## 3 Results and Discussion
### Emissions in Oxy-Combustion of Australian Coal
In the present study, it was also found that in order to achieve the same performance in an existing furnace, the oxygen concentration (about 23%) in oxy-fuel combustion is higher than the oxygen concentration in air, as reported by [PERSON] et al. [38]. Figure 8: Variations in (**a**) CO\({}_{2}\) and O\({}_{2}\) emissions, and (**b**) CO, NO, and SO\({}_{2}\) emissions with 2 nd RFG for Australian coal during oxy-combustion at \(\dot{\rm Q}_{\rm O2}=38.5\) Nm\({}^{3}\)/h. Figure 8b shows a similar trend in NO emissions as that shown in Figure 7b. That is, with increases in the 2 nd RFG flow rate, NO emissions decrease. For Australian coal during oxy-combustion, Figures 7b and 8b also show a similar trend, where in general, SO2 emissions increase as the 2 nd RFG flow rate rises. However, in the flue gas, the concentration of SO2 at Q02 = 38.5 Nm3/h (Figure 8b) is much higher than that at Q02 = 41.5 Nm3/h (Figure 7b), perhaps because SO2 converts into SO3 with greater oxygen supplementation [39]. As shown in Figure 8b, during oxy-combustion with recirculated flue gas, the concentrations of SO2 (in the range of 850-1180 ppm) are about three to four times larger than those (302 ppm) in air-combustion conditions [40], which agrees well with [PERSON] et al. 's study [35]. In addition, increasing the secondary RFG flow rate corresponds to the enhancement of swirling in the swirl burner, leading to better mixing and more complete combustion; therefore, the concentration of CO in the flue gas is reduced. It was also found that at a fixed secondary RFG flow rate, a lower oxygen supply rate at Q02 = 38.5 Nm3/h (Figure 8b) results in a higher CO emission than with a higher oxygen supply rate, at Q02 = 41.5 Nm3/h (Figure 7b), due to the combined effects of increased oxygen concentration in the inlet oxidizer (RFG/oxygen mixture) and strengthened swirling. The emission of NO decreased with increases in the 2 nd RFG flow rate, due to a lower flame temperature. Operating conditions at an oxygen supply rate of 38.5 Nm3/h result in lower NO emissions compared to those with an oxygen supply rate of 41.5 Nm3/h. That is, the NO concentration in the flue gas increased with increases in the O2 concentration in the inlet O2/RFG mixture. This phenomenon is in agreement with the findings in the literature [41; 42; 43]. [PERSON] and [PERSON] [41] reported that a decrease in the concentration of NO under oxy-combustion conditions was mainly due to the reburning of the NO in the recirculated flue gas. [PERSON] et al. [42; 43] found that an increase in the flow rate of recirculated flue gas led to a decrease in NO emissions. In other words, if the flow rate of the recirculated flue gas decreased, the concentration of oxygen in the burner zone increased, which contributed incrementally to NO emissions. Furthermore, the NO concentration under oxy-combustion conditions was 189.5 ppm (at 6% O2), with a secondary RFG flow rate of 170 Nm3/h, while the NO concentration under air combustion conditions was 739 ppm (at 6% O2) [40]. This indicates a 74% NO reduction under oxy-firing conditions compared to the air-firing mode. This significant decrease in NO emissions under oxy-combustion conditions coincides with the findings of [PERSON] et al. [44] and our previous work [31]. To summarize, the results for Australian coal under oxy-combustion conditions with varied RFG flow rates and different amounts of oxygen supply indicate the same temperature distribution trends in both the radiative and convective sections of the furnace. That is, the furnace wall temperatures in the radiative section decreased while the gas temperatures in the convective section increased with increases in 2 nd RFG. In addition, compared to those at Q02 = 41.5 Nm3/h, with a lower oxygen supply (Q02 = 38.5 Nm3/h), the residual oxygen and NO emissions were lower, while SO2, CO2, and CO emissions were higher.