## 3 Results and Discussion
### System Validation with Slaked Lime
#### 3.2.3 PC Characterization  
The results of the FGD experiments for the PC sample indicated that the increase in temperature was more sensitive than the relative humidity in terms of the uptake observed. Consequently, the characterization of PC was based on experimental findings to explain the differences between the reactions of PC with SO2 at 75 \({}^{\circ}\)C and 160 \({}^{\circ}\)C.  
The XRPD patterns of the pristine PC, PC [75 \({}^{\circ}\)C\(\_\)30 RH%], and PC [160 \({}^{\circ}\)C\(\_\)30 RH%] are presented in Figure S10. The unit-cell and structural parameters were determined starting from the trigonal \(R\)-3\(c\) (for calcite) and the trigonal \(P\)-3\(m\)1 (for portlandite) space groups. The PC samples exhibited the characteristic peaks of crystalline portlandite \begin{table}
\begin{tabular}{c c c c c c} \hline
**Sample** & **Weight** & \(\lambda\) & \(t_{b}\) & \(\mu_{max}\) & **Breakthrough Sulfur Capacity** \\ \hline PC & 0.10 g & 16 min & 10 min & 0.027 & 18 mg/g \\
1-PC [75 \({}^{\circ}\)C\(\_\)30 RH\%] & & & & & 19 mg/g \\
2-PC [75 \({}^{\circ}\)C\(\_\)30 RH\%] & 0.14 g & 28 min & 20 min & 0.029 & 18 mg/g \\
3-PC [75 \({}^{\circ}\)C\(\_\)30 RH\%] & & & & & 20 mg/g \\ PC [160\({}^{\circ}\)C\(\_\)30 RH\%] & 0.10 g & 21 min & 20 min & 0.034 & 24 mg/g \\ PC [75\({}^{\circ}\)C\(\_\)45 RH\%] & 0.14 g & 33 min & 30 min & 0.033 & 21 mg/g \\ \hline \end{tabular}
\end{table} 
Table 2: Breakthrough sulfur capacity (mg/g) of PC in different conditions of temperature and RH%. (\(\sim\)96 wt%) and calcite (\(\sim\)3 wt%), as reported in Table S6. No evidence of a loss of crystallinity or the formation of new secondary phases was observed after the SO\({}_{2}\) uptake experiments. In PC samples, the SO\({}_{2}\) uptake was not clearly identified by diffraction data and unit-cell variations (Table S7). Figure S11 illustrates that the PC samples exhibited a multistep weight loss in three distinct temperature ranges, with a total weight loss from 27 wt%, for the samples subjected to a gas flux at different temperatures, to 31 wt%, for the unloaded sample (Figure S11a). The first temperature range, up to approximately 400 \({}^{\circ}\)C, is marked by an endothermic event with a peak centered at approximately 100 \({}^{\circ}\)C (Figure S11c). This phenomenon is commonly associated with a dehydration reaction, which is the result of the release of surface-bonded water molecules. The transitions between the three thermal ranges, beginning at about 400 and 675 \({}^{\circ}\)C, are characterized by an endothermic and an exothermic reaction with peaks centered at approximately 500 and 750 \({}^{\circ}\)C. These reactions typically occur in samples of portlandite and calcite, where the release of hydroxyl and CO\({}_{2}\) molecules occurs during the decomposition processes of these phases in the temperature ranges of 390-485 \({}^{\circ}\)C and 600-870 \({}^{\circ}\)C, respectively [40; 41]. In order to experimentally prove SO\({}_{2}\) chemisorption over the adsorbent surface, further experimental techniques were required for the PC samples. Therefore, EDS and XPS analyses were also performed. The results of the micromorphological investigation indicate that the process of SO\({}_{2}\) adsorption carried out within the reactor did not significantly alter the size of the particles or their state of aggregation. This observation can be clearly discerned by comparing the micrographs of the pristine portlandite (PC) (Figure S12a) with those of [PC 75 \({}^{\circ}\)C\(\_\)30 RH%] and [PC\(\_\)160 \({}^{\circ}\)C\(\_\)30 RH%] (Figure S12b,c). With regard to the microchemical investigation, the SEM-EDS measurements performed on PC samples corroborate their composition with respect to the major and minor components, namely, Ca, Mg, and O (Table S8). Carbon was detected, but its content was not quantified due to the carbon coating procedure used for the sample preparation. In addition to the aforementioned elements, Al, Si, P, and Sr were occasionally identified, just above their respective detection limits.