## 3 Results and Discussion
### Denitration Rates of the Three Biochars
#### 3.2.2 Effect of O\({}_{2}\) Concentration  
Based on the above conclusions, RHA, BC, and SC were selected to investigate the effect of 0.5%-2% O\({}_{2}\) on the denitration rate at the reaction temperatures of 850 \({}^{\circ}\)C, 900 \({}^{\circ}\)C, and 900 \({}^{\circ}\)C, as shown in Figure 4. The initial denitration rates of RHA were 94.2% at 0.5% of O\({}_{2}\) concentration, 92.7% at 1%, and 84.5% at 2%. As shown in Figure 4a, when O\({}_{2}\) concentration was 0.5% and 1%, the denitration rate decreased slowly with time. However, the initial denitration rate was lower when the oxygen concentration was 2%, and its decline was faster, so that a 36% denitration rate was reached after 10 min. As shown in Figure 4b, the denitration rate of BC increased first and then decreased with the increase of O\({}_{2}\) concentration. The initial denitration rates of BC were 84.9% at 0.5%, 98.9% at 1%, and 72.7% at 2% of O\({}_{2}\) concentration, respectively. When the oxygen concentration was low,  
Figure 3: Effect of the reaction temperature on the denitration rate of (**a**) RHA, (**b**) BC, (**c**) SC. nitrogen oxides were adsorbed on the surface of BC, occupying some reaction sites and resulting in a weaker C-NO reaction. When the oxygen concentration was high, more reaction sites caused C-NO to react more easily. The initial denitration rate was lower at 2% oxygen concentration. As shown in Figure 5b, as the oxygen concentration increased, the relative content of CO\({}_{2}\) increased. At 2% oxygen concentration, the reaction produced more CO\({}_{2}\), which suppressed the reduction reaction and decreased CO production. As shown in Figure 4c, the initial denitration rates were almost the same t different oxygen concentrations. The denitration rate declined faster with time under higher O\({}_{2}\) concentrations. Based on the above data, the optimal reaction conditions and denitration rates of the three biochar are shown in Table 2. Both RHA and SC presented the optimal denitration rates when O\({}_{2}\) concentration was 0.5%, whereas the O\({}_{2}\) concentration allowing the optimal denitration rate by BC was 1%. From the above analysis, it appears that the specific surface area and the surface morphology have no important effect on the denitration rate. The SEM pictures of the three biochars show that they had similar surface morphology. The specific surface area and pore volume of BC are smaller than those of the other two biochars. The O element was hardly absorbed on the surface of this biochar at low O\({}_{2}\) concentration, resulting in lower efficiency. At a higher concentration, O\({}_{2}\) reacted with the biochar, which generated large amounts of carbon oxides and inhibited C-NO reduction. Therefore, the optimal \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Reaction Temperature/\({}^{\circ}\)C** & **Oxygen Concentration/\%** & **D/\%** \\ \hline RHA & 850 & 0.5 & 94.2 \\ BC & 900 & 1 & 98.9 \\ SC & 900 & 0.5 & 92.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Optimum reaction conditions and denitration rate of carbon materials. Figure 4: Effect of O\({}_{2}\) concentration on the denitration rate of (**a**) RHA, (**b**) BC, (**c**) SC. O\({}_{2}\) concentration was 1% for BC. In contrast, RHA and SC have a large specific surface area and pore volume, which allowed them to achieve a higher denitration efficiency at a lower O\({}_{2}\) concentrations. BC presented the largest initial denitration rate with the least specific surface area, while RHA and SC behaved the opposite. Therefore, at a high reaction temperature, the specific surface area has no important effect on the denitration rate of biochar. The larger the specific surface area, the lower the O\({}_{2}\) concentration required for the biochar to reach the optimal denitration rate.