## 3 Results
### Mineral Impacts on the Transformation of Fuel Nitrogen  
The ash and sulfur were absent in synthetic coals on account of the none corresponding constituents in raw materials, which is beneficial to probe the influence of certain mineral matters on NO\({}_{x}\) reduction during oxy-fuel combustion compared with air. Figure 8 shows the effects of Fe\({}_{2}\)O\({}_{3}\), Na\({}_{2}\)CO\({}_{2}\), and CaCO\({}_{3}\) on NO\({}_{x}\) emission during air and oxy-fuel combustion. The mineral matters establish the same influence tendency on NO conversion from pyridinic nitrogen of synthetic coal between O\({}_{2}\)/CO\({}_{2}\) and O\({}_{2}\)/N\({}_{2}\) atmospheres as shown in Figure 8a,b. The catalytic effects of sodium (Na), calcium (Ca), and iron (Fe) on the oxidation of pyridinic nitrogen are independent of the combustion atmosphere, and the change from air to oxy-fuel combustion has a negligible relevance to the impacts of mineral addition on the transformation behaviors of pyridinic nitrogen. When the combustion temperature is 1000 \({}^{\circ}\)C, the additive Fe\({}_{2}\)O\({}_{3}\) reduces the NO conversion while Na\({}_{2}\)CO\({}_{3}\) and CaCO\({}_{3}\) can promote the formation of NO. The presence of Fe\({}_{2}\)O\({}_{3}\) possibly promotes the reduction of NO to N\({}_{2}\) via the following reactions [41; 42]:  
\[\mathrm{Fe_{2}O_{3}+3 CO}\to 2\mathrm{Fe}+3\mathrm{CO_{2}}\] (R1)  
\[2\mathrm{Fe}+3\mathrm{NO}\to\mathrm{Fe_{2}O_{3}+1.5N_{2}}\] (R2)  
Figure 7: The particle size dependence of NO and NO\({}_{2}\) emissions at 1200 \({}^{\circ}\)C under the oxy-fuel condition. (**a**) Conversion to NO; (**b**) Conversion to NO\({}_{2}\).  
The conversion of NO from synthetic coal with pyridinic nitrogen has the following sequence: Na\({}_{2}\)CO\({}_{3}\) > CaCO\({}_{3}\) > without-mineral > Fe\({}_{2}\)O\({}_{3}\). However, the addition of these minerals all reduces the NO emission at 1200 \({}^{\circ}\)C with the following sequence of NO conversion from pyridinic nitrogen: without-mineral > Na\({}_{2}\)CO\({}_{3}\) > CaCO\({}_{3}\) > Fe\({}_{2}\)O\({}_{3}\). The contribution of minerals on the transformation of fuel nitrogen is strongly affected by two competitive aspects during oxy-fuel combustion: the catalytic effect on oxidation of fuel nitrogen and the impact on the heterogeneous reduction of NO\({}_{x}\) on char surfaces.  
As Figure 8c,d depicted that the influence of mineral matter on the oxidation of pyrrolic nitrogen differs significantly from the case of pyridinic nitrogen. Only the addition of Fe\({}_{2}\)O\({}_{3}\) leads to a decline of NO conversion from pyrrolic nitrogen, while both Na\({}_{2}\)CO\({}_{3}\) and CaCO\({}_{3}\) facilitate the release of NO. Moreover, the mineral matters generate different influences on the oxidation of pyridinic nitrogen under O\({}_{2}\)/CO\({}_{2}\) and O\({}_{2}\)/N\({}_{2}\) conditions. The conversion of NO from a synthetic coal sample with Na\({}_{2}\)CO\({}_{3}\) additive exceeds that with CaCO\({}_{3}\) in O\({}_{2}\)/CO\({}_{2}\) atmosphere at 1000 \({}^{\circ}\)C, which differs from the air case. Compared with air circumstance, oxy-fuel combustion causes the change of the mineral catalytic effect on the transformation of pyrrolic nitrogen within the organic structure of coal. The combustion temperature and species of mineral matters exhibit various influences on the formation and reduction of NO\({}_{x}\) during the oxy-fuel combustion of coals.