**4. Discussion**

The CaL simulations showed different degrees of conversion for the tested rock sorbents (Figures 16 and 17). As expected, they were the highest for carbonate rocks (except for bituminous limestone); intermediate values were found for marl and bituminous limestone, and the values were lower by an order of magnitude for the remaining sorbents: magnesite and serpentinite. The lowest degree of conversion was determined for basalt.

**Figure 16.** The conversion of carbonate rocks in the CaL process.

**Figure 17.** The conversion of magnesite and non-carbonate rocks in the CaL process.

The decrease in the conversion rate with subsequent CaL cycles was most pronounced for the sorbents with the highest values of this parameter, again with the exception of bituminous limestone (for which a significant decrease was noted between the first and second cycles; Figure 16). In the case of basalt, there were no significant changes in the conversion rate during the simulated cycles.

From the standpoint of the efficiency of the CaL process, an important indicator is the parameter called "cumulative carbonation relative to the initial mass of the sorbent", which, for the 10 analyzed cycles (*n* = 10), could be defined as the degree of carbonation for *n* cycles.

It represents the multiplicity of the captured CO2 relative to the initial sorbent mass. In the course of the analyzed cycles, the cumulative carbonation shows regularities that are similar to the degree of conversion (Figures 18 and 19). This parameter, exceeding 1.0, was also the highest for carbonate rocks (limestones: Stramberk—1.93, Saint Anne Mountain —1.81, Gorazdze—1.65, Podlesie—1.62, and dolomite of Olkusz—1.26); values that were lower by an order of magnitude were achieved for marl, bituminous limestone, and serpentine, and values lower by two orders of magnitude were achieved for magnesite and basalt.

The CaL simulations performed for the thermally pretreated samples also demonstrated different degrees of conversion for the tested rock sorbents (Table 2). As predicted, they were the highest for the Stramberk limestone sample, which was characterized by the highest proportion of calcium carbonate. The decrease in conversion with subsequent CaL cycles was most apparent for the sorbents with the highest values of this parameter.

**Figure 18.** Cumulative carbonation relative to the initial mass of the sorbent—carbonate rocks.

**Figure 19.** Cumulative carbonation relative to the initial mass of the sorbent—non-carbonate rocks.


**Table 2.** Conversion in raw and pretreated limestones.

The cumulative carbonation in the course of the analyzed cycles shows a pattern similar to that of the degree of conversion (Table 3); however, the value of this parameter was more than two times lower than for the raw samples of Stramberk (0.86) and Butkov (0.35). The greatest relative decrease was recorded for the Podlesie limestone, the cumulative carbonation of which decreased by more than three times (from 1.62 for raw sample to 0.5 after thermal treatment) despite the relatively high content of CaCO3.


**Table 3.** Cumulative carbonation relative to the initial weight of the sorbent for raw and pretreated limestones.

The thermal pretreatment was not effective for the examined limestones, as also reported by Manovic et al. [20]; it is believed to be efficient for only some types of natural materials. It is likely that different types of limestone require different pretreatment conditions due to differences in impurities and internal structures [28]. However, this treatment has clear advantages: It is simple and relatively inexpensive compared to other techniques. On the other hand, it should be underlined that this would require additional energy to heat up the sorbent prior to its final use. This may result in a reduction in the power output of a CaL-equipped power generation system. Nonetheless, several studies proved that even if the pretreated limestone shows lower values of initial sorption capacity, this capacity increases over many cycles due to the softening of the hard skeleton. The disadvantage of this refining technique is that although the reactivity increases, the attrition of the particles significantly increases [29].

#### **5. Summary and Conclusions**

The suitability of selected rocks—limestone, dolomite, magnesite, marl, serpentinite, and basalt—was tested for the purpose of CO2 sequestration in the CaL process. TGA-DSC tests were carried out based on a temperature program designed for this purpose. The tests were performed in 10 cycles of alternating calcination and carbonation. During the measurements, changes in the mass of the sample over time (TGA) and heat flow (DSC) were recorded.

CaL simulations showed various degrees of conversion for the tested rock sorbents— the highest values were achieved for carbonate rocks (except for bituminous limestone), intermediate values were achieved for marl and bituminous limestone, lower ones were achieved for the remaining sorbents (magnesite and serpentinite), and the lowest were achieved for basalt.

The decrease in the conversion rate with subsequent CaL cycles was most intense for the sorbents with the highest values of this parameter. In the case of basalt, no significant changes in the conversion rate were observed. The decrease in gas capture efficiency with an increasing number of CaL cycles may be related to the decreasing active sorbent surface due to sintering.

The values of the parameter called "cumulative carbonation relative to the initial mass of the sorbent" corresponded to the multiplicity of the captured CO2 relative to the initial sorbent mass. This parameter, exceeding a value of 1.0, was the highest for carbonate rocks; it achieved lower values for marl, bituminous limestone, and serpentine, as well as—by two orders of magnitude—for magnesite and basalt.

In most of the analyzed samples, the carbonation process was not completed within the assumed time of 10 min. In practice, however, extending the carbonation time could reduce the economics of the capture process.

The simulations of the thermally pretreated samples also demonstrated different degrees of conversion for the tested rock sorbents, which were the highest for the calciumcarbonate-rich Stramberk limestone. The cumulative carbonation of the pretreated samples was more than two times lower than that of the raw ones. The largest relative decrease was recorded in the case of Podlesie limestone, the cumulative carbonation of which decreased by more than three times, despite the relatively high CaCO3 content.

**Author Contributions:** Conceptualization, methodology, validation, formal analysis, investigation, data curation, writing—original draft preparation, writing—review and editing, visualization, K.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Conflicts of Interest:** The author declare no conflict of interest.
