# Nitrogen Oxide Removal by Coal-Based Activated Carbon for a Marine Diesel Engine
## 3 Results and Discussion  
### Effect of AC on Single-Component NO  
The NO adsorption on the surface of AC is the premise of AC denitration. The temperature has a great influence on the AC denitration ability. Therefore, this paper first studied the removal ability of AC on single component NO in the range of 15-300 \({}^{\circ}\)C. As shown in Figure 2, there are four stages according to the temperature, which are 15 \({}^{\circ}\)C, 15-100 \({}^{\circ}\)C, 100-200 \({}^{\circ}\)C, and 200-300 \({}^{\circ}\)C, and the inlet gas is 1000 \(\upmu\)L/L NO/N\({}_{2}\). The outlet NO reaches 970 \(\upmu\)L/L within 10 min at 15 \({}^{\circ}\)C. Modified activated carbon fiber (ACF) was used to investigate ACF's denitration ability at 40 \({}^{\circ}\)C. When the inlet gas is 900 \(\upmu\)L/L NO/N\({}_{2}\), the outlet NO reaches 800 \(\upmu\)L/L within 5 min and then increases slowly [17]. Our results are similar to his. We know that low temperature contributes to the physical adsorption of AC, and generally ACF has a larger specific surface area than AC. So ACF's physical adsorption ability is stronger than AC's. However, the NO adsorption amounts of both ACF and AC are still limited at low temperatures. With the increase in temperature, the physical adsorption ability of AC gradually deteriorates, and the chemisorption ability and catalytic ability gradually improve. Therefore, in order to improve the denitration ability of AC, it is also necessary to optimize its catalytic ability. Figure 2: Effect of activated carbon in NO/N\({}_{2}\) system (1000 \(\upmu\)L/L NO). During the heating process, the outlet NO will increase slightly due to the partial desorption of the adsorbed NO. However, as the temperature increases, the chemisorption ability of AC increases, and more NO is absorbed by chemisorption. Therefore, the outlet NO is decreased. When the temperature is kept constant, the adsorption-activated sites on the surface of the AC gradually decrease over time, leading to an increasing concentration of NO at the outlet. However, as the temperature gradually increases from 15 \({}^{\circ}\)C to 300 \({}^{\circ}\)C, the catalytic ability of AC increases, and so does the denitration efficiency. It can be seen from Figure 2 that the near-stable denitration efficiencies of AC at 100 \({}^{\circ}\)C, 200 \({}^{\circ}\)C, and 300 \({}^{\circ}\)C are 2%, 3.6%, and 5.8%, respectively. The chemisorption and catalytic abilities of AC are closely related to the oxygen-containing functional groups on the surface of AC [18]. The contents of oxygen-containing functional groups on the surface of AC were measured by the Boehm titration method [19; 20; 21] in this paper, and the results are shown in Table 1. It can be seen that the coal-based AC is acidic, and the phenolic hydroxyl group has a relatively higher proportion. Several papers [22; 23; 24] studied the decomposition temperature of different oxygen-containing functional groups on the surface of AC. It can be seen from Figure 2 that CO is generated when the temperature is between 100 \({}^{\circ}\)C and 200 \({}^{\circ}\)C. However, when the temperature is kept at 200 \({}^{\circ}\)C, the CO concentration basically does not change. In the temperature range of 100-200 \({}^{\circ}\)C, it is mainly the carboxyl group decomposing [23]. When the temperature is between 200 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, the experimental phenomenon is similar to that of the third stage, and both the CO concentration and the denitration efficiency are higher. In the temperature range of 200-300 \({}^{\circ}\)C, this is mainly due to the decomposition of the carboxyl group and the lactone group [23; 24]. The decomposition temperature of the phenolic hydroxyl group is generally higher than 600 \({}^{\circ}\)C [24], so it does not decompose under the experimental conditions herein. These oxygen-containing functional groups may react with NO, such as Equation (5) generating CO and Equation (6) generating CO\({}_{2}\) (C\({}_{\text{f}}\) represents the activated sites).