## Materials and Methods
### Experimental Setup  
A lab-scale entrained flow reactor was employed to investigate the NO\({}_{x}\) conversion during the oxy-fuel combustion of synthetic coals as shown in Figure 3, which can realize the rapid combustion of fine coal particles with a preset stable feeding rate. The experimental reactor had an inside diameter of 32 mm and was heated by an electrical heating furnace. The temperature was monitored by an S-type thermocouple. Before the rapid combustion experiments, the reactor was heated to a preset temperature, a sample of 0.1 g was loaded in the micro-feeder in advance, and the system was purged by the desired gas mixture at room temperature. The total flow rate of the mixture gas was 1000 mL-min\({}^{-1}\), while the whole gas mixture was split into two streams. The primary (oxidizer carrying the solid fuel) flow rate was 200 mL-min\({}^{-1}\) and used to entrain the sample into the reactor with a feeding rate of 0.04 g\(\cdot\)min\({}^{-1}\). The oxygen concentration in experiments of oxy-fuel combustion ranged from 10% to 50%. The outlet concentrations of gas species were continuously analyzed by an FTIR gas analyzer Gasmet DX-4000. All experiments were carried out at least three times for repeatability and the results were averaged. Here, a micro-feeder was specially designed to ensure the micro-scale, continuous, and stable feeding of fine solid particles as the enlarged insect depicted in Figure 3. The feeding rate can be adjusted by the flow rate of the stream and the rotation speed of the stepping motor. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Coal Code**} & \multirow{2}{*}{\
\begin{tabular}{c} **BET** \\ **m\({}^{2}\) g\({}^{-1}\)** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Fuel Ratio** \\ **(\(w\)(FC)/\(w\)(V))** \\ \end{tabular} } & \multicolumn{4}{c}{**Proximate Analysis**} & \multicolumn{4}{c}{**Ultimate Analysis**} \\ \cline{4-11}  & & & **w(FC)/\(w\)(V)** & \(w\)(FC) & \(w\)(V) & \(w\)(A) & \(w\)(C) & \(w\)(H) & \(w\)(O)\({}^{a}\) & \(w\)(N) & \(w\)(S) \\ \hline QC & 3.87 & 2.5 & 46.31 & 18.26 & 35.43 & 55.36 & 3.12 & 4.64 & 1.10 & 0.35 \\ PZ & 9.37 & 1.2 & 38.77 & 31.22 & 30.01 & 50.58 & 3.42 & 14.09 & 0.90 & 1.00 \\ M6 & 4.12 & 2.2 & 68.90 & 31.10 & 0.00 & 65.39 & 2.86 & 28.30 & 3.45 & 0.00 \\ M5 & 4.31 & 2.2 & 68.47 & 31.53 & 0.00 & 68.12 & 3.09 & 25.96 & 2.83 & 0.00 \\ \hline \end{tabular}
\end{table} 
Table 1: The proximate and ultimate analyses of real and synthetic coals (wt%, d). Figure 3: The schematic diagram of the experimental system for NO\({}_{x}\) emission during air and oxy-fuel combustion. Due to the content differences of fuel nitrogen in various samples, the comparison between the amounts of nitrogen species cannot reasonably demonstrate the transformation behaviors of various nitrogen functional groups. Here, the conversion ratios of fuel-nitrogen to nitrogen oxides (NO, N\({}_{2}\)O, and NO\({}_{2}\)) were employed to investigate the transformation of fuel nitrogen during air and oxy-fuel combustion. The emission content of nitrogen oxides was calculated from the integral of nitrogen oxide concentration within the duration of a certain time.