## 3 Results and Discussion
### Effect of AC on NO\({}_{x}\)  
There are three ways to generate NO\({}_{\rm x}\), including fuel NO\({}_{\rm x}\), prompt NO\({}_{\rm x}\), and thermal NO\({}_{\rm x}\)[26]. Due to the low nitrogen content in diesel fuel, the fuel NO\({}_{\rm x}\) in diesel exhaust is almost negligible. Prompt NO\({}_{\rm x}\) can be produced at relatively low temperatures, but the proportion is generally small. Thermal NO\({}_{\rm x}\) accounts for the highest proportion, and the higher the temperature is, the more NO\({}_{\rm x}\) will be produced. Figure 6 shows the NO\({}_{\rm x}\) concentration of the reactor inlet at different powers. It can be seen from Figure 3 that the temperature of the exhaust gas increased with the power. Therefore, the concentration of NO gradually increased with the power, and the NO concentration at 15 kW was up to 1079 \(\mu\)L/L, as shown in Figure 6. It is generally believed that NO can be rapidly converted to NO\({}_{\rm 2}\) in the flame zone, and the generated NO\({}_{\rm 2}\) can be preserved after being mixed with a cold gas. Therefore, the ratio of NO\({}_{\rm 2}\):NO was higher in the low-load operation of the diesel engine. It can be seen from Figure 6 that the concentration of NO\({}_{\rm 2}\) was high at a low load and the highest was 72.2 \(\mu\)L/L (3 kW), and then the concentration of NO\({}_{\rm 2}\) gradually decreased with the power. The NO\({}_{\rm x}\) concentration in the diesel exhaust at 15 kW was up to 1101 \(\mu\)L/L. Figure 7 shows the change of NO\({}_{\rm x}\) concentration at the outlet of the AC reactor when urea is not sprayed. The dynamic change process and the outlet NO\({}_{\rm x}\) stability value are shown in Figure 7a,b,  
Figure 5: Effect of AC on CO.  
Figure 6: NO\({}_{\rm x}\) concentration of AC reactor inlet. respectively. During the experiment process, the diesel engine power gradually increased from 0 kW to 15 kW, and each power point was kept for a certain period of time to ensure that the outlet NOx concentration was basically stable. It can be seen from Figure 7 that the NO2 concentration of the outlet was very low at low power (0-6 kW), and close to zero at medium and high power (9-15 kW), indicating that the NO2 removal performance of activated carbon was good. The NO concentration increased gradually with the power in the entire power range. There was a certain degree of NO reduction compared to the inlet NO concentration. The denitration efficiency was 34.5% at 6 kW. The AC reacted with O2 in the exhaust to form the oxygen-containing functional groups, Cf. These oxygen-containing functional groups are the main active sites of AC, and are closely related to the adsorption and catalytic properties of AC [27; 28; 29; 30]. AC can be both a catalyst and a reducing agent in the denitration process. In addition, the reducing CO and CxHy in the exhaust also act as a reducing agent at the surface active sites of the AC. The possible reactions include Equations (6)-(9). \[C+O_{2}\longrightarrow C_{f} \tag{6}\]  
\[CO+NO\alpha\xrightarrow{C_{f}}N_{2}+CO_{2} \tag{7}\]  
\[CxHy+NO\alpha+O_{2}\xrightarrow{C_{f}}N_{2}+H_{2}O+CO_{2} \tag{8}\]  
\[CxHy+NO\alpha+O_{2}\xrightarrow{C_{f}}N_{2}+H_{2}O+CO \tag{9}\]  
Figure 8 shows the change of NOx concentration at the outlet of the AC reactor when urea was injected. Since the urea hydrolysis temperature was above 150 \({}^{\circ}\)C [31; 32] and the exhaust gas temperature of the diesel engine used in this experiment was below 150 \({}^{\circ}\)C when the power is lower than 3 kW (shown as Figure 3), no urea was injected into the reactor below 3 kW. According to the calculation method given in Section 2.2, the injection amounts of urea at different powers in the range of 6 to 9 kW are shown in Table 5. After the injection of urea, it can be seen from Figure 8 that the NO2 concentration at the outlet was essentially zero, while the NO concentration increased with the power. The NOx concentration was reduced to some extent, and the denitration efficiency was 44.8% at 15 kW.