## 3 Results and Discussion  
### Quantitative Analysis of N and S Compositions in Gaseous and Liquid Phases  
In order to analyze the distribution of N and S compositions in closed loop reaction system, the dependence of outlet NO\({}_{2}\) and SO\({}_{2}\) concentrations with the absorption time were investigated, and the results are shown in Figure 2. Due to the high sulfite concentration, both NO\({}_{2}\) and SO\({}_{2}\) were not detected in the outlet flue gas during the initial absorption period of about 200 min, which suggests that nearly all of NO\({}_{2}\) and SO\({}_{2}\) are absorbed. The outlet NO\({}_{2}\) and SO\({}_{2}\) concentrations rapidly increase from 210 min to 230 min and are almost unchanged after 230 min. It is clear that the absorption equilibrium is established after a certain absorption time. These results also show that that the reaction between NO\({}_{2}\) and sulfite may be mainly controlled by diffusion in the initial reaction period, which is due to the sulfite concentration makes reaction rate is much faster than the diffusion rate [22; 26]. Figure 2 also shows that NO, as one of the absorption products, has similar time dependence to NO\({}_{2}\) and SO\({}_{2}\) concentrations. It was found that hardly any NO was detected during initial absorption period of about 200 min. NO concentration gradually increases from about 210 min to 230 min and almost keeps unchanged after 230 min. This can be explained as follows: in the initial absorption period, NO\({}_{2}\) can oxidize sulfite solutions directly (R1). However, with the depletion of sulfite solutions,NO\({}_{2}\) absorption in H\({}_{2}\)O (R2) occurs, and HNO\({}_{2}\) may be formed. Generally, HNO\({}_{2}\) is unstable in its liquid phase and can decompose into NO as R3 [22; 27].  
\[2\text{NO}_{2}+\text{SO}_{3}{}^{2-}+\text{H}_{2}\text{O}\to 2\text{NO}_{2}{}^{-}+ \text{SO}_{4}{}^{2-}+2\text{H}^{+}\] (R1) \[2\text{NO}_{2}+\text{H}_{2}\text{O}\to\text{HNO}_{2}+\text{HNO}_{3}\] (R2) \[3\text{HNO}_{2}\to 2\text{NO}+\text{H}_{2}\text{O}+\text{HNO}_{3}\] (R3)  
NO\({}_{3}{}^{-}\), NO\({}_{2}{}^{-}\), SO\({}_{3}{}^{2-}\), and SO\({}_{4}{}^{2-}\), which are main reactants or products of liquid phase, are synchronously analyzed by IC at the absorption times of 30 min, 60 min, 210 min, 220 min, 230 min and 300 min to better understand the chemical behaviors of NO\({}_{2}\) and SO\({}_{2}\) absorption in sulfite solution. As shown in Figure 3, NO\({}_{3}{}^{-}\), NO\({}_{2}{}^{-}\), and SO\({}_{4}{}^{2-}\) concentrations in absorption solution increase as the absorption proceeds. Compared with NO\({}_{2}{}^{-}\), more NO\({}_{3}{}^{-}\) was formed at the same absorption time, which can be ascribed to the oxidation of NO\({}_{2}{}^{-}\) and the decomposition of unstable HNO\({}_{2}\)[22; 28]. After the absorption time of about 300 min, NO\({}_{2}{}^{-}\) concentration is almost unchanged although NO\({}_{3}{}^{-}\) ions still increase, which is attributed to the fact that nitrite ions are further oxidized into nitrate ions.  
Figure 3 also shows that the sulfite concentration gradually decreases as the absorption proceeds and are almost unavailable after 220 min, which results in a sharp decrease of NO\({}_{2}\) absorption performance. It indicates that sulfite concentration is closely related with the performance of the removal of NO\({}_{2}\), which agrees well with the findings shown in Figure 2. The sulfite is consumed because of its reaction with NO\({}_{2}\) or O\({}_{2}\). For NO\({}_{2}\) absorption in sulfite solution, the sulfate is formed through the reaction between NO\({}_{2}\) or O\({}_{2}\) and sulfite (R1) [29].  
Figure 2: The dependence of outlet NO\({}_{2}\), SO\({}_{2}\), and NO concentrations with the absorption time.