https://doi.org/10.3390/en14164822
“The main disadvantage of CaO is a rapid decline in reactivity under the cyclic carbonation–decarbonation process caused by the following major factors: low Tammann temperature of CaCO3, carbonation process high exothermicity and a big difference in molar volumes of CaO (16.9 cm3/mol) and CaCO3 (36.9 cm3/mol). One of the approaches used to improve the cyclic stability of CaO is the development of CaO-based synthetic sorbents via incorporation of the dopant into CaO. Inertness to CO2 in the temperature range of the CaL process and high sintering temperature (Tammann temperature) are the criteria for the dopant selection. The dopant acts as a spacer, preventing CaO/CaCO3 sintering during cyclic carbonation–decarbonation. The sol–gel method, co-precipitation and flame spray pyrolysis are commonly used to fabricate different CaO-based sorbents [8,9] and, in particular, the Zr-doped CaO sorbents which exhibit superior performance and durability [10,11]. These techniques deal with CaO and the dopant precursors and give rise to nanosized composite sorbents with enhanced CO2 capture performance and cyclic stability. High-energy milling also ensures efficient particle size reduction and effective dispersion, while being easier to implement, thus this method seems to be more promising for the industrial application than the above laboratory methods [12,13,14]. Moreover, high-energy milling is the most obvious approach to fabricate composite materials from chemically inert components. In [14] we used high-energy co-milling of calcium carbonate and baddeleyite (natural zirconia mineral), followed by heat-treatment of the obtained nanopowder to fabricate CaO sorbent doped with well-dispersed CaZrO3 nanoparticles to prevent its sintering. The prepared Zr-doped CaO sorbent was characterized by a high enough steady state CO2 uptake capacity of 8.6 mmol/g under a multi-cycle carbonation–decarbonation process. The Tammann temperature of CaZrO3 is 1218 °C [11]. We used natural zirconia since it is much cheaper than chemically synthesized zirconia and its precursors. Previously, high-energy milling has already been successfully applied to baddeleyite to produce engineering-nanostructured zirconia ceramic with competitive mechanical properties [15].”
Materials and Methods
“Baddeleyite concentrate powder (5 μm, 99.3%, Kovdorsky GOK, Kovdor, Russia) and CaCO3 powder (99.5%, Sigma-Aldrich, Saint Louis, MO, USA) were mechanically mixed to prepare the composite powder with the Zr/Ca molar ratio of 2:10. High-energy milling of the composite powder using distilled water was performed on a planetary mill Pulverisette 7 Premium Line (Fritsch, Idar-Oberstein, Germany) during 5 h. The resulting product was dried in air at 80 °C for 24 h in an OV-11 oven (Jeio Tech Co., Ltd., Seoul, Korea).”
“For all the sorbents, an evaluation of their CO2 uptake capacity in multiple carbonation–calcination cycles was performed (). The pure CaO sorbent showed a rather high initial capacity of 16.1 mmol/g since the stoichiometric capacity of CaO was 17.9 mmol/g. However, after the 3rd cycle, its capacity started to decrease rapidly reaching 1.2 mmol/g in the 24th cycle and then continued to decrease slowly to a steady-state value of about 1.0 mmol/g. Adding ZrO2 resulted in a sorbent initial capacity reduction. The dopant was inactive for CO2 capture which reduced the initial CO2 uptake capacity of the material on a weight basis. The rise in the Zr/Ca molar ratio from 0:10 to 3:10 led to a sorbent initial capacity decrease from 16.1 to 8.1 mmol/g. Although the capacity of the Zr-doped CaO sorbents was initially lower than that of the undoped one, it showed better stability in time with the Zr/Ca molar ratio increase. A decrease in capacity of 15.1, 8.2, 1.9 and 1.8 mmol/g in relation to the initial value was registered in the 50th cycle for sorbents with the Zr/Ca molar ratio of 0:10, 1:10, 2:10 and 3:10, respectively. Among the fabricated sorbents, the one with the Zr/Ca molar ratio of 2:10 (hereinafter referred to as the Zr-doped CaO sorbent) showed the best performance in terms of capacity value and its cyclic stability.”
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Figure 1. The dependencies of the CO
2 uptake capacity of the sorbents with different Zr/Ca molar ratios on the number of carbonation–calcination cycles.”
https://doi.org/10.3390/en13164110
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During the multi-cycle carbonation–calcination process, the Zr-doped CaO sorbent prepared by wet high-energy milling showed the CO
2 uptake capacity steady-state value of 8.6 mmol/g exceeding those of the Zr-doped CaO sorbents with the same Zr/Ca molar ratio previously synthesized using a more laborious sol–gel technique [
14] and a surfactant-template/ultrasound-assisted method [
19].
The microstructure of the fabricated sorbents before and after cyclic tests is illustrated in . a,b show that fresh Zr-doped CaO and undoped CaO sorbents were macroscopically homogeneous and consisted of agglomerates with size of up to several hundred nanometers. Agglomerates were smaller in size and had clearly defined borders in the case of the Zr-doped CaO sorbent. In both sorbents, randomly disposed agglomerates of nanoparticles formed uniformly distributed cavities of arbitrary shape and size. Significant changes in the microstructure of the pure CaO sorbent were observed after the 50th cycle (c). Agglomerates formed a monolithic structure, which led to porosity reduction. On the contrary, the Zr-doped CaO sorbent microstructure had undergone no noticeable changes (d) and was the same as before the 1st cycle.
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“Figure 2. SEM images of the virgin sorbents and the same sorbents after the 50 cycle: (a,c) the pure CaO sorbent; (b,d) the Zr-doped CaO sorbent.”
