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Energy & Fuels 2006, 20, 34-44
Redox Investigation of Some Oxides of Transition-State Metals Ni, Cu, Fe, and Mn Supported on SiO2 and MgAl2O4 Qamar Zafar,*,† Tobias Mattisson,‡ and Bo¨rje Gevert† Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, S-412 96 Go¨teborg, Sweden, and Department of Energy ConVersion, Chalmers UniVersity of Technology, S-412 96 Go¨teborg, Sweden ReceiVed May 9, 2005. ReVised Manuscript ReceiVed October 5, 2005
Chemical-looping combustion (CLC) and chemical-looping reforming (CLR) involve the use of a metal oxide as an oxygen carrier which transfers oxygen from combustion air to the fuel. Two interconnected fluidized beds, a fuel reactor, and an air reactor are used in both processes. In the fuel reactor, the fuel is oxidized by a metal oxide, and in the air reactor, the reduced metal is oxidized back to the original phase. In CLC, a high conversion of the fuel to CO2 and H2O is required in the fuel reactor, whereas only a partial oxidation of the fuel is desired in CLR. Oxides of Ni, Cu, Fe, and Mn supported on SiO2 and MgAl2O4 were prepared by dry impregnation and investigated under alternating reducing and oxidizing conditions in a thermogravimetric analyzer at 800-1000 °C using fuel (10% CH4, 10% H2O, and 5% CO2) and oxidizing gas (5% O2). NiO and CuO supported on both SiO2 and MgAl2O4 showed very high reactivity. However, the reactivity of NiO/SiO2 decreased as a function of the cycle number at 950 °C but was avoided below 850 °C. SiO2-supported Mn and Fe oxides may not be feasible oxygen carriers to be used in the process because of the formation of irreversible silicates at high temperatures. However, iron and manganese oxide supported on MgAl2O4 showed a rather high reactivity during reduction and oxidation and can possibly be used in the process. The amount of material needed in air and fuel reactors was estimated in the range of 127-350 kg/MWth, and the recirculation rate of the oxygen carrier necessary between the two reactors was 1.5-8.0 kg MWth-1 s-1 for CLC.
1. Introduction Because of the fact that carbon dioxide is a well-known greenhouse gas and contributes to global warming, a substantial reduction in atmospheric CO2 emissions is desired. The power industry is a major stationary source of CO2 emissions. Thus, a CO2-free power generation will help to solve the greenhouse problem. The capture and storage of CO2 from the combustion of fossil fuels has been suggested as a way of drastically reducing the emissions of CO2. The major drawback of sequestration of CO2 is the rather high cost of separating CO2 from the flue gases, which may mean a cost in excess of $40/ ton CO2.1 Chemical-looping combustion (CLC) has been proposed as a technique for combusting a fuel with small energy losses when CO2 is separated from the flue gas.2-4 The modification of CLC for the production of hydrogen with carbon dioxide capture, or chemical-looping reforming (CLR), has been suggested by several authors.5-8 Both CLC and CLR utilize a metal oxide to supply oxygen from the combustion air to the fuel. A brief description of each process will be given below. * Corresponding author. Tel: +46-31-7722964. Fax: +46-31-160062. E-mail:
[email protected]. † Department of Chemical and Biological Engineering. ‡ Department of Energy Conversion. (1) Riemer, P. Greenhouse gas mitigation technologies. An oVerView of the CO2 capture, storage and future actiVities of the IEA greenhouse gas R&D programme; 1998. (2) Lyngfelt, A.; Leckner, B.; Mattisson, T. Chem. Eng. Sci. 2001, 56, 3101-3113. (3) Ishida, M.; Zheng, D.; Akehata, T. Energy 1987, 12, 147-154. (4) Anheden, M.; Svedberg, G. Energy ConVers. Manage. 1998, 39, 1967-1980. (5) Mattisson, T.; Lyngfelt, A. Applications of chemical-looping combustion with capture of CO2. Second Nordic Minisymposium on CO2 capture and storage, Go¨teborg, Sweden, 2001.
Figure 1. Chemical-looping combustion.
1.1. Chemical-Looping Combustion. CLC is a combustion technology with an inherent separation of CO2. The system is composed of two fluidized bed reactors, an air reactor, and a fuel reactor, see Figure 1. A metal oxide is used an oxygen carrier, which transfers oxygen from air to fuel, avoiding the direct contact between air and fuel.2-4,9 Fuel is oxidized by the oxygen carrier in the fuel reactor according to
(2n + m)MyOx + CnH2m f (2n + m)MyOx-1 + nCO2 + mH2 O (1) where MyOx is the fully oxidized oxygen carrier and MyOx-1 is (6) Mattisson, T.; Zafar, Q.; Lyngfelt, A.; Gevert, B. Integrated hydrogen and power production from natural gas with CO2 capture. 15th World Hydrogen Energy Conference, Yokohama, Japan, 2004. (7) Zafar, Q.; Mattisson, T.; Gevert, B. Ind. Eng. Chem. Res. 2005, 44, 3485-3496.
10.1021/ef0501389 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/01/2005
Redox InVestigation of Transition-State Metal Oxides
Energy & Fuels, Vol. 20, No. 1, 2006 35
the oxygen carrier in the reduced form. The exit stream from the fuel reactor contains only CO2 and H2O, which means pure CO2 can be obtained by condensing H2O. The reduced metal oxide, MyOx-1, is sent to the air reactor, where it is oxidized according to
MyOx-1 + 1/2O2 f MyOx
(2)
The flue gas stream from the air reactor will have a high temperature and contain N2 and some unreacted O2. This stream could be expanded through a gas turbine to produce electricity. The reaction between the fuel and metal oxide in the fuel reactor may be endothermic as well as exothermic depending on the metal oxide used, while reaction in the air reactor is always exothermic. Since air and fuel go through different reactors and combustion takes place without a flame, NOx formation should be avoided.10 CLC has been successfully demonstrated in a 10 kW prototype of interconnected fluidized beds.11 1.2. Chemical-Looping Reforming. CLC can be adapted for H2 production with some modification.5 The main difference between CLR and ordinary CLC is that less air is fed to the air reactor in CLR, and hence, the fuel will only be partially oxidized in the fuel reactor. Here, the fuel (CH4 in this case) and some steam are added and the metal oxide (MyOx) is reduced to a lower oxidation state (MyOx-1), forming a mixture of CO2, CO, H2, and H2O. The product stream from the fuel reactor is sent to a shift reactor, where CO and H2O will react via
CO + H2O f CO2 + H2
(3)
Thus, a stream mainly composed of H2 and CO2 will be obtained. H2 and CO2 can be separated by physical or chemical absorption, depending on the H2 purity required as well as the pressure. The reduced metal oxide (MyOx-1) is sent into the air reactor, where it reacts with air according to reaction 2 and is oxidized back to its original form (MyOx). 1.3. Oxygen Carriers. Oxygen carriers based mainly on transition-state metals Fe, Ni, Cu, Co, and Mn have been investigated for use in CLC in recent years. In ordinary CLC, the oxygen carrier should be able to oxidize fuel gas completely to CO2 and H2O. For CLR, a full conversion of the fuel is not desired but maximum H2 production is a goal.6,8 The amount of bed material in the reactor system is inversely proportional to the rate of reduction and oxidation, and a smaller solids inventory inventory will be needed for an oxygen carrier with a high rate of reaction. In addition to exhibiting a high reduction and oxidation rate with the fuel and air, an oxygen carrier should have enough mechanical strength to bear the stresses associated with the recirculation between the two reactors. Generally, the active metal oxide is combined with an inert material, for example, SiO2 or Al2O3, which acts as a porous support, providing a high surface area for the reaction. A detailed review of the work in the literature on oxygen (8) Ryden, M.; Lyngfelt, A. Hydrogen and power production with integrated carbon dioxide capture by chemical-looping reforming. 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, 2004. (9) Richter, H. J.; Knoche, K. F. Reversibility of combustion processes. ACS Symp. Ser. 1983, 71-85. (10) Ishida, M.; Jin, H. Ind. Eng. Chem. Res. 1996, 35, 2469-2472. (11) Lyngfelt, A.; Kronberger, B.; Adanez, J.; Morin, J. X.; Hurst, P. The GRACE project. Development of oxygen carrier particles for chemicallooping combustion. Design and operation of a 10 kW chemical-looping combustor. 7th International Conference on Greenhouse Gas Control Technologies, Vancouver, Canada, 2004.
carriers for CLC has recently been presented by Cho.12 Mattisson et al. investigated oxides of Ni, Cu, Mn, and Co on alumina supports prepared by dry impregnation in a thermogravimetric analyzer (TGA) and found that CuO- and NiO-based carriers exhibit high reactivity with the methane while Mn3O4 and CoO were the least reactive oxygen carriers.13 Cho et al. investigated different metal oxides on an Al2O3 support in a laboratory fluidized bed reactor using methane as the fuel and found that NiO and CuO were the most reactive metal oxides. However, CuO agglomerated during the reaction and may not be feasible for use in CLC fluidized beds.14 Ishida and co-workers investigated Fe2O3, NiO, and CoO supported on YSZ (yttria-stabilized zirconia) and NiAl2O4 using H2 and CH4 as the fuel in a TGA. It was found that the reaction temperature had the strongest effect on the reduction rate, while the YSZ content had a large effect on the oxidation rate.15,16 Jin et al. found that there was carbon deposition on NiO when using methane as the fuel. However, the carbon formation was completely avoided by adding water vapor at a ratio of H2O/ CH4 ) 2.0.17 Cho et al. investigated the carbon formation on Ni- and Fe-based oxygen carriers in a fluidized bed using methane. The Ni-based oxygen carrier clearly catalyzed the carbon formation to the largest extent. However, carbon formation was avoided at high degrees of methane conversion to carbon dioxide.18 Ryu et al. investigated a NiO/bentonite oxygen carrier using air as the oxidizing gas and methane as the fuel in a TGA. The metal content in the oxygen carrier and the reaction temperature had a strong effect on the reactivity.19 Adanez et al. studied oxides of Ni, Cu, Mn, and Fe on five different support materials in a TGA and concluded that SiO2, Al2O3, ZrO2, and TiO2 are promising support materials for CuO, Fe2O3, Mn3O4, and NiO, respectively.20 de Diego et al. investigated different compositions and preparation methods for Cu-based oxygen carriers and found that impregnation on a support is an excellent preparation method for obtaining high crushing strength and high redox reactivity for Cu carriers.21 Garcia-Labiano et al. studied the kinetics of reduction and oxidation of CuO (10%) impregnated on alumina using CH4, H2, CO as the fuel gas, and O2 as the oxidizing gas in a TGA. The metal oxide reacted with the fuel with reactivity rates in the order H2 > CO > CH4.22 Villa and co-workers investigated NiO using NiAl2O4 and MgAl2O4 as inert supports. They found that the presence of NiAl2O4 spinel prevents the crystal size growth of NiO and the addition of Mg limits the sintering of the cubic NiO and stabilizes Ni2+ in both spinels and the cubic oxide phase, resulting in improved regenerability upon repeated redox (12) Cho, P. Development and characterization of oxygen carrier materials for chemical-looping combustion. Ph.D. Thesis, Chalmers University of Technology, Go¨teborg, Sweden, 2005. (13) Mattisson, T.; Ja¨rdna¨s, A.; Lyngfelt, A. Energy Fuels 2003, 17, 643651. (14) Cho, P.; Mattisson, T.; Lyngfelt, A. Fuel 2004, 83, 1215-1225. (15) Ishida, M.; Jin, H. J. Chem. Eng. Jpn. 1994, 27, 296-301. (16) Jin, H.; Okamoto, T.; Ishida, M. Energy Fuels 1998, 12, 12721277. (17) Jin, H.; Okamoto, T.; Ishida, M. Ind. Eng. Chem. Res. 1999, 38, 126-132. (18) Cho, P.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2005, 44, 668-676. (19) Ryu, Ho-J.; Bae, D.-H.; Han, K.-H.; Lee, S.-Y.; Jin, G.-T.; Choi, J.-H. Korean J. Chem. Eng. 2001, 18 (6), 831-837. (20) Adanez, J.; de Diego, L. F.; Garcia-Labiano, F.; Gayan, P.; Abad, A. Energy Fuels 2004, 18, 371-377. (21) de Diego, L. F.; Garcia-Labiano, F.; Adanez, J.; Gayan, P.; Abad, A.; Corbella, B. M.; Palacios, J. M. Fuel 2004, 83, 1749-1757. (22) Garcia-Labiano, F.; de Diego, L. F.; Adanez, J.; Abad, A.; Gayan, P. Ind. Eng. Chem. Res. 2004, 43, 8168-8177.
36 Energy & Fuels, Vol. 20, No. 1, 2006
Zafar et al.
Table 1. Metal Oxide Loading and Surface Area of Fresh Oxygen Carriers oxygen carrier
loading (wt %)
sintering temperature (°C)
NiO/MgAl2O4 Fe2O3/MgAl2O4 Mn2O3/MgAl2O4 CuO/MgAl2O4 NiO/SiO2 Fe2O3/SiO2 CuO/SiO2 Mn2O3/SiO2
36.5 31.8 46.0 43.0 34.5 39.4 41.3 47.0
950 950 950 950 950 950 800 950
cycles.23 Recently, Mattisson et al. investigated NiO supported on NiAl2O4, MgAl2O4, TiO2, and ZrO2 in a laboratory fluidized bed reactor. All oxygen carriers showed high reactivity without sintering.24 Johansson et al. studied the effect of sintering temperature, particle size, metal oxide content, and reaction temperature using an oxygen carrier composed of Fe2O3/ MgAl2O4. The rate of reduction was highly dependent on the sintering temperature, while no major difference in rate was seen for different particle sizes. A particle containing 60% Fe2O3 and sintered at 1100 °C was found to be suitable for CLC.25 A limited work has been performed on oxygen carriers directly related to CLR. Mattisson et al. examined thermodynamics and the heat balance of the fuel and air reactors using CuO/SiO2 and NiO/SiO2 and found that higher temperatures and gas conversions can be achieved in the fuel reactor of a CLR system if the steam content in the inlet gas is not more than 40%. This was due to the high-energy-demanding endothermic reaction in fuel reactor.6 Zafar et al. investigated oxides of Ni, Cu, Mn, and Fe on a SiO2 support in a laboratory fluidized bed reactor for CLR application and found that the Ni-based oxygen carrier had the highest selectivity toward hydrogen.7 However, the actual selectivity toward different gaseous products was highly dependent upon the degree of conversion of the NiO. Because of the relatively limited work conducted with SiO2 and MgAl2O4 as supports for oxygen carriers for CLC and CLR, the redox behavior of the most promising active metal oxides with these supports was investigated using methane as the fuel. A TGA was used to obtain reaction rates of the oxygen carriers under both reducing and oxidizing conditions. 2. Experimental Section 2.1. Preparation of Oxygen Carriers. 2.1.1. Oxygen Carriers Using SiO2 as Support. Oxygen carriers of iron, manganese, copper, and nickel on SiO2 were prepared by dry impregnation. The commercial support particles of SiO2 (Sipernat 2200, Degussa AG) were heated at 250 °C for 2 h to evaporate moisture completely. These particles were then exposed to a warm (60 °C) and highly concentrated aqueous solution of metal nitrates. A volume of metal nitrate solution corresponding to the pore volume of SiO2 (1.86 cm3/g) was added to each sample. The oxygen carriers were dried at 110 °C for 3 h to remove pore water, followed by calcination at 500 °C for 3 h to remove NO2 and form metal oxides. With the exception of CuO/SiO2, all samples were sintered for 6 h at 950 °C. The CuO/SiO2 particles were sintered at 800 °C. The reason for the lower sintering temperature is the lower melting point of Cu, which is 1083 °C. The Brunauer-Emmett-Teller (BET) surface area of the oxygen carriers was measured using an ASAP 2010 instrument (Micrometrics), after calcination and (23) Villa, R.; Cristiani, C.; Groppi, G.; Lietti, L.; Forzatti, P.; Cornaro, U.; Rossini, S. J. Mol. Catal. A 2003, 204-205, 637-646. (24) Mattisson, T.; Johansson, M.; Lyngfelt, A. The use of NiO as an oxygen carrier in chemical-looping combustion. Fuel 2005, in press. (25) Johansson, M.; Mattisson, T.; Lyngfelt, A. Ind. Eng. Chem. Res. 2004, 43, 6978-6987.
BET (m2/g) after calcination 6.0 7.5 2.9 115.3 105.5 94.2 93.2
BET (m2/g) after sintering 5.2 4.4 3.2 2.6 39.7 46.7 70.2 11.6
sintering. The metal loadings in addition to the measured BET surface area of the oxygen carriers are shown in Table 1. The sintered oxygen carriers were sieved to get particles in a size range of 180-250 µm. 2.1.2. Oxygen Carriers Using MgAl2O4 as Support. For the MgAl2O4-supported oxygen carriers, the support was prepared by freeze granulation. A water-based slurry of fine MgO and Al2O3 (fine chemical powders,
