Energy & Fuels 2004, 18, 371-377
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Selection of Oxygen Carriers for Chemical-Looping Combustion J. Ada´nez,* L. F. de Diego, F. Garcı´a-Labiano, P. Gaya´n, and A. Abad Instituto de Carboquı´mica (CSIC), Department of Energy and Environment, Miguel Luesma Casta´ n 4, 50015 Zaragoza, Spain
J. M. Palacios Instituto de Cata´ lisis y Petroleoquı´mica (CSIC), Cantoblanco, 28049 Madrid, Spain Received July 28, 2003. Revised Manuscript Received November 3, 2003
Chemical-looping combustion (CLC) has been suggested as an energetically efficient method for capture of carbon dioxide from the combustion of fuel gas. This technique involves the use of an oxygen carrier that transfers oxygen from the air to the fuel, preventing direct contact between them. The oxygen carrier is composed of a metal oxide as an oxygen source, and an inert as a binder for increasing the mechanical strength of the carrier. In this work, 240 samples composed of 40-80% of Cu, Fe, Mn, or Ni oxides on Al2O3, sepiolite, SiO2, TiO2, or ZrO2 were prepared by mechanical mixing as cylindrical extrudates. The samples were sintered at four temperatures between 950 and 1300 °C. The effects of the chemical nature and composition of the carrier and the sintering temperature were investigated by reactivity tests in a thermogravimetric analyzer using CH4 as fuel, and the mechanical strength of the solids. On the basis of these properties, the most promising carriers to be used in a CLC system were selected. The best Cu-based oxygen carriers were those prepared using SiO2 or TiO2 as inert, and sintered at 950 °C. Among the Fe-based oxygen carriers, those prepared with Al2O3 and ZrO2 as inerts showed the best behavior. ZrO2 was the best inert for those Mn-based oxygen carriers. Finally, TiO2 was the best inert for those Ni-based oxygen carriers.
1. Introduction Although the effects of increasing levels of greenhouse gases are difficult to quantify, an apparent rapid climate change has led many countries to adopt different strategies limiting the emissions of these gases. It is generally accepted that the release of carbon dioxide from fossil fuel combustion is the most important greenhouse gas that contributes to global warming. Chemical-looping combustion (CLC) has been suggested as an energetically efficient method for the capture of carbon dioxide from combustion of fuel gas.1,2 This technique involves the use of a metal oxide as an oxygen carrier that transfers oxygen from the air to the fuel, avoiding direct contact between fuel and air. The CLC system is composed of two reactors, an air and a fuel reactor. In the fuel reactor the fuel in gaseous form reacts with the metal oxide:
CH4 (CO, H2) + MeO f CO2 + H2O (CO2, H2O) + Me (1) where Me represents a metal or a reduced form of MeO. This metal or reduced oxide is oxidized in the air reactor through the following reaction:
Me + 1/2O2 f MeO
(2)
* Corresponding author. Tel: 34-976733977. Fax: 34-976733318. E-mail:
[email protected]. (1) Richter, H. J.; Knoche, K. F. Reversibility of Combustion Processes. ACS Symp. Ser. 1983, 235, 71-85. (2) Ishida, M.; Zheng, D.; Akehata, T. Energy 1987, 12, 147-154.
in which the material is regenerated, ready to initiate a second cycle. The flue gas leaving the air reactor will contain N2 and any unreacted O2. The exit gases from the fuel reactor contain CO2 and H2O, which are kept separate from the rest of the flue gas. After condensation of the water almost pure CO2 is obtained, without any energy lost for separation. In addition, in this system the fuel and air go through different reactors with no flame, which makes an opportunity to thoroughly eradicate the generation of NOx.3 Depending upon the metal oxide which is used, reaction 1 can be endothermic or exothermic while reaction 2 is always exothermic. The total amount of heat evolved over the two reactors in the CLC process is the same as in a conventional combustion reactor; however, thermodynamic considerations reveal that the exergy destructions in a CLC process is much lower, giving a chance to increase the net power efficiency.1,2,4 Different metal oxides have been proposed in the literature1,2,5 as possible candidates for CLC process: CuO, CdO, NiO, Mn2O3, Fe2O3, and CoO. In general, these metal oxides are combined with an inert which acts as a porous support providing a higher surface area for reaction, as a binder for increasing the mechanical strength and attrition resistance, and, additionally, as an ion conductor enhancing the ion permeability in the solid.3,6 An oxygen carrier in a CLC power plant must (3) Ishida, M.; Jin, H. Ind. Eng. Chem. Res. 1996, 35, 2469-2472. (4) Anheden, M.; Svedberg, G. Energy Convers. Manage. 1998, 39, 1967-1980.
10.1021/ef0301452 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/09/2004
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show high reaction rates and conversions, resistance against carbon deposition, sufficient durability in successive cycle reactions, and high mechanical strength. Mattisson and co-workers7-9 have investigated the behavior of natural and synthetic iron oxides using CH4 and air in fixed-bed and fluidized-bed reactors. They found higher reaction rates and lower particle breakage in synthetic samples as compared with the performance of natural samples. Ishida and co-workers6,10-14 have investigated the effect of temperature, particle size, gas composition, and pressure on the reduction and oxidation rates and on carbon deposition of Fe, Ni, and Co oxides in a thermogravimetric analyzer, using H2, CO, or CH4 as fuels and air as oxidizing gas. The effect of the inert used as a binder and its concentration were also analyzed.6 They concluded that the carbon deposition and the reaction rates and conversions, in addition to the operating conditions (temperature, particle size, gas composition, total pressure, etc,), depended strongly on the chemical nature of the solid materials.12,13 In this work, 240 samples of potential oxygen carriers for a CLC process, based on four inorganic oxides and five inerts, were prepared by mechanical mixing as cylindrical extrudates. The effects of composition and sintering temperature used in its preparation were investigated through both reactivity tests and mechanical strength measurements. On the basis of these properties, a selection of the most promising carriers to be used in a CLC system was achieved. 2. Experimental Section 2.1. Preparation of Oxygen Carriers. The prepared oxygen carriers are composed of a metal oxide as an oxygen source for the combustion process, and an inert as a binder for increasing the mechanical strength. In addition, during preparation graphite as a high-temperature pore forming additive enhancing chemical reaction was also added. The oxygen carriers were prepared from commercial pure oxides as powders of particle size
