# Nitrogen Oxide Removal by Coal-Based Activated Carbon for a Marine Diesel Engine
## 3 Results and Discussion
### Effect of AC on Single-Component NO\({}_{2}\) and NO\({}_{2}\) + NH\({}_{3}\)  
The exhaust gas of diesel engine contains a certain concentration of NO\({}_{2}\). The effects of AC on single-component NO\({}_{2}\) and NO\({}_{2}\) + NH\({}_{3}\) are the basis for the study of fast SCR reaction Equation (2). Therefore, this section will discuss the effects of AC on single NO\({}_{2}\) and NO\({}_{2}\) + NH\({}_{3}\) separately.  
As shown in Figure 5, the experiment is divided into four stages, namely, 15 \({}^{\circ}\)C (1 st stage), 15-100 \({}^{\circ}\)C (2 nd stage), 100-200 \({}^{\circ}\)C (3 rd stage), and 200-300 \({}^{\circ}\)C (4 th stage). The inlet gas is 500 \(\mu\)L/L NO\({}_{2}\)/N\({}_{2}\). NO\({}_{2}\) is not detected in the outlet during the experiment, which indicates that the NO\({}_{2}\) removal ability of AC is excellent at 15-300 \({}^{\circ}\)C. Interestingly, NO is detected in the outlet and the concentration of NO increases gradually with the temperature. There is a large amount of NO desorbed during the heating process. During the process of heating from 100 \({}^{\circ}\)C to 200 \({}^{\circ}\)C, the concentration of desorbed NO can reach up to 2800 \(\mu\)L/L. When the temperature remains stable at 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, respectively, NO\({}_{2}\) is almost completely converted to NO, which is then desorbed. CO and CO\({}_{2}\) may be produced  
Figure 4: Reaction mechanism of NO reduction with NH\({}_{3}\) on the AC surface.  
Figure 3: Effect of AC in NO/NH\({}_{3}\)/N\({}_{2}\) system (1000 \(\mu\)L/L NO + 1000 \(\mu\)L/L NH\({}_{3}\)).  
when the temperature is above 100 \({}^{\circ}\)C. Since the flue gas analyzer cannot measure CO\({}_{2}\), the amount of CO\({}_{2}\) needs further study. The likely reactions are Equations (8)-(10) [29; 30].  
Then we carried out the experiment with NO\({}_{2}\) + NH\({}_{3}\). As shown in Figure 6, the experiment is divided into five stages, namely, 15 \({}^{\circ}\)C without AC (1 st stage), 15 \({}^{\circ}\)C with AC (2 nd stage), 15-100 \({}^{\circ}\)C with AC (3 rd stage), 100-200 \({}^{\circ}\)C with AC (4 th stage), and 200-300 \({}^{\circ}\)C with AC (5 th stage). The inlet gas is 500 \(\mu\)L/L NO\({}_{2}\) + 500 \(\mu\)L/L NH\({}_{3}\)/N\({}_{2}\). NH\({}_{3}\) can react with NO\({}_{2}\) at room temperature [31]. It can be seen from Figure 6 that NO\({}_{2}\) gradually decreases without AC in the first stage, that is, the reaction takes place between NO\({}_{2}\) and NH\({}_{3}\), and the products include N\({}_{2}\) and NO. When the gas is passed into the AC reactor (2 nd-5 th stages), NO\({}_{2}\) almost completely disappears. During the heating process, NO desorbs, resulting in an increase in the concentration of NO at the outlet. When the temperature is stable, the temperature is higher and the lower the outlet NO is acquired. When the temperature is higher than 200 \({}^{\circ}\)C, the removal efficiency of NOx by AC is more than 95% in the presence of NH\({}_{3}\).  
In summary, this study suggests that when there is no NH\({}_{3}\), AC converts NO\({}_{2}\) into NO, and the concentration of NO in the outlet increases with temperature. When NH\({}_{3}\) is added, AC catalyzes NO\({}_{2}\) and NH\({}_{3}\) to generate N\({}_{2}\), i.e., Equation (11). The catalytic activity of AC increases gradually with the  
Figure 5: Effect of AC on NO\({}_{2}\).  
Figure 6: Effect of AC on NO\({}_{2}\) + NH\({}_{3}\).  
temperature. In addition, the desorbed NO participates in the rapid SCR reaction Equation (2), thereby further improving the denitration ability of AC.  
\[\text{C}_{\text{f}}+\text{NO}_{2}\rightarrow\text{NO}+\text{C}_{\text{f}}\text{-O} \tag{8}\]  
\[\text{C}_{\text{f}}+2\text{NO}_{2}\rightarrow 2\text{NO}+\text{CO}_{2} \tag{9}\]  
\[\text{Cf}+\text{NO}_{2}\rightarrow\text{NO}+\text{CO} \tag{10}\]  
\[8\text{NH}_{3}+6\text{NO}_{2}\rightarrow 7\text{N}_{2}+12\text{H}_{2}\text{O} \tag{11}\]