Role of MgxCa1−xCO3 on the physical–chemical properties and cyclic CO2 capture performance of dolomite by two-step calcination
Introduction
Anthropogenic CO2 release from fossil fuel-fired power plants contributes greatly to global warming [1], [2]. A high-temperature Ca-looping process (CaO + CO2 ↔ CaCO3) using limestone-based sorbents could provide a short-term approach to mitigate climate change [3]. Compared with current low-temperature amine-based solvent systems, this process is significantly more efficient [4]. Furthermore, its feasibility has been successfully demonstrated at the MWth scale of pilot plants [5]. One of the major challenges for the use of lime-based sorbents in cyclic CO2 capture is the rapid reduction in the extent of carbonation reaction [6], which is typically explained by reduction in the surface area and porosity through sintering at high temperatures [7].
There are two approaches to reducing problems related to sintering. One promising method is the development of highly efficient sorbents to improve long-term performance [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. The other is using simple reactivation routes to reduce the rate of decay in reactivity [3], [21], [22], [23]. On the other hand, dolomite (CaMg(CO3)2) is a widely distributed natural CaO sorbent, considered a more efficient precursor than limestone. It has been reported that calcined dolomite (CaO/MgO composites) becomes a more efficient sorbent than calcined limestone (pure CaO) [24]. Sun et al. [25] and Alvarez et al. [26] made similar observations and confirmed the good stability of dolomite. Upon calcination of dolomite, the decomposition of MgCO3 provided additional surface area and pore volume, which allowed CO2 to enter the particle more easily. Additionally, the high-Tammann-temperature of MgO (1276 °C) controlled sintering and stabilized the morphology, mitigating the loss of dolomite CO2 uptake capacity.
Therefore, the use of natural dolomite as precursor along with additional treatments not only may produce a promising sorbent but also could be a cost-effective and eco-friendly route. Li et al. [27] prepared an acetic acid-modified dolomite with improved CO2 capture behavior. Mohammadi et al. [28] developed a microwave heating system to capture CO2 by a packed bed of dolomite. Recently, Herce et al. [29] introduced a novel two-step calcination (half-calcination in CO2 with subsequent flash N2 calcination) to enhance the CO2 carrying capacity of dolomite, but unfortunately they only ascribed this improvement to the higher surface area of the sorbents. There is not enough information about the improved cyclic reactivity in this two-step calcination. It is well known that the thermal decomposition of dolomite proceeds via a single step in a N2 atmosphere and through two distinct stages at high CO2 partial pressures [30], [31]. Experimental studies [30], [31], [32] have also shown that the intermediate products under different atmospheres were also different. Apparently, calcination conditions and the resulting intermediate products can strongly influence the structure and properties of dolomite-based sorbents and, in turn, their cyclic CO2 capture mechanism. Consequently, with the help of clarifying the detailed decomposition process, the ultimate mechanistic understanding of the improved cyclic reactivity in this two-step calcination should be determined.
In this respect, the thermal behaviors and the intermediate products of dolomite in this two-step calcination were fully investigated. We demonstrated that two-step calcination proceeded via an intermediate phase of MgxCa1−xCO3 (calcination in a CO2 atmosphere) prior to the final formation of CaO/MgO (calcination in a N2 atmosphere). Moreover, the effect of the decomposition step of MgxCa1−xCO3 on the physical–chemical properties and the activity of sorbents was determined, which can help to better understand the cyclic CO2 capture mechanism of dolomite.
Section snippets
Sorbents
A natural dolomite was calcined in three different atmospheres. During the first one-step calcination, the complete decomposition process was conducted at 800 °C for 3 h under a 100% N2 atmosphere (designated as D-N8), while the second one-step calcination was at 950 °C for 3 h under a 100% CO2 atmosphere (designated as D-C9.5). The third two-step calcination was initially pretreated at 700 °C for 3 h under a 100% CO2 atmosphere and then at 800 °C for 3 h under a 100% N2 atmosphere (designated as
Characterization of the two-step decomposition process
Fig. 1, Fig. 2, Fig. 3 present TGA and XRD cures of dolomite at three different conditions. First, as shown in Fig. 1a, the decomposition of dolomite in a N2 atmosphere seemed to follow a one-stage decomposition route. The TGA profile showed a major weight loss starting from 400 to 800 °C. A maximum of the DTG peak was observed at 768 °C, which corresponded to 48.1 wt.% loss of CO2. This value approaches the theoretical weight loss of stoichiometric dolomite (47.8 wt.%) based on the Eq. (2) data,
Conclusion
A strong thermal behavior-structure-performance relationship is demonstrated in this work. During the two-step decomposition process, the presence of the Mg-calcite phase hindered the de-mixing of Ca and Mg, which delayed the sintering of CaO particles over repeated carbonation/calcination cycles. As a result, a favorable structure with smaller grains and larger specific surface area and pore volume was obtained. Therefore, D-C7N8 showed a higher and more stable uptake of CO2. In contrast, the
Acknowledgement
This work was supported by financial supports from the National Natural Science Foundation of China (51304197).
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