## 3 Results and Discussion
### Discharge Power in the DBD Reactor  
The discharge power in the DBD reactor has an important influence on the oxidation of NO and SO\({}_{2}\)[30; 31]. The desulfurization and denitration efficiency levels of the system at various discharge power levels are presented in Figure 9. The experimental results show that the desulfurization efficiency is always maintained at 99.7-100%, while the denitration efficiency increases significantly from 14% to 94.5% when the discharge power in the DBD reactor increases from 10.9 W to 26.6 W. With further increases in the discharge power, the denitration efficiency increases slowly.  
SO\({}_{2}\) is very easily absorbed by the alkali solution, while WFGD alone can achieve high desulfurization efficiency; therefore, the discharge power in the DBD reactor has little effect on SO\({}_{2}\) removal. The denitration efficiency is significantly affected by the discharge power, as shown in Figure 9.  
Figure 10 shows the effects of the discharge power on NO, NO\({}_{2}\), and NO\({}_{x}\) concentrations and the oxidation degree of NO\({}_{x}\) at the outlet of the DBD reactor. With increases in DBD power, the concentrations of NO\({}_{x}\) and NO decrease significantly, the concentration of NO\({}_{2}\) increases, and the oxidation degree of NO\({}_{x}\) increases gradually. The preoxidation of DBD means most of the is NO directly oxidized to HNO\({}_{3}\) and HNO\({}_{2}\), then neutralized with the NaOH solution in the scrubber to be removed.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Mass Fraction of NaOH Solution /\%** & 0 & 0.5 & 1.0 & 1.5 \\ \hline
**pH Value** & 7.0 & 13.1 & 13.4 & 13.5 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The pH values of various mass fractions of NaOH solutions.  
Figure 9: Effects of discharge power on desulfurization and denitration efficiency levels (operating conditions: gas flow rate = 5 L/min, scrubbing solution = 1% NaOH solution, spray density = 7.6 m\({}^{3}\)/(m\({}^{2}\cdot\)h), packing type = D\({}_{g}\) 6 mm Dixon ring, packing height = 80 cm).  
In addition, some of the NO is oxidized to NO\({}_{2}\) by DBD. NO\({}_{2}\) is easily removed by the NaOH solution and can react with NO to generate N\({}_{2}\)O\({}_{3}\) and promote the removal of NO, such as in Equations (11)-(18) [12; 35]. When the DBD power increases from 10.9 W to 26.6 W, the oxidation degree of NO\({}_{x}\) increases rapidly from 2.4% to 41.0% and the removal of NO\({}_{x}\) is greatly promoted. Previous research has shown that when the NO\({}_{x}\) oxidation degree is 50% under anaerobic conditions, the absorption rate of NO\({}_{x}\) in NaOH solution is the highest [34]. Under the oxygen enrichment conditions, the oxidation of NO\({}_{x}\) will still occur in the packed scrubber; therefore, when the oxidation degree of NO\({}_{x}\) reaches 41%, high denitration efficiency can be achieved and further increases in the NO\({}_{x}\) oxidation degree have little impact on the denitration efficiency.  
\[2\text{NO}_{2}\rightleftharpoons\text{N}_{2}\text{O}_{4} \tag{11}\]  
\[2\text{NO}_{2}+\text{H}_{2}\text{O}\rightleftharpoons\text{HNO}_{3}+\text{ HNO}_{2} \tag{12}\]  
\[\text{N}_{2}\text{O}_{4}+2\text{NaOH}\rightarrow\text{NaNO}_{2}+\text{NaNO}_{3 }+\text{H}_{2}\text{O} \tag{13}\]  
\[\text{NO}+\text{NO}_{2}\leftrightarrow\text{N}_{2}\text{O}_{3} \tag{14}\]  
\[\text{N}_{2}\text{O}_{3}+2\text{NaOH}\to 2\text{NaNO}_{2}+\text{H}_{2} \text{O} \tag{15}\]  
\[\text{NO}+\text{NO}_{2}+\text{H}_{2}\text{O}\rightleftharpoons 2\text{HNO}_{2} \tag{16}\]  
\[\text{NaOH}+\text{HNO}_{2}\rightleftharpoons\text{NaNO}_{2}+\text{H}_{2} \text{O} \tag{17}\]  
\[\text{NaOH}+\text{HNO}_{3}\rightleftharpoons\text{NaNO}_{3}+\text{H}_{2} \text{O} \tag{18}\]