Article pubs.acs.org/EF
Design of Stable Cage-like CaO/CaZrO3 Hollow Spheres for CO2 Capture Yang Wang,† Wenyu Zhang,† Rui Li,‡ Wubiao Duan,† and Bo Liu*,† †
Department of Chemistry, School of Science, Beijing Jiaotong University, Beijing 100044, People’s Republic of China State Key Laboratory of Solid Waste Reuse for Building Materials, Beijing Building Materials Academy of Science Research, Beijing 100041, People’s Republic of China
‡
ABSTRACT: This paper aims to solve the problem of the reversibility in carbonation/calcination cycles of Ca-based materials. The CaO/CaZrO3 hollow sphere sorbents were prepared using colloidal carbon spheres as a template via the co-adsorption method. The hollow structure sorbents, which contained two kinds of metal ions, were observed by scanning electron microscopy and transmission electron microscopy images, and the components were obtained by X-ray diffraction and energydispersive X-ray analysis. The performances in CO2 capture at different absorption temperatures, calcination temperatures, molar ratios of Ca/Zr, and heating rates compared to calcium oxide were investigated employing the thermogravimetric analyzer. The results showed that the CaO/CaZrO3 sorbent could be fabricated using a trace Zr precursor and the specific prepared sorbent still could maintain a high capture capacity after exceeding a 12 000 min operation, which suggesting the approach that adsorbs both calcium and zirconium ions on the colloidal carbon spheres to form a hollow structure is feasible for enhancing the sintering-resistant properties.
1. INTRODUCTION As a result of the increased rates of deforestation and combustion of fossil fuels, the current CO2 concentration far exceeds its natural fluctuation (d = 180−300 ppm) over the past 800 000 years.1−3 Therefore, the greenhouse effect became more and more serious. As a promising way to deal with global warming and climate change, CO2 capture from postcombustion of fossil fuels has gained increasing interests in recent years.4−7 Among various sorbents, calcium oxide (CaO)based materials, which are a type of high-temperature CO2 sorbent, have attracted tremendous attention, owing to the high theoretical sorption capacity and low cost.8 However, the low reversibility of the carbonation reaction of the calcium-based materials has limited its application. In the past decade, a great deal of studies have been devoted to the improvement of sintering-resistant properties of CaObased sorbents.9−15 Among them, ZrO2 as a stabilizer for CaO sorbents can react at elevated temperatures to form a thermodynamically stable mixture of CaO and CaZrO3 that has received continuous attention.16−21 Broda et al. have reported the synthesis of CaO-based, ZrO2-stabilized CO2 sorbents using a sol−gel technique. The CO2 uptake of the best sorbent could be maintained at a high level after 90 cycles of repeated calcination and carbonation.19 Reddy et al. prepared Zr-stabilized CaO sorbents by various means, including deposition−precipitation, co-precipitation, flame spray pyrolysis, and sol−gel methods. According to the results, the sorbents prepared by sol−gel and flame spray had excellent stability until 1200 carbonation/calcination cycles.21 To our knowledge, the studies regarding Zr-stabilized Ca-based sorbents in recent years had been included in Table 1. Colloidal carbon spheres are largely used as a result of the fact that the preparation procedure is solvent-free and environmentally friendly.22 Natural abundant −OH and C © XXXX American Chemical Society
O groups on the colloidal carbon sphere surface are able to strongly attract electropositive ions in the solution, such as metal ions;23 therefore, it can be applied as a catalyst support and energy-storage material.24 Moreover, it was found that these spheres can be used as a sacrificial template to form hollow structures.25,26 Wang et al. have synthesized hollow sphere CaO for CO2 capture; the hollow structure showed a high capacity and good stability and demonstrated that the hollow structure plays a crucial role in the performance.27 Liu et al. developed a new type of CaO/Ca12Al14O33 hollow sphere sorbent by the sol−gel process; the sorbents exhibit advantageous stable adsorption capacities and durability during multiple cycles of CO2 capture at a high temperature and realistic regeneration conditions.28 In this paper, CaO/CaZrO3 hollow sphere sorbents were prepared using colloidal carbon spheres as the template. To our knowledge, different metal ion co-adsorption on colloidal carbon has not been reported yet, which could, on the one hand, reduce the usage of a metal precursor compared to the sol−gel method but show the same properties when using for CO2 capture. On the other hand, the stable hollow structure contains two kinds of metal ions, which could be a new kind of material to improve the reversibility of the sorbents. In short, this co-adsorption method may introduce a more cost-effective way when preparing rare metal-doping materials, considering the shortage of the limited resources on Earth. Herein, the structure and component of the sorbents were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energydispersive X-ray analysis (EDAX), and then the sorbents were Received: November 26, 2015 Revised: January 19, 2016
A
DOI: 10.1021/acs.energyfuels.5b02781 Energy Fuels XXXX, XXX, XXX−XXX
Article
Energy & Fuels Table 1. Summary of Recent Reports on CaO−ZrO2 Sorbents
CO2 uptake (gofCO2/gofsorbent)
time
sorbent
atmosphere
carbonation temperature (°C)
Lu et al.16 Koirala et al.17 b
2008 2011
Radfarnia and Iliuta18 Broda and Muller19
2012 2014
Zhao et al.20
2014
Zr/Ca = 3:10a Zr/Ca = 3:10a Zr/Ca = 5:10a Zr/Ca = 5:10a Zr/Ca = 3.03:10c Zr/Ca = 20:80d Zr/Ca = 10:90d Zr/Ca = 5:95d 10CaZrO3/90CaOe
30% CO2 100% CO2 100% CO2 100% CO2 100% CO2 20% CO2 20% CO2 20% CO2 15% CO2 100% CO2 15% CO2 100% CO2 15% CO2 100% CO2 99.5% CO2 99.5% CO2 99.5% CO2 99.5% CO2
750 700 700 850 600 650 650 650 650 650 650 650 650 650 700 700 700 700
reference
18CaZrO3/82CaOe 30CaZrO3/70CaOe Reddy et al.21
2014
Zr/Ca Zr/Ca Zr/Ca Zr/Ca
= = = =
8.9e 9.46g 3.57d 1.5a
carbonation time (min)
calcination temperature (°C)
number of cycles
maximum
final
10 30 30 10 30 20 20 20 15 15 15 15 15 15 30 30 30 30
750 700 700 950 750 900 900 900 800 900 800 900 800 900 700 700 700 700
50 100 100 100 15 10 10 10 30 30 30 30 30 30 5 5 1200 1200
0.32 0.34 0.23 0.21 0.19 0.33 >0.49 >0.57 0.373 0.56 0.32 0.43 0.267 0.36 0.183 0.258 0.308 0.226
0.32 0.33 0.23 0.21 0.14 0.21 0.31 0.36 0.365 0.3 0.312 0.31 0.267 0.31
