3.1.6. Serpentinite

The result of the DTA–TG analysis of serpentine sample is shown in Figure 12. Two endothermic peaks were found at temperatures of 623.1 and 701.9 ◦C due to the release of structural water. At a temperature of 834.1 ◦C, a large exothermic peak was visible, which represents the destruction of the serpentine crystalline structure and the formation of forsterite, enstatite, and clinoenstatite. For temperatures higher than 750 ◦C, the TGA analysis showed no significant weight variation. During carbonation, the sample mass changes reached from 1.42% (first cycle) to 0.95% (10th cycle). The efficiency of gas capture decreased at a decreasing rate. The reaction segmen<sup>t</sup> was shortened in successive stages, while the diffusion segmen<sup>t</sup> became more apparent, which confirms the sintering of the sample. About 5 min after the CO2 shut off, there was a slight decrease in the mass of the sample—typical for the first-order reaction—followed by a sharp but slight weight loss due to calcination.

**Figure 12.** TGA and DSC curves for serpentinite.

#### *3.2. Thermal Pretreatment of Sorbents*

Calcium sorbents are characterized by a decreasing activity of capturing CO2 in each subsequent carbonation cycle. After approximately one hundred cycles, the asymptotically decreasing sorbent yield ranges from 7 to 15% for a 10 min carbonation time. It is believed that this is the result of changes in the sorbent's morphology, during which its specific surface area decreases and the micropores disappear. In order to improve the activity of sorbents, the following enhancement techniques are used:


Research performed by Manovic and Anthony [25] and Manovic et al. [26] demonstrated that thermal pre-treatment could be an important method of improving conversion of CaO over long series of cycles. Such a phenomenon might be explained by a theory proposed by Lysikov et al. [27] and developed by Manovic and Anthony [25], according to which the repetitive carbonation/calcination cycles enhance the formation of a skeleton of interconnected CaO. This skeleton acts as the outer layer of the reactive CaO layer and stabilizes the sorption capacity.

The tests by Manovic and Anthony [25] showed that the particles were strongly sintered and that carbonization occurred on the surface of the solid particle. The pretreatment resulted in the formation of an internal skeleton of the sorbent particles and protection of their integrity. When the sorbents are preheated, after the decomposition of CaCO3, ion diffusion continues and stabilizes the skeleton, which, due to its porous structure, is able to maintain significant carbonation (Figure 13).

**Figure 13.** Schematic representation of the pore–skeleton model [25].

Manovic and Anthony [20] studied the improvement of sorbent properties (Kelly Rock limestone) with 54.39% CaO content and loss on ignition (44.20%) through steam reactivation, thermal pre-treatment, and addition of calcium-aluminate-based pellets. The most promising results were obtained for powdered Kelly Rock limestone samples (<50 μm). For the preheating temperatures of 1000 ◦C, the sorbent was self-reactivated for the next 30 carbonation cycles. The highest conversions were obtained at 1000 ◦C, for which 49% conversion was achieved in the last cycle, with an average value of ∼45% for 30 cycles.

Lower conversion values were obtained for samples pretreated at 1100 and 1200 ◦C. However, taking into account the self-reactivation effect, these results may also be promising, especially since heating the sorbent at these temperatures (in combination with granulation) gives better mechanical properties that could prevent its undesirable attrition. Assuming that the pretreatment time is an important parameter,a6h thermal activation study was also carried out on Kelly Rock powdered samples at temperatures of 900–1100 ◦C. It was confirmed that the pretreatment time influences the properties of the sorbent, and that shorter thermal treatment times can positively influence the effectiveness of the sorbent [25].

In this work, based on the results of the preliminary tests, three limestone samples from Stramberk (Czechia), Podlesie (Poland), and Butkov (Slovakia) (Table 1) were selected for further testing. They were characterized by high, medium, and the lowest weight loss, respectively. The tested sorbents were thermally modified by pre-heating the sample for one hour at 1000 ◦C in 100% N2 atmosphere. A further research cycle was carried out according to the procedure described earlier.

The effect of thermal pretreatment is presented for the example of limestone from Stramberk (Czechia) (Figure 14A,B). During the carbonation of the raw sample, CO2 capture was achieved, causing the sample mass to change within a range from 31.36% (first cycle) to 13.68% (10th cycle). The gas capture efficiency decreased with increasing number of CaL cycles. After the first cycle, the gas uptake efficiency decreased to 25.36%, which may be due to the reduction of the active sorbent surface due to sintering. It is noticeable that the reaction time (visible as a steep section of the mass increase) go<sup>t</sup> shorter in subsequent cycles. Simultaneously with the reduction of the reaction section, the extension of the transition section was noticeable, which proves the increasing role of diffusion and confirms the phenomenon of sintering of the sample.

It is noteworthy that, within the assumed time of 10 min, the carbonation process was not completed. This is evident in the sample mass change graph, which shows a sharp rise (reaction stage) moving smoothly (transition stage) towards the diffusion stage. The latter, however, is not observed (no element approaching the horizontal line, mass growth curve). This means that in the case of the Stramberk limestone, carbonation could be carried out for a longer time than assumed in the analyzed simulation. The extension of the carbonation time in this case may be associated with a potential reduction in the economics of the capture process. This issue would require further tests with extended carbonation time in order to calculate the amount of CO2 bound by the sorbent through the diffusion process.

The comparison of the relative mass changes for Stramberk limestone without modification and thermally pretreated samples proves that the end of the reaction stage occurs at a similar temperature—around 644.5 ◦C (Figure 15). The calcination time varies, however, and is shorter for the unmodified sorbent, which is related to the lower content of CO2 blocked in this sample.

**Figure 14.** Relative mass changes for Stramberk limestone: (**A**) unmodified; (**B**) thermally pretreated for 1 h at 1000 ◦C.

**Figure 15.** Relative mass changes for Stramberk limestone at the fifth and sixth calcium looping cycles: (**A**) without modification; (**B**) pretreated for 1 h at 1000 ◦C.
