We have recently developed an alternative reactor concept for CLC which belongs to the option i) group above in which the metal oxide is kept in a doughnut shaped fixed bed that rotates between the different gas streams flowing radially outward through the bed. A schematic drawing of the reactor along the rotation axis is shown in Figure 1. It is believed that a radial gas flow is a r 2011 American Chemical Society
good choice for a CLC process due to gas expansion caused by temperature increase and increase in moles of gas in the system. This could be handled by exploiting the natural increase in bed volume by going from small to large radius. A similar reactor concept was already suggested for “conversion of organic reactants to other organic products” in 1955.10 Rotating bed reactors have also previously been suggested for CO2 temperature swing sorption processes, but, to our knowledge, only at a conceptual level.11 In addition, the use of rotating packed bed (RPB) reactors for CO2 separation using alkanolamine solutions has recently been demonstrated.12 We have in an earlier communication described the basics of our rotating bed reactor system showing that separation of the gases is possible, although some internal gas mixing does occur.13 The present paper results from the first series of experiments carried out at elevated temperatures using methane as fuel. A supported copper oxide (CuO/Al2O3) oxygen carrier has been used. CuO is chosen for two reasons: First because of its fast red-ox kinetics already at relatively low temperatures (90% CO2 capture efficiency based on converted methane. It is natural to believe that the internal gas mixing observed is mainly caused by two factors: 1) by gas diffusion along the interfaces between the parts moving relative to each other inside the reactor and 2) by gas dispersion in the oxygen carrier bed. The former source for mixing should be strongly reduced when up-scaling the reactor since diffusion pathways then is strongly reduced. The second source for mixing will be strongly dependent on the shape of the oxygen carrier material used. Although the reactor has only been operated in periods of up to 6 h, we see that operation is possible over many hours, and also that stable operation can be regained after running into unstable conditions. Stable operation is obtained at relatively high gas velocities where fully reduced oxygen carrier in (the inner) part of the bed is avoided. Potential CO2 purities obtained are in the range 20 to 65% the lack of purity mostly being due to air slippage from the air sector. This seems to be the major drawback of the prototype reactor setup. We believe that significant improvements can be made to further avoid gas mixing in future modified and upscaled reactor versions, mainly because diffusion distances along
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the interfaces then will be significant longer, while the distance between the moving parts still can be kept at a minimum. It is also important to notice that the performance of the rotating bed reactor is strongly dependent on the choice of geometry and design of the individual parts. With the chosen design some adjustments of the performance can be done by varying the process conditions, but the optimal performance is limited by nonadjustable factors. The knowledge gained from the work with this first prototype reactor should be used to design and construct an improved second version reactor with the main focus on high pressure operation at temperatures above 1000 °C, as well as on oxygen carrier stability over prolonged time-on-stream.
’ ASSOCIATED CONTENT
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Supporting Information. Figures. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Phone: +47 90622647. Fax: +47 22067350. E-mail: richard.blom@ sintef.no.
’ ACKNOWLEDGMENT This publication forms a part of the BIGCO2 project, performed under the strategic Norwegian research program Climit. The authors acknowledge the partners: Statoil, GE Global Research, Statkraft, Aker Clean Carbon, Shell, TOTAL, ConocoPhillips, ALSTOM, the Research Council of Norway (178004/I30 and 176059/I30), and Gassnova (182070) for their support. ’ REFERENCES (1) Lewis, W. K.; Gilliland, E. R. Production of pure carbon dioxide. S.O.D. Company, US Patent: 2,665,971, 1954. (2) Ritcher, H.; Knoche, K. Reversibility of combustion process. ACS Symp. Ser. 1983, 235, 71–85. (3) Ishida, M.; Jin, H. A novel combustor based on chemical-looping combustion reactions and its reactions kinetics. J. Chem. Eng. Jpn. 1994, 27, 296–301. (4) Kvamsdal, H. M.; Jordal, K.; Bolland, O. A quantitative comparison of gas turbine cycles with CO2 capture. Energy 2007, 32, 10–24. (5) Erlach, B.; Schmidt, M.; Tsatsaronis, G. Comparison of carbon capture IGCC with pre-combustion decarbonisation and with chemicallooping combustion. Energy 2011, 36, 3804–3815. (6) Ekstrom, C.; Schwendig, F.; Biede, O.; Franco, F.; Haupt, G.; de Koeijer, G.; Papapavlou, C.; Røkke, P. E. Techno-Economic Evaluations and Benchmarking of Pre-combustion CO2 Capture and Oxy-fuel Processes Developed in the European ENCAP Project. Energy Proc. 2009, 1, 4233–4240. (7) Berguerand, N.; Lyngfelt, A. Batch testing of solid fuels with ilmenite in a 10 kWth chemical-looping combustor. Fuel 2010, 89, 1749–1762 and references therein. (8) Pr€ oll, T.; Kolbitsch, P.; Bolhar-Nordenkampf, J.; Hofbauer, H. A novel dual circulating fluidized bed system for chemical looping combustion. Environ. Energ. Eng. 2009, 55, 3255–3266. (9) Jin, H.; Ishida, M. Reactivity study on natural-gas-fueled chemical-looping combustion by a fixed-bed reactor. Ind. Eng. Chem. Res. 2002, 41, 4004–4007. (10) Thayer, C. H. Method and apparatus for conversion of organic reactants to other organic products, US Patent: 2,704,741, 1955. (11) Shimomura, Y. The CO2 wheel: a revolutionary approach to carbon dioxide capture. Modern Power Systems 2003, January, 15–17. 9625
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