Article pubs.acs.org/est
Influence of the Calcination and Carbonation Conditions on the CO2 Uptake of Synthetic Ca-Based CO2 Sorbents Marcin Broda,† Agnieszka M. Kierzkowska,† and Christoph R. Müller*,† †
Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092 Zurich, Switzerland S Supporting Information *
ABSTRACT: In this work we report the development of a Ca-based, Al2O3stabilized sorbent using a sol−gel technique. The CO2 uptake of the synthetic materials as a function of carbonation and calcination temperature and CO2 partial pressure was critically assessed. In addition, performing the carbonation and calcination reactions in a gas-fluidized bed allowed the attrition characteristics of the new material to be investigated. After 30 cycles of calcination and carbonation conducted in a fluidized bed, the CO2 uptake of the best sorbent was 0.31 g CO2/ g sorbent, which is 60% higher than that measured for Rheinkalk limestone. A detailed characterization of the morphology of the sol−gel derived material confirmed that the nanostructure of the synthetic material is responsible for its high, cyclic CO2 uptake. The sol−gel method ensured that Ca2+ and Al3+ were homogenously mixed (mostly in the form of the mixed oxide mayenite). The formation of a finely and homogeneously dispersed, high Tammann temperature support stabilized the nanostructured morphology over multiple reaction cycles, whereas limestone lost its initial nanostructured morphology rapidly due to its intrinsic lack of a support component.
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mechanical mixing,18 which do not allow key structural properties of the material, such as the pore volume or surface area to be adjusted easily. For example, Florin et al.9 synthesized a Ca-based CO2 sorbent via coprecipitation, viz. by bubbling CO2 through an aqueous solution containing Ca(OH)2 and Al(NO3)3. Upon sintering the precipitated materials at temperatures exceeding 800 °C, a reaction between CaO and Al2O3 was observed that resulted in the formation of mayenite (Ca12Al14O33). The cyclic CO2 uptake of the material was tested both in a thermogravimetric analyzer (TGA) and a bubbling fluidized bed reactor. The best sorbent possessed a weight ratio of CaO to Ca12Al14O33 of 85:15 and a CO2 uptake of ∼0.29 g CO2/g sorbent after 15 cycles of repeated calcination and carbonation as determined in a TGA. However, when the corresponding cycling experiments were performed in a fluidized bed reactor, a dramatic decrease in the CO2 uptake of the synthetic material was recorded, reaching only ∼0.13 g CO2/g sorbent after 15 cycles. The potential of advanced synthesis techniques to develop Ca-based sorbents with improved CO2 capture characteristics has been demonstrated recently by employing, for example, the sol−gel technique.10,19,20 Broda et al.10 developed a material that contained 9 wt % Al2O3 and possessed a stable CO2 uptake of 0.51 g CO2/g sorbent after 30 cycles, a value that is approximately
INTRODUCTION In 2010 the global CO2 emissions increased by more than 5% compared to 2009, totalling 33 billion tonnes.1 One possible, midterm solution to reduce the emission of greenhouse gases into the atmosphere is the implementation of CO2 capture and storage (CCS).2 However, using the currently available technology for CO2 capture, i.e. scrubbing with amines, the costs of capturing CO2 are very high. For example, using data from 2011 the cost of CO2 capture using amine scrubbing was estimated to be around 55 $ per ton of CO2 avoided.3 As an alternative to amines, alkaline earth metal-based CO2 sorbents have attracted significant attention recently. A particularly favorable material is CaCO3 due to its low cost and wide availability in naturally occurring minerals, e.g. limestone or dolomite.4 The capture and release of CO2 is described by the following, theoretically, reversible reaction CaO (s) + CO2 (g) ↔ CaCO3 (s)
(1)
The disadvantage of naturally occurring Ca-based materials is the rapid decrease of their CO2 uptake capacity with cycle number due to sintering or pore pluggage.5 To improve the CO2 capture characteristics of limestone, different methods, such as hydration6,7 or thermal pretreatment,8 have been proposed. An alternative strategy is the manufacture of synthetic materials in which CaO is stabilized with an inert, high Tammann temperature (temperature at which sintering starts) matrix, e.g. oxides of Al,9−11 Mg,12−14 or Si.15 However, work in this field has predominantly focused on the use of “simple” synthesis techniques, such as coprecipitation,16 hydrolysis,17 or © 2012 American Chemical Society
Received: Revised: Accepted: Published: 10849
July 10, 2012 September 3, 2012 September 5, 2012 September 5, 2012 dx.doi.org/10.1021/es302757e | Environ. Sci. Technol. 2012, 46, 10849−10856
Environmental Science & Technology
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150% higher than that of Havelock limestone for the same number of cycles. In addition to Al2O3, promising results with respect to the stabilization of Ca-based CO2 sorbents with a high Tammann temperature support have also been achieved for MgO.12−14 For example, Filitz et al.12 synthesized Ca-based CO2 sorbents by coprecipitating solutions of Ca(NO3)3 and Mg(NO3)2 with a mixture of (NH4)2CO3 and NH3aq at a pH value of 10.5. The best sorbent contained 71 wt % CaO and possessed a CO2 uptake of 0.51 g CO2/g sorbent after 15 cycles, thus exceeding the corresponding CO2 uptake of Havelock limestone by almost 100%. Filitz et al.12 also highlighted the influence of the morphology of the sorbent on the CO2 uptake. It was shown that the material with the highest CO2 uptake also possessed the highest BET surface area.12 In a further study, Sultan et al.13 calcined calcium magnesium acetates that contained different ratios of Ca2+ to Mg2+. The best CO2 capture performance over 10 cycles of carbonation and calcination, as determined in a fluidized bed reactor, was reported for a material that contained a molar ratio of CaO:MgO = 7:3. Reducing the molar ratio of Ca2+ to Mg2+ to 1:1 resulted in a markedly lower CO2 uptake, which was attributed to the lower surface area and pore volume of the material, in addition to the effect of the lower CaO content.21,22 In an attempt to improve further the CO2 uptake of calcium acetate, Li et al.23 mixed calcium acetate with MgAl2O4 nanoparticles to stabilize CaO. After 65 cycles, the sorbent containing 32 wt % MgAl2O4 showed the highest CO2 uptake of 0.34 g CO2/g sorbent, corresponding to a CaO conversion of 0.63. Li et al.23 argued that the rod-like shape of MgAl2O4 prevented sintering and, thus, gave a higher CO2 uptake when compared to naturally occurring limestone. The CO2 uptake and applied test conditions, i.e. carbonation/calcination temperature and atmosphere, are summarized in Table S1 (Supporting Information) for the present work and for the studies described above. As summarized above, synthetic sorbents have previously been tested at very different carbonation and calcination conditions, making a comparison between the different materials impossible. Additionally, most synthetic sorbents were in the form of powders, making their use in a fluidized bed, i.e. the type of reactor currently envisaged for practical applications, impossible. In the work reported here a synthetic Ca-based sorbent was developed using a sol−gel technique. The material was of sufficient mechanical strength to allow an assessment of its CO2 uptake characteristics in a fluidized bed. An important facet of this work was to address the influence of the calcination and carbonation conditions, such as the CO2 partial pressure and the calcination temperature, on the CO2 capture characteristics. These measurements allowed us to identify the operating conditions which influenced the CO2 uptake most and to compare our material with previously reported synthetic CO2 sorbents and limestone, which served as a reference material. For the case that calcination was performed at 900 °C in 100 vol. % CO2 using a TGA, the CO2 uptake of the best synthetic sorbent was 0.28 g CO2/g sorbent after 10 cycles, a value which is 75% higher than that of the reference limestone. The significantly improved CO2 uptake of the synthetic sorbent was discussed in light of its thermally stable, nanostructured morphology.
Article
EXPERIMENTAL SECTION
The Ca-based, Al2O3-stabilized CO2 sorbents were synthesized using a sol−gel based technique. Details of the synthesis protocol are provided in the Supporting Information (SI). The synthesized materials were subsequently characterized with regards to (i) surface morphology using scanning electron microscopy (SEM), (ii) surface area and pore volume using N2 adsorption measurements, (iii) crystallinity and chemical composition using X-ray powder diffraction (XRD), and (iv) thermal decomposition and CO2 capture performance using a thermogravimetric analyzer (TGA). Details of the equipment used for particle characterization are given in the SI. The cyclic carbonation and calcination reactions were studied using a thermogravimetric analyzer (TGA, Mettler Toledo TGA/DSC 1). A small amount (
