Article pubs.acs.org/EF
High Temperature CO2 Capture on Novel Yb2O3‑Supported CaOBased Sorbents Yingchao Hu, Wenqiang Liu,* Jian Sun, Xinwei Yang, Zijian Zhou, Yang Zhang, and Minghou Xu* State Key Laboratory of Coal Combustion, School of Energy and Power Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ABSTRACT: Incorporation of CaO in inert solid support has been identified as an effective approach to improve the cyclic CO2 capture performance for CaO-based sorbents. In this work, Yb2O3-supported CaO-based sorbents were fabricated via the wetmixing technique. Elemental dispersion observed by FSEM-EDS showed that the ultrafine active species of CaO particles was finely separated by Yb2O3. Different amounts of Yb2O3 were introduced and 10−15 wt% was found to be the optimal content range for the support to function as the metal skeleton. In comparison with pure CaO, the improved CO2 capture performance was observed for CaYb10 over prolonged carbonation-calcination cycles under a severe test condition. This improvement could be ascribed to the enriched macropores within 50−100 nm, which was identified by the N2 adsorption−desorption analysis. In addition, the microstructure of the synthetic sorbent (analyzed through TEM images) indicated that CaO particles of ∼160 nm were formed with the Yb2O3 nanocrystallines (∼15−25 nm) adhered to the surface functioning as physical barriers and as a result, the sintering was effectively retarded. Generally, the enhanced cyclic CO2 capture performance and the promoted sintering-resistant property of Yb2O3-supported CaO-based sorbents made Yb2O3 a promising inert solid support. 2005, Li et al.9 first fabricated a Ca12Al14O33-supported CaObased sorbent, achieving high capacity of 0.45 g CO2/g sorbent over 13 carbonation/calcination cycles. Since then, Al-based supports including Ca12Al14O33,8,9,21,22,35−38 Ca9Al6O18,39,40 Ca3Al2O6,7,37 and Al2O341,42 have quickly attracted extensive attention and have enhanced the sorbent performance to various degrees. Another widely studied inert support is MgO,10,40,43,44 the effectiveness of which was probably identified for the first time from the favorable CO2 capture performance of calcined dolomite (MgO-supported CaO mixture).45 Ca2MnO4 has been also proved to effectively enhance CaO performance. Luo et al.22 prepared a CaO/ Ca2MnO4 sorbent with the optimal support content of 20 wt%, obtaining the capacity of ∼0.08 g CO2/g sorbent at the 100th cycle with a short carbonation time of 2.5 min and a severe calcination condition. The effectiveness of Ca2MnO4 was further acknowledged by the excellent performance of the naturally occurring manganocalcite.14 Besides, CaZrO3 also exhibited promising prospects in promoting CaO performance within extended operation cycles.46 Except for the aforementioned supports, many other refractory spacers, such as Y2O3,40,47,48 CeO2,20,49 Nd2O3,14 SiO2,15,16,50 CaTiO3,51,52 MgAl2O4,53 La2O322 and Pr6O11,6 have been also reported to function as metal skeletons to resist sintering of CaO-based sorbents. With a high Tammann temperature of 1094 °C,54 Yb2O3 also owns the potential to act as the metal framework and further retard the aggregation of CaCO3/CaO particles. However, up to now, there has been very little work focusing on this inert support. In our recently reported investigation which screened
1. INTRODUCTION The growing anthropogenic CO2 emission has posed a severe threat to the global climate.1 Up to May 2016, the CO2 concentration in the atmosphere has reached 407 ppm, increasing more than 30% over the past five decades.2 Hence, a diversity of measures including carbon capture and sequestration (CCS) has been proposed to mitigate the serious CO2 emission. CO2 removal through solid sorbents has become well-known because of their wide sorption-temperature windows and environmental friendly characteristics.3 Among the various solid sorbents, CaO-based CO2 trappers via cyclic carbonation/calcination processes have drawn extensive attention since they were proposed by Silaban and Harrison in 1995.4 One of the key issues for the ultimate realization of calcium looping in practical application is the capacity-in-loss problem. This phenomenon that the CO2 capture capacity of the CaO sorbents decays with the increase of the cycle numbers are primarily caused by the sintering of CaCO3 particles under the high regeneration temperature (∼900 °C) due to the relatively low Tammann temperature of CaCO3 (∼533 °C).5 Great efforts have been devoted to overcoming this problem, including (i) employing inert solid supports to separate CaO or CaCO3 particles to mitigate agglomeration;6−22 (ii) using other naturally occurring minerals instead of limestone and dolomite;23−25 (iii) acquiring CaO from sintering-resistant precursors;26−29 and (iv) modifying limestone by organic acids.30−34 All of the above-mentioned attempts have achieved success to varying degrees. The incorporation of CaO in inert solid support has been proved to be an effective approach to promote the cyclic CO2 capture performance of CaO-based sorbents. A wide range of refractory supports, such as Al, Mg, Mn, and Zr have been investigated and the synthetic sorbents have acquired remarkably improved CO2 capture performance. As early as © 2016 American Chemical Society
Received: May 17, 2016 Revised: July 21, 2016 Published: August 1, 2016 6606
DOI: 10.1021/acs.energyfuels.6b01185 Energy Fuels 2016, 30, 6606−6613
Article
Energy & Fuels
Figure 1. FSEM-EDS mapping of Ca and Yb of CaYb10. cooling ramp were controlled at 20 °C/min. Thus, 15 carbonation/ calcination tests were repeated to obtain the cyclic CO2 capture performance of the sorbents. A severe experimental condition was also employed to test the performance of typical synthesized CaO-based sorbents. The CO2 concentration during carbonation stage was decreased to 15 vol %. After carbonation, the sample was heated to 900 °C for regeneration while the gas atmosphere was changed to N2. The gas was switched to 15% CO2 once the temperature reached 900 °C. When the 5 min calcination at 900 °C was finished, the atmosphere was again changed to pure N2 and the temperature was cooled down to 650 °C for the next cycle to begin. It is worth noting that because of the apparatus constrains, only 15 vol% CO2 could be chosen for calcination allowing for the CO2 concentration in carbonation stage, although nearly pure CO2 should be employed for calcination in realistic industrial processes. The carbonation conversion in each cycle (Xn, %) and the CO2 capture capacity (Cn, g CO2/g sorbent), as defined in eq 1 and 2, were calculated to assess the performance of the sorbents.
12 inert solid supports,6 the support of Yb2O3 was found to be a good candidate among the studied refractories. But in that study, only a percentage of 25% Yb2O3 was incorporated and the Yb2O3-supported CaO sorbent was merely tested under the mild conditions. The optimal content of Yb2O3 to be incorporated is unclear and the cyclic CO2 capture performance of Yb2O3-supported CaO-based sorbents under a severer condition is also unknown. Therefore, it is still necessary to conduct more detailed investigations on Yb2O3-supported CaO-based sorbents. In the current work, a series of Yb2O3-supported CaO-based sorbents were prepared by a wet-mixing method. The optimal amount of the support was identified and the extended cyclic performance of the sorbent under the severe test conditions was also studied. The dispersion of Yb2O3 among CaO particles and the role of Yb2O3 in retarding the sintering were detailedly discussed with the help of the characterizations of N2 adsorption/desorption, X-ray diffraction (XRD), a field scanning electron microscope (FSEM), and a transmission electron microscope (TEM).
2. EXPERIMENTAL SECTION 2.1. Materials and Sorbent Preparation. Calcium acetate monohydrate (C4H6CaO4·H2O, 98.0% purity, Sinopharm Chemical Reagent Co. Ltd.) was chosen as CaO source and ytterbium(III) acetate tetrahydrate (Yb(C2H3O2)3·4H2O, 99.99% purity, Energy Chemical) was employed as the precursor of the support. The material of ytterbium(III) acetate tetrahydrate could be produced by the acidification of Yb-sources using the industrial waste acetic acid. The Yb2O3-supported CaO-based sorbents were synthesized using the wet-mixing technique, which has been described previously.6,14,55 Appropriate amounts of Ca- and Yb-precursors were added into deionized water and the mixture was stirred at 75 °C in an oil bath until the precursors were completely dissolved. Then, the solution was dried in an oven at 105 °C and subsequently, the final synthetic sorbents were acquired after 30 min precalcination of the dried precursor mixture at 900 °C in air. The sorbent powder was grinded and sieved to
