## 3 Results and Discussion
### Emissions in Oxy-Combustion of Australian Coal  
For Australian coal with an oxygen supply rate of \(\dot{\rm Q}_{\rm O2}\) = 41.5 Nm\({}^{3}\)/h, Figure 7a shows variations in the \(\rm O_{2}\) concentrations and \(\rm CO_{2}\) emissions with the 2 nd RFG flow rate, while Figure 7b shows variations in the CO, \(\rm SO_{2}\), and NO emissions with the 2 nd RFG flow rate. As shown in Figure 7a, for all operating conditions (with various 2 nd RFG flow rates), the residual oxygen concentration in the flue gas ranged from 8% to 10%, while the \(\rm CO_{2}\) concentration in the flue gas ranged between 8% and 90%. On the other hand, as shown in Figure 7b, CO emissions remained nearly constant, with increases in the 2 nd RFG flow rate. With increases in the 2 nd RFG flow rate, the NO emission decreased, due to the decrease in the peak flame temperature, which inhibits thermal NO formation  
Figure 5: Variations in temperature with the 2 nd RFG flow rate for Australian coal under oxy-combustion at \(\dot{\rm Q}_{\rm O2}\) = 41.5 Nm\({}^{3}\)/h, where \(\dot{\rm Q}_{\rm O2}\) is the volume flow rate of pure oxygen. Figure 6: Variations of temperature with the 2 nd RFG flow rate for Australian coal under oxy-combustion at \(\dot{\rm Q}_{\rm O2}\) = 38.5 Nm\({}^{3}\)/h. through the absorption of combustion heat by the relatively cooler flue gas, as well as the reduction in the oxygen concentration in the combustion zone, which reduces the availability of oxygen for the formation of fuel NO. Meanwhile, the SO2 emissions were higher at a higher 2 nd RFG flow rate. In these experiments, the flue gas was recirculated without SO2 removal. Accordingly, there were significant increases in the SO2 concentrations in the flue gas, due to the accumulated effects of flue gas recirculation and the reduced volume of the flue gas [35]. For Australian coal at \(\dot{\text{Q}}_{\text{O2}}\) = 38.5 Nm\({}^{3}\)/h, Figure 8a shows variations in the O2 concentrations and CO2 emissions with the 2 nd RFG flow rate, while Figure 8b depicts variations in the CO, SO2, and NO emissions with the 2 nd RFG flow rate. As shown in Figure 8a, the residual oxygen concentration in the flue gas ranged between 3% and 5%. Meanwhile, the CO2 concentration in flue gas remains at about 94% (average value), and reached 94.8% when the 2 nd RFG flow rate was 170 Nm\({}^{3}\)/h. It is noteworthy that the high CO2 concentration in the flue gas suggests great potential for reducing CO2 emissions through carbon capture and storage, thus mitigating global climate change. Figures 7a and 8a also demonstrate that a higher oxygen supply results in a higher residual O\({}_{2}\) and less CO\({}_{2}\) in the flue gas. The main reason for these results is that the flue gas contains O\({}_{2}\), CO\({}_{2}\), and other gaseous molecules, and CO\({}_{2}\) and O\({}_{2}\) are the two main components. Therefore, Figures 7a and 8a indicate that a lower residual O\({}_{2}\) concentration corresponds to a higher CO\({}_{2}\) concentration [36]. The oxygen concentration in the flue gas at \(\dot{\rm Q}_{\rm O2}=38.5\) Nm\({}^{3}\)/h is lower than that at \(\dot{\rm Q}_{\rm O2}=41.5\) Nm\({}^{3}\)/h; therefore, the CO\({}_{2}\) concentration at \(\dot{\rm Q}_{\rm O2}=38.5\) Nm\({}^{3}\)/h is larger than that at \(\dot{\rm Q}_{\rm O2}=41.5\) Nm\({}^{3}\)/h. In air-firing mode, 20% excess air is typically utilized. Oxy-coal combustion requires excess O\({}_{2}\) to reach a residual O\({}_{2}\) concentration in the flue gas similar to that of air combustion, which ranges from 3% to 5% [37]. It is worth noting that the CO\({}_{2}\) concentration in the flue gas remained at a high average value, about 94%, while the residual O\({}_{2}\) concentration in the flue gas was controlled in the range of 3%-5%. This result is in good agreement with the findings of [PERSON] et al. [37].