## 3 Results
### Comparison of the NO and NO\({}_{2}\) Emissions with Oxygen Content  
The oxygen content in oxy-fuel combustion can be regulated by adjusting the ratio and location of FGR while it also yields an impact on the coal reactivity and NO\({}_{x}\) emission. The effects of oxygen content on NO\({}_{x}\) emissions at the combustion temperature of 1200 \({}^{\circ}\)C are illustrated in Figure 6. The NO conversion increases with the oxygen content, as shown in Figure 6a. The effect of oxygen variation on the NO emission is significant as the O\({}_{2}\) content is below 21%, while the increase extent of NO conversion is slowed down with an O\({}_{2}\) content beyond 21%. [PERSON] et al. [22] and [PERSON] et al. [34] also observed an increase in NO emission with oxygen concentration for the cases of oxy-fuel combustion, while [PERSON] and [PERSON] [33] concluded that the conversion of fuel-nitrogen to NO\({}_{x}\) increased notably with the oxygen content, which is similar to the present study. The oxidation reactions of fuel-N are promoted by increasing oxygen content. More oxygen leads to less CO on char surfaces and the oxidizing atmosphere around the char particles impedes the NO reduction. As for the principal NO\({}_{x}\) precursor, NH\({}_{3}\), the conversion rate of NH\({}_{3}\)-to-NO\({}_{x}\) can be increased obviously with the raised O\({}_{2}\) concentration under oxy-fuel conditions [31]. Whereas, the further access of oxygen content cannot considerably enlarge the NO conversion, which is beneficial for the NO\({}_{x}\) reduction during oxy-fuel combustion. [PERSON] et al. [40] investigated the oxidation of pyridine at 1000 \({}^{\circ}\)C and indicated that the NO conversion cannot continue to increase with the oxygen content, which is consistent with the present experimental result of synthetic coal with pyridinic nitrogen.  
Figure 6b shows the evolution of NO\({}_{2}\) conversion ratio with the oxygen content. The variation of NO\({}_{2}\) conversion is roughly proportional to oxygen content, largely different from the case of NO. The NO\({}_{2}\)/NO\({}_{x}\) ratios present a slightly increasing tendency with the oxygen content in oxy-fuel combustion, which is in accordance with results obtained by [PERSON] et al. [22] The access O\({}_{2}\) can further enhance the conversion of NO to NO\({}_{2}\), which also has a certain association with the slow-down increase of NO. The rise in the concentration of oxidizing radicals from high oxygen inlet content is responsible for the promotion of NO\({}_{2}\) formation as well. The rise of O\({}_{2}\) content not only promotes the formation of NO, but also intensifies the further oxidation of already-formed NO to NO\({}_{2}\). Consequently, compared with NO, the increase of the oxygen partial pressure in the oxidant has a more obvious promotion effect with respect to the NO\({}_{2}\) formation. It can be seen from Figure 6b that, the conversions of NO\({}_{2}\) from synthetic coal \"M5\" are considerably higher than those from \"M6\". Fuel nitrogen of pyrrolic form is inclined to transform into NO\({}_{2}\) under the condition of a high O\({}_{2}\) content in oxy-fuel combustion. The NO conversion from PZ lignite is the highest while the case of NO\({}_{2}\) is different. The conversion of NO\({}_{x}\) (sum of NO and NO\({}_{2}\)) from fuel nitrogen is reduced with an increase in the nitrogen content of dry ash-free basis within coal matrix. Coal with a high level of pyrrolic nitrogen  
Figure 6: The comparison between NO and NO\({}_{2}\) emissions with oxygen content at 1200 \({}^{\circ}\)C under the oxy-fuel condition. (**a**) Conversion to NO; (**b**) Conversion to NO\({}_{2}\).  
favors the formation of NO\({}_{2}\) compared with others. Hence, the coal property and chemical form of fuel nitrogen exhibit distinct influences on the formation and destruction of NO\({}_{x}\) during the oxy-fuel combustion of coal-based fuels.