## 3 Results and Discussion
### NO Formation Characteristics of NH\({}_{3}\)/CH\({}_{4}\) Combustion  
#### 3.2.1 Effect of Proportion of NH\({}_{3}\) in Fuel Gas on NO Production  
In this section, we describe our investigation of the effect of the proportion of ammonia in NH\({}_{3}\)/CH\({}_{4}\) on the emission of NO from combustion at different combustion temperatures. Figure 4 shows the curve of NO production with different proportions of ammonia in fuel  
Figure 3: NO produced at different fuel gas concentrations at 1500 \({}^{\circ}\)C.  
Figure 2: NO produced by NH\({}_{3}\) combustion at different residence times at 1500 \({}^{\circ}\)C.  
gas at different combustion temperatures. The proportion of ammonia gas varied from 30% to 100%; the combustion temperatures were 900 \({}^{\circ}\)C, 1100 \({}^{\circ}\)C, 1300 \({}^{\circ}\)C, and 1500 \({}^{\circ}\)C; and the excess air coefficients were 0.9, 1.0, and 1.1. As shown in Figure 4, when the excess air coefficient was 0.9, the amount of NO production was very low, and it was almost zero above 1100 \({}^{\circ}\)C.  
With the temperature and excess air coefficient set in the experiment, the amount of NO produced by NH\({}_{3}\)/CH\({}_{4}\) blended gas combustion first increased and then decreased with the increase in the proportion of ammonia in the fuel gas, and the emission of NO reached the maximum when the proportion of ammonia was about 40%. [PERSON] et al. [33] and [PERSON] [34] found similar results. In NH\({}_{3}\)-CH\({}_{4}\)-air combustion, NO emissions increased sharply with the increase in NH\({}_{3}\) fuel ratio. However, from a certain point near the NH\({}_{3}\) fuel ratio of 0.65, NO decreased as the NH\({}_{3}\) fuel ratio further increased. [PERSON] [35] showed that high levels of fuel nitrogen led to low fractions of NO. When the excess air coefficient \(a\geq 1\), the main production path of amino NH\({}_{2}\) is [36]  
\[\text{NH}_{3}+\text{OH}=\text{NH}_{2}+\text{H}_{2}\text{O} \tag{1}\]  
\[\text{NH}_{3}+\text{O}=\text{NH}_{2}+\text{OH} \tag{2}\]  
The reaction product, NH\({}_{2}\) free radical, reacts with O atom to produce an important intermediate HNO, which is the main way that HNO is formed under dilute combustion conditions, and the HNO group is subsequently converted to produce NO. When the  
Figure 4: NO production of NH\({}_{3}\)/CH\({}_{4}\) mixed firing under different ammonia ratios. proportion of ammonia is less than 40%, the main rate-limiting reaction of ammonia oxidation path in methane-ammonia flame is [23]  
\[\text{NO}+\text{NH}_{2}=\text{NNH}+\text{OH} \tag{3}\]  
\[\text{NH}_{2}+\text{O}=\text{HNO}+\text{H} \tag{4}\]  
\[\text{HNO}+\text{H}=\text{NO}+\text{H}_{2} \tag{5}\]  
In the ammonia oxidation process, Reactions (3)-(5) exert significant influence on the concentration of H and OH radicals in the methane-ammonia flame. When the methane concentration decreases, the ammonia concentration increases, resulting in higher concentrations of H and OH radicals. This promotes the reverse progression of Reactions (3) and (4), leading to an increase in NO production. When the proportion of ammonia in the fuel gas exceeds 40%, the reducibility of ammonia to NO becomes prominent. Thus, an increase in ammonia concentration promotes the formation of the NH\({}_{x}\) group and facilitates the reduction of NO to N\({}_{2}\). The primary reactions taking place during this process are as follows:  
\[\text{NH}_{2}+\text{NO}=\text{NNH}+\text{OH} \tag{6}\]  
\[\text{NNH}+\text{NO}=\text{N}_{2}+\text{HNO} \tag{7}\]  
Figure 2:0  
Figure 2:1  
Figure 2:2  
Reactions (6) and (8) are the key to the mechanism of thermal NO removal (selective non-catalytic reduction of NO) in ammonia flame chemistry [36], and these NO reburning reactions are not associated with flames containing only trace amounts of ammonia. With the increase in NH\({}_{3}\) proportion and concentration, the positive occurrence of Reaction (6) and (8) is promoted, resulting in the decrease in NO production.