## 3 Results and Discussion
### Mass Yields of PM\({}_{10}\)
#### 3.4.1 PM\({}_{0.5}\)  
As observed in Figures 6 and 7a, the highest yield of PM\({}_{0.5}\) from the AIR condition is created mainly by the highest yield of Ca, and the yields of Na and S from AIR are inhibited compared to oxy-fuel combustion. The increased yield of PM\({}_{0.5}\), like the increased oxygen fraction during oxy-fuel combustion, is mainly caused by the increase in Mg and Ca (Figure 7a). In Section 3.1, it was noted that the OXY21 condition results in the peak size of the ultrafine mode becoming smaller (about 0.07 \(\upmu\)m) compared to the AIR and OXY32 conditions. This is clearly caused by the highest concentrations of Na and S from OXY21 found in Figure 6. The higher content of Na in ultrafine particles generated from lower O\({}_{2}\) fraction of oxy-fuel combustion has also been observed by [PERSON] et al. [28]. **Partitioning of Ca. ** Figure 1 shows that more than half of the Ca occurring in Zhundong coal is organically bound, while \(\sim\)30% of the Ca is HCl-soluble and 10% is residual. The occurrence mode of Mg is similar to Ca. It was noted that 40% of the Ca exists in the inorganic minerals and can be determined by CCSEM, and the data show that \(\sim\)16% of the Ca in minerals was identified as calcite which is HCl-soluble; the others were identified as complex minerals such as dolomite (Ca-Mg), Ca-Mg-S, part of which should also be HCl-soluble. The last 10% of the residual Ca should exist as aluminosilicates containing Ca, Mg, Fe, Na, etc. One of the important serial pieces of research on the thermal utilization of Australian low-rank coal by [PERSON] et al. [44] indicates the reactions of organically bound AAEM during coal combustion as follows:  
\[\text{CM-CH}_{2}\text{-COOX} \rightarrow \text{CM}^{\prime}+\text{CH}_{3}\text{COOX} \tag{1}\] \[\text{CM-COOX} \rightarrow \text{CM-X}+\text{CO}_{2}\] (2) \[\text{CM-X} \rightarrow \text{CM}+\text{X} \tag{3}\]  
where X can be Na, K, Mg or Ca, CM and CM\({}^{\prime}\) represent coal or char matrix. Reaction (1) is insensitive to temperature and can take place at relatively low temperatures (e.g., \(<\)600 \({}^{\circ}\)C). Equation (2) thermally breaks down the carboxylates and releases CO\({}_{2}\). Importantly, reaction (3) is affected by temperature and the bond will be thermally broken at higher temperatures to release AAEM. As a result, the volatilization of organically bound AAEM (e.g., 50% of Ca in Zhundong coal) increases when the char matrix is burned at higher temperatures, generating more ultrafine particles. The 30% HCl-soluble Ca exists mainly as calcite and dolomite. These minerals undergo decomposition (Equations (4) and (5)), vaporization and fragmentation to form ultrafine particles. Figure 7: Absolute mass of inorganic elements in (**a**) PM\({}_{0.5}\) and (**b**) PM\({}_{0.5-10}\) during air and oxy-fuel combustion. Higher particle-burning temperatures (e.g., AIR and OXY32) and lower CO\({}_{2}\) fractions (e.g., OXY32) promote the reactions. \[\text{CaCO}_{3}\rightarrow\text{CaO}+\text{CO}_{2} \tag{4}\]  
\[\text{CaCO}_{3}\text{-MgO}_{3}\rightarrow\text{CaO}\text{-MgO}+2\text{CO}_{2} \tag{5}\]  
Therefore, the release of Mg and Ca from AIR and OXY32 can be expected to be higher than OXY21 and OXY27, creating higher ultrafine particles (Figure 7a). It should be noted that CO\({}_{2}\) participates in the reactions involving both organically bound Ca (Equation (2)) and calcite (Equation (4)). These reactions found in OXY32 with 68% CO\({}_{2}\) should be inhibited compared to AIR, and a much higher yield of Ca is thus found in PM\({}_{0.5}\) from AIR combustion. **Partitioning of Na and S.** [PERSON] et al. [45] reported that after coal combustion, most Na is in the form of either NaCl(g) or Na\({}_{2}\)SO\({}_{4}\)(g), and NaCl will be transferred to Na\({}_{2}\)SO\({}_{4}\) when the content of SO\({}_{2}\) in flue gas is high, according to Equation (7).