Ind. Eng. Chem. Res. 2007, 46, 4633-4638
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Change of CO2 Carrying Capacity of CaO in Isothermal Recarbonation-Decomposition Cycles Anton I. Lysikov, Aleksey N. Salanov, and Aleksey G. Okunev* BoreskoV Institute of Catalysis, NoVosibirsk 630090, Russia
The change of CO2 carrying capacity of CaO sorbents prepared from different precursors has been studied using thermogravimetric analysis in a long series of isothermal recarbonation-decomposition cycles in the temperature range of 750-850 °C. The residual capacity of the CaO sorbents after a large number of cycles was found to depend on the precursor type, the experimental temperature, and the duration of the recarbonation stage. The residual capacities of the CaO derived from the powdered calcium carbonates were much higher than that of the CaO produced from the crystalline CaCO3. A simple tentative model has been suggested, according to which recarbonation-decomposition cycles result in formation of the interconnected CaO network that acts as a refractory support and determines sorption properties of the material. By using a new model, a simple synthesis procedure has been suggested that produces CaO sorbents with high residual CO2 carrying capacities. 1. Introduction Calcium oxide can be used to absorb CO2 in a hydrogen production process now often called sorption-enhanced reforming (SER).1-5 In addition to the conventional catalytic reforming reactions,
negligible for the samples cycled at shorter carbonation times13 and sintering remains the main factor of capacity loss. Recently, two equations have been suggested to describe CaO capacity decay in recarbonation-decomposition cycles. Wang and Anthony14 derived an expression similar to that used for catalyst deactivation through sintering,
CH4 + H2O T 3H2 + CO
aN )
CO + H2O T H2 + CO2 the SER process uses a CO2 absorbent admixture with a steamreforming catalyst for selective removal of the reaction product,
CaO + CO2 f CaCO3 and the absorbent is periodically regenerated:
* To whom correspondence should be addressed. E-mail: okunev@ catalysis.ru.
(1)
where aN is the recarbonation extent in the Nth cycle and k is the only parameter, having the physical meaning of the sintering rate. At kN . 1, the capacity is inversely proportional to the number of cycles and tends to zero. Grasa and Abanades15 modified eq 1 to account for the residual CaO capacity in the long series of recarbonation-decomposition cycles,
aN )
CaCO3 f CaO + CO2 Since du Motay and Mareshal used the SER for hydrogen production for the first time in 1868,6 many research works about the SER have been published. One of the embodiments of the sorption-enhanced steam reforming of the hydrocarbons is a fixed-bed reactor operated cyclically.7 High hydrocarbons conversion, low COx concentrations in the hydrogen rich product gas, and simplicity make this approach promising for fuel processing in fuel-cell-powered generators.8 If the pressureswing adsorption technique is used for the sorbent regeneration, the conditions of cycling would be nearly isothermal.3 A temperature range of 750-850 °C appears to be sufficient to ensure fast CO2 sorption and desorption kinetics.9 Stability of the CO2 carrying capacity of the sorbent is a key issue for the fixed-bed SER technology, and it has to be confirmed for thousands of cycles.10 A drop of the recarbonation extent for a pure CaO in recarbonation/decomposition cycles is well-recognized. The main reasons for CaO capacity decay are pore blockage11 and sorbent sintering.12 The study of Alvarez and Abanades on the evolution of pore-size distribution of the limestone derived CaO revealed that the pore blockage is
1 1 + kN
1 + a∞ 1 + kN 1 - a∞
(2)
where a∞ is the recarbonation extent after an infinitely long number of cycles. Approximation of the large array of the experimental data on recarbonation of the calcinated limestones using eq 2 yielded a∞ ) 0.075.15 Though 2-3 wt % of CO2 carrying capacity seems to be sufficient for the SER process,7 the efficiency of the SER increases at higher CO2 capacity.16 The objective of this work is to investigate the limiting behavior of the capacity in isothermal cycles close to that of the SER for several calcium oxides of different origin. One more objective of the paper is to present a reliable model for CaO sintering in recarbonationdecomposition cycles that simplifies the design of new stable and effective materials. 2. Experiments Monodisperse carbonate particles of 3-4 µm size were produced using the precipitation of CaCO3 from calcium nitrate solution by ammonium carbonate at subzero temperature. The details of the synthesis were described elsewhere.17 The scanning electron microscopy (SEM) image shows that this powdered material consists of cubic and spherically shaped particles. Both
10.1021/ie0702328 CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
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Ind. Eng. Chem. Res., Vol. 46, No. 13, 2007
program and the dynamics of the weight change in a typical run. The morphology of the samples was observed using a Jeol JSM-6460 LV scanning electron microscope. The experimental decay curves were fit using the nonlinear curve fit feature of Origin 7.0 software using the least-squares method. All of the parameters of the fitting formula were independently varied. 3. Results and Discussion
Figure 1. Temperature program and weight change in a typical TG run.
vaterite and calcite phases are present in equal amounts in this sample, as measured using the X-ray diffraction method. The crystalline calcium carbonate sample was prepared from an optically transparent crystal of 2 × 3 × 6 cm3 size originated from Crimea (Ukraine). The piece of the crystal was chopped off, crushed, and sieved to get 80-100 µm fraction of the sample. Other materials, including CaCO3 powder (>98.5% pure, “SoyuzChimProm”, Russia) and Ca(OH)2 powder and Ca(NO3)2‚4H2O (both >99% pure, “Reahim”, Russia), were purchased and used as received. The abbreviations of MC for the monocrystal-derived samples, MD for the monodisperse particles, and CP for the commercial CaCO3 powder have been adopted hereinafter. The cyclic recarbonation and decomposition reactions were experimentally studied in the thermogravimetric analyzer NETZSCH STA 449 C, with a precision of 10-7 g and the baseline stability of 10-6 g/h in isothermal mode. Since the cycle time never exceeded 1 h, the error due to the zero drift was always
